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Site U1316

Site U1316 (proposed Site PORC-4A; 51°22.56′N, 11°43.81′W; 965 m water depth) is located in downslope sediment deposits ~700 m southwest of Challenger Mound (Figs. F6, F7). The principal objectives at Site U1316 included the following:

  • Gain insight into the history of drift deposits on the downslope flank of Challenger Mound and the off-mound transport of mound-related skeletal and nonskeletal grains.
  • Investigate the character and age of the clinoforms observed in the upper seismic Unit P1, which appear to form the basement of Challenger Mound.
  • Evaluate whether the clinoforms contain high concentrations of gas and thus represent a potential hazard for drilling in Challenger Mound. Further drilling at Challenger Mound (Site U1317) was contingent on investigation of the sediments along its flank at Site U1316.

Sediments recovered from Site U1316, located basinward of Challenger Mound, contain a suite of post-, syn-, and premound sediments (Figs. F8, F9). The uppermost lithostratigraphic Unit 1 is 52–58 m thick and is mainly composed of grayish brown silty clay. It is divided into two subunits. Subunit 1A is dominated by dark grayish green silty clay, whereas Subunit 1B contains very dark gray silty clay interbedded with fine sand layers, interpreted as millimeter- and centimeter-scale turbidites. Dropstones are observed throughout Unit 1 in distinct intervals. The base of Unit 1 is defined by an erosive unconformity overlain by a fining-upward sequence of graded fine to very fine sand beds. A 2–4 m thick coral-bearing facies underlies this erosional boundary and defines lithostratigraphic Unit 2. Corals in the basal layer of this unit exhibit floatstone facies, which could have been buried in their growth position. For the uppermost coral horizon a debris flow transport process is suggested by (1) the high fragmentation of corals compared to the coral-bearing horizons below and (2) the fine-grained matrix-supported texture. The age of Unit 2 is mostly Pleistocene, which corresponds to the age of the thick coral mound at Site U1317. Unit 2 rests on lithostratigraphic Unit 3, with a distinct unconformity surface. Unit 3 consists of 92 m thick (Hole U1316C) heterogeneous dark green glauconitic siltstone and is calcareous in the lower part. Dolomite precipitation formed lithified layers at ~72 mbsf.

Calcareous nannofossil data confirm the age of lithostratigraphic Unit 1 as middle–late Pleistocene by the continuous occurrence of Emiliania huxleyi (0.26 Ma–recent). Coral-bearing Unit 2 ranges in age from late Pliocene to Pleistocene, as defined by the Calcidiscus macintyrei and Pseudoemiliania lacunosa Zones. Planktonic foraminifer data are consistent with these biostratigraphic results. More detailed biostratigraphic study will determine the extent of the unconformity, mound growth, and rapid burial. Lithostratigraphic Unit 3, underlying the unconformity, is early–middle Miocene in age.

Magnetic inclinations for lithostratigraphic Unit 1 average 66°, in the neighborhood of the expected inclination (68°) at the site latitude (51.4°N); therefore, Unit 1 is normal polarity and is assigned to the Brunhes Chron (0–0.78 Ma). The Brunhes/​Matuyama boundary is not observed (Fig. F10). Below the unconformity between Unit 2 and Unit 3, magnetic intensities are weaker (typical for carbonate-rich sediments) and inclination data are more scattered and could not be interpreted in terms of magnetic polarity stratigraphy. Extended core barrel (XCB) coring disturbance also degraded the paleomagnetic signal in the cores.

Distinct trends and patterns, which could be related to lithology and seismology, were observed in the physical property measurements. Lithostratigraphic Unit 1 has high magnetic susceptibility and natural gamma radiation typical of relatively siliciclastic rich sediment, both of which have lower values in Units 2 and 3, supportive of the observation of increased carbonate content relative to Unit 1. The downhole increase in density at the Unit 2/3 boundary is the cause of a strong reflection recognized as a regional unconformity in the seismic survey.

After an unsuccessful attempt to log Hole U1316A, triple combination (triple combo) and Formation MicroScanner (FMS)-sonic tool string downhole logs were acquired between 60 and 140 mbsf in Hole U1316C. Density, resistivity, and acoustic velocity logs show a steady downhole increase due to compaction, interrupted by 0.2–5 m thick intervals of higher values indicating the presence of more lithified layers. Photoelectric effect (PEF) values for these layers indicate that they are carbonate rich. These lithified layers are the cause of several strong reflections in the sigmoidal package in the seismic section at this site.

