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doi:10.14379/iodp.pr.349.2014

Site summaries

Site U1431

Background and objectives

Site U1431 (proposed Site SCS-3G) is located near the relict spreading ridge where the youngest crustal magnetic anomalies are observed in the East Subbasin of the SCS (Figs. F4, F8). A positive magnetic anomaly that runs through this site allows regional correlation of crustal ages. This site is also surrounded by abyssal highs in the ocean crust, as well as younger seamounts (Figs. F6, F8B) whose volcanic and/or redepositional events may be recorded by sediments recovered from this site.

The primary objective at Site U1431 was to core into the oceanic basement to determine the age at which seafloor spreading ceased in the East Subbasin. The ~900 m thick package of sediment overlying basement also provides important constraints on the evolution of the ridge and associated late-stage magmatism, deep-marine sedimentary processes, and the paleoceanographic history following the termination of spreading in the SCS. Additionally, coring at this site will allow correlation of biostratigraphic, magnetostratigraphic, and radiometric ages to the observed crustal magnetic anomalies. Physical property and paleomagnetism measurements of basement rock will help to elucidate the cause of the distinct contrasts in the nature of oceanic crust magnetic anomalies of the East and Southwest Subbasins. Furthermore, this site will provide constraints on mantle source, melting, and magmatic processes in the latest stages of basin formation. Physical property measurements of core samples and wireline logging measurements will provide stratigraphic information for correlation with regional seismic profiles. Microbiological sampling will explore the deep biosphere in the SCS to examine how sharp changes in lithology (interfaces) may affect subsurface community structure and function, as well as how posteruption processes might have influenced past ecosystems in the SCS.

Operations

After a 463 nmi transit from Hong Kong averaging 11.0 kt, the vessel stabilized over Site U1431 at 0640 h (UTC + 8 h) on 31 January 2014. We cored five holes at Site U1431 (Table T1). The original operations plan called for one hole to a depth of ~1061 mbsf, which included ~100 m of basement. The plan was modified during transit to include two additional short holes for high-resolution sampling of the upper ~20 m of section. Hole U1431A was successfully cored to 28.4 mbsf and Hole U1431B to 17.0 mbsf. After the first core from Hole U1431C retrieved a split core liner and no mudline, we opted to abandon the hole, which was completed to a depth of 14.2 mbsf, and spudded Hole U1431D. Hole U1431D was cored to 617.0 mbsf when the XCB failed, leaving the cutting shoe, core catcher sub assembly, and breakoff sub in the hole. We abandoned Hole U1431D and switched to the RCB to spud Hole U1431E, which was drilled to 507.0 mbsf, spot cored, and then cored continuously from 575.0 mbsf to total depth at 1008.8 mbsf in igneous basement. After conditioning the hole for logging, two logging runs were performed. The triple combo tool string was run to 463.0 m wireline depth below seafloor (WSF), and the FMS-sonic tool string was run to 444 m WSF with two passes. Total time spent at Site U1431 was 385.7 h (16.1 days).

A total of 122 cores were collected at this site. The advanced piston corer (APC) was deployed 26 times, recovering 225.61 m of core over 228.50 m of penetration (98.7% recovery). The XCB was deployed 48 times, recovering 236.50 m of core over 448.10 m (52.8% recovery). The RCB was deployed 48 times, recovering 243.00 m of core over 443.5 m of penetration (54.8% recovery).

Principal results

The cored section at Site U1431 is divided into 11 lithostratigraphic units, 9 sedimentary and 2 igneous, based mainly on a combination of data from Holes U1431D and U1431E (Fig. F9). Lithostratigraphic Unit I is a 101.16 m thick Pleistocene sequence of dark greenish gray clay and silty clay. Graded silt intervals are abundant and interpreted as turbidites. Discrete volcanic ash layers that are either mafic or felsic in composition and 0.5–5 cm thick occur throughout the unit. This unit is underlain by Unit II (Pliocene–Pleistocene age), which is divided into Subunits IIA (101.16–194.95 mbsf) and IIB (194.95–267.82 mbsf). The 166.66 m of Unit II is dominated by dark greenish gray clay with fewer volcanic tephra layers than Unit I. Subunit IIA is characterized by the presence of clay with nannofossils and calcareous turbidites, which are not found in Subunit IIB. Rare, thin silt turbidites are largely limited to Subunit IIB. Unit III (267.82–326.12 mbsf) is a 58.30 m thick upper Miocene to Pliocene sequence of dark greenish gray clay with modest amounts of interbedded calcareous turbidites. These graded turbidites typically have sandy foraminifer-rich intervals at the base and are interpreted to represent mass wasting events from neighboring seamounts. Unit IV (326.12–412.42 mbsf) is an upper Miocene unit comprising 86.30 m of dark greenish gray clay and silty clay with minor amounts of silt and fine sand interbeds interpreted as turbidites. This unit is much reduced in carbonate content compared to overlying Unit III. Unit V (412.42–603.42 mbsf) is a 191 m thick sequence of upper Miocene dark greenish gray silty sand and interbedded clay with nannofossil ooze. Recovery is low throughout the section, but sandy core catcher samples suggest that many of the unrecovered intervals may consist of sand.

Unit VI (603.42–797.30 mbsf) is readily distinguished from the overlying units by the abundance of greenish black volcaniclastic breccia and sandstone interbedded with minor amounts of claystone. This unit is 193.88 m thick and dated to the late Miocene. The clasts in this breccia are primarily composed of highly vesicular aphanitic basalt and scoria, nonvesicular to sparsely vesicular basalt, basaltic glass shards, and lesser amounts of pumice and mudstone. Major element data indicate that these clasts are characteristic of ocean island basalt (OIB). The breccia beds are typically massive and have erosive bases, indicative of deposition by mass wasting either as debris or grain flows. Based on the composition of the clasts and abundant magmatic mineral fragments, these deposits are likely sourced from the nearby seamounts. Unit VII (797.30–885.25 mbsf), 87.95 m thick and middle to late Miocene in age, is composed of interbedded dark greenish gray sandstone with lesser amounts of siltstone and claystone in a turbidite sequence. It is essentially a less coarse grained equivalent to Unit VI and coarsens uphole through the unit. Unit VIII (885.25–889.88 mbsf) is a 4.63 m thick middle Miocene sequence of massive, dark olive-brown claystone that directly overlies the basalt of Unit IX (889.88–962.51 mbsf). The mudstone represents deep-marine sedimentation. Unit X (962.51–972.00 mbsf) is a 9.49 m thick sequence of lower Miocene yellowish brown claystone and claystone breccia that lies within the volcanic sequence. This unit is underlain by the basalt of Unit XI (972.00–1007.89 mbsf).

Calcareous nannofossils, planktonic foraminifers, and radiolarians recovered at Site U1431 are typical of low-latitude assemblages, characterized by species widely found in the tropical western Pacific region. Calcareous nannofossils are generally poorly preserved and frequent or common in Units I–IV but rare or absent downhole. Planktonic foraminifers are also poorly preserved and vary from frequent to rare in Units I–IV but are absent more frequently in samples from deeper units. Radiolarians are common and well preserved in samples from the uppermost 30 m, absent from 30 to 870 mbsf, and present but poorly preserved in Units VIII and X.

The biostratigraphy of Site U1431 is based on analysis of calcareous nannofossil, planktonic foraminifer, and radiolarian assemblages in all core catcher samples and additional samples from within cores from Holes U1431D and U1431E. The sedimentary succession recovered at Site U1431 spans the lower Miocene through Pleistocene (Fig. F10). Sediment from Units I–VIII is assigned to middle Miocene to Pleistocene calcareous nannofossil Zones NN6–NN21 and planktonic foraminifer Zones M9–Pt1, with no obvious hiatuses. The Pliocene/Pleistocene boundary is located between Cores 349-U1431D-15H and 18H, and the Miocene/Pliocene boundary is located between Cores 31X and 33X. Biostratigraphic control for the upper Miocene section is hampered by a paucity of nannofossils and planktonic foraminifers and poor core recovery in Units IV and V, which are dominated by turbidites. Nevertheless, the middle/late Miocene boundary is placed between Cores 349-U1431E-27R and 33R. In situ calcareous microfossils are absent from the claystones of Unit X; however, radiolarian biostratigraphy indicates that the rock is early Miocene in age (~16.7–17.5 Ma), corresponding to radiolarian Zone RN4. Sedimentation rates varied from ~8 cm/k.y. in the middle to early late Miocene, ~14 cm/k.y. for the remainder of the late Miocene, to ~5 cm/k.y. in the Pliocene–Pleistocene. Extremely low sedimentation rates (<2 cm/k.y.) occurred in the early to earliest middle Miocene during deposition of the claystones of Units VIII and X (Fig. F10).

