Contrasting causes for the demise of Caribbean coral reefs: Terrestrial vs. oceanographic mechanisms

We are currently in the midst of a global 'natural experiment' in which anthropogenic environmental change is forcing biotic response on a global scale. Short term ecological data have only been accumulating for a few decades now (and human impacts have been working for at least a century), and cannot provide the baseline required to provide adequate 'pre-treatment' control for this natural experiment. And obviously, we cannot determine the results of an experiment that remains in progress. Instead, researches can turn to the fossil record and study the results of similar 'natural experiments' in which reef systems responded to long-term regional environmental change. The proposed project is one such study that will provide the information required to better understand the structure and ecological function of coral reef ecosystems as they respond to regional environmental change. There was a major environmental and ecological transition on coral reef ecosystems in the Caribbean coinciding with the Oligocene/Miocene boundary. Approximately half of a diverse Late Oligocene reef-coral biota became extinct at this time, and extensive reef building ceased to occur on a regional scale for at least 20 million years. This interval was also a time of accelerated global environmental changes, including a 'snap' southern hemisphere glaciation and changes in ocean circulation patterns. Superimposed on these global changes were regional effects of tectonic reorganisation that changed the geography and hydrography of river systems in South America. All of these environmental changes are likely to have had impacts on shallow marine ecosystems in the Caribbean, and there is evidence that the Oliogcene/Miocene ecosystem collapse was a result of changing regional water quality, including an increase in regional surface productivity. The objective of the proposed project is to extract geochemical proxy data from a set of exceptionally preserved Early Miocene fossil corals to document environmental conditions in the Southern Caribbean during the Early Miocene. These fossils will be collected from the Cantaure Fm. of the Paraguana Peninsula of Northern Venezuela. This unit is unique because it contains diverse fauna assemblages including reef-corals, mollusks and fish (otoliths and teeth) that are preserved as original aragonite. We know of no other early Miocene fossil reef-corals that have not been altered to calcite during early diagenesis. These fossils provide a unique opportunity to determine environmental conditions in shallow marine coastal ecosystems from the Early Miocene. Sub-seasonally resolved geochemical proxy data will provide time-series information of riverine runoff, SST, sea-surface salinity (SSS), and nutrient chemistry from fossil coral skeletons in order to disentangle land- from sea-based causes of reef decline. Trace elemental and isotopic ratios (e.g. Sr/Ca, U/Ca, Ba/Ca, P/Ca, d18O) in corals will be used as recorders of above-mentioned past oceanographic parameters. Due to their continuous growth, corals can store consecutive time-series information of several hundred years, which can be retrieved at approximately weekly resolution by continuous high-spatial resolution sampling using laser-ablation mass spectrometric analysis (LA-ICPMS). We know that there was a Caribbean-wide decline of extensive reef ecosystems during the Early Miocene, however the precise timing and ecological mechanisms responsible for this decline remain to be fully understood. In the proposed project, we aim to uncover the palaeoenvironmental context for the Early Miocene decline. This essential background will help guide further study of the local-scale details of the event with the overall aim of developing a better understanding of ongoing regional changes of modern Caribbean coral reef systems.