Organic Matter-rich Sediments Research Articles

Cold-subduction biogeodynamics boosts deep energy delivery to the forearc

Metamorphic fluids in subduction zones carry C–H–N–O–P–S species, which are crucial for sustaining subsurface microbial life at shallower crustal depths in the forearc region. Upwards migration of deeply released fluids to shallower levels, where temperatures permit the persistence of microbial life, is recorded by metasomatic rocks formed along the plate interface. Variations in the redox state and component speciation of metamorphic fluids – from local to secular, and highly dependent on thermal gradients and redox state of subduction inputs – may strongly control microbial pathways or even the possibility for metamorphic fluids to sustain microbial communities in the subsurface biosphere at convergent plate margins. We show that metamorphic fluids containing reduced energy sources for microbial life – e.g., CH4, H2 – are common in Phanerozoic, high-pressure/low-temperature plate-interface metasomatic rocks such as jadeitites and albitites worldwide. Based on the stability fields of minerals hosting CH4, H2 and graphite inclusions, we pinpoint the protracted, probably episodic migration of energy sources in the mantle wedge via fluid circulation being mediated by jadeitites from > ca. 35 km depth, and by their retrogressed counterparts forming from between 35–15 km depth. These fluids can cross the so-called biotic fringe – whose limit is the depth corresponding to ca. 122–135 °C (as deep as ca. 13 km depth depending on geothermal gradients) – as suggested by previous documentation of slab-derived fluids reaching subsurface microbial communities. Thermodynamic modeling indicates that cool thermal gradients, possibly combined with increased inputs of organic matter-rich sediments into subduction, favor the abundance of reduced energy sources relative to more oxidized species (e.g., CO2), thus promoting the proliferation of subsurface microbial life at convergent margins.

Water extractable carbon and nitrogen across vegetated and non-vegetated coastal habitats

Many coastal ecosystems have a high carbon (C) and nitrogen (N) accumulation capacity. Extractable C and N have been shown to be useful indicators to assess changes in soil C and N in terrestrial systems but have not been widely used in coastal habitats. Here, we quantified cold and hot water extractable organic C (cweC, hweC) and N (cweN, hweN) and determined specific ultraviolet absorbance (SUVA254) to evaluate how water extractable C and N concentration and chemistry change along the transition from mangrove (vegetated) to tidal flat (non-vegetated) and with sediment depth. Sediment cores (20 cm depth) were collected along transects in a muddy (29–46 % clay + silt, Snells Beach) and sandy (8–11% clay + silt, Hobson Bay) estuary in the Auckland region, New Zealand. Cold water exactable C and N concentrations were similar in the muddy and sandy estuary (7–173 μg C g−1 soil; 1–32 μg N g−1 soil). However, cweC and cweN made up a larger proportion of sediment C and N concentration at the sandy estuary. The hweC and hweN concentrations were significantly higher in the muddy (240–2440 μg C g−1 soil; 25–244 μg N g−1 soil) than sandy estuary (50–960 μg C g−1 soil; 6–119 μg N g−1 soil) suggesting a higher release of C and N from the muddy estuary with organic matter rich sediments. Habitat had a strong effect at both sites with extractable C and N concentrations being higher in mangroves compared to tidal flat. The C:N ratios and SUVA254 values of cweC tended to be lower than hweC across all habitats in both estuaries, suggesting a higher proportion of algal-derived organic matter and a lower proportion of aromatic molecules. Our findings demonstrate that water extractable C and N provide valuable insights into the quality and drivers of C and N in coastal habitats.

Aragonite crystallization in a sulfate-rich hypersaline wetland under dry Mediterranean climate (Laguna Honda, eastern Guadalquivir basin, S Spain)

