Terrestrial and aquatic ecosystem responses to late Holocene climate change recorded in the sediments of Lochan Uaine, Cairngorms, Scotland

Quaternary Science Reviews 29 (2010) 1040–1054

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Terrestrial and aquatic ecosystem responses to late Holocene climate change recorded in the sediments of Lochan Uaine, Cairngorms, Scotland Frank Oldfield a, *, Richard W. Battarbee b, John F. Boyle a, Nigel G. Cameron b, Basil Davis c,1, Richard P. Evershed d, Andrew D. McGovern b, Vivienne Jones b, Roy Thompson e, Rebecca Walker (ne´e Wake) a a

Department of Geography, University of Liverpool, Liverpool L69 3BX, UK Environmental Change Research Centre, University College, London WC1E 6BT, UK Department of Geography, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK d School of Chemistry, University of Bristol, Bristol BS8 1TS, UK e Department of Geology and Geophysics, Edinburgh University, Edinburgh EH9 3JW, UK b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 September 2009 Received in revised form 16 December 2009 Accepted 15 January 2010

We summarise the results of a range of sediment-based studies at Lochan Uaine, a remote corrie lake in the heart of the Cairngorm massif in Scotland. The site lies above the Holocene forest limit and has been minimally affected by human activities. The results presented are mainly based on magnetic measurements, element analysis, granulometry, organic geochemical analysis and pollen analysis carried out over a period of some 15 years. The magnetic properties and element concentrations record a coherent sequence of changes reflecting mainly stages in catchment erosion. In terms of the chronology developed for the sedimentary record from the site, increases in allochthonous, minerogenic sediment delivery to the lake occurred around 1000 BC, AD 330–480 and AD 1260–1410. The only notable change in the pollen diagram records a period of deforestation at lower altitude predating the last of the periods of increased erosion. The organic geochemistry analyses record a series of higher frequency responses in the aquatic ecosystem, already noted in previous papers, e.g. Battarbee et al. (2001). These include fluctuations in organic carbon content and in the concentrations of biomarkers indicative of changing lake productivity. Both the terrestrial and aquatic ecosystem responses are superimposed on a longer-term trend of declining aquatic productivity, progressive catchment weathering and increasing erosion. The sediments of Lochan Uaine thus appear to have recorded complex system responses on three timescales reflecting (a) the long term decline in northern hemisphere insolation during the Holocene, (b) the millennial scale forcing of the kind found in many other mid-late Holocene records and (c) much shorter term, quasicyclic but clearly a-periodic sub-millennial fluctuations. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction As evidence for anthropogenically induced climate change increases, there is growing concern for the impact this will have on both terrestrial and aquatic ecosystems. Part of the value of palaeoenvironmental research lies in the opportunities it provides to assess the effects on ecosystems of past climate change. Lake sediments often contain a record of the effects of changing external

* Corresponding author. Fax: þ44 0151 794 2866. E-mail address: oldfi[email protected] (F. Oldfield). 1 Present address: ARVE Group, Institute for Environmental Science and Technology, Ecole Polytechnique Fe´de´rale de Lausanne, Station 2, 1015 Lausanne, Switzerland. 0277-3791/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2010.01.007

forcing on both terrestrial and aquatic ecosystems, their long term interactions and their response characteristics. High resolution records from the late Holocene are especially valuable as this is the period during which boundary conditions were most closely comparable to those of the recent past and also the period for which we have the most detailed, independent information on past climate change. In sediment sequences from Europe especially, the effects of climate change are often difficult to disentangle from those of human activities since these latter span several millennia in most areas. Sites where the effects of human activities on lake catchments and aquatic ecosystems may be regarded as minimal are therefore of special value. One such is Lochan Uaine. Lochan Uaine, a remote corrie lake in the heart of the Cairngorm massif in Scotland, lies on the eastern slopes of Cairn Toul at an altitude of 910 m (Fig. 1) and with a lake:catchment ratio of 1:10.

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72 5m 75 0m

90

0m

Lochan Uaine 100

0m

120

0m

N

Crags Boulders Heath

Cairngorms

0

0.3 km

Lochan Uaine

BRAEMAR

N

River Dee

Granite

0

5 km

Fig. 1. Location map.

The site is well above the theoretical timberline in the region and would have remained so throughout the Holocene. The highest recorded fossil tree stumps have been found at 790 m (Bennett, 1996). The climate experienced by the lake and its catchment is arctic-alpine in character with the lake frozen for up to three months of the year. The catchment comprises granite bedrock and is characterised by steep bare rock, screes and boulders with thin soils and sparse vegetation. It is a site generally considered to have been minimally affected by human activities until remotely derived industrially generated contaminants began to be deposited in the area from the mid 19th century onwards (Battarbee et al., 2001). The present paper considers rock magnetic, granulometric, geochemical and pollen analytical evidence from the sediments of the lake. The main aim is to explore the record the sediments hold of changing erosion regimes in the catchment and to explore the extent to which these may be linked to changes in climate and/or human activities. Two 8 cm diameter piston cores were obtained in 1993 from the centre of the lake in 15.5 m of water. Both cores, UACT 3 and UACT 4, were extruded vertically and sub-sampled at 2 mm intervals. 210Pb and 14C determinations on core 4 (95 cm long) provided the basis for a chronology (Battarbee et al., 2001) which, using the numerous

tie points between the cores (see below and Fig. 2), can be applied to core 3 (120 cm long). Core 3 is the core from which the samples for magnetic measurements, geochemical and pollen analysis reported here were taken. Subsequently, additional cores were taken. UACT6, obtained in 1997, was a central piston core with a well-preserved sediment/ water interface, taken from below 15.8 m of water. Although less than 50 cm long, the sequence of LOI changes can be correlated closely with those recorded in upper parts of 3 and 4 (Fig. 3). It is the core from which samples for organic geochemistry analyses, reported in McGovern (2000) and considered below, were taken. 2. Core matching and chronology The chronology for core 4 was developed from a sequence of 36 radiocarbon dates on bulk sediment samples and direct gamma assay of 14 samples from the top 12 cm of the core for 210Pb, 137Cs and 241Am (Battarbee et al., 2001). The slow rate of sediment accumulation, coupled with evidence for diffusion, rendered the 137 Cs profile of no chronological value. The roughly monotonic decline in unsupported 210Pb values provided a chronology that differed little depending on the model used for calculation and with

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a

30

Core 3

Loi (%)

25 20 12

10

15

14

18

4

16

2

10

2022

24

26

8

32

28

6

30

34 36

38

40

5 30

44 42

46

48

Core 4

Loi (%)

25 20 12

15

10

10 5 -20

b

30

14 16

2 4

20

18

22

24 26

6 8

32

28

30

38

0

20

40

60

80

100

Core 3

120

42

25

Loi (%)

34 36

28

20 15

20 22

6 8

2

30

32

34

36

38

36

38

40

46 48 44

26 24

14 16 18 10 12

4

10 5 30

Core 4

Loi (%)

25 20

30

6

34

28 2 4

8 10 12

15

14

22

16 18

20

24

32

26

10 5 -20

0

20

40

60

80

100

120

Depth in core 3 (cm) Fig. 2. Core correlation. (a) Loss-on-ignition at 550  C (LOI) vs depth in core 3 aligned to LOI in core 4 vs equivalent depth in core 3. Twenty four (even numbered minima) loss-onignition features in core 3 are labelled (2–48). The uppermost nineteen minima (2–38) can all be seen in core 4. (b) Dry weight vs depth in core 3 aligned to dry weight vs equivalent depth in core 3. The majority of the twenty four loss-on-ignition minima (2–48) of core 3 can be seen as dry weight maxima. The sequence of nineteen dry weight maxima (2–38), of core 3, can be recognised in core 4. Core alignment generated by sequence slotting. See text for details.

