PROVENANCE CHANGE COUPLED WITH INCREASED CLAY FLUX DURING DEGLACIAL TIMES IN THE WESTERN EQUATORIAL ATLANTIC

in the western tropical Atlantic, decreases in the percentage of calci um carbonate in marine sediments are concurrent with increases in clay accumulation rates. In cores with records of percent carbonate extend ing well back into the Pleistocene, a series of low-calcium carbonate events can be seen. Existing C-14 geochronology allows the timing of t he uppermost low-carbonate event to be constrained to the last deglaci al interval and suggests a relatively short duration between about 16 and 14 ka BP (C-14), although more dating is needed to confirm the tim e limits. The bulk compositional changes of the upper low-calcium carb onate zones correspond to changes in clay mineralogy and in Sr isotope composition of the detrital clay fractions. In four cores the kaolini te/chlorite ratio varies systematically along a 1800 km NW-SE transect , starting with low kaolinite/chlorite and low Sr-87/Sr-86 ratios at t he west end of the Vema fracture zone (core RC16-55) and ending with h igh kaolinite/chlorite and high 87Sr/86Sr ratios at the west end of th e Romanche fracture zone (core V26-99). The kaolinite/chlorite ratio a nd Sr isotopic compositions from several samples in these cores also s how a positive co-variation. The changes in composition within the low -carbonate interval correspond to changes in contributions of young, r elatively unweathered (Amazon) and old, highly weathered (Rio Sao Fran cisco) 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 deg laciation, consistent with published oxygen isotope data from the Amaz on fan. However, there are published carbonate records from near Afric a that appear to show a correlative clay-rich layer. Thus, an alternat ive hypothesis is that the gray clay layer could reflect a plume of se diment stirred up from the shelf break during sea level rise associate d with deglaciation. (C) 1998 Elsevier Science B.V. All rights reserve d.