CL5.1.1

Loess-Palaeosol Sequences (LPS) represent the most extensive Quaternary terrestrial archives. Although researchers have long been able to identify short-lived climatic changes in LPS through stratigraphy, until recently we have lacked the tools to 1) identify how continuous loess archives may be, and to what extent short-lived, millennial-timescale climatic events were recorded in loess sediments, and to 2) quantitatively reconstruct past climate parameters from loess proxies. Stratigraphically, the impact of short-lived climatic cycles can be observed in the form of primary loess deposits reflecting cold stadial conditions, intercalated with arctic and boreal brown soils and tundra gley horizons indicating milder interstadials. Short-term establishment and subsequent degradation of an active permafrost layer can also be identified in temperate-latitude loess such as that found in the Rhine Valley of central-western Europe. Recently developed proxy methods can now be used to quantify climatic parameters such as temperature and precipitation in these regions ^1,2. Associated with radiocarbon dating, these new approaches will vastly improve our understanding of continental environmental changes through the Upper Pleistocene, which can now be compared at high temporal resolution with marine and ice core records. In particular, the quantity and stable isotope ratios of crystalline calcite granules (> 0.8 mm), secreted by earthworms (Lumbricus sp.)  at the soil surface, preserve climate information contemporaneous with deposition of the loess sediment.

In this study, we assess the utility of the earthworm calcite granules (ECG) approach by reconstructing temperature and precipitation at high resolution between 50 and 15 ka from two temporally overlapping loess sequences, Schwalbenberg and Nussloch, situated approximately 200 km apart in the German Rhine Valley. ECG counts down the two profiles reveal millennial-timescale climatic variations; high ECG concentrations associated with pedogenetic horizons suggest milder climatic with increasing biological activity and vegetation cover. Using empirical equations based on 1) observations of modern earthworm response to temperature and 2) the linear relationship between ∆¹³C values of plants and precipitation, the stable oxygen and carbon isotope compositions from ECGs can be used as direct proxies for warm season temperature and annual soil moisture, respectively. We embed our climate reconstructions within Bayesian age models based on radiocarbon dating of ECG in order to establish precise correlations between the two sequences and with other climatic archives. We find that ECGs provide valuable proxies able to meaningfully quantify palaeoclimate variations from terrestrial deposits over millennial timescales. Our results further show periods of quasi-simultaneous climatic change in the Northern Hemisphere, closely linking the climatic signatures recorded in the Upper Pleistocene of Schwalbenberg and Nussloch to the Greenland ice core records.

References: 

1. Prud’homme, C. et al. Palaeotemperature reconstruction during the Last Glacial from δ¹⁸O of earthworm calcite granules from Nussloch loess sequence, Germany. Earth Planet. Sci. Lett. 442, 13–20 (2016).

2. Prud’homme, C. et al. δ ¹³C signal of earthworm calcite granules: a new proxy for palaeoprecipitation reconstructions during the Last Glacial in Western Europe. Quat. Sci. Rev. 179, 158–166 (2018).

How to cite: Prud'homme, C., Fischer, P., Jöris, O., Hatté, C., Vinnepand, M., Vonhof, H., Moine, O., Vött, A., and Fitzsimmons, K.: Millennial-timescale reconstruction of Upper Pleistocene temperature and precipitation derived from earthworm calcite granules in western European loess profiles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7071, https://doi.org/10.5194/egusphere-egu21-7071, 2021.

Investigating palaeoclimate records is of major importance for evaluating the impact of past forcing factors on the evolution of ecosystems, megafauna and human dispersal, especially in Southern Patagonia where few records are available. We report on a 40 cm long flowstone core S6, and fragments of flowstone and a stalagmite from Cueva Chica. The samples were radiometrically dated (U-Th & ¹⁴C) to construct age-depth models for the proxy profiles (δ¹³C, δ¹⁸O, and chemical composition). The speleothem proxy data are further informed by both petrographic analysis of the flowstone, and monitoring data. The main objectives of this work are to: i) reconstruct past climate variations using geochemical analyses conducted on the speleothems, and ii) assess the palaeoclimatic context of megafauna extinction in the area. The flowstone core S6 grew discontinuously from ~13 ka to ~1 ka with several possible hiatuses at ~10 ka BP, from 5.7 to 3.0 ka BP, and 2.5 to 1.8 ka BP (interpolated ages). Sample S8 grew from 6.8 to 5.8 ka BP and after 1.2 ka BP. Stable isotopes analyzed at sub-centennial resolution show a 3‰ range for δ¹⁸O, and more than 14‰ for δ¹³C, and the isotope ratios covary along the entire record. These changes are likely caused by kinetic fractionation and prior calcite precipitation (PCP), controlled mostly by changes in moisture availability. The sensitivity of the proxies to hydrological changes and PCP is further tested with indicators using μXRF element data. The multiproxy record from Cueva Chica suggests a wet phase from ~13 to 9 ka BP, likely related to strong westerlies in the Southern Hemisphere, preceded by a short dry/cold spell at ~13 ka BP. This early Holocene wet phase was followed by a colder/drier period from 8.5 to 5.8 ka BP, likely related to weaker westerlies, especially during the mid-Holocene. High precipitation and strong westerlies prevailed from 3.0 to 2.5 ka BP and in Medieval times. Our paleoclimate record implies that the presence of extensive megafauna, the development of Nothofagus forest and human arrival, all occurred during a climatically favorable wet/warm period ca. 13 to 9 ka BP, after the Antarctic Cold Reversal. However, the deterioration of the vegetation cover at the Cerro Benitez coinciding with high δ¹³C values excursions was initiated after ca. 11 ka BP. Previous studies suggest an extinction of major megafauna species (e.g., Mylodon, Smilodon, Panthera onca mesembrina) during this wet/warm period. Such climate-driven changes likely reduced the open ecosystem environment and may have led to local decline of herbivore populations. Later cooling/drying after ca. 9 ka may have contributed to the disappearance of megafauna and other large mamals (e.g., Hippidion Saldiasi).

