Palaeoenvironments of Ancient Humans in Britain: The Application of Oxygen and Carbon Isotopes to the Reconstruction of Pleistocene Environments

3 Palaeoenvironments of Ancient Humans in Britain: The Application of Oxygen and Carbon Isotopes to the Reconstruction of Pleistocene Environments Ian Candy1,*, Mark Stephens2, Jonathan Hancock1 and Ruth Waghorne1 1

Department of Geography, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, United Kingdom 2 School of Geography, The University of the South Pacific, Suva, Fiji *Correspondence and requests for materials should be addressed to Ian Candy (email:[email protected])

2007). A major development in Quaternary studies over the past 30 years has been the study of oxygen and carbon isotopes in terrestrial and freshwater carbonates as a proxy for palaeoenvironmental conditions (see, e.g. Swart et al., 1993; Leng, 2004). This technique has been widely applied to pedogenic carbonates (Cerling et al., 1989; Cerling and Quade, 1993; Andrews et al., 1998; Candy et al., 2006), tufas and travertines (Andrews, 2006), lacustrine carbonates (Marshall et al., 2002; Leng and Marshall, 2004; Marshall et al., 2007) and speleothems (McDermott, 2004). The strength of this approach is that the oxygen and carbon composition of carbonates is strongly controlled by prevailing environmental factors such as temperature and vegetation cover. Consequently, these isotopes provide a powerful technique for understanding past climatic regimes in greater detail. Until recently, however, stable isotopes have not been widely used in the British Quaternary as a means of palaeoenvironmental reconstruction. During the course of the Ancient Human Occupation of Britain (AHOB) project, stable isotopic studies have been routinely applied to a range of terrestrial and freshwater carbonates from deposits of a number of interglacial episodes (Candy et al., 2006; Candy and Schreve, 2007; Rose et al., 2008; Candy, 2009). The purpose of this work has been twofold. First, to study modern carbonates in an attempt to understand how current environmental conditions are preserved within the stable isotopic composition of biogenic and abiogenic carbonates. Secondly, to generate environmental information from carbonates of a range of ages and to use this, along with other proxy information, to produce more detailed palaeoenvironmental reconstructions for early human occupation events. This chapter presents and summarises some of the main findings of the AHOB isotope research. It begins with a discussion of the range of carbonates that occur within British Quaternary sediments and outlines the potential of O and C isotopes in these materials as environmental indicators. The stable isotopic composition of a number of modern carbonates is then reviewed to highlight how isotopic composition relates to environmental conditions. Finally, the application of this technique to two interglacial episodes that are crucial to understanding

Abstract Stable oxygen and carbon isotopes in continental carbonates are used routinely as palaeoenvironmental proxies in Quaternary sequences. In the British Quaternary record, this approach has been relatively under-utilised despite the abundance of a wide variety of carbonate types, including soil and groundwater precipitates, tufa, freshwater and terrestrial mollusc shells and lacustrine carbonates. As part of the Ancient Human Occupation of Britain project, the potential of this approach for understanding the climates and environments of early humans in Britain has been investigated. These studies involved two stages: (1) the analysis of modern carbonates to understand how the stable isotope composition of these materials record modernenvironmental conditions; and (2) the application of these modern analogue studies to the investigation of carbonates from a number of British interglacial episodes. The application of this technique is discussed with reference to two archaeologically significant periods, the Cromerian Complex and the Hoxnian. These examples highlight the potential for using this technique to understand temperatures in past interglacials. The chapter concludes by discussing the significance of these studies to understanding the environments of human occupation. Keywords: Interglacial; Carbonate; Oxygen isotopes; Carbon isotopes.

3.1. Introduction Pleistocene palaeoenvironments in Britain have traditionally been reconstructed using a range of biological and sedimentary proxies (West, 1980; Green et al., 1996, 2006; Bridgland et al., 1999; Murton et al., 2001; Schreve et al., 2002; Preece et al., 2007). These proxies provide a means by which the landscapes, ecosystems and environments of different glacial and interglacial episodes can be reconstructed. In many cases, the environmental data can act as a basis for the quantification of past temperature regimes (Atkinson et al., 1987; Coope, 2001, 2006; Horne,

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the early human occupation of Britain: the Hoxnian (Marine Isotope Stage (MIS) 11) and the Cromerian Complex (MIS 13–19) is discussed.

3.2. Oxygen and Carbon Stable Isotopes in Quaternary Studies 3.2.1. Rationale Oxygen and carbon isotopes are widely used in Quaternary studies as proxies for past climates. This is seen in marine and ice-core records which act as archives of climate change over 105–106 years and allow the reconstruction of past ice volume from marine cores (Imbrie et al., 1984, 1993; Martinson et al., 1987), and air temperature through ice cores (Dansgaard et al., 1993; EPICA, 2004). In terrestrial and freshwater environments, isotope-based environmental reconstructions are typically carried out on carbonate minerals which are found in a range of settings (e.g. speleothems, soils, lake sediments, tufa, molluscs and ostracods). Since these materials are found in a wide range of chemical and geological settings, the techniques are applicable to most regions of the world. The use of stable isotopes as a means of reconstructing past environments is based upon the principle that isotopes of the same element will behave in a chemically identical fashion but, because of differences in mass, will respond in different ways to physical factors such as temperature, evaporation and phase changes. This separation of isotopes on the basis of mass is known as fractionation. The isotopic study of carbonate focuses on the oxygen and carbon isotopes. The most common isotope of oxygen is 16O (99.763%), with the 18O (0.1995%) isotope being much less abundant. Equally, the most abundant carbon isotope is 12C (98.971%) with 13C (1.021%) being much less common. The carbon and oxygen isotopic ratios are measured against, and reported with respect to, a carbonate standard (see below). As the isotopic ratio of the sample is expressed relative to that of a standard, it is shown as either a positive or negative value depending upon whether it is more or less enriched in the heavier isotope (18O or 13C) than the standard. Isotopic values are calculated in the following way: d18 O ¼ 1000  ð18 O=16 Osample 18 O=16 Ostandard Þ=18 O=16 Ostandard In the case of carbonates, the d18O of the sample is quoted relative to the d18O of the Pee Dee Belemnite (PDB) standard, while water samples are quoted relative to the d18O of the Standard Mean Ocean Water (SMOW) standard. A sample with a d18O value of  3% is, therefore, 0.3% or 3% depleted, or deficient, in 18O relative to the standard. Most studies refer to samples being enriched or depleted in the heavier isotope or to samples being isotopically heavier or lighter. During the process of carbonate formation, the oxygen and carbon are taken into the mineral phase from different sources. Oxygen is derived primarily from the oxygen component of H2O, thus the oxygen isotopic

