Oxygen isotope application in paleotemperature reconstruction and water cycle in the deep time

Suitable temperature is the most important environmental parameter for the habitability of the Earth. The oxygen isotope composition of carbonate and phosphate fossils is the most widely used technique for reconstructing deep-time seawater temperatures. However, this technique is subject to uncertainties including diagenetic alteration, the vital isotope fractionation effects, regional variations in seawater oxygen isotope composition, and whether the latter changes with time in the geological past. In this contribution, we evaluated these constraints and solutions for these uncertainties. Our evaluation shows the rigorously screened oxygen isotope data provide the most continuous and reliable paleotemperature record of the Phanerozoic, generally consistent with climate changes indicated by other proxies. Lower temperatures are reconstructed from glaciated intervals (Late Ordovician, Carboniferous-Early Permian, Cenozoic) andhigher temperaturescorrespond to greenhouse intervals (Early Triassic and Middle Cretaceous). However, accurate calculation of absolute temperature is still a challenge due to the intensive dispute over constant or changing δ18O of the hydrosphere. We review and discuss the progress and remaining problems in using oxygen isotopes in deep time to understand surface and deep-water cycling. Because of the nonlinear dependence of water vapor pressure on temperature, it is generally assumed that an increase in temperature enhances the surface water cycle and precipitation. However, the exact mechanism and magnitude of the hydrological cycle response to temperature changes, as well as its spatiotemporal characteristics, are still unclear for deep time. A systematical evaluation of this relationship since the Neogene revealed that the hydrological cycle intensifies during warm climatic conditions, but the response of the hydrologic cycle to climate is not globally uniform and exhibits a marked spatial difference. This pattern is observed as well for older times, e.g., during the Carnian Humid Episode (Late Triassic), the increase in seawater temperature corresponds to a significant dry-wet transition accompanied by negative δ13Ccarb shift and ocean anoxia, suggesting that climate warming led to the intensification of hydrological cycle and deterioration of environmental conditions, which in turn led to a decline in diversity. A similar phenomenon has been noted at the Permian-Triassic boundary. Because of the lack of suitable proxy data to monitor the deep time surface water cycle, it is still unclear whether the extreme climate changes in Phanerozoic are necessarily accompanied by drastic changes in water cycle. On a larger space-time scale, the Earth’s water cycle involves deep processes within the Earth’s interior. The water cycle from the surface to the mantle is a net inflow process; water can reduce the viscosity of asthenosphere, which is a key factor in initiating lithospheric plate tectonics. Sea level lowering caused by the continuous subduction of surface water into the mantle can contribute to continental crust rising above sea level, leading to surface denudation, and weathering, and the initiation of the terrestrial water cycle, eventually leading to a reduction in atmospheric CO2, biogeochemical cycle reorganization, and atmospheric oxidation. The deep-water cycle and its associated plate tectonic processes thus play an important role in shaping the habitability of the Earth. Oxygen is the most abundant element on Earth, and the oxygen exchange during the deep-water cycle is an effective means of tracing the interaction between the lithosphere and hydrosphere, while the oxygen isotope records from different spheres, as well as their coupled or uncoupled variation, provide important clues to the processes of matter and energy cycling between these spheres, as well as their potential causal links with major geological events at the surface, such as major oxidation events, the terrestrialization of plants, and the origin of life.