Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean (original) (raw)

Iron-dependent nitrogen cycling in a ferruginous lake and the nutrient status of Proterozoic oceans

Nature Geoscience, 2017

Nitrogen limitation during the Proterozoic has been inferred from the great expanse of ocean anoxia under low-O 2 atmospheres, which could have promoted NO 3 − reduction to N 2 and fixed N loss from the ocean. The deep oceans were Fe rich (ferruginous) during much of this time, yet the dynamics of N cycling under such conditions remain entirely conceptual, as analogue environments are rare today. Here we use incubation experiments to show that a modern ferruginous basin, Kabuno Bay in East Africa, supports high rates of NO 3 − reduction. Although 60% of this NO 3 − is reduced to N 2 through canonical denitrification, a large fraction (40%) is reduced to NH 4 + , leading to N retention rather than loss. We also find that NO 3 − reduction is Fe dependent, demonstrating that such reactions occur in natural ferruginous water columns. Numerical modelling of ferruginous upwelling systems, informed by our results from Kabuno Bay, demonstrates that NO 3 − reduction to NH 4 + could have enhanced biological production, fuelling sulfate reduction and the development of mid-water euxinia overlying ferruginous deep oceans. This NO 3 − reduction to NH 4 + could also have partly o set a negative feedback on biological production that accompanies oxygenation of the surface ocean. Our results indicate that N loss in ferruginous upwelling systems may not have kept pace with global N fixation at marine phosphorous concentrations (0.04-0.13 µM) indicated by the rock record. We therefore suggest that global marine biological production under ferruginous ocean conditions in the Proterozoic eon may thus have been P not N limited.

Persistent late Tonian shallow marine anoxia and euxinia

Precambrian Research, 2023

The increase in the complexity of eukaryotic life during the Tonian Period (ca. 1000 to 720 Ma) is commonly associated with the oxygenation of the Earth's ocean-atmosphere system, yet the timing and duration of these redox changes remain uncertain. In particular, it is unclear how shallow marine environments, which were likely crucial habitats for early eukaryotic organisms, responded to Neoproterozoic oxygenation. This study uses trace and rare earth element geochemistry to determine shallow marine redox conditions during the deposition of the late Tonian (ca. 760 Ma) Devede Formation, northern Namibia. In this unit, although carbonate phases (microbialite, former aragonite and high Mg calcite marine cements, and primary dolomite marine cements) generally exhibit no Ce/Ce* anomaly, rare negative values (up to 0.55) are consistent with short-lived periods of oxygenation. In contrast, the dominantly positive Eu/Eu* anomalies of the same phases suggest that basinal conditions were anoxic. These phases also exhibit low concentrations of redox-sensitive (U, V, Mo) and chalcophile (Co, Cu, Cd, Zn, Pb) elements, suggesting that euxinic conditions were predominant during the deposition of the Devede Formation. Integration of this data (Ce/Ce* and Eu/Eu*) with that of analogous late Tonian (ca. 840 to 731 Ma) carbonate strata reveals that shallow marine settings were characterised by persistent anoxia and euxinia, suggesting that any potential increase in atmospheric O 2 ca. 800 Ma (e.g. the Bitter Springs Anomaly) was insufficient to facilitate resilient ocean oxygenation. These findings suggest that the late Tonian oceans were likely challenging environments for complex (e.g. eukaryotic) life, and add to a growing body of evidence that the spatially variable redox conditions of late Proterozoic shallow marine settings likely reflect the complex nature of the Neoproterozoic Oxygenation Event.

The co-evolution of the nitrogen, carbon and oxygen cycles in the Proterozoic ocean