This research investigates the influence of water composition, the presence of seasonal algal mats, detrital inputs and the activity of microorganisms on the crystallization of aragonite in the sediments deposited in the hypersaline Laguna Honda wetland (S of Spain). The high alkaline and hypersaline waters (pH > 9.2 and C.E. > 70 mS/cm) of the wetland lake are rich in SO42− (>24,000 mg/l), Cl− (>21,000 mg/l), Na+ (>11,000 mg/l) Mg2+ (>8400 mg/l) and Ca2+ (>1000 mg/l), and are supersaturated for dolomite, calcite and aragonite. Sediments have lower pH values than column waters, oscillating from 8.54 in the low Eh (up to −80.9 mV) central deep sediments and 6.33 in the shallower higher Eh (around −13.6 mV) shore sediments. Erosion of the surrounding olive groves soils produced detrital silicates rich sediments with concretions of carbonate or sulfate. Aragonite (up to 19 %) and pyrite (up to 13 %) are mainly concentrated in the organic matter rich samples from the upper part of the sediment cores, whereas gypsum is preferably concentrated in low organic matter content samples. Mineral crusts containing a MgAl silicate phase, epsomite, halite and gypsum are precipitated on the floating algal mats covering the wetland waters. Floating algal mats deposit increased the organic matter content of the upper sediments which promoted the presence of fermentative microorganisms, sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB) communities and variations of Eh that influence the authigenesis of carbonate and S-bearing minerals. Replacement of poorly crystalline MgSi phases infilling algal cells by aragonite was favored in the organic matter rich sediments with low Eh values and important SRB communities that promoted sulfate bioreduction processes to form pyrite. Aragonite precipitation was favored by the increase of carbonate and bicarbonate concentration produced by the SRB oxidation of organic matter, the CO2 degassing by high summer temperatures and the CO2 uptake by photosynthesis of the algal mats.

A volatile sulfur sink aids in reconciling the sulfur isotope mass balance of closed basin lakes

Sedimentary rocks from the early Eocene Green River Formation comprise the largest known lacustrine oil shale deposits, contain remarkably well-preserved fossils, and provide a unique record of climate evolution across the Early Eocene Climate Optimum, a period of high atmospheric CO2. The depositional environment of these intermountain lakes spanned from relatively fresh and fluvially influenced to expanded and stratified saline closed basin lakes under the influence of the Laramide orogeny and alternating humid and arid climatic conditions. As the surface area of the lakes expanded, alkalinity and salinity increased, with depositional cycles that linked to evaporative cycles and marked by an increasing abundance of organic matter-rich shales (TOC > 10 wt%). Simultaneously, an intriguing 20 ‰ positive shift in the sulfur isotope composition of sulfide minerals and organic sulfur is observed. Given that this trend cannot be simply explained by a change in the source of sulfate delivered to the basin, the evolution of biogeochemical sulfur cycling and the balance of fluxes in response to basin evolution remain unresolved. Here, we combine the sulfur isotope compositions of pyrite, organic sulfur, and carbonate-associated sulfate and molecular proxies of euxinia in samples from the Uinta basin's depocenter. We find that organic matter-rich sediments reflect deposition in a stratified water column with enhanced burial of pyrite and sulfurized organic matter, while organic-lean facies present evidence of, at least transiently, euxinic conditions reaching the photic zone during arid conditions presumably because of evaporation. As the lake became both saline-stratified and euxinic, we observe that δ34S values of all measured sulfur-bearing sedimentary proxies increase and evolve along a 1:1 line, a trend independent of facies that we interpret as reflecting a sulfate-limited system despite saline conditions. The isotopic mass balance of sulfur fluxes implies the existence of a sink of sulfur depleted in 34S that is spatially decoupled from burial in the depocenter. Modeling sulfur biogeochemical processes in a saline stratified lake system allows us to estimate that at least 50 % of the sulfur entering the lake could have been lost from the upper part of the euxinic water column where the fractionation factor imparted by microbial sulfate reduction is expressed. We propose that the overall isotopic enrichment of the system was caused by H2S degassing during arid climate intervals, presumably enhanced by transient water column mixing events. Further, episodic intrusion of euxinic bottom waters into the upper part of the water column might have triggered mass fish and plankton mortality, consequently facilitating the formation of these exceptionally fossiliferous and organic matter-rich rocks. Our study finds that volatile outgassing may be an underappreciated mechanism for the sulfur mass balance of stratified lacustrine systems.

