Abstract We investigate the seasonal evolution of stratification in a deep (100 m), high-altitude... more Abstract We investigate the seasonal evolution of stratification in a deep (100 m), high-altitude (4,730 m a.s.l.) dimictic lake on the Tibetan Plateau using three years of observation of Nam Co. The lake is situated at relatively low latitude (30 °N) where it receives high solar radiative forcing (observed maximum daily average 400 W m−2), yet the annual average air temperature is close to 0 °C at this high altitude. These features make Nam Co distinct from most well-documented dimictic lakes, which are usually located at higher latitude and lower altitude. We classify seasonal stratification into six phases based on strength of stratification, surface temperature relative to the temperature of maximum density, ice-cover, and heat and mixing dynamics, which we use to compare Nam Co with better-documented, higher-latitude dimictic lakes. While the three warm stratification phases (i.e. when water is warmer than the temperature of maximum density) in Nam Co are relatively cold, they are otherwise similar to that observed in high-latitude dimictic lakes. Conversely, two of Nam Co’s cold stratification phases are distinct from that reported in high-latitude lakes. These two phases are characterized by the interplay between the relatively strong radiative forcing and surface heat flux, and include the following: 1) during fall turnover, persistent winds aided by radiatively driven convection prolong vertical mixing (i.e. turnover) and surface heat loss such that the entire water column cools well below the temperature of maximum density (as low as 1 °C); and 2) in contrast, under ice-cover with relatively little snow, the entire water column of the lake warms continuously due to through-ice solar radiative flux. The intense cooling and heating during these two phases counteract each other such that hypolimnetic temperature at spring turnover is similar to that observed in high latitude lakes. Our observations highlight the relative importance of radiatively driven convection on the seasonal stratification dynamics of Nam Co, and underscore that these dynamics must be considered when attempting to predict climate change impacts on high-altitude, low-latitude lakes, including the >1100, largely unstudied, lakes on the Tibetan Plateau.
Changing environmental conditions associated with a predicted change in global temperatures is be... more Changing environmental conditions associated with a predicted change in global temperatures is believed to affect water availability on the Tibetan Plateau, which provides freshwater to approximately 20% of the global population. The reconstruction of past environmental conditions may provide past climatic analogues to modern warming scenarios and help to elucidate the environmental response to climate forcing. One of the most abundant archives of Late Quaternary environmental conditions on the Tibetan Plateau are lacustrine sediments associated with lake systems. However, lake systems are known to reveal complexity in their response to climate forcing resulting in individualized sedimentary patterns for different lake systems. This individuality delimits the potential to infer supra-regional patterns of environmental change. This thesis presents a reconstruction of environmental conditions during the last ~12 ka around Lake Heihai, situated on the northern fringe of the Tibetan Plateau. Special focus is given on analyzing the climatic sensitivity of inorganic sediment fluxes, i.e. fluxes of allogenic minerals originating from the terrestrial surroundings of the lake as well as fluxes of endogenic minerals precipitated in the water column of the lake. A cluster analysis on the mineralogical composition of terrestrial reference samples revealed four major source areas, distributed evenly on two bordering mountain ranges: the Kunlun Mountains in the south and the Burhan Buda Mountains in the north of the lake. Especially sediment source areas on the Kunlun range differ in their main mode of activation, which is predominantly precipitation generated and glacial runoff. An end-member analysis of grain size distributions of the siliciclastic detrital fraction revealed that sediment supply from the Burhan Buda range is a