Biological and granulometric controls on sedimentary organic matter of an intertidal mudflat

A variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year, in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter. Winter and summer were times of relatively fine-grained sediment accumulation, caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations. Stable carbon isotope and Br:C ratios indicated a planktonic source of bulk organic matter. Ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains. Correction for changing grain size during the year showed no change in the organic concentration per unit surface area, in spite of organic matter inputs by in situ primary production, buildup of heterotroph biomass and mucus coatings, and biodeposition of organic-rich seston. There were also no indications of changes in bulk organic quality, measured as hydrolyzable carbohydrates and amino acids, in response to these biological processes. It is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter.     

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

Pre-Cenozoic intra-plate magmatism along the Central Andes (17–34°S): Composition of the mantle at an active margin

Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial ¹⁴³Nd/¹⁴⁴Nd ( 0.5127–0.5128) and ⁸⁷Sr/⁸⁶Sr ( 0.7032–0.7040). The initial ²⁰⁶Pb/²⁰⁴Pb ratios are variable (18.5–19.7) at uniform ²⁰⁷Pb/²⁰⁴Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks (¹⁴³Nd/¹⁴⁴Nd  0.5123, ⁸⁷Sr/⁸⁶Sr  0.704, ²⁰⁶Pb/²⁰⁴Pb  17.5–18.5, and ²⁰⁷Pb/²⁰⁴Pb  15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.

Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism.     

Integrated and Participatory Research Approaches towards Sustainable Livelihoods and Ecosystems in Mountainous Regions

Mountainous regions cover about 27 per cent of the world’s land surface and are home to some 22 per cent of the global population (UNEP 2002). A much greater number of people depend on mountain environments for a wide range of services, including clean water, energy, timber, biodiversity, recreation, and protection from environmental hazards, such as landslides and floods. At the same time, mountain areas are extremely vulnerable ecosystems and under continuous threat of environmental degrad…     

Recent Changes Occurred in the Terminus of the Debris-covered Bilafond Glacier in the Karakoram Himalayas Using Remotely Sensed Images and Digital Elevation Models (1978-2011)

Recent changes occurred in terminus of the debris-covered Bilafond Glacier in the Karakoram Range in the Himalayas, Northern Pakistan was investigated in this research. Landsat MSS, TM and ETM＋ images were used for this study. Digital elevation models derived from ASTER GDEM and SRTM were also utilized. Visible, infrared and thermal infrared channels were utilized in order to get accurate glacier change maps. Three methods were tried to map this debris-covered glacier in this research. The glacier has been mapped successfully and the changes in the glacier terminus from 1978 to 2011 have been calculated. Manual, semi-automatic and thermal methods were found to give similar results. It was found that the glacier has undergone serious ablation during this period despite of the fact that many of the larger glaciers in the Hindu Kush and Karakoram mountain regions in the Upper Indus Basin were reported to be expanding. The terminus has been moved back about 600 meters during this period and there was an abrupt change in the glacier terminus during 1990-2002. We propose that debris thickness is not the only factor that influences the glacier ablation but the altitude of the debris-covered glacier as well. Many glaciers in the Karakoram region reported to be expanding were having higher altitudes compared to the study area.     

Different styles of metasomatic veining in ultramafic xenoliths from the TUBAF Seamount (Bismarck Microplate,Papua New Guinea)

Petrologic, geochemical and isotopic investigations on two ultramafic xenoliths with metasomatic veins from the TUBAF Seamount in the Bismarck Archipelago NE of Papua New Guinea reveal different styles of metasomatic overprinting. The first xenolith, a clinopyroxene–poor spinel lherzolite, was part of the depleted upper mantle. It contains an orthopyroxene-rich vein that formed by hydrous metasomatism at ~ 980 °C and ~ 1.5 GPa. The second xenolith is a clinopyroxene-dominated spinel olivine websterite that formed as a magmatic cumulate at the transition of the upper mantle to the oceanic crust. The websterite contains a vein with orthopyroxenes and clinopyroxenes, which give evidence for high-temperature crystallization at ~ 1300 °C and < 0.36 GPa. Both xenoliths were transported to the seafloor by a Quaternary trachybasalt in a fore-arc position. The vein minerals show a strong affinity to a supra-subduction zone or island arc setting. The REE pattern of the vein in the clinopyroxene–poor lherzolite strongly resembles the one from the host trachybasalt, with a high enrichment of the LREE and a strong to moderate enrichment of the MREE and HREE. Although broadly similar in shape, the REE pattern of the vein in the websterite shows a much weaker enrichment. The same applies to the trace-element patterns, although there are significant differences in the Eu, Zr, Hf and Nb concentrations. The isotope signatures of both veins suggest a derivation from a subducted slab that had been hydrothermally altered by seawater (high ⁸⁷Sr/⁸⁶Sr values).The contrasting crystallization temperatures of the vein minerals as well as their overall geochemical differences indicate that the metasomatic agents responsible for the vein in the websterite were mobilized from a previously depleted source at a much deeper mantle level than those forming the vein of the clinopyroxene–poor lherzolite. The metasomatic agents may also have been mobilized at different times and from different plates, i.e., the deeply subducted Solomon Sea Microplate (for the veins in the websterite) and the shallow dehydrating Pacific Plate (for the veins in the clinopyroxene–poor lherzolite).Metasomatic agents responsible for similar petrologic phenomena, i.e., modal or cryptic metasomatism, may have distinctly different origins and show contrasting histories. A strongly depleted lherzolite may totally lose its initial geochemical signature by the influence of an enriched metasomatic agent, whereas a primarily enriched ultramafic rock, e.g., a websterite, may strongly obscure the trace-element pattern of a less enriched metasomatic vein. Furthermore, the geochemistry of the ultramafic xenoliths may reflect polyphase cryptic and modal metasomatism related to veining and later transport by the hosting melt to the seafloor.     

