美国加州理工学院Thiagarajan Nivedita课题组，利用碳酸盐团块同位素重建了显生宙气候。这一研究成果发表在2024年8月26日出版的国际学术期刊《美国科学院院刊》上。

据悉，海相沉积岩中的氧同位素比值¹⁸O/¹⁶O (δ¹⁸O_VSMOW值)从早古生代至近代增加了约8‰。对这种趋势的解释受到受碳酸盐的形成温度、海水的δ¹⁸O和沉积后成岩作用不确定性的阻碍。碳酸盐团块同位素测量作为一种温度指标，对这个问题提供了限制。在碳酸盐达到同位素平衡内部分布的情况下，该温度计受热力学控制的，并且与碳酸盐生长的水的δ¹⁸O无关；因此，它具有相对严格的化学物理基础，可以应用于水的δ¹⁸O未知的环境中。

该课题组将该技术应用于波罗的海盆地保存异常完好的奥陶纪碳酸盐岩记录，并提出了解释块状同位素结果和重建过去δ¹⁸O_seawater的框架。结果发现，奥陶纪海水的δ¹⁸O值比之前估计的要低，这突出了基于氧同位素重新评估气候记录的必要性，特别是当解释是基于δ¹⁸O_seawater或沉积或成岩温度假设基础上。研究结果强调，早古生代以来，海相沉积岩δ¹⁸O的长期升高是由δ¹⁸O_seawater的增加引起的。这种升高可能是由于整个地球水圈中高温和低温水-岩石相互作用比例的变化所驱动的。

附：英文原文

Title: Reconstruction of Phanerozoic climate using carbonate clumped isotopes and implications for the oxygen isotopic composition of seawater

Author: Thiagarajan, Nivedita, Lepland, Aivo, Ryb, Uri, Torsvik, Trond H., Ainsaar, Leho, Hints, Olle, Eiler, John

Issue&Volume: 2024-8-26

Abstract: The oxygen isotope ratio ¹⁸O/¹⁶O (expressed as a δ¹⁸O_VSMOW value) in marine sedimentary rocks has increased by ~8‰ from the early Paleozoic to modern times. Interpretation of this trend is hindered by ambiguities in the temperature of formation of the carbonate, the δ¹⁸Oseawater, and the effects of postdepositional diagenesis. Carbonate clumped isotope measurements, a temperature proxy, offer constraints on this problem. This thermometer is thermodynamically controlled in cases where carbonate achieves an equilibrium internal distribution of isotopes and is independent of the δ¹⁸O of the water from which the carbonate grew; therefore, it has a relatively rigorous chemical–physics foundation and can be applied to settings where the δ¹⁸O of the water is not known. We apply this technique to an exceptionally well-preserved Ordovician carbonate record from the Baltic Basin and present a framework for interpreting clumped isotope results and for reconstructing past δ¹⁸O_seawater. We find that the seawater in the Ordovician had lower δ¹⁸O_seawater values than previously estimated, highlighting the need to reassess climate records based on oxygen-isotopes, particularly where interpretations are based on assumptions regarding either the δ¹⁸Oseawater or the temperature of deposition or diagenesis. We argue that an increase in δ¹⁸O_seawater contributed to the long-term rise in the δ¹⁸O of marine sedimentary rocks since the early Paleozoic. This rise might have been driven by a change in the proportion of high- versus low-temperature water–rock interaction in the earth’s hydrosphere as a whole.

DOI: 10.1073/pnas.2400434121

Source: https://www.pnas.org/doi/abs/10.1073/pnas.2400434121