- Email: [email protected]
Ostracoda as Proxies for Quaternary Climate Change
Chapter 18
Ostracoda as Proxies for Quaternary Climate Change: Overview and Future Prospects David J. Horne1,2,*, Jonathan A. Holmes3, Julio Rodriguez-Lazaro4 and Finn A. Viehberg5 1
School of Geography, Queen Mary University of London, Mile End Road, London E1 4NS, UK, E-mail address: [email protected]
2
Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK, E-mail address: [email protected] Environmental Change Research Centre, Department of Geography, University College London, Gower Street, London, WC1E 6BT, UK, E-mail address:
3
[email protected] 4
Departamento de Estratigrafı´a y Paleontologı´a, Universidad del Paı´s Vasco UPV/EHU, Apartado 644, E-48080 BILBAO, Spain, E-mail address: julio.
[email protected] 5
Institute of Geology and Mineralogy, University of Cologne, Zuelpicher Str. 49a, D-50674 Cologne, Germany, E-mail address: [email protected]
*Corresponding author: e-mail: [email protected]
ABSTRACT Ostracod crustaceans are excellent Quaternary palaeoclimate proxies. As microfossils they supply evidence of past climatic conditions via indicator species, transfer function and mutual climatic range methods as well as the trace-element and stable-isotope geochemistry of their shells. We provide an overview of 17 contributions to the book Ostracoda as proxies for Quaternary climate change, highlight some of the emerging innovations and concerns, and assess the future prospects for ostracod applications in the field of Quaternary palaeoclimatology. The science of using ostracods as Quaternary palaeoclimate proxies has matured, well beyond the pioneering stage, and their application now needs to be tempered with critical awareness of their limitations. Key areas for future attention include palaeogenetics, improving knowledge of the ecology of living ostracods and the factors influencing their shell geochemistry, the establishment of global distributional databases and, as a necessary corollary to the last-mentioned, a programme of taxonomic harmonisation on a global scale. Finally, we emphasise the need for multi-proxy testing of methods, comparing ostracodinferred climatic parameters with those derived from other proxies such as beetles, chironomids and foraminifera. Keywords: Ostracoda, Pleistocene, Holocene, Palaeoclimate, Proxy
18.1 INTRODUCTION Ostracods, tiny crustaceans that are widespread in marine and continental aquatic environments and have an excellent fossil record from the Ordovician to the Recent, are valuable
Quaternary palaeoclimate proxies. They are, in effect, multiproxy organisms; their fossil shells not only provide wholeanimal evidence of past distributions from which palaeoclimate inferences can be drawn via indicator species, transfer function and mutual climatic range approaches, but also serve as handy packages of biogenic calcite in which stable isotopes and trace elements can be measured and interpreted as proxies, for example, for temperature and salinity. In recent decades, their application to Quaternary palaeoclimate studies has become increasingly common and sophisticated as new techniques have been developed, tested and adopted. In multi-disciplinary studies of Quaternary palaeoenvironments and climate change, ostracods have earned their place alongside other biological proxies that can readily be extracted in large quantities from small sediment samples, such as foraminifera, diatoms, molluscs, beetles, chironomid larvae and pollen. In view of these considerations, the compilation of a series of invited papers in our book Ostracoda as proxies for Quaternary climate change (Horne et al., 2012) is both timely and useful. As originally conceived, this book aimed to combine state-of-the-art reviews with cutting-edge research incorporating new data and interpretations as well as methodological developments, so as to establish a durable benchmark in the science of Ostracoda and Quaternary climate change. We believe that the resultant volume fulfils that aim, but what has emerged is a more mature and critical view of ostracods as palaeoclimate proxies than was envisaged when we began the
Developments in Quaternary Science. Vol. 17, http://dx.doi.org/10.1016/B978-0-444-53636-5.00018-4 ISSN: 1571-0866, # 2012 Elsevier B.V. All rights reserved.
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project. Here we offer a brief overview of the content of the volume, highlighting some of the emerging innovations and concerns, and assess the future prospects for ostracod applications in the field of Quaternary palaeoclimatology.
18.2 INTRODUCTION TO QUATERNARY AND LIVING OSTRACODS The crustacean class Ostracoda is both taxonomically and ecologically diverse. It is estimated that there are about 20,000 living species and more than twice that number have so far been described from their 450-million-year fossil record from the Ordovician to the Quaternary. Today they live in practically every aquatic environment from the deep ocean to freshwater lakes, rivers and temporary ponds, as a consequence of which their fossils offer potential as remarkably precise palaeoenvironmental indicators. In the first chapter of this volume, Rodriguez-Lazaro and Ruiz-Mun˜oz (2012) provide a concise, illustrated overview of ostracod morphology, classification, distribution, fossil record and applications. Mesquita-Joanes et al. (2012) follow with a critical review of recent developments in the study of ostracod ecology, drawing attention to gaps and weaknesses in our present knowledge and suggesting future avenues of research. Together these two chapters, with their extensive reference lists, provide a valuable introduction to the Ostracoda for anyone not already familiar with the group; such readers will find the introductory chapters in the AGU volume The Ostracoda: Application in Quaternary research (Holmes and Chivas, 2000) rich sources of further information.
18.3 OSTRACODS AS PALAEOCLIMATE PROXIES: ECOLOGICAL AND BIOGEOGRAPHICAL APPROACHES It has long been recognised that living marine and freshwater ostracod taxa have geographical distributions that are to a significant extent controlled by temperature, so that their fossil occurrences give some indication of palaeoclimate, if only in the sense of it having been warmer or colder than at present. Brady et al. (1874), for example, recognised the broad palaeoclimatic implications of different Quaternary ostracod assemblages from glacial and interglacial deposits. Subsequent work (e.g. Hazel, 1968) demonstrated the utility of shifts in faunal provinces and the temperature ranges of individual taxa as indicators of climate change. Lord et al. (2012) review such classical approaches, concluding that they remain valid on their own as well as in combination with newer, more sophisticated methods. The value of ostracod species as warm/cold palaeoclimate indicators has increased as knowledge of their modern distribution and ecology has improved. In the deep ocean, ostracods have been used as proxies for major water masses (e.g. North Atlantic Deep Water, Antarctic Bottom Water) that impinge
Ostracoda as Proxies for Quaternary Climate Change
on the ocean floor (e.g. Dingle and Lord, 1990); in the North Atlantic, fossil assemblages indicate ecosystem changes that can be correlated with climatic episodes such as Heinrich events and Bond cycles. Lord et al. also draw attention to the importance of distinguishing between “live” and “dead” components of both modern and fossil assemblages, since empty ostracod shells (being essentially sand-sized articles) are highly susceptible to post-mortem transport, sorting and mixing by current and wave action. Careful taphonomic assessment of fossil ostracod assemblages is a valuable prerequisite to palaeoenvironmental analysis (Boomer et al., 2003). In recent decades, increasingly sophisticated approaches, such as transfer functions, modern analogue techniques and mutual climatic range methods, have been employed for characterising and quantifying Quaternary climate using a variety of proxies such as beetles, pollen, diatoms, molluscs and chironomid larvae as well as ostracods. In fact, ostracods were one of the first biological proxies used to infer quantitative environmental variables in continental fossil records (Delorme et al., 1976, 1977). Such methods require large training data sets of ostracod distribution encompassing wide geographical and environmental gradients. Ostracod transfer functions are reviewed by Viehberg and Mesquita-Joanes (2012), who emphasise the importance of establishing (or at least assuming with justification) that there is a systematic relationship between the ostracod species and the environmental parameter (e.g. mean July air temperature) for which the transfer function is calculated. They summarise examples illustrating the potential utility of both marine and non-marine ostracod transfer functions for factors (such as air and water temperature, water conductivity and water depth) that can be interpreted in terms of climate. The advantages of transfer functions are that they use species assemblages, not just single species, provide error estimates for the inferred variable and do not require the identification of modern analogue assemblages. A disadvantage may be the unjustified belief that a transfer function can accurately reconstruct a single parameter, when species occurrences are in fact influenced by many environmental factors. Viehberg and MesquitaJoanes also warn that random changes in population dynamics (e.g. “ecological drift”) can produce a false impression of changes induced by climatic or other environmental variability. The adequacy of the training sets is a fundamental critical issue; many training sets are too geographically restricted to cover the full environmental/ climatic ranges of the species they contain, and some may not distinguish between “live” and “dead” components of modern assemblages, the latter problem also being discussed by Lord et al. (2012). Probably the best currently available example (as an ideal to emulate) is the Canadian non-marine ostracod data set established by L. D. Delorme in the 1960s and 1970s (Delorme et al., 1977; Delorme, 1989) which comprises records from over 5000 sites and
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distinguishes between ostracods represented by “mature live”, “immature live” and “shells only” records. Nevertheless, it fails to capture the full geographical (and climatic) ranges of many North American species, an issue which is addressed by Curry et al. (2012) who combine it with NANODe (North American Nonmarine Ostracode Database), a data set of ostracod distribution in the United States, to produced NACODe (North American Combined Ostracode Database). A supplementary atlas of selected species distributions based on NACODe is available on http://booksite.elsevier.com/9780444536365. Bunbury (2012) presents additional data from 60 lakes in the Canadian North filling some gaps in Delorme’s coverage, and employs a multivariate ostracod transfer function (Weighted-Averaging Partial Least Squares method) and the Modern Analogue Technique to infer mean July air temperatures in a sediment core from a Canadian (Yukon Territory) lake. The MAT compares fossil and modern assemblages to determine the dissimilarity between samples, and appears to perform slightly better than the transfer function method. Results suggest the Younger Dryas cooling (between 12,900 and 11,500 cal. yr BP according to Greenland ice-core records) was delayed until around 12,000 cal. yr BP in the Yukon Lake, in agreement with estimates from other proxies. Horne et al. (2012) review Mutual Climatic Range (MCR) methods, including the Delorme method (Delorme et al., 1976, 1977), which utilises only those components of a fossil assemblage that are found living together in a modern analogue assemblage today, and the Mutual Ostracod Temperature Range (MOTR) method (Horne, 2007) using all calibrated species in a fossil assemblage. The MOTR method has hitherto relied on the NODE (Nonmarine Ostracod Distribution in Europe) database to provide training data sets for calibration of species according to their mean July and January air temperature ranges; as for NACODe, a supplementary atlas of selected species distributions based on NODE is available on http://booksite.elsevier.com/ 9780444536365. Horne et al. present a revised calibration table that additionally takes account of North American records of certain species that are found on both sides of the Atlantic. In spite of such improvements in training data sets, it is clear that the coverage of many species’ distributions remains incomplete. A new venture aimed at addressing this issue is the OMEGA project (Ostracod Metadatabase of Environmental and Geographical Attributes; Horne et al., 2011), which seeks to integrate data from regional databases such as NANODe, ultimately to provide global coverage (Fig. 18.2). In parallel with this, it is essential to establish taxonomy and nomenclature that are consistent and reliable across the contributing databases. Several ostracod species that are common in Europe, such as Cytherissa lacustris, have wide intercontinental geographical ranges in the northern hemisphere; others, known by different names in different regions, may actually be conspecific. In other cases,
names of species originally described from Europe may have been applied incorrectly to North American taxa and vice versa. Taxonomic harmonisation is considered further below. Horne et al. (2012) make the important point that any single MCR proxy group (e.g. ostracods, beetles, herpetofauna) on its own will probably yield plausible palaeotemperature estimates; such estimates need to be validated, for example, by comparison with those derived from other proxies and other methods. Rigorous multi-proxy comparative testing is needed to improve and validate both the methods and the results obtained from them; ostracods are, in themselves, multi-proxy microfossils, providing material for, e.g., MCR, transfer function, MAT and geochemical methods.
