Subsurface Sediments Research Articles

Chernobyl still with us: 137Caesium activity contents in seabed sediments from the Gulf of Bothnia, northern Baltic Sea

Anthropogenic radionuclides are among those human impacts, which can be seen widely in the marine and terrestrial ecosystems. Fallout from the 1986 Chernobyl nuclear power plant accident has rendered the Baltic Sea as the most polluted marine body in the world with respect to 137Cs.This research investigated sediment cores from 56 sites around the Gulf of Bothnia, Baltic Sea. Radioactivity from 137Cs in sediments has generally declined due to natural/radioactive decay of 137Cs over the last decades. However, 137Cs contents in subsurface sediments remain at elevated levels compared to pre-Chernobyl levels.The highest 137Cs activity contents in subsurface sediments (>4000 Bg kg−1) occur in coastal areas including estuaries. These areas often experience severe anthropogenic pressure. The southern Bothnian Sea, Kvarken archipelago, and southern Bothnian Bay all show elevated 137Cs values in subsurface sediments.Sedimentary 137Cs can also help constrain recent rates of sedimentation. Post-Chernobyl sedimentation rates in the Gulf of Bothnia varied from 0.1 to 4.8 cm/year with an average sedimentation rate of 0.54 cm/year.

Vertical distribution and effect of historical residual organochlorine pesticides on microbial community structure in sediment cores from an abandoned oxidation pond after dredging for 15 years.

The vertical distribution pattern of 19 organochlorine pesticides (OCPs), together with microbial ester-linked fatty acid methyl ester (EL-FAME) profiles were investigated in sediments from an abandoned oxidation pond of Ya-Er lake, China, which had been heavily polluted by hexachlorocyclohexanes (HCHs) and chlorobenzenes in 1980s. Subsurface sediment samples were taken from five sediment cores along the transect running from the lakeshore (0.5~2.7 m in depth) to lakebed (0.1~0.4 m). The total OCP concentration ranged from 29.8 to 941.8 ng g-1 dw. Hexachlorobenzene (HCB), HCHs, and dichlorodiphenyl-trichloroethanes (DDTs) were the three dominant OCP classes, accounting for 26.5-97.4%, 1.8-33.2%, and 0.4-15.5% of the total OCP concentration, respectively. Hot spots of HCB, HCHs, and DDTs were detected at 0.9~2.7 m deep layers of the lakeshore, where was oncethe maindredged sediment backfill site for in-situ remediation of the oxidation pond in 2002-2004. HCHs and HCBstill showed high potential ecological risks. The sources of OCPs were identified and quantified using principal component analysis with absolute principal component scores-multiple linear regression model. The firstthree major sources were persistent residues, recent agricultural input, and historical industrial input, contributing on average 28.2%, 17.9%, and 17.1% of total OCPs, respectively. Redundancy analysis of microbial EL-FAME profiles and nine dominant OCPs revealed that the spatial variation in microbial community structure was significantly corresponded with the OCP composition. This is the first study highlighting the concern on historical industrial inputs of OCPs in subsurface sediments of the lakeshore disposal zone. The findings could help to distinguish the artificial backfill sediments from undisturbed polluted sediments for optimization of further dredging plans.

Subsurface and Infaunal Foraminifera of Kemaman-Chukai Mangrove Swamps, East Coast Peninsular Malaysia

