Dataset: Decadal variations of hypoxia off the Changjiang Estuary since the 1950s: Sedimentary records from benthic foraminifera
Description
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.
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1. Wet sediment samples were dried in an oven at 60℃ to a constant weight, and water content was calculated from weight loss. 10 g of dried sediment was collected for radionuclide analysis. Samples were ground into fine powder using an agate mortar, and stored in sealed containers for 20 days to achieve radioactive equilibrium before measurement. Measurements were conducted using two ultra-low background well-type high-purity Ge γ-ray spectrometers (GWL-120-15-LB-AWT, AMETEK, USA), which simultaneously detected total 210Pb, 137Cs, and 226Ra. 2. 0.3 g of sample was placed in a 50 mL beaker, and added with 15 mL of 30% H2O2 and 15 mL of 1 mol/L HCl to remove organic matter and carbonate minerals, respectively. The sample suspension was transferred to a 1500 mL beaker, and diluted with ~1000 mL of deionized water. After having been left to stand for 12 h, the supernatant was siphoned off. This washing step was repeated until the supernatant was neutral. The sample was then transferred to another 50 mL beaker, with 1–2 drops of sodium hexametaphosphate added as a dispersant, and ultrasonicated for 15 min to disperse sediment particles before instrumental analysis. Grain-size measurements were conducted using a Beckman Coulter LS230 Laser Diffraction Particle Size Analyzer. The detection range was 0.375–2000 μm, and the relative error of mean grain size for replicate measurements was <1%. 3. An Avaatech XRF Core Scanner was utilized to determine the relative contents of elements from Al to U. The smoothed sediment core surface was covered with a special thin film, and measurements were taken at accelerating voltages of 10 kV, 30 kV and 50 kV. 4. Each sample was soaked in a dilute Calgon solution and agitated for 30 min to disaggregate sediment particles, then washed with a 63 μm sieve. The residue was dried in an oven at 45℃. A standard micro-splitter was used to split the dried residue into subsamples containing approximately 200 benthic foraminiferal tests. Benthic foraminifers were hand-picked under a stereomicroscope and placed on the standard 20-square micropaleontological slides for taxonomic identification, following the taxonomies of He et al. (1965), Zheng et al. (1978) and Wang et al. (1988). 5. 1 g of dry sample was ground and placed into a 15 mL centrifuge tube with 1 mol/L ultra-pure HCl added to remove carbonate minerals. The centrifuge tube was put in a water bath at 60℃ for 24 h with periodic oscillation every 2 h to ensure complete reaction. The sample was then rinsed repeatedly with deionized water until the supernatant was neutral, freeze-dried at –20℃, and ground again for TN and TOC measurements. TN and TOC contents were determined using Elementar Vario EL Cube CN Series organic element analyzer. For δ13C analysis, 40 μg of organic carbon was extracted from each pretreated sample and measured using a DELTA plus XP isotope ratio mass spectrometer and a Carlo Erba Instruments Flash 1112 elemental analyzer.
Institutions
- Tongji UniversityShanghai, Shanghai
- Shanghai Ocean UniversityShanghai, Shanghai