Branched GDGT variability in sediments and soils from catchments with marked temperature seasonality

The distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in sediments are used as a proxy measurement to infer changes in past mean annual air temperatures (MAT). When applied to high resolution sedimentary sequences, measurement of brGDGT distributions is employed to reconstruct MAT at subdecadal time scales. In addition, brGDGT proxy estimates are also sometimes purported to be seasonally biased in environments where annual brGDGT production may not be constant during a seasonal cycle. The main aim of this study was to assess the occurrence of seasonality in the production and distribution of brGDGTs and the seasonality bias of the derived temperature proxy. For this purpose, we examined brGDGT distributions and brGDGT-derived MAT estimates in surface soils and settling/suspended particulate matter over one year, from two sites located in the same latitude but at different altitudes, in the Catalan Pyrenees, as well as at one site in southern Norway. These locations have marked seasonal temperature cycles, which were expected to maximize the possibility of detecting any seasonal bias in the production and compositions of brGDGTs. The results show that brGDGT abundance is heterogeneous and increases with soil humidity. The brGDGT distributions and some of the brGDGT-derived proxy measurements in soils are relatively stable throughout the year and do not change significantly in the suspended particulate matter in the river or settling particulate matter in traps. Our study shows that the impact of the seasonality of temperature on brGDGT distribution was absent in the soils studied, regardless of altitude or latitude on a catchment/regional scale. As soils are likely to contain a brGDGT signature which is representative of average environmental conditions in the catchment at least over decades, brGDGT proxy reconstructions derived from soil sources are more suitable to infer variability in environmental parameters over the same timescales (i.e. decades or longer). On shorter timescales (i.e. annual), sediment downcore variability in brGDGTs is likely to be related to changes derived from in situ production and sediment sources.