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doi:10.2204/iodp.proc.303306.214.2010

Site U1302/U1303

The overall objective at Site U1302/U1303 is to explore the record of Laurentide Ice Sheet (LIS) instability at this location close to Orphan Knoll (Fig. F1). Piston cores collected nearby (Cores 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; Hiscott et al., 2001). The correlation of these detrital layers to the classic Heinrich layer stratigraphy and to other central Atlantic detrital layers (e.g., Bond and Lotti, 1995) remains problematic. Oxygen isotope data from planktonic foraminifers imply that these detrital layers are often associated with low-productivity meltwater pulses (Hillaire-Marcel et al., 1994). The Heinrich-type detrital layers usually appear as lighter colored intervals, particularly where they are rich in detrital carbonate. The detrital layers are often evident in magnetic susceptibility and gamma ray attenuation (GRA) density data measured on the shipboard multisensor track (MST). The objective at Sites U1302 and U1303 is to document this manifestation of LIS instability both during and prior to the last glacial cycle, beyond the reach of previously existing piston cores.

Sites U1302 and U1303 are separated by 5.68 km. The move to a different location from Site U1302 was an effort to avoid a debris flow at ~110 mbsf that halted drilling at both sites. An almost complete composite section was constructed at Site U1302 spanning 0–107 meters composite depth (mcd). Although it was not possible to construct a complete composite record at Site U1303, the density and magnetic susceptibility records from Sites U1302 and U1303 are remarkably similar and can be easily correlated. A short segment from one core of Site U1303, together with the record from Site U1302, provides a continuous composite stratigraphic sequence to ~107 mcd.

Sediments at Sites U1302 and U1303 are dominated by varying mixtures of terrigenous components and biogenic debris (primarily quartz, detrital carbonate, and nannofossils); the most common lithologies are clay and nannofossil ooze with silty clay. Dropstones are present throughout the cores. Calcium carbonate content ranges from 1 to 47 wt%. At the base of the sediment sequence, debris flow deposits (106–132 mcd) contain abundant intraclasts in a matrix of sand-silt-clay.

Samples from Site U1302 contain 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. 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 allows identification of glacial and interglacial conditions. The diatom assemblage in the upper 48 m at this site has been studied by Romero.

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 concentration decreases from seawater value to 5.9 mM close to the base of the recovered section, indicating that sulfate reduction is almost complete by 109 mcd.

Benthic foraminifers are rare at this site, although a continuous planktonic oxygen isotope record for the Brunhes Chron has been acquired using N. pachyderma (sinistral) (Hillaire-Marcel et al., pers comm., 2009). Shore-based U-channel paleomagnetic data have shown that the Matuyama/Brunhes boundary is recorded just above the debris flow at ~105 mcd, yielding a mean Brunhes sedimentation rate of 13.5 cm/k.y. (Channell et al., 2009a). Several magnetic excursions are recorded within the Brunhes Chron, including the Iceland Basin excursion at 188 ka, providing further stratigraphic markers and extending the inventory of magnetic excursions in the North Atlantic. The RPI record (Channell et al., 2009a) can be correlated to the oxygen isotope record to provide high-resolution age control throughout the record.

The detrital layer stratigraphy, recognized for the last glacial cycle in conventional piston cores from close to the site (e.g., Cores HU91-045-094P, MD99-2237, and MD95-2024), can be recognized using a variety of parameters including GRA density measurements, Ca/Sr ratios from X-ray fluorescence (XRF) core scanning, and magnetic hysteresis properties. The Ca/Sr ratio derived from XRF core scanning provides an efficient means of recognizing detrital carbonate associated with Heinrich-type detrital layers derived from Hudson Strait (Hodell et al., 2008). Comparison of the Ca/Sr ratio with GRA density and magnetic hysteresis and susceptibility data allows detrital carbonate events to be differentiated from low detrital carbonate events.

The acquisition of oxygen isotope and RPI data from Site U1302/U1303 provides a stratigraphic resolution that permits the proximal record of detrital layers obtained from Orphan Knoll to be correlated to the distal record at Site U1308 (Fig. F1). The correlation of detrital layers across the North Atlantic, particularly beyond the last glacial cycle, has been stymied by the lack of stratigraphic precision that has not allowed unequivocal correlation of detrital layers and hampered the understanding of distribution and provenance of detrital layers in the North Atlantic (e.g., van Kreveld et al., 1996).

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