Bayesian age–depth modelling applied to varve and radiometric dating to optimize the transfer of an existing high-resolution chronology to a new composite sediment profile from Holzmaar (West Eifel Volcanic Field, Germany)

This study gives an overview of different methods to integrate information from a varve chronology and radiometric measurements in the Bayesian tool Bacon. These techniques will become important for the future as technologies evolve with more sites being revisited for the application of new and high-resolution scanning methods. Thus, the transfer of existing chronologies will become necessary because the recounting of varves will be too time consuming and expensive to be funded. We introduce new sediment cores from Holzmaar (West Eifel Volcanic Field, Germany), a volcanic maar lake with a well-studied varve record. Four different age–depth models have been calculated for the new composite sediment profile (HZM19) using Bayesian modelling with Bacon. All models incorporate new Pb-210 and Cs-137 dates for the top of the record, the latest calibration curve (IntCal20) for radiocarbon ages as well as the new age estimation for the Laacher See Tephra. Model A is based on previously published radiocarbon measurements only, while Models B–D integrate the previously published varve chronology (VT-99) with different approaches. Model B rests upon radiocarbon data, while parameter settings are obtained from sedimentation rates derived from VT-99. Model C is based on radiocarbon dates and on VT-99 as several normal distributed tie points, while Model D is segmented into four sections: sections 1 and 3 are based on VT-99 only, whereas sections 2 and 4 rely on Bacon age–depth models including additional information from VT-99. In terms of accuracy, the parameter-based integration Model B shows little improvement over the non-integrated approach, whereas the tie-point-based integration Model C reflects the complex accumulation history of Holzmaar much better. Only the segmented and parameter-based age integration approach of Model D adapts and improves VT-99 by replacing sections of higher counting errors with Bayesian modelling of radiocarbon ages and thus efficiently makes available the best possible and most precise age–depth model for HZM19. This approach will value all ongoing high-resolution investigations for a better understanding of decadal-scale Holocene environmental and climatic variations.</p

Origin and evolution of the Laguna Potrok Aike maar (Patagonia, Argentina)

Laguna Potrok Aike, a maar lake in southern-most Patagonia, is located at about 110 m a.s.l. in the Pliocene to late Quaternary Pali Aike Volcanic Field (Santa Cruz, southern Patagonia, Argentina) at about 52°S and 70°W, some 20 km north of the Strait of Magellan and approximately 90 km west of the city of Rio Gallegos. The lake is almost circular and bowl-shaped with a 100 m deep, flat plain in its central part and an approximate diameter of 3.5 km.Steep slopes separate the central plain from the lake shoulder at about 35 m water depth. At present, strong winds permanently mix the entire water column. The closed lake basin contains a sub saline water body and has only episodic inflows with the most important episodic tributary situated on the western shore. Discharge is restricted to major snowmelt events.Laguna Potrok Aike is presently located at the boundary between the Southern Hemispheric Westerlies and the Antarctic Polar Front. The sedimentary regime is thus influenced by climatic and hydrologic conditions related to the Antarctic Circumpolar Current, the Southern HemisphericWesterlies and sporadic outbreaks of Antarctic polar air masses. Previous studies demonstrated that closed lakes in southern South America are sensitive to variations in the evaporation/precipitation ratio and have experienced drastic lake level changes in the past causing for example the desiccation of the 75 m deep Lago Cardiel during the Late Glacial. Multiproxy environmental reconstruction of the last 16 ka documents that Laguna Potrok Aike is highly sensitive to climate change. Based on an Ar/Ar age determination, the phreatomagmatic tephra that is assumed to relate to the Potrok Aike maar eruption was formed around 770 ka. Thus Laguna Potrok Aike sediments contain almost 0.8 million years of climate history spanning several past glacial-interglacial cycles making it a unique archive for non-tropical and non-polar regions of the Southern Hemisphere. In particular, variations of the hydrological cycle, changes in eolian dust deposition, frequencies and consequences of volcanic activities and other natural forces controlling climatic and environmental responses can be tracked throughout time. Laguna Potrok Aike has thus become a major focus of the International Continental Scientific Drilling Program. Drilling operations were carried out within PASADO (Potrok Aike Maar Lake Sediment Archive Drilling Project) in late 2008 and penetrated ~100 m into the lacustrine sediment.Laguna Potrok Aike is surrounded by a series of subaerial paleo-shorelines of modern to Holocene age that reach up to 21 m above the 2003 AD lake level. An erosional unconformity which can be observed basin-wide along the lake shoulder at about 33 m below the 2003 AD lake level marks the lowest lake level reached during Late Glacial to Holocene times. A high- resolution seismic survey revealed a series of buried, subaquatic paleo-shorelines that hold a record of the complex transgressional history of the past approximately 6800 years, which was temporarily interrupted by two regressional phases from approximately 5800 to 5400 and 4700 to 4000 cal BP. Seismic reflection and refraction data provide insights into the sedimentary infill and the underlying volcanic structure of Laguna Potrok Aike. Reflection data show undisturbed, stratified lacustrine sediments at least in the upper ~100 m of the sedimentary infill. Two stratigraphic boundaries were identified in the seismic profiles (separating subunits I-ab, I-c and I-d) that are likely related to changes in lake level. Subunits I-ab and I-d are quite similar even though velocities are enhanced in subunit I-d. This might point at cementation in subunit I-d. Subunit I-c is restricted to the central parts of the lake and thins out laterally.A velocity-depth model calculated from seismic refraction data reveals a funnel-shaped structure embedded in the sandstone rocks of the surrounding Santa Cruz Formation. This funnel struc

