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Climate and solar variability recorded in Holocene laminated sediments — a preliminary assessment
Quaternary International 68}71 (2000) 363}371
Climate and solar variability recorded in Holocene laminated sediments * a preliminary assessment夽 M.C. Cooper , P.E. O'Sullivan *, A.J. Shine Department Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, UK Loch Ness Project, Loch Ness Centre, Drumnadrochit, Inverness IV3 6TU, UK
Abstract Comparison of the properties of the laminated sediments of Loch Ness, Scotland with a range of climatic and palaeoclimatic indices shows that in a short core spanning the period 1321}1963 AD, the signal of lamination thickness is signi"cantly correlated (p(0.01) with data on the number of ice weeks o! Iceland, the index of the North Atlantic Oscillation (NAO), and with Zurich sunspot number. Power spectra identi"ed in lamination thickness data for both short and long cores indicate the presence of peaks at low frequencies compatible with the NAO, the &&11-year'' sunspot cycle, the Hale (or &&double sunspot'') cycle, and the Gleissberg (88 year) solar cycle. Several peaks also occur at intermediate, and at higher periodicity, however, and general spectral density is weak, so that these results are regarded as preliminary. The sediments of Loch Ness appear, however, to contain a record of the in#uence of the North Atlantic Ocean, and via that, solar variability, upon the climate of Northern Scotland, spanning recent centuries. 2000 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction The North Atlantic ocean is widely recognised as a key sector of the earth's climate. In particular, Broecker (1991) has demonstrated that variability between glacial and interglacial conditions in the Northern Hemisphere is closely related to the strength and direction of the global circulation of Deep Water (&&the Conveyor''), and thus the rate of formation of North Atlantic Deep Water (NADW). On such timescales, deep water formation is orbitally controlled, and the process therefore operates at periodicities of 10}10 years. A number of other cyclic and quasi-periodic factors also in#uence the climate of the North Atlantic region, however, whose periodicity is of the order of 10}10 years. These short-term signals have been associated in the modern instrumental or recent proxy record with a wide range of atmospheric, oceanic, meteorological and biological processes, including (a) accumulation of the Greenland ice cap, (b) changing extent of ice in the Arctic Ocean, the Labrador Sea and
夽
This paper is based on a presentation originally given at the Krakow Workshop of the EDLP, October 1998. * Corresponding author. Tel.: #44-1-752-233-000; fax: #44-1-752233-035. E-mail address: [email protected] (P.E. O'Sullivan).
the Baltic Sea, (c) sea surface temperature and wave height in the North Atlantic, (d) export of atmospheric dust from Africa to the Caribbean, (e) Central England summer temperature, rainfall and pressure, (f) changes in marine plankton and "sh populations in the Eastern North Atlantic, (g) zooplankton abundance in Windermere, UK (h) vegetation dynamics in the England Midlands, and (i) tree ring growth in Northern Europe (O'Sullivan et al., 1999). In order to investigate such changes via the proxy record, it is therefore necessary to study deposits of su$cient resolution to record decadal or centennial trends. High-resolution studies of lake sediments, especially varved sediments, are ideally suited to this kind of investigation. This paper describes an example of this kind of study carried out on one short ((1 m) and two long ('5 m) cores of laminated sediments from Loch Ness, Scotland. The methods employed in order to develop lamination counts, the detailed reasons why it is believed that the "ner of two sets of laminations present (see below) may be regarded as clastic varves, and radiocarbon dating of one of the long cores, are discussed in earlier publications (Cooper, 1998; Cooper and O'Sullivan, 1998; Cooper et al., 1998). The Pb chronology of a further short core, and the mechanism by which clastic varves are formed in Loch Ness, have also recently been described (Jones et al., 1997; Dean et al., 1999).
1040-6182/00/$20.00 2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 0 ) 0 0 0 5 9 - 8
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2. Site description Loch Ness (Fig. 1), whose major limnological and other properties are listed in Table 1, is the largest body of standing water (by volume) in the UK (Maitland, 1981). The Loch is temperate, warm monomictic and unproductive, occupying a glacially scoured, tectonically formed valley ca. 40 km long aligned southwest/northeast, exposing it to prevailing southwesterly gales. Seiches and other wind-driven circulation patterns are common, but
weak strati"cation develops during most summers. During autumn circulation, the entire water column is ventilated (Smith et al., 1981). The Loch may be divided into two basins, either side of Foyers (Fig. 1). In the south, minerogenic, mainly riverborne sediments are found (AJS, personal observation), whereas laminated lacustrine deposits are mostly con"ned to the north, into which fewer rivers discharge directly. Powerful SW winds a!ect the seasonal and spatial distribution of some fractions of the seston, concentrating it at the northern end of the Loch (Jones et al., 1998). The cores employed in this study were therefore collected from this northern basin.
3. Previous work
Fig. 1. Loch Ness, showing location of main coring station, and 200 m isobath.
Table 1 Characteristic physical features of Loch Ness and its catchment (after Maitland, 1981), Asterisks indicate mean values Latitude Longitude UK National Grid Reference Altitude above sea level (m) Length (¸, km) Mean breadth (B, km) Area (A, km) Volume (<, m;10) Catchment area (D, km) Maximum depth (Z , m)
Mean depth (Z, m) Hydraulic retention time (years)
573 15 N* 43 30 W* NH 285 429* 15.8 39 1.45 56.4 7.45 1775 230 132 2.8
All cores were recovered using purpose-built equipment designed by AJS. One metre cores (LNR1 [0.9 m], NESS90 [1.2 m, Jones et al., 1997]) were obtained using a gravity fall sampler, and long cores Ness 3 and Ness 4 (5.75 m in length, 4.25 m of which are laminated) with a device modelled on the Kullenberg corer (Kullenberg, 1947), at UK National Grid Reference NH 572326 (Fig. 1), beneath 204 m of water. After extrusion, cores were sliced vertically, wrapped in polyurethane sheeting, and stored at 53C. The matrix of the Holocene sediments of Loch Ness is composed almost throughout of couplets of alternating clay- and silt-rich laminae (Cooper and O'Sullivan, 1998). Two chaotic zones (10}40 cm thick), which occur in each long core, may represent slumps. Thickness of individual units varies between 0.5 and 0.75 mm, though some reach only ca. 0.25 mm. Clay-rich layers appear dark brown to black, and silt-rich laminae, olive to brown. Superimposed upon the "ner matrix are irregularly spaced, thicker, more prominent silty-clay and silt layers, each decreasing in particle size upwards, which may represent major #oods. Lead-210 dating (in short core NESS90) of one of the most prominent layers to 1869$13 AD, indicates that it correlates with a documented example from 1868 (Jones et al., 1997). All sections examined are poor in organic matter and microfossils, notably diatoms, which are, however, somewhat more numerous in "ne clay-rich laminae, than in corresponding silty layers. They are also more abundant in the lower, Early Holocene parts of the long cores (A.E.S. Kemp, D. Bull and J. Dean, Oceanography Centre, University of Southampton, personal communication). Below the laminated sections are unlaminated clays which on palynological grounds (Cooper et al., 1998) are presumed to be Early Holocene and Late Pleistocene. Data collected by seston trapping in the water column of Loch Ness (Jones et al., 1997) indicates that a major peak in mineral matter (diameter 200 lm) entering the
M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
Loch occurs during January to March, coinciding with the annual maximum of precipitation and stream#ow. This "nding applies especially to the deepest waters below 200 m. During the rest of the year, trapped material consists mostly of allochthonous detrital carbon. Despite its small size, such matter is therefore conveyed to the bottom of the Loch with su$cient rapidity to produce seasonal variations both in deposition rates and in sediment composition. An hypothesis was therefore developed that the pale, silty laminations de"cient in diatoms, represent sediments laid down during late winter/early spring, and darker, clay-rich laminae, in which microfossils are slightly more abundant, the rest of the year. This implies that the "ner of the two sets of laminations constitute clastic varves (see Sturm, 1979), with intermittent, thicker laminae (as in core NESS90) recording the incidence of #oods. This hypothesis has recently been modi"ed to take account of the "nding (Dean et al., 1999) that periodic input from turbidity currents takes place (sixteen events during in the past 72 years), forming pairs of couplets of silt and clay laminae which are di$cult to distinguish from pairs of other laminations. Images of "ner laminations were studied using Xradiographs, and coarser units via "ne-grained, monochrome infra-red photographs of fresh cores (Cooper, 1998). Plots of grey level against depth were constructed, showing the position of laminae as peaks or troughs corresponding respectively to light or dark laminations, and lamination counts prepared using image analysis of consecutive 15 cm sections from one short core (LNR1; Fig. 2), and both long cores (Ness 3 [Fig. 3], and Ness 4). Counts were then "tted together in sequence, and used to develop a #oating &&varve chronology'' which was then tested by radiocarbon dating (Cooper et al., 1998). The results (Fig. 4) indicate that long Core 3 represents some 8500 calendar years, with the basal sediments deposited during the earliest Holocene, and its topmost strata (not included in Fig. 3) dating (by extrapolation from the uppermost radiocarbon date) from ca. 1500 BP. An excellent relationship was therefore found between the time-depth function for this core compiled from
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Fig. 3. Lamination thickness in long core Ness 3.
