ABSTRACT: Comments on "Implications of a late-glacial pollen record for the glacial and climatic history of the Fraser Lowland, British Columbia" by Pellatt et al., 2002.

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Pellatt et al., 2002 inferred from pollen analysis of a bog core in the Fraser Lowland of southwestern British Columbia that the Sumas Stade pre-dates the Younger Dryas interval and that the Younger Dryas in the Fraser Lowland was a time of `progressively warming climate'. Unfortunately, the authors have based these conclusions on outdated correlations that have been replaced by more recent mapping of moraines, stratigraphic and morphologic evidence, combined with large numbers of radiocarbon dates related to Sumas moraines and outwash channels ( [Kovanen and Easterbrook, 2001, Kovanen and Easterbrook, 2002, Kovanen, 2001 and Kovanen, 2002]). Thus, their conclusions conflict with a large amount of geomorphic and stratigraphic evidence demonstrating that the Sumas interval consisted of multiple fluctuations of the Cordilleran Ice Sheet (CIS) during the Allerød and Younger Dryas ( [Easterbrook, 1994, Easterbrook and Kovanen, 1997, Easterbrook and Kovanen, 1998a, Easterbrook and Kovanen, 1998b, Kovanen, 2001, Kovanen, 2002, Kovanen and Easterbrook, 1997, Kovanen and Easterbrook, 2002, Mark and Ojamaa, 1980 and Cameron, 1989]). To better inform readers of the now well-dated glacial history of this area, 14C dates and geomorphic and stratigraphic evidence that were omitted by [Pellatt et al., 2002] are presented here. The incomplete view of the Sumas readvances by [Pellatt et al., 2002] casts doubt on their final conclusions and indicates that their view of Sumas events as strictly pre-Younger Dryas is untenable. What's wrong with the pollen correlations to the Sumas suggested by Pellatt et al.? The main problem with their correlations is that they are based entirely on the outdated view that the entire Sumas can be seen in a single gravel pit (the Bradner pit) in SW British Columbia ([Clague et al., 1997 and Clague et al., 1998]), ignoring two extensive sets of 14C-bracketed moraines younger than the deposits in the Bradner pit (Fig. 1 and Fig. 2) ([Kovanen and Easterbrook, 2001, Kovanen and Easterbrook, 2002, Kovanen, 2001 and Kovanen, 2002]). The Sumas has been conclusively demonstrated to consist of at least four, well-dated advances/stillstands based on morainal morphology and stratigraphy, two of which fall within the Allerød and two within the Younger Dryas ([Kovanen and Easterbrook, 2002, Kovanen, 2001 and Kovanen, 2002]). 14C dates from numerous, peat-filled, outwash channels associated with extensive moraines and deeply incised into underlying glaciomarine drift confirm multiple Sumas advances and allow establishment of a firm Sumas chronology. The southeastern margin of the Pellatt et al. inferred Sumas maximum (their fig. 1) extends into Whatcom County of Washington State, but none of the authors has done any field work in this area and not a single piece of physical evidence is given by the authors for their interpretation. In view of this, their lack of references is puzzling – they only refer to a single paper by [Armstrong et al., 1965] and do not otherwise mention the 35 years of work by Armstrong on the Sumas in British Columbia (e.g., [Armstrong and Brown, 1954, Armstrong, 1960, Armstrong, 1980, Armstrong, 1981, Armstrong and Hicock, 1980a and Armstrong and Hicock, 1980b]), 43 years of work on the Sumas in Washington by Easterbrook (e.g., [Easterbrook, 1963, Easterbrook, 1976, Easterbrook, 1994, Easterbrook and Kovanen, 1997, Easterbrook and Kovanen, 1998a and Easterbrook and Kovanen, 1998b]) nor more than 20 years of work on the Sumas by Hicock ( [Armstrong and Hicock, 1980a and Armstrong and Hicock, 1980b]). They also fail to mention that [Mark and Ojamaa, 1980] also showed geomorphic and stratigraphic evidence for multiple Sumas ice marginal positions. Pellatt et al. ignore the evidence for any of these observable features of multiple Sumas readvances and recognize only the stratigraphy in a single gravel pit that contains deposits of only a single Sumas glacial phase and an earlier glacial phase. Thus, their correlation of pollen records with the glacial record is seriously flawed. (2) Sumas I and II: Fig. 2 shows the position of Sumas I and II ice margins, based on moraine and meltwater channels cut into glaciomarine drift ( [Easterbrook, 1976, Easterbrook and Kovanen, 1998b, Kovanen, 2001, Kovanen, 2002, Kovanen and Easterbrook, 2002 and Mark and Ojamaa, 1980]). Radiocarbon dates from wood and shells in gravel pits and the base of cores in outwash channels yield limiting ages for the Sumas II ice margin ( Fig. 1 and Fig. 2). When ice melted back from the large Sumas II moraine in SW British Columbia, meltwater could no longer reach former outwash channels in front of the moraine. Thus, a basal peat date of 11413±75 14C yr BP from a bog in the abandoned channel dates the end of deposition of the Sumas II moraine. 