|Some important points to consider
when viewing the data
|To many considering this data, the abrupt recent changes in both
taxonomic composition of diatom assemblages and diatom absolute abundances may be viewed
as preservational artifacts. In reality, the slides show absolutely no evidence for silica
dissolution down-core. Delicate forms, for example from the genera Nitzschia and Achnanthes,
are indeed present and well-preserved in many lower levels, although they are never
abundant. Pitting of valve surfaces or the preservation of only the heavily silicified
portions of the valve (cf. the axial area) are consistent with silica dissolution; these
features did not characterize any of the Ellesmere Island material we report upon.
|A second concern, especially relevant to the Col Pond and Elison Lake
records since these are from shallow ponds that freeze solidly to the mud-water interface
during winter, is that the diatom stratigraphies are disturbed. The profiles of 210Pb
from these sites, showing the entire unsupported inventory to be preserved within the
upper 3.0 cm, argue strongly against disturbance of sediment structure. Further, the
abrupt floristic shifts themselves would not have been preserved if the cores had been
|Thus, although we have seriously considered alternative possibilities,
we are left to conclude that diatom algal communities have markedly shifted during the
period of post-Little Ice Age warming. These recent changes are far greater than those
recorded by the diatoms during other periods of known Holocene climatic variability (cf.
early Holocene warming, onset of Neoglaciation), most likely suggesting that both the rate
and amplitude of recent warming is exceptional with the context of the Holocene.
Additional studies showing similar recent trends, from elsewhere in the Canadian Arctic
Archipelago (Wolfe, 1991; Gajewski et al., 1997) confirm that recent ecological
reorganizations are indeed a geographically widespread occurrence. Several papers have
been published relevant to these and other sites studied by the authors; these, as well as
the other works cited above, are listed here.