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Late Holocene coastal sand movements in the Outer Hebrides, N.W. Scotland
Marine Geology 210 (2004) 281 – 306 www.elsevier.com/locate/margeo
Late Holocene coastal sand movements in the Outer Hebrides, N.W. Scotland Sue Dawson a,*, David E. Smith b, Jason Jordan c, Alastair G. Dawson c a
Research Consultant in Quaternary Science, 26 Beverley Road, Leamington Spa, Warwickshire, CV32 6PJ, UK b School of Geography and the Environment, University of Oxford, Mansfield Road, Oxford, OX1 3TB, UK c Centre for Quaternary Science, Geography, Coventry University, Priory Street, Coventry CV1 5FB, UK Received 20 March 2002; received in revised form 10 January 2003; accepted 3 May 2004
Abstract Lithostratigraphical and biostratigraphical investigation of coastal marshes along the Atlantic coast of the Outer Hebrides from Lewis in the north to Barra in the south discloses inland-tapering sand units within marshland areas. The inland extent of each sand unit has been radiometrically dated and the units have been collectively interpreted as a proxy for past coastal storminess. The data appear to indicate that for the study sites investigated, the majority of the sand units were produced during episodes of climate deterioration both prior to and after the well-known period of Medieval warmth (MWP). Many were produced after ca. AD 1400. It is argued that the episodes of sand blow indicated by the deposits may reflect periods of increased cyclogenesis in the Atlantic associated with increased sea ice cover and an increase in the thermal gradient across the North Atlantic region. D 2004 Elsevier B.V. All rights reserved. Keywords: machair; storminess; North Atlantic; Little Ice Age (LIA); Medieval Warm Period (MWP); Outer Hebrides; Scotland
1. Introduction In recent years reconstructions of Late Holocene climate variability of the North Atlantic region have been enhanced considerably as a result of Greenland ice core research (Hammer et al., 1997). In particular, our knowledge of changes in North Atlantic storminess has been transformed as a result of the published chronology of Na+ (sea salt) concentration changes from the GISP2 drill site (Meeker and Mayewski, 2002). Annual changes in sea salt concentration at
* Corresponding author. E-mail address: [email protected] (S. Dawson). 0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2004.05.013
GISP2 calculated for the last 1400 year are interpreted as a regional signal of North Atlantic winter storminess that appears to have undergone a dramatic change at ca. AD 1400– 1420. Winter storminess prior to this time during the Medieval Warm Period (MWP) (Mayes, 2000) appears to have been low. After this time, storminess appears to have increased significantly and to have continued to remain ‘‘high’’ until present. These observations should also be considered within the context of the classic research of Lamb (1979) who argued that the cold conditions of the Little Ice Age (LIA) were associated with an increased meridional thermal gradient across the North Atlantic that, in turn, led to a southward displacement of the North Atlantic storm track. Considered together, the
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results of Lamb (1979) and Meeker and Mayewski (2002) point to an increase in winter storminess across the British Isles after ca. AD 1400 –1420. The chronology of coastal dune formation for the late Holocene has never been resolved adequately. For example, Clarke et al. (2002) in a study of windstorm events along the Aquitaine coast of S.W. France, identified two key periods of coastal sand invasion during the Late Holocene as having occurred between ca. 900– 1300 years ago and between 250 and 500 years ago and attributed to the MWP and the LIA. Similarly, in study of coastal dune development in eastern England, Orford et al. (2000) concluded that the key periods of dune development during the last ca. 2000 years took place between ca. 1500 and 1000 years ago and between ca. 500 and 200 years ago. They further speculated that periods of dune construction may follow intervals of time when relative sea level had been falling. These examples support the view that a major period of coastal sand drift took place during the LIA and that earlier periods of dune building appear to have taken place at different times in different places along the North Atlantic coastal margin. In this paper, we describe a series of Late Holocene coastal windstorm deposits from the Scottish Outer Hebrides that form part of the landward edges of coastal sand accumulations that are intercalated with peat. Radiocarbon dating of peat deposits enclosing the sand units is used to construct a local chronology of Late Holocene windstorm events that are, in turn, used to test the Meeker and Mayewski (2002) hypothesis that an increase in winter storminess took place after ca. AD 1400 –1420 having been preceded by a period of negligible storm activity during the MWP.
2. The study area The Outer Hebrides island chain (Fig. 1) extends from broadly north to south across a latitudinal range of 56– 58jN, thus occupying an area at the heart of middle latitude atmospheric circulation patterns. The islands rise to heights locally in excess of 200 m, but along their western, Atlantic margin they are generally low-lying. Here, the coast is fringed by a largely stabilised dune system consisting of a single low ridge
to seaward and a gently sloping surface to landward, where the system ends amidst peat areas, rocky outcrops and many small lochs. The sand areas are known locally as machair, derived from the Gaelic word for extensive beach or plain and which describes a lowlying, grass-dominated coastal plain composed of calcareous sand with a calcium carbonate content up to ca. 80% (Ritchie and Whittington, 1994). Ritchie (1979) attempted to define the major characteristics of machair in South Uist, however, the term is not restricted to the Outer Hebrides but is often commonly used throughout Scotland and locally in Ireland. The landward limit of the machair can end in a marsh or lochan. It is generally considered to have formed as a result of aeolian processes and has a distinctive dune morphology. Notwithstanding the apparent uniformity of the machair surface, exposures in the dune face to seaward disclose soil and peat horizons which reflect episodes of stability and imply a complex evolution. Studies of sediment exposures and of foreshore peat deposits have been undertaken by several research workers including Ritchie (1979, 1985), Ritchie and Whittington (1994) and Gilbertson et al. (1996, 1999) who have dated episodes of sand encroachment and retreat. It was on the basis of latitudinal extent of the dune system and the evidence for fluctuation in its development as demonstrated by previous workers, that the present study was undertaken. The research concentrated upon sediments within back-barrier, lowlying coastal areas and attempts to determine sediment age through the use of spatially continuous deposits.
3. Research context Studies of machair deposits have contributed to our understanding of landscape evolution, archaeology as well as land use and settlement change (Angus, 1997). The influence of anthropogenic land use changes on machair development were strongly felt during the early 19th century due to the onset of kelp (seaweed) harvesting. This, combined with population growth placed great stresses upon the machair since it represented the majority of cultivable land in the Outer Isles (Angus, 1997). The cropping of marram grass (bent) until it was made illegal in the late 17th century also contributed to machair destabilisation. In addi-
S. Dawson et al. / Marine Geology 210 (2004) 281–306
7°° 10
0
6°° 40
30
20
283
50
N
km
58°°
Key 1. Cliobh 2. Cnip 3. Riof 4. Horgabost 5. Balranald 6. Nunton 7. Kildonan 8. Borve
Lewis 1 23
Harris
4
North Uist
5 0
200 km
6
Benbecula
58°°
7
South Uist
54°°
57°° Barra
8
50°° 8°°
4°°
0°°
Fig. 1. Field site locations, The Outer Hebrides, Scotland.
tion, a fall in the local water table caused by extensive land drainage and reclamation is believed to have caused the machair surface to dry out thus increasing the likelihood of degradation due to windstorm activity. Today the machair is subject to change with erosion and deposition a natural component of the machair system. The system is dynamic by nature and change in the landscape is an inherent part of its existence.
Previous work in the area has generally been concerned with examination of exposures of intertidal sediments along the coast, mainly in the Uists and Barra (Ritchie, 1979, 1985; Ritchie and Whittington, 1994; Gilbertson et al., 1996, 1999). Such coastal exposures are widespread along the island chain and have afforded the opportunity to radiometrically date episodes of sand encroachment and retreat. Over the last three decades this has led to the development of a
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series of models of machair evolution (Ritchie, 1979, 1985; Ritchie and Whittington, 1994). Different models of machair evolution have been developed based upon descriptions of machair sequences, and in particular, the examination of intertidal organic sequences in North and South Uist as well as in adjacent islands. At the end of the last glaciation (Late Devensian) the pre-machair surface was covered by glacially scoured substrate with marshes and lochans occupying the depressions. With the subsequent rise in sea level during Holocene glacially derived sands from offshore were mobilised and mixed with shell debris. This material was reworked into coastal ridges and moved onshore overriding preexisting lacustrine and organic deposits Ritchie, 1966 (Ritchie, 1985). The complex nature of machair evolution was demonstrated by research undertaken in the Uists by Ritchie and Whittington (1994). It was shown that locally variable stratigraphic sequences resulted from non-synchronous sand blow events and highlighted the difficulty in establishing a regional chronology of machair formation. Ritchie and Whittington (1994) showed that the earliest sands represent ‘overlapping landform suites’ which through long term erosion, outcrop today in the present intertidal zone. This led to a revision of models of sand movement on to the coastal plains in the Uists. They showed that the onset of sand movement was sometimes different in adjacent areas, possibly reflecting differences in surrounding topographies or other local effects. It is important to recognise that the pre-sand encroachment surface will be variable across the island chain and also to acknowledge that local coastal configuration will play a part in determining the subsequent pattern of sand encroachment. The sand supply was suggested by Ritchie and Whittington (1994) to be finite. However, this interpretation differs from Mate (1992) who believed that continuous production of offshore biogenic carbonate sand would lead to the continuous formation of machair through time. The cyclical nature of machair stability and instability prior to any human influence on the landscape was demonstrated by Ritchie and Whittington (1994), who identified sand encroachment under scenarios of rising sea levels, coastal erosion and climate changes.
