Paleolimnology in Deep Time: The Evolution of Lacustrine Ecosystems

Most lakes are geologically ephemeral; even the longest-lived individual lakes persist only for tens of millions of years. However there is a continuity to lake systems that transcends the geologically short history of individual lake basins. This continuity comes from the long-term biological evolution of life in freshwater, and fittingly, forms the final subject of this treatment of paleolimnology. Like the oceans, lakes have provided habitats for living organisms for most of the earth’s history. Yet the patterns of aquatic ecosystem evolution in rivers and lakes have differed dramatically from those of the oceans. In large part this can be traced to the fundamentally ephemeral nature of most continental aquatic habitats and the ‘‘disconnectedness’’ in both time and space that exists between individual lakes and rivers compared with the world ocean. This pattern of temporal and spatial patchiness in water body distribution on the continents has shaped the evolution of lacustrine species and communities. Some understanding of this history can be gleaned from the study of modern ecology and molecular genetics of living freshwater organisms. But to understand long-term trends in lacustrine biodiversity and their relationship to the history of the lacustrine environment we must turn to the pre- Quaternary fossil record. Understanding this history, the timing and tempo of major species diversification and extinction events, and the evolution of key ecological innovations is critical for correctly interpreting ancient lake deposits. The fossil record of pre-Quaternary lakes is more difficult to interpret than that of more recent lake basins. Robust phylogenies are largely unavailable for clades of ancient lacustrine fossils, hindering our ability to test hypotheses of evolutionary ecology, although that situation hopefully will improve in coming years. Many major clades of fossil lacustrine organisms are extinct, and ecologies must be inferred from their depositional context. Even for organisms that have close-living relatives, our certainty in making inferences about habitat and relationship with other species weakens as we go back in time. Also the record we have to work with deteriorates with age, the result of (a) a declining volume of lake beds available for study with increasing age, (b) difficulties associated with processing lithified lake beds for their fossil content, and (c) an increasing likelihood of destruction by diagenesis with increasing age.

Paleoecological Archives in Lake Deposits 2: Records from Important Groups

The lacustrine fossil record comprises a mixture of endogenic fossils, such as cladocerans, derived from lakes, and exogenic fossils, such as insects or pollen, which are carried into lakes, by wind and water from surrounding areas. Our primary emphasis here will be on the endogenic fossil record of lakes; we will only briefly consider general aspects of the taphonomy and paleoecological significance of exogenic fossils for terrestrial plant and insect fossils. Information about lake fossils varies greatly between groups. Some taxa, such as diatoms, are virtual workhorses of the field, with numerous investigators, and established methods of sampling, analysis, and interpretation. At the other extreme are organisms such as copepods, which, despite their importance in lacustrine ecosystems, are so poorly fossilized that they are unlikely to ever play a major role in paleolimnology. In between these extremes lie the majority of lacustrine organisms. Many relatively common groups have great potential for paleoecological interpretation, but, for reasons of inadequate study, a lack of researchers, or difficulties in taxonomy, have thus far been little used by paleolimnologists. Major opportunities await new students in the field who are willing to take up the challenges of studying these clades. Despite their importance in lacustrine communities, cyanobacteria remain a relatively unexploited source of information for paleolimnology. Isolated cells have poor preservation potential, and fossil cyanobacterial cells are preserved in Late Quaternary lake muds primarily by their more resistant reproductive spores (akinetes), or occasionally by filaments. Planktonic cyanobacteria are only rarely recorded in older sediments. In contrast, benthic cyanobacterial communities are well represented in ancient lake beds by their constructional deposits, lithified algal mats, stromatolites, and thrombolites. Although their body fossils have been used only rarely to solve paleolimnological problems, planktonic cyanobacteria have great potential for this purpose, given their obvious importance in many lacustrine communities. Relatively resistant akinetes might be very useful for understanding changes in plankton communities, especially in cases where better- studied siliceous microfossils (diatoms and chrysophytes) are not well preserved, for example, in very alkaline lakes. However, almost nothing is known of the taphonomic biases that control the planktonic cyanobacterial fossil record.