Enrichment of Mississippi Valley-type (MVT) deposits in the Tethyan domain linked to organic matter-rich sediments

Enrichment of Mississippi Valley-type (MVT) deposits in the Tethyan domain linked to organic matter-rich sediments

Ecosystem recovery after the Early Jurassic T-OAE in the Châabet El Attaris section of the Tunisian Atlas

The flooding of the Pliensbachian shelf of the North-South Axis of Central Tunisia during the early Toarcian favoured the development of organic-rich facies in the most subsiding areas, such as that represented by the Châabet El Attaris section. The lower Toarcian strata in this section is represented by organic matter-rich sediments (dark laminated marls with low carbonate content) and the negative carbon isotope excursion (CIE) of the Jenkyns Event. The relatively high TOC values, increase of redox sensitive elements, and the absence of benthic organisms confirm the adverse conditions for life at the sea floor (suboxic to anoxic conditions) during the polymorphum Biozone and lower part of the levisoni Biozone. This indicates that the Jenkyns Event is expressed in this setting as the so-called Toarcian Oceanic Anoxic Event (T-OAE). The diversity and abundance of the calcareous nannoplankton was very low during the early Toarcian.After the T-OAE, the decrease in TOC and the low values of the geochemical redox proxies indicate a re‑oxygenation of the bottom waters, parallel with an increase of the carbonate content of the facies. After the negative CIE, the record of benthic macroinvertebrates (such as brachiopods, bivalves, ophiuroids and echinoids) and trace fossils (Zoophycos, Thalassinoides, Planolites, Chondrites, and locally Trichichnus) confirms the re‑oxygenation of the sea floor with increased diversity and abundance from the upper levisoni Biozone and bifrons Biozone. Benthic foraminifera colonization of the sea floor is evidenced by assemblages dominated by infaunal taxa, specially opportunist forms, such as Lenticulina, and progressive increase of diversity and abundance that confirms oxic pore-waters from the levisoni-bifrons biozone boundary. Planktic organisms attest to a progressive recovery of environmental conditions in the photic zone with increasing diversity and abundance of calcareous nannoplankton and radiolarians from the uppermost part of the levisoni Biozone and the bifrons Biozone. A first pulse after the negative CIE is represented by placoliths of the genus Lotharingius (shallow dweller) interpreted as a renewed input of nutrients to surface waters. The subsequent dominance of muroliths, especially of Crepidolithus crassus (a deep dweller), suggests a successive restoration of the deep-photic zone communities. These observations point towards enhanced oxygen availability in deep waters and pore waters due to improved basin ventilation.Taphonomic features of foraminifera pointing to dissolution in the bifrons Biozone are related to a lowered pH below the redox boundary within the sediment. In the same way, the presence of ferruginous moulds of microfossils (gastropods, bivalves, brachiopods, foraminifera, sponge spicules and radiolarians) with framboidal texture, actually preserved as goethite but originally pyrite, confirm the presence of reducing conditions within the sediment below the redox boundary. The record of Trichichnus confirms the reducing conditions in the deepest infaunal environments during the bifrons Biozone and is consistent with the development of pyrite moulds of microfossils early in the diagenesis.

Linking Sediment Gas Storage to the Methane Dynamics in a Shallow Freshwater Reservoir

AbstractFreshwater reservoirs are globally relevant sources of the greenhouse gas methane. Organic matter rich sediments are hot spots of methane production and can store large amounts of methane dissolved in porewater and as free gas. Yet, in situ data on the gas storage as free gas (bubbles) in freshwater sediments are scarce. Here, an acoustic approach was tested and used to map the gas content in the sediment of a shallow temperate reservoir. The sediment gas storage was linked to the methane budget obtained from almost 2 years of in situ monitoring. The emission fluxes were dominated by ebullition and degassing at the reservoir outlet, which combined accounted for 93% of the total methane emissions. 66% of the ebullition variability was explained by a combination of environmental parameters. Mappings of sediment gas content using echo sounder surveys revealed the accumulation of free gas in regions of elevated sediment deposition. Temporally, the gas storage in the sediment was related to methane emissions, in which a period of intensified emissions resulted in a reduction of sediment gas storage. The sediment could store an equivalent of 4 to 13 days of accumulated potential methane production, which could supply the mean ebullition flux for more than 2 months. We suggest that sediment gas storage plays an important role in buffering and modulating methane emissions in aquatic systems and need to be accounted for in process‐based models.

