Holocene tidal levels and sedimentation rates using a diatom-based palaeoenvironmental reconstruction: the Tees estuary, northeastern England

The Holocene 10,4 (2000) pp. 441–452

Holocene tidal levels and sedimentation

rates using a diatom-based palaeo-

environmental reconstruction: the Tees

estuary, northeastern England

A.J. Plater,1 B.P. Horton,2 E.Y. Haworth,3 P.G. Appleby,4

Y. Zong,2 M.R. Wright1 and M.M. Rutherford2

(1Department of Geography, University of Liverpool, PO Box 147, LiverpoolL69 3BX, UK; 2Environmental Research Centre, Department of Geography,University of Durham, Science Laboratories, South Road, Durham DH1 3LE,UK; 3Institute of Freshwater Ecology, Windermere Laboratory, The FerryHouse, Far Sawrey, Ambleside, Cumbria, LA22 0LP, UK; 4Department ofApplied Mathematics and Theoretical Physics, University of Liverpool, PO Box147, Liverpool L69 3BX, UK)

Received 9 October 1998; revised manuscript accepted 26 August 1999

Abstract: An established diatom-based tidal level transfer function is used in combination with the present-day relationship between sedimentation rate and altitude to reconstruct Holocene tidal sediment accretion forthe Tees estuary, northeastern England. The results from five cores reveal two periods of enhanced sedimen-tation, the earlier of which (8000–6000 cal. BP) is related to relatively rapid sea-level rise and increasing tidalrange. The later phase of increased tidal sedimentation also reflects an enhanced marine influence after c. 3000cal. BP, but may also be attributed to climate- and human-induced changes in terrestrial sediment flux to thecoastal zone. Comparison of the reconstructed sediment accretion rates with actual rates calculated from radi-ocarbon and luminescence dated sedimentary horizons reveals that this diatom-based approach overestimatessediment accretion by a factor of three. This overestimation is considered to be due to the contemporarysediment flux being an inappropriate analogue for the mid- to late Holocene rather than to any significantmethodological flaws in the approach.

Key words: Holocene, Tees estuary, transfer function, diatoms, radionuclides, tidal sedimentation, sedimentflux, sea-level change.

Introduction

Much of the current body of palaeoenvironmental research is jus-tified in a reverse-uniformitarianism context, in that the under-standing of the past provides information on the future responseof the coast to global climate and environmental change (Birksand Birks, 1980). In this study, we adopt a conventional uniformi-tarianism approach (Lyell, 1830) to examine temporal variationsin coastal sedimentation during the Holocene by means of present-day tidal dynamics, sedimentary processes and ecological niches.

Rates of sediment accumulation on the tidal flats and saltmarshes of Greatham Creek in the Tees estuary have been estab-lished by previous work using a combined pollution and radio-nuclide (210Pb and 137Cs) approach which revealed a broadly lin-ear relationship between accretion and altitude (Plater et al.,1998). While downcore trends in unsupported 210Pb activity onlyprovide age determinations over a 50–80 year timescale, owing

Arnold 2000 0959-6836(00)HL409RP

to rather low activity levels, these are sufficient to establish bothlinear sedimentation rates (i.e., incremental vertical accretion onthe tidal flat surface, mm yr−1) and dry mass accumulation rates(g cm−2 yr−1) for the present intertidal zone. The latter exhibit ageneral decrease with altitude within the tidal frame as a functionof decreasing tidal inundation frequency and duration (Pethick,1981; Allen, 1990; French, 1993).

Complementary research on the present distribution of diatomspecies on the salt marshes and tidal flats of Greatham Creek hasconfirmed a well-established relationship with altitude, which isrelated to tidal inundation frequency and substrate characteristicssuch as sediment grain size and organic content (Horton, 1997;Zong and Horton, 1999). Consequently, if the diatom taxa in thecontemporary training set and their ecological responses to theenvironmental variables of interest have not changed significantlyover the timespan represented by the fossil assemblage, a transferfunction can be applied to Holocene diatom assemblages to

442 The Holocene 10 (2000)

reconstruct former reference water levels, expressed relative tothe present-day tidal frame, and hence Holocene sea-level trends(Horton, 1997).

The research presented here combines these two lines of evi-dence from the Tees estuary, i.e., the altitudinal control of sedi-ment accretion and the reconstruction of former tidal levels fromdiatom data, in the reconstruction of Holocene rates of sedimentaccumulation according to the fossil diatom assemblage data. Theresults of this approach are then compared with rates of tidal sedi-mentation established for the period between approximately 8100and 2800 cal. BP from radiocarbon and luminescence dated hor-izons from the coastal stratigraphic record in a study of spatialand temporal variations in Holocene sediment flux.

