Bulk Sediment Research Articles

Elutriate preparation affects embryo development test with Paracentrotus lividus: An in-depth study on the differences between two protocols and three different sediment/water mixing times

Coastal areas are under continuous and increasing pressure from different human activities. A mixture of contaminants (e.g. hydrocarbons, pesticides, persistent organic pollutants (POPs), emerging contaminants, and others), originating mainly from populated, industrialised and agricultural areas, can reach the marine environment through different means such as wastewater discharge, soil runoffs, leaching from agriculture, and volatilisation/deposition. In this context, marine sediments have increasingly been considered repositories for a variety of pollutants that can accumulate and be stored for long periods, acting as a secondary source of contaminants during subsequent dredging operation or vessel manoeuvring. Chemical and ecotoxicological analyses of sediments are routinely conducted to evaluate the potential hazard/risk to the environment, either on bulk sediment or elutriate. In general, sediment elutriates are commonly prepared according to ASTM Guide even if alternative protocols are proposed by USACE for the various condition that they have to represent. The goal of the present study was to determine if the toxicological properties of ASTMprepared elutriates are comparable to those obtained from the USACE protocol. Sediment coming from 3 harbours (Olbia, Cagliari, and Toulon), as part of the “Se.D.Ri.Port” Interreg Project, were processed to obtain elutriates according to ASTM Guide and USACE Dredging Elutriate protocol and tested with the sea urchin Paracentrotus lividus embryo development test. Moreover, the significance of different stirring times of water/sediment mixture (1 h, 3 h, and 24 h) was tested with both the ASTM and USACE protocol. In addition to the biological analysis, for each sediment sample, heavy metals concentration, granulometry, and organic matter were determined. Even if for the ports of Toulon and Cagliari, the ASTM and USACE elutriates showed comparable results with P. lividus bioassay, for the port of Olbia the two protocols showed different criticalities. Preliminary results show that for the site Olbia elutriates prepared with the USACE protocol resulted in higher toxicity than elutriates obtained with ASTM (p < 0.001). In conclusion, differences in preparation protocols appear to be significant and can lead to different results in biological testing. To overcome this problem and to obtain more reliable evaluations of risk to the environment, standardisation and regulation must be the next goals in sediment management procedure.

In situ magnetic identification of giant, needle-shaped magnetofossils in Paleocene–Eocene Thermal Maximum sediments

Near-shore marine sediments deposited during the Paleocene-Eocene Thermal Maximum at Wilson Lake, NJ, contain abundant conventional and giant magnetofossils. We find that giant, needle-shaped magnetofossils from Wilson Lake produce distinct magnetic signatures in low-noise, high-resolution first-order reversal curve (FORC) measurements. These magnetic measurements on bulk sediment samples identify the presence of giant, needle-shaped magnetofossils. Our results are supported by micromagnetic simulations of giant needle morphologies measured from transmission electron micrographs of magnetic extracts from Wilson Lake sediments. These simulations underscore the single-domain characteristics and the large magnetic coercivity associated with the extreme crystal elongation of giant needles. Giant magnetofossils have so far only been identified in sediments deposited during global hyperthermal events and therefore may serve as magnetic biomarkers of environmental disturbances. Our results show that FORC measurements are a nondestructive method for identifying giant magnetofossil assemblages in bulk sediments, which will help test their ecology and significance with respect to environmental change.