composition of the carbonate is strongly controlled by the oxygen isotopic composition of the source water. Carbon is derived primarily from dissolved inorganic carbon, which may come from a range of sources. These processes are summarised in the following equation: Ca2þ þ 2HCO 3 ¼ CaCO3 þ CO2 þ H2 O The significance of these principles to palaeoenvironmental reconstruction is that stable isotopic ratios in carbonates can be directly related to important environmental factors. Source water is, in turn, frequently controlled by climatic factors such as air temperature and evaporation, although vital offsets in organisms and salinity levels may also be important (Cerling and Quade, 1993; Darling and Talbot, 2003; Darling, 2004; Leng and Marshall, 2004; Andrews, 2006). The carbon isotopic composition of dissolved inorganic carbon is controlled by biological processes (i.e. photosynthesis), vegetation type and atmospheric CO2 inputs (Cerling et al., 1989; Cerling and Quade, 1993; Leng and Marshall, 2004; Andrews, 2006). Furthermore, temperature-related fractionation of both carbon and oxygen isotopes occurs during the precipitation of the carbonate, although only the fractionation effect associated with oxygen isotopes is significant enough to produce major changes in the isotopic composition of the resulting carbonate (Cerling and Quade, 1993). The environmental factors, together with the fractionation effects, which control the isotopic composition of terrestrial and freshwater carbonates are discussed in the following sections.

3.2.2. Environmental Significance of Oxygen Isotopes in Terrestrial and Freshwater Carbonates The d18O of a carbonate is primarily a function of (1) the d18O composition of the source water from which the carbonate was precipitated and (2) the temperature at which the carbonate was precipitated (Leng and Marshall, 2004; Andrews, 2006; Candy et al., 2006). Most of the carbonates that will be considered in this study have formed from surface, soil or groundwaters, all of which are initially sourced from rainfall. Therefore, to some extent, the d18O of these carbonates will reflect the d18O of precipitation. The d18O of rainfall varies primarily with latitude, but at any specific location the key control on isotopic composition appears to be air temperature, except in the tropics where the net volume of rainfall, the ‘amount effect’, appears to be a major control (Dansgaard, 1964; Rozanski et al., 1993; Darling and Talbot, 2003; Darling, 2004). Sites at which the chemistry of rainfall is routinely monitored show a systematic variation of d18O with air temperature, winter rainfall being isotopically lighter than summer rainfall (Darling, 2004). Clark and Fritz (1997) have estimated that with every 1  C increase in temperature, the d18O of rainfall will become between þ 0.2% and þ 0.7% more enriched. The actual amount of enrichment will vary with both latitude and proximity to the major water source. In temperate latitudes such as Britain, the rainfall percolates through the soil zone, recharging the aquifer. During recharge, subsurface flow and storage, the groundwater

Palaeoenvironments of Ancient Humans in Britain will become mixed, resulting in the seasonally heterogeneous isotopic content of rainfall being transformed into subsurface waters, and finally surface waters, with a homogeneous d18O composition (Stuiver, 1970; Darling, 2004). In general terms, the d18O of groundwater is an approximation for the mean d18O composition of the prevailing precipitation. In Britain, these factors can be clearly observed within the patterns of d18O composition in both rainfall and groundwater (Darling and Talbot, 2003; Darling, 2004). This pattern can be summarised as consisting of a trend of d18O depletion in meteoric waters as you move from west to east (because rainfall is primarily driven by westerly winds from the Atlantic) and from south to north (primarily due to latitudinal variations) (Rozanski et al., 1993; Darling and Talbot, 2003; Darling, 2004). Thus any difference in the d18O of precipitated carbonates from a site on the west coast and one on the east coast could reflect variations in regional patterns of d18O composition of rainfall as well as differences in air temperature. Therefore, comparing the d18O composition of carbonates from different locations within the British Isles is problematic if the aim is to compare or construct regional temperature records. The effect of changing air temperature on the d18O composition of freshwater/terrestrial carbonates can be seen from a number of British lacustrine and tufaceous carbonate sequences. At sites such as Haweswater in northwest England, lake carbonates clearly record the major shifts in air temperature which occurred during the Lateglacial to Holocene transition (Marshall et al., 2002). Cold periods such as the Lateglacial Stadial (Younger Dryas) are characterised by lacustrine carbonates with depleted, or isotopically light, d18O values (typically around  6.5%) while significant enrichment occurs during the Lateglacial Interstadial and at the onset of the Holocene (values around  4%) (Marshall et al., 2002). At tufa sites, such as Wateringbury in Kent, the climate warming that occurred during the onset of the Holocene is characterised by a progressive enrichment in the d18O values of tufa carbonate (Garnett et al., 2004). The d18O records of such sequences are sensitive enough to record even short-lived temperature oscillations such as the 8.2 ka event (Garnett et al., 2004; Marshall et al., 2007). In addition to the temperature control on the d18O composition of ground and surface waters, there is also a temperature-controlled fractionation of oxygen isotopes that occurs during carbonate precipitation. This fractionation effect has been studied by a number of researchers and is quantified in various equations, the most commonly used being those proposed by Anderson and Arthur (1983) based upon marine carbonates: T C ¼ 16:0  4:14ðdc  dwÞ þ 0:13ðdc  dwÞ2 ð3:1Þ and that proposed by Hays and Grossman (1991), which is more specifically relevant to meteoric cements: T C ¼ 15:7  4:36ðdc  dwÞ þ 0:12ðdc  dwÞ2 ð3:2Þ In both equations dc ¼ d18O of the carbonate (PDB), while dw ¼ d18O of the source water (relative to SMOW). The first is based upon data generated by Craig (1965) that included

