Sites U1302 and U1303
The overall objective at Sites U1302 and U1303 was to explore the record of Laurentide Ice Sheet (LIS) instability at this location close to Orphan Knoll (Fig. F1). Piston cores collected at Site U1303 (HU91-045-094P, MD99-2237, and MD95-2024) show the presence of numerous detrital layers within the last glacial cycle, some of which are rich in detrital carbonate (Hillaire-Marcel et al., 1994; Stoner et al., 1995, 1996). The digital image of Core 303-1303B-1H is shown in Figure F2, illustrating that these detrital layers are recognizable visually and in magnetic susceptibility (MS) and gamma ray attenuation (GRA) density measured on the shipboard multisensor track (MST). Isotopic data from planktonic foraminifers indicate that these detrital layers are associated with low-productivity meltwater pulses (Hillaire-Marcel et al., 1994). The objective at Sites U1302 and U1303 is to document this manifestation of LIS instability both during and prior to the last glacial cycle. The two sites are separated by 5.68 km. Drilling revealed a very similar stratigraphic sequence at the two sites. The MST data could be correlated from site to site at fine scale. The rationale for drilling Site U1302 was that the single-channel air gun seismic data (Toews and Piper, 2002) indicated a thicker section above mud waves at Site U1302 than at Site U1303 (the site of the piston cores mentioned above). We moved to Site U1303 after encountering a coarse-grained debris flow at ~105 meters composite depth (mcd) at Site U1302 that caused the cessation of APC penetration. The same debris flow, however, was encountered at Site U1303. The top of the debris flow coincides with a strong reflector in the air-gun seismic data, at ~100 ms two-way traveltime, which can be traced between the two sites.
Five holes, offset from each other by 30 m, were cored with the APC system at Site U1302 with an average recovery of 90.3%. Three holes (Holes 1302A1302C) were cored to total depth (maximum of 107.1 meters below seafloor [mbsf]) (Table T1), which was limited by the presence of a debris flow. Holes U1302D and U1302E consisted only of two cores each to capture the intervals at the top of the succession and provide overlap with coring gaps from the previous holes. Water depth was estimated to be 3555 mbsl based on recovery of the mudline core in Hole U1302D. We cored two APC holes 30 m apart at Site U1303. Penetration depth was limited by the debris interval to 93.9 mbsf in Hole U1303A (73.6% recovery) and 85.7 mbsf in Hole U1303B (83.5% recovery). Water depth at Site U1303 was estimated to be 3518 mbsl based on recovery of the mudline core in Hole U1303B.
An almost complete composite section was constructed at Site U1302 spanning the interval 0107 mcd. It was not possible to construct a complete composite record at Site U1303. However, the density and MS records from Sites U1302 and U1303 are remarkably similar and can be easily correlated. Using a short segment of one core from Site U1303 and the composite record from Site U1302 provides a continuous stratigraphic sequence to ~107 mcd (Fig. F3).
The sediments at Sites U1302 and U1303 are dominated by varying mixtures of terrigenous components and biogenic debris (primarily quartz, detrital carbonate, and nannofossils), so that the most common lithologies are clay, silty clay, silty clay with nannofossils, nannofossil silty clay, silty clay nannofossil ooze, and nannofossil ooze with silty clay. Dropstones are present throughout the cores. Calcium carbonate content ranges from 1 to 47 wt%. The sediments at Sites U1302 and U1303 have been designated as a single unit due to the gradational interbedding of these lithologies at scales of a few meters or less. This unit has been subdivided into two subunits, however, based on evidence for downslope mass flows at the base of the section. Subunit IA (0~106 mcd) is composed of undisturbed sediments, whereas Subunit IB (106132 mcd) contains abundant intraclasts in a matrix of sand-silt-clay and is interpreted to be debris flow deposits.
Samples from Site U1302 reveal rich assemblages of calcareous, siliceous, and organic-walled microfossils. Coccoliths are abundant and well preserved in most samples and permit establishment of biostratigraphic schemes that are complemented by a few datums from diatoms and palynological data. According to these schemes, the composite sequence of Site U1302 covers an interval spanning <1.16 m.y. and approximately the last 0.95 m.y., whereas the composite sequence of Site U1303 probably corresponds to an interval spanning approximately the last 0.85 m.y. (Fig. F4). The mean sedimentation rate at these sites is 13.4 cm/k.y. (Fig. F5). Beyond the biostratigraphic schemes, the micropaleontologic assemblages provide insight into paleoclimatologic and paleoceanographic conditions. In particular, the relative abundance of the planktonic foraminifer Neogloboquadrina pachyderma (sinistral) and some dinocyst assemblages allow identification of glacial and interglacial conditions from some core catcher samples.
