CL5.1.4

Past climatic and environmental changes can be reconstructed thanks to paleoclimatic archives such as ice cores, marine sediment cores, lake sediment cores, speleothems, tree rings, corals, etc. The dating of these natural archives is crucial for deciphering the temporal sequence of events during past climate changes. It is also essential to estimate the absolute and relative errors of such estimated chronologies. This task is, however, complex since it involves the combination of different dating approaches on different paleoclimatic sites and often different types of archives. Here we present Paleochrono, a new probabilistic model to derive a common and probalistically optimal chronology for several paleoclimatic sites with potentially different types of archives. Paleochrono is based on the inversion of an archiving model: a varying deposition rate (also named sedimentation or accumulation rate) and also, for ice cores, a lock-in-depth of air bubbles (since air is not trapped at surface) and a thinning function (since ice undergoes flow). The model integrates several types of chronological information: prior knowledge of the archiving process, independently dated horizons, depth intervals of known duration, undated stratigraphic links between records, and, for ice cores, Δdepth observations (depth differences between synchronous events recorded in the bubbles and ice, respectively). The optimization is formulated as a least-squares problem, assuming that all densities of probabilities are near-Gaussian and that the model is almost linear in the vicinity of the best solution. Paleochrono is the successor of IceChrono, which was dealing only with ice-core records. Paleochrono performs better than IceChrono in terms of computational efficiency, ease of use, and accuracy. We demonstrate the ability of Paleochrono in a new AICC2012-Hulu dating experiment, which combines the AICC2012 dating experiment, based on records from five polar ice cores, with data from two U/Th-dated speleothems from Hulu Cave (China). We analyse the performance of Paleochrono in terms of computing time and memory usage in various dating experiments. Paleochrono is freely available under the MIT open source license.

How to cite: Parrenin, F., Bazin, L., Buizert, C., Capron, E., Chowdry Beeman, J., Corrick, E., Drysdale, R., Kawamura, K., Landais, A., Mulvaney, R., Oyabu, I., and Rasmussen, S.: The Paleochrono probabilistic model to derive a consistent chronology for several paleoclimatic sites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-822, https://doi.org/10.5194/egusphere-egu21-822, 2021.

Dating with ²¹⁰Pb has been instrumental in providing chronologies for sedimentary deposits back to around 100-150 years ago — a key period for recent environmental events such as increased pollution caused by industry developments. However, currently available ²¹⁰Pb age-models often disagree with and/or cannot incorporate other types of dates, depend highly on the correct estimation of background ²¹⁰Pb, often underestimate uncertainties and are limited by unrealistic assumptions of sediment accumulation over time. Plum is open-source Bayesian age-modelling software that solves the above problems. It can integrate other dates, produce realistic uncertainty estimates, and extend ²¹⁰Pb chronologies several decades further back in time. Our method thus enables much more robust chronologies for studies of recent environmental change.

How to cite: Blaauw, M., Aquino-Lopez, M., and Christen, J. A.: Bayesian improvements to 210Pb dating, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1596, https://doi.org/10.5194/egusphere-egu21-1596, 2021.

¹⁴C and ²¹⁰Pb methods are regularly used to determine ages and accumulation rates of peat, fen and lake sediments. The overall aim is to estimate the age of discrete layers, which were analysed for environmental proxies. Ideally, the age-depth models should fit the investigated proxy in terms of resolution and give precise results. Nevertheless, the differences in the nature of dating methods and statistical treatment of data need to be considered.

Both ¹⁴C and ²¹⁰Pb signals are integrated over a considerable period. Moreover, they originate from different sources. ²¹⁰Pb is bound to aerosols and trapped by peat while ¹⁴C is bound from atmospheric CO₂ by photosynthesis. Hence, ²¹⁰Pb gives the time span during which the aerosol has been buried, whereas the ¹⁴C date gives the time of death of a plant.

After the analysis, the results are usually combined into an age-depth model. This process involves statistical treatment of data during which specific assumptions and simplifications are made. Depending on the algorithm, they lead to alterations in modelled ages compared to unmodelled data. Principally it is a desired result–increasing the robustness and decreasing the uncertainty of the age-depth model. In worse cases models alter the modelled ages to an unacceptable extent, which may be overlooked if the results are treated automatically.

