GEUS Bulletin (Geological Survey of Denmark and Greenland)
Not a member yet
521 research outputs found
Sort by
A data set of monthly freshwater fluxes from the Greenland ice sheet’s marine-terminating glaciers on a glacier–basin scale 2010–2020
No full textThe loss of mass from the Greenland ice sheet causes an increasing influx of freshwater to the Greenlandic fjords and the oceans. Freshwater fluxes from marine-terminating glaciers are important to understand fjord circulation and ecosystem dynamics. Here, we present a data set constructed by reformulating existing products into a shared temporal and spatial framework. We combine three publicly available data sets of solid-ice discharge (iceberg), liquid-surface runoff (runoff) and basal melt to present a cohesive overview of the flow of freshwater from marine-terminating glaciers to the Greenlandic fjords. We also calculate glacier drainage basins and compare our findings to previous studies showing that drainage-basin sizes may vary considerably depending on how they were reconstructed. The data set will be a valuable asset to oceanographic, glaciological and marine biological research activities
Preface
No full textThe Palaeogene Skaergaard intrusion, East Greenland, has since its first description by Wager & Deer (1939) been a foremost natural laboratory for the study of low-pressure fractionation of basaltic melt. Ocean floors are composed of basalt and the processes that control compositions of basaltic melts are fundamental to the dynamics of the Earth. This special issue of GEUS Bulletin by Peter Thy, Christian Tegner and Charles E. Lesher is the most recent in a more than eighty year succession of trendsetting works on the evolution of the Skaergaard intrusion and evolution of basaltic melt.
The early sample collections, housed in the universities of Oxford and Cambridge, were used for the development of fractionation models, mapping of the distribution as well as the partition of major and trace elements between melts and liquidus phases in basaltic melts. These and many other studies resulted in the monumental “Layered Igneous Rocks”, edited by L.R. Wager and G.M. Brown (Wager & Brown 1968).
By the 1970s, the original collection in the Oxford University Museum of Natural History and the Sedgwick Museum of Earth Sciences, University of Cambridge had seen extensive use. The science developed and new and more detailed sampling was now required. Fieldwork and sampling in the later part of the 20th century led to a flurry of new studies by more research groups, including Neil Irvine of Carnegie’s Geophysical Laboratory and the University of Oregon group lead by Alex McBirney. Despite all the efforts, no consensus was reached on the many fundamental chemical and physical processes in basaltic magma chambers.
The available collections did not provide sufficiently systematic and detailed information for the modelling of the evolution of the basaltic melt and the genesis of the precious metal deposit that had been discovered in the intrusion.
Exploration drill cores offered the possibility for tight stratigraphic sampling through >1000 m of gabbro. In combination with surface sampling, a new standard profile with superior systematics could be established. In the summer of 2000, the Danish Lithosphere Centre (DLC) organised fieldwork and the transport of the drill cores to the Museum of Natural History in Copenhagen. This special issue is part of the legacy of the DLC and a fulfilment of the then envisaged potential of the Skaergaard intrusion for fundamental studies of processes in basaltic magma chambers.
Thy et al. (2023, this volume) analyse and compile in unprecedented detail the variation in major and trace element compositions of bulk-rock samples and electron microprobe compositions of rock-forming minerals in the Layered Series on the floor of the intrusion. In addition, they include a wealth of information on all samples from the Skaergaard intrusion that are housed in museum, university and survey collections in Denmark, the UK and US, topographic and geological maps, aerial photographs in archives and images of thin sections used in the study.
With all its data and appendices, the monograph is the most comprehensive collection of petrological information for the Skaergaard intrusion and the Layered Series on the floor of the intrusion, ever to have been made available for research. The detailed modelling presented re-evaluates petrogenetic constraints and tests petrogenetic models in the literature.
The modelling is based on liquidus proportions established in experimental studies of appropriate melt compositions. Thy and co-authors conclude that evolution observed in the gabbros of the Layered Series is predominantly controlled by crystal fractionation of bulk liquid. Late in the evolution and above the boundary between Upper Zone a and b, the gabbros crystallised from ponded Fe-rich, immiscible melt. Redistribution of granophyric melts and decoupling of included and excluded trace elements, not only in bulk liquid but also in mush melts, complicates the modelling of trace element distributions in the late and evolved stages of crystallisation in the crystal mushes of the intrusion.
