Twenty‐thousand‐year gap between deglaciation and peat formation on sub‐Antarctic Marion Island attributed to climate and sea level change

Abstract

Radiocarbon dating of basal peats has been a key factor in determining minimum ages for deglaciation on sub‐Antarctic islands. On Marion Island, peat bogs dominate the landscape below 300 m a.s.l., and palynological assessments of peat cores have been used to assess the vegetation history and succession rates as well as the sensitivity of the indigenous flora to climatic change. Initiation of peat on the sub‐Antarctic islands signifies a major landscape change which has previously been linked to the retreat of glaciers. Here we test this hypothesis by comparing previously published and new basal peat ages from Marion Island with cosmogenic isotope dates for deglaciation, and local and regional palaeo‐environmental changes. Results show that, in common with other sub‐Antarctic islands, peat initiation occurred after the Antarctic Cold Reversal (15–13 ka) and through the early Holocene climate optimum. This substantially post‐dates cosmogenic isotope evidence for deglaciation from the basalts which shows that the areas where the peatlands dominate were ice‐free from the start of Marine Isotope Stage (MIS) 2 (~31 ka). This suggests that environmental conditions controlled peat initiation rather than deglaciation. Regional climatic proxies show that during and after MIS 2, extremely low temperatures, extensive sea ice conditions and depressed sea surface temperatures together with lower sea levels at an island scale could have maintained conditions unfavourable for peat initiation at their current locations. On Marion Island, the significant gap of ~20 000 years between the timing of deglaciation and peat formation indicates that the use of peat basal ages as a proxy for the minimum age of deglaciation in the sub‐Antarctic should be used with extreme caution.