Authigenic Calcite Research Articles

Hydrologic and climatic implications of stable isotope and minor element analyses of authigenic calcite silts and gastropod shells from a mid-Pleistocene pluvial lake, Western Desert, Egypt

Authigenic calcite silts at Wadi Midauwara in Kharga Oasis, Egypt, indicate the prolonged presence of surface water during the Marine Isotope Stage 5e pluvial phase recognized across North Africa. Exposed over an area of ∼ 4.25 km 2, these silts record the ponding of water derived from springs along the Libyan Plateau escarpment and from surface drainage. The δ 18O values of these lacustrine carbonates (− 11.3‰ to − 8.0‰ PDB), are too high to reflect equilibrium precipitation with Nubian aquifer water or water of an exclusively Atlantic origin. Mg/Ca and Sr/Ca of the silts have a modest negative covariance with silt δ 18O values, suggesting that the water may have experienced the shortest residence time in local aquifers when the water δ 18O values were highest. Furthermore, intra-shell δ 18O, Sr/Ca, and Ba/Ca analyses of the freshwater gastropod Melanoides tuberculata are consistent with a perennially fresh water source, suggesting that strong evaporative effects expected in a monsoonal climate did not occur, or that dry season spring flow was of sufficient magnitude to mute the effects of evaporation. The input of a second, isotopically heavier water source to aquifers, possibly Indian Ocean monsoonal rain, could explain the observed trends in δ 18O and minor element ratios.

Characterization of mortars used for the construction of Saithidai Heroon Podium (1st century AD) in ancient Messene, Peloponnesus, Greece

The Saithidai Heroon Podium is a Mausoleum dated to the Roman Period located in ancient Messene, Peloponnesus, Greece. Its outer walls were constructed with porous and massive limestone blocks with no mortar in the joints. The core of the podium was filled by rock chips to angular boulders of sandstone, limestone, sand, gravel and earthy groundmass material. The groundmass is mostly composed of quartz and calcite, with minor amounts of clay minerals. Small, irregular lime lumps also occur, composed of authigenic euhedral calcite crystal assemblages and rare sub-microscopic quartz fragments, or rod-like CaO/SiO 2-rich mineral assemblages developed in pore spaces on fine-grained authigenic calcite crystals. The mortar used for the filling material of the Podium was poorly homogenized semi-hydraulic lime derived from the calcination of pure limestone and siliceous limestone fragments, possibly the waste material from the finishing of the building blocks of the Podium walls.

Little Ice Age drought in equatorial Africa: Intertropical Convergence Zone migrations and El Niño–Southern Oscillation variability

High-resolution analyses of the Mg concentration in authigenic calcite in five cores from Lake Edward provide a water balance history of central equatorial Africa spanning the past 1400 yr. A high ratio of Mg to Ca (%Mg) indicates strong droughts in central Africa during the Little Ice Age (A.D. 1400–1750), in contrast to records from Lake Naivasha, Kenya, which suggest a wet Little Ice Age. This spatial pattern in Africa likely arose due to coupled changes in the high latitudes, the position of the Intertropical Convergence Zone, and the El Nino–Southern Oscillation (ENSO) system. Our results further suggest that the patterns and variability of twentieth-century rainfall in central Africa have been unusually conducive to human welfare in the context of the past 1400 yr.

Holocene climate change in the eastern Mediterranean region: a comparison of stable isotope and pollen data from Lake Gölhisar, southwest Turkey

