the mid-holocene climatic transition in the mediterranean: causes and consequences

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2011 21: 3The HoloceneN. Roberts, D. Brayshaw, C. Kuzucuoglu, R. Perez and L. Sadori

The mid-Holocene climatic transition in the Mediterranean: Causes and consequences

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The mid-Holocene climatic transition in the Mediterranean: Causes and consequences

N. Roberts,1 D. Brayshaw,2 C. Kuzucuoglu,3 R. Perez4 and L. Sadori5

AbstractIn the Mediterranean there is often no clear time gap separating an early-Holocene period of nature-dominated environmental change from a human-dominated late-Holocene one. This mid-Holocene ‘mélange’ has been the subject of debates that have often been polarised between support for climatic causation and those favouring anthropogenic explanations for changes in vegetation, river flooding, wildfire regimes, etc. One way to shed light on the causes of mid-Holocene landscape changes is to focus on natural archives, such as lake and cave isotopes, the records of which can be unambiguously attributed to climatic forcing. These primary climate proxies can then be compared and contrasted with secondary or response variables, such as pollen and microcharcoal data, which can be the product of either climate changes or human activity or both, ideally using a multiproxy approach. In addition, synthesised primary palaeoclimate data can be compared with regional-scale climate modelling simulations. Both model output and proxy data suggest an east–west division in Mediterranean climate history. They indicate that the eastern Mediterranean experienced an increase in winter-season precipitation during the early Holocene, followed by an oscillatory decline after ~6 ka BP. In western parts of the Mediterranean, early-Holocene changes in precipitation were smaller in magnitude and less coherent spatially, and maximum increases occurred during the mid Holocene, around 6–3 ka BP, before declining to present-day values. Coincident with and partly stimulated by these climatic changes, complex societies developed across the Mediterranean, particularly in eastern parts of the basin during the Bronze Age. In consequence, by the mid-first millennium bc, human-induced land cover conversion, a drier and more variable climate, and changed fire regimes combined to establish typical sclerophyllous vegetation and landscapes in the circum-Mediterranean region.

Keywordsdata:model comparison, Holocene, Mediterranean, multiproxy, palaeoclimate

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Introduction

In the last two decades an important boost has been given to research investigating Holocene climate change and human exploitation of the environment in the circum-Mediterranean region. This research has been carried out on terrestrial and marine sediment records, natural archives of past climatic changes and human activity. Many Mediterranean pollen records show substantial differences between early- and late-Holocene vegeta-tion development, suggesting a general evolution from wetter to drier climatic conditions (e.g. Sadori and Narcisi, 2001). These have often been interpreted as reflecting aridification phases lead-ing to the establishment of the present-day Mediterranean climate (Jalut et al., 2000). According to an alternative interpretation, the expansion of sclerophyllous Mediterranean woodland since the mid Holocene was instead linked primarily to human impact increasing from the early Neolithic onwards (Pons and Quézel, 1988; Reille and Pons, 1992). Changes in biomass burning, as reconstructed from microcharcoals, contributed to this landscape transformation and were associated in some cases with forest clearance by fire which may have favoured Mediterranean taxa. Research on Holocene environmental changes in the Mediterra-nean area has therefore led to divergent interpretations in terms of natural versus anthropogenic factors (Roberts et al., 2004). These debates concerning environmental changes in the Mediterranean during the mid Holocene have often become polarised between

views supporting climatic causation and those favouring anthro-pogenic explanations.

One way to shed greater light on the causes of mid-Holocene landscape changes is firstly to use those natural archives the records of which can be unambiguously attributed to climatic forcing. In many parts of the Mediterranean, lake-level, lake iso-tope, cave speleothem and deep-sea sedimentary records (e.g. Bar-Matthews et al., 1997; Magny et al., 2002; Roberts et al., 2008) independently suggest a distinction between an earlier part of the Holocene characterised by moister conditions than at pres-ent, favourable to temperate deciduous trees, and a later period with a drier climate particularly in summer, favourable to

1University of Plymouth, UK2University of Reading, UK3CNRS-Paris 1 University, France4Universitat Autònoma de Barcelona, Spain5Sapienza Università di Roma, Italy

Received 26 August 2010; revised manuscript accepted 30 September 2010

Corresponding author:N. Roberts, School of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UKEmail: [email protected]

Holocene Special Issue

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4 The Holocene 21(1)

evergreen sclerophyllous trees and shrubs. In addition, in a few areas such as central and northern Italy, glacier extensions and contractions provide important evidence of Holocene temperature fluctuations (see Giraudi et al., 2011, this issue). These primary climate proxies can be compared and contrasted with secondary or response variables, such as pollen data and geomorphological evidence of river incision and alluviation, which can be the prod-uct of either climate changes or human activity, or a combination of the two, along with additional factors such as ecological dynamics. In some cases, both primary and secondary proxies can be derived from the same sedimentary archive via the multiproxy approach. This avoids the need for intersite correlation, and allows the temporal relationship between potential causes and effects to be tested rigorously. In addition, there are important questions concerning the impact of past climatic change on human societies (e.g. Rosen, 2007), such as the proposed 4.2 ka BP drought (Dalfes et al., 1997; Kuzucuoğlu and Marro, 2007). The best potential for investigating relationships between past societies and their wider environments comes from combining continuous records of proxy vegetation and climate change (e.g. from lake cores) obtained as close as possible to excavated archaeological sites.

