A possibility of 18O-depleted oceans in the Precambrian inferred from triple oxygen isotope of shales and oceanic crust

Oxygen-18/16 ratio recorded in geological media has been utilized to reconstruct surface temperature and discuss associated climatological forcing through Earth’s history. The accuracy of the estimated paleo-temperature through the application of the δ18O record, however, is limited by our understanding of the oceanic 18O/16O ratio in the past. As a longstanding issue, sedimentary rocks in the Precambrian, which are generally depleted in 18O relative to the modern sediments, can be interpreted either to have reflected higher surface temperature when assuming relatively constant oceanic δ18O values, or more 18O-depleted oceans relative to the present day with limited global temperature (climate) deviations. Additional use of a third isotope, 17O, can help constrain the isotopic composition of the porewater during the formation of a rock record as it provides an additional fractionation equation within the same isotopic system. This study utilizes reactive-transport models of triple oxygen isotope (16O, 17O, 18O) for continental weathering and hydrothermal alteration of oceanic crust, built upon the δ18O modeling framework developed by Kanzaki, 2020a, Kanzaki, 2020b. Comprehensive oxygen isotope exchanges in dynamically moving media, both rocks and porewaters, are realized in these models, reflecting spatially-variable and process-dependent alteration conditions (e.g., spreading rate of midocean ridges, uplift rate of continents, porewater advection rate, and isotope exchange kinetics). The models were confirmed to be able to reproduce the signatures of triple oxygen isotope in Phanerozoic shales and oceanic crust. The validated models were then applied to Precambrian shales, which suggests that the Precambrian oceans could have been depleted in 18O, e.g., as low as −12‰ at >2.5 Ga and − 4‰ around 0.5 Ga, relative to the present-day seawater. General absence of positive and near-zero Δ′17O signatures in oceanic crust also suggests that the buffering of oceanic 18O through hydrothermal alteration of the oceanic crust could have been weak. The 18O flux from continental weathering thus could have played a significant role in controlling the 18O/16O ratio of ancient sea and together with climatic forcing could have enabled a secular transition of oceanic δ18O during the Precambrian.