Why so low? Making sense of 15N-depleted nitrogen isotope values in the Late Cretaceous.

Abstract:

Sedimentary organic matter from Cretaceous Oceanic Anoxic Events is characterized by universally low δ¹⁵N values. This observation has generated significant curiosity in the paleoceanographic community and has resulted in several conceptual models that aim to explain the ¹⁵N-depletion. The unifying element in these hypotheses is widespread anoxia and a series of nitrogen and/or metal cycling feedbacks. To address these hypotheses I will examine an interval in the middle Turonian (92.0 to 90.8 Ma) that demonstrates the impacts of redox and climate variability on nitrogen cycle dynamics. New N-isotope measurements from ODP Site 1259, Demerara Rise (DR) reveal δ¹⁵N values that range from +0.2‰ to -3.5‰ and oscillate by 1.5‰ to 3‰ over 1.6 Ma. A new astronomical time scale reveals a strong ~100 ka cyclicity in δ¹⁵N with little variance at ~400 ka through the mid-Turonian. The highest δ¹⁵N values and the largest amplitude ~100 ka cycles are found within the proposed glacial interval and cooler sea surface temperatures. The persistence of the ~100 ka δ¹⁵N cycle and δ¹⁵N maxima suggests a strong link between oceanic ventilation/circulation, climate, and the oscillations in δ¹⁵N are readily explained by variations in oxidation and expansion in the extent of denitrification within anoxic intermediate waters. These data confirm the strong link between water column redox variability and low δ¹⁵N values, but the exact mechanism of the ¹⁵N-depletion remains elusive.