Ola Kwiecien

Soft, unlithified sediments recovered from modern lakes rarely offer clear evidence of diagenetic alterations. Recent work has documented products of early diagenesis in the deep lacustrine setting of Lake Van. Lake Van, cored in 2010 in the frame of the ICDP PALEOVAN project, is a terminal, alkaline lake in Eastern Anatolia, Turkey (McCormack & Kwiecien, 2021). The lake carbonate inventory consists of (1) primary phases: inorganic calcite and aragonite precipitating in surface water, and low-Mg calcite ostracod valves formed at the sediment-water interface; and (2) secondary phases: early diagenetic dolomite forming in the sediment pores and aragonite encrustation of ostracod valves and organic remains. Here we focus on aragonite encrustations.Encrusted grains appear episodically in Lake Van sediments younger than 270 ka, and their occurrence is restricted to two lithologies; homogenous and banded muds, representing lake low-stands, reduced primary productivity/preservation and a well-ventilated water column. Although lake level changes occurred in the past, the water depth of the coring site – today at 350 m – unlikely fell below 200 m.SEM and thin section analyses of the as yet enigmatic encrustations show two generations of aragonite crystals; larger (10 – 20 μm), columnar to blocky ones (inside the closed valves) and a magnitude smaller (1 – 2 μm), columnar ones (outside the valves) intercalated with clay minerals and probably organic matter. The isotopic composition of encrusted valves contrasts with that of inorganic carbonates precipitating in the water column; higher δ18O values support a formation in cold bottom water, higher δ13C values are likely related to microbial activity, however, the nature of this relation is yet unclear. Encrusted valves are often articulated but display different stages of opening. As ostracod valves usually disarticulate within hours to days after the animal’s demise, semi-open valves suggest that the early diagenetic process was – in geological terms – extremely rapid.Our finding calls for care and attention analyzing even sub-recent biogenic carbonates. The episodic and facies-bound occurrence suggests that encrustation is ultimately controlled by environmental factors, yet so far, we were unable to pinpoint these factors or a mechanism responsible for this process. If you are intrigued just like us, do get in touch! ReferencesMcCormack & Kwiecien, 2021. Coeval primary and diagenetic carbonates in lacustrine sediments challenge palaeoclimate interpretations. Scientific Reports

Protracted droughts may have contributed to sociopolitical upheaval and depopulation of cultural centers in the Maya Lowlands during the Terminal Classic Period (~800 to 1000 CE). Regional proxy climate records suggest multiple prolonged drought episodes during the Terminal Classic. The relationship between drought and response of individual sites, however, remains unclear because of large chronological uncertainties and poor temporal resolution of existing local paleoclimate inferences. We present a subannual rainfall record from northwest Yucatán, Mexico, derived from an annually laminated stalagmite spanning 871 to 1021 CE, with ±6-year age uncertainty. Interpretation of the stalagmite oxygen isotope record is supported by modern rain and drip water monitoring. Precisely dated droughts enable detailed analyses of timing and dynamics of regional human-climate interactions. Despite uncertainties in archaeological chronologies, these results suggest political activity at major northern Maya sites, including Chichén Itzá and Uxmal, declined at different times relative to droughts, implying differential cultural responses to climate stress.

