Markus Fuchs

Recognition of insufficient bleaching by small aliquots of quartz for reconstructing soil erosion in Greece

Optical dating of sediments requires sufficient bleaching of the material to be dated. Using single aliquot techniques, the statistical parameter of the coefficient of variation v in combination with the reduction of mineral grains per aliquot enables the detection of insufficiently bleached samples. This was empirically tested on quartz extracts from the Greek study area, which were artificially bleached, differently dosed and then mixed to simulate insufficiently bleached samples. The single aliquot regenerative dose protocol was applied. The test measurements show that for the studied quartz material, the detection of insufficient bleaching is possible if small aliquots of 200–500 grains and a threshold value of v=10% are used. The technique was applied to the age determination of colluvial and alluvial/torrential deposits from the study area, the Basin of Phlious in Greece. The optically stimulated luminescence age data allow reconstruction of the history of Holocene soil erosion, which essentially started in the 7th millennium BC, simultaneous with the onset of agricultural activities in the early Neolithic.

The history of Holocene soil erosion in the Phlious Basin, NE Peloponnese, Greece, based on optical dating

Anthropogenic induced soil erosion has occurred in the Mediterranean during historic and pre historic times, but precise timeframes for periods of past soil erosion are largely missing due to the lack of datable material and suitable dating methods. In this study, optical dating of colluvial sediments from the Phlious Basin, NE Peloponnese, Greece, was applied. The results show that soil erosion derived colluvium formed under a Mediterranean type climate can be successfully dated by optical stimulated luminescence (OSL), and a high-resolution chronology of Holocene soil erosion could be established. Colluvial formation occurred in the early Holocene, but increased distinctly in the seventh millennium, parallel with the onset of the Neolithic (the start of farming activities). Other periods of strong colluviation are the Middle and Late Bronze Age (second millennium), the Roman period (first century to sixth century) and the period since the sixteenth century. Rates of reduced colluviation occurred during the Chalcolithic to the Early Bronze Age (4500–2050 a) and the Early Iron Age (1065–725 a). Based on thin section analysis of the colluvial sediments, no traces of in-situ soil formation and thus no periods of landscape stability were detected for the Holocene. Results show that human activity is the dominant factor for Holocene landscape changes in the area studied.

The temporal and spatial quantification of Holocene sediment dynamics in a meso-scale catchment in northern Bavaria, Germany

The Aufsess River catchment (97 km2) in northern Bavaria, Germany, is studied to establish a Holocene sediment budget and to investigate the sediment dynamics since the early times of farming in the third millennium BCE. The temporal characterization of the sediment dynamics is based on an intensive dating program with 73 OSL and 14 14C ages. To estimate soil erosion and deposition, colluvial and alluvial archives are investigated in the field by piling and trenching, supported by laboratory analyses. The sediment budget shows that 58% of these sediments are stored as colluvium in on- and foot-slope positions, 9% are stored as alluvium in the floodplains and 33% are exported from the Aufsess River catchment. Colluviation starts in the end-Neolithic (c. 3100 BCE), while first indicators of soil erosion-derived alluviation is recorded c. 2–3 ka later. The pattern of sedimentation rates also displays differences between the colluvial and alluvial system, with a distinct increase in the Middle Ages (c. 1000 CE) for the alluvial system, while the colluvial system records low sedimentation rates for this period. A contrast is also observed since Modern times (c. 1500 CE), with increasing sedimentation rates for the colluvial system, whereas the alluvial system records decreasing rates. The different behavior of the colluvial and alluvial systems clearly shows the non-linear behavior of the catchment’s fluvial system. The results further suggest that human impact is most probably the dominant factor influencing the sediment dynamics of the catchment since the introduction of farming.

