Concentrations Of Redox-sensitive Elements Research Articles

Changes of bottom water oxygenation during the last half millennium in the Gulf of Tehuantepec (Eastern Tropical North Pacific): A multiproxy approach

The Gulf of Tehuantepec (GoT), in the Eastern Tropical North Pacific Ocean, is home to one of the largest and more intense Oxygen Minimum Zones (OMZ). To identify the OMZ variability during the last five hundred years in the GoT, we analyzed the temporal variations of benthic foraminiferal (BF) assemblages, enrichment of redox-sensitive elements (Mo, V, Cd, U, and Re) and δ15Nsed, on a laminated sediment core. During the Little Ice Age (LIA; ∼1500 to ∼1860 CE), the BF assemblages showed high dominance, and the most abundant BF species in the sediments were Epistominella sandiegoensis, Takayanagia delicata, and Buliminella tenuata. This assemblage can withstand the lowest dissolved oxygen conditions, indicating an intensified OMZ consistent with the enrichment of Mo, Re, Cd, U, and enhanced denitrification. During the Current Warm Period (CWP; ∼1860 CE to present), the BF assemblages were more diverse, and the most abundant species were Bolivina seminuda, Epistominella sp.1, Gyroidina nitidula, and Suggrunda eckisi, an assemblage less tolerant to low dissolved oxygen conditions, together with lower concentrations of redox-sensitive elements and δ15Nsed, suggest weakening of the reducing conditions within the OMZ. Additionally, our BF data showed evidence of the influence of the Pacific Decadal Oscillation on bottom-water oxygenation. Bottom-water oxygenation changes appear to respond to increased solar forcing caused by Intertropical Convergence Zone migration and strengthened Pacific Walker Circulation, which in turn modulated productivity, organic matter flux to the seabed, and oxygen consumption in the bottom water. Our findings agree with previous studies further north from the study site, indicating these changes' regional scale.

Change of marine redox environment in the western Okinawa Trough since the mid-Holocene: Evidence based on geochemical records

The marine redox environment (MRE) plays a crucial role in the conversion and preservation of seafloor material and is a significant factor in the ocean carbon cycle. The evolution and driving mechanism of MRE are complex and present distinct regional characteristics over different time scales. The Okinawa Trough, situated in the East Asian monsoon realm, is influenced by the Kuroshio Current and flushing water, and its closed geographical pattern and multiple environmental factors make it highly sensitive to climate and marine environment changes. In this study, sediment samples from core GHN056–09 retrieved in the western Okinawa Trough were analyzed to investigate the high-resolution evolution of MRE since the mid-Holocene. The concentration of redox-sensitive elements (V, Co, Ni) and total organic carbon (TOC) were measured. Co/Ni and V/(V+Ni) values in the sediment samples indicated predominantly oxic conditions with matching productivity levels indicated by TOC (in the range of 0.28–1.17%). The oxic condition decreased during 6500–1500 a BP, and a strengthening trend was identified from 1500 a BP to present. By comparing these records with climate and environmental records, we found that weaker El Niño-Southern Oscillation (ENSO) and northward position of Intertropical Convergence Zone (ITCZ) resulted in less monsoon precipitation into the Okinawa Trough. Thinner freshwater lids could enhance seawater mixing at different layers, leading to strong oxic conditions. In contrast, stronger ENSO and southward position of ITCZ controlled by stronger solar insolation supplied more freshwater into the Okinawa Trough. Thicker freshwater barriers blocked the exchange of seawater from bottom to surface, resulting in weak oxic conditions. The trace element records in this study provide robust evidence on the high-resolution variability of MRE driven by the frequency of ENSO, movement of ITCZ, and changes in regional precipitation. Our findings offer new insights into the evolution of MRE in response to global and local climate changes.

Evidence for the early Toarcian Carbon Isotope Excursion (T-CIE) from the shallow marine siliciclastic red beds of Arabia

The Toarcian Oceanic Anoxic Event (T-OAE) and its corresponding Carbon Isotope Excursion (CIE) have been reported widely across the Tethyan region and globally. In Arabia, and based on ammonite dating, the time window of the T-OAE coincided with the deposition of the reddish siliciclastic unit of the Marrat Formation. However, no evidence of the T-OAE/CIE was ever reported from Arabia because these red beds were previously interpreted as continental deposits. Recently, these red beds have been recognized as shallow marine deposits which opened an opportunity to assess the occurrence and expression of T-OAE–CIE in Arabia. In this study, a multiproxy geochemical characterization was performed on the Toarcian Marrat Formation to infer the chemistry of the paleowater column and identify intervals of possible T-OAE/CIE in Arabia. While the low concentrations of redox-sensitive elements (Mo, U, V, Cr) may indicate a shallow oxic marine settings, the coupled negative δ13Corganic excursion and apparent increase in the chemical weathering suggests that the deposition of Marrat red beds coincided with the development of T-CIE and possibly time-equivalent to the T-OAE globally. The origin of reddening is interpreted to have occurred during the middle Marrat deposition due to the stabilization of unstable hydrous iron oxides to hematite under oxic marine conditions. The proposed model further indicates the possible development of source rocks in the deep, anoxic environment counterpart where the T-OAE may be expressed. Since our study documents the first record of the T-CIE and discuss the origin of shallow marine siliciclastic red beds in the Arabian Plate, this will have significant implications for the overall understanding of the T-CIE globally and for hydrocarbon exploration through realizations of potential new source rocks associated with the OAEs in the Toarcian and other time intervals.