consequence of eolian dynamics. The findings suggests a blockage effect of the Kunlun range on southerly air masses, leading to an extended rain shadow north of the Kunlun range. Endogenic mineral formation in Lake Heihai is dominated by carbonate precipitation. The geochemical analysis of carbonate mineral phases apparent within lacustrine sediments revealed, that the main control on the predominant endogenic carbonate mineral phase is the ratio between solute magnesium and calcium (Mg/Ca) in lake waters. The solute lake water Mg/Ca is controlled by solute composition of inflowing waters as well as lake internal Mg enrichment relative to Ca by the removal of Ca due to carbonate precipitation. The solute composition of inflowing waters, in turn, is mainly influenced by weathering intensities of limestone outcrops in the drainage basin of the lake. Changes in the inorganic mineral assemblage of a sediment core from the deepest part of the lake revealed major variations in sediment fluxes during the past ~12 ka induced by varying environmental conditions. After a prolonged dry and cold phase during the Late Glacial, allogenic sediment supply to Lake Heihai is characterized by a sudden intensification of precipitation generated sediment supply around ~10.7 cal. ka BP and a gradual decrease from ~7.9 cal. ka BP to present. This Holocene trend is in phase with known variations in atmospheric circulation systems over monsoonal Asia. The gradual long-term trend to more arid conditions is accompanied by several changes in the predominant endogenic carbonate phase, one of which evidences the onset of permafrost formation at ~3.9 cal. ka BP and hence a complete disappearance of the lake body. The disappearance is synchronous to a phase of weakened monsoonal precipitation as recorded in several continental and terrestrial archives throughout the Asian continent. For the central Kunlun Fault system, this climate event is the driest and coldest of the past 12 ka. However, while allogenic sediment supply is directly affected by climatic variations in the study area, endogenic mineral precipitation is additionally controlled by lake-internal feedbacks not directly influenced by climatic variations, like e.g. geomorphologically controlled changes in the hydrological condition. The observed local paleoenvironmental evolution in the region of Lake Heihai was compared to two well-studied lake systems: Lake Donggi Cona situated on the north eastern Tibetan Plateau and Lake Nam Co on the southern Tibetan Plateau. The supra-regional comparison was based on the allogenic sediment input because of the climatic sensitivity of the allogenic sediment supply system. Variations in allogenic sediment supply were characterized by factor analysis of elemental concentration in core sediments measured by X-Ray fluorescence scans. To account for a potential individual response of lake systems to climate forcing, a phase space reconstruction was carried out on the previously calculated elemental factors. Phase space reconstruction revealed that despite the individuality in the environmental setting of each lake, a similar…
Decadal to sub-decadal variability of inflow, evaporation and biological productivity derived fro... more Decadal to sub-decadal variability of inflow, evaporation and biological productivity derived from Lake Nam Co was used to reconstruct hydrological changes for the past ca. 24 k cal a BP. The timing of these variations corresponds to known climatic shifts on the Northern Hemisphere. After a dry and cold Last Glacial Maximum the lake level of Nam Co initially rose at ca. 20 k cal a BP. Moist but further cold conditions between ca. 16.2 and 14 k cal a BP correspond to Heinrich Event 1. A warm and moist phase between ca. 14 and 13 k cal a BP is expressed as a massive enhancement in inflow and biological productivity and might be associated with a first intensification of the Indian Ocean Summer Monsoon coinciding with the Bølling-Allerød complex. A twostep decrease in inflow and a contemporaneous decline in biological productivity until ca. 11.8 k cal a BP points to cool and dry conditions during the Younger Dryas. Lake levels peak at ca. 9.4 k cal a BP, although hydrological conditions remain relatively stable during the Holocene with only low-amplitude variations observed.
