Redox-stratified seawater during the GOE: Evidences from rare earth elemental and C-O isotopic compositions of Paleoproterozoic BIF and carbonate rocks from the Taihua Group, North China Craton

Abstract

The Paleoproterozoic Great Oxidation Event (GOE) is known for the dramatic change in atmospheric composition that led to oxygen becoming a persistent component of global surface processes. Drastic environmental changes greatly affected ocean and atmosphere chemistry. However, the distribution of dissolved oxygen in the ocean during the GOE is still unclear as rare contemporary sediment-records have been found. In order to identify the redox state of both shallow and deep water, and reveal the ocean redox structure during this time, we present new major and trace elemental and C-O isotopic data of bulk samples of the banded iron formation (BIF), dolomitic marble, and graphite-bearing marble of the early Paleoproterozoic upper Taihua Group in the Lushan area, southern margin of the North China Craton (NCC). The BIF samples exhibit typical major and trace element characteristics akin to very pure marine chemical sediments, such as the shale-normalized positive La, Ga, and Y anomalies, indicative of a marine depositional setting. In these BIF samples, both negative Ce anomalies and the lack of Ce anomalies reflect the presence of both oxic and suboxic/anoxic water in the ocean. Samples of the dolomitic marble are characterized by extremely low amounts of crustal material (such as Al, Ti, Zr, Hf) and high Y/Ho ratios (46–66), also implying a pure marine chemical origin. Excluding some contaminated samples, the rest of the graphite-bearing marble samples display similar features of the dolomitic marble. Significant positive Ce anomalies and high light to heavy REE (Pr/Yb(SN)) ratios in both dolomitic marble and graphite-bearing marble are essentially consistent with the 2.2–2.1 Ga carbonates from the Guanmenshan Formation of the Liaohe Group in the NE Sino-Korean Craton. Combined with the Ce anomalies in the BIF samples, we consider that the seawater when the upper Taihua Group deposited was redox-stratified from oxic shallow water to deeper anoxic water. Besides, the graphite-bearing marble record coeval surface water in the column that was enriched in nutritive materials and beneficial to microbiology and high productivity. Moreover, the early Paleoproterozoic IFs in the Songshan Group and the Lüliang Group in the NCC also exhibit significant both positive and negative Ce anomalies and enrichment of redox sensitive elements. These evidences, therefore, suggest that redox-stratified seawater occurred extensively during the GOE.