Paleoenvironmental significance of benthic foraminiferal associations of the Hauterivian to Barremian marine beds of the Neuquén Basin, Patagonia, Argentina

The upper Hauterivian-lower Barremian sedimentary sequences of the Agua de la Mula Member of the Agrio Formation (Neuquén Basin, Argentina) represent shallow marine environments characterized by mixed siliciclastic‑carbonate deposition. Ten benthic foraminiferal associations based on species composition were defined and evaluated in relation to explanatory variables such as facies, sequence stratigraphy, location and age. Variations in sedimentation rates, food and oxygen availability related to relative sea-level changes largely control distribution of the benthic foraminiferal associations. Four associations (Guttulina sp., Polymorphinids, Spiroplectammina sp.2-Haplophragmoides sp.1, and Planularia madagascariensis) are mostly composed by shallow infaunal, bacterial and detritivorous foraminiferal scavengers that largely occur in sandy facies within 3rd-order regressive systems tracts. These associations are thought to be indicative of well oxygenated and organic matter-rich sediments below the sediment-water interface. The remaining six associations consist of epifaunal, mainly opportunistic foraminifera with active herbivorous and detritivorous trophic strategies. Among them, Epistomina hechti - Epistomina australis and Reinholdella hofkeri associations occur in fine-grained, dark shales throughout 3rd-order transgressive systems tracts, and are considered related to oxygen and food stressed environment. An integrated approach, using benthic foraminifera (alpha diversity analysis, epifaunal/infaunal ratio) and multivariate statistical methods (cluster analysis, RDA, partial-RDA, PERMANOVA, PCoA), allowed to evaluate the paleosynecological significance of benthic associations, and to relate certain paleoenvironmental settings to relative sea-level changes recorded in the Agua de la Mula Member. The application of foraminiferal associations analysis in the paleoenvironmental interpretations led to recognize the influence of major relative sea-level changes in shaping higher-frequency sequences for this sector of the Neuquén Basin. Such approach can also be applied to other sectors of the basin and to high-frequency sedimentary sequences of comparable shallow marine settings.

Near equatorial paleoclimatic evolution and control on organic matter accumulation during the Cenomanian in the Abu Gharadig Basin, southern Tethys: Insights from palynology, organic petrography, and geochemistry

The beginning of the Late Cretaceous (Cenomanian) attested to a major marine transgression interrupted by sea level falls in north Africa, which led to significant environmental perturbation and widespread accumulation of organic matter-rich sediments. During this period, northern Egypt was located at the southern edge of the Tethys Ocean and deposition of thick siliciclastic alternations between shales, siltstones, and sandstones of the Bahariya Formation took place in the BED2 gas field of the Abu Gharadig Basin, north Western Desert. An integrated approach of palynofacies analysis and palynomorph composition, organic petrography, and geochemical analysis of total organic carbon (TOC)/Rock-Eval pyrolysis, carbonate content, XRD analysis, and clay mineralogy of thirty-nine cutting samples obtained from the lower-middle Cenomanian Bahariya Formation was conducted. This approach was used to characterize organic matter and possible occurrence of source rock intervals, paleoenvironmental and paleoclimatic conditions and their role in organic matter accumulation. Moderate to high proportions of pteridophyte and water fern spores and pollen grains of Araucariaceae (Araucariacites), Cheirolepidiaceae (mainly Classopollis), and Gnetalean (Elaterate pollen) and Afropollis versus the sparsely recorded xerophilous ephedroids (Ephedripites) indicate prevalent conditions of a warm and humid climate. This is consistent with clay mineral composition that is dominated mainly by kaolinite and smectite compared to low illite and the lack of chlorite, which indicated a predominantly warm-humid climate state. Palynofacies analysis showed two assemblages; the first one is dominated by phytoclasts and indicates gas-prone Type III kerogen and deposition in a fluvio-deltaic environment. The second palynofacies assemblage is characterized by moderate abundance of phytoclasts and AOM mostly of terrestrial origin, suggesting a mixed Type III/II kerogen and deposition in a marginal to shallow neritic shelf environment. The organic geochemical characterization showed relatively low average values of TOC, S2, and HI, indicating that the Bahariya Formation is characterized by Type III gas-prone kerogen. Thermal maturity of the Bahariya Formation ranges from immature to the early stage of the oil window. Additionally, organic matter characteristics of the Bahariya Formation are qualitatively and quantitatively similar to their equivalents from nearby basins. Low TOC contents are attributed to a high sedimentation rate and an increased terrestrial input of sand- and silt-sized fractions at shallow water column settings that controlled strong dilution and decomposition versus low burial efficiency of the organic matter.

