Site U1312 (proposed Site IRD4A) constitutes a reoccupation of DSDP Site 608 located northeast of the Azores Islands (Portugal) on the southern flank of the King's Trough tectonic complex at a water depth of 3554 m (Fig. F1). Two principal holes (Holes 608 and 608A) were drilled to 515.4 and 146.4 mbsf, respectively, with the variable-length piston coring (VLHPC) system and the XCB during DSDP Leg 94 (JuneAugust 1983) (Ruddiman, Kidd, Thomas, et al., 1987). At this site, a nearly continuous bio- and magnetostratigraphic section of Quaternary to midupper Oligocene sediments was recovered to 455 mbsf (Baldauf et al., 1987). Below this depth, some coring gaps and the presence of a major hiatus representing at least 7.5 m.y. (late Eoceneearly Oligocene) cause the record to be less complete through the Oligocene and into the Eocene. Upper middle Eocene (Zone NP16) sediments lie upon the basaltic basement at 515.4 mbsf. Mean sedimentation rates at DSDP Site 608 are 23 cm/k.y., with the higher values generally occurring in the late Neogene/Quaternary time intervals. Incomplete recovery and the present condition of the existing DSDP cores collected in 1983 do not permit the detailed paleoceanographic studies proposed here. The main objective at Site U1312 was to obtain continuous records of surface and deepwater characteristics and their interactions with ice sheet instabilities during Neogene-Quaternary times. In this context, an important target at this site was the recovery of a complete undisturbed upper Miocene section by means of APC coring.
Two holes were cored with the APC system and nonmagnetic core barrels at Site U1312. Hole U1312A was drilled to a maximum depth of 237.5 mbsf, with a recovery of 104.5% (Fig. F4). In this hole, the "drillover" technique was required for recovery of Cores 306-U1312A-23H through 25H. Because of excessive heave (5 m), initial coring conditions were not optimum. This prevented the recovery of a good mudline in Hole U1312A, and the first several cores (306-U1312A-1H through 3H and 5H) were disturbed by flow-in. Hole U1312B was drilled to a maximum depth of 231.9 mbsf, with a recovery of 102.1% (Fig. F4). In Hole U1312B, a successful mudline was achieved during a period of reduced heave and drillover was required only for the recovery of Core 306-U1312B-25H. Drilling of a third hole was precluded as weather conditions dramatically deteriorated.
The sedimentary succession at Site U1312 consists of Holocene to upper Miocene sediments with varying mixtures of biogenic and detrital components, primarily nannofossils, foraminifers, and clay minerals (Fig. F4). Two lithologic units were distinguished based on sediment color, carbonate content, reflectance values, and the occurrence of detrital components. Unit I (079.70 mbsf; Holocene to Late Pliocene) is dominated by nannofossil ooze, foraminifer nannofossil ooze, nannofossil ooze with silty clay, and silty clay nannofossil ooze. Alternating diffuse color bands occur throughout much of the unit. Most contacts between the various lithologies are gradational and/or bioturbated. Unit I was further subdivided into two subunits. Subunit IA exhibits high-amplitude variations in magnetic susceptibility and carbonate content, whereas in Subunit IB these variations are less distinct. Dropstones are generally rare and small (215 mm in diameter) and are concentrated in the upper 23 m of Subunit IA. Unit II (79.70232.05 mbsf; Late Pliocene to late Miocene) is dominated by nannofossil ooze that exhibits little color change due to a downhole decrease in abundance of both detrital content and diffuse color bands.
Abundant, generally well preserved calcareous nannofossils and planktonic foraminifers occur throughout both holes at Site U1312. Planktonic foraminifer assemblages are mainly composed of temperate to subpolar species, with some sporadic incursions of polar and subtropical taxa. Nannofossil assemblages consist of cosmopolitan species typical of the North Atlantic at mid-latitude. A reliable chronostratigraphic framework spanning from the late Miocene (~11 Ma) to the present was established based on the succession of bio-events identified in the cores (Table T1). Linear sedimentation rates were estimated based on the depth of these events. Average sedimentation rates were low during the late Miocene (12 cm/k.y.), increased in the Early Pliocene (2.56 cm/k.y.), and decreased again in the latest Pliocene and Pleistocene (1.52 cm/k.y.) (Fig. F5). Although calcareous plankton species were usually well preserved, intense fragmentation of planktonic foraminifer shells and overgrown discoasters were observed in the uppermost Miocene (161171 mbsf in Hole U1312A and 165175 mbsf in Hole U1312B), coinciding with an interval of very low sedimentation rates. The occurrence of a hiatus due to carbonate dissolution in this part of the record is probable because several bioevents were observed in the same core. A similar interval with extremely low sedimentation rates and/or a possible hiatus but good carbonate preservation was also observed in the Upper Pliocenelower Pleistocene (3847 mbsf in Hole U1312A and 3241 mbsf in Hole U1312B). Nannofossils recovered from this interval in Hole U1312B indicate an age older than planktonic foraminifers from the same horizon, further suggesting the presence of a hiatus or significant reworking.
