Increasing primary productivity in the oligotrophic Tethyan coastal ocean during the Paleocene-Eocene warming episode

Abstract

The coastal upwelling zones, occupying only ~0.5% of the global ocean, account for ~10% of the global primary productivity. The CO 2 fixation by primary producers amplifies in the upwelling zones during global warming due to the higher nutrient supply. Based on the presumption that the nutrient-deficient coastal ocean is less productive, the state of the oligotrophic coastal ocean is often neglected in the productivity-climate change studies. The present study investigated the changes in the primary productivity, redox condition, and nutrient content, using algal abundance, total organic carbon, and various major, trace, and rare earth elements and yttrium (REY) proxies, of the oligotrophic equatorial eastern Tethyan coastal ocean across the Paleocene-Eocene Thermal Maximum (PETM), a prominent paleo-global warming event. Despite the lower nutrient (lower Ni EF , Cu EF , and Zn EF ) contents, and invariable salinity, p H, and light conditions, the PETM interval shows extensive growth of coralline red algae in the hypoxic-oxic water column. Based on these observations, and inferences drawn from the previous laboratory experiments, conducted on the algal growth in varying p CO 2 by others, we postulate that the increased atmospheric CO 2 concentrations during the PETM probably enhanced the primary productivity of the oligotrophic Tethyan coastal ocean. If so, then the oligotrophic coastal ocean may be considered as an effective CO 2 sink and likely to play a pivotal role in carbon cycle-climate connection studies. • Response of the oligotrophic coastal ocean during global warming is grossly unknown. • Tethyan low-latitude, shallow-marine sections record PETM in carbonate sequence. • Primary productivity (coralline algae) increased during the PETM despite low nutrient. • The higher atmospheric p CO 2 might have triggered the excessive growth of algae. • Oligotrophic coastal ocean can act as an effective carbon sink.