264-5 A new tale of shales: isotopic and molecular approaches for evaluating the biogeochemical control on black shale deposition in the Devonian and Cretaceous.

Abstract:

The iconic black shales that typify the depositional record of the Cretaceous and Devonian during Oceanic Anoxic Events (OAEs) have captivated geologist and geochemists for many years. Diagnosing the causes, durations and environmental impacts of OAEs has been central to contextualizing these events and their relationship to climate variability and accompanying extinctions. The mode of the carbon cycle during the Late Devonian and mid-Cretaceous was tipped toward enhanced production and burial of organic matter under more reducing conditions and catalyzed by non-analog nutrient cycling regimes. Using examples with well-developed temporal chronologies (including astronomical time scales) we will present new and existing carbon, nitrogen and hydrogen [PAC1] isotopes on organic matter from specific compounds, bulk sediments, and other phases. These results paint a picture of or a nitrogen cycle that is dominated by the presence sub-surface waters rich in ammonium and respired carbon-rich that are variably accessed by primary producers. During mixing events, breakdown of water column stratification allowed upward excursions of nutrient-rich chemocline waters that resulted in enhanced organic carbon production that has strong ¹⁵N depletion, and in some cases, ¹³C depletion. Outside of the OAE events, proxy variations tend to follow orbitally controlled periods. What remains unclear is the degree to which these isotope excursions and the organic matter enrichment are caused by external input of terrestrially derived nutrients responding to terrestrial hydroclimate variability or through internally driven oceanic processes (wind-driven upwelling or overturning) occurring at Milankovitch periods. We will present new compound specific hydrogen isotope data on terrestrial leaf wax lipids to explore connections between  terrestrial hydroclimate and marine redox variability in mid-Cretaceous black shales of the North Atlantic.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-20034

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