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shells comprising two different carbonate minerals, calcite and aragonite. As the fractionation effect for calcite and aragonite is slightly different (the d18O of aragonite being around 0.6% more enriched than calcite precipitated at an equivalent temperature (Leng and Marshall, 2004)), the reliability of this equation has been questioned. The temperature-controlled fractionation means that the carbonate mineral becomes depleted in d18O by around 0.25% for every 1  C increase in temperature. This means that in a setting where the d18O of water is constant, a progressive increase in temperature will lead to the derived carbonates becoming more depleted in d18O. The temperature-controlled fractionation, therefore, has the opposite effect to that of increasing air temperature on the d18O composition of rainfall and, consequently, surface water. However, the net result of these two effects is to produce an enrichment in the d18O of carbonates with increasing temperature. This is suggested because (1) the temperature control on fractionation and the air temperature control on the d18O of source water will operate together, and (2) the enrichment in d18O in rainfall/surface water per 1  C is greater than the depletion in d18O per 1  C that occurs during fractionation. This process has been clearly summarised by Andrews (2006, 94), who states that in northwest Europe the mean d18O enrichment in rainfall is approximately 0.58% per 1  C increase, while the depletion in d18O that occurs during carbonate formation is ca.  0.24% per 1  C. Therefore, increasing temperatures will lead to an enrichment of the d18O composition of the derived carbonate; however, the temperature-controlled fractionation will ‘damp’ the effect of air temperature-driven d18O enrichment. Consequently, a 1% enrichment in carbonate d18O will approximate to a 3  C temperature increase, even though such an increase in air temperature would approximate to a 1.8% increase in the d18O composition of rainfall. This quantitative relationship can become more complicated if the surface waters are affected by evaporation prior to carbonate precipitation (Dever et al., 1987; Leng and Marshall, 2004; Andrews, 2006). Evaporation results in the selective removal of 16O over 18O from the water body, leading to the enrichment in 18O of the source water, and the resultant carbonate, without any necessary increase in temperature. Intense periods of soil moisture or surface water evaporation may, therefore, produce carbonates with apparent ‘warm’ signatures, even though no net temperature change occurred.

3.2.3. Environmental Significance of Carbon Isotopes in Terrestrial and Freshwater Carbonates The d13C of terrestrial and freshwater carbonates is primarily a function of the isotopic composition of the dissolved inorganic carbon in the source water, which is controlled by a number of factors (Cerling et al., 1989; Cerling and Quade, 1993; Leng and Marshall, 2004). In Britain the carbon isotopic composition of many soil, tufa and groundwater carbonates is frequently depleted with respect to 13C, primarily due to the input of plant-respired CO2 (Cerling and Quade, 1993; Andrews, 2006; Candy,

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Ian Candy et al. C3 and C4 plants occur, carbon isotopes can provide important environmental indicators as the balance of these respective plant groups will frequently provide an indication of environmental aridity (Cerling and Hay, 1986; Cerling et al., 1991; Cerling and Quade, 1993; Andrews et al., 1998). The British flora, throughout the Quaternary until the onset of agriculture, has been comprised of only C3 plants; consequently, variations in d13C in British carbonates primarily reflects the hydrology of surface waters, rates of degassing and varying biological activity. In British sequences, carbon isotopes provide more information on local conditions than they do on regional climates.

2009). Plants will selectively take up carbon dioxide with 12 C over carbon dioxide with 13C during photosynthesis. As a consequence, CO2 respired in the soil environment will be depleted with respect to d13C. During groundwater recharge, percolating vadose waters will take up plantrespired CO2 from the soil zone which will, in turn, be transferred to springs, streams and rivers (Andrews, 2006). Soil-respired CO2, in regions such as Britain which are dominated by plants using the C3 photosynthetic pathway, typically has a d13C value of around  25% and  32% (as opposed to atmospheric CO2 which has a d13C value of about  7.8%). Diffusion, dissolution and temperature-controlled fractionation result in an offset of around 14% between the d13C of the gaseous CO2 in the soil and the d13C of the precipitated carbonate (Cerling et al., 1989; Cerling and Quade, 1993). In regions such as Britain, soil and groundwater carbonates will typically have d13C values of ca.  10%. In surface water, the d13C of dissolved carbon can be modified by degassing (Dever et al., 1987; Leng and Marshall, 2004) and by the uptake of dissolved carbon from water bodies during photosynthesis by aquatic vegetation (both processes will lead to enrichment of the dissolved CO2 with respect to d13C). In slow moving or stagnant water bodies (such as lakes), the dissolved carbon can become modified through gradual equilibration with atmospheric CO2 ( 6.5% at pre-modern values and  7.8% at modern levels (Friedli et al., 1986)). Consequently, the d13C of many lacustrine carbonates, which were initially fed by groundwater that was strongly depleted in d13C, will frequently be characterised by carbon isotopic values of ca. 0%. In carbonates from temperate latitudes it is typically oxygen isotopes which provide the best environmental records. This is because, as discussed above, the d18O of carbonates will primarily be a function of temperature, either through temperature-controlled isotopic fractionation or the temperature control of the d18O of rainfall. In regions where both

3.2.4. Carbonates in British Quaternary Sequences Although they have received relatively little attention, a wide range of carbonates occurs in British Quaternary sequences, consequently there is a range of material available for stable isotope studies. The main types of carbonate are shown in Fig. 3.1. Typically these carbonate types can be divided into three categories: (1) weathering-zone carbonates; (2) flowing-water carbonates; and (3) still-water carbonates. These categories are not clearly defined and there is a strong degree of overlap between the different groups. Weathering-zone carbonates are those found within soil profiles, such as carbonate nodules (Kemp, 1985; Candy et al., 2006), cemented root features (or rhizoliths) (Klappa, 1980; Candy, 2002) and earthworm granules (Canti, 1998). A number of carbonates occur below the soil zone but have formed in association with the translocation of weathering products. These carbonates typically reflect: (1) the dissolution of carbonate in the soil zone; (2) the downward leaching of carbonate into the sediment body; and (3) the re-precipitation of carbonates at zones of increased porosity or in association with the groundwater table.

1. Soil A

2. Fluvial/spring

3. Lacustrine A

B D C

A D

C

B

D B

C

Soil zone Calcareous till Sand and gravel

5. 4.

Water table Sand and gravel

Bedrock

Fig. 3.1. The main types of freshwater and terrestrial carbonates found in British Quaternary sequences. The background geology, sand and gravel overlain by calcareous till is typical of much of the landscape of East Anglia where these studies have been carried out. 1. Soil carbonates: A, terrestrial mollusc shells; B, earthworm granules; C, carbonate nodules; D, calcified root systems. 2. Fluvial/spring carbonates: A, chara precipitates (stem casts and Oogonia); B, freshwater molluscs; C, ostracods; D, carbonate precipitates on bed/clast surfaces. 3. Lacustrine carbonates: A, freshwater molluscs; B, reworked chara fragments; C, ostracods; D, carbonate marl (precipitated and settled out from the water body). 4. Groundwater precipitates. 5. Vadose precipitates associated with till decalcification.