This study analysed the distribution and abundance of dead and live (Rose Bengal stained) infaunal foraminifera from three short cores taken at three locations in the mangrove swamps of Kemaman-Chukai, Terengganu, Malaysia. Eighteen agglutinated taxa were recorded in assemblages dominated by Arenoparrella mexicana, Haplophragmoides wilberti and Miliammina fusca; and of these, only two taxa were recorded as live. The distribution of subsurface and infaunal foraminifera varied from core to core, as did their depth of occurrence. Core 1 (seaward core) was dominated by sandy deposits, relatively high salinity (32 ppt), and extensive crab mounds, displayed very low numbers of dead foraminifera inconsistently throughout the core, while no infaunal foraminifera were observed, indicating intense bioturbation by mangrove crabs. In Core 2 (middle core), even though the numbers of live foraminifera decreased down-core, the number of dead or subsurface foraminifera were inconsistent, indicating taphonomic loss of the tests. Core 3 (landward core) however, displayed ideal foraminiferal distribution patterns required in the palaeo sea-level reconstruction (with less taphonomic loss and decreasing number of infaunal foraminifera downcore). Because of the similarity displayed in the foraminiferal assemblages in the 0–1 cm and 10–11 cm intervals, the surface sample (0–1 cm) should be an acceptable basis for down-core reconstructions in this study. Live (Rose Bengal stained) infaunal foraminifera, though observed at 40–41 cm depth, are not considered abundant enough to influence the dead assemblage in the subsurface sediment and its applicability for palaeoenvironmental and sea-level reconstructions. Therefore, it is possible for palaeo sea-level to be reconstructed based on foraminiferal assemblages preserved in the Kemaman-Chukai mangrove swamps.

Microbial Involvement in Carbon Transformation via CH4 and CO2 in Saline Sedimentary Pool.

Simple SummaryMethane and carbon dioxide are commonly found in the environment and are considered the most important greenhouse gases. Transformation of these gases is in large carried by microorganisms, which occur even in extreme environments. This study presents methane-related biological processes in saline sediments of the Miocene Wieliczka Formation, Poland. Biological activity (carbon dioxide and methane production or methane oxidation), confirmed by stable isotope indices, occurred in all of the studied Wieliczka rocks. CH4-utilizing microbes constituted 0.7–3.6% while methanogens (represented by Methanobacterium) only 0.01–0.5% of taxa present in the Wieliczka Salt Mine rocks. Water activity was the key factor regulating microbial activity in saline subsurface sediments. Generally, CO2 respiration was higher in anaerobic conditions while methanogenic and methanotrophic activities were dependent on the type of rock.Methane and carbon dioxide are one of the most important greenhouse gases and significant components of the carbon cycle. Biogeochemical methane transformation may occur even in the extreme conditions of deep subsurface ecosystems. This study presents methane-related biological processes in saline sediments of the Miocene Wieliczka Formation, Poland. Rock samples (W2, W3, and W4) differed in lithology (clayey salt with veins of fibrous salt and lenses of gypsum and anhydrite; siltstone and sandstone; siltstone with veins of fibrous salt and lenses of anhydrite) and the accompanying salt type (spiza salts or green salt). Microbial communities present in the Miocene strata were studied using activity measurements and high throughput sequencing. Biological activity (i.e., carbon dioxide and methane production or methane oxidation) occurred in all of the studied clayey salt and siltstone samples but mainly under water-saturated conditions. Microcosm studies performed at elevated moisture created more convenient conditions for the activity of both methanogenic and methanotrophic microorganisms than the intact sediments. This points to the fact that water activity is an important factor regulating microbial activity in saline subsurface sediments. Generally, respiration was higher in anaerobic conditions and ranged from 36 ± 2 (W2200%t.w.c) to 48 ± 4 (W3200%t.w.c) nmol CO2 gdw−1 day−1. Methanogenic activity was the highest in siltstone and sandstone (W3, 0.025 ± 0.018 nmol CH4 gdw−1 day−1), while aerobic methanotrophic activity was the highest in siltstone with salt and anhydrite (W4, 220 ± 66 nmol CH4 gdw−1 day−1). The relative abundance of CH4-utilizing microorganisms (Methylomicrobium, Methylomonas, Methylocystis) constituted 0.7–3.6% of all taxa. Methanogens were represented by Methanobacterium (0.01–0.5%). The methane-related microbes were accompanied by a significant number of unclassified microorganisms (3–64%) and those of the Bacillus genus (4.5–91%). The stable isotope composition of the CO2 and CH4 trapped in the sediments suggests that methane oxidation could have influenced δ13CCH4, especially in W3 and W4.

