CH08: Past Freshwater Changes

1. CH08: Past Freshwater Changes

2. FIGURE 8.1 Fossils of a laggerstatten. Laggerstatten are concentrations of well-preserved fossils that provide unique macrofossil evidence of past species assemblages. Source: Matt Friedman - Specimen is held in the Field Museum, in Chicago.

3. FIGURE 8.2 Mega-droughts over two millennia. Mega-droughts are droughts that last for more than a century. Numerous mega-droughts have been documented from lake sediments in many regions of the world. Note that time runs from right to left in this figure. (a) Lake Naivasha, Kenya; (b) Lake Punta Laguna, Mexico; (c) Lake Titicaca, Bolivia/Peru; (d) Lake lowstands, Califorina, USA; (e) Tree-rings, S. Nevada, USA; (f) Active dunes, Great plains, USA; (g) Moon Lake, N. Dakota, USA. Source: Overpeck et al. (2005). Reproduced with permission from Yale University Press.

4. FIGURE 8.3 Varve record of the end of the Younger Dryas. Varved (annually resolved) lake sediments make possible this highly resolved record of climate change around the Younger Dryas, Bolling, and Allerod events. Source: Landmann et al. (1996).

5. FIGURE 8.4 Milankovitch forcing in lignite deposits. Interbedding driven by orbital cycles is clearly evident in these lake deposits in northern Greece. Source: Cohen (2003).

6. FIGURE 8.5 Monomictic, dimictic, and meromictic lake turnover. Winds, ice, and temperature conditions determine lake turnover, resulting in lakes that turnover once (monomictic), usually in fall; or twice (dimictic)—once in fall and once in spring. Meromictic lakes mix incompletely, resulting in a deep unmixed layer.

7. FIGURE 8.6 Three-dimensional freshwater food chain diagrams. Dr Neo Martinez at the National Center for Ecological Analysis and Synthesis constructs three-dimensional food webs that capture the multiple trophic strategies of freshwater systems. Lower levels of the food chain are interconnected due to multiple midlevel feeders acting on primary producers. Upper levels of the food chain become progressively simpler, often converging on single top carnivores. Source: Courtesy of Paulo C. Olivas.

8. FIGURE 8.7 Freshwater family accumulation in deep time. Source: Cohen (2003).

9. FIGURE 8.8 Freshwater family first and last occurrences. Source: Cohen (2003).

10. FIGURE8.9 Fossil lake and human occupation sites in the Sahara. Fossil lakes (left) in the Sahara indicate that the conditions were much moister there between 2500 and 10,000 years ago. Remains of human occupations and elephants (right) include rock art (symbols), finely worked tools, and signs of hunts. Modern elephant range is shown for comparison. Source: Wilson et al. (2000).