Areas that are now some of the harshest deserts environments were once deep lakes and lush marsh systems. This dramatic change is sometimes difficult to imagine, especially to an unwitting observer standing in the middle of windblown salt flat with no vegetation in sight. However, these landscapes are scattered with distinctive remnants of their curious past, the most widespread and easiest to recognize are paleoshorelines from ancient lakes.
Geography of Ancient Pluvial Lakes in the United States
Ancient pluvial lakes dominated the western landscape of United States, especially those within the Great Basin of North America, throughout the Pleistocene. Rainwater and runoff filled landlocked basins. It is from this rainwater that the pluvial lakes were named; the word pluvial comes from the Latin pluvia, which means “rain”.
How Pluvial Lakes Can Be Indicators of Past Climate Change
Pluvial lakes are useful to geographers and those interested in paleoclimate because of the variety of data contained within the lacustrine record. Shorelines (or more precisely, shorezones) are one of the oldest and most visible remnants as well as the earliest data representation of paleoclimate studied. Shorezones is a better term to describe the zone of multiple shorelines associated with a specific climatic event; the term has not been adopted very widely but is better at describing the actual variability in paleoshorelines.
Close basin hydrologic systems are unique indicators of wet and dry cycles: cooler and wetter conditions fill the lake basin creating higher elevation shorezones while dryer and warmer conditions reduce the size of the lake and lower the elevation of the shorezones.
This is a rather simplistic overview of how pluvial lakes can be indicators of past climate change; in reality, quite a bit of nuances are involved with teasing out the specific inflow and outflow of water within a closed system. For example, variable temperatures can affect surface evaporation. A number of variables must be accounted for in order to account for a lake’s water budget; these variables include the lake area, catchment area, groundwater inflow, runoff variables (including soil type and steepness), evaporation, direct precipitation on the lake, precipitation in the catchment area, groundwater outflow, seasonality, and the effects of isostatic rebound (post-glacial rebound) during times of recession.
Datasets Geographers Use to Understand Paleoclimate
Using this dataset of paleoshorzones, geographers have been mapping lake level fluctuations and interpreting paleoclimate for much of the twentieth century. Many of the large lakes are well known and well mapped including Lake Bonneville and Lake Lahotan. Many smaller lakes are known to exist and have not been fully mapped or described in the scientific literature.
In addition to shorezone mapping, a number of additional datasets have emerged in the last few decades and these are becoming favorites of paleogeographers and paleolimnologists. Along with the physical geographic evidence, living organisms leave traces in ancient lakes. Some of these newer lines of evidence include pollen, siliceous protozoans, biogenic silica, diatoms, ostracods, chrysophyte scales, algal pigments, and even fish faunal remains.
The datasets derived from this plethora of lacustrine records can provide information on much more information on past climate than just temperature and humidity. Information about atmospheric circulation, current flow patterns, climate-ecosystem linkages, and even human settlement patterns can be derived from pluvial lakes.
The wide variability in size and distribution of pluvial lakes can also be useful to geographers. In general, the location of pluvial lakes that are the most sensitive to climatic change (and therefore evidence of climate change) are those in which a small change in climate results in large changes in the limnological environment; this is true for pluvial lakes in extreme environments such as the high Arctic or those at boundaries of ecotones (such as a treelines).
Similarly, the size of pluvial lakes can also showcase different data on paleoclimate. Large lakes are better are recording climatic extremes and climate change on a global scale while shallow lakes are better are indicating seasonal and short-term changes.
Taken together, these datasets of paleoenvironmental variability contained in a variety of pluvial lakes is a treasure-trove of past climate. One of the more important aspects of these datasets is that collectively they form multiple overlapping lines of evidence of past climate and changes in past climate and this can provide a firm baseline for current debates and studies regarding current hypothesis of present-day climate change.
As many scholars of paleoclimate will attest, global warming is not a new phenomenon. Pluvial lakes have shown evidence of at least one major global warming event during an interglacial period. During the middle Holocene, North America experienced a major warming event with drier conditions combined with higher temperatures; this warming event is evidenced by many of the western pluvial lakes drying out completely. The Great Basin became much drier and warmer than it is today, causing major shifts in lake levels, treelines, plant communities, animal distributions and abundances, and even human population response. However, it is important to understand that although global warming has been documented in the past, the causes of climate change are subject to interpretation of the evidence.
The most unambiguous records documenting past climate change are direct records of lake-level fluctuations, most specifically from shorezone evidence and from multiple overlapping proxies that corroborate a climatic hypothesis. Current research in this area is being pursued around the globe.
Sherilyn C. Fritz (1996). Paleolimnological records of climatic change in North America. Limnology and Oceanography 41(5), 882-889
Katrina Moser (2004). Paleolimnology and the Frontiers of Biogeography. Physical Geography V25 No 6. 453-480.