Modelling the Effects of Non-Steady State Transport Dynamics on the Sulfur and Oxygen Isotope Composition of Sulfate in Sedimentary Pore Fluids

Abstract

We present the results of an isotope-enabled reactive transport model of a sediment column undergoing active microbial sulfate reduction to explore the response of the sulfur and oxygen isotopic composition of sulfate under perturbations to steady state. In particular, we test how perturbations to steady state influence the cross plot of δ34S and δ18O for sulfate. The slope of the apparent linear phase (SALP) in the cross plot of δ34S and δ18O for sulfate has been used to infer the mechanism, or metabolic rate, of microbial metabolism, making it important that we understand how transient changes might influence this slope. Tested perturbations include changes in boundary conditions and changes in the rate of microbial sulfate reduction in the sediment. Our results suggest that perturbations to steady state influence the pore fluid concentration of sulfate and the δ34S and δ18O of sulfate but have a minimal effect on SALP. Furthermore, we demonstrate that a constant advective flux in the sediment column has no measurable effect on SALP. We conclude that changes in the SALP after a perturbation are not analytically resolvable after the first 5% of the total equilibration time. This suggests that in sedimentary environments the SALP can be interpreted in terms of microbial metabolism and not in terms of environmental parameters.

The Carbon-Sulfur Link in the Remineralization of Organic Carbon in Surface Sediments

“…It appears that the oxygen isotopic composition of sulfate may increase slightly faster than the sulfur isotopic composition of sulfate at Site 11 (Figure 4A). At one of the sites with piston core data (Site 9; 2,323 mbsl) we note that the cross plot of δ 34 S SO4 vs. δ 18 O SO4 (Figure 4A) comes out of the apparent linear phase, where the sulfur and oxygen isotopic compositions covary, and into the equilibration phase, where δ 18 O SO4 has reset through oxygen isotope equilibrium with water intracellularly and does not change further as the δ 34 S SO4 values continue to increase (Antler et al, 2014;Antler and Pellerin, 2018;Fotherby et al, 2021). The carbon isotope composition of the DIC decreases with depth as expected from the oxidation of organic carbon and the return of 12 C-rich carbon to the DIC pool (Figures 2G,H).…”

The geochemical riddle of “low-salinity gypsum” deposits

“…3). Compared to the range of  34 S SO4 and  18 O values in modern sediment pore waters where microbial sulfate reduction is active (blue area in figure 3) (Fotherby et al, 2021), the range of  34 S SO4 corresponding to Primary Lower Gypsum and Upper Gypsum deposits is limited (Fig. 3).…”

Intensified microbial sulfate reduction in the deep Dead Sea during the early Holocene Mediterranean sapropel 1 deposition

“…Other processes are known to have an effect on the vs. plot such as the sulfide and pyrite oxidation (Balci et al, 2007) and disproportionation (Böttcher et al, 2001, 2005); however, their effect should only decrease the relative estimated calculated rates of MSR. Additionally, it was shown that when the sulfate reduction rates are high (i.e., the vs. slope is low), the effect of transport such as diffusion and sedimentation is negligible on the measured slope (Fotherby et al, 2020). Such a plot can be adapted to the form of (‐ ) vs. (‐ ), where and are the initial SO 4 2− isotope values, thereby negating the “source effect” and allowing data with a linear relationship from various aquatic and subsurface environments beyond the Dead Sea to be compared (e.g., lakes, seas, estuaries, and salt marshes) (Figure 4).…”

“…Other processes are known to have an effect on the δ 18 O SO 4 vs. δ 34 S SO 4 plot such as the sulfide and pyrite oxidation (Balci et al, 2007) and disproportionation (Böttcher et al, 2001(Böttcher et al, , 2005; however, their effect should only decrease the relative estimated calculated rates of MSR. Additionally, it was shown that when the sulfate reduction rates are high (i.e., the δ 18 O SO 4 vs. δ 34 S SO 4 slope is low), the effect of transport such as diffusion and sedimentation is negligible on the measured slope (Fotherby et al, 2020). Such a plot can be adapted to the form of (δ Stable δ 34 S SO 4 and δ 18 O SO 4 from primary gypsum layers found in the Dead Sea sedimentary record were also used to investigate rates of MSR using a similar isotope plot given that the effect of isotope fractionation during gypsum precipitation has a negligible effect on the slope (Torfstein & Turchyn, 2017).…”