Manganese-rich deposits in the Mesoproterozoic Gaoyuzhuang Formation (ca. 1.58 Ga), North China Platform: Genesis and paleoenvironmental implications

Abstract

The redox state of the surface ocean–atmosphere system has long been considered to have been remarkably stable during the mid-Proterozoic (ca. 1800–800 Ma); however, there is a growing body of evidence that supports dynamic redox fluctuations during this time. Sedimentary Mn enrichment in the ca. 1.58 Ga Gaoyuzhuang Formation (North China) may represent an important archive of mid-Proterozoic paleoenvironmental conditions. We present a sedimentological and geochemical study of the Mn-rich carbonate succession in the Gaoyuzhuang Formation in order to further elucidate the genesis and paleoredox implications of this Mn-rich deposit. We suggest that the Mn-rich interval was deposited in deep subtidal environments close to storm wave-base. Manganese occurs either as millimeter-sized Mn-oxide (pyrolusite) nodules or as Mn-rich dolomite, which features moderately positive I/(Ca+Mg) values (0.5–1.7 μmol/mol) and Ce anomalies (1.1–1.2). In contrast, the underlying and overlying carbonates that lack Mn-oxides were deposited in a shallow subtidal environment above fair-weather wave-base. They have relatively low I/(Ca+Mg) values (<0.5 μmol/mol) and negligible Ce anomalies (average Ce/Ce* = 1.05 ± 0.05). Thus, we suggest that during the deposition of the highly Mn-enriched strata shallow seawaters were sufficiently oxygenated to drive active redox cycling of Mn, and that this interval was bracketed by less-oxygenated shallow marine conditions. Given the widespread distribution of the Mn-rich deposits across the North China platform, we argue that the Mn-rich deposits may have recorded a basin-scale, pulsed oxygenation. The association of this interval with enigmatic macrofossils in the overlying strata of the upper Gaoyuzhuang Formation warrants further investigation of the potential link between pulsed episodes of oxygenation and evolutionary advances in the early Mesoproterozoic.