Whether the diagram of intergranular volume versus cementation volume is normative? —Take “depositional and compositional controls on diagenesis of the mixed siliciclastic-volcaniclastic sandstones: A case study of the Lower Cretaceous in Erennaoer Sag, Erlian Basin, NE China” for example

The diagrams of intergranular volume versus cementation volume brought up by Houseknecht (1987) and Ehrenberg (1989), are extensively used around the globe to assess the relative significance of compaction and cementation to porosity reduction in sandstones. However, Houseknecht (1987) overlooked the fact that the bulk sediment volume is dynamically compressed during burial, which directly leads to deviation in evaluating original porosity loss. The present discussion is based on the theoretical and contrastive analysis. These analyses indicate the compactional porosity loss calculated by Ehrenberg's (1989) method is always greater than Houseknecht's (1987) method with a maximum difference of 12.7%. In contrast, the cementation porosity loss using Ehrenberg's (1989) method is always less than Houseknecht's (1987). Besides, the demonstration based on the data of Wei et al. (2020) also supports Ehrenberg (1989). Therefore, the method proposed by Ehrenberg (1989) should be highly recommended to be granted as a reference far and wide.

Controls on the abundance, provenance and age of organic carbon buried in continental margin sediments

Continental margins play a fundamental role in the carbon cycle as primary oceanic locations of organic carbon (OC) burial. However, gaps remain in our understanding of factors controlling the distribution and preservation of organic matter (OM) in these heterogeneous and dynamic systems. In particular, the impact of hydrodynamic processes on the age, abundance, and stable isotopic composition of sedimentary OC is poorly constrained. Here, we characterize the OC present in bulk and grain-size sediment fractions from seven continental margin settings. Our results reveal that hydrodynamic particle sorting processes exert a ubiquitous influence on the radiocarbon age of OC. Both, hydrodynamic characteristics of mineral particles and the nature of their interactions with OM influence sedimentary OC content, whereas no significant influence of either effect is manifested in corresponding δ13C values. Since OC preferentially resides within the fine silt fraction (2-8 μm), and this fraction accounts for a substantial fraction of the bulk sediment mass, translocation and subsequent re-deposition of distant fine silt has the greatest potential to distort local OC signatures relative to those associated with clay or coarse silt fractions. We suggest that the magnitude of differences in 14C-age and OC content among grain-size fractions, determined by the interplay of hydrodynamic sorting and other site-specific processes, allow three different categories of depositional environment to be defined: initial, stable, and mature. Each domain is characterized by different degrees of vertical and lateral OC supply that reflect influences of local biological productivity and carbon export from overlying surface waters and physical forcing that drive hydrodynamic processes. This generic framework may serve as a guide to refine assessment of OC burial and to constrain the magnitude of potential aliasing among co-eval proxy signals in continental margin sedimentary sequences.

Late Jurassic temperatures for the southern Tethyan margin based on belemnites δ18O from the Ouarsenis Mountains, northwestern Algeria

This study presents the first record of stable carbon and oxygen isotopes of well-preserved belemnites and bulk sediment from the Upper Jurassic pelagic carbonates of the Ouarsenis Mountains (northwestern Algeria), which were deposited at the southern margin of the Tethys ocean. Cathodoluminescence and scanning electron microscopy was used to eliminate potentially diagenetically altered specimens (brachiopods, aptychi, and some belemnites). Ultimately, 55 belemnite specimens were used to reconstruct water temperatures for the Middle Oxfordian to the Early Tithonian. Their δ18Ocarb values ranged from −0.35‰ to −3.88‰, suggesting warm and mostly stable temperature conditions during the Late Jurassic. Using a δ18Osea value of 0‰ for the tropical palaeolatitude of 15–20°N, the δ18Ocarb values of the well-preserved belemnite rostra translate into average temperatures of 27.8 °C in the Middle Oxfordian (Gregoryceras transversarium Zone), 27.0 to 27.5 °C in the Late Oxfordian (Epipeltoceras bimmamatum and Idoceras planula zones, respectively), 26.2 °C in the Early Kimmeridgian, and 27.3 °C in the Early Tithonian (Hybonoticeras hybonotum Zone).