The interplay of diffusion and burial and microbial-mediated reactions dictates the profiles of interstitial water chemical species and dissolved gas chemistry at Site U1316. Li and B are apparently being released at depth during silicate diagenesis, whereas Sr is most likely released during carbonate diagenesis deep in Unit 3 or below. Concave-upward curvature in the profiles of minor elements Li, Sr, and B suggests that high rates of sediment burial in the uppermost 80 mbsf dominates diffusion. Interstitial water alkalinity, ammonia, and sulfate profiles indicated two zones of microbiological activity: an upper zone of activity between the surface and 10 mbsf and a lower zone between 80 and 100 mbsf, the latter driven by methane oxidation. Prokaryotes are present in all samples counted, but abundances appear to be low throughout much of Site U1316 (Fig. F11); a zone nearly 30 m thick between 56 and 85 mbsf appears to represent a “dead zone” based on the absence of evidence for cell division. However, prokaryote abundances increase in the zone of apparent methane oxidation coupled with sulfate reduction (Fig. F11). Enhanced ethane/methane ratios suggest preferential removal of methane over ethane through the methane–sulfate transition (82–130 mbsf). Generally, methane concentrations are low within Subunit 3A sediments and only increase to concentrations of 2 mM at 130 mbsf. We found no significant accumulations of gas within the sediments that coincided with the clinoforms in the seismic survey.

Site U1317

Site U1317 is located on the northwest shoulder of Challenger Mound (proposed Site PORC-3A; 51°22.8′N, 11°43.1′W; 781–815 m water depth) (Fig. F7). Scientific drilling of Challenger Mound was the central objective of Expedition 307. Specific objectives of drilling Site U1317 were as follows:

  • Establish whether the mound base is on a carbonate hardground of microbial origin and whether past geofluid migration events acted as a prime trigger for mound genesis.
  • Define the relationship, if any, between the mound-developing event and global oceanographic events that might have formed the erosional surfaces observed in high-resolution seismic profiles.
  • Analyze geochemical and microbiological profiles that define the sequence of microbial communities and geomicrobial reaction throughout the drilled sections.
  • Describe stratigraphic, lithologic, and diagenetic characteristics for establishing the principal depositional model of cold-water coral mounds including timing of key mound-building phases.

Sediments from on-mound Site U1317 can be divided into two lithostratigraphic units; an Upper Pliocene–Pleistocene coral-bearing unit (Unit 1) and a Miocene siltstone (Unit 2) (Figs. F8, F9). Unit 1 consists mainly of coral floatstone and rudstone further characterized by repeated cyclic color changes between light gray and dark green. Sediment carbonate contents and coral preservation relate to color: lighter-colored sediments tend to be more calcareous and commonly exhibit lithification textures and dissolved coral fragments. Corals were dominated by the species Lophelia pertusa. The thickness of the unit increases toward the middle of the mound from 130 m in Hole U1317A to 155 m in Hole U1317E closest to the mound summit (Figs. F8, F12). The horizontal distance between the two holes is <100 m. Coral mound Unit 1 rests on a sharp erosional boundary that appears identical to the boundary between Units 2 and 3 at Site U1316 (Fig. F13). The mound base, at the top of Unit 2, is characterized by firmground exhibiting lighter color probably due to submarine alteration. No lithification feature was recognized at the mound base. Unit 2, 124 m thick (Hole U1317D), consists of glauconitic and partly sandy siltstone, which increases in carbonate content downhole. It is lithostratigraphically correlated with Unit 3 at Site U1316.

