Spatial variations in chemical weathering and CO2 consumption in Nepalese High Himalayan catchments during the monsoon season

The major ion chemistry of the Marsyandi basin and six of its tributaries in the Nepalese Himalaya have been investigated during the monsoon months of 2002. Weekly water samples taken at 10 river monitoring stations in the Annapurna watershed over the course of 4 months provide chemical weathering data for the region at an unprecedented temporal and spatial resolution. The river chemistry of all but one basin is heavily dominated by carbonate weathering which, compared to silicate weathering, contributes 80 to 97% of the total solute load. This prevalence is due to a combination of (a) intrinsically faster dissolution kinetics of carbonates, (b) relatively high runoff and (c) glacial meltwater and low temperatures at high altitudes resulting in enhanced carbonate solubilities. Monitoring stations with headwaters in the Tethyan Sedimentary Series (TSS) are particularly carbonate-rich and slightly supersaturated with respect to calcite through half of the monsoon season. Silicate weathering in the TSS is driven largely by sulfuric acid and therefore does not contribute significantly to the drawdown of atmospheric CO₂. With respect to the tributaries in the Greater Himalayan Sequence (GHS), carbonate weathering is practically as predominant as for the TSS, in spite of the largely felsic lithology of the GHS. Relative to the TSS, the primary proton source in the GHS has shifted, with at least 80% of the protons derived from carbonic acid. Averaged over the whole field area, the CO₂ fluxes, based on silicate-derived Ca and Mg, are considerably lower than the global average. Assuming that this study area is representative of the entire range, we conclude that in situ weathering of the High Himalayas does not represent a significant sink of atmospheric carbon dioxide, despite the presence of a watershed south of the GHS that is characterized by a four times higher CO₂ consumption rate than the global average. Silicate weathering rates of all basins appear to be climate controlled, displaying a tight correlation with runoff and temperature. Given the extremely low chemical weathering under transport-limited conditions in high-altitude crystalline terrains outside of the monsoon season, this would result in virtually no chemical exhumation for 2/3 of the year in such a cold and arid climate, north of the rain shadow cast by the High Himalayas.     

Fossil Hydrophilid Beetles (Coleoptera: Hydrophilidae) of the Late Oligocene Rott Formation (Germany)

<正>Fossil water scavenger beetles(Coleoptera:Hydrophilidae)of the latest Oligocene Rott Formation are revised,based on the examination of the type specimens,as well as numerous additional material from Statz(Los Angeles)and Kastenholz(Bonn)collections.Seven hydrophilid species are recognized,five of which are reliably attributed to the following genera:Berosus morticinus(von Heyden and von Heyden,1866),Paracymus excitatus (von Heyden and von Heyden,1866),Paracymus sp.,Hydrobiomorpha fraterna(von Heyden,1859),and Hydrophilus rottensis(Statz,1939).Coccinella? protogaeae Germar 1837 is attributed to the subtribe Hydrophilina, and Cymbiodyta? austera Statz 1939 to the subfamily Hydrophilinae.Hydrous ebeninus Statz 1939 and Paracymus excitatus(form 3)are excluded from the Hydrophiloidea.Berosus capitatus Statz 1939 is synonymized with Berosus morticinus,and Hydrous neptunus von Heyden and von Heyden 1866 with Hydrobiomorpha fraterna.Lectotypes of Philydrus morticinus von Heyden and von Heyden 1866 and Hydrous ebeninus Statz 1939 are designated.The significance of the hydrophilid fossils for paleoecological reconstructions of the former Rott Lake is briefly discussed.     

Microbially induced cementation of carbonate sands: are micritic meniscus cements good indicators of vadose diagenesis?

