Multi-proxy studies in palaeolimnology

Hilary H. Birks H. John B. Birks Multi-proxy studies are becoming increasingly common in palaeolimnology. Eight basic requirements and challenges for a multi-proxy study are outlined in this essay - definition of research questions, leadership, site selection and coring, data storage, chronology, presentation of results, numerical tools and data interpretation. The nature of proxy data is discussed in terms of physical proxies and biotic proxies. Loss-on-ignition changes and the use of transfer functions are reviewed as examples of problems in the interpretation of data from multi-proxy studies. The importance of pollen analysis and plant macrofossil analysis in multi-proxy studies is emphasised as lake history cannot be interpreted without knowledge of catchment history. Future directions are outlined about how multi-proxy studies can contribute to understanding biotic responses to environmental change. - Ecosystems can be thought of as an almost infinite network of interactions among biotic and abiotic components Communicated by Pim van der Knaap balanced between internal and external driving factors. In a stable ecosystem the interactions are in balance, but when they become unbalanced the character of the ecosystem will change. The change may be small or substantial and may occur suddenly in a short time or slowly over an extended period. A rapid change occurring in the present may be monitored by regular observations. However, many changes have been proceeding over a long period before observation was possible, and some rapid and extensive changes have occurred far back in the past. In order to study the dynamics of these ecosystems we have to look back into the past by using the record of changes in fossil organisms and sediment characteristics (proxy data) to reconstruct past ecosystems and biotic responses. Because of the complex network of interactions throughout the ecosystem, it is desirable to study as many proxies as possible in order to gain a wider overview of the situation than could be acquired from a single proxy (Smol 2002; NRC 2005). Such an investigation is called a multiproxy study. In this essay about multi-proxy studies we shall concentrate on lake-sediment studies (palaeolimnology) in temperate areas, although one should be aware that successful multi-proxy studies have been carried out on peats (e.g. Booth and Jackson 2003; Pancost et al. 2003; Booth et al. 2004; Chambers and Charman 2004; Charman and Chambers 2004; Mighall et al. 2004), dendrochronological series (e.g. McCarroll et al. 2003), archaeological sites (e.g. Clark 1954; Wasylikowa et al. 1985; Davies et al. 2004; Selby et al. 2005), salt-marsh sediments (e.g. Gehrels et al. 2001), freshwater-marsh sediments (e.g. Finkelstein et al. 2005) and marine sediments (e.g. Andersson et al. 2003; Oldfield et al. 2003a; Risebrobakken et al. 2003; Haug et al. 2005), and in tropical (e.g. Verschuren et al. 2000; Velez et al. 2005) and extreme polar (e.g. Birks et al. 2004; Hodgson et al. 2005) environments. The earliest multi-proxy studies, reviewed by Wright (1966) and Birks and Birks (1980), used the palaeolimnological record to test ideas of lake ontogeny and biotic responses over time to external perturbations and internal processes. Although these studies used selected taxa and proxies and there was little or no statistical or numerical analysis, they provided elegant and carefully argued narratives, emphasising limnological processes and the role of catchment changes on lake dynamics. They are major contributions and in many ways they present a challenge to palaeolimnologists today to make further advances in our understanding of lake development and dynamics (Deevey 1984; Likens 1985). In palaeolimnological studies these days, a multi-proxy approach is the norm, but the aims of investigating ecosystem dynamics have turned more towards the reconstruction of past environments and climate changes (Lotter 2003). The synthesis of multi-proxy results in successful studies exceeds the sum of the component parts. However, as knowledge and experience expand, problems have become apparent in the use of some of these component parts for ecosystem reconstruction. Extensive and detailed reviews of multi-proxy studies in palaeolimnology and palaeoecology include Wright (1966), Birks and Birks (1980), Delcourt and Delcourt (1991), Smol (2002), Cohen (2003), Lotter (2003), Pienitz et al. (2004) and NRC (2005). The four volumes on palaeolimnological methods edited by Last and Smol (2001a, b) and Smol et al. (2001a, b) provide detailed accounts of the full range of field and analytical techniques currently available in palaeolimnology. The essential aspect of any multi-proxy study is that several proxies are used simultaneously to address the aims of the project. The methods used will, of course, be related to the research question under investigation. The study of lake sediments can be directed towards reconstructions of the aquatic environment and/or of the terrestrial catchment of the lake, even including the regional landscape beyond the catchment. The factors or processes behind the reconstructed changes (patterns) in the lake ecosystem can be sought in terms of causal processes such as changes in climate, both temperature and precipitation, or human activity that affect most aspects of lake ecosystem functioning. Often, more specific questions are asked concerning both natural and human-induced changes in lake-water quality and catchment characteristics, especially changes in vegetation and the catchment that affect the lake either directly or indirectly (Birks et al. 2000; Lotter and Birks 2003). The results of a multi-proxy study are usually presented and discussed in a descriptive or narrative way (Birks 1993a), using all the lines of evidence to reconstruct various aspects of the past ecosystem and to deduce the range of changes it has undergone. The value of any multiproxy study clearly rests on the reliability of the proxies used to reconstruct the past environmental conditions. Different proxies reflect different environmental factors at a range of spatial scales and consequently show different strengths and weaknesses. By combining proxies, strengths can be exploited and weaknesses can be identified (Mann 2002). However, weaknesses exposed by multi-proxy studies should not be ignored. They demonstrate shortcomings in methodology and resolution, limitations in taxonomic identifications, lack of understanding of the taphonomy of fossils, and gaps in our knowledge of the relationships of proxies, both biological and physical, to environmental factors. Thus important new lines of research may be stimulated. There have been many major advances in palaeolimnology in the last 25 years, as reviewed by Smol (2002). In the context of multi-proxy studies discussed here there have been at least six major areas of development. (1) the study of new proxies such as stable isotopes, near-infrared s (...truncated)