Forschendes Lernen. Konzeptuelle Grundlagen und Potenziale digitaler Medien

Im folgenden Artikel wird dargelegt, was unter dem Konzept des forschenden Lernens zu verstehen ist und welche Einsatzmöglichkeiten sich für das forschende Lernen im Kontext Universität ergeben. Dabei werden speziell diejenigen Merkmale theoretisch herausgearbeitet, die aus Lehrendenperspektive Anknüpfungspunkte für einen prozessbegleitenden Medieneinsatz bieten. Exemplarisch wird eine Lehrveranstaltung angeführt, die auf Fallebene zeigt, wie ein forschungsorientiertes Seminar um digitale Medien angereichert werden kann und welche Chancen und Grenzen sich bei der selbstgesteuerten Verwendung digitaler Medienangebote ergeben. Die Evaluation des Seminars zeigt, dass die Studierenden einen großen Lernzuwachs erleben, jedoch teilweise durch die Offenheit des Lernens überfordert sind. Diese und andere Gründe führen dazu, dass die von Lehrenden vorgeschlagenen Medienangebote selten bis kaum genutzt werden. Zudem sehen Lernende die Qualität virtuell distribuierter Inhalte kritisch, was sich auf die Akzeptanz und die Nutzung der vorhandenen digitalen Medienangebote auswirkt. (DIPF/Orig.

Holocene environmental and climate evolution of central west Patagonia as reconstructed from lacustrine sediments of Meseta Chile Chico (46.5°&thinsp;S, Chile)

Holocene environmental changes in Patagonia were mostly shaped by fluctuating ice cover recession. Consequently, environmental reconstructions are largely based on discontinuous moraine chronologies from valley deposits. Here, we present a 3 m long continuous sediment record recovered from Laguna Meseta (LME), a lake located on Meseta Chile Chico. Its altitude and location relative to the North Patagonian Icefield provide a unique opportunity to reconstruct the glacial history and related environmental dynamics. Our radiocarbon chronology constrains sedimentation to the last ∼10 000 years and provides a minimum age for postglacial ice-free lacustrine conditions, due to a westward retreat of the ice cap. Lacustrine productivity reached its maximum at the start of the lake phase and decreased afterwards. Between 5500 and 4600 cal yr BP, a major shift towards allochthonous sediment accumulation occurred, caused by an abrupt increase in clastic deposition from basaltic lithologies of the Meseta Chile Chico. This episode correlates with the precipitation-driven Mid-Holocene glacier advance of Patagonian glaciers and suggests that conditions were colder and/or wetter on Meseta Chile Chico at that time. After 4600 cal yr BP, these conditions continued to supply LME with clastic sediments until a stepped decrease around 900 cal yr BP. Thereupon, lacustrine productivity distinctly increased and stabilized around 300 cal yr BP. Our findings indicate that changes in sedimentation on Meseta Chile Chico were mainly controlled by regional variability in the precipitation. Furthermore, strong correlation between our records and available proxies for oscillations of the Southern Hemisphere Westerly Winds suggest a pronounced climatic control by this prominent wind system for central west Patagonia during the last 10 000 years.</p