It is generally thought that the sulfate reduction metabolism is ancient and would have been established well before the Neoarchean. It is puzzling, therefore, that the sulfur isotope record of the ...Neoarchean is characterized by a signal of atmospheric mass-independent chemistry rather than a strong overprint by sulfate reducers. Here, we present a study of the four sulfur isotopes obtained using secondary ion MS that seeks to reconcile a number of features seen in the Neoarchean sulfur isotope record. We suggest that Neoarchean ocean basins had two coexisting, significantly sized sulfur pools and that the pathways forming pyrite precursors played an important role in establishing how the isotopic characteristics of each of these pools was transferred to the sedimentary rock record. One of these pools is suggested to be a soluble (sulfate) pool, and the other pool (atmospherically derived elemental sulfur) is suggested to be largely insoluble and unreactive until it reacts with hydrogen sulfide. We suggest that the relative contributions of these pools to the formation of pyrite depend on both the accumulation of the insoluble pool and the rate of sulfide production in the pyrite-forming environments. We also suggest that the existence of a significant nonsulfate pool of reactive sulfur has masked isotopic evidence for the widespread activity of sulfate reducers in the rock record.
Identifying phosphorus (P) sources is critical for solving eutrophication and controlling P in aquatic environments. Phosphate oxygen isotopes (δ18Op) have been used to trace P sources. However, the ...application of this method has been greatly restricted due to δ18OP values from the potential source having wide and overlapping ranges. In this research, P sources were traced by combining δ18Op with multiple stable isotopes of nitrogen (δ15N), hydrogen (δD), and dissolved inorganic carbon (δ13C). Then, a Bayesian-based Stable Isotope Analysis in R (SIAR) model and IsoSource model were used to estimate the proportional contributions of the potential sources in the Tuojiang River. δ18Op was not in equilibrium with ambient water, and statistically significant differences in the δ18Op values were found between the potential sources, indicating that δ18Op can be used to trace the P sources. δ15N, δD, and δ13C could assist δ18Op in identifying the main sources of P. The SIAR and IsoSource models suggested that industrial and domestic sewage was the largest contributor, followed by phosphate rock and phosphogypsum and agricultural sewage. The uncertainty of the calculation results of the SIAR model was lower than that of the IsoSource model. These findings provide new insights into tracing P sources using multiple stable isotopes in watersheds.
Mare volcanics on the Moon are the key record of thermo-chemical evolution throughout most of lunar history
. Young mare basalts-mainly distributed in a region rich in potassium, rare-earth elements ...and phosphorus (KREEP) in Oceanus Procellarum, called the Procellarum KREEP Terrane (PKT)
-were thought to be formed from KREEP-rich sources at depth
. However, this hypothesis has not been tested with young basalts from the PKT. Here we present a petrological and geochemical study of the basalt clasts from the PKT returned by the Chang'e-5 mission
. These two-billion-year-old basalts are the youngest lunar samples reported so far
. Bulk rock compositions have moderate titanium and high iron contents with KREEP-like rare-earth-element and high thorium concentrations. However, strontium-neodymium isotopes indicate that these basalts were derived from a non-KREEP mantle source. To produce the high abundances of rare-earth elements and thorium, low-degree partial melting and extensive fractional crystallization are required. Our results indicate that the KREEP association may not be a prerequisite for young mare volcanism. Absolving the need to invoke heat-producing elements in their source implies a more sustained cooling history of the lunar interior to generate the Moon's youngest melts.
The origin of the tin used for the production of bronze in the Eurasian Bronze Age is still one of the mysteries in prehistoric archaeology. In the past, numerous studies were carried out on ...archaeological bronze and tin objects with the aim of determining the sources of tin, but all failed to find suitable fingerprints. In this paper we investigate a set of 27 tin ingots from well-known sites in the eastern Mediterranean Sea (Mochlos, Uluburun, Hishuley Carmel, Kfar Samir south, Haifa) that had been the subject of previous archaeological and archaeometallurgical research. By using a combined approach of tin and lead isotopes together with trace elements it is possible to narrow down the potential sources of tin for the first time. The strongly radiogenic composition of lead in the tin ingots from Israel allows the calculation of a geological model age of the parental tin ores of 291 ± 17 Ma. This theoretical formation age excludes Anatolian, central Asian and Egyptian tin deposits as tin sources since they formed either much earlier or later. On the other hand, European tin deposits of the Variscan orogeny agree well with this time span so that an origin from European deposits is suggested. With the help of the tin isotope composition and the trace elements of the objects it is further possible to exclude many tin resources from the European continent and, considering the current state of knowledge and the available data, to conclude that Cornish tin mines are the most likely suppliers for the 13th-12th centuries tin ingots from Israel. Even though a different provenance seems to be suggested for the tin from Mochlos and Uluburun by the actual data, these findings are of great importance for the archaeological interpretation of the trade routes and the circulation of tin during the Late Bronze Age. They demonstrate that the trade networks between the eastern Mediterranean and some place in the east that are assumed for the first half of the 2nd millennium BCE (as indicated by textual evidence from Kültepe/Kaneš and Mari) did not exist in the same way towards the last quarter of the millennium.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Ethanol storage and lipid and urea extraction had no effect on bluespotted maskray Neotrygon kuhlii muscle delta13C values whereas urea-removal and ethanol storage increased delta15N values. Results ...presented here show a significant delta15N increase post-urea removal and provide additional support for this approach in future elasmobranch stable-isotope analysis (SIA) studies. Further experimental work on other elasmobranch species is needed to assess extraction and preservation effects on stable-isotope (SI) values.