Core 3 Loss on ignition (% dry weight) 0 5 10 15 20

Core 6 25

0

Loss on ignition (% dry weight) 5 10 15 20 25

30

0 AD 1660

Depth (cm)

10 20

AD 1260

30 40

AD 340

50 60 Fig. 3. The loss-on-ignition (LOI) traces for the upper part of core 3 and core 6. The latter provided all the samples for organic geochemical analysis. The correlation lines are those proposed in McGovern (2000). Approximate ages are those generated by the procedures outlined in the text.

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only the former contributes to the ‘old’ dates, they are likely to be less variable and least influenced by additional old carbon inputs. Provided contamination has been avoided, such samples would likely be those that lie to the ‘young’ side of the mean regression line (cf Oldfield et al., 2003). Fig. 4 shows that 65% of the ‘young’ deviations are for samples from depths that have relatively high loss-on-ignition (LOI) values, which, in turn generally represent sediment in which the organic matter is autochthonous in origin (see below). Developing a chronology using the mean offset value for these samples would not affect the derived dates significantly, but it would make the offset required to link with the 210Pb chronology some 175–200 years less. This approach would give an offset between 250 and 300 years, which is rather more consistent with previous studies (cf Oldfield et al., 1997) than the one derived from the mean regression. In Fig. 3, showing the LOI trace for the upper part of core 3 alongside that for core 6 used for the organic geochemistry analyses, and in subsequent Figures, dates ascribed to selected depths have been derived using the method outlined above. 3. Methods 3.1. Magnetic measurements Two bulk soil samples were taken in the field, one from the upper and one from the lower part of the catchment. Each of these was separated into 13 particle size fractions from <8f to >2f (<4 mm to >4 mm). Samples were dispersed using sodium

Deviation from best fit linear regression* (14C yr) -800

-400

0

400

0 10

20

30

40 Depth (cm)

which the 241Am measurements were in agreement. The radiocarbon dates are also broadly monotonic with relatively few significant age inversions. However, despite this and a mean sediment accumulation rate based on linear regression similar to that inferred from the 210Pb profile, there is an offset of just under 500 years between the uppermost ages derived from radiocarbon and those based on 210Pb. This led Battarbee et al. (2001) to develop a timescale using an offset of 488 years to link the two radiometric chronologies. This dates the depth in Core 4 correlated with the top of Core 3 to AD 1876 and the base of Core 4, at 93.6 cm to 2026BC. The chronology for core 4 was obtained by linear interpolation between these dated depths and transferred to Core 3 by the method described below. Transferring the chronology from Core 4 to Core 3 depends on establishing detailed correlations between the two. We used a practical numerical approach, named sequence slotting, to match the Lochan Uaine sediment cores. The aim of sequence slotting (Gordon, 1982; Thompson and Clark, 1989) is to combine, in an optimal manner, two ordered sequences A ¼ {A1,A2,.,Am} and B ¼ {B1,B2,.,Bn} of observations into a single sequence, while preserving the ordering within each sequence. During the matching no assumption is made about the temporal variation of the measurements within either core. It is simply assumed that subsamples with similar measurements should be close together in the combined sequence. The procedure assumes that there is a welldefined measure of ‘‘local discordance’’, or ‘‘distance’’, between any two sub-samples in either sequence. The ‘‘total discordance’’ of any proposed combined sequence is defined as the sum of the distances between consecutive sub-samples in the pooled sequence, i.e., the combined path length (CPL). The optimal slotting of sequences A and B is that for which this CPL is minimised. Further details about CPL are given in Thompson and Clark (1989). Thirty-eight loss-on-ignition (LOI) (Fig. 2a) and dry weight (Fig. 2b) features can be recognised in core 4. All the features can also be recognised in core 3 and provide a means of transferring the 210 Pb and 14C ages of core 4 to core 3. The mathematical procedure of sequence slotting allows the two cores to be aligned with remarkable fidelity. At many horizons the alignment is good to within 2 mm. The two cores are found to have very similar accumulation patterns though, in the case of core 3, sediments from the last century appear not to have been retrieved. On average core 3 has an accumulation rate of 0.230 mm/year, while core 4 has a rate of 0.233 mm/year. Furthermore the relative accumulation rates of the two cores are found to vary relatively little with age. At first sight the LOI features may look cyclic. However, the peak-to-peak duration of individual loss-on-ignition and dry weight features in fact varies considerably, for example from 68 years for feature 18-19-20 to 275 years for feature 34-35-36. So while the LOI features have an average duration of 210 years they are in no sense truly periodic. The chronology transferred from core 4 was calibrated using CALIB 4 (Stuiver and Reimer, 1993). Since the lowest dated material in core 4 correlates with a depth of c.85 cm in core 3, any dates for the basal 35 cm of the latter core can only be estimated by linear extrapolation and are likely to be less accurate. However, most of the changes considered in this paper lie above this depth. An alternative approach to assessing the offset between the 210 Pb and 14C chronologies would be to use the mean value derived from dates from core 4 that lie to the ‘young’ side of the mean linear regression line. Use of these dates rather than the mean regression line rests on the demonstration that the ‘old’ carbon in such a softwater lake is likely to arise both from delayed incorporation of autochthonous carbon arising initially from aquatic primary productivity, and from terrestrial carbon stored in the catchment and released as a result of soil erosion (Oldfield et al., 1997). Where

1043

50

60

70

80

90

100 * Age = (40.2333 depth) -488 yr Fig. 4. Deviations in uncalibrated radiocarbon ages from the best fit linear regression line (core 4). Solid symbols reflect LOI minima, open symbols, LOI maxima.

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hexametaphosphate (calgon) and ultrasonics. The particle size fractions were obtained using a combination of sieving and pipette extraction from suspension (Oldfield and Yu, 1994). Magnetic measurements have been carried out on all 2 mm slices from core 3 using the following measuring sequence: -

Low field, low frequency susceptibility (c) Anhysteretic Remanent Magnetization (ARM) ‘Saturation’ (¼1 T) Isothermal Remanent Magnetization (SIRM) Stepwise DC demagnetisation of SIRM at 20 mT, 30 mT, 40 mT, 50 mT, 100 mT and 300 mT.