How to cite: Nehme, C., Todisco, D., Breitenbach, S., Couchoud, I., Girault, I., Martin, F., Borrerro, L., Hellstrom, J., TJallingi, R., and Claeys, P.: Climate Variability reconstructed from La Cueva Chica speleothems for the last 13 ka BP: implications for Megafauna in Southern Patagonia, Chile., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10368, https://doi.org/10.5194/egusphere-egu21-10368, 2021.

In the European Alps, the Younger Dryas (YD) was characterized by the last major glacier advance with equilibrium line altitudes being ~220 to 290 m lower than during the Little Ice Age and also by the development of rock glaciers. Dating of these geomorphic features, however, is associated with substantial uncertainties leading to considerable ambiguities on the internal structure of this stadial, the most intensively studied one of the last glacial period.

Our study utilizes a novel paleoclimate archive, coarse crystalline cryogenic cave carbonates (hereafter CCC), that allows to precisely constrain when ~ 0°C conditions prevailed in the shallow subsurface in the past, often related to permafrost thawing events.

Here we presents the first record of CCC from the Dolomites (Southern Alps). In contrast to many studies from Central European caves these speleothems formed not during a major climate warming but within a prominent stadial. ²³⁰Th-dating of the CCC indicates sustained negative temperatures close to  ~0°C between ~12.6 and ~12.2 ka BP at about 50 m below the surface, initiating the slow freezing of dripwater-induced meltwater pockets in perennial cave ice. This in combination with thermal modelling argues for a cooling of ≤ 3°C at the Allerød-YD transition at this high-alpine site in the Southern Alps. Our data suggest that autumns and early winters in the early part of the YD were relatively snow-rich, resulting in a stable winter snow cover at this site. The snow cover insulated the subsurface and allowed the cave interior to remain close to the freezing point (0°C) year-round, promoting CCC formation.

The main phase of CCC precipitation at ~12.2 ka BP coincides with the mid-YD transition recorded in other archives across Europe. Based on thermal modelling we propose that CCC formation at ~12.2 ka BP was most likely associated with a slight warming of approximately +1°C in conjunction with drier autumns and early winters in the second half of the YD. These changes triggered CCC formation in this alpine cave as well as ice glacier retreat and rock glacier expansion in the Alps. Our study demonstrates that CCCs can provide quantitative constraints on paleotemperature and seasonally resolved precipitation changes.

How to cite: Koltai, G., Spötl, C., Jarosch, A. H., and Cheng, H.: Cryogenic cave carbonates in the Dolomites (Northern Italy): insights into Younger Dryas cooling and seasonal precipitation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7889, https://doi.org/10.5194/egusphere-egu21-7889, 2021.

The Toba eruption, marked by the Younger Toba Tuff (YTT), was the largest volcanic event of the Quaternary. Dated to 73.88±0.6 ka BP (2σ), this eruption must have been witnessed by humans globally, even if only through indirect effects of multi-year adverse weather patterns, or atmospheric phenomena. It has been proposed that the YTT acted as trigger or accelerator for Greenland Stadial 20 (GS20). Its global climatic impact is evidenced by data from as far as southwestern USA, Antarctica, and Greenland (Polyak et al. 2017, Svensson et al. 2013). Whether this event also caused a human population bottleneck (Ambrose 2003) remains debated (Petraglia et al. 2007, Ge et al. 2020).

Here we combine stable isotopes with laser ablation based multi-element data to test the impact of the YTT on the East Asian Summer Monsoon using a stalagmite from China.  U-series dating indicates that stalagmite XT5 grew between 87±0.3 to 50±0.4 ka BP. Oxygen and carbon isotope ratios show significant changes linked to GS20, with several large positive excursions between 74 and 70 ka BP suggesting severe droughts. The notion of severe drying is supported by the Sr profile. Several spikes in rare earth elements (REE: Y, La, Ce, Nd, Yb) occurred concurrent with δ¹⁸O, δ¹³C, and Sr increases.

