Bulk Geochemical Data Research Articles

Biomarker records of palaeoenvironmental variations in subtropical Southern Africa since the late Pleistocene: Evidences from a coastal peatland

Southern Africa's unique global position has given rise to a dynamic climate influenced by large sea surface temperature gradients and seasonal fluctuations in the Inter Tropical Convergence Zone. Due to the semi-arid climate of the region, terrestrial palaeorecords are rare and our understanding of the long-term sensitivity of Southern African terrestrial ecosystems to climatic drivers is ambiguous. A 810cm continuous peat core was extracted from the Mfabeni peatland with a 14C basal age of c. 47thousandyears calibrated before present (kcalyr BP), positioning it as one of the oldest known sub-tropical coastal peatlands in Southern Africa. This peat core provides an opportunity to investigate palaeoenvironmental changes in subtropical Southern Africa since the late Pleistocene. Biomarker (n-alkane, n-alkanoic acid and n-alkanol) analysis, in conjunction with previously published bulk geochemical data, was employed to reconstruct organic matter (OM) sources, rates of OM remineralisation and peatland hydrology. Our results showed that the principal OM source into the peatland was emergent and terrestrial plants with exception of shallow lake conditions when submerged macrophytes dominated (c. 44.5–42.6, 29.7, 26.1–23.1, 16.7–7.1 and 2.2kcalyr BP). n-Alkane proxies suggest that local plant assemblages were predominantly influenced by peatland hydrology. By incorporating temperature sensitive n-alkanoic acid and n-alkanol proxies, it was possible to disentangle the local temperature and precipitation changes. We report large variations in precipitation intensities, but subdued temperature fluctuations during the late Pleistocene. The Holocene period was characterised by overall elevated temperatures and precipitation compared to the preceding glacial period, interspersed with a millennial scale cooling event. A close link between the Mfabeni archive and adjacent Indian Ocean marine core records was observed, suggesting the regional ocean surface temperatures to be the dominant climate driver in this region since the late Pleistocene.

A novel method to estimate mineral compositions of mudrocks: A case study for the Canadian unconventional petroleum systems

The mineral composition of mudrocks is an essential attribute in controlling the reservoir quality of unconventional petroleum systems. The present study introduces a semi-quantitative method to estimate mineral phases of mudrocks in various Canadian unconventional hydrocarbon systems using total elemental analysis (inductively coupled plasma-mass spectrometry (ICP-MS)) and Rock-Eval data (total organic carbon (TOC) and mineral carbon (MinC)).This method involves statistical analysis based on a sound knowledge of hydrocarbon source rock inorganic geochemistry. The workflow can be divided into four steps: (i) converting major elements (Si, Al, Fe, K, Na, Ca, Mg, Ti, and P) to their oxides, (ii) inferring modes of occurrence of elements using statistical analysis of geochemical data (major elements, TOC, and MinC), (iii) identifying the mineral types (oxide, aluminosilicates, carbonates, sulfide, and phosphate) according to elemental occurrences and calculating mineral phase concentrations, and (iv) verifying the results by comparing to XRD data on selected samples. The results, especially for brittle minerals such as quartz, carbonates (e.g. calcite, dolomite, and ankerite), and pyrite, show that the estimated mineral compositions correspond closely and consistently with measured mineralogy obtained from XRD. This method takes advantage of bulk geochemical data already available for hydrocarbon potential and chemostratigraphic studies, without devoting additional samples and cost for XRD analysis.

Long-term geochemical evaluation of the coastal Chicot aquifer system, Louisiana, USA

Summary Groundwater is increasingly being overdrafted in the Gulf and Atlantic Coastal regions of the United States. Geochemical data associated with groundwater in these aquifers can provide important information on changes in salinity, recharge, and reaction pathways that can be used to improve water management strategies. Here we evaluated long-term geochemical changes associated with the 23,000 km2 Chicot aquifer system in Louisiana, USA. The Chicot aquifer is currently being overdrafted by about 1,320,000 m3 per day. We compiled selected bulk geochemical data from samples collected from 20 wells in the Chicot aquifer from 1993 to 2015. Oxygen and hydrogen isotope measurements were additionally completed for the 2014 samples. We identified three zones of groundwater with distinctive geochemical character; (1) A groundwater recharge zone in the northern part of the study area with low pH, low salinity, and low temperature relative to other groundwater samples, (2) a groundwater recharge zone in the southeastern part of the study area with low temperature, high alkalinity, and higher Ca and Mg concentrations compared to the other groundwater samples, and (3) groundwater in the southwestern part of the aquifer system with high salinity, high temperature, and a ∼1:1 Na/Cl ratio. The geochemistry of these regions has been relatively stable over the last ∼20 years. However, in the drought year of 2011, the estimated extent of zones with elevated salinity increased substantially. Geochemical evidence suggests that there was increased infiltration of deeper, more salt-rich waters into the shallower Chicot aquifer.

