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

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 L21 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 L21) detected in river water and the high branched GDGT concentration measured in the water of a Swiss peat bog (5,900 ng L21) suggest fluvial transport of branched GDGTs from peats and soils to the oceans. The high crenarchaeol concentration measured in river water (4.4 ng L21) was probably derived from crenarchaeota living in rivers and organic material from soil and peat. The BIT index, d13C 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 d13C and branched GDGT concentrations, indicating that the absolute GDGT concentrations give additional information to the BIT index. Rivers provide large amounts of organic carbon to the marine environment (,4 3 1014 g C yr21; Schlesinger and Melack 1981) and are thus a key component of the global carbon cycle. Analysis of the terrestrial organic matter (TOM) buried in marine sediments is widely carried out to estimate the importance and fate of TOM in carbon cycling. In paleoenvironmental studies, determination of TOM transport can also provide useful information on, for example, the proximity of the continent, river fluxes, and wind strength. The distinction between organic carbon of marine and terrestrial origins is commonly made by analyzing the C : N ratio and the 13C content of bulk organic matter (Meyers et al. 1994). Numerous biomarkers derived from higher plants are also used as proxies for tracing TOM input in marine sediments. However, several problems are inherent to both approaches (Hedges et al. 1997; Pancost and Boot 2004). For instance, the inconstancy of the d13C values of phytoplanktonic organic matter and TOM (e.g., C3 vs. C4 vegetation) renders the estimation of the proportion of terrestrial to marine organic matter in sediments difficult. Similarly, the difference in degradation rates of terrestrial-derived and marine-derived compounds may lead to erroneous estimation of terrestrial input in marine settings. These shortcomings make accurate quantification of TOM input in marine environments challenging (Hopmans et al. 2004, and references therein; Pancost and Boot 2004). In fact, Hedges and coworkers (1997) highlighted the geochemical conundrum, whereby significant fluvial delivery of TOM is not associated with detection of high abundances of TOM in seawater and marine sediments. They suggested that either remineralization of TOM in marine settings is a rapid process or that traditional detection methods are not