Recent morpho-sedimentary processes in Dove Basin, southern Scotia Sea, Antarctica: A basin-scale case of interaction between bottom currents and mass movements

Multibeam bathymetric imagery and acoustic sub-bottom profiles are used to reveal distribution patterns of sub-surface sedimentation in Dove Basin (Scotia Sea). The goals of the study are to determine the imprint of the inflow of deep Antarctic water masses from the Weddell Sea into the Scotia Sea, to establish the factors driving the styles of contourite deposition and to discern the relative contribution of alongslope versus downslope processes to the construction of the uppermost late Quaternary sedimentary record in the basin.The most significant morpho-sedimentary features in Dove Basin are linked to contouritic processes and to mass movements. Plastered drifts on the flanks of the basin constitute the most common contouritic deposits. Basement-controlled drifts on top of structural elevations are common along the central ridge, the central basin plain and scattered along the basin flanks. Sheeted drifts occur on top of adjacent banks or are restricted to the deep basin. In contrast, mounded drifts are poorly represented in Dove basin. A laterally extensive contouritic channel runs along the central ridge. Contouritic channels are also identified in the upper parts of the lateral banks and slopes. Numerous slide scars along the upper parts of the slopes evolve downslope into semitransparent lens-shaped bodies, with occasional development of across-slope channels. Semitransparent lenses occur intercalated within stratified deposits in the slopes of the basin, in the central ridge and in the deepest basin plain.The spatial arrangement of contouritic morphologies points to the influence of the water column structure and the basin physiography. In the eastern sub-basin, two different fractions (lower and upper) of Weddell Sea Deep Water (WSDW) leave an imprint on contourite deposits owing to the sloping interface between the two fractions. Contouritic influence is more subdued in the western sub-basin, and limited to the imprint of the lower WSDW. The upper parts of the surrounding banks are under the influence of deep-reaching Circumpolar waters (i.e., Lower Circumpolar Deep Water), which develops both depositional and erosional morphologies. The cross-section V-shaped morphology of the basin and the common occurrence of structural highs drive the predominance of plastered and basement-controlled drifts in the sediment record. The frequent alternation between contourites and downslope gravity-flow deposits is likely due to different processes associated with over-steepening in the basin, such as basement-controlled steep slopes, deformed drifts atop basement elevations, and the development of thick contouritic piles. Dove Basin is an example of a basin without mounded, plastered or mixed hybrid drifts in the transition between the lower slope and the deep basin, because the upper boundary of the deepest water mass —the Weddell Sea Deep Water— flows shallower along the middle slope. This fact underlines the relevance of the position and depth of water masses in shaping the morphology of the feet of slopes along continental margins.

Microbial Diversity and Function in Shallow Subsurface Sediment and Oceanic Lithosphere of the Atlantis Massif.

ABSTRACTThe marine lithospheric subsurface is one of the largest biospheres on Earth; however, little is known about the identity and ecological function of microorganisms found in low abundance in this habitat, though these organisms impact global-scale biogeochemical cycling. Here, we describe the diversity and metabolic potential of sediment and endolithic (within rock) microbial communities found in ultrasmall amounts (101 to 104 cells cm−3) in the subsurface of the Atlantis Massif, an oceanic core complex on the Mid-Atlantic Ridge that was sampled on International Ocean Discovery Program (IODP) Expedition 357. This study used fluorescence-activated cell sorting (FACS) to enable the first amplicon, metagenomic, and single-cell genomic study of the shallow (<20 m below seafloor) subsurface of an actively serpentinizing marine system. The shallow subsurface biosphere of the Atlantis Massif was found to be distinct from communities observed in the nearby Lost City alkaline hydrothermal fluids and chimneys, yet similar to other low-temperature, aerobic subsurface settings. Genes associated with autotrophy were rare, although heterotrophy and aerobic carbon monoxide and formate cycling metabolisms were identified. Overall, this study reveals that the shallow subsurface of an oceanic core complex hosts a biosphere that is not fueled by active serpentinization reactions and by-products.

