INVESTIGATING REDOX SENSITIVE TRACE METAL UPTAKE IN THE HEEBNER SHALE OF THE LATE PENNSYLVANIAN MIDCONTINENT SEA TO TEST A SUPERESTUARINE DEPOSITIONAL MODEL

During the Late Pennsylvanian, waxing and waning of the Gondwanan ice sheets flooded large parts of the North American interior, creating the Late Pennsylvanian Midcontinent Sea (LPMS). This resulted in alternating deposits of marine and terrestrial deposits termed “cyclothems”. The core shales of cyclothems are black and gray shale deposited during glacio-eustatic highstands. The LPMS core shales are spatially correlative, allowing for detailed investigation of pale-redox sensitive trace metal gradients. Trace metal proxies can provide valuable insight into depositional environments such as the LPMS. One depositional model for the LPMS core shales is a superestuarine model, in which the LPMS experiences anoxic bottom water circulation beneath a stratifying pycnocline. This model predicts that bottom waters become more reducing in shallow, proximal locations, a characteristic not found in modern environments and requiring further testing. Determining the process of uptake for enriched trace metals can test sediment record against the geochemical conditions required for this model.

Recent studies have shown an increasing gradient of enrichment factors of redox sensitive trace metal and Fe/Al ratios across the LPMS towards more proximal locations. This study finds similar trends in an analogous shale and seeks to further investigate the mechanism of trace metal uptake. Two cores of the Heebner Shale were analyzed, showing enriched Al-normalized concentrations of U and Mo compared to Post-Archean Average Shale standard (PAAS). Previous studies suggested that uptake of U and Mo are due to interactions with organic matter. The current study shows correlation of Mo and total organic carbon (R² ~ 0.6 – 0.9), however there appears to be little correlation with U in the proximal core (R² ~ 0.09) and good correlation in the distal core (R² ~ 0.7) suggesting additional mechanisms of uptake in proximal locations. Covariation of Mo_EF and U_EF suggests a weakly restricted basin consistent with particulate shuttling in which redox cycling of metal hydroxides (such as MnOOH and FeOOH) occurs within the water column. Variations of Fe/Al and Mo_EF further support a particulate shuttle, as a key component in Mo fixation. This is consistent with a redox cycling expected for the superestuarine circulation pattern.