Carbon Content Of Sediments Research Articles

A 23000 year old record of paleoclimatic and environmental changes from the eastern Arabian Sea

Abstract Climate of the eastern Arabian Sea is highly influenced by both global climate and Indian monsoon. We produced multi-proxy records of benthic foraminiferal biofacies and stable isotope ratio (δ13C and δ18O), planktic foraminifera and total organic carbon (wt%) content of sediments from core SK291/GC11 in the upper continental shelf off Mangalore, off eastern Arabian Sea (EAS) to decipher paleoenvironmental and paleoclimatic changes during the past ~23,000 years. The monsoon was weak during the Last Glacial Maximum ~23,000–18,000 calibrated years before Present (cal yr BP) when sea level was lower. Globigerinoides ruber populations fluctuate from 30 to 60% in the studied interval with a maximum during ~6000–5000 cal yr BP and a minimum during 19,000–17,000 cal yr BP. The δ18O values of benthic foraminifer Ammonia gaimardii suggest pronounced cooling phases during ~23,000–18,000 cal yr BP, and a short-term cooling at ~6000 cal yr BP. Several intervals of high organic productivity and low oxygen conditions were identified based on benthic foraminiferal biofacies during 23,247–682 cal yr BP. Wind-induced surface mixing was intense during 3200–530 cal yr BP as evidenced by high Globigerina bulloides abundances. The core was affected by moderate chemical weathering and solar variability appears to have been an important forcing factor in shaping climate of the region.

Paleoclimatic reconstruction and the response of carbonate minerals during the past 8000 years over the northeast Tibetan Plateau

The climatic significance of the carbonate content and composition of lake sediments remains unclear. In this study, we analyzed the glycerol dialkyl glycerol tetraethers and mineral contents of Alake Lake sediments to reconstruct temperature and moisture changes over the northeast Tibetan Plateau since 8000 cal yr BP. We identified warm (cold) and wet (dry) conditions during the Roman Warm Period and Holocene Climate Optimum (Early Neoglacial Cooling Period) accompanied by a dominance of aragonite (monohydrocalcite). Based on our results and previous laboratory experiments, we suggest that monohydrocalcite is predominantly deposited in colder, drier, and higher Mg/Ca conditions relative to that of aragonite. The carbonate mineral composition should therefore be fully considered when sediment carbonate contents are used to reconstruct paleotemperature/moisture changes.

How well can near infrared reflectance spectroscopy (NIRS) measure sediment organic matter in multiple lakes?

Loss-on-ignition (LOI) is the most widely used measure of organic matter in lake sediments, a variable related to both climate and land-use change. The main drawback for conventional measurement methods is the processing time and hence high labor costs associated with high-resolution analyses. On the other hand, broad-based near infrared reflectance spectroscopy (NIRS) is a time and cost efficient method to measure organic carbon and organic matter content in lacustrine sediments once predictive methods are developed. NIRS-based predictive models are most robust when applied to sediments with properties that are already included in the calibration dataset. To test the potential for a broad applicability of NIRS models in samples foreign to the calibration model using linear corrections, sediment cores from six lakes (537 samples, LOI range 1.03–85%) were used as reference samples to develop a predictive model. The applicability of the model was assessed by sequentially removing one lake from the reference dataset, developing a new model and then validating it against the removed lake. Results indicated that NIRS has a high predictive power (RMSEP < 4.79) for LOI with the need for intercept and slope correction for new cores measured by NIRS. For studies involving many samples, NIRS is a cost and time-efficient method to estimate LOI on a range of lake sediments with only linear bias adjustments for different records.

Segregation and retention of As, potentially toxic metals, and organic pollutants in a reservoir from the Ohře River (the Czech Republic)

