Impacts of Vegetation Replacement on Organic Carbon Burial in Coastal Wetlands of Sansha Bay, Southeast China
Description
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.