Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Development of the BIT index to trace soil organic matter transported by ice rafted debris

During the Quarternary several rapid and short-lived climatic shifts have taken place in the North Atlantic Ocean (the so called Heinrich events) due to the release of massive icebergs from high latitude ice sheets. Few studies have considered the continental organic matter that may be transported in conjunction by ice rafting to the North Atlantic Ocean floor and which mixes with locally produced marine organic matter. Usually total organic carbon (TOC) and d13C of TOC (d13Corg) are used to infer the origin of organic matter but as a rule they give poor estimates of soil organic matter contributions.

We analysed glycerol dialkyl glycerol tetraethers (GDGTs) in a sediment core from the North Atlantic spanning the last 30 ky to investigate organic matter deposition due to ice rafting. TOC content was low in sediments representing glacial times (0.2 to 0.4%) and even lower in Holocene sediments (<0.2%). d13Corg values varied from -240 in glacial times to -200 at the start of the Holocene (8 ky cal BP; calibrated, Before Present) with negative excursions to -260 during Heinrich events. The d13Corg values correlated with the percentage of 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.

This 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 organic matter to the North Atlantic by ice rafting, in agreement with the d13Corg results. This was confirmed by analyses of 14C-contents of TOC, which indicated substantially older ages than the inferred sediment age. BIT indices and d13Corg show phase offsets during Heinrich events, suggesting differences either in timing of supply or in changing contributions of source areas.

The use of the BIT index thus leads to a substantial improvement in estimating the contribution of soil derived organic matter transported by ice-rafted debris into the oceans. This will improve estimates of terrestrial organic carbon burial into the deep ocean.

Key innovation
Our results show that the BIT index provides a good tracer to infer the presence and relative abundance of soil organic matter in ice rafted debris. GDGTs are relatively newly discovered biomarkers and thus have been rarely used as proxies and therefore this result is highly innovative.

Current status, dissemination and use
This result has been written up as a scientific publication and is currently under review at the journal Organic Geochemistry. This technique may be used by paleoclimatologists and paleoceanographers to reconstruct past fluxes of organic matter from the continent to the deep sea. We are presently using this tool to reconstruct past transport of soil organic matter into marine environments. We have plans to apply it to periods further back in time (i.e. older than the Quarternary) where the presence of ice rafted debris has been poorly documented.

Informations connexes

Reported by

See on map