Accuracy and reproducibility of coral Sr/Ca SIMS timeseries in modern
and fossil corals
Abstract
Coral Sr/Ca ratios provide quantitative estimates of past sea surface
temperatures (SST) that allow for the reconstruction of changes in the
mean state and climate variations, such as the El Nino-Southern
Oscillation, through time. However, coral Sr/Ca ratios are highly
susceptible to diagenesis, which can impart artifacts of 1-2˚C that are
typically on par with the tropical climate signals of interest.
Microscale sampling via Secondary Ion Mass Spectrometry (SIMS) for the
sampling of primary skeletal material in altered fossil corals,
providing much-needed checks on fossil coral Sr/Ca-based
paleotemperature estimates. In this study, we employ a set modern and
fossil corals from Palmyra Atoll, in the central tropical Pacific, to
quantify the accuracy and reproducibility of SIMS Sr/Ca analyses
relative to bulk Sr/Ca analyses. In three overlapping modern coral
samples, we reproduce bulk Sr/Ca estimates within ±0.3% (1s). We
demonstrate high fidelity between 3-month smoothed SIMS coral Sr/Ca
timeseries and SST (R = -0.5 to -0.8; p<0.5). For
lightly-altered sections of a young fossil coral from the
early-20th century, SIMS Sr/Ca timeseries reproduce
bulk Sr/Ca timeseries, in line with our results from modern corals.
Across a moderately-altered section of the same fossil coral, where
diagenesis yields bulk Sr/Ca estimates that are 0.6mmol too high
(roughly equivalent to -6˚C artifacts in SST), SIMS Sr/Ca timeseries
track instrumental SST timeseries. We conclude that 3-4 SIMS analyses
per month of coral growth can provide a much-needed quantitative check
on the accuracy of fossil coral Sr/Ca-derived estimates of
paleotemperature, even in moderately altered samples.