Biogeochemistry of sedimentary organic matter in the Yongjiang River estuary in the southern part of Hangzhou Bay,China,since the Late Pleistocene

A sediment core(YJK19-02)collected from the southern outlet of Hangzhou Bay near the Yongjiang River estuary in East China was analyzed for grain size,lignin,bulk elemental composition,stable carbon isotope,and rare earth elements(REEs)to determine the sources and diagenesis of sedimentary organic matter(OM)of the estuary and adjacent areas since the Late Pleistocene.δ^(13)C values(-24.80‰–-23.60‰),total organic carbon/total nitrogen(TOC/TN)molar ratios(8.00–12.14),and light rare earth element/heavy rare earth element ratios(LREE/HREE=8.34–8.91)revealed the predominance of terrestrial sources of OM,mainly from the Changjiang(Yangtze)River.The lignin parameters of syringyl/vanillyl(S/V=0.20–0.73)and cinnamyl/vanillyl(C/V=0.03–0.19)ratios indicate the predominance of nonwoody angiosperms,and the vanillic acid/vanillin ratios[(Ad/Al)_(V)=0.32–1.57]indicate medium to high degrees of lignin degradation.An increasing trend ofΛ(total lignin in mg/100-mg OC)values from ca.14500 a BP to ca.11000 a BP reflected the increase in temperature during the Late Pleistocene.However,a time lag effect of temperature on vegetation abundance was also revealed.The relatively higher and stableΛvalues correspond to the higher temperature during the mid-Holocene from ca.8500 a BP to ca.4500 a BP.Λvalues decreased from ca.4000 a BP to the present,corresponding to historical temperature fluctuations during this time.Our results show that the vegetation abundance in the Yongjiang River Basin since the Late Pleistocene was related to the temperature fluctuation duo to climate change.

Distribution and sources of sedimentary organic matter in different aquaculture areas of northeastern Zhanjiang Bay using stable carbon and nitrogen isotopes

Zhanjiang Bay is a major aquaculture area in China with many types of mariculture products(such as oysters,fish,and shrimp).The culture area and shrimp output in Zhanjiang Bay are ranked first in China.We investigated the total organic carbon(TOC),total nitrogen(TN),TOC/TN ratio,and stable isotopes(δ^(13)C and δ^(15)N) of the fish and shrimp feed,fish and shrimp feces,and sedimentary organic matter(SOM) in and around different aquaculture areas of northeastern Zhanjiang B ay to study the impact of aquaculture activities on SOM.The average TOC contents of fish and shrimp feed were 39.20%±0.91% and 39.29%±0.21%,respectively.The average TOC content in the surface sediments of the oyster culture area,the mixed(fish and shrimp) culture area,and the cage fish farm area were 0.66%,0.88%±0.10%,and 0.58%±0.19%,respectively,which may indicate that mixed culture had a greater impact on SOM.The relatively high TOC and TN contents and relatively low TOC/TN ratios,and δ^(15)N values in the upper layer of the core sediment in the mixed culture area could also support the significant influence of mixed culture.The average δ^(13)C and δ^(15)N values of fish and shrimp feed were -20.6‰±2.2‰ and 1.8‰±1.2‰,respectively,which were different from the isotopic values of SOM in the study area.δ^(13)C and δ^(15)N values for SOM in different aquaculture areas were different from those of nearby reference stations,probably reflecting the influence of aquaculture.The δ^(13)C and δ^(15)N values in the oyster culture area(-25.9‰ and6.0‰,respectively) seemed to have reduced δ^(13)C and enriched δ^(15)N relative to those of the reference station(-24.6‰ and 5.8‰,respectively).This may reflect the influence of organic matter on oyster culture.The δ^(15)N value of the station in the mixed culture area(7.1‰±0.4‰) seemed to be relatively enriched in δ^(15)N relative to that of the reference station(6.6‰).Sedimentation and the subsequent degradation of organic matter from mixed cultures may have contributed to this phenomenon.The surface sediment at the cage fish farm area seemed to be affected by fish feces and primary production based on the indication of δ^(13)C and δ^(15)N values.The sediment core at the mixed culture region(NS6) had lower TOC/TN ratios and more positive δ^(13)C and δ^(15)N values than the sediment core at the oyster culture area,suggesting a higher proportionate contribution of marine organic matter in the mixed culture area.In summary,oyster culture,mixed culture,and cage fish culture in northeastern Zhanjiang Bay had a certain degree of impact on SOM,and mixed culture had more significant influences on SOM based on the high TOC contents and the significant vertical variations of TOC/TN ratio and δ^(15)N value in the sediment of this area.This study provides new insights into the impact of aquaculture activities on SOM content.

