CGRG Bibliography of Canadian Geomorphology
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Author : Campbell, C.; and Campbell, I.D.
Date : 2002.
Title : Reply to Letter to the Editor on "Late Holocene Lake Sedimentology and Climate Change in Southern Alberta, Canada" by White.
Publication : Quaternary Research
Issue : 58(3):
Page(s) : 398-399.
Abstract
White (this issue) suggests that Campbell (1998), and thus by implication subsequent papers using the same record (Campbell et al., 1998; 2000), misinterpreted the Pine Lake (Alberta) grain size record. Campbell interpreted the grain size record as a function of water through-flow rates, whereby wet-ter climate produced higher flow rates, resulting in a higher rate of input of coarse material and removal of fines from the water column, and thus the deposition of coarser sediment; this record would therefore be mostly a record of spring flood conditions. This interpretation of the grain size record results in an interpreted paleoclimate that is consistent with the historic record for the last century (including the 1930s Dust Bowl period and other historic droughts), and with regional and global trends for the late Holocene. White proposes that, instead, the grain size could be a function of incision of the inflowing streams during warm-dry periods, resulting in erosion and redeposition (in the lake) of previous channel fill sediments. As intriguing as this hypothesis is, we must reject it, as it would suggest an interpretation of regional paleoclimate which is in profound disagreement not only with almost all the regional proxy records (e.g., Vance et al., 1992; Campbell and Campbell, 1997) but also with proxies across the northern hemisphere. To take a recent example, Esper et al. (2002) have gathered long tree-ring series from around the northern hemisphere. These records clearly showa classical Medieval Warm Period ca. 900–1100 A.D., followed by a cool period (including the Little Ice Age) with some fluctuations, yielding to a warming in the past century. Assuming warm in this region means dry (as it does in the historic record), this is substantially the same as Campbell’s original interpretation of the Pine Lake record. White cites Laird et al.’s (1996) study of the paleosalinity of Moon Lake, N.D. This now-classic record, which has been widely cited and indeed republished in conjunction with other records, shows what is most likely climate-induced decadal fluctuations in salinity in response to changes in climate, super-imposed on a longer-term shift in baseline salinity, which, it has recently been suggested, was not climate-mediated at all (Fritz et al., 2000). White appears to be relying on this longer-term shift in baseline when he contrasts the period prior to 1000 yr B.P. to the more recent period. However, there is a great deal of more recent regional literature, using a variety of proxy types, which does not agree with White’s suggested climate history (e.g., Yu and Ito, 1999; Vance and Wolfe, 1996; Smith et al., 1997; Xia et al., 1997; Campbell and Campbell, 2000). White’s hypothesis explaining the variation in grain size in Pine Lake rests on change in the sediment supply to the lake in response to stream incision during dry periods. Certainly Pine Lake is in an area where lake levels are sensitive to changes in climate (Campbell et al., 1994). However, streams in this region are known to have evolved out of phase with their local base levels, out of phase with each other, and out of phase with climate (Rains and Welch, 1988). A decline in the level of Pine Lake would not therefore automatically translate into incision of the inflowing streams (this could occur for a variety of reasons, ranging from the bed armoring suggested by Rains and Welch to the same climate drying that causes the decline in lake level causing the streams to aggrade, rather than incise, due to diminished competence). White suggests that stream incision brought silts rather than clays to the lake, because the clays had already been carried into the lake in previous moist periods and there were no clays left to bring to the lake. He also suggests that during the next moist phase, clays were again available, though he does not suggest why this should be so. He further suggests that ground water would accumulate in swales, whence it would become available to the lake during droughts; it is unclear why this would occur, or why it would be relevant if it did. Further, he suggests that the shoreline sediments (which are generally sandy at Pine Lake) would provide a source of clays during moist periods, but implies they would not do so during dry periods. This interpretation seems unnecessarily complex and would not pro-duce a record such as that found in Pine Lake. It is also at odds with the correspondence between the historic record and the grain size record of the last century. The interpretation proposed by Campbell (1998) agrees with the historic record for the last century, as well as with the general northern hemisphere record of the late Holocene. While the original interpretation of the Pine Lake record is, no doubt in many respects more simplistic than reality, as is any hypothesis, we see no reason to revise it along the lines suggested by White.
Bibliography of Canadian Geomorphology