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Oxygen isotopes as tracers of Mediterranean climate variability: An introduction
Global and Planetary Change 71 (2010) 135–140
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Global and Planetary Change j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g l o p l a c h a
Oxygen isotopes as tracers of Mediterranean climate variability: An introduction C. Neil Roberts a, Giovanni Zanchetta b, Matthew D. Jones c,⁎ a b c
School of Geography, Earth & Environmental Sciences, University of Plymouth, Plymouth, Devon PL4 8AA, UK Dipartimento di Scienze della Terra, University of Pisa, Via S. Maria 53, 56126 Pisa, Italy School of Geography, University of Nottingham, Nottingham, NG7 2RD, UK
a r t i c l e
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Article history: Accepted 22 January 2010 Available online 28 January 2010 Keywords: Mediterranean oxygen isotopes hydrology climate change
a b s t r a c t Oxygen isotopes provide one of the best tracers of the Mediterranean hydrological cycle from source areas in the oceans, through precipitation, to ground- and surface freshwaters. The ratio of oxygen isotopes preserved in sedimentary carbonates also provides a key indicator of natural climatic variability over timescales beyond those based on direct monitoring or historical observations. This set of papers, originally deriving from an ESF MedClivar workshop meeting in Pisa, Italy, critically considers how δ18O data can be used to provide records of Mediterranean climate variability over a hierarchy of timescales. They focus on five main themes, namely 1) water isotopes as tracers for monitoring and modelling patterns of precipitation, 2) lakes and surface waters, and their sedimentary records, 3) groundwaters and cave systems as preserved in speleothems, 4) marine systems, such as deep-sea sediments and surface corals, and 5) intercomparison of different archives and regional-scale data: model comparisons using isotope data bases. Future priorities include improved calibration of proxy-climate data via monitoring of contemporary systems and mass balance modelling, and a focus on synthesising high-resolution climate reconstructions during the last 2000 years. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Given predictions of future climate, changes in rainfall and water resources seem certain to have important socio-economic and political impacts in the Mediterranean region (Giorgi 2006). Understanding the variability of hydro-climate over different time scales is therefore an essential prerequisite for establishing predicted future climate change and its possible impact on human society (Mariotti et al., 2002). The Mediterranean region is both climate-sensitive and has an exceptionally long and rich history of human use, including some the world's most important past civilisations. However, the scale and longevity of human activity around the Mediterranean (e.g. deforestation, erosion) has created serious difficulties in distinguishing climate change from human impact in many proxy records of past environmental change (e.g. via pollen analysis; Roberts et al. 2004). In consequence, we know less about long-term climatic variability in the circum-Mediterranean region than we do about that in other parts of the world with much lower population densities and shorter historical time-depth. Because they vary directly with the physical processes of the hydrological cycle and are not compromised directly by human impact (e.g. pollution), oxygen isotope ratios provide an important hydro-climatic tracer at all time scales - instrumental, historical and geological. Water isotopes have long been used as key tracers in monitoring contemporary patterns of precipitation over the Mediter⁎ Corresponding author. E-mail addresses: [email protected] (C.N. Roberts), [email protected] (G. Zanchetta), [email protected] (M.D. Jones). 0921-8181/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2010.01.024
ranean, particularly in relation to source area characterisation and air mass trajectories (e.g. Rindsberger et al. 1983; Lykoudis and Argiriou 2007). The IAEA Coordinated Research Project on Isotopic Composition of Precipitation in the Mediterranean Basin helped to define the interactions between climatic conditions and isotopes in relation to overall changes in climate currently being experienced in the region (Goucy, 2005). Key indicators of natural climatic variability over timescales beyond those based on direct monitoring or historical observations come from the ratio of stable isotopes of oxygen preserved in materials such as cave carbonates, and lake and marine sediments. During the decade since Goodfriend (1999) reviewed available stable isotope evidence, well over 50 oxygen isotope records have been published from the Mediterranean basin and surrounds. This increasing spatial coverage allows regional syntheses to be made (e.g. Roberts et al., 2008) and helps us to improve the interpretation of past climatic and environmental changes. The collection of papers in this volume derives from a workshop meeting held at the University of Pisa (June–July 2008; http://www. geog.plymouth.ac.uk/research/groups/MedCLIVAR_isotope_workshop. html), with the European Science Foundation as the principal meeting sponsor, under its MedCLIVAR programme. MedCLIVAR is designed to investigate Mediterranean Climate Variability and Predictability, including the past evolution of climate, assessment of climate variability and the mechanisms responsible for it, and identifying trends and providing climate prediction in relation to future emission scenarios. In this introduction we summarise the current state of play regarding the understanding of oxygen isotope variability across the Mediterranean basin today and in the past, and look towards how
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future work may improve our understanding and begin to answer questions that still remain. 2. Water isotopes as tracers for monitoring and modelling patterns of precipitation The IAEA/WMO Global Network for Isotopes in Precipitation allows spatial and temporal patterns in the oxygen isotope values of the Mediterranean precipitation δ18Op to be observed. Weighted average δ18Op values for Mediterranean basin fit into the global gradient, becoming more negative polewards, from c. 4‰ along the south of the Mediterranean basin to around 8‰ in the north (Bowen and Wilkinson, 2002; Bowen and Revenaugh, 2003) (Fig. 1). Values are also more negative towards the east, especially over the high ground of the Anatolian and Iranian plateaux (Fig. 1). Deuterium excess values (d) have also long been recognised as being more positive in the Mediterranean, especially in the eastern basin (Gat and Carmi, 1970) where d = +22‰ compared to the global average of +10‰. Intermediate values can be found in Central Mediterranean (e.g. Longinelli and Selmo, 2003). Dotsika et al. (2010-this issue) observe elevated deuterium excess values in Greek rainfall derived from the Aegean Sea. On top of these large-scale trends are more local variables. The altitude effect, for example, varies across the basin (Goucy, 2005) with values between −0.26 and −0.37‰ per 100 m in Croatia and Slovenia and a range of −0.10 to −0.23‰ per 100 m in Lebanon. Differences have been attributed to a combination of air masses of different origins raining out at different locations and to varying degrees of rainout. Clear differences in isotope values of different air masses had previously been observed in Israel (Rindsberger et al., 1983). Celle-Jeanton et al. (2001, 2004) has provided evidences that single events that originated from the Atlantic Ocean are usually more negative than precipitation mainly originates over the western Mediterranean. Change in monthly values of precipitation over time have also been shown to vary with temperature, e.g. in Ankara (Dirican et al., 2005) and Madrid (Araguas-Araguas and Diaz Teijeiro, 2005). Dotsika et al. (2010-this issue) show that the distance of the observation station from the sea and the altitude are the main factors imprinted in the isotopic signature of precipitation in Greece. Studies from the eastern Mediterranean Sea area (Lykoudis et al., 2010-this issue) show how gridded isotopic data, calculated from empirical regression models along with geostatistical methods, can account for the major factors controlling the isotopic composition of precipitation in space
