Organic Matter-rich Sediments Research Articles

Environmental response to the early Toarcian carbon cycle and climate perturbations in the northeastern part of the West Tethys shelf

Early Toarcian (Early Jurassic; ~183 Ma) climate and carbon cycle perturbations were accompanied by widespread accumulation of bituminous sediments, formed during the Toarcian Oceanic Anoxic Event (T-OAE). On the northwestern Tethyan shelf, organic carbon accumulation was particularly widespread in hydrodynamically restricted regions of the European Epicontinental Basin System (EEBS) where it peaked during the formation of euxinic waters extending into the photic zone. In the adjacent northeastern West Tethys shelf (NE-WTS), an area proximal to the deeper and better ventilated NW Tethys, deposition of strata enriched in organic matter was restricted to depressions of limited spatial extent. While numerous works have addressed the bituminous strata from the EEBS, evolution of environmental conditions and drivers of organic matter accumulation in areas proximal to the open Tethys have received much less attention.Here, we investigated a stratigraphically well-constrained, exceptionally organic matter-rich sediment succession from the NE-WTS deposited during the T-OAE. Organic and isotope geochemical investigations provided insights into the evolution and dynamics of environmental conditions during the T-OAE in a setting proximal to the Tethys.The sedimentary rocks sampled for this study originated from a surface outcrop that experienced weathering-related alteration of its mineral constituents. In particular, sulfides (mainly pyrite) were significantly altered, resulting in a near-quantitative loss of sulfur. In contrast, kerogen as well as bitumen fractions have not been significantly affected by weathering processes and carry a robust paleoenvironmental signal.The new data show that organic carbon accumulation occurred as a direct response to changing environmental conditions that also led to the early Toarcian carbon cycle perturbation. Increase in organic carbon burial occurred in a stepwise manner, following multiple injections of 12C-enriched carbon into the Earth's ocean-atmosphere system. High algal productivity and the subsequent organic matter decomposition by sulfate reducing bacteria led to an expansion of H2S-rich waters into the photic zone, which promoted the growth of Chlorobiaceae. A steady increase in nitrogen isotope values testified to intensified denitrification in an expanded oxygen minimum zone. The resulting decline in the availability of bio-utilizable nitrogen in combination with harsh environmental conditions promoted the proliferation of cyanobacteria as indicated by increasing abundances of 2α‑methyl‑hopanes. Formation of euxinic waters in the NE-WTS was not associated with a freshwater driven salinity stratification. Organic geochemical salinity indicators (methylated chromanes) attest to normal marine salinity, supporting a strong influence of Tethyan water masses.This study enhances our knowledge on the environmental response towards the early Toarcian climate and carbon cycle perturbations in a setting proximal to the Tethys. We demonstrate that photic zone euxinia was a common and particularly widespread feature of the T-OAE and has not been confined to the hydrodynamically restricted EEBS.

Mechanisms of muddy clinothem progradation on the Southwest Louisiana Chenier Plain inner shelf

In both modern and ancient shelf settings, mud-dominated successions commonly contain complex stratigraphic geometries in which low-gradient clinothems feature prominently. Despite their ubiquity, the full range of mechanisms responsible for sediment dispersal and clinothem progradation in such settings is not well understood. Using sediment core data (210PbXS, 137Cs, grain size, porosity, X-radiography) and shallow seismic observations, this study examines the mechanisms of across-shelf sediment transport and clinothem progradation on the muddy Southwest Louisiana Atchafalaya Chenier Plain inner shelf. Observations indicate that rapid transfer of organic matter-rich sediment to the outer topsets and clinothem rollover occurs mainly via hydrodynamic fluid-mud processes during times of high wave-current bed shear stress (e.g., during the passage of storms). Rapid sedimentation, wave perturbation, and the development of biogenic methane within the shallow seabed result in the generation of large internal pore water pressures such that the clinothem rollover and foreset sediments are inherently in a condition of incipient failure. Subsequent basinward sediment transfer to the foresets occurs largely in association with low-gradient (<0.02°) mass-failure events, evidenced by widespread scarping and mudflows on the seabed. These represent an important and as yet unattributed mechanism for clinothem progradation in the study area and are likely to drive basinward sediment transport in other muddy shelf clinothem systems, both modern and ancient.