Fig. 4. Time depth relationship for Loch Ness Core 3 based on lamination counts compared to calibrated radiocarbon dates (calender years).
Fig. 2. Lamination thickness in Loch Ness short (0.9 m) core LNR1. Bold line denotes decadal average.
lamination counts and that based on the radiocarbon dates, implying that the majority of the "ner of the two sets of laminations are indeed varves. The hypothesis (that the pale, silty laminations mainly represent late winter/early spring, and darker, clay-rich
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laminae the rest of the year) was therefore accepted and long core Ness 3 deemed to contain a &&#oating'' varve chronology spanning the second to the 11th calender millennium BP. Preliminary pollen analysis of that core (Cooper et al., 1998) records a vegetation history similar to that well documented for the Holocene of Northern Scotland, which is repeated in Core Ness 4 (O'Sullivan et al., submitted).
4. Palaeoclimatic studies Our subsequent strategy has been to compare the signal of lamination thickness in short core LNR1 with the instrumental, documentary and recent proxy record, in order to try to assess its signi"cance in longer cores
spanning earlier millennia. (Table 2). Climatic and paleoclimatic indices chosen for comparison include: (1) The local instrumental record of annual precipitation, and mean annual temperature, at Fort Augustus and Inverness for the period 1883}1959 (UK Meteorological O$ce, 1996), some of which, especially for temperature, is intermittent. (2) Selected local, UK and European dendrochronologies. (3) Indicators of climatic conditions in and over the North Atlantic ocean, including SST for the area 45}503N, 5}103W (Lamb, 1977), the Koch index of the numbers of weeks per year of ice o! the coast of Iceland (Koch, 1945, in Lamb, 1977) and the index of the North Atlantic Oscillation (NAO; Hurrell, 1995).
Table 2 Comparisons between lamination thickness in Loch Ness short core LNR1 and various instrumental and proxy data Time series Annual precipitation (mm), Fort Augustus, 1883}1959 Annual precipitation (mm), Inverness, 1883}1959 Mean annual temperature (3C), Fort Augustus, 1883}1959 Mean annual temperature (3C), Inverness, 1883}1959 German Oak chronology (1321}1963 AD) Belfast oak chronology (1300}1963 AD) Scottish oak chronology (1300}1963 AD) English Midlands oak chronology (1300}1963 AD) Mar lodge (Scottish Highlands) Larch chronology (1848}1963) North Atlantic SST (1854}1963) No. of ice weeks o! Iceland (1880}1963) Quarterly Index NAO (1869}1959, decadal average)
Sample size (n)
DJF JFM FMA MAM AMJ MJJ JJA JAS ASO SON OND
Sunspot number (1750}1960) UK Meteorological O$ce (1996). Lamb (1977). Baillie (1977a, b; Baillie and Pilcher, pers. comm., 1997). http://www.cgd.ucar.edu/cas/climind/nao}seasonal.html. NOAA (1996).
Correlation coe$cient (r)
77
!0.158
74
!0.0655
Level of signi"cance 15% '10%
47
0.180
20%
57
0.0638
'10%
623
0.0325
'20%
643
0.0134
'20%
643
!0.0100
'20%
643
0.1025
1%
115
0.2676
(1%
107
0.1543
15%
183 90 90 90 90 90 90 90 90 90 90 90 202
0.5234 !0.2874 !0.3616 !0.3253 !0.3386 !0.0506 0.3160 0.5229 0.3125 0.1270 0.0597 0.0247 0.32
(1% (1% (1% (1% (1% '20% (1% (1% (1% '25% '20% '20% 1%
M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
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(4) Data on sunspot frequency for the period 1750}1960 (National Oceanic and Atmospheric Administration [NOAA], 1996). Finally, data on lamination thickness both in short core LNR1, and in long cores Ness 3 and Ness 4, have been analysed for power spectra, by means of Fast Fourier Transformations. 4.1. The local instrumental record of mean annual temperature and precipitation for the period 1883}1959 As shown by the low correlation coe$cients obtained (Table 2), there is no signi"cant statistical relationship between the signal of lamination thickness in core LNR1, and either annual precipitation, or mean annual temperature, at two local stations, one (Fort Augustus) located at the southwest end of the Loch (Fig. 1), the other (Inverness), 15 km to the northeast. It was originally our hypothesis that lamination thickness in the sediments of Loch Ness is related to the amount of annual and especially winter precipitation over the catchment of the Loch, which in turn should in#uence the amount of silt-sized material brought in during late winter #oods. However, comparisons between these two sets of data do not support our theory.
Fig. 5. Data on (a) ring width in European Larch at Mar Lodge, Eastern Scottish Highlands 1845}1963 (NOAA, 1996), compared with (b) lamination thickness in Loch Ness Core LNR1 for the same period. Bold lines denote 5-year running means.
4.2. Dendrochronological data Comparison of lamination thickness in core LNR1 with a number of well-known dendrochronologies (e.g. the German or Belfast Oak chronologies; Hollstein, 1965; Baillie, 1977a), also yields correlations of little or no statistical signi"cance. Signi"cant relationships are generated, however, with an oak chronology from the English Midlands (Baillie and Pilcher, personal communication), and also with that developed by Schweingruber (NOAA, 1996) for the period 1848}1963, based on growth of European Larch (Larix decidua L.) at Mar Lodge, 50 km southeast of Loch Ness (Fig. 5). The sequence of especially thick varves observed for the period 1855}1870 is replicated in the tree ring data, as also are minima in thickness/growth for the intervals 1870}1890, the 1920s, and the 1950s. Thick varves also characterise the 1940s, a decade of wide ring growth (and increased global temperatures; Lamb, 1977). 4.3. Indicators of climatic conditions in and over the North Atlantic ocean Data from core LNR1 were next compared with SST for the North Atlantic ocean between 45}503N, 5}103W for the period 1854}1963 (Lamb, 1977), and the number of weeks per year when ice was observed o! Iceland, for the interval 1770}1963 (Koch, 1945, in Lamb, 1977). Correlation with the "rst, measurements of which were
Fig. 6. Comparison between lamination thickness in Loch Ness core LNR1 and the Koch Index of number of weeks per year of ice o! the coast of Iceland (from Lamb, 1977).
