THE LATE PENNSYLVANIAN MIDCONTINENT SEA OF NORTH AMERICA AS AN ESTUARINE NUTRIENT TRAP

The Late Pennsylvanian Midcontinent Sea of North America as an estuarine nutrient trap

Mariah Peters, University of Cincinnati

Thomas Algeo, University of Cincinnati

Achim Herrmann, Lousiana State University

During glacio-eustatic highstands of the Late Paleozoic Ice Age, large interior regions (> 1×10⁶ km²) of the North American craton were flooded to a depth of up to 100 m, and a strong mid-depth pycnocline was established. The overall circulation pattern within this sea was estuarine, with low-salinity surface waters from its interior reaches flowing oceanward (to the west), and deep waters being laterally advected cratonward (to the east). As with smaller estuarine systems, this circulation pattern resulted in trapping and recycling of nutrients, contributing to high total organic content (TOC) of the sediment (to 40%). In addition, many trace metals (e.g., Mo, U, V, Zn, Cr, Co, Cu, Ni, and Pb) exhibit levels of enrichment (to EFs of >100) that are far higher than those found in most anoxic marine systems. Enrichment levels rise markedly from outer-shelf (Kansas) to inner-shelf regions (Iowa and Illinois). Although deep waters were anoxic throughout the Midcontinent Sea, redox conditions were more reducing in distal deep-shelf areas, as shown by two independent redox proxies (i.e., Fe_T/Al and Corg:P). Because trace-metal enrichment factors increase in the opposite direction (i.e., toward inner-shelf areas), redox conditions cannot have been the sole control on trace-metal patterns. Rather, these extreme enrichment levels are likely to have been the result of trapping and recycling of metals within the deep anoxic “estuarine wedge”. We term this hypothesis the “estuarine wedge enrichment” model.