18.4 OSTRACODS AS PALAEOCLIMATE PROXIES: GEOCHEMICAL APPROACHES Ostracod shell calcite has now been used in geochemical investigations for over 50 years. Most of the work to date has involved analyses of the trace-metal content of ostracod shells or the stable isotopes of oxygen and carbon. To date, most trace-metal investigations have used either magnesium or strontium. A few studies have combined traceelement analyses with stable-isotope determinations, in some cases on the same shells. A minority of investigations have used analysed trace metals other than Mg or Sr and isotopes of strontium, uranium and boron. Holmes and De Deckker (2012) trace the development of geochemical applications of ostracod shells, from the initial confirmation of shell mineralogy, back in the 1950s, through to the nearroutine use of both trace-element and stable-isotope analyses of ostracod shells in recent years. Despite the undoubted value of ostracod shells as sources of calcite for geochemical work in both marine and non-marine waters, Holmes and De Deckker caution against uncritical interpretation of the results and emphasise the need to be aware of areas in which understanding is currently lacking. The state-of-the-art is developed further for trace elements in the chapter by Dettman and Dwyer (2012) and for stable isotopes by Decrouy (2012). Although the basic principles governing trace-element partitioning into ostracod shells are the same regardless of environment, in reality, rather different considerations apply in marine versus non-marine waters. In the oceans, the Mg/Ca ratio of seawater has remained effectively constant during the Cenozoic, so that the only environmental control on the Mg content of ostracod shells is water temperature. Mg/Ca ratios of marine ostracods are therefore valuable palaeothermometers. In non-marine waters, variations in water composition mean that trace-element signatures may be hard to interpret. Existing calibration studies have often yielded contradictory and confusing results, meaning that
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stratigraphical records of trace-metal variations in ostracod shells from non-marine waters such as lakes must be interpreted with care. Dettman and Dwyer summarise the major controls on trace metals in ostracods both from marine and non-marine settings and outline areas in which further understanding is urgently needed, especially for non-marine environments. Decrouy summarises the main controls on oxygen and carbon isotope ratios of ostracod calcite. Most work has focused on non-marine settings, since in ocean sediments, most (but not all) stable-isotope analyses have targeted foraminiferal tests. One of the most intriguing observations, discovered previously and discussed by Decrouy, is the fact that ostracod shells are formed out of oxygen-isotope equilibrium with the water in which they live. Decrouy examines the potential reasons for this disequilibrium and raises the possibility that the magnitude of offset, which seems always to be positive, may be environmentally as well as taxonomically controlled. All geochemical work that utilises ostracods needs to account for the fact that ostracod shell formation is under strong biological control. Moreover, interpretations of geochemical signatures are enhanced if they take into account the ecology and life cycle of the taxa being analysed and, if possible, form part of multiple proxy investigations, where the additional climate proxies may be ostracod-related or derived from other biological or non-biological indicators.
18.5 MULTI-PROXY APPROACHES TO QUATERNARY PALAEOCLIMATE STUDIES As demonstrated above, fossil ostracods can provide several different lines of evidence for past environmental change, based on knowledge of their present-day diversity, biogeography and ecology as well as measurement and interpretation of their trace-element and stable-isotope shell chemistry. Smith and Palmer (2012) explore the use of combined geochemical and palaeoecological analyses in palaeoclimate investigations with reference to four scenarios for palaeolimnological records. The first, in which no significant changes are evident in either species assemblages or shell geochemistry, may reflect stable profundal conditions in a large lake; the implication is that assemblages in smaller lakes may be more likely to respond to hydrological and climatic change and thus yield more useful records. In the second scenario, ostracod assemblages show significant changes through time, while their shell geochemistry exhibits little variability; this is characteristic of groundwater-fed through-flow lakes. Scenario three, with pronounced changes in both species assemblages and shell geochemistry, reflects a more variable waterbody such as a lake with climate-induced changes in water depth and salinity. Finally the fourth scenario, in which shell geochemistry shows marked variability but
Ostracoda as Proxies for Quaternary Climate Change
species assemblages do not, may be indicative of changes in the source of the lake water, such as meltwater influxes. Long palaeolimnological records may incorporate several such scenarios through time, and it is the combination of palaeoecological and geochemical approaches that provides the versatility needed to interpret such complex histories. Complexity presents challenges, nevertheless, and in some circumstances, it may be difficult to distinguish between climatic and other influences. A case in point is provided by Anado´n et al. (2012) who combine ostracod stable-isotope and trace-element shell geochemistry with the MOTR method in order to reconstruct Middle Pleistocene to Holocene palaeoclimate in the Mediterranean region, using a sediment core extracted from an Italian maar lake. They find that local factors (e.g. volcanic–tectonic influences on water chemistry) militate to obscure the climatic signal in ostracod associations and valve geochemistry, and advise that for palaeoclimate studies, it is better to select lacustrine sequences from relatively simple, well-constrained hydrological systems. Many late Holocene/historical records are made even more complex by the influence of human activity on waterbodies. Ostracods provide valuable proxy evidence for both natural (climate-influenced) and human-induced environmental change in the Black, Caspian and Aral seas. Boomer (2012) demonstrates their utility with reference to palaeoecological and shell geochemical studies from each of the seas. Since the mid-twentieth century, excessive abstraction of water from the two rivers supplying the Aral Sea has led to a dramatic lowering of its level to the point at which it now consists of several separate, hydrologically distinct waterbodies, with elevated salinities reflected by their ostracod faunas. However, ostracod shells from sediments cores spanning the interval since the Last Glacial Maximum provide evidence of the sensitivity of the sea to climateinfluenced hydrological changes well before anthropogenic changes became significant. The deep basins of the Caspian Sea may have acted as refugia and centres of evolution since Miocene times, with connections to the Black Sea during intervals of high sea level. During Pleistocene cold phases, ice-dammed lakes may have backed up the usually northward-flowing drainage resulting in the temporary connection of the Black, Caspian and Aral Sea basins. Isolated from the Mediterranean by low eustatic sea level during the Last Glacial Maximum, the Black Sea was connected when sea level overtopped the Bosphorus sill in the early Holocene, allowing marine Mediterranean ostracod taxa to mix with and replace the Ponto-Caspian fauna. Ostracods have a major role to play in studies attempting to evaluate the controversial “Noah’s Flood” hypothesis about nature and timing of the connection, and Boomer demonstrates the use of ostracod shell geochemistry to indicate basinwide hydrological changes, including the effects of a meltwater pulse at around 17,000 BP and a trend towards more
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saline conditions between 8000 and 9000 BP, well before the main connection to the Mediterranean at around 7500 BP. Boomer cites the distinctive nature of the PontoCaspian ostracod fauna, with its many geographically restricted components, taxonomic uncertainties and a shortage of modern autecological data as features that currently limit the palaeoenvironmental and palaeoclimatic interpretation of assemblages. Cronin et al. (2012) reconstruct palaeoclimate for the past 800 years using marine/brackish ostracod palaeoecology and shell geochemistry from cores taken in Biscayne Bay, Florida, in which the link with climate is provided by palaeosalinity variations. In spite of difficulties arising from multiple influences on salinity in the coastal setting, including freshwater runoff, evaporation and groundwater as well as climate (through precipitation variation), they find oscillations reflecting multi-decadal and centennialscale climate variability during the Medieval Climate Anomaly and the Little Ice Age. They also note that some variation in ostracod shell carbon stable-isotope composition may be related to human-induced changes in bay circulation and productivity. The lakes on the Tibetan Plateau have been the focus of several recent studies using ostracods as proxies for Lateglacial and Holocene palaeoenvironmental and palaeoclimatic change. The abundant and diverse waterbodies, with relatively low levels of human impact in most of them, constitute a vast natural laboratory of environmental variability. Mischke (2012) reviews this work and discusses some of the difficulties encountered. Modern distributions of ostracods have been recorded on the plateau from more than 200 waterbodies, including saline as well as freshwater lakes, temporary ponds and flowing waters, providing valuable ecological data and training sets for the development of transfer functions for water conductivity and depth; Mischke advises caution in the use of the latter as the depth distribution of ostracods is strongly influenced by other parameters. The Tibetan Plateau data set has excellent potential to contribute to OMEGA, but, as observed earlier, the need for taxonomic harmonisation will be a significant issue; Mischke gives examples of different taxonomic approaches, with some authors regarding as separate species what others would consider to be ecophenotypic variants of a single species. Oxygen isotope and trace-element analyses of ostracod shells have helped to elucidate the hydrological and climatic histories of several lakes in the region. Echoing comments made by other authors in this volume, Mischke reminds us that the processes governing ostracod shell geochemistry are lakeand catchment-specific, so their interpretation requires the detailed assessment of the sites from which the cores were obtained. One of the lakes, Nam Co, is the focus of a detailed study of Holocene climate change by Wrozyna et al. (2012) using oxygen and carbon isotope measurements in ostracod
valves as well as interpretations of ostracod assemblage taphonomy and palaeoecology. Results confirm a long-term trend of increasing aridity already noted in other Tibetan Plateau lakes, the onset of which is, however, not synchronous across the plateau. Wrozyna et al. stress that the successful use of ostracod shell geochemistry in palaeoenvironmental reconstruction is dependent on understanding the chemical, biological and ecological influences on the chemical composition of valves, and comment that the multi-proxy approach facilitates an overview of the regional situation and an evaluation of the sensitivity of individual proxies to different climatic and environmental variables.
