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doi:10.2204/iodp.pr.307.2005
Site U1316 (proposal 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 (Fig. F5). 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 sigmoid units observed in the upper seismic Unit P1, which appears to form the basement of Challenger Mound.
- Evaluate whether the sigmoid-shaped units 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 sedimentary suite of post-, syn-, and premound growth phases that correspond to three identified lithologic units, Units 1–3, respectively. The uppermost 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 fining-upward sand beds ~0.7–1.0 m thick. 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 ~1 m and 70 cm thick. A coral-bearing facies 10–13 m thick underlies this erosional boundary and defines 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 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.
Biostratigraphic data confirm the age of Unit 1 as middle to late Pleistocene by the continuous occurrence of Emiliania huxleyi (0.26 Ma–recent). The age of Unit 2 is mostly Pleistocene (0.46–1.95 Ma) from the last-appearance datum (LAD) of Discoaster triradiatus to the LAD of Pseudoemiliania lacunosa. A significant hiatus, which includes Moicene microfossils, was recognized above Unit 3. However, an inconsistency arises in age. The top of this unit is dated as early Miocene by nannofossils but Early Pliocene by planktonic foraminifers.
Archive core halves were measured for magnetization after 0, 15, and 20 mT demagnetization steps. 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. F6). 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 1–5 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 the microbial-mediated reactions dictate 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 deeper carbonate diagenesis. 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, probably driven by methane oxidation (Fig. F7). Prokaryotes are present in all samples counted, but abundances appear to be low throughout much of Site U1316 (Fig. F7); 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. 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 wave-shaped reflectors in the seismic survey.
Site U1317 is located on the northwest shoulder of Challenger Mound (proposal Site PORC-3A; 51°22.8'N, 11°43.1'W; 781–815 m water depth) (Fig. F5). 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 deepwater carbonate mounds including timing of key mound-building phases.
Sediments from on-mound Site U1317 can be divided into two lithostratigraphic units; a Pleistocene coral-bearing unit (Unit 1) and a Neogene siltstone (Unit 2) (Fig. F8). Unit 1 consists mainly of coral floatstone, rudstone, wackestone, and packstone and repeats cyclic color changes between light gray and dark green. Sediment carbonate contents relate to color: lighter-colored sediments tend to be more calcareous and commonly exhibit lithification textures. Corals were mostly identified as L. pertusa. The thickness of the unit increases toward the middle of the mound (Fig. F9): 130 m thick in Hole U1317A and thickens to 155 m in Hole U1317E closest to the mound summit (Fig. F10). The horizontal distance between the two holes is <100 m. Coral mound Unit 1 rests a sharp erosional boundary that appears identical to the boundary between Units 2 and 3 at Site U1316 (Fig. F11). The mound base, at the top of Unit 2, is a firmground exhibiting lighter color probably due to submarine weathering. 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 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. The presence of Pseudoemiliania lacunosa (LO = 0.46 Ma) and the absence of Calcidiscus 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 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, for the upper part of Unit 1 (0–11 mbsf) is reworked or in place. Nannofossils from the lower part of Unit 1 (73.0–130.0 mbsf) correspond to the early Pleistocene C. macintyrei Zone (1.59–1.95 Ma). The age of Unit 2 ranges from Early Pliocene to Miocene, as indicated by both nannofossils and planktonic foraminifers. The interval of the hiatus between Unit 1 and 2 was estimated at longer than 1.65 m.y.
Whole-round cores were measured for magnetization after 0, 10, and 15 mT demagnetization steps. Whole rounds were used because (a) three of the holes would not be opened during the expedition, (b) twice as much sediment (compared to the archive half) would give a better signal in these weakly magnetic carbonate-rich sediments, (c) the sediment would be undisturbed by splitting, and (d) 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, but coherent changes in polarity could be observed. In Hole 1317B, 0–62 mbsf is predominantly normal polarity and interpreted as the Brunhes Chron (0–0.78 Ma), and two predominantly reversed 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. F6).