The basalt of Unit IX was encountered at ~890 mbsf in Hole U1431E. Coring continued to ~1008 mbsf, recovering basement basalt separated by an interflow claystone between 3.7 and 9.5 m thick at 962.3 mbsf. In total, 46.2 m of basalt was recovered over a cored interval of 108.4 m, yielding an average recovery of 42.6%. The basalt is divided into 13 igneous lithologic units (Fig. F11) and is mainly composed of massive lava flows (six in Unit IX and two in Unit XI) as thick as ~26.7 m, with limited evidence for pillow basalt flows in between. Because no contacts between flow units were recovered, boundary locations and unit thickness estimates are approximate. The interpretation of igneous lithologic Units 1, 7–10, and 12 as pillow basalts is uncertain and is based on scarce evidence, such as the presence of glassy (curved) chilled margins, ropy flow structure, and a single occurrence of a hyaloclastite breccia.

Most basalt at Site U1431 is aphyric and ranges in grain size from microcrystalline to fine grained, with the groundmass grain size getting coarser (0.7–1 mm) in the cores of the thickest massive lava flows. All basalt has a phase assemblage of plagioclase and clinopyroxene (± olivine) in its groundmass, with 0.1–0.5 mm subhedral-euhedral olivine microphenocrysts present in some igneous lithologic units, resembling a typical mid-ocean-ridge basalt (MORB) crystallization history and, in conjunction with geochemical evidence, indicates that basalt recovered at Site U1431 is representative of typical MORB.

The alteration style of basalt at Site U1431 is typical of MORB. Alteration color is dominated by gray to dark gray-green and yellow to red-brown. Typical secondary minerals include saponite, Fe oxides, carbonate, and celadonite, which represent alteration assemblages at low temperature. Alteration intensity varies from slight to complete, but the majority of the recovered basement rock is moderately altered. There is no systematic change in the alteration nature (e.g., alteration color) with depth that might indicate a transition from more oxidizing to reducing conditions. The strongest alteration occurs in halos flanking veins, which overprints the background pervasive alteration, indicating that the overall distribution of alteration was controlled by fractures and vein structures. Most lithologic basement units include intervals with slight alteration, preserving remarkably fresh olivine crystals that show only limited alteration along their rims and “maschen” fractures.

Fractures and veins occur throughout the basalt in Hole U1431E. These features are randomly oriented, with no obvious offset or thickness variation. A fracture with ~1 cm of normal offset occurs in the interflow claystone of Unit X. The basalt fractures likely formed during cooling of the lava, whereas the fractures in the interflow claystone may suggest slight movement as the lava of Unit IX flowed over it. The major veins are white or reddish brown and filled with carbonate and iron oxides. Arched veins generally occur in sets, often in combination with linear veins, forming a vein network consistent with fractures formed during cooling.

Geochemistry measurements at Site U1431 aimed to characterize the interstitial water chemistry, total organic carbon (TOC), bulk carbonate content, and igneous basement rock. The depth profiles of major elements and nutrients indicate that organic matter diagenesis, biogenic carbonate dissolution and recrystallization, and volcanic ash alteration occurred in the sediment. The interstitial water never reaches complete sulfate reduction in Hole U1431D, with minimum concentrations of ~2.3 mM occurring from ~170 to 260 mbsf. This is consistent with the very low methane concentrations, which range from 1.6 to 4.8 ppmv. Sulfate concentrations gradually increase below 260 mbsf, reaching 24.0 mM at the bottom of Hole U1431D (~600 mbsf). Shore-based isotopic analysis of these interstitial water samples should constrain the source of the sulfate-bearing fluid in Hole U1431D.

Bulk carbonate content varies with depth, ranging from 0 to 47 wt% in Hole U1431D and from 0 to 57 wt% in Hole U1431E. The discrete intervals with higher carbonate content in Hole U1431D correspond to nannofossil ooze beds, whereas in Hole U1431E higher carbonate content is associated with diagenetic carbonate concretions visible in the cores. TOC varies from 0 to 4.7 wt% in Hole U1431D, whereas in Hole U1431E, TOC is lower and ranges from 0 to 0.74 wt%. The TOC to total nitrogen (C/N) ratio is generally <4 at Site U1431, indicating that TOC is derived from a marine source; however, C/N ratios range from 8 to 12 in some intervals of lithostratigraphic Units III and IV in Hole U1431D, which could indicate a mixture of marine and terrestrial organic matter sources.

Major and minor element concentrations measured by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) on Hole U1431D sediment indicate that it is most likely derived from an intermediate igneous source. The basalt recovered from below ~890 mbsf in Hole U1431E has moderately high loss on ignition (LOI) values (0.46–2.85 wt%), but low K₂O (£0.53 wt%) and TiO₂ (1.01–1.77 wt%). The basalt samples of Unit IX contain higher MgO, FeO, and Ni concentrations than those of Unit XI, likely because of olivine accumulation. The basalt major element composition is similar to that of mid-ocean-ridge tholeiite, whereas clasts from the volcaniclastic breccia are alkali basalt with high K₂O (1.08–2.67 wt%) and TiO₂ (2.10–3.13 wt%), probably sourced from the nearby seamounts (Figs. F12, F13).

A total of 105 whole-round samples (5–10 cm long) were collected for microbiological studies from Site U1431. These samples were typically taken adjacent to interstitial water whole-round samples for comparison to interstitial water chemistry when possible. These samples will be analyzed for microbial content based on DNA and lipid properties of the cells present. Subsamples were prepared for fluorescent in situ hybridization and single cell genomics. DNA and lipid samples were preserved at –80×C, whereas fluorescent in situ hybridization samples were preserved at –20×C. Four basalt whole-round samples were selected for cultivation-based studies, with sampled material inoculated into a seawater-based medium containing olivine as a source of energy. An additional 76 samples were collected and prepared for investigation of the microbiology of interfaces using lipid and nucleic acid analyses. These samples were collected mostly in the upper 200 m of Hole U1431D from specific interfaces, including five ash/clay interfaces and ten turbidite/clay interfaces. Selection of these samples was dependent upon recognition of key intervals by the core description team and occurred through consultation between the microbiologists and sedimentologists or petrologists.

Microbiology contamination testing at Site U1431 included the use of perfluorocarbon tracers (PFTs), fluorescent microspheres, and fluid community tracers (FCTs). PFTs were added to the drilling fluid for all APC coring in Holes U1431B–U1431D, as well as for the first four XCB cores in Hole U1431D. Twelve samples were taken from six sediment cores collected over this interval to measure contamination with the PFTs. Microspheres were added to the core catcher before the core barrel was deployed in Hole U1431E over the interval from 651.8 to 952.6 mbsf. Two microsphere samples were collected from each core, one from scraping of the core surface and one as a subsample from the interior of each whole-round sample. In addition, FCT samples were collected from the drilling fluids on a daily basis (for a total of 14) to track the microbial communities typical of seawater and other drilling mud constituents. Microbial community DNA and lipids from these fluids will be compared to those measurements made on the core samples to determine if there are microbes that can be recognized as contaminant taxa.

Variations in the natural remanent magnetization (NRM) intensity at Site U1431 are generally correlated with lithology. Paleomagnetic measurements indicate that the silty clay and clayey silt in Unit I (0–101.16 mbsf) have a mean NRM intensity on the order of 3 × 10^–2 A/m, whereas the clay with nannofossils in Unit II (101.16–267.82 mbsf) has somewhat higher NRM intensity (~6 × 10^–2 A/m). Many discrete peaks of higher NRM values that appear in some depth intervals in both Units I and II can be tied directly to the presence of volcanic ash layers. Magnetic susceptibility data also show positive peaks at these intervals. Overall, magnetic susceptibility and NRM intensity variations through the sedimentary units are closely correlated.

