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Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2 These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

Although previous studies demonstrate the photochemical reduction of uranyl (UO22+) in the presence of various organic compounds, the actual roles of organic molecules as ligands and electron donors during the photoreaction are poorly understood. In this study, photochemical reduction of uranyl is examined with respect to organic ligands as electron donors and complexing agents, the role of titanium oxide (TiO2) nanoparticles as a photocatalyst, and the influence of UV light irradiation with emission peaks in the UV-A, UV-B, and UV-C ranges. Organic compounds with different binding affinities to uranyl such as acetate, ethylenediaminetetracetate (EDTA), oxalate, and hydroquinone were selected. Uranyl solutions prepared with one organic compound into a 1:8 molar ratio were irradiated under anoxic and acidic conditions (O2 < 1 ppm, pH 2.5). Uranyl removal by UV irradiation was better than 70 percent in the presence of oxalate and acetate, followed by hydroquinone (~ 45%) and EDTA (~ 10%). Uranyl removal was nearly constant at the UV-A, UV-B, and UV-C regions in the presence of acetate and oxalate whereas greater removal was observed in the EDTA and hydroquinone solutions exposed to UV-C and UV-A, respectively. These results reveal that uranyl reduction is mediated primarily by TiO2 nanoparticles and is highly dependent on the electrondonor compound. Addition of acetate enhances the uranyl photoreaction in hydroquinone solution. Dissolved EDTA species act as good electron donors at limited EDTA concentrations (1:2 to 1:4 uranyl to EDTA ratios) but at higher concentrations (for example, 1:8), uranyl-EDTA complexes such as [(UO2 2+)HEDTA] compete for the surface sites on the TiO2 nanoparticles, hindering the photoreduction of uranyl. X-ray photoelectron spectroscopy (XPS) of the dried TiO2 powder shows that more than 70 percent of uranium partitioned into the solid phase is present as reduced forms with oxidation states (V) and (IV). The U4f spectra of U partitioned to the solid phase from the photoreaction with acetate reveal the predominance of U(IV) over U(V), whereas U(V) is the dominant oxidation state as a result of the photoreduction with EDTA. Our results suggest that formation of uranium-ligand complexes plays a critical role in controlling the reactivity of uranyl species and the stability of reduced uranium species in the course of the photoreaction. [ABSTRACT FROM AUTHOR]

An origin of the Tinaquillo Peridotite Complex in northern Venezuela, and a model for the tectonic framework of the boundary zone between the southern Caribbean Plate margin and the South American Plate (SAP) during Late Jurassic to Mid-Cretaceous time are developed using newly measured Sr, Nd, Pb and Hf isotopic compositions as well as major and trace element geochemistry for some hornblendite veins and their spinel peridotite host rocks. Depleted geochemical characteristics, principally major element concentrations, and Nd and Hf isotopes (e.g., εNd = + 27; εHf = +50; ca. 3.4 Ga-Nd model age) of the Tinaquillo peridotites suggest a genetic link between this complex and the Guiana Shield farther to the south within Venezuela. Scattered zones within the Tinaquillo peridotite were overprinted by what we interpret as channelized hydrous fluids (<∼3%) derived from the eastward-dipping subduction of the Farallon Plate beneath the SAP in the Late Jurassic, leading to modification of the original Sr, Nd and Hf isotopic compositions as well as the incompatible element concentrations. The hornblendite veins have Sr, Nd, Pb and Hf isotopic compositions, falling within the range of present-day Pacific/Atlantic MORB, but trace element abundance patterns with combinations of arc- and MORB-like characteristics. These results rule out the possibility of local melting of the host peridotites to produce magmas from which the hornblendite veins formed. We suggest that during inception of the westward-dipping subduction of the Protocaribbean Plate immediately following polarity reversal in the Mid-Cretaceous, melts infiltrated the mantle lithosphere and produced the veins. [ABSTRACT FROM AUTHOR]