4. Methodology and techniques The work undertaken in the Outer Hebrides is part of a larger research programme concerned with Holocene relative sea level changes and machair evolution in the Outer Hebrides. The approach adopted in this paper was to examine the evidence for the youngest sand movement identifiable in the stratigraphic record. This was undertaken from strictly comparable locations over as wide a latitudinal extent as possible, using standardised techniques, to enable comparisons to be made between sites and to identify any possible trends. In contrast to many previous studies, which considered machair evolution throughout the entire Holocene, only the most recent episodes, which are enclosed by organic material and in broad terms identifiable from historical records, are described here. Field work combined geomorphological and stratigraphical approaches. Morphological studies involved mapping the selected site areas at a scale of 1:10,000 according to standard geomorphological mapping techniques. Preliminary stratigraphical investigation was undertaken using hand-operated Eijkelkamp gouges to depths of up to 8 m. All boreholes were levelled to local Ordnance Datum (ca. 20 cm above British Ordnance Datum Newlyn; Thompson, 1980). Boreholes were confined to transects which located broadly at right angles to the coastline, across the inland margin of the machair, and onto the peat surface. The inland limit of the uppermost sand unit was thus determined. Sediment samples were taken across the intercalated peats and sand layers using a Stitz piston corer with a 5-cm diameter sampling tube and returned to the laboratory for further analysis. Biogenic samples were taken at the upper and lower boundaries of selected sand units for radiocarbon dating to encompass the chronology of the events. Dates were provided and calibrated by Beta Analytic, Miami (Table 1). In the laboratory, samples for diatoms were prepared according to standard techniques (Barber and Haworth, 1981). Unfortunately, the samples were extremely sparse and viable counts could only be undertaken at sites in Lewis (Kneep) and Harris (Horgabost). Only the presence of species is noted from other sites studied. Where possible, 300 diatoms were counted and expressed as a percentage of total diatom valves (%TDV). Species are categorised
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Table 1 Radiocarbon age determination for sand units in the Outer Hebrides Site
Core/depth and sample thickness
Altitude at surface m OD (Local)
Lab. code
Radiocarbon date 2r
Calibrated date 2r BP (probability method)
Material dated
Stratigraphic position
Horgabost, Harris Horgabost, Harris Cnip, SW Lewis Riof, SW Lewis Riof, SW Lewis Riof, SW Lewis Riof, SW Lewis Balranald, N. Uist Balranald, N. Uist Nunton, Benbecula Nunton, Benbecula Kildonan, S. Uist Borve, Barra
C1 (22 – 24) C1 (64 – 66) K2 (26 – 28) R10 (66 – 68) A7 (22 – 24) R8 (23 – 25) R8 (28 – 30) B4.1 (71 – 73) B5.1 (53 – 55) N5.1 (26 – 28) N5.2 (40 – 42) K4.1 (222) B15.1 (35 – 37)
2.83 2.83 2.68 3.97 3.37 3.39 3.39 4.94 4.49 1.98 1.98 4.17 3.18
Beta-120964 Beta-120965 Beta-127973 Beta-127974 Beta-130800 Beta-130803 Beta-130802 Beta-130797 Beta-130798 Beta-138741 Beta-138742 Beta-138740 Beta-130799
120 F 40 430 F 40 320 F 60 420 F 60 190 F 40 290 F 90 460 F 100 490 F 50 150 F 80 Modern 1140 F 40 950 F 40 670 F 130
AD 1670 to 1945 AD 1413 to 1520 AD 1445 to 1665 AD 1411 to 1532 AD 1722 to 1815 AD 1437 to 1696 AD 1381 to 1643 AD 1387 to 1492 AD 1655 to 1955 N/A AD 800 to 985 AD 1017 to 1164 AD 1150 to 1472
Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat
Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat
according to the halobian classification (Vos and de Wolf, 1993) and nomenclature follows Hartley (1986). Polyhalobian and mesohalobian classes generally represent marine conditions, whilst oligohalobian and Halophobes reflect freshwater habitats. A comparison of sediments from the modern beach environment and the sand blow units was investigated at the sites in SW Lewis and Harris by particle size analysis. Modern samples from the present beach and dune systems were collected and samples were obtained from the cores within the sand units studied. The samples were analysed by the sieving method using a nest of 22 sieves and encompassed a range from 2 to 0.06 mm.
5. Locations examined 5.1. Lewis Along the Atlantic coastline of Lewis, the machair is intermittent, with dune systems being often confined to the mouths of bays or below cliffs. The three locations studied exemplify this, each being confined close to the coast by rising ground inland. 5.1.1. Cnip The site at Cnip (Figs. 1– 3) lies on the Bhaˆltos Peninsula in SW Lewis and is a relatively small area of machair plain flanked to the south by a freshwater loch, Loch na Cuilp, and steep unvegetated cliffs (see
above sand below sand below sand below sand below sand below sand below sand below sand below sand above sand below sand below sand below sand
Fig. 2). The machair area lies directly behind a highly active dune field that adjoins a sandy beach. Lying to the east of the main Cnip site is a small area of marsh (K transect) that offers a more sheltered basin in terms of sediment accumulation. Stratigraphic transects across the smaller basin were undertaken and the stratigraphic diagram is shown in Fig. 3. 5.1.2. Riof The site at Riof lies ca. 500 m to the west of Cnip on the same peninsula and consists of a large expanse of machair and low-lying Carex-Sphagnum marsh (Fig. 4). The Riof site is surrounded by rocky outcrops and hills, and separated from the sea by an extension of the dune/beach system that can be seen at Cnip. Boreholes were put down in two paralel transects from the south of the basin moving seaward to the north. The resultant stratigraphies can be seen in Fig. 4A (R transect) and 4B (A transect). In both localities, the modern surface soil/peat is underlain by sand. The main differences between the stratigraphies of the two sites lie in their proximity to the modern shoreline. The boreholes that were put down at Cnip lie directly behind a dune system, whereas the boreholes at Riof are situated on a machair plain approximately 150 m behind the dune complex. Microfossil work has been carried out on a number of cores from the Riof sites but the preservation of diatom frustules is very poor due to the prevailing acidity of the environment. Diatoms are present in some of the Cnip boreholes. Particle size
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30m
(burial cist)
10
m
m
30
Traigh na Berie Cnip (small marsh) K
m
30
MLWS
B
Cnip (main site) C
MHWS
D E
10m
30m
m 70
50m
c Cuil na ch Lo
(modern grain size samples)
A
R
30m
10m
Riof
70m 50m
A
Loch Baravat (broch)
Loch na Berie
Roads Rivers & streams Buildings Transect & boreholes ( = sampled borehole)
m
30
A
Contours (10m intervals)
10m
50m
Rock outcrop
Extent of peat N
Marsh Beach Break of slope Dune ridge
0 Dunes
Fig. 2. Cnip (Kneep) and Riof (Reef), location and borehole transects.
metres
300
S. Dawson et al. / Marine Geology 210 (2004) 281–306
Seaward (northeast) +3
287
Inland (southwest) K2
K1
320+-60
+2
Metres OD (local)
+1
0
-1
-2
Phragmites peat
Organic sand
-3 1590+-90 Radiocarbon date
0
Metres
Shelly sand
30
Fig. 3. Stratigraphic transect (K) at Cnip (Kneep).
analysis of modern samples from the Riof dunes as well as sand from the sampled cores are shown in Figs. 5 and 6. The particle size curve displays a well sorted uni-modal distribution, for the contemporary samples with a range compatible with aeolian processes (Briggs, 1977). Radiocarbon dates (Table 1) show two broad periods of sand movement. Sand deposition at Riof was taking place after 460 F 100 radiocarbon years BP and 420 F 60 radiocarbon years BP. The upper sand deposition at Cnip occurred after 320 F 60 radiocarbon years BP. 5.2. Harris Harris is similar to Lewis, with machair areas confined to embayments and valley mouths. Here, Horgabost is typical of the sites examined.
5.2.1. Horgabost Liana Horgabost is a large expanse of Phragmites and Sphagnum marsh that is fringed by a 50-m strip of tussock bog on its seaward side (see Fig. 7). There is a ca. 300 m wide area of machair to the north of the site that separates the marsh from the beach and its associated dune system. Flowing from the south of the site along the western margin is a small stream, the Gill Meody, which cuts through the tussock strip, into the machair and through the modern dunes into the sea. Boreholes were taken throughout the Liana Horgabost marsh as well as in the tussock strip (Fig. 8). Directly north of the marsh lies a dune complex, a small, compact machair plain and a machair and tussock bog. This complex is partly reflected in the stratigraphy from the Liana Horgabost site. The upper units of the boreholes contain a decreasing amount of
288
A
R1
Seaward (northwest) R2
R3
R4
R5
R6
R7
R8
R9
R10
4
290 +- 90 BP
Metres OD (local)
3 460 +- 100 BP
2
1
0 0
Metres
50
Sandy peat Phragmites peat
Shelly sand 420 +- 60 BP
Radiocarbon date
Fig. 4. (A) Stratigraphic transect (R) at Riof. (B) Stratigraphic transect (A) at Riof.
420 +- 60 BP
S. Dawson et al. / Marine Geology 210 (2004) 281–306
Inland (southeast)
289
Fig. 4 (continued ).
S. Dawson et al. / Marine Geology 210 (2004) 281–306
290
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Fig. 5. Riof: particle size analysis of modern beach and dunes.
S. Dawson et al. / Marine Geology 210 (2004) 281–306
RIOF R8 196 - 200cm
RIOF R8 20 - 22cm 99.99
100
99.99
90
99.9 99.8
80
99 98
80
99 98
70
95 90
70
95 90
80 70 60 50 40 30 20
60 50 40
10
30
5
Percent of total weight
99.9 99.8
80 70 60 50 40 30 20
60 50 40
10 5
30
20
2 1
10
0.5 0.2 0.1 0.05
10
0
0.01
0
2 1 0.5 0.2 0.1 0.05
20
5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 m .2 .0 .7 .5 .2 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 -1 -1 -0 -0 -0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 63µ <
0.01 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 m .2 .0 .7 .5 .2 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 -1 -1 -0 -0 -0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 63µ <
Sieve Size (Phi)
Sieve Size (Phi)
RIOF R10 136 - 140cm
RIOF R10 36 - 40cm 99.99
100
99.99 99.9 99.8
80
80
99 98
70
95 90
70
95 90
80 70 60 50 40 30 20
60 50 40
10 5
30
2 1 0.5 0.2 0.1 0.05
20 10 0
0.01 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 m .2 .0 .7 .5 .2 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 -1 -1 -0 -0 -0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 63µ <
Percent of total weight
90
99 98
80 70 60 50 40 30 20
60 50 40
10
30
5
20
2 1
10
0.5 0.2 0.1 0.05
0
0.01 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 m .2 .0 .7 .5 .2 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 -1 -1 -0 -0 -0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 63µ <
Sieve Size (Phi)
Sieve Size (Phi)
RIOF A7 162 - 166cm 99.99
90
99.9 99.8
80
99 98
70
95 90 80 70 60 50 40 30 20
60 50 40
10
30
5
Cumulative percent (log scale)
Percent of total weight
100
20
2 1
10
0.5 0.2 0.1 0.05
0
m 5 0 5 0 5 .2 .0 .7 .5 .2 00 25 50 75 00 25 50 75 00 25 50 75 00 25 50 75 00 µ -1 -1 -0 -0 -0 0. 0. 0. 0. 1. 1. 1. 1. 2. 2. 2. 2. 3. 3. 3. 3. 4. <63
Cumulative percent (log scale)
90
99.9 99.8
Cumulative percent (log scale)
Percent of total weight
100
Cumulative percent (log scale)
90
Cumulative percent (log scale)
Percent of total weight
100
291
0.01
Sieve Size (Phi)
Fig. 6. Riof: particle size analysis of sand samples from cores R8, R10 and A7.
