How did the GOE affect the phosphorus cycle? 

We know that atmospheric & marine oxygen levels rose dramatically during the Great Oxidation Event (GOE), but how did this impact the cycling of phosphorus (P), which is a critical nutrient that regulates biological productivity? 

​Previous work (e.g. Reinhard et al. 2017) has used the P content of ancient marine sediments to infer a minimal change in P burial across the GOE; instead, sedimentary P levels slightly increased in the late Neoproterozoic.

Here we'll focus on a more exotic archive of P burial in the ancient ocean: phosphorites. Phosphorites are extremely P-rich sedimentary rocks (several wt. % P or higher) that form under unique environmental conditions. In contrast to P in shales, the temporal distribution of phosphorites resembles the history of atmospheric oxygen - but why? 

Mechanisms for muted Precambrian P burial

In order to thoroughly discuss the impact of the GOE on marine P cycling, we must first consider the proposed mechanisms for muted P burial in Precambrian marine sediments. There are essentially 3 schools of thought that offer mechanisms for low P burial:

(a) Fe scavenging
First proposed by Bjerrum & Canfield in 2002, this model invokes adsorption of P onto Fe oxides as a means of depleting the ocean of P. Recent work has expanded this idea to include the possibility of P adsorption to reduced or mixed-valence Fe minerals. 

(b) High organic burial efficiency (limited P recycling)
An alternative mechanism for P burial invokes high burial efficiency of organic matter (and associated P) due to a paucity of oxidants (i.e. oxygen, sulfate) in the Precambrian ocean (Kipp & Stüeken 2017). This idea has been expanded upon by Laakso et al. (2018; 2020).

(c) Low P burial efficiency (enhanced P recycling)
A third hypothesis (see Poulton 2017) is that - by analogy to modern reducing sediments - P would have been inefficiently buried under anoxic & sulfidic porewater/bottom-water conditions. Importantly, while leading to low sedimentary P levels, this model invokes high P levels in seawater (potentially sustaining high productivity).
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Here we examine the implications of the second model (high burial efficiency) for P cycling in the wake of the GOE

High burial efficiency & its effects on the P cycle

In the high burial efficiency (limited recycling) model, oxidant scarcity in the Archean inhibited the remineralization of organic matter. During the GOE, marine oxidant reservoirs are thought to have expanded. In particular, sulfate evaporites and sulfur isotopes suggest that marine sulfate concentrations rose substantially in the GOE. This would have greatly increased the capacity for organic matter oxidation, meaning that P could have been more efficiently recycled within the ocean. This would have led to a longer marine residence time and a higher steady-state P concentration in seawater, potentially helping to explain the onset of phosphorite deposition at this time. 

In this study we sought to test the hypothesis that growth of the marine sulfate reservoir facilitated phosphorite deposition in the Paleoproterozoic. To do so we conducted a geochemical study of one of Earth's oldest phosphorites. 

The 2.0 Ga Zaonega Formation, Karelia, Russia

In this study we focused on a suite of Paleoproterozoic sedimentary rocks deposited in the Onega Basin. In particular, we utilized drill cores (12AB and 13A shown below) recovered during the FAR-DEEP campaign to study chemostratigraphic trends in the Zaonega Formation (ZF), which contains organic-rich and P-rich mudstones, siltstones and dolostones. Geochronological constraints indicate that the ZF was deposited after the GOE, and a recent study of evaporites in the underlying Tulomozero Formation (TF) indicated that a large seawater sulfate reservoir existed at the time of deposition. We investigated the role of redox fluctuations (namely oxygen and sulfate availability) on sedimentary P enrichment using various geochemical proxies. 

P enrichment in the Zaonega Formation

Both cores (12AB and 13A) contain P-rich horizons, which were identified using P concentrations and P enrichment factors (calculated by normalizing to the detrital tracers Al and Ti). In the figure to the right, the white symbols denote dolostones & colored symbols denote siliciclastics; while detrital normalization leads to over-estimates of P enrichment in chemical sediments (such as dolostones), the inter-bedded mudstones and siltstones also show significant P enrichment relative to continental crust and typical marine sediments (red lines). 

