Present-day Precipitation Research Articles

Oxygen-18 in present-day precipitation on the Tibetan Plateau

The temporal and spatial variations of the δ18O in precipitation on the Tibetan Plateau are analyzed. There is no temperature effect in the southern Tibetan Plateau. Amount effect has been observed at Lhasa station. However, the seasonal variations of the δ18 in precipitation are different from that of precipitation intensity, showing that the precipitation intensity is not a main controlling factor on the stable isotopic compositions in precipitation in the southern Tibetan Plateau. There is notable temperature effect in the middle and northern Tibetan Plateau. The seasonal variations of the δ18O in precipitation are almost consistent with those of air temperature there, indicating that temperature is a main factor controlling the stable isotopic variations in precipitation. A meridional cross-section shows that a notable depletion of the stable isotopic ratio in precipitation takes place in the Himalayas due to very strong rainout of vapor as it rises over the Himalayas, then the δ18O remains basically unchanged although a big temperature fluctuation appears from Tingri to Amdo, and the δ18O in precipitation increases rather than decreases from Tanggula to the northern Tibetan Plateau. Such a spatial distribution is related to the replenishment of vapor with the relatively heavy stable isotopic compositions originated from the inner Plateau.

Climate change and mass movements in the NW Argentine Andes

The chronology of multiple landslide deposits and related lake sediments in the eastern Argentine Cordillera suggests that major mass movements cluster in two time periods during the Quaternary: between 35 000 and 25 000 14C yr BP and after 5000 14C yr BP. The older cluster may correspond to the Minchin wet period (40 000 and 25 000 14C yr BP) identified in tropical and subtropical South America, suggesting a causal relation between enhanced landslide activity and climate change. The younger cluster predates the Titicaca wet period that began at about 3900 14C yr BP which also affected other regions in the Andes and the Amazon Basin. No landslide and associated lake sediments are documented during the Tauca wet period (between 16 000 and 8000 14C yr BP). However, the two clusters correspond to periods where it assumed that the El Niño/Southern Oscillation (ENSO) and tropical Atlantic sea surface temperature dipole (TAD) were active. The analysis of the present-day precipitation patterns in NW Argentina indicates significant spatial and temporal differences between the intra-Andean part of the study area and the Andean foreland. Whereas the TAD seems to consistently increase rainfall, the intensity of precipitation during the El Niño phase of the ENSO is reduced to only 25% of the mean annual average in the intra-Andean basins, whereas the regions east of the Andes receive more than 125%. Similar results, but with an opposite sign, characterize La Niña events. The comparison of this pattern with paleo-precipitation data as inferred from varved lake sediments suggests that increased interannual climate variability and, therefore, increased fluctuations in rainfall and river discharge in narrow valleys may reduce landsliding thresholds.

High-Resolution Modeling of the Advance of the Younger Dryas Ice Sheet and Its Climate in Scotland

Ice-sheet modeling tightly constrained by empirical studies provides an effective framework to reconstruct past climatic and environmental conditions. Scotland was severely affected by the abrupt climate change associated with the Younger Dryas Stade, during which an extensive ice sheet formed across the west highlands after a period of ice-free conditions. Here, a quasi-three-dimensional, time-dependent ice flow/mass-balance model is developed and applied to Scotland at 1 km resolution. The flow model is based on the driving stress approximation with an additional longitudinal correction term, essential at this scale of operation. Surface mass balance is driven by temperature and precipitation changes and further mass wastage is achieved through an empirically defined calving term. The ice dynamics and mass-balance components are coupled through the equation for mass continuity, which is integrated through time over a finite-difference grid which yields the geometric evolution of the ice sheet. Initial experiments reveal the model to be relatively insensitive to internal parameters but highly sensitive to mass balance. Furthermore, these experiments indicate that Scotland is readily susceptible to glaciation with large glaciers building up on the flanks of Ben Nevis after a temperature depression of 2.5°C, under present-day precipitation. The Younger Dryas is modeled using a GRIP temperature series locally adjusted for amplitude and a systematic series of runs enables the isolation of the climate which best matches mapped ice limits. This “optimum-fit” configuration requires an annual temperature cooling of 8°C and the introduction of substantial west–east and south–north precipitation gradients of 40 and 50%, respectively, to the present-day regime. Under these conditions, a series of substantial independent regional ice centers develop in agreement with trimline studies and after 550 years the modeled ice sheet closely resembles the maximum limits as indicated by field mapping. However, modeled ice continues to expand beyond 550 yr, in conflict with the mapped ice limits which suggest a prolonged period of stability. This discrepancy may be explained by the onset of extreme aridity ca. 400 yr into the Stade associated with a southern migration of the Polar Front, leading to a reduction in atmospheric circulation which effectively starved the ice sheet of its moisture source, preventing further expansion. Introduction of an additional 20% reduction in precipitation to the “optimum-fit” regime after 350 yr brings the modeled ice sheet to equilibrium, substantiating this conclusion.