The Physical Environment of Lakes

Before discussing paleolimnological archives, we need to consider those aspects of limnology that regulate how information is produced, transmitted, and filtered through the water column. Although many limnological processes leave behind sedimentary clues of their existence or intensity and are thus amenable to paleolimnological analysis, others leave little or no detectable trace. Our consideration of limnology here emphasizes the former. Throughout the next three chapters we will examine the properties of lakes, the implications of these properties for paleolimnology, and the types of physical, chemical, and biological information that can be transcribed into sedimentary archives. Physical processes in lakes are of interest because they act as intermediary hydroclimate filters between external forcing events of interest, like climate, and the paleolimnological record. For example, understanding the hydrology of a lake is important because water inputs and outputs, which are often controlled by climate, determine lake levels, which in turn are recorded by ancient shoreline elevations, or indirectly by salinity indicators. Light and heat penetration regulate the distribution of organisms and the mixing of the water column, recorded by the distribution of various fossils, sediment types, and geochemical characteristics of sediments. Also, current and wave activity affect the transport of sedimentary particles and therefore the distribution of sediment types around a lake basin. Understanding these physical processes therefore provides us with a means of linking sedimentological, geochemical, and paleobiological records of lake deposits to the external environment. Water enters and exits lakes through a variety of paths that comprise part of the earth’s hydrological cycle. The lake components of this cycle include a series of inputs and outputs of water, which in combination with the morphometry of the lake basin, collectively determine the lake’s level. Inputs include precipitation, surface runoff from rivers, and groundwater discharge into the lake. Outflows include surface outflow, evaporation, evapotranspiration losses from emergent aquatic plants, groundwater recharge, and hydration reactions with underlying sediments. If water inputs and outputs for a lake are equal over a short time span, the lake surface elevation will remain constant. This is approximately the case in most lakes that are surficially open basins.

Paleolimnology at the Regional to Global Scale: Records of Climate Change

Reconstructing climatic change is perhaps the single most common application of paleolimnology. Paleoclimatology is a vast subject, and several entire books have been written on this subject alone (e.g., Crowley and North, 1991; Parrish, 1998; Bradley, 1999). Here we can only touch on some of the more important, interesting, and controversial aspects of climate history that are potentially recorded in lake sediments. As with human impact histories, archives of paleoclimate from individual lakes record responses from both local and regional events (e.g., Giraudi, 1998); teasing the two apart from a single basin often poses a difficult problem. In order to differentiate regional from global-scale changes in climate from lake deposits, it is also necessary that local influences on hydrology, such as drainage diversions, or changes in groundwater flow fields unrelated to climate, be understood. The problem of identifying regionally significant events becomes even more acute when the goals are to assess the rate at which climate changed from lake records or to assess the synchroneity of events between locations. All of these issues accentuate the importance of excellent geochronometry for paleoclimatic interpretation. Also, biological or physical mixing of sediments in any individual core record may mislead us into thinking a change was gradual when in fact it was rapid, whereas unrecognized small-scale unconformities in a single core could mislead us in the opposite direction (Dominik et al., 1992). Conversely, some lakes act to amplify climatic signals, particularly when they cross a threshold of limnological response to some climate variable (for example the transition from closed to open-lake conditions that might accompany an increasing precipitation:evaporation ratio). In this case a ‘‘gradual’’ climatic process might appear rapid from its depositional record. As with human impact studies, a common solution to these problems is to use a comparative-lake and/or comparative-indicator approach, identifying coherent patterns of change in indicators of precipitation, temperature, windiness, or other climate variables of interest throughout a region. This can be done using many of the types of biotic, geochemical, geophysical, or geomorphic indicators we have discussed in chapters 7–11.

Paleolimnology at the Local to Regional Scale: Records of Changing Watersheds and Industrialization

Paleolimnologists have developed an impressive track record documenting the history of human influence on lakes and their surroundings, and using these historical inferences to help policy makers establish lake and ecosystem management goals. Our ability to do this depends on both a comparative analysis of multiple lake records, and a firmly established chronology. The comparative approach to paleolimnology allows us to differentiate local phenomena resulting from peculiarities of study watersheds from regional phenomena. Comparison of records also allows the timing of events to be placed in a regional context, where explanations of processes that affect large areas, like lake acidification, regional patterns of air pollution, or landscape disturbance may be more broadly interpretable. Comparative paleolimnology allows the researcher to study the multiple effects of local to regional-scale phenomena and differentiate them from global phenomena. Closely coupled with our requirement for a comparative approach to paleolimnology is the need to place events in a highly resolved chronology, especially over the past 200 years, the period of greatest interest to understanding major human alternations of the environment. In many parts of the world, including the highly industrialized and relatively well-‘‘monitored’’ environments of North America and Europe, instrumental records of water quality are either spotty or unavailable. Until the 1960s, the number of lakes with regular monitoring programs for even basic limnological parameters was extremely small. And in regions with numerous water bodies, selection criteria for the investigation of lakes often has had more to do with proximity to major research facilities or peculiarities of road access than with the needs of society. Paleolimnological records integrate ecological signals at scales that are relevant to the interests of lake managers, who need to understand the timing and magnitude of human activities. Even when limnological monitoring is available, paleolimnological approaches can answer questions at temporal and spatial scales that are unattainable by the monitoring regime in place. The difficulty of understanding the history of human impacts on ecosystems is particularly acute in underdeveloped regions of the world, where access to monitoring equipment is limited. For lakes in these regions, paleolimnology may provide the only practical and relatively inexpensive means of reconstructing impact histories.