Microfacies and depositional environment of Late Cretaceous to Early Paleocene oil shales from Jordan

The purpose of this study is to better understand the depositional setting of Late Cretaceous to Early Paleocene oil shales from southeast and central-east Jordan. One core from both the Jafr and the Azraq-Hamza Basins was logged, and their lithology, texture, and ichnofabrics were recorded. A total of 79 thin sections were analyzed petrographically, and eight microfacies types recognized. Both cores show lithologic and petrographic similarities. The oil shales are Maastrichtian to Danian in age and can be described as organic matter and calcite-rich mudrocks. The most abundant granular components are foraminifera and various types of phosphatic bio- and lithoclasts. Macrofossils (bivalves, ostracods, echinoderms) were recorded in some intervals. The current results were compared with data from a previous publication on Maastrichtian oil shales from the Jafr Basin. A new model explaining the deposition of the oil shales of the Jafr and Azraq-Hamza Basins is proposed. The onset of the Maastrichtian oil shale deposition in both basins coincides with the early Maastrichtian transgression in this region. The organic matter-rich sediments were deposited in a mid to outer ramp setting below the storm wave base. Younger oil shales of Late Maastrichtian to Danian age were deposited in a shallower environment, below the fair weather wave base. The Cretaceous/Paleogene boundary is marked by a hiatus in both cores. The Danian oil shales show relatively lower “total organic carbon” content than the Maastrichtian ones. The former are believed to have been deposited in more oxygenated bottom waters of a mid-ramp zone.

Study of the formation of the primary 137Cs vertical profile in the organic matter-rich sediments

A laboratory study of the formation of the primary vertical profile of the 137Cs activity concentration in the anaerobic organic matter-rich sediments was carried out. The incubation was performed using sediment baths method (initial thickness of the water layer over the sediments ∼40 cm, initial radiocesium activity concentration in water ∼14.53 Bq·L−1). An exponential profile (R2∼0.999) with the half-height width of ∼0.65 cm was formed after ∼32 days of the incubation. A further course of the experiment (sediments were sampled after 64, 130 and 293 days of the incubation) revealed deviations in the form of the radiocesium activity concentration profile related to the appearance of the additional Gauss factor with the increasing half-height width of ∼1.2, ∼1.3 and ∼3 cm, respectively. In spite of the evident growth of the half-height width, the radiocesium migration rate decreased from 0.02 cm·day−1 for the first 32 days of incubation down to 0.009 cm·day−1 at the end of the incubation experiment. Such a time-dependent decrease in the radiocesium migration is possibly related to the beginning of the second radiocesium migration phase depending on the retardation factor. Besides diffusion, the formation of vertical profiles of radiocesium activity concentrations in sediments was additionally induced by effects of buoyancy of the upper sediment layer due to the seepage of the above lying water into sediments.

Evidence and age estimation of mass wasting at the distal lobe of the Congo deep-sea fan

On continental margins, sulfate reduction occurs within the sedimentary column. It is coupled with the degradation of organic matter and the anaerobic oxidation of methane. These processes may be significantly disturbed by sedimentary events, leading to transient state profiles for the involved chemical species. Yet, little is known about the impact of turbidity currents and mass wasting on the migration of chemical species and the redox reactions in which they are involved. Due to its connection to the River, the Congo deep-sea fan continuously receives huge amount of organic matter-rich sediments primarily transported by turbidity currents, which impact on the development of the associated ecosystems (Rabouille et al., 2017). Thus, it is well suited to better understand causal relationships between sedimentary events and fluid flow path, with consequences on the zonation of early diagenesis sequences.Here, we combined sedimentological observations with geochemical analyses of pore-water and sediment samples to explore how sedimentary instabilities affected the migration of methane and the distribution of organic matter within the sedimentary column. The results unveiled mass wasting processes affecting recent turbiditic and pelagic deposits, and are interpreted as being slides/ slumps and debrites. Two slides were responsible for the exhumation of an organic matter-rich sedimentary block of more than 5m thick and the movement of a methane-rich sedimentary block, while turbidity currents enable the intercalation of sandy intervals within a pelagic clay layer. The youngest slide promoted the development of two Sulfate Methane Transition Zones (SMTZ), and may have possibly triggered a lateral migration of methane. Numerical simulation of the sulfate profile indicates that the youngest sedimentary event has occurred around a century ago. Our study emphasizes that turbidity currents and sedimentary instabilities can significantly affect the transport paths and the distribution of both methane and organic matter in the terminal lobe complex, with consequences on geochemical zonation of the sequential early diagenetic processes within the sedimentary column.

