Quantitative climate reconstructions based on fossil pollen : novel approaches to calibration, validation, and spatial data analysis

Palaeoclimatic reconstructions from fossil proxies have provided important insights into the natural variability of climate in the late Quaternary. However, major challenges remain in ensuring the robustness of these reconstructions. Multiple factors may introduce variability and biases into the palaeoclimatic estimates. For example, quantitative reconstructions use diverse modern calibration data-sets, and a wide variety of numerical calibration methods. While the choice of calibration data-set and calibration method may significantly influence the reconstructions, the comparison and analysis of these data-sets and methods have received relatively little attention. Further challenges are presented by the validation of the prepared reconstructions and the identification of climatic variables which can be robustly reconstructed from a given proxy. In this work, summer temperature reconstructions are prepared based on late-Quaternary pollen sequences from northern Finland and northern Russia, covering the Holocene and the early part of the last glacial period (Marine Isotope Stages 5d c). The major aim of this work is to validate these reconstructions and to identify sources of bias in them. Reconstructions are prepared using a number of different calibration methods and calibration sets, to analyse the between-reconstruction variability introduced by the choice of calibration method and calibration set. In addition, novel regression tree methods are used to test the ecological significance of different climatic factors, with the aim of identifying parameters which could feasibly be reconstructed. In the results, it is found that the choice of calibration method, calibration data-set, and fossil pollen sequence can all significantly affect the reconstruction. The problems in choosing calibration data are especially acute in pre-Holocene reconstructions, as it is difficult to find representative calibration data for reconstructions from non-analogue palaeoclimates which become increasingly common in the more distant past. First-order trends in the reconstructed palaeoclimates are found to be relatively robust. However, the degree of between-reconstruction variability stresses the importance of independent validation, and suggests that ensemble reconstructions using different methods and proxies should be increasingly relied on. The analysis of climatic response in northern European modern pollen samples by regression trees suggests secondary climatic determinants such as winter temperature and continentality to have major ecological influence, in addition to summer temperature which has been the most commonly reconstructed variable in palaeoclimatic studies. This suggests the potential to reconstruct the secondary parameters from fossil pollen. However, validating the robustness of secondary-parameter reconstructions remains a major challenge for future studies.Väitöstutkimuksessa selvitettiin Pohjois-Euroopan ilmaston kehitystä viimeisten noin sadantuhannen vuoden aikana. Tutkimus perustuu Euroopan puoleisen Pohjois-Venäjän tundra-alueelta ja Pohjois-Suomesta kerättyihin kasvifossiiliaineistoihin. Tutkimus edustaa paleoklimatologiaa eli tutkimusalaa, joka pyrkii selvittämään ilmastossa erittäin pitkillä aikaväleillä, tuhansien ja miljoonien vuosien aikana tapahtuneita muutoksia. Tutkimuksessa tehtiin niin sanottuja rekonstruktioita Pohjois-Euroopan ilmastosta ja kasvillisuudesta fossiiliaineistojen perusteella: tuhansien vuosien takaisten ilmastojen ja kasvillisuuden piirteitä arvioitiin tutkimalla eri-ikäisissä maaperän kerrostumissa säilyneitä fossiileita. Tässä tutkimuksessa erityishuomion kohteena olivat fossiiliset siitepölyhiukkaset. Fossiilisten siitepölyjen lajisuhteiden perusteella arvioitiin muinaisten ilmastovaiheiden lämpötiloja käyttämällä erilaisia tilastollisia menetelmiä. Yksi väitöstutkimuksen keskeisistä tuloksista koskee Euroopan puoleisen Pohjois-Venäjän tundra-alueen herkkyyttä ilmaston lämpenemiselle. Tutkimuksessa selvitettiin kasvillisuuden ja lämpötilan muutoksia tällä alueella noin 5000 vuotta sitten, nykyistä selvästi lämpimämmän ilmastovaiheen aikana. Tulosten perusteella lämpötila oli 5000 vuotta sitten noin 2,5 °C nykyistä korkeammalla. Samalla havumetsän pohjoisraja oli n. 150 km pohjoisempana, kaukana nykyisellä tundra-alueella. Myös ikiroudan määrä alueella oli tuolloin huomattavasti nykyistä pienempi. Nämä suuret ympäristömuutokset osoittavat Euraasian pohjoisen tundra-alueen olevan erittäin herkkä ilmaston muutoksille. Pohjoisen Euraasian on ennustettu lämpenevän tämän vuosisadan aikana useilla asteilla, mikä saattaa tämän tutkimuksen tulosten perusteella aiheuttaa suuria muutoksia alueen kasvillisuudessa ja ikiroudassa. Tutkimuksessa analysoitiin lisäksi erilaisia tilastollisia menetelmiä, joita käytetään paleoklimatologiassa ilmastoarvioiden tekemiseen fossiilien perusteella. Tulosten mukaan eri tilastolliset menetelmät näyttävät tuottavan jossain määrin erilaisia ilmastoarvioita. Muinaisten ilmastojen rekonstruktiot olivat kuitenkin pääpiirteiltään samanlaisia riippumatta siitä, mitä tilastollista menetelmää käytettiin. Tämä viittaa siihen, että ilmastojen keskeisimpiä piirteitä voidaan luotettavasti arvioida fossiiliaineistojen perusteella