The papers in this Special Issue present a synthesis and criti-cal evaluation of the mid-Holocene environmental transforma-tion of the Mediterranean, along with its effects on vegetation and on human societies. The papers focus primarily on the period from 8 to 2.5 ka BP; that is, after terrestrial and aquatic systems had stabilised following the last glacial–interglacial transition, but prior to the major increase in human impact dur-ing the late Holocene. Modern global ice volume and sea levels were in place soon after 8 ka BP, and greenhouse-gas concentra-tions were similar to those of pre-industrial levels; but precession-driven seasonal changes in solar radiation were significantly different from today.

Originally prompted by discussions and presentations at a workshop meeting held at Baeza, Spain, in September 2008, these papers comprise principally of multiauthored syntheses around key themes. They include regional summaries of pollen records covering the mid-Holocene climatic transition in Iberia (Perez et al.) and in Italy and the Balkans (Sadori et al.), and of geological records from glacier fluctuations, lake levels and isotopes in Italy (Giraudi et al.). For the eastern Mediterranean Roberts et al. syn-thesise palaeoclimatic records from lake and cave stable isotopes, pollen evidence of vegetation and land cover change, and archae-ological evidence of cultural adaptations to mid-Holocene varia-tions in climate. Along with these four synthetic studies covering the western, central and eastern Mediterranean, this issue includes a number of papers that analyse past environmental changes across the whole region. They include regional-scale numerical palaeoclimate modelling experiments by Brayshaw et al., a synthesis of wildfire history inferred from micro-charcoal records (Vannière et al.), and a summary of Mediterrananean tephrochronology during the Holocene by Zanchetta et al. Tephra layers provide one of the most powerful ways to improve site-based and regional-scale dating of past climatic changes in the central and eastern Mediterranean, using distal volcanic ash deposits ejected during major explosive eruptions.

In addition to these regional syntheses, the issue includes several site-specific studies. Bar-Matthews and Ayalon pro-vide new ultra-high resolution speleothem data from Soreq Cave in Israel spanning the time period between 7 and 4 ka BP,

while Kuzucuoğlu et al. present a study of climate change for the last 6 ka inferred from sediment cores from Tecer lake in central Turkey. Pèlachs et al. use the pollen and organic matter content from Burg lake in the Spanish Pyrenees to infer links to North Atlantic Bond cycles, while Peyron et al. use statisti-cal methods to calibrate pollen data from lake Accesa in cen-tral Italy and Tenaghi Philippon in northern Greece in terms of seasonal climatic parameters. Finally, Mercuri et al. present a series of case studies examining the relationships between mid-Holocene environmental changes and human cultural adaptations, via on-site and off-site records at a number of archaeological sites.

Models of Holocene landscape changeThe Holocene has witnessed a switch from a nature-dominated to a human-dominated Earth system (Messerli et al., 2000). Nat-ural and human agencies can both be causes of landscape distur-bance and instability, linked to decreases in forest cover, increases in soil erosion, etc., which in turn are reflected in proxy data such as decreased arboreal pollen and increased sedi-ment flux. In regions like the temperate forest zone of northwest Europe, identifying the switch from predominantly natural to human-regulated regimes is aided by the fact that there is a clear separation in time between the two principal periods of Holo-cene landscape disturbance (Figure 1a). The opening millennia of the Holocene were a period of dynamic landscape change, with open vegetation giving way to pioneer woodlands (birch, pine, etc.) and then full deciduous forest. Similarly, geomorpho-logical regimes were actively adjusting to the new climate, veg-etation cover and river base-level. These active landscape changes are clearly visible in pollen diagrams and fluvial-lake sedimentary archives, reflecting adjustment to the glacial– interglacial shift in climate. The other principal period of land-scape instability in humid temperate regions of Europe has occurred during the second half of the Holocene, and is associ-ated with increasing human impact, particularly linked to forest clearance for agriculture. The gradual reduction in primary forest cover and increase in arable and grazing land during the last six millennia is evidenced by changes in the ratio of arboreal to non-arboreal pollen (AP/NAP), and has led to significant increases in soil erosion, reflected in increased sediment flux in fluvial and lake records (Dearing, 1994). Between these two phases is a clear time gap during the early–mid Holocene when mesocratic forests were fully developed and most lowland landscapes were geomorphologically stable. These primaeval wildwoods are con-sidered to represent the ‘pre-disturbance’ ecological baseline condition for these regions and often provide the target state for environmental management and habitat restoration.

