Glacial isostasy and plate motion

The influence of glacial-isostatic adjustment (GIA) on the motion of tectonic plates is usually neglected. Employing a recently developed numerical approach, we examine the effect of glacial loading on the motion of the Earth’s tectonic plates where we consider an elastic lithosphere of laterally variable strength and the plates losely connected by low viscous zones. The aim of this paper is to elucidate the physical processes which control the GIA-induced horizontal motion and to assess the impact of finite plate-boundary zones. We show that the present-day motion of tectonic plates induced by GIA is at, or above, the order of accuracy of the plate motions determined by very precise GPS observations. Therefore, its contribution should be considered when interpreting the mechanism controlling plate motion.     

Field significance of performance measures in the context of regional climate model evaluation. Part 1: temperature

A new approach for rigorous spatial analysis of the downscaling performance of regional climate model (RCM) simulations is introduced. It is based on a multiple comparison of the local tests at the grid cells and is also known as “field” or “global” significance. New performance measures for estimating the added value of downscaled data relative to the large-scale forcing fields are developed. The methodology is exemplarily applied to a standard EURO-CORDEX hindcast simulation with the Weather Research and Forecasting (WRF) model coupled with the land surface model NOAH at 0.11 ^° grid resolution. Monthly temperature climatology for the 1990–2009 period is analysed for Germany for winter and summer in comparison with high-resolution gridded observations from the German Weather Service. The field significance test controls the proportion of falsely rejected local tests in a meaningful way and is robust to spatial dependence. Hence, the spatial patterns of the statistically significant local tests are also meaningful. We interpret them from a process-oriented perspective. In winter and in most regions in summer, the downscaled distributions are statistically indistinguishable from the observed ones. A systematic cold summer bias occurs in deep river valleys due to overestimated elevations, in coastal areas due probably to enhanced sea breeze circulation, and over large lakes due to the interpolation of water temperatures. Urban areas in concave topography forms have a warm summer bias due to the strong heat islands, not reflected in the observations. WRF-NOAH generates appropriate fine-scale features in the monthly temperature field over regions of complex topography, but over spatially homogeneous areas even small biases can lead to significant deteriorations relative to the driving reanalysis. As the added value of global climate model (GCM)-driven simulations cannot be smaller than this perfect-boundary estimate, this work demonstrates in a rigorous manner the clear additional value of dynamical downscaling over global climate simulations. The evaluation methodology has a broad spectrum of applicability as it is distribution-free, robust to spatial dependence, and accounts for time series structure.     

Petrology of Lower Cretaceous carbonate mud mounds (Albian, N. Spain): insights into organomineralic deposits of the geological record

The process of organomineralization is increasingly well understood with respect to modern carbonate sediments accumulating adjacent to tropical reef atolls and reef caves. Mineralization related to non-living organic substrates results in autochthonous micrite production (‘automicrites’). ‘Automicrites’ are the main constructive element of Lower Cretaceous (Albian) carbonate mud mounds in northern Spain. These slope mud mounds occur within transgressive and early highstand system tracts encompassing several macrobenthic ecological zones. They are clearly separated from the biocalcifying carbonate factory (Urgonian carbonate platforms), in both space and time. Within these build-ups, most ‘automicrites’ were initially indurated and accreted to form a medium-relief growth framework. ‘Automicrites’ have a uniform, presumably high-Mg-calcite precursor mineralogy. They show an inorganic stable-isotope signature (?¹³C around +3·3‰) within the range of early marine cements, and skeletal compounds lacking major vital effects. Epifluorescence microscopy shows that they have facies-specific fluorescence, which is similar to skeletal compounds of Acanthochaetetes, but clearly different from allomicritic sediment and cements, which are mostly non-fluorescent. The EDTA-soluble intracrystalline organic fraction (SIOF) of Albian automicrites shows an amino acid spectrum that is similar to shallow subsurface samples from their modern counterparts. Gel electrophoresis of the SIOF demonstrates an exclusively acidic character, and a mean molecular size range between 20 and 30 kDa. Experiments in vitro (inhibition tests) indicate that the SIOF has a significant Ca²⁺-binding capacity. Fluorescence and chemical characteristics of SIOF point to a main substance class, such as humic and fulvic acids, compounds that form from pristine organic matter during early diagenesis. Biomarker analyses provide evidence for the crucial role of biodegradation by heterotrophic microorganisms, but no biomarker for cyanobacteria has been found. Primary sources of organic material should have been manifold, including major contributions by metazoans such as sponges. It is concluded that many carbonate mud mounds are essentially organomineralic in origin and that the resulting fabric of polygenetic muds (‘polymuds’) may represent ancestral metazoan reef ecosystems, which possibly originated during the Neoproterozoic.     