Detailed Characterization and Monitoring of a Retrogressive Thaw Slump from Remotely Piloted Aircraft Systems and Identifying Associated Influence on Carbon and Nitrogen Export

Ice-rich permafrost landscapes are sensitive to ongoing changes in climate. Permafrost retrogressive thaw slumps (RTSs) represent one of the more abrupt and prolonged disturbances, which occur along Arctic river and lake shorelines. These features impact local travel and infrastructure, and there are many questions regarding associated impacts on biogeochemical cycling. Predicting the duration and magnitude of impacts requires that we enhance our knowledge of RTS geomorphological drivers and rates of change. Here we demonstrate the utility of remotely piloted aircraft systems (RPAS) for documenting the volumetric change, associated drivers and potential impacts of the largest active RTS along the Old Crow River in Old Crow Flats, Yukon, Canada. RPAS surveys revealed that 29,174 m3 of sediment was exported during the initial evacuation in June 2016 and an additional 18,845 m3 continued to be exported until June 2019. More sediment export occurred during the warmer 2017 summer that experienced less cumulative rainfall than summer 2018. However, several rain events during 2017 were of higher intensity than during 2018. Overall mean soil organic carbon (SOC) and total nitrogen (TN) within sampled thaw slump sediment was 1.36% and 0.11%, respectively. A combination of multispectral, thermal and irradiance (derived from the RPAS digital surface model) data provided detailed classification of thaw slump floor terrain types including raised dry clay lobes, shaded and relatively stable, and low-lying evacuation-prone sediments. Notably, the path of evacuation-prone sediments extended to a series of ice wedges in the northern headwall, where total irradiance was highest. Using thaw slump floor mean SOC and TN values in conjunction with sediment bulk density and thaw slump fill volume, we estimated that 713 t SOC and 58 t TN were exported to the Old Crow River during the three-year study. Findings showcase the utility of high-resolution RPAS datasets for refining our knowledge of thaw slump geomorphology and associated impacts.

Spatial distribution and contamination assessment of heavy metals in surface sediments of Lake Burullus, Egypt

The potentially bio-available heavy metals were assessed in the bulk and the three fine fraction sediments (O3, O4, and O5) from 24 stations across Lake Burullus. We found the same pattern of metal contents in all fractions with the sequence: Fe > Mn > Zn > Cu ≥ Ni > Pb > Cd. The recorded concentrations of Cu, Zn, Ni, Mn, and Fe in Burullus were higher than the coastal area outside the lagoon. Some metals showed trend of different spatial distribution inside the lake; the highest Cu (40.7–82.2 μg/g) and Mn (495.6–690.7 μg/g) was recorded in the middle sector and Fe (1912.6–2017.4 μg/g) recorded in the western sector of the lake. The concentrations of Cu in O4 (41.4–82.2 μg/g) and O5 (28.2–76.2 μg/g) were relatively higher than in the total sediments (5.8–44.8 μg/g); meanwhile, Fe and Mn in fine fractions reached 1.2–1.5 times higher than bulk sediments. Correlation coefficient and principal component analysis (PCA) showed strong association between Cu, Zn, and Ni. On the other hand, there was partial association of Mn with Fe indicating anthropogenic sources from the industrial and agriculture drains. The calculated environmental and biological indices pointed out that the leachable metals in Lake Burullus were classified between unpolluted and moderately polluted. The potential biological adverse effects were in minimum susceptibility because the levels of metals were predominantly below the sediments severe effect levels (SEL) quality guidelines.

Salt tolerance during germination identifies native intertidal plant species at risk under increasing salinity with sea level rise

