Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ15N) a... more Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ15N) and stable organic carbon isotope (δ13Corg) ratios were continuously analysed on a high resolution sediment profile from Lake Sihailongwan (SHL), covering the time span between 16,500 and 9,500 years BP. Strong variations of the investigated proxy parameters are attributed to great climatic fluctuations during the investigated time period. Variations in organic carbon isotope ratios and the ratio of TOC/TN (C/N ratio) are discussed with respect to changing proportions of different organic matter (OM) sources to bulk sedimentary OM. Phases of high TOC content, high TN content, depleted δ13Corg values and high δ15N values are interpreted as times with increased productivity of lacustrine algae in relation to input of terrigenous organic matter. Two distinct phases of enriched nitrogen isotope ratios from 14,200 to 13,700 and 11,550 to 11,050 years BP point towards a reduced phytoplankton discrimination against 15N due to a diminished dissolved inorganic nitrogen pool. The combination of geochemical (TOC, TN, C/N ratio) and isotopic (δ13Corg, δ15N) proxy parameters points to a division of climate development into four stages. A cold and dry stage before 14,200 years BP, a warm optimum stage with high phytoplankton productivity from 14,200 to 12,450 BP, a colder and drier stage from 12,450 to 11,600 BP and a stage of climatic amelioration with high variability in TOC and TN contents after 11,600 BP. These results are discussed in relation to monsoon variability and Northern Hemisphere climate development of the late glacial.
Atmospheric nitrogen compounds play a critical role in tropospheric photochemistry and are crucia... more Atmospheric nitrogen compounds play a critical role in tropospheric photochemistry and are crucial to understand the chemical and physical evolution of atmospheric pollutants in polar areas. Measurements of these species in remote areas are rare, although their relevance is well established. Sampling campaigns of gaseous and particulate atmospheric trace species were performed in Arctic and in Antarctica during three consecutive years (1997-1999), using a proper combination of annular denuders and filter pack. After sampling, the ionic species were extracted with aqueous solutions and analysed by means of ion chromatography. Quality assurance on the sampling and analytical steps allowed accurate and precise measurements of all relevant compounds, which are thought to be important to nitrogen chemistry, at very low concentration levels. In addition, the measurements also included a multistage low-pressure impactor for the collection of particulate matter in different size regions (0.035-15.9 mm). Results obtained from these campaigns demonstrate that the minor components may be measured at levels as low as a few nanograms per cubic meter. The reported concentrations are to be considered among the first observations of nitrogen containing compounds in polar sites.
We studied the internal lake processes that control the spatial distribution and characteristics ... more We studied the internal lake processes that control the spatial distribution and characteristics of modern sediments at the ICDP (International Continental Scientific Drilling Program) deep drilling site in Laguna Potrok Aike, southern Patagonia, Argentina. Sediment distribution patterns were investigated using a dense grid of 63 gravity cores taken throughout the lake basin and 40 additional shoreline samples. Analysis of the surficial sediment distribution points to distinct internal depositional dynamics induced by wind-driven lake internal currents. Distribution maps illustrate that the spatial characteristics of analysed variables are linked to high erosional wave activity. Persistent wave action and littoral erosion along all shores, especially the eastern shore, is caused by prevailing Southern Hemispheric Westerlies. Several sediment variables (grain size, benthic diatoms, total inorganic carbon and calcium) indicate re-suspension of littoral sediment followed by re-distribution to profundal accumulation areas near the eastern shore. Variations within the catchment influence sediment characteristics in the north-eastern bay. That area is characterized by different mineralogical and sedimentological conditions as well as greater accumulation of pollen, inorganic carbon and diatoms. These findings are related to the influence of episodic inflow into this bay. Spatial differences in stable isotope values throughout the lake suggest that ephemeral tributaries around the lake basin may also contribute to the detected spatial sediment variations.
... Andrea Terán-Valdez; Juan M. Guayasamin; Luis A. Coloma. ... The terminology for traits (body... more ... Andrea Terán-Valdez; Juan M. Guayasamin; Luis A. Coloma. ... The terminology for traits (body shape, tail shape, oral disc size etc.) used in the description was taken from Mijares-Urrutia (1998) and McDiarmid and Altig (1999). ...