The increasing δ¹³C and δ¹⁸O values resulted from reduced effective infiltration linked to a longer-term reorganization of the EASM system. The observed REE dynamics and can be interpreted as indicative of large eruption events. Assigning REE spikes to individual eruptions like the YTT remains ambiguous, but agreement with acidity peaks in ice cores (Svensson et al. 2013) suggests that our REE events tracks eruption history over the interval discussed here. Our δ¹⁸O, δ¹³C, and Sr records suggest repeated EASM weakening and regional-scale rainfall reduction in response to volcanic events indicated by REE peaks. Comparison of the REE and Sr profiles further suggests that drying was initiated or exacerbated by these eruptions.

Our new multi-proxy record supports the hypothesis of repeated tropical eruptions that led to several significant weakening episodes of the EASM. The current data do not answer the question whether the YTT initiated GS20 but support the notion of multi-decadal impacts on regional circulation and rainfall across East Asia.

References:

Ambrose S. (2003) Population bottleneck, in: Robinson, R. (Ed.), Genetics, volume 3. Macmillan Reference, New York, 167-171

Ge Y. et al. (2020) Understanding the overestimated impact of the Toba volcanic super-eruption on global environments and ancient hominins. Quat Int. 559, 24-33

Petraglia M. et al. (2007) Middle Paleolithic Assemblages from the Indian Subcontinent Before and After the Toba Super-Eruption. Science 317, 114-116

Polyak V. et al. (2017) Rapid speleothem δ¹³C change in southwestern North America coincident with Greenland stadial 20 and the Toba (Indonesia) supereruption. Geology 45, 843-846

Svensson A. et al. (2013) Direct linking of Greenland and Antarctic ice cores at the Toba eruption (74 ka BP): Climate of the Past 9, 749-766

How to cite: Breitenbach, S. F. M., Finestone, E., Cai, Y., Scott, P., Boivin, N., and Petraglia, M.: Impact of the Toba eruption on rainfall – a speleothem record suggests multi-staged eruption affected EASM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12794, https://doi.org/10.5194/egusphere-egu21-12794, 2021.

Speleothems (secondary cave carbonate deposits) are particularly valuable for studying past climates over a range of temporal and spatial scales, owing to their continuous growth and exceptional viability for radiometric dating. However, the interpretation of many speleothem-based palaeoenvironmental proxies (e.g., stable isotope ratios δ¹⁸O and δ¹³C) in terms of particular environmental or climatic controls remains challenging, and is typically limited to qualitative reconstructions of past environmental conditions and dynamics.

In this study, we develop a novel class of quantitative hydrological proxies by exploring the kinetic behaviour of a suite of first-row transition metals (e.g., Ni, Cu, Co) found in stalagmites. The transport of these elements from the surface to the cave is strongly controlled by the binding to natural organic matter (NOM) present in infiltrating waters. The rate of dissociation of such metal-NOM complexes at the dripwater-stalagmite interface has recently been suggested to determine the availability of these elements for the incorporation into precipitates (Hartland & Zitoun, 2018). The link between NOM-complex dissociation and metal availability for deposition presents an opportunity to quantitatively relate respective carbonate metal concentrations to the time available for complexes to disintegrate and release metals within the thin water films on stalagmite surfaces.

We present preliminary analyses assessing trace metal kinetics and partitioning in monitored natural cave settings, as well as cave-analogue experimental setups. Our findings demonstrate the contrasting controls on the concentrations of different elements in speleothems, and highlight the potential for kinetically-limited elements in stalagmites to enable quantitative estimations of past hydrological variability.

References:

Hartland, A., Zitoun, R. (2018) Transition metal availability to speleothems controlled by organic binding ligands. Geochem. Persp. Let. 8, 22–25.

How to cite: Höpker, S. N., Goswami, B., Grainger, M., Breitenbach, S. F. M., Hansen, M., and Hartland, A.: Partitioning of trace metals in cave (and cave-analogue) carbonate precipitates – towards a quantitative hydrological proxy in stalagmites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13057, https://doi.org/10.5194/egusphere-egu21-13057, 2021.

Oxygen isotopes are commonly used proxies in paleoclimate research, however, a correct interpretation requires a detailed understanding of processes controlling isotope variability for a specific site.  A common interpretation for oxygen isotopes in precipitation across the Asian monsoon region, links the seasonal and interannual variability to changes in the summer monsoon strength.

However, some locations within tropical Asia do not receive rainfall during the summer monsoon season. In central Vietnam most of the annual rainfall falls during autumn instead of summer, making central Vietnam ideal to investigate processes controlling rainwater isotope variability, independent from the summer monsoon. By using rainwater isotopes, collected over five years, and moisture uptake simulations for these time periods, we investigate the seasonal cycle and interannual variability of hydrological processes in central Vietnam.

Our results show that the seasonal variability is dominated by a shift in moisture source from the Indian Ocean in summer to the South China Sea (western Pacific) from autumn to spring. The different source locations are reflected by an increase in δ¹⁸O values from around − 8 to − 10‰ during summer to values between 0 and − 3‰ during winter/spring. Further, we show that the amount effect and the occurrence of tropical cyclones, which are typical for the region, have no effect on a seasonal to interannual scale. Instead, we find that the timing of the seasonal ITCZ migration is a driving component of variability on these time scales.

