J. Vaughn Barrie

Discovery of a “Living Dinosaur”: Globally unique modern hexactinellid sponge reefs off British Columbia, Canada

SummaryGlobally unique hexactinellid sponge reefs occur on the continental shelf off British Columbia, Canada. They cover about 425 km2 of seafloor on the continental shelf off British Columbia (Canada) in water depths between 165 and 240 metres and occur on a low-angle deep shelf, iceberg scoured seafloor, characterized by very low sedimentation rates and very stable environmental conditions. The sponge bioherms are up to 19 metres high with steep flanks, whereas the biostromes are 2–10 metres thick and many kilometres wide. They all consist of dense populations of only seven hexactinellid species. Three of them, all hexactinosan species (Aphrocallistes vastus, Heterochone calyx, Farrea occa) are the main frambuilders, composing a true rigid framework of sponge skeletons encased in a organic rich matrix of modern clay baffled by the sponges. Growth rates of hexactinosan sponges range in the order of 0–7 centimetres per year. The base of the oldest sponge reefs date from approximately 9000 years b.p.Different invertebrate and fish faunas occupy the reefs than occur on adjacent seafloor areas and some species appear to use the sponge reef complex structures as refugia where they can hide.Sidescan sonar data and direct observation by manned submersible clearly show that large areas of sponge reefs have been heavily damaged by seafloor trawling in the past decade.These unique extant siliceous sponge reefs can be used as a modern analogue for a better understanding and interpretation of fossil siliceous sponge reefs, known from many ages and many locations world wide.

Post glacial sea levels on the Western Canadian continental shelf: evidence for rapid change, extensive subaerial exposure, and early human habitation

Grounded piedmont type glaciers inundated and isostatically loaded the deep troughs which indent the Western Canadian continental shelf, as far west as the shelf edge. Glaciers do not appear to have covered the offshore banks east of the Queen Charlotte Islands (Haida Gwaii). Ice retreated from the shelf at approximately 13,500 14C yr B.P. Rapid emergence of the crust followed the ice retreat and resulted in a relative fall of sea level. At 10,400 14C yr B.P. relative sea level on the continental shelf was more than 100 m below that of today and large areas of the Queen Charlotte Basin were subaerially exposed. Eustatic sea-level rise, coupled with subsidence of a glacioisostatic forebulge, allowed sea levels to rise very rapidly, and reach the present shoreline on the Queen Charlotte Islands (Haida Gwaii) by about 9100 14C yr B.P. Dated shoreline deposits (shells), submerged wood remnants, and barnacles on stone tools at elevations between −110 m and +14 m suggest a sea-level rise of 6.3 cm per year between 12,200 and 11,000 calender years. Our reconstructions of the paleogeography and paleoenvironments suggest a hospitable environment for human habitation existed in areas that are now submerged. Stone tools excavated from intertidal deposits support this interpretation. Significant local variations in the depth of synchronous shorelines are described and attributed to localized differences in isostatic load. The documented rates of crustal adjustment are much greater than those used in conventional geophysical (forebulge) models. Regional high-resolution seismic reflection data (3400 line km) shows little evidence for post-glacial faulting and suggest that most crustal adjustments appear to have been isostatically rather than tectonically driven. Subaerial exposure and subsequent sea-level transgression were the dominant post-glacial processes that determined the morphology, texture and paleoenvironment of the Western Canadian continental shelf.

