Alkenone distribution and producers in Canadian lakes: Implications for palaeotemperature reconstructions

Alkenones are long-chain unsaturated ketones produced by a few number of haptophyte algae of the order Isochrysidale. The degree of unsaturation of C₃₇ alkenones has been shown to vary as a function of the environmental temperature, and these lipids have been thus used for decades as a proxy for sea surface temperatures thanks to the unsaturation indices U^K₃₇ ([C_37:2 - C_37:4]/[C_37:2 + C_37:3 + C_37:4]) and U^K'₃₇ ([C_37:2]/[C_37:2 + C_37:3]). More recently, alkenones have been globally reported in freshwater and saline lakes, but their use as a proxy for terrestrial temperatures is hampered by the rather limited knowledge about the environmental factors controlling alkenone distribution and the haptophyte species that produce them. Here, 106 lakes from the Northern Great Plains in Saskatchewan, Canada were investigated for alkenone producers and composition.

The study of surface sediments indicate that 55% of the Canadian lakes contain alkenones, with very high concentrations (up to 2.3 mg/g of sediment) in 7% of the lakes. Statistical analyses reveal that salinity and stratification of the water column play key roles in determining alkenone presence and abundance, which is certainly linked to the ecology of the producing haptophyte species. Moreover, genomic analyses based on surface sediments from some lakes evidence the presence of four distinct species from Group I and Group II haptophytes. Even though calibration of the U^K₃₇ index was not possible from the study of the surface sediments, seven haptophyte strains were isolated from three Canadian lakes, and characterised by genomic analyses as belonging to the clade Isochrysis. The culture of these new species shows that not only the U^K₃₇ and U^K'₃₇, but also alternate indices based on alkenoates (esters with structure close to the alkenones), are temperature dependent.

Taken together, these data bring new information on the production of lacustrine alkenones, and allow us to use these lipids for palaeoenvironmental reconstructions. A short core from one of the survey sites (Humboldt Lake) is being analysed to calibrate the alkenone proxy with historical climate records from the 20^th century, a process which that will allow a longer temperature reconstructions in the Northern Great Plains.