Ecosystemic Postglacial Succession of Nettilling Lake (Baffin Island, Canada) Inferred by Oxygen Isotope Composition of Lacustrine Diatoms

Abstract:

In 2012, a 82 cm long sediment core (Ni2B) was drilled at Nettilling Lake. We use a multi-proxy paleolimnological approach to study the sedimentary records preserved in Nettilling Lake (Baffin Island, Canada) in order to reconstruct the postglacial environmental history of the lake watershed. 31 samples of biogenic silica were purified, their contamination assessed and corrected for and subsequently analysed for the oxygen isotope composition (δ¹⁸O_diatom). Additionally, the diatom assemblage from 35 samples was quantified under the light microscope with x 1000 magnification. Our chronology extends to ~ BC 1200 yrs based on radiometric dating ²¹⁰Pd and ¹⁴C from bulk sediment.

Downcore variations in δ¹⁸O_diatom values show a marine-lacustrine transition. The samples from the marine-brackish zone show a higher isotopic composition (27.5‰, 58.5cm depth, ²middle Holocene²) than the samples from the lacustrine section (21.7‰, 1.5cm, 2002 AD). The transition zone can be distinguished by values between these extremes, too (23.4‰, 33cm, ~1240 BC). This likely reflects changes in the water source, from more isotopically enriched marine water in the past to more depleted and cold lacustrine water. The diatom assemblage reflects the same transition. The marine-brackish zone contains polyhalobous-mesohalobous benthic species (e.g. Trachyneis aspera, Gomphonemopsis aestuarii, G. pseudexigua,Cocconeis scutellum) which have a salinity preference between 35‰ to 5‰, indicating a shallow, littoral environment. The transition zone is characterized by a sharp rise of alkaliphilous freshwater benthic taxa (e.g. Staurosirelle pinnata, Staurosira construens, Staurosira brevistriata). The diatom flora of the upper zone is characterized by halophobous planktonic and benthic species (e.g. Cylotella rossii, Cyclotella pseudostelligera, Tabelaria floculosa, Encyonema silesiacum, Nitzschia perminuta).

The δ¹⁸O_diatom and the diatom assemblage record from Nettilling Lake register changes in the oxygen isotope composition of the lake water. This is the first δ¹⁸O_diatom record documenting a marine-lacustrine transition. Records from Nettilling Lake will provide further evidence for the usefulness of δ¹⁸O_diatom as an important parameter in multi-proxy paleoclimatic reconstruction.