Biomarker and compound-specific isotope records across the Toarcian CIE at the Dormettingen section in SW Germany

The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE; ~ 183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock–Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n-alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ13C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n-C27 (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n-alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ13C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).

Organic carbon accumulation at the northern Gondwana paleomargin (Tunisia) during the Toarcian Oceanic Anoxic Event: Sedimentological and geochemical evidence

During the early Toarcian (Early Jurassic; ~183 Ma) global environmental changes promoted the expansion of oxygen-deficient conditions across vast shelf areas around the globe. Ocean margin deoxygenation favored the preservation of organic matter in shelf sediments and is considered to reflect a Toarcian Oceanic Anoxic Event (T-OAE). Higher-level global drivers controlled temperature fluctuations, sea level evolution, oceanic circulation patterns, marine primary productivity, and stable carbon isotope excursions. Lower-level factors, controlled by individual basin bathymetry and paleogeography, operated on local to regional scale and here regulated organic matter accumulation. Accordingly, the T-OAE has been described as global event with a strong regional character. During the Toarcian, preferentially organic matter-lean sediments have been deposited at the northern Gondwana paleomargin. Enhanced connectivity to the Tethys Ocean and low marine primary productivity are considered to have been major factors preventing the formation of oxygen-deficient conditions and organic matter preservation. We here report on early Toarcian sediments from the Châabet El Attaris Section (Tunisia), located at the northern Gondwana paleomargin that episodically became enriched in organic matter. Bio- and chemostratigraphy both confirm that organic matter-rich sediments preferentially formed during the T-OAE but also predate the T-OAE. Sedimentological and micropaleontological characterization in concert with bulk, molecular and stable isotope geochemical investigations reveal that accumulation of marine organic matter occurred under anoxic, but not under euxinic, conditions. Limitation in the supply of reactive iron regulated organic matter sulfurization indicating that during most of the depositional history the levels of free H 2 S in bottom waters were rather low but limited to the sediment pore water. We conclude that the chemocline was either situated within the sediment or close to the sediment water interface. Spatiotemporally-restricted occurrence of organic matter enriched sediments and anoxic pore waters was promoted by restriction in water column ventilation in a local graben structure. • Organic matter-enriched strata in Tunisia predates but maximizes in the T-OAE. • Organic matter preservation occurred under anoxic but non-euxinic conditions. • Formation of oxygen deficient conditions was promoted by local paleobathymetry and basin restriction.

Postdiagenetic Bacterial Transformation of Nickel and Vanadyl Sedimentary Porphyrins of Organic-Rich Shale Rock (Fore-Sudetic Monocline, Poland).

Nickel and vanadyl porphyrins belong to the so-called fossil geo- or sedimentary porphyrins. They occur in different types of organic matter-rich sediments but mostly occur in crude oils and their source rocks, oil shales, coals, and oil sands. In this study, we aimed to understand the process of bacterial transformation of geoporphyrins occurring in the subsurface shale rock (Fore-Sudetic Monocline, SW Poland). We studied these transformations in rock samples directly obtained from the field; in rock samples treated with bacterial strain isolated from shale rock (strain LM27) in the laboratory; and using synthetic nickel and vanadyl porphyrins treated with LM27. Our results demonstrate the following: (i) cleavage and/or degradation of aliphatic and aromatic substituents of porphyrins; (ii) degradation of porphyrin (tetrapyrrole) ring; (iii) formation of organic compounds containing 1, 2, or 3 pyrrole rings; (iv) formation of nickel- or vanadium-containing organic compounds; and (v) mobilization of nickel and vanadium. Our results also showed that the described bacterial processes change the composition and content of geoporphyrins, composition of extractable organic matter, as well as nickel and vanadium content in shale rock.

