Research paper
Climate and atmospheric circulation changes over the past 1000 years reconstructed from oxygen isotopes in lake-sediment carbonate from Ireland
The Holocene 20(7) 1105–1111 © The Author(s) 2010 Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0959683610369504 http://hol.sagepub.com
Jonathan Holmes,1 Carol Arrowsmith,2 William Austin,3 John Boyle,4 Elizabeth Fisher,4 Richard Holme,4 Jim Marshall,4 Frank Oldfield4 and Kuno van der Post5 Abstract A 1000 year long subdecadal-resolution record of carbonate oxygen isotopes (d18Oc) from Lough-na-Shade, Ireland, provides evidence for changing atmospheric circulation over northwest Europe. The total range of d18Oc values (>5‰) is too large to be explained by changes in water temperature. Moreover, good correlation between the lake record and a previously published d18O time series from an Irish speleothem indicates that the changes in oxygen isotopes are best explained by variations in the isotopic composition of precipitation. The amplitude of change during this period is too large to be explained by shifts in condensation temperature. Instead we suggest that there have been changes in vapour source and transport paths connected with shifts in atmospheric circulation. Changes from a source area from further south within the North Atlantic to one further to the north could explain the prominent positive shift in oxygen-isotope values between the early eighteenth and early nineteenth centuries, for example. Our results also demonstrate the value of a ‘multiple-archive’ approach to deconvolving lake-based carbonate isotope profiles, which are often complex.
Keywords atmospheric circulation, carbonate, Ireland, ‘Little Ice Age’, ostracod, oxygen isotopes
Introduction Oxygen-isotope ratios of lacustrine carbonates (d18Oc values) are often used in palaeoclimate reconstruction. They provide information about past water temperature and water isotope composition, which in turn may be related to climate. In some circumstances, carbonate oxygen-isotope ratios can be used to infer the composition of past precipitation (d18Op), which is also a valuable palaeoclimatic variable (e.g. Hammarlund et al., 2002). Although d18Op values may be strongly correlated with condensation temperature especially in middle and high latitudes, changes in atmospheric circulation are an additional control. Records of past changes in d18Op can therefore yield information about atmospheric circulation as well as air temperature. Inferring past d18Op values from lake sediment carbonates, however, is often fraught with difficulty, owing to multiple controls on the oxygen-isotope composition of lacustrine carbonates. The 18O/16O ratio of lacustrine carbonates is controlled by the 18 O/16O ratio and temperature of lakewater as well as any offsets from oxygen-isotope equilibrium that may arise from mineralogical, kinetic or vital effects. In deep lakes, bottom-water temperatures may remain close to 4°C, such that the isotopic composition of carbonates forming in the hypolimnion is affected only by water composition (e.g. von Grafenstein, 2002). In shallow lakes, temperature changes may modify the water composition signal. Independent palaeotemperature proxies have been used to correct for changes in temperature (e.g. Hammarlund et al., 2002; Marshall et al., 2007). Alternatively, oxygen-isotope analyses can be undertaken on materials such as cellulose that show negligible
temperature dependence in isotope fractionation (e.g. Wolfe et al., 2007). However, the isotopic composition of the water may differ from that of precipitation, owing to mixing of water sources or hydrological effects such as evaporation. Although the latter is most commonly seen in long-residence-time lakes especially in drier environments (e.g. Talbot, 1990), it may also occur under more humid conditions (e.g. Drummond et al., 1995). Temporal changes in evaporative enrichment may therefore bias estimates of past precipitation isotope composition, even in cases where temperature can be assumed to have remained constant or can be reconstructed using independent means. Previous studies have dealt with the potential problem of changing evaporative enrichment in several ways. For example, von Grafenstein (2002) assumed that evaporative enrichment has been negligible in the
1
University College London, UK NERC Isotope Geosciences Laboratory, British Geological Survey, UK 3 University of St Andrews, UK 4 University of Liverpool, UK 5 Medidata Solutions Worldwide, UK 2
Received 11 September 2009; revised manuscript accepted 18 March 2010 Corresponding author: Jonathan Holmes, Environmental Change Research Centre, University College London, Gower Street, London, WC1E 6BT, UK Email:
[email protected]
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Figure 1. Location of Lough-na-Shade and other sites mentioned in text and position of core site and water samples
large, short-residence-time lake that was the subject of his study, and hydrological monitoring supported this assumption. Even in shallow lakes with a short residence time under a humid regime with low ratio of evaporation to precipitation, the water isotope composition may show minimal offset from weighted mean isotope composition of local precipitation. Although this can be confirmed using measurements of the modern isotope composition of lake waters and precipitation, it cannot be assumed to have been true in the past in the absence of independent proxies for effective precipitation. In summary, d18Oc profiles from shallow temperate lakes are often complex signatures. In this paper, we compare a ~1000 year long carbonate oxygenisotope time series from Lough-na-Shade, a small, shallow lake in Ireland, with published values for an Irish speleothem in order to deconvolve the complex shallow-lake record. Although subject to the same fundamental controls on isotope fractionation, speleothems exist in a contrasting hydrological setting to lakes. Similarities and differences in carbonate isotope records from the two archives can therefore be used to determine the relative importance of regional climate versus site-specific factors in controlling the two records. Using such a comparison, we show that there have been significant shifts in the isotope composition of precipitation over northwest Europe over the past few centuries, which are best explained by changes in atmospheric circulation. There is considerable palaeoclimatic interest in the past 1000 years (e.g. Jones et al., 2009). However most reconstructions, at least from the middle and high latitudes, focus on past temperature and there is a pressing need for more studies using proxies, such as oxygen isotopes, that can reveal information about changing atmospheric circulation.
