Permafrost saline water and Early to mid-Holocene permafrost aggradation in Svalbard

Deglaciation in Svalbard was followed by seawater ingression and deposition of marine (deltaic) sediments in fjord valleys, while elastic rebound resulted in fast land uplift and the exposure of these sediments to the atmosphere, whereby the formation of epigenetic permafrost was initiated. This was then followed by the accumulation of aeolian sediments, with syngenetic permafrost formation. Permafrost was studied in the eastern Adventdalen valley, Svalbard, 3–4 km from the maximum up-valley reach of post-deglaciation seawater ingression, and its ground ice was analysed for its chemistry. While ground ice in the syngenetic part is basically fresh, the epigenetic part has a frozen freshwater–saline water interface (FSI), with chloride concentrations increasing from the top of the epigenetic part (at 5.5 m depth) to about 15 % that of seawater at 11 m depth. We applied a one-dimensional freezing model to examine the rate of top-down permafrost formation, which could be accommodated by the observed frozen FSI. The model examined permafrost development under different scenarios of mean average air temperature, water freezing temperature and degree of pore-water freezing. We found that even at the relatively high air temperatures of the Early to mid-Holocene, permafrost could aggrade quite fast down to 20 to 37 m (the whole sediment fill of 25 m at this location) within 200 years. This, in turn, allowed freezing and preservation of the freshwater–saline water interface despite the relatively fast rebound rate, which apparently resulted in an increase in topographic gradients toward the sea. The permafrost aggradation rate could also be enhanced due to non-complete pore-water freezing. We conclude that freezing must have started immediately after the exposure of the marine sediment to atmospheric conditions.</p

Lead content and isotopic composition in submound and recent soils of the Volga upland

Literature data on the historical reconstructions of the atmospheric lead deposition in Europe and the isotopic composition of the ores that are potential sources of the anthropogenic lead in the atmospheric deposition in the lower Volga steppes during different time periods have been compiled. The effect of the increasing anthropogenic lead deposition recorded since the Bronze Age on the level of soil contamination has been investigated. For the first time paleosol buried under a burial mound of the Bronze Age has been used as a reference point to assess of the current contamination level. The contents and isotopic compositions of the mobile and total lead have been determined in submound paleosols of different ages and their recent remote and roadside analogues. An increase in the content and fraction of the mobile lead and a shift of its isotopic composition toward less radiogenic values (typical for lead from the recent anthropogenic sources) has been revealed when going from a Bronze-Age paleosol to a recent soil. In the Bronze-Age soil, the isotopic composition of the mobile lead is inherited from the parent rock to a greater extent than in the modern soils, where the lead is enriched with the less radiogenic component. The effect of the anthropogenic component is traced in the analysis of the mobile lead, but it is barely visible for the total lead. An exception is provided by the recent roadside soils characterized by increased contents and the significantly less radiogenic isotopic composition of the mobile and total lead

1000-580Ma crustal evolution in the northern Arabian-Nubian Shield revealed by U-Pb-Hf of detrital zircons from late Neoproterozoic sediments (Elat area, Israel)