This work addresses the fates of elements from historical mining that peaked in the 16th (Ag, Cu, Pb, and Sn mining) and 20th (U) centuries, polycyclic aromatic hydrocarbons (PAHs), and polar pesticides in the Nechranice Dam Reservoir in the Ohře River. A steady fluvial input from secondary pollution (As, Cu, Pb, Sn, Zn) enabled the study of pollutant segregation during deposition and post-depositional migration in sediments. These elements are present in concentrations close to toxic levels for aquatic life, which will prevent the achievement of a favourable ecological status in the Ohře River, which is similar to many rivers in central and western Europe. The reservoir bottom morphology was acquired by sonar imaging. The sediments were then sampled by extracting 23 gravity cores to describe the distribution of pollution in sediments with sufficient reliability. The inflow delta was characterised by a sub-bottom profiler, aerial imaging, and photogrammetric modelling. X-ray fluorescence spectroscopy and chromatography were used for sediment analysis. The catchment-specific Hg and U pollution peaks provided local chemostratigraphic markers of the pre-dam/reservoir sediment boundary. The Sn-bearing pollutant particles were mostly retained immediately after entering the Nechranice Reservoir. Concentrations of Cu, Pb, and Zn are lower in the reservoir basin than in the inflow delta. The sediments of the reservoir basin with water depths > 20 m are enriched in the top layer with Mn species, which may be relevant for the fates of some pollutants. Arsenic shows a post-depositional enriched peak near the base of the Mn-enriched layer. The PAH concentrations normalised to the total organic carbon content (TOC) in the basin sediments are indirectly proportional to the Mn/Fe ratio, showing their oxidative degradation in the Mn-enriched strata. Our work reveals the important behaviour of pollutants in dam reservoirs, which has not yet been studied adequately to the relevance of the historical burdens in current fluvial systems, secondary fluvial pollution, and future reservoirs with historically polluted catchments. The inflow delta is a highly dynamic sedimentary system responding to changes in water level. The studied reservoir has behaved as a settling system with post-depositional chemical reactions relevant to pollutant fates. The retention of Mn(III,IV) oxides that is considerably enhanced by river damming might play a positive role in the degradation of modern organic pollutants. Arsenic in the sediments undergoes translocation to sharp subsurface peaks.

Characterization of seagrasses at two new locations in the Eastern Tropical Pacific (El Jobo and Matapalito, Costa Rica)

Seagrasses in the Eastern Tropical Pacific (ETP) region are currently only found at a limited number of locations. Seagrass meadows are a threatened coastal habitat worldwide, and there is already a report of seagrass loss in the ETP. Here, we characterize seagrasses at two new locations in Costa Rica, El Jobo and Matapalito, to provide further information on spatial and temporal variability of seagrasses in the ETP. Seagrasses consisted of monospecific Halophila baillonii meadows, which is a small colonizing species with a limited global distribution. Seagrasses were present at Matapalito on all three dates between August 2017 and May 2018, while seagrasses were only found at El Jobo on the first visit. Seagrass biomass at these new locations was 8.4 ± 5.3 g DW m−2 and shoot density 1556 ± 785 shoots m−2. Leaf length was 0.9 ± 0.3 cm, width 0.4 ± 0.1 cm, area 1.2 ± 0.6 cm2, and LAI 0.20 ± 0.15 m2 m−2. Seagrass flowers were not encountered. Biomass Corg percentage was 30 ± 4 % and Corg content was 0.3 ± 0.2 Mg Corg ha-1. Seagrass sediment contained 1.5 ± 0.4 % Corg and 17.4 ± 12.1 % Cinorg. Sediment bulk density was 1.1 ± 0.2 g ml-1 and Corg content was 15.7 Mg Corg ha-1. Halophila baillonii δ13C was -9.4 ± 0.9‰ and δ15N was 4.0 ± 0.8 ‰; while sediment δ13C was -20.1 ± 1.0 ‰ and δ15N was 8.0 ± 1.0 ‰. Seagrass contribution to sediment carbon content was ∼ 50 %. Seagrass morphometrics and carbon content are within reported ranges for Halophila meadows. The reason for lack of seagrasses at El Jobo on subsequent samplings remains unclear. Continued research into seagrass spatial and temporal dynamics in the ETP, and their associated environmental conditions, is currently needed for adequate seagrass conservation and management initiatives in this region.

A quantitative assessment of organic carbon content as a regional sediment-condition indicator

Organic carbon content of sediments, whether directly or indirectly assessed, has often been used as an indicator of marine benthic community condition both in site-specific and regional scale condition assessment studies. The conceptual framework underlying use of this indicator was developed based primarily on site-specific studies. A quantitative analysis of literature data on sediment organic matter impacts in marine systems was conducted to determine whether biotic metrics respond to abiotic indicators of sediment organic content, as predicted by conceptual models, at larger spatial scales. The ability to detect predicted decreases in community metrics (abundance, species richness, species diversity index H', biomass) varied among metrics, with best performance by species richness and H'. There was significant added variation both between and within analytical approaches (loss on ignition, total organic carbon methods), emphasizing the need for careful cross calibration and quality control in studies with multiple laboratory partners. High levels of variability for biotic metrics versus organic carbon metrics appear typical for large scale studies, and organic matter source, site depth, and individual estuarine system differences were important sources of variation. Covariation of organic matter content with percent fine sediments is another known source of variation, but various normalization methods may be inadequate due to inherent sources of variation at estuary level. While likely still useful for point-source studies, multiple major sources of variation appear to limit the usefulness of sediment organic content as a benthic condition indicator at larger spatial scales.