Biostratigraphy based on siliceous fossils was hindered by rare occurrences and dissolution of radiolarians and diatoms. Trace numbers of diatoms are present in the upper ~60 m of both holes, and show an age-depth progression similar to the calcareous nannofossils. Below 60 mbsf, the sediments are almost entirely barren of diatoms. Likewise, radiolarians are found in trace numbers down to a depth of 85.5 mbsf in Hole U1312A and 114 mbsf in Hole U1312B. Only one radiolarian event was observed in Hole U1312A, and only samples at 28.5 and 37.5 mbsf contained rich radiolarian faunas in Hole U1312B.
The magnetic interpretations at Site U1312 were based on measurements of the natural remanent magnetization (NRM) after alternating-field (AF) demagnetization at a peak field of 20 mT. The Brunhes/Matuyama reversal occurs at 18.40 mbsf in Hole U1312A and at 16.95 mbsf in Hole U1312B. The Jaramillo Subchron occurs between 20.90 and 24.80 mbsf in Hole U1312B. In Hole U1312B, the Gauss/Matuyama and Gauss/Gilbert boundaries were tentatively placed at 51.60 and 72.2 mbsf, respectively, although a significant part of the Gauss interval (Chron C2An) is missing because of coring-induced sediment deformation. NRM intensities fall in the range of 105 A/m between ~100 and 210 mbsf in both holes. This range is close enough to the noise level of the magnetometer that establishing a continuous magnetostratigraphy was not possible by shipboard measurements. A long interval of normal polarity at the bottom of Holes U1312A and U1312B (top at 207.6 and 204.7 mbsf, respectively) was tentatively identified as Chron C5n (Fig. F6).
A complete splice of the entire sedimentary section at Site U1312 was difficult to construct because only two holes were cored and much of the upper portion of Hole U1312A was affected by coring disturbance. The remanent magnetic intensity following 20 mT AF demagnetization and the lightness (L*) parameter from color reflectance measurements proved to be the most useful for correlating between holes down to 158.89 mcd (the bottom of Core 306-U1312B-16H). Below 158.89 mcd, stratigraphic correlation was difficult because of the very uniform sediment composition, resulting in few diagnostic variations in physical properties. From 158.89 up to 68.05 mcd, between-hole correlation was good and all core breaks could be filled, resulting in a complete splice. Above this, several gaps occur between core breaks and much of the spliced section is built from Hole U1312B cores, which contained virtually no coring deformation within this interval. From 0 to 40 mcd, L* variations, which mainly reflect the carbonate content, mirror variations observed in the 01.5 Ma portion of benthic oxygen isotope stacks. Sedimentation rates derived from this correlation vary between 0.5 and 3.5 cm/k.y. during the past 1.5 m.y.
Alkalinity of the pore water in the upper 110 m exhibits a continuous trend in increasing values downhole to a depth of ~80 mbsf, followed by a decrease thereafter. In contrast, chlorinity decreases with depth. The highest silica value of ~650 µM was measured at 53.5 mbsf. Barium exhibits its highest values (0.7 µM) at 82 mbsf, just below the boundary between lithologic Units I and II.
Carbonate concentrations in the sedimentary record at Hole U1312A range from 59 to 98 wt% (average = 90.4 wt%). Highest values (9298 wt%) are observed in the lower part of the record (more than ~55 mbsf), whereas the uppermost ~55 m (lithologic Subunit IA) are characterized by lower values and high variability. Two discrete intervals of decreased CaCO3 values (82 wt%) occurred at 81.95 and 110.45 mbsf. A similar trend was observed in the overall CaCO3 concentration at DSDP Site 608. Total organic carbon varies between 0 and 0.9 wt%, with the lowest values (00.1 wt%) found below 85 mbsf and higher and more variable values (0.10.9 wt%) found above. Total nitrogen is low and relatively constant throughout the hole (~0.1 wt%).