Evaluating dose-rate distributions in natural sediments using α-Al2O3:C grains

Abstract Radioactive nuclides in natural sediments are not always distributed homogeneously, and this can lead to variations in dose rate within the sediment on both a macroscopic and microscopic scale. Single-grain dating protocols are expected to allow a detailed examination of the dose distribution in a sample; although the equivalent doses are measured for individual grains, the dose rate is usually assumed to be homogeneous and is obtained from bulk measurements. Then, if an asymmetric dose distribution is observed, it is usually attributed to incomplete bleaching or physical intrusion of older or younger grains. As part of our initial studies on the importance of spatial dose-rate variations, we report here on the first direct measurements to test the dose rate homogeneity in a natural sediment. α -Al 2 O 3 :C grains with diameters of 200– 300 μm were mixed with a loess sediment and the doses to the single grains were measured. Additionally, the homogeneity of the gamma response of the grains was investigated. The results from 26 grains buried in the loess show a distribution with a standard deviation of 27%; this can be compared with a standard deviation of 20% measured using a gamma-irradiated sample, for which the dose should be homogeneous. We conclude that 18% of the dose variation is caused by dose-rate inhomogeneities in the loess sediment.

European glacial dust deposits: Geochemical constraints on atmospheric dust cycle modeling

For a long time global paleodust numerical simulations have greatly underestimated dust sources other than modern deserts. Recent modeling experiments incorporating glaciogenic sources of dust have positively improved the agreement between model and paleodust data. This highlights the importance of accurately representing all areas potentially subjected to deflation during an investigated interval. Geochemical results, obtained from European loess sequences collected along a 50°N transect, combined with dust emission simulations reveal the geographical distribution of the most important European dust sources between 34 ka and 18 ka. We demonstrate that most European dust traveled only a few hundred kilometers or less within the boundary layer from its source before deposition. We conclude that our results encourage acquisition of similar geochemical data for other relevant areas in the world. Further, they could provide critical constraints to benchmark atmospheric models, contributing to improve their performance in simulating dust cycle and associated climate feedbacks.

Climate-controlled landscape evolution in the Western Transverse Ranges, California: Insights from Quaternary geochronology of the Saugus Formation and strath terrace flights

The Las Posas and Ojai Valleys, located in the actively deforming Western Transverse Ranges of California, contain well-preserved flights of strath terraces and Quaternary strata (i.e., Saugus Formation) that when numerically dated elucidate the tectonic, geomorphic, and fluvial histories that sculpted the landscape since ca. 140 ka. This study includes 14 new optically stimulated luminescence and 16 new terrestrial cosmogenic nuclide ages from the late Pleistocene to Holocene that record two regional aggradation events and four intervals of strath terrace formation. Geochronologic data indicate that terrestrial Saugus strata in the Las Posas Valley (Camarillo Member) prograded over marine deposits at ca. 125 and 80 ka and are as young as 60–25 ka, which is an order of magnitude younger than the youngest Saugus strata elsewhere in Southern California. These results highlight the need for precise dating of Saugus strata where identified and utilized to assess rates of tectonic deformation. Based on its compositional character, thickness, stratigraphic relations, and inferred ages, the Camarillo member of the Saugus Formation is correlated with sediments of the Mugu aquifer identified in subcrop throughout the Ventura Basin and thus provides a new regional chronostratigraphic subsurface datum. The aggradation of these sediments and similar deposits in the study area between 13 and 4 ka is subsequent to the transition from humid to semiarid climate correlating to the end of the ultimate and penultimate glacial maximums. Aggradation is inferred to have resulted from increased sediment supply in response transient vegetative conditions and consequent hillslope destabilization. Similar to aggradational events, strath terrace cover sediments ages correlate to dry warm climate intervals, indicating straths in Southern California were cut at ca. 110–100 ka, 50–35 ka, 26–20 ka, and 15–4 ka. These results support recent mathematical and experimental models of strath formation, where increased sediment flux and decreased water discharge enhances lateral erosion rates and inhibits vertical incision. Subsequent incision and strath terrace formation is inferred to occur during intervening wet climate intervals. The correlation of strath terrace ages and aggradational events with environmental changes that are linked to global climate indicates that climate rather than tectonics exhibits first-order control of depositional, denudational, and incisional processes in the Western Transverse Ranges. Moreover, these results provide a chronostratigraphic framework that allows these landforms to be regionally correlated and used to assess rates of active tectonics where geochronologic data are unavailable.

Performance tests using the Lexsyg luminescence reader

This study presents test measurements on a Lexsyg luminescence reader, acquired by the Giessen luminescence group. The reader is of the type Standard, hence designed for routine determination of palaeodoses using quartz or K-feldspars. The tests include measurements of the stimulation powers, preheat temperatures, OSL- and TL-curves, as well as dose recovery tests and calibration measurements using highly sensitised calibration quartz. A comparison of De values determined using a Lexsyg reader with single grain De-values, which have been previously obtained on a Risø reader, is also presented. The results imply that the Lexsyg reader is a highly reliable measurement device with high reproducibility, yielding instrumental uncertainties of around 0.3%.

