ABSTRACT: Reconstructing wetland succession on rebounding coastlines using modern analogues.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Isostatic uplift is an important driver of wetland succession in northern Canada. It provides new shorelines to be colonized by tidal marshes and with continuing uplift, the diminished tidal influence drives non-tidal fens to appear. As peat accumulates in fens, the vegetation becomes isolated from the nutrient-rich substrate. This process leads to the replacement of the fens with oligotrophic (nutrient-poor) raised bogs and forested bogs. During the fen to bog succession, autogenic (autoregulatory) processes such as peat growth and acidification play an important role while the influence of uplift is negligible. These ecosystem changes represent changes in resources the postglacial landscape could have provided to early peoples. In past studies of Hudson and James Bay coastal wetlands, the sequence of paleoecological change has been inferred from pollen assemblages and carbon/nitrogen ratios and was linked to vegetation changes observed on the modern landscape chronosequences. Neither pollen nor carbon/nitrogen ratios were calibrated to modern ecosystems. This has lead to some misinterpretation of the fossil pollen assemblages and thus to incorrect identification of paleoecosystems. For instance, sedge pollen (Cyperaceae) was used as indicator of both tidal marshes and fens which led to an arbitrary discrimination between these ecosystem classes in paleoecological reconstructions. Our research refines the reconstruction of wetland succession in response to isostatic rebound by calibrating the pollen assemblages and carbon nitrogen ratios in a suite of wetlands that represent various successional stages across a landscape chronosequence. These signatures (analogues) can then be used to objectively discriminate between each successional stage in sediment cores. We examined the carbon-nitrogen and palynological signatures of wetlands in the coastal region of eastern James Bay, Quebec. Surface sediments were collected from 29 wetlands that included low and high elevation tidal marsh, fen, and bog. Using discriminant analysis we can successfully differentiate the four wetland types when employing nine palynological indicators. Total dinoflagellate cysts and seaside plantain (Plantago) were important in discriminating low elevation tidal marsh. Pollen from grasses, sedges and arrow grass (Potamogeton-Triglochin type) were good indicators of high elevation tidal marsh. Bayberry (Myrica gale) pollen and, to a lesser extent, moss spores other than Sphagnum, were important in the classification of the fen samples. Sphagnum spores were, as expected, critical indicators for the bog samples. When fossil pollen assemblages from cores are combined with these modern analogues, discriminant analysis assigns them to the modern ecosystem they most resemble. Carbon-nitrogen ratios provide an independent means to validate the pollen-based ecosystems reconstruction. The high carbon-nitrogen ratios of bog samples differentiate it from other wetland ecosystem types.

Bibliography of Canadian Geomorphology