137-13 Early Cretaceous Carbon Isotopic Excursions Were Not Always Anoxic: Evidence from a High-Latitude Barremian – Aptian Mudstone Core in Northern Alaska

Session: Joint SGD-SEPM-IAS Focus on the Sedimentary Record of Climate Change

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Abstract:

High latitude basins have different temperature, light, and oceanographic circulation conditions than mid- and low-latitude basins, where carbon isotopic events (CIEs) have been better documented.  The North Kalikpik 1 well, drilled in Arctic Alaska, cored deep-water mudstones through the Barremian – Aptian pebble shale and gamma-ray zone (GRZ) intervals. 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), major oxides and trace elements, and petrographic descriptions.

 𝛿¹³C_org (organic carbon isotope) results indicate a 14’ (4.3 m) interval that comprises a sharp ~-5‰ decrease followed by a ~+7‰ recovery that we tentatively correlate with Oceanic Anoxic Event (OAE) 1a CIE.  Above this interval, the isotope curve has a more gradual ~-5‰ decrease in isotope values over ~60’ (18.3 m) that may correlate with the Niveau-Fallot (NF) interval above OAE1a.

Correlations between the interpreted OAE 1a CIE interval and other geochemical parameters indicate variable TOC values between 1 – 4 weight %, a sharp increase in HI in the middle part of the CIE, and spikes in U, Mo, and V values at the start of the CIE.  The U and Mo values returned to background levels, while V continued to gradually increase. The NF CIE interval correlates with stable TOC, HI, U, and Mo values over the lowermost ~20 ft (6.1 m), followed by upward increasing values.  V and Hg gradually increased over the entire interval.  Petrographic work through this core shows two distinct microfacies (MF).  MF1 samples are well bioturbated and tiered, while MF2 samples are not bioturbated and have common microbial mats.  In the NF CIE interval, the change from stable values to upward increasing values of TOC, HI, U, and Mo occurred at approximately the same depth that the MF1 microfacies changed to the MF2 microfacies

Extensive bioturbation and low redox proxy values associated with MF1 occurred during the OAE1a CIE and indicate that this high-latitude seafloor was not anoxic during this excursion. In contrast, the subsequent increased TOC, HI and redox proxy values in the upper part of the NF CIE interval correlate with the change to MF2 microfacies and suggest a significant shift to dysoxic/anoxic seafloor conditions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9227

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