Sulfur isotopic studies of pyrite from metasediments in the >2.6 Byr old Deer Lake green-stone sequence, Minnesota, have been conducted in order to evaluate the possible importance of sulfate reducing bacteria in sulfide formation. Pyrite occurs as ovules up to 2 cm in diameter within graphitic slates, and as fine disseminations in metagraywacke units. SEM studies indicate the pyrite is framboidal in morphology. values of pyrite from the Deer Lake sediments range from ?2.3 to 11.1‰, with a peak at ~ +2‰ Isotopic data are consistent with either high temperature inorganic reduction of circulating seawater sulfate, or low temperature bacterial reduction. However, the lack of sulfide bands or massive occurrences in the sediments, the restriction of pyrite mineralization to the sediments, and the absence of evidence for hot spring activity suggest that a diagenetic origin of pyrite is more feasible. Sulfide in such an environment would be produced principally by the action of sulfate reducing bacteria.Results of the study are in agreement with those of Goodwinet al. (1976) who suggest that dissimilatory sulfate reduction was operative in the Archean ocean some 2.75 Byr ago. 相似文献
Simon et al. (2005) reported low Ti3+/Ti4+ values in Ti-rich pyroxenes in the Wark-Lovering rim (WL) of a Leoville CAI (144A) as compared to the interior of the inclusion. These electron microprobe analyses were interpreted as evidence that the growth of the WL rim is the manifestation of an evolution to a more oxidizing environment. Further work by Simon et al. (2007) used XANES analyses to argue for higher Ti3+ abundances and interpreted the data of Simon et al. (2005) as the result of X-ray contamination by neighboring phases, specifically spinel. Late-stage alteration was also included as a possible explanation.To investigate further the oxidation state of Ti in WL rims, we re-analyzed Leoville 144A to obtain a more complete data set of Ti3+/Ti4+ values in the Wark-Lovering rims. We conducted experiments on spinel-mixing to determine whether this was a plausible explanation for the observed paucity of Ti3+ in WL rims. While we found a wider range of Ti3+/Ti4+ in these WL rim data than in our original study, our new data show that the original conclusion that rims are lower in Ti3+/Ti4+ than interiors remains valid. We conclusively rule out spinel-mixing as an explanation for our data, and we see no clear inconsistency between our electron microprobe data and the XANES data. The WL rim of CAI Ef3 was also analyzed by EMPA and compared to the results of Leoville 144A.To predict compositional consequences of this hypothesis, we constructed a reaction space between Ti-rich pyroxene in the WL rim, perovskite, Mg(g), Ca(g), O2(g), and SiO(g). We find the oxidation of Ti3+, coupled with Ti loss via perovskite formation, explains many features of WL rim EMPA analyses. We maintain that the WL rim pyroxenes are compositionally distinct from those in the interior, and are evidence of a more oxidizing environment during WL rim formation. 相似文献
The morphodynamics of shallow, vertically well-mixed estuaries, characterised by tidal flats and deeper channels, have been
investigated. This paper examines what contributes to flood/ebb-dominant sediment transport in localised regions through a
2D model study (using the TELEMAC modelling system). The Dyfi Estuary in Wales, UK has been used as a case study and, together
with idealised estuary shapes, shows that shallow water depths lead to flood dominance in the inner estuary whilst tidal flats
and deep channels cause ebb dominance in the outer estuary. For medium sands and with an artificially ‘flattened’ bathymetry
(i.e. no tidal flats), the net sediment transport switches from ebb-dominant to flood-dominant where the parameter a/h (local tidal amplitude ÷ local tidally averaged water depth) exceeds 1.2. Sea level rise will reduce this critical value
of a/h and also reduce the ebb-directed sediment transport significantly, leading to a flood-dominated estuarine system. A similar
pattern, albeit with greater transport, was simulated with tidal flats included and also with a reduced grain size. This suggests
that analogous classifications for flood/ebb asymmetry of the tide in estuaries as a whole may not represent the local sediment
transport in sufficient detail. Through the Dyfi simulations, the above criterion involving a/h is shown to be complicated further by augmented flow past a spit at the estuary mouth which gives rise to a self-maintaining
