Chronology and palaeoenvironmental reconstruction in the sub-tidal zone: a case study from Hinkley Point

Journal of Archaeological Science 54 (2015) 237e253

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Chronology and palaeoenvironmental reconstruction in the sub-tidal zone: a case study from Hinkley Point Seren Griffiths a, *, Fraser Sturt b, Justin K. Dix c, Benjamin Gearey d, Michael J. Grant c a

Cardiff School of History, Archaeology and Religion, Cardiff University, John Percival Building, Colum Drive, Cardiff, CF10 3EU, UK Faculty of Humanities, University of Southampton, Avenue Campus, Highfield, Southampton, SO17 1BF, UK c Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Waterfront Campus, European Way, Southampton, SO14 3ZH, UK d Department of Archaeology, Connolly Building, Dyke Parade, University College Cork, Cork City, Ireland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 March 2014 Received in revised form 25 November 2014 Accepted 5 December 2014 Available online 18 December 2014

Evidence from the Severn Estuary demonstrates that this region was exploited by Mesolithic huntergatherer-fishers (Bell, 2007). The potential for future archaeological discoveries (Bell, 2007; Webster, 2007: 273; Bell and Warren, 2013: 39), and the well-preserved palaeoenvironmental evidence in the fine-grained and organic sediments of the Somerset, Avon and Gwent Levels (Hosfield et al., 2007a: 40) makes the area of importance for archaeological study. Small quantities of worked flint have been recovered from the foreshore around Stolford, Porlock and Minehead Bay (Mullin et al., 2009; Canti et al., 1995) implying human activity in the present intertidal zone, which is further enhanced by the suggestion of possible deliberate burning of reed swamps (Jones et al., 2005) similar to that postulated in the Severn Estuary (Brown, 2005; Timpany, 2005; Bell, 2007). While considerable research has been carried out within terrestrial and intertidal contexts, remarkably little archaeological work has been undertaken below the mean low water mark (Webster, 2007: 273). The Bristol Channel and Severn Estuary has seen considerable change in sea-level since the Last Glacial Maximum (Long et al., 2002; Philips and Crisp, 2010). Extending our knowledge beyond the intertidal zone is therefore of key importance for understanding the Late Palaeolithic and Mesolithic palaeogeography of the region (Hosfield et al., 2007b). Developments in the recovery of offshore Holocene peat and sediment sequences now permit the production of multi-proxy palaeoenvironmental datasets and landscape reconstructions from submerged sample sites. This paper uses evidence from three cores, recovered from submarine peat deposits at Hinkley Point, Bristol Channel, UK, to explore the issues and challenges associated with producing radiocarbon chronologies from deeply submerged peat sequences within a marine environment. We emphasise the importance of analysis of multiple sequences to construct robust chronologies for local hydrological change and landscape reconstruction (Edwards, 2006). The need for local evidence is critical if we are to move beyond generalised and potentially misleading models of humaneenvironment interaction (Scaife, 2011), because as this case study demonstrates, complex processes and landscape variability might have been features of even highly-localised palaeoenvironments. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Marine Peat Radiocarbon Palaeoenvironmental reconstruction Bayesian

1. Introduction Developments in the statistical treatment of radiocarbon results from palaeoenvironmental sequences, including sophisticated

* Corresponding author. E-mail addresses: griffi[email protected] (S. Griffiths), [email protected] (F. Sturt), [email protected] (J.K. Dix), [email protected] (B. Gearey), [email protected] (M.J. Grant). http://dx.doi.org/10.1016/j.jas.2014.12.008 0305-4403/© 2014 Elsevier Ltd. All rights reserved.

deposit modelling (Bronk Ramsey, 2008; Parnell et al., 2011; Blaauw and Christen, 2011), have resulted in the production of robust chronologies for fluvial (e.g. Howard et al., 2009), lacustrine (e.g. Blockley et al., 2008; Staff et al., 2011), and bog (e.g. Marshall and Whitehouse, 2013) sequences. These approaches can produce revised age estimates for palaeoenvironmental ‘events’ sampled in individual sequences (Gearey et al., 2009; Blaauw and Mauquoy, 2012), and can compare the timing of different events at different sample locations (Gearey et al., 2009; Blaauw et al., 2007).

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Complex chronological modelling represents a development towards understanding the timing and tempo of palaeolandscape changes. Importantly, by critically engaging with the timing of the appearance of different palaeoenvironmental proxies at individual sample sites, such approaches can also help to ‘unpack’ the nature of palaeoenvironmental ‘processes’. In some cases, palaeoenvironmental ‘events’ have, due to poor chronological control or analysis of proxy data, become glossed or reified as ‘mythic’ (Oldfield, 2001; Blaauw, 2012; Blaauw et al., 2007; Lowe and Higham, 1998), achieving conceptual statuses beyond the analysis of evidence from proxy data. As Parnell et al. (2008: 1873) note, events are “… not observable; they are latent and observed through, but not defined by, noisy data. An ‘event’ is thus a theoretical construct”. In order to unpack the timing of ‘events’ at a local scale and characterise ‘processes’ at a regional scale, it is necessary to ensure a robust association between radiocarbon measurements and the point in time at which proxy data were deposited in parent contexts and, of course, whether there was any interval between the formation of proxies and their deposition.

Radiocarbon dating by Accelerator Mass Spectrometry (AMS) has facilitated the measurement of small samples, including individual macrofossil and different carbon fractions (often ‘humin’ and ‘humic acid’ fractions; see Brock et al., 2011 for a review) isolated from peat. The selection of small samples from peat can lead to the dating of material that may have been subject to complex taphonomic processes, post-depositional mixing, redeposition, or diagenesis. In short, whilst the use of AMS measurements allows us to produce measurements on small samples, the technique can lead to the sampling of material that is not of the same radiocarbon age as the formation of the parent horizon from which it was recovered. This may be true even when peat beds indicate in situ formation, as opposed to samples from wetland environments in which much more complex taphonomic processes have been at work, such as fluvial deposits (Chiverrell et al., 2009). An extensive literature considers the relative merits of radiocarbon dating different material from peat from terrestrial wetland sites, including different carbon fractions and plant macrofossils (e.g. Shore et al., 1995; Cook et al., 1998; Waller et al., 2006; Bartley

Fig. 1. Study area. (A) Location of Hinkley Point study area, plotted using the regional scale UKHO derived bathymetry (bin size 20
30 m) of the Bristol Channel and Severn Estuary integrated with the OS Panorama topographic data (50
50 m bin size). Topography reproduced with the permission of Ordnance Survey on behalf of her Majesty's Stationery Office. ©Crown Copyright (2011). Bathymetry reproduced with the permission of British Crown and SeaZone Solutions Limited. All rights reserved. Product Licence 032007.016. (B) Lower Lias Bedrock topography obtained from sub-bottom geophysical survey, along with areas where survey results were obscured by shallow sub-surface gas. Distribution of all boreholes and vibrocores, along with the six that were subject to radiocarbon dating.