American Journal of Science, 2005

Geochemical evidence suggests that there was a delay of several hundred million years between the evolution of oxygenic photosynthesis and the accumulation of oxygen in Earth's atmosphere. The deep ocean appears to have remained euxenic for several hundred million years after the atmosphere became oxygenated. In this paper we examine the possibility that the extraordinary delay in the oxidation of the atmosphere and oceans was caused by a biogeochemical "bottleneck" imposed by metabolic feedbacks between carbon burial, net oxygen production, and the evolution of the nitrogen cycle in the Proterozoic oceans. Whereas under anoxic conditions oceanic ammonium would have been relatively stable, as oxygen concentrations rose, nitrification and subsequent denitrification would have rapidly removed fixed inorganic nitrogen from the oceans. Denitrification would have imposed a strong constraint on the further rise of free oxygen by depriving oxygenic photoautotrophs of an essential nutrient (that is, fixed inorganic nitrogen). To examine the dynamic interactions between oxygen and nitrogen cycling, we developed a five box model that incorporates the salient features of the oxygen, nitrogen and carbon cycles, ocean circulation, and ocean-atmosphere gas-exchange. Model simulations, initiated under anaerobic conditions with no free oxygen in the atmosphere or ocean, are characterized by an initially reduced deep ocean with abundant ammonium, followed by an extended period when neither form of fixed nitrogen is stable, and a fully oxidized phase with abundant nitrate. We infer that, in the process of oxidizing the early Proterozoic ocean, the system had to go through a nitrogen-limited phase during which time export production was severely attenuated. Our studies suggest that the presence of shallow seas with increased organic matter burial was a critical factor determining the concentration of oxygen in the ocean and atmosphere, while the phosphate concentration played a key role in determining the rate of oxygenation of the deep ocean.

Evidence for episodic oxygenation in a weakly redox-buffered deep mid-Proterozoic ocean

Chemical Geology, 2018

Over the last two decades, popular opinion about prevailing conditions in the mid-Proterozoic deep ocean has evolved from fully oxygenated to globally euxinic (sulfidic) to a more heterogeneous, stratified water column with localized pockets of euxinia existing in predominantly iron-rich (ferruginous) deep waters. The Animikie Basin in the Lake Superior region has been essential in shaping our view of marine redox evolution over this time period. In this study, we present a multi-proxy paleoredox investigation of previously unanalyzed strata of the late Paleoproterozoic Animikie Basin using drill cores through the ~1.85 Ga Stambaugh Formation (Paint River Group) in the Iron River-Crystal Falls district of the Upper Peninsula of Michigan, USA. Based on previous tectonic reconstructions and analysis of sedimentary regimes, the Iron River-Crystal Falls section captures strata from among the deepest-water facies of the Animikie Basin. In contrast to previous work on sedimentary rocks in this basin, we find evidence from iron speciation, trace metal, and Mo isotope data for episodes of at least local deep-water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions. While trace-metal enrichments and iron speciation data suggest predominantly anoxic conditions, the occurrence of Mn-rich intervals (up to 12.3 wt % MnO) containing abundant Mn-Fe carbonate, and a wide range of Mo isotope data with extremely negative

Proterozoic ocean redox and biogeochemical stasis

Proceedings of the National Academy of Sciences, 2013

The partial pressure of oxygen in Earth's atmosphere has increased dramatically through time, and this increase is thought to have occurred in two rapid steps at both ends of the Proterozoic Eon (∼2.5-0.543 Ga). However, the trajectory and mechanisms of Earth's oxygenation are still poorly constrained, and little is known regarding attendant changes in ocean ventilation and seafloor redox. We have a particularly poor understanding of ocean chemistry during the mid-Proterozoic (∼1.8-0.8 Ga). Given the coupling between redoxsensitive trace element cycles and planktonic productivity, various models for mid-Proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients, with potential ecological constraints on emerging eukaryotic life. Here, we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates. We find that the metal enrichment record implies a Proterozoic deep ocean characterized by pervasive anoxia relative to the Phanerozoic (at least ∼30-40% of modern seafloor area) but a relatively small extent of euxinic (anoxic and sulfidic) seafloor (less than ∼1-10% of modern seafloor area). Our model suggests that the oceanic Mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of Mo and chromium enrichments through time is consistent with the possibility of a Mo-N colimited marine biosphere during many periods of Earth's history. paleoceanography | geobiology

Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age

Proceedings of the National Academy of Sciences, 2009

Molecular oxygen (O2) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580 -550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic). Here we illustrate how contributions to primary production by anoxygenic photoautotrophs (including physiologically versatile cyanobacteria) influenced biogeochemical cycling during Earth's middle age, helping to perpetuate our planet's intermediate redox state by tempering O 2 production. Specifically, the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained euxinia in the OMZ. On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated sulfide availability and water column oxygenation. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced oceanic redox and the Proterozoic O 2 budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe 2؉ rich) subsurface waters set in motion Earth's transition from its prokaryote-dominated middle age, removing a physiological barrier to eukaryotic diversification (sulfide) and establishing, for the first time in Earth's history, complete dominance of oxygenic photosynthesis in the oceans. This paved the way for the further oxygenation of the oceans and atmosphere and, ultimately, the evolution of complex multicellular organisms.