Ecological response of a glacier-fed peatland to late Holocene climate and glacier changes on subantarctic South Georgia

Abstract

Sedimentary deposits from glacier-fed peatlands provide records of past glacier variability, but the dynamics of these peat-forming ecosystems have rarely been investigated. Through multi-proxy analyses of a 204-cm peat core, we reconstructed the ecological response of a glacier-fed peatland on subantarctic South Georgia to climate and glacier variability over the last 4300 years. A stable peatland with rapid carbon accumulations and dynamic turnovers between brown mosses and monocots persisted between 4300 and 2550 cal yr BP when the up-valley cirque glacier was small under a regional hypsithermal climate. Carbon accumulation rates showed two peak periods driven by climate warming, reaching 140 g C m−2 yr−1 at 4000–3500 cal yr BP and 70 g C m−2 yr−1 at 3200–2700 cal yr BP. Paired δ13C and δ18O data from brown moss cellulose reveal several intervals of glacial meltwater inundation that caused short-term disturbances of the peatland vegetation, indicating that glacial meltwater potentially still affected the peatland ecosystem during this warm period. Moss-dominated vegetation was disrupted and peatland carbon accumulation rates decreased to 15 g C m−2 yr−1 after around 2550 cal yr BP when a cooling-driven glacier advance shifted the erosion and meltwater regime enhancing the glacial sediment influx onto the peatland. Although this enhanced regime of meltwater disturbance has continued since this transition, the brown moss habitat was gradually re-established during the medieval climate warming between 1200 and 600 cal yr BP and then became dominant shortly after that. This re-establishment of brown mosses might have benefited from a period of increased carbon accumulation rates up to 100 g C m−2 yr−1 at 1200–900 cal yr BP that built up the organic matter matrix and stabilized the habitat. We conclude that the ecosystem dynamics of glacier-fed peatlands is strongly shaped by the interplay between regional climate and glacier activity. This study also demonstrates the potential of stable isotope analysis in studying the paleohydrology of non-Sphagnum peatlands.