Linking the Surface and Subsurface in River Deltas—Part 1: Relating Surface and Subsurface Geometries

AbstractRiver deltas are densely populated regions of the world with vulnerable groundwater reserves. Contamination of these groundwater aquifers via saline water intrusion and pollutant transport is a growing threat due to both anthropogenic and climate changes. The arrangement and composition of subsurface sediment is known to have a significant impact on aquifer contamination; however, developing accurate depictions of the subsurface is challenging. In this work, we explore the relationship between surface and subsurface properties and identify the metrics most sensitive to different forcing conditions. To do so, we simulate river delta evolution with the rule‐based numerical model, DeltaRCM, and test the influence of input sand fraction and steady sea level rise (SLR) on delta evolution. From the model outputs, we measure a variety of surface and subsurface metrics chosen based on their applicability to imagery and modeling results. The Kullback‐Leibler (KL) divergence is then used to quantitatively gauge which metrics are most indicative of the imposed forcings. Both qualitative observations and the KL divergence analysis suggest that estimates of subsurface connectivity can be constrained using surface information. In particular, more variable shoreline roughness values and higher surface wetted fraction values correspond to increased subsurface connectivity. These findings complement traditional methods of estimating subsurface structure in river‐dominated delta systems and represent a step toward the identification of a direct link between surface observations and subsurface form.

Response to substrate limitation by a marine sulfate-reducing bacterium.

Sulfate-reducing microorganisms (SRM) in subsurface sediments live under constant substrate and energy limitation, yet little is known about how they adapt to this mode of life. We combined controlled chemostat cultivation and transcriptomics to examine how the marine sulfate reducer, Desulfobacterium autotrophicum, copes with substrate (sulfate or lactate) limitation. The half-saturation uptake constant (Km) for lactate was 1.2 µM, which is the first value reported for a marine SRM, while the Km for sulfate was 3 µM. The measured residual lactate concentration in our experiments matched values observed in situ in marine sediments, supporting a key role of SRM in the control of lactate concentrations. Lactate limitation resulted in complete lactate oxidation via the Wood-Ljungdahl pathway and differential overexpression of genes involved in uptake and metabolism of amino acids as an alternative carbon source. D. autotrophicum switched to incomplete lactate oxidation, rerouting carbon metabolism in response to sulfate limitation. The estimated free energy was significantly lower during sulfate limitation (-28 to -33 kJ mol-1 sulfate), suggesting that the observed metabolic switch is under thermodynamic control. Furthermore, we detected the upregulation of putative sulfate transporters involved in either high or low affinity uptake in response to low or high sulfate concentration.

Bacterial community structure and functional profiling of high Arctic fjord sediments.

Kongsfjorden, an Arctic fjord is significantly affected by the glacier melt and Atlantification, both the processes driven by accelerated warming in the Arctic. This has lead to changes in primary production, carbon pool and microbial communities, especially that in the sediment. In this study, we have examined the bacterial community structure of surface (0-2cm) and subsurface (3-9cm) sediments of Kongsfjorden using the high throughput sequencing analysis. Results revealed that bacterial community structure of Kongsfjorden sediments were dominated by phylum Proteobacteria followed by Bacteroidetes and Epsilonbacteraeota. While α- and γ-Proteobacterial class were dominant in surface sediments; δ-Proteobacteria were found to be predominant in subsurface sediments. The bacterial community structure in the surface and subsurface sediments showed significant variations (p ≤ 0.05). Total organic carbon could be one of the major parameters controlling the bacterial diversity in the surface and subsurface sediments. Functional prediction analysis indicated that the bacterial community could be involved in the degradation of complex organic compounds such as glycans, glycosaminoglycans, polycyclic aromatic hydrocarbons and also in the biosynthesis of secondary metabolites.

