1,720,979 research outputs found
Efstadalsvatn – a multi-proxy study of a Holocene lacustrine sequence from NW Iceland
No full textMulti-proxy data, both lithostratigraphic and biostratigraphic, are presented from Efstadalsvatn, a lake in NW Iceland. The sequence covers the period 10,000 to 3500 14C yr B.P. The biostratgraphic data include the first Icelandic chironomid-based reconstruction of Holocene mean July air temperatures, using a Norwegian training set in the absence of modern Icelandic data. The results show that deglaciation and ecosystem development probably began before 10,000 14C yr B.P. and that July temperatures were around 48C at ca. 9500 14C yr B.P.14 Temperatures then rose to ca. 88C at the time of the deposition of the Saksunarvatn tephra (9100 14C yr B.P.), reaching ca. 108C by 8500 14C yr B.P., high enough for the growth of tree birch, although successful birch colonisation did not take place until 6750 14C yr B.P. There is some evidence for cooling immediately preceding 9100 14C yr B.P. There is little firm biostratigraphic evidence for the 8200 cal. B.P. event, although this may be due to a relatively low resolution pollen sampling interval, but there are changes at this time in the total carbon (TC) and mass susceptibility (MS) data. Optimal temperatures and relative vegetation stability may have occurred between 8000–6100 14C yr B.P. but the chironomid assemblages indicate higher temperatures after 5000 14C yr B.P. This latter interpretation may, however, reflect delayed colonisation of thermophilous taxa and requires further investigation. There is evidence in the lithostratigraphy for greater local terrestrial instability after 6100 14C yr B.P. but it seems unlikely that this led to the redevelopment of ice in the catchment. The biostratigraphic records appear to show a degree of resistence to climate forcing throughout the early and middle Holocene. The new chironomid-based temperature reconstruction needs to be refined by further studies in Iceland, particularly the development of an Icelandic training set, but has already demonstrated the problems of paleoclimatic interpretations based on pollen and/or macrofossil evidence alone
A chironomid-based reconstruction of summer temperatures in NW Iceland since AD 1650
Full text linkFew studies currently exist that aim to validate a proxy chironomid-temperature reconstruction with instrumental temperature measurements. We used a reconstruction from a chironomid percentage abundance data set to produce quantitative summer temperature estimates since AD 1650 for NW Iceland through a transfer function approach, and validated the record against instrumental temperature measurements from Stykkishólmur in western Iceland. The core was dated through Pb-210, Cs-137 and tephra analyses (Hekla 1693) which produced a well-constrained dating model across the whole study period. Little catchment disturbance, as shown through geochemical (Itrax) and loss-on-ignition data, throughout the period further reinforce the premise that the chironomids were responding to temperature and not other catchment or within-lake variables. Particularly cold phases were identified between AD 1683–1710, AD 1765–1780 and AD 1890–1917, with relative drops in summer temperatures in the order of 1.5–2°C. The timing of these cold phases agree well with other evidence of cooler temperatures, notably increased extent of Little Ice Age (LIA) glaciers. Our evidence suggests that the magnitude of summer temperature cooling (1.5–2°C) was enough to force LIA Icelandic glaciers into their maximum Holocene extent, which is in accordance with previous modelling experiments for an Icelandic ice cap (Langjökull)
The ‘4.2 kyr event’ in the British Isles: evidence for an abrupt climate event in the North Atlantic?
No full textPalaeoenvironmental and archaeological data from several regions around the world show evidence of a multi-centennial climatic event occurring approximately 4.2 cal. kyr BP. Abrupt climate change events (ACCs) in the early-Holocene were dominated by meltwater pulse events associated with the final stages of deglaciation, a mechanism unlikely to have driven subsequent ACCs in the mid- and late-Holocene. A study of the ‘4.2 kyr event’ therefore provides an opportunity to study ACCs in the context of comparable environmental conditions to those of the modern day, thus providing valuable lessons for the future. Whilst the climatic change and/or impact of the 4.2 kyr event is clear in certain regions (such as southwest Asia), more work must be done to disentangle the timing and magnitude of change at this time in other regions, including northwest Europe. A more comprehensive reconstruction of the event’s spatial and temporal variability will help determine the likely drivers of this event. Here we present the results of a multi-proxy examination of a peat sequence from Sluggan Moss, Northern Ireland. A range of palaeohydrological proxy analyses have been undertaken, including: humification, plant macrofossil and testate amoebae analyses. Furthermore, stable isotopic analysis (13C and 18O) of Sphagnum ?-cellulose presents an opportunity to examine changes in atmospheric circulation across the 4.2 kyr event. The chronological resolution on the sequence is exceptionally high, providing an excellent opportunity to determine the synchroneity of the climatic signal across the North Atlantic region around 4.2 cal. kyr BP
Was there a ‘4.2 kyr event’ in Great Britain and Ireland? Evidence from the peatland record
No full textPalaeoenvironmental and archaeological data from several regions of the world show evidence of a multi-centennial climatic event c. 4200 cal yr BP. Owing to the absence of continental ice-sheets, studies of this mid-Holocene event provide an opportunity to examine abrupt climatic change within an environmental context comparable to that of the modern day. Whilst the climatic change and/or impact of the event is clear in certain regions, such as western Asia, more work must be done to disentangle the timing and magnitude of changes in other regions, including northern Europe. A more comprehensive reconstruction of the event’s spatial and temporal variability will help determine the likely drivers of the event. Presented here are the results of a temporally-focussed, high-resolution and multi-proxy (testate amoebae, plant macrofossil, peat humification and stable isotopic analyses) examination of peat sequences from northern Ireland, with radiocarbon chronologies supplemented by the region’s excellent tephrochronolog
