Tuesday, June 24, 2014

A post-glacial sea level hinge on the central Pacific coast of Canada

Hakai Protected Area (photo courtesy of Alder Creek)
Duncan McLaren, Daryl Fedje, Murray B. Hay, Quentin Mackie, Ian J Walker, Dan H. Shugar, Jordan B. R. Eamer, Olav B. Lian, Christina Neudorf
Available online 20 June 2014
DOI: 10.1016/j.quascirev.2014.05.023
(Link) open access
[Blog note:  As I've long been curious about the potential of British Columbia's coast as an early route into the Americas, it's wonderful to see this paper. I grew up in Vancouver and have both hiked and kayaked on the Central Coast. A friend of mine, John Clarke, was a renowned mountaineer with many first ascents of mountains in this area.  He was also a historian, public advocate for conservation of the British Columbia Coast Ranges and an advocate for First Nations people.  The Central Coast is one of extraordinary beauty and biodiversity (see for example on this blog Last Wild Wolves of British Columbia's North Coast).  For those who are interested in wilderness conservation, on the bottom of the right margin, I have a number of Twitter links put up by conservation groups.  Relevant to the Central Coast of British Columbia are Save the Great Bear and Haida Gwaii Communities Against Supertankers. I would also mention that Quentin Mackie, an author on the paper, frequently touches on conservation issues and discusses First Nations people and current events on his blog Northwest Coast Archaeology. The Koeye River, recently discussed on his blog, is an area researched in the paper. I'm waiting with expectation as the broader picture of this coastal migration route is reconstructed by these researchers!]
Post-glacial sea level dynamics during the last 15,000 calendar years are highly variable along the Pacific coast of Canada. During the Last Glacial Maximum, the Earth's crust was depressed by ice loading along the mainland inner coast and relative sea levels were as much as 200 m higher than today. In contrast, some outer coastal areas experienced a glacial forebulge (uplift) effect that caused relative sea levels to drop to as much as 150 m below present levels. Between these inner and outer coasts, we hypothesize that there would have been an area where sea level remained relatively stable, despite regional and global trends in sea level change. To address this hypothesis, we use pond basin coring, diatom analysis, archaeological site testing, sedimentary exposure sampling, and radiocarbon dating to construct sea level histories for the Hakai Passage region. Our data include 106 newly reported radiocarbon ages from key coastal sites that together support the thesis that this area has experienced a relatively stable sea level over the last 15,000 calendar years. These findings are significant in that they indicate a relatively stable coastal environment amenable to long-term human occupation and settlement of the area. Our results will help inform future archaeological investigations in the region.
Figure 2.  Location of the study area on the Northwest Coast of North America.

Central Coast of British Columbia, showing the Hakai Protected Area and adjacent areas (map courtesy of Google Maps).

Figure 13.  Stylized cross-section of study area showing the effects of isostatic and eustatic adjustments and the presence of a forebulge on relative sea level through time.

The Hakai West sea level curve (Fig. 12) reveals that relative sea level in the area has been within 10 m of present over the last 15,000 years. The Hakai East sea level curve shows more variation with sea level dropping 15.5 m over the same period. The data presented here demonstrate that a sea level ‘hinge’ existed between regions with higher and lower (than today) relative sea levels on the central Pacific coast of Canada (Fig. 13). The sea level hinge was found to be most stable in the Hakai West region. However, moraines and other glacial features on the landscape reveal that it is likely that much of the Hakai West region was under ice some time before 15,000 BP. During this time, with the increased volume of ice on land it is possible that the sea level hinge was located further offshore.

The Hakai area is a part of a larger region that extends southeast to northwest along the eastern shores of Queen Charlotte Sound, Hecate Strait, Dixon Entrance, and Clarence Channel along which we argue that a similar hinge-like area may be located (Fedje et al., 2004, McLaren, 2008 and McLaren et al., 2011; Shugar et al., in press, this volume). Migration of this hinge through time was dependent on local isostatic and global eustatic factors. The stability of any particular area within this region was dependent on localized factors pertaining to the amount of ice and tectonic activity. It is uncertain whether hinge areas as stable as Hakai West occur elsewhere along the coast.

