POLLEN AND PALYNOFACIES OF ORGANIC-RICH WETLAND DEPOSITS IN THE NORTHERN MARGIN OF THE CONGAREE RIVER FLOODPLAIN, CONGAREE NATIONAL PARK, SC

Nyssa-dominated wetlands (gum swamps) presently occupy embayments along the northern margin of the Congaree River floodplain in Congaree National Park. These arc-shaped wetlands originate from meander scars of the river, but their remoteness from the present river channel and their juxtaposition to the bluffs create a depositional setting with enhanced hydroperiods and significant potential for preservation of phytogenic organic matter, including pollen. Samples macerated for palynological analysis contain a wide variety of pollen, non-pollen palynomorphs, and taphonomic indicators (e.g., palynomorph fragmentation, etching, stain acceptance) that can be used to define at least three distinct zones.

The basal zone is characterized by an assemblage that includes Picea, Tsuga, short-spined Compositae, and Oenothera. This zone is also characterized by various indicators of ponded conditions, such as algal filaments and zygospores; freshwater diatom hash layers; and abundant Nymphaea and Nuphar (water lily) debris including pollen and tissue fragments (especially, asterosclereids and leaf hair bases). These data suggest deposition in an ox-bow lake setting under boreal conditions significantly colder than the present climate, and are consistent with a Pleistocene incision of the northern valley margin.

The middle zone is dominated by Nyssa (gum), Ilex (holly), Carya (hickory), Ulmus (elm), Liquidambar (sweet gum), and Ericaceae pollen, with a significant complement of sedges, ferns, and mosses. This assemblage strongly resembles vegetation occurring today in many warm-temperate, coastal plain river swamps of the Southeast (probably a Nyssa biflora /Acer rubrum / Ilex opaca / Leucothoe axillaris / Carex atlantica assemblage). A significant increase in abundance of charcoal, fungal remains, and degraded tissue fragments suggests a non-ponded wetland now largely controlled by fluctuations in groundwater flow from the bluffs.

The uppermost zone exhibits only minor differences in palynoflora from the middle zone but is significantly different in micropetrographic composition, perhaps reflecting recent climate change or local anthropogenic disruption of drainage and/or groundwater flow.