Uwe Wiechert - Academia.edu (original) (raw)

Papers by Uwe Wiechert

The Tibetan Plateau (TP), including its surrounding mountain ranges, represents the largest store... more The Tibetan Plateau (TP), including its surrounding mountain ranges, represents the largest store of ice outside the polar regions. It hosts numerous lakes as well as the head waters of major Asian rivers, on which billions of people depend, and it is particularly sensitive to climate change. The moisture transport to the TP is controlled by the Indian and Pacific monsoon and the Westerlies. Understanding the evolution of the interaction of these circulation systems requires studies on climate archives in different spatial and temporal contexts. The objective of this study is to learn more about the interannual variability of precipitation patterns across the TP and how different hydrologic systems react to different climatic factors.Aragonite shells of the aquatic gastropod Radix, which is widely distributed in the region, may represent suitable archives for inferring hydrologic and climatic signals in particularly high resolution. Therefore, sclerochronological studies of d18O and d13C ratios in Radix shells from seven lakes were conducted, each representing a different hydrologic and climatic setting, on a transect from the Pamirs across the TP.The shell patterns exhibit an increasing influence of precipitation and a decreasing influence of evaporation on the isotope compositions from west to east. d18O values of shells from lakes on the eastern and central TP (Donggi Cona, Yamdrok Yumco, Tarab Co) mirror monsoon signals, indicated by more negative values and higher variabilities compared to the more western lakes (Karakul, Bangong/Nyak, Manasarovar). In Yadang Co, located on the central southern TP, the monsoon rains did not reach the lake in the sampling year, although it is located in a region which is usually affected by monsoon circulation. The d18O values are used to differentiate the annual hydrological cycle into ice cover period, melt water period, precipitation period and evaporation period. d13C compositions in the shells particularly depend on specific habitats, which vary in biological productivit [...]

Quaternary International, 2020

Abstract The Tibetan Plateau and the Himalayas influence monsoonal circulation patterns and repre... more Abstract The Tibetan Plateau and the Himalayas influence monsoonal circulation patterns and represent the water towers of humanity. Modern coupling of atmospheric circulation and the hydrological cycle over and on the plateau can be observed and delineated, and the factors that control lake levels can be identified. Recent monitoring of lakes has shown that many have grown for decades due to higher meltwater inflow or stronger rainfall from different sources, depending on the location. The long-term dynamics of lakes, however, can be best described by using high-resolution climate archives. We focus here on the often controversial discussion of Holocene lake development and use the Bangong Co drainage basin on the western Tibetan Plateau as a case study. We aim to identify the factors that influence lake level, such as monsoonal or convective precipitation and meltwater. To accomplish this, shells of the aquatic gastropod genus Radix were collected from an early Middle Holocene sediment sequence in the Nama Chu subcatchment of the eastern Bangong Co, and sclerochronlogical isotope patterns for five shells were obtained at a weekly to submonthly resolution. Our data suggest that at ca. 7.5 ka, monsoonal rainfall was higher than it is today. However, summer precipitation was not continuous but occurred as extended moisture pulses. This result implies that the northern boundary of the SW Asian monsoon was similar to that today. We identify convective rainfall events significantly stronger than today and relate these events to higher soil moisture and larger lake surface areas under higher insolation conditions. The regional amount of meltwater corresponds with the westerly derived winter snowfall. Exceptionally heavy δ13C values archived in the shells were likely, at least partly, triggered by biogenic methane production. We suggest that our approach is suitable for studying other lake systems on the Tibetan Plateau from which fossil Radix shells can be obtained.

Climate of the Past Discussions, 2019

Billions of people depend on the precipitation of the Asian monsoons. The Tibetan Plateau and the... more Billions of people depend on the precipitation of the Asian monsoons. The Tibetan Plateau and the Himalayas on the one hand strongly influence the monsoonal circulation pattern and on the other hand represent water towers of humanity. Understanding the dynamics of the Asian monsoons is one of the prime targets in climate research. Modern coupling of atmospheric circulation and hydrological cycle over and on the plateau can be observed and outlined, and lake level controlling factors be identified.