A comparative study of methods for determining carbonate content in marine and terrestrial sediments

Abstract Carbonate can be produced in a variety of environments on the Earth's surface, such as shallow seas and deep oceans, continental lakes, soils and subsurface aquifers. The carbonate content of sediments is an important proxy in paleoceanography and paleoclimatology and is often used to reconstruct paleoenvironmental evolution of the earth. Many methods have been reported to measure the content of carbonate in sediments, but how those methods perform (e.g. degree of accuracy and precision, error-induced factors, operability, cost and time) remains understudied. Here, we employ three widely-used non-instrumental methods, including acid-base titration, gasometric method, and loss on ignition (LOI) based on muffle combustion, together with X-ray diffraction for mineral phase analysis, to compare the carbonate content data of sediments from deep ocean (water depths from 1218 m to 6147 m) of the western Pacific and Cenozoic sedimentary rocks from the Qaidam basin, northern Tibetan Plateau. The results show that data accuracies of most methods are closely related to mineral compositions in sediments (e.g. carbonate species and clay mineral contents) and analytical operations. Non-instrumental methods generally demonstrate low precisions (avg. RSD of 3.36%) in determining sediments with low carbonate contents (

Pharmaceuticals and personal care products in a drinking water resource of Yangtze River Delta Ecology and Greenery Integration Development Demonstration Zone in China: Occurrence and human health risk assessment

The occurrence, partition, and human health risk of thirteen pharmaceuticals and personal care products (PPCPs) have been investigated in surface water, overlying water, pore water and sediment samples from Dianshan Lake of Yangtze River Delta Ecology and Greenery Integration Development Demonstration Zone in China. PPCPs were ubiquitous in aqueous phase and sediments from Dianshan Lake. Sulfamethazine (SMZ) was dominated in surface water and overlying water, while ketoprofen (KPF) was rich in sediment. The total concentration of PPCPs ranged from 0.38–85.27 ng/L, 24.26–130.03 ng/L and 5.39–149.84 μg/kg in surface water, overlying water and sediment, respectively, which were in middle levels compared with these reported in other aquatic environment in China. Naproxen (NPX), sulfadimethoxine (SDM), sulfamethoxazole (SMX) and sulfamethazine (SMZ) in surface water showed a relatively higher level in lake side than those in lake center suggesting that a mixed containment source of human- and animal-derived from the areas around lake. The significant season variations of most PPCPs were mainly attributed to their usage, water temperature and dilution effect. The partition behaviors of PPCPs in sediment-overlying water and sediment-pore water system were mainly affected by their logKow values, and showed weak correlation with total organic carbon (TOC) content in sediment and molecular weights of PPCPs. Preliminary results indicated that PPCPs in Dianshan Lake have not posed a high risk to human health by exposure to drinking water for all age groups. Nevertheless, their potential to cause the mixture toxicity and resistance genes cannot be neglected. This work will contribute to the clear picture of PPCPs contamination in drinking water source in the Demonstration Zone, and provide reliable and simple-to-use information to regulators on the exposure and risk levels of PPCPs, as well as recommendations for future research.

Characteristics of organic matter sources from Guadiana Estuary salt marsh sediments (SW Iberian Peninsula)

