COULD EXHALATION OF HYDROTHERMAL BASIN BRINES TRIGGER GLOBAL ANOXIA?

Most explanations for Paleozoic anoxic events and associated geologic/ biologic/ geochemical/ isotopic anomalies have invoked massive increases in nutrient delivery to the ocean as the stimulus for primary productivity. Coincident increases in marine d13C and d34S have been attributed to enhanced burial of isotopically light organic carbon and sulfur in resulting black shales. Correlative increases in 87Sr/86Sr and black shale metal contents are commonly taken as evidence that the increased nutrient fluxes reflect accelerated continental weathering.

Alternatively, anoxia may be explained by the discharge of hydrothermal basinal brines into the oceans by sedimentary-exhalative (sedex) ore-forming systems. New mass balance arguments based on observations of (1) Paleozoic sedex deposits, (2) evidence from compelling sedex-anoxia correlations, and (3) oceanic box modeling show that the flux of radiogenic Sr from sedex systems had the proper timing, duration, and magnitude to explain sharp 87Sr/86Sr spikes in the Paleozoic record. In addition, metal fluxes associated with brine exhalations surpassed modern riverine fluxes to the ocean, and may account for the high metal contents of black shales deposited during these events.

Similar mass balance observations suggest that brine-associated fluxes of biolimiting nutrients (N, trace metals, reduced C, Si, S, and perhaps P) also surpassed modern riverine fluxes. Significant nutrients may have been carried by liquid petroleum or gas (e.g., C, CH4, N, H2S) that accompanied brine discharge. Overall, the flux of biolimiting nutrients delivered to the ocean by sedex-forming brines may have been sufficient to drive ocean anoxia and the associated biologic, chemical, and climatic changes.

Sedex-associated brine releases occurred periodically through Phanerozoic and Proterozoic time. The extent to which the brine expulsion events inferred from these deposits correlate with anoxia and isotopic or chemical anomalies in the marine record is an important outstanding question that may reveal a fundamental trigger for rapid global change.