292
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Traigh Horgabost 10m
30m
MLWS
(modern grain size samples)
MHWS
10
k oc ss u T
G
m
B
ea ar
50m
D
m
30
C/D C E
A
F
H
I
Liana Horgabost Z
30
50m
m 90
m 110
70m
m
dy eo ll M
Gi
Marsh
Roads Rivers & streams
50m
A
Coniferous trees
0
Buildings
Beach
Contours (10m intervals)
Dunes
Transect & boreholes ( = sampled borehole)
Extent of peat
Break of slope
Tussock marsh
Dune ridge
Machair type grassland Fig. 7. Horgabost, location and borehole transects.
metres
300
N
Rock outcrop
Inland (southwest)
Seaward (northeast) 4.0
C-2
C-1
C1 (25) C2 C1 (37)
C1
C2 (25)
C2 (50)
C3
C4
3.5 3.0 120 +- 40
2.5
430 +- 40
Metres OD (local)
1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -2.5
Phragmites peat
430 +- 4040 Radiocarbon
date
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2.0
Generalised peat Shelly sand
0
Metres
50
-3.0 -3.5
Fig. 8. Stratigraphic transect (C) at Horgabost.
293
294
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shelly sand inland, away from the source of aeolian sedimentation. Dating of this sand unit shows that accumulation was taking place at borehole C1 after 430 F 40 radiocarbon years BP and had halted before 120 F 40 radiocarbon years BP. The date for the initiation of the sand movement is similar to the earlier date from the lower sand unit at Riof (420 F 60 BP). The sand unit was examined for its diatom content to ascertain the dominant mode of deposition and the resultant diatom assemblages are shown in Fig. 9. A mixed assemblage is evident, with species from marine, brackish and freshwater environments, however, the oligohalobous species are in greatest abundance with Cocconeis placentula, Fragilaria construens, Fragilaria pinnata and Pinnularia sp. The brackish species Diploneis interrupta and Navicula peregrina as well as the marine Paralia sulcata suggest a saline influence within the shelly sand. The presence of a limited number of marine species possibly reflects the proximity to marine influence and is not an indication that the sediment was deposited by marine water. An examination of modern machair sand from the beach and dunes at the site provided a similar diatom content, although too sparse to enable a full count, the species broadly suggest a similar sediment source. Particle size analysis was undertaken of modern samples from Horgabost beach and dune (Fig. 10A) as well as sand from the sampled core C1 (Fig. 10B). The particle size curves displays well sorted unimodal distribution for the contemporary samples with a range compatible with aeolian processes (Briggs, 1977). The samples from core C1 exhibit a similar grain size distribution to the present day dunes area and thus suggest an aeolian provenance. 5.3. The lower islands 5.3.1. North Uist (Balranald) The site at Balranald, a local Nature Reserve, is located on the westernmost area of North Uist. The machair areas are fragmentary in this locality, broken up by areas of marsh, loch and rock outcrops. The site occupies a low-lying Phragmites reedswamp marshland area, at the edge of a small loch lying to the east. The marsh lies beyond two dune-fringed areas facing NW and SW, lying either side of a rocky headland and
a gently sloping machair plain. The edge of the machair and the reedswamp area was investigated with a borehole transect and the resulting stratigraphy is shown in Fig. 11. The key feature of the stratigraphy is the distinctive wedge of coarse shelly sand beneath a Phragmites surface peat. The sand thickens to seaward, reaching a thickness of at least 2.5 m, and wedges out landward over a distance of 120 m to ca. 0.20 m. Landward of borehole B6 the sand occurs as occasional grains within the peat. Two cores were selected for dating to determine if the wedge became progressively younger inland. The start of sand deposition was dated to after 490 F 50 radiocarbon years BP at borehole B4, and after 150 F 80 radiocarbon years BP at borehole B5, farther inland near the tip of the wedge of sand. 5.3.2. Benbecula (Nunton) Benbecula is a low-lying island situated between North and South Uist, which rarely exceeds 15 m OD in altitude, and is characterised by numerous lochs. The coastline is dominated by low rock platforms and headlands with small pockets of shingle in hollows on the platform surface. The study site of Nunton lies between two prominent headlands and is located landward of Culla Bay which faces West. The site is a small area of low-lying marsh and lochan lying at the base of a sloping machair surface, approximately 100 m inland of the present coast. A single line of dunes lies behind the modern beach. The marsh surface lies at ca. 2.5 m OD and an extensive deposit of coarse shelly sand lies beneath a thin cover of peat (Fig. 12). In the vicinity of borehole N5 the peat becomes more dissected below the surface. To seaward, at least 3 m of sand is present (N1) whereas, further inland peat dominates the upper 1.5 m of sedimentation (N4). A radiocarbon date obtained from peat directly beneath an inland tapering wedge of sand at N5 gave 1140 F 40 radiocarbon years BP. Peat directly above the wedge of sand was also dated but returned a modern age (probably because here the overlying peat was very shallow, less than 40 cm thick). 5.3.3. South Uist (Kildonan) The Atlantic coast of South Uist consists of wide shelly sand beaches, dunes and machair over a distance of 35 km, separated by occasional rocky head-
S. Dawson et al. / Marine Geology 210 (2004) 281–306
Fig. 9. Diatom diagram, borehole C1, Horgabost. Diatoms are expressed as percentage total valves (TDV) and are based upon a count of 300 valves per level. 295
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Fig. 10. (A) Horgabost particle size analysis: modern beach and dunes. (B) Horgabost particle size analysis: core samples form C1.
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297
Fig. 10 (continued ).
lands. The low-lying areas beyond the gentle machair slopes are generally 1 – 2 m OD and many shallow sand-infilling lochs are present. The site at Kildonan lies landward of a prominent ridge of rock. An extensive machair plain ca. 700 m wide is located beyond the rock ridge with low-lying marshland beyond. The marshland extends up to 2 km inland towards the high ground in the vicinity of Kildonan Glen (Fig. 13). Stratigraphic investigation of the marshland revealed an extensive sand wedge underlying the surface peat (Fig. 13). Beyond the machair slope the sand landward of borehole K3 is underlain by extensive peat deposits. The sand is coarse with many shell fragments and gradually thins towards the
east (K8). The onset of sand deposition was dated in K4 to after 950 F 40 radiocarbon years BP. The sand unit within borehole K4 was examined for its diatom content to try and ascertain the dominant mode of deposition. The sand unit was sparse and there were too few species to enable a full count to 300 species. Nevertheless, a mixed assemblage is present, with species from marine, brackish and freshwater environments. Oligohalobous taxa include Cocconeis placentula, Fragilaria construens, Fragilaria pinnata and Pinnularia sp. The presence of isolated marine taxa probably reflects the individual species being blown into the marsh as they are not dominant.
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Fig. 11. Balranald, North Uist, site location, borehole transect and generalised stratigraphic section.
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Fig. 12. Nunton, Benbecula, site location, borehole transect and generalised stratigraphic section. 299
300
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Fig. 13. Kildonan, South Uist, site location, borehole transect and generalised stratigraphic section.
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Fig. 14. Borve, Barra, site location, borehole transect and generalised stratigraphic section.
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5.3.4. The Isle of Barra (Borve Valley) The mountainous Isle of Barra forms the SW extremity of the arc of islands comprising the Outer Hebrides, with the smaller islands of Vatersay, Sanday and Pabbay to the south. North West facing bays with extensive beach and machair areas form the west coast of Barra. Rock promontories break the western coastline, the Borve valley occupying the head of Traigh Tuath, to the north of Borve Point. The study site in the Borve valley lies beyond the rocky headland extending across an area of marshy low ground which in turn extends ca. 1.5 km east of the present coastline (Fig. 14). Intertidal rock surfaces form the majority of the Borve promontory, and despite no extensive beach/dune complex to seaward, the area has one of the largest machair areas in W. Barra. The stratigraphy of the site is shown in Fig. 14. A radiocarbon date for the top of peat overlain by shelly sand at borehole B15 indicates that the sand reached its inland limit after 670 F 130 radiocarbon years BP.
