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

• 成果类型: Article
• 署名作者: Thiagarajan, Nivedita; Lepland, Aivo; Ryb, Uri; Torsvik, Trond H.; Ainsaar, Leho; Hints, Olle; Eiler, John
• 署名单位: California Institute of Technology; Geological Survey of Norway; University of Tartu; Tallinn University of Technology; Hebrew University of Jerusalem; University of Oslo
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-14886
• DOI: 10.1073/pnas.2400434121
• 发表日期: 2024-09-03
• 关键词: geological water cycle; oceanic-crust; EVOLUTION; temperatures; hydrogen; records

摘要

The oxygen isotope ratio O-18/O-16 (expressed as a delta O-18(VSMOW) value) in marine sedimentary rocks has increased by similar to 8 parts per thousand from the early Paleozoic to modern times. Interpretation of this trend is hindered by ambiguities in the temperature of formation of the carbonate, the delta O-18(seawater), 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 delta O-18 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 delta O-18 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 delta O-18(seawater). We find that the seawater in the Ordovician had lower delta O-18(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 delta O-18(seawater) or the temperature of deposition or diagenesis. We argue that an increase in delta O-18(seawater) contributed to the long-term rise in the delta O-18 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.