Showing papers in "The Holocene in 2015"

Mid-Holocene bottleneck for central European dry grasslands: Did steppe survive the forest optimum in northern Bohemia, Czech Republic?:

Abstract: Revisiting the classical Gradmann’s ‘steppe theory’ for central Europe, we examine whether the early Holocene steppe habitats survived the critical period of maximum Holocene afforestation: the mid-Holocene bottleneck. Despite the undisputable fact that afforestation was a dominant ecological factor in this period, our parallel analyses of pollen and molluscs from sedimentary sequences discovered in the dry lowland area of northern Bohemia, Czech Republic (Zahaji and Suchý potok sites, lower Ohře area) provide strong evidence for uninterrupted local occurrence of steppe grasslands throughout the Holocene. At the onset of the Neolithic agriculture, this area was covered by forest-steppe. Analogously to the present forest-steppe landscapes of eastern Europe and south-western Siberia, dry areas of northern Bohemia were dominated by open-canopy pine–birch forests that enabled continuous survival of many light-demanding plant species from the late Glacial and early Holocene to the Neolithic. Later on, anthropogenic deforestation and livestock grazing created a semi-natural steppe. Our data suggest that this secondary steppe can be viewed as a direct continuation of the late Pleistocene and early Holocene natural steppe rather than a purely cultural steppe developed only after deforestation of a continuously forested mid-Holocene landscape by humans. At the same time, we provide evidence supporting Gradmann’s ‘steppe theory’, assuming that in central Europe, Neolithic farming started in those areas that were not completely forested but contained remnants of natural steppes. This finding has important implications for the interpretation of present biodiversity patterns in central Europe.

Amazonia and the Anthropocene: What was the spatial extent and intensity of human landscape modification in the Amazon Basin at the end of prehistory?

Abstract: The nature and spatial scale of prehistoric human landscape modifications in Amazonia are enduring questions. Original conceptions of the issues by archaeologists published more than 40 years ago posited little human influence because of putative environmental constraints. Empirical data accumulated more recently demonstrated dense, permanent settlements along major watercourses of the central and southern Amazon, and profound landscape alterations in seasonally flooded savanna regions of Bolivia. These results led some investigators to propose that most to all of Amazonia was heavily populated and modified before European arrival, and that prehistoric fires and forest clearing were of such a massive scale that post-Columbian reforestation was a significant contributor to decreasing atmospheric CO2 levels and the onset of the ‘Little Ice Age’. Recent data generated from investigations of soils sampled from underneath standing terra firme forests in parts of the western Amazon indicate ephemeral ancient hu...

Palaeoenvironmental changes in Central Europe (NE Poland) during the last 6200 years reconstructed from a high-resolution multi-proxy peat archive

Abstract: Peat deposits from an ombrotrophic bog (north-eastern Poland) were analysed to reconstruct peatland development and environmental changes. This paper presents reconstructions of hydrological changes and plant succession over the last 6000 years. The methods included the high-resolution analysis of plant macrofossils, pollen and testate amoebae, supported by radiocarbon dating. Three main phases were identified in the history of the bog and surrounding woodland vegetation: 4000–400 BC, 400 BC–AD 1700 and AD 1700–2011. Except for terrestrialisation and the fen-to-bog transition phase, the development of bog vegetation was mainly dependent on the climate until approximately AD 1700. The dominant taxon in Gązwa bog was Sphagnum fuscum/rubellum. Woodland development was significantly affected by human activity at several time periods. The most visible human activity, manifested by the decline of deciduous species, occurred ca. 350 BC, ca. AD 250, ca. AD 1350 and after AD 1700. These events correspond to phases of human settlement in the area. During 400–300 BC, the decline of deciduous trees, primarily Carpinus, coincided with an increase in indicators of human activity and fire frequency. At ca. AD 200, Carpinus and Tilia abundance decreased, corresponding to an increased importance of cereals (Secale and Triticum). Since ca. AD 1350, the impact of Teutonic settlement is apparent, and after AD 1700, deciduous forests largely disappeared.