Physical property measurements at Site U1312 included nondestructive measurements of magnetic susceptibility, density, and natural gamma radiation (Fig. F7). Working sections were used to measure moisture and density (MAD) and compressional P-wave velocity. These properties generally show greatest variability in the upper 40 m, consistent with greater clay content, and generally show lower values with depth, except for density and P-wave velocity, which increase with depth.
Site U1312 accomplished near full recovery of the excellent upper Miocene section first drilled at DSDP Site 608. The sedimentary sequence representing the last ~11 m.y. will allow the study of short- and long-term climate variability and ocean-atmosphere interactions under very different boundary conditions, such as the closure and re-opening of Atlantic/Mediterranean connections at the end of the Miocene (65 Ma), the closing of the Isthmus of Panama (4.53 Ma), and the onset of major northern hemisphere glaciation near 2.5 Ma.Site U1313
IODP Site U1313 (proposed Site IRD3A) constitutes a reoccupation of DSDP Site 607 located at the base of the upper western flank of the Mid-Atlantic Ridge in a water depth of 3426 m, ~240 miles northwest of the Azores Islands. Two holes were drilled at this site during Leg 94 using the VLHPC system and the XCB system (Ruddiman, Kidd, Thomas, et al., 1987). Hole 607 penetrated to a total depth of 284.4 m, and Hole 607A penetrated to a total depth of 311.3 m. The sediments recovered at Site 607 predominantly consist of calcareous biogenic oozes with variable amounts of fine-grained terrigenous material. Based on magneto- and biostratigraphy, the mean sedimentation rate at Site 607 is ~5 cm/k.y. for the PliocenePleistocene time interval.
The rationale for reoccupying this site is essentially the same as for Site U1308 (re-coring of DSDP Site 609) (see Channell, Sato, Malone, and Expedition Scientists, in press). Together Sites 607 and 609 constitute benchmark sites for long-term (millions of years), as well as short-term, surface and deep ocean climate records from the subpolar North Atlantic. These sites, today situated under the influence of the NADW, have been very important for generating benthic 18O, 13C, and CaCO3 records for the Pleistocene (Ruddiman et al., 1989) and Late Pliocene (Ruddiman et al., 1986; Raymo et al., 1989, 2004), for interpreting these records in terms of ice sheet variability and changes in NADW circulation, and for generating orbitally tuned timescales. Site 607, at a water depth of 3427 m, remains the only site in the high-latitude North Atlantic that monitors NADW circulation throughout the Pleistocene.
Reconstruction of sea-surface temperature (SST) in the North Atlantic indicates that the polar front was situated at ~42°46°N during glacial times, extending in an eastwest direction and resulting in a steep southnorth SST gradient (CLIMAP, 1976; Pflaumann et al., 2003). Alkenone SST estimates determined in sediment cores from areas south of and within the Polar Front resulted in very different values for different glacials (Calvo et al., 2001), indicating different climatic conditions (e.g., the location of the Polar Front) during these glacial periods. At the site of Core VM 30-97, located close to Site 607 (Fig. F2), Heinrich events are marked by distinctive detrital carbonate signature and planktonic foraminifer-derived SSTs warmed markedly during the Heinrich events and during the Last Glacial Maximum, in distinct contrast to climate records from the subpolar North Atlantic (Bond et al., 1999b).
Site U1313 (especially in combination with similar records from other Expedition 303/306 sites) will document the evolution of complex surface-temperature phasing over the last few million years. By placing the surface temperature signals into a chronological framework based on a combination of oxygen isotope stratigraphy, detrital carbonate-bearing Heinrich events, and geomagnetic paleointensity, we expect to obtain an optimal reconstruction of the phasing of temperature records and their relationship to ice sheet instability and changes in deepwater circulation on millennial to submillennial timescales.