Study site Lough-na-Shade (54°21′1.53″N, 6°41′24.47″W) is a small (0.3 ha surface area), shallow (maximum depth ~3.5 m) lake in Co. Armagh, N Ireland (Figure 1). It is the sole remaining lake within a series of hollows in undulating drumlinized late Midlandian (Marine isotope stage 2) till underlain by Lower Carboniferous limestone (Weir, 1993). Lough-na-Shade is fed by two small inlets, one of which is certainly artificial and recent, and drained by one small, possibly artificial, outlet (Figure 1). It is therefore assumed that the lake has been more-or-less hydrologically closed for most of its recent history and its hydrology controlled by inflows and outflows of groundwater together with direct precipitation and evaporative loss (Weir, 1993).
Methods A 2 m long core (NSH92) was recovered from the north-central part of the lake in 3.5 m water in August 1992 midway between the lake centre and Phragmites beds to the northwest (Figure 1) using a modified Livingstone piston corer, and transferred to cold (4°C) storage within 24 h. The core was subsampled at 1 cm intervals. For each sample, several grams of wet sediment were washed through a 250 µm mesh and the coarse fraction dried at 105°C. Ostracod valves were picked from the dried >250 µm residue under a low-power (~40 ×) binocular microscope using a fine (4/0) nylon paintbrush. Five to seven adult specimens of the genus Candona (mainly C. candida, but with a few samples of C. neglecta where specimens of the former were absent) lacking signs of dissolution or carbonate overgrowths were selected for stable isotope analysis using conventional methods and results reported as d values relative to the VPDB standard. Although taxon-specific offsets from oxygen-isotope equilibrium are well known for ostracods, these offsets appear to be invariant at genus or even subfamily level (e.g. von Grafenstein, 2002). Since the two ostracod species analysed in this study are closely related members of the same genus, we assume that they show identical offsets from oxygen-isotope equilibrium and so do not distinguish between them in the reporting of our results. The dating of core NSH92 was achieved using pollen stratigraphy and geochemical stratigraphy. The short and variable water residence time of the lake and rapid, recent sedimentation led to very low concentrations of 137Cs and 210Pb, thus rendering them unreliable as chronological indicators. Dating by 14C was precluded by the calcareous nature of the sediment and the lake catchment, and the absence of terrestrial macrofossils. U/Th dating for sediments pre-ad 1700 was precluded by the lack of ‘clean’ carbonates. An age–depth model for core NSH92 was therefore developed using correlation with welldated palynological (Weir, 1993) and geochemical (this study) events as follows. The first stage in dating the sequence involved establishing detailed correlations with an earlier core taken in 1987, using magnetic measurements and pollen analysis. The 1987 core, also from the centre of the lake, provided a high resolution late-Holocene pollen, loss-on-ignition and carbonate record (Weir, 1993), which could be correlated closely with the present data. Second, the bulk geochemistry of the sediments from NSH92 was determined by energy dispersive x-ray fluorescence (XRF) analysis (Boyle, 2000) using a Bruker S2 Ranger X-Ray Spectrometer. From the above, seven dated points were established for core NSH92, as follows.