Geophysical characterization of stratigraphical surfaces: Basin floor and sedimentological architectural elements of Las Tablas de Daimiel (Quaternary of southern-central Spain)

In this paper we analyse and compare the efficiency of electrical resistivity imaging (ERI) and ground-penetrating radar (GPR) as tools for stratigraphic and sedimentological studies. To this end, we carried out borehole drilling and geophysical survey campaigns in two locations in Las Tablas de Daimiel area (Ciudad Real, Spain). In this region, the Quaternary record is build up by siliciclastic deposits (gravel, sand and silt) of fluvial origin and organic matter-rich sediments (peat, clay and silty clay rich in organic matter) and carbonates (biogenic deposits mainly made up of Characeae) deposited in fluvial wetland environments that rest on Ordovician quartzites and Pliocene karstified carbonates.Electrical resistivity imaging (ERI) supported by surface and subsurface (boreholes) lithological information allow to identify the geometry of the basement-Quaternary boundary in two areas. The morphology of this boundary is controlled by fractures and the karstification of the top of the Pliocene limestones.Thirteen GPR profiles (100 and 250MHz antenna) provide information about the morphology and internal structure of the sedimentary units, down to 4m below the surface. The observed features include: onlap of the sediments on the edge of the basin, fossilization of small paleo-reliefs by lacustrine deposits and channel fills in the Holocene deposits, and the sinking-collapse structures in the Neogene substratum.

TOC calculation of organic matter rich sediments using Rock-Eval pyrolysis: Critical consideration and insights

The Total Organic Carbon (TOC) is one of the most important Rock-Eval pyrolysis derived parameters. The present study documents the suppression and underestimation of Rock-Eval TOC as a function of sample amount being used for analysis. For the purpose of study, a manually isolated vitrain band, carbonaceous shale and two shales were analyzed at different weights using a Rock-Eval pyrolyzer. For the vitrain band and the carbonaceous shale samples, a drop in TOC content was observed with increasing sample amount. In each of the case (vitrain and carbonaceous shale), although the S2 pyrograms were fairly similar at successive higher weight fractions, it was observed that increasing portions of the CO2 released during Rock-Eval oxidation and represented by S4 oxidation graphics, was undercounted with increasing sample charge, thereby underestimating the residual carbon (RC) and TOC content. The results indicate that at lower sample weights the entire organic matter is oxidized below 650°C (S4–S5 boundary). However, at higher weights the organic matter is oxidized through a wider temperature range (beyond 650°C) and consequently resulting in release of CO2 through a wider temperature range. The insights developed in the paper indicate that sample amount should be around 5–10mg and 35mg for type III–IV organic matter-bearing coals and carbonaceous shales, respectively. However, when the grain size was kept finer (<63μm), it was observed that sample amounts can be kept around 15mg for type III–IV organic matter-bearing coals. No change in TOC with increasing/decreasing sample charge was observed in case of low TOC shales.

The addition of FeOOH binds phosphate in organic matter-rich sediments

ABSTRACTDue to climate change and anthropogenic nutrients’ runoff into freshwater or shallow lakes, eutrophication caused by phosphorus (P) can be seen in the frequent occurrence of cyanobacterial blooms and excessive growth of macrophytes. Subsequently, decomposition of cyanobacterial bloom biomass (CBB) and macrophytes leads to massive autochthonous organic matter (OM) and creates hypoxia in bodies of water. In this study, we investigated the effects of OM and iron on phosphorus release from lake sediments under anaerobic conditions. As with CBB, the addition of cellulose also enhanced P release from sediments during microcosm experiments, while total phosphorus (TP) concentration in the overlying water displayed an inverse relationship to cellulose amendment, with high TP concentration (0.41 ± 0.07 mg L−1) observed in the treatment of less cellulose amendment (1 g of cellulose). In addition, P release from OM-rich sediments was effectively inhibited when amorphous FeOOH was added to the microcosms. P release was inhibited by 66–92% when the weight ratio between total Fe and total P in sediments varied from 18 to 60. Thus, iron treatment was useful to inhibit P release from OM-rich sediments, and could alleviate eutrophication problems.

