Organic-poor Sediments Research Articles

The effects of sediment types on the structure of bottom invertebrate communities in a small European lowland dam reservoir

The study compares the structure of invertebrate communities inhabiting two types of bottom sediments in Krzynia Reservoir, known also as Lake Krzynia (on the Słupia River, northern Poland). The lake is prone to intensive sedimentation because it is limnic. This limnicity is due to the impact of the river, rather than to surface runoff from the catchment area, where the heavy predominance of natural habitats over transformed ones slows the transport of mineral suspensions from adjacent terrain. In the organic-poor sediments dominated by sand and gravel, the mean density of benthic invertebrates was ten-fold higher than in silty, organic-rich sediments, while invertebrate biomass was only slightly and non-significantly higher in organic-poor sediments than in organic-rich sediments. Simultaneously, the mean invertebrate body weight was markedly lower in organic-poor sediments (0.03 g) than in organic-rich sediments (0.18 g). No significant differences in taxonomic diversity (number and Shannon index of taxa) were found between the studied sediment types in the analysed communities. Among the major invertebrate groups, the Oligochaeta and Mollusca significantly differed in density between sediment types, while the Oligochaeta and Chironomidae differed in biomass. An increased percentage of silt fraction in bottom sediments resulted in a drastic decrease in abundance of benthic fauna, but their biomass declined only slightly. Thus, plans to construct new reservoirs should aim to reduce the rate of silt accumulation in them, in order to preserve their primary function and support biodiversity in transformed river valleys.

Complex pattern of environmental changes and organic matter preservation in the NE Ordos lacustrine depositional system (China) during the T-OAE (Early Jurassic)

The lower Toarcian black shale and mudstone succession of the Anya section in the northeastern Ordos Basin records a climatically controlled evolution of the lacustrine system during the negative carbon isotope excursion (NCIE) of the Toarcian oceanic anoxic event (T-OAE; ca. 183 Ma). In the Anya section, petrography integrated with climate proxies (Sr/Cu and Rb/Sr) and weathering indices (weathering index of Parker, chemical index of weathering and plagioclase index of alteration), suggests that a warm-humid climate with sporadic semihumid-semiarid intervals developed during the early Toarcian and was coincident with strong chemical weathering. Sr/Ba and S/TOC ratios suggest that the Anya section's rocks were generally deposited in freshwater. Four phases (phases A, B, C and D) in lake evolution are distinguished from the variation in petrographical and geochemical proxies providing clues about the drivers of environmental change and organic matter accumulation in the lacustrine basin. Phases A and C have a high preservation of organic matter derived from lake plankton (Type II-III kerogens), were characterized by anoxia, and coincided with high primary productivity. On the contrary, phases B and D indicate a low primary productivity and suboxic-oxic conditions, with a lower organic matter content of predominantly higher land plant origin (Type III kerogen). The accumulation of organic matter was mainly controlled by changes in primary productivity and redox conditions that were tightly linked to the local sedimentary evolution of the Ordos Basin and to global and regional climate. During the early Toarcian NCIE, the accumulation of organic-poor sediment and terrestrial organic matter may have been driven by enhanced seasonality, superimposing on local lake level changes. As a consequence, the peak of the T-OAE carbon cycle perturbation does not coincide with lacustrine black shale deposition and/or peak anoxia in this continental depositional system. This emphasizes the significance of local or regional environmental change on the carbon sequestration within lacustrine systems.

Adsorption and Removal Kinetics of 2,4-Dinitroanisole and Nitrotriazolone in Contrasting Freshwater Sediments: Batch Study.

