2020-2021 isotopic compositions, gas ratios and MATLAB scripts

Understanding the complex processes that lead to 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 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 1.5 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 layerThe text file contains the MATLAB code for the Monte Carlo mixing experiments. It demonstrates the computational process of the Monte Carlo method using July as an example.<br>

理解导致氧耗尽的复杂过程具有挑战性。溶解氧（DO）同位素组成（δ¹⁸O）与其浓度之间的非线性关系，使δ¹⁸O成为研究氧耗尽区O₂循环的独特示踪剂。基于2020年夏季至2021年春季在长江口开展的四次巡航所获DO δ¹⁸O组成数据，我们识别出动态变化的北部氧耗尽区（NODZ，其DO δ¹⁸O值呈季节性波动）及相对稳定的南部氧耗尽区（SODZ）。为量化低氧水混合与呼吸作用对氧耗尽的贡献，我们开展了蒙特卡洛混合实验（Monte-Carlo mixing experiment），涉及高盐低氧水（HSLO）与混合层水（MLW）。研究结果显示：HSLO混合对氧耗尽区氧亏损的贡献占比为59%至100%，其中SODZ的HSLO混合贡献占比超78%；呼吸作用贡献剩余氧亏损，且水柱呼吸（water column respiration，WCR）耗氧量约为沉积物耗氧（sediment oxygen consumption，SOC）的1.5倍。在NODZ中，水柱分层、呼吸作用及海底地形进一步加剧氧耗尽。总体而言，HSLO的输入是长江口氧耗尽的关键触发因素；两个氧耗尽区的时空变化与夏秋季HSLO的入侵和撤退密切相关。本研究凸显了利用DO δ¹⁸O区分并量化海岸氧耗尽区生物与物理过程的重要性。

文件描述：
表S1：本研究数据，含经纬度、采样深度、温度、盐度、δO₂/Ar、δ¹⁸O、[O₂]、[O₂]/[O₂]sat、密度
表S3：混合层以下所有样本的蒙特卡洛混合实验结果，包括不同水体中HSLO占比、混合后[O₂]与δ¹⁸O模拟值、混合导致的氧耗尽及总氧耗尽、混合与呼吸作用对氧亏损的贡献、表观呼吸分馏因子、WCR与SOC对氧亏损的贡献占比，以及混合层以下观测到的盐度、[O₂]与δ¹⁸O值
文本文件含蒙特卡洛混合实验的MATLAB代码，以7月为例演示蒙特卡洛方法的计算过程。<br>