Arctic and subarctic regions are warming at an alarming rate, with consequences for permafrost degradation, greenhouse gas emission, ecosystem destabilization, and infrastructure deterioration. Despite high latitude sensitivity to modern climate change, a substantial gap remains in our understanding of high latitude environmental response to past episodes of climatic change. Peat soils, common across the high latitudes, provide a valuable archive for paleoclimate reconstruction to address this knowledge gap. Many such records exist across interior Alaska and have high potential as paleoclimate archives.

Here we develop a multi-proxy paleoclimate record from a peat bog soil (Terric Hemistel) in the Delta River Valley of interior Alaska to explore mechanisms of peatland initiation and evolution. We aim to develop a comprehensive record related to the formation and evolution of this peat bog and test the hypothesis that increased moisture availability and water perching within the soil, rather than warmer temperatures, initiated peatland development. We evaluate key physical and chemical properties of organic and mineral soil materials from pedon descriptions and samples collected by horizon with depths defined according to organic matter content, bulk density, pH, carbon (C), nitrogen (N), and the carbon to nitrogen (C/N) ratio. We use radiocarbon dates to build an age-depth model and analyze stable isotopic signatures (δ¹³C) of bulk material for moisture and habitat changes, while also evaluating compound-specific isotopic trends (δ¹³C, δD) of plant waxes to track hydrological fluctuations. Finally, we perform analyses of bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) to enable temperature reconstruction.

Our preliminary chronological framework demonstrates the initiation of the peat bog during the Middle Holocene (ca. 6800 cal yr BP) with a consistent growth rate of ~0.15 mm/yr.  At the landscape scale, there is relative stability indicated by pedogenesis through the Holocene, but at the pedon scale there is a shift from concurrent aeolian deposition and mineral soil development to accumulation of organics. This shift in predominant parent material is likely due to a changing moisture regime, inferred to be the result of feedbacks between increased summer precipitation and increased soil water holding capacity. We hypothesize that brGDGTs will show relatively flat temperatures through the Holocene, following other work in the area, while C/N ratios and stable isotope data will indicate pronounced changes in moisture. This multi-proxy approach will help improve models of peatland formation and resolve debate over the response of peatlands to varying climatic conditions (e.g., drier vs. wetter). Such work is particularly important for contextualizing peatland development under ongoing and future climate change. 

How to cite: Kielhofer, J., Patton, N., Baish, C., Thomas, E., and Sion, B.: A multi-proxy paleoclimate record from a peat soil of interior Alaska: insight on mechanisms of Holocene peatland formation and landscape development , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-767, https://doi.org/10.5194/egusphere-egu24-767, 2024.

Synchrotron-based X-ray absorption spectroscopy (XAS), in the context of X-ray florescence (XRF) maps, is a powerful analytical tool that can define element speciation and characterize the local atomic structure of a suitable sample. While this type of analysis is regularly used to provide advances in a variety of scientific fields, including but not limited to material sciences, molecular physics, and medical sciences, it remains an underutilized tool in the (paleo)geosciences.  Here, we show that XAS can advance our knowledge of the Critical Zone in deep time. For this study, we used the X-ray fluorescence microprobe (XFM) beamline above the uranium (U) L_III-edge to create detailed XRF elemental maps. Based on these maps, we used X-ray absorption near edge spectroscopy (XANES) to probe U and Manganese (Mn) speciation of pedogenic carbonate nodules collected from Late Cretaceous paleosols from Big Bend National Park, Texas, USA.

Pedogenic carbonate nodules are derived from soil waters that are oversaturated with respect to calcite. Accumulation of pedogenic calcite primarily occurs in semi-arid environments with mean annual precipitation ranging from 500-1200 mm/yr. Nodules are commonly used for uranium/thorium (U/Th) dating, atmospheric CO₂ concentration(pCO₂) reconstruction, and weathering intensity reconstruction using major and trace element concentrations. As prevalent as nodule research is in these various geological applications, little is known about the compositional variation on a microscale. We present and elucidate the texture, grainsize, and redox-sensitive elemental heterogeneity found within and between carbonate nodules. Preliminary results from U L_III-edge and Mn K-edge XANES showed primarily oxidized U and Mn.  XRF mapping of the samples indicate element discrimination between different carbonate fabrics as well as element zoning. The methods utilized in this study enable us to understand not only the microscale geochemistry of pedogenic carbonate nodules, but to provide an example of how XAS can be beneficial and useful for the paleoclimate and petrography community.