This special issue is a treasure trove for all who study the intrusion and are searching for a data set to be matched with experimental and numeric modelling of low-pressure fractionation of basaltic melt. With this contribution, the Skaergaard intrusion continues to be a foremost natural laboratory for modelling of melts formed in solidifying basaltic magma chambers.
Troels F.D. NielsenEmeritus, GEUS, Denmar
The PGE-Au Mineralisation of the Skaergaard intrusion: precious metal minerals, petrography and ore genesis
No full textThe Skaergaard PGE-Au Mineralisation, alias the Platinova Reef, is hosted in a series of mineralisation levels within a suite of bowl-shaped macrorhythmic layers in the upper Middle Zone of the Skaergaard intrusion. The intrusion is exposed 68°N in East Greenland. The occurrence defines its own type due to its exceptional structure and mineralogy. A wealth of mineralogical data is available in laboratory reports for individual samples and in peer-reviewed publications, but none of these account for the lateral and stratigraphic distribution of PGE and Au parageneses in the gabbros of the intrusion. In this study, we collate and describe the mineralogical data for the first-formed PGE-rich and last-formed gold-rich mineralisation levels and integrate these with petrogenetic models.
Recovery of >4000 grains of precious metal phases allow a detailed study of their distribution and compositions throughout the mineralisation, re-equilibration during cooling, inter-grain relationships and relationships to Cu-Fe sulphides and the gabbroic host rocks. The sulphides are dominated by bornite, chalcocite and minor chalcopyrite. All other sulphides, such as pentlandite, are very rare. Fifty-four different precious metal phases are identified in this study, and include the new IMA approved minerals skaergaardite (PdCu), nielsenite (Pd3Pb) and naldrettite (Pd2Sb). Precious metal phases include (1) intermetallic compounds and alloys of Cu and Pd; (2) intermetallic compounds and alloys of Au and Cu (Ag); (3) sulphides of Pd, Cu (Ag, Cd, Hg, Tl); (4) arsenides of Pd (Pt, Ni) and (5) intermetallic compounds of Pd, Cu with Sn, Pb, Te (Sb, Bi). Skaergaardite (PdCu) is the dominant PGE mineral in the lower and main PGE mineralisation level (Pd5). It is accompanied at the western margin of the intrusions by the sulphides vasilite (Pd16S7) and vysotskite (PdS) but is rare at the eastern margin, which is dominated by plumbide zvyagintsevite (Pd3Pb). Gold phases include a suite of intermetallic compounds and alloys from AuCu3 to native gold and are dominated by tetra-auricupride (AuCu). Gold is concentrated in the tops of individual mineralisation levels and in the uppermost precious metal–bearing mineralisation level, followed by stratiform Cu-rich mineralisation levels.