Study area

Greatham Creek (Figure 1) is the least disturbed of the tidal inletsto the Tees estuary, particularly as it has remained relatively freeof the extensive dredging and navigation works which have com-plicated the recent sedimentary record of the main Tees channel(Berry, 1995). The tidal regime is marginally macrotidal, with aspring tidal range of approximately 4.6 m. Mean tidal level is+0.35 m OD, the mean spring tides reach +2.65 m OD, and thehighest astronomical tidal level is +3.25 m OD (Admiralty TideTables, 1998). While the present-day salt marshes and tidal flatsrecord sedimentation during the last 80 years or so (Berry andPlater, 1998; Plater et al., 1998), the reclaimed lowlands extendthe period of record back to the nineteenth century. Coring fartherafield within the Tees coastal lowlands has revealed a deep(.13 m) record of sedimentation extending back to approximately11 000 years BP in the region of Thornaby-on-Tees (Shennan,1983), although the sedimentary record only extends to approxi-mately 7900 cal. BP in the immediate vicinity of Cowpen Marsh(Plater and Poolton, 1992). As part of the NERC-funded LOIS(land-ocean interaction study) programme, four sites were selec-ted from the coastal lowlands to elucidate the palaeoenvironmen-

Figure 1 The Tees study area, showing the location of deep boreholes (a)and shallow core sampling sites (b).

tal history of the Tees estuary. These sites run in a ‘seaward’to ‘landward’ sequence from Cowpen Marsh (CM01A), throughSaltholme (HFBB11) and Portrack Marsh (PM5), to BillinghamBeck (Figure 1a). At the Billingham Beck site, two cores (BBC6and BBC3) were taken to investigate the nature and sequence ofenvironmental change at the tidal limit of the estuary.

Methodology

Radionuclides

Previous work on tidal accretion in Greatham Creek (Berry andPlater, 1998; Plater et al., 1998) was supplemented in this studyby an additional two cores from Seal Sands Marsh (SSM) and theICI Brine Fields (BF) (Figure 1b). Cores were sampled using a0.5 m × 100 mm diameter PVC tube, and were cut into 20 mmslices when extruded. In addition, 10 ml subsamples were takenfor bulk density determination. Sediment samples were air-driedusing an Edwards Super-Modulo vacuum freeze drier and packedinto Marinelli beakers of known geometry. Radionuclide activitieswere determined using the low-background gamma detectionfacility in the Environmental Radioactivity Research Centre, Uni-versity of Liverpool. Using downcore trends of 137Cs and, in parti-cular, unsupported 210Pb activity, rates of both linear sedimen-tation and dry mass accumulation (linear sedimentation multipliedby dry bulk density) were calculated. The rates based on 210Pbactivity were calculated using the Constant Rate of Supply model(Appleby and Oldfield, 1992), as those given by the ConstantInitial Concentration model were generally highly irregular. Atbest, this approach should provide age determinations for the last150 years or so, but activities in the sediments proved to be ratherlow and the age determinations were limited to 50–80 years(Plater et al., 1998). Downcore variations in sedimentation andaccumulation rates were averaged and are expressed here as meanvalues for the period of sedimentation contained within the coretogether with a combined analytical and temporal variability errorof one standard deviation.

Diatoms

Five cores from four sites in the coastal lowlands of the Teesestuary were sampled for palaeoenvironmental reconstructionbased on their preserved fossil diatom assemblages. These coreswere denoted as CM01A (Cowpen Marsh), HFBB11 (Saltholme),PM5 (Portrack Marsh), BBC6 and BBC3 (Billingham Beck)(Figure 1a). Approximately 0.5 ml of sediment was sampled fromeach stratigraphic unit contained within the cores, with moresamples being taken from thicker units and in the region of strati-graphic transitions. These aliquots were digested in 70–100 ml of20% 100 volume H2O2 by heating gently for up to 24 hours. Sin-gle drops of the digested samples were pipetted onto glass coverslips and dried on a warm hotplate. The cover slips were theninverted and placed onto a glass slide, using Naphrax as themountant. After further gentle heating and cooling, the diatomscontained in each sample were identified at ×630 to ×1000 magni-fication using the keys of Hendey (1964) and Van der Werff andHuls (1958–74), with nomenclature following Hartley (1986).Palaeoenvironmental interpretation was undertaken with referenceto the methods of Vos and de Wolf (1988; 1993) and Denys(1991–92; 1994).

