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 δ 13 C of individual size fractions are therefore an integration of low δ 13 C 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 δ 13 C of many size fractions, the δ 13 C precipitated throughout the growth of the foraminifera can be calculated. Time series of the δ 13 C 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 δ 13 C records indicate similar δ 13 C values during the last glaciation and the late Holocene and a broad deglacial minimum approximately synchronous with a minimum in the δ 13 C 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 δ 13 C values) and/or of the flux of water upwelled into the surface layer. The small (−0.2 to 0.0‰) glacial‐interglacial δ 13 C difference in tropical surface water is also believed to be affected by the chemistry of the underlying source water.