The application of this tripartite temperate-zone model of Holocene landscape development to circum-Mediterranean regions is less straightforward and more contentious for a number of reasons. First, agriculture began in the eastern Mediterranean region around 10 ka BP and spread to most areas of the Mediter-ranean within the following three millennia. In consequence, human transformation of land cover around the Mediterranean began significantly earlier than in northern Europe, and the pre-farming baseline condition has to be moved back in time to the early Holocene. Second, many Mediterranean ecosystems are intrinsically adapted to disturbance, linked to seasonal drought



Roberts et al. 5

and wildfire. In consequence, as in tropical savannas, the idea of a climatic climax vegetation may not be meaningful here (Meadows, 1999). Third, whereas in northwest Europe the last major change in climate occurred at the onset of the Holocene, in many other world regions a major shift in climate took place during the mid Holocene, linked to changes in precessional forcing, and it is now clear that they include the circum-Mediterranean lands. In combination, these facts mean that there is often no clear time gap during separating an early-Holocene period of nature-dominated environmental processes from a human-dominated late-Holocene one (Figure 1, which also uses pollen and microcharcoal data from Sicily to illustrate Holocene changes in Mediterranean land-scape disturbance). The longer timespan of human impact, the intrinsic role of disturbance regimes and a significant mid-postglacial climatic re-organisation have together created what might be described as the mid-Holocene Mediterranean ‘mélange’. Particularly for the period between about 6.5 and 3.0 ka BP, it is hard to distinguish the role of human actions from that of climatic change in Mediterranean environmental processes such as river hydrology and sediment transport (e.g. Kuzucuoğlu et al., 2004 and Wilkinson, 1999; cf. Thorndycraft and Benito, 2006 and Zielhofer et al., 2008).

In reality, many Mediterranean landscapes are the result of synergistic – and in some cases contingent – relationships between people, other animals, plants, climate and other compo-nents of nature. Attempts to explain mid-Holocene landscape changes in terms of purely natural or purely anthropogenic causes alone may therefore be misplaced. The issue is not whether people altered Mediterranean ecosystems during the mid Holocene, but rather at what point this impact became detectable, and what form it took. The papers in this Special Issue attempt to unravel the mid-Holocene Mediterranean ‘mélange’ in a number of ways; by comparing computer model predictions with empirical palaeoclimate data; by using the mul-tiproxy approach to compare primary and secondary climate proxies within the same record; and by using tephrochronology to provide robust time correlations between sequences, and hence test potential cause and effect relationships.

Mediterranean geomorphology, climate and vegetation

The region covered in this Special Issue includes the circum-Mediterranean lands along with those areas of southwest Asia characterised today by a summer-dry Cs-type climate. The geol-ogy and topography of the Mediterranean region has resulted from the northward movement of the African and Arabian tec-tonic plates during the late Cainozoic, and the consequent closure of the Tethys seaway. Orogenesis occurred mainly on the northern side of this collision zone, creating uplifted plateaux and young mountain zones from Iberia and Morocco in the west through Italy, the Balkans, Greece, Anatolia into the Zagros mountains of western Iran (Mather, 2009). This sector also experiences the highest seismic activity anywhere in the Mediterranean basin, and includes areas of active Holocene volcanism in central and southern Italy, the southern Aegean Sea (Thera/Santorini), central Anatolia (Cappadocia) and eastern Anatolia (Van) (see Zanchetta et al., 2011, this issue). Extending into the Levant is the northern-most extension of the Red Sea–East African Rift system, occupied by the Jordan and Ghab valleys and the actively subsiding Dead Sea basin with its hypersaline terminal lake.

During the late-Miocene Salinity Crisis, a dramatic fall in the base level of the Mediterranean Sea led to major river incision and enlargement of the catchment areas draining into the then closed sea. For example, the down-cutting and headward exten-sion of the Nile transformed it into one of the world’s longest rivers, whose flood is now controlled not by winter Mediterra-nean precipitation but by summer rainfall over the Ethiopian Highlands. The Nile Flood, so critical to the development of Ancient Egyptian civilisation, has varied greatly in magnitude during the Holocene, linked to the changing extent and intensity of the Indian Ocean monsoon circulation. Changes in Nile river discharge have also played a key role in altering the hydrography, salinity and circulation within the Levantine and Ionian Sea basins. Deep-sea sediment cores from the eastern Mediterranean include organic-rich sapropel layers, of which the youngest (S1) formed during the early Holocene (Calvert and Fontugne, 2001).

