Spatial distributions of core and intact glycerol dialkyl glycerol tetraethers (GDGTs) in the Columbia River Basin and Willapa Bay, Washington: Insights into origin and implications for the BIT index

Abstract

Branched and isoprenoid glycerol dialkyl glycerol tetraethers (brGDGTs and isoGDGTs, respectively) are used to reconstruct carbon flow from terrestrial landscapes to the ocean in a proxy called the branched vs. isoprenoid tetraether index, or BIT index. The index is based on analysis of core GDGTs (C-GDGTs), that are produced from intact GDGTs (I-GDGTs) upon cell death. GDGTs are a component of the cell membranes of bacteria in soils (br) and archaea (iso) primarily in the marine environment. However, uncertainty as to the identity of the organisms that biosynthesize brGDGTs, the variation in the extent to which GDGTs represent end member carbon pools and the suspected production of brGDGTs and isoGDGTs in both marine and terrigenous environments hinders interpretation of the BIT index. Since brGDGTs remain our only tool for measuring the presence of unknown brGDGT producing organisms, it is particularly important to use the intact form of brGDGTs in living cells (rather than C-brGDGTs in relict biomass) to infer the distributions of the likely source organisms. In situ production within riverine, lacustrine and marine environments is currently thought to be possible, yet few measures of I-brGDGTs are available in these environments to support these claims. Here we assess the spatial distribution of core and intact GDGTs in the Columbia River Basin and nearby Willapa Bay (Washington and Oregon) in order to elucidate source environments and behavior of I-GDGTs vs. C-GDGTs. The presence of I-BrGDGTs throughout the studied soils, rivers and estuaries suggests in situ production across the continuum from soil to marine environments. Likewise, intact crenarchaeol, the marine end member isoGDGT used in the BIT index, was present in all samples. Widespread production of each GDGT class along terrigenous carbon transport paths likely alters the BIT index along this continuum. Core to intact ratios and the weak correlation between I-GDGT derived BIT values and carbon isotope signatures suggest a mixture of allochthonous and autochthonous sources of GDGTs in riverine and marine environments. Our findings highlight the need for further work into the provenance and behavior of GDGTs in order to improve interpretation of the BIT index and other environmental proxies that rely on them.