Geochemical Archives in Lake Deposits

As we saw in chapter 4, the isotopic, elemental, or molecular constituents of a lake and its sediments reflect both external chemical inputs and the lake’s internal biogeochemical cycles. Lake sediment geochemistry is the product of interactions between these external inputs from watershed geology, groundwater, vegetation, and the airshed, and internal lake processes. Both external and internal inputs are heavily influenced by climate, and for the past few thousand years, human activities. With careful consideration of the various information filters affecting their records, geochemists can greatly broaden the scope of questions that can be addressed using paleolimnology. It is of critical importance when interpreting chemical data, that it be placed in the context of other sedimentological or paleontological archives. With modern, automated techniques, it is possible to amass large amounts of geochemical data in a relatively short time, data that can be compiled into deceptively ‘‘simple’’ geochemical profiles. Perhaps more than any other types of indicator, geochemical profiles are often interpreted as standalone records, without reference to petrographical, or even gross lithofacies information. Although it is tempting to read chemical stratigraphies as a direct record of inputs from a watershed or airshed, the signals are blurred by the whole host of messy, internal processes that we have already encountered in the hydroclimate filter: lag and residence time effects, reworking, particle and redox focusing, organismal uptake, and bioturbation. Lake deposits integrate local changes in source conditions, background sedimentation rates, and geochemical focusing processes (Engstrom and Swain, 1986). As a result, different locations within a lake may provide different geochemical histories, and interpretations of an integrated lake history must take into account these internal variations and their probable causes. This is always harder to do with paleolake deposits, where the original basin morphometry and hydrology is obscure. In this chapter we will also consider postdepositional information filters that affect geochemical archives, in particular bioturbation and diagenesis. Because many geochemical components of interest to paleolimnologists are bound to fine-grained particles, they can be readily mixed by bioturbation.

Facies Models at the Lake Basin Scale

Understanding the historical evolution of sedimentation in a lake requires not only a grounding in facies interpretation but also an understanding of the larger-scale, lakewide linkages between deposition and those factors influencing sedimentation. The facies models we examined in chapter 7 can be linked to understand the differences in deposits between lake basins. Basin-scale facies models focus on the major interactions between climate or tectonic/ volcanic activity and sedimentation, attempting to explain why particular facies types develop in particular areas or at particular times in a lake’s history. Here I will focus on a few examples from the most intensively studied depositional settings, including lake types defined by mode of origin and evolution (rifts, glacial lakes, etc.) as well as saline lakes and playas, which share chemical and climatic attributes. Large-scale facies modeling in rift lakes has been driven by a need to understand the occurrence of hydrocarbons in ancient rifts (Lambiase, 1990; Katz, 2001). This in turn spurred a rapid accumulation of seismic reflection and facies data in the East African rift lakes and Lake Baikal (Russia) during the 1980s and 1990s, as well as attempts to synthesize these data and integrate them into general models. As we saw in chapter 2, the evolution of rift basins involves the development of asymmetric half-grabens and, in larger lake systems, the linkage of these half-grabens in a linear chain. As rift basins age, progressive deformation will eventually cause extensive deformation on both sides of the basin, transforming them into asymmetric full grabens, as seen in Lake Baikal today. This pattern of tectonic development has consequences for geomorphology, sediment delivery rates and locations, and sediment composition, that also vary depending on whether the lake basin is relatively full (high-stand conditions) or empty (low-stand) (Rosendahl et al., 1986; Cohen, 1990; Scholz and Rosendahl, 1990; Tiercelin et al., 1992; Soreghan and Cohen, 1996). Large-scale depositional patterns in a rift lake therefore represent an interplay between tectonic and climatic forces, factors that operate on somewhat different time scales.