Wildfires are a rapidly increasing threat to boreal forests. While our understanding of the drivers behind wildfires and their environmental impact is growing, it is mostly limited to the observational period. Here we focus on the boreal forests of southern Siberia and exploit a U–Th-dated stalagmite from Botovskaya Cave, located in the upper Lena region of southern Siberia, to document wildfire activity and vegetation dynamics during parts of two warm periods: the Last Interglacial (LIG; specifically part of the Last Interglacial maximum between 124.1 and 118.8 ka) and the Holocene (10–0 ka). Our record is based on levoglucosan (Lev), a biomarker sensitive to biomass burning, and on lignin oxidation products (LOPs) that discriminate between open and closed forest and hard- or softwood vegetation. In addition, we used carbonate carbon stable isotope ratios (δ¹³C), which reflect a dominant control of the host rock, to evaluate soil respiration and local infiltration changes. Our LOP data suggest that, during the Last Interglacial, the region around Botovskaya Cave was characterised by open forest, which by ca. 121.5 ka underwent a transition from fire-resistant hardwood to fire-prone softwood. The Lev record indicates that fire activity was high and increased towards the end of Last Interglacial just before 119 ka. In contrast, the Holocene was characterised by a closed-forest environment with mixed hard- and softwood vegetation. Holocene fire activity varied but at a much lower level than during the Last Interglacial. We attribute the changes in wildfire activity during the intervals of interest to the interplay between vegetation and climate. The open forests of the Last Interglacial were more likely to ignite than their closed Holocene equivalents, and their flammability was aided by warmer and drier summers and a stronger seasonal temperature contrast due to the increase in seasonal insolation difference compared to the Holocene. Our comparison of the last two interglacial intervals suggests that, with increasing global temperatures, the boreal forest of southern Siberia may become progressively more vulnerable to higher wildfire activity.

The Mediterranean region is highly sensitive to climate change and warms faster than the global average. Models forecast a pronounced drying trend, coupled with an increase in the frequency and intensity of extreme rainfall events. Past Interglacials can be used as analogues to better understand and estimate regional hydroclimatic responses to global warming.The Lake Van (Eastern Anatolia, Turkey) sediment record, ICDP site 5034, serves as a key archive to reconstruct hydrological changes in the Eastern Mediterranean. This terminal lake is the largest soda lake in the world and has experienced significant lake-level changes over Glacial-Interglacial transitions (~105 m above modern lake levels during MIS5e). Sediments covering the transition from MIS6 to MIS5e are finely laminated and, in parts, even annually laminated or varved. Therefore, these sediments enable detailed analyses of hydroclimatic variability during Termination II through XRF, microfacies analysis, stable isotope analysis, and δD on leaf wax biomarkers.Within MIS5e, first results show a significant increase in the bulk organic δ13C signal over a period of several hundred years during a stage associated with higher lake levels. This shift aligns with a change in alkenone composition and precedes a change in stratification, as suggested by a transition from varved to non-varved lithology. The external and internal drivers of these changes are further investigated by XRF core scanning, element mapping, and comprehensive biomarker analyses to explore this proxy behavior.Ultimately, the data obtained will be compared to other lacustrine records, such as the ICDP Core 5017 from the Dead Sea, to contribute to a more comprehensive understanding of regional variations in the hydroclimatic response during this warming phase in the Eastern Mediterranean.

Wildfires are a rapidly increasing threat to boreal forests. Our current understanding of the drivers behind wildfires and their environmental impact, while growing, is based mostly on data that is limited to the observational period. Our most crucial findings reveal that both the LIG and MIS 11 had levoglucosan levels more than double the average observed in other interglacials, indicating a high frequency of wildfire occurrence. Despite similarities in climatic background conditions and a shift from more hardwood (i.e., deciduous) forests to denser softwood-dominated (i.e., coniferous) forests toward the end of each interglacial, the wildfire behaviour differed significantly: both MIS 11 and the Last Interglacial exhibit overall high wildfire activity, but MIS 11 shows a declining trend in wildfires towards its end, while the Last Interglacial experiences a sharp increase in wildfire activity during its final stages. We hypothesise that consistently warmer, more humid conditions throughout MIS 11 allowed for emergence of stable forest ecosystems with a lower susceptibility to wildfires, resulting in lower wildfire vulnerability with declining temperatures closer to the end of the interglacial compared to the LIG. The LIG, especially towards the end, is characterised by shifts towards open forests composed of fire-prone species, which prompted increased wildfire activity additionally boosted by drier and cooler conditions.Our reconstruction provides a framework for understanding long-term interactions between climate and biosphere. By linking interglacial wildfire dynamics with vegetation changes, we highlight how climatic variability modulates ecological resilience and fire susceptibility in boreal regions. This study offers insights into the potential responses of contemporary taiga ecosystems to ongoing anthropogenic climate change, where rising temperatures and shifting precipitation patterns alter biomes and may amplify fire activity.

