LAUSR

Abstract The partial drowning of the Mid-Miocene Kardiva Platform in the Maldives occurred at 13 Ma in the northern Kardiva Channel and at 10.6 Ma in the southern Kardiva Channel. The 13 Ma drowning coincided with the onset of the monsoon-driven currents in the Indian Ocean. The drowning had previously been explained as a consequence of current-induced upwelling and the resultant injection of nutrients into the surface waters and expansion of the oxygen minimum zone (OMZ). However, cores retrieved during IODP Expedition 359 across the drowning successions on the platform and its slope, into the overlying contourite drifts, do not display any of the faunal changes usually associated with such an environmental change. Here we use the δ15N values of sedimentary organic material and the improved total phosphate analysis as geochemical proxies for nutrients and total organic carbon (TOC) as an indicator for ocean anoxia to investigate the causes of platform drowning. This produced two important results, (1) both nutrient proxies, δ15NOM and total phosphate (TPO4) remain unchanged across the drowning successions and into the drift successions, indicating that the nutrient content on the platform and in the Inner Sea did not increase prior or during platform drowning, and (2) little to no TOC is preserved in the drowning successions, providing evidence that the expanding OMZ did not result in ocean anoxia, which could have aided in the demise of the platform. An increase in nutrient content (δ15NOM) is found at 11 Ma, about 2 Myr after the drowning of the Kardiva Platform in the mid-Miocene. Therefore, neither current-induced nutrient injection nor an expansion of the OMZ can be implicated in the platform drowning. Instead, the onset of strong currents seems likely to be the primary cause of the partial platform drowning. A mechanism is envisioned in which flooding of the previously exposed platform is coupled with the onset of a strong current that sweeps the platform of newly produced sediment and inhibits coral larvae settlement.