Climate change and sea level rise are causing an increase in salinity in coastal salt marshes around the world. These changes may alter the germination dynamics of some halophytes, threatening their conservation. Our study area (Atlantic coast of the southwestern Iberian Peninsula) is currently being affected by sea level rise, and increases in sediment salinity have been recorded. We studied the germination responses during and after a wide range of imposed salinities (0-600 mM) of 5 Asteraceae species; 3 colonizing salt marshes (Artemisia caerulescens, Aster tripolium and Inula crithmoides) and 2 (Dittrichia spp.) inhabiting adjacent non-tidal environments. In addition, habitat characteristics (plant cover, sediment redox potential, electrical conductivity, pH, water content and bulk density) associated with each marsh species were measured and compared with the spatial distribution of the Asteraceae species in the field and their capacity to germinate at different salinities. The germination syndrome of Aster tripolium and Inula crithmoides was characterized as germinating in salinity ranging from fresh water to 600 mM salinity (36 ppt). The rare halophyte A. caerulescens and the 2 non-tidal Dittrichia species presented a germination syndrome based on no germination at high salinities and high recovery germination (55-80%). Considering the high sensitivity of A. caerulescens to salinity during germination, active conservation measures are recommended to promote its preservation in the Mediterranean Basin. These include creating new populations and promoting salt marsh connectivity to facilitate the processes of species mobility and marsh migration to uplands where sediment salinity is lower.

Effects of climatic factors on the sedimentation trends of Tarbela Reservoir, Pakistan

The study explores the factors affecting the sedimentation trends in the surrounding catchment area of the Tarbela Dam for the period 1980–2017, specifically rain and temperature. Furthermore, this study also investigates the seasonal effects of sediment loads coming into the reservoir from 2007 to 2012. The statistical techniques were applied for the estimation of the effects of climate (rainfall and temperature) on the sedimentation and the inflow of Tarbela Dam and its surrounding catchment area. The results revealed that sediment deposition pattern points toward the greater bed elevation at 7–12 range lines that lie near the Main Embankment Dam, also that Barandu River carried more bulk of inflow to the Indus, as compared to Siran River and thus contributed a greater amount of sediments to the reservoir. Summer monsoon season also contributed to the greatest bulk of sediments to the Tarbela Dam and its surrounding catchment area. The regression confirmed that for one degree increase in temperature there will be a 7 Million Acre Feet (MAF) increase; for a 1 mm (mm) increase in rainfall there will be a 1 MAF decrease and for a 1 Cusec increase in the inflow coming into the reservoir, there be a 4 MAF increase in sedimentation of the reservoir. Thus, the contribution of the temperature in sediment generation is found to be greater than rainfall. The study recommended that for the proper functioning of Tarbela Dam, the shift in the climate of the Upper Indus Basin (UIB) might be regularly monitored as rising temperatures in UIB would result in greater glacial melt and hence greater sediments to settle down into the reservoirs downstream.

Brittlestar diversity at the dawn of the Jenkyns Event (early Toarcian Oceanic Anoxic Event): new microfossils from the Dudelange drill core, Luxembourg

Abstract Ophiuroids, the slender-armed cousins of starfish, constitute an important component of modern marine benthos and have been used successfully in the exploration of (palaeo)-ecological and evolutionary trends, yet their fossil record is still poorly known. One of the major gaps in the known palaeobiodiversity of this group coincides with a global palaeoenvironmental crisis during the early Toarcian (Early Jurassic, 183 myr ago), known as the Jenkyns Event. Here we describe ophiuroid remains retrieved from a series of samples from the Dudelange (Luxembourg) drill core, which spans the lower part of the Toarcian, between the top of the Pliensbachian and the onset of the Jenkyns Event. A total of 21 species are recorded, including three new genera and 12 new species. Ophiuroid diversity and abundance fluctuate in parallel with depositional facies, with lowest values coinciding with black shales. Highest diversities, including exceptional occurrences of taxa nowadays restricted to deep-sea areas, are recorded from just below the black shales, corresponding to the onset of the Jenkyns Event. Our results show that even small (100 g) bulk sediment samples retrieved from drill cores can yield numerous identifiable ophiuroid remains, thus unlocking this group for the study of faunal change across palaeoenvironmental crises.

