Nitrogen cycle dynamics in the Late Cretaceous Greenhouse

Great attention has been paid to the origin of anomalously low nitrogen isotope values during the Late Cretaceous. Nitrogen isotope values are often as low as −3‰ and are typically less than +2‰, even in relatively organic matter-lean sediments. Here we evaluate nitrogen isotope variability during a relatively quiescent phase of the Late Cretaceous Greenhouse, between Oceanic Anoxic Events 2 and 3, using the black shales of Demerara Rise (DR). Selection of this site allows us to isolate some of the factors that control nitrogen cycle dynamics and contribute to low nitrogen isotope values. New N-isotope measurements from ODP Site 1259 reveal δ¹⁵N values that range from +0.2‰ to −3.5‰ and oscillate by 1.5‰ to 3‰ over 1.6 million years (Ma). Temporal calibration of our data using a new astronomical time scale reveals a strong ∼100 thousand year (ka) eccentricity cyclicity in δ¹⁵N. We attribute this cyclicity to oscillations in the position of the intertropical convergence zone (ITCZ) over DR that modulate upwelling intensity, chemocline depth and the degree of ¹⁵N-depletion. We also recognize a statistically significant correlation (p=0.0022) between the TEX₈₆ indices and δ¹⁵N, with the lowest δ¹⁵N corresponding to the highest TEX₈₆ indices. This relationship suggests that the activity and ecology of ammonia oxidizing Thaumarchaeota and the δ¹⁵N of dissolved inorganic nitrogen utilized by primary producers are linked. We therefore interpret the observed variability in the δ¹⁵N data and TEX₈₆ indices as primarily reflecting fluctuation of upwelling intensity and chemocline depth, and the significant inverse relationship between these data sets suggests that caution should be exercised when interpreting the TEX₈₆ in terms of temperature in similar paleoenvironmental settings.