The final stages in the evolution of the Arabian-Nubian Shield (ANS) during the late Neoproterozoic include the deposition of sub-aerial volcano-sedimentary sequence above exhumed deep-seated plutonic-metamorphic rocks. In southern Israel the sediments in this sequence, known collectively as the Elat conglomerate unit, consist primarily of immature polymictic conglomerates whose composition closely reflects their underlying basement. U-Pb dating and Hf isotopic analysis of single zircons from two samples of the Elat conglomerate is used here to gain a complementary perspective on crustal evolution processes in the northern ANS. The U-Pb age spectra of the detrital zircons bear evidence for two major magmatic cycles separated by a lull; early island-arc magmatism occurred at 880-760Ma and peaked at ∼800Ma, whereas post-collisional granitoids and volcanics invaded the amalgamated arc crust between 660 and 580Ma and peaked at 610-630Ma. Integration with previously published data from Sinai suggests the aforementioned igneous cycles and the intervening lull are of regional significance. Although island-arc igneous activity in the ANS ranged mostly between 880 and 760Ma, the occurrence of detrital zircons with strongly positive e{open} Hf(t) values and U-Pb ages of up to 1.0Ga in the Elat conglomerate suggests that island-arc magmatism in the northern ANS started in the early Tonian, earlier than previously recognized. The youngest zircons in the Elat conglomerate constrain its deposition age to ∼580. Ma. No younger U-Pb zircon ages were obtained for crystalline rocks in this region, our data thus indicates that 580. Ma marks the cessation of igneous activity in the northern ANS. The great majority of detrital zircons in the Elat conglomerate display positive e{open} Hf(t) values, in line with the overall juvenile character of the Shield. Nonetheless, the relatively wide range of e{open} Hf(t) values in zircons representing island-arc magmatism (between +3.6 and +11.6), and the presence of zircons with pre-Neoproterozoic ages and/or negative e{open} Hf(t) values, suggest some contamination of the arc magmas by an older crustal component. The e{open} Hf(t) values in zircons representing the younger, post-collisional magmas are all positive (between +4.9 and 9.2) and resemble those in the preceding arc-derived zircons. This feature is consistent with both reworking of mafic island-arc crust and contribution from mantle-derived melts during the post-collisional stage. Thus, it cannot be deciphered whether the 1000-580Ma igneous activity in the northern ANS represents continuous juvenile crustal addition from the mantle, or subsequent recycling of early-formed arc crust during a later post-collisional phase. © 2012 Elsevier B.V

Hf-in-zircon perspective on crustal growth and recycling in the Arabian-Nubian Shield

1 page(s

Ar-40/Ar-39 geochronology and Sr-Pb isotopic evidence of post-collisional extensional volcanism of the eastern Pontide paleo-arc, NE Turkey

Tertiary aged post-collisional volcanics show stratigraphic, petrographic and age differences in the northern and southern zones of the eastern Pontides. The Eocene sequences in E-W direction are exposed as basin fillings in the southern zone and overlay the Upper Cretaceous and Paleocene aged units. The volcanic rocks in the Gümü..hane and Alucra areas are mainly basalt, basaltic andesite, andesite, trachyandesite and rare rhyolite-obsidien. Petrochemically, the Gümü..hane area volcanics vary from basalt to dacite with low alkali tendency, whereas the Alucra area volcanics from basalt to basaltic-trachyandesite with high alkali tendency. The differences on the element variations of the tholeiitic-alkaline transition and calc-alkaline rocks can be explained by fractionation of clinopyroxene + hornblende + plagioclase ± magnetite ± apatite. The trace element variations show some similarities with enrichment of LILE and negative Nb, Ta and Ti anomalies. Particularly, the enrichments in LILE and a lesser extent in LREE, but the depletions in HFSE indicate that the rocks evolved from a parental magma derived from an enriched source formed by mixing of slab and asthenospheric melts. The chondrite-normalized-REE patterns of these volcanics resemble to each other and spoon-shaped with low to medium enrichment (LaN/LuN=2-35), indicating similar source area for the Gümü..hane and Alucra area volcanics. The 40Ar-39Ar dating of the studied volcanics are between 37.7±0.2 and 44.5±0.2 Ma. The (87Sr/86Sr)i values of the rocks are between 0.70457 and 0.70556, and their 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotopic ratios range 18.59 to 18.73, 15.62 to 15.64 and 38.65 to 38.86, respectively. Pb- and Sr- isotopic ratios may imply that the rocks derived from a source region at the boundary of the mantle-crust, but the samples with low (87Sr/86Sr)i values (<0.705) reflect the mantle source in their origin. The increasing values of SiO2 (wt.%), Sr (ppm) ve (1/Sr)x103 ppm-1 versus (87Sr/86Sr)i values suggest fractionational crystallisation (FC) rather than assimilation (AFC) in their evolution. Based on volcanic variety and distributions together with field and petrological data, volcanic activity in the Eocene time of the eastern Pontide is closely related to extensional tectonic regime with transtansional faults