Response of an ombrotrophic bog to a regional climate event revealed by macrofossil,molecular and carbon isotopic data

We examined the variations in vegetation, organic carbon isotopic compositions and biomarker distributions spanning a well-constrained climatic shift in a Sub-Boreal Dutch raised bog. The macrofossils record a vegetation shift from a predominance of degraded Ericaceae rootlets to a predominance of well-preserved Sphagnum species, first S. cuspidatum, followed by S. imbricatum. This shift, as well as evidence from other macrofossils and fungal spores, indicates that the local climatic conditions changed from relatively dry to relatively wet, with the wettest local conditions occurring at the base of the transition. At this boundary, a positive shift occurs in the δ¹³C values of bulk peat (c. 4‰) and compounds derived from specific peat-forming plants (1 to 2‰). This is attributed to a combination of factors, including selective preservation of certain compound classes, changes in precipitation and its effect on the water table, and plant growth rates. The changes in peat preservation also indicate that redox changes occurred across the climatic change event. Indeed, biomarker transformations thought to reflect past redox conditions or bog water acidity – the 1,2,9-trimethyl 1,2,3,4-tetrahydropicene to ursolic acid ratio, diacid to hydroxy acid ratio, and 17β,21β(H)-homohopane to total homohopane ratio – all exhibited shifts spanning the climatic change transition. Although some of these trends indicate the need for more fundamental research on biomarker transformations in peat, some, such as the 17b,21b(H)-homohopane to total homohopane ratio, show promise as indicators of past bog water conditions that could complement macrofossil analyses.