GEOCHEMICAL CHARACTERIZATION OF THE PERMIAN–TRIASSIC TRANSITION AT OUTCROP, CENTRAL SAUDI ARABIA

This study presents the results of chemostratigraphic analyses and spectral gamma‐ray logging integrated with sedimentological data across the Permian‐Triassic boundary at a measured outcrop section in central Saudi Arabia. The studied section encompasses the uppermost part of the Midhnab Member and the Lower and Upper Khartam Members of the Khuff Formation. Lithofacies were interpreted to have been deposited in subtidal, tidal to supratidal, lacustrine and meandering fluvial / flood plain, marginal marine and lagoonal depositional environments. Integration of bulk geochemical and carbon isotope (δ13C) data allowed the identification of a stratigraphic interval with a negative shift in δ13C ratio values, which was interpreted to correspond to the end‐Permian mass extinction event. The end of this “first negative shift in δ13C values” is taken to mark the Permian‐Triassic boundary. Above this boundary and just below an interval containing scattered thrombolites, a second negative shift in δ13C ratios was observed, and corresponds to an interval with long‐term uranium depletion as indicated by the bulk sediment geochemical and spectral gamma‐ray uranium data.The Permian‐Triassic boundary (PTrB) was placed at the transition between marginal‐marine and subtidal deposits. This stratigraphic position corresponds to the end of the “first negative δ13C shift” and the point of greatest uranium depletion. Although previous studies on outcrops of the Khuff Formation in Saudi Arabia identified a major sequence boundary between the Lower and Upper Khartam Members and interpreted it as the P TrB, no evidence is presented in this study for exposure and dissolution at this surface. Accordingly, the Permian‐Triassic transition is placed in the transgressive portion of the Upper Khartam Member, while the sequence boundary below is interpreted to correspond to the end‐Permian extinction.Correlation of Khuff time‐equivalent units in the Arabian Plate is challenging, and this study will contribute to an improved understanding of this important stratigraphic unit, which contains prolific non‐associated gas reservoirs. The identification of the Permian‐Triassic boundary in central Saudi Arabia will help in the construction of a sequence‐stratigraphic scheme for the Khuff, and with the correlation of lithofacies within this heterogeneous reservoir unit.

Formation and Evolution of a Paleosol across the Lower Silurian‐Lower Permian Boundary in Zunyi District, Northern Guizhou, China and Its Paleoenvironment and Paleoclimate Implications

AbstractThis paper presents a set of bulk geochemical and mineralogical data from a paleoweathering profile located in Zunyi District, Northern Guizhou, China. It was formed at the top of the Hanjiadian Formation of the Lower Silurian. A truncated, argillic, gleyed, kryptic paleospodosol is recognized in the paleoweathering profile. Ratios of immobile elements (Ti/Zr, Ti/Al) and their binary (e.g., Nb vs. Zr/TiO2 and Th/Sc vs. Zr/Sc), triangular diagrams (La‐Th‐Sc, Th‐Sc‐Zr/10, Zr‐Cr‐Ga) reflect that the Gaojiayan paleosol is the product of in‐situ weathering of gray‐green silty mudstone of the underlying Hanjiadian Formation. Mass balance calculations indicate K enrichment and Na enrichment in the upper and lower portions of paleosol, respectively. These findings both are the results of transgression, which brings substantial concentrations of such elements as K, Na, and Sr. In particular, K enrichment is achieved by the illitization of kaolinite. The biological processes of terrestrial vascular plants also enhance K concentration, especially at the top of the paleosol. Na enrichment is a consequence of albitization and/or adsorption by clay minerals through cation exchange. The mass distributions and relative mass changes of rare earth elements (REEs) in the studied profile display characteristics of vertical zonation. Three peaks in total REEs content are observed, indicating two paleoclimatic or paleoenvironmental changes. Mineralogical characteristics indicate that the paleoclimate changed first from warm and humid to cold and dry and later, to dry and lightly warmer. The corresponding soil environment varies from weakly acidic to strongly alkaline and later, to weakly acidic. Mass translocation characteristics of REEs and several transition metals suggest that the Gaojiayan paleosol may have undergone top erosion.

