PS 38-206 - Hydro-climatic and anthropogenic controls on mid- to late Holocene wetland vegetation and sediment accretion regimes at a riparian freshwater marsh in southern Ontario, Canada

Background/Question/Methods

The Lower Great Lakes are surrounded by diverse wetland types including a critical coastal zone supporting local fisheries and some of the highest levels of rare or threatened species in Canada. Riparian zones adjacent to rivers flowing into Lake Erie support marshes and swamps, with mineral and organic sediments, contributing to regional-scale carbon accumulation. These watersheds have been heavily impacted by wetland drainage and forest clearance for agriculture and by the introduction of invasive species since Euro-American settlement. Significant water consumption and degraded water quality due to intensive agricultural operations, particularly in the Long Point Region, have resulted in an increasing interest in wetland restoration for biodiversity conservation, mitigation of nutrient runoff, regulation of streamflow, and natural climate solutions. Paleoecological records have great utility in directing and measuring the success of marsh restoration efforts as they provide baseline information on the key drivers and mechanisms affecting ecosystem change and wetland function. In this study we present a 4-m Late Holocene sedimentary record from the Central Big Creek Marsh in southern Ontario, Canada with the aim of reconstructing pre-settlement environmental conditions for future comparisons with modern and restored wetlands in the watershed.

Results/Conclusions

Our record from Central Big Creek Marsh spans 5,710 cal yr BP with sediment accretion rates between 0.2-2.2 mm yr-1 through the Middle and Late Holocene. These rates increase significantly after European settlement (after1870 AD) with rates up to 6.7 mm yr 1 . The total organic carbon to total nitrogen ratios (TOC/TN) ranged from 15 to 34 until ~ 4,930 cal yr BP, indicating strong influence by terrestrial inputs when lake water levels in Lake Erie began to increase owing to glacial isostatic adjustment. Through the Late Holocene, these ratios fluctuated between 6 and 16 which suggests that organics have been primarily derived from aquatic vegetation. The shifts in accretion rates and TOC/TN are reflected in a pollen-derived record which documents shifts in wetland plant and regional upland forest communities. Pollen data coupled with sediment physical properties were integrated to show a complex hydro-geomorphic context due to underlying glacial and alluvial deposits, major changes at the time of European settlement in terms of sediment accumulation, and changes in vegetation communities. These changes in vegetation and marsh sediment properties through the record provide detailed insights into pre-settlement conditions, as well as the impact of land conversion and recent restoration efforts.