2020-2021 Changjiang estuary dissolved oxygen isotopic compositions and results of Monte-Carlo Mixing experiment

Understanding the complex processes driving oxygen depletion is challenging. The nonlinear relationship between dissolved oxygen (DO) isotopic composition (δ18O ) and its concentration makes δ18O a unique tracer for studying O2 cycling in oxygen depletion zones. Based on the DO δ18O compositions from four cruises conducted from summer 2020 to spring 2021 in the Changjiang Estuary, we identified a dynamic northern oxygen depletion zone (NODZ) with seasonally varying DO δ18O values, and a relatively stable southern oxygen depletion zone (SODZ). To quantify the contributions of low oxygen water mixing and respiration to oxygen depletion, we conducted a Monte-Carlo mixing experiment involving high salinity and low oxygen water (HSLO) and mixed layer water (MLW). Our findings indicated that during summer of 2020, HSLO mixing contributed to 59% to 100% of oxygen deficit in oxygen depletion zones, with SODZ exhibiting a HSLO mixing contribution of more than 78%. Respiration contributed to the remaining oxygen deficit, with water column respiration consuming approximately 2.5 times more oxygen than sediment oxygen consumption. In the NODZ, oxygen depletion was further enhanced by water column stratification, respiration and seafloor topography. Overall, the influx of HSLO was a key trigger for oxygen depletion in the Changjiang Estuary. The spatial and temporal variations of the two oxygen depletion zones were closely linked to the intrusion and retreat of HSLO from summer to autumn. Our findings highlight the importance of utilizing DO δ18O to differentiate and quantify biological and physical processes in coastal oxygen depletion zones.File description：Table S1: Data for this study, including latitude and longitude, sampling depth, temperature, salinity, δO2/Ar,δ18O,[O2],[O2]/[O2]sat, densityTable S3: The results of Monte-Carlo mixing experiments in all samples beneath mixed layer: the fraction of HSLO in different water masses, [O2] and δ18O after mixing from Monte-Carlo mixing experiment. Oxygen depletion caused by mixing and total oxygen depletion. The contribution of mixing and respiration to oxygen deficit. The apparent respiration fractionation factor and the proportion of WCR and SOC to oxygen deficit as well as the observed salinity, [O2] and δ18O beneath the mixed layer