Organic carbon and total nitrogen as well as stable carbon (
δ
13C
org) and nitrogen (
δ
15N) isotopic ratios were determined on suspended matter, settling particles and bottom sediments collected ...from the Changjiang (Yangtze River) Estuary during the summer flood period in 2001. From the results, three sources of suspended matter, riverine, deltaic (salt marshes) and marine (East China Sea — ECS) were distinguished. The resuspended particulate organic matter contents in the water column increased with the amount of total suspended matter (TSM) indicating the role of dispersive and/or turbulent mixing and its control over the high TSM concentration in the Changjiang Estuary. This and an active resuspension/deposition in the estuary determine the nature of organic matter in suspended materials reaching the East China Sea.
The Indian Summer Monsoon (ISM) is a major global climatic phenomenon. Long-term precipitation proxy records of the ISM, however, are often fragmented and discontinuous, impeding an estimation of the ...magnitude of precipitation variability from the Last Glacial to the present. To improve our understanding of past ISM variability, we provide a continuous reconstructed record of precipitation and continental vegetation changes from the lower Ganges-Brahmaputra-Meghna catchment and the Indo-Burman ranges over the last 18,000 years (18 ka). The records derive from a marine sediment core from the northern Bay of Bengal (NBoB), and are complemented by numerical model results of spatial moisture transport and precipitation distribution over the Bengal region. The isotopic composition of terrestrial plant waxes (δD and δ13C of n-alkanes) are compared to results from an isotope-enabled general atmospheric circulation model (IsoCAM) for selected time slices (pre-industrial, mid-Holocene and Heinrich Stadial 1). Comparison of proxy and model results indicate that past changes in the δD of precipitation and plant waxes were mainly driven by the amount effect, and strongly influenced by ISM rainfall. Maximum precipitation is detected for the Early Holocene Climatic Optimum (EHCO; 10.5–6 ka BP), whereas minimum precipitation occurred during the Heinrich Stadial 1 (HS1; 16.9–15.4 ka BP). The IsoCAM model results support the hypothesis of a constant moisture source (i.e. the NBoB) throughout the study period. Relative to the pre-industrial period the model reconstructions show 20% more rain during the mid-Holocene (6 ka BP) and 20% less rain during the Heinrich Stadial 1 (HS1), respectively. A shift from C4-plant dominated ecosystems during the glacial to subsequent C3/C4-mixed ones during the interglacial took place. Vegetation changes were predominantly driven by precipitation variability, as evidenced by the significant correlation between the δD and δ13C alkane records. When compared to other records across the ISM domain, precipitation and vegetation changes inferred from our records and the numerical model results provide evidence for a coherent regional variability of the ISM from the Last Glacial to the present.
•We provide a continuous history of the ISM in the Bengal region for the last 18 ka.•Precipitation and vegetation are reconstructed from alkane δD and δ13C composition.•Paleo-precipitation δD was driven by the amount effect reflecting monsoon intensity.•Paleo-vegetation composition was driven by precipitation variability since 15 ka BP.•Comparison with numerical model results suggests coherent regional ISM variability.
On a global scale tropical SE Asia is critical in terms of fluvial nutrient and sediment input into the ocean as well as in terms of human modifications of the coastal zone altering these inputs. In ...order to obtain information on the biogeochemistry of the Brantas River estuary and adjacent coastal waters of the Madura Strait in eastern Java, Indonesia, a densely-populated area which is strongly affected by human activities in the river catchment, we investigated water, suspended matter and sediments from estuarine and coastal waters and plants and soils from the catchment collected during the dry season. Water samples were analyzed for dissolved nutrients and phytoplankton abundance and composition. Suspended matter, sediment, plant and soil samples were analyzed for carbon, nitrogen, amino acids and stable carbon and nitrogen isotopes. Nutrient concentrations were high in the river and then decreased rapidly seaward. Runoff from agricultural soils may be a major nutrient source. Phytoplankton was dominated by diatoms and most abundant in the estuarine mixing zone. A fairly high phytoplankton abundance despite nitrate-depletion and an N/P ratio <4 in coastal waters indicates additional nitrogen sources. Biogeochemical characteristics and stable isotopes indicate the autochthonous origin of suspended organic matter (OM) during the dry season. Major part of the fluvial input appears to be trapped in the inner estuary. Similar information obtained from the analysis of sediments suggests seasonal differences in the quantity and origin of suspended sediments and OM transported by the river with high inputs into coastal waters during the rainy season, the period of peak river discharge. The amino acid composition in combination with stable isotopes indicates agricultural soils to be a major OM source during that time. A high proportion of amino acid-bound nitrogen in suspended matter and sediments and the presumed sources of dissolved nitrogen other than nitrate to sustain primary productivity in the coastal zone suggest that organic nitrogen may play a more important role for coastal food webs and the nitrogen cycle in tropical regions than previously thought.
