High-Resolution Paleoclimatology - Dendroclimatology - page 7

obtained per annual increment (e.g., Mitsuguchi et al. 1996 ; Quinn and Sampson
2002 ) . Stalagmite research has rarely achieved such detail, with sampling intervals
(in most studies) of a few years at best. However, some studies have established
chronologies by counting annual layers on polished sections under a microscope,
and new analytical approaches (using an electron microprobe, secondary ionization
mass spectrometry [SIMS], or excimer laser ablation-inductively coupled plasma-
mass spectrometry [ELA-ICP-MS]) have made it feasible to identify annual layers
through seasonal changes in trace elements (such as Mg, Ca, Sr, Ba, and U), along
multiple transects of a sample (e.g., Fairchild et al. 2001 ; Desmarchelier et al. 2006 ) .
Image analysis of varved sediments (via impregnated thin sections examined under
a petrographic or scanning electron microscope) can reveal intra-annual sediment
variations that may be associated with seasonal diatom blooms or rainfall events
(Dean et al. 1999 ) . In ice cores, it is now possible to make continuous multipa-
rameter measurements, providing extremely detailed time series (McConnell et al.
2002 a , b). Thus, in most natural archives available for high-resolution paleoclima-
tology, detailed measurements can be made both to define annual layers or growth
increments and to characterize changes therein. However, it is not necessarily the
case that an annual layer fully represents conditions over the course of a year. Much
of the sediment in a varve, for example, may result from brief periods of runoff.
Similarly, annual layers in an ice core represent only those days when snowfall
occurred. Indeed, they may not even do that, if snow was subsequently lost through
sublimation or wind scour. Coral growth increments may result from more continu-
ous growth, and trees may also grow more continuously, at least during the growing
season. Speleothems accumulate from water that has percolated through the overly-
ing regolith, and so short-term variations related to individual rainfall episodes are
likely to be 'smoothed out.' Nevertheless, there is some evidence that extreme rain-
fall episodes can be detected in the carbon isotopes of speleothems in areas where
the throughflow of water is rapid (Frappier et al. 2007 ) .
1.5 Relationships Between Natural Archives and Climate
Extracting a climatic signal from individual archives requires an understanding of
the climatic controls on them. Analysis of the temporal relationships between vari-
ables may provide a statistical basis for calibration, but a theoretical basis for such
a relationship is also required, to direct some light into the statistical black box.
This may require in situ process-based studies to understand the factors control-
ling the proxy signal. Even if such studies are short-term, they can provide valuable
insights into how climate influences the system being studied, and hence improve
our understanding of the paleoclimatic record. For example, studies of meteoro-
logical conditions at the ice-coring site on Sajama, Bolivia, demonstrated strong
seasonality in snow accumulation, with much of the snowfall that accumulated late
in the accumulation season being subsequently lost through sublimation (Hardy
et al. 2003 ) . Consequently, the ice core record is made up of sections of snow that
accumulated for (at most) a few months each year, demonstrating that division of