'Clumped isotope' thermometery in foraminifera

Abstract

Accurate constraints on past ocean temperature and composition are critical for documenting climate change and resolving its causes. Most inorganic and organic geochemical paleothermometers are fundamentally underconstrained because they reflect seawater composition as well as temperature. In addition, some of the most frequently used thermometers — ^(18)O/^(16)O and Mg/Ca ratios in biogenic carbonate — deviate from inorganic calibrations and can exhibit species-specific temperature dependencies. These offsets or ‘vital effects’ are poorly understood and lend an unquantifiable source of uncertainty to reconstructions of past temperature. Here we report ‘clumped isotope’ [1] measurements of benthic and planktic foraminifera from sediment core-tops showing that the proportion of ^(13)C-^(18)O bonds in calcite and aragonite shells exhibits the same temperature dependence as inorganic calcite. The ‘clumping’ of heavy isotopes into bonds with each other is independent of the isotopic composition of the water in which the mineral precipitated, and thus avoids the ambiguity intrinsic to some other thermometers. These observations suggest the clumped isotope thermometer can yield accurate determinations of past seawater temperature, even when applied to the remains of extinct taxa. The lack of discernable ‘vital effects’ in foraminifera and other forms of biogenic calcite and aragonite [1-4] indicates that these organisms precipitate calcium carbonate in isotopic equilibrium with the dissolved inorganic carbon (DIC) pool from which they calcify. The lack of difference between calcite and aragonite may indicate that the ‘clumping’ of these heavy isotopes into bonds with each other reflects the thermodynamically-controlled exchange of stable isotopes amongst DIC species in water, rather than similar exchange equilibria within the solid precipitates [5].