in the western tropical Atlantic, decreases in the percentage of calcium carbonate in marine sediments are concurrent with increases in clay accumulation rates. In cores with records of percent carbonate extending well back into the Pleistocene, a series of low-calcium carbonate events can be seen. Existing C-14 geochronology allows the timing of the uppermost low-carbonate event to be constrained to the last deglacial interval and suggests a relatively short duration between about 16 and 14 ka BP (C-14), although more dating is needed to confirm the time limits. The bulk compositional changes of the upper low-calcium carbonate zones correspond to changes in clay mineralogy and in Sr isotope composition of the detrital clay fractions. In four cores the kaolinite/chlorite ratio varies systematically along a 1800 km NW-SE transect, starting with low kaolinite/chlorite and low Sr-87/Sr-86 ratios at the west end of the Vema fracture zone (core RC16-55) and ending with high kaolinite/chlorite and high 87Sr/86Sr ratios at the west end of the Romanche fracture zone (core V26-99). The kaolinite/chlorite ratio and Sr isotopic compositions from several samples in these cores also show a positive co-variation. The changes in composition within the low-carbonate interval correspond to changes in contributions of young, relatively unweathered (Amazon) and old, highly weathered (Rio Sao Francisco) sources. The provenance pattern allows that this pulse of clay deposition is reasonably interpreted to be a climate signal that could represent increased rainfall in tropical South America during the deglaciation, consistent with published oxygen isotope data from the Amazon fan. However, there are published carbonate records from near Africa that appear to show a correlative clay-rich layer. Thus, an alternative hypothesis is that the gray clay layer could reflect a plume of sediment stirred up from the shelf break during sea level rise associated with deglaciation. (C) 1998 Elsevier Science B.V. All rights reserved.
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