Do red marine carbonates represent oxic environments? New understanding from the Upper Ordovician marine limestone in Tarim Basin, China

Abstract

Marine red beds (MRBs), typically colored by Fe- and Mn-rich minerals, are often interpreted as indicators of bottom water oxygenation. However, their continuous formation requires long-term input of aqueous derived Fe-Mn oxides and/or stable sources of Fe²⁺ and Mn²⁺ ions, challenging the traditional concept of “red equals oxic environment.” This study investigates two coeval Upper Ordovician Sandbian MRB carbonate intervals in the Tarim Basin. Previous studies attributed the pigment origin to hematite. Th/U values and total rare earth element and yttrium (REY) contents increased in the MRB interval, indicating a terrestrial source of iron. Hematite was observed within the intercrystal pores of calcite precipitating from porewater, consistent with high Ni_carb abundance and MREE-enriched bulge pattern in the red intervals. Both sections exhibit high Fe_carb and Mn_carb, pointing to substantial reductive dissolution of Fe and Mn oxides. Simulation shows that isotopic discrimination (Δ¹³C) between carbonate and organic carbon could be biased by increasing benthic Fe-Mn flux, leading to decreased Δ¹³C values in MRB intervals. Ce/Ce∗ values in the high-Mn_carb interval reflect the releasing of Ce by reductive dissolution of Mn oxides. The commonly used carbonate-based redox proxy I/(Ca + Mg) ratio is correlated positively with Mn_carb in the shallower section and with TOC in the deeper section, while it is negatively correlated with TOC in the shallower horizon, suggesting that iodine behavior may be influenced by adsorption and releasing of Mn hydro-oxides and organic matter besides oxygen contents. This study links MRB coloring to Fe and Mn mineral cycling in pore water through reductive dissolution and oxidative precipitation, and highlights potential biases of carbonate-based redox proxies that might be susceptible to other electron acceptors/donors such as Fe-Mn oxides and organic matter, in addition to free oxygen in seawater.