6. Discussion 6.1. Origin of the sand The stratigraphy at the sites examined shows that the machair plain tapers into peat at its inland margin, and though the very tip of the tapering wedges may exhibit discrete thin horizons of sand within the peat, the wedges are generally massive and consist of a single horizon. Otherwise, the surface of the sloping sediment wedges, as far as can be determined, is continuous and the base of the wedges is similarly uncomplicated, although at greater depths other layers of sand occur. The stratigraphical evidence is thus relatively simple. It should be noted that limitations of the equipment precluded detailed investigation further seaward where the sand is more than ca. 3 m thick. Nevertheless, a sustained episode of sand accumulation reaching inland beyond the visible machair and dune area is inferred to have taken place. Microfossil preservation is very poor within the blown sand units dated. Nevertheless, evidence from Harris and South Uist provide information which enable the storm deposits to be broadly characterised. A predominantly fresh-brackish assemblage character-
ises the sand units. This is similar to the diatoms which characterise the modern dune and beach areas at Riof and Horgabost. It should be noted that the dynamic nature of the machair environment will lead to the presence of many allochthonous species while the presence of marine indicators possibly reflect diatoms carried into the back barrier areas by the wind in the vicinity of marine conditions. The absence of a sustained marine signal in the sand precludes a marine water lain source to the deposit. Grain size analysis of both modern samples from beach and dune areas in SW Lewis and Harris shows that the upper sand units studied within the collected cores to be of similar provenance to the present beach and dune areas. The grain size distributions are typical of aeolian processes (Briggs, 1977). 6.2. Chronology Table 1 provides dates for all upper sand blow events throughout the Outer Hebridean island chain. In terms of aspect, all sites studied face between W and NW. The dated storm events for the seven sites exhibit distinctive age clustering. Only three of the dated storm horizons (Nunton, Kildonan and Borve) exhibit standard errors that place the sediments within the MWP while it should also be noted that the mean date of one of these (Borve) dates to before AD 700. The majority of the uppermost dated horizons, however, appear to have been deposited after ca. AD 1300. The stratigraphical evidence disclosed in this study indicates that a sustained period of machair development in which shelly sands encroached inland ended some time between the MWP and perhaps as recently as 200 years ago, although the volume of sand deposited after the end of the MWP was minor at the sites concerned compared with what had accumulated before. Little is known of the prevailing climate in the Outer Isles prior to the MWP although it is generally recognised that the expansion of the Roman Empire and the subsequent Dark Age was a period of climatic deterioration. It is tempting to develop an hypothesis in which the frequency of storm events is related to past episodes of climate change normally described in terms of former air temperature characteristics with greater storm frequencies during periods of cold (e.g.
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the LIA), and periods of reduced storminess during episodes of warmth (e.g. MWP). Such explanations, however, are fraught with difficulty since the westerly winds associated with severe North Atlantic cyclones (storms) are normally associated with higher than average winter temperatures. Equally, it can be argued that the coldest episodes of the LIA (e.g. the 1690s) may have been associated with significantly reduced storminess owing to the expansion of the polar anticyclone (Lamb, 1995). Meeker and Mayewski (2002) provided the most comprehensive record of North Atlantic storminess for the last ca. 1400 years based on measurements of sea salt (Na+) concentrations in the GISP2 Greenland
303
ice core (Fig. 15). Their measurements show that reduced storminess was a feature of the MWP that lasted until ca. AD 1400. They detected a marked increase in storminess frequency (increased Na+ concentration) that commenced at this time and observed that this ‘‘state’’ of increased storminess has lasted until present. The dated sand units are plotted on the same timescale to illustrate the clustering of the dates during the LIA time period. This pattern of storminess change is broadly similar to changes in Icelandic sea ice cover for the last ca. 1000 years (Koch, 1937) (Fig. 16), characterised by reduced/negligible sea ice cover during the MWP and a marked expansion after ca. AD 1400.
Fig. 15. Upper, Concentrations of Na+ (Parts per billion) at GISP 2 drill site, central Greenland, 987 – 1987 AD expressed as departures from long-term average, after Kreutz and Mayewski (1997). Lower, 2 sigma calibrated radiocarbon dates showing the initiation of the uppermost sand deposition at all sites in the Outer Hebrides.
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Fig. 16. Graph showing the number of weeks with ice around Iceland, for 20-year periods from 860 to 939, after Koch, 1937.
Lamb (1971) argued that the expansion of sea ice across the Greenland Sea and around Iceland is likely to have resulted in an increase in the thermal gradient across the North Atlantic region. It is argued here that such increases in the thermal gradient in the North Atlantic atmosphere (and possibly also ocean) provides a simple explanation for the greater frequency of windstorm sediment deposition and machair formation post-AD 1400, and possibly also for the time interval studied here prior to the MWP. Whilst recognising that the mean values for two dates (Nunton N5.2 and Kildonan K4.1) occur clearly within the MWP, it is argued that the overwhelming majority of dated windstorm horizons can be explained by the climate and weather processes described above. Stratigraphical studies from sites along the Atlantic margin of Scotland and Ireland, as well as from the North Sea region provide support for the chronology of sand mobilisation described in this study. Gilbertson et al. (1999) describe late Holocene phases of sand movement 200 – 600, 1300 – 1700 and 3300 – 3800 years ago in the Outer Hebrides utilising the luminescence dating of calcareous machair sands. In northern England, Orford et al. (2000) report IRSL and radiocarbon dating of the majority of coastal dune systems which date within the time period ascribed to the LIA. In western Ireland, remobilisation of sand was noted in back barrier lagoonal sites at ca. 420 radiocarbon years BP to 120 radiocarbon years BP (Delaney and Devoy, 1995).
It is tempting to suggest that some of the recorded windstorm events may be related to anthropogenic activity related to overpopulation and animal grazing pressures characteristic of the 18th and 19th centuries (cf. Angus, 1994; 1997; Hunter, 2000) in the Outer Isles. Although such processes were undoubtedly important in machair destabilization and aeolian sediment transport, we maintain here that the increase in north Atlantic storminess that commenced at ca. AD 1400 – 1420 and has continued until the present, was a key factor in accounting for the ages of the windstorm deposits described from the sites examined within this paper. The cause of a presumed increase in coastal storminess during the LIA was considered by Lamb (1979) to be related to an increased thermal gradient across the polar atmospheric and oceanic fronts in the North Atlantic. He argued that the north –east Atlantic region at this time may have experienced greater cyclonic activity with occasionally intense windstorms amongst which he included the great storms that took place during the winters of AD 1694 – 95, 1697 –98 and 1703 –04. Yet, it is not such a simple matter as to conclude that all coastlines bordering the eastern North Atlantic may have experienced a dramatic increase in storminess. For example, may it have been the case that storminess may have actually decreased during the LIA at latitudes north of the inferred position of the polar atmospheric front.
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7. Summary The paper has examined the sedimentary characteristics of the uppermost sand sediments enclosed within organic deposits at comparable sites across the latitudinal extent of the Outer Hebrides. It is suggested that evidence for storminess may be recorded in the movement of coastal sand dune systems along the Atlantic coastline of the Outer Hebrides, Scotland. It is maintained that the sand movements identified disclose sufficient consistency in their pattern and timing that, notwithstanding the possible influence from anthropogenic activity, sediment supply changes and local site specific effects; episodes of storminess can be determined. These episodes may be related to more regional changes in the vigor and pattern of atmospheric processes. The overall pattern from morphological and stratigraphical evidence is of a concentration of sand blow over the last 2000 years across the Atlantic seaboard of Scotland and immediately adjacent areas. The geological evidence consists of locally persistent landward-tapering layers of blown sand in coastal peat mosses. These layers indicate that in respect of the study sites investigated, the most significant episodes of windstorm deposition pre-date and postdate the MWP. The geological data indicate that windstorm deposition may have occurred during periods of climate deterioration and sea ice expansion across the northern North Atlantic associated with an increased thermal gradient favourable for increased cyclogenesis. The data also appear to indicate that the maximum inland extent of the machair in the Outer Hebrides was not reached until after ca. AD 1400. Dated inferred sand drift episodes across Europe show synchroneity with increased sand mobilisation in SW France, NE England, SW Ireland and the Outer Hebrides implying a regional response to storminess with increased sand invasion during the cool periods of the LIA and the MWP (Lamb, 1995; Gilbertson et al., 1999; Wintle et al., 1998; Wilson et al., 2001).
Acknowledgements This work was carried out as part of contract no. ENV 4-CT97-0488, for the Commission of European Communities, DG XII: Science Research and the
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Environment (Biodiversity and global change unit). The authors would like to thank the following for fieldwork assistance; Robin Cullingford, Robert Devoy, Cesar Andrade, Concecß a´o Freitas, Scott Nichol, Matt Hill, Alaric Rae, Lea Winter and Michael Tooley, and to the many croft and farm owners who allowed access to their land. Graham Corley, Jason Jordan and Sue Dawson produced the figures. This paper is a contribution to International Geological Correlation Programme (IGCP) Project 367.
References Angus, S., 1994. The conservation importance of machair systems of the Scottish Islands, with particular reference to the Outer Hebrides. In: Baxter, J.M., Usher, M.B. (Eds.), The Islands of Scotland: A Living Marine Heritage, pp. 95 – 120 HMSO, Edinburgh. Angus, S., 1997. The Outer Hebrides: The Shaping of the Islands The White Horse Press, Harris. Barber, H., Haworth, E.Y., 1981. A guide to the morphology of the diatom Frustule, with a key to the British Freshwater Genera Freshwater Biological Association, Ambleside 109 pp.. Briggs, D., 1977. Sediments Butterworth, London. Clarke, M., Rendell, H., Tastet, J.-P., Clave´, B., Masse´, L., 2002. Late-Holocene Sand invasion and North Atlantic storminess along the Aquitaine Coast, Southwest France. Holocene 12, 231 – 238. Delaney, C., Devoy, R., 1995. Evidence from sites in western Ireland of late Holocene changes in coastal environments. Marine Geology 124, 273 – 287. Gilbertson, D.D., Kemp, R.A., Grattan, J. (Eds.), 1996. The Outer Hebrides: The Last 14,000 Years. Sheffield Environmental and Archaeological Research Campaign in the Hebrides vol. 2 Academic Press, Sheffield, p. 275. Gilbertson, D.D., Schwenninger, J.L., Kemp, R.A., Rhodes, E.J., 1999. Sand-drift and soil formation along an exposed North Atlantic coastline: 14,000 years of diverse geomorphological, climatic and human impacts. Journal of Archaeological Science 26, 439 – 469. Hammer, C., Mayewski, P.A., Peel, D., Stuiver, M. (Eds.), 1997. Greenland Summit Ice Cores: Greenland Ice Sheet Project 2 and Greenland Ice Core Project. Journal of Geophysical Research (Vol. vol. 102, 26315 – 26886. Hartley, B., 1986. A check-list of the freshwater, brackish and adjoining coastal waters. Journal of the Marine Biological Association of the UK 66, 531 – 610. Hunter, J., 2000. Last of the Free: A Millenial History of the Highlands and Islands of Scotland. Mainstream Publishing, Edinburgh. Koch, L., 1937. The East Greenland Ice. Meddelelser om Grønland, Udgivne af Kommissionen for Videnskabelige Undersøgelser I Grønland, Bd, 130, No. 3.