We test the performance of various age-depth modelling algorithms (OxCal P_Sequence, Bacon, clam, MOD-AGE) on a selected true dataset where ¹⁴C and ²¹⁰Pb data overlap and are used simultaneously. Afterwards, a point estimate is selected and used for proxy analysis on a time scale and for calculation of the accumulation rates. We also check the influence of ²¹⁰Pb calculation method (CRS, ModAge, extrapolation technique) on derived age-depth models.

Together with the thickness of analysed samples the age model provides an information about the time resolution of proxy analysis. While the age-depth curves, except outstanding circumstances, give relatively similar answers within 95% uncertainty ranges, the differences are observed in point estimates and accumulation rate, and they may be relevant for the palaeoenvironmental studies. With this exercise we attempt to assess the uncertainty beyond simple age errors reported from the measurements and age-depth modelling.

How to cite: Piotrowska, N. and Sikorski, J.: Evaluating the age-depth models based on coupled 14C and 210Pb data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13432, https://doi.org/10.5194/egusphere-egu21-13432, 2021.

The extensivity of sand dunes in continental interiors makes the understating of their morphodynamical properties valuable for palaeoenvironmental reconstructions and the interpretation of landscape evolution. Nevertheless, the study of aeolian landscape development at the million-years timescale is hampered by the complex interaction of factors determining dune migration and the inherently self-destructive nature of their chronostratigraphy, thus limiting the applicability of traditional luminescence-based dating methods for configuring processes beyond ~300 Ka. In this study, we present a standalone program that simulates aeolian transport based on luminescence-derived chronologies coupled with numerical modelling of cosmogenic nuclides accumulation. This integrative approach reveals ancient phases of sand irruption and provides a data-based scheme facilitating the morphodynamical study of aeolian processes over multiple timescales. We present a case study of the program application by analyzing data from the Australian Simpson Desert, unfolding several phases of aeolian vitality since the late Pliocene. The synchronicity of the results with drastic changes in environmental settings exemplifies the applicability of process-based modelling in constructing a timeframe of key landscape evolution events in arid environments by studying aeolian landforms. Finally, the relationships between model parameters used to determine environmental settings on sand migration patterns make the program a powerful tool to further investigating triggers and mechanisms of aeolian processes.

How to cite: Vainer, S. and Ben Dor, Y.: Cosmolian: High-resolution Integrated OSL and CN modelling Program for Sand Transportation in Eolian Realms (HIPSTER), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7120, https://doi.org/10.5194/egusphere-egu21-7120, 2021.

On earth, Calcium-41 is produced as a cosmogenic isotope via neutron capture process, leaving a natural isotopic abundance of 10^-15 on earth surface. Calcium is also of vital importance for the metabolism of biological organisms. Consequently, analysis of the long lived radioactive isotope Calcium-41 is of great importance in geoscience, archeology and life sciences. The half-life of Calcium-41 is 1.03 x 10⁵ years. It is a good candidate in dating rock and bone samples ranging from 50,000 to 1,000,000 years old.

The available techniques for trace analysis of Calcium-41 include accelerator mass spectrometry (AMS) and resonance ionization mass spectroscopy (RIMS). The detection limit of RIMS is on the level of 10^-11 due to the interference of Potassium-41, which is difficult to remove from the sample. The analysis with high-energy AMS is more expensive than the table top apparatus, and it also faces similar problem as RIMS method.

We develop an atom trap trace analysis(ATTA) apparatus for Calcium-41 analysis to the sensitivity of 10^-15 abundance level by one hour of single atom counting. ATTA uses laser tuned at the resonant wavelength for a specific element and isotope to slow down and capture single atom by fluorescence radiation. It has a very high selectivity of element and isotope, which is more advantageous than AMS and RIMS to avoid isobar interference. ATTA has been used in analysis of Krypton-81, Argon-39 dating of the hydrological samples. This work on high sensitivity Calcium-41 analysis is very promising in dating the geochemical sample to determine the exposure ages of rocks or in cosmochemistry for investigations on terrestrial ages.

How to cite: Xia, T., Xia, T.-Y., Sun, W.-W., Jiang, W., and Lu, Z.-T.: Analyzing Ca-41 sample at 1E-15 abundance level with cold atom trap techniques, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1959, https://doi.org/10.5194/egusphere-egu21-1959, 2021.

