Ostracode carbonate δ¹⁸O- and δ¹³C-signature of hydrological and climatic changes affecting Lake Neuchâtel, Switzerland, since the latest Pleistocene

Abstract

Stable oxygen and carbon isotope geochemistry of ostracode calcite from a core taken at a site 40 m deep in Lake Neuchâtel, Switzerland, shows distinct and rapid shifts since deglaciation. These shifts are interpreted in terms both of climatic changes and of the impact of major changes in the catchment input. The Aar River, draining a high-altitude Alpine catchment, either entered or bypassed the lake during the latest Pleistocene to the mid-Holocene. During the Preboreal and two short intervals between interpolated ages of 7200 and 6950 yr BP and since 4850 yr BP the Aar River has bypassed the lake.

During times of Aar River input, the isotopic ratios in ostracode valves are approximately 2‰ lower than during periods with only input from the Jura Mountains catchment. Isotopic signatures are interpreted in terms of a four fold chronology provided by pollen stratigraphy and AMS radiocarbon ages: (i) Oldest Dryas, (ii) Bølling/Allerød, (iii) Younger Dryas, and (iv) Holocene.

The interpretation of the isotopic records depends upon a precise sedimentological analysis of the cores. The Oldest Dryas is characterized by clastic rhythmites, whereas the Bølling/Allerød and Younger Dryas to mid-Holocene periods are characterized by thinly-bedded, non-glacial rhythmites. The absence of the Aar River input during the Preboreal, the upper Older Atlantic as well as since 4800 yr BP, is characterized by the deposition of a massive calcareous silt comprising abundant authigenic calcite.

Isotopic signatures of ostracodes from the Oldest Dryas reflect melting of Alpine glaciers and deglaciation conditions. The lowest δ¹⁸O _PDB values of about −11 ‰ are consistent with a mean temperature of annual precipitation (MTAP) of about 5–8 °C lower than that of the Holocene. From mid-Bølling upwards, the record lacks evidence of meltwater from an Alpine ice cap. MATP estimated from the highest Bølling/Allerød δ¹⁸O _PDB values are similar to values estimated for the early to middle Holocene when Aar River water also inflowed into Lake Neuchâtel. A abrupt lowering of δ¹⁸O _PDB values over the Younger Dryas interval is consistent with airmass temperatures 3–4.5 °C lower than that of the Holocene as suggested from other Swiss sites. Evidence of stronger seasonality during the Younger Dryas episode, such as very well-defined laminations, can partly explain the shift to lower δ¹⁸O values. The Holocene shifts in stable isotope ratios, however, are not interpreted in terms of MTAP shifts but rather shifts in the river-input balance.