Precious metal parageneses demonstrate formation and re-equilibration from liquidus to subsolidus temperatures and control by local geochemical environments. The mineralisation is syn-magmatic and the result of fractionation and evolution in the remaining bulk-silicate liquid and crystal mushes. Fractionation led to sulphide saturation and formation of immiscible sulphide melt droplets. This was followed by reaction with mush melts and re-equilibration to lower temperatures, first under the roof and subsequently after slumping to the floor in mushes of macrorhythmic layers. Droplets of sulphide melt formed between 1030–1050°C and trapped precious metals. The subsequent reaction between sulphide melt and interstitial Fe-rich immiscible melt at c. 1015°C, and redistribution to coexisting melt and fluid, led to the separation of PGE, Au and Cu and their up- and inward transport. Magmatic fluids as well as volatile-rich residual silicate melts were retained in gabbros at the margins and resulted in precious metal parageneses in equilibrium with hydrous low-temperature silicate parageneses
X-ray fluorescence (XRF) fingerprinting of Palaeogene deposits in Denmark
No full textIn this study, we test if cost-efficient X-ray fluorescence (XRF) analyses can be used to fingerprint Palaeogene clay and marl deposits in Denmark. A total of 67 samples from key sites in Denmark have been analysed. Our preliminary results indicate that it is possible locally within 10–30 km to distinguish between most of the Palaeogene units, but on a regional scale across Denmark, the units are not unique, and this probably reflects variations in clay mineralogy, grain size and calcareous content. Accordingly, we suggest that a comprehensive reference database is now needed if the full potential of the method is to be utilised, and this will ultimately result in more reliable geological models
Upper Jurassic – Lower Cretaceous of eastern Wollaston Forland, North-East Greenland: a distal marine record of an evolving rift
No full textTwo drill cores covering the Upper Jurassic – Lower Cretaceous succession in Wollaston Forland, NE Greenland, offer an exceptional insight into mud-accumulation in an evolving distal fault block. Previous studies have revealed the presence of long-lasting black mudstone accumulation extending through the oxygen-restricted early rift and rift climax phases (Bernbjerg and Lindemans Bugt Formations). Here, we revisit the depositional evolution recorded in these cores to present a detailed description of the sedimentary succession extending into the late syn-rift settings (Palnatokes Bjerg and Stratumbjerg Formations). The results indicate that the Kimmeridgian – lower Volgian early rift-phase was characterized by suspension settling, laminae-scale event deposition in a tectonically-affected, prodeltaic offshore setting. The event-related depositional processes are expressed by starved wave-ripples, scour-and-fill structures, putative mud floccule ripples, and mud-dominated gravity-flow deposits. During the middle Volgian – Ryazanian rift climax phase, the depositional environment evolved into a narrow half-graben that was detached from the proximal depocentre flanking the coarse sediment fueled deltaic coastline. The correlative sedimentary facies in the detached half-graben are bioclastic and pyrite-rich black mudstones documenting suspension settling and gravity flow/mass wasting deposition in sub-storm wave-base slope and basin-floor environments. Black shale sedimentation ended abruptly in the late Ryazanian when the accumulation of condensed, bioturbated deep sea marls was initiated linked to broader oceanographic reorganization concomitant with waning rift activity in the west. Deposition of red bioclastic mudstones with a common gravity-flow component characterized the Hauterivian, potentially representing final draping of the submerged fault block crest. The top of the cored succession is demarcated by the appearance of dark grey bioturbated mudstones of Barremian age, reflecting the onset of regionally continuous deep-sea mud accumulation in thermally subsidizing basins. Although superficially monotonous, the mudstone-dominated succession reveals a highly dynamic depositional system that reflects shifting marine processes during almost a full rift cycle
Highlighting broad-scale morphometric diversity of the seabed using geomorphons
No full textMorphometric diversity is an important component of overall seabed geodiversity. Automated methods for classification of morphometric features (ridges, peaks, valleys etc.) provide a convenient way of classifying large volumes of data in a consistent and repeatable way and a basis for assessing morphometric diversity. Here, we apply ‘geomorphons’, a pattern recognition approach to morphometric feature classification, to 100 m resolution multibeam bathymetry data in the Barents and Norwegian Seas, Norway. The study area spans depths from a few metres to nearly 6000 m across several geological settings. Ten unique morphometric features are delineated by the geomorphon analysis. From these results, we compute the variety of features per 10 km2. This simple ‘geomorphon richness’ measure highlights broad-scale morphometric diversity across the study area. We compare the richness results with terrain attributes and across physiographic regions. Our results provide new regional insights, which together with more detailed information will help guide follow-up surveys as well as identifying diversity hotspots, which may require special management
Temporal variation of iodine in Danish groundwater
No full textIodine is an essential element for human health, and both high and low iodine intake could have negative health outcomes. The spatial variation of iodine in Danish groundwater has been studied before, but to the author’s knowledge, this is the first time that the temporal variation is characterised. Nationwide data from the Danish groundwater monitoring programme (GRUMO) were analysed between 2011 and 2021, including 2924 samples from 1242 well screens at 893 wells. The sampling frequency varied and so the robust coefficient of variation (rCV) was calculated for 930 (75%) of well screens, and time-series analysis was performed for 23 (2%). Key findings are (1) iodine in Danish groundwater varies over time (0–124%, median = 10%), (2) in one quarter of the well screens rCV exceeds 20% and (3) this variation cannot be attributed solely to analytical uncertainty at 14% of the well screens. The impact of temporal variation of iodine in Danish drinking water of groundwater origin should be evaluated in future exposure or epidemiological studies with respect to the study goal, location and time period. Since the temporal variation could not be quantified over the entire concentration range, monitoring of iodine in Danish groundwater should continue
A whole-rock data set for the Skaergaard intrusion, East Greenland
No full textWe report a compilation of new and published whole-rock major and trace element analyses for 646 samples of the Skaergaard intrusion, East Greenland. The samples were collected in 14 stratigraphic profiles either from accessible and well-exposed surface areas or from drill core, and they cover most regions of the intrusion. This includes the Layered Series, the Upper Border Series, the Marginal Border Series and the Sandwich Horizon. The geochemical data were obtained by a combination of X-ray fluorescence and inductively coupled plasma mass spectrometry. This data set can, for example, be used to constrain processes of igneous differentiation and ore formation.