A transfer function, establishing former tidal levels from thefossil diatom assemblages, has been created using data from theTees estuary, and has been presented by Horton (1997) and Zongand Horton (1999). In these studies, six UK coastal sites wereanalysed systematically to establish the relationship between thedistribution of diatom assemblages within the present-day tidalframe and ground altitude, as illustrated previously by Nelson andKashima (1993), Hemphill-Haley (1996), Shennan et al. (1995;1996) and Zong (1997). Zong and Horton (1999) suggest a strong

A.J. Plater et al.: Holocene tidal sedimentation and diatom-based reconstruction 443

vertical zonation of diatom species from around the mean highwater of neap tides to slightly above highest astronomical tide.This implies that all environmental variables that determine dia-tom distribution are related to tidal submergence which, in turn,are correlated with altitude of the marsh surface. Accordingly,sedentary organisms of the salt marsh are distributed in clearlyidentifiable vertical zones (e.g., tidal flat, low marsh around meanhigh water of spring tides, and high marsh around highest astro-nomical tide), which Horton (1997) and Zong and Horton (1999)used to develop a transfer function to reconstruct Holocene tidallevels (expressed relative to the present-day tidal frame). In thisstudy, the established tidal level transfer function is applied todiatom assemblage data from five cores to produce high-resolutiontidal levels, or depositional altitudes (expressed relative to meansea level at the time of deposition), for former environments ofdeposition preserved in the Holocene stratigraphy.

Results and interpretation

Present-day sedimentation in the tidal frame

The results from the radionuclide analyses reveal a broadly linearrelationship (Altitude = 3.69–5.71 × Accretion Rate) between drymass accumulation rate and altitude within the tidal frame (Table1 and Figure 2). This is considered to be a direct function of morelimited sediment delivery as the frequency and duration of tidalinundation decreases towards the level of the highest tides. Linearsedimentation rates (mm yr−1) show no such relationship (Table1), maybe because the thickness of a sedimentary horizon is moresensitive to changes in the proportion of organic versus minero-genic sedimentation, and is susceptible to post-depositionalchange via compaction (Allen, 1990). The observed decrease insediment accumulation rate with altitude appears to experiencelimited mediation from the biota as the linear relationship spansthe salt marsh/mudflat transition and encompasses such environ-ments as bare mud, pioneer salt marsh and upper salt marsh. How-ever, the lower tidal flat (Seal Sands) exhibits a rather low sedi-mentation rate, perhaps owing to sediment disturbance andredistribution, and the Brine Field site has a sedimentation ratetypical of an upper salt marsh. This latter aspect is probably theresult of the Brine Fields site being enclosed during reclamationworks in the early twentieth century (Plater et al., 1998), whichsignificantly reduced the frequency and duration of tidal inun-dation to extreme flood events and the highest tides.

Holocene palaeoenvironments

The stratigraphy and diatom assemblage data (see Appendix I forprincipal diatom species counts) reveal that Holocene sedimen-tation has taken place in a range of coastal environments fromlower tidal flat to perimarine lake (Figure 3). Minerogenic sedi-mentation has been accomplished mainly by tidal deposition undera variable marine influence. Indeed, this variability is apparent in

Table 1 Mean sedimentation rates for Greatham Creek cores (1s errorbars are a combination of analytical uncertainty and temporal variabilityin sedimentation rate)

Altitude Linear rate Dry mass accumulation rate(m OD) (mm yr−1) (g cm−2 yr−1)

GCS2 +2.86 3.16 ± 0.75 0.15 ± 0.04GCS1 +2.50 3.74 ± 2.65 0.20 ± 0.09SSM +2.00 2.78 ± 1.03 0.30 ± 0.12GCM +1.84 3.29 ± 0.87 0.33 ± 0.09

BF +1.31 1.22 ± 0.53 0.11 ± 0.03SS +0.26 2.50 ± 0.65 0.18 ± 0.05

Figure 2 The relationship between sediment accretion rate (dry massaccumulation) and altitude in the tidal frame, as established from 210Pband 137Cs dating of salt-marsh and mudflat cores (1s error bars are acombination of analytical uncertainty and temporal variability in sedimen-tation rate).

both temporal and spatial terms, particularly along the samplingtransect from Cowpen Marsh to Billingham Beck. The more ‘sea-ward’ of these cores, from Cowpen Marsh (CM01A), exhibits per-haps the most consistent marine influence on sedimentation,although this was interrupted by a phase of organic depositionbetween approximately 6000 and 3700 cal. BP. Tidal-flat and salt-marsh conditions did return to Cowpen Marsh after this period,but these were rather more brackish and freshwater than before.