Rapid sea-level change and coastal evolution on the Pacific margin of Canada

A rapid regression occurred off the northern Pacific margin of Canada approximately between 14,600 and 12,500 14C years B.P., contemporary with the deglaciation of the continental shelf, primarily due to the rapid isostatic rebound. Sea level had reached a maximum lowering of greater than 150 m and remained low until approximately 12,400 14C years B.P., after which a rapid transgression occurred. At 12,500 14C years B.P., the shelf tilt across the northern Pacific margin of Canada ranged between 50 m of submergence at Prince Rupert on the British Columbia mainland to greater than 150 m of emergence on the western edge of the Queen Charlotte Islands. This regressive/transgressive cycle lasted only from 5200 to 5500 14C years, a result of the development and collapse of a glacioisostatic forebulge. Sediment supply, wave action and tidal-current energy are the primary factors that controlled the coastal response to these late Quaternary relative sea-level changes. Plate tectonics, on the other hand, played a secondary role in coastal evolution. During deglacial regression, extensive glacial outwash and glaciomarine deposition occurred except on the outer coast where sediment was transferred to the shelf break. During the transgression of the shelf, the sediment supply was primarily restricted to the erosion of the previously deposited deglacial deposits, resulting in the formation of drowned wave-cut terraces, spit platforms and modification of estuarine environments, as the sea level rose in steps. The present unconsolidated coastline is mostly erosional. During the most recent ENSO event, the sea-level rise of up to 0.4 m resulted in as much as 12 m of coastal retreat.

Sedimentary processes and sediment dispersal in the southern Strait of Georgia, BC, Canada

This paper presents a review of sediment dispersal processes in the Strait of Georgia, based on marine geological studies. Sediment from the Fraser River is dispersed around the Strait through a variety of transport pathways. Most sand and coarser silt fractions settle out and are deposited within a few 100 m of the channel mouths. Both channelled and non-channelled gravity flows probably transport sediment downslope and onto the basin floor. Asymmetric tidal currents force a predominantly northward sediment drift, resulting in a reworked slope off Roberts Bank and a finer-grained depositional slope off Sturgeon Bank. Far-field sediment accumulation is controlled by local morphology and sediment dynamics. Multibeam mapping and seismic profiling reveal that some parts of the basin floor are characterized by bottom sediment reworking and erosion. Given the complexities of sediment dispersal and seafloor reworking, generalizations about sediment dispersal paths and sedimentation rates are difficult. Future understanding will be advanced by the cabled observatory, VENUS, which will enable near real-time monitoring of key processes.

RECENT HEXACTINOSIDAN SPONGE REEFS (SILICATE MOUNDS) OFF BRITISH COLUMBIA, CANADA: FRAME-BUILDING PROCESSES

Abstract Hexactinosidan sponges are important reef-building organisms in Earth history as they are able to create a three-dimensional reef framework and thereby form topographic relief comparable to that produced by scleractinian corals. Study of modern hexactinosidan sponge skeletons from water depths of 165–240 m on the continental shelf off British Columbia, Canada, demonstrate the hitherto undescribed frame-building process that leads to the formation of large and so far unique siliceous sponge reefs in this area. The fundamentals of the frame-building process are based on the production of siliceous envelopes around spicules of dead hexactinosidan sponges. In addition to the development of a three-dimensional reef framework, mound growth is supported by the current baffling effect of the sponges. Fine-grained siliciclastic suspended sediment is trapped and deposited within the gaps in the sponge skeletons and in voids in the reef surface preventing the framework from collapsing as the reef grows. Analogous but tropical examples from the Lower Jurassic of Portugal show that the frame-building potential of hexactinosidan and other siliceous sponges has existed, substantially unchanged, for more than 180 million years. In contrast to well-known fossil mud mounds of various geologic ages, in which the in situ precipitation of automicrite via microbial processes plays a major role, the matrix of the hexactinosidan sponge mounds of British Columbia consists exclusively of baffled fine-grained siliciclastics; automicrite is absent. Existing mud mound classification schemes do not encompass these depositional characteristics, therefore this new type of mound is consequently here classified as a silicate mound.