We review results of magnetooptical studies related to various methods of “spin splitting enginee... more We review results of magnetooptical studies related to various methods of “spin splitting engineering” possible in quantum well (QW) structures made of II–VI diluted magnetic semiconductors (DMS). The experiments made use of unique structures, based on Cd1−x−yMnxMgyTe, with a precise in-plane profiling of either QW width or n-type doping intensity and of structures with digital profiling of the composition of constituent materials in the growth direction. The most typical version of the spin splitting engineering, available also in bulk DMSs, namely the one via an adjustment of (homogeneous) Mn composition, or via the lattice temperature or/and a magnetic field, was used to study magnetooptical properties of QW containing electrons as a function of their g-factor (tuned from ). We also demonstrate three further methods of modifying the spin splitting in low-dimensional structures made of DMSs. They involve: (i) atomically precise control over Mn ion spatial distribution in the growth direction, (ii) reduction of the s–d exchange constant with increasing electron confinement, (iii) modification of Mn spin temperature by the electron gas heated by photo-excited carriers.
The grain size distribution allows characterizing quantitatively the microstructure of an amorpho... more The grain size distribution allows characterizing quantitatively the microstructure of an amorphous solid at different stages of crystallization. We review the theory developed recently for the grain size distribution (GSD) [1] and present two extensions of the model. In the first generalization, we replace the isotropic growth rate by an anisotropic rate that leads to the formation of ellipsoidal grains. Different anisotropic growth mechanisms are considered. We obtain an analytical expression for the GSD when the growth rate leads to a change of volume leaving the shape of grains invariant [2]. In the second generalization, we study how the GSD is affected by replacing the Dirac-type source term of nuclei by a more physical Gaussian-type source. We use that model to analyze the GSD at early stages of crystallization.[4pt] [1] A.V.Teran, R.B.Bergmann and A.Bill, Phys. Rev. B 81, 075319 (2010).[0pt] [2] K.S.Lokovic, R.B.Bergmann and A.Bill, Mater. Res. Soc. Symp. Proc. 1245, A16-07 (2010).
Page 1. J. Limnol., 68(2): 257-273, 2009 DOI: 10.3274/JL09-68-2-10 Seasonal dynamics of stable is... more Page 1. J. Limnol., 68(2): 257-273, 2009 DOI: 10.3274/JL09-68-2-10 Seasonal dynamics of stable isotopes and element ratios in authigenic calcites during their precipitation and dissolution, Sacrower See (northeastern Germany) ...
Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the ef... more Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the effects of climate change and human impact on both lake ecosystem and landscape evolution for this region. Sacrower See, a hardwater lake located in Brandenburg, provides a unique sediment record covering the past 13,000 years which was used to reconstruct climatic and anthropogenic forcing on lacustrine sedimentation. Time control is provided by 12 AMS 14C dates of terrestrial plant remains, the Laacher See Tephra, and the onset of varve formation in AD 1870 (80 cal. BP). Geochemical (including XRF logging of major elements, CNS analyses as well as δ13Corg and δ15N measurements) and pollen analyses allowed detecting detailed environmental changes in the sediment record. During the Younger Dryas cold phase increased soil erosion and hypolimnetic oxygen depletion enhanced the nutrient supply to the lake water causing eutrophic conditions. The beginning of the Holocene is characterized by large changes in C/N ratios, total sulphur, δ13C of bulk organic matter as well as in K, Si, and Ti, reflecting the response of the lake’s catchment to climatic warming. Reforestation reduced the influx of detrital particles and terrestrial organic matter. The first, rather weak evidence of human impact is documented only in the pollen record at 5,500 cal. BP. However, until 3,200 cal. BP sedimentological and geochemical parameters indicate relatively stable environmental conditions. During periods of intense human impact at around 3,200, 2,800, and 900 cal. BP peaks in Ti and K represent phases of increased soil erosion due to forest clearing during the Bronze Age, Iron Age, and Medieval Times, respectively. In general, greater variation is observed in most variables during these perturbations, indicating less stable environmental conditions. The steady rise of biogenic silica accumulation rates during the Holocene reflects an increasing productivity of Sacrower See until diatoms were outcompeted by other algae during the last centuries. The applied multi-proxy approach fosters the interpretation of the sediment record to reveal a consistent picture of environmental change including environmental factors controlling lake ontogeny and the effects of human impact.
Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induc... more Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induced debris flows from those generated by severe storms and associated hyperpycnal currents over the last 7000 years using integrated textural, magnetic, and geochemical signatures. Relative to fine-grained ‘background’ sediments, the majority of Tahoe turbidites exhibit coincident trends of increased mean grain size, increased magnetic susceptibility, decreased TOC, higher δ13Corg and variable C/N. We interpret these characteristics to record the rapid influx of terrigenous sediments within runoff from the watershed triggered by high-intensity storms. Correlation of multiple, individual turbidites between cores suggests a synchronicity of occurrence, supporting the model of extreme hydrologic events as the trigger for most turbidity currents into Lake Tahoe. In contrast, turbidites generated by seismic collapse of steep lake margins would have textural, magnetic and geochemical signatures that would reflect a homogenized mix of autochthonous biogenic debris and multiple older turbidites. Only one of the turbidites in the cores appears to be seismically generated. A second component of this study tested the hypothesis that turbidite clustering reflects phases of increased storminess, paleoprecipitation and lake level. We correlated broad patterns of turbidite frequency in the Tahoe cores with climate proxies from (1) elsewhere in the Tahoe watershed, (2) the western Great Basin (primarily Pyramid Lake) and (3) the San Francisco bay estuary. The reasonable degree of temporal overlap suggests that apparent trends in severe storm frequency recorded by clusters of turbidites provides a measure of long-term regional paleoprecipitation and lake level. A key finding is an extended phase of dryness and a near absence of major storms between ~3000 and ~900 cal yr B.P. in the Tahoe watershed.
Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ15N) a... more Total organic carbon (TOC) content, total nitrogen (TN) content, stable nitrogen isotope (δ15N) and stable organic carbon isotope (δ13Corg) ratios were continuously analysed on a high resolution sediment profile from Lake Sihailongwan (SHL), covering the time span between 16,500 and 9,500 years BP. Strong variations of the investigated proxy parameters are attributed to great climatic fluctuations during the investigated time period. Variations in organic carbon isotope ratios and the ratio of TOC/TN (C/N ratio) are discussed with respect to changing proportions of different organic matter (OM) sources to bulk sedimentary OM. Phases of high TOC content, high TN content, depleted δ13Corg values and high δ15N values are interpreted as times with increased productivity of lacustrine algae in relation to input of terrigenous organic matter. Two distinct phases of enriched nitrogen isotope ratios from 14,200 to 13,700 and 11,550 to 11,050 years BP point towards a reduced phytoplankton discrimination against 15N due to a diminished dissolved inorganic nitrogen pool. The combination of geochemical (TOC, TN, C/N ratio) and isotopic (δ13Corg, δ15N) proxy parameters points to a division of climate development into four stages. A cold and dry stage before 14,200 years BP, a warm optimum stage with high phytoplankton productivity from 14,200 to 12,450 BP, a colder and drier stage from 12,450 to 11,600 BP and a stage of climatic amelioration with high variability in TOC and TN contents after 11,600 BP. These results are discussed in relation to monsoon variability and Northern Hemisphere climate development of the late glacial.
Atmospheric nitrogen compounds play a critical role in tropospheric photochemistry and are crucia... more Atmospheric nitrogen compounds play a critical role in tropospheric photochemistry and are crucial to understand the chemical and physical evolution of atmospheric pollutants in polar areas. Measurements of these species in remote areas are rare, although their relevance is well established. Sampling campaigns of gaseous and particulate atmospheric trace species were performed in Arctic and in Antarctica during three consecutive years (1997-1999), using a proper combination of annular denuders and filter pack. After sampling, the ionic species were extracted with aqueous solutions and analysed by means of ion chromatography. Quality assurance on the sampling and analytical steps allowed accurate and precise measurements of all relevant compounds, which are thought to be important to nitrogen chemistry, at very low concentration levels. In addition, the measurements also included a multistage low-pressure impactor for the collection of particulate matter in different size regions (0.035-15.9 mm). Results obtained from these campaigns demonstrate that the minor components may be measured at levels as low as a few nanograms per cubic meter. The reported concentrations are to be considered among the first observations of nitrogen containing compounds in polar sites.
