Expedition synthesis

Operations during Expedition 349 (26 January–30 March 2014) drilled five sites in the central basin of the SCS (Figs. F1, F2, F8). Sites U1431, U1433, and U1434 were cored into the igneous basement near the fossil spreading center where seafloor spreading terminated, whereas Sites U1432 and U1435 are located proximal to the northern continent/ocean boundary where seafloor spreading started. In total, we recovered 1524 m of sediment and sedimentary rock and 78 m of igneous basement and carried out geophysical logging (triple combo and FMS-sonic tool strings) at Sites U1431 and U1433. The materials recovered and data collected were extensively examined and discussed and allow us to draw the following conclusions concerning the opening history of the SCS, the sedimentary and paleoceanographic responses to the formation of this ocean basin, the mantle source and magmatic processes forming the ocean crust basement, and the geodynamic implications for the tectonics of the region.

Opening history of the South China Sea

Determining the timing of the onset and cessation of seafloor spreading in the SCS were the primary objectives of Expedition 349. At all three sites near the fossil spreading center, we recovered deep-sea claystone near the sediment/igneous basement interface, with interflow claystone between lava flows recovered at Site U1431, and evidence from downhole measurements for an unrecovered interflow sedimentary layer at Site U1433 (Figs. F11, F19, F23). Microfossils within these claystone units provide preliminary age models for the sedimentation history, which in turn helps us to refine the oceanic crustal age models. Relatively unaltered basalt from the ocean crust near the relict spreading center was recovered and will be dated using 40Ar/39Ar methods.

The age at which seafloor spreading ceased in the East Subbasin is estimated at ~16.7–17.5 Ma based on microfossils in the interflow claystone (Fig. F10), or even slightly younger because Site U1431 is ~15 km off the ridge axis. At Site U1433, ~50 km away from the relict spreading center in the Southwest Subbasin, we recorded a biostratigraphic depositional age range from ~18 to 21 Ma for sediment directly above the basement (Fig. F18). Rare and poorly preserved Oligocene to earliest Miocene calcareous nannofossils were also observed in sediment attached to basalt pieces in the upper part of the basement at Site U1433, which could indicate a substantially older age for the basement; however, it is not clear if the nannofossil assemblages are reworked or in situ.

Located just 35 km north of Site U1433, Site U1434 is on the uplifted shoulder of the relict spreading center and is only ~15 km from the spreading axis in the Southwest Subbasin. Here, biostratigraphic age data indicate that the basal sedimentary sequence, lying just above the basaltic basement, is younger than 12 Ma. The uplifted shoulder forms a topographic high that may have contributed to a depositional hiatus or extremely low sedimentation rates between the emplacement of basalt and the earliest sedimentary deposits. We deduce that the cessation age of seafloor spreading in the Southwest Subbasin is somewhere between ~12 and ~21 Ma. Overall, there does not appear to be a large difference in the ages at which seafloor spreading finished between the two subbasins.

We note here that the recorded biostratigraphic age ranges are likely minimum estimates because evidence from paleomagnetic reversals in the basalt units point to prolonged eruption histories at these sites. Postcruise radiometric dating of basement basalt from these sites, plus calibration with magnetic anomalies and paleomagnetic measurements, will constrain the eruptive history of the igneous basement at these sites in the SCS.

Because of a technical error in cementing the final casing string in Hole U1432B, we were unable to reach the oceanic basement near the continent/ocean boundary at the deepest planned drill site. This prevented us from collecting basement samples to directly date the onset of seafloor spreading. However, we were able to drill into one of the most enigmatic structures in the area, a structural high standing along the continent/ocean boundary at Site U1435. Coring here recovered a sharp unconformity, with sediment above dated to ~33 Ma. The sediment above the unconformity records deep marine facies, with sediment below dominated by sandy lithologies of shallow-marine deltaic or coastal facies. The age of this deeper unit, which is composed mainly of poorly sorted sandstone and thin black silty mudstone that are both relatively rich in organic matter (~0.4–1.3 wt%), awaits further shore-based studies as it is nearly barren of calcareous and siliceous marine microfossils. Immediately above the unconformity, drilling recovered abundant recrystallized calcite and dolomite grains, which may have been formed by extremely slow sedimentation or diagenesis. Nevertheless, we interpret this sharp unconformity as the break-up unconformity caused by the initial opening of the SCS, which places the onset of seafloor spreading at ~33 Ma.