The pore water chemistry from Sites U1302 and U1303 is dominated by reactions associated with organic matter degradation, despite the relatively low organic matter content of the sediments (~0.5 wt%). Sulfate concentrations decrease from seawater values to 5.9 mM close to the base of the recovered section indicating that sulfate reduction is almost complete by 109 mcd. Corresponding increases in alkalinity and ammonium downcore are byproducts of organic matter reactions. Alkalinity does not reach concentrations expected for the degree of sulfate reduction indicating alkalinity is being consumed within the cored interval. The decrease in calcium (to a minimum of 5.5 mM, a 52% decrease from seawater values) suggests precipitation of carbonate minerals as one possible explanation for the alkalinity profiles. Strontium concentrations remain at seawater values or lower throughout the cored interval indicating that carbonate dissolution and recrystallization are not important processes at these sites.
Overall, sediments of lithologic Subunit IA from Sites U1302 and U1303 are excellent geomagnetic field recorders as indicated by the fidelity of the shipboard paleomagnetic record. Inclinations vary coherently around those expected for the site latitudes. Declinations show consistent behavior within cores and, when Tensor corrected, between cores. Directional geomagnetic excursions are observed in three replicate sections from Holes 1302C, 1303A, and 1303B at ~31.40 mcd and are interpreted to represent the Iceland Basin Event at ~187 ka. Magnetization intensities are strong and magnetic properties look favorable for shore-based paleointensity studies. Within the debris flow (lithologic Subunit IB), the paleomagnetic record is not of the same quality. There is some evidence for reversed magnetizations within apparently undisturbed sediment within or below the debris flow, possibly denoting the uppermost Matuyama Chron, but the scattered magnetization directions in Subunit IB do not allow further interpretation (Fig. F4). In addition, it appears that the removal of the drill string magnetic overprint is greatly facilitated by the utilization of nonmagnetic core barrels.
The MS records obtained at these sites are strongly influenced by the detrital layer stratigraphy (Fig. F2) superimposed on a pattern of glacialinterglacial variability (Fig. F3). At both sites, natural gamma ray (NGR) variation is consistent with both MS and density measurements (i.e., low MS and density values correspond to low NGR counts: 2022 cps), which suggests these intervals were characterized by carbonate-dominated sedimentation. The downcore MST records not only provide a guide to the glacial-interglacial cycles, possibly back to MIS 17 but also provide a millennial-scale record of LIS instability through recognition of Heinrich-like detrital events. Detrital events in the Heinrich layer 16 (H1H6) interval are easily recognized in the MST data at Sites U1302 and U1303 and can be unambiguously correlated to similar records in neighboring piston cores such as MD95-2024 and MD99-2237 that record the last glacial cycle. The sedimentary succession at Sites U1302 and U1303 provides a record of LIS instability back to at least MIS 17. This record is a proximal analog to the classic Heinrich-layer stratigraphy of the central Atlantic.Site U1304
The objective at Site U1304 was to obtain a deepwater record from the southern edge of the Gardar Drift to compare with the intermediate depth site on the northern part of the Gardar Drift sampled during ODP Leg 162 (Site 983). Site U1304 lies in a partially enclosed basin north of the Charlie Gibbs Fracture Zone (GFZ), 217 km west-northwest of DSDP Site 611. The mean sedimentation rate at Site U1304 (14.9 cm/k.y.) (Fig. F5) is about six times that in the same interval at DSDP Site 611. Excellent preservation of benthic and planktonic microfossils enhances the potential for a high-resolution environmental record. The site will provide a monitor of NADW and sea-surface temperatures (SSTs) and a record of central Atlantic detrital layer stratigraphy.
Four holes were cored with the APC coring system to a maximum depth of 243.8 mbsf at Site U1304 (Fig. F6; Table T1). Overall recovery was 102.6%. Hole U1304C was limited to 69.6 m penetration when operations had to be terminated because of deteriorating weather conditions (heave in excess of 4 m at the rig floor). After waiting more than 3 h for the weather to abate, Hole U1304D was spudded and the interval 052 mbsf drilled before APC coring continued to total depth. The interval from 180.3181.3 mbsf was also drilled in Hole U1304D (i.e., not cored) due to an apparent hard interval impeding APC penetration. Water depth was estimated to be 3065 mbsl based on recovery of the mudline cores in Holes 1304C and 1304D. The drillover technique was utilized in Holes 1304A, 1304B, and 1304D to extend APC coring past initial refusal depth.