Characteristic fabrics such as micrite envelopes, calcified filaments and micritic grain-to-grain bridges are observed in a modern subtidal firmground (Wood Cay, Bahamas) and in a variety of firm- and hardgrounds of Lower Cretaceous and Upper Jurassic platform carbonates (Swiss and French Jura Mountains). Their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands. 'Meniscus-type cements' (to distinguish them from vadose meniscus cements), which clearly formed in subtidal environments, are related to filament calcification, trapping of percolating micrite and microbially induced carbonate formation. Such meniscus-type cements are commonly micritic, but meniscus-shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed. Therefore, an interpretation of vadose early diagenesis should not be based on meniscus cements alone. Similarly, subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure.     

Accretion and stabilisation of the Archaean Zimbabwe Craton (extended abstract)

Geologie en Mijnbouw -     

The Lawn Hill annulus: An Ordovician meteorite impact into water‐saturated dolomite

The Lawn Hill Impact Structure (LHIS) is located 250 km N of Mt Isa in NW Queensland, Australia, and is marked by a highly deformed dolomite annulus with an outer diameter of ~18 km, overlying low metamorphic grade siltstone, sandstone, and shale, along the NE margin of the Georgina Basin. This study provides detailed field observations from sections of the Lawn Hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the Lawn Hill impact. ⁴⁰Ar‐³⁹Ar dating of impact‐related melt particles provides a time of impact in the Ordovician (472 ± 8 Ma) when the Georgina Basin was an active depocenter. The timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed Thorntonia Limestone at the time of impact. The exposed annulus is remarkably well preserved, with preservation attributed to postimpact sedimentation. The LHIS has an atypical crater morphology with no central uplift. The heterogeneous target materials at Lawn Hill were probably low‐strength, porous, and water‐saturated, with all three properties affecting the crater morphology. The water‐saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus, and restricted melt production. The impact timing raises the possibility that the Lawn Hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid‐Ordovician (470 ± 6 Ma).     

Eclogites in the Makuti gneisses of Zimbabwe: implications for the tectonic evolution of the Zambezi Belt in southern Africa

The gneisses of the Makuti Group in north-west Zimbabwe are characterized by complex geometries that resulted from intense non-coaxial deformation in a crustal scale high-strain zone that accommodated extensional deformation along the axis of the Zambezi Belt at c. 800 Ma. Within low-strain domains in the Makuti gneisses, undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages, which record a part of the metamorphic history that predates Pan-African events. Eclogitic rocks can be subdivided into: (1) corona-textured metagabbros that preserve igneous textures, and (2) garnet–omphacite rocks in which primary textures are destroyed. The lenses of eclogitic rocks are enveloped in a mantle of garnet–clinopyroxene–hornblende gneiss, which is a common rock type in the Makuti gneisses. The eclogites preserve multi-staged, domainal, symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression–heating. In the metagabbros, the original clinopyroxene, plagioclase and olivine domains acted separately during the peak of metamorphism, with plagioclase being replaced by garnet and kyanite, and olivine being replaced by orthopyroxene and possibly omphacite. The peak assemblage was overprinted by: (1) the multi-mineralic corona assemblage pargasite–orthopyroxene–spinel–plagioclase replacing garnet–kyanite–clinopyroxene (possibly at c. 19 kbar, 760±25 °C); (2) orthopyroxene–pargasite–plagioclase–scapolite coronas replacing orthopyroxene (15±1.5 kbar, 750±50 °C); and (3) moats of orthopyroxene–plagioclase replacing garnet (10±1 kbar, 760±50 °C). The garnet–omphacite rocks record similar peak conditions (15±1.1 kbar, 760±60 °C). Garnet–clinopyroxene–hornblende–plagioclase gneisses envelop the eclogites and record matrix conditions of 11±1.5 kbar at 730±50 °C using assemblages that are oriented in the regional fabric. These rocks are characterized by decompression-heating textures, reflecting temperature increases during exhumation of the Makuti gneisses. The eclogite facies rocks formed during a collisional event prior to 850 Ma. Their formation could be related to a suture zone that developed along the axis of the Zambezi Belt during the formation of Rodinia (between 1400 and 850 Ma). The main deformation-metamorphism in the Makuti gneisses occurred around 800 Ma and involved extension and exhumation of the high-P rocks (break-up of Rodinia), which experienced a high-T metamorphic overprint. Around 550–500 Ma, a collisional event associated with the formation of Gondwana resulted in renewed burial and metamorphic recrystallization of the Makuti gneisses.     

