Description: The Central Andes of South America host the largest known lithium resources in a confined area, but the primary lithium sources of the salar deposits and the mobilisation process of lithium are still a matter of speculation. Chemical weathering at or near the surface and leaching in hydrothermal systems of the active magmatic arc are considered the two main mechanisms of Li extraction from the source rock. The lithium and strontium isotope composition of typical salar deposits offer insights into the processes on how Li brine deposits in Andean evaporites are formed. Data from the Salar de Pozuelos indicate near-surface chemical weathering in a cold and dry climate as the dominant mobilisation process of Li, with evaporation being responsible for the enrichment. The Cenozoic ignimbrites are the favoured source rock for the Li, with subordinate additions from the Palaeozoic basement. The identification of the source rocks is supported by radiogenic Nd and Pb and stable B isotope data from salar deposits. A comparison with other Li brine and salt deposits in the Altiplano-Puna Plateau and its western foothills places the Salar de Pozuelos as an endmember of Li solubilisation by chemical weathering with only minor hydrothermal mobilisation of Li.

Description: Universität Bremen (1013)

Description: Boron isotope patterns preserved in cap carbonates deposited in the aftermath of the younger Cryogenian (Marinoan, ca. 635 Ma) glaciation confirm a temporary ocean acidification event on the continental margin of the southern Congo craton, Namibia. To test the significance of this acidification event and reconstruct Earth's global seawater pH states at the Cryogenian-Ediacaran transition, we present a new boron isotope data set recorded in cap carbonates deposited on the Yangtze Platform in south China and on the Karatau microcontinent in Kazakhstan. Our compiled d11B data reveal similar ocean pH patterns for all investigated cratons and confirm the presence of a global and synchronous ocean acidification event during the Marinoan deglacial period, compatible with elevated postglacial pCO2 concentrations. Differences in the details of the ocean acidification event point to regional distinctions in the buffering capacity of Ediacaran seawater.

Description: It has long been known that extreme changes in North African hydroclimate occurred during the late Pleistocene yet many discrepancies exist between sites regarding the timing, duration and abruptness of events such as Heinrich Stadial (HS) 1 and the African Humid Period (AHP). The hydroclimate history of the Nile River is of particular interest due to its lengthy human occupation history yet there are presently few continuous archives from the Nile River corridor, and pre-Holocene studies are rare. Here we present new organic and inorganic geochemical records of Nile Basin hydroclimate from an eastern Mediterranean (EM) Sea sediment core spanning the past 28 ka BP. Our multi-proxy records reflect the fluctuating inputs of Blue Nile versus White Nile material to the EM Sea in response to gradual changes in local insolation and also capture abrupt hydroclimate events driven by remote climate forcings, such as HS1. We find strong evidence for extreme aridity within the Nile Basin evolving in two distinct phases during HS1, from 17.5 to 16 ka BP and from 16 to 14.5 ka BP, whereas peak wet conditions during the AHP are observed from 9 to 7 ka BP. We find that zonal movements of the Congo Air Boundary (CAB), and associated shifts in the dominant moisture source (Atlantic versus Indian Ocean moisture) to the Nile Basin, likely contributed to abrupt hydroclimate variability in northern East Africa during HS1 and the AHP as well as to non-linear behavior of hydroclimate proxies. We note that different proxies show variable gradual and abrupt responses to individual hydroclimate events, and thus might have different inherent sensitivities, which may be a factor contributing to the controversy surrounding the abruptness of past events such as the AHP. During the Late Pleistocene the Nile Basin experienced extreme hydroclimate fluctuations, which presumably impacted Paleolithic cultures residing along the Nile corridor.

