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The isotopic composition of Cd buried in marine sediments may preserve valuable palaeoenvironmental information on past ocean redox conditions or biological cycling. It is unclear, however, how the Cd-isotope composition of the sedimentary record reflects these processes. In this study, new Cd-isotope data are presented, along with δ13C, and Cd, Mo and TOC concentrations, from organic-rich mudrocks from the southern Western Interior Seaway (WIS), spanning the Cenomanian–Turonian stages within the Eagle Ford Group of the Maverick Basin, Texas, USA. Relationships between [Cd/TOC], δ114Cd, and MoEF indicate that sedimentary Cd was derived from organic matter with additional contributions from CdS formed in euxinic water masses. Local redox conditions exerted a primary control on the δ114Cd composition of these deposits, with high δ114Cd values attributed to near-quantitative removal from seawater in euxinic environments. Lower δ114Cd values in non-euxinic environments may reflect isotopically light Cd associated with organic material due to partial remineralization. These observations imply that δ114Cd values of samples deposited in demonstrably euxinic conditions may be used to constrain the δ114Cdseawater coming into the Maverick Basin at this time and give a composition of 0.28 ± 0.11‰ (2 SD) for the Early Cenomanian. Samples from an interval of peak-organic carbon burial globally, namely Oceanic Anoxic Event 2 (OAE 2), show anomalous δ114Cd and [Cd] patterns compared to the rest of the data, implying a perturbation to the dissolved Cd pool. The data presented in this study demonstrate that sedimentary Cd isotopes preserve valuable information on the extent of Cd burial into sulfide-bearing sediments at both local and global scales.

We present the first multiple sulfur isotope study (32S, 33S, 34S, 36S) of bulk kerogen sulfur (KS) and disulfides (‘chromium-reducible sulfur’, CRS) from the oil shale of the Umm Rijam Chert-Limestone and Muwaqqar Chalk Marl Formation, Jordan (Late Cretaceous to Early Eocene, appr. 50–70 Ma). Analysis of the sulfur isotopic composition of KS (δ34SKS) shows values ranging from 0.3 to 17.9‰, which are 34S-enriched compared to the δ34SCRS ranging from −23.5 to −3.7‰. Values for CRS and KS are significantly 34S-depleted compared to seawater sulfate sulfur which suggests a major input of early-diagenetic, microbially-generated sulfide. A minor contribution of assimilated seawater sulfate to KS is assumed. The 34S-enrichment of KS compared to CRS can be partly explained by (1) sulfide oxidation to intermediate sulfur species prior to its incorporation into organic material, by (2) fractionations during organic sulfur generation, as well as by (3) a post-depositional timing of formation. Additionally, we hypothesize, based on parallel depth trends of maturity parameters (e.g., vitrinite reflectance) and δ34SCRS and δ34SKS values, that the sulfur isotopic compositions were influenced by thermal maturation (catagenesis). We suggest that the CRS pool comprises a contribution of sulfide released during the thermal decomposition of KS. Overall, our study highlights the importance of organic sulfur in sulfur isotopic studies and the potential of multiple sulfur isotope analyses in maturated sedimentary successions.

The concentrations and isotopic compositions of molybdenum (Mo), zinc (Zn) and cadmium (Cd) in organic-rich marine mudrocks may be used to characterize ocean chemistry in the geological past. These approaches rely on the rarely tested assumption that the geochemical signatures of these metals are not affected by the thermal maturation of the organic matter with which they are associated. We have conducted a series of artificial maturation experiments on two well-known immature organic-rich mudrocks, the Kimmeridge Blackstone Band (Late Jurassic age), and the Posidonia Shale (Early Jurassic age). These pyrolysis experiments allow us to trace changes in the composition of organic matter through varying stages of maturation, and the concentration and isotopic compositions of metals in rock residues and evolved organic fluids. Our results indicate that the thermal maturation of organic matter does not result in significant alteration of the isotopic compositions of Mo, Zn and Cd in the rock residues, which thus retain primary palaeodepositional information. Systematic increases in the concentrations of Mo, Zn and Cd in rock residues with progressively higher thermal maturity are attributed to the loss of substrate mass in the form of fluids released during pyrolysis-induced cracking of kerogen, and to the relatively low concentrations of Mo, Zn and Cd in these fluids. The Mo-isotope compositions of fluids produced during pyrolysis are isotopically similar to the bulk rock; in contrast the isotopic composition of Zn in organic fluids is ∼0.4–0.6‰ lighter than the bulk rock. The progressive loss of organic matter from rock residues during maturation coupled with the increases in metal concentrations leads to an increase of metal/TOC ratios, which may be up to double their original (syn-depositional) value in thermally mature rocks. This observation must be taken into account when using metal/TOC ratios as proxies for oceanic metal inventories throughout geological time. Finally, calculations using the mass of asphaltenes recovered during the pyrolysis experiments suggest that organically bound Mo, Zn and Cd account for several percent of the total rock metal inventory.