Magnetostratigraphic records at Site U1431 indicate the presence of several relatively well defined polarity intervals in the cores. Based on inclination and declination data, the Brunhes/Matuyama Chron boundary (0.781 Ma) is placed at ~46 mbsf in Hole U1431D (Fig. F10). The Matuyama Chron is defined between ~46 and ~135 mbsf. Below ~170 mbsf, the XCB cores are strongly overprinted by a drilling-induced remagnetization that cannot be removed by shipboard thermal demagnetization. The magnetic record improved in the RCB cores of Hole U1431E, allowing tentative correlation of certain parts of the magnetic polarity interval with the geomagnetic polarity timescale in conjunction with biostratigraphic constraints. In particular, the polarity shift from normal to reversed at ~716 mbsf may correspond to the Chron C5n/C5r boundary (11.056 Ma) (Fig. F10).

For basement rock units, the observed paleomagnetic signals cannot be directly linked to the geomagnetic polarity timescale yet because the basalts in lithostratigraphic Units IX and XI were erupted intermittently, and the pelagic clay sediment in lithostratigraphic Units VIII and X may represent a significant time interval. Nevertheless, reliable normal and reversed polarities occur within this interval, which indicates that the eruption of the basalt units may have spanned a significant amount of time on the order of a few thousand to one million years.

Whole-round cores from Holes U1431A–U1431C were measured for P-wave velocity, bulk density, magnetic susceptibility, and NGR. For Holes U1431D and U1431E, measurements were also made on whole-round cores, with additional measurements on split cores and discrete samples, including thermal conductivity, porosity, and bulk, dry, and grain densities. In general, the physical properties correlate with lithology, composition, and induration. In Hole U1431D, bulk density, P-wave velocity, shear strength, NGR, and thermal conductivity increase gradually with depth over the uppermost 150 mbsf (Fig. F9), whereas porosity measured on discrete samples decreases from 84% to 50% over the same depth range. This indicates that sediment compaction dominates the physical property variations above 150 mbsf. Volcanic ash layers in Unit I (e.g., at 25 and 100 mbsf) show relatively high magnetic susceptibility (300–500 SI) values. Below 150 mbsf, a decrease in shear strength may be associated with higher clay content. NGR counts are relatively high from the seafloor to 500 mbsf, which is consistent with the dominance of clay and silt in Units I–V.

Most physical properties show a significant change at ~550–600 mbsf, near the boundary between Units V and VI (Fig. F9). P-wave velocity and porosity increase, whereas NGR values and thermal conductivity are relatively low, a pattern consistent with the dominance of volcaniclastic breccia and sandstone in Unit VI. Layers with higher NGR counts and high magnetic susceptibility values occur at ~660 and ~710 mbsf. These do not correlate with the breccia but correspond to a silt- and/or sandstone probably enriched in magnetic minerals such as magnetite. The basalt units below 889.88 mbsf (Units IX and XI) display the lowest NGR, highest magnetic susceptibility, and largest bulk density values (Fig. F9). The interflow clay (Unit X) between the two basaltic units shows NGR values ~20 times larger than those of the basalt, as well as much lower magnetic susceptibility. Lower magnetic susceptibility and NGR values at the top of the basalt are consistent with higher but still moderate alteration in these basement units.

Two downhole logging tool strings were run in Hole U1431E, the triple combo (NGR, porosity, density, electrical resistivity, and magnetic susceptibility) and the FMS-sonic (NGR, sonic velocity, and electrical resistivity images) tool strings (Fig. F9). The triple combo tool string reached 464 m WSF before a bridge prevented access to the lower part of the borehole. The hole was wider than 17 inches below ~300 m WSF and showed closely spaced variations in borehole width above that depth. These were not ideal conditions for borehole log quality; however, stratigraphic changes are apparent in the NGR and magnetic susceptibility logs. The FMS-sonic tool string reached 444 m WSF, with two passes made above that depth. Downhole temperature measurements of the borehole fluid are consistent with the low geothermal gradient (~15°C/km) established from the advanced piston corer temperature tool (APCT-3) measurements taken on Cores 349-U1431D-4H, 7H, 10H, and 13H (Fig. F14).

Site U1432

Background and objectives

Site U1432 (proposed Site SCS-6A) is located ~60 km south of ODP Site 1148 (Wang, Prell, Blum, et al., 2000; Li et al., 2006; Wang and Li, 2009), just south of the northern continent/ocean boundary on the Chinese continental margin (Figs. F6, F8). This part of the basin shows the deepest basement and is likely the oldest among the subbasins based on magnetic anomalies (Taylor and Hayes, 1980, 1983; Pautot et al., 1986; Briais et al., 1993) (Fig. F4). This site was designed to recover the oldest oceanic crust and the oldest sedimentary rock in the East Subbasin to test the hypothesis that the onset of seafloor spreading in the SCS occurred here first at ~32 Ma. Magnetic anomaly 11, the oldest anomaly interpreted by Taylor and Hayes (1980) and Briais et al. (1993), passes near this site and hence would allow key calibration between ages estimated from magnetic anomalies and those from biostratigraphy, radiometric dating, and magnetostratigraphy.

The true nature of the continent–ocean transition and oceanic basement at this site is still speculative; there could be volcanic extrusions associated with early continental breakup and the onset of seafloor spreading, exhumed lower crustal materials from preferential lower crust extension, exhumed mantle materials, or even Mesozoic rock. Coring at this site was intended to help pinpoint the exact location and tectonic nature of the continent–ocean transition and address key problems in the early tectonic transition from rifting to drifting and associated paleoenvironmental changes, including

1. The age of basement (presumably the oldest oceanic crust) near the continent/ocean boundary;
2. Petrology and geochemistry of basement rock and their bearing on continental breakup, incipient seafloor spreading, and mantle evolution;
3. Physical properties of basement rock and their implications for interpreting sharp magnetic contrasts between different subbasins; and
4. Sedimentary, paleoceanographic, and ecosystem responses to the opening of the SCS.

As a result of operational challenges (see “Operations”), the objectives of sampling basement and basal sediment at Site U1432 were not achieved.

Operations

After a 181 nmi transit from Site U1431 averaging 11.0 kt, the vessel stabilized over Site U1432 at 2337 h (UTC + 8 h) on 16 February 2014. Site U1432 consisted of three holes (Table T1). The first hole was a planned jet-in test to determine the correct casing depth for the 20 inch casing string. The second hole was to consist of a reentry system with three strings of casing to ~900 mbsf, followed by coring to ~1930 mbsf. Because of poor weather conditions, an additional hole was piston cored while waiting on suitable weather to continue the reentry installation.

Hole U1432A was successfully jetted to 62.0 mbsf. A reentry system was then successfully installed to 787.1 mbsf in Hole U1432B. The final cement job on the last casing string compromised the reentry system when the drill string became stuck in the cement. The drill string had to be severed, forcing us to abandon Hole U1432B. Hole U1432C was successfully cored to 110.0 mbsf with the APC. Four downhole temperature measurements were taken in Hole U1432C with the APCT-3. A total of 12 APC cores were collected at this site, recovering 88.74 m of core over 110.0 m of penetration (81% recovery). The total time spent on Site U1432 was 492 h (17.9 days).

Principal results

Hole U1432C consists of 12 cores (Cores 349-U1432C-1H through 12H) that penetrated to 110.0 mbsf. The lithology is dominated by a sequence of dark greenish gray clay and clay with silt, assigned to lithostratigraphic Unit I (Fig. F15). Clay layers are interbedded with very thin bedded (centimeter scale) or laminated silty layers. These layers mostly fine upward and have sharp erosive bases. These graded sequences are generally 10–20 cm thick and are interpreted as distal turbidites. A 2.4 m thick unconsolidated sand layer occurs in the middle of the drilled section. The sand and silt layers represent <5% of the total recovered core and can usually be identified using magnetic susceptibility measurements, as they typically exhibit lower values than the clays. Thin volcanic ash layers (0.5–2.0 cm thick) occur occasionally in some cores.

The age of the sedimentary sequence recovered in Hole U1432C is <0.91 Ma (Middle–Late Pleistocene) based on planktonic foraminifer and calcareous nannofossil biostratigraphy (Fig. F16). Radiolarians are abundant and moderately to poorly preserved in the upper 15 m of the hole but become progressively rarer and more poorly preserved downhole and comprise a Pleistocene–Holocene assemblage. Nannofossil preservation is moderate to good throughout the hole, with considerable reworking of Pliocene and Miocene species above ~50 mbsf. Preservation of planktonic foraminifers is also moderate to good, with evidence of moderate dissolution, as indicated by frequent fragmentation. Planktonic foraminifers are more dominant in sandy intervals that also contain reworked Pliocene species and shallow-water benthic foraminifers.

