Comparison of lignin phenols and branched/isoprenoid tetraethers (BIT index) as indices of terrestrial organic matter in Doubtful Sound, Fiordland, New Zealand

Abstract

The branched/isoprenoid tetraether (BIT) index has recently been proposed as a qualitative proxy for the relative amounts of soil and marine derived organic matter (OM) in marine sediments. We compare BIT values with lignin–phenol CuO oxidation products and bulk OM proxies in surface sediment samples from Doubtful Sound, a fjord in the southwestern end of New Zealand’s South Island, as well as in soils and leaf litter from the local pristine temperate rainforest watershed. The region has extreme rates of annual rainfall (>7 m year −1) and large mass-wasting soil and vegetation events on fjord slopes, which results in relatively high fluxes of terrestrial organic matter (OM terr) to fjord sediments. Sediments contained up to 11.4% w/w organic carbon (OC) with terrestrial to a mixture of terrestrial/marine δ 13C (−24.3‰ to −28.7‰) and C/N (14.7–36.8) signatures. BIT values ranged from 0.24 to 0.93. Lignin abundance in sediments, reported as Λ 8 values (mg 8 lignin–phenols/100 mg OC −1), ranged from 2.7 to 11.2. A weak correlation was found between concentrations of lignin phenols and BIT values ( R 2 0.313). The likely cause of the differences between the two proxies is differences in the relative amounts of soil OM (OM soil) and vegetation (intact and detrital) transported to the fjord by episodic mass-wasting events and gradual accumulation from overland (leaching by rainfall) and riverine sources. Interestingly, the BIT index correlated significantly with bulk sedimentary OM (OM sed) proxies (δ 13C, R 2 0.774) (C/N, R 2 0.629), while lignin phenol values (Λ 8) exhibited a much weaker correlation with δ 13C values ( R 2 0.320) and no correlation with C/N values ( R 2 0.005). This suggests that OM soil represents the dominant fraction of OM terr delivered to fjord sediments.