3. RATIONALES AND OBJECTIVES
4. SCIENTIFIC IMPORTANCE OF CONTINENTAL PALEOCLIMATE RECORDS
Considerable knowledge about past global climates and environments has been acquired in recent years, especially from deep sea cores and from ice cores taken from glaciers in Antarctica, Greenland, and smaller, low-latitude icecaps. Continental records obtained by drilling in lakes and filled lake basins have also contributed to understanding Earth's climate and environmental history. Most of what we know about the history of continental environmental changes is derived from analysis of glacial terrain, arid region landforms and other geomorphological features, and relatively short records obtained by coring trees, lakes, and bogs. These records have proved their value by providing important insights into how global and regional climatic events influence continental ecosystems upon which humanity is most immediately dependent. Compared to those from many marine cores, records obtained from the continents often provide direct evidence of past events on a local or regional scale, with relatively high temporal resolution.
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Figure 1. Example of non-lacustrine, continental paleoclimate record , Time Stream I (0-2,000 yrs). European oak index (dotted line) and Fennoscandian pines (temperature index, °C, solid line), which record an abrupt cold event at AD540, just before the outbreak of the Justinian plague in 542. It is likely that cold conditions caused widespread crop failures leading to famine and plague. Figure provided by M. Baillie; Fennoscandian data courtesy of K. Briffa. |
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Figure 2. Example of non-lacustrine, continental paleoclimate record, Time Stream II (ca.0-250,000 yrs). Magnetic susceptibility and grain-size records from the Chinese loess plateau (Luochuan) as indicators of summer and winter monsoons, respectively (from Xiao et al., 1995). |
4.2 Models and Mechanisms of Climate Change
Climate models have a need for increasingly reliable reconstruction of paleoclimate. High-resolution paleoclimate records from lake sites are still rare; methods of analyses have been inconsistent, processes poorly understood, spatial and time resolution is poor, and telecorrelations among a global network of sites almost non-existent. Groups promoting paleorecords stress the untapped importance of long and short time series from integrated studies of continental records, particularly if they are annually resolved (e.g. Oeschger and Eddy, 1989; Eddy, 1992; Bradley, 1991; Johnson, 1993; PALE Steering Committee, 1994; PAGES, 1995; (PANASH)).
A global network of lacustrine paleoclimate records can be used to establish boundary conditions, record rates of environmental change, and document ecosystem responses to those changes. A well conceived network of continental sites would provide independent tests of general circulation models (GCM's) of the oceans and atmosphere. As part of this global network, databases of specific limnological properties such as lake level (Guiot et al., 1993; Harrison, 1989; Harrison and Digerfeldt) are important.
The transient behavior of abrupt climate changes can be documented on the scale of human generations, but the mechanism forcing such events is largely unknown, except that the surface ocean is probably involved. Partly this is due to a lack of global data. For example, controversy about the structure of the Younger Dryas cold period (11,000-10,000 14C yrs B.P.) centers on a regional mechanism in the North Atlantic (cf. Broecker, 1994). This has spawned a world-wide search for time equivalent signatures. On the other hand, while attention focuses on abrupt changes during deglaciation, ice cores also reveal a tranquil Holocene temperature history. This contrasts with as yet unexplained observations such as rapid Holocene shifts in tropical water budgets and European lake levels. These are likely related to thresholds in monsoon systems, possibly in concert with changes in El Niño-Southern Oscillation (ENSO) behavior. Palynological transects through these contrasting areas, for example from northwest Europe through the Mediterranean to tropical Africa, are needed to decipher the regional signatures of these changes.
Multiproxy lacustrine records are contributing unique information on environmental change in space and time, and are providing goals for subsequent research in oceans and on glaciers. The records from several lakes in north and east Africa, for example, have identified major drying events in the Holocene at 8 ka and 4 ka that provide important insight into past behavior of the Intertropical Convergence Zone (ITCZ), an important component of the global atmospheric circulation system. This has inspired a search for evidence of subtle but important changes in marine records that correspond to the 4 and 8 ka horizons. Palynologists analyzing lacustrine sequences in the tropics of Africa and South America concluded years ago that these regions were 4-5 °C cooler during the last glacial maximum (LGM) than today. However, this conclusion is only now being seriously considered by the paleoceanographic community (Broecker, 1996) after new studies of varved marine basins (Hughen et al., 1996) and isotopic analyses of Peruvian glaciers (Thompson et al., 1995). It is, moreover, consistent with reconstructions derived from noble gas rations in low-latitude groundwater bodies (Stute et al., 1995).
4.3 Spatial Dimensions and Scales
Spatial resolution in reconstructing past environments, especially over the topographically complex continents, is an important factor in understanding how the climate system operates and affects the terrestrial component of planet Earth. To define climate dynamics, several sites over a region must show synchronous changes or closely related temporal trends. Paleorecords from continental archives do not resolve global paleoclimate in the same manner as ice cores, for example, which sample the Earth's atmosphere directly. Instead, they must be studied as archives which store the response of regional ecological and hydrological systems to climate change forcing. Lakes in many parts of the world offer the potential for providing the spatial resolution required for reconstructing the regional scale effects of climate change. Comparison of paleoclimate records from different types of lakes in diverse climatic and vegetational zones are required to fulfill this potential.