How to cite: Wolf, A., Roberts, W. H. G., Ersek, V., Johnson, K. R., and Griffiths, M. L.: Rainwater isotopes in central Vietnam controlled by two oceanic moisture sources and rainout effects, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9545, https://doi.org/10.5194/egusphere-egu21-9545, 2021.

Secondary mineral deposits in caves, such as stalagmites or flowstones, are valuable paleoclimate archives because they have several advantages over other environmental archives. These include stable in-cave conditions, protecting the speleothems from external influences, and the potential to precisely date samples up to 600,000 years using ²³⁰Th/U-dating. [1] Supplementing established climate proxies, such as stable isotopes and trace elements, organic proxies have been increasingly used in recent years to inform on local vegetation and soil dynamics. [2]

Biomass burning events are major sources of atmospheric particulate matter that influences global and local climate. [3] Investigating fire proxies in paleoclimate archives may therefore help determine the interactions of climate, hydrology, and fire activity. Levoglucosan, an anhydrosugar, naturally only originates from the combustion of cellulose and thus constitutes a biomass burning marker. Analysis of levoglucosan in sediments has shown high correlation with traditional burning markers, such as black charcoal. [4] Mannosan and galactosan, both stereoisomers of levoglucosan, are formed during combustion of hemicellulose. Previous work suggests that rather than absolute levoglucosan concentrations the ratio of levoglucosan to its isomers should be considered when characterizing burning events. [5] To date, no data on levoglucosan or its isomers in speleothems has been published, whereas the anhydrosugars are already utilised in other paleoclimate archives, such as sediments and ice cores. [2,3]

We test three approaches (solid phase extraction (SPE), soxhlet extraction and solid/liquid extraction) for the isolation and quantification of anhydrosugars using HILIC-MS instrumentation. As the anhydrosugars are highly polar molecules, extraction from the calcium carbonate matrix and subsequent sample preparation proved challenging. We evaluate the different approaches and compare the resulting concentrations and assumed recoveries. We find that the anhydrosugars do not show significant retention on any of the evaluated SPE materials. While solid/liquid extractions lead to detectable analyte concentrations, soxhlet extractions with methanol or dichloromethane/methanol mixtures are more efficient.

[1] D. Scholz, D. Hoffmann, Quat. Sci. J. 57 (2008) 52–76 [2] A. Blyth et al. Quat. Sci. Rev. 149 (2016) 1-17 [3] P. Yao et al. J. of Glaciology 59 (2013) 599-611 [4] V. O. Elias et al. Geochim. et Cosmochim. Acta 65 (2001) 267-272. [5] D. Fabbri et al. Atmos. Env. 43 (2009) 2286–2295

How to cite: Homann, J., Hoffmann, T., Scholz, D., and Breitenbach, S.: Levoglucosan in speleothems: An evaluation of various sample preparation methods., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1065, https://doi.org/10.5194/egusphere-egu21-1065, 2021.

Soil-derived magnetic particles trapped in speleothems can retain valuable information on the physiochemical conditions of the overlying soil and changes in the hydrological system. However, a direct link between magnetic and isotopic properties of speleothems has been only qualitatively established and is known to vary regionally. Here we investigate two Holocene speleothems from Soreq Cave, Israel and provide evidence for strong coupling over centennial to millennial timescales between the inflow of magnetic particles (quantified using the magnetic flux index, IRM_flux), δ¹³C, and rainfall amounts. The two stalagmites formed at separate intervals: one at ~9.5 ky BP capturing the transition from pluvial Eastern Mediterranean conditions associated with Sapropel 1 (S1) and a second at 5.4 ky BP recording mid-Holocene wet-dry cycles.

The late-Holocene speleothem shows an anomalously high δ¹³C episode that is correlated with extremely low IRM_flux, indicating minimal contribution from overlying soils due to either (1) recently denuded soils, or (2) high overland and vadose runoff. By contrast, the mid-Holocene sample shows saw-tooth cycles in both δ¹⁸O and δ¹³C, which are interpreted as rapid climate fluctuations associated with rainfall changes. IRM_flux during this period varies in-phase with the δ¹³C cycles; however, the peaks in IRM_flux values precede those of the isotope values. The apparent lag in isotopic values may be explained by the faster response of the IRM_flux to increased rainfall resulting from the rapid physical translocation of overlying soil particles via groundwater, compared with slower soil organic matter turnover rates, which may vary on timescales of up to thousands of years.

The separate palaeohydrological scenarios resolved from the two speleothems demonstrate how magnetic data can act as a powerful paleo-hydrology proxy, even in weakly-magnetized speleothems growing under semi-arid conditions.

How to cite: Burstyn, Y., Shaar, R., Keinan, J., Ebert, Y., Ayalon, A., Bar-Matthews, M., and Feinberg, J. M.: Palaeohydrologic interpretations of Holocene wet episodes recorded in stalagmites from Soreq Cave, Israel: Linking the magnetic and isotopic properties of speleothems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9159, https://doi.org/10.5194/egusphere-egu21-9159, 2021.

