Seasonal climate reconstruction using biogenic carbonates from shallow and deep water lake sediments

Carbonate shells from lacustrine organisms provide proxy records of past climatic changes. Their oxygen isotopic composition (δ¹⁸O) is controlled by the δ¹⁸O of the lake water and by water temperature during carbonate precipitation. The lake water δ¹⁸O depends on the δ¹⁸O of precipitation in the catchment, which is positively correlated with air temperature in high latitudes. An increasing air temperature therefore leads to an increase in carbonate δ¹⁸O. However, the proxy interpretation is complicated by the fact that an increasing water temperature during carbonate precipitation leads to a decrease in carbonate δ¹⁸O because of the temperature dependency of isotope fractionation. In the profundal parts of deep lakes, this water temperature change is minimal, and biogenic carbonates mainly reflect lake water δ¹⁸O. However, carbonate remains are often much more abundant in shallow water, where water temperature variations can be large.

In this study, we evaluate the possibility to quantitatively reconstruct seasonal temperature changes by combining sediment cores from littoral and profundal Holocene sediments of Lake Locknesjön, Sweden. We measured the isotopic composition of ostracod and mollusk shells and of encrustations from calcifying algae.

In the shallow water sediments, the differences in the mean carbonate δ¹⁸O between species can mainly be attributed to seasonal water temperature changes. The lowest δ¹⁸O values are observed in Chara encrustations formed during the summer months, and the highest values in adult ostracods, which calcify their valves during the cold season. First isotope measurements on deep water sediments show that the δ¹⁸O is higher and less variable than in the shallow sediments. The offsets between species, and between deep and shallow sediments are not constant throughout the Holocene. Assuming a water temperature near 4 °C, biogenic carbonates in the profundal sediments can give insights into the δ¹⁸O of lake water and might allow – combined with the abundant measurements from the shallow water cores – a quantification of the seasonal temperature range.