At the opposite end of the sampling transect, the cores fromBillingham Beck preserve a record characterized by tidal sedimen-tation during the early Holocene, that is, prior to 7337–7549 cal.BP in BBC3 and 7017–7392 cal. BP in BBC6, but has been moreof a fresh- and brackish-water lagoon for the majority of its depo-sitional history. Indeed, the stratigraphy shows that this rather iso-lated sedimentary system, which is at the limit of the tidal influ-ence, has also periodically undergone the transition to a terrestrialorganic environment during the mid- to late Holocene. Further-more, the more seaward of the two cores (BBC6) underwent thechange to a brackish lagoon at a later date than the more landwardsite (see above), and exhibits a marginally enhanced marineinfluence throughout the period of lagoonal sedimentation.

Although the core from Saltholme (HFBB11) is adjacent to theCowpen Marsh site at the ‘seaward’ extreme of the sampling tran-sect (Figure 1a), the more ‘intermediate’ record from PortrackMarsh (PM5) is characterized by a deeper record of tidal sedimen-tation (Figure 3). This is due to a depth limitation of the Saltholmestratigraphy by the elevation of the underlying Devensian glacialdiamicton and Post-Glacial laminated clays (Plater et al., 2000).Indeed, the Holocene palaeoenvironmental record at a depth ofapproximately 2.00 m in core HFBB11 is very similar to that incore PM5 above a depth of c. 2.50 m. These records both preservea period of salt marsh/perimarine sedimentation (dating fromc. 2800–3253 cal. BP in HFBB11 and c. 2733–2955 cal. BP inPM5) grading upward to a tidal flat with an open marine influence.

In the context of this study, the most important aspect of thepalaeoenvironmental data presented in Figure 3 is that the main


Figure 3 Core stratigraphy and diatom-based palaeoenvironmental reconstructions for CM01A, HFBB11, PM5, BBC6 and BBC3.

body of the Holocene sedimentary record is similar in terms ofdepositional environments to those observed in Greatham Creektoday. In this case, the diatom-based transfer function describedin Horton (1997) and Zong and Horton (1999) can be applied tothese cores to reconstruct the tidal level, or altitude of the site ofdeposition (relative to mean sea level at the time), from the pre-served fossil diatom assemblages.

Holocene reference water levels and sedimentation

rates

The results of the diatom-based transfer function reconstructionof tidal level for the palaeoenvironments of the five coresdescribed above (Appendix II) reveal certain spatial and environ-mental limitations in that the extent of this reconstruction becomesrather tenuous for the Billingham Beck sites (BBC3 and BBC6)which are somewhat distant from the contemporary diatom distri-bution study site. Indeed, the contemporary intertidal environmentof Greatham Creek is different to that of the present-dayBillingham Beck which is at the tidal limit of the Tees estuary.In addition, the upper parts of BBC6 and, in particular, BBC3preserve lagoonal palaeoenvironments which were subject to asignificant freshwater influence. Indeed, this is also true of thelower part of the core from Saltholme (HFBB11), the middle part

of the Portrack Marsh core (PM5), and the more brackish environ-ments preserved in the Cowpen Marsh core (CM01A at depths of1.50, 2.20 and 2.40 m).

For those sites and levels where the transfer function enablesreconstruction of tidal level (depositional altitude), the equationof the regression line for the present-day relationship between alti-tude and sediment accumulation rate (see section on present-daysedimentation and Figure 2) has been combined with the recon-structed tidal level data to give an estimate of Holocene tidal sedi-mentation rates. The reconstructed sedimentation rates areexpressed as downcore trends in dry mass accumulation rate(Figure 4). These trends largely reflect temporal variations in themarine influence. In general, however, the lower parts of four ofthe five cores preserve faster rates of sediment accumulation thanthe upper parts. This is, perhaps, a reflection of decreasing eustaticsea-level rise through the Holocene, with most of these sites exhi-biting an infilling and, therefore, decreasing accretion rate towardsthe present day as sedimentation on the accreting tidal flatbecomes limited by decreasing inundation frequency. The mainexception to this trend, however, is that of core HFBB11 (Figure4b) which exhibits an upward increase in sediment accretion. Thistrend appears also to be present in the upper parts of coresCM01A and PM5 (Figure 4, a and c respectively) where the

