The Holocene record of Loch Etive, western Scotland: Influence ofcatchment and relative sea level changes
N. Nørgaard-Pedersen a, W.E.N. Austin a,⁎, J.A. Howe b, T. Shimmield b
a School of Geography and Geosciences, University of St Andrews, Fife, KY16 9AL, Scotland, UKb Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Argyll, Oban, PA37 1QA, Scotland, UK
Received 18 July 2005; received in revised form 16 December 2005; accepted 12 January 2006
Sea lochs (or fjords) are glacially scoured marinebasins, typically with entrance sills which isolate theirdeep water bodies from the adjacent coastal waters. Inmost cases, glacial erosion through successive glacial/
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interglacial cycles causes the removal of sedimentarysequences in the sea lochs and fjords of NW Europe.The stratigraphic record of these sea lochs thereforelargely preserves a deglacial and Holocene record ofdeposition (e.g. Aarseth, 1997; Howe et al., 2002).Sheltered water and high sedimentation rates poten-tially make sea lochs ideal depositional environmentsfor preserving high-resolution Holocene climaterecords. Because sea loch environments reflect bothchanges in terrestrial run-off, vegetation cover,denudation, and exchange with coastal waters, theyare key areas to understand both long and short termmarine–terrestrial linkages. The Scottish west coast isideally located to monitor both changes in theinfluence of the adjacent North Atlantic Current andthe impact of westerly air flow associated with theNorth Atlantic Oscillation (NAO) (Hurrell and VanLoon, 1997). Long-term variability in these climaticfactors is likely to have had an effect on the sea lochenvironment (Gillibrand et al., 2005) and may,ultimately, be recorded in the sedimentary record.Elsewhere in Europe, for example in the shallow-silledfjords of southern Sweden, a strong relationshipbetween NAO influence and fjordic hydrography isclearly demonstrated (e.g. Nordberg et al., 2000;Filipsson et al., 2004).
In order to extract regional paleoclimatic data fromsea loch records, it is important to understand howlocal controls on sedimentation interact with extra-basinal forcing. The relationship between loch size,basin and sill depth, and the density structure of theloch's vertical water layers, controlled mainly byterrestrial freshwater runoff, determine the extent ofwater exchange between sea lochs and adjacent coastalseas. Previous studies from the west coast of Scotlandprovide a detailed record of late Glacial and Holocenerelative sea level changes from around 12 14C ky BPto the present (e.g. Shennan et al., 1995, 2000;Shennan and Horton, 2002). These relative sea levelchanges (critically controlling changes in sill depth andcross-sectional area) are envisaged to have had a largeimpact on the sea loch circulation. In some instances,this may lead to the formation of isolation basins (i.e.fresh or brackish water bodies that were formerlyconnected to the sea). Recently, Rohling et al. (2004)have shown how global sea level changes can bereconstructed over glacial–interglacial timescales fromthe Red Sea. In this case, a critical sea level control isimparted to the Red Sea planktonic foraminiferaloxygen isotope records by changing water depth overthe entrance sill to the Red Sea (Siddall et al., 2004).Adopting a similar approach to circulation dynamics in
Scottish fjord basins, and isolation basins in particular,may yield palaeoenvironmental records with thepotential to further constrain the rate and timing ofrelative sea level change.
Precipitation in this region of Europe is stronglylinked to westerly air flow. Surface run-off thereforedominates the upland catchments of western Scotland.Changing vegetation cover and land use through theHolocene are additional factors that may have modifiedrun-off rates and rates of catchment erosion andsediment influx to sea lochs.
We have studied records from Loch Etive on the westcoast of Scotland, spanning the last 10,000 yr. LochEtive is unusual in having a very large freshwater influxcompared to other Scottish sea lochs, a very shallowentrance sill dampening the tidal range within the lochand a slow and episodic bottom water renewal (Edwardsand Edelsten, 1977; Edwards and Sharples, 1985).Detailed studies have documented its present dayenvironment, sedimentation rates and geochemical–biological processes (e.g. Shimmield, 1993; Jones andBlack, 2001, Howe et al., 2002). However, untilrecently, very little was known about the Holocenerecord of this and most other Scottish sea lochs. In 2000,inner Loch Etive was mapped with side-scan sonar,boomer seismic profiles were recorded, and gravitycores were taken (Howe et al., 2001, 2002). The presentstudy documents in detail the stratigraphy of twoselected cores from the deepest basin of inner LochEtive. By studying well-dated changes in lithology,geochemistry, and benthic foraminiferal assemblages,our aim is to document the sedimentary environmentand hydrographic pattern through the Holocene and tryto link these records to internal and extra-basinal forcingfactors.
2. Physiography, oceanography and geology
Loch Etive is located 6 km north of Oban, in northernArgyll on the west coast of Scotland. The 1–2 km wideloch extends 30 km over several basins separated byshallow sills (Fig. 1). The entrance sill at Connel Bridgehas a depth of only 7 m below mean high water (MHW)(Edwards and Sharples, 1985). Severe shoaling over theinner basin bedrock sill constrains currents so that theinternal tidal range of the loch is 1.8 m, compared withan external range of 4 m (Edwards and Edelsten, 1977).The sill is composed of granite and is therefore assumedto have remained morphologically constant throughoutthe Holocene. Together, the rivers Awe and Etive bringwater from a large catchment of 1350 km2, providing anannual average discharge of ca. 3×109 m3 of fresh water
Fig. 1. Location of the study area: Loch Etive, W Scotland. The box indicates the location of the map extract shown below with core sites GC004 andGC005 in the ‘Bonawe Deep’. Depth contours are in m (from Edwards and Edelsten, 1977). Black square in Airds Bay indicates locality of thesediment trap.
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to Loch Etive. This large inflow of water, together withthe restrictions to water exchange with the Firth ofLorne, means that the salinity of the surface water in theloch is markedly reduced, except during very dryperiods (Austin and Inall, 2002).