Correlation of cores among holes at Site U1304, utilizing mainly MS and NGR (Fig. F6), provides a continuous stratigraphic sequence to ~258.1 mcd with a single potential break within an 8 m thick diatom mat at ~199.3 mcd. The spliced composite section relies on sections from Holes 1304A and 1304B because good weather conditions during the early occupation of Site U1304 led to excellent recovery and good core quality.
The sediments at Site U1304 are predominantly interbedded diatom oozes and nannofossil oozes with less common intervals of clay and silty clay, which also contain abundant nannofossils and/or diatoms. Calcium carbonate content ranges from 5 to 70 wt% and organic carbon content is low (generally <0.5 wt%). This sedimentary succession has been designated as a single unit because the various lithologies are generally interbedded on a scale of only centimeters to decimeters. Most contacts between nannofossil ooze and clay intervals are gradational, although sharp contacts are also observed. The contacts between diatom ooze beds and the other lithologies are generally sharp. Redeposited beds of silt and sand-sized particles are rare, as are disturbed units related to mass-transport processes (e.g., slumps and debris flows). Thus, the section cored at Site U1304 apparently represents a relatively continuous pelagic section where the sediments record changes in productivity in response to oceanographic and climatic fluctuations.
Recurring laminated diatom sequences are the most prominent feature at Site U1304. The thicker diatom mats are clearly distinguished by very low MS values (Fig. F6). Diatom assemblages are dominated by needle-shaped species of the Thalassiothrix/Lioloma complex. All other groups investigated (coccoliths, planktonic and benthic foraminifers, radiolarians, and palynomorphs) are present in high-to-moderate abundance and are well preserved. Biostratigraphic datums mainly derive from coccoliths and are consistent with datums provided by diatoms, planktonic foraminifers, dinoflagellate cysts, and magnetostratigraphy (Fig. F4). The composite sequence covers the uppermost Pliocene and the entire Quaternary. The microfossil assemblage indicates only minor redeposition.
Preliminary paleoceanographic and paleoclimatologic interpretation of the microflora and microfauna reveals large-amplitude changes in surface water temperature and trophic conditions. Diatom layers were formed during both cold and warm phases, according to the diatom and planktonic foraminiferal assemblages. The presence of the benthic foraminifer Epistominella exigua documents recurring flux pulses of fresh organic matter to the seafloor. A shift from dominance of autotrophic to dominance of heterotrophic dinocyst assemblages is recorded after 1.2 Ma, which may suggest a general change in trophic conditions of the surface ocean.
Site U1304 sediments document an almost continuous sequence including the Brunhes Chron and part of the Matuyama Chron including the Jaramillo Subchron, the Cobb Mountain Subchron, and the top of the Olduvai Subchron (Fig. F4). Short intervals of apparent normal polarity were recognized in the Matuyama Chron below the Cobb Mountain Subchron. Mean sedimentation rates of 17.8 cm/k.y. are estimated for the last 0.78 m.y. and 12.2 cm/k.y. for the interval from 0.78 to 1.77 Ma, with an overall mean sedimentation rate of 14.9 cm/k.y. (Fig. F5).
Site U1304 pore water profiles indicate active sulfate reduction (minimum value of 2.8 mM at 214 mbsf) with corresponding increases in alkalinity and ammonium. Alkalinity values do not reach concentrations expected for the degree of sulfate reduction. Calcium concentrations decrease downcore to 2.7 mM, a ~75% reduction from standard seawater values. The decrease in calcium and consumption of alkalinity suggests active carbonate mineral precipitation. However, Sr concentrations remain at seawater values or lower, indicating that carbonate dissolution and recrystallization reactions are not important processes in the cored interval.
The Quaternary sequence recovered at Site U1304 provides a high-resolution, high-sedimentation-rate (average ~15 cm/k.y.) record of environmental change at a sensitive location close to the subarctic convergence between the surface Labrador Current and the North Atlantic Current. Good preservation of both calcareous and siliceous microfossils, abundant benthic foraminifers, and a high-fidelity magnetostratigraphic record indicate that the environmental record, including the monitoring of NADW, can be placed in a tight chronological framework.Site U1305
Site U1305 is located close to the southwest extremity of the Eirik Drift, 82.2 km south of ODP Site 646. The thickness of the sediments above the midUpper Pliocene seismic reflector R1 (~540 m) is almost twice that at ODP Site 646. The water depth (3459 mbsl) means that the seafloor at Site U1305 lies below the main axis of the Western Boundary Undercurrent (WBUC) and hence preserves expanded interglacial intervals and relatively condensed glacial intervals. The high mean sedimentation rate (>17 cm/k.y. for the Quaternary) promises a high-resolution record of ice sheet instability and changes in surface and deepwater masses.