Environmental release of 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO) is of great concern due to high migration potential in the environment. In the present study we evaluated the adsorption and microbially-mediated removal kinetics of dissolved DNAN and NTO in contrasting freshwater sediments with different total organic carbon (TOC) content. River sand (low TOC), pond silt (high TOC), clay-rich lake sediment (low TOC), wetland silt (high TOC), carbonate sand (low TOC), and iron-rich clay (low TOC) were evaluated. Separate abiotic and biotic bench-top sediment slurry incubations were carried out at 23, 15, and 4 °C for DNAN and NTO. Experiments were conducted over 3 weeks. Time series aqueous samples and sediment samples collected at the end of the experiment were analyzed for DNAN and NTO concentrations. The DNAN compound equilibrated with sediment within the first 2 h after addition whereas NTO showed no adsorption. 2,4-Dinitroanisole adsorbed more onto fine-grained organic-rich sediments (Kd = 2-40 L kg-1 sed-1 ) than coarse-grained organic-poor sediments (Kd = 0.2-0.6 L kg-1 sed-1 ), and the TOC content and cation exchange capacity of sediment were reliable predictors for abiotic DNAN adsorption. Adsorption rate constants and equilibrium partitioning constants for DNAN were inversely proportional to temperature in all sediment types. The biotic removal half-life of DNAN was faster (t1/2 = 0.1-58 h) than that of NTO (t1/2 = 5-347 h) in all sediment slurries. Biotic removal rates (t1/2 = 0.1-58 h) were higher than abiotic rates (t1/2 = 0.3-107 h) for DNAN at 23 °C. Smaller grain size coupled with higher TOC content enhanced biotic NTO and DNAN removal in freshwater environments. Environ Toxicol Chem 2023;42:46-59. © 2022 SETAC.

The origin of abnormally 13C-depleted organic carbon isotope signatures in the early Cambrian Yangtze Platform

A significant negative organic carbon isotope (δ13Corg) excursion occurred in the lower member of the lower Cambrian Qiongzhusi organic-rich shales in the Sichuan Basin, which is also reported in other contemporaneous strata throughout the Yangtze Platform. In this study, 47 lower Cambrian drill core samples were collected to investigate the possible formation mechanism of this negative δ13Corg excursion using multiple geochemical approaches. Primary paleoproductivity, redox conditions and organic carbon isotopic compositions were determined for these core samples of the Qiongzhusi Formation in the Sichuan Basin. The results suggest that the degree of microbially mediated carbon cycling and its contribution to the total sedimentary organic matter has an important influence on organic carbon isotopic compositions. An extensive transgression during the deposition of the lower member of the Qiongzhusi Formation brought abundant nutrients to primary producers and thus stimulated blooms, which is evidenced by the high primary productivity in the lower member of the Qiongzhusi Formation. In addition, marine transgression also provided sulfide to the seawater. Enhanced primary productivity accelerated the consumption of bottom water oxygen and thus promoted the establishment of euxinic bottom water conditions. The euxinic conditions promoted an anaerobic carbon cycle mediated by sulfate-reducing bacteria and chemoautotrophic bacteria, which resulted in the dissolved inorganic carbon reservoir becoming more depleted in 13C. Once these 13C-depleted carbons are utilized by primary producers, the 13C-depleted carbons will participate in the carbon cycle. This series of processes ultimately resulted in lower δ13Corg values within the lower Cambrian Qiongzhusi organic-rich black shales. A shift to positive and rather stable δ13Corg values in the relatively organic-poor sediments indicates a smaller contribution of 13C-depleted bacterial biomass (e.g., sulfate-reducing bacteria and chemoautotrophic bacteria), which is evidenced by more oxygenated ocean bottom conditions in the study area.

Removal of hexavalent chromium by hyporheic zone sediments in an urbanized estuary.

More than 2 million tons of chromium ore processing residue (COPR) waste was disposed of in Hudson County of New Jersey, which was known as the center of the production of chromate in the 20th century. The Cr(VI) removal experiments were conducted with the hyporheic zone (HZ) sediments collected along the shore of an urbanized estuary located in and near Hudson County to investigate the natural remediation of Cr(VI). Fine-grained and organic-rich Passaic River sediments showed the highest removal capacity for Cr(VI), whereas the lowest removal of Cr(VI) occurred in coarse-grained and organic-poor sediments from Newark Bay. In general, Cr(VI) removal increased with higher amounts of sediment organic matter, sulfur, and silt and clay fractions, as well as lower pH conditions. The removal of hexavalent chromium in organic-rich sediments is attributed mainly to the reduction of Cr(VI) to Cr(III), resulting in less reversible immobilization of Cr(VI), while reversible adsorption could also remove Cr(VI). The results suggest that the organic-rich, fine-grained HZ sediments can act as a natural reactive barrier for the remediation of Cr(VI) transport from subsurface to surface water in the estuary. Further research is needed to understand the long-term mobility of Cr along the urban estuary.

