Developing a Modern Lake Geochemical Model for Interpreting Marine from Nonmarine Deposition on Early Earth

The prevailing models for Proterozoic ocean chemistry are developed from archives of shales contentiously interpreted as having either marine or lacustrine origins. Classic examples include facies of the Belt Supergroup and from the McArthur and Hammersley Basins in Australia. The sediments from these settings provide the paleoenvironmental backdrop for evolving atmospheric oxygen availability and biological evolution. In the absence of clear paleogeographic and paleocologic controls, however, connections to the ancient ocean remain enigmatic. Chemical proxies developed for modern lakes may help differentiate between marine and nonmarine deposition, but development and calibration of these approaches is presently inadequate. Whether a given basin was linked to the global ocean has deep implications for interpreting the evolution of the Earth's system history.

We present case studies from modern freshwater lakes (Lake Superior, Lake Tanganyika), hypersaline lakes (Great Salt Lake, Mahoney Lake), and a transitional/restricted marine basin (Black Sea Unit III) as modern analogs for nonmarine source rocks. Our intent is to define a modern geochemical “lake model” developed from diverse major-minor elements that are both redox-sensitive and insensitive and as complemented by organic biomarkers. Bulk major/minor elemental ratios from marine systems track broadly integrated siliciclastic sources that merge toward uniform, average crustal values. In contrast, sediment provenance relationships in lakes reflect the local geology of the watershed. A matrix of organic and inorganic proxies, including total organic carbon (TOC), total inorganic carbon (TIC), sulfur content (pyrite, %S, organic sulfur), metals (e.g., Fe, Mn, Mo, Al, Ti), REE and molecular biomarkers will provide a basis for modern-ancient comparison of lacustrine environments.