Graphite‐schist blocks in the Franciscan Mélange, San Simeon, California: Evidence of high‐P metamorphism

AbstractThe seacliff exposure at San Simeon, California, contains graphite‐schist blocks in a shale‐matrix, an undocumented lithology within the Franciscan mélange. Thirty graphite‐schist blocks were studied to discover all the varieties in this classic locality of mélange. Based on their mineralogical assemblage and composition, and textural characteristics the graphite‐schists in San Simeon are subdivided into two main types (Type I and II) with two subdivisions each (A and B). Type IA and IIA blocks are the most abundant. Type IA graphite‐schists are siltstone/fine greywacke‐like, preserve sedimentary textures, and lack lawsonite. Type IB graphite‐schists are mineralogical and texturally similar to Type IA schists, but are finer grained siltstone and shale. Type IIA graphite‐schists are compositionally layered and contain quartz‐ and albite‐rich layers and dark graphite‐ and intergrown mica/chlorite‐rich layers. Nine out of the 15 Type IIA blocks contain lawsonite. Two Type IIA blocks also contain aragonite (+calcite) in veins. Type IIB graphite‐schists are mostly composed of quartz and minor graphite, intergrown chlorite and white mica, and white mica pseudomorphs after lawsonite. The phengite content of mica in Type IIA blocks is higher than that of mica in Type IA graphite‐schists, confirming they were metamorphosed under high‐P/low‐T conditions. Type IA blocks were recrystallized between 200 and 250 °C at &lt;~3 kbar; whereas, Type IIA blocks were metamorphosed under higher pressure conditions, probably at 250–300 °C and 3–5 kbar. Most likely both types of graphite‐schists were derived from a similar layered siltstone/fine greywacke/shale protolith. Organic matter‐rich sediments deposited in the trench axis were subducted along with oceanic crust during Franciscan subduction. Type I graphite‐schists were subducted to depths &lt;10 km, whereas Type II graphite‐schists were subducted to depths ~15 km where they were underplated under high‐P conditions. The graphitic metasedimentary rocks were juxtaposed with mafic lithologies from the subducted oceanic crust that were metamorphosed to blueschist facies and retrograded to greenstone as they returned to the surface in the subduction channel shear zone.

Nano-Crystal Formation of Tio2Polymorphs Brookite and Anatase Due To Organic—Inorganic Rock–Fluid Interactions

The occurrence of the titania polymorphs brookite and anatase as nano-crystals in organic matter-rich sediments of differing age and thermal maturity has been investigated by means of a multidisciplinary analytical approach (FIB-TEM, organic geochemistry, and petrography). It was the aim of the study to analyze the formation mechanisms, fate and behavior of the titania nano-crystals as a result of organic–inorganic rock–fluid interactions. Brookite nano-crystals have been detected in immature Mediterranean sapropels of Quaternary age, but anatase also occurs in deeper and older black shales (Furongian Alum Shale, Sweden; Devonian to Carboniferous Bakken Shale, Williston Basin, USA). Whereas anatase prevails as single crystals, brookite nano-crystals often are agglomerated. Single brookite nano-crystals from Posidonia Shale (Lower Jurassic, Northern Germany) have increasing crystal diameters with increasing thermal maturity. Exclusively anatase nano-crystals both as single crystals or as agglomerates have been detected at oil–water contacts in oilfields, and along fractures with fluid flow enriched in dissolved organic carbon. Titania nano-crystal precipitation, growth (and agglomeration) takes place in the pore water of micro-environments at low to high temperatures and where low pH is coupled to the occurrence of dissolved organic components. Low sedimentation rates preserving a critical geochemical environment or higher temperatures seem major controls for the precipitation of anatase and its tendency not to agglomerate.

Oceanic response to Pliensbachian and Toarcian magmatic events: Implications from an organic-rich basinal succession in the NW Tethys

