EVIDENCE OF CARBONATE CRASH BETWEEN 13 AND 8 MA IN THE WESTERN EQUATORIAL INDIAN OCEAN ODP SITE 709

The "carbonate crash" signifies a substantial decline in the quantity of carbonate minerals, such as calcium carbonate (CaCO₃), present in marine sediments. Earlier this event was identified in the eastern equatorial Pacific Ocean and the Caribbean Sea between 13 and 9 Ma. In this study, benthic foraminifera abundance data from ODP Site 709C, have been employed to better understand the changes in carbonate deposition and climatic evolution between 13 and 8 Ma. A sudden increase in the oxygen isotope values of Cibicides spp. at 14 Ma from 1.4‰ to 2.6‰ suggests a major expansion of ice volume in Antarctica. This sea-ice expansion has enhanced the influence of cold corrosive Antarctic Bottom Water (AABW) in the Indian Ocean, which can be observed by an increase in the relative abundance of Nuttallides umbonifera, an indicator of AABW between 13.5 and 9 Ma. However, between 13 and 9 Ma, the mass accumulation rates of biogenic carbonate was also low around 0.2 g/cm²/ 1000 yr. The Shannon-Weiner diversity index and Simpson’s index also gradually show an increasing trend from 13 to 8 Ma, suggesting more equable conditions in the western Indian Ocean. From 13 to 9 Ma, the amount of carbon dioxide (CO₂) in the atmosphere rose from 260 ppm to 350 ppm. This increase in atmospheric CO₂ led to oceanic acidification, increasing the concentration of hydrogen ions in seawater and decreased the availability of carbonate ions. Our study suggests the combined effect of corrosive AABW and oceanic acidification between 13 and 8 Ma which further reduced the saturation state of CaCO₃. This reduction in CaCO₃ led to carbonate crash where existing CaCO₃ structure dissolved and the formation of new structures was inhibited. This had significant consequences for marine organisms that rely on CaCO₃ to secrete their shells, leading to broader ecological impacts on marine organisms.