Paleontological Records of Holocene Environmental Changes offshore, Egypt

Paleontological and sedimentological analyses were carried out in sediment core from southeastern Mediterranean Sea, offshore Egypt to reconstruct the past environmental changes in trophic state and temperature during Holocene. To achieve our goals, grain size, total organic carbon, planktic and benthic foraminifera, dinoflagellate cysts have been investigated. Few micropalaentological studies have been done in the studied area and none of them used the combined proxies of benthic foraminifera and dinoflagellate cyst. Theses combined proxies reflects more comprehensive paleoenvironmental view. Biotic and abiotic data have been analysed with multivariate technique including Redundancy Analysis (RDA). Diversity indices such as: Fisher alpha index (α) and Shannon index (H) have been applied. The foraminiferal study yields 9 planktic species and 10 benthic species. Foraminiferal assemblages have low species diversity indices especially at sapropel layer S1. Mediterranean sapropels are layers with elevated organic carbon concentrations that contrast with surrounding organic poor sediments. Sapropels occur periodically in sedimentary sequences of the last millions years, which have been the subject of extensive previous study. Redundancy Analysis (RDA) yields two groups of foraminiferal assemblages depending on the changes of total organic carbon and clay content. The dinocysts study yields 15 species, the majority of cyst types belonging to the order Gonyaulacales. The association of dinoflagellate cyst shows two depositional phases in the sediment. The sapropel layer S1, with anoxic condition and warmer temperature, is recorded at depth 28–46 cm where heterotrophic taxa dominate and the post-sapropel layer is recorded at depth 0–28 cm; where autotrophic taxa dominate.

Paleoenvironmental changes during the last 3000 years in Lake Cari-Laufquen (Northern Patagonia, Argentina), inferred from ostracod paleoecology, petrophysical, sedimentological and geochemical data

South American paleoreconstructions are of global interest because it is the only landmass extending from the tropics to the southern high latitudes and intersecting the entire southern westerly wind belt. In this context, endorheic environments, as Lake Cari-Laufquen Grande (LCLG; 41º35’S, 69º25’W) are excellent sites for paleoenvironmental studies, since they react rapidly to changes in the precipitation/evaporation ratio. In this study, the limnological conditions prevailing during the last 3000 years have been inferred based on a multiproxy analysis of the sedimentary sequence of LCLG (water depth 4 m, core length 505 cm). Today, this is one of the few lakes in Northern Patagonia, providing a unique paleoclimatic and paleoecological lacustrine record. The ostracod assemblages, along with sedimentological, petrophysical and geochemical data, show hydrological changes in Cari-Lauquen Grande basin during the studied period. Our results indicate the continuous presence of a saline to brackish lake. However, changes in ostracod assemblages and sedimentological features reveal variations in the relative salinity of the system. The lake paleosalinity was estimated based on ostracod salinity optima, using a calibration dataset of 29 species and 72 different environments. Intervals of high salinity (24–26 g L-1) were dominated by L. rionegroensis (morph.I) and organic-poor sediments. Periods of lower salinity (14–20 g L-1) favored the occurrence of oligo-mesohaline taxa, such as R. whatleyi, L. rionegroensis (morph.III), Cypridopsis sp., L. patagonica and I. ramirezi. The sediments further display higher values of both TOC and magnetic susceptibility. These salinity changes are interpreted as water level variations, associated in turn to cold-wet and warm-dry periods in northern Patagonia. Our results provide new insights into the late-Holocene environmental history of the region, characterized by a paucity of records. In addition, the ostracod paleoecology modeled using a WA approach allowed quantitative inferences of salinity changes, highlighting their potential in Quaternary paleoclimate research.

Role of Sediment Resuspension on Estuarine Suspended Particulate Mercury Dynamics.

Coastal sediments are an important site for transient and long-term mercury (Hg) storage, and they foster a geochemical environment optimal for Hg methylation. Therefore, efforts have been taken to constrain the role of sediments as a source of methylmercury (MeHg) to the estuarine water column. This study employed the Gust Microcosm Erosion Core system capable of quantifying particle removal from undisturbed cores under measurable shear stress conditions to assess particulate Hg and MeHg exchange between sediments and the water column. Samples were collected from organic-rich and organic-poor sediment types from the mid- and lower Delaware Bay. It was found that bulk sediment samples from organic-rich systems overpredict total Hg and MeHg release to the water column, whereas organic-poor sediments underpredict the exchange. In general, organic-rich sediments in shallow environments have the most impact on surface particle dynamics. There is little evidence to suggest that MeHg formed in the sediments is released to the water column via particulate exchange, and therefore, nonsedimentary sources likely control MeHg levels in this estuarine water column.