AbstractStable isotope and pollen data from Gölhisar Gölü, a small intramontane lake located in southwest Turkey, provide complementary records of Holocene climate change. Modern oxygen and hydrogen isotope water data are used as a means of comparing present‐day isotope composition of the lake water to the past oxygen isotope composition of the lake water as calculated from 18O/16O ratios in calcite precipitated in the summer months. Despite the lake system being chemically dilute, the modern isotope data clearly establish that the lake water is evaporated in relation to its spring input, suggesting that the palaeo data can be interpreted primarily in terms of changing precipitation/evaporation ratios. δ18O and δ13C values from authigenic calcite through the Holocene show predominantly negative values indicating climatic conditions wetter than today. Particularly notable are low (depleted) isotope values during the earliest Holocene (ca. 10 600–8800 cal. yr. BP), a period for which pollen data imply drier conditions than at present. This divergence between pollen‐inferred and stable isotope palaeoclimate data is found in other east Mediterranean lake sediment records, and suggests that vegetation may have taken several millennia to reach climatic equilibrium at the start of the Holocene. Isotopic fluctuations during the early‐to‐mid Holocene (8800–5100 cal. yr. BP) suggest oscillations between aridity and humidity. Higher δ18O and δ13C values for the second half of the Holocene indicate generally drier conditions than during the period before ca.5100 cal. yr BP although there is some evidence for increased humidity coinciding with pollen evidence for increasing human impact and intensification of agriculture, notably during the so‐called Beyşehir Occupation Phase (Classical and early Byzantine periods). The modern trend towards aridity started about 1300 yr ago. Copyright © 2006 John Wiley & Sons, Ltd.

Timing of atmospheric precipitation in the Zagros Mountains inferred from a multi-proxy record from Lake Mirabad, Iran

A sediment core 7.2 m long from Lake Mirabad, Iran, was examined for loss-on-ignition, mineralogy, oxygen-isotopic composition of authigenic calcite, and trace-element composition of ostracodes to complement earlier pollen and ostracode-assemblage studies. Pollen, ostracode-inferred lake level, and high Sr/Ca ratios indicate that the early Holocene (10000 to 6500 cal yr BP) was drier than the late Holocene. Low δ 18O values during this interval are interpreted as resulting from winter-dominated precipitation, characteristic of a Mediterranean climate. Increasing δ 18O values after 6500 cal yr BP signal a gradual increase in spring rains, which are present today. A severe 600-yr drought occurred at ca. 5500 cal yr BP, shortly after the transition from pistachio-almond to oak forest. During the late Holocene, two milder droughts occurred at about 1500 and 500 cal yr BP. Within the resolution of the record, no drought is evident during the collapse of the Akkadian empire (4200–3900 cal yr BP). Rather, a decrease in δ 18O values to early-Holocene levels may indicate the return to a Mediterranean precipitation regime.

Low δ13C in tests of live epibenthic and endobenthic foraminifera at a site of active methane seepage

To investigate the use of benthic foraminifera as a means to document ancient methane release, we determined the stable isotopic composition of tests of live (Rose Bengal stained) and dead specimens of epibenthic Fontbotia wuellerstorfi, preferentially used in paleoceanographic reconstructions, and of endobenthic high‐latitude Cassidulina neoteretis and Cassidulina reniforme from a cold methane‐venting seep off northern Norway. We collected foraminiferal tests from three push cores and nine multiple cores obtained with a remotely operated vehicle and a video‐guided multiple corer, respectively. All sampled sites except one control site are situated at the Håkon Mosby mud volcano (HMMV) on the Barents Sea continental slope in 1250 m water depth. At the HMMV in areas densely populated by pogonophoran tube worms, δ13C values of cytoplasm‐containing epibenthic F. wuellerstorfi are by up to 4.4‰ lower than at control site, thus representing the lowest values hitherto reported for this species. Live C. neoteretis and C. reniforme reach δ13C values of −7.5 and −5.5‰ Vienna Pee Dee Belemnite (VPDB), respectively, whereas δ13C values of their empty tests are higher by 4‰ and 3‰. However, δ13C values of empty tests are never lower than those of stained specimens, although they are still lower than empty tests from the control site. This indicates that authigenic calcite precipitates at or below the sediment surface are not significantly influencing the stable isotopic composition of foraminiferal shells. The comparatively high δ13C results rather from upward convection of pore water and fluid mud during active methane venting phases at these sites. These processes mingle tests just recently calcified with older ones secreted at intermittent times of less or no methane discharge. Since cytoplasm‐containing specimens of suspension feeder F. wuellerstorfi are almost exclusively found attached to pogonophores, which protrude up to 3 cm above the sediment, and δ13C values of bottom‐water‐dissolved inorganic carbon (DIC) are not significantly depleted, we conclude that low test δ13C values of F. wuellerstorfi are the result of incorporation of heavily 13C‐depleted methanotrophic biomass that these specimens feed on rather than because of low bottom water δ13CDIC. Alternatively, the pogonophores, which are rooted at depth in the upper sediment column, may serve as a conduit for depleted δ13CDIC that ultimately influences the calcification process of F. wuellerstorfi attached to the pogonophoran tube well above the sediment/water interface. The lowest δ13C of live specimens of the endobenthic C. neoteretis and C. reniforme are within the range of pore water δ13CDIC values, which exceed those that could be due to organic matter decomposition, and thus, in fact, document active methane release in the sediment.