Four holes (Holes U1313A, U1313B, U1313C, and U1313D) were cored with the APC system and nonmagnetic core barrels to maximum depths of 308.6, 302.4, 293.4, and 152.0 mbsf, respectively (Fig. F7). The average recovery was 103.5%. In Holes U1313A and U1313C, drillover was required for recovery of the last two and four cores, respectively. After completing coring operations in Hole U1313B, the hole was prepared for logging and the triple combo tool string was deployed (including the General Purpose Inclinometer Tool [GPIT] and Multi-Sensor Spectral Gamma Ray Tool [MGT]) to 2.0 m off the bottom of the hole. The entire 300 m sequence was successfully logged.
The Holocene to uppermost Miocene sedimentary succession at Site U1313 consists primarily of nannofossil ooze with varying amounts of foraminifers and clay- to gravel-sized terrigenous components (Fig. F4). Two major lithologic units were identified. Unit I (0111.86 mbsf in Hole U1313A, 0111.28 mbsf in Hole U1313B, 0112.00 mbsf in Hole U1313C, and 0113.14 mbsf in Hole U1313D) consists of Holocene to Upper Pliocene alternating nannofossil ooze, silty clay nannofossil ooze, and nannofossil ooze with clay. Regular occurrences of dropstones in Unit I demonstrate that northern hemisphere ice sheet instability played a role in the sediment's paleoclimate record during the Pleistocene to Late Pliocene. Unit I can be further subdivided into two subunits. Subunit IA exhibits the largest amplitude fluctuations in detrital clay and biogenic carbonate, reflected by distinct color changes and shifts in L* (Fig. F8), weight percent carbonate, gamma ray attenuation (GRA) density, and magnetic susceptibility, whereas Subunit IB is defined by decreased variability in these components. Millimeter- to centimeter-scale pale green color bands are distributed throughout the succession, and a horizon of reworked volcanic ash can be correlated across all holes in Subunit IA. Unit II extends to the bottom of each hole (111.86308.42 mbsf in Hole U1313A, 111.28302.52 mbsf in Hole U1313B, 112.00293.33 mbsf in Hole U1313C, and 113.14153.0 mbsf in Hole U1313D). Unit II is very homogeneous and differs mainly from Unit I in its smaller terrigenous component, which decreases gradually downhole from the unit boundary to ~150 mbsf. Unit II consists of Upper Pliocene to uppermost Miocene nannofossil ooze and is characterized by high and stable carbonate concentrations. Discrete patches and streaks of pyrite occur throughout this unit and are probably related to local reducing conditions associated with organic matter complexes. Pale green color bands are prevalent in the otherwise nearly white sediment.
Site U1313 yielded abundant assemblages of calcareous microfossils spanning the late Miocene to Holocene. Biohorizons based on calcareous microfossils closely match ages based on paleomagnetic data for the Pliocene and Pleistocene, and indicate nearly constant sedimentation rates of 45 cm/k.y. throughout this time interval (Fig. F5). The oldest sediments at Site U1313 are tentatively dated at 6.0 Ma based on a nannofossil last occurrence near the base of Holes U1313A and U1313C, a tentative planktonic foraminifer event at the base of Hole U1313A, and a possible diatom event in Core 306-U1313C-32H (Table T1). Based on these biohorizons, sedimentation rates in the late Miocene are ~1314 cm/k.y. (Fig. F5).
Calcareous nannofossils are well preserved throughout much of the section, although some dissolution and overgrowth is present within the upper Miocene. Pleistocene sediments contain very minor amounts of reworked nannofossils. Planktonic foraminifers are moderately to well preserved and reveal a high faunal diversity including several (sub)tropical species. An incursion of encrusted Neogloboquadrina atlantica (dextral) occurs in the early Pleistocene, making this the youngest occurrence of this species in the mid-latitude North Atlantic.
Radiolarians at Site U1313 reveal great variation in abundance, state of preservation, and faunal associations among the four holes. In general, radiolarians are abundant and well preserved in the upper five to six core catcher samples, whereas dissolution is severe in the lower part. Cycladophora davisiana is found in samples up to 14 cores deeper than its anticipated first occurrence in the North Atlantic at 2.6 Ma. If these occurrences are real and not a result of downhole contamination, then the first occurrence of C. davisiana is ~5.5 Ma.
A diverse warm-water diatom flora is generally present within the upper 4070 mbsf during the PliocenePleistocene intervals, containing a mixture of Arctic and Subarctic, possibly ice-rafted species. However, diatoms are only abundant in the first two core catchers and occur only as traces below 5060 mbsf. When present, warm-water diatoms are generally well preserved, but often fragmented. The diatoms from colder water masses are usually partially dissolved.