scour hole. Simulations of one year of bed evolution in an idealised flat-bottomed estuary, including tidal flow past a spit,
recreate the flood/ebb dominance on either side of the spit and the formation of a scour hole in between. The erosion rate
at the centre of the hole is reduced as the hole deepens, suggesting the establishment of a self-maintaining equilibrium state. 相似文献
Cristobalite is commonly found in the dome lava of silicic volcanoes but is not a primary magmatic phase; its presence indicates that the composition and micro-structure of dome lavas evolve during, and after, emplacement. Nine temporally and mineralogically diverse dome samples from the Soufrière Hills volcano (SHV), Montserrat, are analysed to provide the first detailed assessment of the nature and mode of cristobalite formation in a volcanic dome. The dome rocks contain up to 11 wt.% cristobalite, as defined by X-ray diffraction. Prismatic and platy forms of cristobalite, identified by scanning electron microscopy (SEM), are commonly found in pores and fractures, suggesting that they have precipitated from a vapour phase. Feathery crystallites and micro-crystals of cristobalite and quartz associated with volcanic glass, identified using SEM-Raman, are interpreted to have formed by varying amounts of devitrification. We discuss mechanisms of silica transport and cristobalite formation, and their implications for petrological interpretations and dome stability. We conclude: (1) that silica may be transported in the vapour phase locally, or from one part of the magmatic system to another; (2) that the potential for transport of silica into the dome should not be neglected in petrological and geochemical studies because the addition of non-magmatic phases may affect whole rock composition; and (3) that the extent of cristobalite mineralisation in the dome at SHV is sufficient to reduce porosity—hence, permeability—and may impact on the mechanical strength of the dome rock, thereby potentially affecting dome stability. 相似文献
Mesozoic ribbon radiolarites have no parallel in rocks cored from extant ocean basins; their mineralogy is comparable but their repetitively bedded aspect is unique. The peculiarities of the chert-argillite couplet we relate primarily to variations in surface-water productivity on a tens-of-thousands-of-years scale, to a lesser extent to redeposition from turbidity and particularly bottom currents, and diagenesis, the latter being accentuated by the pressure-temperature gradients imposed during major tectonic events: ribbon cherts are unique to orogenic belts.
We suggest that these peculiar siliceous rocks were formed, as deep-water deposits (traces of sulphate minerals notwithstanding), in small basins of various types; arc-related regions (e.g. the Great Valley Sequence and, possibly, the Franciscan of California) and embryonic oceans dominated by transform faulting such as those of the western Tethys. Given that diatoms have replaced radiolarians in processing most of the World Ocean's dissolved silica and that some Tertiary-Recent diatomites were deposited at comparable rates (expressed as g/cm2 103 years) to Mesozoic ribbon radiolarites it is instructive to discover that certain present-day sites of diatomaceous sedimentation (e.g. Sea of Okhotsk, Bering Sea, Japan Sea and Gulf of California) provide tectonic analogues for Mesozoic siliceous basins.
In these settings, namely arc-related or transform-dominated, the first significant depositional interface probably lay at a depth of 3–3.5 km which was below the Late Jurassic (pre-Tithonian) calcite compensation depth, favouring sedimentation of an entirely siliceous record: thus most Jurassic ophiolites show a basalt-chert contact. The Cretaceous radiolarites of Cyprus and the Middle East which also rest on basalt probably result, however, from local carbonate dissolution in lava-hollows by debouching low-pH hydrothermal fluids; they are not necessarily comparable with their Jurassic counterparts.
The prevalence of mid to Late Jurassic radiolarites in the Californian and Tethyan areas is attributed to heightened fertility and elevated CCD in small ocean basins rather than any factor favouring global radiolarian productivity at this time. Depth and sea-floor topography also controlled facies developments on the continental margins of the western Tethys, shallower-water regions being characterized by pelagic-carbonate sediments.
