Abnormal positive δ~(13)C values of carbonate in Lake Caohai, southwest China, and their possible relation to lower temperature

摘要

Carbon source inputs, CO_2 exchange between atmosphere and lake water, as well as lacustrine productivity are commonly served as the main controls on δ~(13)C values of authigenic carbonates in lake sediments. Generally, δ~(13)C values of recent carbonates in most lakes are lower than +5‰ (all values reported here with respect to the PDB standard), and rather few reach up to +13‰. Extreme positive δ~(13)C values up to +23.10‰ are reported in authigenic carbonate in Lake Caohai, southwest China, and some δ~(13)C values are the most positive values in the previously reported surface water carbonate δ~(13)C values in lakes. Simultaneously, there are two intervals of large carbonate ~(13)C enrichment during the past 500 years, namely 1670-1714 AD and 1788-1881 AD. On the basis of combination with the oxygen isotopic composition of carbonate and Drought/Flood index proxies (D/F), carbon source inputs, CO_2 exchange between atmosphere and lake water can not cause this extreme carbonate ~(13)C enrichment. In addition, the inconsistent between organic matter content and δ~(13)C values of carbonate, lower organic matter δ~(13)C values, as well as the weakegative correlation between δ~(13)C values of organic matter and carbonate during these two intervals confirmed that lacustrine productivity is also not the dominant factor that controls abnormal positive carbonate δ~(13)C values. Temperature variations in Lake Caohai during the past 500 years were reconstructed from co-analysis of δ~(15)O values of carbonate and organic matter cellulose, and periods of abnormal positive δ~(13)C values of carbonate correspond well with the lower temperature periods. The 1670-1714 AD and 1788-1881 AD cold periods were synchronous with the coldest intervals of Little Ice Age (LIA), and also consistent with the Maunder and Dalton sunspot minimum, respectively. Considering these geochemical data together, although there is no direct effect between temperature and carbonate δ~(13)C values, the lower temperature restrains the degradation of organic matter, and bacteria have the chance to participate the carbon isotopic fractionation of organic matter, thus generating the methane (CH4) gas, which has the extreme lower δ~(13)C values up to -60‰, resulting in the extreme enrichment ~(13)C in carbonates due to the preferential synthesis of ~(12)CH_4. The important inputs of bacteria to sedimentary sequence during the periods of 1670-1714 AD and 1788-1881 AD have been further verified by the carbon isotopic fractionation between carbonate and organic matter. Therefore, this mechanism may be responsible for abnormal positive δ~(13)C values in Lake Caohai. Abnormal positive δ~(13)C values of carbonate in Lake Caohai may be indirectly ascribed to the lower temperature, and more attention should be paid to the carbon isotopic composition of carbonates in Lake Caohai in future research. Particular factors affecting carbonate carbon isotopic composition should be taken into account in order to avoid misinterpreting palaeoclimatic reconstructions.