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Figure 4 Reconstructed sediment accretion rates (g cm−2 yr−1) for (a) CM01A, (b) HFBB11, (c) PM5, (d) BBC6 and (e) BBC3. The cross-hatched bars are actual accretion rates from dated sedimentary horizons.


sediments are found at a similar altitude today as those preservedin HFBB11, that is approximately +0.3 to +1.1 m OD. With theaddition of the available radiocarbon ages for organic horizons inthese cores, the apparent correlation based on elevation is con-firmed. The faster sediment accumulation rates below about–1.5 m OD represent a period of tidal sedimentation betweenapproximately 8000 and 6000 cal. BP, while those betweenc. +0.3 and +1.1 m OD would appear to be late Holocene in age,dating from about 3000 cal. BP.

The two periods of enhanced sediment accumulation rate are,of course, the result of changing palaeoenvironments within thetidal frame, which are likely to be driven by enhanced rates ofrelative sea-level rise. While the earlier of these two phases canbe linked to the relatively rapid sea-level rise during the earlyHolocene, the latter is a little more difficult to account for. Whatmay be recorded here is a period of renewed relative sea-levelrise following a phase of stability during the mid-Holocene.Rather than accelerated eustatic rise being the cause of this, thelate-Holocene record of tidal sedimentation may reflect the north-ward migration of a glacial forebulge (Lambeck, 1995). Severalstudies have revealed an inflexion in the relative sea-level curvesfor sites along the east coast of the UK to the south of the Humber(Shennan, 1989; 1992; Long and Shennan, 1993). This representsthe passage of a forebulge crest as crustal uplift on the proximallimb turns to subsidence on the distal limb of the forebulge(Quinlan and Beaumont, 1981). The onset of crustal subsidencemust be accompanied by enhanced relative sea-level rise, bringingwith it an increased marine influence on sedimentation. From therecord preserved in cores CM01A, PM5 and HFBB11, this pass-age of a late-Holocene forebulge crest through the Tees regionprobably dates to approximately 3000 cal. BP. Unfortunately, thisinflexion is not observed in the relative sea-level curve for theTees (Shennan et al., 2000b), perhaps owing to the paucity ofdata for the last 3000 years.

Holocene sedimentation rates from 14C and

luminescence ages

Luminescence and calibrated 14C ages for four of the five Teescores also enable linear sedimentation rates to be calculated forvarious periods during the Holocene (Table 2). Here, the differ-ence in depth between two or more dated horizons has simplybeen divided by the intervening time interval to give an approxi-mate rate of sedimentation in mm yr−1. This makes no allowancefor temporal variations in sedimentation (or periods of erosion)which would be likely to accompany changes in the nature of the

Table 2 Sediment accretion rates calculated from calibrated radiocarbon and luminescence dated sedimentary horizons. The final column provides anapproximate comparison of the reconstructed and calculated accretion rates, i.e., ‘actual’ to ‘reconstructed’ ratio (A/R) (14C ages are mean calibrated ages,luminescence ages are given in italics)

Core Upper age Upper depth Lower age Lower depth (m) Linear rate Dry mass accumulation A/R ratio(cal. BP) (m) (cal. BP) (mm yr−1) (g cm−2 yr−1) (approx.)

CM01A 3691 1.19 5969 2.56 0.601 ± .037 0.087 ± .005 1/3CM01A 5111 3.10 5969 2.56 ` ` N/ACM01A 5111 3.10 5915 3.70 0.746 ± .153 0.109 ± .022 1/3CM01A 5915 3.70 7862 4.97 0.652 ± .090 0.095 ± .013 1/4CM01A 5969 2.56 7862 4.97 1.273 ± .052 0.185 ± .008 1/2HFBB11 2988 1.55 5920 2.26 0.242 ± .030 0.035 ± .004 1/6PM5 2781 2.62 6484 4.35 0.467 ± .045 0.068 ± .007 1/4PM5 6484 4.35 7017 4.54 0.356 ± .047 0.052 ± .007 1/5PM5 2781 2.62 7017 4.54 0.453 ± .034 0.066 ± .005 1/4BBC3 7017 4.99 7496 5.25 0.543 ± .047 0.079 ± .007 1/3BBC3 7496 5.25 7854 6.49 3.464 ± .135 0.504 ± .020 1/1BBC3 7854 6.49 8138 6.59 0.352 ± .072 0.051 ± .010 1/3BBC3 7017 4.99 8138 6.59 1.427 ± .068 0.208 ± .010 1/1

environment of deposition, sediment grain size and sediment com-position.

