Climatic Control of Oceanic Suboxic Zones on Centennial to Million Year Timescales

Suboxic intermediate waters of the world's oceans, though constituting a small fraction of total volume, dominate important redox-sensitive biogeochemical processes. They host a large proportion of marine denitrification (heterotrophic as well as anammox) and thus are a significant control on overall oceanic combined nitrogen balance. Climatic forcing on a variety of time scales can be studied through paleo- records of denitrification intensity. Denitrification increases the 15N/14N ratio (δ15N) of the residual nitrate and this signal is transferred to and preserved in underlying sediments. In the Northern ETP (ETNP), δ15N varies strongly with glacial-interglacial cycles for the past 4 Ma but shows dramatic decrease in average and amplitude of variation prior to establishment of a strong ETP cold tongue. In the Southern ETP off Peru (ETSP), past denitrification intensity has showed a very sharp and early rise at the onset of the last deglaciation. These events in the ETNP and ETSP are not correlated with productivity proxies. These regions also have a high degree of similarity in their δ15N records which is also supportive of common forcing through SAMW ventilation. Together these results indicate important interactions and feedback between global climate and suboxic zone extent and intensity. The Peru system is nevertheless unique as compared to the other large, oceanic suboxic zones in having clear, centennial to millennial-scale oscillation in denitrification intensity especially during the late Holocene. This variability is likely due to the particular ENSO sensitivity of this region and these records are evidence for longer period versions of the known interannual and decadal-scale phenomena. In this case, strong linkage with productivity is evident and suggestive of the mechanism for the fastest responses to future climate change.