In this study, we present lipid biomarker and palynological data for a sediment core from Lake Dojran (Macedonia/Greece), which covers the entire Holocene period. We analyzed vascular plant-derived n-alkanes, combustion-derived polycyclic aromatic hydrocarbons (PAHs), fecal steroids, and bacterial and archaeal glycerol dialkyl glycerol tetraether (GDGT) lipids in concert with microcharcoal and pollen assemblages to reconstruct climatic, environmental, and human impact in the Dojran catchment and the greater Dojran area. Overall, our results suggest a relationship between anthropogenic activity and environmental/climatic change since increased human impact corresponds to phases of higher humidity and high lake levels at Lake Dojran. During the early Holocene, the record reveals increasing temperatures and humidity and concurrent increasing vegetation cover and runoff/soil erosion, respectively. Following a thermal maximum during the middle early Holocene, temperatures decrease gradually until present. The middle-Holocene at Lake Dojran is characterized by relatively stable environmental conditions followed by greater climatic instability and strong anthropogenic overprint during the late-Holocene. The fecal stanol record reveals phases of increased human impact during the early Bronze Age, the late Bonze/early Iron Age, and the Middle Ages. A phase of low stanol and PAH concentrations from the late Iron Age until the early Middle Ages is either related to ecosystem changes and/or changes in settlement pattern since concurrent pollen data indicate intensified land use. Human impact re-intensified during the Middle Ages with some variability probably related to climatic variations of the ‘Medieval Warm Period’ and the ‘Little Ice Age’.
Northern Africa’s past climate is characterized by a prolonged humid period known as the African Humid Period (AHP), giving origin to the “Green Sahara” and supporting human settlements into areas that are now desert. The spatial and temporal extent of climate change associated with the AHP is, however, subject to ongoing debate. Uncertainties arise from the complex nature of African climate, which is controlled by the strength and interactions of different monsoonal systems, resulting in meridional shifts in rainfall belts and zonal movements of the Congo Air Boundary. Here, we examine a ∼12,500-years record of hydroclimate variability from Lake Dendi located in the Ethiopian highlands based on a combination of plant-wax-specific hydrogen (δD) and carbon (δ 13 C) isotopes. In addition, pollen data from the same sediment core are used to investigate the response of the regional vegetation to changing climate. Our δD record indicates high precipitation during peak AHP (ca. 10 to 8 ka BP) followed by a gradual transition toward a drier late Holocene climate. Likewise, vegetation cover changed from predominant grassland toward an arid montane forest dominated by Juniperus and Podocarpus accompanied by a general reduction of understory grasses. This trend is corroborated by δ 13 C values pointing to an increased contribution of C 3 plants during the mid-to late Holocene. Peak aridity occurred around 2 ka BP, followed by a return to a generally wetter climate possibly linked to enhanced Indian Ocean Monsoon strength. During the last millennium, increased anthropogenic activity, i.e., deforestation and agriculture is indicated by the pollen data, in agreement with intensified human impact recorded for the region. The magnitude of δD change (40‰) between peak wet conditions and late Holocene aridity is in line with other regional δD records of East Africa influenced by the CAB. The timing and pace of aridification parallels those of African and Indian monsoon records indicating a gradual response to local insolation change. Our new record combining plant-wax δD and δ 13 C values with pollen highlights the sensitive responses of the regional vegetation to precipitation changes in the Ethiopian highlands.
Atmospheric circulation over the North Atlantic has undergone significant fluctuations during the Holocene. To better constrain these changes and their impacts on the Fennoscandian subarctic, we investigated molecular and inorganic proxies as well as plant wax D/H isotopes (δ D C 28 ) in a Holocene sedimentary record from Lake Torneträsk (Sweden). These data indicate a thermal maximum c . 8100 to 6300 cal. a BP with reduced soil organic matter input, followed by a long‐term cooling trend with increasing soil erosion. δD data suggest a stable atmospheric circulation with predominance of westerly flow and North Atlantic moisture sourcing during the Early and Middle Holocene. A substantial depletion in δD followed by increased flood frequency starting at c . 5300 cal. a BP and intensifying c . 1500 cal. a BP suggests a reorganization of the atmospheric circulation from zonal towards meridional flow with predominantly Arctic Ocean and Baltic Sea moisture sourcing.
