GLS Seminar: Dr. Timothy Lyons, "In Search of Anoxia in the Proterozoic Ocean"

Friday, August 25th, 2006 from 3:30 PM – 4:30 PM, 4069 Derring Hall

Refreshments will be served at 4:30 PM. The public is cordially invited.

In Search of Anoxia in the Proterozoic Ocean

Dr. Timothy Lyons, University of California-Riverside

A growing body of evidence suggests that the global ocean experienced persistent oxygen deficiency and perhaps the establishment of euxinic (anoxic and sulfidic) conditions throughout much of the Proterozoic. Unfortunately, much of this evidence is indirect and can become circular, relying on model predictions of deepwater oxygen availability, interpretations of low sulfate concentrations that may have persisted into the Paleozoic, temporal distributions of iron formations, and patterns of eukaryotic evolution. More direct lines of support, such as organic biomarker records and iron proxies, are limited to narrow geographic and temporal windows, and iron methods may have less value at times of ocean-scale euxinia.

Constraining lateral extents of oxygen deficiency is hampered by our inability to trace paleoenvironments laterally across interbasinal timelines. Instead, proxy results from single basins are commonly extrapolated to interpretations of the world ocean. In such cases, the global relevance of basin-scale observations often hinges on tenuous tectonic arguments for the strength of connection to the open ocean. When interconnectedness can be demonstrated, questions still remain about the ubiquity of euxinia in a heterogeneous ocean—particularly away from the intense upwelling and high primary production of continental margins. Models also require a better understanding of deep ocean ventilation under the Proterozoic climate.

To bridge these gaps, our current efforts are focusing on developing, refining, and applying geochemical tools that speak to global ocean redox. These approaches include molybdenum isotope geochemistry, which will be addressed in complementary presentations by Anbar and Lyons. Molybdenum isotope approaches require independent arguments for local redox and for the extent of basinal Mo drawdown as expressed in ratios of Mo to total organic carbon. These ratios point to the presence or absence of the quantitative Mo uptake required for unambiguous seawater signals. In the Phanerozoic, such Mo limitation occurs most commonly in locally restricted settings with suppressed deepwater renewal, such as the modern Black Sea. In the Proterozoic, lower seawater Mo concentrations and thus more frequent limitations favor the isotope approach. Absent this constraint, however, poorly known fractionation factors must be considered. Furthermore, our isotope mass balance remains incomplete in its treatment of the role of widespread suboxia, which, like euxinia, leads to efficient Mo sequestration. This is an area of ongoing work. Given our present inability to fully resolve suboxic versus euxinic sinks, our current data may underestimate the full extent of oxygen deficiency in the Proterozoic ocean.

Interpreting Mo concentrations is a more direct measure of Mo availability in the water column. At times of widespread euxinia and suboxia, mass balance arguments demand that Mo enrichments be lower than those observed during times of intense but local oxygen deficiency, which typify the Phanerozoic. Our preliminary results suggest that suppressed Mo enrichment in Proterozoic euxinic shales could be a reflection of more generally reducing conditions in the deep ocean.

We are also exploring the significance of iron isotope relationships and manganese concentrations. Published long-term Fe isotope patterns, when interpreted in light of our new results from the Black Sea, suggest oxic to suboxic Fe cycling in shallow water, even in the Archean. Anomalously high isotope values for Proterozoic pyrite could track extreme pyrite burial under pervasively euxinic conditions. Patterns of Mn enrichment and depletion under oxygen deficiency and their potential for delineating broad paleoceanographic conditions are also factoring into our conceptual models. Given the increasing importance of oceanic euxinia in models for Precambrian and Phanerozoic biotic evolution and extinction, the next generation of paleoredox proxy applications must permit unambiguous recognition of paleoenvironments across broad spatial and temporal scales, and these methods must be relevant at times when the rock record is sparse.