A continental-weathering control on orbitally driven redox-nutrient cycling during Cretaceous Oceanic Anoxic Event 2

The Cretaceous period (~145–65 m.y. ago) was characterized by intervals of enhanced organic carbon burial associated with increased primary production under greenhouse conditions. The global consequences of these perturbations, oceanic anoxic events (OAEs), lasted up to 1 m.y., but short-term nutrient and climatic controls on widespread anoxia are poorly understood. Here, we present a high-resolution reconstruction of oceanic redox and nutrient cycling as recorded in subtropical shelf sediments from Tarfaya, Morocco, spanning the initiation of OAE2. Iron-sulfur systematics and biomarker evidence demonstrate previously undescribed redox cyclicity on orbital time scales, from sulfidic to anoxic ferruginous (Fe-rich) water-column conditions. Bulk geochemical data and sulfur isotope modeling suggest that ferruginous conditions were not a consequence of nutrient or sulfate limitation, despite overall low sulfate concentrations in the proto–North Atlantic. Instead, fluctuations in the weathering influxes of sulfur and reactive iron, linked to a dynamic hydrological cycle, likely drove the redox cyclicity. Despite the potential for elevated phosphorus burial in association with Fe oxides under ferruginous conditions on the Tarfaya shelf, porewater sulfide generation drove extensive phosphorus recycling back to the water column, thus maintaining widespread open-ocean anoxia.

Organic Geochemistry and Paleoenvironments of Deposition of the Middle Jurassic Sediments from the Tabas Basin, Central Iran

In an attempt to reconstruct the paleoenvironments of deposition for the Middle Jurassic Baghamshah formation, samples collected from six outcrop sections along the Shotori swell were subjected to detailed geochemical analyses. Bulk geochemical and biological marker data indicate a logical trend of the variation of organic input, salinity, and oxicity within Baghamshah paleoenvironments across the studied area. An increase in terrestrial character from southern end towards the central parts of the Shotori swell parallels with a uniform increase in the oxicity and a decrease in the salinity. The northernmost sections are characterized by less terrestrial impact, reduced oxicity, and elevated salinity compared to the central and southern sections. These variations are interpreted in the framework of past geometric configuration and a hypothetical paleogeomorphologic model is tentatively proposed for the Middle Jurassic of the area. According to these results, the depositional setting of the studied formation decreased in depth from Section-1 towards Section-4, suggesting that the proximity of the latter section to the Yazd Block may have had a strong control over the observed geochemical variations. The terrestrial organic input and the oxicity of the environment are conspicuously low for northern sections and their salinity shows a sharp increase compared to other sections. We hypothesize that a fault plan exists across the northern and southern Shotori Mountains that had played an active role in creating the current geochemical variations.

Early Permian mantle–crust interaction in the south-central Altaids: High-temperature metamorphism, crustal partial melting, and mantle-derived magmatism

Early Permian mafic magmatism, the partial melting of the crust, and high-temperature metamorphism in the Chinese Altai, south-central Altaids, provide an excellent case study of exchange of energy and mass between the mantle and crust. Field and petrographic observations, together with microprobe mineral analyses, have, for the first time, allowed us to identify scapolite-bearing calc-silicate granulites along the southern margin of the Chinese Altai. The conditions of metamorphism were 680–800°C and 6–7 Kbar, based on mineral phase relationship and compositions, as well as the results of previous studies. New LA-ICP-MS zircon U-Pb geochronology demonstrates that the high-T metamorphism in the Chinese Altai was accompanied by the emplacement of leucogranites at approximately 295Ma. SHRIMP II dating results, combined with previously published data, reveal a mafic magmatic event at around 275Ma in a large region of northern Xinjiang, NW China. This mafic magma was derived from N-MORB-like depleted mantle, as deduced from our new bulk geochemical and zircon Hf isotopic data. Synthesizing available geochemical (including isotopic) and geochronological data, we propose a two-stage model of mantle–crust interaction to explain the early Permian geology of northern Xinjiang. The early stage of interaction involved high-temperature metamorphism and the coeval partial melting of crustal rocks, indicating solely a heat exchange between the mantle and crust; in contrast, the later stage involved the exchange of both mass and energy between the mantle and crust.

