Enhanced Mediterranean water cycle explains increased humidity during MIS 3 in North Africa

We report a new fluid inclusion dataset from northeastern Libyan speleothem SC-06-01, which is the largest speleothem fluid inclusion dataset for North Africa to date. The stalagmite was sampled in Susah Cave, a low-altitude coastal site, in Cyrenaica, on the northern slope of the Jebel Al-Akhdar. Speleothem fluid inclusions from the latest Marine Isotope Stage (MIS) 4 and throughout MIS 3 (∼67 to ∼30 kyr BP) confirm the hypothesis that past humid periods in this region reflect westerly rainfall advected through the Atlantic storm track. However, most of this moisture was sourced from the western Mediterranean, with little direct admixture of water evaporated from the Atlantic. Moreover, we identify a second moisture source likely associated with enhanced convective rainfall within the eastern Mediterranean. The relative importance of the western and eastern moisture sources seems to differ between the humid phases recorded in SC-06-01. During humid phases forced by precession, fluid inclusions record compositions consistent with both sources, but the 52.5–50.5 kyr interval forced by obliquity reveals only a western source. This is a key result, showing that although the amount of atmospheric moisture advections changes, the structure of the atmospheric circulation over the Mediterranean does not fundamentally change during orbital cycles. Consequently, an arid belt must have been retained between the Intertropical Convergence Zone and the midlatitude winter storm corridor during MIS 3 pluvials

Clumped isotope thermometry of cryogenic cave carbonates

Abstract Freezing of cave pool water that is increasingly oversaturated with dissolved carbonate leads to precipitation of a very specific type of speleothems known as cryogenic cave carbonates (CCC). At present, two different environments for their formation have been proposed, based on their characteristic carbon and oxygen isotope ratios. Rapidly freezing thin water films result in the fast precipitation of fine-grained carbonate powder (CCC fine ). This leads to rapid physicochemical changes including CO 2 degassing and CaCO 3 precipitation, resulting in significantly 13 C-enriched carbonates. Alternatively, slow carbonate precipitation in ice-covered cave pools results in coarse crystalline CCC (CCC coarse ) yielding strongly 18 O-depleted carbonate. This is due to the formation of relatively 18 O-enriched ice causing the gradual depletion of 18 O in the water from which the CCC precipitates. Cryogenic carbonates from Central European caves were found to have been formed primarily during the last glacial period, specifically during times of permafrost thawing, based on the oxygen isotope ratios and U-Th dating. Information about the precise conditions of CCC coarse formation, i.e. whether these crystals formed under equilibrium or disequilibrium conditions with the parent fluid, however, is lacking. An improved understanding of CCC coarse formation will increase the predictive value of this paleo-permafrost archive. Here we apply clumped isotopes to investigate the formation conditions of cryogenic carbonates using well-studied CCC coarse from five different cave systems in western Germany. Carbonate clumped isotope measurements yielded apparent temperatures between 3 and 18°C and thus exhibit clear evidence of isotopic disequilibrium. Although the very negative carbonate d 18 O values can only be explained by gradual freezing of pool water accompanied by preferential incorporation of 18 O into the ice, clumped isotope-derived temperatures significantly above expected freezing temperatures indicate incomplete isotopic equilibration during precipitation of CCC

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

6.6 Karstification by Geothermal Waters

Thermal waters moving through soluble rock may create voids ranging in sizes from enlarged porosity and cavernosity to extensive two- and three-dimensional cave systems. Hydrothermal caves develop in a number of settings including deep-seated phreatic, shallow phreatic (near-water table), and subaerial (above the thermal water table). Speleogenesis in each setting involves specific mechanisms, resulting in diverse features of cave macro-, meso-, and micromorphology. Mechanisms most characteristic of the hydrothermal speleogenesis are the free convection (in both subaqueous and subaerial conditions) and the condensation corrosion. This chapter describes the morphology of hydrothermal caves

Data on the 14C date obtained from the charcoal figure “Black fox” in Shulgan-Tash (Kapova) cave, Southern Ural, Russia

Shulgan-Tash (Kapova) cave in southern Ural, Russia, is the easternmost European site hosting Late Paleolithic cave art. Most of the 195+ drawings catalogued in the cave are made with red natural pigment (ochre), and only a handful of drawings are made with charcoal, see “Catalogue of images” [4], “Höhlenmalerei im Ural: Kapova und Ignatievka; die altsteinzeitlichen Bilderhöhlen im sudlichen Ural,” [5]. “Constraining the ages of the Late Palaeolithic cave paintings in the Shulgan-Tash cave, Southern Urals, Russia” [6]. None of the charcoal drawings were directly dated by 14C method so far.Black lines delineating a figure similar to the outline of a fox are known in the cave. Here we present data on the 14C AMS date of charcoal with which the lines were drawn. Calibration of the data was performed using the Bomb13NH1 dataset, see “Atmospheric radiocarbon for the period 1950–2010” [7] and the IntCal13 dataset, see “IntCal13 and Marine13 radiocarbon age calibration curves 0–50,000 years cal BP” [8]. The calibrated age distribution has maximum probability density (65.3%) between 1877 and 1918. Keywords: Shulgan-Tash cave, Radiocarbon, Cave ar

Stable isotope composition of the Early–Middle Eocene limestone in the Crimean Piedmont (Republic of Crimea)

Stable isotope compositions of the Early-Middle Eocene limestone were studied in the Prolom quarry (45.105°N; 34.736°E). Samples were collected across a 25 m-thick bed, at 1 m increment. Oxygen and carbon isotope values show little variation across the entire thickness of the bed. The dataset presents the isotope values characteristic of the unaltered limestone forming at shallow depth (nummulite bank) on the Early-Middle Eocene carbonate platform

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