Dissolved oxygen of seawater samples during RV POLARSTERN expedition PS122 – MOSAiC@en

Discrete seawater samples for total dissolved oxygen were collected from the Polarstern (vessel:polarstern:ctd_sbe9plus_321) and Ocean City (vessel:polarstern:ctd_sbe9plus_935) CTD/Rosette systems during the Multidisciplinary drifting observatory for the study of Arctic Climate (MOSAiC) expedition, 20 September 2019 – 14 October 2020. Dissolved oxygen was analyzed on board RV Polarstern during legs 3, 4, and 5 between 6 March 2020 and 17 September 2020. Seawater was drawn from Niskin bottles into Winkler flasks using silicon tubing, filling from the bottom and allowing seawater to overflow by at least two flask volumes (approximately 120 mL). The samples were directly pickled by the addition of 1 mL each of concentrated manganese chloride (3 M) and alkaline (NaOH 8 M) sodium iodide (4 M) solutions to form manganese oxyhydrodoxide precipitate. Samples were stored in the dark and underwater at room temperature until analysis. Dissolved oxygen was determined using an automatic Winkler titration setup with potentiometric detection (Titrino plus 848, Metrohm, Switzerland). Reagents were prepared according to Langdon (2010). Blanks were determined before each run by addition of reagents to MilliQ water. The titrations were performed directly in the flasks used for sampling after the manganese oxyhydroxide precipitate was dissolved by addition of sulphuric acid (5 M). Sodium thiosulphate (~0.2 N) solutions were prepared onboard for the titration and the concentration was determined before each run using an iodate standard (0.01 N, OSIL, United Kingdom). The analytical precision based on duplicate samples analysis was 0.4 ± 2.6 µmol/kg (n = 60).2019年9月20日至2020年10月14日期间，在北极气候多学科漂移观测站（Multidisciplinary drifting observatory for the study of Arctic Climate, MOSAiC）科考航次中，研究人员依托搭载ctd_sbe9plus_321型设备的极星号（Polarstern）科考船与搭载ctd_sbe9plus_935型设备的大洋城号（Ocean City）科考船的CTD/罗塞特采样系统（CTD/Rosette systems），采集了总溶解氧离散海水样品。溶解氧样品的检测于2020年3月6日至2020年9月17日的第3、4、5航段在极星号（Polarstern）科考船上完成。采用硅管将尼斯金采水器（Niskin bottle）采集的海水注入温克勒烧瓶（Winkler flask），从烧瓶底部注液并使海水溢流至少2倍烧瓶容积（约120 mL）。向样品中分别加入1 mL浓度为3 mol/L的氯化锰浓溶液、含8 mol/L氢氧化钠（NaOH）的4 mol/L碘化钠（sodium iodide）碱性溶液，直接完成样品固定，生成羟基氧化锰沉淀（manganese oxyhydrodoxide precipitate）。样品置于避光环境、室温及水下环境中保存，直至完成分析。采用配备电位检测（potentiometric detection）模块的全自动温克勒滴定装置（automatic Winkler titration setup），使用瑞士万通Metrohm Titrino plus 848型设备完成溶解氧含量测定。试剂配制参照Langdon（2010）的方法。每批检测前均以密理博超纯水（MilliQ water）添加试剂制备空白样品，测定空白值。待羟基氧化锰沉淀经5 mol/L硫酸溶解后，直接在采样所用的烧瓶中完成滴定操作。滴定所用的硫代硫酸钠（sodium thiosulphate，约0.2当量浓度）溶液由科考船上配制，每批检测前均采用英国OSIL公司生产的0.01当量浓度碘酸盐标准溶液（iodate standard）标定其浓度。基于60份平行双样分析的结果，本方法的分析精密度为0.4 ± 2.6 µmol/kg。