Abstract Today's knowledge about paleomagnetic secular variations in (southern) Africa is ext... more Abstract Today's knowledge about paleomagnetic secular variations in (southern) Africa is extremely limited. Here, we derive paleomagnetic records from independently radiocarbon dated sediment records from three maars in Madagascar: Andraikiba and Amparihibe are terrestrial maars located on the main island of Madagascar and on a small island in the Northwest, whereas Crater Lake is a maar which has an open connection to the Indian Ocean. Studied through alternating field demagnetization of u-channel samples, characteristic remanent magnetization (ChRM) directions document similar inclination and declination patterns in two of the archives for the past 2,500 years (except for the last 400 cal BP). These new data are the first allowing to test the reliability of previous records which often suffer from low resolution and other obstacles. This will further allow to start to distinguish between robustly confirmed paleomagnetic secular variation data for this region from potentially problematic data. Considering a much lower resolution and a shorter covered time interval of archaeomagnetic data from La Reunion, South Africa, Botswana and Zimbabwe the temporal successions of maxima and minima reveal a coherent picture. Although slightly shifted in time, similarities also exist to the global geomagnetic field reconstruction model SHAWQ2k ( Campuzano et al., 2019 ) which includes the above mentioned data. Surprisingly also similarities, though only in declination, are observed to the CALS3K.4 model ( Korte and Constable, 2011 ) which is used as Northern Hemisphere biased model example. In contrast to this, no declination similarities but remarkable inclination similarities are observed to a lake record from Lake Malawi. An analog inclination pattern is also observed in a record from the Makran Accretionary Wedge which is ∼5,000 km to the north of the investigated sites. Interestingly the spatial distribution of archives showing these inclination similarities resembles the spatial distribution of inclination anomalies detected in model predictions. PSV similarities over such a large area are suggestive of a large-scale core dynamic origin independent of westward drift of non-dipole field components often associated with PSV records. This study emphasizes the potential of maar lakes in Madagascar for paleo reconstructions but also suggests that shallow marine and marine-brackish systems should be avoided if possible when trying to expand the (South) African paleomagnetic database.
This study focuses on the analysis of lake sediments retrieved from the deepest part of Lake Nam ... more This study focuses on the analysis of lake sediments retrieved from the deepest part of Lake Nam Co (Tibetan Plateau). One gravity core of 115 cm length, covering the last ~ 4000 cal BP, was analyzed for geochemical and biological parameters. High organic content at ~ 4000 cal BP and the coinciding presence of pyrite framboids until ~ 2000 cal BP point to hampered decomposition of organic material due to anoxic conditions within the lake sediments. At the same time sedimentological and biological proxies suggest a rather high lake level, but still ~ 5 m below the recent one, with less saline lake water due to enhanced monsoonal activity. During this time a change in the source of organic matter to lowered input of terrestrial components is observed. A rather quick shift to a dry environment with less monsoonal influence and a lake level ~ 15 m lower than today at ~ 2000 cal BP lead to the oxygenation of sediment, the degradation of organic matter and the absence of pyrite. Oscillati...
Abstract We investigate the seasonal evolution of stratification in a deep (100 m), high-altitude... more Abstract We investigate the seasonal evolution of stratification in a deep (100 m), high-altitude (4,730 m a.s.l.) dimictic lake on the Tibetan Plateau using three years of observation of Nam Co. The lake is situated at relatively low latitude (30 °N) where it receives high solar radiative forcing (observed maximum daily average 400 W m−2), yet the annual average air temperature is close to 0 °C at this high altitude. These features make Nam Co distinct from most well-documented dimictic lakes, which are usually located at higher latitude and lower altitude. We classify seasonal stratification into six phases based on strength of stratification, surface temperature relative to the temperature of maximum density, ice-cover, and heat and mixing dynamics, which we use to compare Nam Co with better-documented, higher-latitude dimictic lakes. While the three warm stratification phases (i.e. when water is warmer than the temperature of maximum density) in Nam Co are relatively cold, they are otherwise similar to that observed in high-latitude dimictic lakes. Conversely, two of Nam Co’s cold stratification phases are distinct from that reported in high-latitude lakes. These two phases are characterized by the interplay between the relatively strong radiative forcing and surface heat flux, and include the following: 1) during fall turnover, persistent winds aided by radiatively driven convection prolong vertical mixing (i.e. turnover) and surface heat loss such that the entire water column cools well below the temperature of maximum density (as low as 1 °C); and 2) in contrast, under ice-cover with relatively little snow, the entire water column of the lake warms continuously due to through-ice solar radiative flux. The intense cooling and heating during these two phases counteract each other such that hypolimnetic temperature at spring turnover is similar to that observed in high latitude lakes. Our observations highlight the relative importance of radiatively driven convection on the seasonal stratification dynamics of Nam Co, and underscore that these dynamics must be considered when attempting to predict climate change impacts on high-altitude, low-latitude lakes, including the >1100, largely unstudied, lakes on the Tibetan Plateau.