18.6 FUTURE DEVELOPMENTS: POTENTIAL AND LIMITATIONS OF OSTRACODS IN QUATERNARY PALAEOCLIMATOLOGY 18.6.1
Overview
We set out to compile a volume providing a comprehensive overview of the diversity of ostracod applications in Quaternary palaeoclimate investigations, drawing on the rich experience and expertise of a wide range of international authors. We believe that we have fulfilled this aim, but it is apparent that in its geographical coverage, the content of this volume is focused on the northern hemisphere, with a strong bias towards Europe and North America (Fig. 18.1). This reflects, to a large extent, the state-ofthe-art; it was never our attention deliberately to exclude southern and eastern areas, the book just turned out that way, and here we draw readers’ attention to examples of excellent studies conducted in other parts of the world. In Australia, Gouramanis et al. (2010) used lacustrine ostracod palaeoecology and shell geochemistry to produce a highresolution palaeoclimate record spanning the past 6000 years. Holocene climate-related lake-level and waterchemistry changes were interpreted using ostracod assemblages in lake sediment cores from Lake Titicaca, Bolivia, by Mourguiart et al. (1998). Schwalb et al. (1999) were able to distinguish between local environmental and regional climate signals in their interpretation of ostracod shell stable isotopes from Chilean Altiplano lakes, enabling the recognition of Holocene wet and dry phases, including rapid short-term shifts in the mid- to late-Holocene transition interval. Schwalb et al. (2002) presented extensive data on the ecology and stable-isotope ratios of ostracods inhabiting fresh waters in Argentina, creating a valuable training set for future use in palaeoclimatology based on lake sediment cores. In the East African Rift Valley, Alin and Cohen (2003) used ostracods to reconstruct lake-level changes in Lake Tanganyika, which could be related to late Holocene climate changes; many of their listed taxa are left
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Ostracoda as Proxies for Quaternary Climate Change
FIGURE 18.1 Geographical sites and regions of selected examples and case histories of ostracods mentioned in the text. (A) USA Atlantic coast, Gulf of Alaska and northwest Europe (Lord et al., 2012); (B) British Pleistocene sites (Horne et al., 2012); (C) NACODe coverage region (Curry et al., 2012); (D) Canadian North data set and transfer function application (Bunbury, 2012); (E) North Atlantic core site (Dettman and Dwyer, 2012); (F) Lake Geneva (Decrouy, 2012); (G) lakes in North Dakota (Smith and Palmer, 2012); (H) Black, Caspian and Aral seas (Boomer, 2012); (I) Valle di Castiglione crater lake (Anado´n et al., 2012); (J) Biscayne Bay, Florida (Cronin et al., 2012); (K) Tibetan Plateau (Mischke, 2012; Wrozyna et al., 2012).
in open nomenclature, hinting at the possibility of more sophisticated and extensive analyses when all of their ostracods are identified to species level. A series of papers presenting the results of ostracod studies in Siberia provides modern ecological and stable-isotope data as well as palaeoclimatic interpretations of late Quaternary fossil freshwater ostracod assemblages and shell geochemistry (Wetterich et al., 2005, 2007, 2008a, 2008b, 2008c, 2009). If there is one clear message common to the diverse papers in this volume, it is that the science of using ostracods as Quaternary palaeoclimate proxies has matured to the point at which the various techniques have been developed well beyond the pioneering stage and their application now needs to be tempered with critical awareness of their limitations. Overconfidence is dangerous; it is important not to forget the many underlying assumptions that allow us to proceed with palaeoclimate methods in spite of lacking key information. It is also vital to realise that while mutual climatic range, transfer function, geochemical and other methods may be well-established in the literature, this does not mean that such techniques can simply be used “off the shelf” by non-specialists; their successful application requires skill, experience and (once again) a critical awareness of their limitations. Some of the key areas in
which future research is most needed, and is likely to lead to significant advances in this field, are outlined below.
18.6.2
Ostracod Palaeogenetics
In the final research paper in this volume, Scho¨n et al. (2012) explore the potential for using ostracod DNA to infer palaeoenvironmental and palaeoclimatic changes on various timescales. They consider it unlikely that DNA could be extracted from fossil ostracod shells, soft parts or resting eggs more than about 10,000 years old, but point out that it is feasible to seek evidence of the effects of past climate change in the population genetics of living ostracods. Ancient lakes, such as 28-million-year-old Lake Tanganyika, which are home to diverse species flocks offer some of the best opportunities to pursue such studies.
18.6.3 Improving Knowledge of Ostracod Ecology There is much to learn about ostracod ecology, both marine and non-marine. While significant advances have been made in our understanding of the influences of abiotic factors such as salinity and water temperature, some
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fundamental aspects remain largely unknown. We know little about exactly where (e.g. at the sediment–water interface or interstitially?) and how quickly ostracods moult and calcify their new shells, and lifecycle details are known for only a small proportion of species, all of which information is important for the interpretation of the traceelement and isotopic composition of ostracod shells. Mutual climatic range and transfer function methods assume that factors such as mean January and July air temperature are significant influences on ostracod species’ distributions, yet the biological basis for such assumptions is, as yet, established for only a very few species.
18.6.4
Improving Distributional Data Sets
Many ostracod species have very wide distributions, which means that regional distributional and databases rarely capture fully the climatic ranges of living ostracod species. The OMEGA metadatabase project (Fig. 18.2) addresses this problem by seeking to make appropriate data available on a global scale. In OMEGA, ostracod taxonomic names and the coordinates of localities from which they were recorded are considered to be both data and metadata; their inclusion on the metadatabase will ensure that distributions can be mapped easily and facilitate the answering of such questions as “which regional databases contain data on the species Darwinula stevensoni?” Compiling such a metadatabase is not simply a matter of adding existing data sets together, however, and the global harmonisation of taxonomy and nomenclature is both a necessity and a major challenge.