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Figure 1. (a) Schematic models of Holocene landscape disturbance for the northwest European temperate deciduous forest zone, and for the Mediterranean; (b) Lago di Pergusa, Sicily; non-arboreal pollen (NAP%) and microcharcoal/AP ratio (50–125 μm fraction) (from Sadori and Giardini, 2007; Sadori and Narcisi, 2001)




Increased freshwater influx at this time from the Nile and other sources reduced vertical mixing of water masses and led to marine anoxia. Thus, marine sedimentary records provide clear evidence for a contrast between early- and later-Holocene oceanographic conditions associated with sapropel formation.

A similar state of anoxia exists today in the Black Sea. This deep basin is connected to the Mediterranean (via the Sea of Marmara and the Dardanelles) through the narrow Bosphorus Straight. Even at the present-day, there is only partial mixing of waters between the Mediterranean and Black Seas, so that the latter has a salinity about half of full marine values. Early in the Holocene, global sea levels still lay below the Bosphorus sill threshold (or at least below the level needed for exchange of water masses), and the Black Sea was a freshwater lake. Although Ryan et al. (1997) proposed a spectacu-lar early-Holocene ‘Black Sea Flood’ event, it seems more likely that the initial inflow of Mediterranean waters occurred gradually at around 9400 cal. BP rather than abruptly at a later date (Giosan et al., 2009). Thus the hydrography of both the east Mediterranean and Black Seas have altered substantially during the Holocene.

The modern climate of the Mediterranean is summarised by Harding et al. (2009). It is characterised by autumn–winter–spring season precipitation of cyclonic origin and summer drought (Köppen type Cs). There is an overall precipitation gradient from north to south, with much of North Africa and parts of southeast Spain experiencing low rainfall levels. In Libya and Egypt, the climate of the Sahara desert reaches the coast, and arid climates also occur to the east in the Negev, Sinai and Syrian deserts. Cs-type climates can be divided into eu-mediterranean near to the coast, oro-mediterranean at higher elevations, and continental mediterranean in drier interior regions, such as the Anatolian and Iberian plateaux. Interior areas of southwest Asia can come under the influence of the Siberian High Pressure system in winter, bringing very cold and sometimes snowy weather conditions. Some parts of the Mediterranean, such as the southern flank of the Black Sea, receive summer precipitation and support summer-green forests, while a few of the highest mountains support small glaciers, for example, the Calderone Glacier in the Italian Apen-nines (see Giraudi et al., 2011, this issue).

Winter-season precipitation ultimately derives from North Atlantic sources, but depression tracks often cross southeastwards from northern Europe, especially into the Black Sea. There is also very significant cyclogenesis within the Mediterranean Sea, for example, creating the Genoa and Cyprus low-pressure systems. It is partly because of this that moisture-bearing depressions extend much further inland compared with other Cs-type climate regions of the world, reaching as far east as the Zagros mountains of western Iran. Much of the atmospheric water vapour over the region in fact derives from evaporation of Mediterranean and Black Sea water masses, and is likely to have been affected by regional changes in marine SSTs during the Holocene. The Mediterranean is subject to the effects of the NAO, notably in western parts of the Mediterra-nean. In the east, precipitation and pressure over the southern Levant and Egypt show an inverse correlation with the NAO index – the so-called Mediterranean see-saw (Oldfield and Thompson, 2004). The summer months are dominated by high pressure as the descend-ing limb of the Hadley Cell circulation moves northward from the Sahara to encompass the Mediterranean. The region is not affected directly today by summer monsoon rainfall, but the tropical mon-soons bear upon it indirectly, both through the Nile Flood and through the creation of a northeasterly air flow across the eastern part of the region, circulating around the South Asian Low Pressure centre. Today there is consequently no significant spatial overlap between the zones of winter (cyclonic) and summer (monsoonal) precipitation, unlike, for example, equivalent latitudes in southwest USA and northeast Mexico. However, the Indian and African Mon-soon circulations did extend further north than at present during the early Holocene, and their influence upon the Mediterranean may have been more important at that time (Tzedakis, 2007).

The vegetation zones of the Mediterranean broadly corre-spond to the climatic zones, ranging from summer-green forest in parts of central and northern Italy, through summer-dry scrub and woodland on Mediterranean island ecosystems, to desert vegeta-tion in the south of the region (Zohary, 1973). Typically Mediter-ranean vegetation (Figure 2) is dominated by evergreen shrubs and sclerophyllous trees adapted to the distinctive climatic regime of summer drought and cool wet winters with only sporadic

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Mediterranean zonePollen sites switching pft (6-0 Ka BP)

Figure 2. Modern vegetation zones along with pollen sites that show a shift in inferred plant functional type from temperate mixed forest to xerophytic woodland/scrub between 6 ka and present, reflecting the mid- to late-Holocene extension of sclerophytic vegetation (partly based on Roberts et al., 2004)



Roberts et al. 7

periods of frost. The most favoured time for vegetative growth is spring, when the soil is moist and the temperatures are rising, or autumn, after the first rains. Fire is a regular part of Mediterranean-type ecosystems. The extant flora of the region is a complex mixture of elements contributing to its present-day biodiversity, being a melting pot at the crossroads of quite different biogeo-graphical areas, such as the Euro-Siberian and Irano-Turanian domains. Nowadays, Afro-Tropical elements are less widespread in the basin than in the past. Species present in the Mediterranean area originated in almost all known biogeographic realms of the world and the present-day biological complexity and diversity can be explained looking at the fossil record of the last few mil-lion years (e.g. Bertini, 2010; Sadori et al., 2010b; Suc, 1984), showing that continued species turnover was the rule in mediter-ranean biota (Blondel and Aronson, 1999).