and time thereby improving the coverage of the previously available data sets. It is clear from the above that there are many controls on the δ18Op at any one site in the Mediterranean and a robust understanding of these controls under the current climatic regime is required before any evaluation can be made about how δ18Op values changed in the past. The detailed study of both monthly and daily rainfall data across the Mediterranean region by the IAEA (Goucy, 2005) helps greatly in this regard. With the increased understanding on the controls on δ18Op in the climate cycle, δ18O values for various stages of the water cycle can now be incorporated into earth system models (Schmidt et al., 2005). There is therefore the potential for direct comparison of proxy records and models of past climate scenarios. 3. Isotope records from lakes and tufas Lake isotope stratigraphies offer some of the most widely distributed terrestrial records of past climatic variability in the Mediterranean region. The δ18O signal in lake carbonates and tufas is a function of the lake water isotopic composition and the temperature of the water (Leng and Marshall 2004). The temperature-isotope relationship in lakes and tufas is affected both by direct temperature fractionation effects (ca. −0.24‰/°C for temperatures in the range of interest) and by changes in the δ18O of precipitation, the latter being linked to the isotopic composition of seawater in the vapour source region and to air mass trajectories, temperature and altitude (see discussions above). Lake water isotopic composition can also be modified from that of incoming meteoric precipitation by evaporative effects. In regions of negative hydrological balance many lakes lose water mainly via evaporation and this can cause large positive isotopic shifts in lake waters (Gat 1995). Mediterranean lakes lie across a spectrum that extends from fresh, short residence-time water bodies to saline, hydrologically-closed lake systems. The former have modern δ18O values similar to those of mean seasonally-weighted precipitation in their catchments, while the latter have been isotopically enriched through evaporation. δD vs δ18O cross-plots of modern waters show that only a small minority of Mediterranean lakes lie on the Mediterranean meteoric water line (MMWL; Roberts et al., 2008). Many more lakes lie off this on a local evaporation line, even in systems that are chemically dilute, and their stable isotope hydrology reflects primarily changes in lake water balance (P–E, residence time, etc). By contrast, because tufas form in
Fig. 1. Map of Mediterranean basin, showing interpolated mean weighted δ18O values of precipitation, the zone of eastern Mediterranean sapropel formation, and palaeo-isotope study sites presented in the papers in this volume; 1. Akçakale cave (Jex et al.), 2. Zemeno (Brasier et al.), 3. Grotta della Serratura (Colonese et al.), 4. Lake Pamvotis, Ioannina (Leng et al.), 5. Gölhisar (Leng et al.), 6. Pindal Cave (Moreno et al.), 7. Lake Butrint (Ariztegui et al.), 8. Piànico-Sèllere palaeolake (Mangili et al.), 9. Northern Red Sea corals (Felis and Rimbu), and 10 Nar lake (Leng et al).
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short residence time hydro-systems, these lotic carbonates may record changes in temperature and/or air mass history more faithfully (Andrews et al., 2000). Brasier et al. (2010-this issue) analyse 100 kaold banded freshwater tufas from Greece to show that only part of the full seasonal temperature range was captured, due to non-deposition in mid-winter and mid-summer. Because every lake has a unique isotope hydrology, each one has to be calibrated individually against local climatic parameters and historical instrumental meteorological data, which requires fine-resolution sediment analysis and good dating control. Alternatively or additionally, individual lake isotope hydrologies can be simulated by mass balance modelling (e.g. Jones et al. 2005; Steinman et al. in press). There are currently about 30 basins from around the Mediterranean Sea whose Quaternary sediment records have been analysed for stable isotopes. A few of these extend back to cover pre-Holocene interglacial periods, including Ioannina (Pamvotis) in Greece (Frogley et al., 1999) and the varved Pianico sequence from northern Italy dating to MIS 11 (Mangili et al., 2010-this issue). Like most lake isotope records from the Mediterranean, these derive from endogenic/authigenic carbonates. Leng et al. (2010-this issue) use analysis of paired endogenic and biogenic carbonate (mollusks, ostracods) data from two lakes, Ioannina and Gölhisar in southwest Turkey, along with electron microscopy, to test for detrital carbonate inwash; if undetected, this can contaminate bulk analyses. Ariztegui et al (this volume) describe a high resolution data set from a varved lake sequence over the last 300 years from Lake Butrint, Albania, which shows how complex the forcing mechanisms on lake isotope records can be, and how with careful investigation these different facings can be identified and understood. Roberts et al. (2008) synthesised δ18O data on lake carbonates for the time period since the Last Glacial Maximum in an attempt to identify regionally-coherent isotope trends caused by millennial-scale climatic fluctuations, as part of the collaborative ISOMED project (Lake Isotope Records from the Mediterranean). During MIS2, the δ18O composition of most Mediterranean lake waters was enriched relative to the Holocene, in part because of changes to the isotopic composition of Mediterranean Sea source waters (Kolodny et al. 2005). Maximum isotopic enrichment occurred during the Heinrich 1 cold event (16–17.5 ka) and at the time of the Younger Dryas stadial (11.4 to ∼ 13 ka), when several lakes show independent evidence of saline water conditions consistent with climatic aridity. Almost all Mediterranean lakes saw a shift to more depleted δ18O values during the MIS2-1 climatic transition. This is opposite to the isotopic trend in lakes from Central and Northern Europe over the same time period (e.g. von Grafenstein et al., 1999), but also characterized the MIS 6/5e glacial terminations in Greece (Frogley et al., 1999). This implies that temperature changes were relatively unimportant as a direct driver of Mediterranean lake isotopic records over glacial-interglacial timescales. 4. Speleothem isotope records Carbonate cave deposits provide an excellent source of material for efforts to reconstruct past climate condition on land, which can be accurately and precisely dated by mean of U/Th technique. The latter property is fundamental in solving chronological problems about the climate events and their relation with external forcing. Since the fundamental contribution from southern Levantine caves performed by Bar-Matthews et al. (1996, 1999, 2000) in recent years, stable isotope studies on speleothems in the Mediterranean area have been increasing exponentially (e.g. Frisia et al., 2005, 2006; Drysdale et al., 2006). The ability of the cave systems to record the climate signal with stable isotopes is highly variable and detailed studies on local hydrology and climate condition are necessary (Baker and Bradley, 2010-this issue, Mattey et al., 2008) in order to disentangle the climate signal from in-cave isotope fractionation. In the best cases it is
