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

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.

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

2013

Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic~ 0.55 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− depletion because denitrifiers outcompete sulphate reducers.

Widespread iron-rich conditions in the mid-Proterozoic ocean

Nature, 2011

The chemical composition of the ocean changed markedly with the oxidation of the Earth's surface 1 , and this process has profoundly influenced the evolutionary and ecological history of life 2,3 . The early Earth was characterized by a reducing ocean-atmosphere system, whereas the Phanerozoic eon (less than 542 million years ago) is known for a stable and oxygenated biosphere conducive to the radiation of animals. The redox characteristics of surface environments during Earth's middle age (1.8-1 billion years ago) are less well known, but it is generally assumed that the mid-Proterozoic was home to a globally sulphidic (euxinic) deep ocean 2,3 . Here we present iron data from a suite of mid-Proterozoic marine mudstones. Contrary to the popular model, our results indicate that ferruginous (anoxic and Fe 21 -rich) conditions were both spatially and temporally extensive across diverse palaeogeographic settings in the mid-Proterozoic ocean, inviting new models for the temporal distribution of iron formations and the availability of bioessential trace elements during a critical window for eukaryotic evolution.

Iron‐mediated anaerobic ammonium oxidation recorded in the early Archean ferruginous ocean

Geobiology

The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ15N values found in well‐preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much‐needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ15N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non‐quantitative oxidation of ammonium via the Feammox pathway, a metabolic co‐cycling between iron and nitrogen through ...

A Neoproterozoic Transition in the Marine Nitrogen Cycle

The Neoproterozoic (1000–542 million years ago, Mya) was characterized by profound global environmental and evolutionary changes, not least of which included a major rise in atmospheric oxygen concentrations [1, 2], extreme climatic fluctuations and global-scale glaciation [3], and the emergence of metazoan life in the oceans [4, 5]. We present here phylogenomic (135 proteins and two ribosomal RNAs, SSU and LSU) and relaxed molecular clock (SSU, LSU, and rpoC1) analyses that identify this interval as a key transition in the marine nitrogen cycle. Specifically, we identify the Cryogenian (850–635 Mya) as heralding the first appearance of both marine planktonic unicellular nitrogen-fixing cyanobacteria and non-nitrogen-fixing picocyanobacteria (Synechococcus and Prochlorococcus [6]). Our findings are consistent with the existence of open-ocean environmental conditions earlier in the Proterozoic adverse to nitrogenfixers and their evolution—specifically, insufficient availability of molybdenum and vanadium, elements essential to the production of high-yielding nitrogenases. As these elements became more abundant during the Cryogenian [7, 8], both nitrogen-fixing cyanobacteria and planktonic picocyanobacteria diversified. The subsequent emergence of a strong biological pump in the ocean implied by our evolutionary reconstruction may help in explaining increased oxygenation of the Earth’s surface at this time, as well as tendency for glaciation.

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

Microbial loop of a Proterozoic ocean analogue

2021

Meromictic Lake Cadagno, an ancient ocean analogue, is known for its permanent stratification and persistent anoxygenic microbial bloom within the chemocline. Although the anaerobic microbial ecology of the lake has been extensively studied for at least 25 years, a comprehensive picture of the microbial food web linking the bacterial layer to phytoplankton and viruses, with explicit measures of primary and secondary production, is still missing. This study sought to understand better the abundances and productivity of microbes in the context of nutrient biogeochemical cycling across the stratified zones of Lake Cadagno. Photosynthetic pigments and chloroplast 16S rRNA gene phylogenies suggested the presence of eukaryotic phytoplankton through the water column. Evidence supported high abundances of Ankyra judayi, a high-alpine adapted chlorophyte, in the oxic mixolimnion where oxygenic-primary production peaked. Through the low- and no-oxygen chemocline and monimolimnion, chlorophyte...