The magnetostratigraphy at Site U1313 was constructed on the basis of continuous NRM measurements after AF demagnetization at a peak field of 20 mT. NRM intensities after 20 mT AF demagnetization are in the range of 103 to 104 A/m above 150 mbsf but fall to the range of 105 A/m in the lower part of the section. The sediments provide a good record of the Brunhes, Matuyama, and Gauss polarity intervals down to ~150 mbsf (Fig. F6). Below this depth, the inclination signal is noisier but alternating intervals of normal and reversed polarities can still be defined with confidence down to ~250 mbsf. The magnetostratigraphy is uncertain below this depth, as it varies from one hole to the other, partly because the stronger drill string overprint induced by alloy steel core barrels used in the lowermost part of Holes U1313A and U1313C. The magnetostratigraphy is consistent with the biostratigraphy from the top down to ~220 mbsf. In the underlying sediment, however, the link to the biostratigraphy is not straightforward (Fig. F5).
The four holes cored at Site U1313 provided ample sediment for constructing one complete spliced stratigraphic section and a second nearly complete section. Correlation between holes was excellent in the upper 168.5 mcd because of pronounced variations in nearly all physical properties measured. In particular, L* from color reflectance measurements mimics variations in the global benthic oxygen isotope stack (e.g., Lisiecki and Raymo, 2005), and a preliminary age model was constructed by matching sharp L* variations with glacial and interglacial terminations. Between-hole correlation was more difficult below 168.5 mcd (~151 mbsf) because the sediments are fairly homogeneous calcareous nannofossil ooze.
Apart from general trends, most pore water chemical constituents show a notable change between ~80 and 110 mbsf (i.e., at the lithologic Unit III transition). Alkalinity and Sr2+ increase downhole, whereas the Li+ decreases. The highest dissolved silica content of ~563 µM is measured at 39.3 mbsf. SO42 concentration exhibits a slight downhole decrease from ~26 to 24 µM, whereas the NH4+ shows the opposite trend (126418 µM, with a high value of 615 µM at 47.8 mbsf). Ba2+ shows a more or less uniform concentration (~3.2 µM) throughout the profile.
Carbonate concentrations in the sediments of Hole U1313A range from 31.5 to 96.7 wt% (average = 80.5 wt%). Relatively uniform and high values (>90 wt%) are observed below ~120 mbsf (Unit II), whereas the top ~120 mbsf (Unit I) is characterized by distinct and strong variations (3090 wt%). Maximum amplitude with as much as 60 wt% difference in CaCO3 occurs in the uppermost 40 mbsf, whereas the amplitude of variation is reduced to 4050 wt% from 40 to 120 mbsf. Similar general features of the CaCO3 variability were observed at Site 607. Total organic carbon (TOC) varies between 0 and 0.65 wt%, with the lowest values (<0.1 wt%) below 170 mbsf but higher and more variable values (0.10.65 wt%) above. Total nitrogen is low and relatively constant throughout the hole (0.10.15 wt%). Preliminary results from a limited number of samples (16) show that solvent-extractable organic matter at Site U1313 consists primarily of odd-numbered C25C35 n-alkanes and long-chain C37C40 alkenones. Variations in proportions of these compound classes reflect a change in the organic matter composition with respect to terrigenous and marine sources. Alkenone-derived SSTs show variability from ~13° to 19whereas temperatures of ~20° and 22est Miocene, respectively.
Physical properties measured at Site U1313 include magnetic susceptibility (by multisensor track [MST] and multisensor core logger [MSCL]), GRA density, P-wave velocity, and natural gamma radiation using the MST. In addition, the porosity and density were measured on discrete samples by MAD. Finally, P-wave velocities were measured in the x-direction using the P-wave sensor (PWS3). The results show a large variability of all physical properties in the upper ~120140 m, which is probably related to the variation of clay content in the upper sediments (Figs. F4, F8). Below 120 mbsf, the variability in physical properties is small as a result of the very high carbonate content (>95 wt%).
