Rolf W. Mathewes

Radiocarbon Dating of Pollen by Accelerator Mass Spectrometry

Radiocarbon dating of bulk sediments has been the standard method for establishing chronologies in the studies of lake sediment cores which have contributed significantly to our knowledge of late Quaternary paleo-environments. These bulk sediment dates are presumed to be direct ageindicators for the speciments (e.g., pollen or macrofossils) which are actually being studied. However, several recent studies have reinforced long-standing apprehensions concerning this presumption. In this study, we demonstrate for the first time the radiocarbon dating of pollen concentrate samples by accelerator mass spectrometry. The dates obtained by this method should provide more reliable radiocarbon chronologies for paleo-environmental studies than have been obtainable by bulk sediment dating.

Time of maximum Late Wisconsin glaciation, West Coast of Canada

New data from a deep-sea core in the eastern North Pacific Ocean indicate that the western margin of the Late Wisconsin Cordilleran Ice Sheet began to retreat from its maximum position after 15,600 yr B.P. Ice-rafted detritus is present in the core below the 15,600 yr B.P. level and was deposited while lobes of the Cordilleran Ice Sheet advanced across the continental shelf in Queen Charlotte Sound, Hecate Strait, and Dixon Entrance. The core data are complemented by stratigraphic evidence and radiocarbon ages from Quaternary exposures bordering Hecate Strait and Dixon Entrance. These indicate that piedmont lobes reached the east and north shores of Graham Island (part of the Queen Charlotte Islands) between about 23,000 and 21,000 yr B.P. Sometime thereafter, but before 15,000–16,000 yr B.P., these glaciers achieved their greatest Late Wisconsin extent. Radiocarbon ages of late-glacial and postglacial sediments from Queen Charlotte Sound, Hecate Strait, and adjacent land areas show that deglaciation began in these areas before 15,000 yr B.P. and that the shelf was completely free of ice by 13,000 yr B.P.

Holocene History of Cedar and Native Indian Cultures of the North American Pacific Coast

A comparison of paleobotanical records with archeological and ethnographic evidence from the Pacific Northwest shows a strong correlation between the expansion of Western red cedar (Thuja plicata) in coastal forests between 5000 and 2500 years ago and the evolution of a massive woodworking technology by native cultures. This suggests that an important component of cultural development was environmentally constrained until large cedar trees, the basic resource for canoe-building and plank-house construction, had become available in late Holocene time.

Evidence for a Younger Dryas-like cooling event on the British Columbia coast

Two independent paleoclimatic records from the Pacific coast of Canada indicate that a late-glacial warming trend was interrupted by a return to colder conditions between about 11,000 and 10,200 radiocarbon yr B.P., correlative with the classical Younger Dryas chronozone of the North Atlantic region. Fossil benthic foramnifera from three cores from the continental shelf dated by accelerator mass spectrometry show peak abundances of the cold-water indicator species Cassidulina reniforme at this time. Fossil-pollen spectra from two sites on the Queen Charlotte Islands record a shift from forest to open, herb-rich vegetation after 11,100 yr B.P., probably in response to colder and wetter conditions identified by pollen-climate transfer functions. These preliminary data for a cold oscillation between ca. 11000 and 10000 yr ago in the northeast Pacific argue that this deglacial phenomenon was not restricted to the North Atlantic, but was a hemispheric—and possibly global—event.

Chironomidae (Diptera) remains in surficial lake sediments from the Canadian Cordillera: analysis of the fauna across an altitudinal gradient

The altitudinal distribution of Chironomidae (Diptera) in the southern Canadian Cordillera was analyzed by means of head capsules preserved in surficial sediments of 30 lakes. Taxa characteristic of late-glacial deposits of southern, coastal British Columbia are extant at high elevations, particularly in the Rocky Mountains, and in large, deep, low-elevation lakes. Many chironomid taxa common at low elevations in the southern Canadian Cordillera were not found in alpine and upper subalpine lakes. These faunal differences are probably climatically related. The differences in fauna between high and low-elevation lakes parallel differences between arctic and temperate lakes.

Evidence for Younger Dryas-age cooling on the North Pacific coast of America

Abstract A review of the palynological evidence from the Olympic Peninsula of Washington State north to the Alaska Panhandle strongly supports the existence of a climatic oscillation similar in timing and effect to the Younger Dryas cooling (11-10 ka BP) of Europe and eastern North America. The evidence includes many late-glacial pollen peaks of mountain hemlock (Tsuga mertensiana), an indicator of cool and moist climate, reversals from forest to non-arboreal vegetation, and paleoclimate analysis using pollen-climate transfer functions on the Queen Charlotte Islands. Evidence of cooler ocean waters, based on fossil foraminifera in cores from the continental shelf, also supports an interpretation of a Younger Dryas-age climatic reversal. On the other hand, geological evidence of glacier readvances during the Younger Dryas chronozone is weak and poorly dated. Although more and better-constrained (AMS) dates are needed to confirm the timing of the Pacific Northwest cold oscillation, results so far point to maximum cooling and increased moisture between ca. 10.7-10 ka BP, followed by rapid warming in the early Holocene. Additional late-glacial sites need to be investigated in detail to confirm the geographical pattern of vegetation and climate change during this interval, which is best expressed in hypermaritime and maritime climate regions, similar to the Younger Dryas event around the North Atlantic. These results suggest that the search for causal mechanisms to drive the Younger Dryas cooling cannot be limited to events in the North Atlantic region, but should focus on possible hemispheric or global processes.