Mangroves are one of the most carbon‐dense forests on the Earth and have been highlighted as key ecosystems for climate change mitigation and adaptation. Hundreds of studies have investigated how ...mangroves fix, transform, store, and export carbon. Here, we review and synthesize the previously known and emerging carbon pathways in mangroves, including gains (woody biomass accumulation, deadwood accumulation, soil carbon sequestration, root and litterfall production), transformations (food web transfer through herbivory, decomposition), and losses (respiration as CO2 and CH4, litterfall export, particulate and dissolved carbon export). We then review the technologies available to measure carbon fluxes in mangroves, their potential, and their limitations. We also synthesize and compare mangrove net ecosystem productivity (NEP) with terrestrial forests. Finally, we update global estimates of carbon fluxes with the most current values of fluxes and global mangrove area. We found that the contributions of recently investigated fluxes, such as soil respiration as CH4, are minor (<1 Tg C year−1), while the contributions of deadwood accumulation, herbivory, and lateral export are significant (>35 Tg C year−1). Dissolved inorganic carbon exports are an order of magnitude higher than the other processes investigated and were highly variable, highlighting the need for further studies. Gross primary productivity (GPP) and ecosystem respiration (ER) per area of mangroves were within the same order of magnitude as terrestrial forests. However, ER/GPP was lower in mangroves, explaining their higher carbon sequestration. We estimate the global mean mangrove NEP of 109.1 Tg C year−1 (7.4 Mg C ha−1 year−1) or through a budget balance, accounting for lateral losses, a global mean of 66.6 Tg C year−1 (4.5 Mg C ha−1 year−1). Overall, mangroves are highly productive, and despite losses due to respiration and tidal exchange, they are significant carbon sinks.
The effect of land use on the biogeochemistry of small tropical rivers and their estuaries was studied using the Kallada River and Ashtamudi estuary located in the State of Kerala, India, as a model ...system. Water, suspended matter and sediments collected during the monsoon and intermonsoon periods in 2002 and 2003 were analyzed for dissolved nutrients (nitrate, nitrite, phosphate, silicate) and for phytoplankton abundance and composition, amino acid contents and stable carbon (C)) and nitrogen (N) isotope ratios. Seasonal and spatial variations of dissolved nutrients and suspended matter along the course of the river point to distinct differences in the C and N sources that are controlled by hydrology, geology and land use. Unusually low concentrations of dissolved silicate and suspended matter suggest low erosion rates of the Precambrian basement rocks and the firm lateritic soils in non-agricultural areas. Most dissolved nutrients and suspended particulate organic matter originated from fertilized agricultural soils. The biogeochemistry of sedimentary organic matter indicates that most of the Kallada River load is deposited in the upper Ashtamudi estuary, while the middle and lower parts have a stronger marine influence. The spatio-temporal variation of dissolved and particulate river fluxes clearly indicates an effect of land use and land cover on the biogeochemistry of the Kallada River. While the phosphate yield was high (6 x 10³ mol km⁻² year⁻¹ or 185 kg km⁻² year⁻¹), the N yield was relatively low (10 x 10³ mol km⁻² year⁻¹ or 141 kg km⁻² year⁻¹), which is unlike the situation in many other densely populated regions of tropical Asia.
The biogeochemistry of the Dumai River estuary in eastern Sumatra, Indonesia, was studied in order to obtain information on the sources, transformation, and fate of organic matter. Between October ...and December 2003, water, total suspended matter (TSM), and sediments were sampled along a salinity gradient during four campaigns, and plants and soils were collected from the catchment. Water samples were analyzed for dissolved inorganic nutrients and dissolved organic carbon (DOC). The concentrations of organic carbon (Corg) and total nitrogen (N) and the stable carbon (δ13Corg) and nitrogen (δ15N) isotope distributions were determined in TSM, sediments, plants, and soils. The pH as well as the concentrations of dissolved inorganic nutrients and TSM were very low in the river and increased toward the sea. A maximum DOC concentration of $5,050 \mu mol L^{-1}$ was measured in the river, and concentrations decreased toward the sea. Low-gradient relief and a dense vegetation cover, and hence little weathering and erosion, appear to be responsible for low river loads of dissolved nutrients and TSM in this black-water river. Leaching from extensive peat soils in its catchment may account for the high DOC content of the Dumai River. Peat swamps drained by numerous small rivers are estimated to cover $3.3 times 10^{4} km^2$ in eastern Sumatra, suggesting that leaching of DOC may be a significant source of carbon to the adjacent coastal seas. A comparison with "normal" rivers shows that black-water rivers can export similar amounts of DOC from catchments that are orders of magnitude smaller. Thus, export from small black-water rivers may be quantitatively more significant for the global DOC input into the ocean than previously thought.