S. Griffiths et al. / Journal of Archaeological Science 54 (2015) 237e253

239

Fig. 2. Hinkley Point borehole lithology and stratigraphy.

and Chambers, 1992; Gulliksen et al., 1998; Johnson et al., 1990; Lowe and Walker, 2000; Howard et al., 2009). Radiocarbon results on humin and humic acid fractions from the same peat subsample may be affected by the size fraction dated (Brock et al., 2011), the degree of waterlogging (Shore et al., 1995), and the nature of the waterlogged environment (e.g. Cook et al., 1998; €rnqvist et al., 1992; Johnson et al., 1990; Blaauw et al., 2004; To Howard et al., 2009; Waller et al., 2006). In addition, the nature of pretreatment (e.g. Cook et al., 1998), and sample storage and size (e.g. Nilsson et al., 2001; Wohlfarth et al., 1998) have been shown to impact on results. Different species of plant macrofossil from peat horizons have been demonstrated to be of different ages and are thought to reflect distinct €rnqvist et al., 1992; Shore et al., 1995). taphonomic processes (e.g. To Short-life plant macrofossils of species that may have grown in situ, and which use atmospheric (non-submerged) photosynthetic pathways, provide samples with a robust association with the formation of the peat deposits of interest (Piotrowska et al., 2010/11), though due consideration must again be made of issues relevant to taphonomy. Underlying geology and hydrology have been demonstrated to have important implications on radiocarbon chronologies in terms of ‘hard water offsets’ (e.g. Godwin, 1951; Deevey et al., 1954), and sources of carbon from ground water flow (contrast for example the hydrology of ombrotrophic or minerotrophic bogs; Waddington and Roulet, 1997). For carbon fractions it may not be possible to resolve which chemical or size grade represents the most appropriate measurement for the age of peat formation at specific depths (Brock et al., 2011). As Marshall and Whitehouse (2013: 79) note “… because of the various scientific opinions regarding the fractions of peat that

can be reliably dated and site-specific factors [on terrestrial wetland sites], consistency [should be] sought within [a] radiocarbon dating programme”. In terms of assessing the robustness of the results for building chronologies for submerged marine peats, it is necessary to compare the measurements on different types of samples. Humic acid fractions are often regarded as dating paludification, as a result of in situ breakdown of organic matter, or to be younger than the humin fraction as a result of downward movement of humic acid through profiles (Dresser, 1970; Shore et al., 1995; see Brock et al., 2011 for review). However, in other case studies, humic acid fractions have been found to be older than humin fractions, a pattern which has been suggested to be associated with wetter environments (e.g. Shore et al., 1995: 375 and 379; see also Blaauw et al., 2004: 1541; and their analysis of sites reported in Aaby and Tauber (1975) and Küster (1988)).

Table 1 The stratigraphic sequence offshore of Hinkley Point (see Fig. 2 for details). Unit

Description

1 2 3 4 5 6 7 8 9

Upper silts, sands and gravels Upper peat and organic rich silts Marine silts II Thin intercalated peats and silts Marine silts I Lower peat Lower silts and clay Lower gravel Lower Lias bedrock

240

Table 2 Palaeoenvironmental details from the lower peat (unit 6) sampled in the vibrocores. Full details are given in Sturt et al. 2014. Radiocarbon lab code and sample depth

Position within lithological unit; local pollen assemblage zone (LPAZ)

Palaeoenvironmental details (Sturt et al., 2014)

VC06

GrA-53525 (12.64 mODN to 12.65 mODN) GrA-53526 (12.64 mODN to 12.65 mODN)

Bottom of basal peat unit (unit 6); LPAZ VC06-1 (12.65 to 12.60 m ODN)

GrA-53519 (12.58 mODN to 12.59 mODN) GrA-53520 (12.58 mODN to 12.59 mODN) Beta-288884 (12.58 mODN)

Top of basal peat (unit 6); LPAZ VC06-1 (12.60 to 10.53 m ODN)

GrA-53702 (13.56 mODN to 13.58 mODN)

Incatercalated peat (unit 4)

GrA-53746 (13.74 mODN to 13.75 mODN) GrA-53625 (13.74 mODN to 13.75 mODN) GrA-53628 (13.76 mODN to 13.78 mODN) GrA-53747 (13.80 mODN to 13.81 mODN) GrA-53627 (13.80 mODN to 13.81 mODN)

Top basal peat (unit 6) LPAZ VC15-2 (13.77 to 13.70 m OD) Basal peat (unit 6) LPAZ VC15-2 (13.77 to 13.70 m ODN) Bottom basal peat (unit 6) LPAZ VC15-1 (13.88 to 13.77 m ODN)

13.74 mODN) 13.74 mODN)

Top of peat horizon (unit 6) LPAZ VC25-2 (13.76 to 13.70 m ODN)

13.76 mODN) 13.76 mODN)

Bottom of peat horizon (unit 6) LPAZ VC25-2 (13.76 to 13.70 m ODN)

LPAZ VC06-1. Pollen assemblage dominated by Pinus (42%) and Corylus avellana-type (35e40%), with some Quercus and Ulmus. The peat unit also contained quantities of pre-Quaternary palynomorphs suggesting that fluvial conditions pertained, along with some sediment reworking and/or bedrock erosion. Small numbers of Chenopodiaceae (5%) indicate that this may have been subjected to brackish water ingress or situated close to a salt marsh containing a typical halophyte assemblage. Plant macrofossils from the peat were sparse but included Poaceae indeterminate stem fragments and seeds of Chenopodiaceae, Carex sp, Mentha and Lycopus europaeus, indicative of damp wetland vegetation, along with occasional small wood charcoal fragments. The top of the peat (unit 6), contained a low abundance, low diversity, Foraminifera assemblage characterised by the agglutinated taxa Jadammina macrescens and Haplophragmoides spp, indicative of a saltmarsh environment accumulating toward the upper limit of marine influence. Overlying this unit, characteristic high marsh taxa are replaced by a more diverse assemblage of calcareous Foraminifera, along with low numbers of the agglutinated species Miliammina fusca, collaborating trends seen in the pollen data. These species are typical of a brackish environment (probably sub-tidal) although the presence of some more fully marine species (eg Ammonia batavus) suggests proximity to more open marine conditions. Transition to LPAZ VC06-2 is associated with a significant change from peat to silts (unit 5). The Pinus e Corylus dominance found in LPAZ VC06-1 is now replaced, in VC06-2, by Quercus (up to 38%) and Corylus avellana-type (up to 44%). Pollen assemblage also shows a significant increase in halophytes, including Chenopodiaceae (up to 17%) and a presence of Plantago maritima (along with less diagnostic taxa including Spergularia, Plantago coronopus-type (buck's-horn plantain) and Aster-type (daisies)), indicating stronger brackish or marine conditions. Sharp expansion of pre-Quaternary palynomorphs, as well as dinoflagellates, indicating the inclusion of reworked sediment and/or increased marine conditions. Pollen was absent in the overlying intercalated peat (unit 4) At 13.60 m OD the benthic marine-brackish diatoms Diploneis interrupta and Nitzschia navicularis are present. This implies the onset of marine conditions, most probably saltmarsh vegetation, given the overlying presence of incipient peat layers and its geographical position closer to the shoreline than most of the other boreholes. Throughout the peat, monocotyledon stem and root fragments were noted, including some Phragmites australis culm bases and root fragments. LPAZ VC15-2 shows a reduction in Corylus avellana-type (58%), coupled with decreases in Pinus (from 46%), Dryopteris-type, Pediastrum and pre-Quaternary palynomorphs, and an increase in Quercus (11%) and Ulmus (2%). Marginal aquatic plants increase, including Cyperaceae (10%) and occurrences of Typha latifolia-type, T. angustifolia-type and Menyanthes trifoliata (bogbean). The pollen assemblage in LPAZ VC15-1 is dominated by Corylus avellana-type (up to 69%), indicating it was the most important constituent of woodland growing on, or adjacent to, the site at the time of sediment accretion, along with Pinus (33%). Also present were Ulmus (2%) and Quercus (generally low but with a single peak of 20%), along with a low amount of Poaceae (up to 5%), Cyperaceae (14%) and occasional Typha angustifolia-type. High amounts of Dryopteris-type (56%) and Polypodium vulgare (24%) indicate a high abundance of ferns, while the high numbers of Pediastrum algal cysts (33%) imply a freshwater environment, such as a slow flowing river channel, though the constant presence of pre-Quaternary palynomorphs (16%) implies the incorporation of some reworked sediment sources. The pollen assemblage LPAZ VC25-2 shows a notable change in the pollen flora with increases in Quercus (up to 20%), Ulmus (8%), Corylus avellana-type (35%) and Poaceae (up to 58%), while Pinus, spores and pre-Quaternary palynomorphs now only represent a minor constituent of the pollen assemblage. Increases in Cyperaceae (sedges; 8%) and Typha angustifolia-type (lesser bulrush; up to 7%) attest to local marsh/fen conditions. Phragmites australis stem and culm nodes, and root fragments, were present throughout the peat along with marsh fen taxa including Lycopus europaeus (gypsywort) and Ranunculus subg Batrachium (crowfoot), although this subgenus includes species with both fresh and brackish water preferences. Similarly, Chenopodium rubrum/glaucum (goosefoot) can tolerate both fresh and brackish ground conditions. There are also charred seeds of Cladium mariscus (great fen-sedge) and Sparganium erectum (branched bur-reed), along with charred Poaceae stems. LPAZ VC34-1 dominated by Corylus avellana-type (up to 50%) and Poaceae (up to 62%), with Ulmus (5%), Quercus (21%) and Typha angustifolia-type present throughout. Chenopodiaceae (2e3%) increased slightly towards the top of the peat. Seeds present included Mentha, Lycopus europaeus, Ranunculus sceleratus and R. subg Batrachium, along with Poaceae and Cyperaceae stem fragments, some of which were charred, and frequent small indeterminate wood charcoal. A scarce presence of calcareous Foraminifera (Ammonia spp) was detected within the peat and is likely to reflect inwash or disturbance of the peat as these Foraminifera would usually be dissolved in acidic, organic sediments.