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Figure 3. Age–depth curve for NSH92. See text for further information on age-model construction and dating uncertainties
Figure 2. XRF determinations of total sulfur and lead concentrations for NSH92. Numbered arrows refer to those dating points in the age model that are constrained by geochemical data: the basal sample, marked with an un-numbered arrow, yielded no detectable lead
(1) The top of the core (0 cm) dated to ad 1992, the year of collection. (2) The decline in arable cultivation recorded in the high-resolution pollen diagram was dated by Weir (1993) to ad 1900 by comparison with documented land use history (24 cm). (3) The end of flax (Linum usitatissimum) cultivation identified in the pollen record and dated to ad 1840 (64 cm). (4) The first increase in planted Pinus sylvestris in the region, clearly recognized in the pollen diagram and dated by increment coring at the nearby Lough Gall site to ad 1800 (Molyneux et al., 1972) (79 cm). (5) The peak of hemp (Cannabis sativa) cultivation and retting recorded in the pollen diagram and reinforced by the XRF curve for sulfur (Figure 2). This is dated to ad 1720 (93 cm). (6) The beginning of intensive flax and hemp cultivation recorded in the pollen diagram and dated to c. ad 1675 (102 cm). (7) An estimated age for the beginning of the cereal pollen rise in light of documented land-use history; also the point where lead concentrations increase from post-Roman minimum values (Figure 2). Both the pollen–land-use history correlation and the record of lead deposition (Renberg et al., 2001) date this close to ad 1000 (191 cm) (Figure 2). The age–depth model (Figure 3) shows that NSH92 covers the past c. 1000 years. The ages of the events that describe dating points 1 to 6 inclusive are very precisely determined, and uncertainties in the age assessments for NSH92 are determined mainly by the amount of time represented by the 1 cm stratigraphic increment of the core in which the dating point is found, no more than about 10 years. For dating point 7, there is less certainty in the post-Roman rise in lead concentration, the event on which the dating point is based (Renberg et al., 2001), and this leads to an uncertainty of about a century. The age model is therefore much better constrained for the most recent ~350 years than for the earlier part of the record.
Spot samples of water were collected from the lake, one inlet and its outlet (Figure 1) between June 2006 and January 2007 for D/H and 18O/16O analysis using conventional methods. Results are reported as standard delta (d) values relative to the VSMOW standard.
Results and discussion Water isotopes in precipitation and lake water The spot samples from Lough-na-Shade provide some insight into the controls on the lake’s water isotope composition. Both the lake and the outflow have d18O values about 2‰ greater than the inflow in summer/early autumn, but show negligible 18 O enrichment in winter (Figure 4). Seasonal differences in the lake and its outflow are therefore enhanced compared with the inflow, which shows minimal variation. We have no direct measurements of the isotope composition of rainfall from the Armagh region. However, data from the nearest monitoring stations of Valentia and Carron, which lie c. 360 and 220 km, respectively, to the southwest (Figure 1), provide useful information. At Valentia, the weighted mean annual oxygen-isotope value in precipitation (d18Op) is −5.4‰; lower weighted winter values (−6.2‰) and higher summer values (−4.5‰) are a response to colder and warmer air temperatures coupled with different air mass sources in the respective seasons. At Carron, the weighted mean annual d18Op is c. −6‰; the lower value compared with Valentia is likely a result of increased rain-out at this more inland site. Despite the lack of additional precipitation isotope data from Ireland, oxygen-isotope values of recent groundwater, which are closely related to weighted mean annual values in precipitation, suggest a further progressive decrease in d18Op to the northeast, with values of around −7‰ in the Lough-na-Shade area (Darling et al., 2003). Comparison of d18O of precipitation and shallow groundwater data with oxygen isotope values from Lough-na-Shade and the regional synthesis of Diefendorf and Patterson (2005) suggest that the mean annual values of surface waters show a similar spatial pattern to that in precipitation, but they are slightly 18O-enriched, presumably by evaporation. Overall, the spatial patterns of d18O values in precipitation, groundwater and surface water are consistent with a predominantly southwesterly source for meteoric water at present.
