Carbon Dioxide Content of Mesozoic and Cenozoic Atmospheres using Paleosols

新生代（8 ~ 56 Myr）古土壤碳酸盐碳同位素组成与中生代古土壤碳酸盐碳同位素组成差异显著，分别为- 10 ~ - 12 permil（118个样品）和-6 ~ -8 permil（21个样品）。在这个初步数据集中，只有一个数据点是重叠的。考虑到造成这种明显差异的可能原因，就会得出大气中二氧化碳浓度可能存在的差异。如果这是真的，它可能会导致一种估算大气中P（CO2）至少在200 Ma以前的方法，极大地扩展了我们对古大气化学的认识。到目前为止，对古co2的直接测量只能追溯到0.16 Ma （Barnola, et, 1987）。土壤碳酸盐的同位素组成记录了土壤CO2的同位素组成，而土壤CO2的同位素组成与大气中CO2的浓度直接相关。对于现代土壤，大气对土壤总二氧化碳的贡献几乎可以忽略不计。然而，较高的大气P（CO2）水平使土壤CO2的同位素组成发生显著差异，因此，土壤碳酸盐与土壤CO2形成平衡。本文研究了中生代和新生代古土壤碳酸盐的同位素组成，目的是测定古大气的P（CO2）。初步结果表明，这是一种可行的方法，中生代的二氧化碳水平明显高于今天，约为1500至2500 ppmV。

The carbon isotopic composition of Cenozoic (8 to 56 Myr) paleosol carbonates is significantly different than Mesozoic paleosol carbonates, having C values of about - 10 to - 12 permil (118 samples) and -6 to -8 permil (21 samples), respectively. In this preliminary data set there is only one data point that overlaps. Consideration of the possible causes for this distinct differences leads to a possible difference in the concentration of atmospheric CO2 . If this is true it could lead to a method to estimate P(CO2) in the atmosphere to at least 200 Ma ago, greatly extending our knowledge of paleoatmospheric chemistry. So far, direct measurements of paleo-CO2 measurements go back only 0.16 Ma (Barnola, et al., 1987). The isotopic composition of soil carbonate records the isotopic composition of soil CO2 which is directly related to the concentration of CO2 in the atmosphere. For modern soils the atmospheric contribution to total soil CO2 is almost negligible. However, higher atmospheric P(CO2) levels make a significant difference in the isotopic composition of soil CO2 and hence, soil carbonate formed in equilibrium with soil CO2. In this study we examine the isotopic composition of paleosol carbonates from the Mesozoic and Cenozoic with the aim of determining P(CO2) of the ancient atmosphere. Preliminary results suggest that this is a viable approach and that Mesozoic CO2 levels were significantly higher than today, on the order of 1500 to 2500 ppmV.