Geology of New Zealand Field Trip Guidebook - ResearchGate

nourishes some of New Zealand’s largest extant glaciers. These glaciers have waxed and
waned following the beat of climate change in the southern Pacific.
Figure 5: The orographic effect of the Southern Alps. Peak precipitation can approach 14 m/year!
Note that rainfall is highest on the western side of the divide (baseline zero). This precipitation
nourishes glaciers on both sides of the divide (data from Lowell et al., 2005).
Unlocking the timing of climate change involves dating the waxing and waning of
glaciers by examining the ages of the moraines they left behind as they retreated. Dating
moraines is challenging because there is often not much in the way of organic material
preserved within them, making carbon-14 dating of little use. In addition, the youthfulness of
late Holocene moraines means that they have not yet had time to accumulate significant
quantities of cosmogenic radionuclides that can be used to date these deposits.
One technique, known as lichenometry, is both accurate and simple to apply. It is
particularly useful for dating moraines deposited over the last several centuries or so [see
\Lowell, 2005 #4, at the end of this guidebook]. Lichen is a microbial community composed
of a symbiotic relationship between a fungus and an alga or cyanobacteria. Lichens tolerate
very harsh growing conditions and commonly are the first to colonize rocks in glaciated or
volcanic terrains. Growth rates of the crustose lichen Rhizocarpon geographicum vary
greatly (0.02-2 mm/yr) in this area, depending on substrate and available moisture and
individual thalli (the lichen body) of R. geographicum appear to live for hundreds of years.
The premise behind lichenometry when applied to late Holocene glacial deposits is that
lichens will colonize boulders on a moraine surface shortly after those boulders are deposited.
With time, the diameter of the early colonizers will increase and they’ll be joined by new
recruits of lichens that will continue to colonize available space on boulder surfaces. Only
those early colonizers have sizes that reflect the total time since moraine formation and it is
those thalli that we will need to find on moraine surfaces.
Lichenometry can provide a means of correlating moraines found in nearby valleys,
but its real potential is in dating the age of stabilization of moraine surfaces. To achieve this
potential, a growth curve needs to be developed for moraines in a region, where conditions
are assumed to be similar, although not constant through time. Lowell and colleagues (2005)
accomplished this by measuring the size of R. geographicum thalli on surfaces whose ages
were known by other means (e.g., dendrochronology, radiocarbon dating, or historical
observation). One such growth curve for the Aoraki region is shown in Figure 6.
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