Assessing the effects of sediment inhomogeneity on the core stratigraphy and on proxy records is essential to perform reliable climate reconstructions from marine sediments. Inhomogeneities can stem from sediment mixing (e.g., bioturbation) which destroys the temporally layered climatic information stored in proxy carrier. Thus, in addition to the measurement error, the time-uncertainty in radiocarbon-dated sediments must be taken into account for depth-age modelling in order to obtain an unambiguous time scale.

Here, we present a case-study based on a boxcore (OR1-1218-C2-BC) from the South China Sea (2208 m water depth) covering the last 20 kyr. The boxcore was divided into nine sub-cores by a grid of 3 x 3 (each sub-core is 8 x 8 cm with a length of 34 cm), yielding a total surface area of 576 cm². This sampling scheme offers the possibility for detailed, three-dimensional analyses on small spatial scales. Radiocarbon measurements were performed in every sub-core for seven depth layers, each with a fraction of 200 crushed and well mixed foraminifera (Trilobatus sacculifer, 250 – 350 µm) to study the horizontal sediment heterogeneity. In addition, small sample (5 specimen) replicate radiocarbon measurements from a single sediment sample allow to estimate the age heterogeneity within a 1 cm sediment slice and thus the vertical mixing from bioturbation. The replicate radiocarbon dates suggest a bioturbation depth of around 12 cm; however, the downcore radiocarbon dates show no clear sign of a well-mixed bioturbation layer. Using statistical analysis (e.g., spatial correlation measures and variance analyses), we separate the errors from the radiocarbon measurements, the finite sample size and both the vertical and horizontal heterogeneity. Comparing the radiocarbon dates in the sub-cores indicates a small horizontal heterogeneity compared to the vertical mixing.

The three-dimensional data set allows us to quantify the effect introduced by (post-depositional) sediment mixing on the age-estimate as well as on the proxy signal and to discuss the effects on low-sedimentation climate records. This will provide a better quantification of uncertainties within proxy time series.

How to cite: Zuhr, A., Dolman, A., Groeneveld, J., Ho, S. L., Grotheer, H., and Laepple, T.: Assessing time uncertainty and sediment mixing using three-dimensional high-resolution radiocarbon measurements from a marine boxcore, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8440, https://doi.org/10.5194/egusphere-egu21-8440, 2021.

Our ability to quantify past climate conditions is crucial for predicting future scenarios and landscape evolution. To date, reconstructions of the Earth’s past climate have mostly relied on the use of climate proxies to infer previous surface conditions (e.g. Jones and Mann, 2004 for a review). However, few methods exist that are capable of directly measuring past temperature histories, particularly in terrestrial settings.

The aim of this study is to contribute towards a more detailed understanding of glacial and interglacial temperature fluctuations across the Central and Western Alps, from the Last Glacial Maximum to present day, by constraining past temperatures of exposed bedrock surfaces adjacent to the Gorner glacier in Zermatt, Switzerland. This is done through the recently developed application of feldspar thermoluminescence to surface paleothermometry (Biswas et al., 2018; 2020). The thermoluminescence signal of feldspar, from room temperature to 450°C, is sourced from a continuous distribution of electron traps within the crystal lattice (Biswas et al., 2018). The release of this trapped charge is temperature dependent and thus, at room temperature, results in traps with a range of thermal stabilities with electron residence times ranging from less than a year to several billion years (Aitken 1985). Traps sensitive to typical surface temperature variations (e.g. ∼10°C) have been shown to lie between 200°C and 250°C of the TL glow curve (Biswas et al., 2020). From this temperature range, five thermometers (200°C to 250°C in 10°C intervals) can be used together as a multi-thermometer, and subsequently combined with a Bayesian inversion approach to constrain thermal histories over the last ∼50 kyr (Biswas et al., 2020).

In this study, the preliminary temperature histories of five bedrock samples with independently constrained exposure ages, exposed progressively since the Last Glacial Maximum, will be presented.

References:

Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London.

Jones, P.D., Mann, M.E., 2004. Climate over past millennia. Reviews of Geophysics, 42, 2004.

Biswas, R.H., Herman, F., King, G.E., Braun, J., 2018. Thermoluminescence of feldspar as a multi-thermochronometer to constrain the temporal variation of rock exhumation in the recent past. Earth and Planetary Science Letters, 495, 56-68.