Paleoenvironment, organic matter maturity and the hydrocarbon potential of Menilite shales (Silesian Unit, Polish Outer Carpathians) – Organic and inorganic geochemical proxies

The Oligocene Menilite shales (Silesian Unit, Polish Outer Carpathians), an example of the organic-rich black shale facies of the Paratethys, were sampled in the two sections of Znamirowice and Kobielnik, representing proximal to the land and open-marine environments, respectively. Terrestrial organic matter (OM) imprinted in maceral and biomarker composition was probably supplied by the fresh-water runoff, as suggested by the total organic carbon (TOC)–total sulfur (TS) values and low Sr/Ba ratio in the Znamirowice section. In contrast, normal marine-salinity conditions and the scarcity of higher plant-derived biomarkers or macerals in the Kobielnik section suggest the deposition in a more distal environment. Abundant alginite and bituminite, high TOC content and EF P2O5 values, together with biogenic silica and the occurrence of triaromatic dinosteroids indicate a medium-to-high primary productivity, represented by algal–bacterial communities including silica-secreting organisms.The inorganic proxies show that the analysed rocks were mainly deposited in unrestricted basin probably affected by temporary upwelling. Only a small number of samples approach the restricted basin model. Concentrations of redox-sensitive elements (e.g. Mo, U, V, Ni, Co) corroborating with biomarker proxies (homohopane index, pristane/phytane) allow us to suppose oxic conditions within the water column with periods of oxygen reduction in the bottom waters and mainly dysoxia/anoxia in the sediment during the Menilite shale formation.The studied Menilite shales are source rocks with good-to-excellent hydrocarbon potential, characterised by mostly immature OM. However, the hopane and sterane isomerisation ratios show a more advanced stage of thermal maturation in the Kobielnik section, reaching the threshold of the oil window. In the Znamirowice section the OM is still immature.

Geochemistry, ichnology, and sedimentology of omission levels in Middle Triassic (Muschelkalk) platform carbonates of the Germanic Basin (southern Poland)

Sedimentology and paleoecology of omission or discontinuity surfaces in carbonate strata is well documented, but the geochemistry of such surfaces is poorly recognized. This work describes morphology, fabrics, geochemistry, and biota succession in omission levels from Middle Triassic (Muschelkalk) platformal carbonates in Upper Silesia, southern Poland. Three types of omission levels are distinguished: firmgrounds, hardgrounds, and intraformational conglomerates. (1) Firmgrounds are characterized by burrows of Balanoglossites triadicus . Some burrows are enclosed by a dark and/or (minor) bright diagenetic halo. The prevailing dark color, lower δ 13 C and δ 18 O ratios, and increased concentration of redox-sensitive elements (U, As, Mo, Cu, Zn, Pb, Ni) in the halos compared to the host micrite indicate a significant enrichment in organic matter, which most likely resulted from impregnation of the burrow walls with organic mucus from the burrowing organism, a process observed in modern burrows. The burrow walls were in contact with stagnant saline water (recorded by elevated 87 Sr/ 86 Sr), but were locally bleached by oxygenated fresh seawater pumped by the burrow producer. (2) Hardgrounds show encrustations of the bivalve Placunopsis ostracina and borings Trypanites weisei . In many hardgrounds, the uppermost 5-mm-thick layer has a different color and higher δ 18 O values than the underlying micrite. This heavier δ 18 O signal reflects a higher content of 18 O-enriched early-marine microcrystalline cement precipitated from colder bottom seawater. (3) Intraformational conglomerates consist of flat pebbles derived from reworked firmgrounds and hardgrounds. Many pebbles contain the same encrustations and borings as observed in the hardgrounds, which occur on both sides of the clasts and mimic their shape, indicating that they were developed after pebble formation. All studied types of omission level are laterally discontinuous at some scale and are thus unsuitable for regional correlation. • Firmground burrows are surrounded by dark and/or minor bright early diagenetic haloes. • Dark haloes are enriched in redox-sensitive elements and 12 C and 87 Sr stable isotopes. • These proxies suggest organic matter in burrow walls and stagnant water in burrows. • Hardground top surfaces show discoloration, bedding-parallel halo, and elevated δ 18 O. • This elevated δ 18 O implies early cementation by cold bottom seawater.