Our understanding of global temperature in the recent geological past is predominantly derived from oceanic proxies and modelling reconstructions1–3. Terrestrial proxy data, particularly in continental environments, is sparse and based largely on palaeobotanical and palaeozoological data which can be difficult to accurately date4.We present approximately 30 temperature reconstructions over a six-million-year interval from Botovskaya Cave (N 55.3°, E 105.3°) in Siberia, ca. 300 km west of Lake Baikal. We provide chronological constraint with U-series techniques5 and multi-annual absolute temperature estimates from clumped isotope analyses of speleothems (carbonate cave deposits, e.g. stalagmites and flowstones). Clumped isotope analysis directly infers quantitative paleotemperature estimates, overcoming difficulties associated with conventional stable isotope (δ18O) techniques which require knowledge of the isotopic composition of carbonate precipitation waters – which is often unknown. By targeting subaqueous material, we overcome dis-equilibrium effects which have hindered widespread application of clumped isotopes to speleothems6,7.Our record is the longest palaeotemperature timeseries from continental Eurasia and suggests a ca. 4 – 5°C temperature drop between the Messinian (7.24 – 5.33 Ma) and the present day, coincident with declining atmospheric carbon dioxide8, and in agreement with existing estimates of global temperature over the same interval9,10.References1. Clark, P. U. et al, Global and Regional Temperature Change over the Past 4.5 Million Years. Science (2024).2. Herbert, T. D. et al. Late Miocene global cooling and the rise of modern ecosystems. Nat Geosci (2016).3. Judd, E. J. et al. A 485-million-year history of Earth’s surface temperature. Science, (2024).4. Bradshaw, C. D. et al. The relative roles of CO2 and palaeogeography in determining late Miocene climate: Results from a terrestrial model-data comparison. Climate of the Past (2012).5. Mason, A. J. et al, Simplified isotope dilution approach for the U-Pb dating of speleogenic and other low-232Th carbonates by multi-collector ICP-MS. Geochronology (2022).6. Daëron, M. et al. 13C18O clumping in speleothems: Observations from natural caves and precipitation experiments. Geochim Cosmochim Acta (2011).7. Affek, H. P. et al, Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’ thermometry. Geochim Cosmochim Acta (2008).8. Rae, J. W. B., et al. Atmospheric CO2 over the past 66 million years from marine archives. Annual Review of Earth and Planetary Sciences. (2021).9. Westerhold, T. et al. An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science (2020).10. Pound, M. J. et al, Global vegetation dynamics and latitudinal temperature gradients during the Mid to Late Miocene (15.97-5.33Ma). Earth-Science Reviews (2012).

Paleoclimate studies on the Yucatán Peninsula, Mexico, are important for understanding relationships between past climate change and ancient Maya cultural evolution. The Postclassic Maya capital of Mayapán is a site of interest for such studies, given that evidence of repeated drought conditions corresponded to times of violent conflict, from ~1400 to 1450 CE (Kennett et al., 2022), just before site abandonment in ~1450 and prior to the protracted Spanish conquest of the Yucatán region from 1527 to 1697 CE. As part of further efforts to identify past droughts in northwest Yucatán, we studied a new stalagmite from Cenote Ch’en Mul, located directly beneath the archaeological site of Mayapán. The upper 120 mm section of stalagmite MAYA-22-7 is chronologically constrained by five uranium-series dates between 1618 ± 23 (2σ) and 2001 ± 11, and grew at a near-constant rate of 0.3 mm/yr. Stable isotopes (ẟ18O and ẟ13C) of calcite were measured every 0.1 mm along the upper section growth axis and compared with Colonial-period records of known droughts, famines, and population declines (Hoggarth et al., 2017). The ẟ18O and ẟ13C records capture abrupt increases (1-2 ‰ above baseline), coincident (±5 years) with some of the historical records of extreme drought. One notable example includes ẟ18O values 1.0 ‰ above baseline from 1767 to 1770 CE, indicative of an extended dry period contemporaneous with the “Great Famine” of 1765 to 1773. That dry interval is described in historic records as having been characterized by drought, locusts, famine, widespread epidemics, and mortality, which caused a 39 % population reduction (Hoggarth et al., 2017). Evidence for severe drought is replicated for the interval 1765-1768 in a nearby speleothem record from Belize (Kennett et al., 2012). Furthermore, ẟ13C values demonstrate distinct 2 ‰ shifts, from -10 to -8 ‰, and frequent (3-5 year) repeated oscillations, indicating a strong, rapid response of the overlying ecosystem’s carbon budget to climate variability, perhaps related to the El Niño/Southern Oscillation (ENSO). Ultimately, in combination with other local and regional paleoclimate records, study of MAYA-22-7 will help assess the reliability of stalagmite records to capture historic droughts with high temporal accuracy, document patterns of climate variability (e.g., ENSO-related changes), and track land-use change across cultural transitions, while contributing to our understanding of human-climate-environment interactions in the Maya Lowlands.