The distribution of olive (Olea europaea) and holm oak (Quercus ilex) have sometimes been selected as bio-indicatorsto define the limits of mediterranean-type vegetation. An early example of this use dates back to Pliny the Elder, who was prob-ably the first to define the limits of the Mediterranean using the area of cultivation of olive tree. Another approach to delimiting the Mediterranean area has been to use the presence of plant ‘associations’ including holm or other evergreen oaks (Emberger, 1930). However, a broad range of plants, besides the evergreen sclerophyllous trees and shrubs occurs at different altitudes in the Mediterranean, even within the evergreen formations. A clear example of this variability and mixture is the lowland deciduous oak forests (with an important presence of evergreen trees) of some Italian coastal plains or conifer formations with pine, cypress and cedar that are typical of many Mediterranean coastal zones from the west to the east of the basin.

Not all of the region’s biomes possess pollen records of their vegetation history. For example, they are notably lacking for the arid landscapes of North Africa, and for major alluvial wetlands such as lowland Mesopotamia.

Archaeological periods in the Mediterranean regionThe Fertile Crescent of southwest Asia provided the birthplace of Neolithic farming societies, from where crops and domestic ani-mals spread throughout the Mediterranean basin during the first half of the Holocene. Before 9 ka BP, Neolithic societies devel-oped strategies for the production of domesticated grain (wheat and barley) and animals (sheep, goat, pig and cattle). During this period, small permanent farming settlements spread throughout the ‘Fertile Crescent’ (from the Dead Sea to northern Syria, Iraq and SW Iran). Neolithic farming then progressively spread west-wards, partly by demographic dispersal, partly via adoption of plant and animal domesticates by pre-existing Mesolithic popula-tions, to reach the western Mediterranean by ~7.5 ka BP (Figure 3). In the case of Mediterranean islands such as Crete and Cyprus, the Neolithic settlers were the first permanent human occupants, and they had dramatic impacts on indigenous island floras and faunas, either by introducing animal species from the continent and/or by provoking the extinction of endemic species (e.g. the goat-like deer Myotragus of the Balearics).

In the chronology of prehistoric societies in southwest Asia, the appearance of pottery for storage and cooking ~9 ka BP marks the partition between ‘Pre-Pottery’ and ‘Pottery’ Neolithic. Dur-ing the mid Holocene, production, craft and trade of metal goods

developed. Although long used to differentiate Neolithic sites from Chalcolithic ones, metal artefacts are no longer the main indicator of Chalcolithic societies as the transition from Neolithic to Chalcolithic is now defined as a gradual transformation of rural activities and ways of life towards increasing economic integra-tion and social complexity. This evolution involved wide territo-rial areas, as shown for example by the rapid spread of the Chalcolithic Halaf (sixth millennium bc) and Ubaid (fifth millen-nium bc) ceramic wares in the Near East. From 7.5 to 5 ka BP, Chalcolithic cultures in the Fertile Crescent increased their knowledge, control and social cooperation concerning water, plants, animals and soil resources, enabling higher agricultural production, including irrigation-based systems favouring crop cultivation in low-precipitation areas. This context favoured pop-ulation growth. During the fourth millennium bc, agricultural pro-duction techniques, social structures, policies, urban organisation, etc. developed in Egypt, the Levant and Mesopotamia, and Uruk cultural artefacts reached Anatolia via the Euphrates and Tigris river valleys. In this expansion, the role of metal production, working and trade was fundamental. A transition phase followed, characterised by urban development concomitant with increasing political and economic organisation.

Appearing in a timespan between 3700 and 3000 BC (5.6 to 5 ka BP), Final Chalcolithic/pre-early Bronze Age cities and villages developed over much of the eastern Mediterranean, most under the domination of more or less powerful local rulers. It is during the early Bronze Age (EBA) (third millennium bc) that the first literate societies emerged and that written historical documents started to become available in the eastern Mediterranean. During the third and second millennia bc, diverse Bronze Age cultures rose and expanded in this region, characterised by a strongly structured dynamism: urbanization, increasing city domination over rural territories, grow-ing social complexity, State development, centralized political power managing economic and population resources. The two millennia-long Bronze Age history of these complex societies was punctuated by century-scale political crises, some of them more or less synchronic between neighbouring regions. These regionally synchronic ‘crises’ correspond to the time breaks that divide Near Eastern Bronze Age chronology into three main cultural periods, the dating of which varies according to regions (Figure 3): EBA, middle Bronze Age (MBA) and late Bronze Age (LBA).