Depositional and postdepositional processes of an oil-shale analog at the microstructure scale: The Lower Cretaceous Agrio Formation, Neuquén Basin, northern Patagonia

The Lower Cretaceous Agrio Formation is a source rock considered to be a possible unconventional reservoir. However, no detailed microstructure characterization of this hydrocarbon play has been performed. An exceptional oil-prone level is determined through outcrop information, petrography, x-ray diffraction, geochemical analysis, and x-ray micro–computed tomography (μCT). Microscopic studies reveal a lenticular fabric and calcareous nannofossils suggesting high carbonate productivity in the water column and algal-derived organic matter. The high organic matter content points to anoxic–euxinic conditions consistent with pyrite framboids, authigenic fluorapatite, and an elevated concentration of redox-sensitive elements. However, burrows reveal the activity of infaunal organisms, thus rejecting the idea of persistent bottom-water anoxia. The bioturbated, calcareous plankton and carbonate-rich mudstone represents accumulation in a distal outer ramp setting with calcareous plankton blooms and the co-occurrence of benthic microbial mats. Bottom currents, probably as a consequence of storm-related processes, were involved in detrital transport and oxygenation pulses. A three-dimensional (3-D)–microstructure reconstruction reveals three main μCT density-dependent phases: (1) a low-density phase composed of organic matter and calcite, (2) an intermediate-density phase dominated by calcite and minor organic matter, and (3) a high-density phase composed of authigenic minerals (pyrite, marcasite, and fluorapatite) and silt-sized detrital quartz and plagioclase. This analysis showed that burial diagenesis did not create enough pore space to be evaluated through μCT and that besides mineralogy, particle-size distribution, particle arrangement (sorting), and the local compaction degree affect the 3-D reconstructed structure. The analyzed sample is located toward the carbonaceous end member of the shale spectrum and exemplifies new approaches for the characterization of carbonate oil shales in the unit and similar deposits. From the perspective of unconventional exploration, the evaluation of depositional and postdepositional controls on the resultant microstructure might contribute to a better understanding of potential target intervals within the Agrio Formation.

Geochemical signatures of transgressive shale intervals from the 811 Ma Fifteenmile Group in Yukon, Canada: Disentangling sedimentary redox cycling from weathering alteration

Despite iron and trace element proxies informing much of our insight into Earth’s oxygenation history, the processes that controlled their accumulation and distribution in ancient sedimentary environments are not fully understood. Furthermore, deciphering between primary, depositional signals and oxidative weathering alteration poses a substantial challenge to reconstructing Earth’s redox history using its sedimentary record. Here, we present a multi-proxy geochemical investigation of three transgressive shale intervals from the ca. 811 Ma Reefal assemblage (upper Fifteenmile Group) in Yukon, Canada to help resolve these issues. Systematic stratigraphic trends from low to high FeHR/FeT and FeT/Al compositions across shale intervals represent shoaling of a discrete redoxcline that separated oxygenated surface waters from anoxic, ferruginous deep waters during marine transgression. In some cases, these trends are followed by a symmetric fall, which represents a full “transgressive-regressive cycle” of the redoxcline. The high proportion of glauconite to total illite indicate short-term oscillating redox conditions as the redoxcline migrated across the sediment-water interface during relative sea-level rise. These conditions invigorated dissimilatory iron reduction (DIR) within anoxic sediment pore waters, which released Fe(II) into the overlying water column where it was oxidized to iron oxy(hydr)oxide and shuttled downslope. High rates of DIR within sediment replete with highly reactive iron oxy(hydr)oxide and organic matter removed isotopically light, bio-available iron, rendering the residual sediment depleted in FeT/Al relative to the detrital baseline and enriched in 56Fe. The low iron content of the detrital flux to the basin (Fe/Al ∼ 0.3) rendered the sediment susceptible to changes in its bulk δ56Fe composition by modification of its authigenic components alone and enabled fractionation from DIR to drive bulk-rock δ56Fe values up to 0.8‰. Anomalously high concentrations of redox-sensitive elements in three samples (up to 5792 ppm Cr, 586 ppm Mn, 726 ppm Mo, and 3509 ppm Ni) and their relative distribution patterns show similarities to co-enrichment due to particulate shuttling within a weakly restricted basin. However, SEM-EDS element maps and large-area BSE image mosaics show the remains of framboidal pyrite “ghosts” with secondary enrichment of redox-sensitive elements. Thus, these geochemical trends in redox-sensitive trace metals most likely reflect mobilization and re-accumulation following pyrite dissolution during outcrop weathering. The low primary pyrite content and local transformation of pyrite iron to iron oxy(hdr)oxide suggest that δ56Fe and FeHR/FeT data were unaltered despite indications of pyrite weathering. Ultimately, this dataset elucidates key aspects of sedimentary redox cycling directly preceding evolutionary milestones and a major perturbation to the global carbon cycle and also provides a template for evaluating the effects of outcrop weathering on commonly used sedimentary redox proxies.

