CARBON ISOTOPE COMPOSITION OF TROPICAL SURFACE WATER DURING THE PAST 22,000 YEARS

Carbon isotope values of many species of planktonic foraminifera are size-dependant. Values of delta C-13 of individual size fractions are therefore an integration of low delta C-13 values of calcite precipitated in the juvenile growth stage and high values precipitated in later stages. In general, the selection of a particular size fraction for downcore studies has been subjective. Analysis of different size fractions of planktonic foraminifera, however, yields different time series in downcore studies. By measuring the delta C-13 of many size fractions the delta C-13 precipitated throughout the growth of the foraminifera can be calculated. Time series of the delta C-13 of calcite precipitated in the late growth stages of planktonic foraminifera from two nearby cores from the Caribbean Sea result in a better correlation than time series of constant size fractions. These delta C-13 records indicate similar delta C-13 values during the last glaciation and the late Holocene and a broad deglacial minimum approximately synchronous with a minimum in the delta C-13 records of benthic foraminifera from intermediate-depth Atlantic cores. The deglacial minimum may be a reflection of the isotopic composition of the underlying intermediate water (which may be affected by circulation changes or changes in preformed delta C-13 values) and/or of the flux of water upwelled into the surface layer. The small (-0.2 to 0.0 parts per thousand) glacial-interglacial delta C-13 difference in tropical surface water is also believed to be affected by the chemistry of the underlying source water.