A 550 ka record of aeolian activity near North West Cape, Australia: inferences from grain-size distributions and bulk chemistry of SE Indian Ocean deep-sea sediments

The terrigenous fraction of sediments from a deep-sea sediment core recovered from the northwestern Western Australian continental slope offshore North West Cape, SE Indian Ocean, reveals a history of Western Australian climate throughout the last 550 ka. End-member modelling of a data set of grain-size distributions (n = 438) of the terrigenous sediment fraction allows to interpret the record in terms of aeolian and fluvial sediment deposition, both related to palaeo-environmental conditions in the North West Cape area. The data set can be best described by two aeolian end members and one fluvial one. Changes in the ratio of the two aeolian end members over the fluvial one are interpreted as aridity variations in northwestern Western Australia. These grain-size data are compared with bulk geochemical data obtained by XRF scans of the core. Log-ratios of the elements Zr/Fe and Ti/Ca, which indicate a terrigenous origin, corroborate the grain-size data. We postulate that the mid- to late Quaternary near North West Cape climate was relatively arid during the glacial and relatively humid during the interglacial stages, owing to meridional shifts in the atmospheric circulation system. Opposite to published palaeo-environmental records from the same latitude (20°S) offshore Chile and offshore Namibia, the Australian aridity record does not show the typical southern hemisphere climate variability of humid glacials and dry interglacials, which we interpret to be the result of the relatively large land mass of the Australian continent, which emphasises a strong monsoonal climatic system.

Mineralogical and Geochemical Constraints on the Sediment Sources of Late Stone Age Pottery from the Birimi Site, Northern Ghana

The mineralogy and bulk chemical compositions of 15 Kintampo (Late Stone Age) potsherds from the Birimi site on the Gambaga Escarpment and eight samples of local sediment were determined with the intent of characterizing these wares and identifying the material used in their manufacture. Sediment from clay pits still used by potters north of the escarpment contains iron‐rich laterite clasts (100 × XFeOt = 100 × FeOt/[FeOt + Al2O3 + SiO2] ≥10). Sedimentary clasts in stream sediments are relatively siliceous and iron‐poor (100 × XFeOt < 10). Bulk geochemical data together with the compositions of lithic clasts (laterite, siltstone/sandstone) link the pottery to sediment sources, including escarpment sediments not presently used by Ghanaian potters. Fresh granite clasts found in some of the sherds were not found in the analyzed sediment samples, although some of their distinctive mineralogical constituents (e.g., variably barian alkali feldspar) are present. The analytical data suggest that pots found at Birimi were made locally by mixing escarpment sediment with clay and stream sediment brought in from below the escarpment. This contrasts with present‐day practice, whereby the pots themselves are imported. The place where Birimi pottery was made and the outcrop source of aluminous sediment (mudstone with an “escarpment” trace element signature) used in these wares, however, remain unidentified. © 2013 Wiley Periodicals, Inc.

Thermal maturity of northern Appalachian Basin Devonian shales: Insights from sterane and terpane biomarkers

To better estimate thermal maturity of Devonian shales in the northern Appalachian Basin, eleven samples of Marcellus and Huron Shale were characterized via multiple analytical techniques. Vitrinite reflectance, Rock–Eval pyrolysis, gas chromatography (GC) of whole rock extracts, and GC–mass spectrometry (GCMS) of extract saturate fractions were evaluated on three transects that lie across previously documented regional thermal maturity isolines. Results from vitrinite reflectance suggest that most samples are immature with respect to hydrocarbon generation. However, bulk geochemical data and sterane and terpane biomarker ratios from GCMS suggest that almost all samples are in the oil window. This observation is consistent with the presence of thermogenic gas in the study area and higher vitrinite reflectance values recorded from overlying Pennsylvanian coals. These results suggest that vitrinite reflectance is a poor predictor of thermal maturity in early mature areas of Devonian shale, perhaps because reported measurements often include determinations of solid bitumen reflectance. Vitrinite reflectance interpretations in areas of early mature Devonian shale should be supplanted by evaluation of thermal maturity information from biomarker ratios and bulk geochemical data.