Changing environmental conditions associated with a predicted change in global temperatures is be... more Changing environmental conditions associated with a predicted change in global temperatures is believed to affect water availability on the Tibetan Plateau, which provides freshwater to approximately 20% of the global population. The reconstruction of past environmental conditions may provide past climatic analogues to modern warming scenarios and help to elucidate the environmental response to climate forcing. One of the most abundant archives of Late Quaternary environmental conditions on the Tibetan Plateau are lacustrine sediments associated with lake systems. However, lake systems are known to reveal complexity in their response to climate forcing resulting in individualized sedimentary patterns for different lake systems. This individuality delimits the potential to infer supra-regional patterns of environmental change. This thesis presents a reconstruction of environmental conditions during the last ~12 ka around Lake Heihai, situated on the northern fringe of the Tibetan Plateau. Special focus is given on analyzing the climatic sensitivity of inorganic sediment fluxes, i.e. fluxes of allogenic minerals originating from the terrestrial surroundings of the lake as well as fluxes of endogenic minerals precipitated in the water column of the lake. A cluster analysis on the mineralogical composition of terrestrial reference samples revealed four major source areas, distributed evenly on two bordering mountain ranges: the Kunlun Mountains in the south and the Burhan Buda Mountains in the north of the lake. Especially sediment source areas on the Kunlun range differ in their main mode of activation, which is predominantly precipitation generated and glacial runoff. An end-member analysis of grain size distributions of the siliciclastic detrital fraction revealed that sediment supply from the Burhan Buda range is a consequence of eolian dynamics. The findings suggests a blockage effect of the Kunlun range on southerly air masses, leading to an extended rain shadow north of the Kunlun range. Endogenic mineral formation in Lake Heihai is dominated by carbonate precipitation. The geochemical analysis of carbonate mineral phases apparent within lacustrine sediments revealed, that the main control on the predominant endogenic carbonate mineral phase is the ratio between solute magnesium and calcium (Mg/Ca) in lake waters. The solute lake water Mg/Ca is controlled by solute composition of inflowing waters as well as lake internal Mg enrichment relative to Ca by the removal of Ca due to carbonate precipitation. The solute composition of inflowing waters, in turn, is mainly influenced by weathering intensities of limestone outcrops in the drainage basin of the lake. Changes in the inorganic mineral assemblage of a sediment core from the deepest part of the lake revealed major variations in sediment fluxes during the past ~12 ka induced by varying environmental conditions. After a prolonged dry and cold phase during the Late Glacial, allogenic sediment supply to Lake Heihai is characterized by a sudden intensification of precipitation generated sediment supply around ~10.7 cal. ka BP and a gradual decrease from ~7.9 cal. ka BP to present. This Holocene trend is in phase with known variations in atmospheric circulation systems over monsoonal Asia. The gradual long-term trend to more arid conditions is accompanied by several changes in the predominant endogenic carbonate phase, one of which evidences the onset of permafrost formation at ~3.9 cal. ka BP and hence a complete disappearance of the lake body. The disappearance is synchronous to a phase of weakened monsoonal precipitation as recorded in several continental and terrestrial archives throughout the Asian continent. For the central Kunlun Fault system, this climate event is the driest and coldest of the past 12 ka. However, while allogenic sediment supply is directly affected by climatic variations in the study area, endogenic mineral precipitation is additionally controlled by lake-internal feedbacks not directly influenced by climatic variations, like e.g. geomorphologically controlled changes in the hydrological condition. The observed local paleoenvironmental evolution in the region of Lake Heihai was compared to two well-studied lake systems: Lake Donggi Cona situated on the north eastern Tibetan Plateau and Lake Nam Co on the southern Tibetan Plateau. The supra-regional comparison was based on the allogenic sediment input because of the climatic sensitivity of the allogenic sediment supply system. Variations in allogenic sediment supply were characterized by factor analysis of elemental concentration in core sediments measured by X-Ray fluorescence scans. To account for a potential individual response of lake systems to climate forcing, a phase space reconstruction was carried out on the previously calculated elemental factors. Phase space reconstruction revealed that despite the individuality in the environmental setting of each lake, a similar…
Decadal to sub-decadal variability of inflow, evaporation and biological productivity derived fro... more Decadal to sub-decadal variability of inflow, evaporation and biological productivity derived from Lake Nam Co was used to reconstruct hydrological changes for the past ca. 24 k cal a BP. The timing of these variations corresponds to known climatic shifts on the Northern Hemisphere. After a dry and cold Last Glacial Maximum the lake level of Nam Co initially rose at ca. 20 k cal a BP. Moist but further cold conditions between ca. 16.2 and 14 k cal a BP correspond to Heinrich Event 1. A warm and moist phase between ca. 14 and 13 k cal a BP is expressed as a massive enhancement in inflow and biological productivity and might be associated with a first intensification of the Indian Ocean Summer Monsoon coinciding with the Bølling-Allerød complex. A twostep decrease in inflow and a contemporaneous decline in biological productivity until ca. 11.8 k cal a BP points to cool and dry conditions during the Younger Dryas. Lake levels peak at ca. 9.4 k cal a BP, although hydrological conditions remain relatively stable during the Holocene with only low-amplitude variations observed.
