WHEN CHEMOCLINES COLLAPSE

OAEs have defied our attempts to apply modern, anoxic basin analogs such as the Black Sea. Yet, modern and recent systems provide resolution that is unavailable in the geologic record. Meromictic lakes such as Lake Kivu in East Africa provide unique opportunities to explore the relationship between persistently anoxic conditions at depth and a wide range of proxies that are pertinent to the study of OAEs. Here we explore new organic geochemical and stable isotopic records from Lake Kivu in the East African Rift Basin of Rwanda. Lake Kivu is a stratified half-graben rift basin that lies between Rwanda and the Democratic Republic of Congo on the western arm of the African Rift. The deep waters of Lake Kivu are charged with significant quantities of carbon dioxide and methane and are thought to have episodically overturned in the past, a process which is indicated by pronounced brown sapropel layers that have organic carbon contents that can be higher than 30 wt. %. The upwelling of ¹³C-depleted methane and carbon dioxide induced a 5‰ decrease in the δ¹³C of organic carbon. Overturning also drove significant eutrophication through the delivery of ammonium and phosphate from deep waters. Under these conditions diatoms proliferated and are clearly seen in high concentrations of the diatom-derived carotenoid fucoxanthin. The δ¹⁵N of bulk sediments increases by ~5‰ and was likely the result of the coupling of nitrification and denitrification as the basin overturned. Stable isotopic trends such as these from Lake Kivu are uncommon in the geochemical records of OAEs but bear similarities to the Toarcian, suggesting a similar mechanism.