Organic matter accumulation in the oil shale- and coal-bearing Huadian Basin (Eocene; NE China)

The Huadian Basin, located in northeastern China, is a fault-controlled basin filled by Eocene non-marine sediments assigned to the Huadian Formation. The formation is subdivided from bottom to top into three members: Pyrite Member, Oil Shale Member, and Carbonaceous Shale Member. Using bulk geochemical data from borehole Hd3 and core descriptions from a high number of wells, the factors controlling organic matter accumulation have been evaluated.The organic matter content of the Pyrite Member is relatively low (average TOC: 0.9wt.%). Mudstones deposited in shallow lacustrine environments contain between 1 and 2wt.% TOC (max. 6wt.%) and a kerogen Type II (HI 400–600mg HC/g TOC). An arid climate prevented the accumulation of thick coal.The overlying Oil Shale Member contains 13 oil shale layers, which are mined underground. The Oil Shale Member has been interpreted as a 3rd order sequence. Significant differences exist between oil shales deposited in different systems tracts: 2-m-thick coaly oil shale of low-quality (oil yield 3.5wt.%) developed in the lowstand systems tract. Oil shale, up to 7m thick, of moderate quality (oil yield 4.1–8.0wt.%) was deposited during the transgressive systems tract (TST). Thinner (<3m) high quality oil shale (oil yield 6.4–19.8wt.%) is found in the highstand systems tract (HST) and at the base of the regressive systems tract. The lateral extent of the oil shale layers reaches a maximum in the upper TST and the lower HST. The high number of oil shale layers deposited in an overall deep lacustrine environment notifies frequent changes in base level, controlled by tectonic movements, short-term climatic cycles, or cycles in organic matter production.The Carbonaceous Shale Member representing the balanced-fill to overfilled stage of basin evolution hosts economic coal seams, which are occasionally accompanied by shallow lacustrine shales with abundant hydrogen-rich organic matter. Otherwise non-coal layers are typically low in organic matter (average TOC: 0.9wt.%).In contrast to the Huadian Basin, a very thick coal seam overlain by a single oil shale layer more than 100m thick developed in the Eocene Fushun Basin. This indicates major differences in the evolution of basins with similar tectonic setting and age.

STRATIGRAPHY AND PROVENANCE OF THE AURIFEROUS-URANIFEROUS, FLUVIAL TO SHALLOW-MARINE SINQENI FORMATION, MOZAAN GROUP, NORTHERN KWAZULU-NATAL, SOUTH AFRICA

Research Article| September 01, 2012 STRATIGRAPHY AND PROVENANCE OF THE AURIFEROUS-URANIFEROUS, FLUVIAL TO SHALLOW-MARINE SINQENI FORMATION, MOZAAN GROUP, NORTHERN KWAZULU-NATAL, SOUTH AFRICA N. HICKS; N. HICKS School of Geological Sciences, University of KwaZulu-Natal, Private Bag X 54001 Durban 4000, South Africa, Present address: Council for Geoscience. PO Box 900, Pietermaritzburg, 3200, South Africa. e-mail: nhicks@geoscience.org.za Search for other works by this author on: GSW Google Scholar A. HOFMANN A. HOFMANN School of Geological Sciences, University of KwaZulu-Natal, Private Bag X 54001 Durban 4000, South Africa, Present Address: Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: ahofmann@uj.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information N. HICKS School of Geological Sciences, University of KwaZulu-Natal, Private Bag X 54001 Durban 4000, South Africa, Present address: Council for Geoscience. PO Box 900, Pietermaritzburg, 3200, South Africa. e-mail: nhicks@geoscience.org.za A. HOFMANN School of Geological Sciences, University of KwaZulu-Natal, Private Bag X 54001 Durban 4000, South Africa, Present Address: Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa, e-mail: ahofmann@uj.ac.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2012 Geological Society of South Africa South African Journal of Geology (2012) 115 (3): 327–344. https://doi.org/10.2113/gssajg.115.3.327 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation N. HICKS, A. HOFMANN; STRATIGRAPHY AND PROVENANCE OF THE AURIFEROUS-URANIFEROUS, FLUVIAL TO SHALLOW-MARINE SINQENI FORMATION, MOZAAN GROUP, NORTHERN KWAZULU-NATAL, SOUTH AFRICA. South African Journal of Geology 2012;; 115 (3): 327–344. doi: https://doi.org/10.2113/gssajg.115.3.327 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The Sinqeni Formation of the ~2.98 to 2.87 Ga Pongola Supergroup within the White Umfolozi Inlier in northern KwaZulu-Natal is a fluvial to shallow-marine sequence that has been affected by sea-level changes. Braided alluvial plain conglomerates of the basal Denny Dalton Member are overlain by shallow marine, trough cross-bedded, quartz arenites. A major marine transgression in the Pongola Basin is marked by a conglomerate lag representing a transgressive ravinement surface, which is overlain by banded iron formation, signifying sub-storm wave base deposition on a clastic sediment-starved shelf.Conglomerates of the Sinqeni Formation are locally gold- and uranium-bearing with the ~1 m thick, laterally discontinuous “Mozaan Contact Reef” (MCR) hosting erratic mineralization. Where mineralized, the MCR contains rounded, detrital pyrite grains, derived primarily from erosion of sedimentary pyrites. Three generations of pyrite are recognised, with primary gold hosted as inclusions within compact, rounded detrital pyrite and gersdorffite grains. No uraninite grains are observed in the MCR, with allogenic uraniferous leucoxene and authigenic brannerite forming the major uraniferous component. The uppermost conglomerate of the Sinqeni Formation (CG 4) crops out ~30 m above the Denny Dalton Member conglomerates and hosts uranium precipitated as secondary uraninite and coffinite encrustations around detrital pyrite grains. The conglomerates form a placer deposit which has been subjected to post-depositional alteration. Heavy mineral assemblages as well as bulk geochemical data for conglomerates and quartz arenites suggest a granitoid-greenstone source. Palaeocurrent and mineralogical data suggest a proximal granitoid-greenstone provenance to the west of the White Umfolozi Inlier, whereas equivalent units in the main Pongola Basin have been derived from granitoid-greenstone successions to the north. The reef mineralogy and pyrite geochemistry however, suggest that detritus in the MCR was derived from a variety of sources including granitic basement, Palaeoarchaean greenstones and sulphidic sedimentary rocks of unknown affinity. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Influence of maceral composition on geochemical characteristics of immature shale kerogen: Insight from density fraction analysis

