PEAT PARTICLE SIZE DISTRIBUTION: A PROXY FOR PEAT DEPOSITIONAL ENVIRONMENTS IN MANGROVE SWAMPS OF SOUTH FLORIDA

Detritivores and biogeochemical processes in the taphonomically active zone of peat (i.e. the surficial leaf mat and underlying zone of root intrusion) control particle size distribution of peat. Thus, peat particle size distribution reflects the rate of peat decomposition. Within modern swamps, peat particle size is an important indicator of depositional environments (e.g. freshwater vs marine) and provides one of the few ways to track the history of decomposition. Furthermore, peat particle size distribution can be used as a proxy for rates of decomposition in ancient wetlands, enabling comparisons between ancient and modern wetlands.

We collected short peat cores (15 cm depth) from two sites within a mangrove swamp at Barnes Sound, Key Largo, Florida: a basin site, which is generally flooded, has a thick leaf mat (19-37 stacked leaves) and few macrodetritivore groups; and a fringe site, where the peat surface is generally exposed, has a thin leaf mat (0-4 stacked leaves) and more macrodetritivore groups. For both sites, we used wet sieving of peat samples (2.5 x 2.5 x 2 cm³) to determine peat particle size distribution at four levels of the cores: the surfical leaf mat, immediately below the leaf mat, 5 cm depth, and 10 cm depth. We dried sieved samples from each level for 24hrs at 50°C and weighed them to determine the relative contribution of each size fraction.

The smallest size fraction (106 -150 mm) differed between the two sites. The composition of the smallest size fraction of wet-sieved peat may enable us to identify mangrove depositional environments: fringe cores contained forams, microarthropod bodies/molts, and abundant fecal pellets; basin cores contained small plant particles and snail fecal strings, but lacked fecal pellets and forams. Cohen and Spackman (1977) split peat particles into two size fractions: framework (≥ 0.100 mm) and matrix (< 100 mm). They used point counts of peat thin-sections to determine framework percentages of 40-50% for mangrove peat in South Florida. We found much higher framework percentages (83-94%) using the sieve method. However, our smallest size fractions (106 mm and 150 mm) had high percentages of fecal pellets and debris strings, which may have been dissolved in the Cohen and Spackman (1977) technique required to make thin-sections.