collected ca. 1000 km southwest of Loch Ness, is not signi"cant, but with the second (r"0.5234) it is signi"cant at less than 1% (Fig. 6). The relationship clearly varies from decade to decade (for example, for the period 1800}1840, lamination thickness is low, whereas sea-ice duration is high), but the marked increase in lamination thickness observed for the late 1860s in mirrored by a peak in sea-ice severity for the same period. Similarly, comparison with the North Atlantic Oscillation (NAO; Hurrell, 1995; Fig. 7) shows that high index (i.e. enhanced westerly zonal air#ow) is associated with deposition of thick laminations, whereas low index (i.e. increased anticyclonic conditions, diminished westerly #ow), especially for 1910}1915, coincides with incidence of thin laminae. Signi"cant statistical relationships con"rm a strong positive association (r"0.52, JJA) with summer, and slightly less signi"cant negative correlation (r"!0.36, JFM) with winter (Table 2). Correlations for DJF-MAM (winter and early spring) are negative, whereas those for MJJ-JAS (summer) are positive, and both signi"cant at less than 1%. Those for AMJ (late spring/early summer) and ASO-OND (autumn), are much weaker, and not statistically signi"cant.
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Fig. 7. Comparison between lamination thickness in Loch Ness core LNR1 (thick line) with decadally averaged values of seasonal index of the North Atlantic Oscillation for JJA (thin line). For source of NAO date, see Table 2.
As stated, we originally believed that the presence of laminations in the sediments of Loch Ness is related to annual, and especially winter precipitation over the catchment of the Loch, which in turn in#uences the amount of silt-sized material brought in during late winter #oods. Seston trap data (Jones et al., 1998; Cooper et al., 1998) appear to support our theory. Consequently, we also hypothesised that sequences of thicker laminae are related to periods of increased stream#ow (and hence precipitation), and intervals of deposition of thinner laminations to phases of reduced stream discharge (and hence drier climate). Analysis of trends in the relationship between lamination thickness in core LNR1 and the Koch Index (Fig. 6), and the index of the NAO (Fig. 7) appears partly to support this theory. Thicker laminae are associated in both cases with periods of high index, when in the case of the Koch Index, there is increased duration of sea-ice, and in the case of the NAO, increased surface barometric pressure gradient between the Azores and Iceland, and enhanced zonal westerly air#ow and penetration of cyclonic weather systems into the Norwegian Sea (Mysak et al., 1990; Dickson et al., 1996). Thinner laminae occur during phases of reduced sea-ice severity, and low NAO index (and hence less vigourous westerly circulation), which is, however, also associated with cool summers over Western Europe, e.g. during the 1960s (Dickson, 1997; Dickson et al., 1996). In terms of individual years, there is a much greater a$nity between lamination thickness and the summer index of the NAO, even though the normal maximum of annual precipitation over Loch Ness and its catchment actually occurs during winter. In other words, thickness appears more closely related to climate of summer over Northern Scotland than to that of winter, and is positively correlated with increased severity and duration of pack-ice o! Iceland. Incidence of cool, wet summers, when sea-ice advances, and surface temperature of the North Atlantic declines (cf. Dickson et al., 1996) may thus be an important factor in generating sequences of thicker laminations.
Fig. 8. Comparison of data on (a) lamination thickness in Core LNR1, 1750}1963, (b) sunspot number 1750}1963 (NOAA, 1996), and (c) annual precipitation at Fort Augustus, 1883}1963 (UK Meteorological O$ce, 1996). Bold lines indicate decadal averages.
4.4. Data on sunspot frequency for the period 1750}1960 In Fig. 8 are shown data on (a) lamination thickness in core LNR1 for the interval 1750}1960, (b) sunspot number (NOAA, 1996) for the same period, and (c) precipitation at Fort Augustus for 1883}1959 (UK Meteorological O$ce, 1996). If lamination thickness (a) is compared with sunspot number (b), it can be seen that peaks recorded for the 1890s, and for the 1940s, are indeed associated with increases in varve thickness for the same intervals. Similarly, the sequence of very thick laminations observed for the period 1855}1870 coincides with the second peak of a double sunspot maximum. The pronounced sunspot minimum recorded between 1790 and 1840 is likewise mirrored by a sequence of thin laminae. The statistical relationship between the two sets of data for the period 1750}1960 is r"0.320, which is signi"cant at 1%. The relationship between (b) sunspot number and (c) local precipitation data (which are intermittent) is less strong, with a correlation coe$cient for a three year moving average of r"0.18, signi"cant at only 5%. Several maxima and minima in sunspot number, however, broadly coincide with peaks and troughs in precipitation.
5. Analysis of power spectra 5.1. Short-core LNR1 Fig. 9 illustrates power spectra identi"ed by fast fourier transformation (FFT) of the signal of lamination thickness in Loch Ness short-core LNR1, generated using the program PROFIT (QuantumSoft 4.2, Zurich, 1993} 1995). Several peaks associated with known climatic and palaeoclimatic indices (Table 3) may be identi"ed. Thus, as well as the longer periodicities at 214, 92 and 46 years, those lying between 9.5 and 18 years may be associated
M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
with the &&11-year'' (periodicity 7}17 years) sunspot cycle, and at 27 years, either with harmonics of that function, or with the &&double sunspot'' (or Hale) cycle. According to Stuiver and Braziunas (1995), the 214 year frequency may denote a solar in#uence. Similarly, the peak at 92 years may represent the Gleissberg solar cycle, whose mean amplitude is 88 years (Hoyt and Schatten, 1997). LNR1 also contains higher frequencies, at about the amplitude of the NAO (5.6}6.4 years), and the Quasi-
369
Biennial Oscillation (QBO; Lamb, 1977; 2.9}3.1 years), but these are di$cult to separate from what may be random signals (see below), or from harmonics of the sunspot cycle (Vos, personal communication). Peaks at 14}18 years may also represent a lunar cycle (Stuiver et al., 1995). Several of the above frequencies have also been identi"ed in the Koch index (4.8}5.2 years, 27 years and 88 years; Mysak et al., 1990), and in the NAO (6}10 years, 23 years; Cook et al., 1998; Hurrell and Van Loon, 1997). 5.2. Long-core Ness 3
Fig. 9. Power spectra in data on lamination thickness from Loch Ness short Core LNR1. Bold lines indicate 95% con"dence interval.
Table 3 Peaks in power spectra identi"ed in data on lamination thickness in the sediments of Lock Ness Mean interval (years) LNR1
Ness 3/1
214
342}3 205}6
Interpretation (amplitude, years) Ness 3/2
Ness 3/3
609
166}7 92
46 35 31 27
93, ,
31 22
18 16 14 12.5, 11.6, 9.5, 5}6
161
79 54 42}4 37 33 29 21
5}6
From Stuiver et al. (1995). Hoyt and Schatten (1997). Hurrell (1995).
