The Late Pennsylvanian Midcontinent Sea of North America: A review

The Late Pennsylvanian Midcontinent Sea (LPMS) of North America reached its greatest extent (~ 2.1 × 106 km2) during glacioeustatic highstands from the Middle Pennsylvanian to the Early Permian. At these times, the sea was strongly stratified, with a subpycnoclinal layer that was anoxic and intermittently sulfidic. The development of widespread benthic anoxia in the LPMS was due to a combination of factors, including some found in most modern epicontinental seas, e.g., relatively shallow bathymetry, elevated runoff into a largely landlocked basin, a strong pycnocline, and estuarine-type circulation. However, two factors that contribute significantly to the development of benthic anoxia in such settings, i.e., a shallow marginal sill to limit deepwater renewal, and high marine primary productivity rates to stimulate benthic oxygen demand, were absent in the LPMS. Rather, a key factor controlling benthic redox conditions was lateral advection of “preconditioned” intermediate waters from Panthalassa. As in the modern eastern tropical Pacific, the oxygen-minimum zone (OMZ) may have risen to depths < 100 m in the Late Pennsylvanian eastern tropical Panthalassic Ocean, allowing oxygen-depleted and intermittently denitrified waters to flood deeper basins on the southwestern margin of Laurentia. Slow transit of these waters through the ~ 1000-km-long, stratified Greater Permian Basin Seaway maintained the oxygen-poor status of these waters prior to upwelling out of the Anadarko and Arkoma basins onto the Midcontinent Shelf of the LPMS. Despite low levels of primary productivity and benthic oxygen demand, deepwater anoxia was maintained and even intensified into interior regions of the LPMS due to its strong pycnocline and proximal tapering of the subpycnoclinal layer. The intensity of benthic anoxia in the LPMS was a function of the strength and lateral extent of its pycnocline and, hence, of regional precipitation and continental runoff. Consequently, the LPMS highstand depositional system was highly sensitive to climate fluctuations at intermediate timescales (i.e., hundreds to tens of thousands of years). Controls on benthic redox conditions in the LPMS and similar ancient seas are not well understood owing to a paucity of appropriate modern analogs. Because existing models for anoxia in epicontinental seas do not invoke some of the key controls identified in this study, we propose a new superestuarine circulation model for which the LPMS may be considered the type example.