Abstract Today's knowledge about paleomagnetic secular variations in (southern) Africa is ext... more Abstract Today's knowledge about paleomagnetic secular variations in (southern) Africa is extremely limited. Here, we derive paleomagnetic records from independently radiocarbon dated sediment records from three maars in Madagascar: Andraikiba and Amparihibe are terrestrial maars located on the main island of Madagascar and on a small island in the Northwest, whereas Crater Lake is a maar which has an open connection to the Indian Ocean. Studied through alternating field demagnetization of u-channel samples, characteristic remanent magnetization (ChRM) directions document similar inclination and declination patterns in two of the archives for the past 2,500 years (except for the last 400 cal BP). These new data are the first allowing to test the reliability of previous records which often suffer from low resolution and other obstacles. This will further allow to start to distinguish between robustly confirmed paleomagnetic secular variation data for this region from potentially problematic data. Considering a much lower resolution and a shorter covered time interval of archaeomagnetic data from La Reunion, South Africa, Botswana and Zimbabwe the temporal successions of maxima and minima reveal a coherent picture. Although slightly shifted in time, similarities also exist to the global geomagnetic field reconstruction model SHAWQ2k ( Campuzano et al., 2019 ) which includes the above mentioned data. Surprisingly also similarities, though only in declination, are observed to the CALS3K.4 model ( Korte and Constable, 2011 ) which is used as Northern Hemisphere biased model example. In contrast to this, no declination similarities but remarkable inclination similarities are observed to a lake record from Lake Malawi. An analog inclination pattern is also observed in a record from the Makran Accretionary Wedge which is ∼5,000 km to the north of the investigated sites. Interestingly the spatial distribution of archives showing these inclination similarities resembles the spatial distribution of inclination anomalies detected in model predictions. PSV similarities over such a large area are suggestive of a large-scale core dynamic origin independent of westward drift of non-dipole field components often associated with PSV records. This study emphasizes the potential of maar lakes in Madagascar for paleo reconstructions but also suggests that shallow marine and marine-brackish systems should be avoided if possible when trying to expand the (South) African paleomagnetic database.
This study focuses on the analysis of lake sediments retrieved from the deepest part of Lake Nam ... more This study focuses on the analysis of lake sediments retrieved from the deepest part of Lake Nam Co (Tibetan Plateau). One gravity core of 115 cm length, covering the last ~ 4000 cal BP, was analyzed for geochemical and biological parameters. High organic content at ~ 4000 cal BP and the coinciding presence of pyrite framboids until ~ 2000 cal BP point to hampered decomposition of organic material due to anoxic conditions within the lake sediments. At the same time sedimentological and biological proxies suggest a rather high lake level, but still ~ 5 m below the recent one, with less saline lake water due to enhanced monsoonal activity. During this time a change in the source of organic matter to lowered input of terrestrial components is observed. A rather quick shift to a dry environment with less monsoonal influence and a lake level ~ 15 m lower than today at ~ 2000 cal BP lead to the oxygenation of sediment, the degradation of organic matter and the absence of pyrite. Oscillati...
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