We studied the internal lake processes that control the spatial distribution and characteristics ... more We studied the internal lake processes that control the spatial distribution and characteristics of modern sediments at the ICDP (International Continental Scientific Drilling Program) deep drilling site in Laguna Potrok Aike, southern Patagonia, Argentina. Sediment distribution patterns were investigated using a dense grid of 63 gravity cores taken throughout the lake basin and 40 additional shoreline samples. Analysis of the surficial sediment distribution points to distinct internal depositional dynamics induced by wind-driven lake internal currents. Distribution maps illustrate that the spatial characteristics of analysed variables are linked to high erosional wave activity. Persistent wave action and littoral erosion along all shores, especially the eastern shore, is caused by prevailing Southern Hemispheric Westerlies. Several sediment variables (grain size, benthic diatoms, total inorganic carbon and calcium) indicate re-suspension of littoral sediment followed by re-distribution to profundal accumulation areas near the eastern shore. Variations within the catchment influence sediment characteristics in the north-eastern bay. That area is characterized by different mineralogical and sedimentological conditions as well as greater accumulation of pollen, inorganic carbon and diatoms. These findings are related to the influence of episodic inflow into this bay. Spatial differences in stable isotope values throughout the lake suggest that ephemeral tributaries around the lake basin may also contribute to the detected spatial sediment variations.
... Andrea Terán-Valdez; Juan M. Guayasamin; Luis A. Coloma. ... The terminology for traits (body... more ... Andrea Terán-Valdez; Juan M. Guayasamin; Luis A. Coloma. ... The terminology for traits (body shape, tail shape, oral disc size etc.) used in the description was taken from Mijares-Urrutia (1998) and McDiarmid and Altig (1999). ...
We review results of magnetooptical studies related to various methods of “spin splitting enginee... more We review results of magnetooptical studies related to various methods of “spin splitting engineering” possible in quantum well (QW) structures made of II–VI diluted magnetic semiconductors (DMS). The experiments made use of unique structures, based on Cd1−x−yMnxMgyTe, with a precise in-plane profiling of either QW width or n-type doping intensity and of structures with digital profiling of the composition of constituent materials in the growth direction. The most typical version of the spin splitting engineering, available also in bulk DMSs, namely the one via an adjustment of (homogeneous) Mn composition, or via the lattice temperature or/and a magnetic field, was used to study magnetooptical properties of QW containing electrons as a function of their g-factor (tuned from ). We also demonstrate three further methods of modifying the spin splitting in low-dimensional structures made of DMSs. They involve: (i) atomically precise control over Mn ion spatial distribution in the growth direction, (ii) reduction of the s–d exchange constant with increasing electron confinement, (iii) modification of Mn spin temperature by the electron gas heated by photo-excited carriers.
The grain size distribution allows characterizing quantitatively the microstructure of an amorpho... more The grain size distribution allows characterizing quantitatively the microstructure of an amorphous solid at different stages of crystallization. We review the theory developed recently for the grain size distribution (GSD) [1] and present two extensions of the model. In the first generalization, we replace the isotropic growth rate by an anisotropic rate that leads to the formation of ellipsoidal grains. Different anisotropic growth mechanisms are considered. We obtain an analytical expression for the GSD when the growth rate leads to a change of volume leaving the shape of grains invariant [2]. In the second generalization, we study how the GSD is affected by replacing the Dirac-type source term of nuclei by a more physical Gaussian-type source. We use that model to analyze the GSD at early stages of crystallization.[4pt] [1] A.V.Teran, R.B.Bergmann and A.Bill, Phys. Rev. B 81, 075319 (2010).[0pt] [2] K.S.Lokovic, R.B.Bergmann and A.Bill, Mater. Res. Soc. Symp. Proc. 1245, A16-07 (2010).