Estuaries are dynamic interfaces between land, rivers and the ocean that play major roles in the global carbon cycle. These coastal wetlands store huge amounts of organic carbon (OC), commonly known as “blue carbon” and excellent places to study C cycling. The Guadiana river estuary is among the most important tidal salt marshes in the South - Iberian coastal margin. Here, a detailed organic geochemical study is described that includes the identification of sedimentary OM composition at a molecular and isotopic level. Total organic carbon content (TOC) of core sediments ranged from 0.39 to 2.23% and stable carbon isotope composition (δ13C) also showed a wide range between −22.4 and −27.0‰. A13C depletion trend observed from the surface to bottom in the core sediments profiles, reflects loss of labile biogenic 13C enriched compounds i.e. polysaccharides and a selective preservation of more depleted compounds with depth i.e. lignin and lipids in the core sediments. Series of n-alkanes were found in the range from C10 to C31. Carbon preference index ratio (CPI) calculated for long-chain n-alkanes (C24–C31) that ranged between 1.17 and 1.94 reflecting diverse OM inputs to the sediments. A study of the lignin-derived phenolic composition pointed to a recalcitrant OM derived from both gymnosperm and angiosperm plants. Moreover, high abundance of vinyl phenol and vinyl guaiacol points to a dominant contribution of lignins from grasses and aquatic macrophytes to the sediments. A well-resolved series of long-chain linear alkyl benzenes (LABs from C4 to C22) were recorded in all core sediment samples indicating direct discharge of untreated domestic and/or industrial effluents to the estuary. Polycyclic aromatic hydrocarbons (PAHs) with more than 3 rings reflects the pyrogenic origin of a portion of the sedimentary OM. This study highlights the importance of different OM sources to the lower Guadiana estuarine sediments and contributes to a better knowledge about its origin, dynamics and fate.

Spatial and seasonal variations of organic carbon distributions in typical intertidal sediments of China

Intertidal zones are the main sites for land-ocean interaction and play an important role in transporting and accumulating organic carbon (OC). Understanding sources and preservation of sedimentary OC are vital to better evaluate the processes controlling the OC distribution in intertidal zones. In this study, we present bulk parameters of total organic carbon (TOC), δ13Corg, grain size and biomarker contents in surface sediments of 14 typical intertidal zones along China’s coastline during both the wet season (WS) and the dry season (DS). Statistical analysis of our data set showed that TOC and biomarker contents were generally high in small estuaries, intermediate but more variable in large estuaries and low in non-estuaries; and these distribution patterns broadly correlated with sedimentary clay contents. TOC and biomarker contents were generally higher in the WS than the DS reflecting both OC inputs and preservation. A binary mixing model based on δ13Corg and biomarker proxies revealed that large estuaries were characterized by higher terrestrial OC fractions (62–80%) except for the Yellow River Estuary (24%), while lower terrestrial OC fractions occurred in non-estuaries (13–15%). Variable OC contributions were found in small estuaries, with most sites characterized by low terrestrial OC contributions (3–35%), but high terrestrial OC contributions were found at the Qiantang River Estuary site (59%) and the Jiulong River Estuary site (67%). Carbon stocks in Chinese intertidal sediments are comparable with those in marine sediments from shelf areas and the Exclusive Economic Zone (EEZ). This study suggests that intertidal sediments are potentially an important carbon sink.

Species choice in mangrove reforestation may influence the quantity and quality of long-term carbon sequestration and storage

Despite carbon sequestration being an important service of mangrove ecosystems, many mangrove reforestation projects have little consideration of the carbon sequestration capacity of species to be planted. Species selection is mostly based on growth rate and convenience in planting. In this study, to compare the quantity and quality of carbon stored in soil, four habitats were selected in Haijiang River Estuary, southern China to assess the contribution by different mangrove species to sediment carbon pool. Two 12-year-old mangrove forests of the exotic Sonneratia apetala and native Kandelia obovata, respectively, and the adjacent sandflat and mudflat as unvegetated referencing sites had been studied. The total sedimentary organic carbon and active sedimentary organic content in sediment suggested that after 12 years of growth, (1) mangrove forests significantly increased the organic carbon content of sediment; (2) total organic carbon in the K. obovata forest was higher than that of the S. apetala forest; but (3) the carbon pool of the K. obovata forest was less stable than that of the S. apetala forest. These results corroborated with other studies that the sediment carbon pool of S. apetala forests reached a stable state after 13 years of growth, while that of K. obovata forests gradually stabilised upon long-term (>13 years) growth. Our study confirms that K. obovata is more conducive to capture carbon in long-term mangrove reforestation projects, demonstrating that the provision of this service may not be directly related to apparently relevant plant traits such as growth rate.

