Dataset: Decadal variations of hypoxia off the Changjiang Estuary since the 1950s: Sedimentary records from benthic foraminifera

Hypoxia has become a global marine environmental issue under climate warming. This study focuses on Core YEC1701 (upper 100 cm) from hypoxic area off the Changjiang Estuary, applying 210Pb and 137Cs dating together with lithology, grain size, benthic foraminifera and elemental geochemistry to reveal paleoenvironmental changes and hypoxic history since 1950. Notable findings are as follows: 1. The age model in the upper 100 cm of Core YEC1701 reveals two different sedimentation stages in the Changjiang delta front since 1929: a low sedimentation rate (0.53 cm/yr) from 1929 to 1976, and a high sedimentation rate (1.62 cm/yr) after 1976. 2. The core location has been a delta-front environment since 1950. 3. Hyaline benthic foraminifera (87.06%) dominate the overall assemblage, while the percentages of agglutinated (4.54%) and porcelaneous benthic foraminifera (8.40%) are relatively low. The averages of abundance, number of species and Shannon-Wiener index (H′) are 4182 individuals/40 g, 47 and 3.41, respectively. Based on quantitative analysis, there are a total of 16 dominant species with an average relative abundance of >2% for the overall assemblage. 4. TN contents range from 0.09% to 0.12% (average 0.10%), and TOC contents range from 0.40% to 0.58% (average 0.50%). C/N ratios range from 4.48 to 5.29 (average 4.92). δ13C values range from –23.93‰ to –23.15‰ (average –23.51‰). 5. Bottom-water hypoxia off the Changjiang Estuary has been persistent throughout the study period. From 1950 to 1980, hypoxia was weak and declining; after 1980, hypoxia was severe and intensifying. The details for sample collection are as follows: Core YEC1701 was collected using a gravity corer during a summer cruise by the State Key Laboratory of Marine Geology (Tongji University) in August, 2017, and its total length is 405 cm. The coring site (30°57′3.062″N, 122°44′19.792″E) was located in the muddy delta front and adjacent to the modern core hypoxic area (Li et al., 2002), and the water depth was 22.4 m. The core was split lengthwise in the laboratory: one half was sealed with the preservative film and stored at 4℃ for archiving, and the other (working half) was carefully smoothed with a stainless steel knife to expose sedimentary structures. The working half was photographed, described lithologically, and scanned non-destructively by an X-ray fluorescence (XRF) core scanner before subsampling. Subsamples were collected at 1 cm intervals using a stainless steel cutter, sealed in plastic bags, and stored at 4℃ for subsequent analyses. Note: All the measurements were conducted at the State Key Laboratory of Marine Geology.在气候变暖的背景下，海洋低氧已成为全球性海洋环境问题。本研究以长江口外低氧区的YEC1701岩芯（取其上100 cm段）为研究对象，采用²¹⁰Pb、¹³⁷Cs测年，结合岩性、粒度、底栖有孔虫（benthic foraminifera）及元素地球化学分析，揭示了1950年以来的古环境演变与低氧演化历史。 主要研究结果如下： 1. 对YEC1701岩芯上100 cm段的年龄模型分析显示，1929年以来长江三角洲前缘沉积出现两个截然不同的阶段：1929年至1976年为低沉积速率阶段（0.53 cm/yr），1976年之后为高沉积速率阶段（1.62 cm/yr）。 2. 自1950年以来，该岩芯所处位置始终处于三角洲前缘环境。 3. 玻璃质底栖有孔虫占总组合的87.06%，为优势类群；胶结质底栖有孔虫（4.54%）与瓷质底栖有孔虫（8.40%）占比相对较低。研究对象的个体丰度、物种数及香农-威纳指数（Shannon-Wiener index, H′）平均值分别为4182枚/40g、47种及3.41。经定量分析，总组合中共包含16个优势种，其平均相对丰度均大于2%。 4. 总氮（TN）含量介于0.09%~0.12%之间（平均值为0.10%），总有机碳（TOC）含量介于0.40%~0.58%之间（平均值为0.50%）；C/N比值介于4.48~5.29之间（平均值为4.92）；碳同位素δ¹³C值介于-23.93‰~-23.15‰之间（平均值为-23.51‰）。 5. 研究时段内长江口外底层水低氧现象持续存在：1950年至1980年，低氧程度较弱且呈减弱趋势；1980年之后，低氧程度加剧且不断加重。 样品采集细节如下： YEC1701岩芯由同济大学海洋地质国家重点实验室于2017年8月的夏季科考航次中采用重力取样器采集，岩芯总长度为405 cm。取样站位坐标为30°57′3.062″N、122°44′19.792″E，位于泥质三角洲前缘，毗邻现代核心低氧区（Li等，2002），取样水深为22.4 m。岩芯在实验室中沿纵向劈开：一半用保鲜膜密封后置于4℃冷藏保存归档；另一半（工作半芯）用不锈钢刀具小心刮平以暴露沉积构造。在分样前，工作人员对工作半芯进行拍照、岩性描述，并通过X射线荧光（XRF）岩芯扫描仪进行无损扫描。分样时采用不锈钢取样器以1 cm间隔采集分样样品，封装于塑料袋中，置于4℃冷藏以备后续分析。 备注：所有测试工作均在海洋地质国家重点实验室完成。