Changes in mean temperature of the coldest (T c) and warmest month (T w), annual precipitation (P... more Changes in mean temperature of the coldest (T c) and warmest month (T w), annual precipitation (P ann) and moisture index (α) were reconstructed from a continuous pollen record from Lake Baikal, Russia. The pollen sequence CON01-603-2 (53°57′N, 108°54′E) was recovered from a 386 m water depth in the Continent Ridge and dated to ca. 130–114.8 ky BP. This time interval covers the complete last interglacial (LI), corresponding to MIS 5e. Results of pollen analysis and pollen-based quantitative biome reconstruction show pronounced changes in the regional vegetation throughout the record. Shrubby tundra covered the area at the beginning of MIS 5e (ca. 130–128 ky), consistent with the end of the Middle Pleistocene glaciation. The late glacial climate was characterised by low winter and summer temperatures (T c ~ −38 to −35°C and T w~11–13°C) and low annual precipitation (P ann~300 mm). However, the wide spread of tundra vegetation suggests rather moist environments associated with low temperatures and evaporation (reconstructed α~1). Tundra was replaced by boreal conifer forest (taiga) by ca. 128 ky BP, suggesting a transition to the interglacial. Taiga-dominant phase lasted until ca. 117.4 ky BP, e.g. about 10 ky. The most favourable climate conditions occurred during the first half of the LI. P ann reached 500 mm soon after 128 ky BP. However, temperature changed more gradually. Maximum values of T c ~ −20°C and T w~16–17°C are reconstructed from about 126 ky BP. Conditions became gradually colder after ca. 121 ky BP. T c dropped to ~ −27°C and T w to ~15°C by 119.5 ky BP. The reconstructed increase in continentality was accompanied by a decrease in P ann to ~400–420 mm. However, the climate was still humid enough (α~0.9) to support growth of boreal evergreen conifers. A sharp turn towards a dry climate is reconstructed after ca. 118 ky BP, causing retreat of forest and spread of cool grass-shrub communities. Cool steppe dominated the vegetation in the area between ca. 117.5 ky and 114.8 ky BP, suggesting the end of the interglacial and transition to the last glacial (MIS 5d). Shift to the new glaciation was characterised by cool and very dry conditions with T c ~ −28 to −30°C, T w~14–15°C, P ann~250 mm and α~0.5.
Pollen stratigraphies are the most spatially extensive data available for the reconstruction of p... more Pollen stratigraphies are the most spatially extensive data available for the reconstruction of past land-cover change. Detailed knowledge of past land-cover is becoming increasingly important to evaluate the present trends in, and drivers of, vegetation composition. The European Pollen Database (EPD) was established in the late 1980s and developed in the early 1990s to provide a structure for archiving, exchanging, and analysing Quaternary pollen data from Europe. It provides a forum for scientists to meet and engage in collaborative investigations or data analysis. In May 2007 several EPD support groups were developed to assist in the task of maintaining and updating the database. The mapping and data accuracy work group (MADCAP) aims to produce an atlas of past plant distributions as detected by pollen analyses in Europe, in order to meet the growing need for this data from palaeoecologists and the wider scientific community. Due to data handling problems in the past, a significant number of EPD datasets have errors. The initial task of the work group, therefore, was a systematic review of pollen sequences, in order to identify and correct errors. The EPD currently (January 2009) archives 1,032 pollen sequences, of which 668 have age-depth models that allow chronological comparison. Many errors have been identified and corrected, or flagged for users, most notably errors in the pollen count data. The application of spatial analyses to pollen data is related to the number of data points that are available for analysis. We therefore take this opportunity to encourage the submission of pollen analytical results to the EPD or other relevant pollen databases. Only in this way will the scientific community be able to gain a better understanding of past vegetation dynamics.