A total of 16 whole-round samples (5 cm in length) were taken for interstitial water measurements in Hole U1432C. Geochemical analysis shows that sulfate is completely consumed at ~90 mbsf, coincident with maximum methane concentrations between 4650 and 4750 ppmv just below this depth. The absence of higher hydrocarbons suggests that the methane is primarily microbial in origin. TOC in the hole varies from 0.34 to 0.99 wt%, whereas CaCO₃ concentrations are generally low (<12 wt%).

Five whole-round samples and five interface samples were collected from Hole U1432C for DNA and lipid analysis. The five whole-round samples were also used to inoculate several types of microbiological media to test whether autotrophic and heterotrophic microbes can be grown. For heterotrophic culture enrichments, glucose, acetate, fumarate, and formate were used as sources of carbon and energy. For autotrophic culture enrichments, sodium bicarbonate and hydrogen were used as sources of carbon and energy, respectively. We also collected and preserved 200 mL of drilling fluid for FCT analysis. The microbial communities present in these samples will be compared to those present on the inside and outside of the cores to determine whether microbes in the drilling fluid behave as suitable contaminant tracers.

As seen at Site U1431, the NRM of samples from Hole U1432C contains a vertical component generated by the drilling process, which is easily removed by 5–10 mT alternating field (AF) demagnetization. A polarity reversal at ~105 mbsf is defined as the Brunhes/Matuyama Chron boundary (0.781 Ma) (Fig. F16). In the Brunhes Chron, there are two directional anomaly intervals at ~10 mbsf and between 50 and 70 mbsf. These anomalies could represent authentic magnetic excursions or could be caused by postdepositional disturbances. These magnetostratigraphic results, when combined with the biostratigraphy, indicate a higher sedimentation rate (~13.5 cm/k.y.) in Hole U1432C (Fig. F16) than for the same age interval in Hole U1431D (~5.8 cm/k.y.), which is consistent with its location closer to the continental margin.

Physical property measurements made on whole-round core sections were smoothed using a five-point (10 cm) moving average and combined with discrete sample measurements. Bulk density, P-wave velocity, magnetic susceptibility, NGR, thermal conductivity, and shear strength decrease with depth in the uppermost 50 m of Hole U1432C (Fig. F15), showing an inverse relationship with porosity. Variations in these records are lower below 50 mbsf. This indicates that the compaction effect dominates the physical properties in the uppermost part of Hole U1432C. The 2.5 m thick sand layer near 50 mbsf is clearly delineated by low NGR, low magnetic susceptibility, and higher P-wave velocity (Fig. F15).

Four APCT-3 downhole temperature measurements on Cores 349-U1432C-5H, 7H, 9H, and 11H indicate a geothermal gradient of 85°C/km (Fig. F14). Combining these temperatures with thermal conductivity measurements made on the sediment cores, the preliminary heat flow value at Hole U1432C is 94 mW/m². The geothermal gradient and heat flow values are similar to those at Site 1148, ~60 km to the north-northeast (Wang, Prell, Blum, et al., 2000) (Fig. F14).

Site U1433

Background and objectives

As a result of the marked contrast in magnetic anomaly amplitudes between the Southwest and East Subbasins of the South China Sea (Yao, 1995; Jin et al., 2002; Li et al., 2007, 2008), it is questionable whether rifting and drifting within these two subbasins occurred synchronously and how these subbasins evolved in comparison to the Northwest Subbasin. Site U1433 (proposed Site SCS-4B) is located in the Southwest Subbasin near the relict spreading center and magnetic anomaly C5d identified by Briais et al. (1993) (Figs. F4, F8). Together with Site U1431 in the East Subbasin, coring at Site U1433 should help to explain the sharp differences in magnetic amplitude between the East and Southwest Subbasins and test the hypothesis that in the Southwest Subbasin the breakup from continental rifting to seafloor spreading occurred more recently than in the East Subbasin (Pautot et al., 1986). Coring will help determine the age of this subbasin near the end of the spreading and correlate ages from magnetic anomalies with biostratigraphic, magnetostratigraphic, and radiometric ages. The apparent weak magnetization in basement rock (Li et al., 2008) will be examined through petrological and geochemical analyses and by measurements of magnetic susceptibility and remanent magnetization. The specific objectives at this site were to

1. Determine the termination age of spreading in the Southwest Subbasin and correlate it with ages of regional tectonic events;
2. Test the hypothesis that rifting and subsequent seafloor spreading in the Southwest Subbasin occurred later than that in the East Subbasin;
3. Measure magnetization, mineralization, and geochemical compositions of basement rocks to trace how the mantle evolved through time; and
4. Examine the paleoceanographic and sedimentary response to the opening of the South China Sea.

Operations

After a 334 nmi transit from Site U1432 averaging 11.2 kt, the vessel stabilized over Site U1433 at 0230 h (UTC + 8 h) on 8 March 2014. The original operations plan consisted of drilling one hole to a depth of ~965 mbsf, which included 100 m of basement. This plan was modified during transit in order to eliminate the use of a free-fall funnel and the XCB by coring two holes (Table T1). Hole U1433A was cored using the APC to refusal at 188.3 mbsf. Hole U1433B was drilled to 186.1 mbsf and then cored using the RCB. The sediment/basement interface was encountered at ~798.5 mbsf, and we advanced the hole by rotary coring into basement to a final depth of 858.5 mbsf. After conditioning the hole for logging, we deployed the modified triple combo tool string and the FMS-sonic tool string to 840 m WSF, with multiple passes made in the basement section of the hole with the latter tool.

A total of 94 cores were collected at this site. The APC was deployed 20 times, recovering 168.79 m of core over 188.3 m of penetration (89.6% recovery). The RCB system drilled one 186.1 m interval and collected 74 cores, recovering 443.04 m of core over 672.4 m of penetration (65.9% recovery). The overall recovery at Site U1433 was 71.1%. The total time spent on Site U1433 was 284.5 h (11.85 days).

Principal results

The cored section at Site U1433 is divided into four lithostratigraphic units (three sedimentary and one igneous) based on a combination of data from Holes U1433A and U1433B (Fig. F17). Lithostratigraphic Unit I is a 244.15 m thick sequence of Pleistocene dark greenish gray clay, silty clay, and clay with nannofossils. The clay is interbedded with small volumes of generally thin, graded quartzose silt and nannofossil ooze, both interpreted to be turbidite deposits that comprise <5% of the unit. This unit is underlain by middle Miocene to Pleistocene Unit II (244.15–747.93 mbsf), which is divided into two subunits: IIA (244.15–551.32 mbsf) and IIB (551.32–747.93 mbsf). The entire unit is 503.78 m thick and dominated by dark greenish gray clay with frequent graded carbonate interbeds, largely comprising nannofossil ooze and chalk that are characterized by sharp, erosive bases and gradational, bioturbated tops. In Subunit IIB, carbonate beds are sometimes substantially thicker, up to several meters, rather than <1 m and usually <50 cm in Subunit IIA. The carbonates are turbidite deposits with evidence of resedimentation from shallow-water regions based on the occurrence of benthic foraminifers that dwell in the photic zone. The lowermost sedimentary sequence, Unit III (747.93–796.67 mbsf), is a 48.74 m thick lower to middle Miocene sequence of claystone and claystone with silt. Most of the unit is reddish brown or yellowish brown massive sediment with common burrowing stained black by diagenetic alteration. As in Units I and II, bioturbation of Unit III is consistent with sedimentation at lower bathyal to abyssal water depths (Nereites ichnofacies). Unit III contains sparse, relatively thin calcareous turbidites. There is no evidence for hydrothermal influence on sedimentation or diagenesis despite the fact that it lies directly above the basalt of Unit IV (796.67–857.48 mbsf). Unit III is the product of relatively slow sedimentation in a distal setting at the foot of a continental margin and is similar to the basal sediment at Site U1431 and to “red clay” deposits from the central Pacific (Bryant and Bennett, 1988).

Analysis of calcareous nannofossils, planktonic foraminifers, and radiolarians in core catcher samples and additional samples from split cores indicates that the sedimentary succession recovered at Site U1433 spans the lower Miocene to the Pleistocene (Fig. F18). Age control for the lower to lower middle Miocene section is difficult because of very rare occurrences of microfossils in the brown claystone (Unit III) overlying the basement. Nannofossils in sediment preserved in and around basalt pillows are Oligocene to early Miocene in age, but additional postexpedition analyses are required to determine if these assemblages are reworked or in situ.