Site U1313 (especially in combination with similar records from other Expedition 303/306 sites) will document the evolution of complex surface-temperature phasing over time, addressing questions such as whether the patterns are a peculiarity of the last glaciation, whether they were present in the 41 k.y. world, and whether they appeared at the onset of northern hemisphere glaciation. By placing the surface-temperature signals into a chronological framework based on a combination of oxygen isotopic stratigraphy, detrital carbonate-bearing IRD (Heinrich-type) events, and geomagnetic paleointensity, we expect to obtain an optimal reconstruction of the phasing of the temperature records and its relationship to ice sheet instability and changes in deepwater circulation.Downhole Logging of Hole U1313B
The successful deployment of the triple combo tool string in Hole U1313B provided complete coverage of the 300 m section and provided very good physical property and lithologic information for density, porosity, natural gamma radiation, resistivity, and photoelectric effect. Corresponding core physical property measurements were very consistent with in situ downhole data. Of special note is the dramatically consistent linear correlation of downhole natural gamma radiation (upper 225 mbsf) with the recent Lisiecki and Raymo (2005) benthic oxygen isotope record of the last 5.4 m.y. The consistency of downhole data with both core data and age models will allow mapping of the spliced core record to actual depth, resulting in more accurate sedimentation rate calculations as well as more detailed age-depth models.Site U1314
Site U1314 (proposed Site GAR1B) is located on the southern Gardar Drift in a water depth of 2800 m (Fig. F1). Close to the location of Site U1314, a 33 m Marion Dufresne core (MD99-2253) was collected on the crest of the Gardar Drift in 1999. The MD99-2253 piston core has a high sedimentation rate of ~9 cm/k.y. for the last glacial cycle and well-defined planktonic 18O and geomagnetic paleointensity records. During Leg 162, Sites 983 and 984 were drilled off Iceland on the northern part of the Gardar and Bjorn Drifts, respectively (Fig. F2). These sites have mean Pleistocene sedimentation rates in the 1015 cm/k.y. range and have produced high-resolution climatic and geomagnetic records. Sites 983 and 984, however, are located outside the main IRD belt (Fig. F2) and do not contain a robust detrital carbonate (Heinrich layer) signal. Furthermore, both sites are at shallower water depths (<2000 m) than Site U1314 and therefore monitor intermediate water but not NADW. Site U1314, on the other hand, is located (1) close enough to the IRD belt to record the Heinrich-type detrital layers that monitor ice sheet instability and (2) in a water depth of 2820 m, allowing a high-resolution monitoring of NADW and its short-term (sub)millennial variability.
Three holes (Holes U1314A, U1314B, and U1314C) were cored with the APC system and nonmagnetic core barrels to maximum depths of 258.4, 279.5, and 207.7 mbsf, respectively (Fig. F9). The average recovery was 102.7%. Drillover technique was not required for any of the three holes.
The sedimentary sequence at Site U1314 mainly consists of nannofossil- and clay-rich sediments with minor and varying proportions of diatoms and foraminifers. Only one lithologic unit was defined at Site U1314, which spans from the Late Pliocene to the Holocene (Fig. F4). In particular, two sets of lithologies can be identified: (1) predominantly nannofossil oozes enriched in biogenic (mainly diatoms and foraminifers) and terrigenous (principally clay minerals, quartz, opaque minerals, and calcite) components and (2) terrigenous silty clay with varying proportions of calcareous and siliceous organisms (Fig. F4). The sediment varies in color from very dark gray to light gray to hues of greenish gray. Slight to moderate bioturbation is typical for most of the section. Horizontal and parallel bedding planes and color contacts without erosional relief suggest that there is not visible evidence of significant sediment disturbance by natural processes. Sand- and gravel-sized sediment, common at Site U1314 from 0 to 240 mbsf, provides direct evidence of ice rafting and documents the influence of PliocenePleistocene glaciations in this region. Based on the occurrence of mafic igneous and felsic igneous dropstones, as well as sand-sized, hematite-stained quartz, Iceland and Greenland are probable source areas of the IRD material.
Site U1314 yields abundant assemblages of calcareous and siliceous microfossils spanning the Late Pliocene to the Holocene (Table T1). Sedimentation rates based on microfossil datums and paleomagnetism indicate decreasing rates from ~1111.5 cm/k.y. during the Late Pliocene to ~7.07.5 cm/k.y. during the Pleistocene (Fig. F5). Polar and subpolar species dominate the assemblages, with subordinate amounts of transitional species present as well.
