Interdisciplinary insights into paleoenvironments of the Queen Charlotte Islands/Hecate Strait region




Hetherington, Renée

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Subsequent to the Last Glacial Maximum (LGM), complex coastal response resulted from deglaciation, eustatic sea-level change, and a relatively thin, flexible lithosphere in the Queen Charlotte Islands (QCI) region of northwestern Canada. Presented here is an interdisciplinary study that combines the methodologies and schools of thought from geology, biology, and geography to address a research problem that spans these disciplines, specifically to illustrate the environment, temporal and spatial dimensions of isostatic crustal adjustment and the Late Quaternary coastline of the northeast Pacific continental shelf. Molluscan distribution, lithology, and published sub-bottom profiles are used to deduce sea-levels, outline the influence of glacially-induced crustal displacement, and reconstruct the paleoenvironment of the northeast Pacific Late Quaternary coastline, including the absence of ice and the presence of emergent coastal plains. These data are used to ascertain the region's suitability as a home for an early migrating coastal people. A series of paleogeographic maps and isostatic crustal displacement maps chart the sequence of evolving landscapes and display temporal changes in the magnitudes and extent of crustal flexure as a forebulge developed. The wave-length and amplitude of the glacially-induced forebulge supports thermal and refraction modeling of a thin (~25 km thick) lithosphere beneath Queen Charlotte (QC) Sound and Hecate Strait. Glacial ice at least 200 m thicker than present water depth began retreating from Dixon Entrance after 14,000 and prior to 12,640 14C years BP, generating 50 m of uplift in northern Hecate Strait. The position of the forebulge remained essentially constant after 12,750 14C years BP, implying a fixed ice-front and continued ice presence on the British Columbia (BC) mainland until ~10,000 14C years BP. A 3-dimensional model shows two ice-free terrains emerged: one extended eastward from the QCI, the other developed in QC Sound. By ~11,750 14C years BP a landbridge connected the BC mainland and QCI. Malacological evidence indicates a paucity of Arctic molluscan faima subsequent to glaciation, perhaps a consequence of shallow, narrowed straits, and the presence of ice sheets that interfered with ocean currents. Water temperature, sedimentation rates, turbidity, and photoperiod are factors that limited invertebrate colonization during the Late Pleistocene - Early Holocene. The oldest dated mollusc to colonize QCI region subsequent to LGM was Macoma nasuta at 13,210 14C years BP. Once habitat and sea-surface temperatures were conducive, rates of recolonization appear to be limited only by the availability of ocean currents to bring temperate pelagic larvae into the region from outlying areas. Between ~11,000 and 10,000 14C years BP the appearance of Clinocardium ciliatum and Serripes groenlandicus, concurrent with the disappearance, or significant reduction in number and productivity of temperate intertidal molluscs, indicates the onset of a short interval of cool sea-surface temperatures coincident with the Younger Dryas cooling event. Five molluscan species: Macoma incongrua, Musculus taylori (cf), Mytilimeria nuttallii, Tellina nucidoides, Mytilus edulis/Mytilus trossulus previously categorized as possessing a Recent geologic range were collected in sediments dating older than 10,000 14C years BP. Fossil mollusc shells indicate edible intertidal biomass densities well within commercially harvested levels on southern Moresby Island by 8,800 14C years BP, and on northern Graham Island by 8,990 14C years BP. The presence and productivity of nutritious intertidal molluscs indicates the QCI region had a suitable climate, possessed open ocean conditions, and provided subsistence resources for potential early humans subsequent to at least 13,210 14C years BP. Three-dimensional modeling shows subaerially exposed land that could have been inhabited by plants, animals, including coastal-migrating early humans. Early coastlines that have not been drowned, and which may harbour early archaeological sites, are identified along the western and northern coasts of QCI and the BC mainland.



Paleoecology, British Columbia, Haida Gwaii, Biogeochemical cycles, North Pacific Ocean, Geology