Calcareous nannofossils are generally common to abundant with good preservation in samples from the Pliocene–Pleistocene section but are rare and heavily overgrown or even barren in some upper Miocene and Pliocene samples, especially those from nannofossil ooze/chalk intervals. Planktonic foraminifers also show considerable variations in both abundance and preservation. They are abundant and well preserved in silty layers with numerous small (<150 µm) specimens but poorly preserved and very difficult to identify in lithified intervals. Radiolarians are abundant and well preserved in the Upper Pleistocene section in Hole U1433A, but rare or absent in older sediment sections downhole. In Hole U1433B, samples are barren of radiolarians until the brown claystone of Unit III, in which rare and poorly preserved but biostratigraphically significant specimens occur.

Integration of biohorizons and paleomagnetic datums indicates extremely low sedimentation rates (<0.5 cm/k.y.) during the early to middle Miocene. Sedimentation rates varied from ~5 to 9 cm/k.y. from the late Miocene to early Pleistocene, but then increased sharply to ~20 cm/k.y. since 1 Ma (Fig. F18).

We cored 60.81 m into igneous basement below 796.67 mbsf in Hole U1433B, recovering 29.02 m of basalt (47.7% recovery). This short basement succession was divided into 45 igneous lithologic units, which are grouped into lithostratigraphic Unit IV and are immediately overlain by hemipelagic dark reddish brown to yellowish brown claystone (Unit III) (Fig. F19). The basement at Site U1433 is composed of a 37.5 m thick succession of small pillow basalt lava flows in the top (Fig. F20), with a 23.3 m series of massive basalt lava flows toward the bottom.

The igneous basement begins with a sequence of sparsely to highly plagioclase-phyric pillow basalt with a trace of olivine microphenocrysts. Most of the pillow basalts are nonvesicular to sparsely vesicular, range in grain size from crypto- to microcrystalline, and in many cases have well-preserved glassy chilled margins along both the upper and lower unit boundaries. A few larger lobate flows are present, with flow thicknesses varying from 0.1 to 1.1 m. In two intervals, interpillow hyaloclastite breccia was encountered, with remnants of baked limestone in which Oligocene to early Miocene nannofossils occur. In between these sequences of pillow basalt flows, one 5.2 m thick microcrystalline to fine-grained massive flow was encountered that is sparsely olivine-plagioclase-phyric but has a holocrystalline groundmass with abundant clinopyroxene present in the interstitial spaces. Downhole, the basement is characterized by more massive basalt lava flows as thick as ~12.8 m. These massive flows have similar petrologic characteristics and range from sparsely to highly plagioclase-phyric with minor microphenocrysts of olivine. Toward the interiors of the thickest lava flows the grain size increases to fine grained.

All basalts have a phenocryst phase assemblage of plagioclase ± olivine, whereas the more massive flows also have clinopyroxene in their groundmass. This assemblage resembles a typical MORB crystallization assemblage and, in conjunction with geochemical evidence, we conclude that the basement basalt at Site U1433 is typical MORB.

Alteration is also typical of that of MORB. The basalt ranges from mostly fresh/less altered to moderately altered in intensity, typically as halos in association with cracks and veins, and from gray to dark gray-green and yellow to red-brown in color. Basalt glasses are most abundant near the quenched margin of lava flows and are commonly altered to greenish palagonite, which might indicate alteration from more reducing fluids than that of brownish palagonite. Alteration veins are abundant at the top of the basement cores and decrease with depth, indicating limited downwelling fluid flow, which is also consistent with fewer fractures that occur with increasing depth. Vein filling minerals include carbonate, celadonite, Fe oxide/hydroxides, saponite, smectite, quartz, and some blue minerals that are difficult to identify. Typical secondary minerals include saponite, Fe oxides, carbonate, and celadonite, which represent a low-temperature alteration assemblage.

We measured 240 fractures and veins in the basalt of Hole U1433B. Most of the fractures occur along existing veins without either obvious offset or striations on the surfaces, indicative of drilling-induced fractures, whereas natural fractures are quite rare. In general, veins can be separated into four types with different shapes: single linear, triple-junction, branched, and crosscutting. Arched, crosscutting, and triple-junction veins are usually distributed in pillow basalt layers, whereas single linear, branched, or sinuous veins are usually confined to the massive flows. The highest frequency of veins occurs in the pillow basalts, whereas there are fewer veins within the massive flows. Alteration along fractures and veins produced 1–2 cm wide yellow to brown halos.

At Site U1433, interstitial water sulfate concentration decreases with depth and sulfate is almost completely consumed (<1 mM) below ~30 mbsf. Alkalinity increases with depth, reaching a maximum of 25.8 mM at ~30 mbsf, before gradually decreasing with depth. The depth at which sulfate is consumed and alkalinity reaches its peak corresponds to an increase in methane from ~3 to 1100 ppmv. Below this depth, methane varies between ~22,000 and 93,000 ppmv before it decreases significantly below ~590 mbsf. Ethane and other higher hydrocarbons are also present in low concentrations below ~60 mbsf. This indicates that anaerobic oxidation of methane coupled with sulfate reduction is ongoing in the sediment. Other interstitial water chemistry profiles reflect both lithologic changes and diagenetic processes. TOC varies from 0 to 1.0 wt%, with a general decrease downhole. CaCO₃ content ranges from 0.5 to 77.8 wt%, with the lowest values (generally <15 wt%) in the upper 300 m of the site. Intervals with higher CaCO₃ content below 300 mbsf correspond to carbonate turbidite layers in lithostratigraphic Unit II. Peaks in TOC (1–1.5 wt%) at ~450 mbsf and 540–590 mbsf could reflect an influx of terrestrial organic matter; however, additional shore-based work is needed to confirm this interpretation.

The basalt recovered in Hole U1433B has low LOI values (0.52–2.06 wt%), as well as low K₂O (0.11–0.29 wt%), moderate TiO₂ (1.01–1.77 wt%), and high SiO₂ (48.5–51.1 wt%). In general, concentrations of major elements vary within narrow ranges. When plotted on the alkali vs. silica diagram of volcanic rock types (Le Maitre et al., 1989) (Fig. F12), the data plot within the tholeiite field. As shown in Figure F13, the data from these rocks overlap with but define a much smaller range than those of Indian Ocean and Pacific MORB and are distinct from the Hainan Island OIB and other SCS seamount basalt data fields (Figs. F12, F13). Thus, the basalt samples from Hole U1433B are tholeiites and considered representative of SCS MORB.

At Site U1433, we collected a total of 50 routine 5 cm whole-round samples for microbiological analysis from the seafloor to 790 mbsf. When possible, these samples were collected adjacent to samples for interstitial water measurements. We also collected 164 samples from split cores to study the microbiology of interfaces or coring-induced disturbance. We obtained these samples between 4 and 154 mbsf in Hole U1433A and between 187 and 854 mbsf in Hole U1433B. Most of the samples collected at Site U1433 were preserved for shore-based analysis of DNA, RNA, and lipids. Some portions of the whole-round samples were selected for cultivation-based studies designed to enrich for anaerobic autotrophs and heterotrophs.

We used three methods of contamination testing during coring at Site U1433: PFTs, microspheres, and FCTs. PFT samples were acquired from the outside and inside of 16 cores between 140 and 390 mbsf in Holes U1433A and U1433B. PFTs were not detected in most of the samples collected and analyzed for this tracer regardless of whether the samples were taken from the outside or the inside of the core. Microsphere tracers were used with the RCB coring system in Hole U1433B between 709 and 854 mbsf. Two microsphere samples were taken from each core collected between these depths: one from scrapings of the core surface and one as a subsample from the interior of each whole-round sample. Twenty-four FCT samples were collected either from the drilling fluids that drained from the core liners when cores arrived on the catwalk or from a sampling port near the mud pumps on the rig floor during active coring. The fluids collected for FCT samples correspond to cores obtained from depths ranging between 6 and 824 mbsf.

