An ancient estuarine-circulation nutrient trap: The Late Pennsylvanian Midcontinent Sea of North America

The Late Pennsylvanian Midcontinent Sea (LPMS) of North America, which existed during glacioeustatic highstands of the late Paleozoic ice age, was an immense (>1 × 106 km2) cratonic interior sea exhibiting large-scale estuarine circulation, with a low-salinity surface plume overlying a high-salinity, anoxic, deep water mass. As in river estuaries, these conditions resulted in trapping and recycling of nutrients and organic-reactive elements (e.g., trace metals such as Mo, U, and Zn) in the subpycnoclinal water mass, leading to total organic carbon (TOC) to 40% and peak trace metal enrichment factors (EFs) of 540 (Mo), 73 (U), and 75 (Zn) in the black shale facies of the Hushpuckney Shale. Trace metal EFs increase gradually from distal (Oklahoma) to proximal (Iowa) areas of the Midcontinent Shelf before decreasing abruptly in the Illinois Basin, located to the east of the Mississippi River Arch (MRA). Owing to similar productivity (TOC) and redox (FeT/Al) proxy values on both sides of this arch, the large differences in trace metal EFs are interpreted to reflect divergent deep-water chemistries; specifically, much larger aqueous trace metal inventories on the Midcontinent Shelf than in the Illinois Basin. This condition implies that (1) deep waters of the Midcontinent Shelf and Illinois Basin were physically separated by the MRA, demonstrating its existence as a positive bathymetric feature during the Late Pennsylvanian, and (2) the saltwater wedge to the west of the MRA functioned as a nutrient trap in which organic-reactive trace metals were strongly concentrated through water-column recycling.