Sclerochronological methods are described as a means to reconstruct, from increments recorded on a shell transect (i.e. "sclerochonological growth"), the fluctuations in past environments. This was proposed by an erroneous analogy with tree-ring dendrochronology. However, shells do not grow like trees. Almost all molluscan shells grow by adding increments at the shell edge, while preserving their shape. This is called "terminal growth". The advantage of this property is that there is a generic mathematical model that can quantify the shell expansion (morphological growth). 

Nonetheless, this generic model is not compatible with observed shell incrementation and accretion processes. This is because increment widths should increase geometrically in the mathematical model along a transect. We remarked that despite studying the same object, morphodynamics and sclerochronology have followed divergent paths in past decades, without incorporating advances of the other speciality. 

We have now addressed this problem head-on by developing a new mathematical framework to combine incremental shell growth within shell morphodynamics. This model is designed to be able to confront a theoretical prediction of shell incrementation with a measured one. The method combines morphodynamic modelled shapes with 3D shell scans and increments measured from shell transects. 

Our work in this area highlights several heretofore unrecognized fundamental problems in morphodynamics and sclerochronology which concern the way individual variability is accounted for in both areas. Regarding the reconstruction of environmental trends, we find that the arbitrary choice of one particular "best" shell transect and averaging over groups of individuals can be a source of significant bias. It is time to revise shell science methods to consider the entire incremental geometry (or growth 'ring') so that unbiased estimates of environmental changes can be provided using sclerochonological data. 

How to cite: Guarini, J. and Guarini, J.-M.: Confronting shell morphodynamics and sclerochronology: leaving behind phenomenology to move toward integrative laws of growth, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15863, https://doi.org/10.5194/egusphere-egu21-15863, 2021.

Water activity is the partial pressure of water in a solution, which is a crucial driver of chemical (e.g. dissociation constants) and physical (e.g. evaporation and stable isotope enrichment) properties of water. As such, reconstructing past climate and environmental conditions using lacustrine proxies requires a quantification of past water activity. However, very little research has been conducted on ways to reconstruct this crucial water property. In this research, we present a novel method that potentially can enable reconstructing paleo water activity in closed basin lakes. Closed-basin lakes are lakes with no outlet; thus, the size of the lakes varies as function precipitation and evaporation (P-E), and in turn, can be used to quantitatively reconstruct these conditions. Past climate change altered P-E throughout the geological history causing lakes all around the world to expand (contract), which in turn diluted (concentrated) and reduced (enhanced) lake water activity. To fully understand past hydrological and chemical processes in the lake, it is crucial to quantify the water activity in the lake.

radix sp. is a gastropod that lives at the margins of closed basin lakes. It forms a hard CaCO₃ shell, which at death is deposited in the sediments, remains in the geological record, and informs on past lake levels. We sampled radix sp. shells in elevated shorelines of a closed basin lake in North China and from the modern lake shoreline. Radiocarbon ages of the shells span the past 16 ka and are used to construct the lake level curve for the Late Glacial to Holocene (Goldsmith et al., 2017). During this time period the lake level and volume changed substantially; the Early – Mid Holocene lake area high-stand was x7 larger than the modern lake. On these gastropod shells we measured high resolution profiles (sample/length) of TE/Ca ratios (e.g. Mg/Ca, Sr/Ca) using Laser Ablation ICP-MS. We calculated the dissociation constants of the different TE/Ca ratios using modern shells and modern lake water chemistry.

Our preliminary results show that of all elements analysed, Mg/Ca ratios best track lake volume and therefore can be used to reconstruct lake level. The next step will be using gastropod derived lake water chemistry to reconstruct past water activity. This method entails comparing the TE/Ca ratios of Late Glacial and Holocene gastropods with a hydro-chemical model of the lake, derived from a volume history reconstruction of the lake. The hydro-chemical model assumes a simple addition of river water to the modern lake, until the lake reaches the elevation that each gastropod was sampled at. In tandem, these two data sets constrain water chemistry, and the difference between these two estimates should represent the effect changes in water activity have on the disassociation constant between lake water and shell TE/Ca ratios.

Goldsmith, Y., Broecker, W.S., Xu, H., Polissar, P.J., DeMenocal, P.B., Porat, N., Lan, J., Cheng, P., Zhou, W., An, Z., 2017. Northward extent of East Asian monsoon covaries with intensity on orbital and millennial timescales. Proc. Natl. Acad. Sci. U. S. A. 114, 1817–1821.

How to cite: Goldsmith, Y. and Burstyn, Y.: Towards reconstructing Paleo-water activity using lake level and gastropod TE/Ca ratios, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10548, https://doi.org/10.5194/egusphere-egu21-10548, 2021.

