AN ECCENTRIC NITROGEN CYCLE DURING THE CRETACEOUS?

Oxygen isotope data from mid-Turonian (~91.2 Ma) black shales at Demerara Rise (ODP Site 1259) have been previously interpreted to indicate a transient ~200 ka glaciation during the Cretaceous greenhouse. Here we test for a linkage between the proposed glaciation and marine nitrogen cycle dynamics, as documented for the late Pleistocene glacial/interglacial cycles. New N-isotope measurements from Site 1259a reveal δ¹⁵N values that range from +0.7‰ to -3.5‰ through an interval that encompasses the proposed glaciation event. ¹⁵N-depletion is typical of Cretaceous black shales and consistent with a nitrogen-fixation source for nutrient nitrogen. To provide a temporal framework for interpretation of the δ¹⁵N data, we develop an orbital timescale (OTS) using published high-resolution wt.% CaCO₃ data from Site 1259. Evolutive harmonic analysis reveals persistent and statistically significant wt.% CaCO₃ rhythms. Application of the average spectral misfit method to test a wide range of candidate temporal calibrations for these cycles confirms the presence of a dominant ~400 ka cycle, consistent with the orbital interpretation of Friedrich et al. (2008). However, spectral analysis of the δ¹⁵N data, once adjusted to the wt.% CaCO₃-derived OTS, reveals a strong ~100 ka cycle and little variance at ~400 ka. The highest δ¹⁵N values, and the largest amplitude ~100 ka cycle, are found within the interval of time for which glaciation has been proposed. The δ¹⁵N maxima may be best explained by enhanced oxidation and expansion in the extent of denitrification within anoxic intermediate waters during glaciation. According to the late Pleistocene model for the marine nitrogen cycle, oxygen content of the themocline is enhanced during glacial times due to enhanced oxygen solubility, and the presence of stronger winds that drive ventilation, resulting in a decrease in the global area of marine denitrification. The N-cycle response in the anoxic/euxinic Demerara Rise water column is opposite to that found during the Pleistocene, because advection of oxygen promotes, rather than inhibits denitrification. The persistence of a ~100 ka δ¹⁵N cycle prior to and following the proposed glaciation event suggests that ice sheets may be more important than previously recognized in modulating climate during the Turonian greenhouse.