The outer E–W trending basin is shallower (<70 m)than the inner NE–SW trending basin, reaching amaximum depth of 150 m in the ‘Bonawe Deep’ (focusof this study). The ‘Bonawe Sill’ (ca. 13 m depthbelow MHW) separating the two major basins allowsthe exchange of surface water, but exchange of thedeep water in the inner basin does not occur until thesalinity of the bottom water has been diluted by tidallyinduced turbulent diffusion with the brackish surfacewater. When the bottom water density is less than thatof the mixed water at the sill, exchange is possible.During periods of reduced freshwater input and coldsurface water (e.g. late spring) the density of thesurface waters may exceed the critical value andrenewal of bottom water can take place. Renewalevents happen aperiodically with a mean repetition rateof 16 months (Edwards and Edelsten, 1977; Jones andBlack, 2001; Austin and Inall, 2002). The events,which often take place during the flood of springtides, are characterised by a turbulent plume of densesill water flowing into the deep basin. The penetra-tion of the relatively light old water by the plumerapidly changes the properties of the deep basinwater. In particular, the dissolved oxygen concentra-
tion increases. Deep currents associated with theinflow are fast and turbulent in the neighbourhood ofthe sill, but diminish landwards. It is only duringoverflow that high bottom current velocities exist andthen only at the sillward end of the basin (Edwardsand Edelsten, 1977).
The sedimentation dynamics in the ‘Bonawe Deep’reflects variation in hydrodynamic conditions linked tolonger periods of deep water stagnation, withdominantly riverine suspension load sedimentation,interrupted by relatively short period deep waterrenewal events bringing more salty, oxygen-rich sillwater and coarser sediment to the ‘Bonawe Deep’.During deep water renewal events coincident withspring flood tides, bottom currents measured at about100 m depth are known to reach velocities of up to50–70 cm s−1 (Edwards and Edelsten, 1977). Currentflow of this velocity is easily capable of resuspendingfiner material and transporting coarse sand as bed load(Kennett, 1982). During bottom water renewal events,we expect episodic transport of sandy material fromthe ‘Bonawe Sill’ downslope to the basin. Apart fromsediment available in the sill area where verticalmixing takes place, this process would likely incor-porate resuspension and plume transport of sedimentfrom the basin slope area. The river Awe (draining thefreshwater Loch Awe) has its mouth at the ‘BonaweSill’ and is probably an important local source of bedload and suspended load material that can be
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transported towards the ‘Bonawe Deep’ margin.During stagnation, current velocities (if measurable)are <5 cm s−1 (Edwards and Edelsten, 1977) andoxygen contents decreases to very low values. Beforethe May 2000 bottom water renewal event, dissolvedoxygen concentration in the ‘Bonawe Deep’ reached<1.5 mg oxygen l−1 i.e. <20% saturation (Jones andBlack, 2001).
The region has probably been over-ridden duringsuccessive glaciations over the last 2 Ma. Duringglaciation, ice flowed SW, down the loch from a centralhigh ice field, west of Rannoch Moor (Gray, 1992). LastGlacial Maximum ice withdrew from the region about15 ky BP, but during the Younger Dryas (the ‘LochLomond Readvance’) from ca. 12.5–11.5 ky BP, iceextended through both basins of the loch to the entrancesill at Connel Bridge (Gray, 1975, 1997). Due to theerosive nature of successive ice advances, the sedimen-tary sequence in the loch is only believed to preserve adeglacial-Holocene record for the last 11.5 ky BP. Thisis a widely observed sedimentological response in mid–high latitude fjordic systems (Aarseth, 1997; Dix andDuck, 2000).
A seismic survey of the inner Loch Etive basindocuments a sediment pile of up to 30–50 m thicknessoverlying bedrock (Howe et al., 2002). An upperconformable, well-stratified seismic unit B, <5 m inthickness, that can be traced throughout the innerbasin, is superimposed on a lower seismic unit A withmore variable seismic reflection character and thick-ness (Fig. 2).
Fig. 2. Interpretive line drawing interpretation and boomer seismic reflection pFig. 1). Overlying granitic basement, is seismic unit A of diverse character imarine sediments. Howe et al. (2002) interpret the lower sequence as being thDryas ice retreated up the loch. The transition between glacio-marine and Hseparating the lower sequence from the upper draping sequence.
3. Materials and methods
Two gravity cores, GC004 and GC005, taken fromthe deepest part of Loch Etive, have been studied.GC004 is located at 56°27.348′N 05°11.239′W at awater depth of 148 m in the ‘Bonawe Deep’. GC005 islocated at 56°26.920 05°12.147′W at a water depth of110 m on a terrace between the ‘Bonawe Sill’ and the‘Bonawe Deep’ (Fig. 1). The cores were recovered fromRV Calanus of the Dunstaffnage Marine Laboratory,Oban, in September 2000. A standard British GeologicalSurvey gravity corer was used with a 3 m length barrelfitted with core catcher, a core liner diameter of 8 cm and250 kg weight. The spear core (SP35) was collected inOctober 2002 from the deep basin of Loch Etive, closeto the original position of GC004. The core length is1.55 m with an undisturbed sediment water interfacepreserved, indicating that all surface material was col-lected. Radiocarbon age determinations are presented inTable 1. Fixed volume samples (10 cm3) were takenevery 5 cm along the working half core axis. Thesesamples were used to determine water content, dry bulkdensity, mass specific magnetic susceptibility, grain sizedistribution of the lithogenic fraction, and organic con-tent. Samples of about 10 cm3 for benthic foraminiferalstudies were taken separately every 5 cm from GC004.These samples were wet-sieved (>63 μm), and the resi-due was dried to constant weight at 40 °C and dry-sievedon a >125 μm sieve prior to foraminiferal analysis.
Magnetic susceptibility was measured with a Bar-tington MS2B sensor on fixed volume samples in 5 cm
rofile through core sites GC004 and GC005 in the ‘Bonawe Deep’ (sees superimposed by a stratified seismic unit B, interpreted as Holocenee result of a dynamic glacio-marine environment prevailing as Youngerolocene open marine conditions is indicated by the unconformity (E1)
Table 1AMS 14C dates of mollusc shells in GC004 and GC005
Core no. Depth incore (m)
Dated species Lab. no. Reservoir corrected14C age (14C yr BP) a
Reservoir corrected 14C dates (−400 yr) and the corresponding calibrated ages (yr BP), 1 standard deviation (S.D.) range in brackets. c.c.: corecatcher. Prefix AA indicates the Radiocarbon Laboratory at the University of Aarhus, Denmark, while AAR are from the NERC RadiocarbonLaboratory, East Kilbride, Scotland, and measured at the Arizona AMS facility.a 400 yr reservoir correction.