Changes of sedimentation environment inferred from fallout radionuclides and physical properties of sediment in Sun Moon Lake, Taiwan

This study describes the impact of dam construction at Sun Moon Lake, Taiwan, on lacustrine sedimentation. Analyses of the physical properties of sediment cores show that the surface sediment has two layers: the post-dam layer consisting of high-density, organic-poor sediment, and the underlying pre-dam layer consisting of organic-rich sediment. After correction for decay since initial deposition, the estimated concentration of excess 210Pb in the pre-dam layer was much higher than that in the present surface layer. These results indicate that the dam construction had a large impact on supply of 210Pb and the sedimentation environment in this lake.

Post-depositional evolution over a time scale of 1 million years of eastern Mediterranean organic-rich and organic-poor sediments: new insights on the debromination and layer-silicate markers

Organic matter degradation is the engine behind the biogeochemical evolution of sediments during burial. Previous research has shown that eogenesis is the seat of a complex interplay between organic matter, microbes and the most reactive part of inorganic compounds, such as clay minerals. To explore the variability and stability of bromine and clay minerals as geochemical and mineral tracers, we selected an eastern Mediterranean core that has a high degree of stability in the quality and quantity of organic matter through time at a one-million-year scale and great variability in organic matter content at a 10 ky scale. According to the very low maximal burial depth reached by these sediments (the core length is only 36.5 m), physical parameters, such as temperature and pressure, did not significantly influence the evolution of the studied parameters during the burial history. The bulk clay mineralogy of organic-rich and organic-poor sediments is similar all along the investigated core material; smectite predominates over kaolinite. The only identified authigenic minerals are biogenetic framboidal pyrite and manganese oxides. The X-ray data and the chemical compositions of the smectite are characteristic of a montmorillonite which is representative of a detrital Nile source. At a one-million-year scale, the organic matter content has no significant influence on clay eogenesis, and detrital smectite and kaolinite remain unchanged. Bromine is present in marine organic matter as organobromine compounds. During eogenesis, bromine is released from organic matter as bromide ion, resulting in an increase in the bromide concentration in the pore water with depth. Dissolved bromide can be used as a conservative tracer of the debromination of sedimentary organic matter. For the first time, we established that solid-phase BrOrg is a reliable tracer of debromination rates in marine sediments. The rate of debromination depends on the organic matter content. The rate increases from less than 2.3 × 10−4 μmolBrOrg mol.C−1.y−1to 6.3 × 10−4 μmolBrOrg mol.C−1.y−1when TOC varies from 0.17 to 3%. This increase is related to the development of the bacterial population and provides the basis for further investigation of other oceanic basins. For TOC values >4%, the rate of debromination decreases. We propose that the bioavailability of organic matter is another factor of variability in the debromination rate.

Trace metal distributions in the sediments of the Little Akaki River, Addis Ababa, Ethiopia.

The levels and distribution of trace metals (Cr, Mn, Co, Ni, Cu, Zn, Cd and Pb) at eleven water and sediment stations on the Little Akaki River (LAR) in Addis Ababa, Ethiopia, were determined. The binding forms of the metals in various geochemical fractions of the sediments were also quantified. The molar ratio of the sum of the simultaneously extractable metals (∑SEM) and acid-volatile sulphide (AVS)-as a measure for predicting metal-induced toxicity-was estimated. LAR trace levels in water for Cu, Zn, and, particularly Mn were, in most instances, higher than the recommended guidelines for healthy aquatic ecosystems. Total trace metal (TTM) contents in the LAR sediments at certain stations exceeded "threshold effect concentrations" and even "probable effect concentrations", especially in the cases of Zn, Cu, Ni, Pb, and at all stations for Mn. This became more apparent after applying "normalizations" to the relatively lower TTM adsorption capacities of coarse-grained, organic-poor sediments. Sequential extraction of the sediments showed that trace metals generally have a higher affinity for Fe-Mn oxide and organic matter/sulphidic fractions, followed by the residual fraction. Mn was relatively strongly bound to the exchangeable, carbonate bound fractions, whereas a large proportion of Cr was found in the residual fraction. The Σ[SEM]/[AVS] ratio pointed to potential metal-induced toxicity of sediments collected from seven out of the eleven stations. The results indicate that trace metal pollution pose risks to the health of ecosystems, and to human communities that use the river for a range of different purposes.