Chlorine-36 and the initial value problem

Chlorine-36 is a radionuclide with a half-life of 3.01×105a. Most 36Cl in the hydrosphere originates from cosmic radiation interacting with atmospheric gases. Large amounts were also produced by testing thermonuclear devices during 1952–58. Because the monovalent anion, chloride, is the most common form of chlorine found in the hydrosphere and because it is extremely mobile in aqueous systems, analyses of both total Cl– as well as 36Cl have been important in numerous hydrologic studies. In almost all applications of 36Cl, a knowledge of the initial, or pre-anthropogenic, levels of 36Cl is useful, as well as essential in some cases. Standard approaches to the determination of initial values have been to: (a) calculate the theoretical cosmogenic production and fallout, which varies according to latitude; (b) measure 36Cl in present-day precipitation and assume that anthropogenic components can be neglected; (c) assume that shallow groundwater retains a record of the initial concentration; (d) extract 36Cl from vertical depth profiles in desert soils; (e) recover 36Cl from cores of glacial ice; and (f) calculate subsurface production of 36Cl for water that has been isolated from the atmosphere for more than one million years. The initial value from soil profiles and ice cores is taken as the value that occurs directly below the depth of the easily defined bomb peak. All six methods have serious weaknesses. Complicating factors include 36Cl concentrations not related to cosmogenic sources, changes in cosmogenic production with time, mixed sources of chloride in groundwater, melting and refreezing of water in glaciers, and seasonal groundwater recharge that does not contain average year-long concentrations of 36Cl.

Deuterium and oxygen-18 in present-day precipitation: data and modelling

The cycles of the water isotopic species (HDO and H2 18O) have been incorporated in three atmospheric General Circulation Models (GCMs) with the main objective of predicting the distribution of water stable isotopes in precipitation, The performances of those models are examined for present-day climate through a comparison with existing data available from the IAEA/WMO network established in 1961 and from other sources.

Modelling mass balance on former maritime ice caps: a Patagonian example

Understanding the response of present and former ice caps to climatic fluctuations depends on an effective means of modelling mass balance. Ablation can be reasonably well approximated from surface energy-balance calculations, but changes in snowfall, the primary mass-gain mechanism, are poorly understood. This is a particularly important limitation in those mid-latitude areas where the location of precipitation belts changes from glacial to interglacial conditions. This paper develops a method of modelling the spatial and temporal variability of snowfall in Patagonia. Snowfall is predicted from a vertically integrated moisture-balance equation representing winter, equinoctial and summer seasons. Inputs to the model are sea-surface temperatures and the geostrophic wind at 700 mbar. The dominant process affecting precipitation is the advection of moisture in an air column whose ability to hold moisture is dependent on temperature and altitude. Relative humidity controls evaporation and precipitation, both of which increase with higher wind speeds. Snowfall is calculated from seasonal precipitation and the annual temperature regime, while ablation is estimated from degree-day calculations. The model successfully simulates present-day precipitation and mass-balance patterns in Patagonia. We also run the model for full-glacial conditions with and without an ice cap, revealing the extreme variability of mass balance at different stages of a glacial cycle and the relative importance of altitude/mass-balance feed-back in ice-cap growth. Since the model is driven by wind and temperature fields it has the potential to use the results of GCMs as input.

Modelling mass balance on former maritime ice caps: a Patagonian example

Understanding the response of present and former ice caps to climatic fluctuations depends on an effective means of modelling mass balance. Ablation can be reasonably well approximated from surface energy-balance calculations, but changes in snowfall, the primary mass-gain mechanism, are poorly understood. This is a particularly important limitation in those mid-latitude areas where the location of precipitation belts changes from glacial to interglacial conditions. This paper develops a method of modelling the spatial and temporal variability of snowfall in Patagonia. Snowfall is predicted from a vertically integrated moisture-balance equation representing winter, equinoctial and summer seasons. Inputs to the model are sea-surface temperatures and the geostrophic wind at 700 mbar. The dominant process affecting precipitation is the advection of moisture in an air column whose ability to hold moisture is dependent on temperature and altitude. Relative humidity controls evaporation and precipitation, both of which increase with higher wind speeds. Snowfall is calculated from seasonal precipitation and the annual temperature regime, while ablation is estimated from degree-day calculations. The model successfully simulates present-day precipitation and mass-balance patterns in Patagonia. We also run the model for full-glacial conditions with and without an ice cap, revealing the extreme variability of mass balance at different stages of a glacial cycle and the relative importance of altitude/mass-balance feed-back in ice-cap growth. Since the model is driven by wind and temperature fields it has the potential to use the results of GCMs as input.