‘Soft’ IRM is defined here as the portion of the SIRM that is demagnetized at 20 mT. ‘Hard’ IRM (HIRM) is the portion of the SIRM that remains un-reversed in a field of 300 mT. Both can be expressed on a mass specific basis or as a percentage of SIRM. ARM values have been normalised for the DC bias field and expressed as susceptibility of ARM (cARM). In the case of the particle-sized sub-samples from the catchment, susceptibility, where sufficiently high, was measured at high (4.7 Hz) and low (0.47 Hz) frequency in order to allow calculation of the frequency dependent susceptibility as a mass specific value (cfd) and as a percentage of c (cfd%). Only two reverse fields, 20 mT and 300 mT were used. Susceptibilities were measured using a Bartington MS2 Meter and dual frequency sensor. ARM’s were grown in a Molspin AF-demagnetizer with ARM attachment, using a peak AF field of 0.1 T and a DC bias of 0.04 mT. Isothermal remanences were grown and demagnetized using a Molspin Pulse Magnetizer. All remanences were measured on a Minispin slow speed spinner Fluxgate Magnetometer. Susceptibility values have all been adjusted for the diamagnetic effect of the plastic sample holders and cling-film packing. The measurements allow a wide range of mass specific, quotient and percentage calculations to be made. Because of the small sample size, many of the measurements made, especially of susceptibility and of ARM are close to the noise level of the instruments used. 3.2. Element analysis 140 samples were selected for element analysis using atomic absorption spectroscopy or flame photometry. Sediment was digested using the method of Reeves and Brooks (1978), and stored in polythene bottles. Analysis was performed using a Unicam 939 atomic absorption spectrophotometer. Standard operating conditions were used, diluting where necessary. Potassium chloride (KCl) was added to reduce ionisation effects for aluminum (Al), while lanthanum chloride (LaCl2) was added for calcium (Ca) and magnesium (Mg). A slotted silica tube was used for Pb and Cd to improve sensitivity. K and Na were determined using a Corning 410C Clinical flame photometer. Biogenic silica is estimated by difference. Total silica is assumed to be all sediment not accounted for by LOI and measured elements expressed as oxides. Quartz is assumed equal to the mean estimated concentrations of albite and orthoclase (calculated normatively using the procedure of Boyle, 2001, based on information about the composition of the Cairngorm Granite Thomas et al., 2004). This procedure is reliable where, as in this case, biogenic silica is a major component of the sediment. 3.3. Microprobe analysis A Cambridge Instruments Microscan V electron microprobe was used to establish the major element composition of the non-

biogenic, coarse fraction of the sediment. The Microscan V is a two spectrometer quantitative WDS (wavelength dispersive) instrument. A beam current of 15 nA and an accelerating voltage of 20 kV were used. The instrument was calibrated using a mixture of pure metals, simple silica-compounds and counter dead time. Mineral sediment-grains were first concentrated at University College London (UCL) using the technique developed at UCL (Rose, 1991) for the removal of unwanted components of lake sediments, including organic material and biogenic silica. Then the grains were prepared in Edinburgh for microprobe analysis by being incorporated into araldite resin before being ground and polished. Further details of the sample preparation and calibration procedures used at Edinburgh are set out in Newton et al. (2005). 3.4. Grain size analysis Grain sizes were measured on samples from core 4, in the Grant Institute of Earth Sciences at Edinburgh, by laser diffraction using a Beckmann Coulter LS100 particle size analyser. Scattering of the light, delivered by the laser, was detected by a photo-detector array and converted into a particle size distribution. The Coulter counter measures the grain sizes of particles in suspension in the range 0.4–800 mm. Bulk sediment samples were sieved through a 1000 mm-mesh and the <1000 mm fraction, after gentle dispersion, analysed with the Coulter LS100. 3.5. Pollen analysis Pollen samples were prepared every 4 cm using a standard Acetolysis technique following Hydroflouric Acid digestion (Moore et al., 1991). Samples were mounted in silicone jelly and counted using an Olympus CH2 microscope at 400 magnification (up to 1000 for closer inspection with immersion oil). Over 200 tree pollen grains, or 500 total grains were counted for each sample, with pollen percentages calculated based on the total sum of the terrestrial taxa (including ferns but excluding aquatics). 3.6. Organic geochemistry The present account is based on the unpublished study by McGovern (2000). Analyses of carbon and nitrogen content were carried out on contiguous 2 cm samples by combustion of a small amount of dry sediment. All values are expressed as a percentage of dry sediment. Chlorin concentrations were also determined for each 2 cm sample using approximately 0.2 g of freeze dried sediment. The extraction methods are described in McGovern (2000) following Harris and Maxwell (1995) and analyses were carried out using a Waters 470 Scanning Fluorescence Detector connected to a Waters 501 HPLC pump. Statistical tests on standards measured in each batch of samples show that over 95% of the values are 10%, a level of precision that confirms the validity of the main changes in chlorin concentrations. Concentrations are given as mg/g of total organic matter. 4. Results 4.1. Magnetic measurements Fig. 5 shows the results of the magnetic measurements on the particle sized fractions from the catchment samples. The shaded part of each graph denotes the particle size range comparable to that recorded in the sediments. Concentrations of magnetite-type minerals (indicated here by c, cARM and ‘Soft’ IRM values) peak in the coarse silt fraction. Peak HIRM values, denoting the maximum concentration of hard remanence minerals (haematite

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SAMPLE A (Upper catchment) SIRM ARM (x10-6m3kg-1) (x10-5Am2kg-1) (x10-8Am3kg-1) 0

400 800

0

900 1800

0

110 220

ARM/SIRM 0

0.3 0.6

SIRM/ lf 0

20

40

HIRM

ARM/ lf 0

2

4 6

0

50 100 150

SOFT 0

400

800

IRM reverse % 0

20

40

60

80 100

> -2 UPPER

-2 - -1 -1 - 0 0-1

Phi size

1-2 2-3 3-4 4 5 6 7 8 <8 -20mT

-300mT

SAMPLE B (Lower catchment) SIRM ARM (x10-6m3kg-1) (x10-5Am2kg-1) (x10-8Am3kg-1) 0 > -2 -2 - -1

100

200

0

400 800

0

50

100

ARM/SIRM 0

0.1

SIRM/ lf

0.2 4

6

8

HIRM

ARM/ lf 0.4

0.6

92

96

100

SOFT 0

50 100

IRM reverse % 0

20

40

60

80 100

LOWER

-1 - 0 0-1

Phi size

1-2 2-3 3-4 4 5 6 7 8 <8

-20mT

-300mT

Fig. 5. Selected magnetic measurements for particle-sized sub-samples from two catchment sites. The shaded bands represent the size range found in the lake sediments.

and goethite) occur in the sand fractions which also show peak values for HIRM%. cfd% values (not plotted) are consistently below 5%. cARM/SIRM values range from 0.07 to 0.24 (103). Fig. 6 shows selected magnetic properties for core 3 smoothed with a 5-point running mean. All the concentration indicators show an overall increasing trend from the base of the core. The increases in HIRM are mostly paralleled by similar increases in SIRM. The magnetic properties indicative of concentrations are much lower than the catchment values for similar particle sizes. This is largely explained by the extent to which the sediment is composed of diatoms (Battarbee et al., 2001 and Fig. 7) and, to a lesser extent, organic matter, both of which are diamagnetic. The range of quotient and percentage values are broadly similar to those for the comparable grades in the catchment samples, bearing in mind that, in the case of quotients using the susceptibility values, the small sample volume and the diamagnetic contributions not only from sample pots and packing material, but also from the diatom frustules, make the absolute values plotted subject to some error. Three zone boundaries are shown, each at depths in the sequence where HIRM values increase. These depths are within 1–2 cm of the zone boundaries used to divide the chemical record in Fig. 7. 4.2. Element concentrations The results of the chemical analyses are shown in Fig. 7 alongside the Loss-On-Ignition at 550  C (LOI) data and calculated biogenic silica. McGovern (2000) shows that the carbonate content of the sediments is less than 0.25%. The sediment is dominated by biogenic silica, which contributes more than 50% of the mass.

