Characterization of transport and deposition of terrestrial organic matter in the southern North Sea using the BIT index

Abstract

We have applied a recently introduced proxy, the BIT (branched and isoprenoid tetraether) index, to determine terrestrial organic matter (TOM) transport from the rivers Rhine and Meuse and their tributaries to the southern North Sea. This index is based on crenarchaeol, an isoprenoidal glycerol dialkyl glycerol tetraether (GDGT) predominantly derived from aquatic Crenarchaeota and branched GDGTs produced by soil bacteria. Up to 1.6 ng L-1 of branched GDGTs were measured in seawater, demonstrating the presence of TOM in the southern North Sea. BIT indices were inversely correlated with salinity, indicating that this TOM was recently supplied by rivers. The substantial amount of branched GDGTs (35 ng L-1) detected in river water and the high branched GDGT concentration measured in the water of a Swiss peat bog (5,900 ng L-1) suggest fluvial transport of branched GDGTs from peats and soils to the oceans. The high crenarchaeol concentration measured in river water (4.4 ng L-1) was probably derived from crenarchaeota living in rivers and organic material from soil and peat. The BIT index, δ13C value, and C : N ratio of surface sediments deposited in the southern North Sea were compared to determine TOM deposition. BIT index (0.07 to 0.26) and d13C (220.6 to 222.9%) both showed substantial small-scale differences in TOM deposition in the southern North Sea, but this pattern was not obvious from the C : N ratios. A good correlation was found between δ13C and branched GDGT concentrations, indicating that the absolute GDGT concentrations give additional information to the BIT index.