Deciphering As and Cu cycling in sediment pore waters in a large marina (Port Camargue, southern France) using a multi-tracer (Fe, Mn, U, Mo) approach

The sediments of the Port Camargue marina (South of France) are highly polluted by Cu and As (Briant et al., 2013). The dynamics of these pollutants in pore waters was investigated using redox tracers (sulfides, Fe, Mn, U, Mo) to better constrain the redox conditions.In summer, pore water profiles showed a steep redox gradient in the top 24 cm with the reduction of Fe and Mn oxy-hydroxides at the sediment water interface (SWI) and of sulfate immediately below. Below a depth of 24 cm, the Fe, Mn, Mo and U profiles in pore waters reflected Fe and Mn reducing conditions and, unlike in the overlying levels, sulfidic conditions were not observed. This unusual redox zonation was attributed to the occurrence of two distinct sediment layers: an upper layer comprising muddy organic-rich sediments underlain by a layer of relatively sandy and organic-poor sediments. The sandy sediments were in place before the building of the marina, whereas the muddy layer was deposited later. In the muddy layer, large quantities of Fe and Mo were removed in summer linked to the formation of insoluble sulfide phases. Mn, which can adsorb on Fe-sulfides or precipitate with carbonates, was also removed from pore waters. Uranium was removed probably through reduction and adsorption onto particles. In winter, in the absence of detectable pore water sulfides, removal of Mo was moderate compared to summer.Cu was released into solution at the sediment water interface but was efficiently trapped by the muddy layer, probably by precipitation with sulfides. Due to efficient trapping, today the Cu sediment profile reflects the increase in its use as a biocide in antifouling paints over the last 40 years.In the sandy layer, Fe, Mn, Mo and As were released into solution and diffused toward the top of the profile. They precipitated at the boundary between the muddy and sandy layers. This precipitation accounts for the high (75 μg g−1) As concentrations measured in the sediments at a depth of 24 cm.

Phylogenetic characterization of marine benthic archaea in organic-poor sediments of the eastern equatorial pacific ocean (Odp site 1225)

Phylogenetic characterization of marine benthic archaea in organic-poor sediments of the eastern equatorial pacific ocean (Odp site 1225)

A new approach for molecular characterisation of sediments with Fourier transform ion cyclotron resonance mass spectrometry: Extraction optimisation

Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) can begin to tease apart the molecular character of sedimentary organic matter (SOM). We therefore tested five different solvents (aqueous base, CHCl3, MeOH, pyridine and water) for their ability to extract a representative fraction from two organic rich lacustrine sediments, Mangrove Lake, Bermuda (MLB) and Mud Lake, Florida (MLF). Following comparison using liquid state nuclear magnetic resonance spectroscopy (NMR) and negative ion mode electrospray ionization mass spectrometry (FTICRMS) we found that pyridine was the optimal solvent, extracting a more diverse (10–100× greater integration for carbonyl, amide and amine groups) and a larger number of peaks on average (1375–1450 vs. 380–1450). Comparison of the pyridine extracts between MLB, MLF and two organic poor sediments from the Mississippi River Delta and Bayou Grande (Pensacola, FL) showed that only 4.9% of the molecular formulae were common to all four and that unique formulae made up the highest proportion of the assignments. The use of pyridine for extracting immature (Holocene) SOM for FTICRMS analysis can therefore be widely applied to immature sediments and produce representative spectra.

Iron isotope heterogeneity in pyrite fillings of Holocene worm burrows

We present 54Fe and 56Fe data on pyrite from burrow-like and irregularly shaped pyrite concretions from Holocene postglacial lacustrine clays in the northern Baltic Sea collected using a secondary ion mass spectrometry (SIMS) microanalysis technique. The burrow-like concretions were formed in reducing, mucus-coated worm burrows in oxic surface sediments. Framboidal pyrite in the cores of the burrow-like concretions shows extensively fractionated δ56Fe values down to −3.1‰. The framboids are cemented by poorly crystalline FeS2 with δ56Fe values between −2.1‰ and +1.4‰. The irregularly shaped concretions with microcrystalline textures were formed in organic-poor sediment pore spaces, and display a wide spread of δ56Fe values up to +4.1‰. The measured δ56Fe values reflect the preferential capture of 54Fe to pyrite in the diagenetic sequence and the 56Fe enrichment of remaining pore water. The diagenetic sequence of the pyrite materials is supported by previous petrographical study and 34S/32S microanalysis of the same samples. Our results demonstrate substantial early-diagenetic δ56Fe and δ34S heterogeneity within individual pyrite grains, underlining the necessity of high spatial resolution measurements in studying biological and abiological isotopic signatures.

