Nitrogen isotope evidence for expanded ocean suboxia in the early Cenozoic

Abstract

The million-year variability of the marine nitrogen cycle is poorly understood. Before 57 million years (Ma) ago, the 15N/14N ratio (δ15N) of foraminifera shell-bound organic matter from three sediment cores was high, indicating expanded water column suboxia and denitrification. Between 57 and 50 Ma ago, δ15N declined by 13 to 16 per mil in the North Pacific and by 3 to 8 per mil in the Atlantic. The decline preceded global cooling and appears to have coincided with the early stages of the Asia-India collision. Warm, salty intermediate-depth water forming along the Tethys Sea margins may have caused the expanded suboxia, ending with the collision. From 50 to 35 Ma ago, δ15N was lower than modern values, suggesting widespread sedimentary denitrification on broad continental shelves. δ15N rose at 35 Ma ago, as ice sheets grew, sea level fell, and continental shelves narrowed.

Demise of the Planktic Foraminifer Genus Morozovella during the Early Eocene Climatic Optimum: New Records from ODP Site 1258 (Demerara Rise, Western Equatorial Atlantic) and Site 1263 (Walvis Ridge, South Atlantic)

“…However, their virtual disappearance~160 kyr before the K/X event is significant because this group became virtually absent in the subtropical and temperate Atlantic Sites 1051 and 1263 at the beginning of the EECO either slightly before (Site 1051) or at the K/X event (1263) [64]. Recently published foraminiferal-bound nitrogen isotope data suggest enhanced oxygenation of the thermocline in the early EECO [101]. A combination of increased temperature effect and oxygenation of the thermocline might have played a major role in driving the disappearance of chiloguembelinids inhabiting the OMZ [64].…”

“…Therefore, chiloguembelinids, considered as a eutrophic indicator, may have suffered of reduced food supply besides the warmer temperatures. Interestingly, recently published foraminiferal-bound nitrogen isotope data demonstrate that the extent of water column denitrification, a process that is known to occur only in oxygen depleted waters in the present-day Ocean, and that is inferred by FB (Foraminiferal Bound)-δ 15 N values, started to markedly decline within the EECO at~53 Ma ago [101]. This indicates enhanced oxygenation at the OMZ during the EECO.…”

Size‐Fraction‐Specific Stable Isotope Variations as a Framework for Interpreting Early Eocene Bulk Sediment Carbon Isotope Records

“…Kaiho et al (2006) hypothesized that algal photosymbionts may give these foraminifers an ecological advantage in the nutrient-deficient and thermally stratified ocean waters of the PETM (Bralower, 2002;Carmichael et al, 2017). Consistent with the prevailing nutrient regime, expansion of oceanic suboxia during the late Paleocene-early Eocene, especially across hyperthermal events, has been suggested based on sediment proxies (Nicolo et al, 2010), decreasing I/Ca ratios in foraminiferal tests (Zhou et al, 2014), and foraminifer bound δ 15 N values (Kast et al, 2019).…”

Assessment of C, N, and Si Isotopes as Tracers of Past Ocean Nutrient and Carbon Cycling

“…e) Foraminifera-bound δ 15 N indicating changes in the balance of water column (WC) and sedimentary (sed.) denitrification (Kast et al, 2019 The PETM punctuated a long term, ~2‰ δ 13 C drop between 58 and 52 Ma that may indicate a decrease in organic carbon burial (Komar et al, 2013 Turner, 2014).…”