High molybdenum abundance in the ~2 Ga Zaonega Formation: Implications for seawater following the Lomagundi excursion

Abstract

The Paleoproterozoic era is characterized by fundamental shifts in the redox conditions of Earth’s surface environments. Following the rise of atmospheric O2 at 2.4 Ga, the 2.3–2.0 Ga Lomagundi positive carbon isotope excursion (LIE) has been suggested to reflect atmospheric O2 approaching modern levels [1]. It has been inferred that the waning stage of the LIE was accompanied by a substantial decline in atmospheric O2. The sparse Paleoproterozoic rock record, however, complicates the interpretation of paleoredox proxies. Redox-sensitive elements (RSE; Mo, V, Re, U, and Cr) are frequently used to infer atmospheric and water column redox conditions. Typically, RSE are concentrated in oxygenated waters and sequestered into anoxic sediments, with their abundances in sedimentary successions governed by the global extent of anoxic sinks at deposition. Compilations of sedimentary rock records have demonstrated variations in RSE abundance with low concentrations in the Archean, increasing in the Paleoproterozoic before declining in the Mesoproterozoic [e.g., 2]. The 2.0 Ga Zaonega Formation (ZF) in NW-Russia is a well-preserved volcano-sedimentary succession composed of interlayered organic-rich (10-70 wt.%) mudstones, carbonates and magmatic rocks deposited shortly after the termination of the LIE. These organic-rich rocks are key for understanding the architecture of late Paleoproterozoic redox conditions. Here we report RSE enrichment and isotope data from recent upper ZF drill cores. RSE concentrations are among the highest reported for the Precambrian (Mo ¿ 2000 ppm, V ¿ 8000 ppm, Re ¿ 1.5 ppm), implying oxidative continental weathering, a limited extent for seafloor anoxia, and potentially elevated seawater sulphate compared what was previously suggested at 2 Ga [2]. Our results suggest a significant time-gap between the LIE termination and onset of Mesoproterozoic low-O2 conditions.