Lakes are sensitive to climate change and their sedimentary components play a pivotal role as environmental recorders. In the past, lacustrine carbonates have been utilized in a number of studies attempting at a quantitative reconstruction of rainfall and/or precipitation-evaporation changes based on the biogenic or bulk carbonate δ¹⁸O signature. While these studies are built on sound theoretical grounds of mass balance and kinetic isotopic fractionation, the challenge often overlooked is the mineralogically mixed nature of carbonates comprising the bulk.

We report a case study from Lake Van, the world’s largest alkaline lake. Our time series comprising ca. 140 ka documents not only changing proportions of surface water calcite and aragonite, but also diagenetic bottom-water dolomite and, for the first time in Lake Van, early diagenetic bottom-water aragonite. Importantly, in the Lake Van profile primary and early diagenetic carbonates (in particular aragonite) are concurrent rather than mutually exclusive. A comprehensive comparison of the δ¹⁸O and δ¹³C compositions of singled out water column, biogenic (ostracod valves) and diagenetic carbonates shows, that each of the fractions forms a distinctive cluster characteristic for the depth and timing of their formation. Also, the differences between δ¹⁸O values of concurrent deep-water carbonate phases exceed what is expected from mineral-specific fractionation. Our data suggest that, an uncritical and unchecked application of the isotopic composition of the bulk carbonate fraction in quantitative climate reconstructions can severely compromise the results. We also advocate that, among different carbonate fractions in Lake Van, monospecific biogenic samples most faithfully reflect the oxygen isotopic composition of the lake water contemporaneous to their deposition, while the carbon composition of biogenic samples is additionally influenced by the organism microhabitat.

How to cite: Kwiecien, O. and McCormack, J.: How (not) to quantify hydrological changes using lacustrine carbonates?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10262, https://doi.org/10.5194/egusphere-egu21-10262, 2021.

Hydrogen isotope ratios of leaf waxes are used to reconstruct past hydroclimate because they are correlated to meteoric/growth water hydrogen isotopes. The interpretation of these signatures from ancient sedimentary archives relies on a thorough understanding of the drivers of modern isotope variability. Studies in the high latitudes, regions that are particularly valuable in light of their vulnerability to rapid climate change, are scarce. We studied modern vegetation (22 plants) in two areas in the Northwestern Territories (Canada): Herschel Island and Peel Plateau, to understand the stable isotope variability found in plants of Arctic regions. Bulk biomass stable carbon isotope and radiocarbon composition have been measured as well as fatty acids (wax lipids coating the plant leaves) stable carbon and hydrogen isotopes. Furthermore, lake surface sediments and river bank sediments from the Mackenzie River Delta (surrounded by the same plants) have been similarly studied. Bulk carbon isotope composition of the plants show strong difference between plant type, i.e. herbs, shrubs, lichen and moss, as shown in previous studies. Whereas the commonly used average chain length (ACL) is not useful to differentiate the plants. In term of compound-specific isotope ratios, herbs are generally ²H-enriched in comparison to shrubs as shown in other regions of the world, and the C₂₈ fatty acid present the most differences amongst plant type (from ~ -207‰ for herbs to ~ – 240‰ for shrubs). No major difference between the areas is noted indicating that the ~ 250 km (Herschel Island 69.5⁰N and Peel Plateau 67.3⁰N) have no impact on the hydrogen isotope composition of the fatty acids. As such we decided to compare the plant with the lake surface sediments (from the Mackenzie Delta, located between Herschel Island and the Peel Plateau). Short-chain fatty acids, sourced from organisms growing in the lake, from isolated lakes shows ²H-enriched isotopic values indicating the effect of increased evaporation in the lake during summer plant growth. Whereas long-chain fatty acids do not show any differences and are enriched compared to the shorter-chain (~ -260‰ for long-chain vs ~ -260‰ to - 280‰ for short-chain). In conclusion, differences between plant fatty acids seems to be best represented by the C₂₈ fatty acid, indicating the potential to reconstruct past vegetation and hydrological conditions in the region using lacustrine archives.

How to cite: Lattaud, J., Haghipour, N., Eglinton, T., and Broeder, L.: Stable isotope (carbon, hydrogen) variation in plants and lake surface sediments from northwestern Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8245, https://doi.org/10.5194/egusphere-egu21-8245, 2021.

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are an important tool for reconstructing terrestrial paleotemperatures from lake sediments. In addition to temperature, however, the distribution of these bacterial membrane lipids is influenced by other environmental variables such as pH, conductivity, and dissolved oxygen. Furthermore, though most brGDGT calibrations are performed against mean annual air temperature (MAT), there is considerable evidence that their distributions are more closely tied to warm-season conditions. Here, we present a new method for analyzing brGDGT data that deconvolves the influences of temperature, conductivity, and pH. Additionally, we measure brGDGT distributions in surface sediments from 43 high-latitude lakes with low MAT and high seasonality. In combination with a globally compiled lake sediment dataset, these samples show a clear warm-season bias in brGDGT-derived temperatures. They also show lake water conductivity to be the second-most important variable in controlling brGDGT distributions. We use the compiled dataset and new fractional abundances to generate brGDGT calibrations for warm-season air temperatures and lake water conductivity and pH for use in lake sediments globally.

