| 1. |
- Väliranta, M, et al.
(författare)
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Plant macrofossil evidence for an early onset of the Holocene summer thermal maximum in northernmost Europe.
- 2015
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Holocene summer temperature reconstructions from northern Europe based on sedimentary pollen records suggest an onset of peak summer warmth around 9,000 years ago. However, pollen-based temperature reconstructions are largely driven by changes in the proportions of tree taxa, and thus the early-Holocene warming signal may be delayed due to the geographical disequilibrium between climate and tree populations. Here we show that quantitative summer-temperature estimates in northern Europe based on macrofossils of aquatic plants are in many cases ca. 2 °C warmer in the early Holocene (11,700-7,500 years ago) than reconstructions based on pollen data. When the lag in potential tree establishment becomes imperceptible in the mid-Holocene (7,500 years ago), the reconstructed temperatures converge at all study sites. We demonstrate that aquatic plant macrofossil records can provide additional and informative insights into early-Holocene temperature evolution in northernmost Europe and suggest further validation of early post-glacial climate development based on multi-proxy data syntheses.
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| 2. |
- Helmens, Karin F., et al.
(författare)
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Large shifts in vegetation and climate during the early weichselian (mis 5d c) inferred from multi proxy evidence at Sokli (Northern Finland)
- 2012
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Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791 .- 1873-457X. ; 41, s. 22-38
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Tidskriftsartikel (refereegranskat)abstract
- For decades, detailed studies on Early Weichselian deposits have been made in central Europe. In contrast, these studies are rare in Fennoscandia in northern Europe. We here integrate an extensive multi-proxy data set obtained on sediments of MIS 5d-c age that form part of a long sediment record preserved at Sokli in northern Finland. We make a detailed interpretation of the vegetation and depositional history for MIS 5d-c using pollen, macrofossils, diatoms and other siliceous microfossils, insect remains, and sediment characteristics, and combine these data with recently published estimates on July temperatures based on chironomids and selected plant indicator species in order to make inferences of paleo-climate regimes. The fossil record obtained on the seven meter thick MIS 5d-c deposit at Sokli is exceptionally rich in species due to the large variety of habitats associated with an overall fluvial depositional environment. A braided river system flanked by steppe-tundra vegetation is inferred for MIS 5d. Mean July temperatures of at least 12-14 degrees C are indicated by chironomids and plant indicator species and are in agreement with the presence of conifers and birch trees as recorded by macrofossils. The reconstructed environmental conditions suggest strong continental climate conditions at Sokli during MIS 5d. The gradual infilling of an oxbow lake and subsequent return to stream channel deposition is traced in great detail in the overlying gyttja and gravelly sediment of MIS 5c age. The terrestrial pollen and plant macrofossil record from the gyttja shows the establishment of birch forest followed by the spread of pine and then spruce. Rich plant indicator species assemblages indicate that the boreal environment at Sokli during MIS 5c experienced July temperatures several degrees higher than the present-day value of 13 degrees C. The high summer temperatures and presence of larch suggest more continental conditions. More open vegetation returned at Sokli during late MIS 5c and was followed by glaciation by the Fennoscandian Ice Sheet (MIS 5b). Despite the major differences in zonal vegetation types during MIS 5d-c, differences in reconstructed July temperatures are minor and suggest that winter temperatures combined with precipitation values mostly determined the stadial-interstadial climate variability. The most compelling conclusion from our study is that forest development during MIS 5c was remarkably similar to that recorded for the Eemian (MIS 5e) and Holocene Interglacials at the high-latitude site Sokli, and also to that inferred from MIS 5c sediments on the northern European mainland. Our results question the definition of MIS 5c in the terrestrial record of Europe as an interstadial interval of the last glacial cycle and suggest inclusion of MIS 5c together with the Eemian (MIS 5e) in an interglacial complex.
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| 3. |
- Helmens, Karin F., et al.