Do methanogenic archaea cause reductive pyrite dissolution in subsurface sediments?

Do methanogenic archaea cause reductive pyrite dissolution in subsurface sediments?

Living Benthic Foraminifera from the Surface and Subsurface Sediment Layers Applied to the Environmental Characterization of the Brazilian Continental Slope (SW Atlantic)

Living benthic foraminifera (&gt;63 µm) were studied to characterize the continental slope of the Potiguar Basin (SW Atlantic). Foraminifers from the surface (0–2 cm), subsurface (2–5 cm), and integrated (0–5 cm) sediment layers were analyzed to verify their contribution to environmental characterization. It was also estimated if and which changes occur when the subsurface is added. Sampling stations were distributed in five transects in four isobaths (150, 400, 1000, and 2000 m). Sediment samples were fixed with 4% buffered formaldehyde and stained with Bengal rose. Were recorded 396 species in the surface layer, 228 in the subsurface, and 449 in integrating both layers. This study did not include tubular agglutinated species. The assemblages from 150 m isobath indicated the upper slope, from 400 m indicated the middle slope and the ones from the 2000 m indicated the lower slope. The surface layer’s assemblage at 1000 m isobath was more similar to the middle slope; in contrast, its subsurface layer’s assemblage had more similarity with the lower slope. Rarefaction curves, Permanova, and NMDS routines indicated a high resemblance between surface and integrated layers. Therefore, the first two centimeters were sufficient to characterize this region based on living benthic foraminifera.

Effect of natural rainfall on the migration characteristics of runoff and sediment on purple soil sloping cropland during different planting stages

Abstract Due to the difficulty in monitoring subsurface runoff and sediment migration, their loss loads are still not clear and need further study. This study monitored water and soil loss occurring within experimental field plots for two calendar years under natural rainfall events. The sediment loss load was quantified by considering the corresponding water flow flux and its sediment concentration. The results showed that 60.04% of the runoff and 2.83% of the sediment were lost underground. The annual underground sediment loss reached up to 54.6 kg*ha−1*yr−1. A total of 69.68% of the runoff yield and 67.25% of the sediment yield were produced during the corn planting stage (CPS: March–July). Heavy rain and torrential rain events produced 94.45%, 65.46% of the annual runoff and 94.45%, 76.21% of the sediment yields during the corn-planting stage and summer fallow period (SFP: August–September). The rain frequency, rainfall, and rainfall duration of each planting stage significantly affected the resulting runoff and sediment yield. Measures aimed at the prevention and control of water-soil loss from purple soil sloping land should focus heavily on torrential rain and heavy rain events during the CPS and SFP. This paper aims to provide a practical reference for quantifying the water and soil loss from purple soil sloping cropland.

Comparative insights into genome signatures of ferric iron oxide- and anode-stimulated Desulfuromonas spp. strains

BackgroundHalotolerant Fe (III) oxide reducers affiliated in the family Desulfuromonadaceae are ubiquitous and drive the carbon, nitrogen, sulfur and metal cycles in marine subsurface sediment. Due to their possible application in bioremediation and bioelectrochemical engineering, some of phylogenetically close Desulfuromonas spp. strains have been isolated through enrichment with crystalline Fe (III) oxide and anode. The strains isolated using electron acceptors with distinct redox potentials may have different abilities, for instance, of extracellular electron transport, surface recognition and colonization. The objective of this study was to identify the different genomic signatures between the crystalline Fe (III) oxide-stimulated strain AOP6 and the anode-stimulated strains WTL and DDH964 by comparative genome analysis.ResultsThe AOP6 genome possessed the flagellar biosynthesis gene cluster, as well as diverse and abundant genes involved in chemotaxis sensory systems and c-type cytochromes capable of reduction of electron acceptors with low redox potentials. The WTL and DDH964 genomes lacked the flagellar biosynthesis cluster and exhibited a massive expansion of transposable gene elements that might mediate genome rearrangement, while they were deficient in some of the chemotaxis and cytochrome genes and included the genes for oxygen resistance.ConclusionsOur results revealed the genomic signatures distinctive for the ferric iron oxide- and anode-stimulated Desulfuromonas spp. strains. These findings highlighted the different metabolic abilities, such as extracellular electron transfer and environmental stress resistance, of these phylogenetically close bacterial strains, casting light on genome evolution of the subsurface Fe (III) oxide reducers.