VC15

VC25

VC34

GrA-53517 (13.73 mODN to GrA-53518 (13.73 mODN to Beta-288890 (13.73 mODN) GrA-53521 (13.75 mODN to GrA-53523 (13.75 mODN to Beta-288891 (13.75 mODN)

Beta-288892 (13.62 mODN)

Level within peat (unit 6); LPAZ VC34-1 (13.65 to 13.58 m ODN)

S. Griffiths et al. / Journal of Archaeological Science 54 (2015) 237e253

Core

VC24

Beta-288889 (13.46 mODN) Beta-289591 (13.46 mODN)

Level within peat (unit 6); LPAZ VC24-2, 13.46 to 13.38 m ODN).)

The onset of peat formation contains a sharp change in the pollen assemblage (LPAZ VC24-). Corylus avellana-type (up to 50%) and Poaceae (up to 47%) dominate the assemblage alongside Ulmus (up to 9%) and Quercus (up to 20%). The presence of Potamogeton-type (pondweed), Typha latifolia-type (bulrush) and T. angustifolia-type indicate local freshwater habitats, an interpretation supported by the presence of seeds of Lycopus europaeus, Mentha (mint), Ranunculus lingua (greater spearwort), R. sceleratus (celery-leaved buttercup) and R. subg Batrachium. Between 13.50 and 13.48 m OD remains of Phragmites australis stem/root fragments, some of which were charred, and charred Cyperaceae stems and nutlets, were present. This assemblage indicates that the peat accumulated within predominantly freshwater grassesedge ereedswamp communities. A partial, poorly preserved, but moderately diverse diatom assemblage at the very top of the peat (unit 6), at 13.44 m OD, is dominated by mesohalobous attached and benthic species, such as Achnanthes brevipes, Diploneis didyma and Nitzschia navicularis, along with some polyhalobous diatoms such as Paralia sulcata. At the same level, a Foraminifera assemblage contains agglutinated taxa (eg Jadammina macrescens) commonly associated with high intertidal settings and is dominated by calcareous taxa (Ammonia sp and Haynesina germanica) typical of brackish estuarine settings.

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In the analysis of peat sequences that have been subject to marine inundation (henceforth referred to as marine peats), considerable attention has been paid to the physical relationships of peat deposits to processes of palaeoenvironmental change, specifically sea-level fluctuation (e.g. Waller et al., 2006; Baeteman et al., 2011; Wolters et al., 2010; Waller and Grant, 2012). These approaches have considered factors including the compaction of peat deposits by overlying clastic sediments, erosion of peat surfaces by marine transgression or high-energy events, spatial variability in peat formation, and complex local environmental conditions. A number of studies have identified apparent internal inconsistencies in sequences of dates on samples from marine peats (e.g. Hamilton and Shennan, 2005; Hamilton, 2003; Combellick, 1994; Kidson, 1982; Shennan, 1986: 164; Shennan and Horton, 2002; Haggart, 1988). Shennan et al. (1998: 385) have recognised the difficulties of dating ‘bulk’ peat samples using AMS, and difficulties in interpreting radiocarbon data in terms of regional submergence events (Shennan et al., 1996: 1052). These studies suggest that in the analysis of radiocarbon dates from marine peats, similar consideration should be given as to sample selection as in terrestrial wetland sequences (e.g. Brock et al., 2011). As well as general issues concerning the production of robust chronologies on peat, there may be a number of issues which are specific to the analysis of marine peats. These peats are formed in freshwater/brackish conditions, within low-lying coastal areas, in advance of a marine inundation and subsequent submergence. Peat formation (paludification) is driven by the degree of waterlogging which results from the local elevation of the groundwater table and, particularly for intercalated peats, changes in the rate of sea level rise, sediment supply and accommodation space (Baeteman et al., 2011). There is potential therefore for changes in hydrology to result in mobility of humic and ‘fluvic acid’ fractions up and down the profile with the water table (Shore et al., 1995). In addition, any reservoir offsets in the ground water d as a result of underlying geology or marine reservoirs d could influence the age of the mobile acid carbon fractions extracted from marine peats (Shore et al., 1995). Before attempting any complex modelling of radiocarbon results from marine palaeoenvironmental sequences (Bronk Ramsey, 2008; Parnell et al., 2011), critical considerations therefore need to be made as to the nature of the dated samples (Brock et al., 2011), and their associations with the palaeoenvironmental ‘events’ of interest (Gearey et al., 2009). It is possible that at any particular depth potential radiocarbon samples (both different chemical carbon fractions and macrofossils) are of considerably different radiocarbon ages, resulting from processes such as paludification, post-depositional conditions or taphonomic pathways. The underlying assumption employed in many analyses that all samples from a depth are ‘the same’, or that ‘a depth’ has ‘a single radiocarbon age’ is often demonstrably wrong (e.g. Bayliss et al., 2008). If a sample depth contains plant macrofossils and chemical fractions of different radiocarbon ages, this needs to be critically considered as to how measurements on such samples can be related with palaeoenvironmental proxies, such as pollen and diatoms, which might have results from very distinct taphonomic pathways. The purpose of this paper is the highlight the importance of critical approaches to the construction of palaeoenvironmental chronologies for archaeological analysis. Multiple types of radiocarbon samples may be required to produce chronologies of individual sequences. An individual sequence might not be representative of the temporal development of ‘a site’, even over a relatively small area. In complex burial environments, different sequences might reflect very different processes, including variability with the palaeoenvironment and post-depositional processes. When attempting to provide palaeoenvironmental context