Sedimentary and paleoceanographic responses

Silt and carbonate turbidites

All sites contain deep-marine deposits that likely formed at water depths deeper than 3 km but show significant spatial variations in postspreading sedimentary environment and provenance. Site U1431, in the East Subbasin, records the strongest evidence of hemipelagic, deep-marine gravity flow deposition of material eroded from various (distal) terrestrial sources. Both silt and nannofossil-rich/calcareous turbidites are present, with the former being much more dominant. Silt turbidites are speculated to have been triggered by volcanism and/or earthquake events associated with the Manila subduction zone and/or the Taiwan Orogeny, with sources located to the east and northeast, whereas calcareous turbidites at this site were likely transported from local sources, possibly nearby seamounts topped by carbonate platforms (Fig. F8).

In contrast, Site U1433 in the Southwest Subbasin has nannofossil-rich calcareous turbidites that are more frequent in the cores, with some beds over several meters thick. These turbidites date to the late Miocene. Here the relict spreading center, with well-developed uplifted shoulders and a relatively deep axial trough, likely acted as an effective barrier, preventing terrestrial material from being transported to this site from northerly landmasses such as southern China, the Philippines, or Taiwan. Other possible sources in Borneo or mainland Southeast Asia are more likely. The source of the carbonate turbidites is most likely within the southerly Dangerous Grounds or the Reed Bank area (Fig. F8). Local sources, such as isolated seamounts or abyssal highs, are less likely because they are quite far away from the site or in relatively deep water. Variations in carbonate flux to Site U1433 may reflect subsidence and drowning of the carbonate reefs in the Dangerous Grounds and/or sea level changes.

Variation in the character of the uppermost Pleistocene sediment between Sites U1431 and U1432 also support contrasting provenances in different parts of the basin at that time. The uppermost unit from Site U1431 is dominated by turbidite silt and sand and has frequent ash layers (Fig. F9), whereas the uppermost unit from Site U1432 is mostly clay and shows fewer ash layers over a similar age interval (Fig. F15). At Site U1433, eight geomagnetic excursions have been identified within the Brunhes Chron due to an extremely high sedimentation rate. Based on our current knowledge, two of the excursions have not been reported in previous studies, and postcruise studies will help determine their origin. A sudden increase in sedimentation rates from 5–9 to ~20 cm/k.y. is recorded at ~1 Ma at Site U1433 (Fig. F18) and may reflect some coupling effects of enhanced terrestrial supply and intensified sea level fluctuations associated with the mid-Pleistocene climate transition.

Variations in carbonate compensation depth

Variations in the carbonate compensation depth (CCD) may have had an effect on the massive deepwater carbonate sediments recovered during the expedition, but the mechanisms are not clear. Sediment at Sites U1431 and U1433 was apparently deposited at depths greater than the modern CCD or, alternatively, may have been strongly diluted by variable clastic flux from the continents. However, at Site U1434, sediment shows much less influence from turbidity currents and is rich in calcareous nannofossils. Here, sediment was deposited at slightly shallower water depths on top of a rifted basement high (Fig. F7). The mid-ocean ridge itself was also likely shallower prior to the postspreading subsidence caused by thermal and isostatic processes, as well as the loading from emplacement of a younger seamount on top of the spreading center. Sediment recovered at Site U1434 must have been deposited above the CCD and its compositional changes may reflect CCD variations after tectonic subsidence corrections.