The degree of stability of the shoreline in the Hakai region, and in the Hakai West area in particular, is remarkable. Elsewhere, the interplay between eustatic, isostatic, and tectonic factors tend to result in substantial changes to shoreline elevation through time. This stability means that, in the Hakai region, isostatic rebound was occurring at equal pace with global eustatic sea level rise at the end of the last glaciation. Between 14,000 and 10,000 Cal BP eustatic sea level rise was approximately 1.2 cm per year (Fairbanks, 1989). As relative sea level remained essentially constant, isostatic rebound rates for the Hakai West region must have been comparable. This pattern also suggests that the area has remained relatively tectonically stable over the Holocene accounting for very little change in relative sea level (see Shugar et al., in press, this volume for a discussion of tectonics and sea level change).

Places with stable shorelines allow relatively uninterrupted accumulation of archaeological deposits over long periods of time. In theory, these larger accumulations should be easier to find and they would be expected to retain long records of cultural and ecological information. Places where early archaeological deposits occur may be similar to places that are suitable for coastal habitation today, such as pocket beaches, harbours, and tombolos. This can be contrasted with areas such as Goose Bank, Haida Gwaii, and non-glaciated regions around the globe where late-Pleistocene shorelines are drowned by up to 150 m rendering them very difficult to access, or inland areas such as Kitimat or the Fraser Valley where relative sea level was 200 m higher than today and where significant glaciations occurred up until the end of the Pleistocene.

Relative stability in sea level allows for the establishment of persistent places across the landscape. Of the archaeological sites tested, four show persistent occupation for 10,000 or more years: Namu (ElSx1), Kildidt Narrows (ElTa18), Triquet Island (EkTb9), and Pruth Bay (EjTa15). It is highly likely that there are several other sites in the area with equally long records. This pattern of site re-use and persistence differs from settlement patterns on Haida Gwaii (200 km west of the study area) where early and late period sites tend not to co-occur (Mackie and Sumpter, 2005) and where Holocene sea level rose to 15 m ahht and then fell back to modern levels.

The identification of a sea level hinge is of particular interest for investigations into early period archaeology of the Northwest Coast and the peopling of the Americas (Fedje et al., 2004 and Mackie et al., 2013). Fladmark (1979) presented a compelling argument in which the Northwest Coast is depicted as the most likely route by which early human inhabitants of the Americas circumnavigated the continental ice sheets that covered much of Canada during the Last Glacial Maximum. In their comprehensive review of the timing of the Last Glacial Maximum, Clague et al. (2004) argue that post glacial human occupation of outer coastal areas of Southeast Alaska and British Columbia could have occurred as early as 16,000 Cal BP. Early archaeological sites to the south of the ice sheets including Paisley Caves (Gilbert et al., 2008 and Jenkins et al., 2012) in Oregon, and Manis Mastodon (Gustafson et al., 1979 and Waters et al., 2011) in Washington State, reveal that the western margins of North America was occupied by at least 13,800 Cal BP. The research presented here has revealed potential shoreline targets for archaeological prospection up to 15,000 years old, providing potential for future investigations into the early human occupation of the Americas.

This paper describes a relative sea level history spanning the past 15,000 years for the Hakai Passage region on the central Pacific coast of Canada. Data was gathered using geological and archaeological methods. Overall, the research presented here demonstrates that relative sea level remained remarkably constant through this 15,000 year period despite the large scale changes resulting from global eustatic and regional isostatic processes during the same time period. The evidence reveals that isostatic rebound kept pace with eustatic sea level change and uplift over this period. Part of the reason for this stability is that the study area is located on a sea level hinge between a region with higher relative sea level to the east and lower relative sea level to the west. The sea level history of the study area demonstrates that sea level change in ice-proximal regions can be highly variable and localized (see also Shugar et al. in press, this volume). Attempts to model sea level change in any region along the Pacific coast of Canada and southern Alaska need to take local, regional, and global influences into account. The sea level history presented here will enable research to more effectively target sites that have the potential to lengthen the record of human occupation in the region to early post-glacial times.

Mount Saugstad, east of Calvert Island and the Hakai wilderness (from Coastmountains blog)

No comments:

Post a Comment

Comments have temporarily been turned off. Because I currently have a heavy workload, I do not feel that I can do an acceptable job as moderator. Thanks for your understanding.

Note: Only a member of this blog may post a comment.