Warming the early earth—CO2 reconsidered

Despite a fainter Sun, the surface of the early Earth was mostly ice-free. Proposed solutions to this so-called “faint young Sun problem” have usually involved higher amounts of greenhouse gases than present in the modern-day atmosphere. However, geological evidence seemed to indicate that the atmospheric CO₂ concentrations during the Archaean and Proterozoic were far too low to keep the surface from freezing. With a radiative-convective model including new, updated thermal absorption coefficients, we found that the amount of CO₂ necessary to obtain 273 K at the surface is reduced up to an order of magnitude compared to previous studies. For the late Archaean and early Proterozoic period of the Earth, we calculate that CO₂ partial pressures of only about 2.9 mb are required to keep its surface from freezing which is compatible with the amount inferred from sediment studies. This conclusion was not significantly changed when we varied model parameters such as relative humidity or surface albedo, obtaining CO₂ partial pressures for the late Archaean between 1.5 and 5.5 mb. Thus, the contradiction between sediment data and model results disappears for the late Archaean and early proterozoic.     

Sediment mobility in the Pomeranian Bight (Baltic Sea): a case study based on sidescan-sonar images and hydrodynamic modelling

Sidescan-sonar surveys were performed on a 2×4 km area of seafloor in the southern Baltic Sea (Pomeranian Bight) in 1996 and 1998. Overlapping sub-areas of the individual surveys showing characteristic details were processed into geographically referenced mosaics. Sediment types were identified from echo characteristics and by comparison with granulometric data. The sea bottom covered by the mosaics consists predominantly of sand, with subordinate lag sediments with stones and small ripple fields consisting of coarse sand to fine gravel. A comparison of the two mosaics did not reveal any significant changes of the sea bottom over the 2-year period. Characteristic sedimentary features remained almost unchanged over this period on detailed sonar images of smaller sub-areas. Substantial transport of sandy sediments can thus be excluded in the course of the observation period. Grid files of advective velocity components and orbital velocity of wave motion of a three-dimensional hydrodynamical model for the period from September 1996 to October 1997 were used to estimate the current regime in the study area for the interval between the two sidescan surveys. Comparing critical velocities for the dominant sediment types with the results of the numerical bottom current simulations and the observations from sidescan images, it is apparent that strong current events during the modelled time interval were still too weak to resuspend and transport sand of any grain size, even though maximum current velocities of 30 cm/s at the seafloor were modelled. Only a few patches of newly accumulated (acoustically soft) material (mud, fluff and/or soft plant remnants), with a horizontal extension of about 10 m at a terrain step feature, were recognised in the 1998 mosaic. Our results imply that sand deposits in the southern Baltic Sea can remain stationary over time periods of several years, and that the transport of organic material, nutrients and associated pollutants to depositional areas in deeper water is predominantly accomplished by the movement of material finer than sand. A significant portion of this fine material is evidently transported in the bottom boundary layer under conditions of moderate hydrodynamical forcing.     

The Bering Sea—A dynamic food web perspective

The Bering Sea is a high-latitude, semi-enclosed sea that supports extensive fish, seabird, marine mammal, and invertebrate populations and some of the world's most productive fisheries. The region consists of several distinct biomes that have undergone wide-scale population variation, in part due to fisheries, but also in part due to the effects of interannual and decadal-scale climatic variation. While recent decades of ocean observation have highlighted possible links between climate and species fluctuations, mechanisms linking climate and population fluctuations are only beginning to be understood. Here, we examine the food webs of Bering Sea ecosystems with particular reference to some key shifts in widely distributed, abundant fish populations and their links with climate variation. Both climate variability and fisheries have substantially altered the Bering Sea ecosystem in the past, but their relative importance in shaping the current ecosystem state remains uncertain.     

Scanning electron microscopy,cathodoluminescence, and Raman spectroscopy of experimentally shock‐metamorphosed quartzite

Abstract— We studied unshocked and experimentally (at 12, 25, and 28 GPa, with 25, 100, 450, and 750°C pre‐shock temperatures) shock‐metamorphosed Hospital Hill quartzite from South Africa using cathodoluminescence (CL) images and spectroscopy and Raman spectroscopy to document systematic pressure or temperature‐related effects that could be used in shock barometry. In general, CL images of all samples show CL‐bright luminescent patchy areas and bands in otherwise nonluminescent quartz, as well as CL‐dark irregular fractures. Fluid inclusions appear dominant in CL images of the 25 GPa sample shocked at 750°C and of the 28 GPa sample shocked at 450°C. Only the optical image of our 28 GPa sample shocked at 25°C exhibits distinct planar deformation features (PDFs). Cathodoluminescence spectra of unshocked and experimentally shocked samples show broad bands in the near‐ultraviolet range and the visible light range at all shock stages, indicating the presence of defect centers on, e.g., SiO₄ groups. No systematic change in the appearance of the CL images was obvious, but the CL spectra do show changes between the shock stages. The Raman spectra are characteristic for quartz in the unshocked and 12 GPa samples. In the 25 and 28 GPa samples, broad bands indicate the presence of glassy SiO₂, while high‐pressure polymorphs are not detected. Apparently, some of the CL and Raman spectral properties can be used in shock barometry.