The calculated linear sedimentation rates (Table 2) are rathervariable, although the fastest rates, of the order of 1 mm yr−1,would appear to be between approximately 8000 and 6000 cal.BP. This agrees well with the trends observed in the rates of sedi-ment accumulation described in the previous section. Althoughthe overall trend is somewhat similar, the magnitude is rather dif-ferent. Previous measurements of the bulk density of sedimentbetween depths of approximately 2.60 and 4.60 m in core CM01Arevealed an average value of approximately 1.5 g cm−3. If thisaverage for organic silty-clay is applied for the entire suite ofsamples, dry mass accumulation rates may be estimated for thesetidal sedimentation data. Admittedly, this ignores any significantvariation in the bulk density of these sediments, but the only sig-nificant departure from this average value would be organic unitswhich would exhibit a markedly lower value and, hence, lowerdry mass accumulation rates.

Comparison of the calculated dry mass accumulation rates fromthese chronological data (actual) with those obtained for the sameperiod using the diatom-based transfer function (predicted) revealsthat actual sediment accumulation was less than half that predictedusing the diatom-based method. The only level which gives asimilar result using the two approaches is the period betweenapproximately 7900 and 7500 cal. BP for core BBC3 (Figure 4eand Table 2), although similar orders of magnitude are apparentfor 8100 to 7000 cal. BP for the same core, and for 7900 to 6000cal. BP for core CM01A (Figure 4a and Table 2).

This lack of agreement between the two data sets is disap-pointing, but the observed differences reveal a significant disparitybetween the actual and predicted sediment accumulation rates, theformer being significantly lower in the majority of cases. Indeed,even if a bulk density of double that obtained for the CowpenMarsh core is used as an average value, the main body of actualsediment accumulation rates remains significantly lower.

Discussion

It is clear from the results presented in the preceding section thatthe reconstructive model based on present-day rates of tidal sedi-ment accretion overestimates the actual Holocene rates of massaccumulation by an average factor of three. This is surprising,particularly as dry mass accumulation is used in preference tolinear sedimentation rates as the latter is sensitive to changes in


sediment composition. This overestimation is true of various per-iods of time during the Holocene, and of all the sites sampled inthis study, although there may be a closer match when sedimentaccretion rates are at their fastest, that is at approximately 8000to 6000 cal. BP.

If we consider the transfer function approach to be valid, therewould appear to have been less sediment available for tidal sedi-mentation during the Holocene than today. This would seemreasonable as the supply of sediment to the coastal zone is likelyto have been influenced to a significant degree by the vegetation,clearance and land-use history of the Tees catchment. Althoughearly clearance of the landscape is usually attributed to Neolithiclandinam activity (Roberts, 1989), there is a strong body of palyn-ological evidence dating from the Atlantic period (7000–5000 cal.BP), particularly flint microliths sometimes found in associationwith organo-mineral inwash deposits containing charcoal and pol-len characteristic of forest clearance and disturbed open ground,to suggest high-altitude burning in the northern Pennines and theNorth Yorks Moors during the Mesolithic (Smith, 1970; Tallisand Switsur, 1973; Jacobi et al., 1976; Jones, 1976; Innes andSimmons, 1988; Simmons, 1993). Indeed, Simmons and Innes(1985) proposed that erosion and transport of soils from clearedslopes by increased runoff would have been encouraged by suchactivity. In terms of an acceleration in the rate and extent of clear-ance during the Neolithic, Davies and Turner (1979) note analmost complete lack of clearance activity over much of northernBritain. However, in their study of Blelham Tarn in the Lake Dis-trict, Pennington and Lishman (1984) have identified a stepwiseincrease in runoff and lake sedimentation associated with wetterconditions c. 7400 BP and increased mineral inwash fromapproximately 5000 BP. These two periods of enhanced sedimen-tation are found generally in the Lake District lakes, but Pen-nington (1981) favours a climatic explanation for these inwashphases.