Paleoenvironments and source rock potential of Dorado North well in the Mannar Basin (Indian Ocean)

The Upper Cretaceous to Eocene sedimentary succession of the Dorado North exploration well in the Mannar Basin has been studied for paleoenvironment (organic matter richness, its composition, and depositional environment) and petroleum source rock potential. In this study, drillcore cuttings (n = 142) were analyzed using CHNS elementary analyzer. Results indicated that Lower Campanian sediments were depleted by total organic carbon content (TOC = 0.45%), under arid climate. The Lower Campanian to Upper Maastrichtian sedimentary sequence contains organic carbon rich (TOC = 1.64%) terrestrial (C/N ratio = 27.13) sediments. Consequently, this sedimentary sequence can be identified as a good source rock for gas generation. The formation of organic matter rich terrestrial sediments has been linked to sea-level changes and tectonic activities during the Late Cretaceous. Carbonate carbon contents are rich in the Lower Campanian to Upper Maastrichtian sedimentary sequence that indicates diversification of microorganisms under warm climatic conditions. The Paleocene to Eocene sedimentary sequence indicates poor to fair source rocks for possibly oil generation. However, the depositional environment was drastically changed from oxic (Early-Late Paleocene) to anoxic (Late Paleocene-Late Eocene) conditions. Consequently, the Eocene epoch is characterized by weak oceanic circulation in the Indian Ocean under greenhouse climate. In contrast, the deposition of carbonate platforms since the Late Paleocene (CaCO3 = 29.3%) can be related to the northward movement of the Indian plate into warmer tropical latitudes.

State, source and triggering mechanism of iron and manganese pollution in groundwater of Changchun, Northeastern China.

The present state of iron (Fe) and manganese (Mn) concentration in groundwater of Changchun city located within the Songnen Plain of northeastern China was evaluated in this study. Heavy metal sources, as well as triggering mechanism, were analyzed using a physicochemical, statistical and spatial approach. Results revealed that out of the 2600 samples analyzed, 214 (representing 8.24%) for Fe and 606 wells (representing 23.34%) for Mn exceeded the water standard. Organic matter-rich sediments and Fe-Mn nodules in aquifer and soil serve as sources of Fe and Mn. Organic and inorganic complex formations, as well as long residence time, were found to foster the release of Fe and Mn into groundwater. Additionally, pH and well depth was important in triggering Mn dissolution while groundwater mineralization, depth to the water table and well proximity to the river were found to have minimal/negligible effect on heavy metal mobilization. The removal of Fe and Mn from the water before use was proposed along with the sinking of deeper wells for groundwater exploitation to limit the use of polluted water.

Electron Microscopy Evidence of Zn Bioauthigenic Sulfides Formation in Polluted Organic Matter-Rich Sediments from the Chicamocha River (Boyacá-Colombia)

Electron microscopy and sediment geochemical data from a river basin (the upper Chicamocha river basin, UCRB, Boyacá province, Colombia) affected by anthropogenic activities (wastewater discharges, smelting and agricultural activities) showed the existence of heterogeneously distributed Zn particles in the sediments and sediments with Zn contents above the regional background (42 mg/kg). The objective of this study was to evidence the ZnS sedimentary neoformation in organic matter rich sediments deposited in anthropogenic reservoirs to reveal the processes involved in the sedimentary uptake of Zn from potential pollution sources. The highest Zn concentrations are found in clay minerals and organic matter-rich sediments (up to 427 mg/kg) deposited in slow-flowing reaches of the river associated to La Playa dam. Quartz-rich sediments poor in organic matter deposited in fast flowing segments of the river show very low Zn contents (1–12 mg/kg). Electron microscopy images showed ZnS nanoparticles forming cell-shaped aggregates suggesting that sulfate-reducing microorganisms acted as templates for the partial binding of Zn and for the nucleation and growth of zinc sulfide minerals. A good correlation of Zn with total organic carbon (r = 0.936) and the low potential redox of these sediments (−233 mV) suggest that organic matter was able to maintain oxygen depleted conditions appropriate to the Zn accumulation in the sediments. Our results demonstrate that potentially toxic Zn, originating from anthropic activities, was partially immobilized in organic matter-rich sediments through the precipitation of sulfides.