We conducted paleomagnetic studies at Site U1433 on both sediment and basement cores using pass-through magnetometer measurements on all archive-half sections and AF demagnetization on representative discrete samples. Magnetostratigraphic records at Site U1433 suggest the existence of eight short reversed polarity events within the Brunhes normal chron. These short-lived events most likely represent geomagnetic excursions, as both declination and inclination change. The polarity shifts at depths of ~12, 18, 28, 48, 53, 132, and 152 mbsf match well with known excursion events: Mono Lake (33 ka), Laschamp (41 ka), Blake (120 ka), Iceland Basin (180 ka), Pringle Falls (211 ka), Big Lost (560–580 ka), and marine isotope Stage 17 (670 ka). For two directional anomalies at ~78 and 88 mbsf, there are no counterparts from previous studies, and further shore-based work is needed to confirm the origin of these two anomalies. The Brunhes/Matuyama Chron boundary (0.781 Ma) is tentatively placed at ~188 mbsf in Core 349-U1433A-20H, which indicates a higher sedimentation rate (~23.7 cm/k.y.) for the Middle–Late Pleistocene compared to Sites U1431 and U1432 (Fig. F18). Such a high sedimentation rate facilitates preservation of the short-lived polarity excursions mentioned above.

In Hole U1433B, six major normal chrons are recognized and tied to the geomagnetic polarity timescale using constraints from biostratigraphy. The basal boundaries for the Matuyama Chron (2.581 Ma), Gauss Chron (3.596), and Gilbert Chron (6.066 Ma) are placed at ~350, 420, and 550 mbsf, respectively. The basal age for sediment in Core 349-U1433B-60R is ~11 Ma. Paleomagnetic results for the basalt units show that the upper part of the basement (805–817 mbsf) is dominated by normal polarity. Between ~817 and 830 mbsf, a relatively well defined reversed polarity zone is observed. Below this depth range, the paleomagnetic inclinations display both normal and reversed polarities. Overall, the remanent magnetization of rocks below ~817 mbsf is dominated by reversed polarity. This pattern is similar to that found in the upper part of the basalt units at Site U1431.

Cores from Holes U1433A and U1433B were measured for physical properties on whole-round cores, split cores, and discrete samples. The physical properties correlate well with lithology, composition, and observed lithification. Bulk density, P-wave velocity, shear strength, NGR, and thermal conductivity increase gradually with depth over the uppermost 150 m (Fig. F17), whereas the porosity measured on discrete samples decreases from 90% to 50% over the same depth range. This indicates that sediment compaction dominates physical property variations above 150 mbsf. Below 240 mbsf, variability in porosity, magnetic susceptibility, and NGR values reflects interbedding of carbonate and clay layers (Fig. F17). An increase in P-wave velocity from ~1700 to ~2000 m/s near 550 mbsf coincides with stronger lithification in the deeper sediment. From 680 to 750 mbsf, P-wave velocities measured in the lithified carbonates reach ~2600 m/s, contrasting strongly with those measured in the clay (~2000 m/s) (Fig. F17). The strong reflectors observed in the seismic profile from this site probably result from this contrast in velocity. Magnetic susceptibility gradually increases and NGR decreases in the clays between 750 and 800 mbsf (Fig. F17). The basalts below 800 mbsf display very low NGR and porosity and variable magnetic susceptibility. Some of the fresh, phenocryst-rich basalt has very high magnetic susceptibility and P-wave velocities.

The modified triple combo and FMS-sonic tool strings were run in Hole U1433B. Both tool strings reached 840 m WSF, ~18 m short of the bottom of the hole (Fig. F17). Between 100 and 550 m WSF, there are rapid variations in borehole diameter from ~25 to >43 cm. Below 550 m WSF, the hole diameter is mostly in gauge, with fewer washed out zones. Density and sonic velocity increases from the top of the logs at 100 m WSF downhole to 750 m WSF, caused by compaction and cementation with depth. Superimposed on this trend, excursions to higher velocity and photoelectric factor (PEF) and to lower NGR mark the occurrence of carbonate beds (Fig. F17). This information was used to infer lithology in the unrecovered intervals of Hole U1433B. In the red clay of Unit III, from ~750 to 800 m WSF, high values in the PEF log indicate that hematite and other oxides increase in concentration downhole toward the top of the basalt at ~800 m WSF. Pillow basalt, a massive basalt flow, fractures, and veins are seen in the FMS images in the basement.

Site U1434

Background and objectives

Site U1434 (proposed Site SCS-4E) is located about 40 km northwest of Site U1433 and is directly on the uplifted shoulder of the relict spreading center in the Southwest Subbasin (Figs. F7, F8). This site is also located just south of a large seamount that formed near the relict spreading center after the termination of seafloor spreading. During coring at Site U1433, we decided to use some of our remaining time to core at a second site in the Southwest Subbasin to obtain basement samples more proximal to the extinct spreading center. Site U1434 also offered the opportunity to sample volcaniclastic material from the nearby seamount, which can be compared to the seamounts located near Site U1431 in the East Subbasin.

Sites U1434 and U1433 form a short sampling transect in the Southwest Subbasin (Fig. F7), and with age controls from these two sites, the evolution of the Southwest Subbasin can be better understood. Coring at these sites should help to explain the sharp differences in magnetic amplitude between the East and Southwest Subbasins and test the existing opening models for the Southwest Subbasin (e.g., Pautot et al., 1986). Coring will help determine the age of this subbasin near the end of seafloor spreading and correlate ages from magnetic anomalies with biostratigraphic, magnetostratigraphic, and radiometric ages. The apparent weak magnetization in basement rocks (Li et al., 2008) will be examined through petrological and geochemical analyses and by measurements of magnetic susceptibility and remanent magnetization. Rock samples cored here will place constraints on mantle evolution and oceanic crustal accretion, the terminal processes of seafloor spreading, and the timing and episodes of postspreading seamount volcanism in the area of the relict spreading center.

Operations

After an 18 nmi transit from Site U1433 averaging 10.3 kt, the vessel stabilized over Site U1434 at 0048 h (UTC + 8 h) on 20 March 2014. This site was an alternate site that was originally planned to core from the seafloor with APC/XCB to refusal, drop a free-fall funnel, change to the RCB, and then core 100 m into basement. Because of time considerations, the plan was modified so that we drilled to ~200 mbsf using the RCB and then cored into basement as deeply as time permitted. Logging would then be considered depending on hole depth and condition. Hole U1434A was drilled to 197.0 mbsf and then cored with the RCB (Table T1). Basement was encountered at ~280 mbsf, and the hole was advanced by rotary coring to a final depth of 312.5 mbsf. The hole was terminated because of poor hole conditions and poor recovery. At this site there was one drilled interval of 197.0 m. The RCB was deployed 14 times, recovering 26.43 m of core over 115.5 m of penetration (22.9% recovery).

Principal results

The cored section at Site U1434 is divided into four lithostratigraphic units, three sedimentary and one igneous (Fig. F21). Lithostratigraphic Unit I (197.00–235.10 mbsf) is a 38.1 m thick sequence of upper Miocene dark greenish gray claystone interbedded with black volcaniclastic sandstone and occasional breccia. The fine-grained sediment is mottled greenish and light buff brown, with the browner sediment preferentially found in burrows. This unit is marked by strong bioturbation within the claystone intervals that make up ~40% of the total sediment. The trace fossil assemblages seen within the claystone intervals are consistent with sedimentation in deep water (i.e., lower bathyal or abyssal depths; >2500 m), with assemblages dominated by Chondrites and Zoophycos, although more vertical burrows are also noted. Sandstone beds are typically dark gray or black in color and are composed of volcaniclastic fragments. The volcaniclastic sandstone and breccia are interpreted to be part of the sedimentary apron of a nearby seamount because they contain abundant volcanic glass fragments, scoria, and basalt clasts, as well as isolated crystals of plagioclase, olivine, and biotite.

Lithostratigraphic Unit II (235.10–254.59 mbsf) contains upper Miocene greenish gray nannofossil-rich claystone with very thin claystone with sand interbeds. The color of the sediment varies at the decimeter scale as a result of changes in the carbonate and clay content. The sediment is locally a light greenish gray color, reflecting higher biogenic carbonate content over those intervals. Unit III (254.59–278.27 mbsf) consists of dominantly massive yellowish brown claystone with nannofossil- or foraminifer-rich claystone of latest middle to late Miocene age. This unit is primarily distinguished from Unit II by its color, which tends to be more yellowish or reddish brown compared to the greenish gray tones associated with the overlying unit.

