Extensive sea-floor oxygenation during the early Mesoproterozoic

Abstract

The Mesoproterozoic ocean was previously proposed to be characterized by low oxygen concentrations that may have resulted in a stasis in eukaryotic evolution. Recent studies reveal several pluses of oxygenation during this period that may have facilitated eukaryote evolution. However, few quantitative constraints on the global oxygenation events have hindered the comprehensive understanding of interactions between eukaryotes and environments. Here we present new molybdenum isotope (δ98/95Mo, simplified as δ98Mo) data coupled with sulfur and carbon isotope compositions and trace element contents from the carbonates and mudstones of the Gaoyuzhuang Formation (ca. 1.60–1.54 Ga) in two-well preserved sections in the North China Craton. The δ98Mo data from the two sections show similar trends in stratigraphic variation, with a prominent positive excursion in the Gaoyuzhuang Member III and peak values reaching up to 2.03‰ and 1.92‰, respectively. We developed a revised isotope mass balance model with high manganese (Mn) (hydr)oxide burial rate due to higher manganese concentration in the Mesoproterozoic oceans. Model calculation suggests that at least more than half of global seafloor (>54%) was covered with oxygenated seawater during the deposition of the Gaoyuzhuang Member III. Our estimation points to a significant oxygenation event in the early Mesoproterozoic with magnitude and extent much higher than previously thought. Further, a simple mass-balance calculation suggests that the atmospheric oxygen level was at least > 4% PAL (present atmospheric level) and likely up to 30% PAL at ca. 1.57 Ga. This oxygenation event, together with other environmental and/or developmental factors, may have facilitated a short and rapid evolution of eukaryotes in the early Mesoproterozoic.