How to cite: Lesko, A., Rasbury, T., Tappero, R., Northrup, P., Barker, A., and Dworkin, S.: Using X-ray Fluorescence and Absorption Spectroscopy to Investigate Redox-Sensitive Metals in Late Cretaceous Pedogenic Carbonate Nodules, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-807, https://doi.org/10.5194/egusphere-egu24-807, 2024.

Cosmogenic isotopes of helium and neon are produced at the Earth’s surface and exhibit a wide range of thermal sensitivities in common minerals. We can take advantage of this range of thermal sensitivities to reconstruct past near surface thermal conditions using cosmogenic noble gas observations. For example, cosmogenic noble gases have been used to study past ambient temperatures, changes in snow cover duration, and wildfire histories. Interpreting cosmogenic noble gas observations requires a model of both production and diffusion that predicts cosmogenic noble gas concentrations for different thermal histories. Additionally, models that characterize the diffusion kinetics of helium or neon in a particular mineral sample are often needed, as laboratory-based diffusion experiments demonstrate that helium and neon diffusion kinetics are sample specific and often complex. At present, various codes are available that can carry out pieces of the modeling, but they are generally not interoperable and are often highly specific to a particular past application, limiting the codes’ use for future applications. Here we present progress on creating a general forward modeling framework for inferring thermal histories using cosmogenic noble gas observations, structured around the concept of proxy system modeling. We will describe the architecture of this model framework as well as provide examples of applying it to new and existing cosmogenic noble gas datasets.

How to cite: Tremblay, M. M., Bourikas, T., Bergelin, M., and Balco, G.: A proxy system model framework for reconstructing past environmental conditions with cosmogenic noble gases, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5971, https://doi.org/10.5194/egusphere-egu24-5971, 2024.

The South Pacific Convergence Zone (SPCZ), the largest component of tropical circulation on our planet, extends diagonally across the South Pacific and its displacement controls rainfall variability and tropical cyclone activity. Speleothems that formed during the last glacial period in the Cook Islands offer the opportunity to investigate the strength and position of the SPCZ across both the last glacial termination, and during rapid climate fluctuations (e.g. the Dansgaard-Oeschger Events). We present a high-resolution, multiproxy study of an 80 cm-thick flowstone retrieved in Atiu (Southern Cook Islands) covering the period from 10 to 80 ka. Optical and fluorescence microscopy, synchrotron X-ray fluorescence mapping, stable isotope ratios and LA-ICP-MS trace elements analyses allowed us to characterise the geochemical and optical properties of different calcite fabrics and speleothem facies. The geochemical and textural properties of the flowstone are directly controlled by prior calcite precipitation (PCP), which caused large positive shifts in δ¹³C and δ¹⁸O values, and synchronous peaks in Mg concentration and other hydroclimate-sensitive elements. These rapid shifts are particularly marked between 35 and 45 ka and coincide with several Dansgaard-Oeschger events recorded by the Greenland Ice Cores. The positive shifts in δ¹³C and δ¹⁸O values and Mg concentration are commonly recorded within compact, translucent fabric characterized by low greyscale values. By contrast, shifts to more negative δ¹³C and δ¹⁸O values and low Mg concentrations are associated with a porous, open fabric with a high density of fluid inclusions and high greyscale values. This confirms previous findings from modern Atiuan stalagmites and cave-farmed calcite (Faraji et al., 2022, 2023), thus opening the possibility to utilise greyscale mapping and fabric logs as novel, high-resolution hydroclimate proxies.

References:  Faraji, M., Borsato, A., Frisia, S., Hartland A., Hellstrom, J. (2023). High-resolution and quantitative records of infiltration in the Southern Cook Islands based on multi-element stalagmite data. Quaternary Research, 1-21. https://doi.org/10.1017/qua.2023.51. Faraji, M., Borsato, A., Frisia, S., Mattey, D.P., Drysdale R.N., Verdon-Kidd, D.C., Malcolm, R., Marca, A. (2022). Controls on rainfall variability in the tropical South Pacific for the last 350 years reconstructed from oxygen isotopes in stalagmites from Cook Islands. Quaternary Science Reviews, 289, 107633.

How to cite: Borsato, A., Frisia, S., Sinclair, D., Hellstrom, J., Drysdale, R., and Faraji, M.: High-resolution geochemical and petrographic study of a last glacial Cook Islands flowstone and the hydrological significance of calcite optical properties, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6727, https://doi.org/10.5194/egusphere-egu24-6727, 2024.