Poor preservation and the exclusion of warm-water taxa made it difficult to construct a high-resolution nannofossil biostratigraphy for the mound sediment. The early Pleistocene age of the small Gephyrocapsa Zone (0.96–1.22 Ma) is assigned for the upper part of Unit 1 (0–73.0 mbsf in Hole U1316A) by the high abundance of small Gephyrocapsa. This age assignment is considered tentative as the small Gephyrocapsa Zone is only well preserved in sediment from 0 to 11 mbsf. Alternatively, the presence of P. lacunosa (LO = 0.46 Ma) and the absence of C. macintyrei (LO = 1.59 Ma) indicate that age of the upper part of Unit 1 is between at least 0.46 and 1.59 Ma. The middle–late Pleistocene age of Subzone Pt1b (0–0.65 Ma) is assigned based on planktonic foraminifer biostratigraphy for the upper part of Unit 1 (0–63 mbsf). Further investigation is needed to determine if the sediment, and subsequent nannofossil age assignment, of the upper part of Unit 1 (0–11 mbsf) is reworked or in place. Nannofossils from the lower part of Unit 1 (82.7–130.1 mbsf) correspond to the early Pleistocene C. macintyrei Zone (1.59–1.95 Ma). A Pliocene–Pleistocene age is assigned for the lower part of Unit 1 based on planktonic foraminifers. The age of Unit 2 ranges from early–middle Miocene to possibly early Pliocene, as indicated by nannofossils, and is interpreted from planktonic foraminifers as Miocene. The interval of the hiatus between Units 1 and 2 has a lower limit of 13.5 m.y., as indicated by nannofossil datums.

Whole-round cores were measured for magnetization after 0, 10, and 15 mT demagnetization steps. Whole rounds were used because

  • Three of the holes would not be opened during the expedition,
  • Twice as much sediment (compared to the archive half) would give a better signal in these weakly magnetic carbonate-rich sediments,
  • The sediment would be undisturbed by splitting, and
  • The possible presence of ephemeral magnetic minerals such as greigite could affect results.

Demagnetization tests were run to ensure that only the overprint was removed by bulk demagnetization at 15 mT. Lithostratigraphic Unit 1, the mound sediments, had somewhat scattered inclinations; nevertheless, coherent changes in polarity are observed. Hole 1317B, 0–62 mbsf, is predominantly of normal polarity and interpreted as the Brunhes Chron (0–0.78 Ma) (Fig. F10), and two predominantly reversed polarity intervals occur between 62 and 82 mbsf and 96 and 105 mbsf that are tentatively interpreted as part of the Matuyama Chron. The lower part of Unit 1 below 105 mbsf shows normal polarity, which might correspond to the Olduvai Chron (Fig. F10).

The two lithostratigraphic units at this site exhibit distinctive and contrasting physical properties. In Unit 1, the mound facies, generally low values of natural gamma radiation and magnetic susceptibility are caused by high carbonate content. Some cyclically recurring intervals are characterized by relatively higher natural gamma radiation and magnetic susceptibility, density, and P-wave velocity, indicating a higher clay content (Fig. F12). These intervals could be traced from Hole U1317A upslope through Holes U1317B, U1317C, and U1317E. These intervals coincide with dark green floatstones. Despite this layering observed in the cores, the seismic facies throughout the mound appears acoustically transparent in seismic surveys. Transparency could be caused by the high coral content scattering the seismic waves, irregular surfaces, or low acoustic impedance contrasts between lithologic layers. The lower boundary of Unit 1, the mound base, is characterized by an increase in density, gamma radiation, and magnetic susceptibility. Lithostratigraphic Unit 2 is characterized by very low susceptibility values and eight peaks of high density, P-wave velocity, magnetic susceptibility, and gamma radiation that coincide with more lithified layers and sandier layers.

Triple combo and FMS-sonic tool string downhole logs and a zero-offset vertical seismic profile were run between 80 and 245 mbsf in Hole U1317D. Density, resistivity, and acoustic velocity logs show steady downhole increases due to compaction interrupted by 0.2–5 m thick intervals of higher values, indicating the presence of more lithified layers similar to Hole U1316C. PEF values for these layers indicate that they are carbonate rich. These lithified layers are the cause of the high-amplitude sigmoidal reflectors observed in seismic profiles (in lithostratigraphic Unit 1 and seismic Unit P1) and that are correlated with the aforementioned peaks in physical property measurements. Interval velocities were calculated from the check shot survey. They confirm the values of the acoustic velocity logs but show that the shipboard P-wave velocity measurements made on the cores significantly underestimate in situ velocity (Fig. F14). Prior to drilling, the possibility of a mound velocity of 1850 m/s could be interpreted as partial lithification as the result of biogeochemical processes. Figure F14 shows the mound is, in fact, mostly composed of unlithified sediments with modest velocities.

