Evidence of surficial sediment and water column oxygenation through a carbon isotopic excursion in a high-latitude Early Cretaceous mudstone core in Arctic Alaska

High latitude sedimentary basins have different temperature, light, and oceanographic circulation conditions than mid- and low-latitude basins, where carbon isotopic events (CIEs) have been well documented. The North Kalikpik 1 well, drilled in Arctic Alaska, cored deep-water mudstones through the informal Barremian–Aptian “pebble shale unit” and “gamma-ray zone” (GRZ). This core was investigated for potential differences from CIE models developed in mid- and low-latitude basins. Samples were analyzed for organic and inorganic geochemical parameters, including total organic carbon (TOC), hydrogen index (HI), organic carbon isotopes (δ13Corg), major oxides and trace elements, and petrographic descriptions.

The δ13Corg results indicate a 19’ (5.8 m) interval that consists of a sharp ~4.5‰ decrease followed by a more gradual ~6.7‰ recovery that is tentatively correlated with Oceanic Anoxic Event (OAE) 1a. Above this interval, the isotope curve gradually decreases ~ 3.5‰ over 55’ (16.8 m) that may correlate with the Niveau–Fallot (NF) interval above OAE1a.

Petrographic work shows two distinct microfacies (MF). MF1 samples are well bioturbated and tiered, while MF2 samples are not bioturbated and have visibly reworked microbial mat fragments. Within the NF CIE interval, a change from stable values to upward increasing values of TOC, HI, U, and Mo occurred at approximately the same depth that MF1 changed to MF2.

Extensive bioturbation and low redox sensitive trace element (RSTE) concentrations associated with MF1 within the OAE1a CIE indicate that this high-latitude seafloor was not persistently anoxic during this excursion. In contrast, the subsequent increased TOC, HI, and RSTE concentrations in the upper part of the NF CIE interval correlate with deposition of MF2, suggesting a shift to dysoxic/anoxic seafloor conditions. Water column overturn likely resulted in supplying limited biologic nutrients and sediment reworking as seen in MF2.