Typically, only 25% of the sediment mass is terrigenous mineral matter. Organic matter, measured as LOI, shows a progressive long term increase, interrupted only in the top 15 cm of the record. Through the lower half of the record we also see a progressive decline in Mn and K concentrations while Fe and Mg increase in the upper part of the sequence. The sediment record can be divided into 5 intervals based on chemical composition, as shown on Fig. 7. The top three zone boundaries are each marked by an increase in Mg and Fe concentrations. The change at 65 cm is especially significant, as the mineral matter concentration increase is accompanied by a change in mineral matter quality (see the Mg/Al and Fe/Al ratios, Fig. 7b). 4.3. Microprobe Earlier studies had pointed to the likelihood of recognisable tephra peaks in the sediment samples (McGovern, 2000; Battarbee et al., 2001) Microprobe analysis was undertaken to assess the likelihood of tephra providing an additional chronological tool. Neil Rose provided three samples from the lowermost part of L. Uaine core 3 which were chosen as having the highest concentration of non-biogenic sediment and to lie within one of the zones tentatively identified as containing tephra. The horizons lie between 86.6 and 86.8 cm depth and have an age of around 1730 BC. At least ten grains from each of the three horizons were polished and examined in detail. We found that the chemical pre-treatment procedure had successfully reduced the biogenic component of the coarse fraction and concentrated the non-biogenic grains, as only one out of the 35 grains examined proved to be the remains of

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SIRM 10-8 Am2 kg-1

lf

10-8 m3 kg-1 0

0

20

40

60

80

0

40

80

arm/SIRM 10-3 Am-1

arm

10-8 m3 kg-1 120

0

10

20

30

40

0.1

0.2

0.3

SIRM/ lf 103 Am-1

0.4

1.0

2.0

3.0

4.0

AD 1660 AD 1260

20

Depth (cm)

AD 340

40

60 1040 BC

80

100

120

3300 BC

HIRM 10-5 Am2 kg-1 0

10

arm/ lf

Hard % 20

30

40

0

10

20

30

40

0.2

0.4

Soft IRM 10-3 Am2 kg-1

0.6

0.8

0

10

20

30

Reverse field ratios 0

20

40

60

80

100

0 AD 1660 AD 1260

20 AD 340

Depth (cm)

40

60 1040 BC

80

100

120

3300 BC

Fig. 6. Selected magnetic properties vs depth for core 3. The red lines show data smoothed by a 5-point running mean. Approximate ages are those generated by the procedures outlined in the text.

a diatom (Table 1). Major element compositions, particularly of Si, Al, K, Ca, and Na, allowed the remaining 34 grains to be identified as fragments of albite, orthoclase or quartz. No tephra shards of glass were present. Based on the chemical evidence (Table 1), and the visual evidence (no irregularly shaped, or vesicular shards) the L Uaine coarse fraction appears to be completely dominated by grains of Caingorm granite derived from the local catchment. 4.4. Grain size Both unimodal (Fig. 8a) and bimodal (Fig. 8b) grain-size distributions were measured. The main peak, at a particle diameter of around 16–20 microns, can be attributed mainly to the diatom frustules which dominate the sediment. The secondary peak, at around 200 microns, is likely to be dominated by clastic particles from the catchment or lake shore. The clay fraction (<2 microns), as determined here, typically makes up less than 4% of the total sediment though this is likely to be an underestimate (Hao et al., 2008). The median grain size fluctuates between 13 and 28 microns in the sediment of core 4. The fluctuations correspond very closely with variations in loss-on-ignition content (Fig. 9), high loss-on-ignition values occurring in the coarser sediment horizons. 4.5. Pollen analysis Fig. 10 shows the relative frequencies of selected pollen and spore taxa for the top 56 cm of core 3. The tree pollen record is

dominated by Pinus, together with Coryloid taxa (Corylus avellana or Myrica gale). Other minor tree taxa include Alnus, Quercus and Betula, and a low but significant presence of the moorland shrub Calluna. Overall arboreal pollen percentages range from around 75% in the early part of the sequence, before dropping to modern levels of around 60% between 16 and 20 cm (AD 980–1180). This reduction in woodland cover is the main event shown within the pollen record, and is marked by an increase in Gramineae and then Filicales, and by commensurate declines in Coryloid and Alnus tree taxa. 4.6. Organic geochemistry Selected results from the organic geochemistry analyses on core 6 are shown in Fig. 11 alongside the loss-on-ignition (LOI) profile. LOI and total organic carbon (TOC) closely parallel each other. The C/N ratio shows little change below 5 cm, but above this depth, there are higher values that peak at twice the mean recorded in the lower depths. The chlorin values are rather constant except for three distinctive minima around 34, 18 and 3 cm. These correspond with marked LOI and TOC minima as well as low bulk d13C values (McGovern, 2000; Battarbee et al., 2001). The earliest correlates with the changes between 33 and 30 cm in core 3; the later phases of reduced chlorin concentration correlate with early and late stages in the zone of depressed LOI above 15 cm in core 3. The uppermost dip in chlorin concentrations also coincides with the higher values in the C/N ratio already noted. Chlorins in lake

F. Oldfield et al. / Quaternary Science Reviews 29 (2010) 1040–1054

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a AD 1660 AD 1260

Dept h (cm)

AD 340

1040 BC

3300 BC

b AD 1660 AD 1260

Dept h (cm)

AD 340

1040 BC

3300 BC

Fig. 7. Element analyses vs depth for core 3. Shading identifies the zones discussed in the text. Fig. 8a shows element concentration alongside LOI and calculated biogenic silica (see text). Fig. 8b shows quotients. The bold lines show weathering trends derived using the ALLOGEN model (Boyle, 2007) (see text). Approximate ages are those generated by the procedures outlined in the text.

sediments are early degradation products produced from the chlorophyll of lake biota. They are therefore often regarded as direct proxies for lake productivity. 5. Discussion

likely to have been generated by changes in the balance between autochthonous and allochthonous input. By quoting specific dates we are not seeking to imply unjustified precision, and the possible links with other sequences proposed in the subsequent discussion should be viewed in this light. Given the close tie with the 210Pb chronology, it seems reasonable to suppose that the most recent

5.1. Chronology The chronology used here is subject to uncertainties additional to the familiar ones arising from counting statistics and calibration. In the case of sediments above 85 cm in core 3, some confidence may be derived from both the close agreement between the mean sedimentation rates calculated from both the 210Pb and 14C chronologies, and from the lack of evidence for major variations in sedimentation rate. Nevertheless, some fluctuations in rate are

Table 1 Microprobe mineralogy of individual grains in the coarse fraction of the nonbiogenic extract of Core UACT 4 sediment. Depth (cm)

Orthoclase

Quartz

Albite

Diatom

Tephra

86.4–86.6 86.6–86.8 86.8–87.0

10a 9 2

3 5 3

3 4 5

1 0 0

0 0 0

a

Number of individually identified shards.