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for archaeological sites it is therefore essential that due attention is paid to the complexities of developing robust chronological models. Without such attention, archaeologists risk producing simplistic models of palaeoenvironments that do not attend to local variability, or conflating variability in data from different sites (or sample sequences) which result from post-depositional changes. Such approaches could result in spurious causal narratives describing humaneenvironment interaction, and has significant implications for our understandings of human responses to palaeoenvironmental processes that become reductively glossed as ‘events’. 2. Material and methods Hinkley Point lies on the Somerset coast, on the western edge of Bridgwater Bay, within the Inner Bristol Channel (Fig. 1). The underlying geology in the immediate vicinity is composed of a sequence of limestones, shales and mudstones. The Quaternary geology of Bridgwater Bay is strongly influenced by cyclical changes in sea levels, resulting from eustatic and isostatic processes. Archaeologial research in the area has identified the importance of the inter-tidal zone, especially for earlier prehistory (Webster, 2000; Brown, 2005; Timpany, 2005; Hosfield et al., 2007a, 2007b; Mullin, et al., 2009; Jones, et al., 2005). Changes in sea level are particularly important in providing the environmental context to Mesolithic presence in the the Bristol Channel and Severn Estuary (Long et al., 2002; Philips and Crisp, 2010), both in the contemporary inter-tidal zone and in now submerged areas. A total of 24 boreholes and 63 vibrocores were recovered from the study area by Fugro Alluvial Offshore Limited (FAOL) during late 2009 and early 2010 (Fig. 1). The sequence revealed from the core analysis was broadly consistent across the study area, with nine key

stratigraphic units recognised (Fig. 2; Table 1; Sturt et al., 2014). These included a thin compacted basal peat (unit 6), which was clearly identified in VC06, VC15, VC24, VC25 and VC34, while in VCJ15 a basal organic rich silt was recorded. Within VC06, the unit 6 peat appeared to have an intact regressive contact, grading into the overlying unit 5 estuarine and marine silts. Although changes in the pollen assemblage in this core, between LPAZ VC06-1/2 (Table 2), appeared abrupt, the similar transition recorded in the Foraminifera, across the upper peat e silt boundary, implied that an intact transgressive contact is present (see Waller et al., 2006) with the upper peat capable of providing a meaningful date for the onset of brackish conditions at this locality during the Early Holocene (see Table 2). Sediment d13C and C/N ratios (see Sturt et al., 2014) confirmed a progressive change between these two units implying an unbroken sedimentary transition from freshwater towards marine conditions. All other unit 5e6 contacts were found to contain erosive contacts, indicated by abrupt stratigraphic boundaries and sharp changes in isotope geochemistry. Twenty-six radiocarbon dates have been obtained from the six selected offshore vibrocores (see Sturt et al., 2014 for full details; Fig. 3). Radiocarbon samples were primarily selected to provide a chronology for the onset of formation of an early Holocene peat and the associated palaeoenvironmental landscape changes, which are indicated from a range of palaeoenvironmental proxy data (including pollen, plant macrofossils and diatoms; see Sturt et al., 2014). The end of peat formation in these cores provides termini post quos or dates after fully marine conditions were established. The results also allow a consideration of factors which may affect the ages of potential radiocarbon samples from marine sites. Twelve results were produced as part of a first round of dating, on samples identified as ‘Phragmites australis leaves’, non-rooty ‘plant macrofossils indet.’ and ‘wood indet’. (Dix et al., 2011,

VC06 GrA-53519 GrA-53520 Beta-288884 Beta-288885 GrA-53525 GrA-53526 VC15 GrA-53746 GrA-53625 GrA-53628 -13.80 to -13.81 GrA-53747 GrA-53627 Weighted mean

VC24 Beta-288889 Beta-289591 VC25 -13.73 to -13.74 GrA-53517 GrA-53518 Beta-288890 Weighted mean

-13.75 to -13.76 GrA-53521 GrA-53523 Beta-288891 Humin and macrofossil weighted mean

VC34 Beta-288892 VCJ15 Beta-288886 -11.32 Beta-288887 Beta-289589 -11.34 Beta-289590 Beta-288888 9600

9400

9200

9000

8800

8600

Calibrated radiocarbon date (cal BP) Fig. 3. Probability distributions of calibrated radiocarbon results from horizons with multiple measurements on humin and humic carbon fractions and plant macrofossils from Hinkley Point (Stuiver and Reimer, 1993; Reimer et al., 2013; Bronk Ramsey, 2009; excluding much later result GrA-53702, which is detailed in Table 1). Each distribution represents the relative probability that the measured radiocarbon age occurred at a particular point in time.

Table 3 Radiocarbon samples from Hinkley Point. Results are consistently colour coded in Figs. 3e8 (see Fig. 3 for key). All depths are given in metres Ordnance Datum, Newlyn (mODN). Core

Sample depth (mODN)

VC06 12.58 to 12.59 12.58

12.6

Lab number Dated material

Top basal peat (unit 6) LPAZ VC06-1 … Transition to LPAZ VC06-2 associated with a significant change from peat to silts (unit 5). Basal peat unit (unit 6) at 12.65 to 12.60, LPAZ VC06-1.

GrA-53519

8040 ± 45

GrA-53520 Humic carbon fraction Beta-288884 Wood fragment

28.02 26.80

7905 ± 40 8140 ± 50

Top of peat horizon Dates end of peat Statistically significantly ?dates end peat (upper surface at 12.58) formation different formation 0 (T ¼ 5.0; T 5% ¼ 3.8; n ¼ 1) ?mobile humic acid ? n/a TPQ unknown ‘event’

Beta-288885 Plant macrofossil

28.10

7980 ± 40

Basal peat unit (unit 6) GrA-53525 at 12.65 to 12.60, LPAZ VC06-1. GrA-53526 Incatercalated peat GrA-53702 (unit 4); LPAZ VC15-3

Humin carbon fraction 28.51 Humic carbon fraction Waterlogged Poaceae stem/root

28.00 25.50

Level within peat horizon (upper surface at 12.58, bottom surface at 12.65) 8110 ± 45 Bottom of peat horizon (bottom surface at 12.65) 8605 ± 40 3505 ± 175 Level in upper peat

8150 ± 45 7980 ± 40

Four charred buds and 29.97 three charred bark fragments of Populus spp. Humic carbon fraction 27.39 Humin carbon fraction 27.61

8115 ± 45

8395 ± 45 8525 ± 50

Bottom of lower peat horizon (bottom surface at 13.81)

26.30

7900 ± 50

Level within peat (upper surface at 13.44, lower surface at 13.50)

Beta-289591 Organic sediment 27.50 GrA-53517 Humic carbon fraction 28.63 GrA-53518 Humin carbon fraction 29.18

8130 ± 40 7940 ± 40 8025 ± 40

Beta-288890 Phragmites leaves

27.60

7900 ± 40

GrA-53521

27.81

8065 ± 40

GrA-53523 Humin carbon fraction 26.99 Beta-288891 Phragmites leaves 26.60

7940 ± 40 7920 ± 40

27.90

7950 ± 40

Humic carbon fraction

Beta-288892 Phragmites leaves

Dates peat formation

n/a

?plant macrofossil associated with peat formation

Dates start of peat Statistically significantly formation different (T ¼ 67.0; T0 5% ¼ 3.8; n ¼ 1) Dates peat n/a formation

?dates start peat formation ?mobile humic acid ?plant macrofossil associated with upper peat formation Top of lower peat horizon Dates end of peat Statistically significantly ?mobile humic acid (upper surface at 13.74) formation different ?dates end peat (T ¼ 8.0; T0 5% ¼ 3.8; n ¼ 1) formation Burning horizon within Dates burning n/a Dates burning event lower peat horizon event with in peat horizon

Humic carbon fraction 27.78 Humin carbon fraction 28.29

Bottom basal peat (unit GrA-53747 6) LPAZ VC15-1 GrA-53627 (13.88 to 13.77 m ODN) Beta-288889 Plant macrofossil

VC34 13.62

Interpretation of radiocarbon result

Humin carbon fraction 28.89

13.80 to 13.81

13.75 to 13.76 13.75

Weighted mean

description.