Solar cycle (210) ?Subharmonic of Gleissberg cycle ?Gleissberg cycle (88)
78,
Table 4 Correlation between montly index of the NAO, and precipitation at Fort Augustus and Inverness, 1890}1994 AD (Sources as Table 3)
43}5 34}6 27}8 20}3
For FFT analysis, three 1.5 m subsections were selected in which the most prominent laminations occur (Fig. 4), the "rst representing the upper part of the sequence. These are denoted in Table 4 by Ness 3/1, 3/2 and 3/3. In Ness 3/1, long period frequencies at 343 and 206 years (the second of which may represent a 210 year solar cycle; Stuiver et al., 1995), are followed by shorter intervals at 31, 22, and 5}6 years, the last two of which may, like those recorded in core LNR1, be associated, respectively, with the Hale cycle, and the NAO. In Ness 3/2 and 3/3, the longer periodicities observed in LNR1 and Ness 3/1 at'200 years are absent, except for a peak at 609 years in Ness 3/2. Instead, peaks are observed at 161}167, 93, 78}80, 64, 54, 42}45, 33}36, 27}29 and 20}22 years, several of which are seen in LNR1 and Ness 3/1. However, the FFT results possess, in general, a weak spectral density, and many peaks which are akin to &&noise''. This may be due to lack of pre"ltering of the data, and because the forcing agents (e.g. the sunspot cycle) are non-stationary in time (e.g. the length of the sunspot cycle * de"ned by Lassen and Friis-Christensen (1995) as the time between minima * varies), whereas PROFIT identi"es peaks at standardised intervals. Therefore, the results discussed in this section are not
&&Hale'' cycle (double sunspot) (22) Lunar cycle? (18.6)
Sunspot cycle (7}17) North Atlantic Oscillation (5}12)
January February March April May June July August September October November December Signi"cant at (1%. Signi"cant at (5%.
Fort Augustus
Inverness
0.7364 0.7099 0.7041 0.4509 0.2305 0.2207 0.1198 0.0710 0.4576 0.5036 0.5207 0.5427
0.5652 0.4728 0.4699 0.0649 !0.134 !0.005 !0.0554 !0.1224 0.1541 0.7131 0.1588 0.3745
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M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
very reliable, and represent only a "rst approximation of our studies of this aspect of the Loch Ness record. It is our intention, however, eventually to analyse these data further using more sophisticated techniques.
6. Conclusions The signal of lamination thickness in the recent sediments of Loch Ness would appear to be related to changes in atmospheric circulation over the North Atlantic Ocean, in that signi"cant positive relationships exist both with the seasonal duration of sea-ice o! Iceland, and with the index of the North Atlantic Oscillation. Periods of high index (enhanced westerly air#ow) are associated with greater thickness of laminae, suggesting that they are produced by increased stream#ow resulting from enhanced precipitation over the catchment of the Loch, which in turn brings in greater amounts of the silt-sized material which makes up the paler, &&winter'' laminations. Objections to this statement may state that, as reported above (Table 3), correlations between lamination thickness and local precipitation are low. However, as shown in Table 4, precipitation in the vicinity of Loch Ness is also closely associated with monthly index of the NAO, especially in winter, and especially at Fort Augustus, which being west of Loch Ness, is probably more representative of the catchment than Inverness (Fig. 1). Furthermore, correlation with the quarterly index of the NAO (Table 3) highlights a positive relationship between lamination thickness and summer, and a negative association with winter, suggesting that it is not winter rainfall which contributes to formation of thicker laminae, but summer. Thus our hypothesis that laminations are produced by seasonal increases in precipitation and stream#ow within the catchment of the Loch is upheld, but in a modi"ed form. It appears that the existence of the laminations per se, is due to the normal winter maximum of precipitation, stream#ow and sediment delivery, but that increases in their thickness are due to enhanced summer precipitation. Lamination thickness in core LNR1 is also closely related to the sunspot cycle, particularly to sunspot number, although the e!ect of changing length of the cycle (Lassen and Friis-Christensen, 1995), which we have not so far investigated, may also be important. Periods of increased lamination thickness are related to high sunspot number, and incidence of thinner laminae to low sunspot number, again with a highly signi"cant correlation. This "nding implies that, in Northern Scotland, and for recent centuries at least, high sunspot number is associated with high precipitation. Thus it seems that the laminated sediments of Loch Ness contain an archive of the in#uence of the North
Atlantic Ocean upon the climate of Northern Scotland, in the form, at least for recent decades, of the e!ects of the North Atlantic Oscillation, and of a possible solar in#uence on the long term climate of the region, at least for recent centuries, if not for the entire Holocene. By using high-resolution studies of annually laminated lake sediments, it is possible to detect in continental deposits the signal of submillennial and decadal processes over the North Atlantic ocean which a!ect the climate of at least part of the adjacent land mass of the European continent, in this case Northern Scotland. One goal of future research could be to extend this knowledge to the rest of Europe, beginning with the Atlantic seaboard (O'Sullivan et al., 1999).
Acknowledgements The text of this paper was originally read to the Second Annual Workshop of the European Lakes Drilling Programme (ELDP), KrakoH w, Poland, 23}26th October, 1997. The work described was funded by grants from the University of Plymouth (image analysis), the UK Natural Environmental Research Council Radiocarbon Laboratory Steering Committee (radiocarbon dating), and the SWATCH corporation and Loughborough University of Technology (sediment coring). We are grateful for helpful referee's comments by Dr. Heinz Vos, and the help of Mr. Martin Nicholson with many computing matters.
References Baillie, M.G.L., 1977a. The Belfast oak chronology to AD 1001. Tree Ring Bulletin 37, 1}12. Baillie, M.G.L., 1977b. An oak chronology for South Central Scotland. Tree Ring Bulletin 37, 33}44. Baillie, M.G.L., Pilcher, J.R., 1997. Personal communication regarding the BRITIM tree ring chronology for the English East Midlands. Broecker, W.S., 1991. The great ocean conveyor. Oceanography 4, 79}89. Cook, E.R., D'Arrigo, R.D., Bri!a, K.R., 1998. A reconstruction of the North Atlantic oscillation using tree-ring chronologies from North America and Europe. The Holocene 8, 9}17. Cooper, M.C., 1998. The use of digital image analysis in the study of laminated sediments. Journal of Paleolimnology 19, 33}40. Cooper, M.C., O'Sullivan, P.E., 1998. The laminated sediments of Loch Ness: preliminary construction of a chronology of sedimentation, and its use in assessing Holocene climate variability. Paleogeography Paleoecology Palaeoclimatology 140, 23}31. Cooper, M.C., O'Sullivan, P.E., Harkness, D.D., Lawson, E., Bull, D.J., Kemp, A.E.S., Peglar, S.M., Matthews, N.M., Jones, R.I., Shine, A.J., 1998. Radiocarbon dating of laminated sediments from Loch Ness, Scotland. Radiocarbon 40, 781}793. Dean, J., Kemp, A.E.S., Bull, D., Pike, J., Patterson, G., Zolitschka, B., 1999. Taking varves to bits: scanning electron microscopy in the study of laminated sediments and varves. Journal of Paleolimnology 22, 121}136. Dickson, R.R., 1997. From the Labrador Sea to global change. Nature 386, 649}650.
M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371 Dickson, R.R., Lazier, J., Meincke, J., Rhines, P., Swift, J., 1996. Long term coordinated changes in the convective activity of the North Atlantic. Progress in Oceanography 38, 241}295. Hollstein, E., 1965. Jahrringchronologische Datierung von EichenhoK lzarn ohne Waldkante (Westdeutsche Eichenchronologie). Bonner JahrbuK cher 165, 1}27. Hoyt, D.V., Schatten, K.H., 1997. The Role of the Sun in Climate Change. Oxford University Press, Oxford, 279pp. Hurrell, J.W., 1995. Decadal trends in the North Atlantic Oscillation: regional temperatures and precipitation. Science 269, 676}679. Hurrell, J.W., Van Loon, H., 1997. Decadal variations in climate associated with the North Atlantic Oscillation. Climatic Change 36, 301}326. Jones, R.I., Laybourn-Parry, J., Walton, M.C., Young, J.M., 1997. The forms and distribution of carbon in a deep, oligotrophic lake (Loch Ness, Scotland). Verhandelingen Internationale Verein Limnology 26, 330}334. Jones, V.J., Battarbee, R.W., Rose, N.L., Curtis, C., Appleby, P.G., Harriman, R., Shine, A.J., 1997. Evidence for the pollution of Loch Ness from analysis of its recent sediments. The Science of the Total Environmental 203, 37}49. Koch, L., 1945. The east Greenland ice. Meddelelser om GroK nland, Kjobenhavn 130, 1}375. Kullenberg, B., 1947. The piston core sampler. Svenska Hydrogra"ske Biologiske Kommissionens Skrifter Series 3 (1), 1}46. Lamb, H.H., 1977. Climate: Past Present and Future, Vol. 2, Climatic History and the Future. Methuen, London 835pp. Lassen, K., Friis-Christensen, E., 1995. Variability of the solar cycle length during the past "ve centuries and the apparent association with terrestrial climate. Journal of Atmospheric and Terresterial Physics 57, 835}845. Maitland, P.S., 1981. Introduction and catchment analysis. In: Maitland, P.S. (Ed.), The Ecology of Scotland's Largest Lochs.