Three holes were cored at Site U1305 with the APC system to a maximum depth of 287.1 mbsf (Fig. F7; Table T1) with an average recovery of 104%. Six cores had to be obtained by drillover in Hole U1305A, two in Hole U1305B, and none in Hole U1305C. After completing coring operations, Hole U1305C was prepared for logging and the triple combination (triple combo) tool string was deployed to ~258 mbsf (~29 m from the bottom of the hole). The hole was logged successfully on the first pass, but the tool became stuck when attempting to retrieve it into the drill pipe after a short second pass. The tool was eventually freed, and upon retrieval we discovered that one of the caliper arms had broken off and the logging line had been damaged. Because of heave state (up to 4 m), the hole condition, tool safety, and operational constraints, we decided to forego the deployment of the Formation MicroScanner (FMS)-sonic tool string, concluding operations at Site U1305.
The sediments at Site U1305 are designated as a single unit dominated by varying mixtures of terrigenous components and biogenic material, primarily clay minerals, quartz, detrital carbonate, and nannofossils. Calcium carbonate content ranges from 1 to 49 wt%. The most common lithologies are dark gray to very dark gray silty clay, silty clay with nannofossils, nannofossil silty clay, silty clay nannofossil ooze, and nannofossil ooze with silty clay. In addition, olive-gray sandy silt laminae and centimeter- to decimeter-scale intervals of silty clay with detrital carbonate are present at Site U1305. The sediments are gradationally interbedded at scales of a few meters or less.
Calcareous, siliceous, and organic-walled microfossils show generally good preservation and abundance in the upper ~200 mcd (Fig. F4). However, the abundance of microfossil assemblages is variable below this depth with generally poorer preservation. All microfossil groups investigated are dominated by subpolar to polar assemblages. Planktonic foraminifers show a biomodal test-size distribution. The small test-sized planktonic foraminifers, which are cold-water species, coexist with increased abundance of benthic foraminifers, possibly indicating transport by bottom currents.
The sediments at Site U1305 carry well-defined magnetization components and appear to provide useful records of geomagnetic transitions. Natural remanent magnetization (NRM) intensities are strong both before and after demagnetization and show variability at both the meter scale and throughout the sequence. NRM intensities decrease by ~50% below 166 mcd. Directional magnetization data allow identification of the Brunhes and part of the Matuyama Chron, including the Jaramillo, Cobb, and Olduvai Subchrons (Fig. F4). The Cobb Mountain Subchron and the top of the Olduvai Subchron are less clearly identified because of the incomplete removal of the normal polarity drill string magnetic overprint.
A continuous stratigraphic composite section was constructed to ~295 mcd with a single problematic interval between 197.2 and 206 mcd (Fig. F7). The mean sedimentation rate calculated using biostratigraphic and magnetostatigraphic datums is 17.5 cm/k.y. for the entire section cored at Site U1305 (Fig. F5). Using only paleomagnetic datums results in sedimentation rates that are still relatively uniform with the exception of a greater mean sedimentation rate between 1.07 and 1.19 Ma (from the base of the Jaramillo to the top of the Cobb Mountain Subchrons) that averages 29.3 cm/k.y.
Despite the low organic carbon content (mean = <0.4 wt%), organic matter diagenesis dominates the pore water chemistry. Sulfate decreases linearly downcore and is completely reduced by 58 mbsf. Methane increases immediately below the sulfate reduction zone, reaching a maximum of 46,000 ppmv at 228 mbsf. Ethane fluctuates between 2 and 14 ppmv within the methanogenic zone, but no higher hydrocarbons were detected. Alkalinity increases downcore reaching a maximum of 18.9 mM at the sulfate/methane interface (SMI). Calcium reaches a minimum of 2.58 mM at the same depth, suggesting carbonate precipitation associated with anaerobic methane oxidation at the SMI. Similar to previous Expedition 303 locations, dissolved strontium at Site U1305 is at or below seawater values indicating little or no carbonate dissolution or recrystallization.
Physical property records at Site U1305, in particular MS and density, are highly variable, recording lithologic and mineralogic changes (Fig. F7). Low MS and density values usually coincide with the presence of silt-sized detrital carbonate. Natural gamma radiation increases toward the transition between detrital carbonate and terrigenous-dominated layers, suggesting a relative increase in the clay component. Site U1305 sediments are also characterized by an overall downcore increase in density (from 1.3 to ~1.9 g/cm3), decreasing porosity (from 80% to 62%) and low velocities (15001600 m/s).