(författare)
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Warm summers and rich biotic communities during N-Hemisphere deglaciation
- 2018
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Ingår i: Global and Planetary Change. - : Elsevier BV. - 0921-8181 .- 1872-6364. ; 167, s. 61-73
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Tidskriftsartikel (refereegranskat)abstract
- Detailed studies on fossil remains of plants or animals in glacial lake sediments are rare. As a result, environmental conditions right at the moment of deglaciation of the large N-Hemisphere ice-sheets remain largely unknown. Here we study three deglacial phases of the Fennoscandian Ice Sheet as a unique, repeated element in a long sediment record preserved at Soldl in northern Finland. We summarize extensive multi-proxy data (diatoms, phytoliths, chironomids, pollen, spores, non-pollen palynomorphs, macrofossils, lithology, loss-on-ignition, C/N) obtained on glacial lake sediments dated to the early Holocene (ca. 10 kyr BP), early MIS 3 (ca. 50 kyr BP) and early MIS 5a (ca. 80 kyr BP). In contrast to the common view of an unproductive ice-marginal environment, our study reconstructs rich ecosystems both in the glacial lake and along the shores with forest on recently deglaciated land. Higher than present-day summer temperatures are reconstructed based on a large variety of aquatic taxa. Rich biota developed due to the insolation-induced postglacial warming and high nutrient levels, the latter resulting from erosion of fresh bedrock and sediment, leaching of surface soils, decay of plant material under shallow water conditions, and sudden decreases in lake volume. Aquatic communities responded quickly to deglaciation and warm summers and reflect boreal conditions, in contrast to the terrestrial ecosystem which responded with some delay probably due to time required for slow soil formation processes. Birch forest is reconstructed upon deglaciation of the large LGM ice-sheet and shrub tundra following the probably faster melting smaller MIS 4 and MIS 5b ice-sheets. Our study shows that glacial lake sediments can provide valuable palaeo-environmental data, that aquatic biota and terrestrial vegetation rapidly accommodated to new environmental conditions during deglaciation, and that glacial lake ecosystems, and the carbon stored in their sediments, should be included in earth system modeling.
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| 4. |
- Kaufman, Darrell, et al.
(författare)
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A global database of Holocene paleotemperature records
- 2020
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Ingår i: Scientific Data. - : Springer Science and Business Media LLC. - 2052-4463. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format.
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| 5. |
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| 6. |
- Shala, Shyhrete, et al.
(författare)
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Comparison of quantitative Holocene temperature reconstructions using multiple proxies from a northern boreal lake
- 2017
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Ingår i: The Holocene. - : SAGE Publications. - 0959-6836 .- 1477-0911. ; 27:11, s. 1745-1755
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Shala, Shyhrete, et al.
(författare)
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Evaluating environmental drivers of Holocene changes in water chemistry and aquatic biota composition at Lake Loitsana, NE Finland
- 2014
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Ingår i: Journal of Paleolimnology. - : Springer Science and Business Media LLC. - 0921-2728 .- 1573-0417. ; 52:4, s. 311-329
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a detailed analysis of geochemical and biotic proxies in a lake sediment profile to assess the effects of local and regional environmental drivers on the Holocene development of Lake Loitsana, situated in the northern boreal forest of NE Finland. Multi-proxy studies, in particular those that include a detailed plant macrofossil record, from the part of the northern boreal zone of Fennoscandia which has not been affected by treeline fluctuations, are scarce and few of these records date back to the earliest part of the Holocene. A 9-m sediment sequence of gyttja overlying silts representing the last c. 10,700 cal year, allowed for a high-resolution study with emphasis on the early to mid-Holocene lake history. The lacustrine sediments were studied using lithology, loss-on-ignition and C/N ratios, micro- and macro-fossils of aquatic and wetland taxa, diatoms, chironomids and accelerator mass spectrometry C-14 dating on terrestrial plant macrofossils. Our study shows that the local development at Loitsana was complex and included a distinct glacial lake phase and subsequent drainage, a history of fluvial input affected by nearby wetland expansion, and lake infilling in an eventual esker-fed shallow lake. Enhanced trophic conditions, due to morphometric eutrophication, are recorded as Glacial Lake Sokli drained and open water conditions became restricted to a relatively small Lake Loitsana depression. pH appears to have been stable throughout the Holocene with a well-buffered lake due to the local carbonatite bedrock (Sokli Carbonatite Massif). The fossil assemblage changes are best explained by a complex mixture of drivers, including water-body conditions (i.e. depth, turbidity and turbulence), rate of sediment input, and the general infilling of the lake, highlighting the need to carefully evaluate the possible influence of such local factors as palaeoenvironmental conditions are reconstructed based on aquatic proxies.