18.6.5
Taxonomic Harmonisation
There are many factors influencing the distributions of ostracod taxa, but political boundaries are not among these, except in the sense that ostracod specialists working on one country or region have sometimes named a species without realising that it was already known under a different name somewhere else. As we develop and apply the methods now available for using ostracods as palaeoclimate proxies, it is vital that we seek not only to utilise data sets that are more extensive (both geographically and in climate space) in order to capture more completely the climatic ranges of ostracod taxa, but also to ensure that we are using agreed, consistent taxonomic nomenclature. The integration of non-marine ostracod databases for Canada and the United States by Curry et al. (2012) demonstrates both issues. They note, for example, that the ostracod listed in the Delorme Canadian database as Limnocythere herricki Staplin, 1963 is considered to be synonymous with Limnocythere varia Staplin, 1963. They use the latter name in the combined data set (NACODe), but since the former name
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actually appears first in Staplin’s publication it could be argued that it should be used, due to page priority; the situation is complicated further by Staplin’s cautionary suggestion that his own new species L. varia might simply be a larger variety of Limnocythere illinoisensis Sharpe, 1897. It is important to realise that taxonomic opinions vary and may change in the light of new information; it is essential that distributional databases should retain sufficient information to allow new taxonomic revisions to be applied and accommodated (at the very least, not only the currently accepted name for a species, but also the name under which it was first recorded at any given locality in a database). It is clear that many species occur in both countries and are widespread in North America. Neither NANODe nor the Delorme Canadian database on their own are adequate to delimit the North American distributions of such species as Cypridopsis vidua (O.F. Mu¨ller, 1776) or Fabaeformiscandona rawsoni (Tressler, 1957); on the other hand, the same combined data sets show that certain species are confined to Arctic Canada (e.g. Candona protzi Hartwig, 1898) (see Fig. 6.12 in Curry et al., 2012). The last-mentioned species is of interest because it is also recorded in Europe, where it was originally described and is now assigned by many authors to the candonid genus Fabaeformiscandona. Are the Canadian and European taxa really the same, and, noting that Curry et al. use Fabaeformiscandona for some candonid species, but not this one, which is the correct generic assignment? The abovementioned C. vidua is another European species that appears to be widespread in North America; detailed studies at morphological and molecular levels are needed to ascertain whether this is really a single taxon with very broad environmental and climatic tolerances or represents several distinct, more stenotopic taxa that just happen to look very similar. A European study of mitochondrial DNA in Eucypris virens, a ubiquitous temporary pond species, demonstrated the existence of more than 40 cryptic species, predominantly asexual but including sexual populations from the Mediterranean area, which cannot, however, be distinguished on the basis of shell morphology. This serves to remind us that when working with fossil material we are constrained to use a morphospecies concept, which may not reflect the biological and ecological diversity of the animal in question, especially in the case of parthenogenetic taxa with multiple coexisting clonal lineages. It is arguable whether knowledge of the existence of cryptic species, identified by molecular methods, is a help or a hindrance in palaeoclimatic applications of ostracods. To some extent the same argument applies to the use of “soft parts” (e.g. appendages) to distinguish between living species, given that in the vast majority of cases those working with fossil material can only use
FIGURE 18.2 Distribution of ostracod records in OMEGA; (A) Europe, from NODE database (circles); (B) North America, from NANODe (squares), Delorme Database (circles) and Bunbury (2012) (triangles); (C) (inset) shows detail of Great Lakes region.
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“hard parts” (i.e. shells). The Canadian freshwater ostracod Candona acutula Delorme, 1967 is a useful cold-climate indicator, today restricted to areas with mean January air temperature no higher than 7 C, and (coincidentally) a species that appears to respect the U.S. border since the southern limit of its distribution is latitude 49 N. During Pleistocene cold, glacio-pluvial intervals, it occurred further south, in the Great Salt Lake Basin of Utah (Lister, 1975). Horne et al. (2011) have suggested that it may be synonymous with a European species, Fabaeformiscandona levanderi (Hirschmann, 1912); even if detailed appendage differences eventually show the two species to be distinct, there may be a case for nevertheless regarding them as a single palaeontological morphospecies with palaeoclimatic utility, since they cannot be separated on the basis of their shell morphology alone.
their estimations of mean July air temperature), it inspires confidence in the methods; when they do not, methods and their underlying assumptions are challenged and may need re-thinking.
18.6.6
REFERENCES
Improving Geochemical Methods
Future work will involve additional calibration studies. These are especially important for the trace-element partitioning in non-marine taxa, for which geochemical signatures may be complex. However, additional work is also needed for stable isotopes and for trace metals in marine ostracods in order to enhance the palaeoenvironmental value of these signatures. Finally, the potential for analysing trace metals other than Mg and Sr has not yet been properly investigated. Such metals might include heavy metals derived from anthropogenic pollution and redoxsensitive elements such as iron, manganese and uranium. Although pilot studies have hinted at the value of such metals, a systematic understanding of their partitioning into ostracod shells is currently lacking.
18.6.7
Multi-proxy Testing
Any single proxy method on its own can (and likely to) yield plausible quantitative estimates of palaeoclimatic parameters. The question should always be asked: why should we believe these results? There has been a tendency to accept results uncritically, and the testing of methods has been limited in scope. Several papers in this volume address issues of multi-proxy applications of palaeoclimate methods, leading to a more critical understanding of the efficacy of such methods. Such approaches are to be encouraged and emulated in future. Ostracods are in themselves multi-proxy microfossils, and there is much to be gained from the combination of geochemical techniques with those based on ecology and distribution. Comparisons of ostracod results with those from other palaeoclimate proxies (e.g. diatoms, chironomid larvae, beetles, molluscs, foraminifera, pollen) will be informative and should lead to advances. When two or more proxies agree (in, for example,
ACKNOWLEDGEMENTS We would like to express our gratitude to all of the authors who have contributed to this volume for engaging with the project and for sharing with us, the editors, their exciting research findings and ideas. Each chapter in this volume was peer-reviewed by at least two reviewers, some of whom are acknowledged by their name in individual chapters, while other chose to remain anonymous; we thank them all for responding so positively, efficiently and effectively, often at short notice, to our invitations to review. We are especially grateful to the series editor, Jaap van der Meer, for his patient guidance and encouragement throughout the project.
Alin, S.R. & Cohen, A.S., 2012. Lake-level history of Lake Tanganyika, East Africa, for the past 2500 years based on ostracode-inferred water-depth reconstruction. Palaeogeography, Palaeoclimatology, Palaeoecology 199, 31–49. Anado´n, P., Gliozzi, E. & Mazzini, I., 2012. Geochemical and palaeoecological analyses of Mid Pleistocene to Holocene ostracod assemblages from Valle di Castiglione (Italy): palaeoenvironmental and palaeoclimatic assessment. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 217–240. Elsevier. Boomer, I., 2012. Ostracoda as indicators of climatic and human-influenced changes in the Late Quaternary of the Ponto-Caspian region (Aral, Caspian and Black seas). In: Horne, D.J., Holmes, J.A., RodriguezLazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 205–215. Elsevier. Boomer, I., Horne, D.J. & Slipper, I.J., 2003. The use of ostracods in palaeoenvironmental studies, or what can you do with an ostracod shell? In: Park, L.E. & Smith, A.J. (Eds), Bridging the Gap—Trends in the Ostracode Biological and Geological Sciences, Paleontological Society Papers, 153–179. Brady, G.S., Crosskey, H.W. & Robertson, D., 1874. A Monograph of the Post-Tertiary Entomostraca of Scotland including species from England and Ireland. Monograph of the Palaeontographical Society, v + 232 pp. Bunbury, J., 2012. Distribution of freshwater ostracods in the Canadian North and implications for palaeoclimate reconstructions. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 117–129. Elsevier. Cronin, T.M., Wingard, G.L., Dwyer, G.S., Swart, P.K., Willard, D.A. & Albietz, J., 2012. Climate variability during the Medieval Climate Anomaly and Little Ice Age based on ostracod faunas and shell geochemistry from Biscayne Bay, Florida. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 241–262. Elsevier.
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Curry, B.B., Delorme, L.D., Smith, A.J., Palmer, D.F. & Stiff, B.J., 2012. The biogeography and physicochemical characteristics of aquatic habitats of freshwater ostracods in Canada and the United States. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 85–115. Elsevier. Decrouy, L., 2012. Biological and environmental controls on isotopes in ostracod shells. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 165–181. Elsevier. Delorme, L.D., 1989. Methods in Quaternary Science #7. Freshwater ostracodes. Geoscience Canada 16, 85–90. Delorme, L.D., Zoltai, S.C. & Kalas, L.L., 1976. Freshwater shelled invertebrate indicators of paleoclimate in northwestern Canada during the late glacial. In: Horie, S. (Ed.), Paleolimnology of Lake Biwa and the Japanese Pleistocene (vol. 4) 606–657. Delorme, L.D., Zoltai, S.C. & Kalas, L.L., 1977. Freshwater shelled invertebrate indicators of paleoclimate in northwestern Canada during late glacial times. Canadian Journal of Earth Sciences 14, 2029–2046. Dettman, D.L. & Dwyer, G.S., 2012. The calibration of environmental controls on elemental ratios in ostracod shell calcite: a critical assessment. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 145–163. Elsevier. Dingle, R.V. & Lord, A.R., 1990. Benthic ostracods and deep water-masses in the Atlantic Ocean. Palaeogeography, Palaeoclimatology, Palaeoecology 80, 213–235. Gouramanis, C., Wilkins, D. & De Deckker, P., 2010. 6000 years of environmental changes recorded in Blue Lake, South Australia, based on ostracod ecology and valve chemistry. Palaeogeography, Palaeoclimatology, Palaeoecology 297, 223–237. Hazel, J.E., 1968. Pleistocene ostracode zoogeography in Atlantic coast submarine canyons. Journal of Paleontology 42, 1264–1271. Holmes, J.A. & Chivas, A. (Eds), 2000. The Ostracoda: Applications in Quaternary Research. Geophysical Monograph 131, American Geophysical Union, Washington, 313 pp. Holmes, J.A. & De Deckker, P., 2012. The chemical composition of ostracod shells: applications in Quaternary palaeoclimatology. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 131–143. Elsevier. Horne, D.J., 2007. A Mutual Temperature Range method for Quaternary palaeoclimatic analysis using European nonmarine Ostracoda. Quaternary Science Reviews 26, 1398–1415. Horne, D.J., Curry, B.B. & Mesquita-Joanes, F., 2012. Mutual climatic range methods for Quaternary ostracods. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 65–84. Elsevier. Horne, D.J., Curry, B.B., Delorme, D.L., Martens, K., Smith, A.J. & Smith, R.J., 2011. OMEGA: the Ostracod Metadatabase of Environmental and Geographical Attributes. Joannea Geologie und Pala¨ontologie 11, 80–84.