Data from wireline logging in Hole U1305C span the 95.3265.9 mbsf interval. The triple combo tool string was successfully deployed, yielding downhole records of density, porosity, NGR, electrical resistivity, and photoelectric factor. Density and porosity are generally inversely related to each other, with density increasing downhole and porosity decreasing. Density and porosity data are in some places affected by the large diameter of the hole (up to ~18 in), although these intervals most likely correspond to softer sediments that are more easily washed away. Density and gamma ray logging data show similar downhole trends to those observed in core data, suggesting that it will be possible to correlate core and logging data. The logging data also exhibit stratigraphic trends that appear to be cyclic, possibly caused by temporal changes in lithology. Successful core-log integration will permit an assessment of the origin and significance of the trends in the logging data.
The initial analysis of MST, archive multisensor track (AMST), biostratigraphic, and paleomagnetic data indicates that a complete and continuous high-resolution record (mean sedimentation rate of 17.3 cm/k.y.) covering the uppermost Pliocene and Quaternary has been recovered at Site U1305. The record represents a rich archive of environmental change that will document episodes of instability in the surrounding (Laurentide, Greenland, and Inuitian) ice sheets, the history of surface currents and deepwater currents, and, hence, the strength of the WBUC that contributes to NADW. Good preservation of both planktonic and benthic foraminifers for isotopic analysis and a high-fidelity paleomagnetic record indicate that the environmental record has the necessary attributes for construction of a paleointensity-assisted chronostratigraphy.Site U1306
Site U1306 was placed at the crossing of two seismic lines (Lines 19 and 24) in the multichannel seismic (MCS) network obtained over the Eirik Drift during Cruise KN-166 (Knorr, Principal Investigator [PI]: Greg Mountain) in summer 2002. At this location, mean Upper Pliocene and Quaternary sedimentation rates were estimated to be ~18 cm/k.y. based on identification of seismic reflector R1, which can be correlated to the midUpper Pliocene at ODP Site 646. The placement of the site was designed to yield a high-resolution, high-sedimentation-rate Quaternary environmental record from a water depth (2273 mbsl) within the main axis of the WBUC. Based on a nearby conventional piston core from a similar water depth (Core HU90-013-012) (Hillaire-Marcel et al., 1994), we expect glacial intervals to be expanded relative to interglacial intervals.
Four holes were cored with the APC system at Site U1306 reaching a maximum depth of 309.3 mbsf (Fig. F8; Table T1). Hole U1306D was cored to 180.0 mbsf to provide necessary stratigraphic overlap for the upper portion of the succession. Five intervals totaling 13 m were drilled without coring to adjust the stratigraphic offset between holes or to get through difficult-to-core intervals. Five cores were obtained by drillover. Average recovery was 102.5% for the cored interval.
The sediments at Site U1306 are designated as a single lithostratigraphic unit, composed of Holocene to uppermost Pliocene terrigenous and biogenic sediments, which are gradationally interbedded at scales of a few meters or less. Calcium carbonate content is low ranging from 0.3 to 12.3 wt% (mean = 3.2 wt%). The most common lithologies are silty clay, silty clay with diatoms, nannofossil silty clay, and silty clay nannofossil ooze. Dropstones are present throughout the cored interval, and large dropstones (~4 cm) are common to abundant. Centimeter- to decimeter-scale beds of olive-gray or greenish gray silty clay or clay with high detrital carbonate content are present in all holes at Site U1306, but are thinner and less common than at Site U1305.
Rich assemblages of calcareous, siliceous, and organic-walled microfossils are present at Site U1306 (Fig. F4), although benthic foraminifers are barren in many samples below 175 mcd. Large variations in abundance of microfossils occur downcore. Although preservation is moderate to good in the upper part of the succession, preservation generally decreases below ~170 mcd for calcareous and siliceous microfossils. All samples contain moderately well to well-preserved palynomorphs, but variable numbers of dinocysts, which are abundant only in a few samples. Some redeposition is indicated by the presence of reworked nannofossils and palynomorphs of Cretaceous to Miocene age through the cored interval. The dominant components of each microfossil group reflect cold SSTs for most of the time represented by the sedimentary sequence.
The sediments at Site U1306 carry well-defined magnetization components and document an apparently continuous sequence including the Brunhes Chron and much of the Matuyama Chron. The Jaramillo, Cobb Mountain, and Olduvai Subchrons are clearly identified (Fig. F4). Within the Brunhes Chron, the Iceland Basin Event (~187 ka) was observed in three of the holes.
A continuous stratigraphic composite section was constructed to ~337 mcd (Fig. F8). Below 287 mcd, cores were recovered in two holes only, but the section is complete with only a single tenuous tie near the base of the record. The mean sedimentation rate calculated using biostratigraphic and magnetostatigraphic datums is 15.6 cm/k.y. for the entire section cored at Site U1306 (Fig. F5). Using only paleomagnetic datums, interval sedimentation rates vary between 12.4 and 19.3 cm/k.y.