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| 8. |
- Shala, Shyhrete, 1982-
(författare)
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Palaeoenvironmental changes in the northern boreal zone of Finland: local versus regional drivers
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Multiple proxies derived from the Lake Loitsana sediment sequence (NE Finland) are employed to determine the timing of deglaciation, characterise an early Holocene proglacial lake stage and reconstruct Holocene lake development. Local-scale processes causing shifts in biological assemblages are identified and the most likely Holocene mean July air temperature (Tjul) development is assessed. The study area was deglaciated shortly prior to 10 700 cal. a BP. The sediment record reflects four local events; the presence of a glacial lake, glacial lake drainage and formation of Lake Loitsana, changes in fluvial input due to progressive wetland expansion, and gradual lake infilling. The results suggest that local events have driven changes in biological assemblages through various processes, and that biotic proxies reflect changes in environmental parameters in a highly individual manner. Furthermore, biological assemblages can themselves act as important drivers, influencing the composition of other assemblages. It is suggested that future studies should consider macrophyte abundance and food-web interactions as equally important factors when assessing changes in biological assemblages. Quantitative Tjul reconstructions based on biotic proxies display contrasting trends. While Tjul reconstructions based on pollen found in the Loitsana sequence display relatively low early Holocene values, plant macrofossil and chironomid data reflect warm summer conditions also during the early Holocene, i.e. at the peak of summer insolation. The early Holocene Tjul recorded by terrestrial pollen are affected by local factors possibly combined with a delayed response of the terrestrial ecosystem compared to the aquatic one. This study emphasises the importance of using multiple proxies in palaeoenvironmental studies and shows that local factors have a potential to drive changes in biological assemblages that can affect transfer-function based temperature reconstructions.
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| 9. |
- Shala, Shyhrete, et al.
(författare)
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Palaeoenvironmental record of glacial lake evolution during the early Holocene at Sokli, NE Finland
- 2014
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Ingår i: Boreas. - : Wiley. - 0300-9483 .- 1502-3885. ; 43:2, s. 362-376
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Tidskriftsartikel (refereegranskat)abstract
- The development of a glacial lake impounded along the retreating, northeastern ice margin of the Fennoscandian Ice Sheet during the last deglaciation and environmental conditions directly following the early Holocene deglaciation have been studied in NE Finland. This so-called Sokli Ice Lake has been reconstructed previously using topographic and geomorphologic evidence. In this paper a multiproxy approach is employed to study a 3-m-thick sediment succession consisting of laminated silts grading into gyttja cored in Lake Loitsana, a remnant of the Sokli Ice Lake. Variations in the sediment and siliceous microfossil records indicate distinct changes in water depth and lake size in the Loitsana basin as the Sokli Ice Lake was drained through various spillways opening up along the retreating ice front. Geochemical data (XRF core-scanning) show changes in the influence of regional catchment geochemistry (Precambrian crystalline rocks) in the glacial lake drainage area versus local catchment geochemistry (Sokli Carbonatite Massif) within the Lake Loitsana drainage area during the lake evolution. Principal component analysis on the geochemical data further suggests that grain-size is an additional factor responsible for the variability of the sediment geochemistry record. The trophic state of the lake changed drastically as a result of morphometric eutrophication once the glacial lake developed into Lake Loitsana. The AMS radiocarbon dating on tree birch seeds found in the glaciolacustrine sediment indicates that Lake Loitsana was deglaciated sometime prior to 10 700 cal. a BP showing that tree Betula was present on the deglaciated land surrounding the glacial lake. Although glacial lakes covered large areas of northern Finland during the last deglaciation, only few glaciolacustrine sediment successions have been studied in any detail. Our study shows the potential of these sediments for multiproxy analysis and contributes to the reconstruction of environmental conditions in NE Finland directly following deglaciation in the early Holocene.
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