Submerged late quaternary terrestrial deposits and paleoenvironment of northern Hecate Strait, British Columbia continental shelf, Canada

Abstract Stratigraphic and paleoenvironmental analyses of sediments recovered from cores on the western Canadian continental shelf indicate that areas of the north-central shelf were subaerial and supported terrestrial vegetation during the regional Late Wisconsinan deglaciation. Paleoecological analyses of now submerged shelf sediments reveal a Late Quaternary treeless environment locally covered by wet sedge tundra. Samples from a coeval onshore section on eastern Graham Island indicate that a dwarf shrub tundra with mosses and willows also occupied lowland areas on the Queen Charlotte Islands between 13 and 14 ka. Ice-free subaerial conditions persisted for a period of at least 3200 years on some parts of the emergent shelf, between 13.2 and 10 ka. During the same time period clay-rich glaciomarine sediment with ice-rafted debris was accumulating in the troughs of the shelf. Low relative sea levels during deglaciation on the continental shelf adjacent to the eastern Queen Charlotte Islands suggest that Late Wisconsinan ice was thin and limited in extent in this area. The presence of a habitable lowland environment during this time interval strengthens the view that a coastal migration of humans from Beringia was possible along portions of the exposed continental shelf.

Sponge reefs in the Queen Charlotte Basin, Canada: controls on distribution, growth and development

Sponge reefs in the Queen Charlotte Basin exist at 165–240 m depth within tidally influenced shelf troughs subject to near bottom current velocities of 25–50 cm s−1 where nutrient supply from coastal runoff is augmented by wind-induced upwelling of nutrient rich water from the adjacent continental slope. Large reef mounds to 21 m in elevation affect tidally driven bottom currents by deflecting water flows through extensive reef complexes that are up to 300 km2 in area. Three hexactinellid species construct reefs by building a siliceous skeletal framework through several frame-building processes. These sponge reefs exist in waters with 90 to 150 µM dissolved oxygen, a temperature range of 5.9 to 7.3°C and salinity of 33.2 to 33.9 ‰. Relatively high nutrient levels occur at the reef sites, including silica, which in bottom waters are typically >40 µM and may be up to 80 µM. A high dissolved silica level is potentially an important control on occurrence of these and other dense siliceous sponge populations. The sponge reefs are mainly confined to seafloor areas where exposed iceberg plough marks are common. Sediment accumulation rates are negligible on the relict, glacial surface where the reefs grow, and trapping of flocculated suspended particulate matter by hexactinosidan or framework skeleton hexactinellid sponges accounts for a large proportion of the reef matrix. Suspended sediment concentration is reduced within the nepheloid layer over reef sites suggesting efficient particle trapping by the sponges. The reef matrix sediments are enriched in organic carbon, nitrogen and carbonate, relative to surrounding and underlying sediments. The sponges baffle and trap suspended sediments from water masses, which in one trough have a residence time of approximately 6 days, ensuring a close association of the sponges with the bottom waters. The location of the reef complexes at the heads of canyons provide a means of regionally funnelling particulate material that sponges can trap to enrich their environment with organic carbon and biogenic silica. Like deepsea coral reefs, the sponge reefs are a remote and poorly known ecosystem that can present logistical challenges and survey costs. Also like deep-sea coral reefs, many of the hexactinosidan sponge reefs have been damaged or destroyed by the groundfish trawl fishery.

Contrasting glacial sedimentation processes and sea-level changes in two adjacent basins on the Pacific margin of Canada