A high-precision age estimate of the Holocene Plinian eruption of Mount Mazama, Oregon, USA

Abstract: The climactic eruption of Mount Mazama in Oregon, North America, resulted in the deposition of the most widespread Holocene tephra deposit in the conterminous United States and south-western Canada. The tephra forms an isochronous marker horizon for palaeoenvironmental, sedimentary and archaeological reconstructions, despite the current lack of a precise age estimate for the source eruption. Previous radiocarbon age estimates for the eruption have varied, and Greenland ice-core ages are in disagreement. For the Mazama tephra to be fully utilised in tephrochronology and palaeoenvironmental research, a refined (precise and accurate) age for the eruption is required. Here, we apply a meta-analysis of all previously published radiocarbon age estimations (n = 81), and perform Bayesian statistical modelling to this data set, to provide a refined age of 7682–7584 cal. yr BP (95.4% probability range). Although the depositional histories of the published ages vary, this estimate is consistent with that estimated from the GISP2 ice-core of 7627 ± 150 yr BP (Zdanowicz et al., 1999).

Rethinking the spread of agriculture to the Tibetan Plateau

Abstract: New data from the Tibetan Plateau allow us to understand how populations dealt with the challenges of moving crops into altitudinally constrained environments. Despite the interest in explaining the timing and the mechanisms via which agricultural products spread to the roof of the world, current models for the spread of agriculture to this region have been simplistic and the presence of crop domesticates is often straightforwardly interpreted as indicating the existence of an agricultural system at the site. This is largely due to a fundamental lack of understanding of where crops could be grown in prehistory on the Plateau. Although it has generally been assumed that moving agriculture into this area was challenging, little work has specifically addressed the constraints imposed on humans as they moved crops into this area. Employing an agro-ecological niche model, I formally model the constraints that were faced by humans as they moved a series of crops into the Tibetan Plateau between the 4th and 1st millennium cal. BC. Based on the results of this analysis, I argue that the end of the climatic optimum meant that millet agriculture was no longer a viable strategy in many parts of the Eastern Himalayas. The arrival of frost tolerant crops in the 2nd millennium BC provided new opportunities in the cooler post climatic optimum world. These models further reveal that sites that have been previously considered as engaged directly in agricultural production may have been more distantly connected to an agricultural lifestyle than previously thought.

Last Millennium hydro-climate variability in Central–Eastern Europe (Northern Carpathians, Romania)

Abstract: Proxy-based reconstructions of climate variability over the last millennium provide important insights for understanding current climate change within a long-term context. Past hydrological changes are particularly difficult to reconstruct, yet rainfall patterns and variability are among the most critical environmental variables. Ombrotrophic bogs, entirely dependent on water from precipitation and sensitive to changes in the balance between precipitation and evapotranspiration, are highly suitable for such hydro-climate reconstructions. We present a multi-proxy analysis (testate amoebae, plant macrofossils, stable carbon isotopes in Sphagnum, pollen, spores and macroscopic charcoal) from an ombrotrophic peat profile from the Rodna Mountains (northern Romania) to establish a quantitative record of hydro-climatic changes. We identify five main stages: wet surface mire conditions between AD 800 and 1150 and AD 1800 and 1950, and drying of the mire surface between AD 1300 and 1450, AD 1550 and 1750 and AD 1950 and 2012. Our multi-proxy reconstructions suggest that conditions during the Medieval Climate Anomaly (MCA) period (AD 900–1150) were considerably wetter than today, while during most of the ‘Little Ice Age’ (LIA; AD 1500–1850), they were dry. Mire surface conditions in the Rodna Mountains have dried markedly over the last 40 years mainly as a result of anthropogenic climate change approaching the driest conditions seen over the last 1000 years. There is a marked difference between current hydro-climatic conditions (dry mire) and those of the MCA (wet mire). This implies that for the study region, the MCA cannot provide analogous climatic conditions to the contemporary situation. Our reconstructions are in partial agreement with water table estimates elsewhere in central and eastern Europe but generally contrast with those from NW Europe, especially during LIA. We suggest that these distinctive regional differences result from fluctuations in large-scale atmospheric circulation, which determine the relative influences of continental and oceanic air masses.