Biswas, R.H., Herman, F., King, G.E., Lehmann, B., Singhvi, A.K., 2020. Surface paleothermometry using low temperature thermoluminescence of feldspar. Climate of the Past, 16, 2075-2093.

How to cite: Elkadi, J., Biswas, R., King, G., and Herman, F.: Constraining past bedrock surface temperatures at the Gorner glacier, Switzerland, using feldspar thermoluminescence for surface paleothermometry., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13962, https://doi.org/10.5194/egusphere-egu21-13962, 2021.

Quartz grains are resistant minerals and abundant on Earth´s surface. They have been extensively used for optically stimulated luminescence (OSL) dating of Quaternary sediments from a large range of depositional environments. Recently, it has been demonstrated that the luminescence properties of the quartz can also be a useful tool for provenance analysis, because of the signal properties inherited from its parent rock, weathering conditions, and depositional history (i.e. cycles of erosion, transport, and deposition). These provenance studies are based on the OSL sensitivity (i.e. the light emitted per unit mass per radiation dose) of the first second of the luminesce signal of the quartz, using relatively fast and low-cost measurements. Since laboratories worldwide already have an extensive database with results of quartz signals primarily measured for dating studies, these data could potentially be repurposed for sensitivity analysis.

Here, we investigate the use of OSL quartz signals, originally measured for dating, for now characterizing the quartz OSL sensitivity and their usefulness for provenance analysis. The samples we studied are from Amazon fluvial systems: two Holocene endmembers from the Xingu and Solimões rivers, representatives of cratonic and Andean sediment sources, respectively, and a Pleistocene sample from Içá Formation, a paleo-fluvial system whose provenance is not fully known. First, we evaluate our approach by calculating the OSL quartz sensitivity of all quartz signals (i.e. signals derived from the natural, regenerative, and test doses) measured in a dating sequence with the Single Aliquot Regenerative dose (SAR) protocol. Such analysis gives the basis for deciding which signal, if any, should be prioritized for sensitivity calculation. Then, we compare the OSL sensitivities derived from quartz signals measured using the conventional sensitivity protocol with those measured by the conventional dating protocol. Finally, we deduce the sediment source of the Pleistocene Içá Formation based on the modern analogues (the Holocene endmembers).

Our preliminary results show that: it is feasible to use data from dating sequences for sensitivity calculation; OSL quartz signal derived from the natural test doses (L_n) is the best candidate for sensitivity calculation; the sensitivities provided by our approach are slightly larger than those calculated using the conventional sensitivity protocol; and, the Pleistocene Içá Formation represents a mixed-source (Andean and Cratonic), which is different from the presumed modern analogue represented by the Solimões river, which is dominated by Andean sediments.

How to cite: Souza, P., Pupim, F., del Río, I., Rodrigues, F., Mineli, T., Porat, N., Hartmann, G., and Sawakuchi, A.: Using OSL dating data for quartz provenance analysis in late Quaternary sediments of Amazonia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8099, https://doi.org/10.5194/egusphere-egu21-8099, 2021.

Loess-palaeosol sequences are most important archives for reconstructing terrestrial palaeoenvironments. However, in the European Mediterranean, these archives are surprisingly scarcely investigated. In southern France, respective sediment-soil archives were last investigated in more detail in the middle of the last century, when major construction works in the region provided access to numerous loess exposures. However, this was before the breakthrough of luminescence-dating as a major chronometric method for the Later Pleistocene. Thus, the loess was poorly dated. Later, Mediterranean loess archives attracted fewer researchers than their central-European counterparts. Reasons for this may be that, compared to the loess belt of temperate Europe, Mediterranean loess deposits are often reworked and mixed with slope deposits. Moreover, palaeosols that developed during climate ameliorations of the last glacial period seem to reflect less pronounced temperature and humidity shifts than those in temperate regions. The most prominent palaeosol developed within the last-glacial loess in the Rhône Valley is a brown palaeosol with large carbonate concretions at its base. However, it is usually severely truncated.

We allocated joint research efforts from groups in Germany and France to track last-glacial sedimentation and climate shifts in loess-palaeosol sections along the Rhône Valley, south of the confluence of the River Saone at Lyon. Thereby, optically stimulated luminescence (OSL) dating serves as a vital tool for establishing chronometries for the loess-palaeosol sections in southern France, with first results from the Rhône Valley in Bosq et al. (2018, 2020). We summarize results from two sites that are regarded as key sections for palaeoenvironmental reconstruction along the Rhône Valley as evaluated from inspections during fieldwork, complemented by several smaller sections. Our presentation focusses on challenges met with OSL dating attempts of these Mediterranean archives and first achievements in backing the stratigraphies established by sediment-soil analyses in the field and laboratory by chronometric data.