The response of Earth’s climate system to orbital forcing has been highly state dependent over the past 66 million years.
The states of past climate
Deep-sea benthic foraminifera preserve an ...essential record of Earth's past climate in their oxygen- and carbon-isotope compositions. However, this record lacks sufficient temporal resolution and/or age control in some places to determine which climate forcing and feedback mechanisms were most important. Westerhold
et al.
present a highly resolved and well-dated record of benthic carbon and oxygen isotopes for the past 66 million years. Their reconstruction and analysis show that Earth's climate can be grouped into discrete states separated by transitions related to changing greenhouse gas levels and the growth of polar ice sheets. Each climate state is paced by orbital cycles but responds to variations in radiative forcing in a state-dependent manner.
Science
, this issue p.
1383
Much of our understanding of Earth’s past climate comes from the measurement of oxygen and carbon isotope variations in deep-sea benthic foraminifera. Yet, long intervals in existing records lack the temporal resolution and age control needed to thoroughly categorize climate states of the Cenozoic era and to study their dynamics. Here, we present a new, highly resolved, astronomically dated, continuous composite of benthic foraminifer isotope records developed in our laboratories. Four climate states—Hothouse, Warmhouse, Coolhouse, Icehouse—are identified on the basis of their distinctive response to astronomical forcing depending on greenhouse gas concentrations and polar ice sheet volume. Statistical analysis of the nonlinear behavior encoded in our record reveals the key role that polar ice volume plays in the predictability of Cenozoic climate dynamics.
Oxygen isotope ratios in bivalve shells have long been used as a proxy for environmental change, reflecting both temperature and the oxygen isotope composition of host water. In estuarine systems, ...the oxygen isotope composition of water is complicated by variable mixing between river and seawater, as well as evaporative enrichment. In addition, due to species‐specific variation in temperature‐dependent fractionation into bivalve carbonate, modern calibrations are necessary prior to applications in paleoenvironmental studies. In this study, live specimens of the micromollusc Arthritica helmsi were collected from five sites in the Coorong Lagoon, an estuarine system at the mouth of the River Murray, Australia, on six occasions from November 2016 to May 2018. Whole shell oxygen and carbon isotope compositions (n = 131) were measured alongside monthly temperature and oxygen and hydrogen isotope analyses of waters from the Coorong and neighboring Lake Alexandrina (n = 137). Oxygen and hydrogen isotope ratios in water were mostly controlled by evaporation of source waters, though a period of high river water discharge was reflected in the isotopic values of the Coorong North Lagoon. A species‐specific temperature‐dependent oxygen isotope fractionation equation was calibrated for A. helmsi: T (°C) = (21.39 ± 0.45) − (4.43 ± 0.38) × (δ18Oshell − δ18Owater). This equation is similar to other published paleotemperature equations for biogenic carbonates. These contemporary observations of the isotope hydrology of the Coorong, coupled with our contemporary calibration of oxygen isotope fractionation, lay the foundation for paleoenvironmental studies using bivalves collected from the sediments of the Coorong.
Key Points
Oxygen and hydrogen isotope ratios in waters of the Coorong Lagoons are mostly controlled by evaporation
A new temperature‐dependent oxygen isotope fractionation equation for Arthritica helmsi is developed
Fossil shells of A. helmsi are suitable for paleoenvironmental studies
Recent investigations have revealed significant fractionation of 238U/235U between organic-rich sediments of anoxic marginal seas and seawater, indicating redox-dependent U isotope fractionation. ...This study explores the conditions controlling U isotope fractionation in selected modern anoxic basins (Baltic Sea: Landsort and Gotland Deeps and the Kyllaren fjord in Norway) and compares U with Mo isotope fractionation. Therefore, the concentrations and isotope compositions of dissolved U and Mo from the water column and organic-rich sediments from three currently anoxic basins have been analysed.
The water column samples from the Kyllaren fjord display a moderate depletion of U and a strong depletion of Mo with increasing depth. These variations are associated with a decrease in δ238U and an increase in δ98Mo with depth, from −0.35 ‰ to −0.70 ‰ and from 2.4 ‰ to 2.6 ‰, respectively. From the U isotope composition of the deep euxinic water column of the Kyllaren fjord, a minimum value of Δ238Ured≈0.7 ‰ for the U isotope fractionation during reduction is inferred. Due to the high sedimentation rate in the Kyllaren fjord, surface sediment samples are only moderately enriched in U and Mo (~4μg/g and 6μg/g to 37μg/g, respectively) and display δ238U and δ98Mo averages of −0.26 ‰±0.10 ‰ and 2.18 ‰±0.21 ‰, respectively.