The Bächental bituminous marls (Bächentaler Bitumenmergel) belonging to the Sachrang Member of the Lower Jurassic Middle Allgäu Formation were investigated using a multidisciplinary approach to determine environmental controls on the formation of organic-rich deposits in a semi-restricted basin of the NW Tethys during the Early Jurassic. The marls are subdivided into three units on the basis of mineralogical composition, source-rock parameters, redox conditions, salinity variations, and diagenetic processes. Redox proxies (e.g., pristane/phytane ratio; aryl isoprenoids; bioturbation; ternary plot of iron, total organic carbon, and sulphur) indicate varying suboxic to euxinic conditions during deposition of the Bächental section. Redox variations were mainly controlled by sea-level fluctuations with the tectonically complex bathymetry of the Bächental basin determining watermass exchange with the Tethys Ocean. Accordingly, strongest anoxia and highest total organic carbon content (up to 13%) occur in the middle part of the profile (upper tenuicostatum and lower falciferum zones), coincident with an increase in surface-water productivity during a period of relative sea-level lowstand that induced salinity stratification in a stagnant basin setting. This level corresponds to the time interval of the lower Toarcian oceanic anoxic event (T-OAE). However, the absence of the widely observed lower Toarcian negative carbon isotope excursion in the study section questions its unrestricted use as a global chemostratigraphic marker. Stratigraphic correlation of the thermally immature Bächental bituminous marls with the Posidonia Shale of SW Germany on the basis of C27/C29 sterane ratio profiles and ammonite data suggests that deposition of organic matter-rich sediments in isolated basins in the Alpine realm commenced earlier (late Pliensbachian margaritatus Zone) than in regionally proximal epicontinental seas (early Toarcian tenuicostatum Zone). The late Pliensbachian onset of reducing conditions in the Bächental basin coincided with an influx of volcaniclastic detritus that was possibly connected to complex rifting processes of the Alpine Tethys and with a globally observed eruption-induced extinction event. The level of maximum organic matter accumulation in the Bächental basin corresponds to the main eruptive phase of the Karoo-Ferrar large igneous province (LIP), confirming its massive impact on global climate and oceanic conditions during the Early Jurassic. The Bächental marl succession is thus a record of the complex interaction of global (i.e., LIP) and local (e.g., redox and salinity variations, basin morphology) factors that caused reducing conditions and organic matter enrichment in the Bächental basin. These developments resulted in highly inhomogeneous environmental conditions in semi-restricted basins of the NW Tethyan domain during late Pliensbachian and early Toarcian time.

The hydrological and environmental evolution of shallow Lake Melincué, central Argentinean Pampas, during the last millennium

Summary Lake Melincue, located in the central Pampean Plains of Argentina, is a shallow (∼4 m), subsaline lake (TDS > 2000 ppm), highly sensitive to hydrological changes. The modern shallow lake system is composed of: (a) a supralittoral area, which includes a narrow mudflat, a vegetated mudflat and wetlands subenvironments; and (b) the main water body, comprising lacustrine marginal and inner areas. The development and extension of these subenvironments are strongly conditioned upon lake surface fluctuations. Past environmental changes were reconstructed through sedimentological, physical and geochemical proxy analyses of two short sedimentary cores (∼127 cm). Well-constrained 210 Pb ages profiles were modeled and radiocarbon chronologies were determined, covering a period from ∼AD 800 to the present. The analyzed sedimentary cores from Lake Melincue allowed for the reconstruction of past hydrological scenarios and associated environmental variability, ranging from extremely low lake levels during dry phases to pronounced highstands at wet periods. The paleohydrological reconstruction revealed very shallow conditions in the period between AD 806 and AD 1880, which was registered by massive deposits with low organic matter. Relatively wetter phases disrupting this dry period were represented by organic matter increases. A major wet phase was registered by AD 1454, after the end of the Medieval Climate Anomaly. A subsequent abrupt shift from this wet phase to drier conditions could be matching the transition between the end of the Medieval Climatic Anomaly and the beginning of the Little Ice Age. The occurrence of sedimentary hiatuses between AD 1492 and AD 1880 in Melincue sequence could correspond to intensive droughts during the Little Ice Age. After AD 1880, banded and laminated, autochthonous, organic matter-rich sediments registered an important lacustrine transgression and the onset of a permanent shallow lake, corresponding to the beginning of the current warm period. The uppermost recent fine-grained, low salinity, organic sediments represent a lake transgression occurred in the 1970s, coeval with a general increase in precipitation across southeastern South America. This transgression is registered regionally in other Pampean lakes and in the 20th century instrumental records of Lake Melincue. This paleoenvironmental reconstruction provides a new high-resolution record that registers striking hydroclimatic changes occurred at a regional scale across the Pampean Plains during the last millennium and it contributes to understand the past climatic history in southeastern South America.