Multi-model Bayesian assessment of climate change in the northern annular mode

A typical question in climate change analysis is whether a certain observed climate characteristic, like a pronounced anomaly or an interdecadal trend, is an indicator of anthropogenic climate change or still in the range of natural variability. Many climatic features are described by one-dimensional index time series, like for instance the global mean temperature or circulation indices. Here, we present a Bayesian classification approach applied to the time series of the northern annular mode (NAM), which is the leading mode of Northern Hemisphere climate variability. After a pronounced negative phase during the 1950s and 1960s, the observed NAM index reveals a distinct positive trend, which is also simulated by various climate model simulations under enhanced greenhouse conditions. The objective of this study is to decide whether the observed temporal evolution of the NAM may be an indicator of global warming. Given a set of prior probabilities for disturbed and undisturbed climate scenarios, the Bayesian decision theorem decides whether the observed NAM trend is classified in a control climate, a greenhouse-gas plus sulphate aerosol climate or a purely greenhouse-gas induced climate as derived from multi-model ensemble simulations.The three climate scenarios are well separated from each other in terms of the 30-year NAM trends. The multi-model ensembles contain a weak but statistically significant climate change signal in the form of an intensification of the NAM. The Bayesian classification suggests that the greenhouse-gas scenario is the most probable explanation for the observed NAM trend since 1960, even if a high prior probability is assigned to the control climate. However, there are still large uncertainties in this classification result because some periods at the end of the 19th century and during the “warm” 1920s are also classified in an anthropogenic climate, although natural forcings are likely responsible for this early NAM intensification. This demonstrates a basic shortcoming of the Bayesian decision theorem when it is based on one-dimensional index time series like the NAM index.     

Seasonal prediction of African precipitation with ECHAM4–T42 ensemble simulations using a multivariate MOS re-calibration scheme

A 15 member ensemble of 20th century simulations using the ECHAM4–T42 atmospheric GCM is utilized to investigate the potential predictability of interannual variations of seasonal rainfall over Africa. Common boundary conditions are the global sea surface temperatures (SST) and sea ice extent. A canonical correlation analysis (CCA) between observed and ensemble mean ECHAM4 precipitation over Africa is applied in order to identify the most predictable anomaly patterns of precipitation and the related SST anomalies. The CCA is then used to formulate a re-calibration approach similar to model output statistics (MOS) and to derive precipitation forecasts over Africa. Predictand is the climate research unit (CRU) gridded precipitation over Africa. As predictor we use observed SST anomalies, ensemble mean precipitation over Africa and a combined vector of mean sea level pressure, streamfunction and velocity potential at 850 hPa. The different forecast approaches are compared. Most skill for African precipitation forecasts is provided by tropical Atlantic (Gulf of Guinea) SST anomalies which mainly affect rainfall over the Guinean coast and Sahel. The El Niño/Southern Oscillation (ENSO) influences southern and East Africa, however with a lower skill. Indian Ocean SST anomalies, partly independent from ENSO, have an impact particularly on East Africa. As suggested by the large agreement between the simulated and observed precipitation, the ECHAM4 rainfall provides a skillful predictor for CRU precipitation over Africa. However, MOS re-calibration is needed in order to provide skillful forecasts. Forecasts using MOS re-calibrated model precipitation are at least as skillful as forecast using dynamical variables from the model or instantaneous SST. In many cases, MOS re-calibrated precipitation forecasts provide more skill. However, differences are not systematic for all regions and seasons, and often small.