Rau G H, Takahashi T, Des Marais D J
Institute of Marine Sciences, University of California, Santa Cruz 95064, USA.
Nature. 1989 Oct 12;341(6242):516-8. doi: 10.1038/341516a0.
The stable-carbon isotopic composition of marine organic material has varied significantly over geological time, and reflects significant excursions in the isotopic fractionation associated with the uptake of carbon by marine biota. For example, low 13C/12C in Cretaceous sediments has been attributed to elevated atmospheric (and hence oceanic) CO2 partial pressures. A similar depletion in 13C present-day Antarctic plankton has also been ascribed to high CO2 availability. We report, however, that this high-latitude isotope depletion develops at CO2 partial pressures (pCO2 levels) that are often below that of the present atmosphere (340 microatmospheres) , and usually below that of equatorial upwelling systems (> 340 microatmospheres). Nevertheless, because of the much lower water temperatures and, hence, greater CO2 solubility at high latitude, the preceding pCO2 measurements translate into Antarctic surface-water CO2 (aq) concentrations that are as much as 2.5 times higher than in equatorial waters. We calculate that an oceanic pCO2 level of > 800 microatmospheres (over twice the present atmospheric pCO2) in a warmer low-latitude Cretaceous ocean would have been required to produce the plankton 13C depletion preserved in Cretaceous sediments.
海洋有机物质的稳定碳同位素组成在地质时期发生了显著变化，反映出与海洋生物群吸收碳相关的同位素分馏的显著偏移。例如，白垩纪沉积物中较低的13C/12C被归因于大气（进而海洋）二氧化碳分压的升高。当今南极浮游生物中13C的类似贫化也被归因于高二氧化碳可用性。然而，我们报告称，这种高纬度同位素贫化是在二氧化碳分压（pCO2水平）通常低于当前大气（340微大气压）且通常低于赤道上升流系统（>340微大气压）的情况下发生的。尽管如此，由于高纬度地区水温低得多，因此二氧化碳溶解度更大，之前的pCO2测量结果转化为南极表层水二氧化碳（aq）浓度，比赤道水域高出多达2.5倍。我们计算得出，在温暖的低纬度白垩纪海洋中，要产生保存在白垩纪沉积物中的浮游生物13C贫化，海洋pCO2水平需要>800微大气压（超过当前大气pCO2的两倍）。
