Leg 172 Scientific Report

RESULTS (continued)

Deep Water Sites on the Bermuda Rise and Sohm Abyssal Plain

The sediment drift which comprises the northeast Bermuda Rise is one of the highest resolution archives of paleoclimate and paleo-ocean information known from the open sea. Sedimentation rates as high as 200 m/m.y. are the result of clay and silt advection by deep recirculating gyres, the strength of which is linked to surface ocean and atmospheric conditions. Because much of the clay and silt is derived from eastern Canada, studies of the Bermuda Rise have the potential to link the marine, terrestrial, and atmospheric components of the climate system. Prior to Site 1063, the longest continuous core from the Bermuda Rise was 53 m, terminating in sediments of glacial Stage 6. Accordingly, one of the most important Leg 172 objectives was to recover a complete sequence of sediments back through the origin of Northern Hemisphere glaciation. Such sediments should document any changes in lithology associated with the first glaciation of eastern Canada, which might also account for the base of the acoustically stratified sediments (at 0.3 s two-way traveltime [twt]). In addition, they should document any important changes in deep-ocean circulation patterns associated with that event. Finally, Bermuda Rise sediments should be useful for high-resolution studies of changes in Earth's magnetic field.

When it was clear that we had the extra time, we requested permission to drill a hole on the Sohm Abyssal Plain near the northeast Bermuda Rise. Site 1064 was chosen because it was near some conventional piston cores taken by CSS Hudson, and it appeared to be ideally situated to fulfill two paleoceanographic objectives. One objective of this site was to study the distal turbidites on the abyssal plain to determine the link between their Canadian source and the fine-grained sink on the Bermuda Rise. A second objective of Site 1064 was to provide a pore-water record of O from a deep water location unaffected by climatically controlled changes in the flux of NADW. It was assumed that an abyssal plain site at this latitude would have been continuously bathed by AABW during the past 20 k.y. If this proved to be correct, then the change in O from modern bottom water to glacial-aged pore water at Site 1064 would reflect mostly the secular change in oxygen isotope composition of the ocean.

Site 1063
Site 1063 is located on the northeast Bermuda Rise, in the northern Sargasso Sea, at a water depth of 4584 m (Fig. 2). This location is close to the interface between AABW and NADW, marked by bottom water temperatures of ~2°C. Four holes were cored with total recovery in excess of 100%. The interval 0-201.4 mbsf was triple APC cored and appears to provide a complete composite sequence down to the early Pleistocene. A fourth APC penetrated to 173 mbsf. Double XCB coring extended the site to the early Pliocene with penetration to 418.4 mbsf at Hole 1063A and 342 mbsf at Hole 1063B.

The base of acoustically stratified sediments appears to be late Pliocene to early Pleistocene in age, but the most reflective sediment was deposited beginning ~1 m.y. ago (Fig. 8). These reflectors seem to correlate with late Pleistocene climate change from 40 k.y. to 100 k.y. variability. In spite of the "biscuiting" in the cores drilled with the XCB, good sedimentary structures could still be observed. Overall, an excellent and continuous sedimentary succession highly suitable for paleoceanographic studies was recovered at Site 1063.

The sediments reflect the rapid deposition of clays and silts by deep recirculating gyres in the western North Atlantic. Only one lithostratigraphic unit (Fig. 3) is recognized at Site 1063, characterized by 70%-100% clays alternating with nannofossil-bearing intervals (commonly 15% to 30% but occasionally exceeding 60%). The persistence of one sedimentary unit for more than 3 m.y. suggests that sediment focussing by deep currents at this site has been the dominant depositional process during the late Neogene. In addition to the clays and nannofossils, two other accessory components are silt and biogenic silica. The presence of 15% to 30% biogenic silica (mostly diatoms) within clays and silty clays marks Subunit IA (0-135 mbsf). Within the last glacial maximum, at 4-11 mbsf, a region of black diagenetic mottles containing troilite and hydrotroilite is correlatable with Sites 1061 and 1062 on the Blake-Bahama Outer Ridge. As in the upper sections of other Leg 172 sites, Bermuda Rise sediments contain distinct red lutite beds that are thought to reflect increased advection of fine-grained sediment from the Nova Scotia region during cold climates. Subunit IB also contains these red lutites, and their last appearance at 270 mbsf defines its lower boundary. Between 270 and 420 mbsf, Subunit IC consists of dark greenish gray, homogeneous clays interbedded with light greenish gray carbonate-rich intervals. Erosional events, common at several of the Blake-Bahama Outer Ridge sites, are virtually absent here, with the exception of a thin (10 cm), laminated deposit at 101 mbsf.

Calcareous nannofossils are the dominant microfossils, and they are generally well preserved except for some intervals where they are dissolved. Eleven nannofossil horizons were constrained for the past 3.3 m.y. Foraminifer assemblages are generally dissolved, but despite poor preservation, eight foraminifer horizons were identified. Where diatoms are most common, the sediment has the unusual association of high sonic velocity and low bulk density. It is thought that this stems in part from a structural framework provided by the diatoms and other siliceous microfossils. The diatoms seem to be most prevalent in cold climatic episodes, probably reflecting some combination of higher surface-water productivity and better preservation.

Paleomagnetic results from Site 1063 are exceptional, providing an excellent record of geomagnetic secular variation, geomagnetic excursions, transitional field directions during reversals (Fig. 5), and a detailed magnetostratigraphy down to the Gauss Chron (Fig. 9). In Hole 1063A, the Brunhes/Matuyama reversal boundary (0.78 Ma) is located at 138.5 mbsf, the top and bottom of the Jaramillo (0.99 and 1.07 Ma, respectively) are located at 175.0 and 187.3 mbsf, respectively, and the top of the Gauss (2.581 Ma) is at 347.7 mbsf. Also, clearly identified are the Cobb Mountain Subchron, Olduvai Chron, and the Reunion I and II Subchrons.

There is excellent agreement between the 19 calcareous planktonic biohorizons recognized and the magnetic polarity reversal stratigraphy. The integrated biomagnetostratigraphy indicates a sedimentation rate of 200 m/m.y. from 0 to 0.5 Ma, 140-170 m/m.y. between 0.5 and 1.1 Ma, 100-110 m/m.y. between 1.1 and 2.5 Ma, and 75 m/m.y. in the underlying interval.

The sulfate/methane boundary occurs at 38 mbsf. Methane concentrations below the interface were sufficient to cause significant sediment expansion in the recovered cores. Downhole profiles of interstitial-water calcium, magnesium, and strontium indicate dissolution of biogenic carbonates and probable dolomitization in the zone of methanogenesis. Manganese concentrations (up to 37 mM) in the top of the sulfate reduction zone are higher than at the other sites, which is probably due to the relative proximity of Site 1063 to volcanic sources. Interstitial water profiles indicate decreasing amounts of biogenic silica in the sediment column below 250 mbsf. Chloride concentrations decrease downhole, showing an overall freshening of 3% relative to seawater.

Site 1064
Site 1064 is one sedimentary unit consisting of alternating brown and red clays with exceptionally sharp contacts. The brown clays are probably distal turbidites because of the occurrence of thin basal laminae and scoured contacts. Reddish clays may be turbiditic as well, although there is no evidence of basal scour. The relationship between clays at this site and those on the Bermuda Rise is unclear, because the three APC cores at Site 1064 were insufficient to establish a reliable biostratigraphy.

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