Top basal peat (unit 6) GrA-53746 LPAZ VC15-2 (13.77 GrA-53625 to 13.70 m ODN) Basal peat (unit 6) LPAZ GrA-53628 VC15-2 (13.77 to 13.70 m ODN)

VC25 13.73 to 13.74 13.73

Dated event

age

13.74 to 13.75 13.76 to 13.78

VC24 13.46

d13C (‰) Radiocarbon Parent deposit

Top of peat horizon (upper surface at 13.71)

Bottom of peat horizon (lower surface at 13.77)

Level within peat (upper surface at 13.56, lower surface at 13.65)

Dates start of peat Humin and humic acid formation weighted mean 8454 ± 34 (T’ ¼ 3.7; T0 5% ¼ 3.8; n ¼ 1) Dates peat Statistically significantly formation different (T ¼ 12.8; T0 5% ¼ 3.8; n ¼ 1) ? Dates end of peat Humin, humic acid, and formation plant macrofossil weighted mean 7956 ± 24 Dates reed (T’ ¼ 5.1; T0 5% ¼ 6.0; n ¼ 2) swamp conditions at end of peat formation Dates start of peat formation Humin and plant macrofossil Dates reed weighted mean swamp 7930 ± 29 conditions at (T’ ¼ 0.1; T0 5% ¼ 3.8; n ¼ 1) start of peat formation n/a Dates reed swamp conditions

Calibrated date BP (95% confidence) 9030e8770 8980e8590 9270e8990

9010e8640

9140e8990 9630e9520 4290e3380

9270e9000 9010e8640 9140e8990

Dates start of peat formation

9530e9430

?plant macrofossil associated with upper peat formation ? Dates end of peat formation

8990e8580

9240e8990 8990e8640

?mobile humic acid

9080e8790

?dates end of peat formation

8990e8630

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12.64 to 12.65 VC15 13.56 to 13.58

Palaeo-environmental summary

?dates presence of reed 9000e8630 swamp associated with peat formation 243

(continued on next page)

9470e9260 8640e8420

TPQ unknown ‘event’ ? ?associated with deposit formation ? TPQ unknown ‘event’ 8340 ± 40 7760 ± 50 27.00 31.00 Beta-289590 Organic sediment Beta-288888 Wood fragment

Level within organic silt

? ? ? 7820 ± 40 8540 ± 50 8220 ± 40 27.90 27.20 27.60 Beta-288886 Wood fragment Beta-288887 Organic sediment Beta-289589 Plant macrofossil

Level within organic silt Level within organic silt

? ?

n/a Statistically significantly different (T ¼ 25.2; T0 5% ¼ 3.8; n ¼ 1) Statistically significantly different (T ¼ 80.6; T0 5% ¼ 3.8; n ¼ 1)

Interpretation of radiocarbon result Weighted mean Dated event description. age

Table 3; ‘Beta-’ laboratory code). These samples were prepared and measured by Beta Analytic. Of these samples, ‘plant macrofossils’ may represent short-lived material, as they were differentiated from ‘wood’ fragments. As part of a second phase of analysis, fourteen measurements (Table 3; ‘GrA-’ laboratory code) were produced on material from vibrocores VC06, VC15 and VC24. Short-lived plant macrofossils with atmospheric photosynthetic pathways (Bayliss et al., 2008: xi) were preferentially selected for dating. Where not available, measurements on the humin and humic acid fractions from 1 cm thick subsamples of peat were submitted. While P. australis have intrusive, deep roots, and root penetration has been suggested as the cause of vertical mixing through geoarchaeological profiles (e.g. Howard et al., 2009: 2687; Brock et al., 2011), P. australis leaves should provide accurate estimates for the presence of reed swamp. P. australis has some ability to photosynthesis underwater during episodic flooding events (a gas leaf film facilitates some exchange), but the level of photosynthesis is considerably lower (down to 9%) compared to atmospheric photosynthesis (Colmer et al., 2011). Under normal circumstances underwater photosynthesis would contribute little to the annual total carbon fixation in leaf matter (Ole Pedersen, pers. comm. 2013) and, in cases of permanent inundation, P. australis will die relatively quickly and therefore photosynthesis will cease (Ole Pedersen, pers. comm. 2013). Results on the P. australis leaves therefore provide robust estimates for the age of transitional plant communities, very probably dating conditions associated with a specific phase of peat formation (see the Discussion section below). Measurements from these sample depths were made for loss on ignition and carbonate content (Table 4) in order to explore the potential for significant hard water offsets or the introduction of other carbonaceous material, including inorganic in-washed material (cf. Howard et al., 2009). Plant macrofossils and peat samples for the second round of radiocarbon dating were stored in sterilised glassware, and refrigerated prior to submission to the radiocarbon laboratory at the Centre for Isotope Research, University of Groningen. Plant macrofossil fragments were pre-treated using the acidebaseeacid method (Mook and Waterbolk, 1985). For the measurements on peat, samples were inspected and manually cleaned of rootlets prior to homogenisation and chemical pretreatment, and measurements made on the humic acid fraction (the acid-insoluble/ alkali-soluble fraction), and the humin fraction (the alkali-/acidinsoluble fraction). Samples were combusted and graphatised following Aerts-Bijma et al. (1997, 2001), and measured by AMS (van der Plicht et al., 2000). Results have been calibrated using the program OxCal v4.2.3 (Bronk Ramsey, 1995, 1998, 2001; 2009) and the IntCal13 terrestrial curve (Reimer et al., 2013). The measurements (Table 2) are conventional radiocarbon ages (Stuiver and Polach, 1977). Calibrated date ranges quoted in Table 2 have been calculated using the maximum intercept method (Stuiver and Reimer, 1986). They are

11.34

Table 4 Analysis of % total organic carbon and calcite.

VCJ15 11.28 11.32

Sample depth (mODN) Core

Table 3 (continued )

Palaeo-environmental summary

Lab number Dated material

d13C (‰) Radiocarbon Parent deposit

8700e8540 9560e9470 9310e9020

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Calibrated date BP (95% confidence)

244

Vibrocore

Depth (mODN)

% Total Carbon

% Acidified

% Total Organic Carbon

% Calcite

%Nitrogen

VC15

13.74 13.80 12.58 12.58 12.64 13.73 13.75

5.048 2.813 21.02 21.73 3.04 16.161 12.911

4.582 2.404 31.789 31.32 2.643 20.479 13.238

4.294 2.302 14.486 15.770 2.531 10.044 9.728

6.278 4.260 54.432 49.650 4.241 50.955 26.516

0.42 0.202 1.224 1.33 0.179 0.932 1.066

VC06

VC25

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245

Fig. 4. Probability distributions of radiocarbon dates from VC15, Hinkley Point three models are shown. In Fig. 4a, the preferred model (calculated using the P_Sequence function and using a lower interpolation rate) is shown (with both the 95% probability and 68% probabilities). In Fig. 4b a model of uniform peat formation between the Boundary parameters is shown (with the 95% probabilities). In Fig. 4c a model calculated using the P_Sequence function and a higher interpolation rate is shown (with the 95% probabilities). Full details of the modelling are discussed in the text. For each radiocarbon result two distributions have been plotted; the graph in outline is the radiocarbon calibration, the solid graph is the output from the chronological model. In order to show GrA-53746 on the figure the result has been plotted at 13.755 mODN, slightly lower than the true height from which this sub-sample was extracted (at 13.74 to 13.75 mODN). Graphs in blue are results on humin carbon fractions, graphs in turquoise are results on humic acid fractions from the same peat deposit. Graphs in green are results produced on demonstrably shortlived plant macrofossils from the same sample depths as the duplicate humin and humic acid carbon fractions. Graphs in black are the calibrated weighted mean of these fractions if they are statistically consistent. Red graphs were produced on ‘wood’ and therefore potentially subject to an ‘old wood’ offset (Bowman, 1990), while yellow distributions represent ‘organic sediment’, and graphs in purple ‘plant macrofossils’ that could represent short-lived material. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