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steps and later normalised relative to 10 g dry bulkweight. Additionally, a Bartington MS2E1 surfacescanning sensor was used to measure the magneticsusceptibility at 1cm intervals along the axis of thearchive core half. Water content and dry bulk densitywere determined by weighing the 10 cm3 samples wetand again after drying at 40 °C to constant weight.
Grain size distributions of the lithogenic componentwere measured from dried subsamples with a weight of0.3–0.4 g using a laser diffraction particle size analyser(Coulter Counter LS 230) and following the method ofAustin and Evans (2000).
The chemical elemental composition of the archivehalf of the split sediment cores was analysed semi-quantitatively at 1 cm resolution using a XRF (X-rayFluorescence) spectrometry core scanner at the Univer-sity of Bremen, Germany. Details of the general methodand calibration can be found in Jansen et al. (1998). Inorder to test the validity of these XRF-scan records, 20dry powder samples from GC004 were analysedseparately for comparison by discrete sample XRFspectrometry at the School of Geography and Geos-ciences, University of St Andrews.
Loss on ignition measurements from core GC004were determined on dry sub-samples of 3–4 g followingthe method of Heiri et al. (2001). In addition, total carboncontent was determined from dry homogenised powdersamples of core GC004 by CHN elemental analysis.
Benthic foraminifera were counted from the >125 μmfraction. Generally, the low numbers of foraminifera
encountered (on average 34 specimens/g dry bulksediment) made it impossible to practically count morethan about 100 specimens in most of the samples.
The 137Cs, 226Ra and 210Pb activities were obtainedby counting pressed and sealed 20 g discs of dried,ground sediment using gamma spectroscopy. The excess210Pb activity was obtained by subtracting the 226Raactivity from the total 210Pb activity. The stable Pbisotope ratios (206Pb/207Pb) were obtained from Induc-tively Coupled Plasma Mass Spectrometry (ICP-MS)analysis of dissolved sediment. The samples weredissolved using a CEM MARS 5 digestion system andNIST 918 reference material was used to correct for anymass bias during the analysis. Each sample wasanalysed five times and the mean and the relativestandard deviations reported.
From each core, 8 mollucs samples were selected forAccelerator Mass Spectrometry (AMS)-14C dating. Astandard reservoir correction, R(t), of 400 yr has beenused as the regional average for western Britain(Harkness, 1983: R(t)=405±40 yr). We do not expectany significant influence on R(t) by fossil carbonate‘hard water’ in Loch Etive, as reported elsewhere e.g.from Danish fjords (Heier-Nielsen et al., 1995). The bedrock characteristics, predominance of surface runoff andevidence of continued exchange with coastal waters donot imply any significant change in R(t) through theHolocene. According to Reimer et al. (2002), thedifference, ΔR, between the regional surface ocean 14Cage and the ‘global’ surface ocean 14C age appears to
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have been insignificant for at least the last 6000 yr(ΔR=−33±93 yr). The reported 14C ages in the text arereservoir corrected (radiocarbon years BP (beforepresent=1950)), rounded to the nearest 10 and reportedat one standard deviation (1σ). Calibrated calendar ageshave been obtained from the calibration tables inStuiver et al. (1998) by means of the CALIB 4.0programme using the 10 yr-resolution terrestrialcalibration curve and the ‘intercept method’ (Stuiverand Reimer, 1993). If more than one intercept of themeasured 14C age with the calibration curve occurs, thecalibrated age is given as a time interval. For calculationof linear sedimentation rates, accumulation rates andage/depth models, mean values of calibrated age rangeshave been used. All calendar ages reported in the textare conventional 14C ages which have been marinereservoir corrected and calibrated (ky BP).
4. Sediment properties
4.1. Lithostratigraphy
Based on the visual core description and basicsediment properties data, each core can be subdividedinto four characteristic lithostratigraphic units (I–IV)that can be correlated between the two cores. Basalsediments of core GC004 (Unit I: 235–203 cm) consistof light grey, laminated, well-sorted silt–very fine sand(Fig. 3a). An overlying unit consists of intercalated thinlayers of grey clayey–silty and fine sandy sediments(Unit II: 203–160 cm). From 160–50 cm (Unit III),greenish grey, silty to fine–sandy mud with scatteredshell fragments and fine organic remains occur. Towardsthe core top (Unit IV: 50–0 cm), sediments graduallybecome more fine-grained, dark greenish-grey, rich inorganic remains and watery. Apart from a bivalve shellat the top of GC004, no shell fragments are found above47 cm.
The lowermost unit I (226–197 cm) of core GC005consists of light grey, laminated, well-sorted silt–veryfine sand (Fig. 3b), very similar to the basal unit incore GC004. The basal unit is superimposed byintercalated greenish-grey layers of silty fine sandshowing a faint lamination with scattered pockets ofcoarse sand, shell-rich debris (Unit II: 197–160 cm).From 160–50 cm, a unit (III) of greenish-grey, muddycoarse sand rich in shell fragments and organicremains is found. From 50 cm toward the core top(Unit I), sand content decreases and becomes finer,organic content increases and sediment colourbecomes dark greenish-grey. No shell fragments arefound in the uppermost 25 cm of the core.
4.2. Radiocarbon chronology
A total of 16 bivalve mollusc samples have beenAMS 14C dated from both cores (Table 1, Figs. 3 and 4).The dates indicate that the cored sequences roughlycover the last 9400 14C yr BP (ca. 10.5 ky BP). Apartfrom minor age reversals in the lower part of the cores,increasing ages are found down-core.
From the core catcher sample in GC005 (about236 cm) an age of 9090±90 14C yr BP (10.4–10.2 kyBP) was obtained on a large articulated specimen ofArtica islandica with a largely intact periostracum.Another bivalve at 218 cm in the same coarse silt–finesandy facies gave a slightly older age of 9380±60 14C yrBP (i.e. 11.0–10.3 ky BP). While different burrowingdepths and/or minor reworking may explain the reversal,these radiocarbon ages overlap at two standard devia-tions. The similarity between the sediment facies of thelower part of core GC004 and GC005, the benthicforaminiferal assemblages, and high-resolution magnet-ic susceptibility records suggest that the unit roughlyrepresents the same time period in both cores. However,the age reversal in the lower part of GC004 indicates anapparently younger age (a single valve of Myrteaspinifera yields an age of 7430 14C yr BP at 230 cm).