Pore water chemical profiles at Site U1306 document very similar reactions to nearby Site U1305. Complete sulfate reduction is achieved at shallow depths at Site U1306 (85 mbsf) despite the low organic carbon content (mean = 0.3 wt%). Methane increases below 85 mbsf, reaching a maximum of 46,000 ppmv. Alkalinity reaches a maximum of 18.7 mM at the SMI, whereas calcium concentration attains a minimum value (3.7 mM), indicating carbonate mineral precipitation associated with methane oxidation. From 114 to 258 mbsf, sulfate increases again slightly (1.5 mM). This interval corresponds to pH and iron fluctuations, which are antithetic to each other, and may indicate zones of anaerobic pyrite oxidation. Dissolved strontium remains at or below seawater values, suggesting little or no carbonate dissolution or recrystallization.
Physical property records at Site U1306 are highly variable, recording lithologic and mineralogic changes (Fig. F8). The higher carbonate content in the upper ~100 mcd results in average lower NGR and MS values than in the sediments below. Site U1306 sediments are characterized by an overall downcore increase in density (1.5~1.8 g/cm3) and decreasing porosity (~70%50%).
Based on current knowledge from nearby piston cores, Site U1306 has expanded glacial intervals. This sedimentary pattern is complementary to that at Site U1305 where interglacials are likely to be relatively expanded. The apparently complete Quaternary record recovered at Site U1306 provides a high-resolution, high-sedimentation-rate record of detrital events derived from instability of surrounding ice sheets, and provides a monitor of the activity of the WBUC that supplies a component of NADW to the Labrador Sea. The site appears to have the attributes required for the generation of a well-constrained age model based on oxygen isotopes, micropaleontology, and geomagnetic paleointensity.Site U1307
Site U1307 is positioned in 2575 m of water at common depth point (CDP) 14375 on seismic Line 25a in the MCS network obtained over the Eirik Drift during Cruise KN-166 (Knorr, PI: Greg Mountain) in summer 2002. The location was chosen because of potential access to Pliocene sediments below the Quaternary sequence drilled at Site U1306. A thinner Quaternary sedimentary sequence at Site U1307 allowed the Pliocene sequence to be sampled using the APC.
Two holes were cored with the APC system at Site U1307, reaching a maximum depth of 162.6 mbsf (Fig. F9; Table T1). Two partial strokes of the APC required drilling two intervals in Hole U1307A that were difficult to APC core (50.552.5 mbsf and 73.777.7 mbsf). Five cores were advanced by recovery. Only one core was a partial stroke in Hole U1307B, and no intervals required drillover. Average recovery was 102% for the cored intervals at Site U1307. Coring was terminated due to excessive heave when a passing storm system began to affect drilling operations.
The Lower Pliocene to Pleistocene sedimentary succession at Site U1307 (Fig. F4), which is subdivided into three units, records variations in the input of terrigenous and biogenic components (mostly quartz, detrital carbonate, nannofossils, and foraminifers). Unit I (049.55 mcd) is composed of Holocene and Pleistocene mixtures of foraminifers, silty clay, and nannofossils (silty clay with foraminifers, foraminifer silty clay, and nannofossil silty clay). Minor lithologies include eight discrete foraminifer silty sand and sandy foraminifer ooze beds. Unit II (49.55133.86 mcd) is composed mainly of Pleistocene to Upper Pliocene silty clay with little biogenic component. Unit III (133.86173.6 mcd) is composed of Upper to Lower Pliocene silty clay, silty clay with nannofossils, and nannofossil silty clay. With the exception of the foraminifer sand beds, calcium carbonate content is low (mean = 3.8 wt%).
The abundances of calcareous, siliceous, and organic-walled microfossils at Site U1307 are common to rare with moderate to poor preservation. A possible hiatus (duration ~0.25 m.y.) or a condensed interval (~1.211.45 Ma) is indicated between ~56 and 61 mcd (Figs. F4, F9). The dominant components of each microfossil group reflect subpolar to polar conditions during the Pleistocene. In the lower Upper Pliocene (before 2.74 Ma), the nannofossil assemblage suggests warmer surface water conditions.
Paleomagnetic directional data yield an almost continuous sequence and permit unambiguous identification of the Brunhes, Matuyama, and Gauss Chrons (Fig. F4). Within the Matuyama Chron, the Jaramillo, Olduvai, and Reunion Subchrons are clearly recognized. Within the Gauss Chron, the Kaena and Mammoth Subchrons are also recognized, with the base of the section corresponding to the top of the Gilbert Chron.