Excess protein enabled dog domestication during severe Ice Age winters

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Taide kohtaa geologian : IHME Helsinki 2021 -teos tuo kaukaisen menneisyyden ja tulevaisuuden ensi syksynä Helsinkiin

Otsikkokuvan tiedot Dallas Krentzel: "Ecuadorian Amazon rain forest, looking toward the Andes", Flickr (https://www.flickr.com/photos/31867959@N04/7613943940). Jaettu lisenssillä CC-BY-2.0 (https://creativecommons.org/licenses/by/2.0/)

Chironomid-based temperature reconstruction for the Eemian Interglacial (MIS 5e) at Sokli, northeast Finland

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Machine-learning based reconstructions of primary and secondary climate variables from North American and European fossil pollen data

We test several quantitative algorithms as palaeoclimate reconstruction tools for North American and European fossil pollen data, using both classical methods and newer machine-learning approaches based on regression tree ensembles and artificial neural networks. We focus on the reconstruction of secondary climate variables (here, January temperature and annual water balance), as their comparatively small ecological influence compared to the primary variable (July temperature) presents special challenges to palaeo-reconstructions. We test the pollen-climate models using a novel and comprehensive cross-validation approach, running a series of h-block cross-validations using h values of 100-1500 km. Our study illustrates major benefits of this variable h-block cross-validation scheme, as the effect of spatial autocorrelation is minimized, while the cross-validations with increasing h values can reveal instabilities in the calibration model and approximate challenges faced in palaeo-reconstructions with poor modern analogues. We achieve well-performing calibration models for both primary and secondary climate variables, with boosted regression trees providing the overall most robust performance, while the palaeoclimate reconstructions from fossil datasets show major independent features for the primary and secondary variables. Our results suggest that with careful variable selection and consideration of ecological processes, robust reconstruction of both primary and secondary climate variables is possible.Peer reviewe

New insights from XRF core scanning data into boreal lake ontogeny during the Eemian (Marine Isotope Stage 5e) at Sokli, northeast Finland

Biological proxies from the Sokli Eemian (Marine Isotope Stage 5e) paleolake sequence from northeast Finland have previously shown that, unlike many postglacial records from boreal sites, the lake becomes increasingly eutrophic over time. Here, principal components (PC) were extracted from a high resolution multi-element XRF core scanning dataset to describe minerogenic input from the wider catchment (PC1), the input of S, Fe, Mn, and Ca-rich detrital material from the surrounding Sokli Carbonatite Massif (PC2), and chemical weathering (PC3). Minerogenic inputs to the lake were elevated early in the record and during two abrupt cooling events when soils and vegetation in the catchment were poor. Chemical weathering in the catchment generally increased over time, coinciding with higher air temperatures, catchment productivity, and the presence of acidic conifer species. Abiotic edaphic processes play a key role in lake ontogeny at this site stemming from the base cation- and nutrient-rich bedrock, which supports lake alkalinity and productivity. The climate history at this site, and its integrated effects on the lake system, appear to override development processes and alters its long-term trajectory.Peer reviewe

Calibrating aquatic microfossil proxies with regression-tree ensembles : Cross-validation with modern chironomid and diatom data

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Comparison of quantitative Holocene temperature reconstructions using multiple proxies from a northern boreal lake

Four biotic proxies (plant macrofossils, pollen, chironomids and diatoms) are employed to quantitatively reconstruct variations in mean July air temperatures (T-jul) at Lake Loitsana (northern Finland) during the Holocene. The aim is to evaluate the robustness and biases in these temperature reconstructions and to compare the timing of highest T-jul in the individual reconstructions. The reconstructed T-jul values are evaluated in relation to local-scale/site-specific processes associated with the Holocene lake development at Loitsana as these factors have been shown to significantly influence the fossil assemblages found in the Lake Loitsana sediments. While pollen-based temperatures follow the classical trend of gradually increasing early-Holocene T-jul with a mid-Holocene maximum, the aquatic/wetland assemblages reconstruct higher-than-present T-jul already during the early Holocene, that is, at the peak of summer insolation. The relatively low early-Holocene July temperatures recorded by the pollen are the result of site-specific factors possibly combined with a delayed response of the terrestrial ecosystem compared with the aquatic ecosystem. Our study shows that all reconstructions are influenced at least to some extent by local factors. This finding stresses the need to evaluate quantitatively reconstructed climate values against local lake development and highlights the benefit of using multi-proxy data in Holocene climate reconstructions.Peer reviewe