Palaeoenvironments of Ancient Humans in Britain Numerous carbonate types can form in association with flowing-water processes, most obviously through the development of tufas, either at springs or in riverine environments. In spring settings, tufa formation is typically associated with CO2 degassing as groundwater flows into the subaerial environment (Pentecost, 1993; Andrews, 2006). In riverine settings, carbonate precipitation occurs on the channel bed, frequently as overgrowths on clasts within the river bed substrate which commonly reflect precipitation in association with microbial processes. Abundant biogenic carbonates are also found in such settings, including mollusc shells and ostracods, although these also occur in still-water settings. Along with the range of biogenic carbonates that may form in both flowing- and still-water settings, abiogenic carbonate precipitation may occur in lacustrine environments as a function of the seasonal modification of the water chemistry. In many carbonate-rich lakes, carbonate will precipitate in the water column during the summer months because of the modification of the lake water pH due to enhanced photosynthesis. Photosynthesis, stimulated by increased light and temperature levels during the summer months, leads to the removal of dissolved CO2 from the lake waters, resulting in a reduction in the lakewater acidity. This results in the lake waters becoming supersaturated with respect to carbonate and the precipitation of carbonate in the water column which then falls out of suspension and accumulates on the lake bottom. In general terms, the processes outlined above are likely to be more characteristic of interglacial rather than glacial climates. The majority of these processes are accelerated by increased vegetation and biological activity, which are promoted during interglacial episodes. Furthermore, for these materials to accumulate they typically require stable substrates, that is stable river beds and land surfaces. During glacial episodes, the high rates of weathering and the increased activity of geomorphic systems means that land surfaces, riverine environments and lake basins either undergo large amounts of erosion or high rates of deposition. Consequently, glacial environments are less conducive to the formation and preservation of surficial carbonates than are interglacial environments. As a result, the widespread use of stable isotopes in the reconstruction of British Pleistocene environments will be more useful in understanding the climates of past interglacials than of past glacials.

3.2.5. Stratigraphy of Studied Carbonates Crucial to the successful interpretation of the stable isotope values of Pleistocene carbonates is our ability to relate the formation of the carbonate horizon to a specific climatic period. If the period of carbonate development cannot be correlated with a specific interglacial, then the study of its oxygen- or carbon-isotope composition is meaningless as the derived environmental data will have no stratigraphic context. Therefore, isotopic studies as part of the AHOB project have focused, primarily, on horizons and units which have good stratigraphic contexts. Interglacial beds which contain a range of carbonate phases (e.g. tufa, shells and soil carbonates) but also contain a distinctive biostratigraphic assemblage are, therefore, ideal. Much of

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the work presented below is, therefore, based around type sites or well-studied localities where stable isotope data can be directly correlated with known interglacial events and, frequently, with marine isotopic stages.

3.3. Modern Carbonates in Britain Understanding modern isotopic systems is essential for any study that uses oxygen and carbon isotopes in carbonates to reconstruct past environments (Andrews et al., 1993, 1997; Marshall et al., 2002). In order to highlight the role of environment in controlling isotopic signals in carbonates, the d18O and d13C of modern soil, shell and freshwater (tufa) carbonate was studied and compared with modern climatic conditions. This allows us to establish the isotopic values of carbonates that have formed under modern temperature regimes and to use these as an analogue with which to compare the isotopic composition of Pleistocene carbonates. The stable isotopic composition of modern soil and freshwater carbonates is discussed in the following sections. As the majority of the key Pleistocene sites that are to be studied are found in eastern England the modern carbonates were also taken from this region.

3.3.1. Modern Soil Carbonates Rhizoliths formed in association with modern, decaying root systems were taken from Buckanaye Farm and Corton Cliffs, Suffolk (Fig. 3.2) (Candy, 2009). At Buckanaye Farm, the rhizoliths have formed within shell-rich sands, while at Corton Cliffs the rhizoliths occur within chalk-rich Middle Pleistocene (Lowestoft) till. Further evidence comes from published isotopic data from Holocene soil carbonate found in association with a soil profile formed in limestone-rich outwash gravels at West Tanfield, Yorkshire (Strong et al., 1992). The isotopic values of these carbonates are shown in Fig. 3.3. The d13C of these carbonates range from  8% to  11%, typical of soil carbonates forming under C3 vegetation. The mean d18O values of these carbonates are  5.25% (Buckanaye Farm),  5.48% (Corton) and  6.22% (West Tanfield). Establishing whether the d18O of these carbonates is reliably recording mean annual, mean summer or mean winter temperature is problematic. In Britain soil temperatures, and their variation with air temperatures, are not well known and the relationship between the d18O of rainfall and the d18O of soil moisture is poorly understood. However, if the examples from Buckanaye Farm and Corton are used, then the mean d18O of precipitation in this region is ca.  7.0% and mean annual air temperature is 9–10  C. Using Eq. (3.1) or (3.2) above, a carbonate forming from waters with a d18O value of  7.0% and under a mean annual temperature of 9–10  C would have a d18O composition of between  5.1% and  5.5%. This suggests that modern soil carbonates in eastern England are forming broadly in equilibrium with modern temperature and rainfall conditions. On the basis of experimental data, Cerling and Quade (1993) suggested that soil carbonates form in equilibrium with meteoric waters

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Ian Candy et al. West Runton

Corton Cliffs Elveden West Stow (Beeches Pit)

Legend Modern carbonate sites

Pakefield

Sproughton

Hoxnian carbonate sites

Marks Tey

Cromerian carbonate sites

0

km

Runnymede 50

Buckanaye Farm

Clacton

Swanscombe

Fig. 3.2. The location of the main study sites reported in the text. δ18O (PDB) −8.00

−7.00

−6.00

−5.00

−4.00

Bithynia shells Sproughton Microbial carbonates Sproughton Rhizoliths - Corton Cliffs Rhizoliths - West Tanfield Rhizoliths - Buckanaye Farm

−3.00 0.00 −2.00 −4.00 −6.00 −8.00

δ13C (PDB)

−9.00

−10.00 −12.00 −14.00

Fig. 3.3. Stable oxygen and carbon isotope data from modern soil and freshwater carbonates in eastern England. Corton Cliffs and Buckanaye Farm data are from Candy (2009); West Tanfield data are from Strong et al. (1992). and this would appear to be the case in eastern England. The more depleted soil 18O values from West Tanfield reflect the fact that this site receives rainfall which is more depleted in 18O (ca.  8%) than rainfall in southern England. Using Eq. (3.1) or (3.2), however, the d18O values of these carbonates are still consistent with precipitation in equilibrium with local meteoric waters.