Chironomid-inferred Holocene temperature changes in the South Carpathians (Romania)

Abstract: We present a Holocene summer air temperature reconstruction based on fossil chironomids from Lake Brazi (1740 m.a.s.l.), a shallow mountain lake in the South Carpathians. Summer air temperature rec...

Spatial and temporal distributions of major tree taxa in eastern continental Asia during the last 22,000 years

Abstract: This study investigates the spatial and temporal distributions of 14 key arboreal taxa and their driving forces during the last 22,000 calendar years before ad 1950 (kyr BP) using a taxonomically harmonized and temporally standardized fossil pollen dataset with a 500-year resolution from the eastern part of continental Asia. Logistic regression was used to estimate pollen abundance thresholds for vegetation occurrence (presence or dominance), based on modern pollen data and present ranges of 14 taxa in China. Our investigation reveals marked changes in spatial and temporal distributions of the major arboreal taxa. The thermophilous (Castanea, Castanopsis, Cyclobalanopsis, Fagus, Pterocarya) and eurythermal (Juglans, Quercus, Tilia, Ulmus) broadleaved tree taxa were restricted to the current tropical or subtropical areas of China during the Last Glacial Maximum (LGM) and spread northward since c. 14.5 kyr BP. Betula and conifer taxa (Abies, Picea, Pinus), in contrast, retained a wider distribution during the LGM and showed no distinct expansion direction during the Late Glacial. Since the late mid-Holocene, the abundance but not the spatial extent of most trees decreased. The changes in spatial and temporal distributions for the 14 taxa are a reflection of climate changes, in particular monsoonal moisture, and, in the late Holocene, human impact. The post-LGM expansion patterns in eastern continental China seem to be different from those reported for Europe and North America, for example, the westward spread for eurythermal broadleaved taxa.

Mid- to late-Holocene East Asian summer monsoon variability recorded in lacustrine sediments from Jingpo Lake, Northeastern China

Abstract: The Northeastern China involves complex interactions between the East Asian summer monsoon (EASM) circulation and the polar climate system, and plays a significant role as the bridge communicating ...

Impact of late-Holocene aridification trend, climate variability and geodynamic control on the environment from a coastal area in SW Spain:

Abstract: A detailed pollen analysis has been carried out on two sediment cores taken from a marsh area located in the Donana National Park, southwestern Spain. The studied sedimentary sequences contain a si...

Famine, migration and war: Comparison of climate change impacts and social responses in North China between the late Ming and late Qing dynasties

Abstract: The human-climate-ecosystem interactions in the past were valuable for today’s human beings who face the challenge of global change. The multi-proxy reconstruction of climate change impacts and soc...

Diachronous evolution of sea surface conditions in the Labrador Sea and Baffin Bay since the last deglaciation

Abstract: Assessing changes in sea surface conditions due to the effects of past freshwater outflow through Baffin Bay and Davis Strait to the Labrador Sea, hereafter referred to as the Baffin Bay corridor, ...

Stacking of discontinuous regional palaeoclimate records: Chironomid-based summer temperatures from the Alpine region:

Abstract: Since multi-site reconstructions are less affected by site-specific climatic effects and artefacts, regional palaeotemperature reconstructions based on a number of sites can provide more robust estimates of centennial- to millennial-scale temperature trends than individual, site-specific records. Furthermore, reconstructions based on multiple records are necessary for developing continuous climate records over time scales longer than covered by individual sequences. Here, we present a procedure for developing such reconstructions based on relatively short (centuries to millennia), discontinuously sampled records as are typically developed when using biotic proxies in lake sediments for temperature reconstruction. The approach includes an altitudinal correction of temperatures, an interpolation of individual records to equal time intervals, a stacking procedure for sections of the interval of interest that have the same records available, as well as a splicing procedure to link the individual stacked records into a continuous reconstruction. Variations in the final, stacked and spliced reconstruction are driven by variations in the individual records, whereas the absolute temperature values are determined by the stacked segment based on the largest number of records. With numerical simulations based on the NGRIP δ18O record, we demonstrate that the interpolation and stacking procedure provides an approximation of a smoothed palaeoclimate record if based on a sufficient number of discontinuously sampled records. Finally, we provide an example of a stacked and spliced palaeotemperature reconstruction 15000–90 calibrated 14C yr BP based on six chironomid records from the northern and central Swiss Alps and eastern France to discuss the potential and limitations of this approach.

