A Pulsed Oxygenation in Terminal Paleoproterozoic Ocean: Evidence From the Transition Between the Chuanlinggou and Tuanshanzi Formations, North China

Abstract

AbstractThe mid‐Proterozoic (ca. 1.8–0.8 Ga) witnessed a period of stable carbon cycling and long stasis in eukaryotic evolution, which was commonly ascribed to the persistently low oxygen levels in the atmosphere‐ocean system. Recently, several pulsed marine oxygenations were identified from different continents, and presumed to have facilitated the short‐term diversification of eukaryotes in the early Mesoproterozoic Era. To test this hypothesis, an integrated study of iodine species [I/ (Ca + Mg)], paired carbon isotopes (δ13Ccarb and δ13Corg), and elemental abundances was conducted on the fossil‐bearing Tuanshanzi Formation (∼1.64–1.63 Ga), North China. The near‐zero I/(Ca + Mg) ratios (0.00 ± 0.01 μmol/mol) coupled with low δ13Corg (−30.2 ± 1.4‰) in the strata below the fossil‐bearing interval suggests anoxic conditions in shallow seawater. An increase of I/ (Ca + Mg) up to ∼1.54 μmol/mol coupled with a ∼2‰ negative shift in δ13Ccarb and a ∼11‰ positive excursion in δ13Corg in the fossil‐bearing interval point to an increased oxygen concentration. Above the fossil‐bearing interval, I/ (Ca + Mg) decrease to <0.5 μmol/mol, suggesting deoxygenation and a return to anoxic to suboxic conditions. Combined with relevant data from other mid‐Proterozoic sections of North China and Australia, these data likely indicate dynamic redox conditions in the shallow seawaters of this period, with multiple pulses in oxygenation in otherwise anoxic conditions. These pulsed oxygenations may have facilitated short‐term proliferation and diversification of early eukaryotes in shallow seawaters, but the overall low oxygen levels have impeded their continuous development and the rise to ecological dominance until late Neoproterozoic.