A Unique Benthic Microbial Community Underlying the Phaeocystis antarctica-Dominated Amundsen Sea Polynya, Antarctica: A Proxy for Assessing the Impact of Global Changes

Polynyas in the polar seas are regarded as windows through which ecosystem responses associated with global climate changes are to be noticed. However, little information is available on benthic microbial communities in the Amundsen Sea polynya (ASP), where environmental changes due to global warming are occurring rapidly, from which future climate change-induced ecosystem responses could be assessed. We performed high-throughput sequencing of 16S rRNA genes and quantitative PCR in combination with biogeochemical analyses and metabolic rate measurements to determine the composition, diversity and controls of major microbial communities in sediments of the ASP. A large fraction of the sequenced benthic microbial community (40% on average) in the polynya was uniquely affiliated with the phylum Planctomycetes, whereas Thaumarchaeota (51% on average) predominated in non-polynya areas. The relative abundance of Planctomycetes correlated significantly with organic carbon (Corg) content in the polynya sediment underlying the Phaeocystis-dominated water column. These results suggest that Planctomycetes comprise a major bacterial group utilizing relatively recalcitrant Corg produced primarily by Phaeocystis blooms. In contrast, the predominance of chemolithoautotrohic Thaumarchaeota in the sea-ice zone was attributed to low Corg supply due to low primary productivity in the ice-covered water column. The Planctomycetes-dominated microbial communities in the ASP is in stark contrast to that Proteobacteria (Delta- and Gammaproteobacteria) occupy ecological niches as primary mineralizers of organic materials in most benthic systems in the Southern Ocean, where organic materials in the sediments mostly originate from diatom blooms. Given that microbial communities respond quickly to environmental changes, and that global climate change is proceeding rapidly in the ASP, our results suggest that any modifications in the Planctomycetes-dominated microbial communities will provide valuable insight into changes in organic matter transport to the seafloor that may result from shifts in planktonic communities from Phaeocystis to diatoms associated with climate change.

Estimates of the remineralization and burial of organic carbon in Lake Baikal sediments

Sediment cores collected from several stations throughout Lake Baikal in water depths from 100 m off the Selenga River delta to the deepest basin of the lake (~1640 m), have been analyzed for sedimentary organic carbon, nitrogen, and the remineralized components in pore water. The organic carbon content of surface sediments generally varied from 2.3 to 3.2% by weight, and profiles typically showed an exponential decrease in both organic carbon and nitrogen in the upper 20–30 cm of the sediment column. Steady state models of organic matter diagenesis yield first order decomposition rate constants which range from 0.0009 to 0.022 y−1. The calculated residence times for the metabolizable fraction of the organic matter in these sediments increases roughly with increasing water depth and is on the order of 50–300 years. Pore water concentration profiles were determined for dissolved inorganic carbon, dissolved organic carbon (DOC), methane, and dissolved ammonium. At depth (25–30 cm) methane concentrations ranged from 50 to 800 μmol Lpw−1 and DOC from 400 to 900 μmol Lpw−1. Estimation of carbon recycling rates based upon diffusion along pore water concentration gradients at the sediment-water interface, indicate that combined DOC and methane fluxes generally contribute <15% of the overall turnover of sedimentary organic carbon. Comparisons to Laurentian Great Lakes environments show trends in sediment deposition, organic matter remineralization, and the time scales of carbon recycling across nearly two orders of magnitude with the fraction of organic content buried generally decreasing with decreasing sedimentation rates.

Assessment of Sediment and Benthic Macrofauna Distribution at Dar es Salaam Harbour Channel

Sediment and macrobenthic fauna distribution at Dar es Salaam harbour channel were investigated to attest the influence of sediment grain size distribution and total organic carbon contents on benthic macrofauna dominance and diversity. Sampling campaign was conducted from January to April, 2019 in 25 sampling stations along the channel. Sediment grain sizes ranged between 0.7405 and 0.00273 mm with an average of 0.0804 mm classified as medium silt. Sediment distribution showed an onshore-offshore pattern consisting of fine dark clay-silt sediments in the southern part of the harbour close to Mwalimu Nyerere Bridge, medium (sandy) at the harbour and coarse sandy offshore. Five classes of macrofauna were identified with sedentary Polychaetes (Diopatra cuprea) found in tubes and free forms being dominant in most of the sediment types, however more abundant in clay-silt. High species diversity index values were encountered in sediments with medium total organic carbon (TOC) and sediment grain sizes (sand-silt). Apparently, low species similarity indices were observed in all sediment type indicating significant dissimilarities in species composition among sediment grain sizes. Thus, the distribution of benthic macrofauna species along Dar es Salaam harbour channel is strongly influenced by the sediment grain sizes and organic carbon contents in sediments.