Subtle Cr isotope signals track the variably anoxic Cryogenian interglacial period with voluminous manganese accumulation and decrease in biodiversity

Earth’s atmosphere experienced a step of profound oxygenation during the Neoproterozoic era, accompanied by diversification of animals. However, during the Cryogenian period (720–635 million years ago) Earth experienced its most severe glaciations which likely impacted marine ecosystems and multicellular life in the oceans. In particular, large volumes of Mn and Fe accumulated during the interglacial intervals of the Cryogenian glaciations, indicating large anoxic marine metal reservoirs. Here we present chromium isotope-, rare earth element-, and redox-sensitive trace element data of sedimentary rocks from the interglacial Datangpo Formation deposited between the Sturtian and Marinoan glaciations in South China, in an attempt to investigate the oxidation state of the oceans and atmosphere. Both the Cr isotope and trace element data indicate mainly anoxic water conditions with cryptic oxic surface water incursions after the Sturtian glaciation. Glacial-fed manganese precipitated as manganese carbonate in anoxic basins, and the non-fractionated δ53Cr record of −0.10 ± 0.06‰ identifies anoxic conditions with a cryptic component of slightly fractionated Cr isotope composition in manganese ore, in line with distinctly fractionated Mo isotope composition. Both the manganese carbonate ore and the black shales exhibit very low redox-sensitive element concentrations. Our study demonstrates that the oxygenation of the seawater, and inferably of the atmosphere, at the beginning of the Cryogenian interglacial interval was much subdued. The post-glacial rebound then allowed the Ediacaran biological diversity.

Seasonal variation of arsenic and antimony in surface waters of small subarctic lakes impacted by legacy mining pollution near Yellowknife, NT, Canada

The seasonal variation in lake water arsenic (As) and antimony (Sb) concentrations was assessed in four small (<1.5km2) subarctic lakes impacted by As and Sb emissions from legacy mining activities near Yellowknife, Northwest Territories, Canada. Substantial variation in As concentrations were measured over the two-year period of study in all but the deepest lake (maximum depth 6.9m), including a four-fold difference in As in the shallowest lake ([As]: 172–846μgL−1; maximum depth 0.8m). Arsenic concentrations were enriched following ice cover development in the three shallowest lakes (50–110%) through a combination of physical and biogeochemical processes. Early winter increases in As were associated with the exclusion of solutes from the developing ice-cover; and large increases in As were measured once oxygen conditions were depleted to the point of anoxia by mid-winter. The onset of anoxic conditions within the water column was associated with large increases in the concentration of redox sensitive elements in lake waters (As, iron [Fe], and manganese [Mn]), suggesting coupling of As mobility with Fe and Mn cycling. In contrast, there was little difference in Sb concentrations under ice suggesting that Sb mobility was controlled by factors other than Fe and Mn associated redox processes. A survey of 30 lakes in the region during fall (open-water) and late-winter (under-ice) revealed large seasonal differences in surface water As were more common in lakes with a maximum depth <4m. This threshold highlights the importance of winter conditions and links between physical lake properties and biogeochemical processes in the chemical recovery of As-impacted subarctic landscapes. The findings indicate annual remobilization of As from contaminated lake sediments may be inhibiting recovery in small shallow lakes that undergo seasonal transitions in redox state.

A multi-basin redox reconstruction for the Miocene Monterey Formation, California, USA

Deposition of the Miocene Monterey Formation into numerous basins provides an ideal opportunity to study how preservation of geochemical signatures varies within and among basins as a function of local and global climatic and oceanic conditions. This study presents commonly employed redox indicators, including iron (Fe) speciation and redox-sensitive trace metal concentrations (Mo, U, V), to constrain paleoredox conditions from three Monterey locations: the Santa Barbara Basin, the Santa Maria Basin, and the San Joaquin Basin. The Fe speciation geochemistry of the Santa Barbara and Santa Maria Basin sediments are consistent with a wide range of redox conditions from suboxic to euxinic. These results, in combination with associated phosphorites and trace metal enrichment patterns, indicate oscillatory redox conditions, perhaps with a chemocline that fluctuated above the sediment-water interface on seasonal or multi-year timescales. Overall, these sections were deposited in environments similar to modern phosphorite-forming and upwelling-dominated oxygen minimum zone settings like the Peru Margin. Geochemical evidence indicates that the San Joaquin Basin was generally more restricted and more stably anoxic compared to the other two basins, perhaps more analogous to the modern Cariaco Basin, at least for the studied interval. Iron speciation in the San Joaquin Basin reveals anoxia and euxinia over the entirety of the studied interval, although trace metal concentrations are generally at the low end of those typical of anoxic/euxinic conditions—consistent with a restricted connection with the open ocean. The one exception is an interval c. ~11.6 million years ago (Ma) with elevated concentrations of redox-sensitive elements that may represent a decrease in basin restriction or an increase in water column sulfide concentrations. The present-day California Margin does not achieve euxinic conditions in similar basinal settings comparable with these Monterey sections, suggesting fundamental differences for the Miocene California margin that may carry implications for global-scale oxygen deficiency in Miocene oxygen minimum zones.

