Continental carbonate facies of a Neoproterozoic panglaciation, north-east Svalbard

The Marinoan panglaciation (ca 650 to 635 Ma) is represented in north-east Svalbard by the 130 to 175 m thick Wilsonbreen Formation which contains syn-glacial carbonates in its upper 100 m. These sediments are now known to have been deposited under a CO2-rich atmosphere, late in the glaciation, and global climate models facilitate testing of proposed analogues. Precipitated carbonates occur in four of the seven facies associations identified: Fluvial Channel (including stromatolitic and intraclastic limestones in ephemeral stream deposits); Dolomitic Floodplain (dolomite-cemented sand and siltstones, and microbial dolomites); Calcareous Lake Margin (intraclastic dolomite and wave-rippled or aeolian siliciclastic facies); and Calcareous Lake (slump-folded and locally re-sedimented rhythmic/stromatolitic limestones and dolomites associated with ice-rafted sediment). There is no strong cyclicity, and modern analogues suggest that sudden changes in lake level may exert a strong control on facies geometry. Both calcite and dolomite in stromatolites and rhythmites display either primary or early diagenetic replacive growth. Oxygen isotope values (−12 to +15‰VPDB) broadly covary with δ13C. High δ13C values of +3·5 to +4·5‰ correspond to equilibration with an atmosphere dominated by volcanically degassed CO2 with δ13C of −6 to −7‰. Limestones have consistently negative δ18O values, while rhythmic and playa dolomites preserve intermediate compositions, and dolocretes possess slightly negative to strongly positive δ18O signatures, reflecting significant evaporation under hyperarid conditions. Inferred meltwater compositions (−8 to −15·5‰) could reflect smaller Rayleigh fractionation related to more limited cooling than in modern polar regions. A common pseudomorph morphology is interpreted as a replacement of ikaite (CaCO3·H2O), which may also have been the precursor for widespread replacive calcite mosaics. Local dolomitization of lacustrine facies is interpreted to reflect microenvironments with fluctuating redox conditions. Although differing in (palaeo)latitude and carbonate abundance, the Wilsonbreen carbonates provide strong parallels with the McMurdo Dry Valleys of Antarctica

Cavity-based secondary mineralization in volcanic tuffs of Yucca Mountain, Nevada: a new type of the polymineral vadose speleothem, or a hydrothermal deposit?

Secondary minerals (calcite, chalcedony, quartz, opal, fl uorite, heulandite, strontianite) residing in open cavities in the Miocene rhyolite tuffs of Yucca Mountain, Nevada have been interpreted by some researchers as speleothemic formations, deposited as a result of downward infi ltration of meteoric waters (DOE, 2001, Whelan et al., 2002). The major mineral of the paragenesis, calcite, shows spectacular trend of the textural and crystal morphology change: from anhedral granular occurrences, through (optional) platelet, bladed and scepter varieties, to euhedral blocky morphologies. The trend is consistent with the overall decrease in the supesaturation of the mineral forming solution. Stable isotope properties of calcite evolve from 13C-enriched (δ13C = +4 to +9 ‰ PDB) at early stages of growth to 13C-depleted (-5 to -10 ‰) at late stages. The non-cyclic character of the isotope record and extreme variations of isotopic values argue against the meteoric origin of mineral forming fl uids. The δ13C \u3e4 ‰ PDB require isotope partitioning between dissolved CO2 and CH4, which is only possible in reducing anoxic environment, but not in aerated vadose zone. Fluid inclusions studied in calcite, quartz and fluorite revealed that the minerals were deposited from thermal solutions. The temperatures were higher at early stages of mineral growth (60 to 85°C) and declined with time. Most late-stage calcites contain only all-liquid inclusions, suggesting temperatures less than ca. 35-50°C. Minerals collected close to the major fault show the highest temperatures. Gases trapped in fluid inclusions are dominated by CO2 and CH4; Raman spectrometry results suggest the presence of aromatic/cyclic hydrocarbon gases. The gas chemistry, thus, also indicates reduced (anoxic) character of the mineral forming fluids. Secondary minerals at Yucca Mountain have likely formed during the short-term invasion(s) of the deep-seated aqueous fluids into the vadose zone. Following the invasion, fluids, initially equilibrated with the deep (reduced, anoxic) environment, evolved toward equilibrium with the new environment (cooling, degassing, mixing with shallow oxidizing waters, etc.). While some features of mineralization are compatible with the speleothemic or meteoric infiltration model, most of the evidence does not lend itself to rational explanation within this model

Cavity-based secondary mineralization in volcanic tuffs of Yucca Mountain, Nevada: a new type of the polymineral vadose speleothem, or a hydrothermal deposit?