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Figure 4. Oxygen-isotope values of water samples from the inlet (open circles), lake (crosses) and outflow (filled circles)
Carbonate oxygen-isotope record from NSH92 The NSH92 d18Oc values varied between an overall maximum of +0.7‰ (ad 1190) and a minimum of −4.6‰ (ad 1890) (Figure 5). Between the base of the analysed sequence (c. ad 950) and ad 1500, d18Oc values remained steady at an average of −2.3‰, apart from three sharp positive excursions between ad 1350 and ad 1150. Values rose from about −2.3‰ to a maximum of −1.4‰ and then fell again between ad 1500 and ad 1670. Between ad 1670 and ad 1830, there was a large but steady rise in mean d18O values up to about −0.4‰ around ad 1830 and then a fall to −4.2‰ by ad 1890, with marked short-term fluctuations between ad 1870 and 18 ad 1890. After ad 1890, d Oc values increased, but ostracods are sparse in the upper (post c. ad 1940) part of the core and we have no values above this point.
Carbonate isotopes and controlling factors The ostracod d18Oc values in NSH92 are controlled by water temperature and the oxygen-isotope ratio of the lake water at the time of shell secretion. There is a change in the oxygenisotope ratio of calcite with increasing calcification temperature of between about −0.20 and −0.24‰/°C (e.g. Coplen, 2007; Kim and O’Neil, 1997). The oxygen-isotope ratio of lake water is controlled by the composition of input water, modified to a greater or lesser degree by evaporation, which will cause heavyisotope enrichment compared with the source water, although in shallow, mid-latitude lakes, the 18O/16O ratio of lake water is often close to that of precipitation, providing the residence time is short and the lake subject to minimal evaporative enrichment. Groundwater may also make a significant contribution to a lake’s hydrological budget, although the 18O/16O ratio of shallow groundwater often closely mirrors that of weighted mean annual precipitation. In addition to the effects of water temperature and isotopic composition, ostracod shell calcite shows systematic positive offsets from oxygen-isotope equilibrium; for members of the subfamily Candoninae, this offset is +2.2 ± 0.2‰ (von Grafenstein, 2002). The overall range of d18Oc values in NSH92 is much too large to be explained by variations in water temperature; fluctuations of around 21°C would be required to explain the observed shifts. Other factors must therefore be important. Assuming the 18O/16O ratio of input water was closely related to that of precipitation, changes in the former are a further possible control. In middle and high latitudes, the 18O/16O ratio of precipitation is positively
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correlated with temperature, with a gradient of about 0.7‰/°C in coastal, lowland mid-latitude regions (Dansgaard, 1964). However, if condensation temperature were the only control on the Lough-na-Shade record, the NSH92 values would imply a longterm air temperature range of about 9°C, which is untenable. Moreover, this would likely have been a minimum value, since the effects of changing air temperature would be offset by between about 0.20 to 0.24‰/°C as a result of the change in 18O fractionation from water to calcite, which operates in the opposite direction. Heavy-isotope enrichment of lake water as a result of evaporation may have further complicated the signal and would be expected to work in the same direction as the relationship between air temperature and the 18O/16O ratio of precipitation; i.e. warmer air temperature yields precipitation with higher d18Op values; higher water temperatures further increase the d18O value of lake water as a result of evaporative enrichment. However, Candona candida is an autumn-winter calcifying species (Meisch, 2000) and evaporative enrichment of the lake appears to be limited during the winter months, although we concede that our modern hydrological measurements are currently too few to be able to substantiate this at present. However, as we demonstrate below, there are further reasons for believing that much of the d18Oc record from NSH92 was not significantly affected by evaporation and therefore tentatively conclude that evaporative enrichment was not a major control, although it may have played some part in amplifying any changes resulting from varying air temperature. The 18O/16O ratio of precipitation is, however, only partly explained by changing air temperature; other factors such as air mass source, trajectory and rain-out history may also play a role (e.g. Rozanski et al., 1993). In order to assess whether the NSH92 record is primarily a function of 18O/16O ratio of precipitation, we compare it with an independent record of 18O/16O in speleothem calcite from Crag Cave, southwest Ireland (Figure 1) (McDermott et al., 2001). This 10 000 year sequence (CC3) is thought to be a record primarily of changing d18Op: however as for Lough-Na-Shade, the authors maintain that the variation in d18Oc is too large to be explained solely by variations in air temperature. This contention holds for the last millennium as well as for the earlier part of the record. Comparison of the last 1000 years of CC3 d18Oc values with the NSH92 record (Figure 5) reveals some times of striking similarity as well as intervals where the two sequences are dissimilar: in particular, there is broad similarity at the centennial scale that is especially marked for the past ~400 years (Figure 6). During the short period at the start of the NSH92 record, from ad 960 to 18 ad 1050, variations in d O are similar to those in the CC3 speleothem. However, from ad 1050 to about ad 1440, there is no clear correspondence between the two records, and the three marked positive excursions in d18Oc values in NSH92 are not represented in the speleothem. Between ad 1440 and ad 1600, a long positive excursion is present in both records, although there are differences in detail between the two archives and, in particular, the sharp rise in d18Oc values in CC3 at ad 1440 is not present in NSH92. After ad 1600, however, there is a strong similarity in the two oxygen-isotope profiles. Between ad 1630 and ad 1900, there was a long positive excursion in d18Oc; the overall amplitude of this excursion is larger in NSH92 (4.1‰) than in CC3 (2.8‰), although the amplitude of the decadally smoothed curve in NSH92 is smaller (2.6‰: Figure 2). Given that ostracod-based stable isotope measurements often show quite large short-term variability, comparison of the CC3 sequence with the smoothed NSH92 data, rather than the unsmoothed values, is probably reasonable, in which case the amplitude of this excursion in NSH92 is very similar to that in CC3. The similarity between NSH92 and CC3, especially for the past 400 years, strongly suggests some common control. Given
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Figure 5. (A) Oxygen-isotope values for NSH92 plotted against age: raw values are shown with light solid line, decadally averaged values with heavy solid line. (B) Oxygen-isotope values for carbonate from Crag Cave speleothem CC3 (from McDermott et al., 2001). (C) Multidecadal Winter North Atlantic Oscillation (NAO) reconstruction. (Trouet et al., 2009: data obtained from http://www.ncdc.noaa.gov/paleo/pubs/ trouet2009/trouet2009.html. IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series # 2009-033, NOAA/NCDC Paleoclimatology Program, Boulder CO, USA). In each graph, the smooth curves, shown by heavy dashed lines, were fitted using the method of Penalized Least-squares Splines. This seeks to fit a smooth function through a data series. The spline fits for NSH92 and CC3 oxygen isotope data sets had a damping parameter of 106, which gives an approximate cut-off period of 200 years. For NSH92 the six extreme values from the pre-ad 1600 part of the sequence were removed, since they are deemed not to be part of the general statistical population
the geographical separation of the two sites and the contrasting hydrological settings of the lake and the cave, changes in d18Op are the most likely common control on the d18O values in Loughna-Shade ostracods and the Crag Cave speleothem calcite. Moreover, this overall similarity lends further weight to our contention, discussed earlier, that the d18Oc record from NSH92 has not been significantly affected by evaporation. Differences in
detail between the two sequences may reflect uncertainties in age models or the effects of other factors that may operate in a different way at each site; for example, changing ground-surface temperature may have had a greater influence at the lake site than in the cave. We note that age uncertainties are likely to be greatest in the poorly constrained earlier part of NSH92, as discussed above. We have shown above that the changes in d18Op
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Figure 6. Correlation between oxygen-isotope values for carbonate from Lough-na-Shade core NSH92 and Crag Cave speleothem CC3. For the purposes of correlation, both time series were resampled for the period ad 960 to ad 1940 at decadal intervals with a cubic spline using the AnalySeries 2.0 software for Macintosh OS X (Paillard et al., 1996). (A) Pre-ad 1600. The open symbols represent values that correspond to the positive d18Oc excursions in NSH92. For further information, see text. The r2 value excludes the data from the intervals covered by these excursions. (B) Post-ad 1600
that are implied by both CC3 and NHS92 are too large to be explained by the effects of varying air temperature alone. Changes in atmospheric circulation, which may have altered the source and trajectory of air masses that brought moisture to each site, must therefore have been important.
