SITE 1089

Site 1089 is located in the southernmost Cape Basin in the southeast Atlantic Ocean, just north of the Agulhas Ridge. High rates of sedimentation (8-13 cm/k.y.) result in an expanded sedimentary sequence that is ideally suited for studying environmental changes in response to climate variability on orbital and suborbital time scales. Given these high sedimentation rates, Site 1089 is the Southern Hemisphere analog to the North Atlantic drift deposits drilled during Legs 162 and 172, and it will be useful for determining the response of the Southern Ocean to orbital forcing and the phase relationships to climate change in the North Atlantic. The high accumulation rates at Site 1089 will also permit detailed correlation between marine sediment analyses and ice-core records, especially the Vostok ice-core signal that has now been extended to 420 ka (Petit et al., 1997).

A 264.9-m-thick sedimentary section spanning the interval from the Holocene to late Pliocene (~2.4 Ma) was recovered at Site 1089. The sediments are predominantly composed of diatoms, nannofossils, and terrigenous mud in varying proportions. Calcium carbonate (CaCO3) contents in Hole 1089A average 27.0 wt% and range from 0.6 to 69.3 wt%, whereas total organic carbon (TOC) varies between 0 and 0.82 wt% with an average value of 0.43 wt%. Although Site 1089 is deep (4624 m), benthic foraminifer abundance is fairly constant downhole to about 220 mbsf, below which it goes to zero. It should be possible, therefore, to produce a continuous stable isotope stratigraphy in the upper 220 mbsf. No major lithologic boundaries occur within Site 1089, and only one lithologic unit was identified.

The upper 100 m of the section, representing approximately the Brunhes Chron (0.78 Ma to present), is nearly complete. Variations in spectral color reflectance, CaCO3, bulk density, and magnetic susceptibility permit the prediction of glacial and interglacial Stages 1 to 19, constrained by diatom and calcareous nannofossil biostratigraphy. With four holes drilled to more than 118 mbsf at Site 1089, a continuous spliced record was constructed to a depth of 94 mcd by aligning features in the records of closely spaced physical properties measurements. Core logging data obtained at 2- to 6-cm sampling intervals show cyclic variations at Milankovitch frequencies as well as variations at higher (sub-Milankovitch) frequencies. Postcruise analysis of these signals will be useful for delineating climatic variability on these time scales. Preservation of remanent magnetization is good in the upper 100 m and preliminary results are encouraging for constructing the first detailed Southern Hemisphere record of geomagnetic paleointensity during the Brunhes Chron.

A series of 3- to 15-m-thick deformed sediment units, possibly slump or slide deposits, were cored between 95 and 156 mcd. Soft-sediment deformation is manifested in the sediment by dipping and/or contorted beds, sharp color contacts, and microfaults. The Brunhes/Matuyama boundary occurs in the interval from 105 to 114 mbsf in Hole 1089B; however, the transition is not well preserved because of soft-sediment deformation that affects the interval from the lowest Brunhes Chron to the top of the Jaramillo Subchron. However, even within the deformed sediment interval, laminations and burrow structures are preserved and suggest that the stratigraphic section is relatively intact. Postcruise analysis may allow us to piece together a composite section that eliminates several of the deformed intervals.

Polarity transitions in cores from Hole 1089B define the lower boundary of the Jaramillo Subchron at 151.6 to 153.6 mbsf, and the upper and lower boundaries of the Olduvai Subchron at 213.8 to 215.8 mbsf and 225.5 to 227.6 mbsf, respectively. We place the Pliocene/Pleistocene boundary at ~229 mcd at Site 1089.

Biomagnetostratigraphy provides an age-depth relationship that indicates continuous sedimentation at Site 1089 since the late Pliocene (~2.4 Ma). Sedimentation rates average ~128 m/m.y. in the upper 94 mcd (~0.7 Ma), ~180 m/m.y. between 94 and 156 mcd (~0.65 to 1 Ma), ~110 m/m.y. from 156 to 230 mcd (1-1.8 Ma), and ~84 m/m.y. from 230 to 280.6 mcd (1.7-2.4 Ma). The middle interval of relatively high sedimentation rates (between 95 and 154 mcd) corresponds to the disturbed section, but the superposition of biostratigraphic datums is as expected in this interval. Within the last 400 k.y. (upper 60 mbsf), sedimentation rates were higher during interglacial than during glacial intervals, probably because of enhanced carbonate production and/or preservation during interglacials (Howard and Prell, 1994).

Pore-water profiles from Site 1089 indicate reducing conditions such that sulfate reduction is complete by 50 mbsf and methane concentrations are high below this depth in the hole. The Ca2+ profile shows a dramatic decrease in the sulfate reduction zone reaching a minimum at 50 mbsf. This Ca2+ decrease results in unusually high Mg/Ca ratios because Mg2+ concentrations remain at near-seawater values.

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