Differences in the age of lithostratigraphic unit IIbetween the two cores, suggest either the developmentof a diachronic facies or the dating of reworked material.The preservational state of the two Nucula spp. bivalvesdated from this unit (202 cm and 163 cm) in core GC004is evidently better (well-preserved periostracum) thanthe mixed bivalve fragment sample (165 cm) and M.spinifera specimen (abraded surface, 185 cm) that wasdated in GC005 in the correlatable unit. It is thus likely,that the older ages obtained in the coarser-grained part ofcore GC005 are due to dating of reworked older shellmaterial.
4.3. Age models and sedimentation rates
Age models have been constructed for the two coresusing average calendar age estimates (Table 1, Fig. 4)and linear interpolation between dating points. In orderto deal with conflicting dates the following steps havebeen undertaken. An average age for the two lowermostdates in GC005 (218 cm and 236 cm) has been assignedto the midpoint between the two dates (227 cm: 10.35 kyBP) and this age has been used as a control point insteadof the two dated horizons. The date at 230 cm in GC004has not been included (see discussion above). Instead,the average basal age of core GC005 (10.35 ky BP) hasalso been assigned to the base of core GC004 according
Fig. 3. Compilation of basic sedimentological parameters for (a) core GC004 and (b) GC005. Beside the lithological log, molluscan AMS 14C datesare indicated (reservoir corrected by 400 yr). The relative content of sand (63–2000 μm), silt (2–63 μm) and clay (<2 μm) are indicated (decalcifiedand organic content digested sediment). High-resolution magnetic susceptibility record (continous line); open symbols represent dry mass (10 g)normalised values (MSnorm) at 5 cm intervals. The Ca and Fe content records are based on high-resolution (1 cm intervals) XRF split coremeasurements (continuous line) and discrete-sample XRF measurements on dry powdered sediment (dots). The carbon content record (dots) iscompared to loss on ignition records LOI-450 (450 °C) and LOI-850 (850 °C); all at 5 cm intervals. Characteristic lithological units (I–IV) referred toin the text are indicated.
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to the correlation of physical properties discussed above.The two dates obtained from the 95 cm level in GC004on Nucula sp. and M. spinifera shells gave nearlyidentical ages (2690±45 14C yr BP and 2930±60 14C yr
BP) and have been assigned an average age of 2810 14Cyr BP (3.0 ky BP). Both core tops have been assigned azero age, confirmed by the modern age of the molluscdate at 1 cm in GC004. It is likely, however, that some
Fig. 4. (a) Age–depth plots for cores GC004 and GC005 based on calibrated radiocarbon ages. Linear sedimentation rates (cm ky−1) are indicated bythe inclination of the lines connecting fix points. One date from 230 cm in GC004 (shown in brackets) is considered to be too young (see text fordiscussion) and is consequently omitted from the age–depth model. (b) Corresponding accumulation rate (g cm−2 yr−1) records of GC004 andGC005 based on the age models shown and bulk sediment dry density values. (c) Sediment accumulation rates for the deep basin of Loch Etivecalculated from 137Cs (utilising the maximum discharge of Sellafield 137Cs in 1975) and 210Pb activities within a sediment core (Shimmield, 1993).(d) Pb-isotope ratios and Pb concentration profiles from spear core SP35 (preserved sediment water interface) and gravity core GC004 from LochEtive indicating a loss of 25–30 cm of core top from the gravity core. The Pb-isotope and Pb concentration profiles of both cores are plotted on thedepth scale of spear core SP35, so that the profiles of core GC004 are matched to SP35 in order to estimate the loss of core-top material.
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core top loss and mixing of the watery and organic-richsurface sediment has taken place in both gravity cores.206Pb/207Pb ratios of GC004, when compared with206Pb/207Pb ratios in nearby spear core SP35, indicatethat around 25 cm of the top of GC004 (Fig. 4d) and35 cm of GC005 were lost. In the discussion of theHolocene evolution of Loch Etive, we rely on the agemodel established for core GC004 and correlatespecific lithological units to core GC005.
Average linear sedimentation rates (cm ky−1) for thetwo cores are about 23 cm ky−1 (Fig. 4). Accumulationrates for the main Holocene sequence are in the range0.01–0.03 g cm−2 yr−1. However, assuming some lossof surface sediment during the coring process, the linearsedimentation rate/accumulation rate estimates for thelate Holocene unit IV are probably too low. 210Pb and137Cs based accumulation rate estimates for sites nearbysuggest considerably higher present day accumulation
Fig. 5. Grain size distribution data (Coulter Counter) of organiccarbon-digested and decalcified sediments for (a) GC004 and (b)GC005. The lithogenic fraction grain size distribution curves havebeen stacked to correspond to their core depth and the volume-% scaleis indicated for the core top sample (0–1 cm) only.
Fig. 6. Grain size distribution of the suspended load accumulatingthrough the year 2000 in a sediment trap (RE6) from Airds Bay(56°26.878′N 05°14.793′W, 69 m water depth) in Loch Etive,immediately west of the ‘Bonawe Sill’. The composition of thelithogenic component (organic carbon- and calcium carbonate freefraction) and the bulk sample are shown for comparison.
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rates (0.06–0.07 g cm−2 yr−1) in the ‘Bonawe Deep’(Shimmield, 1993).
4.4. XRF Ca and Fe records
The XRF core scan data of the Ca and Fedistributions are shown in Fig. 3 (including additionaldiscrete powder sample data from GC004). The discretesample data for Ca verify good correlation to the corescan data (r2 =0.88), whereas the Fe data do not agree as
well (r2 =0.37). One obvious reason for the limitedreliability of the scan record could be that unitscharacterised by a higher sand content are not ideal forXRF core scanning.
4.5. Grain size distribution
The grain size distribution of the lithogenic sedimentfraction and derived grain size parameters (mean andmodal grain size, weighted mean of sortable silt, clay/silt/sand content) of the two cores are shown in Fig. 5.Due to the composite bimodal character of most of thesamples, the mean grain size parameter is not well-suited for characterising the sediment. Therefore modalvalues are plotted beside the mean grain size and theweighted mean of the sortable silt (10–63 μm) fraction.Mode #1 is equivalent to the grain size value of the mostfrequent grain size class (highest peak), whereas Mode#2 is equivalent to the grain size value of the mostprominent secondary peak.