3.3.2. Freshwater Carbonates To illustrate the role of environment in controlling the isotopic composition of freshwater carbonates, mollusc shells and carbonate clast coatings were studied from the bed of the

River Gipping at Sproughton, Ipswich (Fig. 3.2). Given the factors that control the d18O composition of carbonates, it is important to understand the temperature regime and d18O chemistry of the water body from which these minerals precipitate. Water temperature data from the Environment Agency indicates that the thermal regime of River Gipping water varies throughout the year in association with air temperatures (mean annual temperature ¼ 9.5  C, mean July temperature ¼ 17  C, mean January temperature ¼ 4.5  C). Water samples taken from the Gipping during different periods of a single year (2007) show very little seasonal variation in d18O values (mean ¼  6.50, July ¼  6.27, November ¼  6.69, January ¼  6.20, April ¼  6.38). This homogeneous river-water signal is consistent with

Palaeoenvironments of Ancient Humans in Britain British river systems as they are fed primarily by groundwater, the d18O of which is typically homogeneous and reflects the mean d18O composition of rainfall. The River Gipping is regularly dredged; carbonates on the river bed have therefore precipitated within a three year period (the time since the previous dredging). Across the river bed a large number of clasts, typically flint, have thin crusts of carbonate on their upper surfaces. Twenty carbonate-coated clasts were sampled from the modern river bed, all of which displayed a thin (< 1 mm) veneer of calcite on the upper surface. Loose material was removed from the surface of these clasts and consolidated carbonate material was sampled using a rotary drill. The 20 samples yielded mean d18O values of  6.95% (1s ¼ 0.40) and mean d13C values of  7.73% (1s ¼ 1.34) (Fig. 3.3). Fifteen shells from modern individuals of the freshwater mollusc Bithynia tentaculata were also sampled from the River Gipping site. These shells, like the shells of many freshwater molluscs, are made of aragonite. Due to the ca. 0.6% enrichment in d18O that occurs during the precipitation of aragonite relative to calcite, the Bithynia shells will have different d18O values to the freshwater carbonate clast coatings, even if they precipitated under identical environmental conditions. The isotopic composition of Bithynia shells yielded a mean d18O value of  5.88% (1s ¼ 0.29) and a mean d13C value of  10.95% (1s ¼ 1.23) (Fig. 3.3). In general, the values obtained from the River Gipping are consistent with (1) our current understanding of modern freshwater carbonates and (2) the temperature and water chemistry for this system. The d13C values of both the clast coatings and the shells are strongly depleted with respect to d13C. This is primarily because river waters record the take-up of CO2 from the soil zone during groundwater recharge. However, in the case of the molluscs, uptake of carbon will also be from aquatic vegetation which uses the C3 photosynthetic pathway and will, therefore, also be strongly depleted in d13C. The d18O composition of both sets of carbonates suggests that they have precipitated in equilibrium with the River Gipping waters during summer months. Using the mean water d18O composition of the River Gipping and temperature equation (3.2), the carbonate clast coatings appear to have formed under a temperature of ca. 17  C, consistent with mean July and August water temperatures at the site. Temperature equation (3.2) (Hays and Grossman, 1991) is not appropriate for Bithynia shells as they are composed of aragonite; however, temperatures of formation were calculated by (1) using a laboratory-based aragonite temperature/fractionation equation (White et al., 1999) and (2) by adding 0.6% (a general figure of difference between aragonite and calcite d18O values) to each of the shell values and then using the Hays and Grossman (1991) equation. Both of these methods indicate aragonite precipitation under thermal regimes consistent with summer temperatures within the River Gipping. Work by White et al. (1999) indicates that precipitation took place at temperatures slightly above 17  C, while the use of the Hays and Grossman equation on corrected values indicates precipitation at temperatures between 15.5 and 16.5  C. The suggestion that these carbonates are recording summer temperatures is consistent with the factors that control

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their precipitation. During summer months the potential for carbonate precipitation will be increased due to lower flow, increased evaporation and increased photosynthesis. The form of the carbonate clast coatings is characteristic of microbial carbonates; these are carbonates precipitating in association with microbial communities that live on the upper surface of clasts on the beds of streams and rivers. Precipitation typically occurs during summer months because many microbial communities are made up of cyano-bacteria which are photosynthetic. During summer months accelerated photosynthesis leads to the removal of CO2 from the water body altering the pH in the area of the clast surface and leading to carbonate precipitation. It is, therefore, more reasonable for such carbonates to reflect summer temperatures, rather than mean annual or winter temperatures. The d18O composition of Bithyniatentaculata shells from the River Gipping is consistent with the d18O composition of both Bithynia and Valvata shells from the Thames at Runnymede (Davies et al., 2000). As the River Thames at this location has a very similar thermal regime and isotope chemistry to the River Gipping, this highlights the importance of summer temperatures in controlling the d18O composition of these shells. 3.3.3. Summary of Modern Isotopic Studies The study of modern soil and freshwater carbonates highlights the potential of the study of oxygen and carbon isotopes in Pleistocene carbonates. Soil carbonates appear, at the sites which were studied, to form in equilibrium with modern meteoric waters and to be useful indicators of the general temperature regime of the site. The d18O composition of freshwater carbonates appears to be strongly controlled by summer temperatures; winter temperatures and mean annual temperatures are of lesser significant. These modern studies are crucial to the isotopic study of any Pleistocene carbonates as they provide a record of how isotopes, primarily d18O, record modern air and water temperatures. These examples can, therefore, provide the modern analogues against which Pleistocene isotopic datasets can be compared. Deviations in the isotopic composition of both terrestrial and freshwater carbonates may, therefore, be explained in terms of either changes in temperature regimes or meteoric water chemistry.

3.4. The Oxygen and Carbon Isotopic Record of British Interglacials In terms of understanding the ancient human occupation of Britain, the Cromerian Complex and the Hoxnian are two key interglacials periods. The Cromerian Complex includes a number of distinct interglacial episodes spanning the early Middle Pleistocene (780–450 ka, MIS 19, 17, 15, 13) and possibly even the late Early Pleistocene (Turner, 1996; Preece and Parfitt, 2000). Deposits of this period contain evidence for the oldest known human occupation in Europe north of the Alps (Parfitt et al., 2005). Deposits of the Hoxnian interglacial, which are routinely correlated with MIS 11 (ca. 410 ka) (Bridgland,