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possible to reach sub-annual resolution, even if Mediterranean records with this resolution are so far restricted to the last few thousand years (Orland et al., 2009). During the Holocene a strong similarity in the general δ18O trend has been observed between western and eastern Mediterranean at Corchia and Soreq caves (BarMatthews et al., 2000; Zanchetta et al., 2007); in particular both records have clearly recorded a significant increase in precipitation between ca 9 and 7 ka. So far this feature did not appear so evident in northern Italian speleothems (McDermott et al., 1999; Frisia et al., 2005). However, different speleothems do show an increase of calcite δ18O values during the second half of the Holocene, probably as a consequence of the progressive decrease of summer insolation in northern latitudes. Carbon isotope composition has also emerged has an interesting proxy for effective precipitation, as the contribution of isotopically 13C-depleted CO2 from soil decreases when effective precipitation declines (e.g. Genty et al., 2003; Hodge et al., 2008) or, in some cases, simply changes the residence time of water in the soil (e.g. Bar-Matthews et al., 2000; Frisia et al., 2006). In drier parts of the Mediterranean region, cave speleothems have often been inactive during the Holocene and experienced their last phase of growth during the last glacial period. At this time, lower temperatures and evaporation rates altered the hydrological flux through cave systems. The cave speleothem sequence from northern Iberia described by Moreno et al. (2010-this issue) is an example of this type, and it records the sequence of climatic adjustments driven from the circum-North Atlantic basin during the last glacial-interglacial transition before calcite growth ceased at the end of the Pleistocene. The recent development of “clumped isotope” palaeo-thermometry represents a further powerful application of stable isotopes in speleothem research. This approach, coupled with fluid inclusion studies, can offer the opportunity to reconstruct palaeotemperatures and therefore the origin of meteoric precipitation thanks to the possibility of obtaining the original meteoric precipitation lines. Using this approach Affek et al (2008) have suggested that at Soreq Cave during the late Holocene d-excess in precipitation was within the range of modern values, whereas during the early Holocene the dexcess was significantly lower indicating significantly higher humidity. Offset between modern temperature cave values and “clumped isotope” temperatures suggest that any correction is potentially site specific and for each cave a separate calibration is necessary. 5. Marine isotope records The Mediterranean is a semi-enclosed sea whose only connection with the open Atlantic Ocean is through the narrow Strait of Gibraltar. The Mediterranean Sea itself is sub-divided into a western and an eastern basin, separated by the Straits of Sicily. The limited communication with the open ocean means that climatic signals are recorded rapidly and in amplified fashion in properties such as salinity and stable isotope composition (Rohling et al., 2009). The strong net evaporative loss from the Mediterranean surface leads to salinity values in excess of 39 psu in the eastern part of the sea and to δ18Owater values enriched by up to 1.5‰ relative to the world ocean. Meteoric water isotopic composition is partly dependent on the δ18O composition of the sea water from which it originally derived. Although the region's atmospheric precipitation ultimately has a North Atlantic origin, there is significant local cyclogenesis within the Mediterranean (e.g. Genoa basin). Evaporation of Mediterranean marine waters makes a significant contribution to the isotopic composition of atmospheric moisture; around 40% is of “local” origin according to modelling reported by Bard et al. (2002). Changes to the isotopic mass balance of the Mediterranean Sea are therefore important not only as a recorder of past climate variations, but also because they influenced the isotopic composition of meteoric waters recorded in continental palaeoclimatic archives (e.g. speleothems, lakes) as discussed by Spötl et al., (2010-this issue) for the early
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Holocene. On glacial-interglacial timescales, Mediterranean hydrography was substantially altered by the lowering of global sea levels of up to 130 m, which reduced exchange of water through the Strait of Gibraltar to about half of modern values (Rohling et al., 2009). This in turn increased the residence time of Mediterranean waters and led to a significant increase in salinity and δ18Owater values. Planktonic foraminifera from the Atlantic Iberian margin and the western Mediterranean show δ18O values 2.0 to 2.5‰ more positive at the Last Glacial Maximum (Roucoux et al., 2001; Kallel et al., 2004). In the Levantine basin of the Eastern Mediterranean planktonic isotope values show an even larger shift of up to 4‰ between glacial and Holocene times (Emeis et al., 2000). During the last Glacial-Interglacial climatic transition, oceanic waters became more negative as glacial ice masses melted, and the difference in isotope values was enhanced in the Mediterranean basin due to increased freshwater input and enhanced circulation during interglacial periods. This negative shift in oceanic waters affected the isotopic composition of precipitation falling across the whole region. The Mediterranean is characterized by periodic, widespread deposition of organic-rich sediments or ‘sapropels’. During the formation of the most recent sapropel I (9.5–6.5 ka BP), waters below about 350 m depth in the entire eastern Mediterranean became anoxic (Fig. 1), linked to increases in freshwater influx and primary productivity (Rohling 1994; Ariztegui et al., 2000). One significant source of freshwater inflow into the Mediterranean Sea came via the river Nile, whose flood discharge was significantly increased due to enhanced monsoonal rains over East Africa. During the early Holocene δ18O values of East Mediterranean atmospheric precipitation may have been about 2‰ lighter than at the present-day due to isotopicallydepleted Mediterranean Sea surface waters. Source area changes had less effect in the western Mediterranean because the isotopic composition of North Atlantic waters did not alter significantly during the last ∼ 10 ka (Duplessy et al., 1992; Kallel et al., 2004). Although Mediterranean seawater is too cold for surface-water corals to form, the immediately adjacent Red Sea contains warmer waters and supports the world's most northerly shallow-water coral communities. Felis and Rimbu (2010-this issue) provide an overview of recent research into coral isotopes from the northern Red Sea for the last 250 years, and show a clear causal link to North Atlantic (NAO) and Arctic Oscillations (AO). Red Sea marine records have also shown that summer monsoonal precipitation did not extend as far north as the Mediterranean Sea during the early Holocene (Arz et al., 2003). 6. Other oxygen isotope archives and multi-archive comparison The range of potential stable isotope archives available for the Mediterranean region is wide, for example, animal teeth and bones (Delgado Huertas et al., 1995; Pearson et al., 2007). Many of these archives remain under-exploited as sources of information regarding past environmental conditions. Groundwater studies showing changes through time are limited by the fact that isotopes are often used as a tracer rather than a climate proxy and not all waters are dated. Bajjali and Abu-Jaber (2001) describe changes in the meteoric water line slope (which decreased), and d value (which increased) since the Pleistocene from groundwaters in Jordan which they suggest was caused by a reduction in relative humidity at the precipitation source as well as in the area of rain-out. The controls on the δ18O of land snails has been further investigated (Zanchetta et al., 2005) following the studies of Goodfriend (1990) which showed a significant shift in δ18O across the Negev desert between 6000 and 4000 radiocarbon years ago. Land snails have also been used from stratified archaeological sites (Colonese et al., 2010this issue). There is a rich archaeological archive in the Mediterranean basin, and although archaeological sites do not usually provide the continuous records of change possible from lake sediment and speleothems, they potentially provide important snapshots of envi-