Functional traits as a new approach for interpreting testate amoeba palaeo-records in peatlands and assessing the causes and consequences of past changes in species composition

Abstract: Subfossil remains of various groups of organisms preserved in peat and sediment archives are commonly used to infer past environmental changes using transfer functions based on species composition. However, the changes in community structure can also be explored using the functional trait approach. Investigation of functional traits in palaeoecological records can inform about the mechanisms through which abiotic variables such as temperature or moisture impact communities. Moreover, changes in functional traits provide information about changes in ecosystem functioning and can potentially lead to the reconstruction of past processes at the ecosystem scale. Here, we use five key functional traits of arcellinid testate amoebae (TAs), a group of protozoa that are key actors in the microbial foodwebs in peatlands. We apply this approach to the subfossil TA Holocene record of four geographically independent peatlands from Alaska, Switzerland, Poland and Russia. We found that species with larger shells were frequently eliminated from the communities most likely as a result of a switch towards drier conditions. However, when conditions were wetter, species with large shells and species with small shells could coexist because they differed in their trophic niche (i.e. preys). Our results show direct but site-specific links between TA trait data and the depth to water table and pH data inferred from TA species composition. This suggests that past environmental changes influenced both species composition and community function in these ecosystems. Overall, this study demonstrates that species- and trait-based approaches yield complementary information on past environmental changes. For instance, while taxonomic approaches reveal the changes in community composition over time, investigation of traits informs both on the causes and the consequences of these changes on ecosystem functioning.

Towards more robust chronologies of coastal progradation: Optically stimulated luminescence ages for the coastal plain at Moruya, south-eastern Australia

Abstract: Accurate chronologies are fundamental for detailed analysis of palaeoenvironmental conditions, archaeological reconstructions and investigations of Holocene coastal morphological changes. Chronolog...

The onset of the palaeoanthropocene in Iceland: Changes in complex natural systems

Abstract: This study was financially supported by the Carnegie Trust for the Universities of Scotland, the National Science Foundation of America (through grant 1202692 ‘Comparative Island Ecodynamics in the North Atlantic’ and grant 1249313 ‘Tephra layers and early warning signals for critical transitions’) and the Leverhulme Trust (Study Abroad Fellowship to AJD).

Using adjusted Blue Intensity data to attain high-quality summer temperature information: A case study from Central Scandinavia

Abstract: The inexpensive Blue Intensity proxy has been considered a complement or surrogate to maximum latewood density (MXD), but is associated with biases from differential staining between sapwood and heartwood and also between deadwood samples and living-wood samples that compromise centennial-scale information. Here, we show that, with some minor adjustments, ΔBlue Intensity (ΔBI) is comparable with MXD or ΔDensity (Δ = the difference or contrast between latewood and earlywood density) in dendroclimatological reconstructions of summer temperatures in the Central Scandinavian region, using Pinus sylvestris L. (Scots pine), on annual and multi-centennial timescales. By using ΔBI, this bias is significantly reduced, but the contrast between earlywood and latewood in BI is altered with degree of staining, while for density it is not. Darker deadwood samples have a reduced contrast compared with the lighter living-wood samples that make ΔBI and ΔDensity chronologies diverge. Here, we quantify this behaviour in BI ...

