Dolomite formation in the Miocene Kardiva platform, Maldives archipelago: a tale of closed-system and open-system dolomitization by current pumping of seawater

ABSTRACT Dolomites from two IODP Expedition 359 sites on the northeastern margin of the Miocene Kardiva platform, Maldives archipelago, were examined to explore the question of open-system versus closed-system dolomitization in a drowned carbonate platform. The uppermost ∼ 130 m of platform-margin carbonate at site U1465 contains ≤ 12% dolomite except for five, meters-thick intervals with up to 65% dolomite. All U1465 dolomites consists of decimicron-size euhedral cement crystals and mimetically replaced peloids and coralline red-algal clasts. The abundance and the petrographic features of these dolomites are similar to periplatform and slope dolomite in many other settings that have been interpreted as the product of hydrologically closed-system diagenesis. In contrast, the recovered platform-margin deposits at site U1469 are > 99% dolomite. Those dolomites are partially fabric retentive with fine-to-medium crystalline, planar subhedral to euhedral crystal mosaics of replacive dolomite and dolomite cement, all with Sr contents that average 256 ppm. Their characteristics are comparable to Neogene platform carbonates universally interpreted to signify hydrologically open-system dolomitization. Sr-isotope ages indicate Miocene dolomitization at both sites after platform drowning, and δ18Odolomite values are compatible with dolomitization by cold (10° to 15° C) seawater when the platform margin was, on average, ∼ 400 m below sea level (mbsl). A current-pumping mechanism for the advection of seawater into the top of the platform at site U1469 is proposed and tested with a computational fluid-flow simulation. Current pumping occurs when strong ocean-bottom currents flow over sedimentary bedforms and generate lateral pressure differences along the sediment–water interface. The pressure differentials drive seawater through the underlying sediments. The flow simulation shows that the ocean currents that swept large sediment sand waves over and off the drowned Kardiva platform for many millions of years could have vigorously pumped Miocene seawater to sub-seafloor depths of many tens of meters. Mass-balance considerations suggest complete dolomitization of the upper 20 m of the platform within 500 ky or less. The current-pumping mechanism could drive dolomitization and mineralogical stabilization below hiatal surfaces, or very slowly accumulating sediments, in any marine setting characterized by strong bottom currents. The greater acidity of Miocene seawater relative to younger seawater possibly made the pumping, by any means, of Miocene seawater of normal salinity an effective dolomitizing agent at site U1469 and in undrowned Miocene platforms globally.