Reconstruction of deglacial N- and O- cycling in the intermediate Pacific

Abstract

Global warming, along with increased consumption of fossil fuels, and artificial chemical nitrogenous fertilizers is accelerating the ongoing oceanic oxygen loss and the expansion of oxygen deficient zones around the world. Understanding the changes in the past nutrient cycling in the low oxygen environments can provide important insights in the future trends of nitrate and redox conditions. In this study, bottom-water nitrate levels from four different oxygen deficient zones in the Pacific ocean is reconstructed for the last 20, 000 years using the pore density of unicellular eukaryotes. To do this, a novel automated image analysis technique developed using scannning electron microscopy and artificial intelligence. Furthermore, the redox conditions in these oxygen deficient zones are studied. The study reveals spatial and temporal variations in bottom-water nitrate levels and redox conditions. Changes in the composition of intermediate, and deep water masses, water-column denitrification, primary productivity, and dissolved oxygen concentrations can influence bottom-water nitrate at the study sites.