Sedimentological Archives in Lake Deposits

Lake sediments are both repositories and sources of information about lake history. Depositional products tell us about the mechanisms of transport or accumulation of important geochemical and fossil archives, but important clues about that history are imbedded in the pattern of sedimentation itself. Geologists have recognized this fact since the earliest paleolimnological studies. Although he would certainly not have called himself a paleolimnologist, Charles Lyell’s (1830) classic studies and interpretation of the depositional environments of the Eocene Paris Basin set the tone for a time-honored approach to the study of ancient lake deposits. Lyell recognized that understanding the physical, chemical, and biological attributes of lakes that affect sedimentation, obtained through modern observation, must be applied to a four-dimensional (spatial plus time) analysis of sedimentary deposits and depositional history. However, not everything we need to know or every process we need to invoke will necessarily arise from our short-term observations of modern lakes. Events that are unlikely to occur in the course of a brief, several-year experiment or period of monitoring may become virtual certainties over the long history of some lakes and may leave a sedimentary archive of which we have little prior understanding from modern studies (Dott, 1983). Furthermore, the sedimentary response that we observe to some external forcing event may differ depending on the time scale over which we observe the response (Dearing, 1991). Consider a hill slope that is undergoing accelerated erosion, and that is producing an accumulation of sediment in a downstream channel as a result of land-clearing activities. Initially there may be no response in terms of sedimentation rate in the downstream lake; all of the sediment is being held in temporary storage. This process may occur over time scales of a few decades. At some later time a triggering event, perhaps a series of abnormally high rainfall and discharge years, causes this sediment to be released to the lake, now at an accelerated rate. This becomes a sedimentary response that the paleolimnologist can record. But, over geological time scales of millennia or longer, the original process may be modified, and new ones may gain in importance.

The Biological Environment of Lakes

Biological processes form the basis for a rich source of information for paleolimnologists. Populations of organisms are sensitive to variations in their external environment, and this sensitivity can be recorded as proportional changes in fossil abundances, evolutionary change, or extinction. Variations in lake temperature or water chemistry below the threshold of geochemical archives would normally go unrecorded in lake deposits were it not for fossils capable of registering these changes. Biotic systems are also the most complex components of lake systems, involving numerous species, their interactions with each other, and with their external environment. As a result, the interpretation of lacustrine fossil records is rarely straightforward, and must be viewed in the context of complex ecological dynamics, unfolding against a background of environmental and evolutionary change. In this chapter we will consider the biotic structure of lakes from a paleolimnological perspective, focusing on organisms and ecological interactions likely to be preserved in a lake’s fossil record. A transect running downslope and offshore from the shoreline will almost invariably reveal a change in habitat and lake organisms (see figure 3.2). In the shallow, littoral zone, high rates of photosynthesis can normally be supported, as light is not a limiting factor for growth. A high diversity of autotrophic and heterotrophic (consuming) organisms is encountered here. Near the shoreline, a fringe of emergent or submerged macrophytes is often present, either attached to the substrate, or floating nearshore. These plants form a substrate for many attached (epiphytic) or crawling organisms. On wave-swept, rocky, or sandy coasts macrophytes may be absent, but abundant algae or photosynthetic bacteria may be present, attached to rock surfaces (epilithic), or adhering to sand grains. In the sublittoral zone, light penetration is reduced, and large macrophytic plants are absent, but lower levels of benthic primary production may persist from algal or bacterial growth. Although algae are frequently found below the photic zone, because of circulation or settling, they are not photosynthesizing under such conditions. In the aphotic, profundal zone food resources are provided exclusively through secondary productivity, consumption of settling detritus (or the organisms that feed on such detritus), and microbial food resources.

Lakes as Archives of Earth History

For several months each year I work in central Africa collecting sediment cores and fossils from a large rift lake, Lake Tanganyika. Periodically my nonscientist friends ask me why I do this. They usually mean both ‘‘why would someone collect mud from the bottom of a lake’’? and perhaps as an even greater challenge to my sanity, ‘‘why would one travel halfway around the world to do this’’? The answer to these questions (and the theme of this book) is deceptively simple. Paleolimnologists study lake deposits because they provide science with archives of earth and ecosystem history that are both highly resolved in time and of long duration. In the particular case of Lake Tanganyika, this combination, in principle, permits us to study events as closely spaced in time as annual events over the lake’s 10-million-year history. Few other records of earth history beyond those found in lake muds provide this combination of duration and resolution. The range of questions that can be examined with these archives is enormous. Paleolimnologists provide constraints on the timing of past climate change, determine rates of evolutionary change in species, and investigate the timing of pollutant introduction into watersheds. One might reasonably ask what good could come from trying to synthesize these disparate questions. I believe that the unifying factor behind all of these fields of study lies in the character of lake sediment archives. Lakes are attractive targets for study by such different fields of investigation because of the special nature of their depositional environment. Considering the contents of lake archives and their characteristics is the best place to start thinking about what makes paleolimnology a distinctive discipline. . . . What Are Lake Archives? . . . An archive is a singularly appropriate term to describe the foundations of paleolimnological research. The term archive can refer both to a historical record, its content, and to the place where such records are housed, its container.