More significantly, Pennington and Lishman (1984) note thedeposition of a fine-detritus mud throughout the Lake District, thebase of which corresponds with the earliest evidence of significantforest clearance dated to c. 2500–2300 BP. In Upper Teesdale,significant opening of the tree canopy occurred during the BronzeAge c. 3300 BP (Turner et al., 1973; Chambers, 1978), but amore extensive phase of clearance in northern England and south-ern Scotland took place during the Iron Age c. 2200 BP(Chambers, 1978; Turner, 1981; Bell and Walker, 1992). Theconsequent release of soil at this time is also recorded in alluvi-ation of the river valleys of northern England from c. 2200 BPonward (Macklin and Lewin, 1991). Perhaps the most significantperiod of forest clearance on a grand scale was the result ofRoman occupation in northern Britain c. 1800 BP (Barber etal., 1993).

From the above discussion, it is clear that sediment flux duringthe latter part of the Holocene and, indeed, the present day, fol-lowing further clearance and the use of intensive agricultural prac-tices, is an inappropriate analogue for sediment supply to the coas-tal zone for the entire Holocene period. Consequently, thepredicted rates of sediment accretion exceed those calculated fromthe dated sedimentary horizons. However, the enhanced supply ofsediment from forest clearance during the Mesolithic may explaincloser agreement between the predicted and actual rates of sedi-ment accretion between 8000 and 6000 cal. BP and, indeed, theacceleration in sediment accumulation between c. 7800 and 7500cal. BP recorded in cores BBC3, CM01A and, somewhat later,PM5. Furthermore, the acceleration in sediment accretion afterapproximately 3000 cal. BP may not be due to renewed sea-levelrise alone, but also to an increase in terrestrial sediment flux fromthe Bronze Age onward.

Changes in tidal dynamics may also be responsible for higherrates of tidal sedimentation between c. 8000 and 6000 BP. Shen-

nan et al. (2000a) suggest that the spring tidal range off Tees-mouth was approximately 50–60% of the present range at c. 8000cal. BP, increasing to approximately 90% by c. 6000 cal. BP.Franken (1987) and Hulsen (1994) noted that a similar increasein tidal range on the Dutch coast was accompanied by increasedtidal asymmetry and, hence, enhanced tidal sediment accumu-lation, leading to the infilling of coastal lagoons in Zeeland byc. 6500 cal. BP (Vos and van Heeringen, 1997). The faster ratesof sediment accretion reconstructed for the Tees during the sameperiod may be attributed to increased tidal asymmetry as theamplified tidal range is accompanied by increased velocity of thetidal wave and reduced bottom friction in the deeper water(Franken, 1987; Hulsen, 1994). It is, therefore, not surprising thatthe approach used here significantly overestimates the actual sedi-ment accumulation rates because the present-day tidal dynamicswould be an inappropriate analogue for much of the Holocene.The observed disparity between the actual and predicted sedimentaccumulation rates would appear to support significant changes intidal dynamics during the Holocene period, which may be ratherappropriate considering the marked reduction in accommodationspace for the tide within the Tees estuary as it has infilled.

The alternative approach is to question the validity of the recon-structive model used in this study. First, the sedimentation ratemodel may overestimate mass accumulation rates at lower depo-sitional altitudes, particularly as there is no obvious mediationby the biota in this mudflat environment. The linear relationshipbetween sediment accretion and altitude may, therefore, breakdown where either the vegetation does not disrupt the tidal flowor the diatoms and euglenids provide no additional stabilizationof the tidal-flat surface. Indeed, the observed rate of sedimentaccretion of Seal Sands is much lower than that of the mudflat(see section on present-day sedimentation). Consequently, therewould be much less of a disparity between the actual and pre-dicted rates of sediment accumulation for the lower tidal-flatenvironments. Unfortunately, this does not account for the dispari-ties observed for the higher-altitude sites of deposition, and thereis every reason to expect the established relationship to hold trueover the altitude range of c. +3.07 to +1.86 m OD. However,biotic mediation of tidal sedimentation may be more importantthan this study has assumed.

Second, caution should be exercised when applying transferfunctions since several characteristics within the contemporarydata may affect the accuracy of tide-level reconstructions. Forexample, there was an uneven spatial sampling within the contem-porary data with respect to altitude. The majority of sites werefound above mean high water of neap tides (MHWNT) and, there-fore, predictions for reference water levels below MHWNT maybe unreliable (Horton, 1997). Furthermore, the transfer functionused here calibrates the diatom data to one environmental variable(altitude), which may then misrepresent the influence of otherenvironmental factors and give anomalous results (Le, 1992;Huntley, 1993).

