Osumilite–melt interactions in ultrahigh temperature granulites: phase equilibria modelling and implications for the P–T–t evolution of the Eastern Ghats Province,India

The exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine‐grained mineral intergrowths comprising cordierite + K‐feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close‐to‐isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post‐peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter‐clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar^?1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step‐wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post‐peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close‐to‐isobaric cooling.     

Transient Structures and Stream Interaction Regions in the Solar Wind: Results from EISCAT Interplanetary Scintillation, STEREO HI and Venus Express ASPERA-4 Measurements

We discuss the detection and evolution of a complex series of transient and quasi-static solar-wind structures in the days following the well-known comet 2P/Encke tail disconnection event in April 2007. The evolution of transient solar-wind structures ranging in size from <10⁵ km to >10⁶ km was characterised using one-minute time resolution observation of Interplanetary Scintillation (IPS) made using the European Incoherent SCATter (EISCAT) radar system. Simultaneously, the global structure and evolution of these features was characterised by the Heliospheric Imagers (HI) on the Solar TERrestrial RElations Observatory (STEREO) spacecraft, placing the IPS observations in context. Of particular interest was the observation of one transient in the slow wind, apparently being swept up and entrained by a Stream Interaction Region (SIR). The SIR itself was later detected in-situ at Venus by the Analyser of Space Plasma and Energetic Atoms (ASPERA-4) instrument on the Venus Express (VEX) spacecraft. The availability of such diverse data sources over a range of different time resolutions enables us to develop a global picture of these complex events that would not have been possible if these instruments were used in isolation. We suggest that the range of solar-wind transients discussed here may be the interplanetary counterparts of transient structures previously reported from coronagraph observations and are likely to correspond to transient magnetic structures reported in in-situ measurements in interplanetary space. The results reported here also provide the first indication of heliocentric distances at which transients become entrained.     

Cassini spectra and photometry 0.25-5.1 μm of the small inner satellites of Saturn

The nominal tour of the Cassini mission enabled the first spectra and solar phase curves of the small inner satellites of Saturn. We present spectra from the Visual Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS) that span the 0.25-5.1 μm spectral range. The composition of Atlas, Pandora, Janus, Epimetheus, Calypso, and Telesto is primarily water ice, with a small amount (∼5%) of contaminant, which most likely consists of hydrocarbons. The optical properties of the “shepherd” satellites and the coorbitals are tied to the A-ring, while those of the Tethys Lagrangians are tied to the E-ring of Saturn. The color of the satellites becomes progressively bluer with distance from Saturn, presumably from the increased influence of the E-ring; Telesto is as blue as Enceladus. Janus and Epimetheus have very similar spectra, although the latter appears to have a thicker coating of ring material. For at least four of the satellites, we find evidence for the spectral line at 0.68 μm that Vilas et al. [Vilas, F., Larsen, S.M., Stockstill, K.R., Gaffley, M.J., 1996. Icarus 124, 262-267] attributed to hydrated iron minerals on Iapetus and Hyperion. However, it is difficult to produce a spectral mixing model that includes this component. We find no evidence for CO₂ on any of the small satellites. There was a sufficient excursion in solar phase angle to create solar phase curves for Janus and Telesto. They bear a close similarity to the solar phase curves of the medium-sized inner icy satellites. Preliminary spectral modeling suggests that the contaminant on these bodies is not the same as the exogenously placed low-albedo material on Iapetus, but is rather a native material. The lack of CO₂ on the small inner satellites also suggests that their low-albedo material is distinct from that on Iapetus, Phoebe, and Hyperion.     

The use of macroalgal species richness and composition on intertidal rocky seashores in the assessment of ecological quality under the European Water Framework Directive

