Climatic determinants of Holocene peatland initiation in South Florida

An overview of basal ages of peatlands in Southern Florida shows two episodes of peatland initiation, the first between 4.5-6.5 kiloyear before present (ka) and a second between 2.0-3.5 ka. Here we test three hypotheses that may explain these two episodes of peatland initiation: (1) a gradual increase in precipitation throughout the Holocene, (2) decreased drainage due to Holocene sea level rise and, (3) increased climate variability from the mid to the late Holocene. The three hypotheses were tested by means of specific forcings in a model of peat accumulation and decomposition in Southern Florida. The magnitudes of these forcings were based upon climatic conditions for this region as derived from multiple independent proxy datasets. The model results suggest that long-term average precipitation was sufficient for peat development throughout the Holocene. Hence, the onset of peatland development at 6.5 ka cannot be explained by increased precipitation alone. Although sea level rise could explain this first period of peatland initiation, it could not account for the decline in peatland initiation after 4.5 ka. Instead, this period of reduced peatland initiation between 3.5-4.5 ka may be explained by an increase of multidecadal variability in precipitation. Multidecadal droughts may decrease the hydroperiod and make peatlands vulnerable to erosion and fires. The results further suggest that ample precipitation and the relatively high sea level of the late Holocene began to alleviate the impact of these multidecadal droughts after 3.5 ka, explaining the second episode of peatland initiation in Southern Florida. We conclude that the role of multidecadal climate variability is crucial to understand peatland development in Southern Florida.