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Lister, K.H., 1975. Quaternary freshwater Ostracoda from the Great Salt Lake Basin, Utah. The University of Kansas Paleontological Contributions, Paper 78, 34 pp., 5 pls. Lord, A.R., Boomer, I., Brouwers, E. & Whittaker, J.E., 2012. Ostracod taxa as palaeoclimate indicators in the Quaternary. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 37–45. Elsevier. Mesquita-Joanes, F., Smith, A.J. & Viehberg, F., 2012. The ecology of Ostracoda across levels of biological organisation from individual to ecosystem: a review of recent developments and future potential. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 15–35. Elsevier. Mischke, S., 2012. Quaternary ostracods from the Tibetan Plateau and their significance for environmental and climate-change studies. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change. Developments in Quaternary Science 17, 263–279. Elsevier. Mourguiart, P., Corre`ge, T., Wirrman, D., Argollo, J., Montenegro, M.E., Pourchet, M. & Carbonel, P., 1998. Holocene palaeohydrology of Lake Titicaca estimated from an ostracod-based transfer function. Palaeogeography, Palaeoclimatology, Palaeoecology 143, 51–72. Rodriguez-Lazaro, J. & Ruiz-Mun˜oz, F., 2012. A general introduction to ostracods: morphology, distribution, fossil record and applications. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 1–14. Elsevier. Scho¨n, I., Eggermont, H., Verheyen, E. & Martens, K., 2012. Palaeogenetics for ostracods (Crustacea, Ostracoda). In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 297–304. Elsevier. Schwalb, A., Burns, S.J. & Kelts, K., 1999. Holocene environments from stable isotope stratigraphy of ostracods and authigenic carbonate in Chilean Altiplano lakes. Palaeogeography, Palaeoclimatology, Palaeoecology 148, 153–168. Schwalb, A., Burns, S.J., Cusminsky, G., Kelts, K. & Markgraf, V., 2002. Assemblage diversity and isotopic signals of modern ostracodes and host waters from Patagonia, Argentina. Palaeogeography, Palaeoclimatology, Palaeoecology 187, 323–339. Smith, A.J. & Palmer, D.F., 2012. The versatility of Quaternary ostracods as palaeoclimate proxies: comparative testing of geochemical, ecological and biogeographical approaches. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 183–203. Elsevier. Viehberg, F. & Mesquita-Joanes, F., 2012. Quantitative transfer function approaches in palaeoclimatic reconstruction using Quaternary ostracods. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 47–64. Elsevier. Wetterich, S., Schirrmeister, L. & Pietrzeniuk, E., 2005. Freshwater ostracodes in Quaternary permafrost deposits in the Siberian Arctic. Journal of Paleolimnology 34, 363–376.
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Wertterich, S., Schirrmeister, L., Meyer, H. & Siegert, C., 2007. Thermokarst lakes in Central Yakutia (Siberia) as habitats of freshwater ostracods and archives of palaeoclimate. In: Kane, D.L. & Hinkel, K.M. (Eds). Ninth International Conference on Permafrost, Institute of Northern Engineering, University of Alaska Fairbanks, Alaska, USA, pp. 1945–1950. Wetterich, S., Herzschuh, U., Meyer, H., Pestryakova, L., Plessen, B., Lopez, C.M.L. & Schirrmeister, L., 2008a. Evaporation effects as reflected in freshwaters and ostracod calcite from modern environments in Central and Northeast Yakutia (East Siberia, Russia). Hydrobiologia 614, 171–195. Wetterich, S., Kuzmina, S., Andreev, A.A., Kienast, F., Meyer, H., Schirrmeister, L., Kuznetsova, T. & Sierralta, M., 2008b. Palaeoenvironmental dynamics inferred from late Quaternary permafrost deposits on Kurungnakh Island, Lena Delta, Northeast Siberia, Russia. Quaternary Science Reviews 27, 1523–1540.
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Wetterich, S., Schirrmeister, L., Meyer, H., Viehberg, F.A. & Mackensen, A., 2008c. Arctic freshwater ostracods from modern periglacial environments in the Lena River Delta (Siberian Arctic, Russia): geochemical applications for palaeoenvironmental reconstructions. Journal of Paleolimnology 39, 427–449. Wetterich, S., Schirrmeister, L., Andreev, A.A., Pudenz, M., Plessen, B., Meyer, H. & Kunitsky, V.V., 2009. Eemian and Late Glacial/Holocene palaeoenvironmental records from permafrost sequences at the Dmitry Laptev Strait (NE Siberia, Russia). Palaeogeography, Palaeoclimatology, Palaeoecology 279, 73–95. Wrozyna, C., Frenzel, P., Daut, G., Ma¨usbacher, R., Zhu, L. & Schwalb, A., 2012. Holocene lake level changes of Lake Nam Co, Tibetan Plateau, deduced from ostracod assemblages and d18O and d13C signatures of their valves. In: Horne, D.J., Holmes, J.A., Rodriguez-Lazaro, J. & Viehberg, F. (Eds), Ostracoda as Proxies for Quaternary Climate Change, Developments in Quaternary Science 17, 281–295. Elsevier.
Ostracoda as Proxies for Quaternary Climate Change: Overview and Future Prospects David J. Horne1,2,*, Jonathan A. Holmes3, Julio Rodriguez-Lazaro4 and Finn A. Viehberg5 1
School of Geography, Queen Mary University of London, Mile End Road, London E1 4NS, UK, E-mail address: [email protected]
2
Department of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK, E-mail address: [email protected] Environmental Change Research Centre, Department of Geography, University College London, Gower Street, London, WC1E 6BT, UK, E-mail address:
3
[email protected] 4
Departamento de Estratigrafı´a y Paleontologı´a, Universidad del Paı´s Vasco UPV/EHU, Apartado 644, E-48080 BILBAO, Spain, E-mail address: julio.
[email protected] 5
Institute of Geology and Mineralogy, University of Cologne, Zuelpicher Str. 49a, D-50674 Cologne, Germany, E-mail address: [email protected]
*Corresponding author: e-mail: [email protected]
ABSTRACT Ostracod crustaceans are excellent Quaternary palaeoclimate proxies. As microfossils they supply evidence of past climatic conditions via indicator species, transfer function and mutual climatic range methods as well as the trace-element and stable-isotope geochemistry of their shells. We provide an overview of 17 contributions to the book Ostracoda as proxies for Quaternary climate change, highlight some of the emerging innovations and concerns, and assess the future prospects for ostracod applications in the field of Quaternary palaeoclimatology. The science of using ostracods as Quaternary palaeoclimate proxies has matured, well beyond the pioneering stage, and their application now needs to be tempered with critical awareness of their limitations. Key areas for future attention include palaeogenetics, improving knowledge of the ecology of living ostracods and the factors influencing their shell geochemistry, the establishment of global distributional databases and, as a necessary corollary to the last-mentioned, a programme of taxonomic harmonisation on a global scale. Finally, we emphasise the need for multi-proxy testing of methods, comparing ostracodinferred climatic parameters with those derived from other proxies such as beetles, chironomids and foraminifera. Keywords: Ostracoda, Pleistocene, Holocene, Palaeoclimate, Proxy
18.1 INTRODUCTION Ostracods, tiny crustaceans that are widespread in marine and continental aquatic environments and have an excellent fossil record from the Ordovician to the Recent, are valuable
Quaternary palaeoclimate proxies. They are, in effect, multiproxy organisms; their fossil shells not only provide wholeanimal evidence of past distributions from which palaeoclimate inferences can be drawn via indicator species, transfer function and mutual climatic range approaches, but also serve as handy packages of biogenic calcite in which stable isotopes and trace elements can be measured and interpreted as proxies, for example, for temperature and salinity. In recent decades, their application to Quaternary palaeoclimate studies has become increasingly common and sophisticated as new techniques have been developed, tested and adopted. In multi-disciplinary studies of Quaternary palaeoenvironments and climate change, ostracods have earned their place alongside other biological proxies that can readily be extracted in large quantities from small sediment samples, such as foraminifera, diatoms, molluscs, beetles, chironomid larvae and pollen. In view of these considerations, the compilation of a series of invited papers in our book Ostracoda as proxies for Quaternary climate change (Horne et al., 2012) is both timely and useful. As originally conceived, this book aimed to combine state-of-the-art reviews with cutting-edge research incorporating new data and interpretations as well as methodological developments, so as to establish a durable benchmark in the science of Ostracoda and Quaternary climate change. We believe that the resultant volume fulfils that aim, but what has emerged is a more mature and critical view of ostracods as palaeoclimate proxies than was envisaged when we began the
Developments in Quaternary Science. Vol. 17, http://dx.doi.org/10.1016/B978-0-444-53636-5.00018-4 ISSN: 1571-0866, # 2012 Elsevier B.V. All rights reserved.
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project. Here we offer a brief overview of the content of the volume, highlighting some of the emerging innovations and concerns, and assess the future prospects for ostracod applications in the field of Quaternary palaeoclimatology.
18.2 INTRODUCTION TO QUATERNARY AND LIVING OSTRACODS The crustacean class Ostracoda is both taxonomically and ecologically diverse. It is estimated that there are about 20,000 living species and more than twice that number have so far been described from their 450-million-year fossil record from the Ordovician to the Quaternary. Today they live in practically every aquatic environment from the deep ocean to freshwater lakes, rivers and temporary ponds, as a consequence of which their fossils offer potential as remarkably precise palaeoenvironmental indicators. In the first chapter of this volume, Rodriguez-Lazaro and Ruiz-Mun˜oz (2012) provide a concise, illustrated overview of ostracod morphology, classification, distribution, fossil record and applications. Mesquita-Joanes et al. (2012) follow with a critical review of recent developments in the study of ostracod ecology, drawing attention to gaps and weaknesses in our present knowledge and suggesting future avenues of research. Together these two chapters, with their extensive reference lists, provide a valuable introduction to the Ostracoda for anyone not already familiar with the group; such readers will find the introductory chapters in the AGU volume The Ostracoda: Application in Quaternary research (Holmes and Chivas, 2000) rich sources of further information.
