THE INFLUENCE OF GLOBAL AND REGIONAL ENVIRONMENTAL CHANGE ON CENOZOIC CARBONATES IN SE ASIA

The SE Asian carbonate record allows insight into the poorly known response of equatorial marine systems to regional and global change during the Cenozoic. There is a marked change from larger benthic foraminifera to corals as dominant producers in SE Asia around the Oligo-Miocene boundary. The Early Miocene acme of coral development in SE Asia lags Oligocene coral development in the Caribbean and Mediterranean. Changing CO₂, oceanography, nutrient input and precipitation patterns are inferred to be the main cause of this lag. Moderate, although falling level of CO₂, Ca²⁺ and Ca\Mg when combined with the reduced salinities in humid equatorial waters probably all contributed to reduced aragonite saturation hindering reefal development compared with warm more arid regions during the Oligocene. By the Early Miocene, atmospheric CO₂ levels had fallen to pre-industrial levels. Although this was a relative arid phase globally, in SE Asia palynological evidence indicates the Early Miocene experienced everwet, but more stable and less seasonal conditions than periods before or after. Tectonic convergence truncated deep through-flow of cool nutrient-rich currents from the Pacific to Indian Ocean around the beginning of the Miocene, thereby directly, and perhaps indirectly (though less seasonal conditions) reducing nutrients. Aragonitic reefs were promoted where previously the waters had been more acidic, more mesotrophic, more turbid, and less aragonite saturated. Extensive reefal development resulted in an order of magnitude expansion of shallow carbonate areas through buildups and pinnacle reefs in the Early Miocene. Tectonics via increased habitat partitioning and reducing distances to other coral-rich regions may also have contributed to enhanced reefal development. Implications of this study are that with anthropogenically induced environmental changes it will be the diverse reefs of SE Asia that are likely to be amongst the first and hardest hit as oceanic aragonite saturation decreases and terrestrial nutrient runoff increases.