The EC Water Framework Directive (WFD) suggests using abundance and species composition of intertidal seaweed communities for ecological quality classification of rocky seashores. There are two difficulties with this. According to WFD all sensitive species should be present on a shore. There is no accepted list of sensitive seaweed species and those which may be sensitive in one location may not be so in another. Second, natural successions can result in very large abundance changes of common species, e.g. from almost completely fucoid-dominated shores to almost totally barnacle-dominated shores, without any change in ecological quality. Studies have shown that numerical species richness, not the list of actual species present, is broadly constant in the absence of disturbance. The ephemeral species, possibly the sensitive members of the community, change regularly in such a way as to conserve species richness. It is proposed that species richness on a defined length of shore be used as a criterion of ecological quality. A database of species found on over 300 shores in the British Isles, under strictly controlled sampling conditions, has given ranges of values of species richness to be expected and has allowed for variations in these values due to sub-habitat variability, wave exposure and turbidity to be factored in. A major problem in applying such a tool is the lack of expertise of many workers in critical identification of seaweed species. A reduced species list has been extracted from the database using species commonly present and identifiable with reasonable certainty. A numerical index of ecological quality is proposed based on scores for various aspects of the physical nature of the habitat combined with a score for species richness which may be based on the reduced species list. The scoring system also uses further aspects of community structure, such as ecological status groups and the proportions of rhodophyta, chlorophyta and opportunist species. For this system to be effective there has to be close control of the way in which sampling is carried out to ensure a uniform level of thoroughness.     

A century of change: Reconstructing the biogeochemical history of Hubbard Brook

Ecosystems constantly adjust to altered biogeochemical inputs, changes in vegetation and climate, and previous physical disturbances. Such disturbances create overlapping ‘biogeochemical legacies’ affecting modern nutrient mass balances. To understand how ‘legacies’ affected watershed-ecosystem (WEC) biogeochemistry during five decades of studies within the Hubbard Brook Experimental Forest (HBEF), we extended biogeochemical trends and hydrologic fluxes back to 1900 to provide an historical framework for our long-term studies. This reconstruction showed acid rain peaking at HBEF in the late 1960s-early 1970s near the beginning of the Hubbard Brook Ecosystem Study (HBES). The long-term, parabolic arc in acid inputs to HBEF generated a corresponding arc in the ionic strength of stream water, with acid inputs generating increased losses of H⁺ and soil base cations between 1963 and 1969 and then decreased losses after 1970. Nitrate release after disturbance is coupled with previous N-deposition and storage, biological uptake, and hydrology. Sulfur was stored in soils from decades of acid deposition but is now nearly depleted. Total exports of base cations from the soil exchange pool represent one of the largest disturbances to forest and associated aquatic ecosystems at the HBEF since the Pleistocene glaciation. Because precipitation inputs of base cations currently are extremely small, such losses can only be replaced through the slow process of mineral weathering. Thus, the chemistry of stream water is extremely dilute and likely to become even more dilute than pre-Industrial Revolution estimates. The importance of calculating chemical fluxes is clearly demonstrated in reconstruction of acid rain impacts during the pre-measurement period. The aggregate impact of acid rain on WEC exports is far larger than historical forest harvest effects, and even larger than the most severe deforestation experiment (Watershed 2) at HBEF. A century of acid rain had a calcium stripping impact equivalent to two W2 experiments involving complete deforestation and herbicide applications.     

A high-resolution record of climate variability and landscape response from Kettle Lake,northern Great Plains,North America

A decadal-scale multiproxy record of minerals, pollen, and charcoal from Kettle Lake, North Dakota provides a high-resolution record of climate and vegetation change spanning the entire Holocene from the northern Great Plains (NGP) in North America. The chronology is established by over 50 AMS radiocarbon dates. This record exhibits millennial-scale trends evident in other lower-resolution studies, but with much more detail on short-term climate variability and on the rapidity and timing of major climatic shifts. As a proxy for precipitation, we utilize the rate of endogenic carbonate sedimentation, which depends on groundwater inflow, which in turn depends on precipitation. Independent cluster analyses of mineral and pollen data reveal major Holocene mode shifts at 10.73 ka (ka = cal yr BP), 9.25 ka, and 4.44 ka.The early Holocene, 11.7–9.25 ka, was generally wet, with perhaps a trend to higher evaporation associated with warming temperatures. A switch from calcite to aragonite deposition associated with a severe, but brief drought occurred at 10.73 ka. From 10.73 ka to 9.25 ka, climate was generally humid but punctuated at 100–300 yr intervals by brief droughts, including the most severe drought of the entire Holocene at 9.25 ka. This event was coeval with the 9.3–9.2 ka event in the Greenland ice cores and observed at a number of sites worldwide. In contrast, the prominent 8.2 ka event in Greenland is not remarkable at Kettle Lake. The prominence of the 9.25 event locally in the NGP may be due to a major drawdown and northward retreat of Lake Agassiz at this time, reducing its mesoclimatic effect on the NGP and thrusting the region into an insolation controlled regime.The mid-Holocene, 9.25–4.44 ka, was characterized by great variability in moisture on a multi-decadal scale, with severe droughts alternating with more humid periods. The high abundance of the weedy but drought intolerant Ambrosia generally during the mid-Holocene and specifically during the multi-decadal drought periods is seemingly paradoxical, but can be explained by high interannual variability of moisture overlaid on the multi-decadal variability.The late Holocene, 4.44 ka–present, was also characterized by multi-decadal variability in moisture, but was generally wetter than the mid-Holocene and the magnitude of variability was less. The trends in wet-dry mineral, pollen, and charcoal proxies were similar to the mid-Holocene, but late Holocene mineral-pollen assemblages are distinct from mid-Holocene. The shift to wetter climate in the late Holocene was more gradual than the abrupt shift to arid conditions 9.25 ka, which may explain the asymmetric retreat and readvance of forest along the eastern margin of the NGP.Precipitation variations in the NGP have been linked with Pacific and Atlantic sea-surface temperatures, and mid-Holocene drought in the NGP has been linked with sustained La Niña-like conditions in the Pacific. These linkages may explain the decadal- and millennial-scale variations seen in the NGP, but cause of the prominent century-scale variations remains elusive.     