Abstract Variations in the relative proportions of individual macerals in shales can significantly influence the geochemical characteristics of bulk organic matter. Density fractions of kerogen from the thermally immature New Albany Shale (Devonian and Mississippian) with contrasting maceral compositions exhibit strong geochemical differences. The parental shale is characterized by a vitrinite reflectance (Ro) of 0.45%, a total organic carbon content of 13 wt.%, and a sulfur content of 6.2 wt.%. Organic matter is dominated by amorphinite and alginite, with vitrinite and inertinite accounting only for 1% by volume. Alginite-dominated density fractions (density ca. 1.0–1.15 g/cm3) contain significantly more aliphatic hydrogen, a stronger carboxyl/carbonyl contribution, and reduced Fourier transform infrared spectroscopy absorbance in the 1000 to 1100 cm− 1 region assigned to ether bonds (C O C), as compared to the amorphinite-dominated density fraction (density ca. 1.2–1.6 g/cm3). Aromaticity generally increases from alginite-dominated to amorphinite-dominated fractions. Density fractions dominated by amorphinite are more deuterium-depleted (δDn values of nonexchangeable hydrogen up to − 105‰) than alginite-rich density fraction (δDn values reach − 90‰). In contrast, changes in relative proportions of alginite and amorphinite in the New Albany Shale do not significantly affect the amount of isotopically exchangeable hydrogen in total hydrogen (i.e., hydrogen exchangeability). Alginite is relatively 13C-enriched, whereas density fractions having a high content of amorphinite are relatively 13C-depleted. Our results suggest that even small bulk geochemical and isotopic differences can gain relevance after deconvolution from maceral-related variability. The masking influence of maceral abundance patterns must be considered when interpreting bulk geochemical data as paleoenvironmental proxies. The findings of this study on shale and Type II kerogen are relevant for all types of kerogens in sediments and rocks.

Tracing soil organic carbon in the lower Amazon River and its tributaries using GDGT distributions and bulk organic matter properties

In order to trace the transport of soil organic carbon (OC) in the lower Amazon basin, we investigated the distributions of crenarchaeol and branched glycerol dialkyl glycerol tetraethers (GDGTs) by analyzing riverbed sediments and river suspended particulate matter (SPM) collected in the Solimões-Amazon River mainstem and its tributaries. The Branched and Isoprenoid Tetraether (BIT) index, a proxy for river-transported soil OC into the ocean, was determined from the distributions of these GDGTs. The GDGT-derived parameters were compared with other bulk geochemical data (i.e. C:N ratio and stable carbon isotopic composition). The GDGT-derived and bulk geochemical data indicate that riverine SPM and riverbed sediments in the lower Amazon River and its tributaries are a mixture of C3 plant-derived soil OC and aquatic-derived OC. The branched GDGTs in the SPM and riverbed sediments did not predominantly originate from the high Andes soils (>2500m in altitude) as was suggested previously. However, further constraint on the soil source area of branched GDGTs was hampered due to the deficiency of soil data from the lower montane forest areas in the Andes. Our study also revealed seasonal and interannual variation in GDGT composition as well as soil OC discharge, which was closely related to the hydrological cycle. By way of a simple binary mixing model using the flux-weighted BIT values at Óbidos, the last gauging station in the Amazon River, we estimated that 70–80% of the POC pool in the river was derived of soil OC. However, care should be taken to use the BIT index since it showed a non-conservative behaviour along the river continuum due to the aquatic production of crenarchaeol. Further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil-derived GDGT signals are transformed in large tropical river systems through their transport pathway to the ocean.