The suspended load accumulating through the year2000 was collected in a sediment trap (RE6) from AirdsBay (56°26.878′N 05°14.793′W, 69 m water depth)immediately west of the ‘Bonawe Sill’. The mean grainsize of the lithogenic fraction was a fine–medium-sizesilt, with a broad modal peak around 10 μm (Fig. 6).Therefore, the annually integrated suspended load grainsize distribution is very similar to the core top sedimentdata in GC004.
4.6. Benthic foraminifera (GC004)
In general, the content of benthic foraminifera/g bulksediment is low in GC004 (<50 specimens/g). In thelower part of the core, the content of calcareous
Fig. 7. Benthic foraminiferal assemblage record (%) and radiocarbon age of GC004. The relative proportion of agglutinated, calcareous and porcelaneous specimens are indicated to the right.
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foraminifera reaches maxima of about 40 specimens/g,whereas agglutinated types are less abundant (maximum10–20 specimens/g) (Figs. 7 and 8). From about 80 cmto 50 cm core depth, there is a consistent decline inabundance of calcareous specimens, whereas thecontent of agglutinated specimens in the same intervalincreases toward a peak value of about 70 specimens/g.The uppermost 50 cm of the core is totally barren ofcalcareous specimens. The poor preservational state ofthe calcareous foraminifera in the unit directly below thebarren core top indicates that carbonate dissolution is alikely reason for their absence. The downward decline inorganic cemented agglutinated specimens may be due to
Fig. 8. Records of sedimentological and faunal parameters in GC004 shown veLoch Etive is based on estimates of the RSL trend fromW Scotland (ShennanOD in the mid-Holocene for Loch Etive is based upon shoreline mapping (Smtext for discussion) is described at the top of the diagram. Characteristic litho
postdepositional disintegration; a common phenomenonobserved in agglutinated foraminifera records (e.g.Brodniewicz, 1965).
The benthic foraminifera record of GC004 (Fig. 7)exhibits a diverse assemblage of calcareous speciesbelow 205 cm. It is dominated by Nonionella turgida,Elphidium sp., Rosalina sp., Cibicides lobatulus andQuinqueloculina seminulum. At 202 cm in a thin clayeysilt layer, a quite unique assemblage clearly dominatedby Q. seminulum is found. The unit between 200 and170 cm has a more diverse assemblage of calcareousspecies such as Q. seminulum, Elphidium sp., Rosalinasp., C. lobatulus, Ammonia batavus, and N. turgida. The
rsus age (cal. yr BP). The approximate relative sea level (RSL) curve foret al., 2000; Shennan and Horton, 2002). The maximum RSL of 8–9 mith et al., 2000). The general character of the catchment vegetation (seestratigraphical units (I–IV) referred to in the text are indicated.
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Unit from 170 to 50 cm is less diverse, with a cleardominance of A. batavus, C. lobatulus, Textularia bockiand Eggerelloides scaber. In the upper 50 cm of thecore, no calcareous foraminifera are found and onlyagglutinated taxa such as E. scaber, Reophax sp. andAmmobaculites sp. have been identified. In general, theforaminiferal assemblages closely match the changes inlithology (see Section 4.1).
5. Discussion
5.1. Depositional context
Our stratigraphic data from cores GC004 and GC005indicate that Loch Etive has experienced marineconditions since at least 9400 14C yr BP (ca. 10.5 kyBP). The majority of the up to 50 m thick sediment fill inLoch Etive was therefore probably deposited duringYounger Dryas ice sheet retreat from its maximumextension at the mouth of Loch Etive, a few km west ofConnel (Gray, 1975, 1992; Howe et al., 2002). Thisrelatively short period was likely characterised by alarge sediment supply to Loch Etive, dominated by ice-proximal, glacio-fluvial processes and mass-flow depo-sition (cf. Seramur et al., 1997). The withdrawal ofglacier ice exposes landscapes that are in an unstable ormetastable state, and consequently liable to modifica-tion, erosion and sediment release at rates greatlyexceeding background denudation rates (Ballantyne,2002). In the mountainous landscape surrounding LochEtive, denudation rates were probably very high beforevegetation spread and stabilised steep ground (cf.Brazier et al., 1988). Dates from basal samples oflake mud from Rannoch Moor (about 20 km northeastof Loch Etive) clustering around 10,000 14C yr BP(11.5 ky BP) (Lowe and Walker, 1992) suggests thatregional deglaciation of the area, including LochEtive, occurred before 11.5 ky BP, during the late partof the Younger Dryas stadial.
The <5 m thin, upper conformable seismic unit B(Fig. 2), which can be followed over the entire innerLoch Etive Basin as a drape over the thick deglacialcomposite sequence (unit A), is of Holocene age andmarine. The distinct boundary (E1,) separating the twoseismic units, likely represents the transition fromproximal glacio-marine sedimentation to normal marinesuspension-load sedimentation in Loch Etive. Howe etal. (2002) suggest that a two-stage deglaciation processtook place with a transition from a glacio-lacustrinesetting during Younger Dryas ice retreat to an openmarine fjord environment in the early Holocene(assuming that sea level rose during the deglaciation
phase). However, as a result of glacio-isostasy, relativesea level (RSL) in this part of W Scotland fell during thedeglaciation phase. The first regional RSL rise tookplace at about 9000 14C yr BP (10 ky BP), reaching amaximum of about 8–9 m above present in this areaduring the mid-Holocene (Shennan et al., 2000; Smith etal., 2000). The presence of marine molluscs and benthicforaminifera throughout the Holocene records of the‘Bonawe Deep’ indicate that fully marine conditionswere established in the inner Loch Etive Basin from thevery earliest Holocene. If a short lacustrine phaseoccurred during the deglaciation (cf. Howe et al., 2002),it can only have been possible by a damming of theentrance to Loch Etive (e.g. by a terminal moraine).Further investigations, with sediment coring across theboundary between seismic units A and B, will benecessary to determine the exact nature of thedeglaciation of Loch Etive.