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Kreutz, K.J., Mayewski, P.A., 1997. Bipolar changes in atmospheric circulation during the Little Ice Age. Science 277, 1294 – 1296. Lamb, H.H., 1971. Climate, History and the Modern World Methuen Press, London. Lamb, H.H., 1979. Climatic variations and changes in the wind and ocean circulation: the Little Ice Age in the Northeast Atlantic. Quaternary Research 11, 1 – 20. Lamb, H.H., 1995. Climate, History and the Modern World, 2nd edition Routledge, London. Mate, I.D., 1992. The theoretical development of the machair in the Hebrides. Scottish Geographical Mag. 108, 35 – 38. Mayes, J., 2000. Medieval warm period. In: Hancock, P.L., Skinner, B.J. (Eds.), The Oxford Companion to the Earth Oxford Univ. Press, Oxford. Meeker, L.D., Mayewski, P.A., 2002. A 1400-year high resolution record of atmospheric circulation over the North Atlantic and Asia. Holocene 12, 257 – 266. Orford, J.D., Wilson, P., Wintle, A.G., Knight, J., Braley, S., 2000. Holocene coastal dune initiation in Northumberland and Norfolk, eastern UK: climate and sea-level changes as possible forcing agents for dune initiation. In: Shennan, I., Andrews, J. (Eds.), Holocene Land – Ocean Interaction and Environmental Change around the North SeaSpecial Publications - Geological Society (London) (Vol. vol. 166, 197 – 217. Ritchie, W., 1966. The Postglacial rise in sea-level and coastal
changes in the Uists. Trans. Inst. British Geographers 39, 79 – 86. Ritchie, W., 1979. Machair chronology and development in the Uists and adjacent islands. Proceedings of the Royal Society of Edinburgh 77, 107 – 122. Ritchie, W., 1985. Inter-tidal and sub-tidal organic deposits and sea level changes in the Uists, Outer Hebrides. Scottish Journal of Geology 21, 161 – 176. Ritchie, W., Whittington, G., 1994. Non-synchronous aeolian sand movements in the Uists: the evidence of the intertidal organic and sand deposits at Cladach Mor, North Uist. Scottish Geographical Magazine 110, 40 – 46. Thompson, K.R., 1980. An analysis of British monthly mean sea level. Geophysical Journal of the Royal Astronomical Society 63, 57 – 73. Vos, P.C., de Wolf, H., 1993. Diatoms and a tool for reconstructing sedimentary environments in coastal wetlands; methodological aspects. Hyrobiologia 269/270, 285 – 296. Wilson, P., Orford, J.D., Knight, J., Bradley, S.M., Wintle, A.G., 2001. Late Holocene (post-4000 yrs BP) coastal development in Northumberland, northeast England. Holocene 11, 215 – 229. Wintle, A.G., Clarke, M.L., Musson, F.M., Orford, J.D., Devoy, R.J.N., 1998. Luminescence dating of recent dune formation on Inch Spit, Dingle Bay, southwest Ireland. Holocene 8, 331 – 339.
Late Holocene coastal sand movements in the Outer Hebrides, N.W. Scotland Sue Dawson a,*, David E. Smith b, Jason Jordan c, Alastair G. Dawson c a
Research Consultant in Quaternary Science, 26 Beverley Road, Leamington Spa, Warwickshire, CV32 6PJ, UK b School of Geography and the Environment, University of Oxford, Mansfield Road, Oxford, OX1 3TB, UK c Centre for Quaternary Science, Geography, Coventry University, Priory Street, Coventry CV1 5FB, UK Received 20 March 2002; received in revised form 10 January 2003; accepted 3 May 2004
Abstract Lithostratigraphical and biostratigraphical investigation of coastal marshes along the Atlantic coast of the Outer Hebrides from Lewis in the north to Barra in the south discloses inland-tapering sand units within marshland areas. The inland extent of each sand unit has been radiometrically dated and the units have been collectively interpreted as a proxy for past coastal storminess. The data appear to indicate that for the study sites investigated, the majority of the sand units were produced during episodes of climate deterioration both prior to and after the well-known period of Medieval warmth (MWP). Many were produced after ca. AD 1400. It is argued that the episodes of sand blow indicated by the deposits may reflect periods of increased cyclogenesis in the Atlantic associated with increased sea ice cover and an increase in the thermal gradient across the North Atlantic region. D 2004 Elsevier B.V. All rights reserved. Keywords: machair; storminess; North Atlantic; Little Ice Age (LIA); Medieval Warm Period (MWP); Outer Hebrides; Scotland
1. Introduction In recent years reconstructions of Late Holocene climate variability of the North Atlantic region have been enhanced considerably as a result of Greenland ice core research (Hammer et al., 1997). In particular, our knowledge of changes in North Atlantic storminess has been transformed as a result of the published chronology of Na+ (sea salt) concentration changes from the GISP2 drill site (Meeker and Mayewski, 2002). Annual changes in sea salt concentration at
* Corresponding author. E-mail address: [email protected] (S. Dawson). 0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2004.05.013
GISP2 calculated for the last 1400 year are interpreted as a regional signal of North Atlantic winter storminess that appears to have undergone a dramatic change at ca. AD 1400– 1420. Winter storminess prior to this time during the Medieval Warm Period (MWP) (Mayes, 2000) appears to have been low. After this time, storminess appears to have increased significantly and to have continued to remain ‘‘high’’ until present. These observations should also be considered within the context of the classic research of Lamb (1979) who argued that the cold conditions of the Little Ice Age (LIA) were associated with an increased meridional thermal gradient across the North Atlantic that, in turn, led to a southward displacement of the North Atlantic storm track. Considered together, the
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results of Lamb (1979) and Meeker and Mayewski (2002) point to an increase in winter storminess across the British Isles after ca. AD 1400 –1420. The chronology of coastal dune formation for the late Holocene has never been resolved adequately. For example, Clarke et al. (2002) in a study of windstorm events along the Aquitaine coast of S.W. France, identified two key periods of coastal sand invasion during the Late Holocene as having occurred between ca. 900– 1300 years ago and between 250 and 500 years ago and attributed to the MWP and the LIA. Similarly, in study of coastal dune development in eastern England, Orford et al. (2000) concluded that the key periods of dune development during the last ca. 2000 years took place between ca. 1500 and 1000 years ago and between ca. 500 and 200 years ago. They further speculated that periods of dune construction may follow intervals of time when relative sea level had been falling. These examples support the view that a major period of coastal sand drift took place during the LIA and that earlier periods of dune building appear to have taken place at different times in different places along the North Atlantic coastal margin. In this paper, we describe a series of Late Holocene coastal windstorm deposits from the Scottish Outer Hebrides that form part of the landward edges of coastal sand accumulations that are intercalated with peat. Radiocarbon dating of peat deposits enclosing the sand units is used to construct a local chronology of Late Holocene windstorm events that are, in turn, used to test the Meeker and Mayewski (2002) hypothesis that an increase in winter storminess took place after ca. AD 1400 –1420 having been preceded by a period of negligible storm activity during the MWP.
2. The study area The Outer Hebrides island chain (Fig. 1) extends from broadly north to south across a latitudinal range of 56– 58jN, thus occupying an area at the heart of middle latitude atmospheric circulation patterns. The islands rise to heights locally in excess of 200 m, but along their western, Atlantic margin they are generally low-lying. Here, the coast is fringed by a largely stabilised dune system consisting of a single low ridge
to seaward and a gently sloping surface to landward, where the system ends amidst peat areas, rocky outcrops and many small lochs. The sand areas are known locally as machair, derived from the Gaelic word for extensive beach or plain and which describes a lowlying, grass-dominated coastal plain composed of calcareous sand with a calcium carbonate content up to ca. 80% (Ritchie and Whittington, 1994). Ritchie (1979) attempted to define the major characteristics of machair in South Uist, however, the term is not restricted to the Outer Hebrides but is often commonly used throughout Scotland and locally in Ireland. The landward limit of the machair can end in a marsh or lochan. It is generally considered to have formed as a result of aeolian processes and has a distinctive dune morphology. Notwithstanding the apparent uniformity of the machair surface, exposures in the dune face to seaward disclose soil and peat horizons which reflect episodes of stability and imply a complex evolution. Studies of sediment exposures and of foreshore peat deposits have been undertaken by several research workers including Ritchie (1979, 1985), Ritchie and Whittington (1994) and Gilbertson et al. (1996, 1999) who have dated episodes of sand encroachment and retreat. It was on the basis of latitudinal extent of the dune system and the evidence for fluctuation in its development as demonstrated by previous workers, that the present study was undertaken. The research concentrated upon sediments within back-barrier, lowlying coastal areas and attempts to determine sediment age through the use of spatially continuous deposits.
3. Research context Studies of machair deposits have contributed to our understanding of landscape evolution, archaeology as well as land use and settlement change (Angus, 1997). The influence of anthropogenic land use changes on machair development were strongly felt during the early 19th century due to the onset of kelp (seaweed) harvesting. This, combined with population growth placed great stresses upon the machair since it represented the majority of cultivable land in the Outer Isles (Angus, 1997). The cropping of marram grass (bent) until it was made illegal in the late 17th century also contributed to machair destabilisation. In addi-
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7°° 10
0
6°° 40
30
20
283
50
N
km
58°°
Key 1. Cliobh 2. Cnip 3. Riof 4. Horgabost 5. Balranald 6. Nunton 7. Kildonan 8. Borve
Lewis 1 23
Harris
4
North Uist
5 0
200 km
6
Benbecula
58°°
7
South Uist
54°°
57°° Barra
8
50°° 8°°
4°°
0°°
Fig. 1. Field site locations, The Outer Hebrides, Scotland.
tion, a fall in the local water table caused by extensive land drainage and reclamation is believed to have caused the machair surface to dry out thus increasing the likelihood of degradation due to windstorm activity. Today the machair is subject to change with erosion and deposition a natural component of the machair system. The system is dynamic by nature and change in the landscape is an inherent part of its existence.