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1994; Bowen, 1999; Rowe et al., 1999; Preece et al., 2007), contain some of the most important Middle Palaeolithic sites in Britain including Swanscombe, Clacton and Beeches Pit (West Stow). In terms of reconstructing interglacial palaeoenvironments and understanding past climate systems, both of these periods are highly significant. Many long climate records, such as those from ice cores from east Antarctica (EPICA, 2004) and marine isotope records (e.g. Flower et al., 2000) indicate that interglacials MIS 19–13 were characterised by climatic conditions that were significantly cooler than the Holocene. Equally, there is a significant body of evidence to suggest that MIS 11 was a period of extreme climate warmth, during which temperatures reached levels comparable to those of the Mid-Pliocene (Burckle, 1993; Howard, 1997; Guo et al., 1998; Hearty et al., 1999; Becquey and Gersonde, 2002; Bowen, 2003; Kukla, 2003). It has been proposed that during this extreme warming both the Greenland and West Antarctic ice sheets collapsed generating global sea-levels between 13 and 20 m higher than the present (Hearty et al., 1999; Bowen, 2003). The palaeoclimates of these periods have significant implications for our understanding of human occupation in Britain. If correct, the suggestion of cooler interglacials during the early Middle Pleistocene implies that the earliest humans in Europe north of the Alps would have had to colonise a landscape and ecosystem in a climate that was significantly cooler than today. Within the Hoxnian, however, humans would have been occupying Britain during a period of enhanced and extended warmth, with major implications for the ecosystem of that time. Deposits of both of these periods contain a wide range of terrestrial and freshwater carbonates that may act as a basis for palaeoenvironmental interpretations. Furthermore, as deposits of these interglacials frequently contain a range of other climatic indicators, it is possible to use the isotopic evidence as part of a multi-proxy environmental reconstruction. The following sections will report the results of isotopic studies from two Cromerian Complex sites and five Hoxnian sites, all in eastern England.

3.5. Carbonates from the Cromerian Complex Both Pakefield and West Runton (Fig. 3.2) are considered to relate to early in the Cromerian Complex as they both contain teeth of the small mammal Mimomys savini (Preece and Parfitt, 2000, 2008). Oxygen and carbon stable isotopic analysis has also been carried out in later Cromerian Complex deposits, but this work is beyond the remit of this chapter (see Holmes et al., 2009). Cromerian deposits at Pakefield contain humanly struck flint flakes and cores which, in association with the biostratigraphy and lithostratigraphy, provide the earliest known evidence for human occupation in Europe north of the Alps (Parfitt et al., 2005). Cromerian deposits at West Runton have not yielded any known archaeology; however, this is the stratotype for the Cromerian (West, 1980) and, in the context of understanding interglacial climates of the early Middle Pleistocene, is an important site.

3.5.1. Pakefield The Cromerian deposits at Pakefield comprise a fluvial sequence consisting of (1) overbank fines (the ‘Rootlet Bed’) and (2) channel-fill deposits (Blake, 1877, 1890; West, 1980; Lee et al., 2006). Both of these facies contain humanly worked flints (Parfitt et al., 2005). It has long been recorded that deposits of the ‘Rootlet Bed’ are rich in carbonate nodules, frequently termed ‘race’ (West, 1980). However, detailed sedimentology and micromorphology has shown that these nodules are pedogenic and represent carbonate precipitation in soil profiles during the accumulation of the floodplain sediments (Candy et al., 2006). Significantly, however, the Pakefield soil carbonates comprise large (20–30 mm), densely cemented nodules which are very different from carbonate cement rhizoliths described from modern soil profiles in eastern England (Candy et al., 2006). Candy et al. (2006) described the Pakefield soil carbonates as soil calcretes and indicated that their formation requires an annual soil moisture deficit. They also argued that calcrete development at Pakefield requires a different precipitation regime from modern-day eastern England, where rainfall is evenly distributed throughout the year, and have proposed a regime more like that of southern Europe, with a pronounced dry season during summer months. A stable isotopic study was carried out through the best developed soil profile within the ‘Rootlet Bed’. Soil carbonate samples, nodules and calcified rootlets were taken at 50 mm intervals throughout this profile and analysed for oxygen and carbon isotopic composition (Fig. 3.4A). The results, described in Candy et al. (2006), show mean d18O values of  4.09% (1s ¼ 0.18) and mean d13C values of  9.24% (1s ¼ 0.38). There is a progressive enrichment in both carbon and oxygen isotopes up-profile by ca. 0.4% (oxygen) and 1.0% (carbon). The significance of these results is best seen through a comparison with the isotopic composition of modern soil carbonates (Fig. 3.4B). The d13C of both the modern and the Pakefield soil carbonates is consistent with values expected in British soil carbonates. They are all strongly depleted with respect to d13C, reflecting the dominance of plant-respired CO2 in the soil zone. The most marked difference between the datasets lies within the d18O values with Pakefield carbonates being, on average, 1.1–1.4% more enriched than modern soil carbonates from eastern England. The most basic explanation of this difference is that the Pakefield soil carbonates formed under a warmer climate than that which occurs in modern day eastern England. Given the role of temperature in controlling the d18O of rainfall/meteoric water and the fractionation of oxygen isotopes during carbonate precipitation (an approximate enrichment of 0.3 per 1  C), it could be argued that these carbonates were precipitated under a climate 4–5  C warmer than modern day eastern England. This suggestion is based on a number of assumptions, the most fundamental of which is that there is no major modification of the d18O of soil water prior to carbonate precipitation. As Candy et al. (2006) have suggested that the Pakefield soil carbonates formed under conditions of a net soil moisture deficit, it is possible that soil moisture evaporation could have been a significant process during

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Fig. 3.4. Stable oxygen and carbon isotope data from a soil profile within the ‘Rootlet Bed’, Pakefield, Suffolk (Candy et al., 2006). (A) Variation of isotope data within the soil profile of the ‘Rootlet Bed; (B) comparison of oxygen and carbon isotopic data from the ‘Rootlet Bed’ and modern soil carbonate data from elsewhere in eastern England. See caption for Fig. 3.3. calcrete growth (Dever et al., 1987). As evaporation leads to the selective removal of H216O relative to H218O, the net result of this process will be to overestimate the temperature under which the carbonate has formed. It is unlikely that the whole difference in d18O between the modern and Cromerian carbonates is a function of evaporation as the amount of enrichment is too great; however, it is possible that the interglacial represented by the Pakefield deposits may have been only 2–3  C warmer than present. The suggestion, based on stable isotopic analysis, that climates during the ‘Pakefield interglacial’ were several degrees warmer than the present is supported by biological proxies found within the sediments. Hippopotamus fossils and remains of plant species such as Ilex and Hedera indicate mild winters with no evidence for harsh continental winters (West, 1980; Parfitt et al., 2005). Plant macrofossils such as Trapa natans and Salvinia

natans suggest mean summer temperatures of > 18  C, at least 1–2  C warmer than the present (Parfitt et al., 2005). Furthermore, the fossil coleopteran assemblage suggests that mean warmest month temperatures, based on the Mutual Climatic Range (MCR) technique, were ca. 21  C (range 18–23  C) (Parfitt et al., 2005; Coope, 2006). All of this is consistent with the isotopic data from the soil carbonates.