Climate and abrupt vegetation change in Northern Europe since the last deglaciation

Abstract: A long-standing question in palaeoecology has been to determine the importance of climate driving vegetation change since the last deglaciation. Here, we investigate the local-to-regional dynamics of vegetation change during the Lateglacial and the Holocene in Northern Europe. We extracted sites from the European Pollen Database and used the squared-chord distance (SCD) dissimilarity metric to identify time periods of high pollen assemblage turnover representing periods of abrupt vegetation change. In addition, a set of generalized additive mixed models were applied to investigate the underlying dynamics of two periods of higher rates of turnover: the Younger Dryas–early Holocene transition (YD-EH; 11.6–9.0 kyr) and early–middle Holocene (EMH; 9.0–6.0 kyr). Results revealed a high frequency of turnover events between 12.75–11.5, 10.75–11, 10.25–10, 7.75–7.25, 3.25–3.0 and 1.75–.25 kyr. Furthermore, there was a strong linear relationship between pollen assemblage turnover and large directional temperature ...

Evidences for centennial dry periods at ~3300 and ~2800 cal. yr BP from micro-facies analyses of the Dead Sea sediments

Abstract: Laminated lake sediments from the Dead Sea basin provide high-resolution records of climatic variability in the eastern Mediterranean region, which is especially sensitive to changing climatic cond...

Using multi-proxy palaeoecology to test a relict status of refugial populations of calcareous-fen species in the Western Carpathians

Abstract: The distribution pattern of relict and specialised species in calcareous fens was revealed to be non-stochastic, with ancient fens harbouring more of these species than younger ones. This phenomenon could be caused by long-lasting in situ survivals over millennia, but direct palaeoecological evidence is lacking. We addressed the question whether at least some ancient calcareous fens indeed retained open-fen patches throughout the Holocene, using a palaeoecological approach involving proxies with different taphonomies (pollen, vascular plants, bryophytes, molluscs). We identified three old fens in the Western Carpathians, where several postglacial relict species have recently been found, and we reconstructed their histories with respect to sedimentary processes, vegetation structure and dynamics of relict species. The development at all the sites started with a (semi)-open fen community dominated by sedges and brown mosses. The site with the highest recent number of relict species was reconstructed to harbour open patches continually since the late Glacial to the present, including the middle Holocene when open-fen patches were restricted. By contrast, at the site with the lowest recent number of relict species, a large sedimentary hiatus suggested peat mineralisation or erosion that prevented the survival of light-demanding species. At all the sites, characteristic snails of European Glacial periods occurred during fen initiation, but disappeared around the early/middle Holocene transition. The probability of a relict species being present in a modern fen community increases with fen age, but it also depends on the continual existence of open-fen patches and peat accumulation throughout the middle Holocene.

Pre-Columbian land use in the ring-ditch region of the Bolivian Amazon

Abstract: The nature and extent of pre-Columbian (pre-AD 1492) human impact in Amazonia is a contentious issue. The Bolivian Amazon has yielded some of the most impressive evidence for large and complex pre-Columbian societies in the Amazon basin, yet there remains relatively little data concerning the land use of these societies over time. Palaeoecology, when integrated with archaeological data, has the potential to fill these gaps in our knowledge. We present a 6000-year record of anthropogenic burning, agriculture and vegetation change, from an oxbow lake located adjacent to a pre-Columbian ring ditch in north-east Bolivia (13°15′44″S, 63°42′37″W). Human occupation around the lake site is inferred from pollen and phytoliths of maize (Zea mays L.) and macroscopic charcoal evidence of anthropogenic burning. First occupation around the lake was radiocarbon dated to ~2500 calibrated years before present (BP). The persistence of maize in the record from ~1850 BP suggests that it was an important crop grown in the rin...

Vegetation, climate and environmental history of the last 4500 years at lake Shkodra (Albania/Montenegro)

Abstract: Three parallel overlapping cores have been taken in the Albanian side of Lake Shkodra (Albania/Montenegro). The chronological frame of the record, spanning approximately the last 4500 years, has been assessed using four radiocarbon dates and four well-known tephra layers of Italian volcanoes. Multidisciplinary analyses turned out to be decisive to understand environmental, climatic changes and human impact. Here, we focus on palynology. The humidity at Shkodra was always enough to allow the developing of a luxuriant arboreal vegetation. The pollen percentage diagram does not record important changes in terrestrial plants percentages. Arboreal pollen (AP) shows only a rather slight decrease, with ‘natural forests’ replaced by intensive cultivation of chestnut and walnut in the last seven/eight centuries. The rather minimal changes in composition and dominance are because of the fact that the pollen rain comes from different vegetation belts, from the Mediterranean to the alpine one. Two major periods of hu...