References

Bosq, M., Bertran, P., Degeai, J.-P., Kreutzer, S., Queffelec, A., Moine, O., Morin, E., 2018. Last Glacial aeolian landforms and deposits in the Rhône Valley (SE France): Spatial distribution and grain-size characterization. Geomorphology 318, 1–20. doi:10.1016/j.geomorph.2018.06.010

Bosq, M., Kreutzer, S., Bertran, P., Degeai, J.-P., Dugas, P., Kadereit, A., Lanos, P., Moine, O., Pfaffner, N., Queffelec, A., Sauer, D., 2020. Chronostratigraphy of two Late Pleistocene loess-palaeosol sequences in the Rhône Valley (southeast France). Quaternary Science Reviews 245, 106473. doi:10.1016/j.quascirev.2020.106473

How to cite: Kadereit, A., Pfaffner, N., Kreutzer, S., Wang, T., Cornu, S., Zipf, L., Moine, O., Bosq, M., Bertran, P., and Sauer, D.: First results of optically stimulated luminescence (OSL) analyses from loess-palaeosol sections along the Rhône Valley, France., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14759, https://doi.org/10.5194/egusphere-egu21-14759, 2021.

Ruddons Point, situated on the Firth of Forth coastline, Scotland, is a laterally extensive terrace of glacial and marine sediment deposits which are raised above current sea level. The deposits are situated near to Kincraig Point, a key site that records a series of stepped erosional platforms carved into the local tuff and agglomerate bedrock, representing a series of post Last Glacial Maximum (LGM) paleoshorelines. The raised marine deposits of Ruddons Point are composed of sand, shell, gravel and cobble horizons, which have been deposited unconformably on glacial tills and clays. 1km inland, incised by a small stream (the Cocklemill Burn), numerous cut banks reveal further raised deposits. Previous attempts to date these sediments have produced conflicting depositional histories, with no clear correlation to the Kincraig Point paleoshorelines or those dated further west along the Forth Valley. To this end, a multidisciplinary study was conducted to provide a detailed geochronology and interpretation of this diverse field area, ranging from the dating of glacial clays, thought to be deposited during the early phases of sedimentation leading up to the LGM, through to establishing the timing of marine sediment deposition in the Mid to Late Holocene. Geophysical electrical surveys were employed to supplement the subsurface investigations of the shore and adjacent saltmarsh and to aid construction of a sedimentary deposition model. Optically stimulated luminescence dating and radiocarbon dating were carried out on selected samples through the raised deposits and subsurface cores to provide a chronometric framework. Results demonstrate that subsurface clay horizons date to ~29 ka, indicating deposition at the onset of the last glacial. Inland raised deposits along the Cocklemill Burn preserve periods of sedimentation ranging from ~8.5 ka to ~4.5ka, and suggest a punctuated rise in sea level during the Mid Holocene. The nearshore raised sands and gravels of Ruddons Point are dated to ~3.5ka, deposited during a period of relative sea level (RSL) fall in the Late Holocene. The study concludes that the combination of detailed fieldwork, geophysical survey and dating allow the scope for complex landscape changes to be identified and further the understanding of the interplay between glacial isostatic adjustment and RSL changes across the postglacial coastlines of Fife.

How to cite: Boyd, S., Bates, R., Kinnaird, T., Raub, T., and Srivastava, A.: Investigating glacial and postglacial sediment deposition and relative sea level changes at Ruddons Point, Fife, Scotland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7238, https://doi.org/10.5194/egusphere-egu21-7238, 2021.

Multi-proxy studies (including sedimentological, pedological, radiocarbon and optically stimulated luminescence dating methods) were used to establish origin and chronology of depositional processes in the type section Mierzyn, central Poland. The investigated key site is located in the extraglacial zone of the Last Glaciation, ca. 130 km to the south from the Last Glacial Maximum in the Luciąża river valley area. In the studied profile (16 m thick) two lithofacial complexes were identified. The lower, fluvio-aeolian complex consists of silty-sandy sediments (1.6 m) deposited. The final phase of fluvio-aeolian deposition is expressed by initial pedogenic processes. Above is located aeolian complex (13 m of thickness). Three aeolian units are separated by two palaeosols.