Water column samples from the Baltic Sea display a minor decrease in the U and Mo concentrations and only negligible U and Mo isotope fractionation compared to open seawater. The sediment layers from the Baltic Sea (Landsort Deep), which were deposited under mostly anoxic conditions, are moderately enriched in U (up to 8μg/g) and highly enriched in Mo (up to 222μg/g). However, U and Mo isotopic compositions are significantly lighter (with δ238U of −0.36 ‰±0.08 ‰ and δ98Mo of −0.04 ‰±0.11 ‰) than those of typical organic-rich sediments from anoxic basins.
These findings reveal that compared to Mo, the U isotopic composition of sediments in restricted and strongly euxinic basins like the Kyllaren fjord is significantly more dependent on (1) the extent of U removal from the water column, which is lower than that of Mo, and on (2) the sedimentation rate (i.e., the fraction of authigenic U relative to detrital U in the sediment), which is also lower compared to that of Mo. In more open and only temporarily euxinic basins such as the investigated basins of the Baltic Sea, strong Mo is coupled with weak U isotope fractionation between water and sediment. These signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light Mo during frequently occurring flushing events with oxygen-rich water from the open sea. This implies that Mo and U isotope signatures of sediments only record paleo-water column redox conditions of restricted basins if the water column was permanently stratified.
•Dissolved uranium of restricted anoxic basins is isotopically lighter than open ocean U.•Uranium of restricted anoxic basins is isotopically lighter than their respected sedimentary U′.•Uranium isotope fractionation in restricted anoxic basins depends on the extent of U removal from the water column.•The sedimentation rate in those restricted anoxic basins strongly affects the U isotope composition of the sediments.
The rise of atmospheric oxygen fundamentally changed the chemistry of surficial environments and the nature of Earth's habitability
. Early atmospheric oxygenation occurred over a protracted period ...of extreme climatic instability marked by multiple global glaciations
, with the initial rise of oxygen concentration to above 10
of the present atmospheric level constrained to about 2.43 billion years ago
. Subsequent fluctuations in atmospheric oxygen levels have, however, been reported to have occurred until about 2.32 billion years ago
, which represents the estimated timing of irreversible oxygenation of the atmosphere
. Here we report a high-resolution reconstruction of atmospheric and local oceanic redox conditions across the final two glaciations of the early Palaeoproterozoic era, as documented by marine sediments from the Transvaal Supergroup, South Africa. Using multiple sulfur isotope and iron-sulfur-carbon systematics, we demonstrate continued oscillations in atmospheric oxygen levels after about 2.32 billion years ago that are linked to major perturbations in ocean redox chemistry and climate. Oxygen levels thus fluctuated across the threshold of 10
of the present atmospheric level for about 200 million years, with permanent atmospheric oxygenation finally arriving with the Lomagundi carbon isotope excursion at about 2.22 billion years ago, some 100 million years later than currently estimated.
Uranium isotope variations (δ238U) recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time. However, the effects of diagenetic ...alteration on the U isotopic composition in carbonate sediments, which are crucial to understand the accurate reconstruction of marine δ238U, are currently poorly constrained. Here we examine the effects of the aragonite-to-calcite transition in the Pleistocene Key Largo Limestone of South Florida, and assess the effects of vadose meteoric, phreatic meteoric, and phreatic marine diagenesis on U isotope fractionation in carbonate sediments from the Bahamas Transect, including the well-studied Clino, Unda, and ODP Site 1006 drill cores.
Our results suggest that early diagenetic processes in Bahamas carbonate sediments fractionate U isotopes by an average of 0.27 ± 0.14‰ (1 SD) heavier than contemporaneous seawater. Downcore variations of δ238U in slope and basin sediments display little, if any, correlation with U concentration and common geochemical indicators of diagenesis (δ13C, δ18O, Mn/Sr, Mg/Ca, Sr/Ca), enrichments of redox-sensitive elements, or rare earth elements anomalies. We propose two possible mechanisms to interpret the positive change in the δ238U during carbonate diagenesis: authigenic enrichment of isotopically positive U(IV) in carbonates and preferential incorporation of isotopically positive aqueous U(VI) species into carbonates. These processes likely operate during early (syndepositional) diagenesis on the banktop. Further diagenesis during deeper burial is limited by the low solubility of U(IV) under reducing pore water conditions.
The early diagenetic behavior of U isotopes in Bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past. We suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater δ238U from ancient carbonates. Furthermore, early diagenesis induces significant statistical variability in sediment δ238U values, pointing to the need for large, high resolution data sets in order to average out stochastic variations in individual bulk sediment samples.
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