ronment, including δ18O values, in the past. Pustovoytov et al. (2007) describe techniques for obtaining δ18O values from Lithospermeae fruit which may have larger regional potential in the future. It is beyond the scope of this summary to attempt a Mediterraneanwide multi-archive oxygen isotope comparison. However first order similarities are evident between different records. The δ18O depletion recorded in marine cores during the first half of the Holocene has a parallel in East Mediterranean lake and cave records (Verheyden et al., 2008; Bar-Matthews et al. 2003; Roberts et al., 2008), and appears to have resulted from wetter climatic conditions than today, although changing rainfall seasonality may also have been significant (Stevens et al., 2001). Jones et al. (2007) used isotope mass balance modelling to calculate that early Holocene precipitation in central Turkey was around ∼20% higher than in recent millennia, broadly consistent with the estimate of Bar-Matthews et al. (2003) from Soreq cave carbonate isotopes. The freshwater lid and anoxia in the East Mediterranean Sea during Sapropel I must therefore have been caused by increases in local rainfall and runoff from northern shores, as well as by higher discharge from the Nile and North African wadis. By contrast, a pattern of isotopic depletion is less evident from lakes and caves surrounding the western Mediterranean Seas, which remained well-vented during the early Holocene. This suggests that there may have been a NW–SE contrast in climate history across the Mediterranean during the Holocene. 7. Linking present and past: calibration of different archives As can be seen in the summaries above, the production of palaeoisotope records is now relatively quick and cheap and has become a technique applied to many palaeoclimate archives. Work is now moving towards understanding better the controls on these records. Unlike biological proxies such as diatoms or pollen, transfer function or modern analogue techniques cannot be widely applied to stable isotope records. Records from all archive types are heavily dependent on site specific controls, such as basin morphology or geographical location. This means that a good understanding of oxygen isotope hydrology at each site is required before past inferences can be drawn. There are three main techniques for fully understanding isotope systems at a given site; monitoring of the contemporary system, calibration of recent archive material and modelling of the physical and chemical processes involved. Using these techniques are, rightly, now becoming essential elements in site-based isotopic reconstructions of past climate and hydrology. Published examples of isotope calibration from the Mediterranean include speleothem δ18O data compared against local climate data from Gibraltar, where Mattey et al. (2008) showed a strong relationship between measured δ18Op and reconstructed drip water δ18O values, and from Turkey (Jex et al., 2010-this issue). Baker and Bradley (2010-this issue) use the same northeast Anatolian record to compare a new modelling approach to understanding δ18O variability in speleothems. Lemcke and Sturm (1997) and Jones et al. (2007) have attempted to interpret Pleistocene-Holocene shifts in lake isotope records using mass-balance models. Jones et al. (2005) used a coupled calibration and modelling approach to investigate controls on the late Holocene δ18O record from Lake Nar. In this case, calibration results indicated a hydrologically more sensitive system than was suggested by modelling which required changes in more than one climate parameter to drive the observed δ18O change. Jones and Imbers (2010-this issue) show that theoretical lake models driven by measured δ18Op and weather data describe lake isotope variability measured in the field, suggesting these models have further potential for quantifying palaeoclimatic change. 8. Conclusions Recent years have seen a rapid growth in isotope-based studies of climatic change and variability in the Mediterranean region. The challenge now is to translate these data into a form that can be used
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for analysis of regional-scale hydro-climatic dynamics. There are several steps that would help this to be achieved. Firstly, methods for numerically calibrating palaeo-isotope data need to be improved and harmonised in order to derive quantitative values for past temperature, precipitation, and relative humidity. For example, there are currently only a handful of Mediterranean lakes whose isotopic records have been translated into climatically-relevant parameters. Monitoring of modern hydro-climatic conditions in the same lake or cave systems that preserve longer term isotope-climate records can also provide key insights into the role of seasonality preserved in climate archives. A particular challenge for calibration against instrumental data is the spatial inhomogeneity of precipitation (and associated isotopes), compared to spatially less variable temperature or relative humidity. Secondly, it would be desirable to improve both palaeo and modern isotope data coverage in a number of regions of the Mediterranean, such as North Africa. Thirdly, isotope records are being compiled into data bases using common formats that allow comparison with regional-scale climate modelling experiments. This process is already underway for isotopes in precipitation (via IAEA GNIP), marine waters and deep-sea sediment cores (e.g. http://www.giss.nasa.gov/data/o18data/), and lake records (e.g. ISOMED; Roberts et al 2008). Most palaeo-isotope data sets are currently based on multi-millennial timescales; e.g. since the LGM. In future it would be desirable to establish similar data bases for welldated records from speleothems, corals and lakes for the last ∼2000 years at decadal or better resolution, in order to compare them against data from historical sources and observational records (e.g. Luterbacher et al., 2006). Finally, palaeo-δ18O data should be evaluated in their own right in terms of the water cycle, and not just as dummies for other climatological parameters. There is an improving understanding of the hydro-meteorological factors that determine isotopic signals over different temporal and spatial scales. This in turn requires that there is a continual and improved dialogue between palaeo-, contemporary and modelling communities working on Mediterranean climate change. 9. Dedication We dedicate this set of papers to Professors Antonio Longinelli (University of Parma) and Roberto Gonfiantini (CNR Instituto di Geoscienze e Georisorse, Pisa), who have greatly advanced our knowledge of stable isotope hydrology as a result of their research over many decades. Acknowledgements We are pleased to acknowledge the assistance of the ESF MedClivar programme, University of Pisa (Dipartimento di Scienze della Terra and Dipartimento di Biologia), CNR Instituto di Geoscienze e Georisorse, Pisa, Regione Toscana, University of Plymouth (especially the Geography cartographic unit), and University of Nottingham. We thank referees for their careful reviews of manuscripts. References Affek, H.P., Bar-Matthews, M., Ayalon, A., Matthews, A., Eiler, J.M., 2008. Glacial/ interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’ thermometry. Geochimica et Cosmochimica Acta 72, 5351–5360. Andrews, J.E., Pedley, H.M., Dennis, P.F., 2000. Palaeoenvironmental records in Holocene Spanish tufas: a stable isotope approach in search of reliable climatic archives. Sedimentology 47, 961–978. Araguas-Araguas, L.J., Diaz Teijeiro, M.F., 2005. Isotope composition of precipitation and water vapour in the Iberian Peninsula. In: Goucy, L. (Ed.), Isotopic composition of precipitation in the Mediterranean Basin in relation to air circulation patterns and climate, IAEA-TECDOC-1453, Vienna. Ariztegui, D., Asioli, A., Lowe, J.J., Trincardi, F., Vigliotti, L., Tamburini, F., Chondrogianni, C., Accorsi, C.A., Mazzanti, M.B., Mercuri, A.M., van der Kaars, S., McKenzie, J.A., Oldfield, F., 2000. Palaeoclimate and the formation of sapropel S1: inferences from
