Australia: The Land Where Time Began

A biography of the Australian continent 

Neoarchaean Sulphate Concentrations in Seawater from Sulphur Isotopes in Massive Sulphide Ores

In the oceans of the present sulphate concentration is about 28,000 μmol/L, a concentration that is maintained by a balance between the removal by pyrite burial and deposition, and by the oxidative weathering and erosion of sulphate minerals from evaporite deposits that are then transported to ocean (Halvey, Peters & Fischer, 2012). In the Neoarchaean, about 2.4 Ga, the sulphate concentrations in the oceans were much lower than at the present as the partial pressure of oxygen in the atmosphere at that time, prior to the increase of oxygen concentrations, was much lower than at the present (Lyons & Gill, 2010). From 3.85-2.5 Ga the spread of δ³⁴S values in sedimentary sulphides was limited, suggesting microbial sulphate reduction was occurring , and if its role in the marine sulphur cycle of the Archaean was important it must have occurred at concentrations of sulphate that were 200 μmol/L or less (Habicht et al., 2002). In this article the authors¹ have used sulphur isotope systematics from volcanogenic massive sulphide ore deposits at Kidd Creek, Ontario, to obtain constraints on the concentrations of sulphate in seawater that are not dependent on biological considerations. When they compared these values with metal and sulphur budgets from present-day hydrothermal settings they were able to estimate that 2.7 Ga the sulphate concentrations in seawater were about 80 μmol/L. At these levels the sulphate residence time would be on the order of 200,000 years, which is considered a long enough period to make sulphate a conservative compound in the open ocean, though the period is short enough to suggest that a significant sink on a global scale was associated with microbial sulphate reduction accompanying hydrothermal sulphur fluxes.

Sources & Further reading

1. Jamieson, J. W., B. A. Wing, J. Farquhar, and M. D. Hannington. "Neoarchaean Seawater Sulphate Concentrations from Sulphur Isotopes in Massive Sulphide Ore." Nature Geosci 6, no. 1 (01//print 2013): 61-64.