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F. Oldfield et al. / Quaternary Science Reviews 29 (2010) 1040–1054

4 CLAY

SILT

SAND

CLAY

SILT

SAND

Volume (%)

3

2

1

0

b

5

Volume (%)

4 3 2 1 0 0.5

1

5 10 50 100 Particle diameter (microns)

500 1000

Fig. 8. Particle size histograms from core 4, illustrating the range of particle size distributions to be found in the Lochan Uaine sediment cores.

dates are the most reliable. In the discussion that follows, little use has been made of the extrapolated chronology below 85 cm in core 3 save to indicate the likely time-span recorded. 5.2. The nature, origin and survival of the magnetic minerals in the sediments

15 26

10

loi (%)

20

Before using the magnetic properties in core 3 as indicators of changes in catchment erosion, alternative sources of variation in the magnetic properties of lake sediments need to be considered. Externally derived minerals are present as a result of detrital input from the catchment and atmospheric deposition from sources outside the catchment. Both types of source are capable of generating a range of magnetic mineral types and magnetic grain size assemblages. Atmospheric deposition has been shown to contribute magnetic minerals to the sediment record of remote

32

28

34

30

(a)

36 38

60

65

70

75

80

85

90

80

85

90

30 20 (b)

10

Median (microns)

Depth (cm)

60

65

70

75

Depth (cm) Fig. 9. (a) Loss-on-ignition and (b) Median particle size through loss-on-ignition cycles 26–38 in core 4.

lakes in the Cairngorm region only from the beginning of the 20th century (Oldfield and Richardson, 1990). It cannot therefore have contributed to the sequence of changes considered here. Magnetic minerals are also generated within the lake and sediments as a result of bacterial activity, notably the production of magnetosomes by magnetotactic bacteria (see e.g. Vali et al., 1987). This process gives rise to Stable Single Domain (SSD) magnetite with grain diameters mainly ranging from 0.02 to 0.1 mm diameter. Among the possible contributors to magnetic mineral assemblages formed authigenically within the lake sediments, greigite, a ferrimagnetic iron sulphide (Fe3S4) has been detected with increasing frequency as a result of its distinctive magnetic properties (se e.g. Snowball and Torri, 1999). In addition to the effects of deposition from allochthonous sources, as well as biogenic and authigenic magnetic mineral formation, magnetic properties may also be strongly influenced by reductive diagenesis (see e.g. Anderson and Rippey, 1988). This leads to the dissolution of the minerals, first the fine then the coarser grained ferrimagnets and eventually the hard remanence antiferromagmetic minerals too. Finally, it must also be borne in mind that although non-magnetic minerals make no contribution to remanence, they affect measurements of magnetic susceptibility and hence also influence the quotients in which susceptibility values are included. In order to develop any robust interpretation of the magnetic measurements presented here, it is first necessary to consider the source of the magnetic minerals in light of the possibilities outlined above. Greigite, where it makes a significant contribution to the magnetic mineral assemblage, is normally detectable in routine measurements by high SIRM/c values (>40 mAm2) independent of any increase in ‘hard’ IRM, but associated instead with a very steep acquisition and/or loss of IRM between 40 and 100 mT. This latter characteristic shows up as a widening of the gap between the lines representing percentage reverse saturation at each of these fields. In the present suite of measurements, SIRM/c values exceed 18 mAm2 only where the reverse field measurements indicate high contributions from haematite. The detailed reverse field demagnetisation data, not shown here, include no sign of divergence in the ‘back-field’ curves. We therefore conclude that greigite does not contribute to the magnetic properties in core 3. The contribution to the magnetic properties of sediments by biogenic magnetite resulting from the formation of magnetosomes by magnetotactic bacteria is difficult to quantify, but its relative importance can often by evaluated qualitatively by considering the ARM values and the quotients derived from them. Where strong biogenic contributions have been inferred (see e.g. Oldfield, 1994) and, in several cases now, confirmed by high resolution Transmission Electron Micrography (Gibbs, 2000; Oldfield et al., 2009) the strongest evidence is provided by the cARM/SIRM quotient values. In the case of magnetosome dominated assemblages, values often exceed 2  103 and usually exceed 0.8. In the case of core 3, the mean values for cARM/SIRM average around 0.25. The rare values that exceed 0.4, are for individual depths scattered throughout the core and are therefore more likely to reflect the effects of instrumental noise on ARM measurements than any sustained contribution from magnetotactic bacteria. Even if SIRM values are halved to reflect the maximum effect of any additional contribution of haematite to SIRM, the mean quotient values still fall short of those indicating significant magnetosome contributions. Further evidence for the relative unimportance of magnetosomes comes from a comparison between the sediment and catchment values. cARM/SIRM values range between 0.07 and 0.24 in the catchment samples. The higher values, which come from the fine sand and silt grades are close to the mean values in the sediments. This stands in sharp contrast to other lakes from highland regions, such as Blelham Tarn (van der Post et al., 1997), Brothers

F. Oldfield et al. / Quaternary Science Reviews 29 (2010) 1040–1054

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Lochan Uaine Core 3 Pollen

Aq ua t

ic s

Aquatics

Fe rn s

Ferns

H er bs

Herbs

Fa gu C s al un a vu lg C ar or is yl Er oid ic H ac ed ae Ju era n Sa ipe heli li ru x So x s rb Va us c C cin ar iu C yop m he h y An nop llac t o e Ar hem dia ae te is ce ae C mi ru si c a G ife ra ra m e in ea e G <4 ra 0J m H in m yp ea Li erz e > gu ia 4 0 Pl liflo se J m an r la Pl tag ae go a o R nta lan um go c R ex /me eol an c d at R un risp ia- a an c u m u a Fi unc lus s jor lip u -t U en lus ype m du -tr b G ell la cop al ife hy llu Se ium rae s la Fi gin lic e al lla es Po ly Pt po er di Sp idiu um h m C ag yp nu Ty era m ph ce Po a a ae t n M am gus yr og ti Tr iop eto folia ee hy n -t yp s llu an m e d sp sh ic ru at bs um

U lm Q us u Al erc nu us s

Be tu Pi la nu s

Trees and shrubs

0 5

Depth (cm)

10 15

AD 1180

20

AD 980

25 30 35 40 45 50 55 60

20 40 20 20 20 20 Analysis: B.A.S Davies, Department of Geography, University of Newcastle, UK

20

20 40 60 80

20

20

Fig. 10. Pollen diagram (4 cm sample interval) from the top 56 cm of core 3. Approximate ages are those generated by the procedures outlined in the text.

Water (Oldfield and Wu, 2000) and Ponsonby Tarn (Oldfield et al., 1999) where clear distinctions between fine grained assemblages in catchments and sediments point to important magnetosomes contributions to the latter (see also Oldfield, 2007). Dissolution of magnetic minerals is associated with a decline in concentrations and an apparent coarsening of the residual ferrimagnetic grain size assemblage as the finest grains are dissolved preferentially. Alongside this effect, there is an increase in at least the relative importance of ‘hard remanence’ magnetic minerals since these are more resistant to the early stages of reductive diagenesis. The magnetic signatures generated by these effects are common to marine sediments (Oldfield et al., 1995; Robinson et al., 2000), lake sediments (Anderson and Rippey, 1988) and stored sediments undergoing dissolution of magnetic minerals (Oldfield et al., 1992). In the present suite of measurements, there is no down-core decrease in cARM/SIRM (indicative of a coarsening in magnetic grain size) or increase in HIRM% (indicative of preferential survival of imperfect antiferromagnets). We therefore conclude

0

Loss on ignition (% dry sediment) 10 20 30

that the evidence does not support the proposition that the downcore declines in concentration indicated by the overall trends in c, SIRM, ARM, ‘soft’ and ‘hard’ IRM reflect progressive dissolution. Moreover, similar down-core declines in Fe and Mg concentrations in the upper part of the sequence suggest that the trends are not confined to the magnetic measurements alone. 5.3. The magnetic properties as erosion indicators In light of the above, we conclude that it is valid to regard the magnetic signatures in core 3 as reflecting the input of minerals from the catchment as a result of erosion. The magnetic properties of the particle-sized sub-samples from the catchment show that peak magnetic concentrations are in fractions coarser than 8 mm. This suggests that the characteristics of the magnetic detrital input from the catchment reflect mainly primary, unweathered material (including haematite) rather than finer grained secondary weathered products. Mean cfd values of around 4%, where they can be

TOC (% dry sediment) 0

4

8

Chlorins/mg gTOC-1

C/N weight ratio 12 0

10

20

0

0.3

0.6

0.9

0 AD 1660

Depth (cm)

10

20

AD 1260

30 AD 340 40

50 Fig. 11. Core 6: loss-on-ignition, total organic carbon, carbon/nitrogen (C/N) ratios and chlorin concentrations. Approximate ages are those generated by the procedures outlined in the text.

