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Copenhagen, Denmark

Van der Meer Q.H.A.,VU University Amsterdam | Klaver M.,VU University Amsterdam | Waight T.E.,Oster Voldgade | Davies G.R.,VU University Amsterdam

Petrological, whole rock major element and mineral chemical analysis of mantle xenoliths from the Venetia kimberlite pipes (533. Ma) in South Africa reveals an apparently stratified cratonic mantle beneath the Central Zone of the Limpopo Mobile Belt (LMB) that separates the Kaapvaal and Zimbabwe Cratons. Combined pressure-temperature (P-T) data and petrographic observations indicate that the mantle consists of an upper layer of Low-T coarse-equant garnet + spinel lherzolite (~. 50 to ~. 130. km depth). This layer is underlain by a region of mixed garnet harzburgites and garnet lherzolites that are variably deformed (~. 130 to ~. 235. km depth). An equilibrated geotherm did not exist at the time of kimberlite eruption (533. Ma) and a localised heating event involving the introduction of asthenospheric material to the High-T lithosphere below 130. km is inferred. Low-T garnet-spinel lherzolites are highly melt depleted (40% on average). In contrast, the High-T lithosphere (mostly at diamond stable conditions) consists of a mixed zone of variably sheared and melt depleted (30% on average) garnet harzburgite and mildly melt depleted (20% on average) garnet lherzolite. The chemistry of the High-T xenoliths contrasts with that of minerals included in diamond originating from the same depth. Inclusions suggest diamond crystallisation in a more melt depleted lithosphere than represented by either Low- or High-T xenoliths. High-T xenoliths are proposed to represent formerly melt depleted lithosphere, refertilised by asthenosphere-derived melts during the diapiric rise of a proto-kimberlitic melt pocket. This process is coupled to the positive temperature perturbation observed in the High-T xenoliths and may represent a common process in the lower lithosphere related to localised but intense tectono-magmatic events immediately preceding kimberlite eruption. The presence of clinopyroxene, garnet and abundant orthopyroxene in the Low-T lherzolite implies a history of melt depletion followed by metasomatic addition of Si-Al-Ca, forming high-temperature orthopyroxene from which clinopyroxene and garnet exsolved. Si enrichment is a characteristic feature of the majority of the Kaapvaal Craton to the south of the LMB but not of the Zimbabwe Craton to the north, implying a Kaapvaal origin. The provenance of the High-T lithosphere beneath the LMB is less well constrained as it is intensely modified by kimberlitic magmatism and diamond inclusion chemistry does not show significant systematic variation across the cratons. The presence of rare, mildly silica enriched high-temperature harzburgites suggests that a Kaapvaal origin for the entire lithosphere beneath the LMB is most likely. © 2013 Elsevier B.V. Source

Lindstrom S.,Geological Survey of Denmark | van de Schootbrugge B.,Goethe University Frankfurt | Dybkjaer K.,Geological Survey of Denmark | Pedersen G.K.,Oster Voldgade | And 3 more authors.

Profound changes in both marine and terrestrial biota during the end-Triassic mass extinction event and associated successive carbon cycle perturbations across the Triassic-Jurassic boundary (T-J, 201.3 Ma) have primarily been attributed to volcanic emissions from the Central Atlantic Magmatic Province and/or injection of methane. Here we present a new extended organic carbon isotope record from a cored T-J boundary succession in the Danish Basin, dated by high-resolution palynostratigraphy and supplemented by a marine faunal record. Correlated with reference C-isotope and biotic records from the UK, it provides new evidence that the major biotic changes, both on land and in the oceans, commenced prior to the most prominent negative C-isotope excursion. If massive methane release was involved, it did not trigger the end-Triassic mass extinction. Instead, this negative C-isotope excursion is contemporaneous with the onset of floral recovery on land, whereas marine ecosystems remained perturbed. The decoupling between ecosystem recovery on land and in the sea is more likely explained by long-term flood basalt volcanism releasing both SO2 and CO2 with short- and long-term effects, respectively. © 2012 Geological Society of America. Source

Van As D.,Geological Survey of Denmark | Hubbard A.L.,Aberystwyth University | Hasholt B.,Oster Voldgade | Mikkelsen A.B.,Oster Voldgade | And 2 more authors.

