Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)

Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic. Received: 7 July 1998 / Accepted: 4 November 1998     

Stratigraphic evidence for an Early Aptian sea-level fluctuation: the Graua Limestone of south-eastern Ethiopia

The occurrence of a thin band of marine limestone (Graua Limestone) within a thick succession of fluviatile sandstones in south-eastern Ethiopia is direct evidence of flooding of part of the East African craton (Horn of Africa). According to the presence of abundant orbitolinid foraminifers (Palorbitolina lenticularis Blumenbach andPraeorbitolina cormyi Schroeder), the age of the Graua Limestone can be referred to the Early Aptian. Stratigraphy and palaeogeographic reconstructions for the Early Cretaceous in the surrounding regions (Kenya, Somalia, Yemen, Socotra, Oman, Syria, Lebanon, Israel, Egypt and Libya) show that the Early Aptian transgression was of regional extent. Our data seem to confirm that this transgression was of relatively short duration. This pulse cannot be related to tectono-eustatic mechanisms, which are too slow. A short-lived event should be invoked: either a regional tectonic pulse or the desiccation of the proto-South Atlantic.     

Impact of grazing by benthic eukaryotic organisms on the nitrogen sediment–water exchange in the North Sea

The metabolic inhibitor cycloheximide was used to estimate the influence of primarily unicellular eukaryotes (heterotrophic protozoa) on nutrient recycling in different types of sediments in the North Sea. Fluxes of dissolved inorganic nitrogen across the sediment–water interface were measured in undisturbed sediment cores (controls) and compared to fluxes in sediment cores with cycloheximide added. If eukaryotes play an important role in nutrient recycling, one would expect to find lower nitrogen sediment–water effluxes in cores with cycloheximide due to the inactivation of eukaryotes. This important role hypothesised for eukaryotes was not generally observed: Only at four of the nineteen stations were ammonium effluxes significantly higher in controls than in cores with cycloheximide, and at five stations nitrate effluxes were significantly higher in the controls than in the cores with cycloheximide. Eukaryotic activity apparently contributed to the sediment–water exchange of ammonium through mineralisation of organic matter, nitrification and the subsequent release of ammonium and nitrate at these stations. At most other stations no differences were obtained between controls and cores with cycloheximide. This suggests that bacteria were the most important nutrient mineralisers at these stations at the time of the cruises.     

Ecosystem engineering by hummock-building earthworms in seasonal wetlands of eastern South Africa: Insights into the mechanics of biomorphodynamic feedbacks in wetland ecosystems

This paper resolves the origin of clay hummock micro-topography in seasonal wetlands of the Drakensberg Foothills, providing a review and appraisal of previously-suggested mechanisms of hummock formation in the context of new field and laboratory data. Field surveys revealed neo-formation of clay hummocks in a river channel that had been abandoned in c.1984. Fresh earthworm castings were located atop hummocks protruding from inundated abandoned channel margins. Earthworm castings, and sediment cores taken in hummocks and adjacent hollows, were analysed for soil-adsorbed carbon and nitrogen using an HCN analyser, and for ²¹⁰Pb activity using alpha-geochronology. ²¹⁰Pb activity profiles suggest relative enrichment of the isotope in hummocks, and relative depletion in adjacent hollows. Earthworm castings are characterised by very high ²¹⁰Pb activity, as well as high C and N contents. Hummocks have significantly higher C and N contents than adjacent hollows. Results suggest that it is the foraging activity of earthworms in litter-rich seasonal wetland hollows, and repeated excretion of castings atop adjacent hummocks, that is responsible for the elemental enrichment observed. The paper presents a conceptual model of hummock formation in wetlands through interactions between hydrogeomorphology and earthworm activity, and illustrates a mechanism of biogeomorphic inheritance through which ordered patterns of preferential flow can emerge in ecosystems. Further implications of hummock formation and nodal accumulation of nutrients are considered in relation to wetland resilience and regulatory ecosystem service provision.© 2018 John Wiley & Sons, Ltd.     