СОВРЕМЕННЫЕ ПРОБЛЕМЫ ИССЛЕДОВАНИЯ ЦИКЛА УГЛЕРОДА В АЗОВСКОМ МОРЕ

A review of the carbon cycle studies in the coastal waters of the World Ocean was carried out. The authors of the literature reviews emphasize the importance of regional studies to refine estimates of the contribution of coastal waters to the global carbon balance, as well as their response to global climate change. In addition, understanding the carbon cycle in coastal waters is necessary to address a range of environmental, social and economic issues. This work presents the results of the synthesis of scientific knowledge about different elements of the carbon cycle in the Sea of Azov: primary production of organic matter (OM), OM and calcium carbonate content in water and bottom sediments of the sea, lateral sources of organic carbon and carbonates. The highly productive Sea of Azov was mainly considered earlier in terms of its fish resources. Current understanding of the balance of organic carbon, the saturation of the waters of the Sea of Azov with free carbonic acid, and the exchange of CO2 with the atmosphere are based on estimates made in the middle of the 20th century, while changes in the ecosystem of the Sea of Azov associated with climate fluctuations and anthropogenic stress have been well documented since the mid-1980s. The following questions become relevant. Does the Sea of Azov continue to be a source of CO2 into the atmosphere? If so, why is this happening and what are the main factors; how this process is distributed in time and space; what should be expected in the future? If this is not the case, then it is important to understand what and why influenced it the most? A number of scientific hypotheses are formulated that require further development.

Examining the correlation between quantifiable SVOCs and organic carbon content or particulate size in benthic sediments as a function of ocean stratum.

The raw analytical concentration values of semi-volatile organic compounds (SVOCs) in ocean sediments do not necessarily reflect the bioavailable fractions of SVOCs in the sediments due to factors such as the total organic carbon content of sediments (TOC) and the percentage of fine particles in sediments (% fines) as they are believed to affect the extraction efficiency of SVOCs from sediments. Corrective actions are therefore taken to minimize their influence on measurements. In doing so, a broad and uniform correlation is assumed to exist between the 'native' levels of SVOCs and TOC or % fines across ocean strata. However, the validity of this blanket assumption is not yet verified. In this study, we examined the strength of the assumption using DDTs, PAHs and PCBs levels in sediments from Santa Monica Bay (SMB), California, USA. (The distribution patterns of these SVOCs in SMB are known and reproducible for quality assurance.) As our results show, a uniform correlation between SVOC levels and TOC or % fines across strata is mostly absent. For example, PAH and PCB levels show negative correlation with TOC or % fines, and only in canyon sediments, DDT levels correlate positively with both TOC and % fines across at least three strata. Furthermore, the distribution of PAH molecules appears to be controlled by molecular size with smaller PAHs being found almost exclusively in the canyons. Our finding here, being the first of its kind, suggests that more work is needed to clarify the reporting of SVOC levels in ocean sediments.

The influence of fluids on the species composition and preservation of microfossils in biogenic carbonate sediments by hydrothermal cluster “Pobeda” (Mid-Atlantic ridge)

The species composition, distribution and preservation of carbonate shells of nanno and microfossils of biogenic bottom sediments within the hydrothermal cluster Pobeda (1707.451708.7 n) were studied on the basis of materials obtained in flight No. 37 of NIS Professor Logachev in the Russian exploration area (Mid-Atlantic ridge). Physical-chemical parameters and the total carbonate content of sediments were measured, their changes in column sections were revealed. For complexes of coccoliths studied precipitation is attributed to the top (acme) biostratigraphic zone of Emiliania huxleyi. It is established that within the zone of hydrothermal activity (ore cluster Pobeda) the total number of microorganisms decreases and their species diversity decreases, as a rule, in the lower part of the columns. Down the column section also decreases the Eh, pH and carbonate precipitation. These changes are explained by the influence of diffuse hydrothermal fluids.

