Comment on "Donders, T.H. 2014. Middle Holocene humidity increase in Florida: climate or sea-level? Quaternary Science Reviews 103:170-174."

Donders (2014) has recently proposed that the climate of Florida became progressively wetter over the past 5000 years in response to a marked strengthening of the El Niño regime. This reconstruction is largely based on a re-analysis of pollen records from regions north of Lake Okeechobee (Fig. 1) using a new set of pollen transfer functions. Donders concluded that a latitudinal gradient in precipitation prevailed across Florida since the mid Holocene, but the overall trend was toward progressively wetter conditions from 5000 cal BP to the present.

Donders (2014) also proposed that this climatic trend extended across South Florida despite contrary paleo-records from the Everglades. In particular he singled out the Northeast Shark River Slough (NESRS) record of Glaser et al. (2013) as an atypical local signal of paleo-environmental change that was biased by a misinterpretation of the ecology of pine and Amaranthaceae (Amaranth family). In response to this direct critique of our paleo-environmental interpretation, we wish to point out that:

1. Our interpretation of the NESRS sedimentary sequence (site 4, Fig. 1A) was based on multiple lines of evidence that all indicate a shift from wetter to drier (i.e. less wet) conditions that occurred after 2800 cal BP.
2. A similar climatic shift from wetter to less wet conditions was reported for this time interval from other sites in the Everglades (Willard et al., 2006 and Willard and Bernhardt, 2011) and also from the Caribbean region to the east (Hodell et al., 1991,Hodell et al., 1995 and Higuera-Gundy et al., 1999).
3. The NESRS site is located in an area where runoff collected from a 10,000 km²wetland prior to 1900 AD when most of the drainage from the Everglades was channeled to the sea through the narrow Shark River Slough. The sedimentary sequence at the NESRS site should therefore contain an integrated record of hydrological change across the pre-historical Everglades (Glaser et al., 2012). In addition this essentially non-forested wetland is well suited to accumulate a representative sample of the regional pollen rain in its sediments (e.g. Jaccobson and Bradshaw, 1981 and Prentice, 1985).