The role of organic carbon input, sedimentation rate and bottom water oxygenation (BWO) for organic matter (OM) degradation was investigated using surface sediments collected from above, within and ...below the oxygen minimum zone (OMZ) within varying sedimentary regimes along the Pakistan continental margin. For this, samples from 47 box cores and multi cores were analyzed for their bulk component (organic carbon, total nitrogen, carbonate, opal) and amino acid concentrations. The results are compared to plankton net samples from the Arabian Sea, sinking particles collected at the Pakistan continental margin, cultured sedimentary deep-sea bacteria, and data from the deep Arabian Sea basins. High organic carbon and amino acid concentrations were found in the laminated sediments of the OMZ, in bioturbated sediments immediately below the OMZ, and below regions of high productivity in the western Arabian Sea. The contribution of amino acids to organic carbon and total nitrogen as well as the molar contribution of certain non-protein amino acids do not mirror these differences in OC contents. No clear trend in relation to sediment structure or water depth is observed. In contrast to these established indicators of OM degradation, the molar contribution of the aromatic amino acid tyrosine (Tyr) exhibits a distinct pattern. Its contribution is relatively high in sediments at depths above the OMZ and further increases towards the center of the OMZ. Below the OMZ, it decreases continuously with increasing water depth. In addition, for samples from similar depths and bottom water oxygen contents Tyr shows systematic differences between varying sedimentary regimes. The lack of a distinct trend for the established indicators does not support the idea of a predominant control of BWO on quantitative OM preservation. Nevertheless, the variability of Tyr in relation to water depth or the sedimentary regime suggests that (i) the productivity-related OC input, (ii) the bulk accumulation rate, and (iii) BWO influence the alteration of the sedimentary OM by controlling its oxygen exposure time.
The South China Sea (SCS), characterized by a large continental shelf, is located at the edge of the Asian monsoon domain. In this study, two marine sediment cores from the northern SCS (NSCS) ...continental slope were investigated to construct composite vegetation and precipitation isotopic composition records based on the δ13C and δD values of plant-wax n-alkanes throughout the Holocene (last 11,200 years; i.e. 11.2 ka). The composite δ13Cwax record indicates an overall predominance of C3 vegetation over the last 11.2 ka. Before 8 ka BP, higher δ13Cwax values are attributed to preferential wax input from grassland and wetland biomes on the exposed continental shelf. After the inundation of the shelf by eustatic sea level rise until ca. 8 ka BP grassland and wetland biomes suffered a major size reduction and arboreal vegetation became better represented in the δ13Cwax record. The composite temperature corrected δDwax-T record suggests that moisture source variability drove precipitation isotopic composition changes during the Holocene. Lower δDwax-T values before 8.3 ka BP are interpreted as a larger moisture contribution by Pacific Ocean tropical cyclones, whereas higher δDwax-T values after 8.5 ka BP are interpreted as a larger moisture contribution from the Indian Ocean summer monsoon. Higher incidence of tropical cyclones in the NSCS during the Early Holocene was related to a temporary westward shift of the Western Pacific Warm Pool and enhanced insolation over the Northern Hemisphere. Both external and internal forcing mechanisms regulated moisture source changes in East Asia during the Holocene.
•Sea level change was the main driver of lowland vegetation cover in the Holocene.•Tropical cyclones are a large and often overlooked moisture source in East Asia.•Tropical cyclone incidence in Southeastern China-Taiwan was higher ca. 11.2–8.3 ka BP.•External and internal forcing mechanisms drive moisture source changes in East Asia.
Suspended matter, settling particles and sediments from the continental margin off the São Francisco, the largest river of eastern Brazil, were analyzed for amino acids, hexosamines and ...carbohydrates. Their concentrations and spectral distributions are comparable to those observed in other marine regions and exhibit variations which are related to the degree of dilution with lithogenic material. Preferential degradation and transformation of organic constituents within the water column during settling affect the nature and quantity of organic matter. The variations in the amino acid spectra indicate that high bottom water oxygen contents and low sedimentation rates are the factors responsible for severe degradation of organic matter at the sediment–water interface.
