This article is taken from:
Davies, B.J., 2022. CRYOSPHERIC GEOMORPHOLOGY: Dating Glacial Landforms I: archival, incremental, relative dating techniques and age-equivalent stratigraphic markers. (link)
Trees colonising recently deglaciated land surfaces, especially on markers of ice advance such as moraines, provide a means of dating surfaces that are too young to date reliably by other means (Coulthard and Smith, 2013; Koch, 2009; Smith and Laroque, 1996). This has been termed ‘dendroglaciology’ (Masiokas et al., 2009).
By using long-established tree-ring dating principles (Shroder Jr, 1980), it is possible to assign ages to landforms and landscape processes. Commonly, this involves dating trees killed directly or indirectly by a glacial advance, and possibly being reworked into moraines, establishing the age of trees growing directly on glacial landforms, or dating growth irregularities in ice-marginal trees affected by glaciers (Figure below) (Coulthard and Smith, 2013), such as tilting or scars.
This method works well in temperate regions where glaciers extend down into well-forested areas during the Late Holocene, such as New Zealand, North America and Patagonia, and operates over a scale of centuries (Barclay et al., 2009; Wiles et al., 1999).
Glacially killed trees
Glaciers may kill trees by expanding into forests, shearing tree trunks, burying trees beneath glacial sediment, or partially burying them in outwash sediments (Figure 5) (Barclay et al., 2009; Jackson et al., 2008; Koehler and Smith, 2011). Tree stumps may be protected in downstream low-pressure zones and downstream of bedrock protrusions (Coulthard and Smith, 2013). If the tree stump remains in a growth position, the kill date indicates the glacier position at a specific time. This requires a tree-ring master chronology to determine the tree age (ibid.).
Alternatively, mats of organic woody material may be reworked into moraines (Coulthard et al., 2013), where they can be radiocarbon dated (Koehler and Smith, 2011), or if they are sufficiently well preserved, provide floating chronologies that can be related to existing ‘master’ tree-ring chronologies (Wiles et al., 2011).
Case study: Icy Bay, Alaska
The dating potential and precision is dependent on preservation, especially of the outer rings and bark. For example, at Icy Bay in southern Alaska, USA, subfossil logs were found in growth position; other logs were reworked into till. These logs were radiocarbon dated to find the timing of glacier advance (Barclay et al., 2006). Tree-ring samples were collected from each log, and samples were analysed to determine the preservation of the last years of growth.
Cross-dating was attempted for spruce and hemlock logs with >65 rings. Samples were initially cross-dated with other subfossil logs, and then compared with a master chronology from Sitka spruce growing on nearby glacial outwash. Together, these data extended the glacial history of Icy Bay back to 3800 years ago, and revealed a number of advance-retreat cycles (Barclay et al., 2006).
Glacially damaged trees
Glacier advances may scar or cause growth irregularities in ice-marginal trees. Glacial landforms or ice-marginal trees may be shunted, tilted or otherwise moved by glacier fluctuations. Trees that are tilted may continue to grow, and the irregularity in their annual rings can be dated. Trees may also be scarred by surface abrasion from glacier advances, and the date of this scar can be calculated from the development of callous tissue on tree stems (Coulthard and Smith, 2013).
Assessing the age of trees growing on glacial landforms, such as terminal or lateral moraines, can provide a minimum exposure age for the moraine (Wiles et al., 1999). This method has a dating precision of around 10 years, and the age of the oldest tree provides a minimum estimate for the surface age (Coulthard and Smith, 2013). Limitations of the technique include that the ecesis time (time between surface exposure and tree germination) is challenging to estimate (Koch, 2009), and it makes the assumption that the oldest tree has been sampled. It is therefore recommended to sample a number of trees at a site, particularly if the site is well forested and the oldest tree difficult to locate (Coulthard and Smith, 2013).
This surface-dating application of dendroglaciology works well on glacial landforms that date from the last few centuries. For example, it has been widely applied to date “Little Ice Age” moraines around the North Patagonian Icefield and South Patagonian Icefield (e.g. Boninsegna et al., 2009; Koch and Kilian, 2005; Masiokas et al., 2009; Warren et al., 2001; Winchester et al., 2014, 2001; Winchester and Harrison, 2000). Here, most tree-ring chronologies use the South American beech (Nothofagus sp.), the conifer Pilgerodendron uviferum or Fitzroya cupressoides.
Quality assurance protocols for dendroglaciology
High-quality moraine ages from dendrochronology / dendroglaciology would have:
- an ecesis time and growth rate that has been calculated and provided
- a clear sample context
- the age of the tree should be clearly calculated
Selected references and further reading
Barclay, D.J., Wiles, G.C., Calkin, P.E., 2009. Tree-ring crossdates for a First Millennium AD advance of Tebenkof Glacier, southern Alaska. Quaternary Research 71, 22-26.
Boninsegna, J.A., Argollo, J., Aravena, J.C., Barichivich, J., Christie, D., Ferrero, M.E., Lara, A., Le Quesne, C., Luckman, B.H., Masiokas, M., Morales, M., Oliveira, J.M., Roig, F., Srur, A., Villalba, R., 2009. Dendroclimatological reconstructions in South America: A review. Palaeogeography Palaeoclimatology Palaeoecology 281, 210-228.
Coulthard, B.L., Smith, D.J., 2013. DENDROCHRONOLOGY A2 – Elias, Scott A, in: Mock, C.J. (Ed.), Encyclopedia of Quaternary Science (Second Edition). Elsevier, Amsterdam, pp. 453-458.
Koch, J., Kilian, R., 2005. ‘Little Ice Age’ glacier fluctuations, Gran Campo Nevado, southernmost Chile. Holocene 15, 20-28.
Koch, J., 2009. Improving age estimates for late Holocene glacial landforms using dendrochronology–Some examples from Garibaldi Provincial Park, British Columbia. Quaternary Geochronology 4, 130-139.
Masiokas, M.H., Luckman, B.H., Villalba, R., Delgado, S., Skvarca, P., Ripalta, A., 2009. Little Ice Age fluctuations of small glaciers in the Monte Fitz Roy and Lago del Desierto areas, south Patagonian Andes, Argentina. Palaeogeography, Palaeoclimatology, Palaeoecology 281, 351-362.
Shroder Jr, J.F., 1980. Dendrogeomorphology: review and new techniques of tree-ring dating. Progress in Physical geography 4, 161-188.
Smith, D., Laroque, C., 1996. Dendroglaciological dating of a Little Ice Age glacial advance at Moving Glacier, Vancouver Island, British Columbia. Géographie physique et Quaternaire 50, 47-55.
Warren, C., Benn, D., Winchester, V., Harrison, S., 2001. Buoyancy-driven lacustrine calving, Glaciar Nef, Chilean Patagonia. Journal of Glaciology 47, 135-146.
Wiles, G.C., Barclay, D.J., Calkin, P.E., 1999. Tree-ring-dated ‘Little Ice Age’ histories of maritime glaciers from western Prince William Sound, Alaska. The Holocene 9, 163-173.