CL5.1.4

In the search for very old ice, finding the age of the ice is a key parameter necessary for its interpretation. Most ice core dating methods are based on chronological markers that require the ice to be in stratigraphic order. However, the oldest ice is likely to be found at the bottom of ice sheets, where the stratigraphy is disturbed, or in ablation areas, where the classical methods cannot be used. Absolute dating techniques have recently been developed to provide new constraints on the age of old ice. In particular, ⁸¹Kr measurements provide strong dating constraints for the old ice cores. Still, these measurements are limited in deep ice cores because of the large sample size required (5-6 kg). In addition to ⁸¹Kr dating, we discuss here the analytical performances of a new technique for ⁴⁰Ar dating, which allows us to provide a reliable age with 80g of ice rather than 800g, as previously published. Finally, we present two applications for the ⁸¹Kr and ⁴⁰Ar dating on the bottom of the TALDICE and Dome C ice cores.

How to cite: Landais, A., Orsi, A. O., Fourré, E. F., Jacob, R., Crotti, I., Ritterbusch, F., Lu, Z.-T., Yang, G.-M., and Jiang, W.: Absolute dating of deep ice cores with argon and krypton isotopes., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13260, https://doi.org/10.5194/egusphere-egu22-13260, 2022.

Radiocarbon (¹⁴C) dating is the standard method for obtaining the age of marine sediments of Holocene and late Pleistocene age. For accurate calibrations, however, this tool relies on precise knowledge of the local radiocarbon reservoir age of the surface ocean, i.e. the regional difference (ΔR) from the average global marine calibration dataset. This parameter has become impossible to measure from modern material samples because of ¹⁴C contamination from extensive testing of thermo-nuclear bombs in the second half of the twentieth century. The local reservoir age can thus only be calculated from the radiocarbon age of samples collected before AD 1950 or from sediment records containing absolute age markers, derived from e.g. tephrochronology or paleomagnetism.

Knowledge of the marine reservoir age around Greenland is sparse and relies on work by a few studies, represented by measurements clustered in local patches. In this study we add new radiocarbon measurements on samples from historical mollusk collections from Arctic expeditions of the late 19^th and early 20^th Century. The 92 new samples are from central east Greenland and the entire western Greenland coast. Although the new data is mostly coastal, it includes a few deeper sites from the Labrador Sea and northeastern North Atlantic Ocean, where deep waters were found to be very young. Together with existing measurements, the new results are used to calculate average ΔR values for different regions around Greenland, all in relation to Marine20, the most recent radiocarbon calibration curve. Despite the significant addition of new measurements, very few data exist for southeastern Greenland, while no data at all is available for the Arctic Ocean coast in northern Greenland.

How to cite: Pearce, C., Özdemir, K. S., Forchhammer, R. C., Detlef, H., and Olsen, J.: The radiocarbon reservoir age of coastal Greenland waters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8868, https://doi.org/10.5194/egusphere-egu22-8868, 2022.

The eastern Mediterranean region is located at divergent climatic zones and contrasting precipitation regimes of the humid Mediterranean climate and hyper-arid Saharo-Arabian desert belt. Important sedimentary archives from lakes allow past hydroclimatic variability to be reconstructed using multiple proxies. This can provide useful insight into potential future water budget scenarios. However, problems associated with chronological uncertainty can prevent insight into regional climatic (a)synchronies. The use of isochronous chronological markers of tephra (volcanic ash) can be a powerful tool in correlating palaeoclimatic records, particularly over vast distances with the development of cryptotephra analyses (non-visible volcanic glass shards).

            The TephroMed project aims to precisely synchronise two key ICDP palaeoclimatic records from eastern Mediterranean through the use of tephrostratigraphic investigations: to the north, in the Anatolian region, Lake Van (PALEOVAN, Litt et al., 2014) and to the south, in the Levant, the Dead Sea (DSDDP, Stein et al., 2011). Both records have undergone lake level reconstructions, indicating contrasting past regional responses to large-scale climatic events (e.g. Finne et al., 2019; Neugebauer et al., 2015). Though both records are dated through absolute and relative methods (radiocarbon, U-Th, varve counting, wiggle-matching), inherited large chronological uncertainties do not allow detailed insight into the potential climatic time-transgressive nature between the two sites. Yet, both records have tephra deposits within their lacustrine sediments, highlighting the potential to facilitate the alignment of both records using tephra (Neugebauer et al., 2021).