Solute transport by surface runoff from low‐angle slopes: theory and application

The removal of chemicals in solution by overland flow from agricultural land has the potential to be a significant source of chemical loss where chemicals are applied to the soil surface, as in zero tillage and surface‐mulched farming systems. Currently, we lack detailed understanding of the transfer mechanism between the soil solution and overland flow, particularly under field conditions. A model of solute transfer from soil solution to overland flow was developed. The model is based on the hypothesis that a solute is initially distributed uniformly throughout the soil pore space in a thin layer at the soil surface. A fundamental assumption of the model is that at the time runoff commences, any solute at the soil surface that could be transported into the soil with the infiltrating water will already have been convected away from the area of potential exchange. Solute remaining at the soil surface is therefore not subject to further infiltration and may be approximated as a layer of tracer on a plane impermeable surface. The model fitted experimental data very well in all but one trial. The model in its present form focuses on the exchange of solute between the soil solution and surface water after the commencement of runoff. Future model development requires the relationship between the mass transfer parameters of the model and the time to runoff to be defined. This would enable the model to be used for extrapolation beyond the specific experimental results of this study. The close agreement between experimental results and model simulations shows that the simple transfer equation proposed in this study has promise for estimating solute loss to surface runoff. Copyright © 2000 John Wiley & Sons, Ltd.     

Assessing Possibilities and Limitations for Biomarker Analyses on Outcrop Samples: A Case Study on Carbonates of the Shibantan Member (Ediacaran Period,Dengying Formation,South China)

Abstract: The Shibantan Member (Dengying Formation, South China) represents one of only two carbonate settings with Ediacara-type organisms and offers a rare opportunity to study the biogeochemistry of these ecosystems. To evaluate possibilities and limitations for future biomarker studies on fossil-bearing outcrop samples of the Shibantan Member, we analysed the spatial distribution of hydrocarbons in extractable organic matter (i.e. bitumen) on a millimetre scale. Our study demonstrates that the sample and most likely also other rocks from the same setting are contaminated with petroleum-derived compounds that bear the potential for erroneous interpretations in palaeo-reconstructions. The contamination was revealed by distribution patterns and amounts of extractable n-alkanes and acyclic isoprenoids. The contamination is linked to the external weathering surfaces but also to cracks within the rock, and the extent most likely depends on concentration gradients between these contamination sources. Here we show that contamination can successfully be distinguished from syngenetic signals obtained from non-extractable organic matter (i.e. kerogen) using catalytic hydropyrolysis (HyPy). However, we observed that decalcification is necessary to achieve sufficient yields of kerogen-bound hydrocarbons and to avoid artificial alteration of the biomarker signals due to matrix effects.     

Late Hercynian polymetallic vein-type base-metal mineralization in the Iberian Pyrite Belt: fluid-inclusion and stable-isotope geochemistry (S–O–H–Cl)

Late Variscan vein-type mineralization in the Iberian Pyrite Belt, related to the rejuvenation of pre-existing fractures during late Variscan extensional tectonism, comprises pyrite–chalcopyrite, quartz–galena–sphalerite, quartz–stibnite–arsenopyrite, quartz–pyrite, quartz–cassiterite–scheelite, fluorite–galena–sphalerite–chalcopyrite, and quartz–manganese oxide mineral assemblages. Studies of fluid inclusions in quartz, stibnite, and barite as well as the sulfur isotopic compositions of stibnite, galena, and barite from three occurrences in the central part of the Iberian Pyrite Belt reveal compelling evidence for there having been different sources of sulfur and depositional conditions. Quartz–stibnite mineralization formed at temperatures of about 200 °C from fluids which had undergone two-phase separation during ascent. Antimony and sulfide are most probably derived by alteration of a deeper lying, volcanic-hosted massive sulfide mineralization, as indicated by δ³⁴S signatures from ?1.45 to ?2.74‰. Sub-critical phase separation of the fluid caused extreme fractionation of chlorine isotopes (δ³⁷Cl between ?1.8 and 3.2‰), which correlates with a fractionation of the Cl/Br ratios. The source of another high-salinity fluid trapped in inclusions in late-stage quartz from quartz–stibnite veins remains unclear. By contrast, quartz–galena veins derived sulfide (and metals?) by alteration of a sedimentary source, most likely shale-hosted massive sulfides. The δ³⁴S values in galena from the two study sites vary between ?15.42 and ?19.04‰. Barite which is associated with galena has significantly different δ³⁴S values (?0.2 to 6.44‰) and is assumed to have formed by mixing of the ascending fluids with meteoric water.     

Spatial variability of phytoplankton, nutrients and new production estimates in the waters around Svalbard

Phytoplankton dynamics and carbon input into Arctic and sub-Arctic ecosystems were investigated around Svalbard, in summer 1991. Phytoplankton biomass, species composition and dissolved nutrient concentrations were analysed from water samples collected along seven transects. Phytoplankton biomass was low especially to the north (Chlorophyll-a mean 0.3 pg 1- '), where flagellates dominated the communities and only ice-diatoms were present. To the west, the phytoplankton composition was representative of a summer Atlantic community, in a post-bloom state. Zooplankton grazing, mainly by copepods, appeared to be the main control on biomass to the west and north of Svalbard.
In the Barents Sea (east of Svalbard), an ice edge bloom was observed (Chlorophyll-a max. 6.8 pgl-') and the ice edge receded at a rate of approximately 1 1 km day-'. The phytoplankton community was represented by marginal ice species, especially Phaeocystis poucherii and Chaeroceros socialis. South of the ice edge, Deep Chlorophyll Maxima (DCM) were observed, as surface waters became progressively nutrient-depleted. In these surface waters, the phytoplankton were predominantly auto- and heterotrophic flagellates.
Carbon production measurements revealed high net production (new and regenerated) to the north of the Barents Sea Polar Front (BSPF); it was especially high at the receding ice edge (reaching 1.44gC m-'day-'). To the south, a low level of production was maintained, mainly through regenerative processes.