7000 year record of lake-level change on the northern Great Plains: A high-resolution proxy of past climate

Mineralogical and paleobotanical characteristics of the sedimentary fill in a shallow, saline lake in southeastern Alberta indicate that drought intervals are aperiodic and that climatic extremes unprecedented in historic time occurred earlier during the Holocene, when century-long intervals of repeated, intense droughts alternated with long periods when droughts were rare. Low-water intervals (signifying periods of drought) are recorded by carbonate-rich laminae containing abundant plant fossils indicative of hypersalinity. In contrast, highstands of relatively fresh water (outlining moist periods) are represented by massive, silicate-rich sediment with lower numbers of halophytic indicators.

High-resolution Holocene vegetation history and climate from Hole 1034B, ODP leg 169S, Saanich Inlet, Canada

Abstract High-resolution pollen analysis of laminated marine sediments from ODP Hole 1034B in Saanich Inlet, British Columbia reveals changes in vegetation and inferred climate during the Holocene. Four main pollen zones are discerned using constrained cluster analysis. Although the timing of major vegetation changes at the Saanich Inlet is similar to other study sites in the Pacific Northwest, the composition of pollen assemblage zones is different from the mainland sites. Vegetation assemblages reconstructed from the pollen and spore record include a Douglas-fir (Pseudotsuga menziesii) parkland with abundant grass (Poaceae) and bracken (Pteridium) between 11,450 and 8300 BP (all ages are calibrated calendar years), oak (Quercus) savanna or parkland with high grass and bracken (8300–7040 BP), a mixed deciduous/coniferous forest with oak, western hemlock (Tsuga heterophylla) and Douglas-fir (7040–5750 BP), and the development of modern coastal temperate forest with the marked expansion of cedar (Cupressaceae), western hemlock, spruce (Picea) and Douglas-fir (5750–1050 BP). Climatic periods inferred from the cores include an early Holocene warm/dry interval (11,450–8300 BP), a warm period with mild winters (8300–7040 BP), a period of transitional mid-Holocene climate (7040–5750 BP), and the advent of a relatively cool/wet neoglacial climate after 5750 BP. Modern conifer forests and oak savannas became established by about 3800 BP. The Saanich Inlet pollen record indicates that vegetation and inferred climate change was particularly rapid between 8700 and 8300 BP when grass and bracken abruptly decrease and oak becomes a significant component of the paleovegetation. Because neoglacial conditions have prevailed from 3800 years to present in the Pacific Northwest, factors other than climate, such as anthropogenic modification of the landscape, may be responsible for the persistence of oak savannas.

Early Holocene thermal maximum in western North America: New evidence from Castle Peak, British Columbia

Conifer logs and branches of early Holocene age are common on the surface and in sediments above timberline at Castle Peak in the southeastern Coast Mountains of British Columbia. A study of this wood and associated peat and colluvium has shown that local timberline from 9.1 to 8.2 ka was at least 60 m, and perhaps more than 130 m, higher than today. Mean growing-season temperature at Castle Peak during this period thus may have been 0.4-0.8°C warmer than at present. This is consistent with theoretical considerations based on Milankovitch forcing of climatic change and is supported by other paleoecological data from the southern Canadian Cordillera and adjacent northwestern United States. A generally warm climate may have persisted until about 5-6 ka, followed by late Holocene cooling.

Chironomidae (Diptera) and postglacial climate at Marion Lake, British Columbia, Canada

Abstract Chironomid (midge) remains analyzed from an 8.95-m-long sediment core from Marion Lake reveal successional changes over the last 12,000 yr since deglaciation. A late-glacial Heterotrissocladius -dominated association characterizes the earliest sediments. Succeeding this community, at the onset of the Holocene, is a Tanytarsini-dominated assemblage. The transition between these communities occurs during a time of rapid climatic amelioration, preceding an early Holocene xerothermic interval. The late-glacial fossil fauna is suggestive of more northerly affinities. Similar sequences have been reported in late-glacial lake sediments elsewhere in North America and in Europe. The composite picture of these chironomid assemblages suggests the gradual retreat of a formerly widely distributed, cold stenothermous fauna. The response of the chironomid community accords well with paleoclimatic inferences based on pollen data at the lake. Subsequent postglacial changes are less pronounced and most are probably attributable to the gradual shallowing of the basin. Chironomid remains from shallow, weakly stratified lakes may yield valuable paleoclimatological data.