Isotopic constraints on fluid evolution and precipitation mechanisms for the Boléo Cu–Co–Zn district, Mexico

Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boleo district of the Miocene Santa Rosalia basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boleo Formation is characterized by modified marine carbon (δ13CPDB=−6.0 to +4.4‰) and oxygen (δ18OSMOW=+19.5 to +26.2‰) isotope composition due to the influx of 13C- and 18O-depleted fluvial water. Sulfate sulfur isotope composition (δ34SCDT=+17.21 to +22.3‰ and δ18OSMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ34S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ34S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ13C=−11.6 to −3.2‰ and δ18O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ13C=−7.9 to +4.3‰ and δ18O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.

Stable isotopic evidence for Neogene surface downdrop in the central Basin and Range Province

Authigenic calcite and smectite stable isotopic records were determined for five Neogene terrestrial stratigraphic sections from the central Basin and Range Province and surrounding regions. We recognized a general pattern of increasing authigenic mineral δ 18 O values with decreasing age since the middle Miocene throughout the region. We suggest this pattern results from regional elevation decrease and associated changes in atmospheric circulation patterns. An anomalous pattern of decreasing δ 18 O values with decreasing age is observed in the Miocene Horse Spring Formation exposed in the Lake Mead Basin, which is contemporaneous with local magmatic activity. We suggest this pattern results from surficial uplift of local highlands feeding water into the Horse Spring depositional system. The topographic histories suggested here provide insight into the response of the land surface to crustal processes. Neogene extensional deformation in the central Basin and Range Province resulted in a net regional elevation decrease.

Methanogenic calcite, 13C-depleted bivalve shells, and gas hydrate from a mud volcano offshore southern California

Methane and hydrogen sulfide vent from a cold seep above a shallowly buried methane hydrate in a mud volcano located 24 km offshore southern California in 800 m of water. Bivalves, authigenic calcite, and methane hydrate were recovered in a 2.1 m piston core. Aragonite shells of two bivalve species are unusually depleted in 13C (to −19‰ δ13C), the most 13C-depleted shells of marine macrofauna yet discovered. Carbon isotopes for both living and dead specimens indicate that they used, in part, carbon derived from anaerobically oxidized methane to construct their shells. The δ13C values are highly variable, but most are within the range −12‰ to −19‰. This variability may be diagnostic for identifying cold-seep–hydrate systems in the geologic record. Authigenic calcite is abundant in the cores down to ∼1.5 m subbottom, the top of the methane hydrate. The calcite is depleted in 13C (δ13C = −46‰ to −58‰), indicating that carbon produced by anaerobically oxidized methane is the main source of the calcite. Methane sources include a geologic hydrocarbon reservoir from Miocene source rocks, and biogenic and thermogenic degradation of organic matter in basin sediments. Oxygen isotopes indicate that most calcite formed out of isotopic equilibrium with ambient bottom water, under the influence of gas hydrate dissociation and strong methane flux. High metal content in the mud volcano sediment indicates leaching of basement rocks by fluid circulating along an underlying fault, which also allows for a high flux of fossil methane.