Bulk Geochemical Data of Fossil Wood from the Middle Jurassic Clays of Poland

Abstract Macroscopic observations, microscopic studies and literature data revealed that Middle Jurassic wood from Częstochowa area has a different state of preservation, and various types and degrees of mineralization and oxidation. Obtained results of organic matter fractionation illustrate a clear domination of polar fraction in the obtained extracts revealing low thermal maturity stage. Total organic carbon (TOC) values for analysed samples are in a wide range from 1.06% to 68.50%. The highest amount of TOC were measured in not or poorly mineralized wood samples but most of them are mineralized wood fragments, showing the TOC values in the range of 2% - 10%. Percentage content of carbonate in fossil wood constitute in a wide range from less than 1% CaCO3 to above 85% CaCO3. The resulting percentage of the total sulfur content is very varied and do not show convergence with other data such as TOC, carbonate content, etc and is most probable connected with pyritisation range. Unlike the Middle Jurassic clay samples, where long-chain and short-chain n-alkanes occur in similar concentrations, in wood samples always short-chain n-alkanes dominated, in the range from 15 to 23 carbon atoms in molecule. The values of the CPI are generally higher than 1 which indicates the contribution of organic matter derived from higher plant waxes, which are characteristic of e.g. needles from gymnosperm plants. Under the influence of post - diagenetic oxidation in mineralized wood samples distribution of n-alkanes is changing. Diaster-13(17)-enes with 28 and 29 carbon atoms in molecule are present in the wood samples, while those with 29 atoms strongly prevail.

MIL 03443, a dunite from asteroid 4 Vesta: Evidence for its classification and cumulate origin

Abstract– The absence of dunite (&gt;90 vol% olivine) in the howardite, eucrite, and diogenite (HED) meteorite suite, when viewed with respect to spectroscopic and petrologic evidence for olivine on Vesta, is problematic. Herein, we present petrologic, geochemical, and isotopic evidence confirming that Miller Range (MIL) 03443, containing 91 vol% olivine, should be classified with the HED clan rather than with mesosiderites. Similarities in olivine and pyroxene FeO/MnO ratios, mineral compositions, and unusual mineral inclusions between MIL 03443 and the diogenites support their formation on a common parent body. This hypothesis is bolstered by oxygen isotopic and bulk geochemical data. Beyond evidence for its reclassification, we present observations and interpretations that MIL 03443 is probably a crustal cumulate rock like the diogenites, rather than a sample of the Vestan mantle.

Geochemistry and petrogenesis of mafic magmatic rocks of the Jharol Belt, India: geodynamic implication

The belt consisting of deep-water meta-sediments of the Aravalli Supergroup hosts numerous mafic and ultramafic rocks occurring at two different structural levels. These are the basal Bagdunda volcanics and the Gopir magmatic rocks represented by Gopir dikes and flows associated with the ultramafic rocks along the Kaliguman lineament. The Bagdunda and Gopir mafic volcanic rocks are sub-alkaline, varying in composition from high Mg-tholeiite to basaltic komatiite. They are predominantly LREE depleted, but some flat REE patterns are also observed. Bulk geochemical data, especially the incompatible trace elements discount the possibility of crustal contamination. Gopir dikes are highly enriched in terms of their REE [(Ce/Sm) N and (Ce/Yb) N ] and other incompatible trace elements and closely resemble the basal Aravalli volcanics. Their ratio (rock)/ratio (PM) and normalized multi-element abundance patterns depict their overall enriched nature compared to N-MORB and primitive mantle. The Gopir and Bagdunda volcanics reflect trace element characteristics transitional between E-MORB and OIB. Gopir dikes show remarkable similarity with continental tholeiite and Continental Flood Basalts (CFBs) with negative Nb, P and Ti anomalies and low Nb/Ce ratio unlike the volcanics of both the suites. Petrogenetic modeling based on the compositionally corrected [Mg] and [Fe] abundances for Gopir dikes, Gopir and Bagdunda volcanics indicates (a) their derivation from non-pyrolitic sources (b) their derivation from sources that were variably enriched in [Fe/Mg] ratios with large variation in their [Fe] contents and (c) olivine was a major phase to fractionate, followed by a lesser amount of clinopyroxene in the case of Gopir volcanics, whereas in Bagdunda volcanics a combination of olivine and clinopyroxene fractionation is suggested. Based on the geochemical data, supported by field evidence, we propose a geodynamic model for the development of the Jharol Belt in which we suggest that the basement rock, i.e. the Banded Gneissic Complex (BGC) started rifting probably under influence of a mantle plume during the late Archaean–early Proterozoic period. During the opening of the basin, magma derived from asthenospheric mantle reached the surface contemporaneously with sedimentation. The first phase of volcanism is represented by Bagdunda volcanics. With continuous rifting, the crust became highly attenuated and facilitated asthenospheric upwelling, causing high degrees of melting (indicated by a large volume of mafic and ultramafic rocks) during the second phase of magmatism (Gopir volcanics) that occurred at later stages of Jharol sedimentation. Coeval melting of the sub-continental lithosphere under the adjoining BGC craton probably caused the emplacement of dikes along with the Gopir volcanics. At this stage of progressive rifting, an oceanic crust very similar to that in marginal basins developed in the Jharol Belt.