Towards the end of the second millennium bc, most LBA cul-tures in the Near East came to a brutal end. In ancient texts, the synchronic fall of the last LBA Kingdoms and Empires correspond to population movements and wars (e.g. the ‘Sea Peoples’). The following period of political disorder and economic disorganization lasted longer in some regions (e.g. Anatolia) than in others (e.g. Egypt). After this disruption, renewed Kingdoms and Empires rose again during the Iron Age (first millennium bc), expanding over territories in some cases wider than the previous Bronze Age ones.

In the central and western Mediterranean, cultural develop-ments proceeded at a slower pace after the cultural florescence of the Neolithic with its spectacular megalithic monuments, for example, on Malta. Although Chalcolithic-Eneolithic and Bronze Age cultures in Iberia and Italy show impressive evidence of trade and metallurgy, complex literate urban societies did not develop prior to the mid-first millennium bc, as they did further to the east. The appearance of complex urban societies in the central and western Mediterranean was linked primarily to the establishment of Greek and Phoenician trading colonies such as Massilia (Marseille).




Date DateBP Iberia N Africa Italy Balkans Aegean Anatolia Levant Egypt Mesopotamia BC

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(Uruk, Jamdat Nasr)

Figure 3. Summary Mediterranean archaeological periods by region, 9000–2300 BP



Roberts et al. 9

Figure 3 presents a summary of archaeological periods on a region by region basis between 9000 and 2300 BP (7000–300 bc). As this shows, cultural changes during the mid-Holocene envi-ronmental transition were strongly time-transgressive across the Mediterranean. Thus the Bronze Age started around 5.3 ka BP in the east, but began around 1000 years later (4.3 ka BP) in the west. However, by the end of the time period shown in Figure 3 (~2.3 ka BP) cultural changes were synchronised across the whole Mediterranean basin, associated with the wide spread of Greek and Phoenician colonies, and the rise of Rome and Carthage near the geographical centre of the Mediterranean basin. Based on this, it might be expected that the impact of agricultural and then com-plex State-based societies on the natural environment should also have been time-transgressive from east to west during the mid Holocene. In principle this hypothesis should be testable using proxy data such as pollen records. In practice human landscape disturbance was patchy, so that the timing of first deforestation varied locally as well as regionally across the Mediterranean, as several papers in this Special Issue demonstrate.

Understanding Mediterranean climate dynamics during the HoloceneThe global climate system has changed markedly over the Holo-cene period. Over the last ~12 ka, the seasonality of Northern Hemisphere insolation reduced with time associated with a sum-mer cooling trend during the same period (Renssen et al., 2009). In the tropics, this is consistent with a southward movement of the boreal summer ITCZ (e.g. Braconnot et al., 2007; Haug et al., 2001) and increased aridity over subtropical Africa and Asia. There is good evidence to indicate that changes in climate across the mid-Holocene transition were far from gradual in many regions. In particular, the southward shift of the boreal summer ITCZ and the weakening of the West African monsoon system, associated with the end of the African Humid period, appears to occur abruptly around 6 ka BP (DeMenocal et al., 2000; Renssen et al., 2003). Debret et al. (2009) further suggest that the dominant millennial-scale frequencies of climate variability may have changed during this mid-Holocene period, with the later period becoming increasingly dominated by the ‘internal’ variability of the coupled ocean–atmosphere system.

The climate history of the Mediterranean during the mid-Holocene transition bears a rather close overall resemblance to that in lower-latitude regions of Africa and Asia, particularly in the eastern part of the Mediterranean basin; that is, changing from wetter to drier conditions through time during the Holocene. In many ways this is a surprising conclusion, because the source of the enhanced early-Holocene precipitation in the eastern Medi-terranean was not from the tropics (Rohling and Hilgen, 1991; Tzedakis, 2007). The summer monsoon did extend much further north into the Saharan and Arabian deserts prior to ~6 ka BP (Gasse and Roberts, 2004), but its direct influence does not appear to have reached the Mediterranean coast (Arz et al., 2003). Fur-ther east, in Oman and the southern Zagros, and perhaps also in the Tibesti and Ahaggar highlands of the central Sahara, there is a possibility that the zones of cyclonic and monsoonal precipitation overlapped or even interacted during the early Holocene (Nicholson, 2000). However, around the shores of the Mediterranean Sea, higher early-Holocene rainfall would have been linked to changes in the behaviour and character of cyclonic depressions. At present

these are active primarily during the winter season, and one intriguing question is whether summer cyclonic rainfall was more common in the past. Changes in Mediterranean cyclogenesis would potentially have been influenced by lower SSTs and evapo-ration during the period of sapropel formation. Rohling and Hilgen (1991) proposed that summer cyclonic depressions would have lowered the excess of evaporation from the eastern Mediter-ranean relative to that from the western basin as they picked up additional moisture along their eastward path. On the other hand, climate modelling experiments by Brayshaw et al. (2011, this issue) found increased winter rainfall in the eastern Mediterra-nean under early-Holocene conditions but no indication of sum-mer precipitation. Those changes are consistent with a southward shift in the North Atlantic storm track and an intensification over the Mediterranean, although the associated atmospheric circulation changes are a subject of some debate in the literature (Brayshaw et al., 2010; Gladstone et al., 2005).