Previous work in the area has generally been concerned with examination of exposures of intertidal sediments along the coast, mainly in the Uists and Barra (Ritchie, 1979, 1985; Ritchie and Whittington, 1994; Gilbertson et al., 1996, 1999). Such coastal exposures are widespread along the island chain and have afforded the opportunity to radiometrically date episodes of sand encroachment and retreat. Over the last three decades this has led to the development of a
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series of models of machair evolution (Ritchie, 1979, 1985; Ritchie and Whittington, 1994). Different models of machair evolution have been developed based upon descriptions of machair sequences, and in particular, the examination of intertidal organic sequences in North and South Uist as well as in adjacent islands. At the end of the last glaciation (Late Devensian) the pre-machair surface was covered by glacially scoured substrate with marshes and lochans occupying the depressions. With the subsequent rise in sea level during Holocene glacially derived sands from offshore were mobilised and mixed with shell debris. This material was reworked into coastal ridges and moved onshore overriding preexisting lacustrine and organic deposits Ritchie, 1966 (Ritchie, 1985). The complex nature of machair evolution was demonstrated by research undertaken in the Uists by Ritchie and Whittington (1994). It was shown that locally variable stratigraphic sequences resulted from non-synchronous sand blow events and highlighted the difficulty in establishing a regional chronology of machair formation. Ritchie and Whittington (1994) showed that the earliest sands represent ‘overlapping landform suites’ which through long term erosion, outcrop today in the present intertidal zone. This led to a revision of models of sand movement on to the coastal plains in the Uists. They showed that the onset of sand movement was sometimes different in adjacent areas, possibly reflecting differences in surrounding topographies or other local effects. It is important to recognise that the pre-sand encroachment surface will be variable across the island chain and also to acknowledge that local coastal configuration will play a part in determining the subsequent pattern of sand encroachment. The sand supply was suggested by Ritchie and Whittington (1994) to be finite. However, this interpretation differs from Mate (1992) who believed that continuous production of offshore biogenic carbonate sand would lead to the continuous formation of machair through time. The cyclical nature of machair stability and instability prior to any human influence on the landscape was demonstrated by Ritchie and Whittington (1994), who identified sand encroachment under scenarios of rising sea levels, coastal erosion and climate changes.
4. Methodology and techniques The work undertaken in the Outer Hebrides is part of a larger research programme concerned with Holocene relative sea level changes and machair evolution in the Outer Hebrides. The approach adopted in this paper was to examine the evidence for the youngest sand movement identifiable in the stratigraphic record. This was undertaken from strictly comparable locations over as wide a latitudinal extent as possible, using standardised techniques, to enable comparisons to be made between sites and to identify any possible trends. In contrast to many previous studies, which considered machair evolution throughout the entire Holocene, only the most recent episodes, which are enclosed by organic material and in broad terms identifiable from historical records, are described here. Field work combined geomorphological and stratigraphical approaches. Morphological studies involved mapping the selected site areas at a scale of 1:10,000 according to standard geomorphological mapping techniques. Preliminary stratigraphical investigation was undertaken using hand-operated Eijkelkamp gouges to depths of up to 8 m. All boreholes were levelled to local Ordnance Datum (ca. 20 cm above British Ordnance Datum Newlyn; Thompson, 1980). Boreholes were confined to transects which located broadly at right angles to the coastline, across the inland margin of the machair, and onto the peat surface. The inland limit of the uppermost sand unit was thus determined. Sediment samples were taken across the intercalated peats and sand layers using a Stitz piston corer with a 5-cm diameter sampling tube and returned to the laboratory for further analysis. Biogenic samples were taken at the upper and lower boundaries of selected sand units for radiocarbon dating to encompass the chronology of the events. Dates were provided and calibrated by Beta Analytic, Miami (Table 1). In the laboratory, samples for diatoms were prepared according to standard techniques (Barber and Haworth, 1981). Unfortunately, the samples were extremely sparse and viable counts could only be undertaken at sites in Lewis (Kneep) and Harris (Horgabost). Only the presence of species is noted from other sites studied. Where possible, 300 diatoms were counted and expressed as a percentage of total diatom valves (%TDV). Species are categorised
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Table 1 Radiocarbon age determination for sand units in the Outer Hebrides Site
Core/depth and sample thickness
Altitude at surface m OD (Local)
Lab. code
Radiocarbon date 2r
Calibrated date 2r BP (probability method)
Material dated
Stratigraphic position
Horgabost, Harris Horgabost, Harris Cnip, SW Lewis Riof, SW Lewis Riof, SW Lewis Riof, SW Lewis Riof, SW Lewis Balranald, N. Uist Balranald, N. Uist Nunton, Benbecula Nunton, Benbecula Kildonan, S. Uist Borve, Barra
C1 (22 – 24) C1 (64 – 66) K2 (26 – 28) R10 (66 – 68) A7 (22 – 24) R8 (23 – 25) R8 (28 – 30) B4.1 (71 – 73) B5.1 (53 – 55) N5.1 (26 – 28) N5.2 (40 – 42) K4.1 (222) B15.1 (35 – 37)
2.83 2.83 2.68 3.97 3.37 3.39 3.39 4.94 4.49 1.98 1.98 4.17 3.18
Beta-120964 Beta-120965 Beta-127973 Beta-127974 Beta-130800 Beta-130803 Beta-130802 Beta-130797 Beta-130798 Beta-138741 Beta-138742 Beta-138740 Beta-130799
120 F 40 430 F 40 320 F 60 420 F 60 190 F 40 290 F 90 460 F 100 490 F 50 150 F 80 Modern 1140 F 40 950 F 40 670 F 130
AD 1670 to 1945 AD 1413 to 1520 AD 1445 to 1665 AD 1411 to 1532 AD 1722 to 1815 AD 1437 to 1696 AD 1381 to 1643 AD 1387 to 1492 AD 1655 to 1955 N/A AD 800 to 985 AD 1017 to 1164 AD 1150 to 1472
Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat
Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat Peat
according to the halobian classification (Vos and de Wolf, 1993) and nomenclature follows Hartley (1986). Polyhalobian and mesohalobian classes generally represent marine conditions, whilst oligohalobian and Halophobes reflect freshwater habitats. A comparison of sediments from the modern beach environment and the sand blow units was investigated at the sites in SW Lewis and Harris by particle size analysis. Modern samples from the present beach and dune systems were collected and samples were obtained from the cores within the sand units studied. The samples were analysed by the sieving method using a nest of 22 sieves and encompassed a range from 2 to 0.06 mm.
5. Locations examined 5.1. Lewis Along the Atlantic coastline of Lewis, the machair is intermittent, with dune systems being often confined to the mouths of bays or below cliffs. The three locations studied exemplify this, each being confined close to the coast by rising ground inland. 5.1.1. Cnip The site at Cnip (Figs. 1– 3) lies on the Bhaˆltos Peninsula in SW Lewis and is a relatively small area of machair plain flanked to the south by a freshwater loch, Loch na Cuilp, and steep unvegetated cliffs (see
above sand below sand below sand below sand below sand below sand below sand below sand below sand above sand below sand below sand below sand
Fig. 2). The machair area lies directly behind a highly active dune field that adjoins a sandy beach. Lying to the east of the main Cnip site is a small area of marsh (K transect) that offers a more sheltered basin in terms of sediment accumulation. Stratigraphic transects across the smaller basin were undertaken and the stratigraphic diagram is shown in Fig. 3. 5.1.2. Riof The site at Riof lies ca. 500 m to the west of Cnip on the same peninsula and consists of a large expanse of machair and low-lying Carex-Sphagnum marsh (Fig. 4). The Riof site is surrounded by rocky outcrops and hills, and separated from the sea by an extension of the dune/beach system that can be seen at Cnip. Boreholes were put down in two paralel transects from the south of the basin moving seaward to the north. The resultant stratigraphies can be seen in Fig. 4A (R transect) and 4B (A transect). In both localities, the modern surface soil/peat is underlain by sand. The main differences between the stratigraphies of the two sites lie in their proximity to the modern shoreline. The boreholes that were put down at Cnip lie directly behind a dune system, whereas the boreholes at Riof are situated on a machair plain approximately 150 m behind the dune complex. Microfossil work has been carried out on a number of cores from the Riof sites but the preservation of diatom frustules is very poor due to the prevailing acidity of the environment. Diatoms are present in some of the Cnip boreholes. Particle size
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30m
(burial cist)
10
m
m
30
Traigh na Berie Cnip (small marsh) K
m
30
MLWS
B
Cnip (main site) C
MHWS
D E
10m
30m
m 70
50m
c Cuil na ch Lo
(modern grain size samples)
A
R
30m
10m
Riof
70m 50m
A
Loch Baravat (broch)
Loch na Berie
Roads Rivers & streams Buildings Transect & boreholes ( = sampled borehole)
m
30
A
Contours (10m intervals)
10m
50m
Rock outcrop
Extent of peat N
Marsh Beach Break of slope Dune ridge
0 Dunes
Fig. 2. Cnip (Kneep) and Riof (Reef), location and borehole transects.
metres
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Seaward (northeast) +3
287
Inland (southwest) K2
K1
320+-60
+2
Metres OD (local)
+1
0
-1
-2
Phragmites peat
Organic sand
-3 1590+-90 Radiocarbon date
0
Metres
Shelly sand
30
Fig. 3. Stratigraphic transect (K) at Cnip (Kneep).
analysis of modern samples from the Riof dunes as well as sand from the sampled cores are shown in Figs. 5 and 6. The particle size curve displays a well sorted uni-modal distribution, for the contemporary samples with a range compatible with aeolian processes (Briggs, 1977). Radiocarbon dates (Table 1) show two broad periods of sand movement. Sand deposition at Riof was taking place after 460 F 100 radiocarbon years BP and 420 F 60 radiocarbon years BP. The upper sand deposition at Cnip occurred after 320 F 60 radiocarbon years BP. 5.2. Harris Harris is similar to Lewis, with machair areas confined to embayments and valley mouths. Here, Horgabost is typical of the sites examined.