A Holocene sedimentary record from Bosten Lake, China

A 9.24 m long sediment record from the Kaidu River delta margin of Bosten Lake, northwestern China, provides evidence for changes in water balance of the lake during the past 8600 calendar years. Quantitative and qualitative analyses of detrital components, authigenic calcite, TOC and Sr/Ca molar ratios as well as comparisons with selected ostracod assemblages show that lake level fluctuations at minimum 5–6 m display changes in effective moisture supply, mainly driven by the west wind system. Distinct periods of negative water balances and thus dry conditions are dated to 8.5–8.1, 7.3–7.1, around 6.0, at 5.4–5.0, 4.6–4.3, 3.9–3.6, 3.4–3.2, 2.6–2.5, 1.9–1.1, 0.5–0.3 and 0.18–0.05 cal. ky BP and match the regional and global pattern of climate shifts during the Holocene. Temporal shifts of moisture availability during mid-Holocene time were recognised as well and may be attributed to regionally different airflow conditions. The last 150 years of rising lake levels are accompanied by strong eutrophication as a result of increased human impact.

Methane-induced early diagenesis of foraminiferal tests in the southwestern Greenland Sea

Planktic and epibenthic foraminiferal δ 13C records at Site PS62/015-3 (southwestern Greenland Sea) reveal a series of transient events of extreme 13C depletion down to − 6‰ during the period 90–40 ka. Scanning electron microscope studies of the ultrastructures of foraminiferal tests suggest that 13C depleted specimens are affected by some 10–20% overgrowth by authigenic calcite contributing to the light δ 13C signal. Incremental-leaching experiments and census counts of pristine versus overgrowth-affected specimens show that the 13C depleted foraminiferal tests incorporate a primary δ 13C signature most likely ranging from + 1‰ to − 1.7‰ and a post-depositional δ 13C signature around − 17‰ to − 19.5‰. Extremely low values of productivity and organic carbon in Late Quaternary sediments along the east Greenland margin preclude organic matter as potential source of the isotopically light carbon. In contrast, thermal instability of clathrates and subsequent aerobic oxidation of (highly 12C enriched) methane in pore and ocean water provide a compelling mechanism to account for the negative δ 13C excursions of both primary and post-depositional carbonates. Here, pore water methane may have led to a supersaturation of 13C depleted bicarbonate and precipitation of isotopically light authigenic calcite on and in foraminiferal tests, a feature that may serve as a tracer to former sites of clathrate destabilization.

Oxygen and hydrogen isotope ratios in freshwater chert as indicators of ancient climate and hydrologic regime

The oxygen and hydrogen isotopic composition of Eocene and Miocene freshwater cherts in the western United States records regional climatic variation in the Cenozoic. Here, we present isotopic measurements of 47 freshwater cherts of Eocene and Miocene age from the Great Basin of the western United States at two different sites and interpret them in light of regional climatic and tectonic history. The large range of δ 18O of terrestrial cherts measured in this study, from 11.2‰ to 31.2‰ (SMOW: Standard Mean Ocean), is shown to be primarily the result of variations in δ 18O of surface water. The following trends and patterns are recognized within this range of δ 18O values. First, in Cenozoic rocks of northern Nevada, chert δ 18O records the same shift observed in authigenic calcite between the Eocene and Miocene that has been attributed to regional surface uplift. The consistent covariation of proxies suggests that chert reliably records and retains a signal of ancient meteoric water isotopic composition, even though our analyses show that chert formed from warmer waters (40°C) than coexisting calcite (20°C). Second, there is a strong positive correlation between δ 18O and δD in Eocene age chert from Elko, Nevada and Salina, Utah that suggests large changes in lake water isotopic composition due to evaporation. Evaporative effects on lake water isotopic composition, rather than surface temperature, exert the primary control on the isotopic composition of chert, accounting for 10‰ of the 16‰ range in δ 18O measured in Eocene cherts. From authigenic mineral data, we calculate a range in isotopic composition of Eocene precipitation in the north-central Great Basin of −10 to −14‰ for δ 18O and −70 to −100‰ for δD, which is in agreement with previous estimates for Eocene basins of the western United States. Due to its resistance to alteration and record of variations in both δ 18O and δD of water, chert has the potential to corroborate and constrain the cause of variations in isotope stratigraphies.