Spatial characterization of the late Sinemurian (Early Jurassic) palaeoenvironments in the Lusitanian Basin

The upper Sinemurian of the Lusitanian Basin (Portugal) is characterized by the deposition of carbonates (dolomitic limestones, limestones), sometimes enriched in organic matter (total organic carbon up to 22%). The main goal of this study is to understand the distribution of carbonates and organic matter-rich sediments in space and time, and to characterize the context of their deposition at the basin scale. Three sections located along a proximal–distal transect in the Lusitanian Basin and dated from the oxynotum to raricostatum ammonite zones (upper Sinemurian) have been studied for their microfacies and sedimentary structures, and correlated by means of ammonite and calcareous nannofossil biostratigraphy, and sequence stratigraphy. The proximal part of the basin is dominated by carbonates whereas organic-rich sediments and marl-limestone alternations are common in the distal part. Microfacies analysis shows a trend from high- to low-energy environments, which reflects a deepening trend in most of the basin within the raricostatum ammonite Zone. A forced regression took place in the earliest Pliensbachian, which is likely related to tectono-eustasy. This particular surface can be correlated at the regional level, to several basins of the western Tethys.The presence of black shales in the distal part of the basin and of framboidal pyrite in the proximal part is interpreted as resulting from an episode of pervasive dysoxia/anoxia at the basin scale during this time interval. The basin recorded conditions varying from dysoxic/anoxic phases in times of water column stratification, to full oxygenation in times of intense mixing by storms, attested by the presence of storm-related sedimentary structures in some carbonate-rich deposits. The occurrence of upper Sinemurian sediments rich in organic matter in other basins, in England and Spain, in the same time interval (upper Sinemurian, raricostatum ammonite Zone) suggests that palaeoclimatic and palaeogeographic conditions were prone to the development of a supraregional dysoxia/anoxia.

Alternative food sources of native and non-native bivalves in a subtropical eutrophic lake

The aim of this study was to evaluate the consumption of alternative food sources (water column and sediment) and the potential competition under different conditions between Diplodon parallelopipedon (native) and Corbicula fluminea (non-native). We evaluated filtration and ingestion rates of water column at 6 and 24 h, held with and without organic matter and different phytoplankton types: needle-shaped green algae (Ankistrodesmus sp.) and filamentous cyanobacteria (Planktothrix agardhii) dominated communities. Our results confirmed higher filtration and ingestion rates per biomass unit for C. fluminea in the presence of sediment without organic matter. However, when we compared the filtration of bivalves held in sediment with organic matter, D. parallelopipedon rates were not significantly different from C. fluminea values. Moreover, in the presence of filamentous cyanobacteria, only C. fluminea filtration and ingestion rates decreased significantly. Our experimental results and previous field evidence concur that C. fluminea were not able to outcompete the native bivalves in organic matter rich sediment and cyanobacteria blooms conditions. The differential responses to the eutrophication process between exotic and native bivalves, apparently favoring the later, might buffer the potential competition interactions allowing their coexistence.

Influence of tidal regime, diurnal phase, habitat and season on feeding of an intertidal crab

AbstractThe relationship between diet and feeding activity of intertidal crabs, and environmental cycles (tidal, daily and seasonal), habitat and level of the intertidal zone (high/low) was studied using Neohelice granulata (Brachyura, Varunidae) as a model. This is a semi‐terrestrial burrowing crab occupying different habitats in the Southwestern Atlantic coasts and estuaries from bare low intertidal mudflats to high intertidal salt marshes, and from fine, organic matter rich sediment to very coarse sediment with low content of organic matter. The study was carried out in two contrasting habitats of three sites with diverse sets of physical and biological conditions. Diet and feeding of adult N. granulata were indirectly studied through the proportion of food items and the presence/absence of food in crab stomachs, respectively. This species has a dual mode of feeding: predominantly herbivorous (live plants or plant litter in salt marshes) or deposit feeder (superficial sediment and detritus in mudflats), but the quantity and quality of ingested food varies among habitats and sites. A trend to omnivory (including algae and conspecifics) was detected in relation to low quality of resources. Feeding activity modulated by a complex interaction of factors varied according to spatial and/or temporal changes in some natural cycles. Males and non‐ovigerous females fed preferably after dark and during submersion periods, but also after emersion periods if mudflat sediment remained wet; salt marsh crab feeding is somewhat independent of light and tidal cycles. Ovigerous females almost never fed. Both diet and feeding activity of this crab seem to be flexible traits adapted to different combinations of physical and biological factors.

Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist

This paper exposes the unique set of attributes explaining why precious opal has formed in such abundance in central Australia, and almost nowhere else on Earth. The Early Cretaceous history of the Great Artesian Basin is that of a high-latitude flexural foreland basin associated with a Cordillera Orogen built along the Pacific margin of Gondwana. The basin, flooded by the Eromanga Sea, acted as a sink for volcaniclastic sediments eroded from the Cordillera's volcanic arc. The Eromanga Sea was shallow, cold, poorly connected to the open ocean, muddy and stagnant, which explains the absence of significant carbonates. Iron-rich and organic matter-rich sediments contributed to the development of an anoxic sub-seafloor in which anaerobic, pyrite-producing bacteria thrived. Rich in pyrite, ferrous iron, feldspar, volcanic fragments and volcanic ash, Lower Cretaceous lithologies have an exceptionally large acidification potential and pH neutralisation capacity. This makes Lower Cretaceous lithologies particularly reactive to oxidative weathering. From 97 to 60 Ma, Australia remained at high latitude, and a protracted period of uplift, erosion, denudation and crustal cooling unfolded. It is possibly during this period that the bulk of precious opal was formed via acidic oxidative weathering. When uplift stopped at ca 60 Ma, the opalised redox front was preserved by the widespread deposition of a veneer of Cenozoic sediments. On Earth, regional acidic weathering is rare. Interestingly, acidic oxidative weathering has been documented at the surface of Mars, which shares an intriguing set of attributes with the Great Artesian Basin including: (i) volcaniclastic lithologies; (ii) absence of significant carbonate; (iii) similar secondary assemblages including opaline silica; (iv) similar acidic oxidative weathering driven by very similar surface drying out; and, not surprisingly, (v) the same colour. This suggests that the Australian red centre could well be the best regional terrestrial analogue for the surface of the red planet.

Organofacies and paleoenvironment of the Oligocene Maikop series of Angeharan (eastern Azerbaijan)

The Maikop Formation, deposited in eastern Azerbaijan during Oligocene and Early Miocene times, contains prolific source rocks with primarily Type II organic matter. Paleontological analyses of dinoflagellate cysts revealed a Lower to Upper Oligocene age for the investigated succession near Angeharan. A major contribution of aquatic organisms (diatoms, green algae, dinoflagellates, chrysophyte algae) and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n-alkane distribution patterns, composition of steroids and δ13C of hydrocarbon biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulfur bacteria. Higher inputs of terrigenous organic matter occurred during deposition of the Upper Oligocene units of the Maikop Formation from Angeharan mountains. The terpenoid hydrocarbon composition argues for angiosperm dominated vegetation in the Shamakhy–Gobustan area.High primary bioproductivity resulted in a stratified water column and the accumulation of organic matter rich sediments in the Lower Oligocene units of the Maikop Formation. Organic carbon accumulation during this period occurred in a permanently (salinity-) stratified, mesohaline environment with free H2S in the water column. This is indicated by low pristane/phytane ratios of all sediments (varying from 0.37–0.69), lower methylated-(trimethyltridecyl)chromans ratio in the lower units and their higher contents of aryl isoprenoids and highly branched isoprenoid thiophenes. Subsequently, the depositional environment changed to normal marine conditions with oxygen deficient bottom water. The retreat of the chemocline towards the sediment–water interface and enhanced oxic respiration of OM during deposition of the Upper Oligocene Maikop sediments is proposed.Parallel depth trends in δ13C of total OM, n-alkanes, isoprenoids and steranes argue for changes in the regional carbon cycle, associated with the changing environmental conditions. Increased remineralisation of OM in a more oxygenated water column is suggested to result in low TOC and hydrocarbon contents, as well as 15N enriched total nitrogen of the Upper Oligocene units.

Gas emissions due to magma–sediment interactions during flood magmatism at the Siberian Traps: Gas dispersion and environmental consequences

