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.
Physical Transport Processes that Affect the Distribution of Oil in the Gulf of Mexico: Observations and Modeling