Late glacial to Holocene climate and sedimentation history in the NW Black Sea

Gravity cores from the continental slope in the northwestern Black Sea were studied using high-resolution stable isotope, grain size and XRF-scanning data. The measurements provide a 30 000 years AMS 14C-dated record of variations in the hydrological regime of the Black Sea and give insight into changing paleoenvironments in the surrounding areas. Stable climatic conditions during the Last Glacial Maximum were followed by a series of meltwater pulses most likely originating from the Scandinavian ice sheet between 18 000 and 15 500 yr BP. 1 1 All dates are calibrated to calendar years B.P. (yr BP). This meltwater input rose the level of the Caspian Sea to a point that Caspian water could spill into the Black Sea via the Manych-depression north of the Caucasian mountains. High-frequency oscillations in the XRF-data during this period suggest a probable link to the arctic climate regime. Later, during the Bølling/Allerød and the early Holocene, prevailing high temperatures led to authigenic calcite precipitation through increased phytoplankton activity, interrupted by the Younger Dryas and the “8200 yr BP cold event” with dominant clastic sedimentation.

Sedimentary geochemical evidence for recent eutrophication of Lake Chenghai, Yunnan, China

Geochemical anomalies and stable isotope ratios (δ18O, δ13C) in authigenic carbonates and organic matter (δ13C) from a 660-year sediment core from Lake Chenghai, southern China, provide a continuous history of recent lake eutrophication. The multi-proxy geochemical and isotopic record can be divided into a three-part history of contrasting limnological development, including: (1) a clear-water, oligotrophic open lake system (1340 and 1690 AD); (2) an environmentally unstable, hydrologically closed, oligotrophic lake system (1690–1940 AD); and (3) an increasingly eutrophic, closed lake system marked by higher organic matter, nitrogen, CaCO3, and pigment concentrations, and lower δ18O and δ13C values in authigenic calcite (1940–1999 AD). The unanticipated lowering of δ18O and δ13C of authigenic calcite during eutrophication is thought to be the result of disequilibrium water–carbonate fractionation of oxygen and carbon isotopes during periods of elevated primary production, pH, and [CO3 2−] activities in the water column. The recent eutrophication of Lake Chenghai indicated by these geochemical proxies is essentially simultaneous with large-scale human migration and the application of agricultural fertilizers in the catchment area during the 20th century.

Vegetation and sedimentation in the Lake Edward Basin, Uganda–Congo during the late Pleistocene and early Holocene

New palynological and sedimentological data obtained from the basal 3 m of core E96-2P from Lake Edward, Uganda–Congo record conditions wetter than present in the Edward basin from ∼11 000 to 6700 yr BP, in phase with other climate and vegetation records of northern hemispheric East Africa. Dominant pollen taxa include Celtis spp., Alchornea spp., Olea spp., and Moraceae indicating a moist semi-deciduous tropical forest. More xeric indicators such as Amaranthaceae and Asteraceae together with Poaceae comprise less than 5% of the pollen sum throughout this interval as compared with between 44 and 50% during a lake lowstand at ∼2000 cal yr BP and at the core top (near modern). The differences between these two assemblages suggest a ∼25 to 60% increase in annual precipitation during the early- to mid-Holocene as compared to modern (1500–2000 vs. 1200 mm/yr today). Early Holocene sediments in E96-2P are composed of laminated diatom oozes with moderately high total sulfur concentrations (2.8–4.7%) and no authigenic calcite, also indicative of conditions wetter than present. Between ∼9000 and 6700 yr BP, palynological and sedimentary proxies indicate sub-millennial-scale events related to changes in riverine discharge and runoff in the Edward basin. We attribute the variability in runoff, and hence precipitation, to Holocene variability in Indian or Atlantic Ocean SSTs or to shifts in the relative contribution of Indian and Atlantic moisture sources to the western Rift of equatorial Africa.