18.3 OSTRACODS AS PALAEOCLIMATE PROXIES: ECOLOGICAL AND BIOGEOGRAPHICAL APPROACHES It has long been recognised that living marine and freshwater ostracod taxa have geographical distributions that are to a significant extent controlled by temperature, so that their fossil occurrences give some indication of palaeoclimate, if only in the sense of it having been warmer or colder than at present. Brady et al. (1874), for example, recognised the broad palaeoclimatic implications of different Quaternary ostracod assemblages from glacial and interglacial deposits. Subsequent work (e.g. Hazel, 1968) demonstrated the utility of shifts in faunal provinces and the temperature ranges of individual taxa as indicators of climate change. Lord et al. (2012) review such classical approaches, concluding that they remain valid on their own as well as in combination with newer, more sophisticated methods. The value of ostracod species as warm/cold palaeoclimate indicators has increased as knowledge of their modern distribution and ecology has improved. In the deep ocean, ostracods have been used as proxies for major water masses (e.g. North Atlantic Deep Water, Antarctic Bottom Water) that impinge
Ostracoda as Proxies for Quaternary Climate Change
on the ocean floor (e.g. Dingle and Lord, 1990); in the North Atlantic, fossil assemblages indicate ecosystem changes that can be correlated with climatic episodes such as Heinrich events and Bond cycles. Lord et al. also draw attention to the importance of distinguishing between “live” and “dead” components of both modern and fossil assemblages, since empty ostracod shells (being essentially sand-sized articles) are highly susceptible to post-mortem transport, sorting and mixing by current and wave action. Careful taphonomic assessment of fossil ostracod assemblages is a valuable prerequisite to palaeoenvironmental analysis (Boomer et al., 2003). In recent decades, increasingly sophisticated approaches, such as transfer functions, modern analogue techniques and mutual climatic range methods, have been employed for characterising and quantifying Quaternary climate using a variety of proxies such as beetles, pollen, diatoms, molluscs and chironomid larvae as well as ostracods. In fact, ostracods were one of the first biological proxies used to infer quantitative environmental variables in continental fossil records (Delorme et al., 1976, 1977). Such methods require large training data sets of ostracod distribution encompassing wide geographical and environmental gradients. Ostracod transfer functions are reviewed by Viehberg and Mesquita-Joanes (2012), who emphasise the importance of establishing (or at least assuming with justification) that there is a systematic relationship between the ostracod species and the environmental parameter (e.g. mean July air temperature) for which the transfer function is calculated. They summarise examples illustrating the potential utility of both marine and non-marine ostracod transfer functions for factors (such as air and water temperature, water conductivity and water depth) that can be interpreted in terms of climate. The advantages of transfer functions are that they use species assemblages, not just single species, provide error estimates for the inferred variable and do not require the identification of modern analogue assemblages. A disadvantage may be the unjustified belief that a transfer function can accurately reconstruct a single parameter, when species occurrences are in fact influenced by many environmental factors. Viehberg and MesquitaJoanes also warn that random changes in population dynamics (e.g. “ecological drift”) can produce a false impression of changes induced by climatic or other environmental variability. The adequacy of the training sets is a fundamental critical issue; many training sets are too geographically restricted to cover the full environmental/ climatic ranges of the species they contain, and some may not distinguish between “live” and “dead” components of modern assemblages, the latter problem also being discussed by Lord et al. (2012). Probably the best currently available example (as an ideal to emulate) is the Canadian non-marine ostracod data set established by L. D. Delorme in the 1960s and 1970s (Delorme et al., 1977; Delorme, 1989) which comprises records from over 5000 sites and
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distinguishes between ostracods represented by “mature live”, “immature live” and “shells only” records. Nevertheless, it fails to capture the full geographical (and climatic) ranges of many North American species, an issue which is addressed by Curry et al. (2012) who combine it with NANODe (North American Nonmarine Ostracode Database), a data set of ostracod distribution in the United States, to produced NACODe (North American Combined Ostracode Database). A supplementary atlas of selected species distributions based on NACODe is available on http://booksite.elsevier.com/9780444536365. Bunbury (2012) presents additional data from 60 lakes in the Canadian North filling some gaps in Delorme’s coverage, and employs a multivariate ostracod transfer function (Weighted-Averaging Partial Least Squares method) and the Modern Analogue Technique to infer mean July air temperatures in a sediment core from a Canadian (Yukon Territory) lake. The MAT compares fossil and modern assemblages to determine the dissimilarity between samples, and appears to perform slightly better than the transfer function method. Results suggest the Younger Dryas cooling (between 12,900 and 11,500 cal. yr BP according to Greenland ice-core records) was delayed until around 12,000 cal. yr BP in the Yukon Lake, in agreement with estimates from other proxies. Horne et al. (2012) review Mutual Climatic Range (MCR) methods, including the Delorme method (Delorme et al., 1976, 1977), which utilises only those components of a fossil assemblage that are found living together in a modern analogue assemblage today, and the Mutual Ostracod Temperature Range (MOTR) method (Horne, 2007) using all calibrated species in a fossil assemblage. The MOTR method has hitherto relied on the NODE (Nonmarine Ostracod Distribution in Europe) database to provide training data sets for calibration of species according to their mean July and January air temperature ranges; as for NACODe, a supplementary atlas of selected species distributions based on NODE is available on http://booksite.elsevier.com/ 9780444536365. Horne et al. present a revised calibration table that additionally takes account of North American records of certain species that are found on both sides of the Atlantic. In spite of such improvements in training data sets, it is clear that the coverage of many species’ distributions remains incomplete. A new venture aimed at addressing this issue is the OMEGA project (Ostracod Metadatabase of Environmental and Geographical Attributes; Horne et al., 2011), which seeks to integrate data from regional databases such as NANODe, ultimately to provide global coverage (Fig. 18.2). In parallel with this, it is essential to establish taxonomy and nomenclature that are consistent and reliable across the contributing databases. Several ostracod species that are common in Europe, such as Cytherissa lacustris, have wide intercontinental geographical ranges in the northern hemisphere; others, known by different names in different regions, may actually be conspecific. In other cases,
names of species originally described from Europe may have been applied incorrectly to North American taxa and vice versa. Taxonomic harmonisation is considered further below. Horne et al. (2012) make the important point that any single MCR proxy group (e.g. ostracods, beetles, herpetofauna) on its own will probably yield plausible palaeotemperature estimates; such estimates need to be validated, for example, by comparison with those derived from other proxies and other methods. Rigorous multi-proxy comparative testing is needed to improve and validate both the methods and the results obtained from them; ostracods are, in themselves, multi-proxy microfossils, providing material for, e.g., MCR, transfer function, MAT and geochemical methods.
18.4 OSTRACODS AS PALAEOCLIMATE PROXIES: GEOCHEMICAL APPROACHES Ostracod shell calcite has now been used in geochemical investigations for over 50 years. Most of the work to date has involved analyses of the trace-metal content of ostracod shells or the stable isotopes of oxygen and carbon. To date, most trace-metal investigations have used either magnesium or strontium. A few studies have combined traceelement analyses with stable-isotope determinations, in some cases on the same shells. A minority of investigations have used analysed trace metals other than Mg or Sr and isotopes of strontium, uranium and boron. Holmes and De Deckker (2012) trace the development of geochemical applications of ostracod shells, from the initial confirmation of shell mineralogy, back in the 1950s, through to the nearroutine use of both trace-element and stable-isotope analyses of ostracod shells in recent years. Despite the undoubted value of ostracod shells as sources of calcite for geochemical work in both marine and non-marine waters, Holmes and De Deckker caution against uncritical interpretation of the results and emphasise the need to be aware of areas in which understanding is currently lacking. The state-of-the-art is developed further for trace elements in the chapter by Dettman and Dwyer (2012) and for stable isotopes by Decrouy (2012). Although the basic principles governing trace-element partitioning into ostracod shells are the same regardless of environment, in reality, rather different considerations apply in marine versus non-marine waters. In the oceans, the Mg/Ca ratio of seawater has remained effectively constant during the Cenozoic, so that the only environmental control on the Mg content of ostracod shells is water temperature. Mg/Ca ratios of marine ostracods are therefore valuable palaeothermometers. In non-marine waters, variations in water composition mean that trace-element signatures may be hard to interpret. Existing calibration studies have often yielded contradictory and confusing results, meaning that
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stratigraphical records of trace-metal variations in ostracod shells from non-marine waters such as lakes must be interpreted with care. Dettman and Dwyer summarise the major controls on trace metals in ostracods both from marine and non-marine settings and outline areas in which further understanding is urgently needed, especially for non-marine environments. Decrouy summarises the main controls on oxygen and carbon isotope ratios of ostracod calcite. Most work has focused on non-marine settings, since in ocean sediments, most (but not all) stable-isotope analyses have targeted foraminiferal tests. One of the most intriguing observations, discovered previously and discussed by Decrouy, is the fact that ostracod shells are formed out of oxygen-isotope equilibrium with the water in which they live. Decrouy examines the potential reasons for this disequilibrium and raises the possibility that the magnitude of offset, which seems always to be positive, may be environmentally as well as taxonomically controlled. All geochemical work that utilises ostracods needs to account for the fact that ostracod shell formation is under strong biological control. Moreover, interpretations of geochemical signatures are enhanced if they take into account the ecology and life cycle of the taxa being analysed and, if possible, form part of multiple proxy investigations, where the additional climate proxies may be ostracod-related or derived from other biological or non-biological indicators.