3.5.2. West Runton The Cromerian deposits known as the ‘Freshwater Bed’ that are exposed at West Runton have a long and detailed history of study; they are thought to record the infilling of a still/sluggish water body on an active floodplain (Rose et al., 2008; Gibbard et al., in press). The lower part of the deposit is characterised by diamictons and units of

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pebbly sand which reflect bank collapse and episodic flooding events. The upper part of the deposit is significantly finer with a greater degree of sorting, essentially reflecting a standing water body on a stabilised floodplain with sedimentation being dominated by material falling out of suspension (Rose et al., 2008). The sedimentology of the unit is highly variable, both laterally and vertically. The lower 0.5 m of the deposit is rich in freshwater mollusc shells, the characteristics of which support the above suggestion. Davies et al. (2000) carried out a detailed isotopic study on shells of Valvata piscinalis from the lower 0.5 m of the Freshwater Bed in an exposure produced by the excavation of a mammoth skeleton in 1996. A further isotopic study was carried out as part of the AHOB project on an exposure 55 m to the west of the mammoth excavation (Rose et al., 2008). Due to the relative scarcity of molluscan shells in the upper part of this profile, this study again focused on the lower 0.5 m of the Freshwater Bed. The Davies et al. (2000) dataset generated a mean d18O value of  5.82% (1s ¼ 0.55) and a mean d13C value of  11.06% (1s ¼ 1.18). The Rose et al. (2008) dataset generated a mean d18O value of  5.54% (1s ¼ 0.45) and a mean d13C value of  10.01% (1s ¼ 0.78). Although subtle differences exist between the descriptive statistics of these datasets, there is a strong degree of overlap and no statistical difference exists. Furthermore, slight differences in isotopic values are not unrealistic as the lateral variability of the Freshwater Bed means that even though the two study sites occur at the same vertical position it does not mean that they represent identical stratigraphic units. The d13C values of the two datasets are consistent with those of modern freshwater molluscs from rivers Gipping and Thames. There is also good consistency between the d18O of the modern datasets and those of the West Runton assemblage. Realistically, when analytical uncertainties are considered, the descriptive statistics of the modern dataset and the Davies et al. (2000) dataset are indistinguishable. The implication of these datasets is that summer temperature conditions in modern-day eastern England and during the accumulation of the Freshwater Bed at West Runton are indistinguishable from one another. This interpretation is consistent with the large body of biological indicators that are found within this unit. Coleopteran assemblages from the Freshwater Bed provide MCR temperatures for the warmest month of 16–19  C and for the coldest month of  3 to þ 5  C (Coope, in press), which is consistent with modern-day July (16–17  C) and January (3–4  C) temperatures. Despite over 100 years of study, there is no evidence from the West Runton Freshwater Bed of exotic taxa which would indicate climates any warmer than those of the present day (Parfitt, 2008).

proxy data from each site. These studies provide two important conclusions. First, the earliest known evidence for humans in Europe north of the Alps is found in association with evidence for climates that were significantly warmer than the present day (and under climates characterised by a strongly seasonal precipitation regime). Second, these two sites show that interglacials of the Cromerian Complex contain both isotopic and biological evidence for climates that were as warm as or warmer than the present day. This is in strong contrast with records such as EPICA and SPECMAP which suggest that interglacials of the early Middle Pleistocene were significantly cooler than the Holocene.

3.6. Carbonates from the Hoxnian Interglacial Sediments at a large number of Hoxnian sites also contain carbonate deposits. These precipitates are from a variety of origins including soil, groundwater, tufaceous and lacustrine. Carbonates were sampled from five different sites within eastern and southeastern England (Fig. 3.2): pedogenic carbonates from Swanscombe (Kemp, 1985) and Elveden (Ashton et al., 2005), groundwater and tufaceous carbonates from Clacton (Bridgland et al., 1999) and West Stow (Preece et al., 2007) and lacustrine carbonates from Marks Tey (Turner, 1970; Rowe et al., 1999). These carbonates all occur at the same broad stratigraphic position within the Hoxnian (pollen zone HoII to HoIII) (Candy, 2009). This means that the isotopic composition of all five carbonate horizons will correspond to the same time period and, therefore, the environmental records from each site can be put together to form a regional climatic reconstruction.

3.6.1. Pedogenic Carbonates from Swanscombe and Elveden Carbonate-cemented nodules and rhizoliths have been recorded from Hoxnian palaeosols at Swanscombe and Elveden (Kemp, 1985; Ashton et al., 2005). These features are relatively small with maximum rhizolith length and maximum nodule diameter of ca. 10 mm. Candy (2009) suggested that they are consistent with calcic features that are currently forming in the lower calcareous horizons of modern British palaeosols. They do not reflect calcrete-style pedogenesis and, therefore, do not reflect any major shift in soil moisture budget (Candy, 2009). The soil carbonates from Swanscombe have a mean d18O value of  5.16% (1s ¼ 0.40) and a mean d13C of  9.56 (1s ¼ 0.12). The soil carbonates from Elveden have a mean d18O value of  4.32% (1s ¼ 0.17) and a mean d13C of  7.56 (1s ¼ 1.50).

3.5.3. Summary of Cromerian Complex Climates Stable isotopic studies indicate that at Pakefield Cromerian Complex climates were several degrees warmer than the present day, and that at West Runton they were similar to the present day. These isotope-based reconstructions are supported by the detailed biological

3.6.2. Groundwater Carbonates from Clacton and West Stow Interpreting the palaeoenvironmental significance of groundwater carbonates is not straightforward, primarily

Palaeoenvironments of Ancient Humans in Britain because it is difficult to distinguish between those that have formed contemporaneously with the interglacial deposits within which they are found, and those that developed perhaps several hundred thousand years later. At West Stow, the groundwater carbonates occur in a conformable relationship with a Hoxnian tufa unit and have been reworked into overlying cold climate slope deposits (Preece et al., 2007; Candy, 2009). It is, therefore, considered reasonable that these precipitates are Hoxnian in age. Less confidence can be placed upon the stratigraphic context of the precipitates at Clacton; they are tentatively placed in the Hoxnian but this correlation is not robust. The tufaceous groundwater carbonates from West Stow have a mean d18O value of  4.67% (1s ¼ 0.20) and a mean d13C of  8.81% (1s ¼ 0.58). The groundwater carbonates from Clacton have a mean d18O value of  4.90% (1s ¼ 0.07) and a mean d13C of  8.97% (1s ¼ 0.33).