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Lomond, Awe, Ness, Morar and Shiel, Monographiae Biologicae No. 44. Dr. W. Junk bv, The Hague, pp. 1}27. Mysak, L.A., Manak, D.K., Marsden, R.F., 1990. Sea-ice anomalies observed in the Greenland and Labrador Seas during 1901}1984 and their relation to an interdecadal Arctic climate cycle. Climate Dynamics 5, 111}133. National Oceanic and Atmospheric Administration, NOAA, 1996. http://www.ngdc.noaa.gov O'Sullivan, P.E., Nicholson, M.J., Cooper, M.C., 1999. High resolution proxy study of Late Holocene climatic change on the Atlantic seaboard of Europe * a discussion of possibilities. Terra Nostra 99/10, 72}75. O'Sullivan, P.E., Cooper, M.C., Shine, A.J., Huckerby, E., Matthews, N.M., Satter, N.C.J., Henon, D.N., Jones, V.J., Williams, T.S.C., Nicholson, M.J., Sandford, R.A., Morris, A., 2000. Long term response of Loch Ness, Scotland, to changes in inputs from its catchment. Submitted to Verhandlungen Internationale Vereinigung Limnologie 27. Smith, I.R., Lyle, A.A., Rosie, A.J., 1981. Comparative physical limnology. In: Maitland, P.S. (Ed.), The Ecology of Scotland's Largest Lochs: Lomond, Awe, Ness, Morar & Shiel. Monographiae Biologicae No. 44. Junk, The Hague, pp. 29}65. Stuiver, M., Braziunas, T.F., 1995. Evidence of solar activity variations. In: Bradley, R.S., Jones, P.D. (Eds.), Climate Since A.D. 1500. Routledge, pp. 593}605. Stuiver, M., Grootes, P.M., Braziunas, T.F., 1993. The GSP2 dO climate record of the past 16,500 years and the role of the sun, ocean and volcanoes. Quaternary Research 44, 341}354. Sturm, M., 1979. Origin and composition of clastic varves. In: SchluK chter, C. (Ed.), Moraines and Varves. Balkema, Rotterdam, pp. 281}285. United Kingdom Meteorological O$ce, 1996. Data on mean annual rainfall and temperature at Fort Augustus and Inverness supplied on written application.
Climate and solar variability recorded in Holocene laminated sediments * a preliminary assessment夽 M.C. Cooper , P.E. O'Sullivan *, A.J. Shine Department Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, UK Loch Ness Project, Loch Ness Centre, Drumnadrochit, Inverness IV3 6TU, UK
Abstract Comparison of the properties of the laminated sediments of Loch Ness, Scotland with a range of climatic and palaeoclimatic indices shows that in a short core spanning the period 1321}1963 AD, the signal of lamination thickness is signi"cantly correlated (p(0.01) with data on the number of ice weeks o! Iceland, the index of the North Atlantic Oscillation (NAO), and with Zurich sunspot number. Power spectra identi"ed in lamination thickness data for both short and long cores indicate the presence of peaks at low frequencies compatible with the NAO, the &&11-year'' sunspot cycle, the Hale (or &&double sunspot'') cycle, and the Gleissberg (88 year) solar cycle. Several peaks also occur at intermediate, and at higher periodicity, however, and general spectral density is weak, so that these results are regarded as preliminary. The sediments of Loch Ness appear, however, to contain a record of the in#uence of the North Atlantic Ocean, and via that, solar variability, upon the climate of Northern Scotland, spanning recent centuries. 2000 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction The North Atlantic ocean is widely recognised as a key sector of the earth's climate. In particular, Broecker (1991) has demonstrated that variability between glacial and interglacial conditions in the Northern Hemisphere is closely related to the strength and direction of the global circulation of Deep Water (&&the Conveyor''), and thus the rate of formation of North Atlantic Deep Water (NADW). On such timescales, deep water formation is orbitally controlled, and the process therefore operates at periodicities of 10}10 years. A number of other cyclic and quasi-periodic factors also in#uence the climate of the North Atlantic region, however, whose periodicity is of the order of 10}10 years. These short-term signals have been associated in the modern instrumental or recent proxy record with a wide range of atmospheric, oceanic, meteorological and biological processes, including (a) accumulation of the Greenland ice cap, (b) changing extent of ice in the Arctic Ocean, the Labrador Sea and
夽
This paper is based on a presentation originally given at the Krakow Workshop of the EDLP, October 1998. * Corresponding author. Tel.: #44-1-752-233-000; fax: #44-1-752233-035. E-mail address: [email protected] (P.E. O'Sullivan).
the Baltic Sea, (c) sea surface temperature and wave height in the North Atlantic, (d) export of atmospheric dust from Africa to the Caribbean, (e) Central England summer temperature, rainfall and pressure, (f) changes in marine plankton and "sh populations in the Eastern North Atlantic, (g) zooplankton abundance in Windermere, UK (h) vegetation dynamics in the England Midlands, and (i) tree ring growth in Northern Europe (O'Sullivan et al., 1999). In order to investigate such changes via the proxy record, it is therefore necessary to study deposits of su$cient resolution to record decadal or centennial trends. High-resolution studies of lake sediments, especially varved sediments, are ideally suited to this kind of investigation. This paper describes an example of this kind of study carried out on one short ((1 m) and two long ('5 m) cores of laminated sediments from Loch Ness, Scotland. The methods employed in order to develop lamination counts, the detailed reasons why it is believed that the "ner of two sets of laminations present (see below) may be regarded as clastic varves, and radiocarbon dating of one of the long cores, are discussed in earlier publications (Cooper, 1998; Cooper and O'Sullivan, 1998; Cooper et al., 1998). The Pb chronology of a further short core, and the mechanism by which clastic varves are formed in Loch Ness, have also recently been described (Jones et al., 1997; Dean et al., 1999).
1040-6182/00/$20.00 2000 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 0 ) 0 0 0 5 9 - 8
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2. Site description Loch Ness (Fig. 1), whose major limnological and other properties are listed in Table 1, is the largest body of standing water (by volume) in the UK (Maitland, 1981). The Loch is temperate, warm monomictic and unproductive, occupying a glacially scoured, tectonically formed valley ca. 40 km long aligned southwest/northeast, exposing it to prevailing southwesterly gales. Seiches and other wind-driven circulation patterns are common, but
weak strati"cation develops during most summers. During autumn circulation, the entire water column is ventilated (Smith et al., 1981). The Loch may be divided into two basins, either side of Foyers (Fig. 1). In the south, minerogenic, mainly riverborne sediments are found (AJS, personal observation), whereas laminated lacustrine deposits are mostly con"ned to the north, into which fewer rivers discharge directly. Powerful SW winds a!ect the seasonal and spatial distribution of some fractions of the seston, concentrating it at the northern end of the Loch (Jones et al., 1998). The cores employed in this study were therefore collected from this northern basin.