Organic matter in a low productivity anoxic intraplatform basin in the Triassic Tethys

This paper presents isotopic, bulk geochemical and biomarker data measured on organic matter accumulated in a narrow extensional basin developed at the oceanward margin of the huge Triassic carbonate platform in the Alps–Appenines domain. The integration of isotope signatures, organic petrographical and biomarker evidence together with the composition of kerogen pyrolysates suggests immature organic matter predominantly of algal origin with a minor, but not negligible, higher plant derived and moderate bacterial contribution for the entire sequence. The mineral sources are dominated by platform-derived subtidal Dachstein limestone with a minor palaeosol input and a moderate contribution of autochtonous quartz. Nevertheless, parallel variations observed in the mineral content, as well as in the amount and the quality of the organic matter reflect variations in the palaeoenvironment. The increased humidity, existing in the period of the accumulation of the upper section of the sequence, led to the restriction of dolomitization. A slightly greater higher plant derived contribution, in this section, is evidenced by the composition of bitumen and the results on GC and GC/MS analyses on the non-aromatic hydrocarbon fraction of bitumen. Moreover, the climate-induced weathering enchanced the primary productivity and resulted in a pronounced increase in the TOC content. The average estimated value of the planktonic productivity is about three times higher for the calcite-rich sequence than the dolomite-rich one, being 44.2 and 15.3 tC org/m 2/Ma, respectively. The low to moderate planktonic productivity shows that anoxic conditions, observed for the entire succession, are a consequence of the stagnant water stratification rather than high planktonic productivity. Depth trends in the data measured on kerogens (HI, OI, δ 13Corg values, composition of pyrolysate) together with the δ 18O excursions and Δ δ 13C values appear to be controlled by sea-level fluctuations. Consistent with the high abundance of alkyltiophenes in kerogen pyrolysates, the high S org/C ratios (ranging between 0.05 and 0.10) suggest the importance of natural sulfurization in the formation of the sulfur-rich type-II-S kerogen occurring in all of the samples.

Variation in marine total organic carbon through the type Kimmeridge Clay Formation (Late Jurassic), Dorset, UK

Bulk geochemical data from the NERC Rapid Global Geological Events project are used to appraise the pattern of variation in mean organic content through the cymodoce to rotunda zones of the type Kimmeridge Clay Formation in southern Dorset. There is a symmetrical stratigraphic pattern in mean and modal total organic carbon (TOC) values, which increase from 1–2% at the top and base to a peak of 8–9% in the middle of the formation. To remove distortion caused by the strong positive skewness of the TOC values, the mean TOC per zone was recalculated for the first mode of the TOC distribution only, and combined pragmatically with pyrolysis estimates of the reactive (marine) TOC fraction to derive marine TOC values; these also vary symmetrically from c . 0.5% to peak values of c . 5.0%. The mean TOC is negatively correlated with sedimentation rate, indicating that dilution is a significant controlling variable. Mean palaeoproductivity was assessed using modern marine sediment relationships between carbon burial efficiency, sedimentation rate (derived from RGGE cyclostratigraphic analyses), the likely dissolved oxygen range, and water depth. Best estimates of palaeoproductivity range from 40 to 150 g C m −2 a −1 (low to moderate by comparison with present-day shelves) and are positively but nonlinearly correlated with mean δ 13 C TOC values.