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Tree-ring study sites: subfossil wood

Subfossil wood is an important source for developing annually resolved and absolutely dated tree-ring chronologies, as it enables their extension over large parts of the Holocene. In combination with core or disc samples from living trees, historical timber and archaeological remains, subfossil wood provides an ideal archive to reconstruct ecological and climatological conditions over past centuries to millennia.

The following examples from different regions (i.e., French gravel pits, Alpine glaciers, Siberian rivers and Zurich construction site) and periods (i.e., covering the entire Holocene) illustrate the unique palaeo-potential of subfossil wood.

Gravel pits
gravel pits Recent archaeological prospections of several gravel pits along the Seine, Aube, Marne and Rhine in the northeastern French Champagne and Lorraine regions recovered many buried pine and oak trees. So far, several hundred samples of subfossil stems have been radiocarbon-dated and dendrochronologically analyzed.

Mounting evidence suggests additional subfossil material could be detected if future research is coordinated more systematically between different groups and regions. Subfossil wood from the first millennium AD provides a good test case to evaluate the dating precision of tree-ring chronologies prior to the documentary evidence for this period, if  high-resolution measurements reveal a rapid 14C increase in atmospheric radiocarbon concentration from AD 774-775.

glacier moraine Alpine glaciers are strong indicators for mid- to long-term climate variability, and increasing temperatures since around the early-19th century have caused the current glacier retreat across the European Alps. During the last ~150 years, about 50% of the glacier area in Alps has disappeared. The Great Aletsch Glacier, the largest and best documented glacier in the World, has a long history of advances and retreats recorded by radiocarbon and tree-ring dating, moraine investigations and annual measurements since 1892, which subsequently allowed a unique mass-balance model to be developed.

Even more fascinating is, however, the (ideally in situ) sampling of stumps that attest to formerly overridden forests. This picture represents the collection of disc samples from subfossil wood recently released by the Morteratsch glacier in the southeastern Swiss Alps, Canton Grison (kindly provided by M. Schmidhalter, Dendrolab-Valais, Brig, Switzerland). Location of the 1850s side moraine and the glacier tongue indicates the massive ice loss during the last ~160 years.

 Siberian shorelines
Lena Delta shorelines

The Lena Delta in northeastern Siberia – the World’s largest delta – constitutes a unique sink for boreal forest trees well beyond their northern distribution limit of growth. A complex mixture of recent, dry-dead and subfossil wood, ranging from small branches to large trees, often side-by-side with mammoth (Mammuthus primigenius) macrofossils, has been ultimately deposited after its waterway journey.

The endless shorelines and abundant erosion slopes provide fascinating insight into a remarkably well-preserved archive of subfossil wood. The picture shows examples of different driftwood specimens (fresh floating, dry-dead deposited and subfossil buried, with and without root collar) deposited near or within a sandy erosion slope that is typical for the Lena Delta. The picture was taken near the Samoylov Research Station, an area representative for the most active part of the delta formed during the past ~9’000 years. (Photo W. Tegel)

The "Binz material"
Binz material

This picture represents one pine out of the discovery of 256 Late Glacial pine trees in the city of Zurich in 2013 (herein referred to as the Binz material), for which first radiocarbon dates now suggest a time window roughly between 14’000 and 11’000 BP. We expect this exceptional find to extend the worldwide longest, absolutely dated and continuous tree-ring record by almost two millennia until around 14’000 BP, thus covering a particularly interesting climatological transition from the Last Ice Age into the Early Holocene – a period for which proxy evidence is generally scarce.