Following the identification of P-rich intervals (denoted in this and subsequent figures with grey shading), our question became: what conditions enabled sedimentary P enrichment in these horizons?

Evidence for redox fluctuations

The P-rich horizons have elevated total organic carbon (TOC), total sulfur (TS) and redox-sensitive element (RSE) concentrations (here showing selenium for example) compared to non-P-rich horizons. Other work has shown that this sulfur exists predominantly as sulfide minerals, indicating greater rates of sulfate reduction at these times. Elevated sulfate reduction rates are consistent with a larger basinal sulfate reservoir and/or enhanced export production. 

Note: red shaded region marks a petrified asphalt spill onto the seafloor, which is characterized by a massive organo-siliceous rock (sometimes called "shungite"). We do not consider this interval representative of typical depositional conditions in the ZF. 

Fe enrichment indicates presence of sulfide

Additional evidence for sulfate reduction comes from Fe/Al ratios. In modern settings, Fe/Al ratios become enriched when hydrogen sulfide is present in sediment porewaters or bottom-waters (i.e. sulfidic conditions). In sulfidic regions of the Black Sea, this leads to Fe/Al values of 0.6-1.2, compared to 0.5 in typical marine sediments. 

In the ZF, the samples with Fe/Al values consistent with sulfidic porewaters/bottom-waters also have the highest P concentrations. This could suggest that P-rich intervals were also time of greater sulfate availability and thus elevated sulfate reduction (=sulfide production). 

Mo/U trend suggests weak restriction

Lastly, we consider an additional piece of environmental context: the degree of watermass exchange with the open ocean. Extensive evaporite deposits in the TF and positive sulfur isotope ratios in sulfides of the ZF suggest that the Onega Basin was moderately to severely restricted from the open ocean at times. Following work on Paleozoic black shales, use Mo/U co-variance to assess the degree of basinal restriction. We find that Mo/U enrichment indeed shows a pattern consistent with weak to strong basinal restriction.

P-rich intervals likely had high sulfate

Using the pieces of evidence outlined above, we can put together a consistent picture of environmental conditions and P enrichment during deposition of the ZF. We find that the P-rich horizons likely were deposited in times when sulfate was more abundant in the basin. If seawater sulfate concentrations indeed approached modern magnitude, this would implicate a similar increase in marine residence time to 1-10 Myr timescales. Such a long residence time makes it unlikely that these stratigraphic trends represent global fluctuations in seawater sulfate, since deposition of the ZF may have occurred in a few Myrs. Instead, we find it more likely that the sulfate-rich intervals resulted from enhanced communication with the open ocean. 

Thus, not only did expansion of the marine sulfate reservoir play an important role in enabling biomass recycling and sedimentary P enrichment; basinal hydrography also played an important role in regulating sulfate supply to the Onega Basin. 

Growth of the marine sulfate reservoir may have enabled phosphorite deposition after the GOE

Zooming out again to the secular trend of phosphorite deposition, we postulate that waxing and waning of the marine sulfate reservoir may have exerted a first-order control on phosphorite deposition across Earth history.

​Here we have presented evidence from a single Paleoproterozoic phosphorite; future studies of other phosphorites can help to confirm the role of sulfate in controlling sedimentary P enrichment. This applies not only to P cycling in the wake of the GOE, but also to phosphorite deposition in the late Proterozoic. 

Future work should aim to resolve the precise role of sulfate in enabling phosphogenesis, specifically by determining whether it was increased P recycling or proliferation of sulfide-oxidizing bacteria in sediments (or both) that enabled extensive phosphorite deposition.

Acknowledgements

Melanie Mesli, Timmu Kreitsmann, Brett Smith, Andy Schauer, Scott Kuehner, Bruce Nelson, Eva Stüeken

Funding

For more details, download our paper here: