A study of vital effects on multiple geochemical proxies recorded in a Porites coral from Great Barrier Reef
Abstract
The kinetic and biological processes, known collectively as vital effects, have been widely-recognized to appreciably limit the accuracy of paleoclimate reconstructions from geochemical proxies applied to coral skeletons. Several lines of evidence, based on experimental and culture studies, indicate that vital effects could originate from various sources, including 1) kinetic disequilibria due to a faster rate of skeletogenesis than that of inorganic precipitation and that required for maintaining a chemical/isotope exchange equilibrium; 2) closed system processes in which the Rayleigh fractionation plays important roles; and 3) variations in the chemical composition (e.g. pH, saturation state, etc.) of the calcifying fluid that is different from that of the ambient seawater. In this work, we measured trace element concentrations, including boron(B), lithium(Li), magnesium(Mg), strontium(Sr), barium(Ba), uranium(U) and calcium(Ca), oxygen (δ18O), Mg (δ26Mg) and B (δ11B) isotope compositions of a Porites coral to understand the origin(s) of the ';vital effects' evident in each proxy. Furthermore, with this increased understanding we explore the potential to extract precise environmental variables from a combination of these proxies. Analyses were made on a tropical Porites coral that was collected from Rib Reef within Australia's Great Barrier Reef (GBR). Long term monitoring of the GBR provides valuable records of in situ sea surface temperature (SST), salinity and other variables, against which geochemical proxies can be compared. A series of powder samples were milled at ~1 mm intervals along the coral's axis of maximum growth providing ~monthly temporal resolution across a ~2-year growth period. Aliquots of the aragonite powders were cleaned and prepared using established methods for trace metal, B isotope and Mg isotope analysis [Foster, 2008; Wang et al., 2013a; Wang et al., 2013b]. Resulting B/Ca, Li/Ca, Mg/Ca, Ba/Ca, U/Ca, δ18O and δ26Mg values all showed annual cycles and a correlation (of variable quality) with SST, i.e., B/Ca, Li/Ca, Sr/Ca, Ba/Ca and U/Ca have a positive correlation with SST, whereas Mg/Ca, δ18O and δ26Mg values have a negative correlation. The observed variations in trace elemental ratios can be understood by a Rayleigh process, which depends on SST and the amount of precipitation. Using the method of Gaetani et al. [2011], precise SSTs can be obtained within 1oC from Mg/Ca, Sr/Ca and Ba/Ca, and partition coefficients for elements other than Mg, Sr and Ba can be inferred. In contrast, δ18O and δ26Mg values are inconsistent with Rayleigh processes, but expectedly depend on growth rates and pH values of the calcifying fluid. Support for this comes from δ11B data, which suggest a very limited pH range of 8.44×0.02 over the two-year period that indicates a strict control on the internal pH of the calcifying fluid.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMPP31A1846W
- Keywords:
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- 0419 BIOGEOSCIENCES Biomineralization;
- 4916 PALEOCEANOGRAPHY Corals;
- 1041 GEOCHEMISTRY Stable isotope geochemistry;
- 1065 GEOCHEMISTRY Major and trace element geochemistry