With only two holes drilled, it was impossible to construct a complete spliced record for Site U1307. However, several long intervals of overlap between holes allowed segments to be correlated between holes (056.5 mcd, 76.4104.7 mcd, and 104.7146.2 mcd), which were then appended in the record (Fig. F9). The mean linear sedimentation rate calculated using biostratigraphic and magnetostatigraphic datums is 4.8 cm/k.y. (Fig. F5). Using only magnetostratigraphic datums, interval sedimentation rates vary between 2.7 and 7.6 cm/k.y.
As for the other Eirik Drift sites, pore water geochemical profiles reflect the influence of organic matter remineralization reactions. Sulfate decreases linearly from the uppermost sample to 79 mbsf, where it remains at or below ~1 mM. The methane profile is atypical, decreasing from 200 ppmv in the uppermost sample near the sediment/water interface to a low of ~30 ppmv at 54 mbsf and then increasing again below the sulfate reduction zone to a high of 26,000 ppmv. Calcium and strontium attain minimum values (5.5 mM and 76 mM, respectively) at the base of the sulfate reduction zone where alkalinity reaches a maximum (10 mM), suggesting carbonate mineral precipitation.
Physical property records at Site U1307 document high-frequency changes in sediment composition (Fig. F9). The variability in sediment composition recorded in MS, NGR, and density likely reflect changes in paleoceanographic conditions in the overlying and surrounding water masses and ice sheets at a range of timescales. Site U1307 sediments are also characterized by an overall downcore increase in density (from 1.55 to ~1.76 g/cm3) and variable but generally decreasing porosity (from ~70% to 40%).
Site U1307 demonstrates that the Pliocene sediments of the Eirik Drift are located at penetration depths achievable with the APC. Apart from one possible hiatus (in the 1.21.4 Ma interval), the sedimentary record at Site U1307 is apparently continuous (Fig. F4) with a mean sedimentation rate of ~5 cm/k.y. (Fig. F5). The base of the recovered section correlates to the uppermost Gilbert Chron, indicating that the record extends back to ~3.6 Ma (Fig. F4). The sediments from Site U1307 will provide information on the history of bottom and surface currents, the Laurentide and Greenland ice sheets, and age control for seismic reflectors that will provide constraints on the sedimentary architecture of the Eirik Drift. The PlioceneQuaternary history at this site can be placed into a tight age model, as the sediments have the attributes required for high-resolution chronostratigraphy based on paleontologic, isotopic, and magnetic methods.Site U1308
Site U1308 constitutes a reoccupation of DSDP Site 609 (Fig. F1). During DSDP Leg 94 (JuneAugust 1983), two principal holes (Holes 609 and 609B) were drilled with the variable-length hydraulic piston corer (VLHPC) and XCB. Two cores were collected from Hole 609A to recover the mudline, and seven XCB cores were collected from Hole 609C to recover the 123190 mbsf interval. Samples from DSDP Site 609 have played a major role in driving some of the most exciting developments in paleoceanographic research during the last 1015 years, such as the recognition and understanding of Heinrich layers, the recognition of the 1500 y pacing in hematite-stained grains and Icelandic glass, and the correlation of ice core 18O to SST proxies. The majority of the analyses from Site 609 have dealt with the record younger than MIS 6, partly because of the lack of a continuous pristine composite record. A primary objective at Site U1308 was to recover a demonstrably complete composite record, hence, considerably enhance the potential for PlioceneQuaternary climatic records from this site.
Six holes were cored with the APC system at Site U1308: (1) Hole U1308A to 341.mbsf, (2) Hole U1308B to 198.3 mbsf, (3) Hole U1308C to 279.9 mbsf, (4) Hole U1308D to 6.7 mbsf, (5) Hole U1304E to 193.0 mbsf, and (6) Hole U1308F to 227.0 mbsf (Fig. F10; Table T1). Water depth was estimated to be 3871 mbsl. Average recovery was 95.4% for the cored intervals. Sea swell up to 6 m affected Hole U1308A core quality below ~170 mbsf. We therefore waited ~16 h for the swell to abate before coring Hole U1308B. Coring in Hole U1308B was terminated after three successive poor-recovery cores suggested accumulation of debris either in the hole or in the bottom hole assembly (BHA). However, we could not spud Hole U1308C because of incomplete firing of the APC indicating the problem was bit obstruction. Holes U1308C, U1308D, and U1308E were cored after clearing the bit, but core recovery was not optimal (<100%) due to loss of core from the base of core liners and (possibly related) crushed core liners. The bit was cleared again before coring Hole U1308F with an average recovery of 100.7%, although crushed liners were still commonplace.