Changes in Holocene climate and the intensity of Southern Hemisphere Westerly Winds based on a high-resolution palynological record from sub-Antarctic South Georgia

Abstract: Sub-Antarctic South Georgia is a key region for studying climate variability in the Southern Hemisphere, because of its position at the core of the Southern Hemisphere Westerly Wind belt and between the Antarctic Circumpolar Current and the Polar Frontal Zone. Here, we present a 5.8-m long high-resolution pollen record from Fan Lake on Annenkov Island dominated by local sub-polar vegetation, with Acaena and Poaceae being present throughout the last 7000 years. Palynological and sedimentological analyses revealed a warm late Holocene ‘climate optimum’ between 3790 and 2750 cal. yr BP, which was followed by a gradual transition to cool and wet conditions. This cooling was interrupted by slightly warmer environmental conditions between 1670 and 710 cal. yr BP that partly overlap with the Northern Hemisphere ‘Medieval Climate Anomaly’. Increases in non-native and long-distance pollen grains transported from South America (e.g. Nothofagus, Podocarpus) indicate that stronger Southern Hemisphere Westerly Winds over South Georgia possibly occurred during some ‘colder’ phases of the late Holocene, most notably between c. 2210 and 1670 cal. yr BP and after 710 cal. yr BP.

Radiocarbon dating of mangrove sediments to constrain Holocene relative sea-level change on Zanzibar in the southwest Indian Ocean

Abstract: Mangrove sedimentary deposits are sensitive to changes in sea level and can be used to reconstruct mid- to late Holocene sea-level fluctuations in intermediate and far-field locations, distant to t...

Lake Belau and its catchment (northern Germany): A key archive of environmental history in northern central Europe since the onset of agriculture:

Abstract: The environmental history since the onset of agriculture was reconstructed from sediments and soils in the catchment area of Lake Belau (Schleswig-Holstein, northern Germany). The established chron...

Late Holocene fire and vegetation reconstruction from the western Klamath Mountains, California, USA: A multi-disciplinary approach for examining potential human land-use impacts

Abstract: The influence of Native American land-use practices on vegetation composition and structure has long been a subject of significant debate. This is particularly true in portions of the western Unite...

Interpreting interfluvial landscape transformations in the pre-Columbian Amazon:

Abstract: Despite evidence for the protracted presence of humans in the Amazon Basin, its vast interfluvial habitats are frequently depicted as having survived until recently as ‘wild’ landscapes with neithe...

Early Holocene woodland vegetation and human impacts in the arid zone of the southern Levant

Abstract: Palynological archives dating from the Pleistocene–Holocene transition are scarce in the arid zone of the southern Levant. Anthracological remains (the carbonized residues of wood fuel use found in archaeological habitation sites) provide an alternative source of information about past vegetation. This paper discusses new and previously available anthracological datasets retrieved from excavated habitation sites in the southern Levant dating to the Pre-Pottery Neolithic (PPN) period. The available evidence indicates the existence of distinct arboreal floras growing in different ecological niches, which occupied areas that today are either treeless or very sparsely wooded. The anthracological data provide independent confirmation of the hypothesis that early Holocene climate in the southern Levant was significantly moister than at present. Clear North–South and East–West precipitation and associated woodland composition gradients are evidenced. Far from deducing widespread anthropogenic degradation of the ...

Multi-centennial scale SST and Indian summer monsoon precipitation variability since the mid-Holocene and its nonlinear response to solar activity

Abstract: Indian Summer Monsoon (ISM) shows a weak correlation with solar variability in the 20th century. However, such climatological observations on solar activity–monsoon relationship are very short and ...

Measuring the skill of variance-scaled climate reconstructions and a test for the capture of extremes:

Abstract: Climate reconstructions produced using regression, with a proxy as the independent variable, are inevitably biased towards the mean, exhibit reduced variance and underestimate extremes Scaling the