Abstract During the late Wisconsin Fraser Glaciation on the Pacific Margin of Canada, ice moved offshore from the Coast Mountains of the Canadian Cordillera and south into the Strait of Georgia, reaching a maximum extent at about 14 000 14C bp. Most of the strait was ice-free by 11 300 14C bp. Deglaciation was very rapid with regional downwasting and widespread stagnation. This resulted in a stratigraphy of thick till (30–60 m), overlain by ice-proximal glacimarine sediments and a thin and discontinuous ice-distal glacimarine unit. Glaciation of Queen Charlotte Basin reached a maximum sometime after 21 000 14C bp. Deglaciation in this region began sometime after 16 000 to 15 000 14C bp and ice had retreated fully onto mainland British Columbia by 13 500 14C bp. Deglaciation was rapid, with the eastward retreat of an ice shelf. This resulted in a stratigraphy of a till up to 50 m in thickness, usually turbated by iceberg scour and overlain in some areas by thin, ice-proximal glacimarine sediments and much thicker (20 m) widespread ice-distal glacimarine sediments. A significant difference between these two regions is the deglacial relative sea-level history. Rapid regression of the outer Queen Charlotte Islands shelf occurred between approximately 14 600 and 12 500 14C bp, primarily due to rapid isostatic rebound and contemporaneous with deglaciation of the continental shelf. Sea-level reached a maximum lowstand of greater than 150 m and remained low until approximately 12 400 14C bp. In Georgia Basin, sea-level was at a relative high stand of 50 to 200 m during initial deglaciation, falling to between 0 to 50 m below present sometime after 10 000 14C bp. We suggest that rapid emergence on the northern margin of the outer shelf was due to forebulge effects. Further, the very limited extent of glacial ice on the Queen Charlotte Islands and the exposure to the open Pacific forced the retreat of the Cordilleran ice-sheet margin eastwards thereby resulting in dominantly ice-distal glacimarine sedimentation. In contrast, the initial relative sea-level highstand during deglaciation between the Vancouver Island and Cordilleran glaciers in the Strait of Georgia resulted in significant ice-proximal deposition and limited ice-distal glacimarine deposition.

Paleoenvironments of the Strait of Georgia, British Columbia during the last deglaciation: microfaunal and microfloral evidence

The Strait of Georgia is situated between Vancouver Island and the western Canadian mainland. It was deglaciated quickly, around 12,500 14C yr BP, by in situ downwasting. The subsequent marine invasion has left glacimarine sediments with foraminifers, diatoms and dinocysts characterizing successive paleoenvironments recognizable basinwide. The first paleoenvironment represents near-ice conditions under the influence of the turbid meltwater plume marked by low algal productivity and few foraminifers, mostly Elphidium excavatum. Just off the plume, the second environment shows a peak of Nonionellina labradorica possibly resulting from an observed sudden increase in diatom numbers, mostly ice-dwelling forms implying a long annual sea-ice cover. Further away, Cassidulina reniforme increases, N. labradorica decreases while other calcareous foraminifer species colonize the area; there are fewer ice diatoms, more marine planktonic diatoms and dinocysts become more abundant. Finally, after 12,000 14C yr BP, calcareous foraminifers are gradually replaced because of dissolution of calcium carbonate by a low-diversity arenaceous assemblage. At the same time, abundant planktonic diatoms and dinocysts indicate warmer surface temperature. The succession of fossils and environments compares well with modern, horizontal, off-glacier successions observed in fjords of the European Arctic.

The Queen Charlotte Fault, British Columbia: seafloor anatomy of a transform fault and its influence on sediment processes

The Queen Charlotte Fault Zone (QCFZ) off western Canada is the northern equivalent to the San Andreas Fault Zone, the Pacific–North American plate boundary. Geomorphologic expression and surface processes associated with the QCFZ system have been revealed in unprecedented detail by recent seabed mapping surveys. Convergence of the Pacific and North American plates along northern British Columbia is well known, but how the QCFZ accommodates this convergence is still a subject of controversy. The multibeam sonar bathymetry data reveal, for the first time, evidence of a fault valley with small depressions on the upper slope, offshore central Haida Gwaii (Queen Charlotte Islands). The depressions form where strike-slip right-step offsets have realigned the fault due to oblique convergence. Core stratigraphy and radiocarbon dating of sediments within the fault valley and small depressions suggest that these features are recent in origin. In addition, the development of the fault valley and dislocation of submarine canyons control sediment migration from the continental shelf through to the lower slope. This interpretation of the geomorphic expression of major plate tectonic processes along the QCFZ can now be tested with new surveys subsequent to the October 2012 magnitude 7.7 earthquake.