The chronostratigraphic data, together with thesediment properties and characteristic benthic forami-niferal assemblages of the two cores studied, allow us tocorrelate the Holocene stratigraphic units, assign crudeage models to the cores, and interpret the stratigraphicdata in terms of possible forcing factors. We expect thatthe hydrodynamic environment at the core sites has beeninfluenced mainly by the change in relative sea level,controlling the frequency and dynamics of deep waterrenewal events in the inner Loch Etive basin (seebelow). Climate controlled precipitation, however, mayhave been able to modulate the circulation dynamics,with periods of increased precipitation leading toenhanced freshwater runoff and a more restrictedcirculation. Changes in vegetation cover may haveinfluenced catchment area denudation and sedimenttransport to the sea loch. Vegetation changes werecontrolled mainly by climate in the early-mid Holoceneand by human deforestation and land use changesduring the late Holocene.
5.2. Relative sea level influence
RSL changes, as a consequence of glacio-isostaticrebound and global glacio-eustatic sea level risesubsequent to deglaciation, have almost certainly hada profound influence on the circulation of the ‘shallow-silled’ Scottish sea lochs. The main features of the well-dated W Scottish RSL changes are a rapid fall of sealevel before 10,000 14C yr BP, an almost stationary levelin the early Holocene, a rise to a mid-Holocenemaximum at about 6000 14C yr BP and then a gradualfall through the late Holocene (Shennan et al., 1995,2000). There exists no dated local sea level curve for
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Loch Etive. However, the ‘Main Postglacial Shoreline’(Sissons et al., 1966), which is generally regarded todayas the highest Holocene raised shoreline in westernScotland, reaches an altitude of about 14 m OD in outerLoch Etive (Gray, 1974). This altitude, however, cannotbe taken as a direct measure of RSL, since Gray's (1974)and other early shoreline-based isobase maps did nottake into account the different tidal levels at whichdifferent features formed, or the different tidal rangesaround Scotland (I. Shennan, pers. comm., 2002).Recently revised isobase maps for the Main PostglacialShoreline indicate, however, a RSL of about 8–9 m ODfor Loch Etive (Smith et al., 2000). This isobase map isalso consistent with the RSL estimates obtained byShennan et al. (2000), Shennan and Horton (2002) formost of the radiocarbon-dated Main Postglacial Shor-elines in western Scotland. There are no good estimatesof RSL for the early Holocene in Loch Etive. The initialstages of Younger Dryas ice wastage are marked in outerLoch Etive by the occurrence of a well-preserved systemof kame terraces. The fluvial outwash plain, Moss ofAchnacree, north of Connel Bridge, which partly blocksthe entrance to Loch Etive grades westward to a level of12–13 m OD indicating that RSL was at least as low asthis during the Younger Dryas stadial (Gray, 1992). Sealevel rose in the early–mid Holocene, obliterating mostearly Holocene shoreline features. A qualified estimate,taking the RSL trend from W Scotland (Shennan et al.,2000; Shennan and Horton, 2002) into consideration,would be that the early Holocene sea level in Loch Etivewas somewhere between the present level and themaximum value (8–9 m) obtained for the mid-Holocene.
While we do not use the stratigraphic data in thisstudy to constrain RSL, they can, potentially, give usclues to RSL changes. If we assume that sill morphologyhas not changed dramatically during the Holocene, andRSL was about 8–9 m higher in the mid-Holocene, itwould have had important implications for sea lochcirculation. Tidal amplitudes during the mid-Holocenewould have been higher inside Loch Etive (M. Inall,pers. comm., 2001), because slower, deeper flow overthe sill would generate less friction. Today friction overthe shallow sill results in the reduced tidal range insideLoch Etive (1.8 m) compared to the range outside(4.0 m). Deep water ventilation would be improved fortwo reasons. Firstly, by a greater volume exchange oneach tidal cycle, and secondly, because slower flow overthe sill may be more efficient at generating internal tidesand waves which cause diffusive mixing in the deepwaters. Enhanced diffusive mixing (breaking internalwaves) would also result in increased deep water renewal
frequency, as deep water density would be reduced morequickly by the enhanced internal wave field (e.g. Inalland Rippeth, 2002).
5.3. Stages of Holocene evolution
GC004 and GC005 can be subdivided into charac-teristic stratigraphic units that appear to be correlatablebetween the two cores and to represent the generallyconformable Holocene stratigraphy of the inner LochEtive basin sediment drape. Core GC005, locatedsomewhat shallower and closer to the ‘Bonawe Sill’,is evidently coarser-grained than core GC004 (Fig. 5).This suggests that sediment bed-load transport period-ically came from the sill region, and that lateral changesin sediment composition in the Holocene strata reflectthe position of sediment sources and bottom currenttransport capability. Based on the composite stratigra-phy of both cores, four characteristic depositional units(I–IV, Fig. 8) can be linked to the Holocene evolution ofthe Loch Etive environment.
5.3.1. Unit I: very early Holocene (10.5–9 ky BP)The basal unit (Unit I) is of very early Holocene age,
about 10.5–9.0 ky BP. The deposits of laminated, well-sorted, coarse silt–fine sand indicate rapid deposition bybottom currents of moderate to high velocity. A diverseassemblage of calcareous benthic foraminifera with adominance of Elphidium sp., N. turgida, C. lobatulus,Rosalina sp., Q. seminulum and Bolivina sp. indicatesfully marine conditions with frequent bottom waterrenewal events. This faunal data is consistent with RSLsomewhat higher than at the present day. A pollen studyof GC004 (S. Harker, pers. comm., 2002) providesindependent stratigraphic evidence for a very earlyHolocene basal age (similar to the basal age of GC005).Unit I contains a pollen assemblage dominated by herbs,with less tree pollen compared to the overlying units IIand III. A similar transition of pollen types is seen in thepollen diagrams from Rannoch Moor (Clasgour B andC, Gleann Fuar: Haggart and Bridge, 1992), withGramineae and Betula as dominant components prior toabout 9000 14C yr BP (10.3 ky BP). These earlyHolocene pollen assemblages indicate a surroundinglandscape with pioneer vegetation and a limited treecover. During this period, shortly after the YoungerDryas stadial, denudation rates were likely high in thesurrounding area, and rivers and surface run-off mayhave carried a large suspended load to Loch Etive(Brazier et al., 1988; Ballantyne, 2002). Bottom currentactivity, however, may have prevented the accumulationof very fine-grained material at the core sites. It is also
68 N. Nørgaard-Pedersen et al. / Marine Geology 228 (2006) 55–71
possible that the well-sorted, silty to fine–sandysediment originates from bottom current reworking ofunconsolidated glacial marine sediment belonging toseismic unit Awithin Loch Etive. This process may haveformed the unconformity between seismic unit A and B(Fig. 2).