Uranium isotope variations in a dolomitized Jurassic carbonate platform (Tithonian; Franconian Alb, Southern Germany)

The uranium (U) isotopic composition (δ238U) of limestones is increasingly used to quantitatively track changes in paleoredox conditions of the global ocean. However, many limestones have undergone significant dolomitization during diagenesis. To assess the potential impact of diagenetic changes on the U isotope composition of dolomitized rocks, we examined the uranium isotopic composition of a Jurassic carbonate platform of the Franconian Alb in Southern Germany. This platform underwent dolomitization during shallow burial due to the supply of magnesium-rich fluids in the form of slightly modified seawater. This type of dolomitization is common in the geologic record. The carbonate platform of the Franconian Alb can therefore serve as an example for many dolomitized carbonate platforms.A positive correlation between the concentrations of redox sensitive elements (e.g., Re and V) and δ238U confirms that the authigenic uptake of U under reducing conditions preferentially incorporates U enriched in 238U. This positive correlation between redox sensitive elements and δ238U does not change within the dolomitized interval, suggesting that the δ238U is not altered during shallow burial dolomitization of limestones. Therefore, our results indicate that dolostones can retain information about secular variations in seawater δ238U.The diagenetic uptake of U shifts δ238U of bulk carbonate sediments to values heavier than contemporaneous seawater. The magnitude of this offset correlates with the level of authigenic enrichment of redox sensitive elements in our sample set. Samples with high Re and V concentrations and high Re/Mo ratios are the most enriched in 238U. This positive correlation could be used to constrain the amount of diagenetic offset of bulk δ238U values from seawater. This is particularly important in carbonate systems that had a primary calcitic mineralogy. Primary calcite precipitates have low U concentrations and so small amounts of 238U uptake during diagenesis can have a significant impact on the bulk composition.The samples with the most depleted δ238U values have high Mo concentrations and low Re/Mo ratios. As carbonate sediments are generally Mo poor, the high Mo concentrations suggests that the depleted δ238U values might be linked to a manganese cycle that operated across the sediment-water interface during the deposition of this Jurassic carbonate platform.

Early diagenesis and trace element accumulation in North American Arctic margin sediments

Concentrations of redox-sensitive elements (S, Mn, Mo, U, Cd, Re) were analyzed in a set of 27 sediment cores collected along the North American Arctic margin (NAAM) from the North Bering Sea to Davis Strait via the Canadian Archipelago. Sedimentary distributions and accumulation rates of the elements were used to evaluate early diagenesis in sediments along this section and to estimate the importance of this margin as a sink for key elements in the polar and global oceans. Distributions of Mn, total S and reduced inorganic S demonstrated that diagenetic conditions and thus sedimentary carbon turnover in the NAAM is organized regionally: undetectable or very thin layers (<0.5cm) of surface Mn enrichment occurred in the Bering–Chukchi shelves; thin layers (1–5cm) of surface Mn enrichment occurred in Barrow Canyon and Lancaster Sound; and thick layers (5–20cm) of surface Mn enrichment occurred in the Beaufort Shelf, Canadian Archipelago, and Davis Strait. Inventories of authigenic S below the Mn-rich layer decreased about fivefold from Bering–Chukchi shelf and Barrow Canyon to Lancaster Sound and more than ten-fold from Bering–Chukchi shelf to Beaufort Shelf, Canadian Archipelago and Davis Strait. The Mn, total S and reduced inorganic S distributions imply strong organic carbon (OC) flux and metabolism in the Bering–Chukchi shelves, lower aerobic OC metabolism in Barrow Canyon and Lancaster Sound, and deep O2 penetration and much lower OC metabolism in the Beaufort Shelf, Canadian Archipelago, and Davis Strait. Accumulation rates of authigenic S, Mo, Cd, Re, and U displayed marked spatial variability along the NAAM reflecting the range in sedimentary redox conditions. Strong relationships between the accumulation rates and vertical carbon flux, estimated from regional primary production values and water depth at the coring sites, indicate that the primary driver in the regional patterns is the supply of labile carbon to the seabed. Thus, high primary production combined with a shallow water column (average 64m) leads to high rates of authigenic trace element accumulation in sediments from the Bering–Chukchi shelves. High to moderate primary production combined with deep water (average 610m) leads to moderate rates of authigenic trace element accumulation in sediments from Lancaster Sound. Low to very low primary production combined with moderate water depths (average 380m) leads to low rates of authigenic trace element accumulation in sediments in the Beaufort Shelf, Davis Strait and Canadian Archipelago. Authigenic Mo accumulation rates show a significant relationship with vascular plant input to the sediments, implying that terrestrial organic matter contributes significantly to metabolism in Arctic margin sediments. Our results suggest that the broad and shallow shelf of the Chukchi Sea, which has high productivity sustained by imported nutrients, contributes disproportionately to global biogeochemical cycles.

