Reconstructing sea-surface temperatures and compositions for the tropical Caribbean through LA-ICPMS analysis of Dichocoenia corals
Abstract
Climate change is increasing the thermal stress on modern coral reefs, and has heightened interest in obtaining high-resolution temperature records associated with past reef building episodes to investigate the ability of corals to adapt to changing temperature regimes. Laser ablation ICPMS allows multi-element sampling across coral skeletons, and thus, can provide continuous temperature and compositional records of seawater across multiple years. Nonetheless, many shallow-water coral skeletons are exceedingly porous and fast growing (e.g. Porites), which creates challenges for effective laser sampling. For this study, we explored changes in the chemical compositions of skeletons of a sparsely porous and slow-growing coral genus: Dichocoenia. We analyzed two samples for concentrations of U, Sr, and Mg normalized to Ca. One was collected live in 1974 from near Key Largo; the other was collected from 6.2 Ma strata in the Dominican Republic. By analyzing for several elements with differing ocean residence times we have attempted to compare both sea-surface temperatures and seawater composition between the late Miocene and the present in the Caribbean region. Most of the Mg/Ca, U/Ca, and Sr/Ca values for both corals vary consistently with changes predicted by a Rayleigh fractionation model of aragonite growth (Gaetani et al., 2011, GCA). Skeleton segments with compositions that were not consistent with this model had significantly elevated Mg/Ca, probably resulting from the presence of calcite or particulate matter. Plots of Sr/Ca against Mg/Ca excluding the high-Mg data largely overlapped for the modern and fossil corals, although the modern data were offset to higher Mg/Ca and had a wider range of Mg/Ca than the fossil data. Assuming that aragonite growth rates linearly scale to temperature, vital effects on element concentrations in intra-coralline fluids and element partitioning are identical for the modern and fossil coral, these data suggest that the Late Miocene sea-surface temperatures for the Dominican Republic were roughly 2 degrees C cooler (20 to 28) than those of the 1960s and early 1970s near Key Largo assuming historical monthly averages of 21 to 31 degrees C. Plots of U/Ca against Mg/Ca for the modern coral were displaced to higher U/Ca values compared to the fossil data. This offset is consistent with a roughly 30% higher concentration of U in ocean water today compared to the Late Miocene, which could be a reflection of the relatively short residence time for U in ocean water and greater modern erosion rates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGC51H1123R
- Keywords:
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- 1050 GEOCHEMISTRY / Marine geochemistry;
- 1635 GLOBAL CHANGE / Oceans;
- 4220 OCEANOGRAPHY: GENERAL / Coral reef systems;
- 4916 PALEOCEANOGRAPHY / Corals