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reliably measured in the catchment samples, suggest that secondary magnetic minerals are not completely absent in the catchment regolith, although the sediment sample sizes are too small to permit cfd measurements. All the above lines of argument support the use of the magnetic measurements in core 3 as indicators of catchment erosion predominantly of silt and fine sand size particles derived mainly from unweathered material. Of the various properties measured, we use the saturation and ‘hard’ remanence values (SIRM and HIRM), especially the latter which may be viewed as the least ambiguous, since it is even less likely to have been affected by any of the other processes discussed above than are the measurement reflecting ferrimagnetic concentrations. We therefore use these values below as a basis for inferring changes in surface processes within the catchment. The correlation coefficient (r) between HIRM and SIRM is 0.878, between SIRM and Fe and Mg, 0.818 and 0.690 respectively; between HIRM and Fe and Mg, 0.770 and 0.645. In view of the tendency for HIRM% to increase with increased particle size from 16 mm upwards in the catchment samples, any increases in the sediment record might indicate coarser grades of mineral input from the catchment. There are two horizons of sustained rapid up-core increase in SIRM and HIRM, at ca 64 cm and 15 cm. There is also a significant increase in HIRM around 30 cm. These three horizons correspond with the upper three zone boundaries in the chemical stratigraphy outlined below. 5.4. Chemical stratigraphy The prolonged progressive changes in sediment composition shown in Fig. 7 are best interpreted with reference to the minerals likely to control them. The Cairngorm Granite comprises subequal proportions of orthoclase, plagioclase and quartz, with ca 5% biotite and trace quantities of apatite, zircon and magnetite (Thomas et al., 2004) in addition to haematite which the magnetic measurements show to be many times more abundant than magnetite in the catchment and sediment samples considered here. In such a system, Na will be predominantly in plagioclase, K in orthoclase (with only a minor contribution from biotite), Mg in biotite and its dissolution products, and Al primarily in the two feldspars. Changes in the relative proportions of these minerals in the lake sediment can arise from two entirely different mechanisms. First, weathering leads to long term soil composition change as the more susceptible minerals are leached from the soil. Second, widely differing particle size distributions for the minerals leads to a very strong particle size control over sediment composition. The first of these effects is likely to be greatest for biotite which has the most rapid dissolution kinetics of the principle minerals, though even in this case the effect will be subtle across only 5000 years. However, detection of change in biotite concentration using element concentrations is problematic; any Fe and Mg released is likely to be retained in secondary mixed layer clay minerals (Allen et al., 2001), while K loss is masked by the dominant contribution of K by orthoclase feldspar. Dissolution of the feldspar is also expected. Though this should be less substantial than for biotite, it is more easily detected owing to the minimal capture of dissolved Na and K by soils and sediment. To test whether a chemical weathering depletion signal is likely to be present at Lochan Uaine, the ALLOGEN model (Boyle, 2007) has been used to predict the rate of mineral change. ALLOGEN was run using a granite composition inferred from the description of Thomas et al. (2004) (Quartz 35%, orthoclase 30%, albite 30%, biotite 5%), and assuming a mean annul temperature of 2.7  C and precipitation of 929 mm/

yr. Fig. 7b shows the model output superimposed on the data for element/aluminium ratios. Both Na/Al (albite dissolution) and K/ Al (orthoclase dissolution) show good agreement for the simulated fine silt and clay fractions, comparable to the particle size properties of the sediment. The Mg/Al and Fe/Al simulations show the same trend as the data, but poorer agreement. Unlike Na/Al and K/Al, where the dominant cause of change is loss of the alkalis, increasing Mg and Fe concentration are largely due to a transfer from primary biotite to the finer clay fraction, a process which is harder to simulate reliably. Thus the longer-term trends in Na, K, Mg and Fe can plausibly be attributed to effects of progressive chemical weathering. The alternative possibility, that they could also be the result of a progressive fining of the sediment is not supported by the particle size data (Fig. 9 and unpubl.). What cannot be attributed to weathering are the step changes at various depths in the record; these can most easily be explained in terms of changes in particle size due to altered soil erosion or particle transport. Thus, the five compositional zones on Fig. 7 provide information about catchment change; these are now discussed in detail. Zone 1 (118–97 cm). This interval has the lowest concentration of organic matter, and highest concentration of elements associated with terrigenous mineral matter. Biogenic silica makes up ca 60% of the material. The composition is typical for upland impoverished lake systems. Zone 2 (97–65 cm). This interval is characterised by significantly higher LOI, and lower element concentrations. At the base of this unit the change could be attributed to reduced mineral matter supply from the catchment, as LOI rises. Later in the interval element concentrations climb again suggesting enhanced supply of mineral matter from the catchment. Zone 3 (65–32 cm). The abrupt increase in Fe and Mg is one of the most striking step changes in the whole record. No other elements show sustained change. A minimum in LOI values coincides with the start of the increases in Fe and Mg but values recover rapidly thereafter. There is no change in biogenic silica. The changes recorded can only be attributed to a mechanism altering the supply of either biotite or its derived secondary minerals. The absence of any change in feldspar contribution (Na and K) argues against a major change in soil erosion rate. Instead, a change in source (part of catchment, or depth in soil profile) or drainage connectivity must have taken place, allowing improved delivery of the fine fraction. This boundary coincides with the first significant increase in inferred haematite concentrations as well as with a clear minimum in median grain size and arise dry weight percentages, though only in the case of the haematite signature does the change lead to a sustained shift in values. Zone 4 (32–15 cm). A further stepped increase in Fe and Mg takes place, accompanied this time by Mn. This is accompanied, initially, by a decline in LOI and an increase in HIRM. This is most easily interpreted as enhanced catchment erosion. Through the interval other element concentration rise, and for the first time in the record the biogenic silica concentration shows a sustained decline. Similar changes in median grain size and dry weight to those noted above occur once more with the same contrast between relatively sustained higher values for magnetic concentrations and oscillating values for other parameters. There is no evidence for an increase in total sediment accumulation rate, suggesting that any enhancement in catchment erosion is accompanied by reduced diatom productivity. Zone 5 (15–0 cm). The base of this zone is characterised by a sharp increase in terrigenous mineral concentrations, diluting organic matter and biogenic silica. The pattern of changes in HIRM, dry weight, LOI and median grain size roughly mirror those recorded at the two preceding boundaries.