In contrast to previous palaeoclimate model simulations for the Mediterranean which mostly use coarse global-scale climate models, the regional-scale climate model study of Brayshaw et al. (2011, this issue) indicates significant changes in precipitation during the Holocene compared with present day. Significantly, the strongest fractional change in precipitation over the Mediter-ranean simulated by the model is an increase during the early–mid Holocene (6 ka plus 8 ka BP average) in the eastern part of the basin, centred on western-central Turkey, in good accord with pal-aeoclimate data (Figure 4d–f). By the mid–late Holocene (2 ka plus 4 ka BP average), modelled precipitation decreases in the eastern Mediterranean, but there are suggestions of an increase in the western part of the basin (Figure 4a–c). In addition, the model experiments exhibit significant spatial heterogeneity in moisture precipitation changes across the Mediterranean basin; in other words, rainfall changes were patchy in their distribution. This ‘patchiness’ is particularly marked in the western Mediterranean for the early- to mid-Holocene climate simulations (6 ka plus 8 ka BP). Spatial heterogeneity is likely to be related to the complex land–sea geography and orography of the Mediterranan basin, and it helps to explain why different sites within a region can have markedly different climate histories, for example, in the Italian peninsula.

Figures 4 and 5 show data-model intercomparisons in space and time, respectively, for the Holocene compared with present; model protocols and output are described in Brayshaw et al. (2011, this issue). Palaeoenvironmental data derive first from inferred changes in lake hydrology (isotopes, salinity, water level; based on Roberts et al., 2008 and studies in the present issue) and cave speleothems (isotopes). In addition these can be com-pared with pollen-inferred changes in plant functional type, in particular a switch from deciduous woodland to sclerophyllous woodland and scrub (Figure 2). These derive from pollen sites described in Roberts et al. (2004) supplemented by sequences reported in the current issue (Perez et al., Roberts et al., Sadori et al.). Encouragingly, there is a good overall correspondence between trends and patterns in model outputs and proxy climate data. For the early–mid Holocene (6 ka plus 8 ka BP), both model and data indicate an increase in precipitation in the eastern Medi-terranean, more or less in the same area as marine sapropel depo-sition. In the western Mediterranean both model and proxy data show that changes were smaller in amplitude and patchier in their geographical extent. Thus, for example, in central Italy, cave spe-leothem and lake level/isotope records do not show a coherent




pattern at this time (see Giraudi et al., 2011, this issue, for further discussion). By the mid–late Holocene, model and data indicate a different pattern. The stacked lake isotope record of inferred P-E indicates a decline after ~6 ka BP in the eastern Mediterranean, but at the same time in the west, lake isotope-inferred precipita-tion levels rise to reach maximum values between ~6 and ~2 ka BP. This is in good accord with model output (Figure 5) and also with pollen-derived climate reconstructions (e.g. Cheddadi et al., 1998), with the model appearing to show a see-saw effect between eastern and western Mediterranean during the mid-Holocene tran-sition. The modelled amplitude of Holocene precipitation change (<7%) is smaller than site-specific reconstructions which indicate >20% increase during the early Holocene (Jones et al., 2007). However, the latter assume constant temperatures, while the

former is averaged over large areas so is likely to underestimate the maximum amplitude of change. There are also some areas where data and model do not agree, such as western Iran, for which the numerical models indicate relative aridity during the early Holocene. Perhaps significantly, the pollen and lake isotope records in this region fail to agree, and their interpretation has been a matter of lively debate (summarised in Roberts et al., 2011, this issue).

