Tephra‐linked peat humification records from Irish ombrotrophic bogs question nature of solar forcing at 850 cal. yr BC

This paper investigates evidence for palaeoclimatic changes during the period ca. 1500–500 cal. yr BC through peat humification studies on seven Irish ombrotrophic bogs. The sites are well‐correlated by the identification of three mid‐first millennium BC tephras, which enable the humification records at specific points in time to be directly compared. Phases of temporarily increased wetness are suggested at ca. 1300–1250 cal. yr BC , ca. 1150–1050 cal. yr BC , ca. 940 cal. yr BC and ca. 740 cal. yr BC . The last of these is confirmed to be synchronous at five sites, suggesting external forcing on a regional scale. The timing of this wet‐shift is constrained by two closely dated tephras and is demonstrated to be distinct from the widely reported changes to cooler/wetter conditions associated with a solar minimum at 850–760 cal. yr BC , at which time the Irish sites appear instead to experience drier conditions. The results suggest the possibility of either non‐uniform responses to solar forcing in northwest Europe at this time, or the existence of unrelated climate events in the early first millennium BC . The findings caution against the correlation of loosely dated palaeoclimate data if the effects of forcing mechanisms are to be understood. Copyright © 2006 John Wiley & Sons, Ltd.     

Separating climatic and possible human impacts in the early Holocene: biotic response around the time of the 8200 cal. yr BP event

The early Holocene is characterised by rapid climate change events, which in the North Atlantic region are often associated with changes in thermohaline circulation. Superimposed on this in northwest Europe is localised evidence for human impact on the landscape, although separating climatic and anthropogenic mechanisms for environmental change is often difficult. Biotic and sedimentological evidence from a lacustrine sequence from the Inner Hebrides, Scotland, shows a considerable reduction in inferred local woodland centred upon 8250 cal. yr BP. These data correlate precisely with a distinctive rise in the charcoal:pollen ratio and hence suggest a possible Mesolithic human impact upon the vegetation around this time. A quantitative temperature reconstruction from chironomid analyses from the same sequence, supported by sedimentological data, indicates that the fall in arboreal pollen taxa occurred as climate warmed significantly during the early Holocene. This warming was followed by a significant cold event, with mean July temperatures reduced by 2°C, that lasted for at least 320 years ca. 7790–7470 cal. yr BP. Woodland recovered during this phase suggesting that the vegetation during the 8250 cal. yr BP interval was likely to have been responding to human activity, and not climate, and hence it is possible at specific sites to separate the influence of these key drivers of environmental change. Copyright © 2006 John Wiley & Sons, Ltd.     

Post‐glacial vegetation reconstruction and a possible 8200 cal. yr BP event from the low arctic of continental Nunavut,Canada

Climate models suggest that the global warming during the early to mid‐Holocene may have partly resulted from the northward advance of the northern treeline and subsequent reduction of the planetary albedo. We investigated the Holocene vegetation history of low arctic continental Nunavut, Canada, from a radiocarbon‐dated sediment core from TK‐2 Lake, a small‐lake ca. 200 km north of the limit of the forest‐tundra. The pollen and loss‐on‐ignition data indicate the presence of dwarf shrub tundra in the region since the beginning of organic sedimentation at ca. 9000 cal. yr BP with dominance of Betula, especially since 8700 cal. yr BP. At 8100–7900 cal. yr BP the dominance of the shrub tundra was punctuated by a transient decline of Betula and coincident increases of Ericaceae undiff., Vaccinium‐type, and Gramineae. This suggests an abrupt disturbance of the Betula glandulosa population, approximately simultaneously with the sudden 8200 cal. yr BP event in the North Atlantic. However, in the absence of other sites studied in the area, linkage to the 8200 cal. yr BP event remains tentative. The lack of any evidence of forest‐tundra in the region constrains the northern limit of the mid‐Holocene advance of the forest‐tundra boundary in central northern Canada. Consequently, our results show that the climate models imposing a mid‐Holocene advance of the limit of the forest‐tundra to the arctic coast of Canada may have overestimated the positive climatic feedback effects that can result from the replacement of tundra by the boreal forest. Copyright © 2003 John Wiley & Sons, Ltd.     

Evidence for climatic variability and its impact on human development during the Neolithic from Loughmeenaghan,County Sligo,Ireland

High‐resolution pollen and geochemical analyses conducted on a sediment profile from a small lake in County Sligo, Ireland, revealed that human development during the Neolithic was influenced by pronounced climatic oscillations. The primeval woodland around the lake experienced a considerable transformation coinciding with the elm decline at 3810 BC. The subsequent increase in summer temperatures and decrease in precipitation favoured wheat cultivation in the lake's catchment area, which was practised for approximately 140 years. A shift towards pastoral farming took place with the establishment of exceptionally dry conditions between 3650 and 3560 BC, when lake level and influx of allochthonous material were notably low. The onset of cool and wet conditions at the transition from the Early to Middle Neolithic possibly caused the initial decline of human activity in the area. Periods of particularly high precipitation during the Middle and early Late Neolithic contributed to the abandonment of the area by the first farmers. Comparison of the proxy record from the study site with other palynological and archaeological records from Ireland suggests that climatic variability on the decadal to centennial scale represented a primary control on the nature and duration of farming practices during the Neolithic. Copyright © 2012 John Wiley & Sons, Ltd.     