Description: The strontium (Sr), neodymium (Nd) and lead (Pb) isotope signatures of suspended particulate matter (SPM) in rivers reflect the radiogenic isotope signatures of the rivers' drainage basin. These signatures are not significantly affected by weathering, transport or depositional cycles, but document the sedimentary contributions of the respective sources. We report new Sr, Nd and Pb isotope ratios and element concentrations of modern SPM from the Brazilian Amazon River basin and document the past evolution of the basin by analyzing radiogenic isotopes of a marine sediment core from the slope off French Guiana archiving the last 40 kyr of Amazon River SPM, and the Holocene section of sediment cores raised between the Amazon River mouth and the slope off French Guiana. The composition of modern SPM confirms two main source areas, the Andes and the cratonic Shield. In the marine sediment core notable changes occurred during the second phase of Heinrich Stadial 1 (i.e. increased proportion of Shield rivers SPM) and during the last deglaciation (i.e. increased proportion of Madeira River SPM) together with elsewhere constant source contributions. Furthermore, we report a prominent offset in Sr and Nd isotopic composition between the average core value (εNd: −11.7 ± 0.9 (2SD), 87Sr/86Sr: 0.7229 ± 0.0016 (2SD)) and the average modern Amazon River SPM signal (εNd: −10.5 ± 0.5 (2SD), 87Sr/86Sr: 0.7213 ± 0.0036 (2SD)). We suggest that a permanent change in the Amazon River basin sediment supply during the late Holocene to a more Andean dominated SPM was responsible for the offset.

Description: The dataset contains B, Li, Mg and Sr concentrations and isotopic compositions of black-smoker, acid-sulfate fluids and "hybrid-types" as well as of fresh and altered rocks from different vent fields (DESMOS and North Su) within the Manus Basin, Papua New Guinea. The data is used to understand the controls of their compositional variability. In particular, the formation of acid-sulfate and hybrid smoker fluids is still poorly understood, and their high Mg concentrations are explained either by dissolution of Mg-bearing minerals in the basement or by mixing between unmodified seawater and magmatic fluids. Mg isotope ratios of the acid-sulfate fluids from Manus Basin are seawater-like, which supports the idea that acid-sulfate fluids in this study area predominantly form by mixing between unmodified seawater and a Mg-free magmatic fluid. Changes in the B, Li, and Sr isotope ratios relative to seawater indicate water-rock interaction in all acid-sulfate fluids. Further, the combination of δ7Li with B concentrations of the same fluids links changes in δ7Li to changes in (1) basement alteration, (2) water-to-rock ratios during water-rock interaction and/or (3) the reaction temperature. These isotope systems, thus, allow tracing of basement composition and acid-sulfate driven alteration of the backarc crust, and help increase our understanding of hydrothermal fluid-rock interactions and the behavior of fluid-mobile elements.

Description: The dataset contains boron (B) concentrations and isotope ratios of hydrothermal vent fluids and volcanic rocks from different vent fields within the Manus Basin, Papua New Guinea, and Nifonea volcano, New Hebrides back-arc. The fluids from these settings show a range of salinities, gas contents, acidities, and host rock compositions; many of them are influenced by phase separation and by addition of magmatic volatiles (both CO2 and SO2). Previous studies of hydrothermal vents in arc/back-arc settings suggest that B contents and isotopic composition of vent fluids are controlled by interactions between seawater, basement and sediments, and propose that phase separation and magmatic fluids play only a subordinate role. In our study, we demonstrate that vent fluids with minor magmatic input indeed reflect the interaction between seawater and oceanic crust. In contrast, the low-salinity Nifonea fluids and some of the acid-sulfate fluids from the Manus Basin have higher B contents as expected, whereas other volatile-rich fluids from the Manus Basin show B depletions. The lack of correlation between B contents and the intensity of magmatic fluid influx (CO2 and SO2) may indicate that magma degassing is not responsible for the B enrichments or depletions in these vent fluids. B enrichments might be related to preferential partitioning of B into the vapour phase during phase separation under PT-conditions well above the two-phase curve and critical line (i.e. T 〉〉 Tcritical, P 〉〉 Pcritical). However, this cannot explain the low B concentrations in the vapour-rich vent fluids from the Manus Basin and the low B isotope ratios in the Nifonea fluids. Instead, we propose that B concentrations and isotope ratios in submarine vent fluids largely depend on the residence and reaction time of the vent fluid in the subsurface. In general, all vent fluids are still influenced by water-rock interaction during hydrothermal circulation. However, vent fluids with short residence times define a trend towards lower B concentrations and isotope ratios, which can be explained by mixing between hydrothermal and magmatic fluid, which is similar to the composition of the host rock. In contrast, the B signature of the magmatic fluid can be overprinted due to preferential mobilization of B from the oceanic crust into vapour-rich fluids at longer reaction times. Thus, B may provide a tool for estimating the extent of B leaching and hence hydrothermal alteration in the subseafloor.