3. Previous work
Fig. 1. Loch Ness, showing location of main coring station, and 200 m isobath.
Table 1 Characteristic physical features of Loch Ness and its catchment (after Maitland, 1981), Asterisks indicate mean values Latitude Longitude UK National Grid Reference Altitude above sea level (m) Length (¸, km) Mean breadth (B, km) Area (A, km) Volume (<, m;10) Catchment area (D, km) Maximum depth (Z , m)
Mean depth (Z, m) Hydraulic retention time (years)
573 15 N* 43 30 W* NH 285 429* 15.8 39 1.45 56.4 7.45 1775 230 132 2.8
All cores were recovered using purpose-built equipment designed by AJS. One metre cores (LNR1 [0.9 m], NESS90 [1.2 m, Jones et al., 1997]) were obtained using a gravity fall sampler, and long cores Ness 3 and Ness 4 (5.75 m in length, 4.25 m of which are laminated) with a device modelled on the Kullenberg corer (Kullenberg, 1947), at UK National Grid Reference NH 572326 (Fig. 1), beneath 204 m of water. After extrusion, cores were sliced vertically, wrapped in polyurethane sheeting, and stored at 53C. The matrix of the Holocene sediments of Loch Ness is composed almost throughout of couplets of alternating clay- and silt-rich laminae (Cooper and O'Sullivan, 1998). Two chaotic zones (10}40 cm thick), which occur in each long core, may represent slumps. Thickness of individual units varies between 0.5 and 0.75 mm, though some reach only ca. 0.25 mm. Clay-rich layers appear dark brown to black, and silt-rich laminae, olive to brown. Superimposed upon the "ner matrix are irregularly spaced, thicker, more prominent silty-clay and silt layers, each decreasing in particle size upwards, which may represent major #oods. Lead-210 dating (in short core NESS90) of one of the most prominent layers to 1869$13 AD, indicates that it correlates with a documented example from 1868 (Jones et al., 1997). All sections examined are poor in organic matter and microfossils, notably diatoms, which are, however, somewhat more numerous in "ne clay-rich laminae, than in corresponding silty layers. They are also more abundant in the lower, Early Holocene parts of the long cores (A.E.S. Kemp, D. Bull and J. Dean, Oceanography Centre, University of Southampton, personal communication). Below the laminated sections are unlaminated clays which on palynological grounds (Cooper et al., 1998) are presumed to be Early Holocene and Late Pleistocene. Data collected by seston trapping in the water column of Loch Ness (Jones et al., 1997) indicates that a major peak in mineral matter (diameter 200 lm) entering the
M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
Loch occurs during January to March, coinciding with the annual maximum of precipitation and stream#ow. This "nding applies especially to the deepest waters below 200 m. During the rest of the year, trapped material consists mostly of allochthonous detrital carbon. Despite its small size, such matter is therefore conveyed to the bottom of the Loch with su$cient rapidity to produce seasonal variations both in deposition rates and in sediment composition. An hypothesis was therefore developed that the pale, silty laminations de"cient in diatoms, represent sediments laid down during late winter/early spring, and darker, clay-rich laminae, in which microfossils are slightly more abundant, the rest of the year. This implies that the "ner of the two sets of laminations constitute clastic varves (see Sturm, 1979), with intermittent, thicker laminae (as in core NESS90) recording the incidence of #oods. This hypothesis has recently been modi"ed to take account of the "nding (Dean et al., 1999) that periodic input from turbidity currents takes place (sixteen events during in the past 72 years), forming pairs of couplets of silt and clay laminae which are di$cult to distinguish from pairs of other laminations. Images of "ner laminations were studied using Xradiographs, and coarser units via "ne-grained, monochrome infra-red photographs of fresh cores (Cooper, 1998). Plots of grey level against depth were constructed, showing the position of laminae as peaks or troughs corresponding respectively to light or dark laminations, and lamination counts prepared using image analysis of consecutive 15 cm sections from one short core (LNR1; Fig. 2), and both long cores (Ness 3 [Fig. 3], and Ness 4). Counts were then "tted together in sequence, and used to develop a #oating &&varve chronology'' which was then tested by radiocarbon dating (Cooper et al., 1998). The results (Fig. 4) indicate that long Core 3 represents some 8500 calendar years, with the basal sediments deposited during the earliest Holocene, and its topmost strata (not included in Fig. 3) dating (by extrapolation from the uppermost radiocarbon date) from ca. 1500 BP. An excellent relationship was therefore found between the time-depth function for this core compiled from
365
Fig. 3. Lamination thickness in long core Ness 3.
Fig. 4. Time depth relationship for Loch Ness Core 3 based on lamination counts compared to calibrated radiocarbon dates (calender years).
Fig. 2. Lamination thickness in Loch Ness short (0.9 m) core LNR1. Bold line denotes decadal average.
lamination counts and that based on the radiocarbon dates, implying that the majority of the "ner of the two sets of laminations are indeed varves. The hypothesis (that the pale, silty laminations mainly represent late winter/early spring, and darker, clay-rich
366
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laminae the rest of the year) was therefore accepted and long core Ness 3 deemed to contain a &&#oating'' varve chronology spanning the second to the 11th calender millennium BP. Preliminary pollen analysis of that core (Cooper et al., 1998) records a vegetation history similar to that well documented for the Holocene of Northern Scotland, which is repeated in Core Ness 4 (O'Sullivan et al., submitted).
4. Palaeoclimatic studies Our subsequent strategy has been to compare the signal of lamination thickness in short core LNR1 with the instrumental, documentary and recent proxy record, in order to try to assess its signi"cance in longer cores
spanning earlier millennia. (Table 2). Climatic and paleoclimatic indices chosen for comparison include: (1) The local instrumental record of annual precipitation, and mean annual temperature, at Fort Augustus and Inverness for the period 1883}1959 (UK Meteorological O$ce, 1996), some of which, especially for temperature, is intermittent. (2) Selected local, UK and European dendrochronologies. (3) Indicators of climatic conditions in and over the North Atlantic ocean, including SST for the area 45}503N, 5}103W (Lamb, 1977), the Koch index of the numbers of weeks per year of ice o! the coast of Iceland (Koch, 1945, in Lamb, 1977) and the index of the North Atlantic Oscillation (NAO; Hurrell, 1995).
Table 2 Comparisons between lamination thickness in Loch Ness short core LNR1 and various instrumental and proxy data Time series Annual precipitation (mm), Fort Augustus, 1883}1959 Annual precipitation (mm), Inverness, 1883}1959 Mean annual temperature (3C), Fort Augustus, 1883}1959 Mean annual temperature (3C), Inverness, 1883}1959 German Oak chronology (1321}1963 AD) Belfast oak chronology (1300}1963 AD) Scottish oak chronology (1300}1963 AD) English Midlands oak chronology (1300}1963 AD) Mar lodge (Scottish Highlands) Larch chronology (1848}1963) North Atlantic SST (1854}1963) No. of ice weeks o! Iceland (1880}1963) Quarterly Index NAO (1869}1959, decadal average)
Sample size (n)
DJF JFM FMA MAM AMJ MJJ JJA JAS ASO SON OND
Sunspot number (1750}1960) UK Meteorological O$ce (1996). Lamb (1977). Baillie (1977a, b; Baillie and Pilcher, pers. comm., 1997). http://www.cgd.ucar.edu/cas/climind/nao}seasonal.html. NOAA (1996).