Methane distribution in Holocene marine sediments in the Bornholm Basin, southern Scandinavia

The Baltic Sea is an ideal natural laboratory to study the methane cycle in the framework of diagenetic processes. In this paper we present preliminary geological mapping results from project Baltic Gas, a research project with the overall aim to contribute to the development of a scientific basis for long term sustainable use and protection of the Baltic Sea ecosystem. The Baltic Sea is a marginal sea with a strong permanent haline stratification, which leads to oxygen-poor bottom waters, and which is sometimes interrupted by oxygen-rich saltwater flowing in from the North Sea. The history of the Baltic Sea has resulted in deposition of organic-rich Holocene marine sediments that overlie glacial, late-glacial and early Holocene organic-poor sediments.

PRESERVATION OF GIANT ANOMALOCARIDIDS IN SILICA-CHLORITE CONCRETIONS FROM THE EARLY ORDOVICIAN OF MOROCCO

The recently discovered Fezouata Biota, from the Early Ordovician (late Tremadocian to late Floian) of Morocco, preserves a diverse soft-bodied fauna. While preservation is mostly of Burgess Shale-type, giant anomalocaridids also occur in siliceous concretions. Petrographic and geochemical analyses of these concretions reveal their growth history and the circumstances that led to the fossilization of nonbiomineralized anatomy within them. The large ( .1 m) concretions are homogeneous in composition and geochemical characteristics, suggesting rapid, pervasive growth of mineral frameworks during decay of the large animals at, or near, the sediment-water interface. Concretions are comprised of ultrafine-grained (2–20 mm) authigenic quartz, Fe chlorite, and calcite, a composition unlike other described marine concretions. Abundant pyrite, now represented by oxide pseudomorphs, grew adjacent to the anomalocaridid carcasses, but rarely within the matrix of the concretions. This distribution indicates that sulfate reduction around the carcasses was vigorous within otherwise organic-poor sediments resulting in the establishment of prominent chemical gradients around the giant anomalocaridids that led to early precipitation of mineral overgrowth around nonbiomineralized tissues. Rapid precipitation of intergrown silica and Fe chlorite required an abundant source of silica, iron, and aluminum. These ions were likely derived from dissolution of volcanic ash in the sediments. Limited intergrown calcite (d 13C avg. 212.2%o ,n 5 23) precipitated from bicarbonate that was generated largely by sulfate reduction of organic tissues of the carcasses. Whereas Burgess Shale-type preservation of fossils in the Fezouata biota required suppression of degradation, exceptional preservation of anomalocaridids within the siliceous concretions resulted from extensive microbial decomposition of a large volume of organic tissues. Rapid mineralization was facilitated by localization of microbial activity around the large carcasses and must have required an unusually reactive sediment composition.

Mechanisms of damage to corals exposed to sedimentation

We investigated the mechanisms leading to rapid death of corals when exposed to runoff and resuspended sediments, postulating that the killing was microbially mediated. Microsensor measurements were conducted in mesocosm experiments and in naturally accumulated sediment on corals. In organic-rich, but not in organic-poor sediment, pH and oxygen started to decrease as soon as the sediment accumulated on the coral. Organic-rich sediments caused tissue degradation within 1 d, whereas organic-poor sediments had no effect after 6 d. In the harmful organic-rich sediment, hydrogen sulfide concentrations were low initially but increased progressively because of the degradation of coral mucus and dead tissue. Dark incubations of corals showed that separate exposures to darkness, anoxia, and low pH did not cause mortality within 4 d. However, the combination of anoxia and low pH led to colony death within 24 h. When hydrogen sulfide was added after 12 h of anoxia and low pH, colonies died after an additional 3 h. We suggest that sedimentation kills corals through microbial processes triggered by the organic matter in the sediments, namely respiration and presumably fermentation and desulfurylation of products from tissue degradation. First, increased microbial respiration results in reduced O(2) and pH, initiating tissue degradation. Subsequently, the hydrogen sulfide formed by bacterial decomposition of coral tissue and mucus diffuses to the neighboring tissues, accelerating the spread of colony mortality. Our data suggest that the organic enrichment of coastal sediments is a key process in the degradation of coral reefs exposed to terrestrial runoff.

