Investigating bacterial 3-hydroxy fatty acids as new indicators of past air temperature in lake sediments from New Zealand

Abstract

Sedimentary lipid biomarkers are important indicators to reconstruct past air and water temperatures in lakes, particularly for paleoreconstructions beyond monitored periods. Currently, glycerol dialkyl glycerol tetraethers (GDGTs) are widely used as temperature proxies in these settings, but uncertainties remain around biological sources and non-thermal impacts (e.g. pH, redox and relative contributions of in situ production and terrestrial supply). An alternate novel terrestrial biomarker proxy for temperature reconstructions has been developed based on the relative distributions of 3-hydroxy-fatty acids (3-OH-FAs), sourced from the outer membrane of Gram-negative bacteria. Preliminary studies in contemporary soils, stalagmites and lake sediments correlating bacterial 3-OH-FA compositions to air temperature and pH have shown promising results. However, their performance under discrete physical and chemical conditions remains largely unexplored. In this study, we have analysed 3-OH-FA distributions in surface sediments of 143 lakes across New Zealand. The lakes span a large diversity of lake and catchment sizes, water depths, geographic locations (latitude and altitude), productivity and trophic level, catchment vegetation and land-use. We have also studied the community composition and diversity of Gram-negative bacteria in the surface sediments of the studied lakes. 3-OH-FAs were detected in the sediments of all lakes and generally showed a correlation with monitored air temperatures. The strongest correlation was shown in a sub-set of remote lakes along an altitudinal transect with limited human impact where the ratio of anteiso to normal C17 3-OH-FAs (RAN17 index) and iso to normal C17 homologues (RIN17 index) correlate with mean annual air temperature (MAAT) with R2 of 0.73 and 0.64, respectively. In contrast, the RIN13 index, which was previously proposed for lake sediments, showed no correlation with MAAT. For the whole dataset comprising all studied lakes, only the RAN17 index correlates with MAAT with R2 of 0.46. Therefore, 3-OH-FAs have great potential as new indicators applicable to the reconstruction of past air temperatures from sediments of New Zealand’s lakes.