‘Bond cycles’ recorded in terrestrial Pleistocene sediments of southwestern British Columbia,Canada

Recent data from exposures of terrestrial Pleistocene sediments in the Fraser Lowland of southwestern British Columbia reveal at least two ‘Bond cycles’ within Oxygen Isotope Stage 2. The maximum of the Coquitlam Stade coincides with the timing of Heinrich event H2, the Port Moody Interstade with Dansgaard–Oeschger (D–O) interstade 2, the maximum of the Vashon Stade with H1, and the Fort Langley interval with D–O interstade 1. The Sumas Stade apparently preceded H0 (Younger Dryas) but could have been in response to the same climatic signal. The timing of Sumas advances may be explained by a combination of glacio-isostatic rebound, destabilisation of the ice margin, and rapid movement over a short distance on soft muddy beds of a rising sea floor, thereby leading the timing of North Atlantic events by hundreds of years. In contrast, Coquitlam and Vashon advances were mainly over permeable glaciofluvial sediments and because of this their maxima probably did not precede the timing of H2 and H1. The Port Moody Interstade coincided with the global Last Glacial Maximum, due in part to the moderating effect of moist summer storms in a southward-shifted jet stream that influenced the Fraser Lowland at that time. Copyright © 1999 John Wiley & Sons, Ltd.     

Forecasting Groundwater Temperature with Linear Regression Models Using Historical Data

Although temperature is an important determinant of many biogeochemical processes in groundwater, very few studies have attempted to forecast the response of groundwater temperature to future climate warming. Using a composite linear regression model based on the lagged relationship between historical groundwater and regional air temperature data, empirical forecasts were made of groundwater temperature in several aquifers in Switzerland up to the end of the current century. The model was fed with regional air temperature projections calculated for greenhouse‐gas emissions scenarios A2, A1B, and RCP3PD. Model evaluation revealed that the approach taken is adequate only when the data used to calibrate the models are sufficiently long and contain sufficient variability. These conditions were satisfied for three aquifers, all fed by riverbank infiltration. The forecasts suggest that with respect to the reference period 1980 to 2009, groundwater temperature in these aquifers will most likely increase by 1.1 to 3.8 K by the end of the current century, depending on the greenhouse‐gas emissions scenario employed.     

Multi‐dimensional analyses of the SEAM controlled source electromagnetic data—the story of a blind test of interpretation workflows

Using a subset of the SEG Advanced Modeling Program Phase I controlled‐source electromagnetic data, we apply our standard controlled‐source electromagnetic interpretation workflows to delineate a simulated hydrocarbon reservoir. Experience learned from characterizing such a complicated model offers us an opportunity to refine our workflows to achieve better interpretation quality. The exercise proceeded in a blind test style, where the interpreting geophysicists did not know the true resistivity model until the end of the project. Rather, the interpreters were provided a traditional controlled‐source electromagnetic data package, including electric field measurements, interpreted seismic horizons, and well log data. Based on petrophysical analysis, a background resistivity model was established first. Then, the interpreters started with feasibility studies to establish the recoverability of the prospect and carefully stepped through 1D, 2.5D, and 3D inversions with seismic and well log data integrated at each stage. A high‐resistivity zone is identified with 1D analysis and further characterized with 2.5D inversions. Its lateral distribution is confirmed with a 3D anisotropic inversion. The importance of integrating all available geophysical and petrophysical data to derive more accurate interpretation is demonstrated.     