Pelagic brownish to reddish or yellowish brown claystone

At Sites U1431, U1433, and U1434, we recovered a series of reddish brown to yellowish brown claystone and claystone with silt directly above the basement basalt units. At Site U1431 in the East Subbasin, this lower middle Miocene (~12–14 Ma) unit is only ~5 m thick and is massive and homogeneous in the lower part and laminated with silty claystone in the upper part (Figs. F9, F11). Another similarly colored lower Miocene (~16–18 Ma) claystone ~10 m thick is interbedded within the basalt lava sequence but is less homogeneous, showing evidence for local mass wasting, and is characterized mostly by claystone with minor intraformational breccia or conglomerate containing rare basalt clasts that have well-developed reaction rims.

At Site U1433 (Figs. F17, F19) in the Southwest Subbasin, the yellowish brown claystone and claystone with silt is ~49 m thick and is dated to the middle to late Miocene (~10–18 Ma). Although mostly massive, there are intervals of graded and bioturbated clayey siltstone and silty claystone. At both Sites U1431 and U1433, only siliceous microfossils (radiolarians) are found in this unit, most likely because calcareous microfossils have been dissolved because of deposition below the CCD. At Site U1434 (Figs. F21, F23), ~24 m of yellowish or reddish brown claystone with variable amounts of nannofossils and foraminifers lies immediately above the basaltic basement. This sequence is dated as latest middle to late Miocene in age (~10–12 Ma) and is massive and structureless, with very little evidence for current influence during sedimentation. In contrast to the other sites, this unit at Site U1434 is characterized by a lack of radiolarians, with abundant foraminifers clearly visible on the core surface.

The reddish brown to yellowish brown claystone units at Sites U1431, U1433, and U1434 were deposited at somewhat different time intervals and water depths but all immediately overlie the basaltic basement. The estimated sedimentation rates are quite low, varying from <0.5 cm/k.y. at Site U1433 (Fig. F18) to <2.0 cm/k.y. at Site U1431 (Fig. F10) and ~1.6 cm/k.y. at Site U1434 (Fig. F22). We interpret the claystone to be largely a product of deep-sea pelagic and hemipelagic sedimentation at very low sedimentation rates. The yellowish brown to dark brown color reflects enhanced oxidation of the clastic components of the sediment, which is a typical feature of regions of slow sedimentation and oxygenated bottom water, such as in many parts of the Pacific, where “red clays” are widespread (Bryant and Bennett, 1988).

Although lacking sedimentary evidence, there may have been some hydrothermal influence on the deposition or later alteration after sedimentation as a result of fluid flow and geochemical exchange. Logging in this unit at Site U1433 showed distinctly high NGR logs of potassium, uranium, and thorium over these intervals (Fig. F17). NGR values decrease slightly, whereas PEF increases with depth toward the claystone/basalt boundary and reaches peaks within the top of the basalt unit. This could reflect an increasing abundance of hematite and other oxides in the claystone. The increased concentration of such oxides could result from hydrothermal processes and fluid flow, as well as chemical exchange between the basalt and overlying sediment.

Mantle and crustal processes

Seamount volcanism and terminal processes of the extinct spreading center

Sites U1431 and U1434 are located very close to seamounts developed along and on top of the relict spreading center (Fig. F8). Well-recovered volcaniclastic sandstone and breccia may therefore reveal the history of seamount volcanism following the end of seafloor spreading in the SCS. At Site U1431, we recovered ~280 m of dominantly greenish black volcaniclastic breccia and sandstone, interbedded with minor amounts of claystone (Fig. F9) dated to the late middle Miocene to early late Miocene (~8–13 Ma). This would indicate an approximately 5 m.y. period of extensive seamount volcanic activity that started nearly 4 m.y. after the cessation of seafloor spreading. Volcaniclastic breccia layers comprise much of the section, are generally thicker bedded in the upper parts of this unit, and have normal grading and erosive bases, indicative of deposition by mass wasting either as debris or grain flows.