Clay mineral transformations in anthropic organic matter-rich sediments under saline water environment. Effect on the detrital mineral assemblages in the Upper Chicamocha River Basin, Colombia

The results of this study showed how clay mineral assemblages from alluvial sediments were used to deduce the environmental impact of anthropogenic modifications in the Upper Chicamocha River Basin (UCRB) in Colombia. Two dams were investigated: La Playa dam (with anthropogenic input from farm activities and wastewater) and Sochagota Lake dam (with input from natural hydrothermal waters) and found to contain organic-rich deposits modified under highly saline, eutrophic conditions. Results from X-ray diffraction (XRD), scanning electron microscope (SEM) and analytical high resolution electron microscope AEM-HRTEM analyses indicated that the mineral assemblage in the sediments trapped in La Playa dam in the upper part of the basin is characterized by the presence of Fe-bearing smectite (up to 0.4 atoms per formula unit (a.p.f.u.)), whereas sediments downstream the Sochagota Lake dam are characterized by the presence of illite and dispersed fragments of pyrite framboids. Intense eutrophication due to organic compounds and nutrient loads from urban wastewater and agricultural activities in the area above La Playa dam created a reducing environment in which sediments promoted Fe-uptake in authigenic smectite and efficiently trapped it in the sediments. Periodic release of water from Sochagota Lake into the Chicamocha River allowed the incorporation of detrital illite grains and fragments of pyrite framboids (formed by low temperature illitization and Fe-uptake in the lake sediments) into the alluvial sediments in the lower section of the UCRB.

Transformation of S-Bearing Minerals in Organic Matter-Rich Sediments from a Saline Lake with Hydrothermal Inputs

Geothermal systems can provide significant amounts of hydrothermal sulfur to surface waters, increasing salinity and avoiding some of the common anthropic uses. The objective of this study was to investigate the sedimentary neoformation of S-bearing phases in organic matter-rich sediments from a saline lake with hydrothermal inputs (Sochagota Lake, Colombia). Detrital kaolinite and quartz are the main minerals of the materials deposited in the Sochagota Lake. Neoformed clay minerals (illite and illite-dioctahedral vermiculite mixed layers) are concentrated in the central and northern part of the lake in sediments with high organic matter content. The most organic matter-rich materials are characterized by S-bearing minerals: mackinawite, pyrite, and elemental sulfur (S°). FESEM, high-resolution transmission electron microscopy (HRTEM), EDS, and Raman microspectrometry have revealed the presence of cell-shape aggregates of mackinawite nanoparticles filling the inner part of plant fragments, indicating that microorganisms were involved in the hydrothermal sulfur uptake. The alteration of mackinawite in free sulfide excess environment produced the formation of framboidal pyrite. The evolution to conditions with the presence of oxygen favored the formation of complex S° morphologies.

Thermal Effects of Magmatism on Surrounding Sediments and Petroleum Systems in the Northern Offshore Taranaki Basin, New Zealand

In the past two decades, numerical forward modeling of petroleum systems has been extensively used in exploration geology. However, modeling of petroleum systems influenced by magmatic activity has not been a common practice, because it is often associated with additional uncertainties and thus is a high risk associated with exploration. Subsurface processes associated with volcanic activity extensively influence all the elements of petroleum systems and may have positive and negative effects on hydrocarbon formation and accumulation. This study integrates 3D seismic data, geochemical and well data to build detailed 1D and 3D models of the Kora Volcano—a buried Miocene arc volcano in the northern Taranaki Basin, New Zealand. It examines the impact of magmatism on the source rock maturation and burial history in the northern Taranaki Basin. The Kora field contains a sub-commercial oil accumulation in volcanoclastic rocks that has been encountered by a well drilled on the flank of the volcano. By comparing the results of distinct models, we concluded that magmatic activity had a local effect on the thermal regime in the study area and resulted in rapid thermal maturation of the surrounding organic matter-rich sediments. Scenarios of the magmatic activity age (18, 11 and 8 Ma) show that the re-equilibration of the temperature after intrusion takes longer (up to 5 Ma) in the scenarios with a younger emplacement age (8 Ma) due to an added insulation effect of the thicker overburden. Results of the modeling also suggest that most hydrocarbons expelled from the source rock during this magmatic event escaped to the surface due to the absence of a proper seal rock at that time.