Trace and rare earth element characteristics in Fe-Mn carbonates associated with stratiform Ag-Pb-Zn mineralization from the Lengshuikeng ore district, Jiangxi Province: Implications for their genesis and depositional environment

We performed a systematic trace and rare earth element analysis for the bedded Fe-Mn carbonate rocks related to the stratiform Ag-Pb-Zn mineralization in the Lengshuikeng ore district, Jiangxi Province, South China. Three types of Fe-Mn carbonates are distinguished, namely, the massive, breccia, and vein types. Both carbonate and silicate fractions in the samples are analyzed for their trace and rare earth element concentrations using a step acid-leaching technique. Our results show that the carbonate fractions in the massive type samples have the lowest REE concentrations but pronounced positive Eu and Y anomalies with Eu/Eu* value from 1.3 to 6.2 and Y/Ho value from 40.1 to 59.5, and similar characteristics are also shown for the silicate fractions in the massive type samples (Eu/Eu*=1.0–6.7, Y/Ho=20.7–55.1). These REE characteristics are similar to those of Sedex type massive sulfide deposits worldwide, and we suggest that the massive type Fe-Mn carbonate rocks were likely formed from an exhalative volcanic-hydrothermal fluid feeding the depression basin of a volcanic lake. The high concentrations of redox-sensitive elements and ratios such as U/Th, V/Cr and V/(V+Ni) indicate a dysoxic environment for the Fe-Mn carbonate deposition. In contrast, the breccia type and vein type Fe-Mn carbonate samples show different trace and rare earth element features from those of massive type samples, and they are more similar to the volcanic rocks and magmatic-hydrothermal fluids in the Lengshuikeng ore district and may reflect strong overprinting from volcanic and sub-volcanic magmatism related to the porphyry type mineralization in the district.

Mineralogy and geochemistry of Palaeozoic black shales from Peninsular Malaysia: Implications for their origin and maturation

Palaeozoic black shale-bearing formations comprise 25% of the sedimentary cover in Peninsular Malaysia. However, nothing has been published regarding their geology, composition and maturation. Representative samples from different ages and localities of the black shales were subjected to detailed mineralogical and geochemical investigations during this study to determine their origin. These investigations also provide an opportunity to trace the changes in depositional environments and paleoclimate during the Palaeozoic as well as the nature of unknown and hidden basement rocks in Malaysia.This paper reports nineteen Palaeozoic formations in Malaysia that contain black shales that are classified into seven categories based on their age. These are the Cambrian-Ordovician, Ordovician-Devonian, Silurian-Devonian, Devonian, Carboniferous, Carboniferous-Permian and Permian black shales. Although illite was identified in all black shales, other clay minerals including kaolinite and smectite-illite mixed layers show significant variations among the studied samples. The Al2O3/TiO2 ratios (13–33), SiO2 contents of probable magmatic source rocks (54–69wt.%) and Zr-TiO2 bivariate diagram suggest a mixture of felsic and intermediate source rocks for the studied black shales. This indicates that the hidden basement rocks in Malaysia are dominated by felsic igneous rocks. Variations in clay minerals, SiO2 and Al2O3/TiO2 ratios and upper continental crust (UCC)-normalized REE patterns suggest a local provenance for each locality rather than regional and common source rocks for all black shales. Al2O3/SiO2 ratios vary from one age to the other ranging between 0.09 and 0.37. This indicates changes in the degree of weathering of the sediment-source rocks of these shales and suggests climate changes in the Peninsular Malaysia during the Palaeozoic from relatively wet conditions during the Cambrian to relatively dry conditions during the Ordovician followed by relatively wet conditions extends from the Silurian to the Carboniferous, which followed by relatively dry conditions during the formation of Carboniferous-Permian black shales and finally to relatively wet conditions during the Permian. The V/Ni, V/(V+Ni) and V/(V+Cr) ratios as well as the positive Ce anomalies indicate that these black shales were deposited under reducing conditions. The high V/Ni ratios (>3) also suggest a purely marine source input for the organic matter. Devonian black shales show relatively high concentrations of redox-sensitive elements, which may suggest the possible existence of Devonian anoxia as well as the Devonian-Carboniferous (D-C) boundary in Malaysia. Based on the abundance of illite (>85%) and illite crystallinity (IC) values (27–46°Δ2θ), the majority of the Palaeozoic black shales in Peninsular Malaysia are probably anchimetamorphic and over-matured.