Palaeoecological,archaeological and historical data and the making of Devon landscapes. I. The Blackdown Hills

This paper presents the first systematic study of the vegetation history of a range of low hills in SW England, UK, lying between more researched fenlands and uplands. After the palaeoecological sites were located bespoke archaeological, historical and documentary studies of the surrounding landscape were undertaken specifically to inform palynological interpretation at each site. The region has a distinctive archaeology with late Mesolithic tool scatters, some evidence of early Neolithic agriculture, many Bronze Age funerary monuments and Romano‐British iron‐working. Historical studies have suggested that the present landscape pattern is largely early Medieval. However, the pollen evidence suggests a significantly different Holocene vegetation history in comparison with other areas in lowland England, with evidence of incomplete forest clearance in later‐Prehistory (Bronze?Iron Age). Woodland persistence on steep, but poorly drained, slopes, was probably due to the unsuitability of these areas for mixed farming. Instead they may have been under woodland management (e.g. coppicing) associated with the iron‐working industry. Data from two of the sites also suggest that later Iron Age and Romano‐British impact may have been geographically restricted. The documented Medieval land management that maintained the patchwork of small fields, woods and heathlands had its origins in later Prehistory, but there is also evidence of landscape change in the 6th–9th centuries AD. We conclude that the Blackdown Hills area was one of many ‘distinctive subregions’, which due to a combination of edaphic, topographic and cultural factors could qualify as an eco‐cultural region or ‘pays’. It is argued that the use of such eco‐culturally distinctive regions or pays can provide a spatial and archaeological framework for palaeoecology, which has implications for landscape research, designation and heritage management.     

Watershed studies at the Hubbard Brook Experimental Forest: Building on a long legacy of research with new approaches and sources of data

The Hubbard Brook Experimental Forest (HBEF) was established in 1955 by the U.S. Department of Agriculture, Forest Service out of concerns about the effects of logging increasing flooding and erosion. To address this issue, within the HBEF hydrological and micrometeorological monitoring was initiated in small watersheds designated for harvesting experiments. The Hubbard Brook Ecosystem Study (HBES) originated in 1963, with the idea of using the small watershed approach to study element fluxes and cycling and the response of forest ecosystems to disturbances, such as forest management practices and air pollution. Early evidence of acid rain was documented at the HBEF and research by scientists at the site helped shape acid rain mitigation policies. New lines of investigation at the HBEF have built on the long legacy of watershed research resulting in a shift from comparing inputs and outputs and quantifying pools and fluxes to a more mechanistic understanding of ecosystem processes within watersheds. For example, hydropedological studies have shed light on linkages between hydrologic flow paths and soil development that provide valuable perspective for managing forests and understanding stream water quality. New high frequency in situ stream chemistry sensors are providing insights about extreme events and diurnal patterns that were indiscernible with traditional weekly sampling. Additionally, tools are being developed for visual and auditory data exploration and discovery by a broad audience. Given the unprecedented environmental change that is occurring, data from the small watersheds at the HBEF are more relevant now than ever and will continue to serve as a basis for sound environmental decision-making.