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Contents lists available at ScienceDirect
Global and Planetary Change j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g l o p l a c h a
Oxygen isotopes as tracers of Mediterranean climate variability: An introduction C. Neil Roberts a, Giovanni Zanchetta b, Matthew D. Jones c,⁎ a b c
School of Geography, Earth & Environmental Sciences, University of Plymouth, Plymouth, Devon PL4 8AA, UK Dipartimento di Scienze della Terra, University of Pisa, Via S. Maria 53, 56126 Pisa, Italy School of Geography, University of Nottingham, Nottingham, NG7 2RD, UK
a r t i c l e
i n f o
Article history: Accepted 22 January 2010 Available online 28 January 2010 Keywords: Mediterranean oxygen isotopes hydrology climate change
a b s t r a c t Oxygen isotopes provide one of the best tracers of the Mediterranean hydrological cycle from source areas in the oceans, through precipitation, to ground- and surface freshwaters. The ratio of oxygen isotopes preserved in sedimentary carbonates also provides a key indicator of natural climatic variability over timescales beyond those based on direct monitoring or historical observations. This set of papers, originally deriving from an ESF MedClivar workshop meeting in Pisa, Italy, critically considers how δ18O data can be used to provide records of Mediterranean climate variability over a hierarchy of timescales. They focus on five main themes, namely 1) water isotopes as tracers for monitoring and modelling patterns of precipitation, 2) lakes and surface waters, and their sedimentary records, 3) groundwaters and cave systems as preserved in speleothems, 4) marine systems, such as deep-sea sediments and surface corals, and 5) intercomparison of different archives and regional-scale data: model comparisons using isotope data bases. Future priorities include improved calibration of proxy-climate data via monitoring of contemporary systems and mass balance modelling, and a focus on synthesising high-resolution climate reconstructions during the last 2000 years. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Given predictions of future climate, changes in rainfall and water resources seem certain to have important socio-economic and political impacts in the Mediterranean region (Giorgi 2006). Understanding the variability of hydro-climate over different time scales is therefore an essential prerequisite for establishing predicted future climate change and its possible impact on human society (Mariotti et al., 2002). The Mediterranean region is both climate-sensitive and has an exceptionally long and rich history of human use, including some the world's most important past civilisations. However, the scale and longevity of human activity around the Mediterranean (e.g. deforestation, erosion) has created serious difficulties in distinguishing climate change from human impact in many proxy records of past environmental change (e.g. via pollen analysis; Roberts et al. 2004). In consequence, we know less about long-term climatic variability in the circum-Mediterranean region than we do about that in other parts of the world with much lower population densities and shorter historical time-depth. Because they vary directly with the physical processes of the hydrological cycle and are not compromised directly by human impact (e.g. pollution), oxygen isotope ratios provide an important hydro-climatic tracer at all time scales - instrumental, historical and geological. Water isotopes have long been used as key tracers in monitoring contemporary patterns of precipitation over the Mediter⁎ Corresponding author. E-mail addresses: [email protected] (C.N. Roberts), [email protected] (G. Zanchetta), [email protected] (M.D. Jones). 0921-8181/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2010.01.024
ranean, particularly in relation to source area characterisation and air mass trajectories (e.g. Rindsberger et al. 1983; Lykoudis and Argiriou 2007). The IAEA Coordinated Research Project on Isotopic Composition of Precipitation in the Mediterranean Basin helped to define the interactions between climatic conditions and isotopes in relation to overall changes in climate currently being experienced in the region (Goucy, 2005). Key indicators of natural climatic variability over timescales beyond those based on direct monitoring or historical observations come from the ratio of stable isotopes of oxygen preserved in materials such as cave carbonates, and lake and marine sediments. During the decade since Goodfriend (1999) reviewed available stable isotope evidence, well over 50 oxygen isotope records have been published from the Mediterranean basin and surrounds. This increasing spatial coverage allows regional syntheses to be made (e.g. Roberts et al., 2008) and helps us to improve the interpretation of past climatic and environmental changes. The collection of papers in this volume derives from a workshop meeting held at the University of Pisa (June–July 2008; http://www. geog.plymouth.ac.uk/research/groups/MedCLIVAR_isotope_workshop. html), with the European Science Foundation as the principal meeting sponsor, under its MedCLIVAR programme. MedCLIVAR is designed to investigate Mediterranean Climate Variability and Predictability, including the past evolution of climate, assessment of climate variability and the mechanisms responsible for it, and identifying trends and providing climate prediction in relation to future emission scenarios. In this introduction we summarise the current state of play regarding the understanding of oxygen isotope variability across the Mediterranean basin today and in the past, and look towards how
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future work may improve our understanding and begin to answer questions that still remain. 2. Water isotopes as tracers for monitoring and modelling patterns of precipitation The IAEA/WMO Global Network for Isotopes in Precipitation allows spatial and temporal patterns in the oxygen isotope values of the Mediterranean precipitation δ18Op to be observed. Weighted average δ18Op values for Mediterranean basin fit into the global gradient, becoming more negative polewards, from c. 4‰ along the south of the Mediterranean basin to around 8‰ in the north (Bowen and Wilkinson, 2002; Bowen and Revenaugh, 2003) (Fig. 1). Values are also more negative towards the east, especially over the high ground of the Anatolian and Iranian plateaux (Fig. 1). Deuterium excess values (d) have also long been recognised as being more positive in the Mediterranean, especially in the eastern basin (Gat and Carmi, 1970) where d = +22‰ compared to the global average of +10‰. Intermediate values can be found in Central Mediterranean (e.g. Longinelli and Selmo, 2003). Dotsika et al. (2010-this issue) observe elevated deuterium excess values in Greek rainfall derived from the Aegean Sea. On top of these large-scale trends are more local variables. The altitude effect, for example, varies across the basin (Goucy, 2005) with values between −0.26 and −0.37‰ per 100 m in Croatia and Slovenia and a range of −0.10 to −0.23‰ per 100 m in Lebanon. Differences have been attributed to a combination of air masses of different origins raining out at different locations and to varying degrees of rainout. Clear differences in isotope values of different air masses had previously been observed in Israel (Rindsberger et al., 1983). Celle-Jeanton et al. (2001, 2004) has provided evidences that single events that originated from the Atlantic Ocean are usually more negative than precipitation mainly originates over the western Mediterranean. Change in monthly values of precipitation over time have also been shown to vary with temperature, e.g. in Ankara (Dirican et al., 2005) and Madrid (Araguas-Araguas and Diaz Teijeiro, 2005). Dotsika et al. (2010-this issue) show that the distance of the observation station from the sea and the altitude are the main factors imprinted in the isotopic signature of precipitation in Greece. Studies from the eastern Mediterranean Sea area (Lykoudis et al., 2010-this issue) show how gridded isotopic data, calculated from empirical regression models along with geostatistical methods, can account for the major factors controlling the isotopic composition of precipitation in space
