Assessing Sedimentary Boundary Layer Calcium Carbonate Precipitation and Dissolution Using the Calcium Isotopic Composition of Pore Fluids

Abstract

We present pore fluid geochemistry, including major ion and trace metal concentrations and the isotopic composition of pore fluid calcium and sulfate, from the uppermost meter of sediments from the Gulf of Aqaba (Northeast Red Sea) and the Iberian Margin (North Atlantic Ocean). In both the locations, we observe strong correlations among calcium, magnesium, strontium, and sulfate concentrations as well as the sulfur isotopic composition of sulfate and alkalinity, suggestive of active changes in the redox state and pH that should lead to carbonate mineral precipitation and dissolution. The calcium isotope composition of pore fluid calcium (δ44Ca) is, however, relatively invariant in our measured profiles, suggesting that carbonate mineral precipitation is not occurring within the boundary layer at these sites. We explore several reasons why the pore fluid δ44Ca might not be changing in the studied profiles, despite changes in other major ions and their isotopic composition, including mixing between the surface and deep precipitation of carbonate minerals below the boundary layer, the possibility that active iron and manganese cycling inhibits carbonate mineral precipitation, and that mineral precipitation may be slow enough to preclude calcium isotope fractionation during carbonate mineral precipitation. Our results suggest that active carbonate dissolution and precipitation, particularly in the diffusive boundary layer, may elicit a more complex response in the pore fluid δ44Ca than previously thought.

Trace Element Geochemistry in North Pacific Red Clay Sediment Porewaters and Implications for Water‐Column Studies

“…An alternative explanation is that the observed drop in concentrations between the overlying water and first porewater sample is a sampling artifact. However, field and laboratory experiments do not suggest Rhizon samplers introduce an error in $${{\text{SO}}_{4}}^{2-}$$ sampling (James et al., 2021; Steiner et al., 2018). A pressure artifact is also a possibility but the profiles do not change as a function of bottom depths; Mo and U concentrations at Stations 1 and 3 are similar yet Station 3 was almost as deep as Station 2, while Station 1 was shallower and similar in depth to Stations 4 and 5.…”

Pathways of carbonate sediment accumulation in a large, shallow lake

“…In addition, we assume that the nm-scale flakes of smectite could provide compartmentalized spaces in which high local supersaturations can be achieved when metastable Mg-bearing calcite locally dissolves, eventually leading to the reprecipitation of Mg-rich carbonate, including protodolomite. Whether these dissolution-reprecipitation phenomena happen in the water column or at the sediment/water interface (where aerobic oxidation of organic matter lowers the pH) should be the subject of future studies; nevertheless, the processes are rather complex and even advanced stable isotope studies may return ambiguous results (James et al, 2021).…”