We estimate the fluxes of extremely reduced gas emissions produced during the emplacement of the Siberian Traps large igneous province, due to magma intrusion in the coaliferous sediments of the Tunguska Basin. Using the results of a companion paper (Iacono-Marziano et al., accepted for publication), and a recent work about low temperature interaction between magma and organic matter (Svensen et al., 2009), we calculate CO–CH4-dominated gas emission rates of 7×1015−2×1016g/yr for a single magmatic/volcanic event. These fluxes are 7–20 times higher than those calculated for purely magmatic gas emissions, in the absence of interaction with organic matter-rich sediments. We investigate, by means of atmospheric modelling employing present geography of Siberia, the short and mid-term dispersion of these gas emissions into the atmosphere. The lateral propagation of CO and CH4 leads to an important perturbation of the atmosphere chemistry, consisting in a strong reduction of the radical OH concentration. As a consequence, both CO and CH4 lifetimes in the lower atmosphere are enhanced by a factor of at least 3, at the continental scale, as a consequence of 30 days of magmatic activity. The short-term effect of the injection of carbon monoxide and methane into the atmosphere is therefore to increase the residence times of these two species and, in turn, their capacity of geographic expansion. The estimated CO and CH4 volume mixing ratios (i.e. the number of molecules of CO or CH4 per cm3, divided by the total number of molecules per cm3) in the low atmosphere are 2–5ppmv at the continental scale and locally higher than 50ppmv. The dimension of the area affected by these high volume mixing ratios decreases in the presence of a lava flow accompanying magma intrusion at depth. Complementary calculations for a 10-yr duration of the magmatic activity suggest (i) an increase in the mean CH4 volume mixing ratio of the whole atmosphere up to values 3–15 times higher than the current one, and (ii) recovery times of 100yr to bring back the atmospheric volume mixing ratio of CH4 to the pre-magmatic value. Thermogenic methane emissions from the Siberian Traps have already been proposed to crucially contribute to end Permian–Early Triassic global warming and to the negative carbon isotopic shift observed globally in both marine and terrestrial sediments. Our results corroborate these hypotheses and suggest that concurrent high temperature CO emissions also played a key role by contributing to increase (i) the radiative forcing of methane and therefore in its global warming potential, and (ii) the input of isotopically light carbon into the atmosphere that generated the isotopic excursion. We also speculate a poisoning effect of high carbon monoxide concentrations on end-Permian fauna, at a local scale.

Alkylated phenol series in lacustrine black shales from the Nördlinger Ries, southern Germany

Several series of alkylated phenols were detected for the first time in the extractable bitumens of organic matter-rich sediments from the Nördlinger Ries (southern Germany). Most abundant and significant constituents comprise those with n-octadecyl, n-eicosanyl, phytanyl, and iso-pentadecyl and anteiso-pentadecyl substituents. The structures of these compounds are suggested from mass spectrometric and retention time data and coinjection with synthetic standards. Diagenetic alteration of phenolic algal lipids is suggested as a possible way to the formation of these compounds in the Nördlinger Ries sediments.

Trace metal concentrations in tidal flat coastal sediments, Yamaguchi Prefecture, southwest Japan

Geochemical investigations of tidal flat coastal sediments at Ogori, Ozuki, and Kasado in Yamaguchi Bay of southwest Japan were conducted to determine their metal concentrations and to assess contamination levels, compared with sediment quality guidelines (SQG) and several pollutant indicators. Selected major oxides, trace elements, and total sulfur (TS) were determined by X-ray fluorescence. pH values of most samples were alkaline, indicating anoxic conditions. Average abundances of As, Pb, Zn, Cu, Ni, and Cr in Ozuki sediments were 11, 27, 109, 21, 19, and 52 mg/kg, respectively, compared to 9, 29, 80, 16, 18, and 42 mg/kg at Ogori and 12, 27, 151, 34, 30 and 80 mg/kg at Kasado, respectively. Average concentrations of As, Zn, and Cu in all samples and TiO(2), Fe(2)O(3), and P(2)O(5) at Kasado were greater than those of the upper continental crust. Contamination levels were assessed based on SQG, contamination factors (CF), pollution load index (PLI), enrichment factor (EF), and index of geoaccumulation (Igeo). According to the SQG of the US EPA, the sediments were heavily polluted with respect to As, whereas Zn, Cu, Ni, and Cr were classed as moderately polluted. The elevated CF values of As, Pb, and Zn identify moderate to considerable contamination, indicating that these metals are potentially toxic in the study area. Based on PLI and EF, the study sites are moderate to moderately severe polluted with As and Pb, moderately polluted with Zn, and weakly contaminated to noncontaminated with Cu, Ni, and Cr. The highest Igeo values for As, Pb, and Zn in the surface and core sediments reflected the tendency of metal contamination that seems to be related to their fine-grained nature, organic matter-rich sediments, and anthropogenic point sources. Trace metal contents were strongly correlated with Fe(2)O(3) and TiO(2), suggesting that Fe oxyhydroxides and detrital clastic load play a role in controlling abundances in the study area.