Here, we present new major and minor element volcanic glass chemical data from several tephra layers from both Lake Van and the Dead Sea ICDP cores. New geochemical data from selected visible tephra layers in Lake Van are given. The cryptotephra results from the Dead Sea show particular significant findings with volcanic glass derived from potentially several volcanic regions within the Mediterranean (e.g. Anatolia, Italy). This new data can help to facilitate a chronological alignment between the Dead Sea, Lake Van and other important climatic archives in the Mediterranean. In addition, it highlights the importance of distal records in understanding past volcanic eruptions. As a result of these findings, we can now start to answer questions associated with regional expression of past climatic events and their temporal transgression.

References

Finné, M., Woodbridge, J., Labuhn, I., Roberts, C.N., 2019. Holocene hydro-climatic variability in the Mediterranean: A synthetic multi-proxy reconstruction. Holocene 29(5), 847–863

Litt, T., Anselmetti, F.S., 2014. Lake Van deep drilling project PALEOVAN. Quat. Sci. Rev. 104, 1-7.

Neugebauer, I., Brauer, A., Schwab, M.J., Dulski, P., Frank, U., Hadzhiivanova, E., Kitagawa, H., Litt, T., Schiebel, V., Taha, N., Waldmann, N.D., DSDDP Scientific Party, 2015. Evidences for centennial dry periods at ~3300 and ~2800 cal. yr BP from micro-facies analyses of the Dead Sea sediments. Holocene 25, 1358-1371.

Neugebauer, I., Müller, D., Schwab, M.J., Blockley, S., Lane, C.S., Wulf, S., Appelt, O., Brauer, A., 2021. Cryptotephras in the Lateglacial ICDP Dead Sea sediment record and their implications for chronology. Boreas 50 (3), 844-861.

Stein, M., Ben-Avraham, Z., Goldstein, S.L., 2011. Dead Sea deep cores: A window into past climate and seismicity. Eos, Transactions American Geophysical Union 92, 453-454

How to cite: Kearney, R., Schwab, M. J., Neugebauer, I., Pickarski, N., and Brauer, A.: The TephroMed project: Precise synchronising of two key palaeoclimatic ICDP records of the eastern Mediterranean using tephra, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-468, https://doi.org/10.5194/egusphere-egu22-468, 2022.

Coral microatolls allow for the reconstruction of relative sea level (RSL) and the inference of tectonic deformation along tropical coastlines over the Holocene. Microatolls track RSL with unparalleled vertical precision, and their annual banding allows us to count years precisely over an individual coral’s lifetime; however, RSL histories reconstructed from multiple corals depend on accurate and precise radiocarbon (¹⁴C) or uranium-thorium (²³⁰Th) ages.

We collected coral microatoll slabs from sites in Ilocos Region, northwestern Luzon, Philippines, and dated them with ¹⁴C and ²³⁰Th techniques. Notably, initial RSL reconstructions for some sites disagreed markedly depending on the dating technique used. Attempts to replicate geochronologic analyses have shown that the coral skeletons are susceptible to diagenesis, complicating efforts to accurately determine coral ages.

We are developing a strategy to overcome this limitation. We extracted multiple samples from each microatoll slab for paired ¹⁴C and ²³⁰Th dating. The number of annual bands separating any dated sample was used to further constrain the age of the coral; by subtracting the number of years from each dated sample, samples taken from different parts of the slab can produce independent estimates of the outermost preserved band. After excluding anomalously young replicate ¹⁴C ages and samples flagged as partly calcified by x-ray diffraction, we find that ²³⁰Th ages from a single coral disagree at 4σ in 4 of 8 cases, whereas calibrated ¹⁴C dates overlap at 2σ in 8 of 9 cases for an arbitrary radiocarbon marine reservoir correction, ∆R = 0 yr.

Using OxCal and the Marine20 calibration curve, we apply Bayesian statistics to combine ¹⁴C and ²³⁰Th ages, to estimate ∆R, and to determine the coral ages using the best available data. We further analyze the ∆R value for each coral, and account for overdispersion and underdispersion, whilst generating a ∆R value per site, and an overall ∆R value (inclusive of all sites). We find no statistically significant difference in ∆R for each site, and we calculate an overall ∆R of -155 ± 117 yr for sites in Ilocos Region since the mid-Holocene, though century-scale variability in ∆R may occur.