Lake‐level fluctuations at Lake Bourget (eastern France) around 4500–3500 cal. a BP and their palaeoclimatic and archaeological implications

On the basis of sedimentological analysis of two cores taken at Chatillon, Lake Le Bourget (northern French Pre‐Alps), and well dated by radiocarbon dates in addition to tree ring dates obtained from an archaeological layer, this paper presents a high‐resolution lake‐level record for the period 4500–3500 cal. a BP. The collected data provide evidence of a complex palaeohydrological (climatic) oscillation spanning the ca. 4300–3850 cal. BP time interval, with major lake‐level maxima at ca. 4200 and 4050–3850 cal. a BP separated by a lowering episode around 4100 cal. a BP. The lake‐level highstands observed at Chatillon between 4300 and 3850 cal. BP appear to be synchronous with (i) a major flooding period recorded in deep cores from the large lakes Le Bourget and Bodensee, and (ii) glacier advance and tree line decline in the Alps. Such wetter and cooler climatic conditions in west‐central Europe around 4000 cal. a BP may have been a nonlinear response to decrease and seasonal changes in insolation. They may also provide a possible explanation for the general abandonment of prehistoric lake dwellings north of the Alps between 4360 and 3750 cal. a BP. Copyright © 2012 John Wiley & Sons, Ltd.     

Problems with identifying the ‘8200‐year cold event’ in terrestrial records of the Atlantic seaboard: a case study from Dooagh,Achill Island,Ireland

The Northern Hemisphere cooling event 8200 years ago is believed to represent the last known major freshwater pulse into the North Atlantic as a result of the final collapse of the North American Laurentide ice sheet. This pulse of water is generally believed to have occurred independently of orbital variations and provides an analogue for predicted increases in high‐latitude precipitation and ice melt as a result of anthropogenically driven future climate change. The precise timing, duration and magnitude of this event, however, are uncertain, with suggestions that the 100‐yr meltwater cooling formed part of a longer‐term cold period in the early Holocene. Here we undertook a multiproxy, high‐resolution investigation of a peat sequence at Dooagh, Achill Island, on the west coast of Ireland, to determine whether the 8200‐year cold event impacted upon the terrestrial vegetation immediately ‘downwind’ of the proposed changes in the North Atlantic. We find clear evidence for an oscillation in the early Holocene using various measures of pollen, indicating a disruption in the vegetation leading to a grassland‐dominated landscape, most probably driven by changes in precipitation rather than temperature. Radiocarbon dating was extremely problematic, however, with bulk peat samples systematically too young for the North Atlantic event, suggesting significant contamination from downward root penetration. The sustained disruption to vegetation over hundreds of years at Dooagh indicates the landscape was impacted by a long‐term cooling event in the early Holocene, and not the single century length 8200‐year meltwater event proposed in many other records in the North Atlantic region. Copyright © 2006 John Wiley & Sons, Ltd.     

Evidence for high-frequency late Glacial to mid-Holocene (16,800 to 5500 cal yr B.P.) climate variability from oxygen isotope values of Lough Inchiquin, Ireland

A 7.6-m core recovered from Lough Inchiquin, western Ireland provides evidence for rapid and long-term climate change from the Late Glacial period to the Mid-Holocene. We determined percentage of carbonate, total organic matter, mineralogy, and δ¹⁸O_calcite values to provide the first high-resolution record of climate variability for this period in Ireland. Following deglaciation, rapid climate amelioration preceded large increases in GISP2 δ¹⁸O_ice values by ∼2300 yr. The Oldest Dryas (15,100 to 14,500 cal yr B.P.) Late Glacial event is documented in this record as a decrease in δ¹⁸O_calcite values. Brief warming at ∼12,700 cal yr B.P. was followed by characteristic Younger Dryas cold and dry climate conditions. A rapid increase in δ¹⁸O_calcite values at ∼10,500 cal yr B.P. marked the onset of Boreal warming in western Ireland. The 8200 cal yr B.P. event is represented by a brief cooling in our record. Prior to general warming, a larger and previously undescribed climate anomaly between 7300 and 6700 cal yr B.P. is characterized by low δ¹⁸O_calcite values with high-frequency variability.     

Reconstruction of climatic and environmental changes in NW Romania during the early part of the last deglaciation (15,000–13,600 cal yr BP)

High resolution pollen, plant macrofossil, charcoal, mineral magnetic and sedimentary analyses, combined with AMS ¹⁴C measurements, were performed on multiple sediment sequences along a transect through the former crater lake Preluca iganului in northwestern Romania in order to reconstruct the climatic and environmental changes during the early part of the Last Termination. Lake sediments started to accumulate at 14,700 cal yr BP. Initially the upland vegetation consisted of an open forest with mainly Betula and Salix and few Pinus sp., but from 14,500 cal yr BP onwards, Pinus mugo, P. sylvestris and Populus and later on also Larix became established around the lake. Between 14,150 and 13,950 cal yr BP, Pinus cembra seems to have replaced P. mugo and P. sylvestris. At 13,950 cal yr BP the tree cover increased and Picea appeared for the first time, together with Pinus cembra, P. mugo and Larix. From 13,750 cal yr BP onwards, a Picea forest developed around the site. Based on the combined proxy data the following climatic development may be inferred: 14,700–14,500 cal yr BP, cooler and wet/humid; 14,500–14,400 cal yr BP: gradually warmer temperatures, wet/humid with dry summers; 14,400–14,320 cal yr BP: warm and dry; 14,320–14,150 cal yr BP: cooler and wet/humid; 14,150–14,100 cal yr BP: warm and dry; 14,100–13,850 cal yr BP: warmer and wet/humid; <13,850 cal yr BP: warm and dry. The tentative correlation of this development with the North Atlantic region assumes that the period >14,700 cal yr could correspond to GS-2a, the time span between 14,700 and 14,320 to GI-1e, the phase between 14,320 and 14,150 cal yr BP to GI-1d and the time frame between 14,150 and 13,600 cal yr BP to the lower part of GI-1c.