A role for hydrocarbon fluid flow in the initial growth phase of Challenger Mound is not obvious from either lithostratigraphy or the initial geochemistry and microbiology results. We found no significant quantities of gas in the mound or in the subbasal mound sediments, nor were carbonate hardgrounds observed at the mound base. Overall indexes of microbial abundance in the mound were low (Fig. F11). In short, Challenger Mound is not a model for a fully microbial origin of Phanerozoic carbonate mounds. Rather, mound geochemistry reflects the interplay of sediment trapping and deposition, mineral-controlled diagenesis, and reactions driven by microbial activities. Methane, sulfate, and alkalinity profiles reflect zones of microbially mediated organic mineralization. The concave-downward profiles for sulfate and the convex-upward curvatures for alkalinity between 0 and 50 mbsf indicate active sulfate reduction. Magnesium also shows a loss, as evidenced in the decreased Mg/Ca ratio at these depths (Fig. F15). With the distinct increase in Sr in this interval, we propose that aragonite dissolution is releasing Sr to the interstitial pore fluids. Concurrently, dolomite or some other Mg-bearing carbonate mineral (calcian dolomite) is precipitating and removing Mg. Below the mound base, the methane and sulfate profiles between 150 and 200 mbsf define the zone of anaerobic oxidation of methane coupled to sulfate reduction (Fig. F15). Increasing Ca and decreasing Mg suggest that dolomite is forming within these deeper Miocene sediments.

Site U1318

Site U1318 (proposed Site PORC-2A; 51°26.16′N, 11°33.0′W; 423 m water depth) is located on the eastern slope of Porcupine Seabight on the southwest continental margin of Ireland (Figs. F1, F6, F9) and is upslope from the Belgica mound province, including Challenger Mound. The principal objective at Site U1318 was to recover sediments from the three seismic units (P1–P3) of the southern Belgica mound province (Fig. F3). Complete data from the seismically low-amplitude layer (Unit P2) would refine the paleoenvironmental history of when Challenger Mound growth initiated.

Sediments from Site U1318 were divided into three lithostratigraphic units based on sediment color, erosional surfaces, and biostratigraphy (Figs. F8, F9). Unit 1 is 78.9–82.0 m thick and consists of grayish brown silty clay with black mottled structure. This unit was divided into three subunits (1A, 1B, and 1C) on the basis of relative dominance of laminated or bioturbated textures. Dropstones are common in Subunit 1A, and the base of Subunit 1C is defined by a sharp erosional boundary.

Unit 2 (4–6 m thick) mainly consists of olive-gray medium–fine sand interbedded with dark yellowish brown silty clay. The sand beds are normal graded with sharp lower and upper boundaries. Pebble-sized clasts, with diameters up to 3 cm, are found in both sand and clay horizons. The base of this unit is a 5–10 cm thick conglomerate that shows normal grading and contains black apatite nodules and was interpreted as a basal conglomerate on an unconformable boundary with Unit 3.

The top of Unit 3 (84.2–86.7 mbsf) is marked by a 10–20 cm thick phosphatic shell bed (bivalves), and shells are common in the uppermost 20 m of Unit 3. Unit 3 is 155 m thick (Hole U1318B), mainly siltstone, and is divided into three subunits. Subunit 3A is characterized by frequent intercalation of sandstone and is differentiated from darker-colored Subunit 3B, which rarely contains sandy layers. The boundary between Subunits 3A and 3B is marked by a distinct erosional surface. The boundary between seismic Units P2 and P1 is roughly correlated to the boundary between lithostratigraphic Subunits 3B and 3C, where no evident unconformity was found (Fig. F9).

Unit 1 is younger than 0.26 Ma, as indicated by the abundant occurrence of E. huxleyi, and correlates to Unit 1 at Site U1316. Nannofossils from Unit 2 indicate the early Pleistocene small Gephyrocapsa Zone (0.96–1.22 Ma). The interval of the hiatus between Units 1 and 2 was estimated as at least 0.7 m.y. The age of Subunit 3A is older than 3.6 Ma and nannofossil data indicate a clear hiatus between Units 2 and 3 of at least 2.4 m.y. Nannofossil data also indicates that Subunits 3B and 3C are middle to lower Miocene in age (13.6–15.6 Ma).