Using a multi-proxy approach to locate the elusive Phoenician/Persian anchorage of Tel Akko (Israel)

Previous geoarchaeological research on the Akko coastal plain have contributed to the understanding of the ancient coastal interface and added evidence as to the location/shift of the ancient anchorages dating from the Middle Bronze Age (beginning of the 2nd Millennium BC) to the Early Hellenistic period (mid of the 2nd century BC) of the ancient site of Tel Akko. The present research provides new insights into the environmental changes and likely anchorage sites along the western edge of Tel Akko in the 1st Millennium BC (Iron Age II and III, periods associated with the Phoenician mariners and Persian army incursion). Our approach for locating the anchorage is based on a detailed investigation of subsurface sediments combining sedimentological and faunal analysis and radiocarbon dating of cores as well as identification of ceramic sherds found in the cores, and ground penetrating radar (GPR) and electrical resistivity tomography (ERT) surveys. Paleoenvironmental changes are compared and contrasted with the results of the archaeological investigations on the tell and in its vicinity. Our new data demonstrates that the Phoenician/Persian maritime interface of Tel Akko was mainly oriented toward the southwestern area of the tell where a natural anchorage was likely to have been located. At that time, the water depth in this area was ca. 2m, allowing for the anchorage of seagoing vessels. Increasing sediment deposition lead to the deterioration of direct, and eventual loss of access, to the sea. These conditions initiated the abandonment of the tell in the Early Hellenistic period as well as the westward shift to habitation on the peninsula, now the ‘Old city of Akko’, the Crusaders' Saint Jean d’Acre.

Methanogenic archaea use a bacteria-like methyltransferase system to demethoxylate aromatic compounds

Methane-generating archaea drive the final step in anaerobic organic compound mineralization and dictate the carbon flow of Earth’s diverse anoxic ecosystems in the absence of inorganic electron acceptors. Although such Archaea were presumed to be restricted to life on simple compounds like hydrogen (H2), acetate or methanol, an archaeon, Methermicoccus shengliensis, was recently found to convert methoxylated aromatic compounds to methane. Methoxylated aromatic compounds are important components of lignin and coal, and are present in most subsurface sediments. Despite the novelty of such a methoxydotrophic archaeon its metabolism has not yet been explored. In this study, transcriptomics and proteomics reveal that under methoxydotrophic growth M. shengliensis expresses an O-demethylation/methyltransferase system related to the one used by acetogenic bacteria. Enzymatic assays provide evidence for a two step-mechanisms in which the methyl-group from the methoxy compound is (1) transferred on cobalamin and (2) further transferred on the C1-carrier tetrahydromethanopterin, a mechanism distinct from conventional methanogenic methyl-transfer systems which use coenzyme M as final acceptor. We further hypothesize that this likely leads to an atypical use of the methanogenesis pathway that derives cellular energy from methyl transfer (Mtr) rather than electron transfer (F420H2 re-oxidation) as found for methylotrophic methanogenesis.

Biogeography, Assembly Patterns, Driving Factors, and Interactions of Archaeal Community in Mangrove Sediments.

