The temperature and carbonate ion influence on Pleistocene high latitude planktonic foraminiferal carbon isotopic records

Abstract:

Establishing a credible record of the carbon isotopic composition of high latitude surface ocean DIC over ice ages has been an enormous challenge, because the possible archives of this important variable in deep sea sediments all incorporate complex effects of the biomineralization process. For example, culture experiments (by Spero and colleagues) demonstrate a strong temperature and carbonate ion effect on the carbon isotopic composition of G. bulloides--the taxon of planktonic foraminifera that is most abundant in the majority of subpolar sediment sequences. Here we capitalize on the fortuitous observation of exceptionally strong covariation between the oxygen and carbon isotopic composition of G. bulloides in multiple sediment sequences from the Benguela upwelling region. The covariation is most clear during Marine Isotopic Stage 3 (an interval when the isotopic composition of the seawater was least variable) and undoubtedly results from the precipitation of tests under variable conditions of temperature and carbonate ion. The unusually clear isotopic relationship in planktonic foraminifera observed off Namibia constitutes a field calibration of the biomineralization effects observed in culture, and we apply it to previously published high latitude carbon isotopic records throughout the Southern Ocean. We find that many of the excursions toward lower planktonic foraminiferal δ¹³C that have been interpreted previously as the upwelling of nutrient rich water during deglaciations are better explained as increases in upper ocean temperature and carbonate ion. Conversely, the excursions toward high δ¹³C during ice age intervals that have been interpreted previously as increased export production (purportedly stimulated by dust) are also better explained by temperature and carbonate ion variability. After removal of the inferred temperature and carbonate ion signal from the planktonic foraminiferal time series, the residual is essentially (but not exactly) the same as the deep ocean carbon isotopic variability recorded in co-occurring benthic foraminifera. These reinterpretations bear not only on the assumed venting of carbon through the high latitude surface ocean, but also on the "preformed" carbon isotopic variability throughout the interior of the Pleistocene oceans.