ALBERT

Description: Time-scale uncertainties between paleoclimate reconstructions often inhibit studying the exact timing, spatial expression and driving mechanisms of climate variations. Detecting and aligning the globally common cosmogenic radionuclide production signal via a curve fitting method provides a tool for the quasi-continuous synchronization of paleoclimate archives. In this study, we apply this approach to synchronize 10Be records from varved sediments of Lakes Tiefer See and Czechowskie covering the Maunder-, Homeric- and 5500 a BP grand solar minima with 14C production rates inferred from the IntCal13 calibration curve. Our analyses indicate best fits with 14C production rates when the 10Be records from Lake Tiefer See were shifted for 8 (−12/+4) (Maunder Minimum), 31 (−16/+12) (Homeric Minimum) and 86 (−22/+18) years (5500 a BP grand solar minimum) towards the past. The best fit between the Lake Czechowskie 10Be record for the 5500 a BP grand solar minimum and 14C production was obtained when the 10Be time-series was shifted 29 (−8/+7) years towards present. No significant fits were detected between the Lake Czechowskie 10Be records for the Maunder- and Homeric Minima and 14C production, likely due intensified in-lake sediment resuspension since about 2800 a BP, transporting old 10Be to the coring location. Our results provide a proof of concept for facilitating 10Be in varved lake sediments as novel synchronization tool required for investigating leads and lags of proxy responses to climate variability. However, they also point to some limitations of 10Be in these archives mainly connected to in-lake sediment resuspension processes.

Description: Wildfire occurrence is influenced by climate, vegetation and human activities. A key challenge for understanding fire-climate-vegetation interactions is to quantify the effect vegetation has in mediating fire regime. Here, we explore the relative importance of Holocene land cover and dominant functional forest type, and climate dynamics on biomass burned in temperate and boreo-nemoral regions of Central and Eastern Europe over the past 12 ka BP years. We used an extensive data set of Holocene pollen and sedimentary charcoal records, in combination with climate simulations and novel statistical modelling. Biomass burned was highest during the early Holocene and lowest during the mid Holocene in all three ecoregions, but diverged more markedly over the past 3–4 ka BP. Although the climate was an important driver of fire hazard during the warm and dry early Holocene, tree cover was consistently the strongest predictor of past biomass burning. In temperate forests, biomass burned was high at ~ 45 % tree cover and decreased strongly towards 60 % tree cover. In needleleaf dominated forests, biomass burned was highest at ~ 60–65 % tree cover and abruptly declined at 〉 65 % tree cover. Biomass burned also increased when arable lands and grasslands reached ~ 15–20 %, although this relationship was highly dynamic depending on land use intensity throughout ignition and fuel type and availability. Our observations cover the full range of Holocene climate variability and land cover changes and illustrates that percentages of land cover is a key predictor of the probability of fire occurrence over timescales of centuries to millennia. We suggest that long-term fire risk may be effectively reduced through land cover management, given that land cover has controlled fire regimes under the dynamic climates of the Holocene.

Description: Timescale uncertainties between paleoclimate reconstructions often inhibit studying the exact timing, spatial expression and driving mechanisms of climate variations. Detecting and aligning the globally common cosmogenic radionuclide production signal via a curve fitting method provides a tool for the quasi-continuous synchronization of paleoclimate archives. In this study, we apply this approach to synchronize 10Be records from varved sediments of Tiefer See and Lake Czechowskie covering the Maunder, Homeric and 5500 a BP grand solar minima with 14C production rates inferred from the IntCal13 calibration curve. Our analyses indicate best fits with 14C production rates when the 10Be records from Tiefer See were shifted for 8 (−12∕ + 4) (Maunder Minimum), 31 (−16∕ + 12) (Homeric Minimum) and 86 (−22∕ + 18) years (5500 a BP grand solar minimum) towards the past. The best fit between the Lake Czechowskie 10Be record for the 5500 a BP grand solar minimum and 14C production was obtained when the 10Be time series was shifted 29 (−8∕ + 7) years towards present. No significant fits were detected between the Lake Czechowskie 10Be records for the Maunder and Homeric minima and 14C production, likely due to intensified in-lake sediment resuspension since about 2800 a BP, transporting old 10Be to the coring location. Our results provide a proof of concept for facilitating 10Be in varved lake sediments as a novel synchronization tool required for investigating leads and lags of proxy responses to climate variability. However, they also point to some limitations of 10Be in these archives, mainly connected to in-lake sediment resuspension processes.

