Chromium isotope heterogeneity on a modern carbonate platform

Chromium isotope (853Cr) values of marine sedimentary rocks are increasingly utilised for monitoring changes in redox conditions in the Earth's ancient oceans and atmosphere. Carbonate rocks are of particular interest as they are relatively common in the sedimentary record and have been shown under laboratory conditions to be capable of recording the Cr isotope composition of precipitating fluids. However, little data from appropriate modern analogues is currently available to constrain the variability in 853Cr values within a carbonate platform and slope system and assess the preservation potential of marine 853Cr values during burial, lithification and diagenesis. In this paper we investigate the heterogeneity of 853Cr values on a modern marine carbonate platform in the context of carbonate sediment deposition, mineralogy, and the influence of burial and early lithification. We present new 853Cr values from carbonate sediments in the upper 200 m of ODP Leg 166 Site 1003 taken from the western margin of the Great Bahama Bank (GBB) and compare these data to modern carbonate sediments accumulating on the GBB platform-top. Shallow-water surface sediments record a range of heavy 853Cr values from +0.77 to +1.07%o while drill core samples record a larger range of 853Cr values from +0.51 to +0.97%o. Down-core variability in 853Cr values occurs systematically with changes in sediment composition and mineralogy, where more positive 853Cr values occur in drill core samples that preserve proportionally more banktop-derived carbonate and less well developed diagenetic textures. These values decrease down-core where proportionally more pelagic carbonate exists, and with increasing neomorphism of primary aragonite to low-Mg calcite. The mineralogical dependence of 853Cr values at Site 1003 appears to be related to variations in relative sea-level across the platform and the degree of lithification and diagenetic recrystallization with depth, rather than in response to changes in the 853Cr composition of the overlying water column. As the GBB represents a reasonable modern analogue for many Phanerozoic and Precambrian carbonate successions, the preliminary framework presented here may have significant implications for the interpretation of vertical profiles of 853Cr values in paleo-redox studies. In particular, smaller scale variability in ancient carbonate sections can be more simply explained by local variations in depositional environment and diagenetic regime rather than fluctuations in water column redox conditions.