Variability of deep carbon sources in Mexican geothermal fluids

A sampling campaign for collecting geothermal fluids was carried out between 2014 and 2016 at the Cerro Prieto (CP), Los Azufres (LA), Los Humeros (LH), and Las Tres Vírgenes (LTV) geothermal fields (GFs) in Mexico. The aim of this study was to determine the CO2 carbon origin and behavior in geothermal reservoirs. The LA-GF and LH-GF are situated in an arc magmatism context related to the subduction of the Cocos plate under the North American one. The CP-GF and LTV-GF are located in an extensional pull-apart context related to the San Andreas Fault System and the opening of Gulf of California. The most interesting observation of this study was that the measured isotopic composition of CO2 (13C/12C and 18O/16O ratios) is distinct and unique for each sampled geothermal field. The C isotopic variability is controlled mainly by its sources (i.e., δ13C is −6 ± 2‰ in the mantle; −20‰ in carbon organic matter-rich sediment; and 0 ± 2‰ in marine carbonates) and possibly secondary processes such as calcite precipitation. The δ13C in the CP-GF and LA-GF are compatible with a dominant mantle carbon source. LTV-GF and LH-GF show lower (−10.7 ± 0.5‰) and higher (−3.5 ± 0.9‰) values than that of the mantle respectively. The lower δ13C of carbon in LTV-GF fluids could be due to calcite precipitation (carbonate scaling), a well-known phenomenon in this field. However, calcite precipitation should lower the CO2/3He ratios from an initial mantle value of 2 × 109. Calculated CO2/3He ratios of 1.72 and 5.49 × 1010 in LTV-GF fluids suggest that the addition of sedimentary carbon rather than calcite precipitation might be the main process. Based on the isotopic composition of helium (3He/4He ratios, or R, from 3.2 to 7.9 times the atmospheric ratio, Ra = 1.384 × 10−6) and carbon (δ13C from −11 to −2‰ vs PDB) in the fluids, the δ13C variability could be caused by the addition of carbon organic matter-rich sediments from an ancient subducting plate in the LTV-GF and from leaching of marine carbonates located underneath the basement in the LH-GF. The oxygen isotopic variability is related to the degree of isotopic re-equilibration between steam (H2O) and CO2, which, in the case of fluids extracted from production wells, may be incomplete.

Continuous accumulation of organic matter-rich sediments associated with Oceanic Anoxic Event 1a in the El Pujal section, Organyà Basin, Catalunyà Spain and its relation to episodic dysoxia

A detailed lithostratigraphic and geochemical analysis was performed on a 13.77 m sequence consisting of a succession of limestones, argillaceous limestones and marlstones at the El Pujal section in the Organyà Basin of the south-central Spanish Pyrenees. Biostratigraphic analysis reveals the presence of the foraminifer Leupoldina cabri positioning the sequence within OAE 1a. Corroborating evidence from stable carbon isotope data permits the correlation of the El Pujal section with other Tethyan sections, namely Roter Sattel (Switzerland), Cismon (Italy) and El Pui (Spain) to within carbon isotope segment C5. Geochemical results reveal periodic increase of redox sensitive trace elements (RSTEs: V, Ni, Co, Cr, Cu, Mo), biolimiting (P, Fe) and terrestrially derived elements (Al, Si, Ti) at argillaceous limestone and marlstone intervals, concurrent with episodes of enhanced organic matter preservation (TOC >1.18%). Further evidence from microfacies characterized by higher pyrite concentration, impoverished benthic fauna and lower degree of bioturbation are in accord with geochemical proxies. Biomarker data shows n-alkanes distributions ranging from nC10 to nC31, suggesting that the organic matter (OM) is a mixture of in-situ produced and terrestrially derived. Calculated average chain length (ACL nC25-nC33) shows lower values in intervals associated with argillaceous limestones and marlstones, suggesting wetter climate during these periods. The results imply that fluvial influences controlled by regional hydrologic variability related to climatic fluctuations probably exacerbated the already sluggish basinal conditions through the development of density stratification in the water column. Integration of the data from the El Pujal section shows that changes in paleoproductivity, terrigenous input, and redox conditions characterize the deposition of OAE 1a in the Organyà Basin without the hallmark of an anoxic phase. Conditions remained similar to those recorded in the preceding sequence at El Pui, thus revealing continuous deposition of OM under impoverished oxygen conditions, punctuated by episodes of dysoxia.