Third, the issue of mismatching timescales must not be over-looked. The uniformitarianism approach presented here is ques-tionable where a significant disparity exists between the present-day and Holocene Tees estuaries in terms of both morphologyand dynamics. In addition, while the contemporary relationshipbetween altitude and sediment accumulation rate is based on therecords for the last 50 to 80 years, the ‘actual’ rates of sedimen-tation are averages for c. 500–3000 year periods. Consequently,the magnitude-frequency characteristics of these two time periodsmay be quite different. Indeed, the significance of temporal vari-ations in sedimentation rate, or even erosion of the sedimentaryrecord, is much greater for the longer periods of time. Nonethe-less, Shennan (1995) was unable to identify any such trend inlinear sedimentation rate with age for Holocene minerogenic sedi-ment sequences for UK coastal lowlands.


In addition to the above, autocompaction of the sediment col-umn should be considered, particularly for sediments with a sig-nificant thickness of overburden and for sedimentary units with ahigh organic content. If, for example, the vertical spacing betweentwo radiocarbon dated sediment horizons was once double thatobserved today, the linear sedimentation rates should be a factorof two greater. This would then enable an enhanced degree ofagreement between the predicted and actual rates of sedimen-tation. This may be true of linear sedimentation rates, but the bulkdensity would be sensitive to such compression and, hence,decompressing the sediment column would reduce the bulk den-sity by a similar factor, and the resulting dry mass accumulationrates should remain unchanged, that is double linear sedimentationrates multiplied by a halved bulk density. Thus, linear sedimen-tation rates are susceptible to autocompaction, but dry massaccumulation rates are more robust. Simple comparison of bulkdensity measurements for the Holocene and present-day tidal sedi-ments would reveal any significant autocompaction effects.Hence, bulk density measurements should be considered as anintegral part of any palaeoenvironmental reconstruction.

Conclusions

The results of this study raise a number of interesting questionsconcerning Holocene sediment flux and relative sea-level rise inthe Tees region.

The combined use of present-day tidal sediment accumulationrates and a diatom-based tidal level transfer function reveals twoperiods of enhanced sediment accretion: an earlier period relatedto relatively rapid sea-level rise and possible changes in tidaldynamics (increasing tidal range and asymmetry) betweenapproximately 8000 and 6000 cal. BP, and a later one linked toan enhanced marine influence after approximately 3000 cal. BP.This second phase may be controlled by crustal subsidence fol-lowing the northward migration of a glacial forebulge through theregion during the late Holocene.

Appendix I

Principal diatom species preserved in cores CM01A, HFBB11, PM5, BBC6 and BBC3. Data are expressed as total valves recorded ateach level.

Comparison of the reconstructed rates of sediment accumu-lation with those calculated from radiocarbon and luminescencedated sedimentary horizons reveals that the former are about afactor of three greater than those actually observed for the differ-ent sample sites at different times during the Holocene, althoughthe disparity is less marked during the period of relatively rapidsediment accretion between approximately 8000 and 6000 cal. BP.This disparity is unlikely to be related to autocompaction of thesediment column, or to application of the model beyond its estab-lished limits, but may be related to changes in the tidal regime ofthe Tees during the Holocene and temporal variations in sedimentflux from the terrestrial system following climate- and/or human-induced changes in catchment vegetation and land use. Indeed,Mesolithic forest clearance may have enhanced terrestrial sedi-ment supply to the coastal zone during the early to mid-Holocene,hence a closer agreement between the predicted and actual ratesof sediment accumulation, but it is more likely that widespreadclearance activity from the Bronze Age onward contributed toaccelerated coastal sedimentation during the late Holocene.Hence, this rather classical approach to the study of Holocenesediment flux reveals important temporal variations which wouldotherwise remain undetected owing to the limited chronologicaldata which often characterizes palaeoenvironmental investi-gations.

Acknowledgements

The authors should like to thank all those who have contributedto this study, in particular Jim Innes, Peter Vos, Mark Weldon,Alison Berry, Vicky Holliday, Nicola Shenton, Kelly Swales, IanShennan, Chris Evans, Efren Santa Maria and Anna Pretious. Spe-cial thanks are extended to Sandra Mather for her expertise inproducing the figures. This research was undertaken as part of theNERC-funded LOIS special topic (grant no. LOEPS 33,GST/02/0738). This is LOIS publication number 701.



Appendix II

Reconstructed tidal level and sediment accumulation rate usingthe tidal level transfer function on diatom data from coresCM01A, HFBB11, PM5, BBC6 and BBC3. (Tidal level isexpressed relative to mean sea level at the time of deposition.)


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Holocene tidal levels and sedimentation rates using a diatom-based palaeoenvironmental reconstruction: the Tees estuary, northeastern England

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