Global Change Modulated Asian Inland Climate Since 7.3 Ma: Carbonate Manganese Records in the Western Qaidam Basin

Late Cenozoic drying of the Asian inland has not only exerted a profound impact on the regional environment but also affected global climate as an important source of global atmospheric dust. Continuous and accurately dated sediment records from the Asian interior are pivotal to a better understanding of the evolutionary history of Asian inland drying and the associated driving mechanisms. In this study, we present a continuous record of climate change in the Asian interior spanning the past 7.3 Myr, reconstructed by the redox evolution of a paleolake in the western Qaidam Basin, NE Tibetan Plateau. The paleolake redox conditions are linked to the oxygen concentration of lake bottom water and lake level, and were revealed by the manganese (Mn) concentration in the carbonate fraction (leached by the diluted acetic acid) of the carbonate-rich lacustrine sediments retrieved from two drill-cores (SG-1 and SG-1b). The reconstructed regional climate in the western Qaidam Basin shows long-term fluctuations, consistent with the secular evolution of the coeval global climate, especially the sea surface temperature variation in the high latitude North Atlantic. Three transitions of the paleolake hydrochemical system occurred at 6.2, 5.3, and 2.6 Ma, with a short drying stage at 6.2–5.3 Ma and prolonged Quaternary drying since 2.6 Ma. We argue that drying of the Asia interior has been dominantly forced by global cooling, in particular, the high-latitude cooling of the Northern Hemisphere.

Secular oxidation of Archean upper mantle caused by increasing crustal recycling

The redox evolution of Archean mantle impacted Earth differentiation, mantle melting and the nature of chemical equilibrium between mantle, ocean and atmosphere of the early Earth. However, how and why it varies with time remain controversial. Archean mantle-derived volcanic rocks, especially basalts are ideal lithologies for reconstructing the mantle redox state. Here we show that the ~3.8-2.5 Ga basalts from fourteen cratons are subdivided geochemically into two groups, B-1, showing incompatible element depleted and modern mid-ocean ridge basalt-like features ((Nb/La)PM ≥ 0.75) and B-2 ((Nb/La)PM < 0.75), characterized by modern island arc basalt-like features. Our updated V-Ti redox proxy indicates the Archean upper mantle was more reducing than today, and that there was a significant redox heterogeneity between ambient and modified mantle presumably related to crustal recycling, perhaps via plate subduction, as shown by B-1 and B-2 magmas, respectively. The oxygen fugacity of modified mantle exhibits a ~1.5-2.0 log units increase over ~3.8-2.5 Ga, whereas the ambient mantle becomes more and more heterogeneous with respect to redox, apart from a significant increase at ~2.7 Ga. These findings are coincident with the increase in the proportions of crustal recycling-related lithologies with associated enrichment of associated incompatible elements (e.g., Th/Nb), indicating that increasing recycling played a crucial role on the secular oxidation of Archean upper mantle.

Interaction of Corroding Iron with Eight Bentonites in the Alternative Buffer Materials Field Experiment (ABM2)

Bentonite, a common smectite-rich buffer material, is in direct contact with corroding steel in many high-level radioactive waste repository designs. The interaction of iron with the smectite-rich clay may affect its swelling and sealing properties by processes such as alteration, redox reactions and cementation. The chemical interactions were investigated by analysing the Fe/clay interfaces of eight bentonite blocks which had been exposed to temperatures up to 130 °C for five years in the ABM2 borehole at the Äspö Hard Rock Laboratory managed by the Swedish Nuclear Fuel and Waste Management Co (SKB). Eleven interface samples were characterised by high spatial resolution methods, including scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and μ-Raman spectroscopy as well as by “bulk” methods X-ray diffraction, X-ray fluorescence and 57Fe Mössbauer spectrometry. Corrosion induced an iron front of 5–20 mm into the bentonite, except for the high-Fe bentonite where no Fe increase was detected. This Fe front consisted mainly of ferric (oxyhydr)oxides in addition to the structural Fe in the smectite fraction which had been partially reduced by the interaction process. Fe(II) was also found to extend further into the clay, but its nature could not be identified. The consistent behaviour is explained by the redox evolution, which shifts from oxidising to reducing conditions during the experiment. No indication of smectite alteration was found.

Revisiting stepwise ocean oxygenation with authigenic barium enrichments in marine mudrocks

There are current debates around the extent of global ocean oxygenation, particularly from the late Neoproterozoic to the early Paleozoic, based on analyses of various geochemical indices. We present a temporal trend in excess barium (Baexcess) contents in marine organic-rich mudrocks (ORMs) to provide an independent constraint on global ocean redox evolution. The absence of remarkable Baexcess enrichments in Precambrian (&gt;ca. 541 Ma) ORMs suggests limited authigenic Ba formation in oxygen- and sulfate-deficient oceans. By contrast, in the Paleozoic, particularly the early Cambrian, ORMs are marked by significant Baexcess enrichments, corresponding to substantial increases in the marine sulfate reservoir and oxygenation level. Analogous to modern sediments, the Mesozoic and Cenozoic ORMs exhibit no prominent Baexcess enrichments. We suggest that variations in Baexcess concentrations of ORMs through time are linked to secular changes in the marine dissolved Ba reservoir associated with elevated marine sulfate levels and global ocean oxygenation. Further, unlike Mo, U, and Re abundances, significant Baexcess enrichments in ORMs indicate that the overall ocean oxygenation level in the early Paleozoic was substantially lower than at present.

Redox control on the tungsten isotope composition of seawater

Free oxygen represents an essential basis for the evolution of complex life forms on a habitable Earth. The isotope composition of redox-sensitive trace elements such as tungsten (W) can possibly trace the earliest rise of oceanic oxygen in Earth’s history. However, the impact of redox changes on the W isotope composition of seawater is still unknown. Here, we report highly variable W isotope compositions in the water column of a redox-stratified basin (δ186/184W between +0.347 and +0.810 ‰) that contrast with the homogenous W isotope composition of the open ocean (refined δ186/184W of +0.543 ± 0.046 ‰). Consistent with experimental studies, the preferential scavenging of isotopically light W by Mn-oxides increases the δ186/184W of surrounding seawater, whereas the redissolution of Mn-oxides causes decreasing seawater δ186/184W. Overall, the distinctly heavy stable W isotopic signature of open ocean seawater mirrors predominantly fully oxic conditions in modern oceans. We expect, however, that the redox evolution from anoxic to hypoxic and finally oxic marine conditions in early Earth’s history would have continuously increased the seawater δ186/184W. Stable W isotope compositions of chemical sediments that potentially preserve changing seawater W isotope signatures might therefore reflect global changes in marine redox conditions.