Page 1. J. Limnol., 68(2): 257-273, 2009 DOI: 10.3274/JL09-68-2-10 Seasonal dynamics of stable is... more Page 1. J. Limnol., 68(2): 257-273, 2009 DOI: 10.3274/JL09-68-2-10 Seasonal dynamics of stable isotopes and element ratios in authigenic calcites during their precipitation and dissolution, Sacrower See (northeastern Germany) ...
Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the ef... more Lacustrine sediments in north-eastern Germany have rarely been used as archives to address the effects of climate change and human impact on both lake ecosystem and landscape evolution for this region. Sacrower See, a hardwater lake located in Brandenburg, provides a unique sediment record covering the past 13,000 years which was used to reconstruct climatic and anthropogenic forcing on lacustrine sedimentation. Time control is provided by 12 AMS 14C dates of terrestrial plant remains, the Laacher See Tephra, and the onset of varve formation in AD 1870 (80 cal. BP). Geochemical (including XRF logging of major elements, CNS analyses as well as δ13Corg and δ15N measurements) and pollen analyses allowed detecting detailed environmental changes in the sediment record. During the Younger Dryas cold phase increased soil erosion and hypolimnetic oxygen depletion enhanced the nutrient supply to the lake water causing eutrophic conditions. The beginning of the Holocene is characterized by large changes in C/N ratios, total sulphur, δ13C of bulk organic matter as well as in K, Si, and Ti, reflecting the response of the lake’s catchment to climatic warming. Reforestation reduced the influx of detrital particles and terrestrial organic matter. The first, rather weak evidence of human impact is documented only in the pollen record at 5,500 cal. BP. However, until 3,200 cal. BP sedimentological and geochemical parameters indicate relatively stable environmental conditions. During periods of intense human impact at around 3,200, 2,800, and 900 cal. BP peaks in Ti and K represent phases of increased soil erosion due to forest clearing during the Bronze Age, Iron Age, and Medieval Times, respectively. In general, greater variation is observed in most variables during these perturbations, indicating less stable environmental conditions. The steady rise of biogenic silica accumulation rates during the Holocene reflects an increasing productivity of Sacrower See until diatoms were outcompeted by other algae during the last centuries. The applied multi-proxy approach fosters the interpretation of the sediment record to reveal a consistent picture of environmental change including environmental factors controlling lake ontogeny and the effects of human impact.
Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induc... more Sediment cores from Lake Tahoe permit the discrimination of turbidites initiated by seismic-induced debris flows from those generated by severe storms and associated hyperpycnal currents over the last 7000 years using integrated textural, magnetic, and geochemical signatures. Relative to fine-grained ‘background’ sediments, the majority of Tahoe turbidites exhibit coincident trends of increased mean grain size, increased magnetic susceptibility, decreased TOC, higher δ13Corg and variable C/N. We interpret these characteristics to record the rapid influx of terrigenous sediments within runoff from the watershed triggered by high-intensity storms. Correlation of multiple, individual turbidites between cores suggests a synchronicity of occurrence, supporting the model of extreme hydrologic events as the trigger for most turbidity currents into Lake Tahoe. In contrast, turbidites generated by seismic collapse of steep lake margins would have textural, magnetic and geochemical signatures that would reflect a homogenized mix of autochthonous biogenic debris and multiple older turbidites. Only one of the turbidites in the cores appears to be seismically generated. A second component of this study tested the hypothesis that turbidite clustering reflects phases of increased storminess, paleoprecipitation and lake level. We correlated broad patterns of turbidite frequency in the Tahoe cores with climate proxies from (1) elsewhere in the Tahoe watershed, (2) the western Great Basin (primarily Pyramid Lake) and (3) the San Francisco bay estuary. The reasonable degree of temporal overlap suggests that apparent trends in severe storm frequency recorded by clusters of turbidites provides a measure of long-term regional paleoprecipitation and lake level. A key finding is an extended phase of dryness and a near absence of major storms between ~3000 and ~900 cal yr B.P. in the Tahoe watershed.
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