quoted in the form recommended by Mook (1986), rounded outwards to 10 years. All other ranges are derived from the probability method (Stuiver and Reimer, 1993). Bayesian age-depth modelling of the results from the three analysis cores d VC15 (Fig. 4), VC06 (Fig. 5) and VC25 (Fig. 6) d was undertaken using OxCal v4.2.3 (Bronk Ramsey, 2008; cf. Parnell et al., 2011). The posterior density estimates outputs from the Bayesian models described above are by convention quoted in italics. The models are defined by the OxCal v4.2.3 command query

keywords and the brackets shown in the figures. OxCal commands are given in bold. Several modelling approaches were applied to the cores to explore their impact on output (Table 5). Our preferred models for these sequences apply the P_Sequence age-depth function, reflecting the potentially random or Poisson nature of peat formation (Bronk Ramsey, 2008), with the prior for the deposition rate defined as log_10(k/k_0) where k_0 ¼ 1. This allows k to take any value from 0.01 to 100, interpolation was set to every 10 cm. The generic code for these models takes the form:

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This modelling approach reflects the relatively limited depth of the peat units sampled in the cores modelled here (the deepest is 8 cm), and that dated samples are located in all cases very close to or at the upper and lower peat horizon interfaces. Other modelling approaches, including applying a higher interpolation rate (every 1 cm), and a uniform deposition rate between the Boundary parameters for each sequence were undertaken as part of the analysis. The outputs of these approaches are shown in Figs. 4e6 and the posterior density estimates shown in Table 5. We prefer the models with a Poisson peat formation and lower interpolation, as the number of dated events in each of these sequences is relatively few, and the total depth of peat limited. In practice, because of these factors the various modelling approaches presented here

Fig. 5. Probability distributions of radiocarbon dates from VC06, Hinkley Point; three models are shown. In Fig. 5a the preferred model (calculated using the P_Sequence function and using a lower interpolation rate) is shown (with both the 95% probability and 68% probabilities). In Fig. 5b a model of uniform peat formation between the Boundary parameters is shown (with the 95% probabilities). In Fig. 5c a model calculated using the P_Sequence function and a higher interpolation rate is shown (with the 95% probabilities). Full details of the modelling are discussed in the text. For purposes of presentation GrA-53526 is plotted at 12.65 mODN rather than the true sample height of 12.64 to 12.65 mODN. GrA-53520 is plotted at 12.575 mODN and Beta-28884 at 12.595 mODN rather than the true sample height of 12.58 to 12.59 mODN (see Fig. 4).

S. Griffiths et al. / Journal of Archaeological Science 54 (2015) 237e253

have limited impact on the outputs (see Table 5). As shall be seen in the Results and Discussion sections below, in the sequences presented here, the critical factors for producing robust palaeoenvironmental reconstructions and archaeological interpretations of palaeoenvironmental change, are the recognition that different chemical fractions or plant macrofossils from the same horizon may have different radiocarbon ages. The nature of samples submitted for palaeoenvironmental reconstruction is of fundamental importance in archaeological interpretations, especially in environments which might result in complex taphonomic or diagenetic processes.

247

3. Results Multiple results from the same horizons are shown in Fig. 7. Weighted means have been taken prior to calibration using the received protocol for radiocarbon dates (Ward and Wilson, 1978) and the OxCal function R_Combine (e.g. Bronk Ramsey, 2009) where results from a depth are statistically consistent. These are shown in black. Duplicate measurements on humin and humic carbon fractions from the bottom 1 cm of peat from VC15 (Table 2 and Fig. 7) are statically consistent, and could be of the same actual age (T0 ¼ 3.7;

Fig. 6. Probability distributions of radiocarbon dates from VC25, Hinkley Point; three models are shown (with both the 95% probability and 68% probabilities). In Fig. 6a the preferred model (calculated using the P_Sequence function and using a lower interpolation rate) is shown (with the 95% probabilities). In Fig. 6b a model of uniform peat formation between the Boundary parameters is shown (with the 95% probabilities). In Fig. 6c a model calculated using the P_Sequence function and a higher interpolation rate is shown. Full details of the modelling are discussed in the text. For purposes of presentation GrA-53521 is plotted at 13.76 mODN rather than the true sample height of 12.75 to 12.76 mODN (see Fig. 4).

248

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Table 5 Posterior density estimates from the models incorporating results on humin fractions, and statistically consistent Phragmites australis. leaves, humin and humic acid fractions (Figs. 4e6) or from the analysis model as a whole (Fig. 8). The results from the preferred model are highlighted in the column in grey. Core

Parameter name, parameter description and figure in which calculated

Posterior density estimate from the preferred models (Figs. 4e6; % probability) cal BP

Posterior density estimate from the uniform deposition rate models (Figs. 4e6; % probability) cal BP

Posterior density estimate from the higher interpolation rate models (Figs. 4e6; % probability) cal BP

n/a

Start Hinkley Point Lower Peat Dates start of peat formation (Fig. 8) VC06StartPeatFormation Dates start of peat formation in VC06 (Fig. 5) GrA-53519 Dates upper peat formation (Fig. 5)

10100e9390 (95%) 9810e9530 (68%) 9360e8990 (95%) 9170e9030 (68%) 9010e8760 (95%) 8990e8860 (46%) or 8840e8780 (23%) 9010e8810 (95%) 9000e8860 (68%) 9150e8970 (93%) 9090e8990 (68%) 8980e8600 (95%) 8900e8680 (68%) 9830e9450 (95%) 9710e9550 (68%) 9000e8690 (95%) 8930e8750 (68%) 9210e8990 (95%) 9130e9050 (61%) 9530e9430 (95%) 9520e9450 (68%)

n/a

n/a

VC06

VC15

VC25

VC34

VC24

n/a

Beta-288885 Dates plant macrofossil at depth in core (Fig. 5) GrA-53525 Dates lower peat formation (Fig. 5) VC06EndPeatFormation Dates end of peat formation in VC06 (Fig. 5) VC15Start lower peat formation Dates start of peat formation in VC15 (Fig. 4) GrA-53625 Dates upper peat formation (Fig. 4) GrA-53628 Dates burning event at depth in model (Fig. 4) bottom lower peat VC15 Weighted mean of GrA-53627 and GrA-53747 Dates bottom peat formation in VC15 (Fig. 4) VC15End lower peat formation Dates end of peat formation in VC15 in preferred model (Fig. 4) VC25StartPeatFormation Dates start of peat formation in VC25 (Fig. 6) Top of peat horizon Weighted mean of GrA-53517, GrA-53518 and Beta-288890 Dates top peat formation (Fig. 6) Bottom peat (humin and macro) Weighted mean of GrA-53523 and Beta-288891 Dates bottom peat formation at this depth (Fig. 6) VC25EndPeatFormation Dates end of peat formation in VC25 (Fig. 6) Beta-288892 Dates presence of transitional plant communities (Phragmites australis leaves) and peat formation (Fig. 8) Beta-288889 Dates presence of transitional plant communities (Phragmites australis leaves) and peat formation (Fig. 8) End Hinkley Point Lower Peat Dates end of peat formation (Fig. 8)

