Nitrogen Isotopes as a Paleoredox Proxy: Co-Variation of Nitrogen Isotopes and Redox States through Glacial-Interglacial Cycles in the Black Sea

Nitrogen fixation and denitrification fuel the export flux of organic matter to the ocean interior. The nitrogen isotopic values of these two species represent end-members in a mixing curve, and their fluxes within the ocean are controlled by microbial processes linked to oxygen concentrations. The relative contribution of these two sources of nitrogen in phytoplankton is translated to the sedimentary organic matter. Provided that the sediment record is well-preserved, the nitrogen isotopic profile at a given location reflects history of the relative contributions of nitrogen fixation to denitrifiation and thus the past redox environment.

We have previously determined that the nitrogen isotopic value of bulk sediment could possibly serve as a proxy for the redox state of the water column at the time of deposition, and proposed a correlation between δ¹⁵N and O₂ concentrations. The relatively fast and extreme changes in water column redox chemistry in the Black Sea on glacial-interglacial time scales provide an opportunity to test our model. The Black Sea is unique in that the water column switches from an anoxic, stratified marine water body in the interglacial periods to an oxic freshwater lake in glacial phases in a cyclical pattern.

Our data indicate that higher δ¹⁵N values occur during the transition from one redox end-member to another, implying a relative increase in denitrification, while the periods of fully oxic and completely anoxic conditions are characterized by low sedimentary δ¹⁵N, due to predominant nitrification and complete nitrate utilization, respectively. This study strongly suggests that the pre-anthropogenic nitrogen cycle in the Black Sea was strongly dependent on the oxygen content of the water column on glacial-interglacial time scales, and that the changes in nitrogen cycling as a result of changes in redox can be identified in the sedimentary record.