Abstract. A new high-resolution pollen and NPP (non-pollen palynomorph) analysis has been performed on the sediments of Lake Dojran, a transboundary lake located at the border between Greece and the former Yugoslav Republic of Macedonia (FYROM). The sequence covers the last 12 500 years and provides information on the vegetational dynamics of the Late Glacial and Holocene for the southern Balkans. Robust age model, sedimentological diatom, and biomarker analyses published previously have been the base for a multi-perspective interpretation of the new palynological data. Pollen analysis revealed that the Late Glacial is characterized by steppic taxa with prevailing Amaranthaceae, Artemisia and Poaceae. The arboreal vegetation starts to rise after 11 500 yr BP, taking a couple of millennia to be definitively attested. Holocene vegetation is characterized by the dominance of mesophilous plants. The Quercus robur type and Pinus are the most abundant taxa, followed by the Quercus cerris type, the Quercus ilex type and Ostrya–Carpinus orientalis. The first attestation of human presence can be presumed at 5000 yr BP from the contemporary presence of cereals, Juglans and Rumex. A drop in both pollen concentration and influx together with a δ18Ocarb shift indicates increasing aridity and precedes clear and continuous human signs since 4000 yr BP. Also, a correlation between Pediastrum boryanum and fecal stanol suggests that the increase in nutrients in the water is related to human presence and pasture. An undoubted expansion of human-related plants occurs since 2600 yr BP when cereals, arboreal cultivated and other synanthropic non-cultivated taxa are found. A strong reduction in arboreal vegetation occurred at 2000 yr BP, when the Roman Empire impacted a landscape undergoing climate dryness in the whole Mediterranean area. In recent centuries the human impact still remains high but spots of natural vegetation are preserved. The Lake Dojran multi-proxy analysis including pollen data provides clear evidence of the importance of this approach in paleoenvironmental reconstruction. Cross-interpretation of several proxies allows us to comprehend past vegetation dynamics and human impact in the southern Balkans.
Greater understanding of Holocene climatic and environmental variability and processes, as well as about feedback and forcing mechanisms of the climate system is crucial for the assessment of both natural and anthropogenic future climate and environmental changes. Compared to prior epochs in earth’s history, the climate of the Holocene is traditionally regarded as relatively stable. However, Holocene climate also showed significant fluctuations although perturbation were smaller in magnitude compared to Pleistocene. These fluctuations can be assessed by organic geochemical molecular and isotope analyses of lake sedimentary organic matter (OM) that have the potential to reveal a variety of information regarding physical, chemical and biological changes and processes of the lake, its environment, and the climate. Therefore, within the scope of this thesis, sedimentary archives from selected lakes from the Sub-Artic (Lake Tornetrask), the Mediterranean (Lake Dojran), and the African tropics (Lake Dendi) were analyzed using various analytical methods including the analysis of lipid biomarker and compound specific leaf wax stable isotopes, as well as palynological, microcharcoal, and inorganic sedimentological analyses. All three lakes are situated in key regions for the understanding of northern hemispheric Holocene climate variability and natural/anthropogenic forcing and feedback mechanisms:
To constrain changes in atmospheric circulation patterns and their effects on the environment in the Fennoscandian sub-arctic, lipid biomarker, inorganic proxies, and compound specific δD analysis are applied to a Holocene sedimentary record from Lake Tornetrask (NW Sweden). Owing to its climate being influenced by both the North Atlantic and the polar frontal zone, northern Fennoscandia can be regarded as a key region to better understand the regional expression and potential threshold effects of insolation-forced migrations of atmospheric circulation systems. The results indicate a non-linear reorganization of the atmospheric circulation expressed as a change from zonal towards more meridional flow starting at ~4,000 and intensifying ~2,000 cal yrs BP.