Changes in climate and atmospheric circulation Previous studies have revealed a link between d18Op and atmospheric circulation in mid-latitude areas over the whole Holocene (e.g. Amundson et al., 1996; Edwards et al., 1996; Hammarlund et al., 2002) and on shorter timescales (e.g. Burnett et al., 2004; Friedman et al., 2002; Rademacher et al., 2002), although the temporal averaging in palaeo-archives and the integrated monthly nature of most precipitation sampling tends to obscure details of the relationships. However, in an event-based precipitation record for eastern England, Heathcote and Lloyd (1986) showed that
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d18Op varies significantly with moisture source. Rainfall associated with westerly airflow and a source area in the northern North Atlantic (cold sea-surface temperatures, SSTs) had d18Op values in the sampling period ~4‰ more negative than precipitation associated with easterly airflow and continental European source, or from southwesterly winds and a source further south in the North Atlantic (warmer SSTs): correlations with air temperature were, however, weak. Assuming the same general controls operate in North-Central Ireland as in Eastern England, the major changes in the NSH92 and CC3 oxygen-isotope records could be explained mainly by changes in dominant moisture sources from a northern North Atlantic source during times of more negative d18Oc values, to a more southerly North Atlantic or continental European source during more positive d18Oc phases. The major control on interannual climate variations over the North Atlantic, together with western and central Europe, is the North Atlantic Oscillation (NAO), an index of atmospheric pressure difference between Iceland and the Azores. During times of positive NAO, especially during winter, westerly storm tracks are stronger and winters warmer over Europe, whereas negative NAO intervals are associated with weaker westerlies and colder winters over northwest Europe (e.g. Hurrell, 1995). Baldini et al. (2008) reported a positive correlation between winter NAO and d18Op over Europe, which is especially strong over central Europe where it appears to operate via the temperature effect on d18Op (negative NAO leads to colder winter air temperatures over central Europe, and hence more negative d18Op values). However, there is no clear relationship between long-term NAO variations (e.g. Trouet et al., 2009) and the d18Oc records from NSH92 and CC3 for the past millennium (Figure 5), although we note that the peak in d18Oc values in both NSH92 and CC3 in the first half of the nineteenth century coincided with an especially pronounced cold period of the ‘Little Ice Age’. This specific interval has been explained by anomalous pressure conditions over the North Atlantic leading to a reduction in northward heat transport in the ocean and cooling in the Northeast Atlantic/North Sea/Norwegian Sea (van der Schrier and Barkmeijer, 2005). The advection of cold air led to the cold winters of this ‘Little Ice Age’ interval together with enhanced storm activity and more variable storm tracks, but also warming in the Sargasso Sea and other parts of the western North Atlantic. It is possible that sea-surface warming in the water vapour source region (which would lead to less fractionation during vapour formation) coupled with an increase in the balance of moisture from more southerly sources might explain the increase in d18Op values seen in the Irish records during the late eighteenth and early nineteenth centuries, reinforced by a shift to more positive NAO index values (Figure 5), despite the fact that the ‘Little Ice Age’ was cold. For much of the rest of the record, however, there is no relationship with NAO suggesting that controls on atmospheric circulation were complex. Our data additionally suggest that there must have been significant changes in the gradient of the relationship between d18OP and air temperature over the past millennium. The abrupt positive excursions in NSH92 between ad 1180 and 1350 are totally absent from CC3. However, only the most recent of these is well constrained by more than one data point, so the earlier ones may be insignificant. Changes in the hydrological regime of the lake could explain such excursions in oxygen-isotope values. The peak d18O values in the lake must relate to events that would not have affected the speleothem; brief but intense dry periods leading to enhanced evaporative enrichment might be one possibility, although in the absence of further data, such a mechanism remains speculative.
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Conclusions The 1000 year long oxygen-isotope record from Lough-na-Shade shows large-amplitude variability, which we interpret primarily as a function of changing isotopic composition of precipitation, especially for the latter 400 years of the record. This is confirmed by the correspondence between the lake record and the oxygen-isotope values for an Irish speleothem. Recent variations in d18Op values are much too large to be explained by changes in condensation temperature alone. Instead, we suggest that there have been major changes in atmospheric circulation over the past few centuries, which have led to changes in the source and/or transport paths of rain-bearing depressions. We find no simple relationship between d18Op values from Ireland and long-term changes in the NAOindex, which describes the dominant mode of atmospheric variability over the North Atlantic, and suggest that controls on long-term changes in d18Op values over Ireland must have been complex. Our results further show that comparison of lake-sediment oxygen-isotope records with isotope time series from independent archives – in this case a speleothem – can help deconvolve potentially complex signals from the former, and allow separation of regionally significant climatic events from basin-specific hydrological changes.
Acknowledgements We thank Frank McDermott for supplying data from CC3, Valérie Trouet for permission to use the NAO reconstruction, Ben Goldsmith for collection of water samples from Lough-na-Shade and Mary Cassidy for help with collection of rainfall samples from Carron. We acknowledge funding from the UK NERC TIGGER programme grant GST/02/701.
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