T0 5% ¼ 3.8; n ¼ 1; Ward and Wilson, 1978). We address in detail the associations of different carbon fractions and plant macrofossils in the Discussion section below. A weighted mean taken prior to calibration indicates peat initiation begun here in 9530e9430 cal BP (95% confidence; 8454 ± 34 BP; R_Combine “bottom lower peat VC15”). The humin and humic acid fractions from the upper 1 cm sample from the lower peat deposit in VC115 are statistically significantly different at 95% (T0 ¼ 8.0; T0 5% ¼ 3.8; n ¼ 1), with the humic acid measurement (GrA-53746) producing an earlier range. The measurement on charred Populus sp. buds and bark from VC15, which directly dates a burning event, calibrates to 9140e8990 cal BP (95% confidence; GrA-53628; Table 2). From the bottom of the peat deposit in vibrocore VC25, the humic acid result (GrA-53521; Fig. 7) is not statistically consistent with either the result produced on Phragmites australis leaves (Beta-288891) or the result produced on the humin fraction (GrA53523). The humin fraction and P. australis measurement are

9370e9000 9180e9030 9000e8720 8930e8770

(95%) (68%) (95%) (68%)

9350e8950 9150e9000 9050e8750 9000e8750

(95%) (68%) (95%) (68%)

9010e8800 8970e8850 9260e8970 9120e9000 8990e8690 8910e8740 9750e9580 9720e9620 8990e8630 8830e8650 9190e8990 9120e9000 9530e9430 9520e9460

(95%) (95%) (95%) (68%) (95%) (68%) (95%) (68%) (95%) (68%) (95%) (68%) (95%) (68%)

9050e8800 9000e8850 9250e8950 9100e8950 9000e8700 9000e8750 9830e9450 9710e9560 9000e8660 8930e8730 9210e8990 9130e9010 9530e9430 9520e9450

(95%) (68%) (95%) (68%) (95%) (68%) (95%) (68%) (95%) (68%) (95%) (68%) (95%) (68%)

8980e8600 (95%) 8900e8680 (68%)

8950e8560 (95%) 8780e8580 (68%)

8980e8590 (95%) 8890e8670 (68%)

9340e8670 (95%) 9070e8740 (68%) 8940e8640 (95%) 8870e8820 (14%) or 8800e8650 (55%)

9290e8730 9210e8870 8930e8630 8850e8640

9470e8670 9070e8740 8940e8640 8870e8650

8990e8700 (95%) 8980e8830 (58%) or 8780e8740 (10%) 8970e8300 (95%) 8860e8550 (68%) 8990e8650 (95%) 8980e8820 (45%) or 8800e8720 (23%) 8980e8600 (95%) 8960e8920 (8%) or 8870e8830 (8%) or 8780e8630 (52%) 8830e8150 (95%) 8740e8470 (68%)

8990e8700 (95%) 8990e8830 (68%)

8990e8690 (95%) 8980e8740 (68%)

8920e8340 (95%) 8810e8470 (68%) n/a

8960e8130 (95%) 8890e8570 (68%) n/a

n/a

n/a

n/a

n/a

(95%) (67%) (95%) (68%)

(95%) (68%) (95%) (68%)

statistically consistent, and a weighted mean taken prior to calibration indicates reed swamp and peat inception in VC25 in 8990e8630 cal BP (95% confidence; 7930 ± 29 BP; T0 ¼ 0.1; T0 5% ¼ 3.8; n ¼ 1; Ward and Wilson, 1978; R_Combine “bottom peat VC25 (humin and macro)”). From the top 1 cm of the peat in VC25 (Fig. 7), measurements on Phragmites australis leaves and humin and humic acid fractions are statistically consistent (T0 ¼ 5.1; T0 5% ¼ 6.0; n ¼ 2; Ward and Wilson, 1978), indicating that peat formation ceased in 8990e8640 cal BP (95% confidence; 7956 ± 24 BP; R_Combine “top peat VC25”), and was followed by a transition towards the onset of fully marine conditions. From vibrocore VC06, humic and humic acid measurements from the bottom 1 cm of the peat deposit are statistically significantly different (T0 ¼ 67.0; T0 5% ¼ 3.8; n ¼ 1; Ward and Wilson, 1978), with the humic acid fraction (GrA-53526) again earlier than the humin fraction (GrA-53525; Fig. 7). From the upper most

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249

Fig. 7. Measurements on different chemical fractions and plant macrofossils from the same depths within different cores. Graphs in black are the calibrated weighted mean of these results if they are statistically consistent (Ward and Wilson, 1978).

1 cm subsample from the peat, all three radiocarbon dates are statistically inconsistent with each other result. The result on the unidentified wood fragment (Beta-288884), dated in the first round, is excluded as it may include an inbuilt old-wood offset (Bowman, 1990) and the nature of the association of this macrofossil with deposit formation at this depth is unclear. In this case, the humic acid fraction is younger than the humin fraction (the only instance of this from this analysis). From the first round of radiocarbon work, a possible short-life ‘plant macrofossil’ from higher up in the sequence at 12.6 mODN (Ordnance Datum Newlyn), might be associated with later peat formation at the sample site, produced a younger date range of 9010e8640 cal BP (95% confidence; Beta-288885; Table 2). 4. Discussion The outputs from the models (Figs. 4e8) are summarised in Table 5. As noted above, our preferred models (shown in Figs. 4a, 5a and 6a), apply the P_Sequence age-depth function with a low interpolation calculation, reflecting the dated sample frequency and the depth of peat in each core. These models have exclude results on humic acid fractions where they are inconsistent with other measurements from the same horizons; we discuss below why we believe these treatments represent the best available interpretation of these data. The close proximity of the dated samples has been used to estimate separate parameters for the date of peat onset and cessation (cf. Parnell et al., 2008: 2951). While alteration in the vertical height of these estimates, as a result of compaction, must be considered if these data are employed as regional sea-level index points (SLIPs; cf. Kuchar et al., 2012; Edwards, 2006), the data themselves should provide robust estimates for the timing of the start and end of peat formation in the individual cores (see discussion below). This model estimates that the total duration of the results which might be associated with the phase of lower peat formation occurred over 650e1130 years (95%

probable, or 780e1020 years 68% probable; “Duration Hinkley lower peat”; Fig. 8). The palaeoenvironmental proxies from each core emphasise the potential for highly localised histories sampled in individual cores (Sturt et al., 2014). The results suggest that a complex wetland environmental resource would have been available for Mesolithic populations in this part of Bridgwater Bay, and that in this context changes in hydrology were part of ongoing processes of sea-level change, rather than a cataclysmic event-like inundation. The differences between inconsistent duplicate humin and humic acid measurements are shown in Fig. 9. These data emphasise that excluding either humin or humic measurements when duplicate data are not statistically consistent, may represent a significant simplification of peat formation processes (Blaauw and Christen, 2005); such differences may be indicative of site-specific paludification processes or post-depositional preservation conditions. In this case, the Phragmites australis leaves from the Hinkley Point sequences estimate the presence of transitional reed swamp wetlands and the formation of in situ peat deposits prior to marine transgression (Waller et al., 2006; Wolters et al., 2010). Fragile P. australis leaves are unlikely to survive significant relocation €rnqvist et al., 1992). When a non-erosive contact is found results (To on P. australis provide robust termini post quos for the ensuing marine conditions (Waller et al., 2006). Given the association of the results on P. australis leaves, these measurements and the statistically consistent measurements on the humin chemical fraction, provide the most robust estimates for the peat horizons from which samples were recovered (Fig. 3). In the preferred age-depth treatments of the data, the results on humic acid samples have therefore been excluded where they are inconsistent with humin fractions (Figs. 4e6). Even if the downward movement of plant macrofossils and humin had occurred, perhaps as a result of root penetration, an associated downward movement of humic acids might be expected, and such a process