Fluid evolution and mineral formation in the Beregovo gold-base metal deposit, Transcarpathia, Ukraine

The Beregovo vein-type gold-base metal deposit is hosted by volcanic sedimentary sequences of the Pannonian basin in the Carpathian Neogene volcanic belt. Four stages of mineralization were distinguished: (1) sulfide, (2) quartz-barite, (3) carbonate-quartz, and (4) carbonate-goethite. Homogenization temperature, salinity, and chemical and isotopic composition of fluid inclusions in sphalerite, galena, barite, quartz, and carbonates, combined with the delta 18 O values of quartz, carbonates, and barite, suggest the presence of three major fluid types: (1) deep, relatively high temperature (250 degrees C) and moderate salinity (about 8 wt % NaCl equiv) Na-K-Cl fluid with relatively high delta 18 O (5ppm), delta D (-60ppm), and delta 13 C (-5ppm) values; (2) high-salinity (up to 14 wt % NaCl equiv) Ca-Mg-Na-Cl formation water with higher delta D values (-40ppm); and (3) near-surface steam-heated (about 200 degrees C) sulfate-bicarbonate-dominated fluid delta 18 O from -5 to -1ppm, and delta D from -110 to -56ppm). Low delta 13 C values (-25ppm) for CO 2 in type 2 and 3 fluids reflect a significant contribution of carbon from organic-rich sedimentary wall rocks.The delta 34 S values of sulfide minerals range from -2 to +2.5 for pyrite, -1 to +6 for sphalerite, and -2 to +8 per mil for galena. Two sources of sulfur are identified for the Beregovo sulfide ores. Ores with delta 34 S values close to 0 per mil may have been magmatic and those with higher delta 34 S values (up to 8ppm) perhaps resulted from organic matter (5ppm) in the wall rocks.Two main isotopically and genetically different barite generations were found: hypogene barites with delta 34 S values from 13 to 26 per mil, and supergene barites with values from 4.0 to 5.5 per mil. None of the barites had isotopically equilibrated with the sulfide minerals. If equilibrium in the isotopic exchange between sulfate and water were realized, the delta 18 O values of the fluid would range from -5.0 to -2.0 and -10.5 to -5.9 per mil for the two generations, respectively. For comparison, oxygen isotope composition for present-day precipitation is about -10 per mil.

Long quaternary record in eastern Washington, U.S.A., interpreted from multiple buried paleosols in loess

Loess that is from a few meters to 75 m thick covers an area of more than 20,000 km 2 on the Columbia Plateau in Washington, Idaho and Oregon. The region of deepest loess is in eastern Washington and is called “The Palouse”. The Palouse is downwind of the Channeled Scabland, through which massive outburst floods from Glacial Lake Missoula flowed repeatedly during glacial stages of the Pleistocene. The deepest stratigraphic section yet studied is in a remnant of the once continuous loess cover that is surrounded by Scabland channels. A normal over reverse magnetic polarity zonation in this section supports an age of more than 790,000 yr, but 15–35 m of loess may be present beneath this section, so a total of 1.5-2 million yr is possible. Upward fining of texture in some layers in the loess at this site and stratigraphic evidence at nearby sites suggest that at least some pulses of loess deposition were triggered by episodes of cataclysmic floods. Nineteen or more individual paleosols can be recognized in 26 m of section based on field morphology, physical and chemical properties, and micromorphology. The paleosols consist of calcic, petrocalcic and duripan horizons, many of which are associated with cambic horizons. Less strongly developed soils in this and other sections have been obscured by partial overlap of soil development in an episodically rising loess landscape. Paleosols in the loess at other sites reflect a dry-to-moist climatic gradient from west to east across the region during the Pleistocene that was grossly similar to today's. Accumulation of pedogenic carbonates and silica dominates in a western zone where the present-day mean annual precipitation is less than about 450 mm. Translocation of silicate clays and leaching of carbonates dominate in a central steppe zone where precipitation is between 450 and about 700 mm. Fragipans are common in an eastern zone along the steppe-forest transition at present-day precipitation of over 700 mm. Reconnaissance study suggests that these climatic-pedogenic zones have been somewhat stable during many of the episodes of soil development preserved in the loess, although some paleosols have different features from the majority of the paleosols in that zone, e.g., paleosols with fragipans in a sequence of paleosols with argillic horizons, which suggests that some episodes of soil development took place under different climatic conditions.