To establish stratigraphic framework of depositional and pedogenic processes, four samples for radiocarbon dating from palaeosols and twelve samples for OSL dating from sandy units were collected. The obtained results reveal very good agreement of both absolute dating methods. It led to reconstruct chronology of main palaeoenvironmental changes. The fluvio-aeolian complex and the lowermost part of aeolian complex (below the lower palaeosol) were deposited in the Oldest Dryas in relatively cool and dry climate conditions. The amelioration of climate in the Bølling interstadial caused development of pedogenic processes expressed by 0.3 m thick palaeosol. Main part of aeolian complex (10 m of thickness) was deposited in the Older Dryas. The upper palaeosol developed in the Allerød interstadial as a result of the next amelioration of the climate. During the Younger Dryas was deposited the uppermost part of aeolian complex.

Classic development of fluvial to- aeolian succession in the Mierzyn site as well as detailed chronology based on two independent absolute age methods reveal that it can be treated as stratotype for the Late Glacial and correlated with other type sections in the Central and Western Europe.

Ackowledgments

Presented results were obtained with support of Polish National Science Centre, contract number 2018/30/E/ST10/00616.

How to cite: Piotr, M., Robert, S., Zdzisław, J., Paweł, Z., Jerzy, R., Marcin, K., Jacek, S., Grzegorz, P., Michał, Ł., Agnieszka, S., and Konrad, T.: Multi-proxy studies of the Late Glacial fluvio-aeolian succession in the type site Mierzyn, central Poland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4891, https://doi.org/10.5194/egusphere-egu21-4891, 2021.

Annually laminated sediments, also called varves, are valuable natural archives to reconstruct past environments and climate. Until now, the most common and reliable procedure to count varves has been to produce overlapping thin sections of the entire sediment sequence and counting in the microscope — a process that can take months to complete. Replacing this laborious method has been a long ongoing process within the varve community, and a task that we now may be getting with advancements in analytical tools. This study assesses the use of CT scanning to produce varve chronologies, applying it to the ferruginous sediments of Lake Sagtjernet in southeastern Norway — the first non-glacial varved lake sediment sequence in Norway continuously covering the last 4300 years.

Microfacies analyses of the sediments show that the varves are formed by cyclical deposition of iron and manganese. Oxygen measurements through 2013-2014 show permanent anoxic bottom waters while the seasonal turnover only reaching a depth of c. 6 m (out of a total 12 m depth). Combined with measurements of iron from the water column (highly enriched in the bottom waters) we suggest to classify Lake Sagtjernet as a ferruginous meromictic lake.

Varve counting on CT scans resulted in a 4300-year chronology, which we compared to an independent radiocarbon chronology (based on 17 ¹⁴C dates, radionuclide and ²¹⁰Pb analyses). Our results show that all of the varve ages fall within the 95% confidence interval of the radiocarbon chronology. However, some sections of the sediments with lower concentrations of iron and manganese illustrate vague boundaries between laminae in the CT scans — increasing age uncertainties in the chronology. These age uncertainties can be reduced by using XRF scanning or thin sections in parallel with CT scans to evaluate the boundaries. Based on these results, we conclude that CT scanning is a fast and non-destructive method for producing varve chronologies.

How to cite: Ballo, E. G., Bajard, M., Støren, E., and Bakke, J.: Using CT scans to count varves in lake sediments. Application to Lake Sagtjernet, southeastern Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10075, https://doi.org/10.5194/egusphere-egu21-10075, 2021.

The Scandinavian Ice Sheet responded time-transgressively to the Younger Dryas (Greenland Stadial 1) cold event with large regional variations. Around Trondheimsfjorden in central Norway, the Tautra Moraines and the Hoklingen Moraines have long been assumed to have formed by glacial readvances during this event, as they have been dated to c. 12.7 and 11.6 cal. ka BP respectively (Olsen et al., 2015), mainly based on radiocarbon dating of often marine fossils. The Tautra Moraines, being the outer ridges of the two, should thus represent the maximum ice-sheet extent in this region during the Younger Dryas.