Calcareous nannofossils are abundant and generally well preserved throughout the section. Minor amounts of Cretaceous and Paleogene reworked nannofossils occur in all holes. Samples with increased amounts of reworked material typically contain coarser sediment and reduced abundances of in situ nannofossils. Late Pliocene discoasters are rare but present and can be used biostratigraphically, even though they are considered warm-water species. Generally well preserved planktonic foraminifers are the dominant component in the sand fraction of most core catcher samples, with lower proportions of benthic foraminifers, ostracodes, siliceous microfossils, and IRD. The fauna consists of species typical for transitional to subpolar provinces in the Pleistocene and Pliocene. Neogloboquadrina pachyderma (sinistral) is dominant in several of the glacial samples.
Radiolarians at this site show a great variation in species and abundances among the three holes. The state of preservation is generally good in all holes. C. davisiana is found in most samples, with a first common occurrence in Samples 306-U1314A-25H-CC and 306-U1314B-24H-CC. This species has a first common abundance in the North Atlantic at ~2.6 Ma. Cycladophora sakaii is commonly found in Samples 306-U1314A-25H-CC and 306-U1314B-25H-CC. In the North Pacific, C. sakaii evolves into C. davisiana at ~2.6 Ma. This is the first documented occurrence of C. sakaii in the North Atlantic. The last occurrence of Spongaster tetras? (2.6 Ma) is found in Samples 306-U1314A-28H-CC and 306-U1314B-27H-CC.
Abundant and diverse boreal to subarctic diatom flora, with a minor input of warm-water species, are present in all holes. The exception is the interval around Samples 306-U1314A-13H-CC to 15H-CC, where few or rare diatoms coincide with high content of siliciclastic material. The preservation is generally moderate to good, with a deteriorating trend downhole, as well as poor preservation coinciding with lithic-rich intervals. The flora is dominated by long pennate specimens, as well as resting spores of Chaetoceros and fragments of big Coscinodiscus species.
The magnetostratigraphy at Site U1314 was constructed on the basis of continuous NRM measurements after AF demagnetization at a peak field of 20 mT. NRM intensities after 20 mT AF demagnetization are in the range of 101 to 102 A/m. These values are considerably greater than at Sites U1312 and U1313, owing to a higher magnetic mineral content. Site U1314 provides a very good record of the Brunhes, Matuyama, and the upper part of the Gauss (Fig. F6). The Brunhes/Matuyama reversal occurs at 57.3±0.1 mbsf in Hole U1314A, 56.6±0.1 mbsf in Hole U1314B, and 57.7±0.1 mbsf in Hole U1314C. The deepest magnetic polarity interval recorded at Site U1314 corresponds to the top normal interval of the Gauss (Subchron 2An.1n; 2.58 Ma). Several short geomagnetic intervals are present in the paleomagnetic record, such as the Cobb Mountain and Reunion events. The magnetostratigraphy is consistent with the biostratigraphy throughout the section.
Stratigraphic correlation was straightforward at Site U1314 because most of the sediment physical properties show prominent short-wavelength amplitude variations related to changes in lithology. We relied mainly on between-hole correlation of distinctive magnetic susceptibility and natural gamma ray variations for depth shifting the cores. These correlations were confirmed to be consistent with geomagnetic polarity reversals recorded in the paleomagnetic inclination. The resulting mcd scale is well resolved, and the spliced section is complete down to 281 mcd. Because of core disturbance in the upper part of Hole U1314A, the splice in the interval 0188.30 mcd was built from Holes U1314B and U1314C, with the exception of a short interval (65.8069.50 mcd) where an undisturbed section of Core 306-U1314A-8H was incorporated into the composite section. From 188.30 to 300 mcd, the splice was constructed from Holes U1314A and U1314B because Hole U1314C was only drilled to 222 mcd. The two deepest cores (306-U1314B-29H and 30H), which span an interval not cored in Holes U1314A or U1314C, were appended to the splice. A growth factor of 1.08 is calculated by linear regression for the three holes at Site U1314, indicating an 8% increase in mcd relative to mbsf.
The ionic composition of the pore waters at Site U1314 was measured between Cores 306-U1314A-1H and 12H. Pore water alkalinity increases downhole from 5.34 to 7.46 mM. Ca2+ and Mg2+ concentrations decrease downhole from 8.4 to 5.2 mM and 48.7 to 37.4 mM, respectively. Fe2+ concentrations are variable with the lowest value (6.5 µM) measured at 36.4 mbsf, which coincides with more abundant darker lithologies. Ba2+ concentration is higher at Site U1314 than at the IRD sites, ranging from 17 to 18.3 µM. Mn2+ concentration ranges from 46.8 to 15.9 µM and shows a rapid decrease downhole between 17.4 and 36 mbsf. H4SiO4 concentration increases with depth from 487.6 to 571.9 µM.