Distribution of iridium and associated geochemistry across the Triassic–Jurassic boundary in sections at Kuhjoch and Kendlbach, Northern Calcareous Alps, Austria

Samples from strata spanning the Triassic–Jurassic boundary at the GSSP at Kuhjoch and at Kendlbachgraben were studied by NAA, XRF and combustion analysis to determine Ir levels and associated geochemistry. The results are compared to previously determined carbon isotope stratigraphy at these sections. Ir concentrations in the limestones of the Kössen Formation at Kuhjoch are very low (<10pg/g) below the top of the formation, rising to 26pg/g, in the T-bed at the top of the Eiberg Member. The Tiefengraben Member of the Kendlbach Formation exhibits higher concentrations of Ir in general relative to the strata below. The shift to higher levels is abrupt at the base of the member, coinciding with a decrease in carbonate content. Concentrations of 60 to 80pg/g are typical through the entire thickness of the Schattwald Beds and into the gray Tiefengraben Member strata, peaking at 145pg/g. Concentrations decline to 30–40pg/g above 680cm from the formation base, coinciding with increasing carbonate content. The analyses from the Kendlbachgraben section compare well with those from Kuhjoch, with similar difference in Ir concentration between the Kössen and Kendlbach formations. In both sections, the initial increase in Ir corresponds to the initial carbon isotope excursion. Concentrations of redox-sensitive elements indicate transient reducing conditions during deposition of the uppermost Kössen Formation, but oxidizing conditions during Tiefengraben Member deposition. The Al/Ti ratio indicates more intense weathering during deposition of the lowermost 20cm of the Tiefengraben Member, but otherwise consistently humid to sub-humid climate prevailed during deposition. The primary control on Ir concentration in the sampled section is formation lithology, although there are variations within the Tiefengraben Member that are independent of carbonate content. Enrichment of Ir at the top of the T-bed is associated with a redox boundary, but the cause of other variations is undetermined.

Radiogenic Nd isotope labeling of the northern NE Atlantic during MIS 2

Paleoceanographic reconstructions rely on chemical proxies which are controlled by physical, chemical, and biological marine parameters. The accurate interpretation of proxy records relies on the integrity of proxy-environmental relationships through time, and under changing conditions. In this study we closely examine paleo controls on authigenic Nd isotope records from five cores in the northern NE Atlantic, approximating a depth-transect, allowing spatial and temporal relationships to be reconstructed. We compare our Nd isotope records with other paleocirculation proxies, and consider the sedimentalogical controls on Nd isotope signals, by comparing ice-rafted detritus lithology and counts, detrital sediment chemistry and redox sensitive element concentrations measured on foraminifera authigenic coatings. With this suite of geochemical and sedimentalogical data we show that Nd isotope records in the northern NE Atlantic were labeled by radiogenic sediments, however this modification did not occur in the pore-waters of each core, but instead likely reflects changes in the Nd isotopic composition of deep-waters caused by the input of ice-rafted sediment during Heinrich events and the last glacial maximum. This study has implications for understanding how localized changes in the Nd isotope signal can set a watermass end-member composition, decoupling chemical proxy-circulation relationships locally, but providing a signal which can be potentially traced along the deep-water flowpath. Such scenarios must be considered in future interpretations of glacial Nd isotope records taken from within the ice-rafted detritus belt and downstream along watermass flowpaths.

Comparison of isotopic and geochemical characteristics of sediments from a gas- and liquids-prone wells in Marcellus Shale from Appalachian Basin, West Virginia

The Middle Devonian age Marcellus Shale contains one of the largest shale gas plays in North America. Hydrocarbon production in the eastern part of the play is mostly “dry gas,” consisting of essentially pure methane. Production of natural gas liquids (condensate) increases toward the west, which is the area currently, being targeted by developers. Two Marcellus Shale cores from West Virginia were analyzed to compare the isotopic and geochemical characteristics of a liquids-prone well (WV-7) in Wetzel County with a gas-prone well (WV-6) in Monongalia County. The contrasts between the cores indicate that the conditions of the Marcellus Shale deposition were different between the two sites. The dominant organic matter preserved in each core is isotopically different; δ13Corg values are lighter on average in WV-6 compared with WV-7. A possible explanation is that a larger fraction of terrestrial organic matter was preserved in the WV-6 core, whereas WV-7 may contain a greater percentage of marine organic matter. Clastic-influx proxies (e.g. Ti/Al, Ca/Al and Mg/Al) also suggest that the WV-6 core site received a higher siliciclastic input compared to WV-7, consistent with a more proximal location to dry land and the delivery of greater amounts of terrestrial organic matter. Depleted δ13Ccarb values, low concentrations of redox sensitive elements (e.g. V, Cr, Ni and U), and high variability δ15N values in the WV-6 core all suggest the presence of higher dissolved oxygen concentration and short term shifts in an oxic/anoxic boundary near the sediment–water interface during deposition. These lines of evidence indicate that the depositional conditions were favorable for the accumulation of predominantly gas-prone Type III kerogen in the Marcellus Shale at the WV-6 site. In contrast, the Marcellus Shale at the WV-7 site was deposited in a more distal area that received a low terrestrial sediment supply, organic matter primarily derived from marine algae, and bottom water conditions that were dominantly anoxic. Such conditions were favorable for the accumulation of Type II kerogen that has a greater capacity to generate liquid hydrocarbons. Differences between the liquids-prone and gas-prone parts of the Marcellus Shale play have been largely ascribed to depth-of-burial and thermal maturation history; this study indicates that depositional environment and sedimentary facies may have played significant roles as well.

