A High-resolution Sr/Ca, Ba/Ca, Rare Earth Elements (REEs) and δ18O Coral Record from the Andaman Sea: Implications for the South Asian Summer Monsoon Variability
Abstract
The surface water hydrography of the Bay of Bengal and the Andaman Sea is dominated by rainfall and river runoff during the summer monsoon season, offering a unique prospect for understanding the past variability of the South Asian Summer Monsoon (SAM) over the ocean and the land. Tropical shallow-water corals record a continuous, well-dated, and high (monthly) resolution geochemical signal of sea surface temperature (SST), sea surface salinity (SSS) and water chemistry in their aragonitic skeletons. A 70-cm-long core was recovered from a dead Porites sp. coral head washed onto a beach at the eastern coast of Middle Andaman Island, facing the Andaman Sea. A preliminary coral age was established by AMS 14C dating, suggesting that the coral grew during the 19th century, at the transition from the Little Ice Age to the observational period. The coral growth rate was 1.8 cm/year as estimated from X-radiograph images of the slabbed core, which revealed clear pairs of high-density/low-density bands that likely represent annual growth increments. We have micro-sampled the upper part of this coral at 1.4 mm resolution, which provides approximately twelve samples per annual growth increment. Prior to measuring Sr/Ca, Ba/Ca, Rare Earth Elements (REEs), δ18O and δ13C in this coral, several cleaning protocols were tested in order to assess their effectiveness for removing clay particles and Fe-Mn oxidative coatings. Coral Sr/Ca appears to be an excellent proxy for SST and coral δ18O is a function of both SST and sea-surface water δ18O composition (δ18OSW). By subtracting the Sr/Ca based SST component from the δ18O signal, we were able to reconstruct δ18OSW at nearly bimonthly resolution and to infer SSS trends during the approximately twenty years of coral growth. The latter should reflect past patterns of rainfall and freshwater input in this area, which can also be inferred from coral-derived Ba/Ca ratios and REEs distributions. The preliminary results of this study demonstrate that the combined use of high-resolution δ18O, Sr/Ca, Ba/Ca and REEs systematics can provide quantitative constraints on seasonal changes of SAM intensity prior to instrumental observations in this understudied region of the Indian Ocean.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP42B..06Y
- Keywords:
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- 0419 Biomineralization;
- BIOGEOSCIENCESDE: 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHYDE: 4994 Instruments and techniques;
- PALEOCEANOGRAPHY