Description: While of higher plant origin, a specific source assignment of sedimentary leaf wax n-alkanes remains difficult. In addition, it is unknown how fast a changing catchment vegetation would be reflected in sedimentary leaf wax archives. In particular, for a quantitative interpretation of n-alkane C and H isotope ratios in terms of paleohydrological and paleoecological changes, a better understanding of transfer times and dominant sedimentary sources of leaf wax n-alkanes is required. In this study we tested to what extent compositional changes in leaf wax n-alkanes can be linked to known vegetation changes by comparison with high-resolution palynological data from the same archive. We analyzed leaf wax n-alkane concentrations and distributions in decadal resolution from a sedimentary record from Trzechowskie paleolake (TRZ, northern Poland), covering the Late Glacial to early Holocene (13 360–9940 yr BP). As an additional source indicator of targeted n-alkanes, compound-specific carbon isotopic data have been generated in lower time resolution. The results indicated rapid responses of n-alkane distribution patterns coinciding with major climatic and paleoecological transitions. We found a shift towards higher average chain length (ACL) values at the Allerød–Younger Dryas (YD) transition between 12 680 and 12 600 yr BP, coevaled with a decreasing contribution of arboreal pollen (mainly Pinus and Betula) and a subsequently higher abundance of pollen derived from herbaceous plants (Poaceae, Cyperaceae, Artemisia), shrubs, and dwarf shrubs (Juniperus and Salix). The termination of the YD was characterized by a successive increase in n-alkane concentrations coinciding with a sharp decrease in ACL values between 11 580 and 11 490 yr BP, reflecting the expansion of woodland vegetation at the YD–Holocene transition. A gradual reversal to longer chain lengths after 11 200 yr BP, together with decreasing n-alkane concentrations, most likely reflects the early Holocene vegetation succession with a decline of Betula. These results show that n-alkane distributions reflect vegetation changes and that a fast (i.e., subdecadal) signal transfer occurred. However, our data also indicate that a standard interpretation of directional changes in biomarker ratios remains difficult. Instead, responses such as changes in ACL need to be discussed in the context of other proxy data. In addition, we find that organic geochemical data integrate different ecological information compared to pollen, since some gymnosperm genera, such as Pinus, produce only a very low amount of n-alkanes and for this reason their contribution may be largely absent from biomarker records. Our results demonstrate that a combination of palynological and n-alkane data can be used to infer the major sedimentary leaf wax sources and constrain leaf wax transport times from the plant source to the sedimentary sink and thus pave the way towards quantitative interpretation of compound-specific hydrogen isotope ratios for paleohydrological reconstructions.