How to cite: Raberg, J. H., Blumm, A., Harning, D. J., Crump, S. E., de Wet, G., Dildar, N., Kopf, S., Geirsdóttir, Á., Miller, G. H., and Sepúlveda, J.: Revised brGDGT fractional abundances and warm-season temperatures strengthen relationships between brGDGTs and environmental variables, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8362, https://doi.org/10.5194/egusphere-egu21-8362, 2021.

In the Caucasus Mountains, the role of human influences and climate changes on steppes expansion over the Holocene is still discussed because this region is poorly documented. This study investigates (1) modern pollen-vegetation relationships in Armenia and (2) changes in vegetation, human activity and climate in the Holocene record of Vanevan peat (south-eastern shore of Lake Sevan) located in Armenia. The last 9700 years are recorded in the Vanevan core. We used a multiproxy approach including XRF, Pollen, Non-Pollen Palynomorphs (NPPs) and branched glycerol dialkyl glycerol tetraethers (brGDGTs) to reconstruct changes in vegetation, human impact and climate. The combination of these proxies is innovative and aims to distinguish the impact of human activities and climate change on vegetation. Modern pollen assemblages from semi-desert/steppe regions of Armenia show an abundance of Chenopodiaceae while meadows steppes, subalpine and alpine meadows are dominated by Poaceae. The Holocene vegetation at Vanevan is characterized by steppes dominated by Poaceae, Artemisia and Chenopodiaceae. However, several arboreal taxa, such as Quercus, Betula, Carpinus betulus and Ulmus, are more developed on slopes between 8600 and 5100 cal BP. Regarding the human impact, the presence of agriculture is attested since 5200 cal BP, largely increases during the last 2000 years cal BP (high percentages of Cerealia-type pollen) and correlates with the occupation periods reported in archeological studies. Palaeoclimate changes at Vanevan are estimated from (1) water level changes (2) temperature reconstructions based on brGDGTs (3) climate reconstructions based on pollen (through a multi-method approach: Modern Analogue Technique, Weighted Averaging Partial Least Squares regression, Random Forest, and Boosted Regression Trees). Climate reconstructions based on pollen and brGDGTs are rare and the multi-method approach using pollen data is innovative in the region. The results of Vanevan give evidence of high temperatures from 7900 to 5100 cal BP and arid events at 6000, 5000-4500 and 4200 cal BP, in agreement with other regional records.

How to cite: Robles, M., Peyron, O., Brugiapaglia, E., Ménot, G., Dugerdil, L., Ollivier, V., Develle, A.-L., Tozalakyan, P., Meliksetian, K., Sahakyan, K., Sahakyan, L., Perello, B., Badalyan, R., Colombié, C., and Sébastien, J.: Holocene vegetation, climate and human impact in steppes around Lake Sevan (Armenia) based on a multiproxy approach: Pollen, NPPs and brGDGTs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15206, https://doi.org/10.5194/egusphere-egu21-15206, 2021.

How to cite: Wang, T., Bell, B., Fletcher, W., Wogelius, R., and Zielhofer, C.: Quantifying past changes in Holocene ultraviolet-absorbing compounds (UACs) from Cedrus pollen in Lake Sidi Ali, Morocco, Africa based on fourier-transform infrared spectroscopy (FTIR), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10727, https://doi.org/10.5194/egusphere-egu21-10727, 2021.

The circum-Mediterranean region is characterized by high climatic diversity derived from its orographic heterogeneity and the influence of global marine and atmospheric circulation patterns. The region also has a long and dynamic history of human occupation dating back to ~ 8000 years BP.  The complexity of this area is a challenge for reconstructing the dynamics of the vegetation through the Holocene. Rule-based approaches to reconstructing changing vegetation patterns through time are insufficient as they require the imposition of subjective boundaries between biomes and can be affected by known biases in pollen representation.  We have developed and tested a new method that characterises biomes as a function of observed pollen assemblages based on a similarity index, conceptually related to the likelihood function, which takes account of within-biome variability in taxon abundances. We use 1181 modern pollen samples from the EMBSeCBIO database and assign these samples to biomes as represented in a map of potential natural vegetation that was developed using machine learning. The method was applied down-core to reconstruct past vegetation changes. Preliminary results show that this new methodology produces more accurate biome assignments under modern conditions (<80% accuracy) and more stable down-core reconstructions, apparently reducing the "flickering switch" problem found when using the traditional biomisation method for this purpose. Climate-induced vegetation changes are observable on a sub-regional scale in the Eastern Mediterranean through the Holocene. Most of the records show a change from humid to more arid biomes between 4000 and 3000 years BP. However, they are distinct subregional patterns in the expression and timing of wetter conditions during the Holocene. Mountain regions appear to show more muted changes during the Holocene, although there are biome shifts everywhere across the Pleistocene-Holocene transition.

How to cite: Cruz-Silva, E., Harrison, S. P., Marinova, E., and Prentice, I. C.: Vegetation dynamics of the eastern Mediterranean region during the Holocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6518, https://doi.org/10.5194/egusphere-egu21-6518, 2021.