Additionally, to improve the reliability of our dates, our final dating strategy in OxCal is to apply the previously determined ∆R, to a code that places the corals in sequence (based on precise elevation measurements, morphological similarities, and coral die-down events), along with the ¹⁴C dates that are dated to the outermost preserved band.

How to cite: Mitchell, A., Lim, J., Gopal, A., Meltzner, A., Chan, A., Sarkawi, G., Li, X., Cantillep, A. M., Sarmiento, L. F., Komori, J., Yu, T.-L., Shen, C.-C., Gong, S.-Y., Weil-Accardo, J., Maxwell, K., Lin, K., Lu, Y., Wang, X., and Ramos, N.: Geochronologic methods for dating coral microatolls in the Philippines, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11178, https://doi.org/10.5194/egusphere-egu22-11178, 2022.

We investigated sediments from three different depositional environments along the northern Argentine continental margin to assess the main processes controlling sediment deposition since the last glacial period. Further, we evaluated how different depositional conditions affect (bio)geochemical processes within sediments. Sediment cores were collected during expedition SO260 in 2018[1]. Two sites are located at ~1100 m water depth north and south of the Mar del Plata Canyon (N- and S-Middle Slope Site). Another site is situated at the lower continental slope at 3600 m water depth (Lower Slope Site). Reliable age constraints of sediments deposited during the last glaciation at the Argentine margin are difficult to obtain due limited amounts of carbonate. We overcame this issue by combining radio-isotope analyses (¹⁴C,²³⁰Th_ex) with sedimentological, geochemical and magnetic data demonstrating that all sites experienced distinct changes over time.

Both, N- and S-Middle Slope Sites, record at least the last 30 ka. The S-Middle Slope Site is dominated by continuously organic carbon-starved and winnowed sandy deposits, which according to geochemical and magnetic data leads to insignificant sulfate reduction and sulfidation of iron (oxyhydr)oxides. Glacial sedimentation rates at the Middle Slope increase northwards suggesting a decrease in bottom-current strength. The N-Middle Slope Site records a transition from the last glacial period, dominated by organic carbon-starved sands, to the early deglacial period when mainly silty and organic carbon-rich sediments were deposited between 14-15 ka BP. Concurrently, glacial sedimentation rates of ~50 cm/ka significantly increased to 120 cm/ka. We propose that this high sedimentation rate relates to lateral sediment re-deposition by current-driven focusing as response to sea level rise. Towards the Holocene, sedimentation rates strongly decreased to 8 cm/ka. We propose that the distinct decrease in sedimentation rates and change in organic carbon contents observed at the N-Middle Slope Site caused the nonsteady-state pore-water conditions and deep sulfate-methane-transition (SMT) at 750 cm core depth. The Lower Slope Site records the last 19 ka. Continuously high terrigenous sediment input (~100 cm/ka) prevailed during the Deglacial, while sedimentation rates distinctly decreased to ~13 cm/ka in the Holocene. Here, pore-water data suggest current steady-state conditions with a pronounced SMT at 510 cm core depth. Our study confirms previous geochemical-modelling studies at the lower slope, which implied that the observed SMT fixation for ~9 ka at specific depth relates to a strong decrease in sedimentation rates at the Pleistocene/Holocene transition[2].

During the Holocene, total organic and inorganic carbon contents, inorganic carbon mass accumulation rates and XRF Si/Al ratios (preserved diatom flux) increase at our sites. We relate this to increased primary production in surface waters and less terrigenous input along the continental margin. Our multidisciplinary approach presents improved age constraints at the northern Argentine Margin and demonstrates that lateral/vertical sediment transport and deposition was strongly linked to Glacial/Interglacial variations in bottom currents, seafloor morphology, sea level and sediment supply. The dynamic depositional histories at the three sites still exert a significant control on modern sedimentary (bio)geochemical processes.

[1]Kasten et al. (2019).Cruise No. SO260. Sonne-Berichte.

[2]Riedinger et al. (2005).Geochim. Cosmochim. Acta. 69.