Multiple sulfur isotope chemostratigraphy across the Permian–Triassic boundary at Chaotian, China: Implications for a shoaling model of toxic deep‐waters

AbstractPrevious studies on multiple sulfur isotopes (32S, 33S, and 34S) in sedimentary pyrite at the end‐Permian suggested a shoaling of anoxic/sulfidic deep‐water contributing to the extinction. This scenario is based on an assumption that the sedimentary sulfur cycle was largely controlled by benthos activity, though a stratigraphic correlation between the sulfur records and ichnofabrics of the sediments at the end‐Permian has not yet been examined. We report the multiple sulfur isotopic composition of pyrite in the Permian–Triassic boundary interval at Chaotian, South China. Our data can be generally explained by a mixing of sulfur in sulfide from two different sources: one produced via sulfate reduction in an open system with respect to sulfate and the other produced in a closed system. In particular, the former with the substantially low δ34S (&lt;−40 ‰) and high ∆33S (up to +0.100 ‰) values was likely produced via water‐mass sulfate reduction or via sulfate reduction in oxic sediments with common burrows. The frequent occurrence of small pyrite framboids (mostly &lt;5 μm in diameter) in the Lopingian (Late Permian) Dalong Formation of deep‐water facies supports the enhanced water‐mass sulfate reduction in an anoxic deep‐water mass. The negative ∆33S values are observed only in the oxic limestones, and no substantial ∆33S change is observed across the extinction horizon despite of the disappearance of bioturbation. Our results are apparently inconsistent with the previous shoaling model. We expand the model and infer that, when the deep‐water was sulfidic and its shoaling rate was high, a substantial amount of hydrogen sulfide (H2S) was supplied onto the shelf via the shoaling; that resulted in the positive ∆33S value of the bulk sediments. The observed ∆33S variation on a global scale suggests a substantial variation in H2S concentration and/or in upwelling rate of shoaling deep‐waters during the Permian–Triassic transition.

Topographic, lithospheric and lithologic controls on the transient landscape evolution after the opening of internally-drained basins. Modelling the North Iberian Neogene drainage

The opening of internally-drained (endorheic) sedimentary basins often leads to a major drainage change, re-excavation of the basin sedimentary infill, and transient landscape. The timing of such basin openings can be dated only in exceptional cases in which the youngest sedimentary infill remains preserved. For this reason, the processes and timing involved in their transient landscape evolution are poorly known. We explore the role of erodibility, basin geometry and flexural isostasy during the capture of internally-drained basins by means of numerical modelling techniques constrained by recent terrace cosmogenic dating and geomorphological analysis, addressing the issue as to why the Duero and Ebro rivers, draining two Cenozoic sedimentary basins in N Iberia with similar geographical dimensions and drainage histories, have undergone a markedly different erosion evolution leading to distinctly different present morphology. To evaluate how these intrinsic parameters affect the transient landscape evolution, we design a synthetic scenario inspired by those basins. The results show that, once a basin becomes externally drained, its drainage integration and erosion rates are strongly dependent on (1) the basin elevation above the base level; (2) the width of the topographic barrier, (3) its erodibility; and (4) the rigidity of the lithosphere. The results show that transient landscape evolution can last for tens of millions of years even in absence of tectonic activity and changes in base level or climate. Basins isolated by wide and resistant barriers such as the Duero Basin may undergo a multi-million-year time lag between drainage opening and basin-wide incision. In the case of the Duero Basin, this delay may explain the paradoxical time lag between the last lacustrine bulk sedimentation dated at 9.6 Ma and the onset of widespread basin incision variously estimated at 3.7 to 1 Ma.

Chronology of geomorphological changes in a cantabrian mountain valley over the last 20,000 years.

