Core and intact polar glycerol dialkyl glycerol tetraethers (GDGTs) in Sand Pond, Warwick, Rhode Island (USA): Insights into the origin of lacustrine GDGTs

Abstract

Branched and isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs) are abundant in lake sediments and have potential use for determining past changes in climate and limnology. However, emerging evidence suggests that both classes of GDGTs can be associated with both allochthonous soil organic matter and in situ production, potentially complicating interpretation of these compounds in lacustrine sedimentary environments. Here, we investigate the “dual source” issue surrounding lacustrine GDGTs – with an emphasis on the branched GDGTs – by studying both core and intact polar GDGTs within a pond and its watershed in Warwick, RI. Significant differences in both absolute concentrations of branched GDGTs and the percent of branched GDGTs present as IPLs between the soils and shallow lake sediments suggests in situ production of these compounds within the lake environment. Principal components analysis of GDGT lipid concentrations and environmental parameters in Sand Pond sediments and soils highlights two prominent trends in the data, one related to the absolute concentrations of “type I” branched GDGTs (brGDGT-I, Ib, Ic) and the majority of the isoprenoidal GDGTs, and another related to the concentration of the more methylated “type III” branched GDGTs (brGDGT-III, IIIb, IIIc) and isoprenoidal GDGT-0. The latter trend exhibits a subsurface maximum in the sediment column, potentially indicating in situ production of more highly methylated branched GDGTs and GDGT-0 within the lacustrine sediment column. Trends in the distributions of branched GDGTs (as reflected by the MBT and CBT indices) imply mixing of two distinct sources of branched GDGTs (soil-derived and in situ), or alternatively, two or more different microbial producers. Both possibilities present a challenge to the application of brGDGT-based paleoenvironmental proxies in lakes. This study highlights the importance of determining the sources of GDGTs to the lacustrine environment prior to paleoenvironmental application.