Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

Abstract

Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic B0.55 Ma, despite oxygenation of Earth's atmosphere nearly 2 Gyr earlier. Marine sediments from the intervening period suggest predominantly ferruginous (anoxic Fe(II)-rich) waters, interspersed with euxinia (anoxic H 2 S-rich conditions) along productive continental margins. Today, sustained biotic H 2 S production requires NO 3 À depletion because denitrifiers outcompete sulphate reducers. Thus, euxinia is rare, only occurring concurrently with (steady state) organic carbon availability when N 2 -fixers dominate the production in the photic zone.Here we use a simple box model of a generic Proterozoic coastal upwelling zone to show how these feedbacks caused the mid-Proterozoic ocean to exhibit a spatial/temporal separation between two states: photic zone NO 3 À with denitrification in lower anoxic waters, and N 2 -fixation-driven production overlying euxinia. Interchange between these states likely explains the varying H 2 S concentration implied by existing data, which persisted until the Neoproterozoic oxygenation event gave rise to modern marine biogeochemistry.

The evolution of Earth's biogeochemical nitrogen cycle

“…The observations of sulfide-driven denitrification (Section 2.1, Shao et al, 2010), combined with the significantly higher metabolic energy gain from nitrate reduction compared to sulfate reduction (Froelich et al, 1979) has led to the hypothesis that euxinia and nitrate are mutually exclusive (Boyle et al, 2013). In other words, one should never see evidence of both in the same sample.…”

“…Aerobic nitrogen cycling is commonly inferred where δ 15 N is greater than ~+2‰ (horizontal dashed line). Data points in the top right quadrant violate the model of Boyle et al (2013) Figure 4: Illustration of the marine nitrogen cycle. Essential metal cofactors are in bold.…”

Paleobiological Perspectives on Early Eukaryotic Evolution

“…Thus, we might expect that in Proterozoic oceans with persistent subsurface anoxia, cyanobacteria and other photosynthetic bacteria would dominate primary production (Johnston et al 2009), in part because of low fixed-nitrogen abundances in surface waters (Fennel et al 2005). Indeed, Boyle et al (2013) have argued that, in Proterozoic oceans, sulfidic subsurface water masses could only develop beneath surface waters dominated by nitrogen-fixing primary producers. Conversely, biogeochemical study of Silurian microbial mats shows a strong presence of eukaryotic biomarkers (Bauersachs et al 2009), suggesting that mat-seals provide imperfect barriers to the burial of eukaryotic lipids.…”

Spatial and temporal trends in Precambrian nitrogen cycling: A Mesoproterozoic offshore nitrate minimum

“…This may in turn have delayed the radiation of photosynthetic eukaryotes, which are most productive under a steady supply of nitrate and are incapable of N 2 fixation (Anbar and Knoll, 2002). Nitrate scarcity could have created a positive feedback on trace metal scavenging because it may have spurred microbial sulfate reduction and consequently the expansion of euxinic environments (Boyle et al, 2013). This would have prolonged nitrate limitation until the extent of euxinia contracted (Sperling et al, 2015) possibly through a Neoproterozoic expansion of oxic waters (Canfield et al, 1996;Ader et al, 2014;Thomson et al, 2015).…”