ABSTRACTArchaea are a major part of Earth’s life. They are believed to play important roles in nutrient biogeochemical cycling in the mangrove. However, only a few studies on the archaeal community in mangroves have been reported. In particular, the assembly processes and interaction patterns that impact the archaeal communities in mangroves have not been investigated to date. Here, the biogeography, assembly patterns, and driving factors of archaeal communities in seven representative mangroves across southeastern China were systematically analyzed. The analysis revealed that the archaeal community is more diverse in surface sediments than in subsurface sediments, and more diverse in mangroves at low latitudes than at high latitudes, with Woesearchaeota and Bathyarchaeota as the most diverse and most abundant phyla, respectively. Beta nearest-taxon index analysis suggested a determinant role of homogeneous selection on the overall archaeon community in all mangroves and in each individual mangrove. In addition, the conditionally rare taxon community was strongly shaped by homogeneous selection, while stochastic processes shaped the dominant taxon and always-rare taxon communities. Further, a moderate effect of environmental selection on the archaeal community was noted, with the smallest effect on the always-rare taxon community. Mangrove location, mean annual temperature, and salinity were the major factors that greatly affected the community composition. Finally, network analysis revealed comprehensive cooccurrence relationships in the archaeal community, with a crucial role of Bathyarchaeota. This study expands the understanding of the biogeography, assembly patterns, driving factors, and cooccurrence relationships of the mangrove archaeal community and inspires functional exploration of archaeal resources in mangrove sediments.IMPORTANCE As a key microbial community component with important ecological roles, archaea merit the attention of biologists and ecologists. The mechanisms controlling microbial community diversity, composition, and biogeography are central to microbial ecology but poorly understood. Mangroves are located at the land-ocean interface and are an ideal environment for examining the above questions. We here provided the first-ever overview of archaeal community structure and biogeography in mangroves located along an over-9,000-km coastline of southeastern China. We observed that archaeal diversity in low-latitude mangroves was higher than that in high-latitude mangroves. Furthermore, our data indicated that homogeneous selection strongly controlled the assembly of the overall and conditionally rare taxon communities in mangrove sediments, while the dominant taxon and always-rare taxon communities were mainly controlled by dispersal limitation.

Natural and anthropogenic organic matter inputs to intertidal deposits of the urbanized Arctic region: A multi-proxy approach

Growing anthropogenic activity in the Arctic, especially near regional municipal and transportation hubs, threatens coastal environments with significant changes and contamination. A detailed study of a wide set of organic-geochemical proxies in surface and subsurface sediments and soils around the typical urbanized Arctic area (Tromsø, Norway) was performed to investigate natural and anthropogenic input of hydrocarbons. The sites were located close to main areas of human activity and differed in the level of anthropogenic pressure: oil terminal, boat harbor and recreation beach zone in the city center, and a remote, conditionally-clean area. The samples from each site were collected along four subtidal – intertidal – supratidal transects. GC–MS data on the distribution of more than 110 compounds were obtained including n-alkanes, isoprenoids, terpanes, parent polycyclic aromatic hydrocarbons (PAHs), and alkyl PAHs. Organic matter embedded in a mineral matrix was more thermally mature than its fresh non-bound fraction. Biomarker distribution and diagnostic ratios attested to the predominantly local origin of near-shore organic matter formation (phytoplankton, algae, high foliar plants, grass). PAH composition revealed a significant input of pyrogenic components from liquid and solid fuel combustion to the surface coarse-grained sediments. Background bacterial activity was evidenced by the high content of biohopanes and hopenes in less polluted sediments and soils. The presence of an unresolved complex mixture and even n-alkanes in sediment profiles from the most polluted sites attested to the active biodegradation that, together with the erosion of the upper layers by tidal currents, reduces the toxic compound penetration deeper into the sediments.

Low Light Availability Reduces the Subsurface Sediment Carbon Content in Halophila beccarii From the South China Sea.