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. F10). These intervals could be traced from Hole U1317A upslope through Holes U1317B, U1317C, and U1317E. These intervals coincide with dark green floatstones. The corresponding seismic facies is acoustically transparent, despite the layering observed in the cores. Transparency could be caused by the high coral content scattering the seismic waves or by the lack of horizontally contiguous facies that would provide internal reflectors throughout the mound. 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. These layers can be correlated with high-amplitude sigmoidal reflectors observed in seismic profiles.
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 a steady downhole increase due to compaction interrupted by 1–5 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. Interval velocities were calculated from the check shot survey. They confirm the values of the acoustic velocity logs but show that the physical property measurements made on the cores significantly underestimate in situ velocity (Fig. F12).
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 activity and abundance in the mound were low (Fig. F7). In short, Challenger Mound is not a model for a microbial origin of Phanerozoic carbonate mounds. Rather, it is the subtle intertwining of carbonate diagenesis and microbial sulfate reduction that provides the highlights of the chemical and microbiolgical investigations on the mound site. Sulfate, ammonium, and alkalinity profiles reflect zones of microbially mediated organic mineralization. The concave-downward profile for sulfate between 10 and 50 mbsf concurrent with the convex-upward curvature for alkalinity indicates active sulfate reduction. Magnesium also shows a loss, clearly shown in the decreased Mg/Ca ratio at these depths (Fig. F13). With the slight increase in Sr, we propose that aragonite dissolution is releasing Sr to the interstitial pore fluids. Concurrently, dolomite or some other high-Mg content carbonate mineral precipitates and removes Mg. Decomposition of organic matter (organoclastic) by sulfate reduction may be driving this process by (1) producing CO2, which enhances aragonite weathering, and (2) increasing the overall dissolved inorganic carbon concentration. Interestingly, this dolomite formation (or high-Mg carbonate mineral formation) must be occurring in a sulfate-rich zone. Deeper in the methane zone below 150 mbsf there is also evidence for dolomite formation. A broad transition of methane and sulfate between 150 and 200 mbsf defines the zone of anaerobic oxidation of methane coupled to sulfate reduction (methylotrophic).
Site U1318 (proposal 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 (Fig. F1) 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 (Fig. F11). The uppermost 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/bioturbated textures. Dropstones are common in Subunit 3A. Across a distinct erosional surface, 4–6 m thick Unit 2 underlies Unit 1. Unit 2 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. Dropstones, 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. It was interpreted as a basal conglomerate on an unconformable boundary with Unit 3. The top of Unit 3 is marked by a 10–20 cm thick oyster bed, and oysters 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 3B and 3C is marked by a distinct erosional surface. Unit 3 tends to become more calcareous downhole.
Unit 1 is younger than 0.26 Ma, as indicated by the abundant occurrence of E. huxleyi, and corresponds to Unit 1 at Site U1316. Nannofossils from Unit 2 indicate the early Pleistocene small Gephyrocapsa Zone (0.96–1.22 Ma), which was also found in the upper part of Unit 1 (mound section) at Site U1317. The interval of the hiatus between Unit 1 and 2 was estimated at older than 0.7 ma. The age of Unit 3 ranges from Pliocene to Miocene. Nannofossil data indicate a clear hiatus between Units 2 and 3.
Archive halves were measured for magnetization after 0, 15, and 20 mT demagnetization steps. 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. F6). 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, dropstones, and oyster bed of lithostratigraphic Unit 2 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, whose identification 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 oyster bed at the top of Unit 3 is rich in uranium (as seen in the natural gamma radiation logs), which tends 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 nearly nonchanging, 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 low-water stand (e.g., the Messinian Salt Crisis in the late Miocene). Buried and repeating trends in the sequence of terminal electron acceptors are also seen in the interstitial water profiles of Mn, Fe, and sulfate. We observe a peak in dissolved Mn underlain by dissolved peaks in Fe and a decrease in sulfate concentrations. This indicates the classic sequence of Mn reduction, Fe reduction, followed by sulfate reduction. This sequence occurs at the surface and then repeats at 80 mbsf and then again at 180 mbsf. These represent buried redox fronts. Prokaryote abundances (Fig. F7) 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 must 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 concentration between Unit 1 (200 Unit 3 (900 ring 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.