Fractionation of Cr isotopes between seawater and carbonate sediment in the Caribbean Sea

Please join us for a graduate student seminar this Friday October 25 at 3:30 pm in rm 155 Geology presented by Jyotsnamani Mohanta, PhD Candidate:

Fractionation of Cr isotopes between seawater and carbonate sediment in the Caribbean Sea

Chromium isotopes are a promising proxy for reconstructing past redox conditions in the oceans, and marine carbonate sediment is being investigated as a substrate that might record these redox-driven changes in seawater Cr isotope values. Analysis of shallow water carbonate sediment from the modern Caribbean Sea showed difference of –0.45‰ (Δ53Cr(CaCO3-SW)) between seawater (δ53Cr=1.14‰) and bulk carbonate sediment (δ53Cr=0.69 ‰), implying partial Cr(VI) reducion to Cr(III) during its removal from seawater. The studied carbonate sediment is composed of green algae (δ53Cr=0.55‰), forams (δ53Cr=0.74‰), molluscs (δ53Cr=0.44‰), red forams (δ53Cr=0.58‰), and red algae (δ53Cr=0.85‰). Based on these values and their corresponding Cr concentrations, a mass balance calculation reveals that organisms producing high magensium calcite dominate seawater-derived Cr in the sediment (red algae and forams), despite the fact that aragontitic green algae are the dominant carbonate producers by volume in the sediment. Bulk carbonate sediment from different locations yields a range of range of δ53Cr values from 0.49‰ to 0.96‰, despite the fact that seawater is presumed to be constant in all of the studied locations along the Yucatan coast (1.14‰). The variation in bulk sediment δ53Cr values cannot be due to variations in carbonate producers or mineralogy, but could be related to different rates of Cr(VI) uptake from seawater, and local factors affecting Cr isotopic fractionation during uptake, which could include carbonate mineralogy, grain size and the age of the sediment. The basis of this interpretation is that in all cases studied, thus far, carbonate sampled from live producers, e.g., corals (δ53Cr= -0.21‰ to -0.48‰), Halimeda opuntia (green algae) (δ53Cr =0.33‰), Amphiroa tribulus (red algae) (δ53Cr= -0.06‰), exhibit lower Cr concentrations and lower δ53Cr values than bulk carbonate sediment, suggesting that there is an additional flux of isotopically heavy Cr into sediment after it is released by the organisms to form a sediment.