Distribution, sources and burial fl ux of sedimentary organic matter in the East China Sea

Distribution of total organic carbon(TOC),total nitrogen(TN),stable isotope ratio of organic carbon and nitrogen(δ13 C andδ15 N)in 31 surface sediments were examined to quantitatively discriminate the source and burial of sedimentary organic matter(SOM)in the East China Sea(ECS).Results show that high content of TOC and TN occurred in the southern inner shelf and the northeast area due to the transport of organic matter(OM)from the Changjiang(Yangtze)River southwards by the Zhejiang Fujian Coastal Current(ZFCC)and the delivery from the old Huanghe(Yellow)River estuary by the Subei Coastal Current(SbCC),respectively.The signifi cant relationship of TOC to clay provided evidence that the fi ne-grained sediment was the primary carrier for SOM from the inner shelf to open sea.The TN varied proportionally to the TOC with the neglected intercept,suggesting that most of nitrogen measured was related to the SOM and the infl uence of the sorption of inorganic nitrogen was insignifi cant.The seaward enrichment ofδ13 C and seaward depletion of C/N andδ15 N illustrated the decrease of terrestrial OM(TOM)and the increase of marine OM(MOM).The close of average C/N ratio to the Redfi eld ratio indicated the minor role of terrestrial plant debris in the SOM.Distribution of bulk OM properties was ascribed to the terrestrial and marine input by the shelf circulation and phytoplankton with the contributions of 76.1%MOM and 23.9%TOM to SOM,respectively.The burial fl ux of OC ranged within 0.34–7.56 mg/(cm 2∙a)(averaged 2.80 mg/(cm 2∙a))and enriched along the shore and in the fi ne-grained area,manifesting the signifi cant input of the land-based sources and the eff ect of shelf mud depositional process on the fate of SOM.The sharp seaward decrease of burial fl uxes of terrestrial and marine OC gave evidences that the majority of organic carbon were settled in the inner shelf with the small fraction of them further transported eastwards.

Source and composition of sedimentary organic matter in the head of Three Gorges Reservoir: a multiproxy approach using δ^(13)C,lignin phenols, and lipid biomarker analyses

A multiproxy approach including bulk organic carbon, δ^(13) C, lignin phenols, and lipid biomarker analyses were applied to characterize the source and composition of sedimentary organic matter in the head part of Three Gorges Reservoir. OM consisted of both natural(autochthonous and allochthonous) and anthropogenic inputs.The natural OM included input from vascular plants,especially non-woody angiosperms. The allochthonous input included plankton and microbial production, likely due to localized eutrophication. Anthropogenic inputs likely derived from petroleum input and/or urban activities.Other anthropogenic inputs were untreated sewage waste.These influences were concluded to be regionally specific point sources of pollution based on relative distributions and on the fact that molecular characteristics of sedimentary OM were not distributed smoothly along a gradient.

The Distribution of Sedimentary Organic Matter and Implication of Its Transfer from Changjiang Estuary to Hangzhou Bay, China

In this study, a comparison was made between the Changjiang Estuary and the Hanghzou Bay, in terms of the sources and diagenesis of the sedimentary organic matter (OM). To achieve this purpose, surface sediments from the estuary and bay were analyzed for lignin-derived phenols, stable carbon isotope and TOC/TN (total organic carbon/total nitrogen) molar ratio. The signal of land-derived OM decreased, and the vanillic acid to vanillin ratio, (Ad/Al)v, increased with increasing distance from the Changjiang Estuary and the Hangzhou Bay. These results corresponded with the contribution of the terrigenous OM from the rivers to the coastal zone, and the predominance of marine OM farther offshore, and that the land-derived OM underwent decomposition during transport along the estuary and bay. It should be noted that besides the Qiantang River, Hangzhou Bay is also receiving more than half of its materials from Changjiang Estuary, which flows into the Hangzhou Bay at the north, and leaves via the southern part of the bay. This important aspect of the hydrological cycle in Hangzhou Bay corresponded to higher Λ (total lignin in mg/100 mg OC), higher TOC and C/N ratios and more elevated (Ad/Al)v and (Ad/Al)s values in the bay than the Changjiang Estuary, thus, rendering the bay as a site for the accumulation and rapid cycling of terrigenous OM.

Stability of sedimentary organic matter: Insights from molecular and redox analyses

Sedimentary organic matter(SOM)affects the stability of the aquatic carbon pool.The degradation process of SOM is complex for its multifaceted composition.The concentration and properties of SOM affect its steady state,yet the transformation processes of SOM in lakes remain unclear.Here we show the molecular and redox perspectives of SOM stability in polluted sediments with high organic matter content and diverse vegetation.We find significant differences in carbon fractions across various sites.The origin of the organic matter,determined using excitation-emission matrix spectra,influences the consistency of organic matter composition and biochemical degradation in lacustrine sediment.We also observe that sulfur-containing substances decrease carbon chain length and reduce organic matter stability.Fourier-transform ion cyclotron resonance mass spectrometry shows that sulfur-containing substances decrease the degree of saturation and cause reduction.In contrast,nitrogen-containing compounds increase the modified aromaticity index and humin content,enhancing organic carbon complexity and stability(p<0.05).These results complement the characteristics and transformations of SOM.In a broader perspective,this study contributes to laying the foundation for understanding SOM stability in the carbon cycle and its future effects.