Mesozoic ribbon cherts thus provide a tantalizing record of local palaeoceanographic parameters, palaeotectonic regimes and sedimentary environments but they tell us little about the behaviour of the World Ocean during Jurassic and Cretaceous time. 相似文献
Understanding land use/land cover (LULC) effects on tropical soil infiltration is crucial for maximizing watershed scale hydro-ecosystem services and informing land managers. This paper reports results from a multiyear investigation of LULC effects on soil bulk infiltration in steep, humid tropical, and lowland catchments. A rainfall simulator applied water at measured rates on 2 × 6 m plots producing infiltration through structured, granulated, and macroporous Ferralsols in Panama's central lowlands. Time-lapse electrical resistivity tomography (ERT) helped to visualize infiltration depth and bulk velocity. A space-for-time substitution methodology allowed a land-use history investigation by considering the following: (a) a continuously heavy-grazed cattle pasture, (b) a rotationally grazed traditional cattle pasture, (c) a 4-year-old (y.o.) silvopastoral system with nonnative improved pasture grasses and managed intensive rotational grazing, (d) a 7 y.o. teak (Tectona grandis) plantation, (e) an approximately 10 y.o. secondary succession forest, (f) a 12 y.o. coffee plantation (Coffea canephora), (g) an approximately 30 y.o. secondary succession forest, and (h) a >100 y.o. secondary succession forest. Within a land cover, unique plot sites totalled two at (a), (c), (d), (e), and (g); three at (b); and one at (f) and (h). Our observations confirmed measured infiltration scale dependency by comparing our 12 m2 plot-scale measurements against 8.9 cm diameter core-scale measurements collected by others from nearby sites. Preferential flow pathways (PFPs) significantly increased soil infiltration capacity, particularly in forests greater than or equal to 10 y.o. Time-lapse ERT observations revealed shallower rapid bulk infiltration and increased rapid lateral subsurface flow in pasture land covers when compared with forest land covers and highlighted how much subsurface flow pathways can vary within the Ferralsol soil class. Results suggest that LULC effects on PFPs are the dominant mechanism by which LULC affects throughfall partitioning, runoff generation, and flow pathways. 相似文献
Climate-driven alterations of hydro-meteorological conditions can change river flow regimes and potentially affect the migration behaviour of fishes and the productivity of important fisheries in the Amazon basin, such as those for the continental-scale migratory goliath catfishes (Brachyplatystoma, Pimelodidae). In this study, we investigated hydrologic responses to climate change using a hydrologic model forced with climate inputs, which integrate historical (2001–2010) observations and general circulation model (GCM) projections under the emission scenario Representative Concentration Pathway 8.5. We developed an empirical model to characterize future (2090–2099) climate-change impacts on goliath catfish spawning migrations as a function of river flow depth dynamics at the upstream elevational limit of spawning (250 m) in headwater basins of the Amazon. The model results revealed spatially variable impacts of climate change on the catfish spawning migrations. The Marañón, Ucayali, Juruá, Purus, and Madeira basins had a predicted increase in the annual mean (3–8%) and maximum (1.1–4.9%) spawning migration rate (i.e., the fraction of fish that migrate to the spawning grounds in a day), mainly due to the lengthened rising phase of flow-driven migratory events during wet seasons. The Caquetá-Japurá, Putumayo-Içá, Napo, and Blanco rivers had predicted decreases (3–7%) in the mean migration rate because of decreases in the length of the rising season of flow depth and the frequency of migratory events. The predicted timing of fish spawning migrations (quantified by the temporal centroid of migration rates) was delayed by 7–10 days in the west-central and southwest regions and was 8 days earlier in the northwest and northcentral areas, due to changes in the onset of the rising season. We established a river depth baseline that controls the onset of goliath catfish spawning migration. This depth varies between 0.9–5.6 m across study sites. We found that the estimated depth baseline was most sensitive to uncertainties in river width and cross-sectional channel shape. These results may help inform sustainable adaptation strategies for ecosystem conservation and local fisheries management in the Amazon basin. 相似文献
Headwater forest ecosystems of the western USA generate a large portion of the dissolved organic matter (DOM) transported in streams across North America. Land cover changes that alter forest structure and species composition affect the quantity and composition of DOM transferred to aquatic ecosystems. Clear-cut harvesting affects ~1% of the forest area of North America annually, leaving most western forests in varying stages of regrowth and the total area of old-growth forest is decreasing. The consequences of this widespread management practice on watershed carbon cycling remain unknown. We investigated the role of land cover change, because of clear-cut harvesting, from mixed-species old-growth to lodgepole pine-dominated second-growth forest on the character and reactivity of hillslope DOM exports. We evaluated inputs of DOM from litter leachates and export of DOM collected at the base of trenched hillslopes during a 3-year period (2016–2018) at the Fraser Experimental Forest in north-central Colorado, USA. Dissolved organic carbon and total dissolved nitrogen were higher in lateral subsurface flow draining old- versus second-growth forest. Fluorescence spectroscopy showed that the DOM exported from the old-growth forest was more heterogeneous and aromatic and that proteinaceous, microbially processed DOM components were more prevalent in the second-growth forest. Biological oxygen demand assays revealed much lower microbial metabolism of DOM in litter leachate and subsurface exports from the old-growth forest relative to second growth. Old-growth and second-growth forests are co-mingled in managed ecosystems, and our findings demonstrate that land cover change from a mixture of conifer species to lodgepole pine dominance influences DOM inputs that can increase the reactivity of DOM transferred from terrestrial to aquatic ecosystems. 相似文献