Connecting Satellite Observations with Water Cycle Variables Through Land Data Assimilation: Examples Using the NASA GEOS-5 LDAS

A land data assimilation system (LDAS) can merge satellite observations (or retrievals) of land surface hydrological conditions, including soil moisture, snow, and terrestrial water storage (TWS), into a numerical model of land surface processes. In theory, the output from such a system is superior to estimates based on the observations or the model alone, thereby enhancing our ability to understand, monitor, and predict key elements of the terrestrial water cycle. In practice, however, satellite observations do not correspond directly to the water cycle variables of interest. The present paper addresses various aspects of this seeming mismatch using examples drawn from recent research with the ensemble-based NASA GEOS-5 LDAS. These aspects include (1) the assimilation of coarse-scale observations into higher-resolution land surface models, (2) the partitioning of satellite observations (such as TWS retrievals) into their constituent water cycle components, (3) the forward modeling of microwave brightness temperatures over land for radiance-based soil moisture and snow assimilation, and (4) the selection of the most relevant types of observations for the analysis of a specific water cycle variable that is not observed (such as root zone soil moisture). The solution to these challenges involves the careful construction of an observation operator that maps from the land surface model variables of interest to the space of the assimilated observations.     

Authigenic carbonate mineral formation in the Pagassitikos palaeolake during the latest Pleistocene, central Greece

The Pagassitikos Gulf in Greece is a semi-enclosed bay with a maximum depth of 102 m. According to the present-day bathymetric configuration and the sea level during the latest Pleistocene, the gulf would have been isolated from the open sea, forming a palaeolake since ~32 cal. ka b.p. Sediment core B-4 was recovered from the deepest sector of the gulf and revealed evidence of a totally different depositional environment in the lowest part of the core: this contained light grey-coloured sediments, contrasting strongly with overlying olive grey muds. Multi-proxy analyses showed the predominance of carbonate minerals (aragonite, dolomite and calcite) and gypsum in the lowest part of the core. Carbonate mineral deposition can be attributed to autochthonous precipitation that took place in a saline palaeolake with high evaporation rates during the last glacial–early deglacial period; the lowest core sample to be AMS ¹⁴C dated provided an age of 19.53 cal. ka b.p. The palaeolake was presumably reconnected to the open sea at ~13.2 cal. ka b.p. during the last sea-level rise, marking the commencement of marine sedimentation characterised by the predominance of terrigenous aluminosilicates and fairly constant depositional conditions lasting up to the present day.     

The northern boundary of the Jan Mayen microcontinent, North Atlantic determined from ocean bottom seismic, multichannel seismic, and gravity data

The Jan Mayen microcontinent was as a result of two major North Atlantic evolutionary cornerstones—the separation of Greenland from Norway (~54 Ma), accompanied by voluminous volcanic activity, and the jump of spreading from the Aegir to the Kolbeinsey ridge (~33 Ma), which resulted in the separation of the microcontinent itself from Eastern Greenland (~24 Ma). The resulting eastern and western sides of the Jan Mayen microcontinent are respectively volcanic and non-volcanic rifted margins. Until now the northern boundary of the microcontinent was not precisely known. In order to locate this boundary, two combined refraction and reflection seismic profiles were acquired in 2006: one trending S–N and consisting of two separate segments south and north of the island of Jan Mayen respectively, and the second one trending SW–NE east of the island. Crustal P-wave velocity models were derived and constrained using gravity data collected during the same expedition. North of the West Jan Mayen Fracture Zone (WJMFZ) the models show oceanic crust that thickens from west to east. This thickening is explained by an increase in volcanic activity expressed as a bathymetric high and most likely related to the proximity of the Mohn ridge. East of the island and south of the WJMFZ, oceanic Layers 2 and 3 have normal seismic velocities but above normal average crustal thickness (~11 km). The similarity of the crustal thickness and seismic velocities to those observed on the conjugate M?re margin confirm the volcanic origin of the eastern side of the microcontinent. Thick continental crust is observed in the southern parts of both profiles. The northern boundary of the microcontinent is a continuation of the northern lineament of the East Jan Mayen Fracture Zone. It is thus located farther north than previously assumed. The crust in the middle parts of both models, around Jan Mayen island, is more enigmatic as the data suggest two possible interpretations—Icelandic type of oceanic crust or thinned and heavily intruded continental crust. We prefer the first interpretation but the latter cannot be completely ruled out. We infer that the volcanism on Jan Mayen is related to the Icelandic plume.