Chemocline collapse in Lake Kivu as an analogue for nitrogen cycling during Oceanic Anoxic Events

One of the outstanding issues in the study of nutrient dynamics during Oceanic Anoxic Events (OAEs) is the preservation of ¹⁵N-depleted nitrogen isotope signals, which are not observed in analogous modern environments. The favored mechanism for δ¹⁵N values below 0‰ is utilization of excess ammonium derived from underlying anoxic waters by primary producers. However, there are few modern examples available for testing this hypothesis, due to the overall well-oxygenated state of modern oceans. Here, we present sedimentary δ¹³C_org and δ¹⁵N_bulk data, as well as pigment biomarker ratios, from the sediments of Lake Kivu, a meromictic lake in East Africa. This system serves as a potential analogue for the development of strongly depleted δ¹⁵N_bulk in OAE black shales. We focus on intervals of organic carbon enrichment (sapropels) that are interpreted to have been the result of lake water column mixing events. Sediment δ¹³C_org and δ¹⁵N_bulk decrease by up to 6 and 4‰ respectively near the base of sapropel layers. These negative excursions provide evidence for the mixing of ¹³C-depleted dissolved inorganic carbon and ammonium in excess of photosynthetic need, derived from the deeper anoxic basin of the lake. Support of this model is also provided by photosynthetic pigment biomarkers which show significant fluctuations in phototrophic ecology consistent with water column mixing and changes in nutrient status. Further evidence for the shallowing of the chemocline during sapropel deposition is found in the presence of bacteriochlorophyll derivatives produced by phototrophic sulfide oxidizers.