Sources and transport of terrestrial organic matter in Vancouver Island fjords and the Vancouver–Washington Margin: A multiproxy approach using dCorg, lignin phenols, and the ether lipid BIT index

The Branched and Isoprenoid Tetraether (BIT) index (a ratio of marine and terrestrial glycerol dialkyl glycerol tetraether [GDGT] membrane lipids) was recently introduced as a proxy for the fraction of fluvially derived organic carbon (OC) buried in marine sediments. We investigate the source and distribution of terrestrial OC in marine sediments in a large spatial data set that encompasses the Washington–Vancouver Island Margin, Vancouver Island fjords, and Puget Sound with the goal of comparing the BIT index to other traditional measures of terrestrial OC (d13C of OC [dCorg] and lignin phenols). dCorg values and lignin concentrations indicate that the fjords are a significant trap for undegraded, wood-derived terrestrial material, with surface sediments containing up to 76% terrestrial OC. These proxies also show significant terrestrial OC contributions to margin sediments (up to 54%), with most likely an angiosperm source from the Columbia River. In contrast to dCorg values and lignin concentrations, BIT index values indicate extremely low terrestrial carbon throughout the study area (BIT index values range from 0 to 0.28) and are uncorrelated with the other proxies. Though the BIT index did not correlate with other proxies, margin GDGT concentrations varied offshore similarly to other terrestrially derived OC and are well-correlated with other proxies. We suggest that the BIT index is best interpreted as a proxy for soilor peat-derived OC. The majority of terrestrially derived OC delivered to sediment in this area has a lignin-rich, nonsoil source. The disparity between the BIT index and other proxies implies that the BIT index significantly underestimates terrestrial relative to marine OC input to sediments in areas in which riverine OC input is not soil or peat derived. Burial of organic carbon (OC) on continental margins is estimated to account for the majority of the total carbon burial in the ocean and is a significant sink for terrestrial OC (Hedges and Keil 1995). Understanding the sources and fate of this terrestrial OC in marine sediments is thus of importance in understanding global biogeochemical cycles. In this study, we use two traditional proxies for terrestrial OC input to sediments—the d13C of bulk sedimentary OC (dCorg) and the concentration of lignin phenols—to examine the sources and quantity of terrestrial OC in sediments throughout the Washington–Vancouver Island Margin, Vancouver Island fjords, and Puget Sound. We compare these results to those obtained using a recently introduced proxy for terrestrial OC in marine sediments, the Branched and Isoprenoid Tetraether (BIT) index (Hopmans et al. 2004), to evaluate this proxy in a new location. The Columbia River is the dominant source of terrestrial OC to the Washington Margin and Cascadia Basin. Transport of the Columbia River plume brings fluvially derived material northward, toward the Olympic Peninsula and Vancouver Island (Sternberg 1986; Ridge and Carson 1987). Previous work in this region used dCorg, carbon : nitrogen (C : N) ratios, lignin phenols, and n-alkanes to demonstrate that terrestrial OC contributed between 15% and 60% of the total OC in margin sediments (Prahl et al. 1994). The coast of Vancouver Island is indented by a series of temperate fjords. Terrestrial carbon enters fjords from rivers at the head of each fjord, as well as from non–point source debris inputs from the sides of the fjords. Temperate fjords are thought to potentially trap significant amounts of carbon as a result of the high input of refractory (terrestrial) OC. Nuwer and Keil (2005) demonstrated the importance of fjords in storing terrestrial OC, estimating that fjord sediments may be responsible for trapping at least 12% of the total OC buried in continental margins over the last 100,000 years. Analysis of dCorg and C : N ratios of fjord sediments demonstrated that the fjord sediments can contain up to ,85% terrestrial OC (Nuwer