Reinvestigation of the classic late‐glacial Bølling Sø sequence, Denmark: chronology, macrofossils, Cladocera and chydorid ephippia

Abstract

The late-glacial Bølling period was first identified by Johs. Iversen on the basis of pollen results from Lake Bølling Sø in Denmark. Because there were no radiocarbon dates from the sequence the Bølling Chronozone (12 000-13 000 14 C yr BP) was later established on the basis of dates from other sites. A new project is reinvestigating the sediments from the Bølling Sø sequence with AMS radiocarbon dating and multiproxy analyses. Here we present results of AMS radiocarbon dating, macrofossil analyses, cladoceran analyses (Cladocera concentrations and chydorid ephippia) and Pediastrum analyses (concentrations). The AMS dates on land plant remains show that the lower part of the sequence is around 12 500 14 C yr BP, and thus clearly pre-dates the Allerød chronozone. However, construction of a chronology for the sequence was problematic, partly because of reworking of macroscopic plant remains. The climate ameliorated after glacial conditions to such an extent that growth of plants could begin at ca. 12 500 14 C yr BP, but the results of multiproxy analyses show little evidence for a further warming period during the pre-Allerød part of the sequence. Lake productivity was low, and tree birch rare or maybe absent. This may reflect widespread occurrence of dead ice, unstable soils, heavy in-wash of minerogenic matter to the lake, resulting in turbid water and rapid sedimentation. The early pioneer vegetation was characterised by Salix polaris and Dryas octopetala, and by herbs. The Allerød Chronozone, and especially its initial part, appears to have been relatively warm but reduced cladoceran concentrations and increased proportion of chydorid ephippia suggest that climate cooled in the middle Allerød and that the late Allerød was colder than the early part. The early Younger Dryas was probably colder than the late Younger Dryas. Clear warming is apparent at the beginning of the Holocene, where the first macrofossil evidence of trees (Betula pubescens, Populus tremula) is found.

Younger Dryas Cladocera assemblages from two valley mires in central Poland and their potential significance for climate reconstructions

“…This has been recognised for the subbeen recognised for the subrecognised for the subrecognised for the subfor the subfossil remains of Cladocera: species preferring warmer conditions occurred sometimes during the YD (Szeroczyńska, 2006). These temperaThese temperatemperature fluctuations have also been recorded from other sites in Europe (Szeroczyńska, 1998a;Hofmann, 2000;Lotter et al, 1997Lotter et al, , 2000Bennike et al, 2004). This may indicate that, during the YD, there were short phases of milder climate conditions.…”

“…Yet, during the YD, there was also a short warmer episode with a high diversity of cladocerans and the presence of species preferring warm water, such as Camptocercus rectirostris, Pleuroxus spp., and Alonella nana (Szeroczyńska, 1998a;Hoff-243 man, 2000;Lotter et al, 2000;Bennike et al, 2004). A rapid warming occurred over the final 300 years of the late YD, reflected by a rise in the July temperatures of the order of 5°C (from 10-13°C to 16°C; Ralska-Jasiewiczowa et al, 1998;Starkel, 2002).…”

Environmental changes during the last millennium recorded in subfossil Cladocera, diatoms and sediment geochemistry from Lake El Sol (Central Mexico)

“…The fourth Cladocera taxa pres ent in the sed i ments of Lake El Sol was Chydorus cf. sphaericus, which is one of the spe cies most tol er ant of un fa vour able en viron men tal con di tions, es pe cially cold wa ter and nu tri ent scarcity (Sarmaja-Korjonen, 2004;Bennike et al, 2004).…”

Living on the good soil: relationships between soils, vegetation and human settlement during the late Allerød period in Denmark

“…Kolstrup (2007) demonstrated cover sand activity in western Denmark during the Allerød, which suggests that the woods, at least in western Denmark, must have had areas of open, sparsely vegetated ground that could have provided the source material. This interpretation is supported by several detailed plant macrofossil analyses from Bølling Sø, none of which have found tree birch during the Lateglacial period (Brandt 1954;Stockmarr 1975;Usinger 1985;Bennike et al 2004b). Nor have combined analyses of pollen and plant macrofossils from two localities in southern Denmark been able to show such an early immigration of tree birch; both show that it did not become established until the middle of the Allerød period (Mortensen et al 2011;Mortensen et al in press).…”