Triple oxygen isotope investigation of fine-grained sediments from major world's rivers: Insights into weathering processes and global fluxes into the hydrosphere

Continental weathering is accompanied by formation of clays and other secondary minerals and their delta O-18 and Delta O-17 values should hence reflect to some extent signatures of meteoric water (delta O-18(MW)) and mean annual temperatures (MAT). Our ability to extract climate information from weathered products across the geologic history relies on analytical methods tested and calibrated against modern climate conditions. We here present triple-oxygen isotope analyses of clay-size sediments from 45 rivers worldwide, as well as delta O-18 analyses of corresponding silt- and sand-rich detrital fractions, which altogether cover about 25% of the continental area that drained into the oceans, extending from the tropics to polar regions. The majority of studied clays closely approximate weathering products, always having high-delta O-18 signatures regardless of the bedrock type, and in equilibrium with local meteoric waters. Silts are only similar to 1.9 parts per thousand lighter on average due to greater detrital dilution. Overall, bulk clays from across different climatic regions do not vary much isotopically; an observation which we attribute to opposing effects of temperature on clay-water fractionation and hydrologic relationship between temperature and delta O-18(MW). Mathematical inversion of measured clay delta O-18 and Delta O-17 values (corrected for detrital contribution) into MAT and delta O-18(MW), compiled for each studied watershed, returns satisfactory estimates. Globally, triple O isotopes in clays appear to be water-dominated, being controlled almost exclusively by delta O-18(MW) at respective temperature of weathering, with minor effects related to evaporation. Using sand from rivers, correlation of delta O-18 silts with detrital proportions, and estimated surface outcrop of different rock types, we additionally arrive at a +11.5 parts per thousand estimate for the exposed silicate crust undergoing weathering. Globally-averaged, sediment-flux weighted clay delta O-18 and Delta O-17 values are +14.80 parts per thousand and 0.164 parts per thousand, respectively. These values are significantly skewed toward O isotope signatures for the southeast Asia and western Pacific regions, characterized by very high sediment fluxes to the ocean. Using both clayand silt-size fractions, the total weighted silicate weathering delta O-18 signature exported to the world's ocean is 2.59 parts per thousand, almost 50% higher the previous estimate, yielding an ice-free world hydrosphere estimate of 0.78 parts per thousand. Overall, the modern river clays represent a snapshot of modern weathering conditions on continents, and associated first-order climatic signatures related to MAT and delta O-18 of the hydrosphere. This implies that measured increase in delta O-18 and stepwise decrease in Delta O-17 in shales in the geologic record capture: evolving global hydrologic cycle upon continental emergence, decrease in global MAT or diagenetic conditions, and decreasing ocean mass via rehydrating of the mantle by subduction of hydrated low-delta O-18, high-Delta O-17 slabs. (C) 2019 Elsevier B.V. All rights reserved.