New detailed biome reconstructions are proposed in East Africa from modern pollen data derived fr... more New detailed biome reconstructions are proposed in East Africa from modern pollen data derived from 150 sites located in northern Kenya (40 sites), north-western Uganda (51 sites) and southern Tanzania (59 new sites presented as pollen diagram), which are representative of the major vegetation associations occurring in seven phytogeographical regions, mosaics or centres of endemism. We use the standard biomisation method previously published for the African continent, but we reconsider the taxa assignment to plant functional types. We include in this approach all identified taxa (408) except aquatics, ferns and exotic taxa. The method is validated by comparison with local vegetation data and we show that 124 (82.6%) sites are assigned to the correct biome and that for all the biomes under investigation, the number of correct assignments always exceeds the number of incorrect ones. When an incorrect biome reconstruction occurs, mainly toward drier biomes, this is generally linked to the local open/degraded structure of the original vegetation or to the occurrence of a mosaic of open/closed vegetation. In turn, most of the reconstructions of more humid/closed biomes than the corresponding local vegetation (8.6%) remain unexplained. A comparison of our reconstructed biomes with the main East African vegetation types of White's map indicates that 121 (80.6%) sites are assigned to the correct biomes. However, the majority of sites are incorrectly reconstructed compared to Olson and IGBP maps from satellite data, mainly due to incorrect allocation of the land cover classes compared to the potential vegetation. The application of this method to our pollen data set demonstrates that modern pollen assemblages can successfully reconstruct the main modern East African vegetation types.
The Last Glacial Maximum has been one of the first foci of the Paleoclimate Modelling Intercompar... more The Last Glacial Maximum has been one of the first foci of the Paleoclimate Modelling Intercomparison Project (PMIP). During its first phase, the results of 17 atmosphere general circulation models were compared to paleoclimate reconstructions. One of the largest discrepancies in the simulations was the systematic underestimation, by at least 10°C, of the winter cooling over Europe and the Mediterranean region observed in the pollen-based reconstructions. In this paper, we investigate the progress achieved to reduce this inconsistency through a large modelling effort and improved temperature reconstructions. We show that increased model spatial resolution does not significantly increase the simulated LGM winter cooling. Further, neither the inclusion of a vegetation cover compatible with the LGM climate, nor the interactions with the oceans simulated by the atmosphere-ocean general circulation models run in the second phase of PMIP result in a better agreement between models and data. Accounting for changes in interannual variability in the interpretation of the pollen data does not result in a reduction of the reconstructed cooling. The largest recent improvement in the model-data comparison has instead arisen from a new climate reconstruction based on inverse vegetation modelling, which explicitly accounts for the CO2 decrease at LGM and which substantially reduces the LGM winter cooling reconstructed from pollen assemblages. As a result, the simulated and observed LGM winter cooling over Western Europe and the Mediterranean area are now in much better agreement.
In order to improve the reliability of climate reconstruction, especially the climatologies outsi... more In order to improve the reliability of climate reconstruction, especially the climatologies outside the modern observed climate space, an improved inverse vegetation model using a recent version of BIOME4 has been designed to quantitatively reconstruct past climates, based on pollen biome scores from the BIOME6000 project. The method has been validated with surface pollen spectra from Eurasia and Africa, and applied to palaeoclimate reconstruction. At 6 cal ka BP (calendar years), the climate was generally wetter than today in southern Europe and northern Africa, especially in the summer. Winter temperatures were higher (1–5°C) than present in southern Scandinavia, northeastern Europe, and southern Africa, but cooler in southern Eurasia and in tropical Africa, especially in Mediterranean regions. Summer temperatures were generally higher than today in most of Eurasia and Africa, with a significant warming from ∼3 to 5°C over northwestern and southern Europe, southern Africa, and eastern Africa. In contrast, summers were 1–3°C cooler than present in the Mediterranean lowlands and in a band from the eastern Black Sea to Siberia. At 21 cal ka BP, a marked hydrological change can be seen in the tropical zone, where annual precipitation was ∼200–1,000 mm/year lower than today in equatorial East Africa compared to the present. A robust inverse relationship is shown between precipitation change and elevation in Africa. This relationship indicates that precipitation likely had an important role in controlling equilibrium-line altitudes (ELA) changes in the tropics during the LGM period. In Eurasia, hydrological decreases follow a longitudinal gradient from Europe to Siberia. Winter temperatures were ∼10–17°C lower than today in Eurasia with a more significant decrease in northern regions. In Africa, winter temperature was ∼10–15°C lower than present in the south, while it was only reduced by ∼0–3°C in the tropical zone. Comparison of palaeoclimate reconstructions using LGM and modern CO2 concentrations reveals that the effect of CO2 on pollen-based LGM reconstructions differs by vegetation type. Reconstructions for pollen sites in steppic vegetation in Europe show warmer winter temperatures under LGM CO2 concentrations than under modern concentrations, and reconstructions for sites in xerophytic woods/scrub in tropical high altitude regions of Africa are wetter for LGM CO2 concentrations than for modern concentrations, because our reconstructions account for decreased plant water use efficiency.
The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes... more The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years. Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this remote basin during colonial and post-colonial times.