Analysis of calcareous nannofossils and planktonic foraminifers in core catcher samples and additional samples from split cores indicates that the sedimentary succession recovered at Site U1434 spans the uppermost middle to upper Miocene, with the base of the sequence younger than 11.9 Ma (Fig. F22). Calcareous nannofossils are generally common to abundant but decrease in abundance downhole, and preservation is poor to moderate. Planktonic foraminifers vary from common to absent, with good to poor preservation, but are frequently fragmented. Radiolarians are present in only one sample. Although none of the species present are biostratigraphic index taxa, the radiolarian assemblage is consistent with the late Miocene age inferred from nannofossils and foraminifers. Correlation of microfossil biohorizons and paleomagnetics data suggest a sedimentation rate of ~1.6 cm/k.y. for the sediment sequence recovered at Site U1434 (Fig. F22).

We cored 30.28 m into igneous basement below 278.37 mbsf in Hole U1434A, recovering 3.05 m of basalt (10.1% recovery). This basement succession is divided into seven igneous lithologic units, which are grouped into lithostratigraphic Unit IV (Fig. F23). The basement at Site U1434 comprises a succession of small pillow basalt flows or a thicker autobrecciated pillow lava flow with three occurrences of hyaloclastite breccia. The igneous basement comprises angular to subangular basalt fragments that are aphyric and have glassy to aphanitic groundmasses. The only phenocrystic phase is olivine, which appears as sparse euhedral-subhedral microphenocrysts throughout the core. The groundmass contains abundant plagioclase microlites, growing in spherulitic and variolitic patterns, with the majority of the groundmass consisting of variably altered mesostasis. Clinopyroxene is only observed in a few thin sections, growing in patches and filling the interstitial spaces between plagioclase microlites. Most of the basalt is nonvesicular to sparsely vesicular. The hyaloclastite breccia contains abundant fresh volcanic glass shards in a mostly clay and/or carbonate matrix. All basalts have phase assemblages of olivine ± plagioclase, and some slightly coarser basalt pieces also have clinopyroxene in their groundmass. These resemble typical MORB crystallization assemblages.

Basalt alteration at Site U1434 is typical of that of MORB. The recovered basalt is slightly to moderately altered. Pillow basalt pieces are altered in zones, with alteration color ranging from dark gray in the interior to light yellow-brown in halos along the outer rims. Typical secondary minerals include saponite and other clay minerals, Fe oxides, carbonate, and celadonite, which constitute a low-temperature alteration assemblage. Fresh basalt glass exists near some of the pillow basalt margins and in the clasts of hyaloclastite breccia. Those basalt glasses are partially altered to orange and brown palagonite. Most of the vesicles are empty or only partially filled with Fe oxide, saponite, celadonite, and/or carbonate. Only two alteration veins were observed in this short basement section.

Most fractures observed in the sedimentary sequence at Site U1434 are drilling induced. One fracture shows some offset that suggests it occurred prior to consolidation of the sediments. Fractures are rare in the small amount of basalt recovered at this site. Several linear veins are present and filled with carbonate or Fe oxide.

We measured alkalinity and pH on five interstitial water samples taken from 207.9–264.8 mbsf in Hole U1434A. Alkalinity increases from 0.5 at 208 mbsf to 3.5 at 257 mbsf and then decreases in the two samples below that depth. The pH decreases from 7.9 at 208 mbsf to 7.2 at 265 mbsf and then increases slightly just above basement. Methane was detected in very low concentrations (<3.1 ppmv) in the headspace gas samples taken at this site. CaCO₃ content is low (<10 wt%) in lithostratigraphic Units I and II. Samples from the base of Unit III are higher in CaCO₃ (15–30 wt%). TOC is also low (<0.5 wt%), with the highest values near the base of the sedimentary section. ICP-AES major and trace element results from Site U1434 indicate that the basalt has somewhat higher K₂O than those at Sites U1431 and U1433 but is still tholeiitic in composition and similar to MORB (Figs. F12, F13).

We collected six routine 5 cm whole-round samples for microbiological analysis from depths of 208–275 mbsf in Hole U1434A. These samples were collected adjacent to samples for interstitial water measurements so that microbiological data and water chemistry data are proximal. We also collected and preserved 13 samples from either the split cores on the sampling table or from basement samples shortly after the samples were retrieved from the catwalk to investigate the microbiology of interfaces. The whole-round and split core samples were preserved for shore-based characterization of the microbial communities (i.e., DNA, RNA, lipids, and cultivation-based studies).

We also collected samples for measuring contamination testing tracers, including microspheres and FCTs. Microsphere tracers were used with the RCB coring system in Hole U1434A by adding them to the core catcher for cores collected between 208 and 303 mbsf. Two microsphere samples were collected from each of the cores collected between those depth intervals: one from scrapings of the core surface and one as a subsample from the interior of each whole-round sample. Microscopic counts of the microspheres in these samples will be performed in shore-based laboratories. Six FCT samples were collected from drilling fluids that drained from the core liners when cores arrived on the catwalk or from a sampling port near the mud pumps on the rig floor during active coring. The fluids collected for FCT samples correspond to cores obtained from depths ranging between 208 and 293 mbsf. Microbial community DNA and lipids from FCT samples will be compared to the same measurements made on the core samples to determine if the drilling fluids contain microbes that can be regularly tracked as recognizable contaminant taxa.

We conducted paleomagnetic studies at Site U1434 on both sediment and basement cores using pass-through magnetometer measurements on archive-half sections. NRM intensity ranges from 0.001 to 0.1 A/m for the sediment units and increases to several A/m for the basalt units, suggesting that the basalt contains more iron oxides than the sediment. Because of the poor recovery at the site, only fragmentary patterns of magnetic polarity are observed. Available biostratigraphic data allow us to tentatively correlate certain parts of the magnetic polarity interval recorded in the sediments with the geomagnetic polarity timescale. Near the base of the sedimentary sequence, biostratigraphy indicates an age <11.9 Ma, which we use to correlate the negative inclinations at ~278 mbsf to Chron C5r (11.056–12.049 Ma). The long, dominantly positive inclinations between ~250 and 270 mbsf may represent the long normal Subchron C5n (9.984–11.06 Ma), the short normal polarity zone between ~235 and 240 mbsf appears to have recorded Chron C4An (8.771–9.015 Ma), and the normal polarity zone between 205 and 210 mbsf can be tentatively assigned to Chron C4n around 7.15 Ma (Fig. F22).

Cores from Hole U1434A were measured for physical properties on whole-round cores, split cores, and discrete samples. In general, the physical properties correlate well with lithology, composition, and the observed lithification. Because of the low recovery rate, measurements of physical properties show significant discontinuities between intervals. In Hole U1434A, the observed range of values for magnetic susceptibility (30 × 10^–5 to 80 × 10^–5 SI) and NGR (25–45 cps) in Units I–III are typical for clay material, which dominates the sediment layers (Fig. F21). The low NGR value in Unit IV corresponds to the basalt layer. The magnetic susceptibility values in the basalt ranges from 10 × 10^–5 to 90 × 10^–5 SI, which is much lower than what is typical for basalt (Fig. F21). The high grain densities in the claystone of Units II and III suggest the presence of heavy minerals, such as hematite. The porosity measured on discrete samples increases from 40% to 60% with depth, which may be correlated to the lithification and composition of the claystone.

Site U1435

Background and objectives

Coring at Site U1435 (proposed Site SCS-6C) became a high priority after failing to achieve our basement objectives at Site U1432 when the final cementing operations compromised the reentry system. This site was originally added as an alternate because of the high risk of being unable to reach basement at Site U1432. The site is located on the continental side of the continent/ocean boundary and is fundamentally different from Site U1432 but nonetheless does have the potential to provide information about the breakup process. Site U1435 is located on a structural high at the transition between the extended continental crust and the oceanic crust (Figs. F6, F8). Similar conspicuous structural high features can be found on the continent/ocean boundary in many other seismic profiles crossing the SCS northern margin and therefore appear to represent tectonic structures typical of the area. The formation mechanism and nature of this structural high was still speculative; it could have been a volcanic extrusion associated with continental extension at the onset of seafloor spreading, lower crust material emplaced from preferential lower crust extension, exhumed mantle material, or a structural high composed of older (Mesozoic) sedimentary rock. Coring at this location was designed to help pinpoint the exact nature of this structure and improve our understanding of early continental breakup, the rift-to-drift transition, and seafloor spreading processes.