East Siberia is an ideal area for investigating the relationships between modern pollen assemblages and vegetation under the extremely cold and dry climate conditions. These relationships are the basis of paleovegetation and paleoclimate reconstructions from fossil pollen records. Pollen productivity estimates (PPE) are required for reliable pollen-based quantitative vegetation reconstructions. Here, we present a new pollen dataset of 48 moss/soil and 24 lake surface sediment samples collected from Chukotka and Yakutia. Generally, tundra and taiga vegetation sites can be well distinguished in the surface pollen assemblages from East Siberia. Moss/soil and lake samples have mostly similar pollen assemblages but contents of some pollen taxa may vary significantly in different sample types. We classified drone images based on field survey to obtain high-resolute vegetation data. Pollen counts in moss/soil samples and vegetation data can? be used in the Extended R-Value (ERV) model to estimate the relevant source area of pollen (RSAP) and the PPEs of major plant taxa. The result of PPE calculation for most common taxa (Alnus, Betula, Cyperaceae, Ericaceae, Larix, Pinus and Salix) can be used to improve vegetation reconstructions.

How to cite: Geng, R., Andreev, A., Kruse, S., Zhao, Y., Herzschuh, U., Pestraykova, L., and Zakharov, E.: Modern pollen-vegetation-climate relationships and pollen productivity estimates for common East Siberian taxa, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12546, https://doi.org/10.5194/egusphere-egu21-12546, 2021.

Analyses of the regional controls representing climate, vegetation and human activities on modern burnt area in the Iberian Peninsula show that the vegetation properties that determine fuel availability are major influences on the occurrence of fire. This finding opens up the possibility of using pollen data to reconstruct past changes in fire regimes. We could then make use of the much greater abundance of pollen data compared to other sources of information on past fire regimes to constrain biomass-burning feedbacks to the carbon cycle and climate. We applied Tolerance-Weighted Averaging Partial Least-Squares (TWA-PLS) to derive quantitative relationships between pollen-taxon and charcoal abundances from 15 entities from the Iberian Peninsula, using core-top charcoal data and a generalized linear model of present-day fire probability to provide conversion factors between the relative scale of charcoal abundance and the absolute scale of fire. We show that pollen taxon abundance has good predictive power for fire (r² = 0.56) and that the contribution of specific taxa to the prediction makes sense in terms of their ecological adaptations to fire. We apply the TWA-PLS quantitative relationship to predict changing fire regimes across the Iberian Peninsula through the Holocene. Results show that fire change synchronous with climate warming events.

How to cite: Shen, Y., Harrison, S., and Prentice, C.: Reconstructing Holocene vegetation-fire regimes in the Iberian Peninsula, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3848, https://doi.org/10.5194/egusphere-egu21-3848, 2021.

Forest fires are an important factor in the global carbon cycle and high latitude ecosystems. Eastern Siberian tundra, summergreen larch-dominated boreal forest on permafrost and evergreen boreal forest have characteristic fire regimes with varying fire intensities. Yet, it is unknown which role fire plays in long-term climate-vegetation-permafrost feedbacks and how high-latitude fire regimes and ecosystems will change in a warmer world. To learn from fire regime shifts during previous interglacials, prior to human presence, we use lake-sedimentary charcoal as proxy for high-intensity forest fires and monosaccharide anhydrides (i.e. levoglucosan, mannosan, galactosan: MA) as molecular proxies for low-temperature biomass burning, typical for surface fires in modern larch forest. However, MA pathways from source to sink and their stability in sediments are very poorly constrained. Recently, Dietze et al. (2020) found MA in up to 420 kyr old sediment of Lake El’gygytgyn (ICDP Site 5011-1), NE Siberia, suggesting that they are suitable proxies for fires in summergreen boreal forests. Surprisingly, the ratios of the MA isomers were exceptionally low compared to published emission ratios from modern combustions.

To understand what MA from Arctic lake sediments tell us, we have analyzed the MA and charcoal composition in modern lake surface sediments of Lake El’gygytgyn and three East Siberian lakes and we compare them to late glacial-to-interglacial El’gygytgyn records. The three Siberian lakes were chosen to represent spatial analogues to the El’gygytgyn conditions during MIS 5e and 11c. We discuss first results of the modern sediments in context of recent MODIS- and Landsat-based fire extents and biome-specific land cover data, a wind field modelling using climate data over eastern Siberia, and lake-catchment configurations from TDX-DEM analysis to assess potential fire proxy source areas and regional-to-local transport processes. Thereby, we provide insights into the meaning of sedimentary fire proxies, crucial for a sound reconstruction of long-term fire regime histories.

How to cite: Dietze, E., Mangelsdorf, K., Weise, J., Matthes, H., Lisovski, S., and Herzschuh, U.: The potential and limitations of long-term fire regime reconstructions in Eastern Siberia based on sedimentary charcoal and low-temperature fire markers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8974, https://doi.org/10.5194/egusphere-egu21-8974, 2021.