Model and data display different degrees of temporal variability (Figure 5), but this is primarily because most computer model simu-lations are driven by long-term forcings (such as changes in seasonal insolation), not by shorter-term ones (such as volcanic eruptions) and this lack of multi-year variability is further compounded by the relatively simple ocean model used in the simulations. The models

50°N(e) Model Precipitation change (MH - PreIndustrial) Oct - May (%)

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–50 –20 –10 –5 –2 2 5 10 20 50

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–50 –20 –10 –5 –2 2 5 10 20 50

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50°N(d) Model Precipitation change (MH - PreIndustrial) Oct - May (mm day–1)

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50°N(a) Model Precipitation change (LH - PreIndustrial) Oct - May (mm day-1)

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–2 –1 –0.5 –0.25 –0.1 0 0.1 0.25 0.5 21 –2 –1 –0.5 –0.25 –0.1 0 0.1 0.25 0.5 21

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50°N(c) LH Isotope records

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50°N(f) MH Isotope records

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0° 10°E 20°E 30°E 40°E 50°EWetterNo changeDrier

Lakes Speleothems

Early Holocene sapropel zone

Figure 4. Holocene model:data intercomparison for the Mediterranean. (a–b) and (d–e) Precipitation differences from present (pre-industrial) using the regional numerical model downscaling of Brayshaw et al. (2011, this issue) October to May, precipitation units: mm/day, percentchange relative to present-day; (a) and (b) mean of experiments for 2 ka and 4 ka BP combined relative to present (LH, late Holocene); (d) and (e) mean of experiments for 6 ka and 8 ka BP combined relative to present (MH, mid Holocene). In (a) and (d) stippled areas shows regions where the changes are statistically significant at 90%. (c) and (f) show proxy climate data from cave and lake isotopes and lake hydrology for inferred changes in precipitation 2 to 4 ka BP (LH) and 6 to 8 ka BP (MH) compared with present-day (partly based on Roberts et al., 2008)



Roberts et al. 11

are therefore designed to capture multi-millennial, not centennial or decadal, trends. The higher temporal precision of palaeoclimate data imply that long-term trends in climate were oscillatory rather than gradual. Mayewski et al. (2004) and Wanner et al. (2008) examined Holocene climate variability and pointed out a number of periods of rapid climate change and major atmospheric circulation re-organisation. This certainly appears to be the case in the Mediterra-nean, as demonstrated by high-resolution studies from Soreq Cave, Israel, by Bar-Matthews and Ayalon (2011, this issue), which are consistent with a strong ~1500-yr periodicity to climatic oscillations.

ConclusionWhere does this leave the mid-Holocene Mediterranean ‘mélange’? Although many significant gaps remain unfilled, a synthesis of primary proxy climate data from lake and cave iso-tope records and regional-scale climate model simulations, sug-gests an east–west division in Mediterranean climate history. They indicate that the eastern Mediterranean experienced an increase in winter-season precipitation during the early Holocene, followed by an oscillatory decline after ~6 ka BP. In western parts of the Mediterranean, early-Holocene changes were smaller in magnitude and less coherent spatially, and maximum increases in precipitation occurred during the mid Holocene, around 6–3 ka

BP, before declining to present-day values. The dividing line between these two climate domains appears to run through the Balkans, southern Italy and – presumably – Tunisia, although good data are lacking from much of North Africa. Based on these climate reconstructions, the ‘mediterraneanization’ of terrestrial ecosystems might be expected to have been registered earlier in the eastern than in the western Mediterranean. In practice, as pol-len records show, the impact of regional-scale climate changes was superimposed on local conditions with differing levels of sensitivity to climate. For example, in well-watered parts of cen-tral Italy some areas of deciduous oak woodland remained intact even after the mid-Holocene transition to drier climatic condi-tions (see Sadori et al., 2011, this issue).

Coincident with and partly stimulated by these climatic changes, complex societies developed across the Mediterranean, particularly during the Bronze Age. The ‘mediterraneanization’ of landscape ecologies may have been initiated primarily by changed climatic regimes after ~6 ka BP, but during subsequent millennia, human-induced land cover conversion became an increasingly significant agent in landscape metamorphosis. By the mid-first millennium bc, increased human impact and a drier and more variable climate had combined to create typical sclero-phyllous vegetation and landscape ecosystems around much of the Mediterranean basin.

0 2 4 6

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. BP

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stacked lake isotope record stacked lake isotope record

modelled precipitation change (%)

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Figure 5. Temporal changes in inferred precipitation from model simulations of Brayshaw et al. (2011, this issue) and palaeoclimate data from the stacked and normalised lake isotope record (see Roberts et al., 2011, this ssue, for details). Western Mediterranean lake sites used are Sidi Ali and Tigalmamine (Morocco), Castiglione and Frassino (Italy) and Medina (Spain) (Roberts et al., 2008). For model simulations, western Mediterranean = 0–20°E and eastern Mediterranean = 20–40°E, in both cases 30–45°N. Numbers next to the model data points indicate the statistical likelihood precipitation being different to ‘pre-industrial’ conditions (1 = time period is statistically certain to be different from pre-industrial, 0 = time period is statistically certain to be the same as pre-industrial)




Acknowedgements

We are grateful to the Universidad Internacional de Andalucia, who supported an environment workshop meeeting at Baeza in 2008 on this theme. We would also like to thank Aidan Dodson, Tamar Hodos, Jamie Quinn, Roger Matthews, Nicoletta Momi-gliano, Brian Rogers, Alessandro Vanzetti and Carlos Fernandez Gonzalez for assistance.

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