SULFUR ISOTOPE SYSTEMATICS IN A LOW SULFATE, PERMANENTLY ANOXIC LAKE

Sulfur isotope records of sedimentary sulfates and sulfides have been used extensively to reconstruct ancient marine sulfate concentrations and the redox evolution of the ocean-atmosphere system. Though many researchers have argued that seawater sulfate concentrations have been low relative to the modern ocean for much of Earth history, very little work has been done to characterize sulfur isotope systematics in modern, low sulfate systems. In the present study, we examine the sulfur isotope composition of water column sulfate and hydrogen sulfide, and iron monosulfide and pyrite within sediments deposited in both the oxic and anoxic portions of Lake McCarrons, MN—a permanently anoxic lake with low sulfate concentrations.

Surface water sulfate concentrations were ~275mM but dropped sharply below the chemocline (11.5m) to ~125mM as bacterial sulfate reduction occurred within the anoxic bottom waters. d³⁴S_sulfate values above the chemocline were relatively constant with depth (~5‰), but increase dramatically in the lower 4 meters of anoxic bottom waters. The sulfur isotope composition of the sulfide buried below the anoxic water column approached the sulfur isotope composition of the surface water sulfate resulting in a small D³⁴S_{sulfate-sulfide} value (~5‰). By contrast, the sulfur isotope composition of pyrite buried at the relatively shallow, oxic site was ³⁴S-depleted relative to the anoxic site resulting in a larger D³⁴S_{sulfate-sulfide} value (~11‰). This preliminary work highlights the relationship among d³⁴S_sulfate, d³⁴S_sulfide, and water column depth within an anoxic, low-sulfate system and the importance of understanding how the sulfate reservoir evolves within a spatial context and the implications for the sulfur isotope composition of pyrite that is ultimately buried in these systems.