3.6.3. Lacustrine Carbonates at Marks Tey The site of Marks Tey contains a thick sequence of lacustrine clays deposited from the end of MIS 12 (the Anglian Stage) through a large part of MIS 11 (the Hoxnian) (Turner, 1970; Rowe et al., 1999). For a large part of this sequence the lacustrine sediments are finely laminated, these sedimentary structures being interpreted as tripartite varves. Each varve triplet is composed of a diatom bloom lamina (spring), a carbonate precipitate lamina (summer) and a detritus lamina (autumn/winter). The carbonates are precipitated during summer months as a result of increased photosynthesis, the removal of CO2 from the water column and the consequent change in water pH. Thin section analysis of these materials indicates that shell fragments and ostracods, which may complicate the isotopic signal of the lacustrine carbonates, are absent. The lacustrine carbonates from Marks Tey have a mean d18O value of  4.37% (1s ¼ 0.40) and a mean d13C of 0.62% (1s ¼ 0.39).

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3.6.4. Significance of the Stable Isotopic Composition of Hoxnian Carbonates The stable isotopic composition of all Hoxnian carbonates are shown in Fig. 3.5. The pedogenic and groundwater carbonates from Hoxnian sites have d13C values consistent with precipitation in association with soil and groundwaters that are dominated by soil-respired CO2 and are, consequently, strongly depleted in d13C (Cerling et al., 1989; Candy, 2009). The lacustrine carbonates from Marks Tey are relatively enriched in d13C. This is not uncommon in lake waters which have long residence times allowing the water to equilibrate with atmospheric CO2 generating a ‘heavier’ isotopic signal (Leng and Marshall, 2004). The d18O values of the Hoxnian carbonates are best seen in the context of data from modern and Cromerian (Pakefield) carbonates (Candy et al., 2006; Candy, 2009) (Fig. 3.6). The modern soil carbonates provide an indication of the d18O composition of soil precipitates forming under current temperature/climatic regimes, while the Pakefield carbonates provide an indication of the d18O composition of soil carbonates forming under climates several degrees warmer than the present day. What is noticeable about this comparison is that the Hoxnian dataset is more enriched in d18O than modern day soil carbonates but not as strongly enriched in d18O as the Cromerian deposits. The most basic interpretation of this is that the temperature regime of the Hoxnian may have been slightly warmer than the present day but not as warm as the Mediterranean-style climate of the Cromerian interglacial represented by the deposits at Pakefield. Interpreting the d18O values of the Marks Tey carbonates is more difficult as currently no convincing modern analogue exists. Candy (2009) suggested that such an interpretation is relatively consistent with palaeoenvironmental records of biological proxies from Hoxnian sites elsewhere in England (see Preece et al., 2007, for an in-depth discussion). Although occasional thermophilous species occur in these deposits, such as the European pond tortoise,

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Fig. 3.6. A comparison of the stable isotope composition of modern, Cromerian and Hoxnian carbonates (from Candy et al., 2006; Candy, 2009). Emys orbicularis, which requires summer temperatures of > 18  C to successfully incubate its eggs (Stuart, 1979), there is an absence of the diverse range of exotic taxa that characterise ‘warm’ interglacials such as the Ipswichian (MIS 5e) (Coope, 1974; Keen et al., 1999), and the Cromerian Complex, at sites such as Pakefield (Parfitt et al., 2005), Sugworth (Gibbard and Pettit, 1978; Osborne, 1979) and Broomfield (Gibbard et al., 1996). Many authors have discussed whether there is evidence for the Hoxnian being characterised by a ‘wetter’ climate than the Holocene, but stable isotopic data in a temperate mid-latitude region cannot provide evidence for palaeo-precipitation levels.

3.6.5. Summary of Palaeoclimates of the Hoxnian Two major conclusions can be derived from the stable isotopic analysis of Hoxnian-age carbonates. The d18O composition of Hoxnian carbonates is compared with that of modern soil carbonates and carbonates formed under interglacials that were significantly warmer than the present day. This comparison indicates that climates during the Hoxnian may have been warmer than the present but not exceptionally warm by the standards of other interglacials. Human occupation during this interglacial, therefore, occurred during climates that were broadly comparable to those experienced during the Holocene. The isotopic studies show no evidence for the exceptional warmth that has been claimed for MIS 11 from a range of proxy evidence. This is consistent with other evidence from northwest and western Europe.

3.7. Summary and Conclusions In the past, oxygen and carbon isotope studies have contributed relatively little to our understanding of British Pleistocene interglacial palaeoenvironments. It is hoped that the isotope work carried out during the course of the AHOB project has highlighted two main points for

future isotope-based research. First, that British Pleistocene deposits contain a wide range of carbonates, both terrestrial and freshwater, that can act as the basis for stable-isotope studies. Secondly, that the stable isotopic composition of these carbonates, particularly d18O, can provide important information on past temperature regimes. In particular, stable isotopes are an important tool when they can be used as part of multi-proxy environmental reconstruction, combining isotopic, biological and sedimentary proxies of past climates. It is hoped that the isotopic studies carried out as part of the AHOB project will act as the basis for future research which focuses on using stable isotopes to further understand interglacial climates. In particular, two key avenues of research are identified: the isotopic record of the last interglacial, and the use of the d18O composition of freshwater shells as a palaeotemperature tool. Deposits of the Ipswichian (MIS 5e) are rich in exotic taxa and it is generally accepted that this interglacial episode was several degrees warmer than the Holocene. If this is the case then the carbonates that formed during this period should have a distinctive isotopic signal (possibly comparable to that of sites like Pakefield). During the AHOB project little isotopic work was carried out on deposits of this period, primarily because no associated archaeology is known to exist in Britain. However, given the importance of understanding the way that landscapes, ecosystems and hydrology operate in Britain under warmer climates, a detailed investigation of MIS 5e carbonates could be highly significant. The potential of using the isotopic composition of freshwater shells as palaeotemperature indicators has been highlighted by the study of modern shells and the detailed work on fossil shells from West Runton. The advantage of studying the d18O composition of shells is that they appear to provide an indication of summer temperatures. Furthermore, even poorly preserved or relatively small interglacial deposits may contain enough shells to carry out a detailed isotopic study, even though they may not preserve sufficient pollen, beetles or vertebrates to generate other palaeoenvironmental

Palaeoenvironments of Ancient Humans in Britain information. Future studies need to focus on the isotopic composition of shells from interglacial deposits with well-constrained summer temperature reconstructions in order to understand how the d18O composition of fossil shells varies with past temperature regimes.

Acknowledgements The authors would like to thank the large number of researchers who supplied carbonates from archived material, including David Keen, Richard Preece, Charles Turner, Nick Ashton and Simon Parfitt. Dave Lowry of the Department of Earth Sciences (RHUL) is gratefully thanked for running large numbers of samples over the years, while Adrian Palmer is also thanked for the production of a large numbers of thin sections as part of this work. We also thank Dave Horne and Ian Boomer for constructive comments on the text. This chapter is a contribution to the Ancient Human Occupation of Britain Project funded by the Leverhulme Trust.

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