Our current understanding of past changes in East Asian monsoon climate variability is largely based on loess-paleosol sequences and speleothem δ¹⁸O records. Although most records have provided insight on the strength of summer monsoon precipitation, past temperature variations over East Asia are so far relatively poorly understood. Here we quantitatively reconstruct a high-resolution land temperature for central China covering the past 250 kyr, based on soil bacterial lipid signatures, co-called branched glycerol dialkyl glycerol tetraethers (brGDGTs) preserved in a loess-paleosol sequence at Mangshan on the eastern Chinese Loess Plateau (CLP). We find that the brGDGT-based temperature is dominated by precession cycles (21 kyr), except during the last deglaciation when temperature change was mainly driven by obliquity due to low eccentricity, consistent with model outputs. Notably, our record provides first evidence of extreme cooling events during glacial times when summer insolation decreases to a critical value as predicted by model simulations. Furthermore, the degree of cyclization (DC) of brGDGTs, which is suggested to indicate moisture availability, also shows clear precession cycles but does not show extreme events in response to the critical low insolation values. Our results thus indicate that the warm season temperature and moisture are mainly forced by precession, and extreme cooling events could be triggered by low summer insolation during glacial times.

How to cite: Lu, H., Guo, J., Fuchs, L., Yin, Q., Sun, Y., and Peterse, F.: Insolation and ice volume induced extreme cooling events in East Asia during glacial times , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6742, https://doi.org/10.5194/egusphere-egu24-6742, 2024.

Stalagmites (carbonate deposits in caves) are excellent archives of palaeoenvironmental changes, as they are not only amenable to high-precision radiometric dating but host numerous proxy variables that reflect past changes in climate conditions related to hydrology, temperature, and atmospheric dynamics. In most stalagmites, the convex shape of the apex leads to rapid runoff of dripwater, which results in a thin film of water on top of the stalagmite, fast CO₂ degassing, and depletion of the dissolved inorganic carbon (DIC) pool. These kinetic dynamics lead to carbon and oxygen isotope fractionation, hampering quantitative estimation of the carbonate formation temperature and reconstruction of original dripwater composition using, e.g., clumped isotope systematics (Affek et al. 2008; Affek & Zaazur 2014). Equilibrium conditions can be achieved however in subaqueously-formed speleothems (Daëron et al. 2019).

Whether stalagmites that possess a ‘drip cup’, i.e., a concave apex, show near-equilibrium conditions and constitute archives of past temperature should be testable using isochronous clumped isotope analyses. We hypothesized that the deeper water body inside the ‘drip cup’ limits CO₂ degassing and DIC depletion, enabling near-equilibrium conditions compared to the outer slope of the rim wall surrounding the ‘drip cup’ that is likely affected by kinetic fractionation. We tested this hypothesis using multiple isochronous samples from stalagmite MAYA-22-7, taken at Cenote Ch’en Mul, Mayapán, Yucatán (México). The actively growing stalagmite was collected in 2022. Air temperature in the cave chamber is very stable, at 25.7 ± 0.6°C year-round. The stalagmite shows a prominent and several-mm-deep ‘drip cup’ in its lower growth section. U-series dating showed that this interval is ca. 500 years old. Very thin growth layers inside the ‘drip cup’ and thick growth layers at the rim wall indicate different growth conditions across the stalagmite at the time of formation, likely with suppressed DIC depletion and reduced kinetic isotope fractionation in the ‘drip cup.’ Δ₄₇ values were predicted to be highest inside the drip cup (if kinetic isotope fractionation increases with distance from the apex) or to remain constant (if kinetic effects are negligible). We milled seven subsamples from growth layers of the same age for Δ₄₇ analysis. Each sample integrates several years (< 10) because of the required sample mass (~ 4.5 mg). Clumped isotopes were measured at Northumbria University on a NU Instruments Perspective IRMS. We discuss the clumped isotope dynamics along a single growth interval and implications for palaeotemperature reconstructions. Finding near-equilibrium conditions inside the ‘drip cup’ would offer the opportunity to reconstruct past temperatures, if not continuously, then at least for some time intervals.

References

Affek, H.P., Bar-Matthews, M., Ayalon, A., Matthews, A., Eiler, J.M., 2008. Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’ thermometry. Geochim. Cosmochim. Acta 72, 5351–5360

Affek, H.P., Zaarur, S., 2014. Kinetic isotope effect in CO₂ degassing: insight from clumped and oxygen isotopes in laboratory precipitation experiments. Geochim. Cosmochim. Acta 143, 319–330

Daëron, M., Drysdale, R.N., Peral, M., Huyghe, D., Blamart, D., Coplen, T.B., Lartaud, F., Zanchetta, G., 2019. Most Earth-surface calcites precipitate out of isotopic equilibrium. Nature Comms. 10:429

How to cite: Breitenbach, S. F. M., Longman, J., Umbo, S., Modestou, S., Kwiecien, O., Carolin, S. A., James, D. H., Peraza Lope, C., Brenner, M., and Hodell, D.: Stalagmites with ‘drip cups’ – are they suitable for palaeotemperature reconstructions?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12108, https://doi.org/10.5194/egusphere-egu24-12108, 2024.