5.2.1. Horgabost Liana Horgabost is a large expanse of Phragmites and Sphagnum marsh that is fringed by a 50-m strip of tussock bog on its seaward side (see Fig. 7). There is a ca. 300 m wide area of machair to the north of the site that separates the marsh from the beach and its associated dune system. Flowing from the south of the site along the western margin is a small stream, the Gill Meody, which cuts through the tussock strip, into the machair and through the modern dunes into the sea. Boreholes were taken throughout the Liana Horgabost marsh as well as in the tussock strip (Fig. 8). Directly north of the marsh lies a dune complex, a small, compact machair plain and a machair and tussock bog. This complex is partly reflected in the stratigraphy from the Liana Horgabost site. The upper units of the boreholes contain a decreasing amount of
288
A
R1
Seaward (northwest) R2
R3
R4
R5
R6
R7
R8
R9
R10
4
290 +- 90 BP
Metres OD (local)
3 460 +- 100 BP
2
1
0 0
Metres
50
Sandy peat Phragmites peat
Shelly sand 420 +- 60 BP
Radiocarbon date
Fig. 4. (A) Stratigraphic transect (R) at Riof. (B) Stratigraphic transect (A) at Riof.
420 +- 60 BP
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Inland (southeast)
289
Fig. 4 (continued ).
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Fig. 5. Riof: particle size analysis of modern beach and dunes.
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RIOF R8 196 - 200cm
RIOF R8 20 - 22cm 99.99
100
99.99
90
99.9 99.8
80
99 98
80
99 98
70
95 90
70
95 90
80 70 60 50 40 30 20
60 50 40
10
30
5
Percent of total weight
99.9 99.8
80 70 60 50 40 30 20
60 50 40
10 5
30
20
2 1
10
0.5 0.2 0.1 0.05
10
0
0.01
0
2 1 0.5 0.2 0.1 0.05
20
5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 m .2 .0 .7 .5 .2 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 -1 -1 -0 -0 -0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 63µ <
0.01 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 m .2 .0 .7 .5 .2 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 -1 -1 -0 -0 -0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 63µ <
Sieve Size (Phi)
Sieve Size (Phi)
RIOF R10 136 - 140cm
RIOF R10 36 - 40cm 99.99
100
99.99 99.9 99.8
80
80
99 98
70
95 90
70
95 90
80 70 60 50 40 30 20
60 50 40
10 5
30
2 1 0.5 0.2 0.1 0.05
20 10 0
0.01 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 m .2 .0 .7 .5 .2 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 -1 -1 -0 -0 -0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 63µ <
Percent of total weight
90
99 98
80 70 60 50 40 30 20
60 50 40
10
30
5
20
2 1
10
0.5 0.2 0.1 0.05
0
0.01 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 5 0 m .2 .0 .7 .5 .2 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 .2 .5 .7 .0 -1 -1 -0 -0 -0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 63µ <
Sieve Size (Phi)
Sieve Size (Phi)
RIOF A7 162 - 166cm 99.99
90
99.9 99.8
80
99 98
70
95 90 80 70 60 50 40 30 20
60 50 40
10
30
5
Cumulative percent (log scale)
Percent of total weight
100
20
2 1
10
0.5 0.2 0.1 0.05
0
m 5 0 5 0 5 .2 .0 .7 .5 .2 00 25 50 75 00 25 50 75 00 25 50 75 00 25 50 75 00 µ -1 -1 -0 -0 -0 0. 0. 0. 0. 1. 1. 1. 1. 2. 2. 2. 2. 3. 3. 3. 3. 4. <63
Cumulative percent (log scale)
90
99.9 99.8
Cumulative percent (log scale)
Percent of total weight
100
Cumulative percent (log scale)
90
Cumulative percent (log scale)
Percent of total weight
100
291
0.01
Sieve Size (Phi)
Fig. 6. Riof: particle size analysis of sand samples from cores R8, R10 and A7.
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Traigh Horgabost 10m
30m
MLWS
(modern grain size samples)
MHWS
10
k oc ss u T
G
m
B
ea ar
50m
D
m
30
C/D C E
A
F
H
I
Liana Horgabost Z
30
50m
m 90
m 110
70m
m
dy eo ll M
Gi
Marsh
Roads Rivers & streams
50m
A
Coniferous trees
0
Buildings
Beach
Contours (10m intervals)
Dunes
Transect & boreholes ( = sampled borehole)
Extent of peat
Break of slope
Tussock marsh
Dune ridge
Machair type grassland Fig. 7. Horgabost, location and borehole transects.
metres
300
N
Rock outcrop
Inland (southwest)
Seaward (northeast) 4.0
C-2
C-1
C1 (25) C2 C1 (37)
C1
C2 (25)
C2 (50)
C3
C4
3.5 3.0 120 +- 40
2.5
430 +- 40
Metres OD (local)
1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -2.5
Phragmites peat
430 +- 4040 Radiocarbon
date
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2.0
Generalised peat Shelly sand
0
Metres
50
-3.0 -3.5
Fig. 8. Stratigraphic transect (C) at Horgabost.
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shelly sand inland, away from the source of aeolian sedimentation. Dating of this sand unit shows that accumulation was taking place at borehole C1 after 430 F 40 radiocarbon years BP and had halted before 120 F 40 radiocarbon years BP. The date for the initiation of the sand movement is similar to the earlier date from the lower sand unit at Riof (420 F 60 BP). The sand unit was examined for its diatom content to ascertain the dominant mode of deposition and the resultant diatom assemblages are shown in Fig. 9. A mixed assemblage is evident, with species from marine, brackish and freshwater environments, however, the oligohalobous species are in greatest abundance with Cocconeis placentula, Fragilaria construens, Fragilaria pinnata and Pinnularia sp. The brackish species Diploneis interrupta and Navicula peregrina as well as the marine Paralia sulcata suggest a saline influence within the shelly sand. The presence of a limited number of marine species possibly reflects the proximity to marine influence and is not an indication that the sediment was deposited by marine water. An examination of modern machair sand from the beach and dunes at the site provided a similar diatom content, although too sparse to enable a full count, the species broadly suggest a similar sediment source. Particle size analysis was undertaken of modern samples from Horgabost beach and dune (Fig. 10A) as well as sand from the sampled core C1 (Fig. 10B). The particle size curves displays well sorted unimodal distribution for the contemporary samples with a range compatible with aeolian processes (Briggs, 1977). The samples from core C1 exhibit a similar grain size distribution to the present day dunes area and thus suggest an aeolian provenance. 5.3. The lower islands 5.3.1. North Uist (Balranald) The site at Balranald, a local Nature Reserve, is located on the westernmost area of North Uist. The machair areas are fragmentary in this locality, broken up by areas of marsh, loch and rock outcrops. The site occupies a low-lying Phragmites reedswamp marshland area, at the edge of a small loch lying to the east. The marsh lies beyond two dune-fringed areas facing NW and SW, lying either side of a rocky headland and
a gently sloping machair plain. The edge of the machair and the reedswamp area was investigated with a borehole transect and the resulting stratigraphy is shown in Fig. 11. The key feature of the stratigraphy is the distinctive wedge of coarse shelly sand beneath a Phragmites surface peat. The sand thickens to seaward, reaching a thickness of at least 2.5 m, and wedges out landward over a distance of 120 m to ca. 0.20 m. Landward of borehole B6 the sand occurs as occasional grains within the peat. Two cores were selected for dating to determine if the wedge became progressively younger inland. The start of sand deposition was dated to after 490 F 50 radiocarbon years BP at borehole B4, and after 150 F 80 radiocarbon years BP at borehole B5, farther inland near the tip of the wedge of sand. 5.3.2. Benbecula (Nunton) Benbecula is a low-lying island situated between North and South Uist, which rarely exceeds 15 m OD in altitude, and is characterised by numerous lochs. The coastline is dominated by low rock platforms and headlands with small pockets of shingle in hollows on the platform surface. The study site of Nunton lies between two prominent headlands and is located landward of Culla Bay which faces West. The site is a small area of low-lying marsh and lochan lying at the base of a sloping machair surface, approximately 100 m inland of the present coast. A single line of dunes lies behind the modern beach. The marsh surface lies at ca. 2.5 m OD and an extensive deposit of coarse shelly sand lies beneath a thin cover of peat (Fig. 12). In the vicinity of borehole N5 the peat becomes more dissected below the surface. To seaward, at least 3 m of sand is present (N1) whereas, further inland peat dominates the upper 1.5 m of sedimentation (N4). A radiocarbon date obtained from peat directly beneath an inland tapering wedge of sand at N5 gave 1140 F 40 radiocarbon years BP. Peat directly above the wedge of sand was also dated but returned a modern age (probably because here the overlying peat was very shallow, less than 40 cm thick). 5.3.3. South Uist (Kildonan) The Atlantic coast of South Uist consists of wide shelly sand beaches, dunes and machair over a distance of 35 km, separated by occasional rocky head-
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Fig. 9. Diatom diagram, borehole C1, Horgabost. Diatoms are expressed as percentage total valves (TDV) and are based upon a count of 300 valves per level. 295
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Fig. 10. (A) Horgabost particle size analysis: modern beach and dunes. (B) Horgabost particle size analysis: core samples form C1.
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297
Fig. 10 (continued ).
lands. The low-lying areas beyond the gentle machair slopes are generally 1 – 2 m OD and many shallow sand-infilling lochs are present. The site at Kildonan lies landward of a prominent ridge of rock. An extensive machair plain ca. 700 m wide is located beyond the rock ridge with low-lying marshland beyond. The marshland extends up to 2 km inland towards the high ground in the vicinity of Kildonan Glen (Fig. 13). Stratigraphic investigation of the marshland revealed an extensive sand wedge underlying the surface peat (Fig. 13). Beyond the machair slope the sand landward of borehole K3 is underlain by extensive peat deposits. The sand is coarse with many shell fragments and gradually thins towards the
east (K8). The onset of sand deposition was dated in K4 to after 950 F 40 radiocarbon years BP. The sand unit within borehole K4 was examined for its diatom content to try and ascertain the dominant mode of deposition. The sand unit was sparse and there were too few species to enable a full count to 300 species. Nevertheless, a mixed assemblage is present, with species from marine, brackish and freshwater environments. Oligohalobous taxa include Cocconeis placentula, Fragilaria construens, Fragilaria pinnata and Pinnularia sp. The presence of isolated marine taxa probably reflects the individual species being blown into the marsh as they are not dominant.
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Fig. 11. Balranald, North Uist, site location, borehole transect and generalised stratigraphic section.