Organic geochemistry of an Upper Jurassic – Lower Cretaceous mudstone succession in a narrow graben setting, Wollaston Forland Basin, North-East Greenland
No full textThe Oxfordian–Ryazanian was a period of widespread deposition of marine organic-rich mudstones in basins formed during the early phases of the rifting that heralded the formation of the present-day North Atlantic. Occasionally, uninterrupted deposition prevailed for 20 million years or more. Today, mudstones of this time interval are found on the shelves bordering the North Atlantic and adjacent areas from Siberia to the Netherlands. Here, we report data on two fully cored boreholes from Wollaston Forland (North-East Greenland, approx. 74° N), which represent an uninterrupted succession from the upper Kimmeridgian to the Hauterivian. The boreholes record basin development at two different positions within an evolving halfgraben, located at the margin of the main rift, and thus partially detached from it. Although the overall depositional environment remained an oxygen-restricted deep-shelf setting, rifting-related changes can be followed through the succession. The Kimmeridgian was a period of eustatic highstand and records the incipient rifting with a transgressive trend straddling the transition to the lower Volgian by a gradual change from deposits with high levels of total organic carbon (TOC) and kerogen rich in allochthonous organic matter to deposits with lower TOC and a higher proportion of autochthonous organic matter. This is followed by a slight regressive trend with lower TOC and increased proportions of allochthonous organic matter until rifting culminated in the middle Volgian–Ryazanian, indicated by increasing autochthonous organic matter and higher TOC, which prevailed until basin ventilation occurred towards the end of the Ryazanian. The properties of the reactive kerogen fraction remained rather stable irrespective of TOC, underlining the effect of terrigenous matter input for TOC. These variations are also captured by biological markers and stable carbon isotopes. The deposits are very similar to equivalent successions elsewhere in the proto-North Atlantic region, albeit the proportion of terrigenous kerogen is greater
Transport of nitrate-containing groundwater to coastal areas through buried tunnel valleys, Denmark
No full textNitrogen impact on the aquatic environment, including coastal areas, is too high in many countries worldwide, particularly in areas with intensive agriculture. Efficient mitigation initiatives demand that important pathways and the fate of nitrate in the hydrological cycle are known. In this study, we focus on groundwater nitrate contamination in two near-shore catchment areas in north-west Denmark. Groundwater in the area is mainly located in buried tunnel valleys, which are subsurface structures eroded by meltwater during Pleistocene glaciations in former glaciated areas. Groundwater samples from the aquifers inside the buried valleys reveal the presence of up to 120 mg/l nitrate down to 10 m below sea level and about 1 km down from the stream outlet towards the coast. We interpret the complex tunnel-valley infill to be responsible for the spatial heterogeneity of the groundwater geochemistry, where sandy geological windows create localised hydraulic pathways and complex redox structures. Groundwater and stream water chemistry in the study area clearly demonstrate the role of groundwater in nitrate transport within the catchment as well as the direct pathway to the coast bypassing the stream and riverine systems. Our results show that the buried tunnel valleys potentially contribute to submarine groundwater discharge and therefore could be responsible for a hitherto unaccounted input of nitrogen to the marine environment