5.3.2. Unit II: early Holocene (9–7 ky BP)According to the delimiting radiocarbon dates in
GC004, Unit II represents the period 8140–6870 14C yrBP (ca. 9–7 ky BP). The older ages obtained from coreGC005 are believed to be due to reworked older shellmaterial (reflected by the poorer preservation of the shellmaterial). Unit II is composed of intimately layered siltyand fine sandy strata with small pockets of coarse-grained sand in GC005. The reduction in grain size fromthe basal unit below is less prominent in GC005, but asignificant decrease of magnetic susceptibility values isobserved in both cores, possibly indicating a majorchange in sediment influx related to catchment land-scape evolution or current regime (changing sea lochcirculation). The lower part of unit II (GC004) containsa benthic foraminiferal assemblage dominated by Q.seminulum; with the abundance of A. batavus and E.scaber increasing upward. These assemblage changesfrom Unit I support the argument that sea lochcirculation underwent a significant change at this time.The fine-grained character of the lower part of Unit IIsuggests a reduced influx of terrigenous sedimentsupply and/or a decrease of bottom currents able totransport sandy material to the core sites. The consistentupward increase in grain size toward the overlying unit,on the other hand, could be related to the rapidtransgression that took place prior to the mid-Holocenesea level high stand, causing more frequent/vigorousbottom water renewal events.
5.3.3. Unit III: mid-early to late Holocene (7–1 ky BP)Unit III, representing the period 6870–1000 14C yr
BP (ca. 7–1 ky BP), shows in both cores a bimodalgrain size distribution with incursions of coarse sandmixed with suspension load fine silt material and shelldebris. The coarse mode is considerably coarser in coreGC005 closer to the ‘Bonawe Sill’, again supporting asediment source from that area. During the transitioninto the late Holocene a progressive fining of the coarsemode is observed. These grain size characteristics canbe explained by the influence of RSL, resulting fromthe mid-Holocene maximum and subsequent fall duringthe late Holocene. The benthic foraminiferal assem-blages of Unit III, dominated by C. lobatulus and A.batavus, suggest marine conditions and vigorous
bottom water circulation. The bimodal sediment com-position probably originates from bioturbation mixingof discretely deposited laminae of fine and coarsecharacter. The difference in coarse-mode grain sizecharacteristics between sites GC005 and GC004suggests that current velocities and bed load transportduring deep water renewal events dropped markedlydownslope towards the ‘Bonawe Deep’. During presentday deep water renewal events, there is also a dramaticdecrease of bottom current velocities landward fromthe ‘Bonawe Sill’ (Edwards and Edelsten, 1977). Thecoarse mode grain sizes, however, are considerablycoarser during this period compared to the core-topsediments, suggesting that current velocities duringrenewal events were considerably higher during themid Holocene. The higher sand content may addition-ally be explained by more frequent renewal eventsduring which the winnowing of fines occurred, butbioturbation has subsequently reintroduced fine grainsize components throughout this unit. The fine grainsize mode, dominated by fine silt in both cores, mostlikely reflects the background suspension load depos-ited during extensive periods between deep waterrenewal episodes. This grain size component is quitesimilar to the fine mode observed in the core topsediments and in a sediment trap recovered nearby(Fig. 6).
The occurrence of the coarsest grain size, peakmagnetic susceptibilities, and peak abundances ofepibenthic species e.g. C. lobatulus in the mid-Holocene at about 7–6 ky BP, support the argumentthat deep water renewal events were most frequent/vigorous during this period of maximum RSL. Highersalinities than present day are also supported bypreliminary oxygen isotope data on C. lobatulus andA. batavus (unpubl. data, N. Nørgaard-Pedersen andW.E.N. Austin). Towards the late Holocene, the finingof grain sizes, increase in organic content, increase ofagglutinated species e.g. E. scaber and increasingdissolution of calcareous shells, suggest that theprogressive lowering of RSL at this time caused areduction in deep water renewal into the inner basin ofLoch Etive. Moreover, a regionally identified increaseof Calluna vulgaris (heather) pollen at the expense oftree pollen during the late Holocene (Haggart andBridge, 1992; Harker, pers. comm., 2002) indicatesdeforestation and increasing podzolization of soilprofiles. A debate continues regarding the role andsignificance of human impact (e.g. pastoral land use),versus changing climatic conditions (increasing wet-ness from about 5–4 ky BP), in driving vegetationchange at this time (cf. Macklin et al., 2000).
69N. Nørgaard-Pedersen et al. / Marine Geology 228 (2006) 55–71
From the mid-Holocene onwards, the gradual in-crease in agglutinated taxa and associated increase inorganic carbon content make the interpretation of theforaminiferal assemblages difficult. Changing land useshould have increased denudation rates within thecatchment area and may well have supplied theincreasing organic input. However, falling RSL through-out this period would have restricted circulation withinthe inner basin of Loch Etive, lowering bottom wateroxygen concentrations and may have increased thepreservation of organic material.