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Boundary End d Hinkley Point Lower Peat R_Date Beta-289591? [P:100] R_Date Beta-288889 [A:95] Phase VC24 R_Date Beta-288892 [A:101] Phase VC34 Prior VC25EndPeatForm r ation [A:112] Prior VC25StartPeatForm r ation [A:101] Phase VC25 Prior VC06EndPeatForm r ation [A:101] Prior VC06StartPeatForm r ation n [A:100] 100] Phase VC06 Prior VC15End lower peat form r ation [A:103] 3 Prio Pri or VC15 VC15Start lower pea peat form forrmation [A:89]] Phase VC15 Phase Hinkley Point Lower Peat Bound Boun dary Star Start Hinkley Point Lower wer Peat Sequence [Amodel:100] 11000

10500

10000

9500

9000

8500

Posteri r or density estimate (cal BP) P Fig. 8. A model estimating the total duration the results suggest for lower peat formation from Hinkley Point. The model uses posterior density estimates calculated in Figs. 4e6.

still does not account for the generally older results on these fractions (Dresser, 1970; Bayliss et al., 2008). Shore et al. (1995: 375) suggested that one cause of older humic acid fractions, in contrast with humin fractions from the same depth, could be the result of upward movement of humic acid. Such a model could be consistent with an increasingly waterlogged environment, and increasingly brackish conditions associated with sea-level change and elevated ground water levels (cf. Waddington and Roulet, 1997), though this effect does not appear to be systematic. Old humic acid effects could also be exacerbated by low energy submerged conditions over a long duration (cf. Avrahamov et al., 2013). Alternative interpretations are possible; the percentage carbon data (Table 4), suggests that there may be some temporal variability in which fractions represent the most ‘appropriate’ age estimates for sample depths. For example, the data for the upper sample from VC06 (Table 4) suggests this horizon may be subject to increased presence of carbonates, which might suggest distinct preservation conditions at this depth. The data presented here emphasise that site-specific considerations are required in the sampling and analysis of submerged marine peats. Analyses of submarine peat deposits at least need to consider current best practice associated with dating terrestrial

wetland peat sequences (e.g. Brock et al., 2011; Marshall and Whitehouse, 2013). Given the evidence that plant macrofossils and chemical fractions from the same horizon may produce different radiocarbon ages, we emphasise that archaeologists investigating sea-level change may need to date multiple chemical fractions from peat. To produce robust interpretations of processes of change operating across landscapes d rather than extrapolating the conditions at single sample spots d analysis of multiple cores will be required. This is especially pertinent when potentially highly complex and highly localised processes, such as hydrological change associated with sea-level change, are investigated. 5. Conclusions Palaeoenvironmental reconstruction provides an essential context for human activity, especially in earlier prehistory, where sites are relatively rare, or in regions which have been subject to significant Holocene environmental change. Potential radiocarbon samples from individual horizons in a submerged marine peat may have different radiocarbon ages as a result of highly specific formation processes, and geochemical and hydrological preservation conditions. This has been little explored in the context of submerged marine peats, and more study of such cases is required.

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251

Difference VC25 humin_humic Difference VC25 weighted mean_ humic Phase VC25 bottom peat Difference VC06 top peat Phase VC06 top peat Difference VC06 bottom peat Phase VC06 bottom peat Difference VC15 Phase VC15 top lower peat

-800

-600

-400

-200

0

200

400

600

Interval (years) Fig. 9. Probability distributions of the differences between statistically inconsistent radiocarbon measurements on duplicate humin and humic acid carbon fractions (shown in Fig. 3) from the same peat subsample. The distributions have been calculated using the Difference function in OxCal v4.2.3. (Bronk Ramsey, 2009), taking the age of the humic fraction from duplicate the age of the humin fraction. In most cases this presents a positive distribution because the humic acid results are older than the humin carbon fractions.

When attempting to produce chronologies of marine peat formation, measurements on multiple radiocarbon dates d including multiple carbon fractions from the same horizon and/or sequences of short-life plant macrofossils with atmospheric photosynthesis pathways d should be undertaken. The available data from Hinkley Point could indicate that complex carbon cycling processes occurred, as part of in situ paludification and as a result of post-depositional burial environments. The results also indicate potential complex landscape mosaics with diverse ecotones in autogenic peat bogs and reed swamps, as a result of variability in the timing of environmental change d even across relatively small areas. In such case studies, localised regions of tussock development can result in highly variable niches over relatively small regions, and systems which do not necessarily include simple linear changes. When attempting to reconstruct palaeolandscapes rather than producing models of chronological change for individual sample points, we must be recognise that different environmental conditions (e.g. reed swamp and minerogenic mire) may have been present at the same time as part of a complex vegetation mosaic. This may be especially so along marine-terrestrial gradients, which are subject to local geomorphological variations. In order to produce a developed impression of change across palaeolandscapes, and how these processes could have impacted on local Mesolithic populations, multi-sequence investigations should be undertaken (for example Yendell, 2012; Stevens et al., 2012). As Scaife (2011, 115) notes, rather than regarding palaeolandscapes as ‘pristine’ climax communities, vegetation should be regarded “… more realistically as a polyclimax, with different ecological niches (soil type, moisture, aspect and so on) giving rise to different dominant taxa and communities.” Evidence for diverse hydrological conditions in multiple cores d even in relatively close proximity d might be better regarded as reflecting unstable ‘poly-transitional’ communities, which were complex, variable, and reflect changes occurring at various rates and scales. In terms of the archaeological context (Thomas, 1990), understanding variability matters because it is against these

reconstructions that the pre-inundation palaeolandscapes are populated (e.g. Leary, 2011; Sturt et al., 2013). If palaeoenvironmental reconstructions of pre-inundation sample sites are presented using ‘top down’ models, which do not represent the specific conditions at sample sites, we risk abstracting models of processes and variability. As part of these reconstructions critical consideration of the nature of dated materialeits association, measurement, modelling and interpretation d is of vital importance. Charred plant remains from within the cores, and dated in VC15, along with evidence for Mesolithic presence within the wider landscape (Bell, 2007; Canti et al., 1995), strongly indicates that Mesolithic humans would have made use of the current sub-tidal zone around Hinkley Point. As such, the ability to better characterise rates of environmental change provides vital landscape context for Mesolithic activity. As Bell and Warren (2013), and Sturt and van de Noort (2013) argue, this is a key step towards developing improved accounts of land and seascape use in prehistory. It is through building up knowledge of regional sequences that archaeologists can move away from overly generalised accounts of the impact of large-scale processes, such as sea-level and climate change, on past populations. Funding Funding for this work was provided by EDF Energy. Acknowledgements This work has been funded by EDF Energy and represents the development of archaeological assessment work as part of the Hinkley Point C consents process. The authors wish to acknowledge the support of Rebecca Calder (EDF Consents Manager) throughout this process; her humour and patience have been much appreciated. The project was managed on behalf of EDF by AMEC to which we owe Sean Steadman unreserved thanks. Shir Akbari, NOCS, undertook the Total Organic Carbon and Carbonate analysis. Alex

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Bayliss is thanked for providing invaluable comments on an earlier version of this paper. Two reviewers are thanks for their comments on this paper.

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