This is an in-depth study of a mixed deposit of terrace and alluvial fan in a valley of the Cantabrian Mountains that aims to characterize environmental conditions during the period following the Last Glacial Maximum (LGM) at the end of the Pleistocene and the entire Holocene, including human influence to the present day. In this perspective, the lithostratigraphy of different layers was described, the granulometric analysis performed in situ and in the laboratory, the organic bulk sediment dated by radiocarbon and weight, and other empirical studies, such as paleomagnetism were performed. The results reveal a temporal sequence of deposition from around 19.4 ky BP to the present showing different sedimentary environments. Paleoclimatical scientific literature in sorrounding areas complemented the interpretation of the results e.g. pleistocene glaciers, speleothems, palynology and geomorphology. We conclude a completely diferent evolution of studied section between Late-Pleistocene and Holocene. Bottom layers, composed of coarse boulders, are interpreted in the context of a second glacial push after the Local Last Glacial Maximum under conditions of torrentiality. In the upper layers, in the Holocene, there was an increase in sedimentation from slopes through alluvial and colluvial cones, and rivers lost their capacity to transport these sediments. Important environmental changes in the Middle Holocene, 5.5 ky BP, were detected with possible evidence of human action. A decrease from that moment in the deposited sediment caliber was verified.

Patterns in forage fish mercury concentrations across Northeast US estuaries

Biogeochemical conditions and landscape can have strong influences on mercury bioaccumulation in fish, but these effects across regional scales and between sites with and without point sources of contamination are not well understood. Normal means clustering, a type of unsupervised machine learning, was used to analyze relationships between forage fish (Fundulus heteroclitus and Menidia menidia) mercury (Hg) concentrations and sediment and water column Hg and methylmercury (MeHg) concentrations, ancillary variables, and land classifications within the sub-watershed. The analysis utilized data from 38 sites in 8 estuarine systems in the Northeast US, collected over five years. A large range of mercury concentrations and land use proportions were observed across sites. The cluster correlations indicated that for Fundulus, benthic and pelagic Hg and MeHg concentrations were most related to tissue concentrations, while Menidia Hg was most related to water column MeHg, reflecting differing feeding modes between the species. For both species, dissolved MeHg was most related to tissue concentrations, with sediment Hg concentrations influential at contaminated sites. The models considering only uncontaminated sites showed reduced influence of bulk sediment MeHg for both species, but Fundulus retained sediment drivers at some sites, with dissolved MeHg still highly correlated for both. Dissolved organic carbon (DOC), chlorophyll, land use, and other ancillary variables were of lesser importance in driving bioaccumulation, though DOC was strongly related within some clusters, likely in relation to dissolved Hg. Land use, though not of primary importance, showed relationships opposite to those observed in freshwater, with development positively correlated and forests and agriculture negatively correlated with tissue concentrations across clusters and species. Clusters were composed of sites from geographically distinct systems, indicating the greater importance of small scale drivers of MeHg formation and uptake into the food web over system or region-wide influences.

The Influence of Hydrothermal Fluids on the REY-Rich Deep-Sea Sediments in the Yupanqui Basin, Eastern South Pacific Ocean: Constraints from Bulk Sediment Geochemistry and Mineralogical Characteristics

Rare earth elements (REEs) and yttrium (Y), together known as REY, are extremely enriched in deep-sea pelagic sediments, attracting much attention as a promising new REY resource. To understand the influence of hydrothermal processes on the enrichment of REY in deep-sea sediments from the eastern South Pacific Ocean, we conducted detailed lithological, bulk sediment geochemical, and in situ mineral geochemical analyses on gravity core sample S021GC17 from the Yupanqui Basin of eastern South Pacific. The REY-rich muds of S021GC17 are dark-brown to black zeolitic clays with REY contents (ΣREY) ranging from 1057 to 1882 ppm (average 1329 ppm). The REY-rich muds display heavy rare earth elements (HREE) enriched patterns, with obvious depletions in Ce, and positive anomalies of Eu in Post-Archean Australian Shale (PAAS)-normalized REE diagrams. In contrast, the muds of S021GC17 show light rare earth elements (LREE) enriched patterns and positive anomalies of Ce and Eu in the seawater-normalized REE diagrams. Total REY abundances in the core show positive correlations with CaO, P2O5, Fe2O3, and MnO concentrations. In situ analyses of trace element contents by laser ablation-inductively coupled plasma–mass spectrometry (LA-ICP-MS) demonstrate that bioapatite fossils contain high REY concentrations (998 to 22,497 ppm, average 9123 ppm), indicating that they are the primary carriers of REY. The in situ Nd isotope values of bioapatites are higher than the average values of seawater in Pacific Ocean. Fe–Mn micronodules are divided into hydrogenetic and diagenetic types, which have average REY concentrations of 1586 and 567 ppm, respectively. The high contents of Fe-Mn-Ba-Co-Mo, the positive correlations between ΣREY and Fe-Mn, the ratios of Fe/Ti and Al/(Al + Fe + Mn), and the LREE-enriched patterns in the REY-rich muds, combined with high Nd isotope values shown by bioapatite fossils, strongly indicate that the hydrothermal fluids have played an important role in the formation of the REY-rich sediments in the eastern South Pacific Ocean.

