'Clumped isotope' thermometery in foraminifera
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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].