WHERE DID ALL THE HALIMEDA GO?; THE ABSENCE OF THICK HALIMEDA PACKSTONES IN THE BAHAMIAN FOSSIL RECORD

Halimeda is an abundant calcareous alga in tropical carbonate environments capable of generating between 3-19 new crops per year (Hillis, 1991) or over 2m/1000y of vertical accumulation. We would therefore expect Halimeda to be well represented in the fossil record of the Bahamas. Halimeda is present in the fossil record beginning in the Cretaceous and experienced a major period of diversification in the Paleogene, The most extensive record of the genus is in Egypt and Morocco where thick sequences of Halimeda rich units have been described (Dragastan and Soliman, 2002; Dragastan and Herbig, 2007). Ironically Halimeda does not have a rich fossil record in pre-Holocene sequences of the Bahamas. On San Salvador fossil Halimeda has been found in Pigeon Creek quarry, and has been documented from stratigraphic sections probably representing MIS 5e (Mylroie et al, 2012). However, the abundance of fossil Halimeda segments present are considerably less than those of the very thick Halimeda packstones/wackestones observed by Andersen and Boardman (1989) and Colby and Boardman (1989), which date to the mid Holocene. The presence of Halimeda appears to be a function of nutrients provided by lagoonal grasses or mangrove swamps. Another argument for the lack of Halimeda bioherms and/or packstones is a biogeographical one. In essence, Halimeda opuntia, which forms bioherms may actually be invasive. H. opuntia is part of a clade of Indo-Pacific species and phylogenetic analysis indicates that the species originated within that basin and had to be dispersed to the Atlantic (Verbruggen et al. 2009). One scenario that has been proposed is that introduction was via early inter-oceanic shipping. Other factors limiting the fossil record of Halimeda relate to diagenetic processes, particularly early ones, preferentially preserving some grains while destroying others (Kidwell and Bosence 1991). In particular, back-reef sites are especially unfavorable for preserving Halimeda and the grains experience dissolution due to intense microboring and pore-water undersaturation(Perry, 2000). Future studies investigating the taphonomy and alteration of Halimeda segments have the potential to clarify why there is a preservational bias of Halimeda in the fossil record of the Bahamas.