How to cite: Melcher, A., Miramontes, E., Geibert, W., Henkel, S., Wilckens, H., Pape, T., Köster, M., Volz, J., Frederichs, T., Bozzano, G., Chiessi, C., Chidolue, N. C., Ngui, O. S., Schwenk, T., and Kasten, S.: Strong changes in depositional conditions during the Late Glacial and the Holocene along the northern Argentina Continental Margin: a multiproxy approach., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8328, https://doi.org/10.5194/egusphere-egu22-8328, 2022.

Many classical models of landscape evolution in South Africa have previously relied on large-scale, predominantly qualitative, field observations. In recent decades, however, the development of the accelerator mass spectrometer (AMS) has allowed for greater use of cosmogenic nuclide analyses in landscape evolution studies to quantify rates of denudation and establish timescales of landscape development. In South Africa, various field areas and isotopes have been studied to understand the development of the landscape on Quaternary and longer timescales. The aim of our study is to use a cosmogenic nuclide (¹⁰Be) to investigate the development of geographically separate parts of the South African landscape, and so contribute towards the growing database of landscape evolution rates across southern Africa. Samples of granitic bedrock have been collected along the Olifants River (local/original names: Lepelle, Obalule or iBhalule) in the Kruger National Park in the subtropical east and are being compared to samples of similar composition from the Orange River (local/original names: Gariep, Senqu,) near the Augrabies Falls National Park in the arid west. Both rivers have similar multi-channel morphologies (e.g. mixed bedrock-alluvial anabranching).  A comparison of erosion rates along these otherwise similar rivers at opposite sides of the country will enable an investigation of the effects of climatic differences on erosion rates. Results will allow us to test previous, largely qualitative hypotheses of landscape evolution using state-of-the-art cosmogenic nuclide data analysed at the African continent’s only AMS facility.

How to cite: Khosa, R., Tooth, S., Mbele, V., and Pickering, R.: A Tale of Two Rivers: Comparing erosion rates from two sides of the South African landscape, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-312, https://doi.org/10.5194/egusphere-egu22-312, 2022.

Our ability to quantify past climate conditions is crucial for understanding and predicting future climate scenarios as well as landscape evolution. One of the most drastic climatic changes in Earth’s history was the Last Glacial Maximum (LGM) where a significant area of the planet’s surface was covered in ice (Clark et al., 2009). However, most reconstructions of the Earth’s past climate rely on the use of climate proxies (e.g. Jones and Mann, 2004 for a review), which are particularly poorly preserved in terrestrial settings previously covered by ice- thus limiting the applicability of existing methods.

Here, we apply feldspar thermoluminescence (TL) surface paleothermometry (Biswas et al., 2018; 2020) to better constrain the temperature history of exposed bedrock surfaces since the Last Glacial Maximum to present day. 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. Feldspar TL paleothermometry is a recently developed technique that exploits the dependence of trapped charge on temperature (Biswas et al., 2018). The trapped charge is sourced from feldspar’s crystalline lattice. While a TL signal can be extracted between room temperature and 450°C, traps sensitive to typical surface temperature variations (e.g.10°C) are found between 200°C and 250°C (Biswas et al., 2020). As a result, 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 last50 kyr (Biswas et al., 2020).

The temperature histories of bedrock samples collected down two vertical transects adjacent to the Gorner (Switzerland) and the Mer de Glace (France) glaciers, which have been exposed progressively since the LGM, will be presented. Preliminary results suggest a temperature difference of ∼10 °C in both locations, which is promising and in agreement with past surface temperatures obtained from other studies.

References:

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.

Clark, P. U., Dyke, A. S., Shakun, J. D., Carlson, A. E., Clark, J., Wohlfarth, B., Mitrovica, J. X., Hostetler, S. W., and McCabe, A. M., 2009. The Last Glacial Maximum. Science, 325 (5941), 710-714.

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

How to cite: Elkadi, J., Biswas, R. H., Visnjevic, V., Magnin, F., Lehmann, B., King, G. E., and Herman, F.: Constraining Last Glacial Maximum bedrock surface temperatures in the Western Alps using thermoluminescence paleothermometry., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11906, https://doi.org/10.5194/egusphere-egu22-11906, 2022.