Long records from lake sediments are an underused archive that fill a gap in the global coverage of paleorecords. Ice cores are limited to polar regions and high-altitude ice caps. Tree rings are time-limited and are most useful across temperate latitudes. Lake sediments record the information in a variety of components that indirectly represent the atmosphere (precipitation, seasonality, temperature, variability, wind, storms, drought), the terrestrial ecosystem (pollen, insects, other fossils, organic matter, fire recurrence, volcanic ash events, flood recurrence, soil development, weathering) or the aqueous system (salinity; composition; evaporation; circulation and mixing; microbial, algal, and fish biota; aquatic plants; productivity; endemism; chemical sedimentation; organic matter; environmental isotopes; and others).
Lakes are widely distributed on the continents and come in a large selection of sizes, depths, chemistries, salinities, origins, and settings. Lacustrine freshwater giants such as the 70,000 km2 Lake Bonneville, may dwindle to hypersaline pools such as the Great Salt Lake, within a few centuries. Such transitions may be documented in only centimeters of sediment. Crater lakes capture rain like giant pluviometers monitoring the atmosphere, whereas other types of lakes such as shallow Lake Eyre, Australia, integrate the evolution of surface and groundwaters across half a continent.
4.4 Time Scales and Age Resolution
Lacustrine records usually can provide the temporal resolution of relevance on human time scales (Duplessy and Overpeck, 1996). Because of relatively fast sedimentation rates encountered in many lakes, opportunities exist to calibrate the sediment record of the past one to two centuries directly against historical and instrumental records of environmental change. Moreover, rapid sedimentation rates allow for quantification of the rate of abrupt environmental change. High-resolution records on the continents derived from lake sediments, for example, frequently show evidence of cyclic behavior with periodicities ranging from a few years to a few decades or centuries. While their existence and significance are still being evaluated, they offer the potential for providing new insights into the importance of processes such as ENSO and solar variability on global climate at time scales of immediate relevance to humankind. Although high resolution is possible for any length of lacustrine record, high-resolution methods are particularly needed for all or critical parts of long (> 250,000 yrs) records.
Annually laminated lacustrine sediments (varves) are part-icularly important for reasons ranging from the exact timing of clim-atic changes (e.g. Hajdas et al., 1995; Watts et al., 1996) to calibration of the radiocarbon time scale (Stuiver et al., 1991). Varves contain a wealth of annually resolved paleoenvironmental information (Anderson and Dean, 1988). Not only do records from such sediments address questions of inter-annual climate variability, but in some cases, they reveal details concerning seasonal changes (e.g. Zolitschka and Negendank, 1996).
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Figure 3. SM: Sporer Minimum |
Long records from lakes offer the potential for determining onshore records of long-term climatic changes, such as those associated with cyclic variation in the earth's orbit (Milankovich cycles). Potential differences in response to orbital forcing between oceanic and continental climates are important aspects of the climate system that need to be understood. The difference between the Devil's Hole record (Winograd et al., 1992) and the marine oxygen isotope record (Imbrie et al., 1984) is an outstanding example. Lacustrine records that span several 100,000-yr climatic cycles are relatively rare compared to those for the post-glacial period. Long records from single sites also have the advantage of minimizing the effect of non-climatic variables on the cycle-to-cycle paleoclimate signal of climate proxies.
Some large lakes of tectonic origin (such as Lake Baikal, the East African rift lakes, and the Bogota basin) may preserve records that span millions of years; significantly longer than the longest ice-core records and rivaling some marine records. In addition to paleoclimate records, sediments in these lakes may provide information about tectonic processes, such as rift formation.
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Figure 4. Example of lacustrine paleoclimate record, Time Stream II (ca. 0-250,000 yrs). Biogenic silica, a measure of diatom productivity, from Lake Baikal compared to SPECMAP oxygen isotope record and modeled maximum summer temperatures, from Colman et al. (1995). |
4.5 Societal Relevance
In many cases, lacustrine sediments record the impact of climate change as well as the change itself. Responses of terrestrial ecosystems, the hydrological system, and landscapes to climate change are clearly one of the most important aspects of global change research, in terms of relevance to human activities. For example, recent research (Bradbury et al., 1993; Markgraf and Kenny, 1996) indicates that vegetation and lakes may actually respond much more quickly to climate change than has been assumed previously. Aquatic vegetation may even record seasonal limnological changes (e.g. Haworth, 1984; Simola et al., 1981).
Lacustrine records of past environmental change are especially useful for gauging environmental response to human impact (e.g. Hollander et al., 1992), and comparing this response to "natural" changes that occurred prior to significant anthropogenic forcing (so called "baseline" conditions). The potential exists for quantifying the integrated watershed system response to anthropogenic and climate forcing, in terms of local and regional vegetation, soil development, water chemistry, and aquatic biota. This is possible because lacustrine records often provide several quantitative, independent, environmental proxies that may be used for reconstructing these and other conditions. These proxies include plant micro- and macro-fossils, soil minerals, aquatic microfossils, and authigenic minerals.
Lacustrine records can provide valuable information on past variability not only in climate and vegetation, but also in geological hazards such as earthquakes and volcanoes. Earthquakes, for example, may trigger turbidites in an otherwise quiet depositional setting. Volcanic ash layers record past eruptions and can be examined carefully for any associated changes in aquatic microfossils, pollen, or other indicators of environmental change. It may be possible to discern the effects of eruptions on regional ecosystems and environments from evidence preserved in lake sediments (e.g., vegetation changes, water chemistry changes, changes in aquatic ecosystems, such as diatoms). When such catastrophic change occurs, the rapidity of the ecosystem response and rebound is important.
Continental records not only provide information about climatic changes on temporal and spatial scales that are pertinent to human activity, but they also document the impact of those changes on the surrounding environment. It is this aspect of global change-namely the regional response and threshold behavior-which is most difficult to estimate from global climate modeling, but which concerns populations and societies.
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