PALEOCEANOGRAPHY, VOL. 10, NO. 2, PAGES 179-196, APRIL 1995 Late Pleistocene oxygen isotope records of biogenic silica from the Atlantic sector of the Southern Ocean A. Shemesh Department of Environmental Sciences andEnergy Research, Weizmann Institute of Science, Rehovot, Israel L. H. Burckle and J. D. Hays Lamont-Doherty EarthObservatory of Columbia University, Palisades, New York Abstract. We determined the isotopic composition of oxygen in marinediatoms in eightdeep- sea cores recovered from theAtlanticsector of the Southern Ocean. The analytical reproducibility and core-to-core consistency of the isotopic signal suggests that diatom •i•80 can be used as a newpaleocenographic toolto reconstruct past variations in surface water characteristics andto generate •80-isotope-based stratigraphy forthe Southern Ocean. The data indicate that diatom •i•80 reflects sea surface temperature and seawater isotopic composition and that diatoms retain their isotopic signal on timescales of a least 430ka.The •i•80 analyses of different diatom assemblages revealthattheisotopic signal is free of species effects and thatthe common Antarctic species have thesame water-opal fractionation. The transition fromthelastglacial maximum (LGM) to the Holocene is fullyrecorded in high sedimentation rate cores. An •80 enrichment during theLGM, a post-LGM meltwater spike and aninput of meltwater during the lateHolocene arethemain isotopic features observed in down core records. Theorigin of this meltwater wasverylikely melting icebergs and/or continental iceor by melting sea ice that had accumulated snow. The most pronounced meltwater effects arerecorded in cores thatare associated withtheWeddel gyre. Ourresults provide thebasis for extending isotope studies to oceanic regions devoid of carbonate; further, isotopic stratigraphies maybe constructed for records and regions where theywere previously notpossible. Introduction Paleoceanographic studies based on oxygen isotope compositionof planktonic and benthic foraminifera are well establishedfor Quaternary and Tertiary deep sea sediments. Wide areas of the ocean floor, however, lack suitable calcareous microfossils due to dissolution or unfavorable ecological conditions; nevertheless such sediments frequently contain abundant diatoms and radiolaria. This is true in areas of upwelling and deep vertical mixing, such as the Southern Ocean, the northwest and equatorial Pacific Oceanand coastal upwelling zones. The determination of the oxygen isotopic composition of marine biogenic silica in these areasprovides basic information (temperature, isotopic composition of seawater,etc.) required to reconstruct paleoenvironmental and climate and ocean circulationchanges over geologic time. South of the Polar Front Zone (PFZ), diatoms dominate the phytoplankton community. Because these algae are light- limited, their distributionis largely limited to the uppermost layer of the ocean. The typical depth of the photic zone at polar regions is considered to be about 100 m. However, self- shading generated by algae and chlorophyll degradation products reduces the depth of the photic zone. It has been Copyright 1995 by theAmerican Geophysical Union. Paper number 94PA03060. 0883-8305/95/94PA-03060510.00 observedthat polar blooms usually do not occur when the mixed layer is greater than 40-50 m [Sakashaug and Holm- Hansen, 1984]. In general, most of the diatom population lives between 10 and 40 m. Therefore their oxygen isotopic composition is not susceptibleto uncertainty in "depth habitat"and is a particularly suitable monitorof the surface mixed layer. Southern Ocean diatoms which survive to sediment burial are generally the result of a seasonal bloom that occurs with the melt back of sea ice in the late springto early summer. Thus, their isotopiccomposition has a strong seasonal (and sea ice free) bias and allows reconstruction of sea surface temperature for bloom conditions rather than averagedannual variations. The major problem confronting all workers who have attempted to measure the fi•80 of biogenic silica is the isotopic exchange of silica oxygen with opaline water- oxygen during laboratory dehydration processes. Measurement of fi•80of biogenic silica in radiolaria revealed that radiolaria contain 10-16% in weight of loosely bound water that interferes with the isotopic analysis of silica [Mopper and Garlick, 1971]. Mikkelsen et al. [1978] obtained a silica fi•80 profile that parallels the planktonic fi•80 curve in Pacific equatorial core PLDS-72 by applying improved oxidation and dehydration techniques. They concludedthat fi•80 of biogenic silica (diatoms) can be usedas a paleothermometer and that no isotopic re-equilibration betweensilica and bottom waterstakesplace on timescales of 10-20 ka. Parallel trends in the isotopic recordsof biogenic