The upper Miocene through Quaternary sedimentary succession at Site U1308 (Fig. F4), which is subdivided into two units, records variations in the input of terrigenous and biogenic sediments (primarily nannofossil ooze, nannofossil silty, and silty clay). Unit I (0196.85 mcd) is composed of a HoloceneUpper Pliocene sequence of interbedded biogenic and terrigenous sediments with dropstones. Subunit IIA (196.85262.14 mcd) is composed of Upper Pliocene nannofossil ooze interbedded with terrigenous sediment-rich layers but at a lower frequency than Unit I. Subunit IIB (262.14355.89 mcd) is entirely composed of lowermost Upper Pliocene to uppermost Miocene nannofossil ooze.
Diverse assemblages of calcareous, siliceous, and organic-walled microfossils were recovered at Site U1308 (Fig. F4). Calcareous microfossils are abundant with good preservation in the upper ~200 mcd grading to moderate preservation below this depth. Siliceous microfossils are rare to common and moderately preserved above ~255 mcd (Upper PliocenePleistocene) with radiolarians locally abundant only in the middle part of the cored sequence. Siliceous microfossils are barren below 255 mcd. The concentration of terrestrial palynomorphs is low. Dinocysts are common to abundant in the upper 200 mcd but less common below. Microfossil assemblage changes observed at Site U1308 document the onset of Northern Hemisphere glaciation, as well as seasonal changes in bioproductivity and hydrographic fronts.
Paleomagnetic directional data document an apparently continuous sequence of polarity transitions. Identification of the Brunhes, Matuyama, and Gauss Chrons are unambiguous (Fig. F4). The Gilbert Chron is tentatively recognized in the lower part of Hole U1308A. The Jaramillo, Cobb Mountain, Olduvai, Reunion, Kaena, and Mammoth Subchrons are also clearly identified.
Six holes were cored at Site U1308 to ensure the complete recovery of the stratigraphic section to 247 mcd (Fig. F10). The unusually large number of holes was required because of poor recovery and core disturbance due to excessive heave and crushed core liners. There is one problematic interval between ~186 and ~196 mcd where inclined bedding and sharp lithologic contacts suggest a possible break in continuity of sedimentation. The mean linear sedimentation rate calculated using magnetostatigraphic datums is ~8.3 cm/k.y. for the last ~3.5 m.y. (Fig. F5). Prior to that time, the mean sedimentation rate was ~3.3 cm/k.y. based on biostratigraphic markers (Fig. F5).
Interstitial water sulfate decreases downhole to 9 mM, but complete sulfate reduction is not achieved within the cored interval. Unlike all other Expedition 303 sites, strontium increases with depth to a maximum of 1592 mM at Site U1308. The nearly linear strontium increase and the corresponding increase in Sr/Ca ratios indicate that recrystallization (not dissolution) of biogenic carbonate is occurring. The approximately linear downhole decreases in magnesium and potassium and increase in calcium below ~100 mbsf are consistent with the alteration of volcanic material and/or basement below the cored interval.
Physical property records at Site U1308 document long-term changes in sediment composition, which likely reflect fundamental changes in North Atlantic climate. The NGR and lightness (L*) records from lithologic Unit I (0197 mcd) show a strong glacialinterglacial variability (Fig. F10). In Subunit IIA (197262 mcd), MS and NGR values decrease both in absolute value and variability and L* increases (Fig. F10). NGR shows a fairly abrupt change in absolute values and variability at the Unit II/I boundary at ~197 mcd (~2.74 Ma). In the white nannofossil ooze of lithologic Subunit IIB (262356 mcd), MS and NGR values are significantly lower and less variable than in Subunit IIA. Data for Subunit IIB (262356 mcd) are not shown in Figure F10, as the composite record does not extend below Subunit IIA.
The six holes at Site U1308 have been pieced together to produce a complete composite section to ~247 mcd. The base of the composite section correlates to the middle part of the Gauss Chron at ~3.2 Ma. A discontinuous record was recovered below this level to 356 mcd in upper Miocene white nannofossil ooze. The Upper Pliocene to Quaternary composite section will provide a means of studying the evolution of NADW, the extension of the central Atlantic detrital layer (Heinrich-type) stratigraphy beyond the last glacial cycle, and the 1500 y cycle in the petrologic characteristics of IRD. The mean sedimentation rate for the composite section (7.6 cm/k.y.) indicates that these studies can be carried out at moderately high resolution. Good preservation of benthic and planktonic calcareous and siliceous microfossils indicates that the site will yield high-quality environmental and isotopic records. The pristine magnetostratigraphic record indicates that the site has good potential for the generation of a PAC that will place the environmental record into a global millennial-scale stratigraphic framework.
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