A laboratory experiment on the behaviour of soil-derived core and intact polar GDGTs in aquatic environments

We have performed incubation experiments in order to examine the behaviour of soil-derived branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids upon entering an aquatic environment and to evaluate the processes that potentially take place during their fluvial transport from land to sea. We incubated a soil from the Rakaia River catchment on the South Island of New Zealand using Rakaia River water and ocean water collected near the river mouth as inocula for a period of up to 152 days. The concentrations, as well as the relative distribution of brGDGTs derived from intact polar (living; IPL) lipids and core (fossil; CL) lipids remained unaltered over the course of the experiment. Although the stability of the brGDGTs may be a consequence of the higher than natural soil : water ratio used in the laboratory experiment, the substantial increase (27-72 %) in the total pool of isoprenoid GDGTs (isoGDGTs) in all incubation setups, including the control using distilled water, indicates that entering an aquatic environment does influence the behaviour of soil-derived GDGTs. However, the availability of water appears to be more important than its properties. As a consequence of increasing isoGDGT concentrations, a decrease in Branched and Isoprenoid Tetraether (BIT) index values - a proxy for the relative input of fluvially discharged soil material into a marine system - became evident after an incubation period of 30 days, with a maximum final decrease of 0.88 to 0.74 in the experiment with river water. The relative distribution within the isoGDGT pool shows changes with time, suggesting that isoGDGT producers may either have different rates of membrane adaptation or production/degradation, or that preferential release from the soil matrix or a shift in source organism(s) may take place. While the apparent stability of soil brGDGTs during this incubation experiment reinforces their potential as tracers for land-sea transport of soil organic carbon and their use in paleoclimate reconstructions, the distributional differences between GDGTs in river water and nearby soil, as well as in river and ocean water, indicate that further research is needed to pinpoint the sources of GDGTs that are ultimately discharged to the oceans and are subsequently archived in continental margin sediments.