Carbon isotope composition of tropical surface water during the past 22,000 years

摘要

Carbon isotope values of many species of planktonic foraminifera are size‐dependant. Values of δ13C of individual size fractions are therefore an integration of low δ13C 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 δ13C of many size fractions, the δ13C precipitated throughout the growth of the foraminifera can be calculated. Time series of the δ13C 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 δ13C records indicate similar δ13C values during the last glaciation and the late Holocene and a broad deglacial minimum approximately synchronous with a minimum in the δ13C 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 δ13C values) and/or of the flux of water upwelled into the surface layer. The small (−0.2 to 0.0‰) glacial‐interglacial δ13C difference in tropical surface water is also believed to be affected by the chemistry of the unde