Variations in climatic conditions from the Cayman Islands through stable isotope and element analysis from corals and sediment cores; a 500,000 year record

Abstract

The Caribbean region is particularly important to understanding global climate change and feedback systems because the tropics are the primary source of heat and water vapor for the atmosphere. The Caribbean region, however, is a relatively understudied area in terms of tracking climate change through time. The Cayman Islands, specifically, have little documentation of climate change before the 1980’s, apart from anecdotal records of past storms. Climate change has been increasingly studied in recent years due to the current and proposed future impacts of such changes on global environments. Numerous proxies (δ18O, δ13C, Sr/Ca, Mg/Ca, U/Ca, Sr-U, Li/Ca, Li/Mg, Mg/Li, Ba/Ca, B11/Ca, trace and rare earth elements) for past environmental conditions (sea surface temperature (SST), salinity, photosynthetic light activity, water depth, upwelling, riverine run-off, atmospheric moisture variability) have been developed for use with coral skeletons and sediment cores. The application of these proxies, however, is complicated as many factors that may control the incorporation of these proxies into the geologic records (vital effects, geographic location, sampling analytics) are still debated. Interpretations based on geochemical proxies (stable isotope, element/Ca ratios, and elemental concentrations) derived from sediment cores and modern and fossil corals (Orbicella annularis and Montastrea cavernosa) from the Cayman Islands over the last 500,000 years record alternating cool (SST < 28.5°C; current average water temperature for Grand Cayman) and warm (SST >28.5°C) periods in the Caribbean Sea. Coral skeletons from Grand Cayman and Cayman Brac are the baseline for the development of an oxygen isotope geothermometer that accurately reconstructs SST within the range of measured Caribbean water temperatures. Using this δ18Ogeothermometer, these coral skeletons record two cool periods, one warm period, and one mild period over the last ~540 years around the Cayman Islands. These temperature periods correlate with climate change in the wider Caribbean region. Oxygen isotopes compositions and elemental concentrations from sediment cores in North Sound, Grand Cayman’s largest lagoon, record five periods of climate (SST and atmospheric moisture variability) change over the last ~6000 years. These climate periods correlate to phases of climatic variability in the Caribbean and at higher latitudes in the Northern Hemisphere. The global nature of these climate periods can be related to the movements of the Intertropical Convergence Zone and the phase of the North Atlantic Oscillation. Although it has been shown that modern corals can reliably reconstruct SST, application to older corals is more complicated. Determination of SST from older corals is only possible if their aragonitic skeletons have undergone little to no diagenetic alteration. For corals from the Pleistocene Ironshore Formation (Units A-F; 80 to 500 ka), SST calculations are only viable if the coral skeleton has >95wt% aragonite, no cements, Mg/Ca ratios <12.0 mmol/mol, Sr/Ca ratios >8.0 mmol/mol, δ18OVSMOW values >25.1‰, and δ13CVPDB values >–3.0‰. Based on these criteria the corals from Units A-C (229 to 500 ka) cannot be used, whereas those from Units D-F (125 to 80 ka) produce reliable SST records. The temperature profiles developed from Units D-F correlate with temperature reconstructions from other localities during the Pleistocene (e.g., the Caribbean, North Atlantic, Coral Sea, South China Sea, and Antarctica). This work is significant as tracking changes in climate from the past may provide indicators for future climate trends.