14C dates place the Sumas II in the late Allerød, perhaps equivalent to the intra-Allerød Cold Period. These dated Sumas phases are not included in the `Sumas' events of Pellatt et al., making their correlations incomplete. (3) Sumas III: When the ice melted back from the large Sumas II moraine (the youngest Sumas phase recognized by Pellatt et al.), meltwater streams built the broad Abbotsford outwash plain (up to 12 km wide) inside the Sumas II moraine. The outwash plain terminates at SIII moraines 20 km from the SII terminal position (Fig. 2 and Fig. 3), demonstrating that ice must have retreated from the area behind the Sumas II moraine before deposition of the Abbotsford outwash plain and the SIII moraines. The significance of the geomorphic position of the outwash plain is that the CIS must have retreated 20 km before building the SIII morainal complex, which must be younger than the SII moraine ([Kovanen and Easterbrook, 2002 and Kovanen, 2002]). The geomorphic relations between SII and SIII are apparent on shaded digital topographic models ( Fig. 3A) but were not considered by Pellatt et al. in interpretation of their youngest Sumas ice marginal position. The retreat from the Sumas II moraine and development of the Abbotsford outwash plain coincides with the warming trend suggested by Pellatt et al. between 11000 and ~10400 14C yr BP, although allowing time for the lag of vegetation after a climate change, it probably began slightly earlier. (4) Sumas IV, the youngest Sumas phase, is based on a prominent morainal remnant and outwash terrace near Sumas (Fig. 2 and Fig. 3) with limiting dates of 10245±90 and 10265±65 14C yr BP ([Easterbrook and Kovanen, 1998b]) from cores at the base of a bog in a kettle at Pangborn Lake ( [Easterbrook, 1976, Easterbrook, 1994, Kovanen and Easterbrook, 2002, Kovanen, 2001 and Kovanen, 2002]). Till rests on outwash and part of a moraine is draped across a Sumas III meltwater channel ( Fig. 3B), indicating a readvance of Sumas ice that may correlate with the cool interval between 10400 and 10000 yr BP suggested by Pellatt et al., following deposition of Sumas III moraines and outwash ( [Kovanen and Easterbrook, 2002]). The evidence for the Sumas III and IV ice margins and radiocarbon dates have been ignored by Pellatt et al., who limit their correlations to the single gravel pit, giving the reader a false impression of the number and timing of Sumas glacial events.(5) A climatic origin (direct or indirect) for the multiple Sumas glacial events has been demonstrated ([Kovanen and Easterbrook, 2002]). Pellatt et al. concluded that the inferred vegetation assemblage during most of the Younger Dryas indicates a progressive warming climate with a cool interval between 10400 and 10000 yr BP. This is in contrast to the estimated sea surface temperature (SST) record west of Vancouver Island ( Fig. 1) by [Kienast and McKay, 2001] who showed that the SST dropped from ~9°C during the Allerød to ~6°C during the Younger Dryas. They also recorded a SST drop of ~1.5°C during the Allerød which they suggested may correspond to the intra-Allerød Cold Period. [Mathewes, 1993] and [Friele and Clague, 2002] also showed paleoecologic and morainal evidence for cooling on the British Columbia coast between 10700 and 10000 14C yr BP in direct contrast to the conclusions of Pellatt et al. Other factors not considered by Pellatt et al. are (1) the possibility that vegetation was not sensitive enough to archive the complete climate history (or represents a complacent environment) and (2) the time lag between climatic change and the response of vegetation to it. In addition, their pollen spectra indicated an increase in moisture and only slight warming for pollen zones MiL-3 (11310–10350 yr BP) and MiL-4 (10350–10000 yr BP). This increase in moisture supply may have helped fuel the Sumas glacial advances. Hopefully, the information presented here will improve correlation of climatic changes inferred from pollen and SSTs with observable, 14C-dated moraines in the Fraser Lowland that show Allerød and Younger Dryas oscillations of remnants of the CIS ice during the Sumas interval.

Bibliography of Canadian Geomorphology