18.5 MULTI-PROXY APPROACHES TO QUATERNARY PALAEOCLIMATE STUDIES As demonstrated above, fossil ostracods can provide several different lines of evidence for past environmental change, based on knowledge of their present-day diversity, biogeography and ecology as well as measurement and interpretation of their trace-element and stable-isotope shell chemistry. Smith and Palmer (2012) explore the use of combined geochemical and palaeoecological analyses in palaeoclimate investigations with reference to four scenarios for palaeolimnological records. The first, in which no significant changes are evident in either species assemblages or shell geochemistry, may reflect stable profundal conditions in a large lake; the implication is that assemblages in smaller lakes may be more likely to respond to hydrological and climatic change and thus yield more useful records. In the second scenario, ostracod assemblages show significant changes through time, while their shell geochemistry exhibits little variability; this is characteristic of groundwater-fed through-flow lakes. Scenario three, with pronounced changes in both species assemblages and shell geochemistry, reflects a more variable waterbody such as a lake with climate-induced changes in water depth and salinity. Finally the fourth scenario, in which shell geochemistry shows marked variability but
Ostracoda as Proxies for Quaternary Climate Change
species assemblages do not, may be indicative of changes in the source of the lake water, such as meltwater influxes. Long palaeolimnological records may incorporate several such scenarios through time, and it is the combination of palaeoecological and geochemical approaches that provides the versatility needed to interpret such complex histories. Complexity presents challenges, nevertheless, and in some circumstances, it may be difficult to distinguish between climatic and other influences. A case in point is provided by Anado´n et al. (2012) who combine ostracod stable-isotope and trace-element shell geochemistry with the MOTR method in order to reconstruct Middle Pleistocene to Holocene palaeoclimate in the Mediterranean region, using a sediment core extracted from an Italian maar lake. They find that local factors (e.g. volcanic–tectonic influences on water chemistry) militate to obscure the climatic signal in ostracod associations and valve geochemistry, and advise that for palaeoclimate studies, it is better to select lacustrine sequences from relatively simple, well-constrained hydrological systems. Many late Holocene/historical records are made even more complex by the influence of human activity on waterbodies. Ostracods provide valuable proxy evidence for both natural (climate-influenced) and human-induced environmental change in the Black, Caspian and Aral seas. Boomer (2012) demonstrates their utility with reference to palaeoecological and shell geochemical studies from each of the seas. Since the mid-twentieth century, excessive abstraction of water from the two rivers supplying the Aral Sea has led to a dramatic lowering of its level to the point at which it now consists of several separate, hydrologically distinct waterbodies, with elevated salinities reflected by their ostracod faunas. However, ostracod shells from sediments cores spanning the interval since the Last Glacial Maximum provide evidence of the sensitivity of the sea to climateinfluenced hydrological changes well before anthropogenic changes became significant. The deep basins of the Caspian Sea may have acted as refugia and centres of evolution since Miocene times, with connections to the Black Sea during intervals of high sea level. During Pleistocene cold phases, ice-dammed lakes may have backed up the usually northward-flowing drainage resulting in the temporary connection of the Black, Caspian and Aral Sea basins. Isolated from the Mediterranean by low eustatic sea level during the Last Glacial Maximum, the Black Sea was connected when sea level overtopped the Bosphorus sill in the early Holocene, allowing marine Mediterranean ostracod taxa to mix with and replace the Ponto-Caspian fauna. Ostracods have a major role to play in studies attempting to evaluate the controversial “Noah’s Flood” hypothesis about nature and timing of the connection, and Boomer demonstrates the use of ostracod shell geochemistry to indicate basinwide hydrological changes, including the effects of a meltwater pulse at around 17,000 BP and a trend towards more
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saline conditions between 8000 and 9000 BP, well before the main connection to the Mediterranean at around 7500 BP. Boomer cites the distinctive nature of the PontoCaspian ostracod fauna, with its many geographically restricted components, taxonomic uncertainties and a shortage of modern autecological data as features that currently limit the palaeoenvironmental and palaeoclimatic interpretation of assemblages. Cronin et al. (2012) reconstruct palaeoclimate for the past 800 years using marine/brackish ostracod palaeoecology and shell geochemistry from cores taken in Biscayne Bay, Florida, in which the link with climate is provided by palaeosalinity variations. In spite of difficulties arising from multiple influences on salinity in the coastal setting, including freshwater runoff, evaporation and groundwater as well as climate (through precipitation variation), they find oscillations reflecting multi-decadal and centennialscale climate variability during the Medieval Climate Anomaly and the Little Ice Age. They also note that some variation in ostracod shell carbon stable-isotope composition may be related to human-induced changes in bay circulation and productivity. The lakes on the Tibetan Plateau have been the focus of several recent studies using ostracods as proxies for Lateglacial and Holocene palaeoenvironmental and palaeoclimatic change. The abundant and diverse waterbodies, with relatively low levels of human impact in most of them, constitute a vast natural laboratory of environmental variability. Mischke (2012) reviews this work and discusses some of the difficulties encountered. Modern distributions of ostracods have been recorded on the plateau from more than 200 waterbodies, including saline as well as freshwater lakes, temporary ponds and flowing waters, providing valuable ecological data and training sets for the development of transfer functions for water conductivity and depth; Mischke advises caution in the use of the latter as the depth distribution of ostracods is strongly influenced by other parameters. The Tibetan Plateau data set has excellent potential to contribute to OMEGA, but, as observed earlier, the need for taxonomic harmonisation will be a significant issue; Mischke gives examples of different taxonomic approaches, with some authors regarding as separate species what others would consider to be ecophenotypic variants of a single species. Oxygen isotope and trace-element analyses of ostracod shells have helped to elucidate the hydrological and climatic histories of several lakes in the region. Echoing comments made by other authors in this volume, Mischke reminds us that the processes governing ostracod shell geochemistry are lakeand catchment-specific, so their interpretation requires the detailed assessment of the sites from which the cores were obtained. One of the lakes, Nam Co, is the focus of a detailed study of Holocene climate change by Wrozyna et al. (2012) using oxygen and carbon isotope measurements in ostracod
valves as well as interpretations of ostracod assemblage taphonomy and palaeoecology. Results confirm a long-term trend of increasing aridity already noted in other Tibetan Plateau lakes, the onset of which is, however, not synchronous across the plateau. Wrozyna et al. stress that the successful use of ostracod shell geochemistry in palaeoenvironmental reconstruction is dependent on understanding the chemical, biological and ecological influences on the chemical composition of valves, and comment that the multi-proxy approach facilitates an overview of the regional situation and an evaluation of the sensitivity of individual proxies to different climatic and environmental variables.
18.6 FUTURE DEVELOPMENTS: POTENTIAL AND LIMITATIONS OF OSTRACODS IN QUATERNARY PALAEOCLIMATOLOGY 18.6.1
Overview
We set out to compile a volume providing a comprehensive overview of the diversity of ostracod applications in Quaternary palaeoclimate investigations, drawing on the rich experience and expertise of a wide range of international authors. We believe that we have fulfilled this aim, but it is apparent that in its geographical coverage, the content of this volume is focused on the northern hemisphere, with a strong bias towards Europe and North America (Fig. 18.1). This reflects, to a large extent, the state-ofthe-art; it was never our attention deliberately to exclude southern and eastern areas, the book just turned out that way, and here we draw readers’ attention to examples of excellent studies conducted in other parts of the world. In Australia, Gouramanis et al. (2010) used lacustrine ostracod palaeoecology and shell geochemistry to produce a highresolution palaeoclimate record spanning the past 6000 years. Holocene climate-related lake-level and waterchemistry changes were interpreted using ostracod assemblages in lake sediment cores from Lake Titicaca, Bolivia, by Mourguiart et al. (1998). Schwalb et al. (1999) were able to distinguish between local environmental and regional climate signals in their interpretation of ostracod shell stable isotopes from Chilean Altiplano lakes, enabling the recognition of Holocene wet and dry phases, including rapid short-term shifts in the mid- to late-Holocene transition interval. Schwalb et al. (2002) presented extensive data on the ecology and stable-isotope ratios of ostracods inhabiting fresh waters in Argentina, creating a valuable training set for future use in palaeoclimatology based on lake sediment cores. In the East African Rift Valley, Alin and Cohen (2003) used ostracods to reconstruct lake-level changes in Lake Tanganyika, which could be related to late Holocene climate changes; many of their listed taxa are left
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FIGURE 18.1 Geographical sites and regions of selected examples and case histories of ostracods mentioned in the text. (A) USA Atlantic coast, Gulf of Alaska and northwest Europe (Lord et al., 2012); (B) British Pleistocene sites (Horne et al., 2012); (C) NACODe coverage region (Curry et al., 2012); (D) Canadian North data set and transfer function application (Bunbury, 2012); (E) North Atlantic core site (Dettman and Dwyer, 2012); (F) Lake Geneva (Decrouy, 2012); (G) lakes in North Dakota (Smith and Palmer, 2012); (H) Black, Caspian and Aral seas (Boomer, 2012); (I) Valle di Castiglione crater lake (Anado´n et al., 2012); (J) Biscayne Bay, Florida (Cronin et al., 2012); (K) Tibetan Plateau (Mischke, 2012; Wrozyna et al., 2012).
in open nomenclature, hinting at the possibility of more sophisticated and extensive analyses when all of their ostracods are identified to species level. A series of papers presenting the results of ostracod studies in Siberia provides modern ecological and stable-isotope data as well as palaeoclimatic interpretations of late Quaternary fossil freshwater ostracod assemblages and shell geochemistry (Wetterich et al., 2005, 2007, 2008a, 2008b, 2008c, 2009). If there is one clear message common to the diverse papers in this volume, it is that the science of using ostracods as Quaternary palaeoclimate proxies has matured to the point at which the various techniques have been developed well beyond the pioneering stage and their application now needs to be tempered with critical awareness of their limitations. Overconfidence is dangerous; it is important not to forget the many underlying assumptions that allow us to proceed with palaeoclimate methods in spite of lacking key information. It is also vital to realise that while mutual climatic range, transfer function, geochemical and other methods may be well-established in the literature, this does not mean that such techniques can simply be used “off the shelf” by non-specialists; their successful application requires skill, experience and (once again) a critical awareness of their limitations. Some of the key areas in
which future research is most needed, and is likely to lead to significant advances in this field, are outlined below.