Cave monitoring programs have provided a wealth of information on processes that affect speleothem proxy systems such as stable isotopes and trace elements. Prior carbonate precipitation (PCarbP), i.e. the precipitation of a carbonate mineral from supersaturated water occurring in the epikarst or at the ceiling of the cave, is one of the dominant processes that controls the dripwater d¹³C, Mg/Ca, Sr/Ca, Ba/Ca as well as U/Ca. Enhanced PCarbP is associated with reduced infiltration or a decrease in CO₂ partial pressures of the infiltrating water. Often this process can be linked to the precipitation - evaporation balance (P-E) or rainfall variability. The type of carbonate polymorph that precipitates, calcite or aragonite, determines how dripwater trace element to Ca (Me/Ca) ratios evolve as a consequence of PCarbP. This has led to distinguish prior calcite precipitation (PCP) from prior aragonite precipitation (PAP) since calcite trace element partitioning coefficients (D_Me) differ from aragonite D_Me. Determining accurate calcite and aragonite D_Mefor Mg, Sr, Ba, and U is crucial to identify the PCarbP mode (i.e., PCP or PAP) and increase our confidence in interpreting speleothem trace element variability.

Using an autosampler we were able to collect dripwater samples from Grotte de Piste, Morocco at unprecedented 4-day resolution covering the transition from the wet winter to the dry summer season. The drip site has high drip rates during the wet winter but progressively dries out during summer. We observe a strong significant positive correlation between dripwater Ca concentrations and drip rate (r=0.996, p<0.01) indicating that PCarbP is the dominant process affecting dripwater Ca concentrations. By studying the evolution of dripwater Ca and trace element concentrations we show that a single drip site can be affected by both PCP as well as PAP. From the dripwater element compositions we determine new aragonite D_Me values. As opposed to D_Mg, D_Sr and D_U, it appears that aragonite D_Ba is very similar to calcite D_Ba. Barium thus seems least sensitive to mineral phase changes associated with PC_CarbP compared to Mg, Sr, U. However, calcite or aragonite D_Me values should not be considered constants and may vary with solution composition, dripwater saturation index and temperature. Aragonite D_Ba for example increases with dripwater saturation index. It is therefore possible that D_Ba might differ within the epikarst as compared to the cave ceiling or the speleothem surface unless the dripwater saturation index remains low at the speleothem surface. Barium concentrations in aragonite speleothems with low growth rates may be most suitable to trace combined PCP and PAP variations as they precipitate from dripwater with a low saturation index.

How to cite: Wassenburg, J. A., Samanta, A., Sha, L., Lee, H., Scholz, D., Cheng, H., Ait Brahim, Y., Budsky, A., and Breitenbach, S. F. M.: Effects of prior calcite and prior aragonite precipitation on cave dripwater compositions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13434, https://doi.org/10.5194/egusphere-egu24-13434, 2024.

Speleothems are valuable paleoenvironmental archives that can grow continuously over thousands of years and are particularly favorable due to reliable dating using the ²³⁰Th/U-method. As the caves in which these mineral deposits are formed are largely closed systems and the speleothems resist chemical alteration, it is possible to analyse organic substances in addition to established proxies such as stable isotopes and trace elements. [1] The analysis of organic markers has only recently come into focus but shows promising results for the reconstruction of past climatic conditions, especially regarding vegetation changes and paleofires.  [2]

Polycyclic Aromatic Hydrocarbons (PAHs) are ubiquitous organic compounds consisting of one or more fused aromatic rings. They are formed, for example, during the incomplete combustion of biomass and enter caves with the infiltrating water, where they are incorporated into speleothems. [3] Alongside other proxies such as levoglucosan (combustion product of cellulose), they can provide information about past vegetation and fire activity.

As only a small fraction of PAHs reaches the speleothems, suitable sample preparation methods are required to maximize the amount of analyte while keeping the required amount of sample low. We present a rapid and easy extraction method using an ultrasonic bath and the addition of keepers to reduce the loss of low molecular weight PAHs like naphthalene during the evaporation process.

The developed method, as well as a new and fast GC-HRMS method, was applied to a particularly old flowstone from the Conturines Cave in northern Italy, dating back to the Pliocene. 

