Turonian Sea Level and Paleoclimatic Events in Astronomically Tuned Records From the Tropical North Atlantic and Western Interior Seaway

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• Sageman, Bradley B.
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• Meyers, Stephen R.

Abstract

Multiple hiatuses punctuate the Middle to Upper Turonian Substages of the Western Interior Basin (WIB, USA), interrupting an otherwise highly resolved and relatively conformable Upper Cretaceous stratigraphic record. To determine the duration and possible causes of these hiatuses, while simultaneously reconstructing Turonian carbon cycling, we develop a new astronomically tuned carbon isotope (δ13C) chemostratigraphy from time-equivalent organic carbon-rich shales of the tropical North Atlantic (Demerara Rise—Ocean Drilling Program Leg 207). A Gaussian kernel smoothing and cross-correlation technique is also introduced to quantitatively correlate the astronomically tuned Demerara Rise δ13C chemostratigraphy to coeval records globally. This analysis reveals a general consistency among ages for Turonian δ13C excursions from astronomical and radioisotopic timescales. It also indicates that a positive oxygen isotope excursion at Demerara Rise, previously interpreted to signify cooling, is synchronous (~±200 ka) with the Ogbourne Hardground in the English Chalk, the Bass River—Magothy sequence boundary, and an expansive mid-Turonian hiatus in the WIB. However, the event is not concurrent with maximum regression in the WIB. Trends in δ13C, and additional geochemical data from Demerara Rise (weight percent total organic carbon, weight percent carbonate, and C/N), are consistent with oceanic upwelling conditions modulated by monsoonal winds, which drove significant organic carbon burial instep with several prominent positive δ13C excursions (e.g., Hitchwood #2-3). Moreover, δ13C and weight percent total organic carbon preserve strong obliquity power (~1 Ma). This suggests that higher cross-equatorial insolation gradients resulting from changes in Earth's axial tilt intensified monsoonal winds, strongly influencing tropical Atlantic organic carbon burial and Turonian carbon cycling dynamics following Ocean Anoxic Event 2.Key Points Astronomically tuned carbon isotope (δ13C) chemostratigraphy yields Turonian Stage (L. Cretaceous) duration from Demerara Rise (DR) Obliquity modulated tropical Atlantic monsoonal winds intensified upwelling, anoxia, and organic carbon burial at DR through Turonian Stage Mid-Turonian upper woolgari hiatus in Western Interior Basin is coeval with evidence for cooling at DR via δ13C kernel smoothing correlation