Widespread deposition of organic-rich shales during the Late Cretaceous Oceanic Anoxic Event 2 (OAE 2, ca. 94 Ma) occurred during a period of significant global paleo-environmental and geochemical change. It has been proposed that an increase in nutrient input to the ocean during OAE 2 was the key mechanism that generated and sustained high rates of organic-matter burial over time scales of 103–105 yr. Zinc is a bio-essential micronutrient and the proportion of Zn burial in oxic sediments relative to burial in organic-rich continental margin sediments is reflected in its seawater isotope composition. The first Zn-isotope records dating from the Cretaceous are presented here from three coeval carbonate successions: two from Europe (southern England and southern Italy) and one from southern Mexico. The new data show reproducible stratigraphic Zn-isotope patterns in spatially and lithologically diverse carbonate successions. Excursions to lower Zn-isotope values may be linked to the input of magmatic Zn, changes in the proportion of Zn burial into organic-rich sediments, and the liberation of previously buried Zn during an episode of widespread seafloor re-oxygenation during OAE 2 (the Plenus Cold Event).

Sediment petrographic studies combined with carbon and oxygen stable-isotope analyses are considered to be powerful tools in deciphering the depositional and diagenetic history of carbonate rocks. These studies have been found valuable for understanding the environmental controls and global changes of climate and oceanography. Two cores of Eocene bituminous marls (OS-22 and OS-23) from central Jordan with thicknesses of 222.4 m and 256.3 m, respectively, were logged. A total of 103 thin sections were analyzed for sediment petrographic investigations. Another 295 bulk rock samples from Core OS-23 were selected for stable-isotope analysis (δ13C, δ18O). The two cores show similar lithologic variations that correspond to third-order sea-level changes. The sediment petrographic study reveals the presence of six major microfacies that range from mudstones to grainstones. The distribution of these six types of microfacies in the two cores indicates an overall shallow-marine environment in an interior carbonate-platform setting. Within this environment, conditions fluctuated between open and restricted marine. The stratigraphic variability of the carbon-isotope data of Core OS-23 reflects a highly dynamic depositional system that exhibits a variable rate of organic matter accumulation in the sediments, which can be directly linked to the interaction between primary organic-matter burial and oxidative weathering of reworked organic matter. Within the overall diagenetically controlled δ18Ocarb profile an excursion is observed for a specifically enriched organic matter interval. This excursion is believed to coincide with the Middle Eocene Climatic Optimum (MECO), suggesting that next to seawater, porewaters were influenced by this event and making it the first record of this event in the region.

Three Si isotope materials have been used for an inter-laboratory comparison exercise to ensure reproducibility between international laboratories investigating natural Si isotope variations using a variety of chemical preparation methods and mass spectrometric techniques. These proposed standard reference materials are (i) IRMM-018 (a SiO2 standard), (ii) Big-Batch (a fractionated SiO2 material prepared at the University of California Santa Barbara), and (iii) Diatomite (a natural diatomite sample originally deposited as marine biogenic opal). All analyses are compared with the international Si standard NBS28 (RM8546) and are in reasonable agreement (

Silicon isotope ratios (28Si, 29Si and 30Si) can be measured with high precision by multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS). However, the problematic extraction of silicon from geological materials has been a major disadvantage in previous silicon isotope studies with conventional gas source mass spectrometry, whereas available silicon isotope results obtained by MC-ICP-MS techniques have been mainly restricted to waters and high purity silica. We show here that high yields of silicon (>97%) can be achieved from samples ranging from pure silica to basaltic compositions (45–52 wt.% SiO2) via a three-step digestion and purification procedure. Silicon isotope measurements, performed with a Finnigan Neptune MC-ICP-MS used in medium-resolution mode (resolving power: 2500), indicate that polyatomic interferences can be resolved and that both δ29Si and δ30Si can be determined with high accuracy and precision on interference-free peak plateaux in the mass spectrum. Instrumental blanks (20–65 mV) were reduced to acceptable values with a Cetac Aridus desolvating device fitted with a sapphire injector in the torch. Sensitivity in medium-resolution mode is in the range of ∼6 V per μg g−1 for 28Si. δ29Si and δ30Si have been determined for silicon isotope standards IRMM-018 (δ30Si = −1.75‰), IRMM-018-76 (δ30Si = −1.42‰), Diatomite (δ30Si = 1.34‰) and Big Batch (δ30Si = −10.52‰), for USGS standards BHVO-2 (δ30Si = −0.09‰) and AGV-2 (δ30Si = −0.01‰), and for Aldrich pure silica powder (δ30Si = −0.32‰). Precision on δ30Si is 0.18–0.41‰ (2 s.d.). Our combined procedure for sample preparation followed by high-resolution MC-ICP-MS analysis facilitates straightforward and safe measurement of silicon isotope ratios in silicate materials.

Sample preparation methods for MC-ICP-MS silicon-isotope measurements often involve a cation-exchange purification step. A previous study has argued that this would suffice for geological materials, as the occasional enrichment of anionic species would not compromise silicon-isotope analysis. Here we report significant offsets in MC-ICP-MS silicon-isotope measurements induced by the presence of sulfur. We show that offsets in δ30Si become significant above SO4/Si ratios (wt.) of 0.02, reaching up to ca. +1.4‰ at SO4/Si ratios above ∼0.2. This finding is particularly relevant for studies of sulfur-rich waters and silicified rocks where alteration was accompanied by sulfur-enrichments. We propose an additional purification step to remove sulfur from solid sample material.