and time thereby improving the coverage of the previously available data sets. It is clear from the above that there are many controls on the δ18Op at any one site in the Mediterranean and a robust understanding of these controls under the current climatic regime is required before any evaluation can be made about how δ18Op values changed in the past. The detailed study of both monthly and daily rainfall data across the Mediterranean region by the IAEA (Goucy, 2005) helps greatly in this regard. With the increased understanding on the controls on δ18Op in the climate cycle, δ18O values for various stages of the water cycle can now be incorporated into earth system models (Schmidt et al., 2005). There is therefore the potential for direct comparison of proxy records and models of past climate scenarios. 3. Isotope records from lakes and tufas Lake isotope stratigraphies offer some of the most widely distributed terrestrial records of past climatic variability in the Mediterranean region. The δ18O signal in lake carbonates and tufas is a function of the lake water isotopic composition and the temperature of the water (Leng and Marshall 2004). The temperature-isotope relationship in lakes and tufas is affected both by direct temperature fractionation effects (ca. −0.24‰/°C for temperatures in the range of interest) and by changes in the δ18O of precipitation, the latter being linked to the isotopic composition of seawater in the vapour source region and to air mass trajectories, temperature and altitude (see discussions above). Lake water isotopic composition can also be modified from that of incoming meteoric precipitation by evaporative effects. In regions of negative hydrological balance many lakes lose water mainly via evaporation and this can cause large positive isotopic shifts in lake waters (Gat 1995). Mediterranean lakes lie across a spectrum that extends from fresh, short residence-time water bodies to saline, hydrologically-closed lake systems. The former have modern δ18O values similar to those of mean seasonally-weighted precipitation in their catchments, while the latter have been isotopically enriched through evaporation. δD vs δ18O cross-plots of modern waters show that only a small minority of Mediterranean lakes lie on the Mediterranean meteoric water line (MMWL; Roberts et al., 2008). Many more lakes lie off this on a local evaporation line, even in systems that are chemically dilute, and their stable isotope hydrology reflects primarily changes in lake water balance (P–E, residence time, etc). By contrast, because tufas form in
Fig. 1. Map of Mediterranean basin, showing interpolated mean weighted δ18O values of precipitation, the zone of eastern Mediterranean sapropel formation, and palaeo-isotope study sites presented in the papers in this volume; 1. Akçakale cave (Jex et al.), 2. Zemeno (Brasier et al.), 3. Grotta della Serratura (Colonese et al.), 4. Lake Pamvotis, Ioannina (Leng et al.), 5. Gölhisar (Leng et al.), 6. Pindal Cave (Moreno et al.), 7. Lake Butrint (Ariztegui et al.), 8. Piànico-Sèllere palaeolake (Mangili et al.), 9. Northern Red Sea corals (Felis and Rimbu), and 10 Nar lake (Leng et al).
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short residence time hydro-systems, these lotic carbonates may record changes in temperature and/or air mass history more faithfully (Andrews et al., 2000). Brasier et al. (2010-this issue) analyse 100 kaold banded freshwater tufas from Greece to show that only part of the full seasonal temperature range was captured, due to non-deposition in mid-winter and mid-summer. Because every lake has a unique isotope hydrology, each one has to be calibrated individually against local climatic parameters and historical instrumental meteorological data, which requires fine-resolution sediment analysis and good dating control. Alternatively or additionally, individual lake isotope hydrologies can be simulated by mass balance modelling (e.g. Jones et al. 2005; Steinman et al. in press). There are currently about 30 basins from around the Mediterranean Sea whose Quaternary sediment records have been analysed for stable isotopes. A few of these extend back to cover pre-Holocene interglacial periods, including Ioannina (Pamvotis) in Greece (Frogley et al., 1999) and the varved Pianico sequence from northern Italy dating to MIS 11 (Mangili et al., 2010-this issue). Like most lake isotope records from the Mediterranean, these derive from endogenic/authigenic carbonates. Leng et al. (2010-this issue) use analysis of paired endogenic and biogenic carbonate (mollusks, ostracods) data from two lakes, Ioannina and Gölhisar in southwest Turkey, along with electron microscopy, to test for detrital carbonate inwash; if undetected, this can contaminate bulk analyses. Ariztegui et al (this volume) describe a high resolution data set from a varved lake sequence over the last 300 years from Lake Butrint, Albania, which shows how complex the forcing mechanisms on lake isotope records can be, and how with careful investigation these different facings can be identified and understood. Roberts et al. (2008) synthesised δ18O data on lake carbonates for the time period since the Last Glacial Maximum in an attempt to identify regionally-coherent isotope trends caused by millennial-scale climatic fluctuations, as part of the collaborative ISOMED project (Lake Isotope Records from the Mediterranean). During MIS2, the δ18O composition of most Mediterranean lake waters was enriched relative to the Holocene, in part because of changes to the isotopic composition of Mediterranean Sea source waters (Kolodny et al. 2005). Maximum isotopic enrichment occurred during the Heinrich 1 cold event (16–17.5 ka) and at the time of the Younger Dryas stadial (11.4 to ∼ 13 ka), when several lakes show independent evidence of saline water conditions consistent with climatic aridity. Almost all Mediterranean lakes saw a shift to more depleted δ18O values during the MIS2-1 climatic transition. This is opposite to the isotopic trend in lakes from Central and Northern Europe over the same time period (e.g. von Grafenstein et al., 1999), but also characterized the MIS 6/5e glacial terminations in Greece (Frogley et al., 1999). This implies that temperature changes were relatively unimportant as a direct driver of Mediterranean lake isotopic records over glacial-interglacial timescales. 4. Speleothem isotope records Carbonate cave deposits provide an excellent source of material for efforts to reconstruct past climate condition on land, which can be accurately and precisely dated by mean of U/Th technique. The latter property is fundamental in solving chronological problems about the climate events and their relation with external forcing. Since the fundamental contribution from southern Levantine caves performed by Bar-Matthews et al. (1996, 1999, 2000) in recent years, stable isotope studies on speleothems in the Mediterranean area have been increasing exponentially (e.g. Frisia et al., 2005, 2006; Drysdale et al., 2006). The ability of the cave systems to record the climate signal with stable isotopes is highly variable and detailed studies on local hydrology and climate condition are necessary (Baker and Bradley, 2010-this issue, Mattey et al., 2008) in order to disentangle the climate signal from in-cave isotope fractionation. In the best cases it is