At Site U1434, volcaniclastic sandstone and minor breccia encountered at the top of the cored interval of the hole show coarse grain sizes and poor sorting (Fig. F21) and are indicative of a relatively local provenance, most likely the adjacent seamount ~15 km to the north (Fig. F8). The age of this unit is late Miocene (younger than 9 Ma). Its upper boundary was not cored, but the penetration rates during drilling of the interval above suggest well-lithified volcaniclastic breccia and sandstone at shallower depths (<200 mbsf). Volcaniclastic material is absent from the sediment deposited between ~12 and 9 Ma in the yellowish brown claystone unit directly overlying the basement basalt. This indicates that this seamount volcanism was not active until ~9 Ma but then was active for at least 2 m.y. based on the cored section. This also puts a time period of ~7 m.y. between the cessation of the seafloor spreading and the initiation of seamount activities in the Southwest Subbasin. This is comparable to late Miocene and Pliocene seamount volcanism found in the extreme southwest of the oceanic basin (Li et al., 2013). Further postcruise sedimentological and geochemical studies will refine the ages and nature of these seamount activities and reveal how magma sources at the dying spreading center evolved through time.

Mantle evolution and magma processes revealed by oceanic basalt

We successfully cored into oceanic basement in the SCS for the first time and recovered basalt at Sites U1431, U1433, and U1434 (Figs. F11, F19, and F23). This allows for the study of the mantle evolution and magmatic processes in this young ocean basin. Despite their apparent differences in groundmass grain sizes, all basalts have variable phase assemblages of plagioclase, olivine, and clinopyroxene, typical of MORB. This interpretation is also supported by geochemical evidence (Figs. F12, F13). Coring at Site U1431 recovered massive basalt lava flows with limited evidence for pillow basalt fragments on top of the section (Fig. F11). Most basalt is aphyric and ranges in grain size from microcrystalline to fine grained, with some medium- to coarse-grained intervals occurring in the interiors of the thickest massive lava flows. These basalts were altered in a low-temperature and oxidative environment, with long intervals only slight affected, providing ample material for postcruise radiometric age dating.

The Site U1433 basement section shows more abundant, small pillow basalt lava flows at the top and a few massive basalt lava flows toward the bottom of the hole (Fig. F19). This basalt ranges from sparsely to highly plagioclase-phyric. Alteration of the basalt at this site is low in intensity and typical secondary minerals represent a low-temperature and both oxidative and nonoxidative alteration assemblage. The contrast in alteration style is interpreted to indicate less active fluid activities at Site U1433 compared to the other sites, possibly caused by the cover of clayey sediment with low permeability.

The basement unit at Site U1434 is comprised of a succession of small pillow lava flows, or a thicker autobrecciated pillow lava flow, with three occurrences of hyaloclastite breccia (Fig. F23). The basalts are aphyric with glassy to microcrystalline groundmasses. The phenocrysts observed are olivine and plagioclase. The secondary mineral assemblage indicates slight to moderate alteration under low temperature, limited fluid flow, and oxidative conditions.

A total of 34 basement basalt samples were analyzed for concentrations of major and trace elements from the three sites. Almost all samples are tholeiitic basalt (Fig. F12) with extremely low K2O (<0.3 wt%), in contrast to the much higher K2O contents (>1.0 wt%) in the alkali basalt clasts in the younger volcaniclastic breccia. Overall, all concentrations of major elements in the basalt overlap with, but define a much smaller range than those known from compilations of Indian Ocean and Pacific Ocean MORB data, and are distinct from the OIB data fields from nearby Hainan Island and other seamounts in the SCS (Figs. F12, F13). Further postcruise geochemical studies will trace the mantle source and magmatic processes involved during and after opening of the SCS in detail.

