Upper Ordovician marine red limestones, Tarim Basin, NW China: A product of an oxygenated deep ocean and changing climate?

摘要

Marine red beds (MRB) have been commonly interpreted as products of bottom water oxygenation (or ventilation) and coeval perturbations of the global carbon cycle during greenhouse climates. The Sandbian (Upper Ordovician) successions at Dawangou and Nanyigou in NW Tarim, China are characterized by deep-shelf nodular red limestones. Using ultraviolet-visible-near infrared (UV-VIS-NIR) diffuse reflectance spectroscopy (DRS), field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray spectrometer (EDS), and paired inorganic and organic carbon isotope systematics, this study attempts to reveal the origin of these MRBs and coeval carbon cycling during deposition. Both the DRS and FESEM results consistently show that these red limestones are exclusively impregnated with amorphous to poorly crystalline, submicron ferric oxides (mostly hematite), constituting the dominant components of the red pigment. Hematite aggregates (or lumps) occur either in the calcite crystal interstices and pores or within the lattice structure of calcite crystals, implicating early diagenetic or syndepositional origins. Paired organic and inorganic carbon isotope data through the MRB successions demonstrate a time-specific Sandbian positive carbon isotope plateau (similar to 1 to 2%o VPDB) in both fractions as reported elsewhere around the world. This may have intitially resulted from enhanced burial of organic matter, then a drawdown of atmospheric pCO(2) levels, and subsequent climate cooling. This scenario agrees with the decreasing isotope difference (Delta C-13 = delta C-13(carb )- delta C-13(org); similar to 26 parts per thousand VPDB on average) in the Sandbian MRBs relative to the underlying strata (similar to 29 parts per thousand VPDB) from both sections. A cooling climate could have accelerated oceanic ventilation rates and led to increased oxygenation of deep-shelf water masses, eventually facilitating the widespread deposition of MRBs near continental margins. The persistence of a cool climate and the expansion of oxygenated deep-shelf water masses onto the shelf that initiated during the Darriwilian may have stimulated the rapid biodiversification of marine organisms during the Sandbian.