Precambrian Research, 2015

Ediacaran carbonates from the shallow-water section near Xiaofenghe (Three Gorges area, Hubei Pro... more Ediacaran carbonates from the shallow-water section near Xiaofenghe (Three Gorges area, Hubei Province) on the Yangtze Platform, South China have been studied to understand post Marinoan changes in seawater chemistry. Major and trace element abundances and Sr–Nd isotopic compositions were obtained on acetic acid leachates of carbonate sediments from both the Doushantuo Formation (Members D1–D4) and the overlying Dengying Formation. C and O isotopic compositions of these samples were analysed using the phosphoric acid method. With the exception of the D1 cap carbonates (87Sr/86Sr = 0.7088–0.713), most samples show low Mn/Sr and 87Sr/86Sr (0.7078 in D2–D4 and 0.709 in the Dengying Formation) similar to seawater values inferred from Ediacaran carbonates elsewhere. These data, together with the absence of a correlation between 87Sr/86Sr and δ18Ocarb, indicate minor diagenetic fluid overprinting. The initial ɛNd values of the carbonate leachates show a limited variation range from −4.3 to −7.5 with the highest values in D1. These data overlap with published data for Doushantuo sediments in South China, and possibly indicate an isotopically homogenous Nd source in the source area during most of the Ediacaran. D1 samples of cap carbonates display negative δ13Ccarb values of −2.0 to −3.6 (VPDB) and δ18Ocarb values ranging from −6.5 to −7.3 (VPDB), whereas the overlying D2 strata show positive δ13Ccarb values around 6. Carbonate rocks from D3 and D4 show a large variation of δ13Ccarb from −1.4 to 9.4, with some samples having unusually high δ13Ccarb values coupled to low TOC contents. Most carbonate leachates at the base of the Doushantuo display superchondritic Y/Ho and variable HREE enrichment (Pr/Yb < 1), similar to modern seawater. This suggests a similar seawater chemistry as under modern open ocean conditions at the beginning of the Ediacaran, whereas lower Y/Ho in D3 correlate with flat REE + Y patterns, indicating a change to variable freshwater–seawater mixing and suppression of HREE/LREE fractionation as in modern estuaries. Carbonate rock samples from D4 are rich in clay minerals (up to 15%), of possible authigenic origin, such as nontronite and saponite. Formation of these clay minerals may only be possibly under extreme alkaline and saline conditions. The D4 carbonate leachates show the highest Y/Ho and lowest Ce/Ce* values and have unusually high δ13Ccarb values, which may also support the conditions of extreme shallowing and perhaps temporarily restricted basins that led to a rise in salinity due to high evaporation rates. Ce anomalies in carbonate leachates show strong variation throughout the profile. A secular increase of seawater oxygenation through the Ediacaran is not obvious. However the most pronounced Ce anomalies can be found in the upper Doushantuo (Ce/Ce* as low as 0.64).

CATENA

The Atacama Desert is the oldest and driest non-polar desert on Earth. Millions of years of hyper... more The Atacama Desert is the oldest and driest non-polar desert on Earth. Millions of years of hyperaridity enabled salt accumulations through atmospheric deposition. These salts can serve as proxies to decipher the interaction between water and soil as well as to understand the habitability with changing environmental settings. Therefore, we investigated four soil profiles regarding their mineralogy, salt abundance, and sulfate stable isotopic composition. The profiles were located along an elevation transect in the hyperarid region southeast of Antofagasta, Chile. The two lower sites situated on the distal parts of inactive alluvial fan deposits were subject to occasional fog occurrences. The upper steeper-sloped sites experienced no fog but are subject to minimal erosion. In all soil profiles, sulfates are the dominant salts showing a downward transition from gypsum to anhydrite that is accompanied by an increase of highly soluble salts and a decrease of sulfate δ 34 S and δ 18 O values. These trends are consistent with downward directed water infiltration during rare rain events causing salt dissolution followed by precipitation within the deeper soil column. This conclusion is also w supported by our Rayleigh fractionation model. We attribute the presence of anhydrite at > 40 cm depth to the cooccurrence of nitrate and chloride salts, which decreases water activity during sulfate precipitation and therefore drives anhydrite formation. Along the elevation transect, the total salt inventories of each profile show a trend for nitrates and chlorides concentration decreasing with elevation. This observation together with the sulfate stable isotopes indicates a fog-independent source and suggests remobilization of soluble salts through enhanced washout from hillslopes to alluvial fans. These findings are essential for assessing the longterm regional habitability of hyperarid environments and have also relevance for Mars.