Correlation coe$cient (r)
77
!0.158
74
!0.0655
Level of signi"cance 15% '10%
47
0.180
20%
57
0.0638
'10%
623
0.0325
'20%
643
0.0134
'20%
643
!0.0100
'20%
643
0.1025
1%
115
0.2676
(1%
107
0.1543
15%
183 90 90 90 90 90 90 90 90 90 90 90 202
0.5234 !0.2874 !0.3616 !0.3253 !0.3386 !0.0506 0.3160 0.5229 0.3125 0.1270 0.0597 0.0247 0.32
(1% (1% (1% (1% (1% '20% (1% (1% (1% '25% '20% '20% 1%
M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
367
(4) Data on sunspot frequency for the period 1750}1960 (National Oceanic and Atmospheric Administration [NOAA], 1996). Finally, data on lamination thickness both in short core LNR1, and in long cores Ness 3 and Ness 4, have been analysed for power spectra, by means of Fast Fourier Transformations. 4.1. The local instrumental record of mean annual temperature and precipitation for the period 1883}1959 As shown by the low correlation coe$cients obtained (Table 2), there is no signi"cant statistical relationship between the signal of lamination thickness in core LNR1, and either annual precipitation, or mean annual temperature, at two local stations, one (Fort Augustus) located at the southwest end of the Loch (Fig. 1), the other (Inverness), 15 km to the northeast. It was originally our hypothesis that lamination thickness in the sediments of Loch Ness is related to the amount of annual and especially winter precipitation over the catchment of the Loch, which in turn should in#uence the amount of silt-sized material brought in during late winter #oods. However, comparisons between these two sets of data do not support our theory.
Fig. 5. Data on (a) ring width in European Larch at Mar Lodge, Eastern Scottish Highlands 1845}1963 (NOAA, 1996), compared with (b) lamination thickness in Loch Ness Core LNR1 for the same period. Bold lines denote 5-year running means.
4.2. Dendrochronological data Comparison of lamination thickness in core LNR1 with a number of well-known dendrochronologies (e.g. the German or Belfast Oak chronologies; Hollstein, 1965; Baillie, 1977a), also yields correlations of little or no statistical signi"cance. Signi"cant relationships are generated, however, with an oak chronology from the English Midlands (Baillie and Pilcher, personal communication), and also with that developed by Schweingruber (NOAA, 1996) for the period 1848}1963, based on growth of European Larch (Larix decidua L.) at Mar Lodge, 50 km southeast of Loch Ness (Fig. 5). The sequence of especially thick varves observed for the period 1855}1870 is replicated in the tree ring data, as also are minima in thickness/growth for the intervals 1870}1890, the 1920s, and the 1950s. Thick varves also characterise the 1940s, a decade of wide ring growth (and increased global temperatures; Lamb, 1977). 4.3. Indicators of climatic conditions in and over the North Atlantic ocean Data from core LNR1 were next compared with SST for the North Atlantic ocean between 45}503N, 5}103W for the period 1854}1963 (Lamb, 1977), and the number of weeks per year when ice was observed o! Iceland, for the interval 1770}1963 (Koch, 1945, in Lamb, 1977). Correlation with the "rst, measurements of which were
Fig. 6. Comparison between lamination thickness in Loch Ness core LNR1 and the Koch Index of number of weeks per year of ice o! the coast of Iceland (from Lamb, 1977).
collected ca. 1000 km southwest of Loch Ness, is not signi"cant, but with the second (r"0.5234) it is signi"cant at less than 1% (Fig. 6). The relationship clearly varies from decade to decade (for example, for the period 1800}1840, lamination thickness is low, whereas sea-ice duration is high), but the marked increase in lamination thickness observed for the late 1860s in mirrored by a peak in sea-ice severity for the same period. Similarly, comparison with the North Atlantic Oscillation (NAO; Hurrell, 1995; Fig. 7) shows that high index (i.e. enhanced westerly zonal air#ow) is associated with deposition of thick laminations, whereas low index (i.e. increased anticyclonic conditions, diminished westerly #ow), especially for 1910}1915, coincides with incidence of thin laminae. Signi"cant statistical relationships con"rm a strong positive association (r"0.52, JJA) with summer, and slightly less signi"cant negative correlation (r"!0.36, JFM) with winter (Table 2). Correlations for DJF-MAM (winter and early spring) are negative, whereas those for MJJ-JAS (summer) are positive, and both signi"cant at less than 1%. Those for AMJ (late spring/early summer) and ASO-OND (autumn), are much weaker, and not statistically signi"cant.
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M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
Fig. 7. Comparison between lamination thickness in Loch Ness core LNR1 (thick line) with decadally averaged values of seasonal index of the North Atlantic Oscillation for JJA (thin line). For source of NAO date, see Table 2.
As stated, we originally believed that the presence of laminations in the sediments of Loch Ness is related to annual, and especially winter precipitation over the catchment of the Loch, which in turn in#uences the amount of silt-sized material brought in during late winter #oods. Seston trap data (Jones et al., 1998; Cooper et al., 1998) appear to support our theory. Consequently, we also hypothesised that sequences of thicker laminae are related to periods of increased stream#ow (and hence precipitation), and intervals of deposition of thinner laminations to phases of reduced stream discharge (and hence drier climate). Analysis of trends in the relationship between lamination thickness in core LNR1 and the Koch Index (Fig. 6), and the index of the NAO (Fig. 7) appears partly to support this theory. Thicker laminae are associated in both cases with periods of high index, when in the case of the Koch Index, there is increased duration of sea-ice, and in the case of the NAO, increased surface barometric pressure gradient between the Azores and Iceland, and enhanced zonal westerly air#ow and penetration of cyclonic weather systems into the Norwegian Sea (Mysak et al., 1990; Dickson et al., 1996). Thinner laminae occur during phases of reduced sea-ice severity, and low NAO index (and hence less vigourous westerly circulation), which is, however, also associated with cool summers over Western Europe, e.g. during the 1960s (Dickson, 1997; Dickson et al., 1996). In terms of individual years, there is a much greater a$nity between lamination thickness and the summer index of the NAO, even though the normal maximum of annual precipitation over Loch Ness and its catchment actually occurs during winter. In other words, thickness appears more closely related to climate of summer over Northern Scotland than to that of winter, and is positively correlated with increased severity and duration of pack-ice o! Iceland. Incidence of cool, wet summers, when sea-ice advances, and surface temperature of the North Atlantic declines (cf. Dickson et al., 1996) may thus be an important factor in generating sequences of thicker laminations.
Fig. 8. Comparison of data on (a) lamination thickness in Core LNR1, 1750}1963, (b) sunspot number 1750}1963 (NOAA, 1996), and (c) annual precipitation at Fort Augustus, 1883}1963 (UK Meteorological O$ce, 1996). Bold lines indicate decadal averages.
4.4. Data on sunspot frequency for the period 1750}1960 In Fig. 8 are shown data on (a) lamination thickness in core LNR1 for the interval 1750}1960, (b) sunspot number (NOAA, 1996) for the same period, and (c) precipitation at Fort Augustus for 1883}1959 (UK Meteorological O$ce, 1996). If lamination thickness (a) is compared with sunspot number (b), it can be seen that peaks recorded for the 1890s, and for the 1940s, are indeed associated with increases in varve thickness for the same intervals. Similarly, the sequence of very thick laminations observed for the period 1855}1870 coincides with the second peak of a double sunspot maximum. The pronounced sunspot minimum recorded between 1790 and 1840 is likewise mirrored by a sequence of thin laminae. The statistical relationship between the two sets of data for the period 1750}1960 is r"0.320, which is signi"cant at 1%. The relationship between (b) sunspot number and (c) local precipitation data (which are intermittent) is less strong, with a correlation coe$cient for a three year moving average of r"0.18, signi"cant at only 5%. Several maxima and minima in sunspot number, however, broadly coincide with peaks and troughs in precipitation.