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possible to reach sub-annual resolution, even if Mediterranean records with this resolution are so far restricted to the last few thousand years (Orland et al., 2009). During the Holocene a strong similarity in the general δ18O trend has been observed between western and eastern Mediterranean at Corchia and Soreq caves (BarMatthews et al., 2000; Zanchetta et al., 2007); in particular both records have clearly recorded a significant increase in precipitation between ca 9 and 7 ka. So far this feature did not appear so evident in northern Italian speleothems (McDermott et al., 1999; Frisia et al., 2005). However, different speleothems do show an increase of calcite δ18O values during the second half of the Holocene, probably as a consequence of the progressive decrease of summer insolation in northern latitudes. Carbon isotope composition has also emerged has an interesting proxy for effective precipitation, as the contribution of isotopically 13C-depleted CO2 from soil decreases when effective precipitation declines (e.g. Genty et al., 2003; Hodge et al., 2008) or, in some cases, simply changes the residence time of water in the soil (e.g. Bar-Matthews et al., 2000; Frisia et al., 2006). In drier parts of the Mediterranean region, cave speleothems have often been inactive during the Holocene and experienced their last phase of growth during the last glacial period. At this time, lower temperatures and evaporation rates altered the hydrological flux through cave systems. The cave speleothem sequence from northern Iberia described by Moreno et al. (2010-this issue) is an example of this type, and it records the sequence of climatic adjustments driven from the circum-North Atlantic basin during the last glacial-interglacial transition before calcite growth ceased at the end of the Pleistocene. The recent development of “clumped isotope” palaeo-thermometry represents a further powerful application of stable isotopes in speleothem research. This approach, coupled with fluid inclusion studies, can offer the opportunity to reconstruct palaeotemperatures and therefore the origin of meteoric precipitation thanks to the possibility of obtaining the original meteoric precipitation lines. Using this approach Affek et al (2008) have suggested that at Soreq Cave during the late Holocene d-excess in precipitation was within the range of modern values, whereas during the early Holocene the dexcess was significantly lower indicating significantly higher humidity. Offset between modern temperature cave values and “clumped isotope” temperatures suggest that any correction is potentially site specific and for each cave a separate calibration is necessary. 5. Marine isotope records The Mediterranean is a semi-enclosed sea whose only connection with the open Atlantic Ocean is through the narrow Strait of Gibraltar. The Mediterranean Sea itself is sub-divided into a western and an eastern basin, separated by the Straits of Sicily. The limited communication with the open ocean means that climatic signals are recorded rapidly and in amplified fashion in properties such as salinity and stable isotope composition (Rohling et al., 2009). The strong net evaporative loss from the Mediterranean surface leads to salinity values in excess of 39 psu in the eastern part of the sea and to δ18Owater values enriched by up to 1.5‰ relative to the world ocean. Meteoric water isotopic composition is partly dependent on the δ18O composition of the sea water from which it originally derived. Although the region's atmospheric precipitation ultimately has a North Atlantic origin, there is significant local cyclogenesis within the Mediterranean (e.g. Genoa basin). Evaporation of Mediterranean marine waters makes a significant contribution to the isotopic composition of atmospheric moisture; around 40% is of “local” origin according to modelling reported by Bard et al. (2002). Changes to the isotopic mass balance of the Mediterranean Sea are therefore important not only as a recorder of past climate variations, but also because they influenced the isotopic composition of meteoric waters recorded in continental palaeoclimatic archives (e.g. speleothems, lakes) as discussed by Spötl et al., (2010-this issue) for the early
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Holocene. On glacial-interglacial timescales, Mediterranean hydrography was substantially altered by the lowering of global sea levels of up to 130 m, which reduced exchange of water through the Strait of Gibraltar to about half of modern values (Rohling et al., 2009). This in turn increased the residence time of Mediterranean waters and led to a significant increase in salinity and δ18Owater values. Planktonic foraminifera from the Atlantic Iberian margin and the western Mediterranean show δ18O values 2.0 to 2.5‰ more positive at the Last Glacial Maximum (Roucoux et al., 2001; Kallel et al., 2004). In the Levantine basin of the Eastern Mediterranean planktonic isotope values show an even larger shift of up to 4‰ between glacial and Holocene times (Emeis et al., 2000). During the last Glacial-Interglacial climatic transition, oceanic waters became more negative as glacial ice masses melted, and the difference in isotope values was enhanced in the Mediterranean basin due to increased freshwater input and enhanced circulation during interglacial periods. This negative shift in oceanic waters affected the isotopic composition of precipitation falling across the whole region. The Mediterranean is characterized by periodic, widespread deposition of organic-rich sediments or ‘sapropels’. During the formation of the most recent sapropel I (9.5–6.5 ka BP), waters below about 350 m depth in the entire eastern Mediterranean became anoxic (Fig. 1), linked to increases in freshwater influx and primary productivity (Rohling 1994; Ariztegui et al., 2000). One significant source of freshwater inflow into the Mediterranean Sea came via the river Nile, whose flood discharge was significantly increased due to enhanced monsoonal rains over East Africa. During the early Holocene δ18O values of East Mediterranean atmospheric precipitation may have been about 2‰ lighter than at the present-day due to isotopicallydepleted Mediterranean Sea surface waters. Source area changes had less effect in the western Mediterranean because the isotopic composition of North Atlantic waters did not alter significantly during the last ∼ 10 ka (Duplessy et al., 1992; Kallel et al., 2004). Although Mediterranean seawater is too cold for surface-water corals to form, the immediately adjacent Red Sea contains warmer waters and supports the world's most northerly shallow-water coral communities. Felis and Rimbu (2010-this issue) provide an overview of recent research into coral isotopes from the northern Red Sea for the last 250 years, and show a clear causal link to North Atlantic (NAO) and Arctic Oscillations (AO). Red Sea marine records have also shown that summer monsoonal precipitation did not extend as far north as the Mediterranean Sea during the early Holocene (Arz et al., 2003). 6. Other oxygen isotope archives and multi-archive comparison The range of potential stable isotope archives available for the Mediterranean region is wide, for example, animal teeth and bones (Delgado Huertas et al., 1995; Pearson et al., 2007). Many of these archives remain under-exploited as sources of information regarding past environmental conditions. Groundwater studies showing changes through time are limited by the fact that isotopes are often used as a tracer rather than a climate proxy and not all waters are dated. Bajjali and Abu-Jaber (2001) describe changes in the meteoric water line slope (which decreased), and d value (which increased) since the Pleistocene from groundwaters in Jordan which they suggest was caused by a reduction in relative humidity at the precipitation source as well as in the area of rain-out. The controls on the δ18O of land snails has been further investigated (Zanchetta et al., 2005) following the studies of Goodfriend (1990) which showed a significant shift in δ18O across the Negev desert between 6000 and 4000 radiocarbon years ago. Land snails have also been used from stratified archaeological sites (Colonese et al., 2010this issue). There is a rich archaeological archive in the Mediterranean basin, and although archaeological sites do not usually provide the continuous records of change possible from lake sediment and speleothems, they potentially provide important snapshots of envi-