Present-day precipitation of lead and zinc from groundwaters

Small quantities of galena and sphalerite are currently precipitating 26m below the surface, at the Vinemount quarry, Ontario, Canada from sulfurous spring waters, issuing from carbonates of the Lockport Formation. At the spring orifices, these sulfides are present in both fracture coatings and an associated black sludge. These precipitates are overlain by a crust rich in native sulfur. The mineralogy of the precipitates, and the chemistry of waters taken from three consistent spring horizons, indicates that downward-moving meteoric groundwaters leach metals from the host rocks and precipitate galena and sphalerite on encountering H2S-rich waters at the water table. While our observation of the transport and deposition of lead and zinc by shallow groundwaters lends support to intra-karstic models for the direct precipitation of galena and sphalerite, it also enables us to propose that many carbonate-hosted Pb-Zn deposits, of differing theories of origin, may have been enriched during periods of uplift.

Origin, age and movement of pore water in argillaceous Quaternary deposits at four sites in southwestern Ontario

Hydrogeologic information in the form of textural properties, field- and laboratory-determined hydraulic conductivity values, vertical hydraulic gradients from piezometer nests, and major-ion and isotopic contents ( 18O, 2H, 3H, 13C, 14C), was obtained from four sites located in southwestern Ontario on thick deposits of clayey till and glaciolacustrine clay. At each site the piezometers were installed from the water-table zone, situated 1–4 m below the surface, to a maximum depth between 20 and 35 m below the surface. Values of hydraulic conductivity determined in the laboratory by consolidometer and triaxial methods are in the order of 10 −8–10 −7 cm/s and are similar to values calculated from rates of water-level rise in piezometers. Mean values of average vertical groundwater velocity, calculated with the measured values of hydraulic conductivity, porosity and hydraulic gradient, using the Darcy equation for a saturated non-fractured porous medium, range from 0.13 to 0.26 cm/yr. Tritiated pore waters only occur within 3–6 m of ground surface, thereby indicating that groundwaters below this depth recharged prior to at least 1952. 18O in the pore waters exhibit a distinctive regular shift with depth from water-table values of −9 to −10‰ (SMOW), characteristic of present-day precipitation, to values between −14 and −17‰ at depths of 20–40 m, which are characteristic of much cooler waters. Corrected 14C ages of the groundwater are greater than 8000 yr. B.P., thereby suggesting that the groundwaters at depth entered these deposits during or after the formation of these deposits approximately 11,000 to 14,000 yr. ago. 18O, 2H and Cl − concentration profiles were simulated with a one-dimensional model for transport by advection and diffusion in a saturated porous medium. Reasonable agreement between the model and the field profiles was obtained with values of effective diffusion coefficients of 3.0 · 10 −6 cm 2/s for 18O, 2H and Cl − and with groundwater velocities between 0.03 to 0.05 cm/yr. Based on these results, it is concluded that the pore water in these deposits is a mixture of late Pleistocene and modern waters and that the distribution of 18C, 2H and Cl − in these deposits is influenced predominantly by molecular diffusion, apparently more so than by hydraulic flow.

Alteration of Crude Oil by Waters and Bacteria--Evidence from Geochemical and Isotope Studies

Oils from the Mississippian Mission Canyon 5 (MC5) carbonates of Saskatchewan, on the northern flank of the Williston basin, have been changed from sweet, medium- to high-gravity crudes, to sour nonproducible tars where the MC5 subcrop trend is intersected by a zone of fresh meteoric formation water. These waters are recognized by lower salinities and oxygen isotopic compositions which resemble water from present-day precipitation. The alteration of petroleum by fresh water may involve three processes: inorganic oxidation, water washing, and biodegradation. Inorganic oxidation (by molecular oxygen) is restricted largely to near-surface reservoirs and is not documented by the present study of crudes from depths of 3,500-6,000 ft. Water washing has occurred where formation waters undersaturated with hydrocarbons have dissolved the more soluble lighter hydrocarbons in the reservoired oils. This has left more sulfurous, heavier crudes with lower solution gas-oil ratios. Bacterial degradation has occurred within the area of freshwater invasion, presumably through the introduction of microorganisms, molecular oxygen, and nutrients. Normal paraffins and smaller naphthenes have been preferentially degraded and nonhydrocarbon NSO's and asphaltenes produced with resulting low-wax, heavier, more sulfurous crudes. Isotope data provide additional evidence of bacterial activity in the subsurface. In the study area, water washing and biodegradation have reduced crude oil gravities by more than 20 degrees API and increased the sulfur content from less than 1 percent to more than 3 percent.