Transport of terrestrial organic matter to the deep North Atlantic Ocean by ice rafting

Total organic carbon (TOC), δ13C values of TOC (δ13Corg) and glycerol dialkyl glycerol tetraethers (GDGTs) were analysed for a sediment core from the North Atlantic covering the last 30 kyr to investigate organic matter deposition due to ice rafting. TOC content was low in sediments representing glacial times (0.2–0.4%) and even lower in Holocene sediments (< 0.2%). The δ13Corg values varied from −24‰ in glacial times to −20‰ at the start of the Holocene (8 ky cal BP), with negative excursions to −26‰ during Heinrich events. The δ13Corg values correlated non-linearly with % ice rafted debris in the sediments, suggestive of supply of continental organic matter by ice rafting. GDGT analysis revealed varying amounts of soil-derived branched GDGTs and the marine isoprenoid GDGT, crenarchaeol, which is expressed in the branched isoprenoid tetraether (BIT) index. The BIT index was relatively high (0.3) in sediments deposited during the glacial compared to those laid down at the start of the Holocene (0.1), suggesting enhanced delivery of terrestrial OM to the North Atlantic by ice rafting, in agreement with the δ13Corg results. This was confirmed by analysis of the 14C content of TOC, which indicated substantially older ages (1–9 kyr) than the sediment age inferred from planktonic foraminifera. BIT indices and δ13Corg values show phase offsets during Heinrich events, suggestive of differences either in timing of supply or of different sources of organic matter, i.e. soil organic matter and ancient mature sedimentary organic matter. Alternatively it may reflect changing contributions of source areas.