Mesoproterozoic oxygenation event: From shallow marine to atmosphere

Abstract

Oxygen availability is crucial for the evolution of eukaryotes in geological history, but the redox conditions of the mid-Proterozoic atmospheric-oceanic system remain vigorously debated. Previous geochemical studies suggested a very low mid-Proterozoic atmospheric oxygen level of probably below 0.1−1% of the present atmospheric level (PAL) with weakly oxygenated surface ocean, which may have suppressed the early evolution of eukaryotes in the ocean. A recent study, however, revealed a pulsed oxygenation event that was recorded in shallow marine carbonate of the middle Gaoyuzhuang Formation (at ca. 1.57 Ga, North China) with atmospheric pO2 of up to ≥4% PAL based on iodine speciation. This event is associated with the earliest known multi-cellular eukaryotic fossils. To elucidate whether this pulsed oxygenation was limited to shallow local seawaters or could also reflect an increase in atmospheric oxygen, chromium (Cr), and carbonate-associated sulfate (CAS), sulfur isotopes in the middle Gaoyuzhuang Formation were studied in two sections of the North China Platform. The results show a positive shift in authigenic Cr isotope from −0.18‰ to +0.66‰, which was followed by a decline to −0.16‰. This suggests a rise in atmospheric oxygen sufficient to initiate oxic Cr cycling and isotope fractionation (pO2 > 0.1%−1% PAL). This positive Cr isotope excursion was associated with a transient positive shift in δ34SCAS from ∼20‰ to ∼50‰ and a subsequent decline to ∼10‰, which provides independent evidence of atmospheric oxygenation. This oxygenation may have enhanced oxidative terrestrial weathering and increased sulfate input to the ocean, thus stimulating bacterial sulfate reduction and pyrite burial in deep anoxic seawaters. This is broadly consistent with previous results of carbonate I/(Ca+Mg) and Ce anomaly but also reveals a lag between shallow seawater and atmospheric oxygenation. Fluctuated redox conditions and decoupled oxygenation between shallow seawater and atmosphere during the mid-Proterozoic may help to account for the current debate.

Enhanced Weathering Triggered the Transient Oxygenation Event at ∼1.57 Ga

“…Major elements (Fe, Al, P), total organic carbon (TOC), total sulfur (TS), ε Nd (t), Hg concentration and Hg isotopes are Wang et al, 1985). (c) Simplified geological map of the study area, showing locality of the studied Gan'gou section in Yanqing County, north of Beijing (modified from Xie et al, 2022).…”

“…(b) A generalized Mesoproterozoic paleogeographic map of the central North China Platform (modified after Wang et al., 1985). (c) Simplified geological map of the study area, showing locality of the studied Gan'gou section in Yanqing County, north of Beijing (modified from Xie et al., 2022).…”

“…The ∼1.57 Ga oxygenation event is characterized by high I/(Ca + Mg) values, positive Cr isotopes coupled with a negative carbon isotope (δ 13 C) excursion that was interpreted as the result of dissolved organic carbon (DOC) oxidation in response to increased oxygen (Shang et al., 2019; Xie et al., 2022). Although this oxygenation event is currently only documented from North China, the high I/(Ca + Mg) values up to 3.5 μmol/mol (Shang et al., 2019) and positive ẟ 53 Cr values up to 0.66‰ (Xie et al., 2022) from multiple shallow‐marine, carbonate‐rich sections require an increase of atmospheric O 2 , which implies its global significance. Closely associated with this oxygenation event is the occurrence of decimeter‐scale multicellular eukaryotes, suggesting a possible causal relationship between oxygenation and early evolution of eukaryotes (Shang et al., 2019; K. Zhang et al., 2018; Zhu et al., 2016).…”

“…One of the intriguing phenomena for the ∼1.57 Ga oxygenation event is its transient nature: geochemical proxies indicative of oxygenation quickly return back to mid‐Proterozoic background values (Shang et al., 2019; Xie et al., 2022). In spite of the general consensus that enhanced organic carbon/pyrite burial is the most efficient way for oxygen increase in Earth's surface environment (Berner, 2003), the cause(s) for such transient oxygenation events in the mid‐Proterozoic remain uncertain.…”

“…The rest of data are all from this study. Some δ 53 Cr auth data from the Jixian Section were also plotted in this figure to show the secular variation (cf Xie et al, 2022)…”