Secondary minerals (calcite, chalcedony, quartz, opal, fluorite, heulandite, strontianite) residing in open cavities in the Miocenerhyolite tuffs of Yucca Mountain, Nevada have been interpreted by some researchers as "speleothemic" formations, deposited as aresult of downward infiltration of meteoric waters (DOE, 2001, Whelan et al., 2002). The major mineral of the paragenesis, calcite,shows spectacular trend of the textural and crystal morphology change: from anhedral granular occurrences, through (optional)platelet, bladed and scepter varieties, to euhedral blocky morphologies. The trend is consistent with the overall decrease in thesupesaturation of the mineral forming solution. Stable isotope properties of calcite evolve from 13C-enriched (δ13C = +4 to +9 ‰ PDB)at early stages of growth to 13C-depleted (-5 to -10 ‰) at late stages. The non-cyclic character of the isotope record and extremevariations of isotopic values argue against the meteoric origin of mineral forming fluids. The δ13C >4 ‰ PDB require isotope partitioningbetween dissolved CO2 and CH4, which is only possible in reducing anoxic environment, but not in aerated vadose zone.Fluid inclusions studied in calcite, quartz and fluorite revealed that the minerals were deposited from thermal solutions. Thetemperatures were higher at early stages of mineral growth (60 to 85oC) and declined with time. Most late-stage calcites containonly all-liquid inclusions, suggesting temperatures less than ca. 35-50oC. Minerals collected close to the major fault show the highesttemperatures. Gases trapped in fluid inclusions are dominated by CO2 and CH4; Raman spectrometry results suggest the presenceof aromatic/cyclic hydrocarbon gases. The gas chemistry, thus, also indicates reduced (anoxic) character of the mineral formingfluids.Secondary minerals at Yucca Mountain have likely formed during the short-term invasion(s) of the deep-seated aqueous fluidsinto the vadose zone. Following the invasion, fluids, initially equilibrated with the deep (reduced, anoxic) environment, evolvedtoward equilibrium with the new environment (cooling, degassing, mixing with shallow oxidizing waters, etc.). While some featuresof mineralization are compatible with the "speleothemic" or "meteoric infi ltration" model, most of the evidence does not lend itself torational explanation within this model

Moisture availability in the southwest United States over the last three glacial-interglacial cycles

The projected long-term drying of the southwest (SW) United States in response to climate warming raises a sobering alarm for this already water-limited region, yet the climatic controls on moisture availability over longer time scales remain a topic of debate. Here, we present a 350,000-year record of past water table fluctuations in Devils Hole 2 cave that are driven by variations in recharge amount to the local groundwater flow system. Because of the unprecedented length and precision of our record, we can observe variations in regional moisture availability over the last three glacial-interglacial cycles at a millennial-scale resolution. The timing of past water table rises and falls (\u3e9 m in amplitude) closely coincides with the expansion and reduction of Northern Hemisphere ice volume, which in turn influences the position and intensity of westerly winter storms on orbital time scales. Superimposed on this long-term trend are millennial-scale highstands recorded during the last glaciation that coincide with North Atlantic Heinrich events. Earlier millennial-scale highstands provide the first evidence of multiple short-lived wet periods in the SW United States linksed to coeval cooling intervals in the North Atlantic during marine isotope stages 6 and 8. The Devils Hole 2 water table record is currently the longest independently dated paleomoisture record in the SW United States and thus provides a critical testbed to examine the controls on regional moisture availability over larger time scales

Moisture availability in the southwest United States over the last three glacial-interglacial cycles

The projected long-term drying of the southwest (SW) United States in response to climate warming raises a sobering alarm for this already water-limited region, yet the climatic controls on moisture availability over longer time scales remain a topic of debate. Here, we present a 350,000-year record of past water table fluctuations in Devils Hole 2 cave that are driven by variations in recharge amount to the local groundwater flow system. Because of the unprecedented length and precision of our record, we can observe variations in regional moisture availability over the last three glacial-interglacial cycles at a millennial-scale resolution. The timing of past water table rises and falls (\u3e9 m in amplitude) closely coincides with the expansion and reduction of Northern Hemisphere ice volume, which in turn influences the position and intensity of westerly winter storms on orbital time scales. Superimposed on this long-term trend are millennial-scale highstands recorded during the last glaciation that coincide with North Atlantic Heinrich events. Earlier millennial-scale highstands provide the first evidence of multiple short-lived wet periods in the SW United States linksed to coeval cooling intervals in the North Atlantic during marine isotope stages 6 and 8. The Devils Hole 2 water table record is currently the longest independently dated paleomoisture record in the SW United States and thus provides a critical testbed to examine the controls on regional moisture availability over larger time scales

Travertines of the South-Eastern Gorny Altai (Russia): Implications for Paleoseismology and Paleoenvironmental Conditions

The south-eastern Gorny Altai is one of the most hazardous seismogenic area in the north of Central Asia. We present a synthesis of field, 230Th-U geochronological, mineralogical and geochemical data collected on seven Quaternary travertines. All travertines occur within the zones of active faults that border the Chuya and Kurai intermontane basins. Travertine cement mainly comprises calcite (with minor amounts of aragonite), which cements alluvial, alluvial fan, and colluvial deposits. The results of 230Th-U dating suggest that deposition of the travertines was triggered by large paleoearthquakes in the last eight thousand years. Several stages of travertine formation with ages 9–11 ka BP correspond to the known period of strong paleoseismicity in the region (8–16 ka BP). The 123 ka BP travertine resulted from a slip triggered by the Middle Pleistocene deglaciation, while that of 400 ka BP represents seismic motions likely associated with the main Cenozoic orogenic phase. All travertine forming events fall within warm and wet climatic phases (interglacials). Large earthquakes activated faults and caused a rapid rise along them of ambient-temperature bicarbonate groundwater, which was previously sealed in deep-seated Upper Neoproterozoic–Paleozoic limestone-dolostone aquifers. Rapid CO2 degassing of the spring water was the most important control of calcite or aragonite precipitation. Such travertines represent an important tool for paleoseismological research in seismically active regions