Lake Van in Eastern Anatolia (Turkey) is the world's largest alkaline lake and the fourth largest terminal lake. A long, almost continuous, and partly annually laminated sedimentary profile, recovered during the ICDP PALEOVAN campaign in 2010 provided valuable information about the climatic, vegetation, tectonic, and volcanic history of the region over the last ca. 600 ka (Litt & Anselmetti 2014 and references therein). Individual proxy records suggest that Lake Van and its vicinity experienced significant hydroclimatic changes in concert with glacial/interglacial cycles and Dansgaard/Oeschger oscillations. The results of the original PALEOVAN project constituted an excellent framework and motivated further problem-oriented research. The last 10 years have seen a number of inspiring publications testing strengths and limitations of carbonate-based proxies in lakes and deepening our understanding of forcing mechanisms and rates of ecological changes.Here we summarise the highlights of Lake Van research to date and offer a perspective on exciting ongoing work. Systematic analyses of the sedimentary carbonate inventory of Lake Van put the mineralogy of lacustrine carbonates in the limelight and revealed caveats of geochemical analyses on bulk samples (McCormack et al., 2018, 2019). Moreover, these studies stretched the envelope of dolomite-forming environments and early diagenetic products (McCormack & Kwiecien, 2021; McCormack et al., 2023). The established climatic context (Pickarski et al., 2015a, 2015b) and annually laminated nature of Lake Van sediments enabled the quantitative estimation of the time vegetation takes to recover after volcanic eruptions and elucidated the effect tephra deposition has on in-lake productivity (Pickarski et al., 2023). Lake Van sediments keep on giving, with ongoing projects targeting the timing and mechanism of the lake closure and high-resolution comparisons of the impact of interglacial warming on (pan)regional hydroclimate. So perhaps 'The Best Of' is yet to come!ReferencesLitt & Anselmetti 2014. Lake Van deep drilling project PALEOVAN. Quaternary Science ReviewsMcCormack & Kwiecien, 2021. Coeval primary and diagenetic carbonates in lacustrine sediments challenge palaeoclimate interpretations. Scientific ReportsMcCormack, et al., 2019. Refining the interpretation of lacustrine carbonate isotope records: Implications of the mineralogy-specific Lake Van case study. Chemical GeologyMcCormack et al., 2018. Controls on cyclic formation of Quaternary early diagenetic dolomite. Geophysical Research LettersMcCormack, et al., 2023. Hydrochemical mixing‐zones trigger dolomite formation in an alkaline lake. Sedimentology.Pickarski et al., 2023. Volcanic impact on terrestrial and aquatic ecosystems in the Eastern Mediterranean, Communications Earth & EnvironmentPickarski et al., 2015b. Vegetation and environmental changes during the last interglacial in eastern Anatolia (Turkey): a new high-resolution pollen record from Lake Van. Palaeogeography, Palaeoclimatology, PalaeoecologyPickarski et al., 2015a. Abrupt climate variability of eastern Anatolia vegetation during the last glacial. Climate of the Past

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.ReferencesAffek, 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, 5351–5360Affek, 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, 319–330Daëron, 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