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, 116 (35), 17207-17212

A Productivity Collapse to End Earth's Great Oxidation

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A Productivity Collapse to End Earth's Great Oxidation

Malcolm S W Hodgskiss et al. Proc Natl Acad Sci U S A.

Abstract

It has been hypothesized that the overall size of-or efficiency of carbon export from-the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma). However, the timing, tempo, and trigger for this decrease remain poorly constrained. Here we test this hypothesis by studying the isotope geochemistry of sulfate minerals from the Belcher Group, in subarctic Canada. Using insights from sulfur and barium isotope measurements, combined with radiometric ages from bracketing strata, we infer that the sulfate minerals studied here record ambient sulfate in the immediate aftermath of the GOE (ca. 2,018 Ma). These sulfate minerals captured negative triple-oxygen isotope anomalies as low as ∼ -0.8‰. Such negative values occurring shortly after the GOE require a rapid reduction in primary productivity of >80%, although even larger reductions are plausible. Given that these data imply a collapse in primary productivity rather than export efficiency, the trigger for this shift in the Earth system must reflect a change in the availability of nutrients, such as phosphorus. Cumulatively, these data highlight that Earth's GOE is a tale of feast and famine: A geologically unprecedented reduction in the size of the biosphere occurred across the end-GOE transition.

Keywords: Great Oxidation Event; Proterozoic; nutrient limitation; primary productivity; triple-oxygen isotopes.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Location of the Belcher Islands, sampling site, and sulfate/evaporite occurrences. (A) Stratigraphic column of the Belcher Group, indicating the intervals at which gypsum pseudomorphs (gypsum pseudo.), halite casts, microbarite, and macrobarite were observed. Refer to SI Appendix, Fig. S1 for photographs of each type of occurrence. The Kasegalik Formation, deposited in a sabkha environment, was host to the largest amount of evaporite casts and SO4 minerals observed in the Belcher Group. Higher in the stratigraphy, the McLeary Formation contains 1 stratigraphic horizon with a small amount of gypsum pseudomorphs. Finally, the Costello Formation contains centimeter-scale barite crystals along a single stratigraphic horizon on eastern Tukarak Island. (B) Map of North America with Belcher Islands in the black box. (C) Map of Belcher Islands. Macrobarites are indicated by a red circle. (D) Macrobarites, with Canadian penny for scale.
Fig. 2.
Fig. 2.
Geochemical and GPP model results. (A) Triple-oxygen isotope results from the Costello Formation macrobarites (red circles) together with results from previously reported values (gray circles) that span the GOE and mid-Proterozoic (ref. and references therein and ref. 46). (B) Box and whisker plots for low and high pO2 scenarios during the GOE and low pO2 post-GOE, showing the decline in GPP across the end-GOE. GPP fell dramatically across the end-GOE transition, even if pO2 levels remained constant, and remained at very low levels through the mid-Proterozoic. Bottoms and tops of boxes correspond to 25th and 75th percentiles, respectively. Lower and upper whiskers correspond to 2.5th and 97.5th percentiles, respectively. The solid horizontal lines indicate medians, and dashed horizontal lines indicate means. Dotted vertical lines represent potential SO4 ages based on different genetic models of barite formation.
Fig. 3.
Fig. 3.
Sulfur and barium isotope chemistry of Costello Formation macrobarites. (A) A δ34S-Δ33S cross-plot shows that Costello Formation macrobarites are consistently depleted in Δ33S relative to modern seawater, while they overlap with modern values in δ34S. δ34S measurements also overlap with CAS measurements of the Kasegalik Formation (gray field; ref. 42). A comparison of syn- to post-GOE sulfate minerals shows that post-GOE samples are comparatively depleted in Δ33S, possibly due to a change in sulfur inputs rather than microbial sulfur recycling. (B) δ138/134Ba measurements of Costello Formation macrobarites compared with potential barite sources and post-Marinoan barites (38). The blue bar represents the typical analytical uncertainty on individual measurements. The range of values and small variance suggest a marine source of Ba for the Costello Formation macrobarites.

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