Persistent oxygenation of the Mesoproterozoic shallow ocean: evidence from the <i>c.</i> 1.44-billion-year-old Tieling Formation, North China Craton

The Mesoproterozoic era experienced dynamic variability in atmosphere–ocean oxygen levels. Recent studies suggest that a pulse of oxygenation is recorded in the c. 1.44 Ga Tieling Formation but the duration of this event remains unclear. Here, we report relatively high-resolution carbonate carbon isotope ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> 13 C), rare earth element (REE) and I/(Ca + Mg) data from the Tieling Formation at Jixian, North China. The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> 13 C values reveal a limited range, mainly between −2 and +1‰, and systematic stratigraphic trends. The lower member of the formation is characterized by manganiferous carbonates with light and middle REE-enriched patterns and relatively high but scattered I/(Ca + Mg) values. We interpret these geochemical signals in terms of the early diagenetic reduction of primary Mn oxides. The upper member, by contrast, exhibits stratigraphic covariation between negative Ce anomalies and I/(Ca + Mg) ratios that we attribute to sea-level change rather than oxygenation. The geochemical data from both lower and upper members support vigorous oxidative Mn cycling, thus suggesting that an oxygenated shallow ocean probably persisted for millions of years in the Mesoproterozoic.