CALCIUM ISOTOPE RATIOS SHOW SPATIAL PATTERNS OF OPEN-SYSTEM MARINE DIAGENESIS IN MALDIVES CARBONATE SEDIMENTS (Invited Presentation)

Sedimentary carbonates offer us a rich but complicated geochemical archive of ancient marine conditions, where many diagenetic processes are known to occur. Based on previous work in the Bahamas (Higgins et al., 2018, GCA), calcium isotope ratios have shown the potential to identify open- versus closed-system marine diagenesis (i.e. neomorphism or recrystallization with a high or low water:rock ratio) and to explain stratigraphic patterns of other geochemical proxies that are unrelated to changing seawater composition. However, the role of the geometry and development of the Bahamas platform in shaping these observed patterns has not been fully assessed. We use a comparison with another shallow-water carbonate depositional system, the Maldives archipelago, to understand the interplay of platform architecture and marine diagenesis.

In 2015, IODP Expedition 359 recovered core samples from platform, slope, and pelagic drift deposits in the Maldives spanning from the latest Oligocene to the modern. Previous work has examined the transformation of the platform architecture over this interval (Betzler et al., 2018, PEPS) and the similarities and divergences of carbon and oxygen isotope ratios from the Maldives relative to deep-sea pelagic records (Swart et al., 2019, Sedimentology). In this context, we present calcium isotope ratios measured in homogenized, bulk samples across several of these cores. In contrast to the Bahamas, slope and drift deposits show very little variation in calcium isotope ratios, but partially dolomitized platform facies show much higher ratios (δ^44/40Ca values), indicating pervasive open-system marine diagenesis. These relationships suggest that the interactions of sea level and platform geometry may provide important first-order control on the extent of alteration in these environments. By comparing the spatial pattern of open-system diagenesis in the Bahamas and Maldives, this analysis provides lessons about how to target and test for the most useful application of geochemical proxies within shallow-water sedimentary carbonate systems.