Eutrophication, dredging, agricultural and urban runoffs, and epiphyte overgrowth could reduce light availability for seagrass. This may affect “blue carbon” stocks in seagrass beds. However, little research is available on the effect of light intensities on carbon sequestration capacity in seagrass beds, especially small-bodied seagrasses. The dominant seagrass Halophila beccarii, a vulnerable species on the IUCN Red List, was cultured in different light intensities to examine the response of vegetation and sediment carbon in seagrass beds. The results showed that low light significantly reduced leaf length and above-ground biomass, while carbon content in both above-ground and below-ground tissues were not affected. Low light reduced both the above-ground biomass carbon and the total biomass carbon. Interestingly, while under saturating light conditions, the subsurface and surface carbon content was similar, under low light conditions, subsurface sediment carbon was significantly lower than the surface content. The reduction of subsurface sediment carbon might be caused by less release flux of dissolved organic carbon from roots in low light. Taken together, these results indicate that reduced light intensities, to which these meadows are exposed to, will reduce carbon sequestration capacity in seagrass beds. Measures should be taken to eliminate the input of nutrients on seagrass meadows and dredging activities to maintain the “blue carbon” storage service by enhancing light penetration into seagrass.

Seasonality in Saharan Dust Across the Atlantic Ocean: From Atmospheric Transport to Seafloor Deposition

AbstractSaharan dust is transported in great quantities from the North African continent every year, most of which is deposited across the North Atlantic Ocean. This dust impacts regional and global climate by affecting the atmospheric radiation balance and altering ocean carbon budgets. However, little research has been carried out on time series of Saharan dust collected in situ across the open Atlantic. Here, we present a unique three‐year time series of Saharan dust along a trans‐Atlantic transect, sampled by moored surface buoys and subsurface sediment traps. Results show a good correlation between the particle‐size distributions of atmospheric dust and the lithogenic particles settling to the deep ocean floor, confirming the aeolian origin of the lithogenic particles intercepted by the subsurface sediment traps, even in the distal western part of the Atlantic Ocean. Dust from both dry and wet deposition as collected by the sediment traps, shows increased deposition fluxes and coarser grain size in summer and/or autumn that coincides with increased precipitation at the sampling sites as derived from satellite data. In contrast, both buoys that sampled dust during transport at sea level show little seasonal variation in both concentration and particle size, as the large amounts of dust transported in summer and early autumn at high altitudes are far above their sampling range. This implies that wet deposition in summer and autumn defines the typical seasonal trends of both the dust deposition flux and its particle‐size distribution observed in the sediment traps.

Luminescence dating of holocene siliciclastic sediments in eastern Dahomey Basin, southwestern Nigeria

Several attempts at reconstructing geological settings and palaeoclimatic changes of the siliciclastic deposits of the Dahomey Basin, SW Nigeria, using relative age dating and correlation methods, have resulted in serious discrepancies on the ages. Therefore, a chronology framework established by an absolute age dating method is requisite to constrain the geological interpretation. This research focuses on quartz optically stimulated luminescence (OSL) dating of the upper siliciclastic sediments to help bridge the lacuna that arose from previous relative geologic dating. Ten sub-surface sediment samples were collected from the eastern part of the basin, and quartz OSL dating using single-aliquot regenerative-dose protocol was conducted for all the samples. The OSL signals appear well bleached prior to deposition and the OSL ages are reliable and robust. Through the application of OSL, the age framework of the uppermost part of sediments in the study area was established. The OSL dating results revealed that these depositional periods fall within the Holocene and are concentrated during two groups: 3.52 ka–1.55 ka, and 0.64 ka–0.05 ka. The samples with ages of 3.52 ka–1.55 ka distribute in the belt-like inland zone approximately parallel to the coastline. This deposition episode appears to be caused by the decrease in relative sea level during late Holocene. Thus, this study sheds light on the understanding past coast dynamics in the region.