CHARACTERIZING THE TAPHONOMICALLY ACTIVE ZONE IN SUBTROPICAL PEAT FROM BARNES SOUND, KEY LARGO, FLORIDA: IMPLICATIONS FOR USING TAPHONOMIC FACTORS TO IDENTIFY ANCIENT MANGROVE MIRES

Processes in the taphonomically active zone at the mire surface influence the organ composition and particle size of buried peat. Surficial mangrove peat from Barnes Sound, Key Largo, Florida illustrates the link between macro- and micro-detritivores and peat composition. Unlike previously studied mangrove mires, Barnes Sound has a thick coherent leaf mat (~2.5 cm thick, 19 – 37 stacked leaves). The coherent leaf mat overlies a layer of fragmented leaves intruded by living rootlets, which extends to a depth of approximately 5 cm. Below this depth, the peat consists primarily of living and dead roots and rootlets, large pieces of wood, and fecal pellets. At Barnes Sound, the percentage of small particles (particles with one dimension < 250 μm) increased with depth from 5% to 20%, and the percentage of large particles (particles with one dimension ≥ 2 mm) decreased with depth from 80% to 25%.

Taphonomic analysis of 100 leaves from the coherent leaf mat at Barnes Sound revealed herbivory in the canopy as well as detritivory and fungal and microbial decomposition in the peat. Fifteen percent of leaves had scraped surfaces on one side, indicative of herbivory by mangrove crabs in the canopy. Abundant fecal pellets (106 μm – 2 mm) throughout the cores indicate the importance of detritivory by microarthropods and insects. Shells belonging to the detritivorous neogastropod Melampus coffeus occurred in the core, but leaf deterioration by M. coffeus, which results in partial skeletonization of leaves, is hard to distinguish from skeletonization by canopy herbivores or other detritivores. Although we found no carb burrows at the study site, 8% of leaves showed edge-feeding patterns consistent with leaf consumption by detritivorous mangrove crabs.

Detritivorous crabs contribute to the destruction of leaves in many modern mangrove mires. The presence of exceptionally thick leaf mats at Barnes Sound may be due to the scarcity of detritivorous mangrove crabs at this locality. If detritivorous crabs are reduced or absent, thick leaf mats can accumulate in saltwater mires. Our results suggest that thick leaf mats could have accumulated in ancient saltwater mires that lacked crab macro-detritivores. Low shoot-to-root ratios and the presence of thick leaf mats cannot confidently be used as taphonomic indicators of freshwater peat.