ronment, including δ18O values, in the past. Pustovoytov et al. (2007) describe techniques for obtaining δ18O values from Lithospermeae fruit which may have larger regional potential in the future. It is beyond the scope of this summary to attempt a Mediterraneanwide multi-archive oxygen isotope comparison. However first order similarities are evident between different records. The δ18O depletion recorded in marine cores during the first half of the Holocene has a parallel in East Mediterranean lake and cave records (Verheyden et al., 2008; Bar-Matthews et al. 2003; Roberts et al., 2008), and appears to have resulted from wetter climatic conditions than today, although changing rainfall seasonality may also have been significant (Stevens et al., 2001). Jones et al. (2007) used isotope mass balance modelling to calculate that early Holocene precipitation in central Turkey was around ∼20% higher than in recent millennia, broadly consistent with the estimate of Bar-Matthews et al. (2003) from Soreq cave carbonate isotopes. The freshwater lid and anoxia in the East Mediterranean Sea during Sapropel I must therefore have been caused by increases in local rainfall and runoff from northern shores, as well as by higher discharge from the Nile and North African wadis. By contrast, a pattern of isotopic depletion is less evident from lakes and caves surrounding the western Mediterranean Seas, which remained well-vented during the early Holocene. This suggests that there may have been a NW–SE contrast in climate history across the Mediterranean during the Holocene. 7. Linking present and past: calibration of different archives As can be seen in the summaries above, the production of palaeoisotope records is now relatively quick and cheap and has become a technique applied to many palaeoclimate archives. Work is now moving towards understanding better the controls on these records. Unlike biological proxies such as diatoms or pollen, transfer function or modern analogue techniques cannot be widely applied to stable isotope records. Records from all archive types are heavily dependent on site specific controls, such as basin morphology or geographical location. This means that a good understanding of oxygen isotope hydrology at each site is required before past inferences can be drawn. There are three main techniques for fully understanding isotope systems at a given site; monitoring of the contemporary system, calibration of recent archive material and modelling of the physical and chemical processes involved. Using these techniques are, rightly, now becoming essential elements in site-based isotopic reconstructions of past climate and hydrology. Published examples of isotope calibration from the Mediterranean include speleothem δ18O data compared against local climate data from Gibraltar, where Mattey et al. (2008) showed a strong relationship between measured δ18Op and reconstructed drip water δ18O values, and from Turkey (Jex et al., 2010-this issue). Baker and Bradley (2010-this issue) use the same northeast Anatolian record to compare a new modelling approach to understanding δ18O variability in speleothems. Lemcke and Sturm (1997) and Jones et al. (2007) have attempted to interpret Pleistocene-Holocene shifts in lake isotope records using mass-balance models. Jones et al. (2005) used a coupled calibration and modelling approach to investigate controls on the late Holocene δ18O record from Lake Nar. In this case, calibration results indicated a hydrologically more sensitive system than was suggested by modelling which required changes in more than one climate parameter to drive the observed δ18O change. Jones and Imbers (2010-this issue) show that theoretical lake models driven by measured δ18Op and weather data describe lake isotope variability measured in the field, suggesting these models have further potential for quantifying palaeoclimatic change. 8. Conclusions Recent years have seen a rapid growth in isotope-based studies of climatic change and variability in the Mediterranean region. The challenge now is to translate these data into a form that can be used
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for analysis of regional-scale hydro-climatic dynamics. There are several steps that would help this to be achieved. Firstly, methods for numerically calibrating palaeo-isotope data need to be improved and harmonised in order to derive quantitative values for past temperature, precipitation, and relative humidity. For example, there are currently only a handful of Mediterranean lakes whose isotopic records have been translated into climatically-relevant parameters. Monitoring of modern hydro-climatic conditions in the same lake or cave systems that preserve longer term isotope-climate records can also provide key insights into the role of seasonality preserved in climate archives. A particular challenge for calibration against instrumental data is the spatial inhomogeneity of precipitation (and associated isotopes), compared to spatially less variable temperature or relative humidity. Secondly, it would be desirable to improve both palaeo and modern isotope data coverage in a number of regions of the Mediterranean, such as North Africa. Thirdly, isotope records are being compiled into data bases using common formats that allow comparison with regional-scale climate modelling experiments. This process is already underway for isotopes in precipitation (via IAEA GNIP), marine waters and deep-sea sediment cores (e.g. http://www.giss.nasa.gov/data/o18data/), and lake records (e.g. ISOMED; Roberts et al 2008). Most palaeo-isotope data sets are currently based on multi-millennial timescales; e.g. since the LGM. In future it would be desirable to establish similar data bases for welldated records from speleothems, corals and lakes for the last ∼2000 years at decadal or better resolution, in order to compare them against data from historical sources and observational records (e.g. Luterbacher et al., 2006). Finally, palaeo-δ18O data should be evaluated in their own right in terms of the water cycle, and not just as dummies for other climatological parameters. There is an improving understanding of the hydro-meteorological factors that determine isotopic signals over different temporal and spatial scales. This in turn requires that there is a continual and improved dialogue between palaeo-, contemporary and modelling communities working on Mediterranean climate change. 9. Dedication We dedicate this set of papers to Professors Antonio Longinelli (University of Parma) and Roberto Gonfiantini (CNR Instituto di Geoscienze e Georisorse, Pisa), who have greatly advanced our knowledge of stable isotope hydrology as a result of their research over many decades. Acknowledgements We are pleased to acknowledge the assistance of the ESF MedClivar programme, University of Pisa (Dipartimento di Scienze della Terra and Dipartimento di Biologia), CNR Instituto di Geoscienze e Georisorse, Pisa, Regione Toscana, University of Plymouth (especially the Geography cartographic unit), and University of Nottingham. We thank referees for their careful reviews of manuscripts. References Affek, H.P., Bar-Matthews, M., Ayalon, A., Matthews, A., Eiler, J.M., 2008. Glacial/ interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’ thermometry. Geochimica et Cosmochimica Acta 72, 5351–5360. Andrews, J.E., Pedley, H.M., Dennis, P.F., 2000. Palaeoenvironmental records in Holocene Spanish tufas: a stable isotope approach in search of reliable climatic archives. Sedimentology 47, 961–978. Araguas-Araguas, L.J., Diaz Teijeiro, M.F., 2005. Isotope composition of precipitation and water vapour in the Iberian Peninsula. In: Goucy, L. (Ed.), Isotopic composition of precipitation in the Mediterranean Basin in relation to air circulation patterns and climate, IAEA-TECDOC-1453, Vienna. Ariztegui, D., Asioli, A., Lowe, J.J., Trincardi, F., Vigliotti, L., Tamburini, F., Chondrogianni, C., Accorsi, C.A., Mazzanti, M.B., Mercuri, A.M., van der Kaars, S., McKenzie, J.A., Oldfield, F., 2000. Palaeoclimate and the formation of sapropel S1: inferences from
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