Evolution of nitrogen cycling and primary productivity in the tropics during the Late Ordovician mass extinction

Abstract

The late Ordovician is characterized by significant environmental changes in global climate, oceanic redox conditions, as well as a major disaster for marine organisms—the Late Ordovician mass extinction (LOME) (ca. 445 Ma). During this event, planktonic and benthic faunas as well as microbial communities were substantially influenced, and changes in the microbial ecosystems could provide useful clues for the dynamics of marine nutrients and primary production. Here, we utilize new nitrogen isotope (δ15Nbulk) and abundance data, as well as organic carbon accumulation rates (OCARs) from an Ordovician-Silurian (OS) deep marine section, Tianba, deposited under consistently anoxic water columns in the Yangtze Shelf Sea (South China), to unravel the evolution of nitrogen cycling and primary productivity. The OCARs for Tianba section were the lowest during the sea level lowstands and Hirnantian glaciation maximum but were high at the eve and aftermath of the Hirnantian glaciation maximum, consistent with the OCARs dynamics for another anoxic deep-water section located in Laurentia Block in the tropical ocean. Low OCARs mainly demonstrate that the marine primary productivity was low, with N2-fixers populating the ecosystems during the Hirnantian glaciation maximum as evidenced by the spatial distribution of δ15Nbulk values in the paleotropical waters. However, the high OCARs mainly imply high marine primary productivity right before and after the Hirnantian glaciation maximum in the paleotropics, which were likely fueled by fixed nitrogen sourced from upwellings. The spatial heterogeneity in δ15Nbulk values for the top of Wufeng Formation and coeval strata in the paleotropics suggests that fixed nitrogen species (including NH4+ and NO3−) in surface waters may vary in space. The cooccurrence of zooplankton extinction with spatial variation of δ15Nbulk values suggests that the dynamics of surface waters redox conditions could have played a significant role in the extinction pattern of planktonic faunas during the LOME-1.