Characterising carbon cycle perturbations in the Cenomanian western interior seaway of North America

Abstract

As current global temperatures continue to rise at an accelerated pace, it is becoming increasingly important to examine how the planet has responded to previous periods of global warming in order to gain valuable insights into how the Earth system may react in the future. The mid-Cretaceous (~101-91 Ma) provides an excellent case study of a prolonged greenhouse climatic state, where high rates of oceanic crust production and enhanced large igneous province-related volcanism released vast quantities of greenhouse gases into the atmosphere. Consequently, global temperatures reached their highest level for the past 450 Myr during the Cenomanian to Turonian stages, and peak sea levels led to the development of epeiric seas such as the Western Interior Seaway (WIS), which extended southwards from the northern Boreal Ocean, across central North America, and into the Tethys Ocean. Superimposed upon this greenhouse climate were a series of even more extreme climate perturbations, of which Oceanic Anoxic Event 2 (OAE 2), spanning the Cenomanian-Turonian boundary (94.1 Ma), was the most prominent, globally widespread, extreme climatic event of the Late Cretaceous. OAE 2 represents a major global carbon cycle perturbation, during which enhanced marine productivity coupled with the expansion of oxygen minimum zones across global oceans led to the extensive deposition of organic-rich black shale deposits. An earlier Mid-Cenomanian Event (MCE I; ~96.5 Ma) is taken to be a precursor to OAE 2, as it marks the beginning of a ~2 Myr oceanographic reorganisation during which bottom waters started to become increasingly oxygen-depleted. MCE I is identified on a global scale by a distinct dual-peaked positive carbon isotope excursion. However, with little evidence of widespread organic-rich mudrock deposition, most studies of MCE I are from carbonate-rich successions, thus limiting the range of analyses that can be undertaken.
Here, a high-resolution, integrated organic and inorganic multi-proxy study of the Rebecca K Bounds-1 core from western Kansas has been undertaken in order to elucidate environmental and oceanographic changes during MCE I and the interval leading up to OAE 2 in the central-eastern WIS. MCE I is identified in an organic-rich sequence through a dual-peaked, positive organic carbon isotope excursion (MCE 1a and MCE 1b, respectively), and is determined to coincide with the establishment of a fully connected seaway that extended from the northern Boreal Ocean to the Tethyan Ocean in the south via the WIS in the mid-Cenomanian, as evidenced by diversified palynological, foraminiferal, and geochemical changes between MCE 1a and MCE 1b. Periodically enhanced planktic productivity is linked to ~50 kyr obliquity cycles across MCE I through cyclostratigraphic analysis. This heightened productivity is attributed to the strengthening of meridional winds during obliquity maxima, which may have increased upwelling-related nutrient input into Tethyan surface waters in equatorial regions. Palynological, redox-sensitive trace metal, lipid biomarker, and sedimentological data reveal that a complex dynamic oceanographic system prevailed in the central region of the WIS, with mixed early Cenomanian Tethyan-Boreal waters eventually being replaced by a northward migrating Tethyan water mass in the mid- to late Cenomanian. This situation prevailed until OAE 2 set in, when a rapid southward incursion of Boreal waters extended as far as the southern margin of the seaway. This multi-faceted study provides a new in-depth, high-resolution example of how Earth systems and environmental conditions reacted to periods of stressed, greenhouse climatic conditions.

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ORCID for Ian Harding: orcid.org/0000-0003-4281-0581

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