18.6.2
Ostracod Palaeogenetics
In the final research paper in this volume, Scho¨n et al. (2012) explore the potential for using ostracod DNA to infer palaeoenvironmental and palaeoclimatic changes on various timescales. They consider it unlikely that DNA could be extracted from fossil ostracod shells, soft parts or resting eggs more than about 10,000 years old, but point out that it is feasible to seek evidence of the effects of past climate change in the population genetics of living ostracods. Ancient lakes, such as 28-million-year-old Lake Tanganyika, which are home to diverse species flocks offer some of the best opportunities to pursue such studies.
18.6.3 Improving Knowledge of Ostracod Ecology There is much to learn about ostracod ecology, both marine and non-marine. While significant advances have been made in our understanding of the influences of abiotic factors such as salinity and water temperature, some
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fundamental aspects remain largely unknown. We know little about exactly where (e.g. at the sediment–water interface or interstitially?) and how quickly ostracods moult and calcify their new shells, and lifecycle details are known for only a small proportion of species, all of which information is important for the interpretation of the traceelement and isotopic composition of ostracod shells. Mutual climatic range and transfer function methods assume that factors such as mean January and July air temperature are significant influences on ostracod species’ distributions, yet the biological basis for such assumptions is, as yet, established for only a very few species.
18.6.4
Improving Distributional Data Sets
Many ostracod species have very wide distributions, which means that regional distributional and databases rarely capture fully the climatic ranges of living ostracod species. The OMEGA metadatabase project (Fig. 18.2) addresses this problem by seeking to make appropriate data available on a global scale. In OMEGA, ostracod taxonomic names and the coordinates of localities from which they were recorded are considered to be both data and metadata; their inclusion on the metadatabase will ensure that distributions can be mapped easily and facilitate the answering of such questions as “which regional databases contain data on the species Darwinula stevensoni?” Compiling such a metadatabase is not simply a matter of adding existing data sets together, however, and the global harmonisation of taxonomy and nomenclature is both a necessity and a major challenge.
18.6.5
Taxonomic Harmonisation
There are many factors influencing the distributions of ostracod taxa, but political boundaries are not among these, except in the sense that ostracod specialists working on one country or region have sometimes named a species without realising that it was already known under a different name somewhere else. As we develop and apply the methods now available for using ostracods as palaeoclimate proxies, it is vital that we seek not only to utilise data sets that are more extensive (both geographically and in climate space) in order to capture more completely the climatic ranges of ostracod taxa, but also to ensure that we are using agreed, consistent taxonomic nomenclature. The integration of non-marine ostracod databases for Canada and the United States by Curry et al. (2012) demonstrates both issues. They note, for example, that the ostracod listed in the Delorme Canadian database as Limnocythere herricki Staplin, 1963 is considered to be synonymous with Limnocythere varia Staplin, 1963. They use the latter name in the combined data set (NACODe), but since the former name
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actually appears first in Staplin’s publication it could be argued that it should be used, due to page priority; the situation is complicated further by Staplin’s cautionary suggestion that his own new species L. varia might simply be a larger variety of Limnocythere illinoisensis Sharpe, 1897. It is important to realise that taxonomic opinions vary and may change in the light of new information; it is essential that distributional databases should retain sufficient information to allow new taxonomic revisions to be applied and accommodated (at the very least, not only the currently accepted name for a species, but also the name under which it was first recorded at any given locality in a database). It is clear that many species occur in both countries and are widespread in North America. Neither NANODe nor the Delorme Canadian database on their own are adequate to delimit the North American distributions of such species as Cypridopsis vidua (O.F. Mu¨ller, 1776) or Fabaeformiscandona rawsoni (Tressler, 1957); on the other hand, the same combined data sets show that certain species are confined to Arctic Canada (e.g. Candona protzi Hartwig, 1898) (see Fig. 6.12 in Curry et al., 2012). The last-mentioned species is of interest because it is also recorded in Europe, where it was originally described and is now assigned by many authors to the candonid genus Fabaeformiscandona. Are the Canadian and European taxa really the same, and, noting that Curry et al. use Fabaeformiscandona for some candonid species, but not this one, which is the correct generic assignment? The abovementioned C. vidua is another European species that appears to be widespread in North America; detailed studies at morphological and molecular levels are needed to ascertain whether this is really a single taxon with very broad environmental and climatic tolerances or represents several distinct, more stenotopic taxa that just happen to look very similar. A European study of mitochondrial DNA in Eucypris virens, a ubiquitous temporary pond species, demonstrated the existence of more than 40 cryptic species, predominantly asexual but including sexual populations from the Mediterranean area, which cannot, however, be distinguished on the basis of shell morphology. This serves to remind us that when working with fossil material we are constrained to use a morphospecies concept, which may not reflect the biological and ecological diversity of the animal in question, especially in the case of parthenogenetic taxa with multiple coexisting clonal lineages. It is arguable whether knowledge of the existence of cryptic species, identified by molecular methods, is a help or a hindrance in palaeoclimatic applications of ostracods. To some extent the same argument applies to the use of “soft parts” (e.g. appendages) to distinguish between living species, given that in the vast majority of cases those working with fossil material can only use
FIGURE 18.2 Distribution of ostracod records in OMEGA; (A) Europe, from NODE database (circles); (B) North America, from NANODe (squares), Delorme Database (circles) and Bunbury (2012) (triangles); (C) (inset) shows detail of Great Lakes region.
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“hard parts” (i.e. shells). The Canadian freshwater ostracod Candona acutula Delorme, 1967 is a useful cold-climate indicator, today restricted to areas with mean January air temperature no higher than 7 C, and (coincidentally) a species that appears to respect the U.S. border since the southern limit of its distribution is latitude 49 N. During Pleistocene cold, glacio-pluvial intervals, it occurred further south, in the Great Salt Lake Basin of Utah (Lister, 1975). Horne et al. (2011) have suggested that it may be synonymous with a European species, Fabaeformiscandona levanderi (Hirschmann, 1912); even if detailed appendage differences eventually show the two species to be distinct, there may be a case for nevertheless regarding them as a single palaeontological morphospecies with palaeoclimatic utility, since they cannot be separated on the basis of their shell morphology alone.
their estimations of mean July air temperature), it inspires confidence in the methods; when they do not, methods and their underlying assumptions are challenged and may need re-thinking.
18.6.6
REFERENCES
Improving Geochemical Methods
Future work will involve additional calibration studies. These are especially important for the trace-element partitioning in non-marine taxa, for which geochemical signatures may be complex. However, additional work is also needed for stable isotopes and for trace metals in marine ostracods in order to enhance the palaeoenvironmental value of these signatures. Finally, the potential for analysing trace metals other than Mg and Sr has not yet been properly investigated. Such metals might include heavy metals derived from anthropogenic pollution and redoxsensitive elements such as iron, manganese and uranium. Although pilot studies have hinted at the value of such metals, a systematic understanding of their partitioning into ostracod shells is currently lacking.
18.6.7
Multi-proxy Testing
Any single proxy method on its own can (and likely to) yield plausible quantitative estimates of palaeoclimatic parameters. The question should always be asked: why should we believe these results? There has been a tendency to accept results uncritically, and the testing of methods has been limited in scope. Several papers in this volume address issues of multi-proxy applications of palaeoclimate methods, leading to a more critical understanding of the efficacy of such methods. Such approaches are to be encouraged and emulated in future. Ostracods are in themselves multi-proxy microfossils, and there is much to be gained from the combination of geochemical techniques with those based on ecology and distribution. Comparisons of ostracod results with those from other palaeoclimate proxies (e.g. diatoms, chironomid larvae, beetles, molluscs, foraminifera, pollen) will be informative and should lead to advances. When two or more proxies agree (in, for example,
ACKNOWLEDGEMENTS We would like to express our gratitude to all of the authors who have contributed to this volume for engaging with the project and for sharing with us, the editors, their exciting research findings and ideas. Each chapter in this volume was peer-reviewed by at least two reviewers, some of whom are acknowledged by their name in individual chapters, while other chose to remain anonymous; we thank them all for responding so positively, efficiently and effectively, often at short notice, to our invitations to review. We are especially grateful to the series editor, Jaap van der Meer, for his patient guidance and encouragement throughout the project.
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Chapter 18
Overview and Future Prospects
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