[1] A. Blyth et al. Quat. Sci. Rev. 149 (2016) 1-17. [2] J. Homann et al. Biogeosciences. 20 (2023) 3249–3260. [3] Y.Sun et al. Chemosphere 230 (2019) 616–627.

How to cite: Schäfer, J., Spötl, C., and Hoffmann, T.: Improved analysis of polycyclic aromatic hydrocarbons (PAHs) in speleothems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13463, https://doi.org/10.5194/egusphere-egu24-13463, 2024.

Sedimentary records covering the Late Pleistocene show glacial-interglacial and millennial temperature changes accompanied with, for instance, rainfall and vegetation changes at the global and regional scales. However, such records are missing for the islands of Macaronesia.

Here we generate three sedimentary records over the last 10,000 to 45,000 years from the islands of Tenerife, Gran Canaria, and La Gomera using glycerol dialkyl glycerol tetraethers (GDGTs). At the global scale, air temperature and soil pH influence GDGT distributions in soils, although these biomarkers also react to other environmental factors (e.g., land use, vegetation, and soil moisture and chemistry) and shifts in bacterial and archaeal communities. Accordingly, we examined several GDGT-based proxies, notably those using bacterial branched GDGTs (brGDGTs), to assess their applicability in the Canary Islands.

Our preliminary results show drastic downcore and inter-site changes in GDGT distributions, with brGDGT-based air temperature ranges larger than 10 °C over the last 10,000 to 45,000 years when applying global calibrations at the three study sites. Air temperatures and soil pH inferred from brGDGTs decrease in Tenerife and La Gomera over the end of the African Humid Period, which suggests an effect of reduced rainfall on brGDGTs, possibly accompanied with a shift in bacterial communities. Air temperatures inferred from brGDGTs show a general increase over the last 27,000 years in Gran Canaria, whereas cyclization and isomerization indices of brGDGTs suggest typically opposite changes in soil pH, in disagreement with global-scale patterns from surficial soils. Our GDGT-based records also show a few drastic increases in archaeal GDGT abundances relative to the full GDGT pool after the Last Glacial Maximum, notably in Gran Canaria and La Gomera, partly related to the rainfall increase during the African Humid Period.

Our preliminary application of GDGT-based proxies in the Canary Islands reveals complex environmental influences on soil bacterial and archaeal lipids, which may also be related with local climate and vegetation dynamics. Our preliminary study also motivates a follow-up GDGT study in surficial soils from the Canary Islands to establish, for instance, a brGDGT-temperature calibration suitable for this archipelago.

How to cite: Davtian, N., Castilla-Beltrán, A., Martín Ramos, M. P., Fernández-Palacios, E., Criado, C., Nogué, S., Villanueva, J., Fernández-Palacios, J. M., and de Nascimento, L.: Assessing the potential of bacterial and archaeal membrane lipids (GDGTs) to reconstruct Late Pleistocene and Holocene climatic changes in the Canary Islands, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17926, https://doi.org/10.5194/egusphere-egu24-17926, 2024.

Terrestrial paleoclimate records from arid southwestern Asia are relatively sparse. Therefore, the regional impact of abrupt glacial climate variability remains poorly constrained for much of the Western Asia, particularly winter (wet season) dynamics during Marine Isotope Stage 3. Here, we present the first paleoclimate reconstructions of Southwestern and Central Iran, which span the interval ~50-30 ka, based on speleothem δ¹⁸O and δ¹³C. Stable-isotope signals in the two stalagmites are generally uncorrelated and do not exhibit a consistent response to Greenland stadials or interstadials; however, both show a positive δ¹⁸O excursion that coincides with Heinrich event 4. This behavior contrasts with that observed in northern Iran, Anatolia, and Levant for the last glacial period. We explore the potential mechanisms for intermittent coupling of speleothem δ¹⁸O across Iran through isotope-enabled atmospheric modelling outputs, from which we utilize the spatial δ¹⁸O gradient as a proxy for wintertime westerly vs. southerly jet strength. Our results suggest that during Heinrich event 4 and several Greenland stadials, stronger westerly winds enhanced Mediterranean moisture contributions to both sites and reduced aridity in southern Iran. We emphasize the importance of analyzing spatial trends in speleothem δ¹⁸O to interpret atmospheric dynamics, rather than relying on time series from single sites.

How to cite: Soleimani, M., Baker, J., Nadimi, A., Dublyansky, Y., Koltai, G., and Spötl, C.: A novel proxy approach to constraining atmospheric modes in central and southwestern Iran during Marine Isotope Stage 3, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18601, https://doi.org/10.5194/egusphere-egu24-18601, 2024.