Abstract
Carbonate precipitation is a major process in the global carbon cycle. It was recently proposed that authigenic carbonate (carbonate precipitated in situ at the sediment–water interface and/or within the sediment) played a major role in the carbon cycle throughout Earth’s history. The carbon isotopic composition of authigenic carbonates in ancient oceans have been assumed to be significantly lower than that of dissolved inorganic carbon (DIC) in seawater, as is observed in the modern oceans. However, the δ13Ccarb values of authigenic carbonates in the past has not been analyzed in detail. Here, we report authigenic carbonates in the uppermost Guadalupian (Middle Permian) rocks at Chaotian, Sichuan, South China. Monocrystalline calcite crystals <20 mm long are common in the black mudstone/chert sequence that was deposited on a relatively deep anoxic slope/basin along the continental margin. Textures of the crystals indicate in situ precipitation on the seafloor and/or within the sediments. The calcite precipitation corresponds stratigraphically with denitrification and sulfate reduction in the anoxic deep-water mass, as indicated by previously reported nitrogen and sulfur isotope records, respectively. Relatively high δ13Ccarb values of the authigenic carbonates (largely −1 ‰) compared with those of organic matter in the rocks (ca. −26 ‰) suggest that the main carbon source of the carbonates was DIC in the water column. The calcite crystals precipitated in an open system with respect to carbonate, possibly near the sediment–water interface rather than deep within the sediments. The δ13Ccarb values of the carbonates were close to the δ13CDIC value of seawater due to mixing of 13C-depleted remineralized organic carbon (that was released into the water column by the water-mass anaerobic respiration) with the large DIC pool in the oceans. Our results imply that δ13Ccarb values of authigenic carbonates in the anoxic oceans might have been systematically different from the values in the oxic oceans in Earth’s history, controlled by the depth of the redoxcline in the water column and sediments. If our model is correct, authigenic carbonates with relatively high δ13Ccarb values in the ancient anoxic oceans may have had a less substantial influence on the bulk δ13Ccarb values in geologic records than has been previously suggested.
Highlights
Carbonate precipitation is a major process in the global carbon cycle and is thought to have been closely related to climate changes throughout Earth’s history (e.g., Walker et al 1981; Kasting 1993; Ridgwell and Zeebe 2005)
The δ13CDIC value in the deep water is lower than the value of the surface oceans as previously suggested at Chaotian (Saitoh et al 2013b), our results demonstrate that the δ13CDIC value in the deep-water mass was not decreased remarkably by the addition of remineralized carbon released from the suspended organic matter by the watermass anaerobic respiration
Our results suggest that the forms of authigenic carbonate precipitation in the oxic and anoxic oceans in the past may have been substantially different
Summary
Carbonate precipitation is a major process in the global carbon cycle and is thought to have been closely related to climate changes throughout Earth’s history (e.g., Walker et al 1981; Kasting 1993; Ridgwell and Zeebe 2005). In a general model of the global carbon cycle, the input of carbon to the oceans is balanced by its output via two major pathways, normal marine carbonate precipitation and organic carbon burial (Shackleton and Hall 1984). Biotic and/or abiotic carbonate precipitation of dissolved inorganic carbon (DIC) in the oceans exhibits little carbon isotope fractionation, whereas organic matter generated by biological carbon fixation is depleted in 13C relative to seawater DIC. On the basis of a compilation of δ13Ccarb values in geologic records from numerous previous studies, carbonate precipitation is suggested to have accounted for up to 80 % of total carbon removal from the ocean during most of Earth’s history (Hayes et al 1999; Shields and Veizer 2002)
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