Do Skeletal Density Changes Within the Tissue Layer of Corals Affect Paleoclimate Reconstructions?

Joanna Sarah Griffiths, Louisiana State University, Biological Sciences, Baton Rouge, LA, United States, Kristine L DeLong, Louisiana State University, Geography and Anthropology, Baton Rouge, LA, United States, Terrance Quinn, University of Texas at Austin, Institute for Geophysics, Jackson School of Geosciences, Austin, TX, United States, Frederick W Taylor, Institute for Geophysics, Austin, TX, United States, K. Halimeda Kilbourne, University of Maryland Center for Environmental Science Appalachian Laboratory, Frostburg, MD, United States and Amy J Wagner, UNC - Wilmington, Wilmington, NC, United States
Abstract:
Sea surface temperature (SST) reconstructions from coral geochemistry provide information on past climate variability; however, not all coral studies agree on a common calibration slope. Therefore, understanding the impacts of coral skeletal growth on strontium-to-calcium ratios (Sr/Ca) and oxygen isotopic ratios (δ18O) is necessary to ensure accurate calibrations. The study of Gagan et al. (2012) suggests that for the Pacific coral genera Porites, SST calibrations for coral Sr/Ca and δ18O need to be adjusted to account for skeletal density changes in the tissue layer, which may attenuate the seasonal cycle in coral geochemistry. We attempt to duplicate those results and density patterns in several Porites lutea colonies from two locations, yet our results do not show an increase in density in the tissue layer. Another study with Montastraea faveolata reveals reduced seasonality in coral Sr/Ca compared to slower-growing Siderastrea siderea in close proximity and same water depth, suggesting the faster growing M. faveolata geochemistry may be attenuated. By measuring skeletal density changes by micromilling a standard volume throughout the tissue layer and immediately below, we find no pattern of skeletal accumulation in the tissue layer of multiple colonies of M. faveolata and S. siderea from different locations. We conclude that these species lay down all of their skeletal material at the skeleton surface, thus skeletal density changes in the tissue layer do not account for reduced seasonality. We propose that time averaging occurs in M. faveolata as a result of the coral polyp’s deep calyces mixing time intervals in the adjacent thecal wall in which micromilling for geochemical analysis produces a sample area that contains several growth increments. Our results show that skeletal density growth effects cannot be applied to all coral genera and paves the way for new research on calyx depth as an alternative explanation for differences in coral calibration slopes.
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