Florida

Because much of North America was ice-covered during the last glacial period, there are few sites where long records (extending back tens of thousands of years) have been recovered. In Florida, lower sea level and generally drier conditions led to lower water tables during the Wisconsin glaciation, so only the deepest lakes seem to have sedimentary records from that period (Watts and Hansen, 1994). Lake Tulane in south-central Florida is one example; an 18.5 m core appears to span the last -50 ka (though dating of the record before -35 ka is somewhat problematic). The pollen record from this site (Fig. 9.25) shows pronounced oscillations in the percentage of Pinus (pine) pollen, which varies in opposition to the percentages of

pollen alternating with intervals when Quercus (oak) and Ambrosia (ragweed) pollen percentages were high. Also shown is the relationship between pine pollen and Heinrich events, represented here by the fraction of lithic grains in samples from North Atlantic marine sediment core DSDP 609. Dating of the lake sediments prior to -35 ka B.P. is uncertain.There appears to be a correspondence between the timing of Heinrich events and pine pollen increases, though the mechanism linking these two records is not clear (Grimm et o/„ 1993).

pollen alternating with intervals when Quercus (oak) and Ambrosia (ragweed) pollen percentages were high. Also shown is the relationship between pine pollen and Heinrich events, represented here by the fraction of lithic grains in samples from North Atlantic marine sediment core DSDP 609. Dating of the lake sediments prior to -35 ka B.P. is uncertain.There appears to be a correspondence between the timing of Heinrich events and pine pollen increases, though the mechanism linking these two records is not clear (Grimm et o/„ 1993).

Quercus (oak) and Ambrosia-type (ragweed and marsh-elder). This is interpreted as a shift from pine to oak-savanna or open grassland-type vegetation, with abrupt transitions between the two (Grimm et al., 1993). The changes represent a shift in moisture availability, with the pine phase indicating wetter intervals separating drier episodes. Of particular interest is the similarity in timing of the pine expansion with Heinrich events in the North Atlantic (see Section 6.10.1), indicating the possibility that the terrestrial and marine events are causally linked. However, the mechanism of such a linkage is not clear. Heinrich events represent large-scale discharge of icebergs into the North Atlantic following prolonged periods of SST cooling. Cooler and/or wetter conditions in Florida (represented by increases in pine pollen) are related to cool waters in the Gulf of Mexico, which shifts the P-E balance towards positive values. One scenario is that the large-scale reorganization of North Atlantic circulation associated with Heinrich events led to cooler conditions in the Gulf of Mexico (via a reduction in the inflow of warm Caribbean/Gulf Stream waters). Alternatively, the drainage of glacial meltwater through the Mississippi River to the Gulf may have cooled surface waters in the Florida region. Neither scenario is clearly supported by the existing literature, so the exact mechanism linking the North Atlantic and Florida records remains enigmatic. Indeed, Watts and Hansen (1994) suggest that the higher pine percentages may be related to warmer SSTs in the Gulf, leading to stronger convective activity and higher rainfall amounts. This is one area where modeling could shed some light on the various alternatives. Further studies of other long continental records from along the Atlantic and Caribbean coasts are also needed to help resolve the matter. Because lake sediments have the potential for much higher resolution records than marine sediments, there are good prospects that detailed palynological studies can resolve in considerable detail the evolution of climatic changes in North America prior to, during and after glacial stage Heinrich events.

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