For the reconstruction of the climatic, environmental, and human impact on the southern Balkan Peninsula lipid biomarker, microcharcoal, and pollen analyses are applied to a Holocene sedimentary record from Lake Dojran (Macedonia/Greece). The southern Balkan region played a key role in the early migration of the Neolithic lifestyle to Central Europe and is thus very suitable for studies of human-environment forcing and feedback mechanisms. The results suggest a relationship between anthropogenic activity and centennial to millennial scale environmental/climatic changes, since increased human impact corresponds to phases of higher humidity and high lake levels at Lake Dojran.
To detect changes in atmospheric circulation, hydrology, and vegetation in East Africa, associated with the African Humid Period (AHP), lipid biomarker and compound specific δD and δ13C analysis are applied to sedimentary OM from Lake Dendi (Ethiopia). Due to its location in proximity of the Congo Air Boundary (CAB) and the Intertropical Convergence Zone (ITCZ), the Dendi region can play a crucial role in the understanding of past changes in atmospheric circulation pattern of the tropical regions. The results indicate a rapid re-strengthening of the monsoonal circulation in the Early Holocene followed by Peak AHP conditions between ~9,800 yrs cal BP and ~8,000 yrs cal BP. Subsequently a moderate decrease in precipitation and a shift in moisture sources due to weakening monsoonal systems and associated shifts of the ITCZ and the CAB have been detected.
Together, the lakes datasets suggest a thermal maximum and a northernmost position of the atmospheric circulation systems in the Early Holocene followed by a long-term trend of decreasing temperatures and environmental changes in accordance with decreasing NH summer insolation. Despite some differences in nature and timing, all tree records further indicate a southwards migration and weakening of NH atmospheric circulation systems over the course of the Holocene with significant phases of climatic/environmental changes around 4,500 yrs cal BP and 2,000 yrs.
Abstract. A new high-resolution pollen and NPPs (Non-Pollen Palynomorphs) analysis has been performed on the sediments of Lake Dojran, a transboundary lake located at the border between Greece and Former Yugoslavian Republic of Macedonia (FYROM). The sequence covers the last 12 500 years and provides information on vegetational dynamics of the Late Glacial and Holocene for southern Balkans. A robust age-model, sedimentological, diatom, and biomarker analyses published previously have been the base 5 for a multi-perspective interpretation of the new palynological data. Pollen analysis revealed that the Late Glacial is characterized by steppic taxa with prevailing Amaranthaceae, Artemisia and Poaceae. The arboreal vegetation starts to rise after 11 500 yr BP, taking a couple of millennia to be definitively attested. Holocene vegetation is characterized by the dominance of mesophilous plants. Quercus robur type and Pinus are the most abundant taxa followed by Quercus cerris type, Quercus ilex type and Ostrya/Carpinus orientalis. The first attestation of human presence can be presumed at 5000 yr BP for the 10 contemporary presence of cereals, Juglans and Rumex. A drop of both pollen concentration and influx together with a δ18Ocarb shift indicates increasing aridity and precedes clear and continuous human signs since 4000 yr BP. Also a correlation between Pediastrum boryanum and fecal stanol suggests that the increase of nutrient in the water is related to human presence and pasture. An undoubted expansion of human-related plants occurs since 2600 yr BP when cereals, arboreal cultivated and other synanthropic non-cultivated taxa are found. A strong reduction in arboreal vegetation occurred at 2000 yr BP, when the strong 15 Roman Empire impacted on a landscape undergoing climate dryness in the whole Mediterranean area. In recent centuries the human impact still remains high but spots of natural vegetation are preserved. The Lake Dojran multi-proxy analysis including pollen data provide a valuable contribution to the palaeoenvironmental reconstruction and the comprehension of the past vegetation dynamics of southern Balkans.
