Late Miocene Ice Rafting in the Iceland Sea. Evidence from coarse fraction analysis of ODP Site 907

Abstract

A high-resolution sedimentary record from ODP Leg 151 Hole 907A has been investigated with sedimentological and geochemical methods to study the paleoenvironmental evolution of the Iceland Sea over a 1-Myr time interval in the Late Miocene. Climate deterioration characterizes the Neogene period, culminating in a transition to bipolar glaciations in the Late Pliocene (Thiede and Myhre, 1996). Ice rafting has been reported in the Iceland Sea back to ~8 Ma (Fronval and Jansen, 1996). Despite evidence of older IRD pulses from several other high northern latitude sites (e.g. Helland and Holmes, 1997, Winkler et al., 2002), IRD analyses has not been carried out further back in time. This thesis extends the IRD record beyond 8 Ma and reveals continuous ice rafting in the Iceland Sea between 11 to 9.98 Ma as suggested by the common occurrence of coarse-fraction terrigenous sediment (>125 μm) throughout the studied interval in ODP Hole 907A. Surface texture analysis of sand-sized quartz grains (250–1000 μm) indicates iceberg transport of IRD to the Iceland Sea during the early Late Miocene. The IRD observed in this study has been related to small-scale glaciers on Greenland large enough to reach sea level in the Late Miocene. This is supported by a similar source proposed for IRD at Irminger Basin ODP Site 918 at the same time (Helland and Holmes, 1997). A southwards flowing East Greenland Current-precursor was presumably established (Wei, 1998) and may have transported icebergs to the study site. In addition, coarse fraction analysis reveals a biological turnover at 10.4 Ma, which coincides with changes observed in diatom and dinoflagellate cyst assemblages at ODP Site 907 (Stabell and Koç, 1996; Schreck et al., 2013). The biological turnover corresponds to maxima in quartz grain abundance and a significant coarsening of the bulk sediment grain-size, which may reflect intensified ice rafting at around 10.4 Ma. The timing of the changes observed in ODP Hole 907A correspond favourably to a short-term global Miocene cooling event (Mi-6 event, 10.7–10.4 Ma, Westerhold et al., 2005), thus, this thesis provides the first direct indications for the Mi-6 event in the high northern latitudes.