Changes in mean temperature of the coldest (T c) and warmest month (T w), annual precipitation (P... more Changes in mean temperature of the coldest (T c) and warmest month (T w), annual precipitation (P ann) and moisture index (α) were reconstructed from a continuous pollen record from Lake Baikal, Russia. The pollen sequence CON01-603-2 (53°57′N, 108°54′E) was recovered from a 386 m water depth in the Continent Ridge and dated to ca. 130–114.8 ky BP. This time interval covers the complete last interglacial (LI), corresponding to MIS 5e. Results of pollen analysis and pollen-based quantitative biome reconstruction show pronounced changes in the regional vegetation throughout the record. Shrubby tundra covered the area at the beginning of MIS 5e (ca. 130–128 ky), consistent with the end of the Middle Pleistocene glaciation. The late glacial climate was characterised by low winter and summer temperatures (T c ~ −38 to −35°C and T w~11–13°C) and low annual precipitation (P ann~300 mm). However, the wide spread of tundra vegetation suggests rather moist environments associated with low temperatures and evaporation (reconstructed α~1). Tundra was replaced by boreal conifer forest (taiga) by ca. 128 ky BP, suggesting a transition to the interglacial. Taiga-dominant phase lasted until ca. 117.4 ky BP, e.g. about 10 ky. The most favourable climate conditions occurred during the first half of the LI. P ann reached 500 mm soon after 128 ky BP. However, temperature changed more gradually. Maximum values of T c ~ −20°C and T w~16–17°C are reconstructed from about 126 ky BP. Conditions became gradually colder after ca. 121 ky BP. T c dropped to ~ −27°C and T w to ~15°C by 119.5 ky BP. The reconstructed increase in continentality was accompanied by a decrease in P ann to ~400–420 mm. However, the climate was still humid enough (α~0.9) to support growth of boreal evergreen conifers. A sharp turn towards a dry climate is reconstructed after ca. 118 ky BP, causing retreat of forest and spread of cool grass-shrub communities. Cool steppe dominated the vegetation in the area between ca. 117.5 ky and 114.8 ky BP, suggesting the end of the interglacial and transition to the last glacial (MIS 5d). Shift to the new glaciation was characterised by cool and very dry conditions with T c ~ −28 to −30°C, T w~14–15°C, P ann~250 mm and α~0.5.
Pollen stratigraphies are the most spatially extensive data available for the reconstruction of p... more Pollen stratigraphies are the most spatially extensive data available for the reconstruction of past land-cover change. Detailed knowledge of past land-cover is becoming increasingly important to evaluate the present trends in, and drivers of, vegetation composition. The European Pollen Database (EPD) was established in the late 1980s and developed in the early 1990s to provide a structure for archiving, exchanging, and analysing Quaternary pollen data from Europe. It provides a forum for scientists to meet and engage in collaborative investigations or data analysis. In May 2007 several EPD support groups were developed to assist in the task of maintaining and updating the database. The mapping and data accuracy work group (MADCAP) aims to produce an atlas of past plant distributions as detected by pollen analyses in Europe, in order to meet the growing need for this data from palaeoecologists and the wider scientific community. Due to data handling problems in the past, a significant number of EPD datasets have errors. The initial task of the work group, therefore, was a systematic review of pollen sequences, in order to identify and correct errors. The EPD currently (January 2009) archives 1,032 pollen sequences, of which 668 have age-depth models that allow chronological comparison. Many errors have been identified and corrected, or flagged for users, most notably errors in the pollen count data. The application of spatial analyses to pollen data is related to the number of data points that are available for analysis. We therefore take this opportunity to encourage the submission of pollen analytical results to the EPD or other relevant pollen databases. Only in this way will the scientific community be able to gain a better understanding of past vegetation dynamics.