Abrupt sand-dune accumulation at the northeastern margin of the Tibetan Plateau challenges the wet MIS3a inferred from numerous lake-highstands.

Over the Tibetan Plateau and adjacent regions, numerous 14C-based lake records revealed a ubiquitous wet climatic period during 40–25 ka (late MIS 3), which is in contradiction with the global pattern of generally cold and dry climates. This paper focuses on OSL dating results of a large set of sand dunes and alluvial sediments (50 OSL ages) from the Qinwangchuan (QWC) Basin at the northeast edge of the Tibetan Plateau, with the aim to test the validity of the anomalous wet condition for the late MIS 3 interval, evidenced by numerous lake highstands. The abrupt sand dune accumulation as indication of increased aridity in the study area was OSL dated to ~40–13 ka. This dry climatic inference of the sand dune system from QWC apparently shows no wet MIS 3a event. Thus, the anomalous wet conditions revealed by high lake levels for the late MIS 3 phase may not be a universal phenomena across entire western China.

Late Pleistocene structural evolution of the Camarillo fold belt: Implications for lateral fault growth and seismic hazard in Southern California

The Camarillo fold belt in the Western Transverse Ranges poses a significant seismic hazard to nearly one million people living in Southern California, yet few published geologic or geochronological data from this fold belt exist. The Camarillo fold belt is composed of several south-verging, west-plunging anticlines that characterize the western extent of the Simi fault zone, which extends for 40 km through urbanized Ventura and Los Angeles Counties. Surface and subsurface geologic data are utilized to accurately construct five cross sections within discrete structural domains to assess the local style of deformation, and to quantify the magnitudes of fault slip, fault- and fold-related uplift, and percent shortening. Eight new optically stimulated luminescence (OSL) dates from three paleoseismic trenches and six numerical dates from a previous study were utilized to quantify the timing and rates of deformation on discrete faults and folds. The onset of deformation in the Camarillo fold belt is everywhere younger than ca. 125 ka and locally as young as ca. 25 ka. Quaternary deformation occurs on reactivated steeply dipping (70°) Miocene faults, with shortening being largely accommodated in a narrow zone (<2 km) of folding where fold-related uplift is typically twice as great as hanging-wall uplift in response to fault slip. A minimum fault slip rate between 0.8 mm/yr and 1.4 mm/yr, a recurrence interval of 715–1100 yr, and a maximum Mw 6.8 earthquake for faults within the Simi fault zone is estimated, which are comparable to other better-studied fold belts in Southern California. A model of punctuated lateral fault propagation is proposed to explain westward growth of the Simi fault, which occurs in discrete pulses that are separated by intervals of fault displacement accumulation and fold amplification during constant-fault-length conditions. Lateral fault growth is limited in space and time by an orthogonal north-striking fault set, which juxtaposes a series of west-plunging anticlines that decrease in structural relief and age toward the west.

Geoarchaeological Investigation at the Intermediate Bronze Age Negev Highlands Site of Mashabe Sade

Massive settlement activity characterizes the arid Negev Highlands during the Intermediate Bronze Age (ca. 2500–1950 BCE). However, the underlying subsistence basis of this population is poorly understood. Recent microarchaeological work at Iron Age sites in the Negev Highlands has shown the potential for recovering direct evidence for subsistence practices through analysis of the microscopic plant remains in degraded animal dung. Following these methods, this paper reports new macro- and micro-archaeological results of two sites near Mashabe Sade: a central Intermediate Bronze Age site, and for comparison, an ephemeral site in the immediate vicinity. At the central site, dated to the Intermediate Bronze Age by pottery and Optically Stimulated Luminescence (OSL), evidence is absent for any sort of food production. In contrast, identification of ancient livestock dung at the ephemeral site suggests that it was sustained by animal husbandry— yet the OSL results suggest these degraded dung deposits date to the Iron Age. Taken together, the Intermediate Bronze Age results from Mashabe Sade bolster arguments suggesting that central sites were supported mainly by trade and other alternative subsistence practices.