Carbonate concentrations in the sediments of Hole U1314A range from 3.7 to 70.5 wt%. The average carbonate value linearly shifts from 20 wt% at 250 mbsf toward 40 wt% near the top. CaCO3 values show a good correlation to L* data throughout the section. TOC varies between <0.1 and 0.5 wt% (average = 0.2 wt%). The downhole TOC variation is similar to the variability in magnetic susceptibility, suggesting that organic carbon at Site U1314 is primarily of terrigenous origin. Based on initial results from eight samples, solvent-extractable organic matter at Site U1314 consists mainly of long-chain, odd-numbered C25C35 n-alkanes and long-chain C37C40 alkenones. Except for one sample from the Late Pliocene, all Pleistocene samples show a distinct prevalence of n-alkanes relative to alkenones, confirming the assumption of a mainly terrigenous origin of the organic matter at Site U1314. Alkenone-derived SSTs vary between ~10° and 13
Physical property measurements at Site U1314 included nondestructive measurements of magnetic susceptibility, density, P-wave velocity, and natural gamma radiation (Fig. F9). Working sections were used to measure MAD and compressional P-wave velocity. These properties are positively correlated consistent with the terrigenous nature of the sediment. The magnetic susceptibility records, for example, show a highly variable record attributed to lithologic and/or mineralogic changes; multiple excursions toward high values are generally associated with IRD layers.
At Site U1314, a complete Upper Pliocene to Holocene sequence, characterized by high sedimentation rates of 7 to >11 cm/k.y., was recovered. Because of its location close to the IRD belt and within NADW, as well as its high potential for paleomagnetic and isotopic age control, this section will be used to establish a high-resolution (millennial to submillennial) environmental record of sea-surface and bottom water characteristics and a detrital (Heinrich type) stratigraphy for the past ~2.7 m.y.Site U1315
Site 642, located on the Vøring Plateau in a water depth of ~1280 m, was first drilled during ODP Leg 104 (Eldholm, Thiede, Taylor, et al., 1987). In Hole 642E, a 1229 m deep sequence was drilled that is composed of upper Eocene to Quaternary biogenic and terrigenous sediments with volcaniclastics in the lower part (Units I to IV; 0315 mbsf) and Eocene tholeiitic (upper series) and andesitic (lower series) basalt flows with interbedded volcaniclastic sediments (3151229 mbsf). The location of Site 642 was revisited during Expedition 306 (Site U1315A; 67°12.74' N, 02°56.24' E).
The primary objective at this site is to document bottom water temperature variations and monitor subbottom diffusion over a 5 y period. Bottom water temperature and salinity variations are monitored with instruments that sit in the water column by way of an elevated reentry cone. Diffusion of the thermal wave through the subsurface is monitored with a 150 m thermistor string deployed in a cased and CORKed borehole. Hole U1315A was drilled to a depth of 179.07 mbsf and cased with 103/4 inch casing. The base of the casing was cemented, and the casing string was displaced with bentonite mud. The thermistor string is attached to Spectra rope using friction tape, cable ties, and marine duct tape to carry the weight of the load and the 250 lb sinker bar. The thermistor string is connected to a data logger and external battery at the top.Downhole Logging of Hole 642E
To assess current background thermal conditions in the region, a downhole record of temperature from nearby Hole 642E was obtained using the LDEO Temperature/Acceleration/Pressure (TAP) tool in combination with the triple combo. In addition, the FMS-sonic tool string was deployed. At a depth of 588 mbsf, an impassable obstruction was reached and downhole logging had to be stopped. The TAP tool indicated a bottom water temperature at the seafloor of ~0.2had a very steep gradient (~2500low gradient of ~22cursion to ~42and will allow the correlation to existing core data and filling in the gaps (~60% of the formation). In combination with detailed FMS resistivity measurements and imaging and sonic data, it will be possible to obtain reliable permeability estimates. Understanding the permeability will allow better understanding of fluid flow and temperature gradients observed in the borehole.
Next Section | Table of Contents