Earthen ridges in coastal sand dunes of the Caribbean coast, Venezuela: Anthropogenic or natural?

AbstractEarthen ridges that occur in the active Los Médanos de Coro dune fields of the Venezuelan Caribbean coast have been attributed either to anthropogenic or natural origins. Anthropogenic explanations have focused on their location, geometry, structure, and compaction, in the context of archaeological sites and artifacts throughout the dunes, to suggest an origin as causeways built by the extinct Caquetío people and/or their pre‐Hispanic ancestors. To assess this topic, geomorphic, physicochemical, and elemental analyses were carried out on the ridges and interdunal beaches and playas. The configuration, elevation, and composition of the ridges, along with three bulk sediment 14C dates that range 1900–3000 year BP, are here found to be most compatible with a natural origin as exhumed paleochannels. Past human occupants may, however, have used the features as causeways.

Effects of salt marsh plants on mobility and bioavailability of REE in estuarine sediments

Sediments colonised by three halophyte species, Spartina maritima (Curtis) Fernald, Halimione portulacoides (L.) Aellen and Sarcocornia fruticosa (L.) Scott) and bulk sediment from a SW European salt marsh (Tagus estuary, Portugal) were subjected to sequential extractions and analysed to assess the rare earth elements (REE) geochemical fractionation and to evaluate the plants' role in the mobility and bioavailability of these elements. The results showed that REE were mainly bound to the residual (yttrium and heavy-REE) and carbonate (middle-REE and heavy-REE) fractions, followed by the reducible and oxidisable (light-REE and middle-REE) fractions, while the easily soluble fraction was negligible. This fractionation evidenced a sediment REE mobility mainly dependent not only on carbonates but also on FeMn oxyhydroxides and on organic matter content. On the other hand, REE associated with the reducible and oxidisable fractions, and particularly the redox-sensitive Ce, may become more available, due to the redox condition seasonal changes that occur in salt marshes' sediments promoted by the plants' activity. Moreover, this study demonstrated that the REE bioavailability depends not only on the sediments' characteristics and the plants' seasonal activity but also on the specificity of each element, as demonstrated by the different fractionation patterns observed in the various sedimentary fractions.

An Assessment of xsBa Flux as a Paleoproductivity Indicator and Its Water‐Depth Dependence in the Easternmost Equatorial Pacific Ocean

AbstractWe present the first regional‐scale records of biogenic Barium (xsBa) fluxes in the Panama basin of the eastern‐equatorial margin of the Pacific Ocean in order to assess xsBa as a paleoproductivity proxy. Measurements of xsBa from 13 cores that range in water depths from about 700 to 3,000 m show an increase in 230Th‐normalized xsBa mass accumulation rates (MARs) with increasing water depth during both marine oxygen isotope stages (MIS) 1 and 2. The correlation of xsBa MARs with depth are strong despite differences in bulk sediment MARs and differing degrees of sediment redistribution. We interpret the increasing xsBa with water depth as likely due to the continued decomposition and remineralization of falling and/or resuspended biogenic particles. xsBa does not seem to be affected by diagenetic sulfate reduction in most of the cores. Calculated estimates of xsBa preservation in the sediment pile are high and fluctuate between 45% and 52% throughout the last 25 kyr. Although xsBa fluxes can be a robust indicator of paleoproductivity, caution is needed if (a) there is evidence of sulfate reduction in sediments being analyzed, and (b) one is trying to quantify differences in paleoproductivity among sites that are located at different depths in the water column.