New detailed biome reconstructions are proposed in East Africa from modern pollen data derived fr... more New detailed biome reconstructions are proposed in East Africa from modern pollen data derived from 150 sites located in northern Kenya (40 sites), north-western Uganda (51 sites) and southern Tanzania (59 new sites presented as pollen diagram), which are representative of the major vegetation associations occurring in seven phytogeographical regions, mosaics or centres of endemism. We use the standard biomisation method previously published for the African continent, but we reconsider the taxa assignment to plant functional types. We include in this approach all identified taxa (408) except aquatics, ferns and exotic taxa. The method is validated by comparison with local vegetation data and we show that 124 (82.6%) sites are assigned to the correct biome and that for all the biomes under investigation, the number of correct assignments always exceeds the number of incorrect ones. When an incorrect biome reconstruction occurs, mainly toward drier biomes, this is generally linked to the local open/degraded structure of the original vegetation or to the occurrence of a mosaic of open/closed vegetation. In turn, most of the reconstructions of more humid/closed biomes than the corresponding local vegetation (8.6%) remain unexplained. A comparison of our reconstructed biomes with the main East African vegetation types of White's map indicates that 121 (80.6%) sites are assigned to the correct biomes. However, the majority of sites are incorrectly reconstructed compared to Olson and IGBP maps from satellite data, mainly due to incorrect allocation of the land cover classes compared to the potential vegetation. The application of this method to our pollen data set demonstrates that modern pollen assemblages can successfully reconstruct the main modern East African vegetation types.
The Last Glacial Maximum has been one of the first foci of the Paleoclimate Modelling Intercompar... more The Last Glacial Maximum has been one of the first foci of the Paleoclimate Modelling Intercomparison Project (PMIP). During its first phase, the results of 17 atmosphere general circulation models were compared to paleoclimate reconstructions. One of the largest discrepancies in the simulations was the systematic underestimation, by at least 10°C, of the winter cooling over Europe and the Mediterranean region observed in the pollen-based reconstructions. In this paper, we investigate the progress achieved to reduce this inconsistency through a large modelling effort and improved temperature reconstructions. We show that increased model spatial resolution does not significantly increase the simulated LGM winter cooling. Further, neither the inclusion of a vegetation cover compatible with the LGM climate, nor the interactions with the oceans simulated by the atmosphere-ocean general circulation models run in the second phase of PMIP result in a better agreement between models and data. Accounting for changes in interannual variability in the interpretation of the pollen data does not result in a reduction of the reconstructed cooling. The largest recent improvement in the model-data comparison has instead arisen from a new climate reconstruction based on inverse vegetation modelling, which explicitly accounts for the CO2 decrease at LGM and which substantially reduces the LGM winter cooling reconstructed from pollen assemblages. As a result, the simulated and observed LGM winter cooling over Western Europe and the Mediterranean area are now in much better agreement.
In order to improve the reliability of climate reconstruction, especially the climatologies outsi... more In order to improve the reliability of climate reconstruction, especially the climatologies outside the modern observed climate space, an improved inverse vegetation model using a recent version of BIOME4 has been designed to quantitatively reconstruct past climates, based on pollen biome scores from the BIOME6000 project. The method has been validated with surface pollen spectra from Eurasia and Africa, and applied to palaeoclimate reconstruction. At 6 cal ka BP (calendar years), the climate was generally wetter than today in southern Europe and northern Africa, especially in the summer. Winter temperatures were higher (1–5°C) than present in southern Scandinavia, northeastern Europe, and southern Africa, but cooler in southern Eurasia and in tropical Africa, especially in Mediterranean regions. Summer temperatures were generally higher than today in most of Eurasia and Africa, with a significant warming from ∼3 to 5°C over northwestern and southern Europe, southern Africa, and eastern Africa. In contrast, summers were 1–3°C cooler than present in the Mediterranean lowlands and in a band from the eastern Black Sea to Siberia. At 21 cal ka BP, a marked hydrological change can be seen in the tropical zone, where annual precipitation was ∼200–1,000 mm/year lower than today in equatorial East Africa compared to the present. A robust inverse relationship is shown between precipitation change and elevation in Africa. This relationship indicates that precipitation likely had an important role in controlling equilibrium-line altitudes (ELA) changes in the tropics during the LGM period. In Eurasia, hydrological decreases follow a longitudinal gradient from Europe to Siberia. Winter temperatures were ∼10–17°C lower than today in Eurasia with a more significant decrease in northern regions. In Africa, winter temperature was ∼10–15°C lower than present in the south, while it was only reduced by ∼0–3°C in the tropical zone. Comparison of palaeoclimate reconstructions using LGM and modern CO2 concentrations reveals that the effect of CO2 on pollen-based LGM reconstructions differs by vegetation type. Reconstructions for pollen sites in steppic vegetation in Europe show warmer winter temperatures under LGM CO2 concentrations than under modern concentrations, and reconstructions for sites in xerophytic woods/scrub in tropical high altitude regions of Africa are wetter for LGM CO2 concentrations than for modern concentrations, because our reconstructions account for decreased plant water use efficiency.
The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes... more The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years. Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this remote basin during colonial and post-colonial times.
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