Early Holocene climate variability and the timing and extent of the Holocene thermal maximum (HTM) in northern Iceland
No full textThe magnitude and timing of Holocene maximum warmth in the Arctic and sub-Arctic has been the subject of considerable recent interest, particularly in the context of future climate change. Although lying at a crucial location in the North Atlantic close to significant atmospheric and oceanic boundaries, terrestrial Holocene climatic data from Iceland are few and predominantly derive from glacial and palaeoecological evidence. Here we present new datasets from Tröllaskagi, based on chironomid-inferred temperatures (CI-T), using sub-fossil chironomids from the same lake sediments supplemented by pollen data. July air temperatures have been derived using an Icelandic training set, and the data suggest optimal temperatures at sea level up to 1.5 °C above current levels around 8 k cal. yr BP, a time when birch woodland was well developed in Tröllaskagi, but when woodland had still not fully developed in the more isolated NW peninsula. Our data thus suggest that optimal summer warmth did not occur in Iceland until 8 kcal. yr BP at the earliest, possibly lasting until 6.7 kcal. yr BP. The amount of warming for July was therefore at least 1.5 °C, but possibly up to 2–3 °C higher than the 1961–1990 average on the basis of the tree-line data. Comparison with data from elsewhere in adjacent Arctic regions, Greenland and Eastern Arctic Canada show peak warmth to be later in Iceland, and less pronounced. It also appears that there were enhanced temperature gradients during the first half of the Holocene between the two study areas Tröllaskagi and the NW Peninsula and that they influenced patterns of vegetation colonisation, with current spatial temperature patterns only developing as Holocene climate deteriorated after around 6 kcal. yr BP
Lacustrine evidence of early-Holocene environmental change in Northern Iceland: a multiproxy palaeoecology and stable isotope study
No full textEarly Holocene warming in Iceland caused rapid glacial ice melt which led to exposed landscapes on which soils developed and floras quickly established. Our records from northern Iceland suggest temperatures were up to 2-2.5°C warmer than present throughout the first two millennia post deglaciation (~10,500 to 8,500) on a background of soil and catchment development before catchment conditions started to stabilise. The warming trend over this period was not uniform however, but punctuated by a series of relatively short lived climatic events. Specifically inwash events are suggested by the δ13Corganic, %TOC and C/N data around 9600 cal BP and 8250 cal BP and are correlated by two independent sites. There is also evidence from the δ18Ocarbonate and δ13Ccarbonate records which suggests that progressive evaporation of the lakes in the region occurred from ~8200 cal BP, the timing of which accords well with other isotopic records of drier conditions from around the North Atlantic
Regional climate change from peat stratigraphy for the mid- to late Holocene in central Ireland
No full textAt the millennial scale bog surface wetness (BSW) records show a clear Holocene climate event stratigraphy,with major phases comparable with other regional climate proxies such as chironomid inferredtemperature records. Moving towards the centennial scale, however, and towards the limits of chronologicalcertainty within the records, regional differences are apparent which likely reflect the moreheterogeneous precipitation patterns which occur on shorter timescales. The BSW data presented in thispaper are reconstructed from testate amoebae assemblages from central Ireland and are compared withother regional records. The results suggest that this region is in phase with the North of Ireland in termsof timings and durations of climate change, but the comparisons are less clear with a stacked and tunedrecord from Northern Britain that shows an apparent offset compared to the Irish records. This mayreflect variations in past regional precipitation or be a function of the tuning and stacking process. Thebroad phases of comparison between the Irish records, and the extension of the central Irish record backto 6000 cal BP, allow comparisons with low frequency temperature reconstructions from chironomids,which also show a broad level of correlation, with cooler temperatures relating to wetter BSW atcentennial to millennial timescales
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Climatic variability during the last millennium in Western Iceland from lake sediment records
Full text linkThe aim of this research was to create a decadal scale terrestrial quantitative palaeoclimate record for NW Iceland from lake sediments for the last millennium. Geochemical, stable isotope and chironomid reconstructions were obtained from a lake sequence constrained by tephra deposits on the Snæfellsnes peninsula, W Iceland. Obtaining a quantitative record proved problematic, but the qualitative chironomid record showed clear trends associated with past summer temperatures, and the sedimentological records provided evidence for past changes in precipitation, mediated through catchment soil inwash. When the full range of chronological uncertainty is considered, four clear phases of climatic conditions were identified: (1) a relatively warm phase between AD 1020 – 1310; (2) a relatively stable period between AD 1310 and AD 1510, cooler than the preceding period, but still notably warmer that the second half of the millennium; (3) a consistent reduction of temperatures between AD 1560 – 1810, with the coolest period between AD 1680-1810; (4) AD 1840-2000 has temperatures mainly warmer than in the preceding two centuries, with a rising trend and increased variability from c. AD 1900 onwards. The reconstructions show clearly that the first half of the millennium experienced warmer climatic conditions than the second half, with a return to the warmer climate only occurring in the last c. 100 years. Much of the variability of the chironomid record can be linked to changes in the North Atlantic Oscillation (NAO). The reconstructions presented can track low frequency and long-term trends effectively and consistently but high resolution and calibrated quantitative records remain more of a challenge – not just in finding optimal sedimentary deposits, but also in finding the most reliable proxy. It is this that presents the real challenge for Holocene climate reconstruction from this key area of the North Atlantic
- …