This ice-front position established a pro-glacial lake west of present-day Leksvik village on the Fosen peninsula (Selnes, 1982), which covered the Lomtjønnin lakes and Lomtjønnmyran fens, and drained through a spillway via Lake Rørtjønna. Some 20 km inland (northeast) from this location, inside the Tautra Moraines, the location of the Damåsmyran bog was covered by the ice sheet at that time.

By examining sediments from these sites for occurrences of volcanic ashes (visible and cryptotephra), combined with radiocarbon dating, we find that the ice front remained at the Tautra Moraines until the late Younger Dryas, contrary to the previous chronology (and overriding the suggested formation age of the Hoklingen Moraines). These findings comply with several recent reconstructions of the deglaciation at other sites in western (Lohne et al., 2012; Mangerud et al., 2016) and southern Norway (Romundset et al., 2019) and are a strong example of the usefulness of tephrochronology in the reconstruction of past ice-sheet dynamics.

References

Lohne, Ø.S., Mangerud, J. & Svendsen, J.I. (2012) Timing of the Younger Dryas glacial maximum in Western Norway. Journal of Quaternary Science, vol. 27, pp. 81–88.

Mangerud, J., Aarseth, I., et al. (2016) A major re-growth of the Scandinavian Ice Sheet in western Norway during Allerød–Younger Dryas. Quaternary Science Reviews, vol. 132, pp. 175–205.

Olsen, L., Høgaas, F. & Sveian, H. (2015) Age of the Younger Dryas ice-marginal substages in Mid-Norway—Tautra and Hoklingen, based on a compilation of 14C-dates. Norges geologiske undersøkelse Bulletin, vol. 454, pp. 1–13.

Romundset, A., Lakeman, T.R. & Høgaas, F. (2019) Coastal lake records add constraints to the age and magnitude of the Younger Dryas ice-front oscillation along the Skagerrak coastline in southern Norway. Journal of Quaternary Science, vol. 34, pp. 112–124.

Selnes, H. (1982) Paleo-økologiske undersøkelser omkring israndavsetninger på Fosenhalvøya, Midt-Norge. Thesis at the Department of Botany, University of Trondheim.

How to cite: Larsson, S. A., Wastegård, S., and Høgaas, F.: Tephrochronological evidence of a later Younger Dryas ice-sheet maximum in central Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11404, https://doi.org/10.5194/egusphere-egu21-11404, 2021.

Uranium-uranium (²³⁴U-²³⁸U) disequilibrium dating can determine the age of secondary carbonates over greater time intervals than the well-established ²³⁰Th-²³⁴U dating method. Yet it is rarely applied due to unknowns in the initial d²³⁴U (d²³⁴U_i) value, which result in significant age uncertainties. In order to understand the d²³⁴U_i in Devils Hole 2 cave, Nevada, we have determined 110 d²³⁴U_i values from phreatic calcite using ²³⁰Th-²³⁴U disequilibrium dating. The sampled calcite was deposited in Devils Hole 2 between 4 and 590 ka, providing a long-term look at d²³⁴U_i variability over time. We then performed multi-linear regression among the d²³⁴U_i values and correlative d¹⁸O and d¹³C values. The regression can be used to estimate the d²³⁴U_i value of Devils Hole calcite based upon its measured d¹⁸O and d¹³C values. Using this approach and the measured present-day d²³⁴U values of Devils Hole 2 calcite, we calculated 110 independent ²³⁴U-²³⁸U ages. In addition, we used newly measured d¹⁸O, d¹³C, and present-day d²³⁴U values to calculate 10 ²³⁴U-²³⁸U ages that range between 676 and 731 ka, thus allowing us to extend the Devils Hole chronology beyond the ²³⁰Th-²³⁴U-dated chronology while maintaining an age precision of ~2 %. Our results indicate that calcite deposition at Devils Hole 2 cave began no later than 736 ± 11 kyr ago. The novel method presented here may be applied to future speleothem studies in similar hydrogeological settings, given appropriate calibration studies.

How to cite: Li, X., Wendt, K. A., Dublyansky, Y., Moseley, G. E., Spötl, C., and Edwards, R. L.: Novel method for determining 234U-238U ages of Devils Hole 2 cave calcite (Nevada), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1955, https://doi.org/10.5194/egusphere-egu21-1955, 2021.

