Interpreting lake isotope records of Holocene environmental change in the Eastern Mediterranean
Introduction
Lake records provide continuous, potentially high resolution, terrestrial archives of environmental change. They, therefore, play an important role in the reconstruction of past climate and environment and allow comparison with other continuous archives such as marine sediments and ice cores. The use of oxygen isotopes recorded from biogenic or sedimentary hosts within lake sediments has become an increasingly common technique (Leng and Marshall, 2004) and a large number of lake isotope records from across the Mediterranean basin have now been published (Roberts and Jones, 2002). If fully understood, these records have the potential to provide a regional picture of hydrological and climatic change. However, because there are multiple controls over the isotopic composition of lake waters, the interpretation of these records is far from straight forward and requires careful analysis.
Here we review oxygen isotope records from lake carbonates (δ18Oc) and their interpretations from lakes in the Eastern Mediterranean with reference to three key Holocene records; Lake Zeribar in western Iran (Stevens et al., 2001), Lake Van in eastern Turkey (Lemke and Sturm, 1997) and Eski Acıgöl (Roberts et al., 2001) in the Cappadocian region of central Turkey. These records have been interpreted in different ways and raise questions as to the “real” meaning of such δ18Oc records.
The values of δ18Oc precipitated from lake waters are determined by the temperature and isotopic value of the water (δl). Recent reviews by Leng and Marshall (2004) and Leng et al. (2006) discuss the controls on these systems in detail and so they will only be briefly outlined here. In open lake systems, with short-residence times, lake waters generally have similar oxygen isotope values to the weighted average of precipitation. δl changes through time are caused by changing temperature, seasonality of precipitation or changes in the location and/or δl of the rainfall source area (Gat et al., 2001). Increase in temperature will lead to a net movement to more positive δ18Oc (precipitation values change by +0.7‰ per 1 °C increase in temperature although this is offset by a −0.24‰ per 1 °C increase in temperature in the fractionation from δl to δ18Oc (Stuiver, 1970)). Winter (cold) precipitation tends to have more negative isotope values and a change to more winter-dominated precipitation will therefore push δl towards lower values. Sites further along given rain tracks tend to have more negative values as the heavier isotope (18O) is preferentially rained out.
In closed systems, with a long residence time, evaporation plays a further role in changing δl (Li and Ku, 1997) by preferentially removing the lighter isotope (16O) from the system and therefore shifting δl to more positive values. In regions of the world where evaporation exceeds precipitation, such as large parts of the Eastern Mediterranean, this is an important driver of lake isotope change.
Section snippets
Site descriptions and isotope records
The location of the three sites on which discussion in this paper is focussed can be seen in Fig. 1 and the respective Holocene isotope records are shown in Fig. 2. In all cases, stable isotope measurements were taken from bulk sediment samples rather than biogenic hosts such as molluscs or ostracods. The three sites represent a range of lake sizes, shapes and hydrological states (Table 1 and discussion below).
Eski Acıgöl (38°33′N, 34°32′E) is a former crater lake lying within a larger caldera
Interpretations
The records from Lake Van and Eski Acıgöl have previously been interpreted as changes in the precipitation evaporation ratio (P:E). Periods of net water loss (E>P) lead to removal of the lighter isotope resulting in more positive isotope values. Alternatively during periods of P>E, isotope values become more negative. In both cases, the early–mid Holocene isotope values are relatively negative compared to the late Holocene and are therefore interpreted as representing wetter climatic
Discussion
Although every lake must be interpreted on its own merits, two broad schools of interpretation have emerged from these three sites. All records have relatively negative δ18Oc values in the early–mid Holocene with a shift to more positive values by the late Holocene. The record from Zeribar has been interpreted from pollen and other proxies as showing a generally dry early Holocene with a wetter mid-Holocene (Stevens et al., 2001). Changes in δ18Oc were interpreted as the result of changes in
Palaeoclimatic implications
Previous interpretations of Eastern Mediterranean climate based on pollen records (e.g. van Zeist and Bottema, 1991, Roberts and Wright, 1993) led to the conclusion that modern levels of moisture availability were not achieved before mid-Holocene times. An increasing number of isotope records, including those from Van and Eski Acıgöl, and also non-lacustrine evidence such as speleothems and marine sapropel layers (e.g. Bar-Matthews et al., 1997; Ariztegui et al., 2000), now suggest an opposite
Conclusions
We have attempted to assess critically the range of different interpretations of isotope data from three lakes in the Eastern Mediterranean and compared them against other proxy records from the same sites and elsewhere in the region. Lake isotope systems are clearly complex and each record must be interpreted on its own merits. However, previous interpretations of these systems may have been oversimplified based on general assumptions for given lake types, or driven by interpretations of other
Acknowledgements
We thank members of the ISOMED working group for early discussions regarding some of the topics discussed here. Details can be found at www.geog.plymouth.ac.uk/research/groups/is18omed.htm. Jamie Quinn is thanked for the drafting of Fig. 1. An anonymous reviewer is thanked for constructive comments which improved the quality of the manuscript.
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