F. Oldfield et al. / Quaternary Science Reviews 29 (2010) 1040–1054

5.5. The history of erosion As noted above, there are three depths in the sequence where HIRM values increase significantly, around 65 cm, 33–30 cm, and, most notably, at w14 cm. The first and third of these coincide with increases in SIRM, and all three correspond with changes in sediment chemistry. In terms of the flux of detrital magnetic minerals, each succeeding episode seems to have been more severe than the previous one. These results point to at least three phases of greater erosive input to the sediments which are otherwise dominated by organic matter and biogenic silica, both predominantly autochthonous in origin (Battarbee et al., 2001). The combined evidence from chlorin, d13C and TOC measurements suggests that the later periods of increased erosive, minerogenic input also saw a shift in the balance between catchment-derived and autochthonous organic matter that was partly accounted for by a reduction in within-lake productivity. These episodes also correspond with minima in the Carbon Preference Index (CPI) (Bray and Evans, 1961) which declines overall towards the sediment surface (McGovern, 2000). This points to the delivery of increasingly mature organic matter from the catchments. The declining CPI values through time suggest that successive erosive episodes may have yielded organic matter from progressively greater depths in the regolith. The chronology of sedimentation derived from a combination of 210 Pb and 14C dates suggests that the main erosive periods were initiated around 1000 BC, AD 330–480 and AD 1260–1410. The first date is within 1–2 centuries of the deterioration in climate noted by many authors (e.g. Beer and van Geel, 2008) the second corresponds with the date of the post-Roman climatic deterioration (cf Lauritzen and Lundberg, 1999) and the final dates lie at the transition from the Medieval Climatic Optimum to the Little Ice Age (Grove, 1988). These dates have often been identified with widescale climatic shifts, especially in the North Atlantic region (e.g. Berglund, 2003). Erosion at high elevations in the Cairngorms results from debris flows, slope wash resulting from snow-melt (Ferguson, 1984; Ballantyne, 2002) and, on the ridge crests, wind action (Burges, 1951; Bayfield, 1984). All are linked closely to climate and are likely to be exacerbated during periods of greater storminess, snowfall and/or freeze-thaw action. The inferred erosive episodes have a less regular and much longer interval than the fluctuations in aquatic productivity inferred from sediment chemistry and biology (Battarbee et al., 2001). It is important to note that the pollen changes above 20 cm, already highlighted, begin between AD 980 and AD 1180, before the inferred changes in erosion regime. The decline in alder and hazel and expanding grasslands points to deforestation at lower altitudes and it is noteworthy that there are some parallels with changes in the upper parts of pollen diagrams from Speyside published by Birks (1970) and O’Sullivan (1973) though neither diagram has a sufficiently detailed chronology to confirm any correlation. Brooks and Birks (2001), on the basis of chironomid-inferred temperature reconstructions for the same core from Lochan Uaine, place the depth over which this change occurs within the Medieval Climatic Optimum and this is consistent with the present chronology. The increases in first Filicales then Pteridium that follow the initial deforestation may point to some abandonment of cleared land under deteriorating climatic conditions. 5.6. Changes in aquatic productivity Battarbee et al. (2001) discuss the quasi-cyclic oscillations in LOI and biological indicators that they interpret as evidence for fluctuations in aquatic productivity on sub-millennial timescales. Other

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studies (e.g. McGovern, 2000) have provided additional evidence in support of this interpretation. Several sediment characteristics show maximum values associated with peaks in LOI. These include, total organic carbon (TOC), chlorin concentrations (derived from aquatic algae), short chain (C17) n-alkanes (McGovern, 2000), also indicative of an algal origin, and chironomid head capsules. Lighter d13C values are characteristic of the periods with high LOI (Battarbee et al., 2001) and this also points to higher contributions from aquatic algae. As noted above, there is evidence for changes in the maturity of allochthonous organic matter in the sediments linked to the degree of catchment erosion. In the foregoing analysis of sources of ‘old’ carbon in the sediments, it was shown that, on average, the organic matter resulting from within-lake productivity diluted 14C activity less severely than that derived from the catchment and that the latter was responsible for most of the examples of more extreme dilution (see above and Fig. 4). In light of the foregoing discussion of the evidence for periods of increased catchment erosion, it is important to note that the first episode of severely reduced chlorin concentration correlates with onset of the erosive episode around AD 340–480. Since the chlorin concentrations are calculated on the basis of TOC, the link between evidence for catchment erosion and reduced within-lake productivity is not an artefact of the basis of calculation. It is worth noting that each of the erosion episodes inferred from the magnetic record begins in a period of low LOI, with many of the associated characteristics already described above. This suggests that there are intervals, as around AD 340–480, when both the aquatic and terrestrial ecosystems are responding to deteriorating climatic conditions. Whereas the effects on the erosion regime appear to persist, aquatic productivity appears to recover to levels comparable to those predating the perturbation. The main exception to this pattern occurs in the sediments spanning the last ca 750 years. Whereas the major phase of ecosystem change beginning around AD 340 affects both the terrestrial and aquatic ecosystems in a way that is consistent with an interpretation in terms of climatic deterioration, the nature of the shorter term, a-periodic and probably less severe oscillations in aquatic productivity as recorded, for example, in the LOI variability present throughout cores 3 and 4, leaves open the question of whether they reflect responses that track fluctuations in climate and thus map onto a pattern of variable forcing, or whether they reflect a recurrent response to and recovery from brief perturbations that trigger, but do not map onto the quasi-cyclic changes. 5.7. The last 750 years The higher values for erosion indicators, both magnetic and chemical, that characterise the last w750 years of the record, begin with a steep increase above 15 cm, dating from the 13th century. Values then decline, followed by a second increase from the early 15th century onwards. LOI and biogenic silica decline, recover then decline again. The initial shifts date from the period of transition from the Medieval Warm Period to the onset of the Little Ice Age (LIA) in the northern hemisphere and they are broadly contemporaneous with evidence for strong volcanic forcing followed by a downturn in solar radiation (Goosse et al., 2008). The 13th century also saw the first evidence for the southward advance of pack ice in the N. Atlantic as well as for glacier advance in Iceland and Greenland (Grove, 1988). The second phase of increased erosion lies firmly within the period of the LIA. LOI and TOC values do not fully recover at any stage in the sequence after AD1220. This is not supported by the more diagnostic indicators of aquatic productivity obtained from core 6 where the decline of LOI values begins at 20 cm. The restoration of chlorin concentrations between 16 and 6 cm in core 6 is as strong as in previous episodes (Fig. 11),

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and the n-alkane data also point to high algal contributions up to the same depth (McGovern, 2000). Above this, these productivity indicators plunge to a final minimum that persists to within less than 2 cm of the surface. It is only within this latest period of minimum algal productivity that C/N ratios point to dominance of the organic matter by higher plants from terrestrial sources (Fig. 11). Although precise correlation between cores 4 and 6 is not possible for this most recent period, it is significant that the two topmost 14C dates from levels in core 4 spanning the 16th and 17th century on the chronology proposed, gave uncalibrated ages of 1415  60 BP (at 8.0–8.2 cm) and 1695  50 BP (at 5.8–6.0 cm), consistent with the previous inference that the most severe dilution of 14C arises from catchment inwash. The various lines of evidence for the last 750 years show that the initial climatic deterioration at the onset of the LIA strongly affected both surface processes and aquatic productivity. Recovery of the latter took place during the early part of the period, but during the later, more severe phase from the beginning of the 16th century, the response of both terrestrial and aquatic ecosystems was the most extreme recorded over the whole of the 5000 year long record. 5.8. A long term trend The episodes of both terrestrial and aquatic ecosystem response noted in the preceding sections are superimposed on a clear, long term tendency for the magnetic erosion indicators in the sediment record to increase through time. The chemical stratigraphy suggests that progressive weathering is at least partly responsible for this. However there is also a steady decline in biogenic silica only partially offset by increased organic matter up to 16 cm. Given the inferred links with climate, it is possible that the long term changes are also a response to the overall decline in solar radiation at this latitude throughout the Holocene (Bradley, 2003). Several other mid-Late Holocene climate reconstructions from northern Europe and the north Atlantic record a similar overall decline (Heikkila¨ and Seppa¨, 2003; Jansen et al., 2008).

