A diagenetic origin for isotopic variability of sediments deposited on the margin of Great Bahama Bank, insights from clumped isotopes

Abstract The clumped isotope temperature proxy has been used to investigate the diagenetic history of carbonate sediments in two cores recovered during ODP Leg 166 on the margin of Great Bahama Bank. While periplatform sediments constitute a tempting archive of paleo ocean chemistry as they are unlikely to be subducted, their primary limitation is a well-documented susceptibility to post-depositional diagenetic reworking. The crystallization temperatures reconstructed using the clumped isotope proxy, as well as the mineralogy and δ 13 C and δ 18 O values have been used to determine the relative effects of sediment mixing and sediment recrystallization. Results show that as sediments undergo diagenetic alteration at the seafloor, their initially “warm” clumped isotope composition is overprinted at cooler benthic temperatures. This process appears to occur in an environment with sufficient fluid exchange to overprint carbon isotopes; an observation confirmed in a separate study by analyses of calcium, a similarly rock-buffered element. This early reactive exchange between carbonates and fluids is likely driven by the conversion of metastable aragonite to calcite. This partially occurs within the “flushed zone”, where porewater compositions remain compositionally similar to seawater throughout the upper ∼40 meters of the sediment column. Sediments dominated by open system isotopic compositions correspond to a period of minimal sediment accumulation between 2 and 3 Ma. More deeply buried Miocene sediments of the more platform-proximal Site 1003 show evidence of subsequent recrystallization, incorporating the warmer geothermal TΔ 47 values, and more as well as modified water δ 18 O values, likely driven by co-evolving porewater and carbonate oxygen isotopes. Reconstructed water δ 18 O values of these deeper sediments at Site 1003 are considerably more positive than the measured modern values, suggesting that porewater δ 18 O values were more positive during the Miocene. Sediments deposited at the platform distal Site 1006 between the early and middle Miocene did not show evidence for this deeper recrystallization. Differences in diagenetic behavior between the two sites cannot be solely accounted for by differences in sediment accumulation rate. To illustrate this, time-integrated models were constructed which simulated the burial of identically reactive material through the depositional history of each site, the sediments deposited at Site 1006 appear to “stabilize” after an initial phase of neomorphism, whereas the more platform proximal Site 1003 continues to recrystallize during deeper burial, apparently in the presence of a parent fluid with a more positive δ 18 O value than observed today. We conclude that despite consisting of the same end-member sediment sources, and being spatially separated by less than 30 km, the difference in clumped and oxygen isotopic composition between Sites 1003 and 1006 can be predominantly attributed to differences in the rate and duration of recrystallization during burial.