Operations

After a 336 nmi transit from Site U1434 averaging 8.5 kt, the vessel stabilized over Site U1435 at 1524 h (UTC + 8 h) on 24 March 2014. Because we anticipated shallow sediment cover (~10 m), we conducted a 3.5 kHz sonar survey to select a location with maximum sediment thickness to help stabilize the drill string when trying to penetrate basement with thin sediment cover in rough weather. After reaching basement, the plan was to core as deeply into basement as time permitted. Sediment thickness was significantly greater than expected based on the seismic interpretation. Hole U1435A was cored with the RCB to a final depth of 300.0 mbsf when time allocated to the expedition expired, never reaching igneous basement (Table T1). The RCB was deployed 32 times, recovering 171.37 m of core over 300.0 m of penetration (57.1%).

Principal results

The cored section at Site U1435 is divided into three sedimentary lithostratigraphic units (Fig. F24). Unit I (0–77.65 mbsf) is a sequence of Oligocene–Pleistocene greenish gray nannofossil-rich clay and clayey nannofossil ooze, together with manganese nodules. The unit is divided into Subunits IA and IB based on variations in the nannofossil content of the sediment. Subunit IA (0–36.04 mbsf) is Miocene to Pleistocene in age and consists of manganese nodules underlain by clayey nannofossil ooze. The manganese nodules have a lobate appearance and are typically associated with very low sedimentation rates. The massive clayey nannofossil ooze has a few Planolites trace fossils visible on the cut surface of the core. Subunit IB (36.04–77.65 mbsf) is Oligocene in age and consists of mostly greenish gray nannofossil-rich clay and lesser quantities of greenish gray clay. There are interbedded silty clay and clay with silt intervals, but deeper in the section the sediment becomes more calcareous, primarily through an increase in the proportion of nannofossils. The sediment is heavily bioturbated with trace fossils of the Nereites ichnofacies.

Unit II (77.65–275.54 mbsf) is a 197.89 m thick sequence of pre-Oligocene thick-bedded and mostly medium-grained dark gray silty sandstone, with very little carbonate and minor sandy siltstone and conglomerate. The sandstone is better cemented than the Unit I nannofossil-rich clay and increases in lithification downhole. Units I and II are separated by a hard carbonate rock that likely represents a hiatus. The sandstone is moderately well sorted and is characterized by dispersed carbon fragments, shell fragments, and current lamination that is largely disrupted by bioturbation, with burrows typical of the Cruziana ichnofacies indicative of shallow-marine conditions. Several whole bivalves and gastropods occur in the sandstone. Unit III (275.54–300.00 mbsf) is a 24.46 m thick sequence of dark gray silty sandstone, silty mudstone, and minor conglomerate. The unit is distinguished from Unit II by being generally finer grained.

We analyzed all core catcher samples and additional samples from split cores for calcareous nannofossils, foraminifers, and radiolarians at Site U1435. Based on nine nannofossil and four planktonic foraminifer bioevents, the sedimentary sequence above 77.65 mbsf is assigned an age spanning the early Oligocene (<33.43 Ma) to the Pleistocene (>0.12 Ma), with possible unconformities or condensed sections existing between the upper Oligocene and middle Miocene, between the upper Miocene and lower Pliocene, and between the upper Pliocene and Middle Pleistocene. Based on a limited number of bioevents, sedimentation rates during the Oligocene were ~0.5 cm/k.y. (Fig. F25).

Samples from 77.65 to 300 mbsf are barren of nannofossils, radiolarians, and planktonic foraminifers. A few long-ranging, shallow-water benthic foraminifers occur in samples from ~200 to 250 mbsf. Although these specimens are not useful for age control, they indicate a depositional environment of brackish water to shallow marine for Unit II, consistent with the deltaic setting inferred from the sedimentology and trace fossil assemblage.

A small number of deformation structures are present in the sedimentary rock of Site U1435. Most of the fractures are drilling induced, and in the black mudstones near the base of the section, these induced fractures developed along the bedding. Two normal fault structures are found in the sandstone, each composed of several fractures that have little offset. No deformation or thickness changes occur in the rock of the hanging walls and footwalls, indicating that these faults occurred at a later stage and did not control sedimentation. One linear white carbonate vein was found in the sandstone. Bedding is generally horizontal or subhorizontal in Unit I, but toward the base of Units II and III the strata are inclined to a significant degree (>25°). These dips are not depositional and are interpreted to reflect rotation caused by normal faulting, possibly during formation of the structural high on which the site is located.

Downhole interstitial water concentrations of chloride, bromide, and sodium are variable and slightly higher than modern seawater; however, the Na/Cl ratio is ~0.85 throughout the sampled interval, which indicates that the interstitial water is of typical marine origin. Only very low concentrations (<10 ppmv) of methane and ethane were detected in the headspace gas samples from Hole U1435A. CaCO₃ content in the upper part of the hole is higher than that of the lower part, which corresponds to the change from nannofossil ooze and nannofossil-rich clay in lithostratigraphic Unit I to sandstone in Unit II. Despite variable TOC with depth, the ratio of TOC to total nitrogen (C/N) suggests that TOC is dominated by a terrestrial organic matter source, with lower input from marine organic matter.

We collected 25 routine 5 cm whole-round samples for microbiological analyses from 37 to 299 mbsf in Hole U1435A. These samples were collected adjacent to samples for interstitial water measurements so that microbiological data and water chemistry data are proximal. The whole-round samples were preserved for shore-based characterization of microbial communities (i.e., DNA, RNA, lipids, and cultivation-based studies).

We also collected samples for measuring contamination testing tracers, including microspheres and FCTs. Microsphere tracers were used with the RCB in Hole U1435A by adding them to the core catcher for Cores 349-U1435-5R through 32R (37–299 mbsf). Two microsphere samples were taken from each of the cores collected between those depth intervals: one from scrapings of the core surface and one as a subsample from the interior of each whole-round sample. Microscopic counts of the microspheres in these samples will be performed in shore-based laboratories. Five FCT samples were collected from drilling fluids that drained from the core liners when cores arrived on the catwalk or from a sampling port near the mud pumps on the rig floor during active coring. The fluid collected for FCT samples corresponds to cores obtained from between 90 and 273 mbsf. Microbial community DNA and lipids from FCT samples will be compared to the same measurements made on the core samples to determine if the drilling fluids contain microbes that can be regularly tracked as recognizable contaminant taxa.

We performed measurements of NRM on all archive-half cores from Hole U1435A. We subjected these cores to AF demagnetization up to 20 mT in order to establish a reliable magnetostratigraphy at the site and to observe the magnetic properties of the different lithologies recovered. Because of time constraints, we were unable to perform measurements and demagnetization on discrete samples taken from the working halves. Overall, paleomagnetic data at Site U1435 are reasonably robust and provide magnetic information about the recovered sediment. Several relatively well defined polarity intervals are identified in the downhole magnetostratigraphic records, despite some samples showing unstable and ambiguous magnetization. Based on biostratigraphic data, we were able to tentatively correlate certain parts of the magnetic polarity interval recorded in the sediment with the geomagnetic polarity timescale. Assuming no significant hiatus between the marine Oligocene nannofossil-rich clay (Subunit IB) and the sandstone of Unit II, the Chron C16n/C15r boundary (36.05 Ma) is tentatively placed at ~280 mbsf. This interpretation indicates relatively high sedimentation rates for the sandstone of Unit II (~5 cm/k.y.) (Fig. F25).

Cores from Hole U1435A were measured for physical properties on whole-round cores, split cores, and discrete samples. Thermal conductivity was measured with a needle probe in soft sediment and then with a contact probe in the sedimentary rock. The physical properties correlate with lithology and observed lithification. P-wave velocity increases gradually with depth over the uppermost 150 m (Fig. F24), whereas porosity measured on discrete samples decreases from 65% to 30% over the same depth range, reflecting sediment compaction. Bulk density, NGR, magnetic susceptibility, and thermal conductivity show a sharp increase near 78 mbsf at the lithostratigraphic Unit I/II boundary between the nannofossil-rich clay and sandstone (Fig. F24). A significant increase in P-wave velocity and thermal conductivity is observed near 170 mbsf, which is associated with stronger lithification of the sandstone. Magnetic susceptibility and NGR values decrease with depth below 270 mbsf, which corresponds to the change from a dominance of sandstone in Unit II to mudstone in Unit III (Fig. F24).

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