5.3.4. Unit IV: late Holocene (1–0 ky BP)At about 1 ky BP, a marked reduction occurs in the
mean grain size deposited at both core sites. It isnoticeable, however, that whereas the coarse modeclearly decreases upward in GC005 and almost dis-appears in GC004, the fine mode shows an upwardincrease in grain size in both cores. The mass specificmagnetic susceptibility values also increase markedly inthe upper part of the cores representing the early part ofthe last millennium (in the older part of the core, it tendsto be positively correlated with sand content). Theseobservations suggest a marked decrease of bottomcurrent velocity/deep water renewal frequency, and achange in the composition of the fine backgroundsuspension material. Organic material deposition in-creased during the late Holocene and calcareous benthicforaminifera as well as bivalves disappear completelybecause of CO2-aggressive bottom waters (from organicmaterial decomposition). Agglutinated species such asE. scaber, Reophax sp. and Ammobaculites sp. dominatethe benthic foraminiferal assemblages, suggesting a lowdissolved oxygen environment associated with highorganic detritus accumulation. The present day pattern ofrenewal events in the inner Loch Etive is infrequent (1–2yr) and the stratigraphic evidence suggests that similarconditions prevailed during the last millennium. Thepresent day sediment accumulation rate, based on 210Pband 137Cs profiles of well-preserved core tops from thedeep part of Loch Etive, is about 0.06–0.07 g cm−2 yr−1
(Shimmield, 1993). This is about five times higher thanthe average value obtained for the Holocene section inGC004 (Fig. 4). It is likely that some core top loss tookplace in both gravity cores studied here (25 to 35 cmbased on 206Pb/207Pb ratios; Fig. 4c,d) and therefore wedo not rely on accumulation rates based on thestratigraphy of the uppermost part of the two cores. Asignificant increase in sedimentation rates may beexplained by the effect of human activity and defores-tation during the last millennium. From local pollenrecords we know that after ca. 1000 yr BP the landscape
in the Oban region became largely deforested (Macklinet al., 2000). During the 1800s, the shoreline of LochEtive was almost completely deforested by intensivecharcoal production for the iron furnace at Taynuilt byBonawe (a possible source of magnetic minerals). Thesecatchment changes likely caused a further increase in soilerosion and denudation rates, possibly leading to anenhanced sediment influx to the inner basin of LochEtive.
6. Synthesis and concluding remarks
The ‘Bonawe Deep’ sediment and faunal recordsappear to reflect changes in hydrodynamic conditionswhich responded to Holocene RSL in a predictablemanner (Fig. 8). The frequency and dynamics of deepwater renewal events within the inner basin of LochEtive are critically controlled by factors such as waterdepth and freshwater input across the sill region(Edwards and Edelsten, 1977; Inall et al., 2004). Duringthe early Holocene, immediately after the retreat of theYounger Dryas ice, fully marine conditions prevailedand continued to the present day. As the global sea levelovertook glacio-isostatic readjustment in this region ofW Scotland, RSL reached a maximum at about 7–6 kyBP. At this time our faunal and sedimentological recordssuggests frequent and vigorous deep water renewal.Falling late Holocene sea levels have reduced theexchange with coastal waters and are reflected in theproxy records.
Major changes within the catchment may haveinfluenced sediment delivery to Loch Etive, in particularduring the very early Holocene (post Younger Dryasvegetation recovery) and during the late Holocene (e.g.human land-use changes). While the ‘Bonawe Deep’may seem like the ideal sediment trap, with the potentialfor the recovery of very high resolution records, netHolocene accumulation in the deepest part of the basinappears to be <3 m. One explanation, considering thenarrow trench geometry of the ‘Bonawe Deep’ (Howe etal., 2001), is that the sediment fill has repeatedly beenwinnowed during powerful deep water renewal events.These events were likely most frequent during the mid-Holocene RSL high, when vigorous bottom currentswould have cascaded downslope from the sill region.The fining in grain size between basin margin coreGC005 and basin deep core GC004 agrees with modernobservations of reduced bottom current velocities awayfrom the sill region during deep water renewal episodes(Edwards and Edelsten, 1977). Given the moderncirculation, we would expect expanded Holocenesequences, less influenced by episodic winnowing and
70 N. Nørgaard-Pedersen et al. / Marine Geology 228 (2006) 55–71
composed of finer sediments, further away from the sillregion. However, the seismic records of Howe et al.(2002) indicate a relatively uniform thickness of theHolocene sediment drape over much of the inner basinof Loch Etive.
The dominance of sill depth and RSL changes incontrolling the circulation dynamics of the inner LochEtive basin make it difficult to decipher the role of otherpossible forcing mechanisms. For example, the postu-lated increased wetness during the fifth millennium BP(Birks, 1972; Walker et al., 1992) may have impactedcirculation by reducing the exchange of water across thesill (e.g. Edwards and Edelsten, 1977). Indirectly,climate changes influenced the vegetation cover of thecatchment area, providing yet another factor in sourcesediment supply. Increasing anthropogenic influencesduring the late Holocene may further obscure theprimary signal arising from natural climate variabilityat these particular sites.
This study presents the first detailed record ofHolocene palaeoenvironments obtained from a Scottishsea loch. The constraints imposed by RSL change onbasin hydrography suggest that restricted fjord basins,which occur in many formerly glaciated regions, canprovide valuable insights to the processes pre-dating theeventual isolation of such basins in response to glacio-hydro-isostatic changes (e.g. Lloyd, 2000). Furtherwork to develop reliable temperature and salinityproxies from Scottish fjords will potentially allow thereconstruction of former RSL and climatic changes, in asimilar manner to recent work on the Red Sea (Rohlinget al., 2004; Siddall et al., 2004). Long-term variabilityin Northern Hemisphere climatic factors directlyinfluence sea loch environments (Gillibrand et al.,2005) and the potential exists, at sites of rapid sedimentaccumulation, to reconstruct these changes at millennial,centennial and possibly decadal resolution.
Acknowledgements
This work is part of the HOLSMEER projectsupported by the EC Framework V programme and aspart of the SAMS, NERC-funded, Northern Seasprogramme. We wish to thank the Captain and Crewof RV Calanus of the Dunstaffnage Marine Laboratory,Oban. John Derrick (BGS) is thanked for his technicalassistance during coring. Radiocarbon datings weresupported by a NERC grant to W.E.N. Austin (samplesprepared at the NERC Radiocarbon Laboratory, EastKilbride) as well as by the HOLSMEER project(samples measured at the Aarhus University AMSLaboratory). M. Furze (Univ. Bangor) is acknowledged
for the identification of bivalve shell species. A BremenUniversity ‘Paleostudies’ grant to W.E.N. Austin and N.Nørgaard-Pedersen supported XRF logging of split coresections. Angus Calder (School of geography andGeoscience, Univ. St Andrews) did XRF determinationon dry powder samples. T. Brand (Dunstaffnage MarineLaboratory) did CHN measurements. We thank I.Shennan (Univ. Durham) and M. Inall (DunstaffnageMarine Laboratory) for information on relative sea levelchanges and its impact upon sea loch circulation. P.Cundill and S. Harker (Univ. St Andrews) are thankedfor access to unpublished pollen data of GC004.
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