Speleothem deposits can provide a wealth of critical, detailed paleoclimate information from low and mid-latitude terrestrial environments. A key strength of these archives is that they may be dated precisely using U-Th techniques. Yet, depending on the cave environment, overlying geology, seepage water flow characteristics, and speleothem growth habit, accurate, precise dating of speleothems can be challenging. For example, contamination by Th-bearing detritus degrades precision and accuracy when model-based corrections for initial ²³⁰Th are applied, and partial dissolution or secondary infilling of porosity can also lead to inaccurate ages, thereby confounding interpretations of paleoclimatic change. Here we present a new chronology from a Holocene stalagmite, WMC2, from White Moon Cave in the Santa Cruz Mountains of California, USA, that exhibits multiple challenges. Stalagmite WMC2 was not active at the time of collection, but it was in situ, with a top age of 3267 ± 28 yrs BP 1950.  WMC2 calcite has relatively high U (3-7 ppm), however, the stalagmite contains sporadically distributed sub-millimeter pockets of silicate detritus, leading to 100-fold differences in common Th (²³²Th) concentrations in dating samples (i.e., >80 to <1ppb). Additionally, ages that appear to be anomalously young are associated with zones containing high densities of fluid inclusions, suggesting possible secondary calcite growth. We overcome these challenges using a combination of micro-CT imaging, transmitted-light microscopy and assessing replicate samples.  Micro-CT provides a non-destructive method for imaging the internal structure of the stalagmite, allowing for the sampling of dense, pure calcite. Using this approach, we are able to avoid sub-millimeter pockets of silicate detritus that are not visible from the cut surface of the sample, thereby reducing ²³²Th concentrations and associated initial ²³⁰Th corrections, and obtaining more precise and accurate ages. Dating replicate samples from individual growth bands can confirm or refute whether diagenesis suspected from petrographic study has measurably affected U-Th ages since corrupt ages should scatter more than expected from analytical errors alone. We use our carefully screened ages for WMC2 to evaluate various age modeling approaches typically used for stalagmite proxy records, including those that apply Monte Carlo methods and Bayesian approaches. By employing multiple techniques to optimize stalagmite dating samples, reliable, precise U-Th ages (median 2s ~30 yr) may be obtained from stalagmites previously deemed too flawed for accurate dating, thereby broadening our ability to develop accurately dated speleothem paleoclimate records.

How to cite: Oster, J., de Wet, C., Neal, K., and Sharp, W.: Using Micro-CT and petrographic analysis to select optimal U-Th samples from challenging stalagmites., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3322, https://doi.org/10.5194/egusphere-egu21-3322, 2021.

The Outer Western Carpathians is a region in Poland with an exceptionally high number of landslides. Besides geohazard issues, these landslides are frequently an archive of the past environmental changes thanks to organic and mineral material accumulated in numerous landlocked pits where small peat bogs formed. We investigated a large landslide body developed on the northside part of Mt Śnieżnica. No historical information exists related to the instability of this region. After finding and examining 20 pits, we selected three for detailed analyses. We extracted three cores with the use of a peat sampler. The cores were 10 cm in diameter and up to 4,2 m deep. Compared to other landslides in this region, the depth of peat bogs is substantial and suggests that the landslide is a minimum of several thousand years old. Laboratory analyses included: bulk density measurement, geochemical analyses (organic matter, SiO2ter, SiO2biog, CaCO3, TOC, TIC, C, N, S, Na, K, Mg, Ca, Fe, Mn, Cu, Zn, and Pb contents) of biogenic deposits (405 samples), AMC radiocarbon dating of plant tissue (20 pieces), macroscopic charcoal (420 samples each representing 1 cm slice of the core). The aim of the present study is 1) to reconstruct environmental conditions in the area, 2) dating of the landslide formation in relation to climate change during the Holocene, and the main events after its triggering: changes in vegetation cover, hillslope instability, e.g., soil erosion, hydroclimate fluctuations, and human impact. Our results fill the gap in environmental reconstruction encountered by other authors investigating this part of the Outer Western Carpathians and push forward the ongoing discussion on the environmental conditions during the Holocene in this region.

How to cite: Pawlik, L., Okupny, D., Kroh, P., Cybul, P., and Stachowicz-Rybka, R.: Holocene environmental reconstructions based on organic and mineral deposits of Mt Śnieżnica landslide, Outer Western Carpathians, Poland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6666, https://doi.org/10.5194/egusphere-egu21-6666, 2021.