5. Analysis of power spectra 5.1. Short-core LNR1 Fig. 9 illustrates power spectra identi"ed by fast fourier transformation (FFT) of the signal of lamination thickness in Loch Ness short-core LNR1, generated using the program PROFIT (QuantumSoft 4.2, Zurich, 1993} 1995). Several peaks associated with known climatic and palaeoclimatic indices (Table 3) may be identi"ed. Thus, as well as the longer periodicities at 214, 92 and 46 years, those lying between 9.5 and 18 years may be associated
M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
with the &&11-year'' (periodicity 7}17 years) sunspot cycle, and at 27 years, either with harmonics of that function, or with the &&double sunspot'' (or Hale) cycle. According to Stuiver and Braziunas (1995), the 214 year frequency may denote a solar in#uence. Similarly, the peak at 92 years may represent the Gleissberg solar cycle, whose mean amplitude is 88 years (Hoyt and Schatten, 1997). LNR1 also contains higher frequencies, at about the amplitude of the NAO (5.6}6.4 years), and the Quasi-
369
Biennial Oscillation (QBO; Lamb, 1977; 2.9}3.1 years), but these are di$cult to separate from what may be random signals (see below), or from harmonics of the sunspot cycle (Vos, personal communication). Peaks at 14}18 years may also represent a lunar cycle (Stuiver et al., 1995). Several of the above frequencies have also been identi"ed in the Koch index (4.8}5.2 years, 27 years and 88 years; Mysak et al., 1990), and in the NAO (6}10 years, 23 years; Cook et al., 1998; Hurrell and Van Loon, 1997). 5.2. Long-core Ness 3
Fig. 9. Power spectra in data on lamination thickness from Loch Ness short Core LNR1. Bold lines indicate 95% con"dence interval.
Table 3 Peaks in power spectra identi"ed in data on lamination thickness in the sediments of Lock Ness Mean interval (years) LNR1
Ness 3/1
214
342}3 205}6
Interpretation (amplitude, years) Ness 3/2
Ness 3/3
609
166}7 92
46 35 31 27
93, ,
31 22
18 16 14 12.5, 11.6, 9.5, 5}6
161
79 54 42}4 37 33 29 21
5}6
From Stuiver et al. (1995). Hoyt and Schatten (1997). Hurrell (1995).
Solar cycle (210) ?Subharmonic of Gleissberg cycle ?Gleissberg cycle (88)
78,
Table 4 Correlation between montly index of the NAO, and precipitation at Fort Augustus and Inverness, 1890}1994 AD (Sources as Table 3)
43}5 34}6 27}8 20}3
For FFT analysis, three 1.5 m subsections were selected in which the most prominent laminations occur (Fig. 4), the "rst representing the upper part of the sequence. These are denoted in Table 4 by Ness 3/1, 3/2 and 3/3. In Ness 3/1, long period frequencies at 343 and 206 years (the second of which may represent a 210 year solar cycle; Stuiver et al., 1995), are followed by shorter intervals at 31, 22, and 5}6 years, the last two of which may, like those recorded in core LNR1, be associated, respectively, with the Hale cycle, and the NAO. In Ness 3/2 and 3/3, the longer periodicities observed in LNR1 and Ness 3/1 at'200 years are absent, except for a peak at 609 years in Ness 3/2. Instead, peaks are observed at 161}167, 93, 78}80, 64, 54, 42}45, 33}36, 27}29 and 20}22 years, several of which are seen in LNR1 and Ness 3/1. However, the FFT results possess, in general, a weak spectral density, and many peaks which are akin to &&noise''. This may be due to lack of pre"ltering of the data, and because the forcing agents (e.g. the sunspot cycle) are non-stationary in time (e.g. the length of the sunspot cycle * de"ned by Lassen and Friis-Christensen (1995) as the time between minima * varies), whereas PROFIT identi"es peaks at standardised intervals. Therefore, the results discussed in this section are not
&&Hale'' cycle (double sunspot) (22) Lunar cycle? (18.6)
Sunspot cycle (7}17) North Atlantic Oscillation (5}12)
January February March April May June July August September October November December Signi"cant at (1%. Signi"cant at (5%.
Fort Augustus
Inverness
0.7364 0.7099 0.7041 0.4509 0.2305 0.2207 0.1198 0.0710 0.4576 0.5036 0.5207 0.5427
0.5652 0.4728 0.4699 0.0649 !0.134 !0.005 !0.0554 !0.1224 0.1541 0.7131 0.1588 0.3745
370
M.C. Cooper et al. / Quaternary International 68}71 (2000) 363}371
very reliable, and represent only a "rst approximation of our studies of this aspect of the Loch Ness record. It is our intention, however, eventually to analyse these data further using more sophisticated techniques.
6. Conclusions The signal of lamination thickness in the recent sediments of Loch Ness would appear to be related to changes in atmospheric circulation over the North Atlantic Ocean, in that signi"cant positive relationships exist both with the seasonal duration of sea-ice o! Iceland, and with the index of the North Atlantic Oscillation. Periods of high index (enhanced westerly air#ow) are associated with greater thickness of laminae, suggesting that they are produced by increased stream#ow resulting from enhanced precipitation over the catchment of the Loch, which in turn brings in greater amounts of the silt-sized material which makes up the paler, &&winter'' laminations. Objections to this statement may state that, as reported above (Table 3), correlations between lamination thickness and local precipitation are low. However, as shown in Table 4, precipitation in the vicinity of Loch Ness is also closely associated with monthly index of the NAO, especially in winter, and especially at Fort Augustus, which being west of Loch Ness, is probably more representative of the catchment than Inverness (Fig. 1). Furthermore, correlation with the quarterly index of the NAO (Table 3) highlights a positive relationship between lamination thickness and summer, and a negative association with winter, suggesting that it is not winter rainfall which contributes to formation of thicker laminae, but summer. Thus our hypothesis that laminations are produced by seasonal increases in precipitation and stream#ow within the catchment of the Loch is upheld, but in a modi"ed form. It appears that the existence of the laminations per se, is due to the normal winter maximum of precipitation, stream#ow and sediment delivery, but that increases in their thickness are due to enhanced summer precipitation. Lamination thickness in core LNR1 is also closely related to the sunspot cycle, particularly to sunspot number, although the e!ect of changing length of the cycle (Lassen and Friis-Christensen, 1995), which we have not so far investigated, may also be important. Periods of increased lamination thickness are related to high sunspot number, and incidence of thinner laminae to low sunspot number, again with a highly signi"cant correlation. This "nding implies that, in Northern Scotland, and for recent centuries at least, high sunspot number is associated with high precipitation. Thus it seems that the laminated sediments of Loch Ness contain an archive of the in#uence of the North
Atlantic Ocean upon the climate of Northern Scotland, in the form, at least for recent decades, of the e!ects of the North Atlantic Oscillation, and of a possible solar in#uence on the long term climate of the region, at least for recent centuries, if not for the entire Holocene. By using high-resolution studies of annually laminated lake sediments, it is possible to detect in continental deposits the signal of submillennial and decadal processes over the North Atlantic ocean which a!ect the climate of at least part of the adjacent land mass of the European continent, in this case Northern Scotland. One goal of future research could be to extend this knowledge to the rest of Europe, beginning with the Atlantic seaboard (O'Sullivan et al., 1999).
Acknowledgements The text of this paper was originally read to the Second Annual Workshop of the European Lakes Drilling Programme (ELDP), KrakoH w, Poland, 23}26th October, 1997. The work described was funded by grants from the University of Plymouth (image analysis), the UK Natural Environmental Research Council Radiocarbon Laboratory Steering Committee (radiocarbon dating), and the SWATCH corporation and Loughborough University of Technology (sediment coring). We are grateful for helpful referee's comments by Dr. Heinz Vos, and the help of Mr. Martin Nicholson with many computing matters.
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