Geophysical constraints and geodynamic implications

During the expedition, a suite of physical property measurements and color spectra scanning was completed on whole-round cores, split cores, and discrete samples. Downhole logging at the two deepest sites provided in situ constraints on the sediments and rocks, which is particularly important over unrecovered intervals. These data will aid in future geophysical interpretations of geophysical data from the SCS.

Compaction and consolidation are controlling factors for physical property variations in the sediment, as measured porosity often has good inverse correlation with other measurements, such as bulk density, P-wave velocity, shear strength, NGR, and thermal conductivity. Increased compaction and lithification also lead to sharp velocity contrasts and differentiate velocities of different lithologies that would otherwise be similar. These observations explain the strong seismic reflectivity of strata in the bottom unit of the sedimentary cover at Site U1433, where lithified carbonates show much higher velocities than interbedded claystone (Fig. F17).

NGR downhole logging in the igneous basement at Site U1433 helped define two interflow sedimentary layers between lava units through identification of high peaks in NGR (Fig. F17). This was particularly useful because coring did not fully recover these layers. Cores show only traces of sediment attached to basalt pieces. Gamma ray logging also helped constrain the alternating nature of carbonate with low NGR values and clay layers with high NGR values and is therefore valuable in delineating turbidites.

Magnetic susceptibility was measured on both whole-round cores and the split archive halves. In addition, remanent magnetization, which is proportional to the magnetic susceptibility, was measured with the pass-through magnetometer on all archive halves and on representative discrete samples from the working halves. We found that magnetic susceptibility of the basalt varies from ~10 × 10–5 to 2000 × 10–5 SI (Figs. F9, F17, F21). The values are much higher in relatively unaltered intervals within the more massive basalt flows compared to small lava flow units that are typically more prone to alteration. For now it remains uncertain how the entire basaltic layer of the oceanic crust behaves with respect to magnetic susceptibility, since we only recovered its very top. Overall, we did not observe major differences in the measured magnetic susceptibilities between the East and Southwest Subbasins, whereas differences in magnetic susceptibilities have been previously predicted or suspected by the contrast in surface magnetic anomalies (Li et al., 2008). Therefore, other mechanisms such as spreading rate, thermal disturbance, compositional variation, and posteruption alteration are needed to further explain the observed surface magnetic contrast.

Interplay between microbiology, fluid flow, geochemistry, and tectonics

A suite of samples collected at all of the Expedition 349 sites for microbiological analysis will allow examination of how microbial community features are linked to large-scale geological processes characteristic of the SCS that are representative of subseafloor settings worldwide. Samples were collected to examine the importance of ammonia-oxidizing archaea in the sediment in which archaeal biomarkers are preserved, what these archaea indicate about paleoceanographic conditions, and whether Bdellovibrio and similar organisms (bacterial predators) exist in the seafloor under contrasting fluid flow and heat flow conditions. These microbiological samples were collected proximal to samples for interstitial water chemistry analysis to help us reconstruct the environmental conditions where these cells are present.

We collected samples allowing us to test the hypothesis that the habitats at key interfaces, such as where volcanic ashes or turbidites overlie fine-grained sediment, provide optimum conditions for microbial colonization and survival. In addition to helping to explain the explicit conditions under which microbial life in the deep subseafloor may thrive, these sites provide several different environments that link to large-scale processes such as volcanism, tectonism, and turbidity flows. There is the potential to assess how regional and continental scale events related to erosion, seafloor spreading, and subduction zones can dictate life at the smallest scale.

Samples from most sites drilled during Expedition 349 show low levels of hydrocarbon gases; however, at Sites U1432 and U1433, sediment samples showed evidence of moderately high concentrations of methane. Ethane and propane concentrations increased with depth at these sites. Evidence of higher hydrocarbon contents in this deep-sea sediment is worth additional study, and their presence suggests factors that should be considered regarding the biogeochemistry of these deep subseafloor systems and how they respond to regional tectonics and depositional settings.