Sedimentary and Geochemical Data of Short Cores in Coastal Wetlands of Sansha Bay, Southeast China

Coastal wetlands are essential yet vulnerable carbon sinks; however, the impacts of vegetation replacement on carbon dynamics remain less understood. This study investigates the sources, burial rates, and historical trends of sedimentary organic carbon (OC) across various wetland types, including mangroves, Spartina alterniflora (S. A.), and barren tidal flats in Sansha Bay, Fujian Province, Southeast China. We conducted all experiments in the State Key Laboratory of Marine Geology, Tongji University. Six sediment cores were retrieved from diverse vegetation types in the intertidal wetlands of Sansha Bay between July 27 and August 2, 2018 . A 1-meter-long PVC pipe was vertically driven into the sediment to obtain core samples. Due to variations in sediment composition, the lengths of the cores ranged from 72 to 92 cm. Sampling locations included mangrove areas (SD-3, SD-8), S. A. dominated zones (SD-4, SD-7), and barren mudflats (SD-1, SD-2). Following longitudinal splitting and photographic documentation, cores were subsampled at 1 cm intervals, with one half archived at 4°C for future analysis. Grain size analysis: Sediment samples were analyzed for grain size at 2 cm intervals. A 0.5 g subsample was treated with 15 ml of 30% hydrogen peroxide and heated at 60°C for 4 hours to remove organic matter. This was followed by treatment with 15 ml of 10% hydrochloric acid at 60°C for additional 4 hours to eliminate carbonates. The residual sample was rinsed to achieve a neutral pH, then dispersed using sodium hexametaphosphate and sonicated for 5 minutes. Grain size analysis was conducted using a Beckman Coulter LS230 laser particle size analyzer, which has a measurement range of 0.04-2000 μm and a relative error of less than1%. Total organic carbon (TOC), total nitrogen (TN) and δ13C Analysis: For the organic geochemical analysis, sediments were subsampled at 1–2 cm sections. Carbonates were removed using 10% hydrochloric acid, followed by neutralization and grinding to a particle size of less than 200 mesh. The concentrations of TOC and TN were determined using a Vario EL cube elemental analyzer, while δ¹³C values were measured with a MAT253 isotope ratio mass spectrometer. Radionuclide Analysis: To correct for sediment compaction effects, water content was measured by drying samples at 60°C. After sealing, samples sat for at least 20 days to allow for 222Rn ingrowth, this allowed us to determine supported 210Pb activity by measuring daughter products of 226Ra. Gamma-ray measurements were conducted by using an ORTEC low-background well-type HPGe gamma-ray detector (GWL-120-15-LB-AWT). Analytical precision and methodological adherence were consistent with Wang et al. (2019). This analysis yielded activity measurements for supported 210Pb (based on photo peaks at 295.0 and 351.9 keV), total 210Pb (at 46.5 keV) and 137Cs (at 661.6 keV). Excess 210Pb (210Pbex) activity was determined by subtracting the supported 210Pb from total 210Pb.滨海湿地是兼具关键生态功能与天然脆弱性的碳汇；然而，植被更替对碳动态的影响仍有待深入阐明。本研究针对中国东南部福建省三沙湾的不同湿地类型——包括红树林、互花米草（Spartina alterniflora，简称S. A.）及光滩——探究了沉积有机碳（OC）的来源、埋藏速率及历史演化趋势。本研究所有实验均于同济大学海洋地质国家重点实验室完成。 2018年7月27日至8月2日期间，研究人员从三沙湾潮间带湿地的不同植被覆盖区域采集了6根沉积柱样。采用1米长的PVC管垂直插入沉积物中以获取柱样。由于沉积物组成存在差异，柱样长度介于72至92厘米之间。采样点位涵盖红树林区域（SD-3、SD-8）、互花米草优势区（SD-4、SD-7）以及光滩（SD-1、SD-2）。柱样经纵向劈开并拍照记录后，以1厘米为间隔进行分样，其中一半样品于4℃下保存以备后续分析。 粒度分析（Grain size analysis）：沉积物样品以2厘米为间隔开展粒度测试。称取0.5克分样，加入15毫升30%过氧化氢溶液，于60℃下加热4小时以去除有机质；随后加入15毫升10%盐酸溶液，于60℃下继续加热4小时以去除碳酸盐。将残留样品冲洗至中性pH值，随后用六偏磷酸钠分散并超声处理5分钟。粒度分析采用贝克曼库尔特（Beckman Coulter）LS230激光粒度分析仪完成，该仪器的测量范围为0.04~2000 μm，相对误差小于1%。 总有机碳（TOC）、总氮（TN）及δ¹³C分析：开展有机地球化学分析时，沉积物以1~2厘米为间隔进行分样。采用10%盐酸去除碳酸盐，随后经中和处理并研磨至粒径小于200目。总有机碳与总氮的浓度采用Vario EL cube元素分析仪测定，而δ¹³C值则通过MAT253同位素比率质谱仪完成检测。 放射性核素分析（Radionuclide Analysis）：为校正沉积物压实效应，通过60℃烘干法测定样品含水量。样品密封后静置至少20天以完成²²²Rn的生长富集，借此通过测定²²⁶Ra的子体产物来确定支持性²¹⁰Pb的活度。γ射线测量采用ORTEC低本底井型HPGe γ射线探测器（GWL-120-15-LB-AWT）完成。分析精度与实验方法规范性均符合Wang等人（2019）的研究标准。本次分析获得了支持性²¹⁰Pb（基于295.0 keV与351.9 keV处的光电峰）、总²¹⁰Pb（46.5 keV处）以及¹³⁷Cs（661.6 keV处）的活度数据。过剩²¹⁰Pb（²¹⁰Pbex）的活度通过总²¹⁰Pb活度减去支持性²¹⁰Pb活度计算得到。