ARCTIC SULFUR CYCLE FROM SOURCE TO SINK: STABLE SULFUR ISOTOPIC INVESTIGATIONS OF AN ACIDIC POND IN SOUTHWEST GREENLAND

A field site near Kangerlussuaq in southwestern Greenland, informally named triangle pond, was investigated based on the discovery of efflorescent sulfate blooms and unusually acidic chemistry compared with near-neutral lakes in close proximity. The primary sulfate mineral in soil/crust samples is gypsum (15 to 40 wt %) and jarosite is a notable secondary sulfate mineral (5 to 12 wt %). Studies of the δ³⁴S values of sulfate blooms, pond sediment, and nearby exposed Paleoproterozoic bedrock are in progress. Sequential sulfur extractions were performed and targeted the water soluble sulfate, acid volatile sulfide, and chromium reducible sulfide.

Concentric rings of evaporitic crusts show decreasing δ³⁴S values extending radially outward from the edge of the pond. Sulfate values from the pond sediment were lower than (δ³⁴S = -18.0 to -26.8‰) sulfates from the crust and near surface samples (δ³⁴S = -1.8 to -14.1‰). The difference in sulfate values between the pond sediment and surface crusts reflects active microbially mediated sulfur cycling within a small closed drainage basin for which primary sulfide deposits are 1) being oxidized in the pond water, 2) deposited as sulfides in the pond sediment after microbial sulfate reduction, and 3) oxidized at the surface as the pond water recedes during times of lowstand pond water levels.

Results show elevated sulfide concentrations not only in the bedrock samples and sediment, but also in the crust and near surface samples. These findings are consistent with metagenomic sequencing indicating complete sulfur oxidation and reduction pathways from a rich diversity of microorganisms including assimilatory and dissimilatory sulfur reducing bacteria and sulfur oxidizing lithotrophs.

The sulfur cycling investigation of triangle pond details the microbial influence on a cold and arid region of the Earth that has many similarities to current investigations on the surface of Mars; for example, the ancient lacustrine environment of Gale Crater. The sulfur mineralogy and isotopic trends of triangle pond reflect the strong influence of redox conditions, microbial activity, and concurrent iron cycling. This study extends ongoing mineralogical and metagenomic analyses of the study site and aids in identifying the complete sulfur pathway from source to sink within a Mars analog system.