High-Resolution Paleoclimatology - Dendroclimatology - page 6

establishing a chronology has been far more challenging. Recent analytical
improvements using continuous flow isotope mass spectrometry have made feasi-
ble the almost continuous sampling of wood, so that annual changes in isotopic
properties can be identified, even in wood that appears to be undifferentiated in
its growth structure (Evans and Schrag 2004 ; Poussart et al. 2004 ) . This technique
opens up the possibility of using trees for paleoclimatic reconstruction in regions
that were hitherto unavailable. However, replication of samples from nearby trees is
still necessary to reduce chronological uncertainties in these newer records.
In the case of most other high-resolution proxies, replication is rarely carried
out. This is generally related to the cost of sample recovery (in terms of logistics
or time) or because of the analytical expense of duplicating measurements. Most
coral records, for example, are based on single transects through one core, though
the veracity of the chronology may be reinforced through the measurement of mul-
tiple parameters, each of which helps confirm the identification of annual layering
in the coral. Similarly, in ice cores, multiparameter glaciochemical analyses can
be especially useful in determining a secure chronology (McConnell et al. 2002 a ;
Souney et al. 2002 ) . In addition, in some locations more than one core may be
recovered to provide additional ice for analysis and to help resolve uncertainties in
chronology (Thompson 1993 ) . It may also be possible to identify sulfate peaks in the
ice, related to explosive volcanic eruptions of known age. Such chronostratigraphic
horizons can be very helpful in confirming an annually counted chronology (Stenni
et al. 2002 ) . Varved sediments are sometimes analyzed in multiple cores, but sample
preparation (such as impregnation of the sediments with epoxy, thin section prepa-
ration, etc.) is expensive and very time-consuming, so duplication is not commonly
done. Where radioactive isotopes from atmospheric nuclear tests conducted in the
late 1950s and 1960s can be identified in sediments (and in ice cores), such horizons
can be useful time markers. Tephra layers (even finely dispersed cryptotephra) can
be useful in confirming a sedimentary chronology if the tephras can be geochem-
ically fingerprinted to a volcanic eruption of known age (e.g., Pilcher et al. 2005 ) .
Finally, where annual layer counting is not feasible—as in many speleothems—
radioactive isotopes ( 210 Pb, 14 C, and uranium-series) can be used to obtain mean
deposition/accumulation rates, though there may have been variations in those rates
between dated levels.
1.4 High-Resolution Sampling
Advances in analytical techniques have now made sub-annual sampling and mea-
surements fairly routine in most high-resolution proxies. Whereas tree rings were
generally measured in terms of total annual increments, densitometry now enables
measurements of wood density and incremental growth in early and latewood sec-
tions of each annual ring. Image analysis provides further options in terms of
analyzing cell growth parameters (Panyushkina et al. 2003 ) . Isotopic dendroclimatic
studies require subannual sampling resolution to determine growth increments. In
corals, such detailed sampling is now routine; often 10 or more samples will be