Table 2 Summary of the results of counting whole and broken valves of the diatom Brachysira vitrea in order to test the hypothesis that diatom breakage was responsible for the links between LOI and median particle size shown in Fig. 10. Whole valves Broken valves Total valves % Broken LOI max no. LOI min no. 25 27 26 26 20 28 30 25 20

34 55 31 40 42 34 32 35 45

59 82 57 66 62 62 62 60 65

57.6 67.1 54.4 60.6 67.7 54.8 51.6 58.3 69.2

26 25 24 23 22 21 20 19 18

fluctuations in the mean size of the diatom assemblages between LOI peaks and troughs may be responsible for the particle size-LOI correlation is not supported by diatom counts which point to little variation between LOI peaks and troughs (Battarbee et al., 2001). Such a linkage could also arise as a result of catchment-based processes, for example, a shift in the balance between the amount of clastic material contributed by coarse, and probably abraded ‘lag’ sands around the shoreline and finer grained, less abraded silt inwashed from the catchment during periods of enhanced erosion. All the evidence on both short and longer timescales suggests that low LOI and increased catchment erosion are correlated. Moreover, the geochemical evidence suggests that at one level at least increased erosion is accompanied by a decline in mean grain size. For this mechanism to work the single peak in grain size in those samples with an unimodal grain size assemblage (Fig. 8a) would reflect both diatoms and eroded catchment material within the 15–20 microns range. One of the main processes responsible for increased erosion may have been enhanced snowfall and subsequent melt-water flows (cf Ferguson, 1984). This proposed mechanism would work best if the coarser lake margin material had been selectively quartz-enriched and depleted in magnetic minerals.

5.9. The LOI-particle size link 6. Conclusions One of the most distinctive and puzzling aspects of the record is the strong link between particle size and LOI (Fig. 9). Two possible explanations are explored below. The strong influence on the grain size record from diatom frustules raise the possibility that the explanation may be found in terms of within-lake processes. The breakage of diatom frustules during storm events could be the primary cause of the grain-size fluctuations. The times of smaller grain sizes might be explained by unusually high wind speeds in winter storms as would occur when Lochan Uaine lay directly under the path of Atlantic storm tracks, perhaps a reflection of shifts in the NAO. LOI often reflects increased lake productivity. At Uaine this association is supported by increases in chironomid biomass at times of high LOI. Similar relationship between head capsule numbers (caused by increased lake productivity which results in an increase in the chironomid biomass) and LOI have been found at the nearby high altitude lake of Lochnagar (Dalton et al., 2005), and at Loch Ruthven near Inverness (Brooks, pers. com., and unpublished data). In an attempt to test this hypothesis, calculations of whole to broken diatom valves were made using Brachysira vitrea and spanning the sequence of LOI peaks and troughs numbered 18–26 on the core 4 LOI diagram (Fig. 2). A randomisation test (computer intensive version of a t-test that makes no assumptions about the distribution of the data) confirms that there is no significant difference in the level of breakage of B. vitrea between LOI maxima and minima (Table 2). The alternative possibility, that, irrespective of breakage,

1. The magnetic properties of the late Holocene sediments in core 3 from Lochan Uaine are dominated by detrital input from the lake catchment. 2. Changes in magnetic properties, notably SIRM and HIRM can be interpreted as signatures of erosive input to the sediments. They probably reflect mainly silt and fine sand size material inwashed during periods favourable to increased rates of allogenic sediment delivery. 3. Three such periods are notable over the last 3 k years, beginning around 1000 BC, AD330 and AD 1260. Each of these changes corresponds to changes in chemical stratigraphy that can also be interpreted as reflecting shifts in erosion regimes. The two later episodes correlate with geochemical evidence for temporarily reduced lake productivity. Given the chronological uncertainties, it seems likely that each of these episodes occurred during times of deteriorating climate and they reflect responses to this both by surface processes operating within the catchment and by the aquatic biota within the lake. 4. As well as indicating changes in erosional regime coinciding with the magnetic evidence for increase detrital input, the chemical stratigraphy also records the progressive effects of weathering, notably of biotite, over the whole period of accumulation. 5. The erosive episodes appear to increase in severity, culminating in the latest one, believed to reflect the impact of the

F. Oldfield et al. / Quaternary Science Reviews 29 (2010) 1040–1054

6.

7.

8.

9.

10.

Little Ice Age. Pollen changes occurring just before this reflect land cover changes at lower altitude resulting from human activity. The millennial scale fluctuations in erosive input are superimposed on an overall trend of increasing catchment input and declining biogenic silica. This may reflect, in part, the effects of long term weathering. However, in view of the inferred links with climate and in the absence of human impact in the catchment, this trend, like several others from the N hemisphere, may be linked to the overall climate trend reflecting the decline in solar input to mid-high latitudes in the northern hemisphere during the Holocene. The variability linked to catchment changes has a lower frequency signal than that expressed in the LOI record, which appears to be one of the indicators of changing aquatic biomass and organic matter content that reflect within-lake changes in productivity. Coincident with each inferred increase in catchment erosion, there are indications of reduced within-lake productivity, but whereas the catchment-derived signatures tend to be persistent and, in the long term, incremental, aquatic productivity signatures are not, save to some degree in the uppermost part of the sequence, post ca AD 1260. This suggests that both interlinked parts of the lake catchment system are responding, at times synchronously, to climate forcing, but with quite different response thresholds, degrees of inertia and capacity for recovery. The two patterns of change interact contrapuntally. The close link between lower median particle size and lower LOI values, and vice versa remains difficult to explain. It may reflect a shift in the balance between coarse clastic input from the lake shore, possibly linked to water level changes, and finer inwash from the catchment during periods of increased erosion. Overall, the sediments of Lochan Uaine appear to have recorded complex system responses on three timescales reflecting (a) the long term decline in northern hemisphere insolation during the Holocene, (b) millennial scale forcing possibly contemporary with that found in many other mid-late Holocene records and (c) much shorter term, quasi-cyclic but clearly a-periodic sub-millennial fluctuations. Such superimposed variability makes lake sediments of this type elegant and sensitive registers of the impacts of past climate variability within both terrestrial and aquatic systems and of the responses of each to externally driven perturbations. However, much more work is needed to elucidate the process interactions responsible for the changes recorded.

Acknowledgements The initial research reported here was funded by NERC under the ‘TIGGER’ initiative. We are grateful for much help with the figures from Sandra Mather and Suzanne Yee. References Allen, C.E., Darmody, R.G., Thorn, C.E., Dixon, J.C., Schlyter, P., 2001. Clay mineralogy, chemical weathering and landscape evolution in Arctic–Alpine Sweden. Geoderma 99, 277–294. Anderson, N.J., Rippey, B., 1988. Diagenesis of magnetic minerals in the recent sediments of a eutrophic lake. Limnology and Oceanography 33, 1476–1492. Ballantyne, C.K., 2002. Geomorphological Changes and Trends in Scotland: Debris Flows. Scottish Natural Heritage Commissioned Report F00AC107A. SNH, Perth. Battarbee, R.W., Cameron, N.G., Golding, P.G., Brooks, S.J., Switsur, R., Harkness, D., Appleby, P.G., Oldfield, F., Thompson, R., Monteith, D.T., McGovern, A., 2001. Evidence for Holocene climate variability from the sediments of a Scottish remote mountain lake. Journal of Quaternary Science 16, 339–346. Bayfield, N.G., 1984. The dynamics of heather (Calluna Vulgaris) stripes in the Cairngorm Mountains, Scotland. Journal of Ecology 72, 515–527.

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