Decadal- to Orbital-Scale Links Between Climate, Productivity and Denitrification on the Peru Margin

Denitrification is the predominant global loss term for combined nitrogen and can exert a major control on its oceanic inventory, global productivity and atmospheric CO₂. Our prior work demonstrates that proxy records for changing denitrification, oxygenation and productivity in the recent geological past in the Arabian Sea exhibit unprecedented similarity with abrupt climate fluctuations recorded in high-latitude ice-cores. Since the Peru Margin and Arabian Sea together constitute almost two-thirds of global marine water-column denitrification, changes in concert in these two regions could potentially have effected rapid global climate changes through an oceanic mechanism. The Peru Margin is intimately coupled to the Equatorial Pacific, source of El Ni&ño-La Niña SST, productivity and precipitation anomalies. Here, biogeochemical cycles are especially sensitive to abrupt climatic changes on decadal time-scales by virtue of this ENSO coupling. The purpose of our research is to investigate whether longer changes in tropical Pacific oceanography represent a 'scaling up' of anomalous ENSO conditions, modulated by both internal (e.g. nutrient inventory or WPWP heat budget) and external (e.g. orbital) forcing throughout the last glacial/inter-glacial cycle. Here we present first results of a detailed investigation of recently-recovered sediments from ODP Site 1228 on the Peru margin upper continental slope, in an attempt to capture some of the essential aspects of ENSO-like variability. Despite the existing availability of high quality sediment cores from this margin, little detailed paleoclimatic information currently exists because of poor sedimentary carbonate preservation (exacerbated post-recovery) which has limited generation of essential chronostratigraphic controls. Instead, we rely on the development and novel application of compound-specific AMS dating verified and supplemented by intermittent foraminiferal and bulk-carbon AMS dates, a magnetic paleo-intensity record and tephra layers to tie our records to established global chronologies for abrupt climate change. Based on this age model, we present records of nitrogen isotopic values (δ¹⁵N), chlorin and alkenone abundances, and alkenone-derived (U^k'₃₇) SSTs for the last glacial-interglacial cycle. We extrapolate these new nitrogen isotopic results in the context of global marine denitrification. By constraining the loss term for marine nitrate at decadal-to-millennial timescales within the principal major regions of global denitrification, we make a first attempt to reconcile the records of atmospheric CO₂ trapped in ice-cores with such rapid changes in global nutrient inventory.