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In the absence of diagnostic fossils, identifying Precambrian lake sediments requires us to bridge organic geochemistry with an understanding of inorganic elemental cycles and their local versus global inventories. We focus on modern lakes as a baseline for the geochemistry of Precambrian shales and specifically for fingerprinting possible nonmarine contributions. Our interests include the Proterozoic when the proxy expressions of low-sulfate ocean chemistry may well have mimicked lacustrine conditions.

We have chosen four systems that capture the geochemical expressions of pure lacustrine end members as well as records of fresh-to-saline and oxic-to-anoxic environmental change. These gradients allow us to define and refine geochemical proxies, including novel tracers of the secondary overprints the mark marine-nonmarine transitions.

1a. Lake Champlain

The Lake Champlain region (NY and VT) transitioned from freshwater to marine back to freshwater as a consequence of isostatic adjustments during post-glacial sea level rise. The marine stage, or Champlain Sea, developed into modern freshwater Lake Champlain approximately 9 kyr ago--juxtaposing two well established end-member environments: modern marine conditions (with known sulfate concentration and isotopic composition) and a large (1100 km2) freshwater lake.

We collected two piston cores in August 2009 in collaboration with Dr. Thomas Cronin (USGS, Reston) and Dr. Patricia Manley (Middlebury College, VT)--both experts in the region. Our encouraging chemical analyses led to an invitation from Dr. Cronin to sample sediments drilled through an earlier sequence from the glacial freshwater lake to the Champlain Sea. Sediment splits were collected in September 2009 in collaboration with Dr. John Rayburn (SUNY New Paltz, NY) and Dr. David Franzi (Plattsburgh State University, NY) and are currently being processed.

Results from the Champlain Sea to Lake Champlain transition reveal pronounced geochemical contrasts, including a shift from a carbonate-rich marine stage to carbonate-lean lacustrine deposition and a prominent maximum in pyrite concentration at the marine-freshwater contact. The chemical record, coupled with changes in physical properties and microfossil abundances, define a tantalizing biogeochemical interface between methane-charged lake sediment ([CH4] upwards of 1.5 mM) and overlying sulfate-rich marine pore waters. We continue to explore the signals of redox sensitive metals (e.g., Mo and Fe) coupled with provenance indicators (Al/Ti) to optimize or geochemical indices of marine versus nonmarine deposition.

1b. St. Lawrence Seaway

Complementary to the temporal gradients of the Lake Champlain region, recent sediments from the St. Lawrence capture the spatial gradient from fresh to marine waters. The salinity of the St. Lawrence Seaway grades from open marine in the Gulf of St. Lawrence to the fresh waters of the Great Lakes. Our specific goal is to evaluate C/S and trace metal relationships relative to the abundance of the 24-n-propylcholestane marine organic biomarker--a potentially robust indicator of open-to-restricted marine deposition as far back as the Neoproterozoic and possibly earlier. It is not known, however, whether this compound is/was produced in transitional aquatic settings such as lagoons and estuaries. Confirming the absence of 24-n-propylcholestane in a variety of lacustrine environments will validate its utility as a marine-only tracer.

Core-top sediments (upper 40 cm) from the St. Lawrence River and Estuary and the Gulf of St. Lawrence were obtained from Dr. Charles Gobeil (University of Quebec) in November 2009. C-org/S-total increases downstream from the river to the gulf--likely reflecting an increase in contributions of seawater sulfate. We extracted the sediments for biomarkers, and we are currently screening for 24-n-propylcholestane. We will also complement the carbon-sulfur data with Mo, Fe and Al concentrations.

2. The Great Salt Lake

Lake Bonneville was a deep (>300 m) freshwater lake that covered more than 50,000 km2 of northwestern Utah approximately 17,000 years ago. The hypersaline Great Salt Lake is the modern remnant of this ancient aquatic environment. Paleoenvironmental reconstructions record multiple climatically driven cycles between deep freshwater, saline-hypersaline and marsh conditions; the most dramatic fluctuation in lake height occurred during the Bonneville flood 14.5 ka when lake level dropped approximately 100 m. Evaporative concentration and eventual desiccation resulted in the precipitation of massive evaporite beds, now sandwiched between freshwater and overlying saline stages. These samples allow us to explore geochemical fluctuations within a lake that has no historical connection with the ocean yet has transitioned from dilute (Lake Bonneville) to hypersaline (modern) waters.

Core samples from the Great Salt Lake were collected from the LaCore repository at the University of Minnesota (August 2008). Our sample suite, drilled as part of the Global Lakes Drilling Program, represents approximately 160,000 years of continuous deposition, spanning two extreme events in the hydrologic balance: freshwater Lake Bonneville and the modern playa lake.

Initial results indicate concentration enrichments in total organic carbon and total sulfur within the sapropels deposited during the evaporation events. Pyrite-sulfur isotope values increase through the Lake Bonneville section, suggesting near-quantitative precipitation within a low sulfate reservoir, and a return to much lower isotope values within the modern hypersaline sediments marks deposition below sulfate-rich bottom waters. Our remaining work includes metal concentrations (Fe, Mo and Al).

3. The Black Sea

The Black Sea was a freshwater lake in the Late Pleistocene when sea level fell below the Bosporus connection to the Mediterranean Sea. Depositional conditions evolved into a brackish marine setting during the Holocene rise in sea level. As the largest anoxic basin in the modern world, the Black Sea is often treated as an analog for laminated black shale deposition in the past.

Sediments--available through our ties with former post-doc Dr. Silke Severmann (now an Assistant Professor at Rutgers University) and her participation in the Meteor M72/5 Expedition--will be extracted for solid-phase sulfur species and their isotopic compositions. Metal analysis will be accomplished in collaboration with the Severmann research group.

4. Mahoney Lake, southern British Columbia

This meromictic, highly sulfidic lake is providing an excellent end member for euxinic lacustrine deposition. NASA funded our initial sampling and analyses; however, a recent return to the lake funded partially through this award has added a rigorous geomicrobiologial and microelectrode component through collaborations with Caltech, Harvard, ASU, the University of Vermont, and WashU. Details are available on request.