Evidence for large methane releases to the atmosphere from deep-sea gas-hydrate dissociation during the last glacial episode.

Past atmospheric methane-concentration oscillations recorded in polar ice cores vary together with rapid global climatic changes during the last glacial episode. In the "clathrate gun hypothesis," massive releases of deep-sea methane from marine gas-hydrate dissociation led to these well known, global, abrupt warmings in the past. If evidence for such releases in the water column exists, however, the mechanism and eventual transfer to the atmosphere has not yet been documented clearly. Here we describe a high-resolution marine-sediment record of stable carbon isotopic changes from the Papua Gulf, off Papua New Guinea, which exhibits two extremely depleted excursions (down to -9 per thousand ) at approximately 39,000 and approximately 55,000 years. Morphological, isotopic, and trace metal evidence dismisses authigenic calcite as the main source of depleted carbon. Massive methane release associated with deep-sea gas-hydrate dissociation is the most likely cause for such large depletions of delta(13)C. The absence of a delta(13)C gradient in the water column during these events implies that the methane rose through the entire water column, reaching the sea-air interface and thus the atmosphere. Foraminiferal delta(18)O composition suggests that the rise of the methane in the water column created an upwelling flow. These inferred emission events suggest that during the last glacial episode, this process was likely widespread, including tropical regions. Thus, the release of methane from the ocean floor into the atmosphere cannot be dismissed as a strong positive feedback in climate dynamics processes.

Stable oxygen and carbon isotopes in Late Glacial–Holocene freshwater carbonates from Belarus and their palaeoclimatic implications

Four sets of δ 18O and δ 13C curves were derived from Late Glacial to mid-Holocene spring and lacustrine carbonates from Belarus. Stratigraphic division was carried out by palynological studies and 14C-dating. It was revealed that variations of isotopic compositions and the dynamics of terrestrial vegetation were affected to some extent by common climatic forcing: birch expanded during relatively warm and/or arid phases (higher δ 18O and/or δ 13C), fir spread during relatively cold and/or humid phases (lower δ 18O and/or δ 13C), and grasses were abundant during relatively cold (lower δ 18O) or arid phases (higher δ 13C). A comparison of four pairs of δ 18O and δ 13C curves allowed the recognition of several palaeoclimatic events at a regional scale. Two main cold events interrupted by a warmer interval occurred in the Younger Dryas. A short-term episode of active permafrost decay at the end of the Younger Dryas and in the very beginning of the Preboreal caused an abrupt rise of lake levels followed by their rapid fall. A shift toward a lower precipitation–evaporation balance and lower lake levels took place roughly in the middle of the Boreal. A trend of decreasing temperatures throughout most of the Boreal culminated in a short-lived very cold phase at ∼8100 14C year BP or ∼9050 cal. 14C year BP, according to the proposed age model. In the Atlantic 1 warmer phases were coincident with the phases of the lower precipitation–evaporation balance. An episode of significant short-term warming occurred in the last third of the Atlantic 1. Correlation of the δ 18O curve derived from authigenic calcites precipitated between the beginning of the Preboreal 2 and the end of Atlantic 1 in the Ptich river valley with that from the GISP2 Greenland ice core allowed good matching of the trends and spikes (i.e. of the air temperature changes) under assumption that the ages of the Ptich pollen-based time model are 600–1000 years too old.

