HOLOCENE AND ANTHROPOCENE OOID PRECIPITATION AS A POSSIBLE NATURAL ARCHIVE FOR SHORT-TERM CHANGES IN OCEAN GEOCHEMISTRY AND CLIMATE

Marine calcareous ooids are near-spherical grains, usually 0.25 to 2.00 mm in diameter, consisting of a nucleus, which is commonly a mineral grain or a shell fragment, overgrown by successive laminae, or cortex, of calcium carbonate. The cortex precipitates successively around the nucleus as the grain is held in suspension in agitated shallow-marine waters supersaturated with respect to dissolved calcium carbonate. Such conditions exist locally along arid and sub-tropical shorelines and in isolated tidally influenced carbonate platforms such as the Bahama Banks in the Western Atlantic. The purpose of our research is to test to what extent ooids can be used for study of changes in ocean geochemistry over the last few hundreds to thousands of years. This is largely a new field of research that potentially can add information to questions addressed in Holocene and Anthropocene climate research. For our pilot study ooids were collected from present-day foreshore and shoreface environments in Joulters Cay, Andros Island, Bahamas. Lithified ooid sands of Holocene age were collected from backshore cliff sections and studied for comparison. Ooids have been studied by means of Scanning Electron Microscopy (SEM-EDX) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) in order to characterize their structure and geochemical trace element composition. Trace elements were measured using an Analyte G2 193 nm laser and a Bruker Elite quadrupole ICP-MS. Profiles and mapping were ablated using a scan speed of 8 µm per second, a laser repletion rate of 8 Hz and a spot size of 10x50 µm (profiles) and 20x20 µm (mapping). Trace element concentrations were calculated using MACS3 as the calibration standard and Ca43 as the internal standard. Our SEM studies reveal abundant traces of different morphology from endolithic bacteria in the surface of all studied ooids, independent of sampling site, and a cyclic distribution of these traces through the cortex (time), suggesting separate periods of ooid growth and ooid rest. LA-ICP-MS reveals distinct changes in the concentration of several trace elements between sets of cortex laminae, for example in Ba, Mg and U. Our presentation will present the project and the planned research as well as discuss plausible causes for the observed cyclicity in trace element concentrations.