Dual clumped isotopes reveal an out-of-equilibrium state in shallow-water carbonate sediments on Great Bahama Bank

Abstract

While non-skeletal carbonate grains and muds precipitated on shallow-water platforms have been widely used to reconstruct past ocean temperatures and chemistry, the question remains as to what degree the geochemical signatures of carbonate sediments are in equilibrium with their original environments. To evaluate the extent of equilibrium, we have applied the dual clumped isotope proxy (Δ₄₇ and Δ₄₈) to surface sediments (aragonite > 90 %, n = 150) ranging from mudstones (< 63 μm) to non-skeletal grainstones from the Great Bahama Bank. While there is no statistical difference in the mean Δ₄₇ values of the various facies, there are very large ranges of Δ₄₇ values within each type (∼ 0.1 ‰) equivalent to a temperature uncertainty of 25 °C. Our Δ₄₈ data reveals an out-of-equilibrium state where the muddy sediments have more positive values than the equilibrium, while the grainy facies are more negative. The positive Δ₄₈ disequilibrium in the muds is proposed to be a result of the photosynthetic removal of CO₂ caused by the activity of cyanobacteria turns promotes the precipitation of calcium carbonate in the water column (whitings area). In contrast, the grainstones, that are mainly composed of peloids, show a negative Δ₄₈ disequilibrium resulting from the CO₂ absorption by a complex mixture of biogeochemical processes. Our findings highlight the importance of kinetic processes in shaping non-skeletal carbonate factories and defining their geochemistry.