Description: While of higher plant origin, a specific plant source assignment of sedimentary leaf wax n-alkanes remains difficult. Recent compilations of global plant data sets have demonstrated an overlapping and non-systematic production of different chain-length homologues among different classes of terrestrial vegetation. Further, n-alkane distributions can change within the same species due to environmental changes. In addition, it is unknown how fast a changing catchment vegetation would be reflected in sedimentary leaf wax archives. However, in particular for a quantitative interpretation of n-alkane C and H isotope ratios in terms of paleohydrological and paleoecological changes, a better understanding of transfer times and dominant sedimentary sources of leaf wax n-alkanes is required. In this study we aim to identify the major leaf wax contributors to a Central European lacustrine system. Therefore, we tested to what extent leaf wax n-alkane compositional changes (expressed through compound concentration ratios, such as nC27 vs. nC31, average chain length ACL, etc.) can be linked to known vegetation changes, specifically during the Younger Dryas cold period (YD), by comparison with high-resolution palynological data from the same archive. We analysed leaf wax n-alkane concentrations and distributions in decadal resolution from a sedimentary record from Trzechowskie paleolake – TRZ – (Northern Poland), covering the Late Glacial to early Holocene (13,360–9,940 yrs BP). As additional source indicator of targeted n-alkanes, compound specific carbon isotopic data have been generated in lower time resolution. The results showed rapid responses of n-alkane distribution patterns coinciding with major climatic and paleoecological transitions. We find a shift towards higher ACL values at the Allerød/YD transition between 12,680 and 12,600 yrs BP, coeval with a decreasing contribution of arboreal pollen (mainly Pinus and Betula) and a subsequently higher abundance of pollen derived from herbaceous plants (Poaceae, Cyperaceae, Artemisia), as well as shrubs and dwarf shrubs Juniperus and Salix. The termination of the YD was characterized by a successive increase of n-alkane concentrations coinciding with a sharp decrease of ACL values between 11,580–11,490 years BP, reflecting the expansion of woodland vegetation at the YD/Holocene transition. Centennial reversals to longer chain lengths during the Allerød could possibly be linked to Greenland Interstadial 1b (GI-1b). A similar pattern during the early Holocene has more likely been triggered by rapid ecological responses in course of warming, rather than to reflect a local impact of a Preboreal Oscillation or 11.4 yr event. Another gradual increase in ACL values after 11,200 yrs BP, together with decreasing n-alkane concentrations, most likely reflects the early Holocene vegetation succession with a decline of Betula. These results show, that n-alkane distributions reflect vegetation changes and that a fast (i.e. subdecadal) signal transfer occurred. However, our results also indicate that a standard interpretation of directional changes in biomarker ratios remains difficult. Instead, responses such as changes of ACL need to be discussed in context of other proxy data. In addition, we find that organic geochemical data integrate different ecological information compared to pollen, since some gymnosperm species, such as Pinus, produce only very low amount of n-alkanes and thus their contribution may be largely absent from biomarker records. Our results demonstrate that a combination of palynological and n-alkane data can be used to infer the major sedimentary leaf wax sources and constrain leaf wax transport times from the plant source to the sedimentary sink and thus pave the way towards quantitative interpretation of compound specific hydrogen isotope ratios for paleohydrological reconstructions.

Description: Wildfire occurrence is influenced by climate, vegetation and human activities. A key challenge for understanding the risk of fires is quantifying the mediating effect of vegetation on fire regimes. Here, we explore the relative importance of Holocene land cover, land use, dominant functional forest type, and climate dynamics on biomass burning in temperate and boreo-nemoral regions of central and eastern Europe over the past 12 kyr. We used an extensive data set of Holocene pollen and sedimentary charcoal records, in combination with climate simulations and statistical modelling. Biomass burning was highest during the early Holocene and lowest during the mid-Holocene in all three ecoregions (Atlantic, continental and boreo-nemoral) but was more spatially variable over the past 3–4 kyr. Although climate explained a significant variance in biomass burning during the early Holocene, tree cover was consistently the highest predictor of past biomass burning over the past 8 kyr. In temperate forests, biomass burning was high at ∼45 % tree cover and decreased to a minimum at between 60 % and 70 % tree cover. In needleleaf-dominated forests, biomass burning was highest at ∼ 60 %–65 % tree cover and steeply declined at 〉65 % tree cover. Biomass burning also increased when arable lands and grasslands reached ∼ 15 %–20 %, although this relationship was variable depending on land use practice via ignition sources, fuel type and quantities. Higher tree cover reduced the amount of solar radiation reaching the forest floor and could provide moister, more wind-protected microclimates underneath canopies, thereby decreasing fuel flammability. Tree cover at which biomass burning increased appears to be driven by warmer and drier summer conditions during the early Holocene and by increasing human influence on land cover during the late Holocene. We suggest that long-term fire hazard may be effectively reduced through land cover management, given that land cover has controlled fire regimes under the dynamic climates of the Holocene.