Effects of Fe oxides on organic carbon variation in the evolution of clayey aquitard and environmental significance

As an important part of groundwater systems, aquitards may have a considerable impact on the quality of groundwater in an aquifer. Organic carbon (OC) serves as an important component in biogeochemical processes which affects the hydrochemical composition of pore water; the association of OC with (Iron) Fe-containing minerals is recognized as an important stabilization mechanism for OC. However, the characteristics of the process forming complexes between OC and Fe oxides in subsurface aquitards and the contribution of OC to aquifers has been poorly understood. In this study, the content, speciation and reaction types of OC and Fe oxides were investigated to reveal the characteristics and mechanisms of OC-Fe interaction in subsurface aquitards at different depths in two typical sedimentary facies of the Jianghan Plain in central China. The total organic carbon (TOC) contents of aquitard sediments in alluvial-lacustrine and residual slope facies were 7.25 ± 5.10 and 2.79 ± 0.86 mg g−1, respectively. In general, the proportion of the heavy fraction of OC (HFOC) to TOC gradually increased with increasing depth because consumption of the light fraction of OC (LFOC) that caused more HFOC to remain in sediment. On average, Fe bound-OC contributed 30.3% and 31.6% of TOC by adsorption and coprecipitation, respectively. The adsorption-stabilized OC has not changed obviously but the coprecipitation-stabilized OC increased gradually with depth. Coprecipitation stabilized more OC in a stable environment (residual slope facies) when compared with an unstable environment (alluvial-lacustrine facies), which can be supported by the greater average ratio of FePP bound-OC:TOC ratio and increased enrichment of carboxylates and aromatics in a stable environment. The transformation of OC-Fe complexes could affect the transport of As, Cr and ammonium which chemically bind to organic matter (OM) and Fe minerals from sediment to pore water by reductive dissolution. The findings of this study are helpful in understanding the interaction between OC and Fe oxides in subsurface aquitard environments, in which reaction products may affect adjacent aquifers.

Quantifying the carbon content of aeolian sediments: Which method should we use?

Total organic carbon (TOC) and inorganic carbon (carbonates) are important soil factors and paleoenvironmental proxies used worldwide. During past decades, several different methods have been proposed for determining these parameters from various sediment archives. Although the automatic elemental analyzer is gradually replacing the traditional quantification methods, it is critical to assess the reliability of all currently used methods to ensure accurate regional comparisons, carbon stock estimations and long-term carbon-cycle dynamics modelling. In this study, three most commonly used methods for determining TOC and carbonate content are compared using sediment samples from a sand-paleosol sequence located in northeastern China. Pearson's correlation between results of the different TOC and carbonate quantification methods produced high correlation coefficients, confirming substantial consistency among them. The automatic carbon analyzer yielded more accurate values with smaller uncertainties, while results of the general chemical method and loss-on-ignition (LOI) produced relatively reliable values with more data variability. Meanwhile, combustion at 500 °C to quantify the TOC content yielded significantly higher values than the automatic carbon analyzer and chemical methods, indicating that LOI method is not as reliable as previously thought. We attribute the overestimation to unsuitable SOM to TOC conversion factors and/or the presence of volatile minerals in the samples. In contrast with other studies, chemical wet oxidation of the sand-paleosol sediments did not significantly overestimate TOC, which commonly occurs in modern soils and lacustrine sediments. Our results suggest that the reliability of different methods for measuring carbon content could be affected by the sediment's origin and environment. Assessment of the results of the different TOC and carbonate quantification methods on additional types of sediment is needed so that researchers can use the most appropriate method for their study and make appropriate paleoenvironmental reconstructions given the sediments and methods used.

Environmental trends between 2400 and 1200 BP in Barrancas, Argentinean Puna: Impacts on local resource variability and socioeconomic organization

AbstractThis paper presents a multiproxy analysis—geomorphology, organic matter, and carbonate content, diatoms, pollen, and magnetic properties of sediments—of the Barrancas Pueblo profile, located in Barrancas, Jujuy, Argentina (S 23°18′08,7; W 66°05′15,2; 3666 m.a.s.l.) to explore local environmental change over the last few millennia. This study is part of a broader investigation of the environmental conditions that facilitated and/or triggered the development of a mixed herding and hunting economic strategy during the late Holocene. The results suggest ongoing local moisture availability for most of the late Holocene; between 2400 and 1500 BP there was a stable, low energy environment that supported a vegetated floodplain, resulting in high availability of pasture and water. However, throughout most of the studied period, there were punctual arid episodes and erosion of the river catchment. High environmental variability post‐3000 cal BP could have put a premium on strategies to reduce the risk associated with resource unpredictability, such as economic specialization and intensification and storage practices designed to control and mitigate resource variability. This process could have ultimately lead to the consolidation of the Andean pastoralist way of life ca. 3000–2500 BP.