Chironomid larvae stimulate biogeochemical and microbial processes in a riverbed covered with fine sediment

We determined if the influence of chironomid larvae on biogeochemical and microbial processes in fine surface sediments of a riverbed depended on the characteristics of the sedimentary habitat. The influence of chironomids was measured on sediment reworking, biogeochemical processes and bacterial characteristics in infiltration sediment columns having three different surface sediments (with different organic matter characteristics). The results showed that chironomids exhibited similar bioturbation activities (construction and irrigation of U-shaped tubes in the upper layers of sediments) and significantly stimulated O2 uptake and percentage of active bacteria in all sediment treatments. In contrast, the release of NH4 + in anoxic sediment layers was strongly increased by chironomids in the presence of organicrich sediments, whereas they had no effect on this process with organic-poor sediments. Our results indicated that the influence of chironomids on biogeochemical processes in river bed sediments are linked to the redox conditions of the system. Moreover, when the effects of chironomid larvae are compared to those measured with tubificid worms in the same experimental conditions, we conclude that the mode of bioturbation (sediment reworking, biogenic structure building, burrowing depth, bioirrigation) can also determine the impact of benthic animals on microbial activities and biogeochemical processes in the sediment.

Trace element and Nd-isotopic evidence for sediment sources in the mid-Proterozoic Vindhyan Basin, central India

Ancient sediments reflect the compositions of old eroded continental crustal material and preserve geochemical signatures of paleo-tectonic and volcanic events. Geochemical study of such sediments provides insight into tectonic events that shaped the face of the Earth. Nd isotopes are best suited for such studies because rare earth elements (REE), specifically Sm and Nd are not fractionated during post-depositional processes such as erosion, transportation and diagenesis of organic-poor sediments and hence reflect the source composition of these sediments. Sedimentation in the Proterozoic Vindhyan Basin of central India with a ∼4 km thick succession of unmetamorphosed and mostly undeformed sediments lasted over 1000 Ma from ∼1630 Ma up to the Precambrian–Cambrian boundary. These sediments provide an ideal opportunity to study the evolution of ancient continental crust as well as the tectonic evolution of Peninsular India. The occurrence of porcellanites in the lower part of the basin indicates active volcanism during deposition of the Vindhyan sediments. These tuffs show large ion lithophile element (LILE) enrichment as well as depletions in Nb and Ta in primitive mantle-normalized plots, characteristic of an arc volcanic source. In combined trace element ratio plots, these volcanic rocks indicate mixing between mantle-derived melts and the continental crust and overlap with the composition of global arcs. Their initial ɛ Nd at 1630 Ma, ranging from −5.7 to −0.9, is similar to those observed in ‘Andean-type’ continental margin arcs. Trace element concentration patterns as well as Nd isotopic compositions of the clastic and non-clastic sediments of the Vindhyan Basin, from one of the type-localities of the Vindhyan stratigraphy in the Son valley, are vastly different from those of the Bundelkhand granite, which is the basement for these sediments, indicating that these granitic rocks were not a major source of sediments for the Vindhyan Basin. Our Nd isotopic study clearly indicates that sedimentation in the Vindhyan Basin, which lasted over ∼1 Ga, was not continuous. The boundaries between the Semri and Kaimur Groups as well as the Rewa and Bhander Groups are characterized by abrupt changes in the Nd isotopic compositions indicating changes in provenance, with sediments in the Kaimur and Bhander Groups being derived from more juvenile sources compared to the Semri and Rewa Groups, respectively. This inference is also supported by changes in T DM and f Sm/Nd for the sediments across these boundaries. The strikingly different trace element patterns of the Rewa Group sediments compared to the underlying Kaimur sediments indicates that sediment sources also changed across the Kaimur–Rewa boundary. Multiple changes in provenance in the Vindhyan sediments indicate that this basin was tectonically active during the entire span of sedimentation. While there is some uncertainty in determining the exact sources of sediments for the Vindhyan Basin, we argue on the basis of our geochemical, in particular Nd-isotopic ratios data, and available paleocurrent data that majority of the sediments were derived from a now-extinct Andean-type arc source in the south that resulted from a southerly dipping subduction prior to the collision of the Bhandara and Bundelkhand cratons. According to this reconstruction, the Vindhyan Basin was initiated as a foreland Basin after the collision of these two cratons.