This study uses data from six on-ice weather stations, calibrated MODIS-derived albedo and proglacial river gauging measurements to drive and validate an energy balance model. We aim to quantify the record-setting positive temperature anomaly in 2010 and its effect on mass balance and runoff from the Kangerlussuaq sector of the Greenland ice sheet. In 2010, the average temperature was 4.9 A°C (2.7 standard deviations) above the 1974-2010 average in Kangerlussuaq. High temperatures were also observed over the ice sheet, with the magnitude of the positive anomaly increasing with altitude, particularly in August. Simultaneously, surface albedo was anomalously low in 2010, predominantly in the upper ablation zone. The low albedo was caused by high ablation, which in turn profited from high temperatures and low winter snowfall. Surface energy balance calculations show that the largest melt excess (∼170%) occurred in the upper ablation zone (above 1000 m), where higher temperatures and lower albedo contributed equally to the melt anomaly. At lower elevations the melt excess can be attributed to high atmospheric temperatures alone. In total, we calculate that 6.6 ± 1.0 km3 of surface meltwater ran off the ice sheet in the Kangerlussuaq catchment in 2010, exceeding the reference year 2009 (based on atmospheric temperature measurements) by ∼150%. During future warm episodes we can expect a melt response of at least the same magnitude, unless a larger wintertime snow accumulation delays and moderates the melt-albedo feedback. Due to the hypsometry of the ice sheet, yielding an increasing surface area with elevation, meltwater runoff will be further amplified by increases in melt forcings such as atmospheric heat. © Author(s) 2012. Source

Bruun T.B.,Copenhagen University | Egay K.,University Malaysia Sarawak | Mertz O.,Oster Voldgade | Magid J.,Copenhagen University
Agriculture, Ecosystems and Environment

Oil palm plantations are spreading rapidly throughout Southeast Asia and in some countries, they are promoted as carbon sinks compared to the swidden cultivation systems that they often replace. However, little is known about the impacts of this land use change on soil organic carbon (SOC) stocks or soil quality. This study uses resampling of archived soil samples to investigate the sensitivity of permanganate oxidizable carbon (Pox-C) concentration to a change in land use from swidden cultivation to small-scale oil palm plantation on an Ultisol in Sarawak, Malaysia. Furthermore, the results of two different methods of calculating SOC stocks are compared - namely the fixed depth approach and the equivalent soil mass approach, which is sensitive to changes in soil bulk density. Results show that using a method that is sensitive to changes in bulk density is important as the soil bulk density increases upon establishment of oil palm. Thus, topsoil carbon stocks significantly decreased 3-8 years after oil palm establishment as measured by the equivalent soil mass approach, but only marginally and insignificantly decreased according to the fixed depth approach. After 15 years of oil palm, carbon stocks were 40% lower according to the fixed depth approach but 50% lower when using the equivalent soil mass approach. Importantly, the resampling of geo-referenced soil gives more consistent data, and lends credibility to the observation of large reductions in SOC stocks. The concentration of Pox-C in the 0-10. cm layer declines exponentially as oil palm plantations age and can serve as an indicator of change in the soil ecosystem brought about by the investigated land use transition. Pox-C is not more sensitive to this change than standard SOC analyses, but it may serve as an inexpensive, fast and field-suitable means of estimating the SOC status of different land use systems. © 2013 Elsevier B.V. Source

Rasmussen K.,Oster Voldgade | D'haen S.,Oster Voldgade | Fensholt R.,Oster Voldgade | Fog B.,Oster Voldgade | And 4 more authors.
Regional Environmental Change

The Sahel has been the object of intensive international research since the drought of the early 1970s. A considerable part of the research has focused on environmental change in general and land degradation, land cover change and climate change in particular. Rich and diverse insights from many different scientific disciplines about these three domains have been put forward. One intriguing feature is that an agreement on the overall trends of environmental change does not appear to emerge: questions such as whether the Sahel is greening, cropland is encroaching on rangelands, drought persists remain contested in the scientific literature, and arguments are supported by contrasting empirical evidence. The paper explores the generic reasons behind this situation in a systematic manner. We distinguish between divergences in interpretations emerging from (1) conceptualizations, definitions and choice of indicators, (2) biases, for example, related to selection of study sites, methodological choices, measurement accuracy, perceptions among interlocutors, and selection of temporal and spatial scales of analysis. The analysis of the root causes for different interpretations suggests that differences in findings could often be considered as complementary insights rather than mutually exclusive. This will have implications for the ways in which scientific results can be expected to support regional environmental policies and contribute to knowledge production. © 2015 Springer-Verlag Berlin Heidelberg Source

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