Constraining the Baltic Sea sediment chronology using tephrochronology

Precise and accurate sediment chronologies are essential for studies of past climate change. In the brackish-marine environment of the Baltic Sea, radiocarbon dating is complicated by varying influx of old carbon and reservoir age changes, challenging the construction of a robust age model. One approach to overcome the problem is well-dated isochronous tie-points like tephra. Here, we present for the first time four cryptotephra horizons in the interval 4500−2000 cal. a BP from sediments of the Western Gotland Basin containing tephra populations from the Hekla-4, Hekla-S, and Hekla-3 eruptions and the Glen Garry and Grákolla tephras from eruptions of Askja and Torfajökull, respectively. In combination with new and published radiocarbon ages, we quantify 14C age offsets for these snapshots in time. In homogeneous sediments, which contain old bulk organic carbon, ages reveal divergences of up to 1200 years. In time intervals not influenced by influx of old carbon, we define a regional marine reservoir age of 250 ± 50 yr. Based on the tephra ages as tie points and applying this reservoir age we calculate a revised age model for the Western Gotland Basin with reduced dating uncertainties that shifts previous chronologies by ∼200 years toward older ages.