Isotopic evidence of the origin of mineralized waters from the Central Carpathian Synclinorium, SE Poland

Stable isotopes of hydrogen and oxygen were determined in 45 samples of water (27 samples of oil-associated waters, 17 samples of mineral waters used by spas, 1 sample of surface river water) from the Central Carpathian Synclinorium, covering a stratigraphic range of flysch sediments from Upper Cretaceous to Oligocene. Moreover, oxygen isotope compositions of authigenic calcite (vein and cement) from core samples of four boreholes were made to evaluate isotopic equilibrium between waters and diagenetic carbonates as a function of temperature. The saline and brackish waters (TDS from≅1 g/l to 48.9 g/l) considered here, generally belong to four hydrogeochemical classes: Na-Cl, Cl-HCO3-Na, HCO3-Cl-Na and HCO3-Na. Their isotopic composition causes them to fall to the right of Global Meteoric Water Line (GMWL) showing enrichment in 18O and 2H. On the other hand, relative to Standard Mean Ocean Water (SMOW) they are depleted in 2H and both depleted and enriched in 18O. The observed isotopic composition can be explained by the three-component mixing of surface water, diagenetically modified sea water (kind of connate water) and metamorphic water. The mixing is accompanied by an exchange of oxygen isotopes between water and carbonate cements causes 18O enrichment of interstitial waters. The contribution of isotopic exchange between water and clay minerals in shales was evaluated only theoretically basing of the literature.

Increased aridity during the early Holocene in West Greenland inferred from stable isotopes in laminated-lake sediments

Palaeoclimatic inferences from the Greenland ice cores suggest that Holocene climate was relatively uniform. The ice core records primarily reflect hemispheric temperature-related climate processes whereas lakes tend to reflect regional scale climatic variability. Stable isotopes in closed-basin lakes are sensitive to changes in effective precipitation. Here we report stable isotope evidence (δ 18O and δ 13C records from authigenic calcite in laminated lake sediments) from two oligosaline, closed-basin lakes in southern West Greenland (Søndre Strømfjord). The stable isotope profiles indicate a dynamic early to mid-Holocene climate as a strongly negative precipitation-evaporation balance from ∼7000 to 5600 cal yr BP lowered lake levels throughout the region. This interpretation of the stable isotope data is supported by a survey of modern lake-water isotopic composition and fossil shorelines around many closed-basin saline lakes which indicate higher lake levels in the past. The increased aridity during the early Holocene resulted from a shift in regional circulation patterns in the Davis Straits–Baffin Bay area. Enhanced stability of the West Greenland depression and the situation of a regional trough over the Søndre Strømfjord area caused weakened westerly airflow and strengthened outflow (i.e. winds blowing off the ice sheet), resulting in reduced precipitation in the region. Decreased precipitation, coupled with higher insolation, probably caused the negative precipitation–evaporation balance.

Spatial and temporal variation of Cenozoic surface elevation in the Great Basin and Sierra Nevada

The surface uplift of mountain belts caused by tectonism plays an important role in determining the long-term climate evolution of the Earth. However, the general lack of information on the paleotopography of mountain belts limits our ability to identify the links and feedbacks between topography, tectonics, and climate change on geologic time-scales. Here, we present a δ 1 8 O and δD record of authigenic minerals for the northern Great Basin that captures the timing and magnitude of regional surface uplift and subsidence events in the western United States during the Cenozoic. Authigenic calcite, smectite, and chert δ 1 8 O values suggest the northern Great Basin region experienced ∼2km of surface uplift between the middle Eocene and early Oligocene followed by ∼1 to 2km of surface subsidence in the southern Great Basin and/or Sierra Nevada since the middle Miocene. These data when combined with previously published work show that the surface uplift history varied in both space and time. Surface uplift migrated from north to south with high elevations in southern British Columbia and northeastern Washington in the middle Eocene and development of surface uplift in north and central Nevada in the Oligocene. This pattern of north to south surface uplift is similar to the timing of magmatism in the western Cordillera, a result that supports tectonic models linking magamtism with removal of mantle lithosphere and/or a subducting slab.