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Fig. 12. Nunton, Benbecula, site location, borehole transect and generalised stratigraphic section. 299
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Fig. 13. Kildonan, South Uist, site location, borehole transect and generalised stratigraphic section.
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Fig. 14. Borve, Barra, site location, borehole transect and generalised stratigraphic section.
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5.3.4. The Isle of Barra (Borve Valley) The mountainous Isle of Barra forms the SW extremity of the arc of islands comprising the Outer Hebrides, with the smaller islands of Vatersay, Sanday and Pabbay to the south. North West facing bays with extensive beach and machair areas form the west coast of Barra. Rock promontories break the western coastline, the Borve valley occupying the head of Traigh Tuath, to the north of Borve Point. The study site in the Borve valley lies beyond the rocky headland extending across an area of marshy low ground which in turn extends ca. 1.5 km east of the present coastline (Fig. 14). Intertidal rock surfaces form the majority of the Borve promontory, and despite no extensive beach/dune complex to seaward, the area has one of the largest machair areas in W. Barra. The stratigraphy of the site is shown in Fig. 14. A radiocarbon date for the top of peat overlain by shelly sand at borehole B15 indicates that the sand reached its inland limit after 670 F 130 radiocarbon years BP.
6. Discussion 6.1. Origin of the sand The stratigraphy at the sites examined shows that the machair plain tapers into peat at its inland margin, and though the very tip of the tapering wedges may exhibit discrete thin horizons of sand within the peat, the wedges are generally massive and consist of a single horizon. Otherwise, the surface of the sloping sediment wedges, as far as can be determined, is continuous and the base of the wedges is similarly uncomplicated, although at greater depths other layers of sand occur. The stratigraphical evidence is thus relatively simple. It should be noted that limitations of the equipment precluded detailed investigation further seaward where the sand is more than ca. 3 m thick. Nevertheless, a sustained episode of sand accumulation reaching inland beyond the visible machair and dune area is inferred to have taken place. Microfossil preservation is very poor within the blown sand units dated. Nevertheless, evidence from Harris and South Uist provide information which enable the storm deposits to be broadly characterised. A predominantly fresh-brackish assemblage character-
ises the sand units. This is similar to the diatoms which characterise the modern dune and beach areas at Riof and Horgabost. It should be noted that the dynamic nature of the machair environment will lead to the presence of many allochthonous species while the presence of marine indicators possibly reflect diatoms carried into the back barrier areas by the wind in the vicinity of marine conditions. The absence of a sustained marine signal in the sand precludes a marine water lain source to the deposit. Grain size analysis of both modern samples from beach and dune areas in SW Lewis and Harris shows that the upper sand units studied within the collected cores to be of similar provenance to the present beach and dune areas. The grain size distributions are typical of aeolian processes (Briggs, 1977). 6.2. Chronology Table 1 provides dates for all upper sand blow events throughout the Outer Hebridean island chain. In terms of aspect, all sites studied face between W and NW. The dated storm events for the seven sites exhibit distinctive age clustering. Only three of the dated storm horizons (Nunton, Kildonan and Borve) exhibit standard errors that place the sediments within the MWP while it should also be noted that the mean date of one of these (Borve) dates to before AD 700. The majority of the uppermost dated horizons, however, appear to have been deposited after ca. AD 1300. The stratigraphical evidence disclosed in this study indicates that a sustained period of machair development in which shelly sands encroached inland ended some time between the MWP and perhaps as recently as 200 years ago, although the volume of sand deposited after the end of the MWP was minor at the sites concerned compared with what had accumulated before. Little is known of the prevailing climate in the Outer Isles prior to the MWP although it is generally recognised that the expansion of the Roman Empire and the subsequent Dark Age was a period of climatic deterioration. It is tempting to develop an hypothesis in which the frequency of storm events is related to past episodes of climate change normally described in terms of former air temperature characteristics with greater storm frequencies during periods of cold (e.g.
S. Dawson et al. / Marine Geology 210 (2004) 281–306
the LIA), and periods of reduced storminess during episodes of warmth (e.g. MWP). Such explanations, however, are fraught with difficulty since the westerly winds associated with severe North Atlantic cyclones (storms) are normally associated with higher than average winter temperatures. Equally, it can be argued that the coldest episodes of the LIA (e.g. the 1690s) may have been associated with significantly reduced storminess owing to the expansion of the polar anticyclone (Lamb, 1995). Meeker and Mayewski (2002) provided the most comprehensive record of North Atlantic storminess for the last ca. 1400 years based on measurements of sea salt (Na+) concentrations in the GISP2 Greenland
303
ice core (Fig. 15). Their measurements show that reduced storminess was a feature of the MWP that lasted until ca. AD 1400. They detected a marked increase in storminess frequency (increased Na+ concentration) that commenced at this time and observed that this ‘‘state’’ of increased storminess has lasted until present. The dated sand units are plotted on the same timescale to illustrate the clustering of the dates during the LIA time period. This pattern of storminess change is broadly similar to changes in Icelandic sea ice cover for the last ca. 1000 years (Koch, 1937) (Fig. 16), characterised by reduced/negligible sea ice cover during the MWP and a marked expansion after ca. AD 1400.
Fig. 15. Upper, Concentrations of Na+ (Parts per billion) at GISP 2 drill site, central Greenland, 987 – 1987 AD expressed as departures from long-term average, after Kreutz and Mayewski (1997). Lower, 2 sigma calibrated radiocarbon dates showing the initiation of the uppermost sand deposition at all sites in the Outer Hebrides.
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Fig. 16. Graph showing the number of weeks with ice around Iceland, for 20-year periods from 860 to 939, after Koch, 1937.
Lamb (1971) argued that the expansion of sea ice across the Greenland Sea and around Iceland is likely to have resulted in an increase in the thermal gradient across the North Atlantic region. It is argued here that such increases in the thermal gradient in the North Atlantic atmosphere (and possibly also ocean) provides a simple explanation for the greater frequency of windstorm sediment deposition and machair formation post-AD 1400, and possibly also for the time interval studied here prior to the MWP. Whilst recognising that the mean values for two dates (Nunton N5.2 and Kildonan K4.1) occur clearly within the MWP, it is argued that the overwhelming majority of dated windstorm horizons can be explained by the climate and weather processes described above. Stratigraphical studies from sites along the Atlantic margin of Scotland and Ireland, as well as from the North Sea region provide support for the chronology of sand mobilisation described in this study. Gilbertson et al. (1999) describe late Holocene phases of sand movement 200 – 600, 1300 – 1700 and 3300 – 3800 years ago in the Outer Hebrides utilising the luminescence dating of calcareous machair sands. In northern England, Orford et al. (2000) report IRSL and radiocarbon dating of the majority of coastal dune systems which date within the time period ascribed to the LIA. In western Ireland, remobilisation of sand was noted in back barrier lagoonal sites at ca. 420 radiocarbon years BP to 120 radiocarbon years BP (Delaney and Devoy, 1995).
It is tempting to suggest that some of the recorded windstorm events may be related to anthropogenic activity related to overpopulation and animal grazing pressures characteristic of the 18th and 19th centuries (cf. Angus, 1994; 1997; Hunter, 2000) in the Outer Isles. Although such processes were undoubtedly important in machair destabilization and aeolian sediment transport, we maintain here that the increase in north Atlantic storminess that commenced at ca. AD 1400 – 1420 and has continued until the present, was a key factor in accounting for the ages of the windstorm deposits described from the sites examined within this paper. The cause of a presumed increase in coastal storminess during the LIA was considered by Lamb (1979) to be related to an increased thermal gradient across the polar atmospheric and oceanic fronts in the North Atlantic. He argued that the north –east Atlantic region at this time may have experienced greater cyclonic activity with occasionally intense windstorms amongst which he included the great storms that took place during the winters of AD 1694 – 95, 1697 –98 and 1703 –04. Yet, it is not such a simple matter as to conclude that all coastlines bordering the eastern North Atlantic may have experienced a dramatic increase in storminess. For example, may it have been the case that storminess may have actually decreased during the LIA at latitudes north of the inferred position of the polar atmospheric front.
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7. Summary The paper has examined the sedimentary characteristics of the uppermost sand sediments enclosed within organic deposits at comparable sites across the latitudinal extent of the Outer Hebrides. It is suggested that evidence for storminess may be recorded in the movement of coastal sand dune systems along the Atlantic coastline of the Outer Hebrides, Scotland. It is maintained that the sand movements identified disclose sufficient consistency in their pattern and timing that, notwithstanding the possible influence from anthropogenic activity, sediment supply changes and local site specific effects; episodes of storminess can be determined. These episodes may be related to more regional changes in the vigor and pattern of atmospheric processes. The overall pattern from morphological and stratigraphical evidence is of a concentration of sand blow over the last 2000 years across the Atlantic seaboard of Scotland and immediately adjacent areas. The geological evidence consists of locally persistent landward-tapering layers of blown sand in coastal peat mosses. These layers indicate that in respect of the study sites investigated, the most significant episodes of windstorm deposition pre-date and postdate the MWP. The geological data indicate that windstorm deposition may have occurred during periods of climate deterioration and sea ice expansion across the northern North Atlantic associated with an increased thermal gradient favourable for increased cyclogenesis. The data also appear to indicate that the maximum inland extent of the machair in the Outer Hebrides was not reached until after ca. AD 1400. Dated inferred sand drift episodes across Europe show synchroneity with increased sand mobilisation in SW France, NE England, SW Ireland and the Outer Hebrides implying a regional response to storminess with increased sand invasion during the cool periods of the LIA and the MWP (Lamb, 1995; Gilbertson et al., 1999; Wintle et al., 1998; Wilson et al., 2001).
Acknowledgements This work was carried out as part of contract no. ENV 4-CT97-0488, for the Commission of European Communities, DG XII: Science Research and the
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Environment (Biodiversity and global change unit). The authors would like to thank the following for fieldwork assistance; Robin Cullingford, Robert Devoy, Cesar Andrade, Concecß a´o Freitas, Scott Nichol, Matt Hill, Alaric Rae, Lea Winter and Michael Tooley, and to the many croft and farm owners who allowed access to their land. Graham Corley, Jason Jordan and Sue Dawson produced the figures. This paper is a contribution to International Geological Correlation Programme (IGCP) Project 367.
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