An integrated Markov chain Monte Carlo algorithm for upscaling hydrological and geochemical parameters from column to field scale

Predicting and controlling the concentrations of redox-sensitive elements are primary concerns for environmental remediation of contaminated sites. These predictions are complicated by dynamic flow processes as hydrologic variability is a governing control on conservative and reactive chemical concentrations. Subsurface heterogeneity in the form of layers and lenses further complicates the flow dynamics of the system impacting chemical concentrations including redox-sensitive elements. In response to these complexities, this study investigates the role of heterogeneity and hydrologic processes in an effective parameter upscaling scheme from the column to the landfill scale. We used a Markov chain Monte Carlo (MCMC) algorithm to derive upscaling coefficients for hydrological and geochemical parameters, which were tested for variations across heterogeneous systems (layers and lenses) and interaction of flow processes based on the output uncertainty of dominant biogeochemical concentrations at the Norman Landfill site, a closed municipal landfill with prevalent organic and trace metal contamination. The results from MCMC analysis indicated that geochemical upscaling coefficients based on effective concentration ratios incorporating local heterogeneity across layered and lensed systems produced better estimates of redox-sensitive biogeochemistry at the field scale. MCMC analysis also suggested that inclusion of hydrological parameters in the upscaling scheme reduced the output uncertainty of effective mean geochemical concentrations by orders of magnitude at the Norman Landfill site. This was further confirmed by posterior density plots of the scaling coefficients that revealed unimodal characteristics when only geochemical processes were involved, but produced multimodal distributions when hydrological parameters were included. The multimodality again suggests the effect of heterogeneity and lithologic variability on the distribution of redox-sensitive elements at the Norman Landfill site.

Carbon cycling and burial in New Zealand's fjords

Understanding carbon cycling in continental margin settings is critical for constraining the global carbon cycle. Here we apply a multiproxy geochemical approach to evaluate regional carbon cycle dynamics in six New Zealand fjords. Using carbon and nitrogen concentrations and isotopes, lipid biomarkers, and redox-sensitive element concentrations, we show that the New Zealand fjords have carbon-rich surface sediments in basins that promote long-term storage (i.e., semirestricted basins with sediment accumulation rates of up to 4 mm yr−1). Using δ13C distributions to develop a mixing model, we find that organic carbon in fjord sediments is well-mixed from marine and terrestrial sources in down-fjord gradients. This is driven by high regional precipitation rates of >6 m yr−1, which promote carbon accumulation in fjord basins through terrestrial runoff. In addition, we have identified at least two euxinic subbasins, based on uranium, molybdenum, iron, and cadmium enrichment, that contain >7% organic carbon. Because the strength and position of the Southern Hemisphere westerly winds control precipitation and fjord circulation, carbon delivery and storage in the region are intimately linked to westerly wind variability. We estimate that the fjord region (759 km2) may be exporting up to 1.4 × 107 kgC yr−1, outpacing other types of continental margins in rates of carbon burial by up to 3 orders of magnitude.

Trace elements geochemistry of kerogen in Upper Cretaceous sediments, Chad (Bornu) Basin, northeastern Nigeria: Origin and paleo-redox conditions

Trace element contents in isolated kerogen from Upper Cretaceous sediments within Gongila and Fika formations in the Chad (Bornu) Basin, northeastern Nigeria were determined using Inductively-coupled plasma mass spectrometer (ICP-MS), in order to infer the origin of the organic matter and the paleo-redox conditions during their sedimentation. The concentrations of the elements in the kerogen samples varied from 1.01 to 24,740ppm. The distribution of elements shows that Fe is the most abundant element in Chad (Bornu) Basin kerogen, followed by Ce. Among the biophile elements, V is the most abundant, followed by Ni and Co in that order. Statistical evaluation of the elemental composition data shows that As, Ce, Pb, V, Cr, Fe, Co, Ni and U exhibit good positive correlations with each other. Molybdenum, on the other hand displays no obvious correlation with most of the trace elements determined including TOC, but has good positive correlation with TS and negative correlation with Tmax, Ce and Th, which suggests that the concentration of Mo decreases with increasing maturity and vice versa. Some trace element concentrations and their ratios suggest mixed marine and terrigenous source input for the organic matter (kerogen) in Chad (Bornu) Basin. More so, the concentrations of redox-sensitive elements, such as V, Ni, Cu, Cr Mo and Mn, in the kerogen samples suggest dysoxic bottom water conditions within the Gongila and Fika sediments. Cross-plots of V and Ni and V/(V+Ni) ratio also indicate that the organic matter of these samples was deposited in slightly reducing environments.