Geochemistry, Geophysics, Geosystems, 2015

To evaluate the interlaboratory mass bias for high-precision stable Mg isotopic analysis of natur... more To evaluate the interlaboratory mass bias for high-precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3& in 26 Mg/ 24 Mg, >4 times of uncertainties) different for some mafic samples. The interlaboratory mass bias does not correlate with matrix element/Mg ratios, and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well-calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias.

Mineralogical and geochemical data was determined on variably serpentinized harzburgites and carb... more Mineralogical and geochemical data was determined on variably serpentinized harzburgites and carbonate-veined serpentinites from the Chimaera seeps, Turkey, an active continental serpentinization system. Mineralogical descriptions are based on macroscopic observations and detailed polarization microscopy. Bulk rock sulphur contents and stable sulphur isotope compositions of sulphide and sulphate fractions were conducted by wet chemical sulphur extraction by chromium reduction under nitrogen atmosphere and subsequent isotope analyses on an isotope ratio mass spectrometer combined with an elemental analyzer. Calculated errors are from duplicate extractions and analyses. Mineral chemical data of sulphide minerals and opaque phases of the studied samples were determined by electron microprobe analyses on a JEOL 8200 Superprobe at the Freie Universität Berlin. The general analytical procedure is described in Schwarzenbach et al. (2021).

Geochemistry, Geophysics, Geosystems, 2013

Geochimica et Cosmochimica Acta

Introduction: To determine the oxygen isotopic composition of the present day solar wind is one o... more Introduction: To determine the oxygen isotopic composition of the present day solar wind is one of the key goals of the Genesis solar wind (SW) collection mission, as the solar wind is a proxy for the solar nebula. In order to increase the number of atoms to be analysed and hence analytical precision on O isotopes, SW ions in the mass range up to 28 amu were accelerated and focussed onto a "concentrator target" by an electrostatic mirror. However, the implanted SW ions on this target are isotopically fractionated due to a combination of effects [1]. This fractionation is expected to vary as a function of the radial position on the target. In order to correct the O isotope values in the concentrator targets this fractionation will be determined experimentally for Ne isotopes and by modelling the expected difference between Ne and O fractionation factors. Ne isotopes are most suitable for this purpose because i) it is an abundant element in the Sun, ii) its isotopic composit...

$Research paper thumbnail of Crystallographic control on lithium isotope fractionation in Archean to Cenozoic lithium-cesium-tantalum pegmatites$

Geology, 2016

The age distribution of lithium-cesium-tantalum (LCT) pegmatites largely overlaps with major phas... more The age distribution of lithium-cesium-tantalum (LCT) pegmatites largely overlaps with major phases of collisional orogenic events and assembly of super-continents. Some of the largest known LCT pegmatite deposits formed in very short intervals, 2.7-2.5 and 1.9-1.8 billion years ago (Ga), corresponding to two major pulses of continental crust growth. However, the exact process of generation and segregation of large volumes of Li-bearing pegmatite liquids, perhaps involving disequilibrium fractional crystallization and leaving residual melts enriched in fluxing elements such as B, F, H2O, Li, and P, remains largely obscure. The new data on Li contents and isotope compositions in major mineral phases from temporally and geographically separated pegmatite bodies document extreme variations in δ 7 Li values among individual large LCT pegmatite bodies, in particular Archean occurrences. The observed >10‰ variations in δ 7 Li values for the same mineral phases from different localities (i.e., beryl, petalite, spodumene, lepidolite, amblygonite, muscovite) contrast with globally homogeneous Li isotope systematics of major mineral phases from unmodified mantle rocks. Consistent Li isotope offsets between coexisting mineral phases are best explained by Li isotope fractionation as a function of the bond length between Li and neighboring ions (O, OH, F). We suggest that spatially distinct Li isotope patterns act as fingerprints for different pegmatites and can be explained by the pre-existing Li isotope differences of their crustal sources at the time of pegmatite formation owing to differences in crustal age and evolution. This would imply secular evolution of the continental crust over Earth history toward present-day globally broadly uniform crustal 7 Li/ 6 Li ratios (δ 7 Li ~0‰). The differences among Archean occurrences could reflect possible Archean paleogeography and perhaps be linked with different thermal regimes of individual cratons as a consequence of variations in crustal thickness. One possible application of the new data set may be in source verification of gemquality stones by using Li isotopes.