Magnetic inclinations for lithostratigraphic Unit 1 are close to the expected inclination (68°) at the site latitude (51.4°N); therefore, Unit 1 is assigned to the Brunhes Chron (0–0.78 Ma). The normal polarity of Unit 1 is truncated by a hiatus, identified by lithostratigraphy and biostratigraphy, so the base of the Brunhes is not represented (Fig. F11). Below the hiatus in lithostratigraphic Unit 3, due to weakened magnetic susceptibilities and intensities, inclination data are more scattered and could not be interpreted in terms of magnetic polarity stratigraphy. It is noteworthy that, although the silty clay sediments are calcareous, carbonate content is not high enough to dilute magnetic susceptibility to the extent observed. Therefore, either lithostratigraphic Unit 3 has a much lower siliciclastic content than is supposed or the magnetic minerals in the unit have been dissolved.

Major changes in physical properties that can be directly related to reflectors in the seismic section were observed at lithostratigraphic unit boundaries. The sand layers, silty clays, and dropstones of lithostratigraphic Unit 2 and the bivalve bed of Unit 3 create a high-amplitude reflector in the seismic profile, and this erosive reflector has been tentatively identified as the upslope continuation of the mound base reflector. The enigmatic low-amplitude seismic package (P2 in Fig. F3), the identification of which was one of the main aims of drilling this site, corresponds to homogeneous calcareous silty clays. Lithostratigraphic Subunit 3C, below 192 mbsf, is characterized by a slight general increase in density in combination with some high-density thin beds and corresponds to high-amplitude, high-frequency parallel reflectors that can be traced along the seismic profile to the sigmoid unit at Site U1316.

Triple combo and FMS-sonic tool string downhole logs were acquired between 70 and 240 mbsf in Hole U1318B. The downhole logs are characterized by low-amplitude variations in lithostratigraphic Subunits 3A and 3B (92–192 mbsf) and by increased velocity and thin lithified layers in Subunit 3C (below 192 mbsf). The hiatus represented by the bivalve bed at the top of Unit 3 is rich in uranium (as seen in the natural gamma radiation logs), which is known to accumulate at hiatuses and condensed intervals.

Periods of rapid sedimentation overlying hiatuses have profoundly affected the chemistry and microbial activity of the Site U1318 sediments. The dominance of burial over diffusion within the upper Unit 1 sediments is strikingly shown by the near-seawater concentrations of Li and Sr in the upper 50 m of drift sediments. Below 50 mbsf both elements linearly increase with depth indicating a source for both elements deeper than our maximum sampling depth. Chlorinity, as meticulously measured by high-precision hand titration, exhibits a constant concentration of 570 mM in Unit 1. However, we observed a distinct increase in chloride concentrations between 80 and 150 mbsf of as much as 580 mM (at 140 mbsf). This excursion may be correlated to a major oceanographic change in seawater salinity. Buried and repeating trends in the classic sequence of microbially mediated terminal electron acceptor processes of Mn reduction and Fe reduction, followed by sulfate reduction, are manifested in the interstitial water profiles of Mn, Fe, and sulfate. We observe a peak in dissolved Mn underlain by peaks in Fe and a decrease in sulfate concentrations. This sequence occurs at the surface and repeats at 80 and 180 mbsf. These may be interpreted as buried redox fronts. Prokaryote abundances (Fig. F10) are greater than predicted from the global depth-abundance curve in the sediments of Subunits 1B and 1C but drop precipitously below the hiatus in Unit 2. Deep dolomitization appears be occurring based on the decrease in Mg at depths below 200 mbsf. This is consistent with microscopic and X-ray diffraction identification of dolomite in these sediments. Si exhibits striking change in the interstitial water concentrations between Unit 1 (200 µM) and Unit 3 (900 µM) sediments. This probably reflects a change in siliceous facies occurring over the hiatus at 82 mbsf. Total carbonate concentrations also change dramatically at this boundary, increasing from 10 wt% in Unit 1 to a peak of 40 wt% throughout Subunit 3A.