Reconstructing Western Boundary Current Stability in the North Atlantic Ocean for the Past 700 Kyr From Globorotalia truncatulinoides Coiling Ratios

AbstractDown‐core changes in the coiling direction of Globorotalia truncatulinoides in the northwestern subtropical Atlantic (KNR140‐37PC and Ocean Drilling Program Sites 1063, 1059, 1056, 1058) provide a tracer for the hydrographic conditions in the western boundary current over the past 700 kyr (Marine Isotope Stage, MIS, 1–17). A consistent association between percent G. truncatulinoides (sinistral) abundances, total test counts, and bulk sediment CaCO3 content is established by MIS 11 suggesting a response to ocean‐atmosphere interactions during the mid Brunhes event. Commencing with MIS 11, interglacial maxima are associated with high total test counts and either distinct sinistral test minima (MIS 9e, 11c) or maxima (MIS 1, 5e, 7a). High sinistral test abundances with relatively high test counts is similar to the late Holocene relationship at the study sites. Low sinistral test abundances despite high test counts means that coiling ratios are dominated by dextral forms. We interpret this pattern to indicate more intense flow in the subtropical gyre either via the western boundary current drawing toward the gyre center, or a more northern influence of the North Equatorial and Antilles Currents. This suggests that the western boundary current may have been more intense during MIS 11c and MIS 9e then during MIS 7a, MIS 5e, and MIS 1 consistent with climate warm anomalies in northern Europe at these times. Regardless of the mechanism, the observation that minima and maxima in sinistral test abundances are prolonged at these times indicates that the western gyre boundary remained stable during relative warm intervals.

Geochemistry and pollution status of surface sediments of Loktak Lake, Manipur, India

Surface sediments (n = 29) from Loktak Lake, the largest freshwater lake in northeastern India, were studied for geochemistry, mineralogy and the heavy metal pollution of the lake sediments was assessed. The XRD spectra of bulk sediments showed the dominance of quartz and clay fractions showed the dominance of illite and chlorite. The major oxides consist dominantly of SiO2, Al2O3 and FeO with low concentrations of K2O, Na2O and CaO. The depletion of CaO, Na2O and K2O compared to upper continental crust, post-Archean average shale and North-American shale composite indicated moderate to strong weathering, recycling of the source rock and their removal during transportation. The chemical index of alteration values (76.86–84.98) also suggested moderate to extreme chemical weathering of the Loktak Lake catchment lithologies. The mean concentrations (in mg/kg) of Zn (117 ± 23.45), Co (34 ± 13.67), Fe (7 ± 1.38 wt%), Cu (67 ± 12.87), Cr (282 ± 45.25) and Ni (194 ± 30.12) are about 2–4 times higher than that of the upper continental crust, except for Mn (425 ± 168.99 mg/kg) and Pb (17 ± 7.47 mg/kg). The heavy metal concentrations follow the trend of Fe > Mn > Cr > Ni > Zn > Cu > Co > Pb. The assessment of heavy metal pollution using enrichment factor, geo-accumulation index and modified degree of contamination suggested non-contamination to moderate contamination of sediments. According to sediment quality guidelines, the sediment-dwelling biota could be affected by the higher concentrations of Ni and Cr. The mean-effect range median-quotient suggests medium–high to high potential of the effects of heavy metal pollution. The uniformly higher concentration of heavy metals in general and Ni and Cr, in particular, indicated the dominance of natural (ophiolitic) sources with no or little, if any, anthropogenic influence.