Somerset Island Research Articles

Transitional occupations of southeastern Somerset Island, Nunavut

Four transitional period sites on Somerset Island compare closely with High Arctic sites attributed to Independence II. The four sites are situated at elevations between Pre-Dorset and Early Dorset occupations in the same area. The sites are described, with an emphasis on architectural characteristics. Vertical slab midpassage structures are present at all sites along with artifact assemblages containing shallowly side-notched bifaces, "cloven hoof" lance heads, a closed socket harpoon head, burin-like tools, narrow microblades, and needles with gouged eyes. Some sites also have exterior hearths and possible caches. Radiocarbon dates place the occupations within the transitional period (i.e. 800-500 B.C.). As during Independence II, the sites are of various size (three small ones and a large one), and are in association with polynyas.

Changes in mercury and cadmium concentrations and the feeding behaviour of beluga ( Delphinapterus leucas) near Somerset Island, Canada, during the 20th century

Beluga ( Delphinapterus leucas) continues to be an important food species for Arctic communities, despite concerns about its high mercury (Hg) content. We investigated whether Hg and cadmium (Cd) concentrations had changed during the 20th century in beluga near Somerset Island in the central Canadian Arctic, using well-preserved teeth collected from historical sites (dating to the late 19th century and 1926–1947) and during subsistence hunts in the late 1990s. Mercury concentrations in both historical and modern teeth were correlated with animal age, but 1990s beluga exhibited a significantly more rapid accumulation with age than late 19th century animals, indicating that Hg concentrations or bioavailability in their food chain had increased during the last century. The geometric mean tooth Hg concentration in modern 30 year old animals was 7.7 times higher than in the late 19th century, which corresponds to threefold higher concentrations in muktuk and muscle. Teeth from 1926 to 1947 were similar in Hg content to the late 19th century, suggesting that the increase had occurred sometime after the 1940s. In contrast, tooth Cd was not correlated with animal age and decreased during the last 100 years, indicating that anthropogenic Cd was negligible in this population. Late 19th century beluga displayed a greater range of prey selection (tooth δ 15N values: 15.6–20.5‰) than modern animals ( δ 15N: 17.2–21.1‰). To prevent this difference from confounding the temporal Hg comparison, the Hg–age relationships discussed above were based on historical animals, which overlapped isotopically with the modern group. Tooth δ 13C also changed to isotopically more depleted values in modern animals, with the most likely explanation being a significant shift to more pelagic-based feeding. Industrial Hg pollution is a plausible explanation for the recent Hg increase. However, without further investigation of the relationship between the range exploitation of modern beluga and their possible exposure to regional marine food chains with (naturally) higher Hg contents than their historical counterparts, we cannot unequivocally conclude that the increase was anthropogenically driven.

The temporal evolution of North American kimberlites

North American kimberlite magmatism spans a period of time in excess of 1 billion years from Mesoproterozoic kimberlites in the Lake Superior and James Bay Lowlands region of Ontario to Eocene kimberlites in the Lac de Gras field, N.W.T. Based on a compilation of more than 150 robust radiometric age determinations, several distinct kimberlite emplacement patterns are recognized. In general, the temporal pattern of kimberlite emplacement in North America can be broadly subdivided into five domains: (1) a Mesoproterozoic kimberlite province in central Ontario, (2) an Eocambrian/Cambrian Labrador Sea Province in northern Québec and Labrador, (3) an eastern Jurassic Province, (4) a central Cretaceous corridor and (5) a western mixed domain that includes two Type-3 kimberlite provinces (i.e. multiple periods of kimberlite emplacement preserved in the Slave and Wyoming cratons). For some provinces the origin of kimberlite magmatism can be linked to known mantle heat sources such as mantle plume hotspots and upwelling asthenosphere attendant with continental rifting. For example, the timing and location of Mesoproterozoic kimberlites in North America coincides with and slightly precedes the timing of 1.1 Ga intracontinental rifting that culminated in the Midcontinent Rift centered in the Lake Superior region. Many of the kimberlites in the Eocambrian/Cambrian Labrador Sea province were emplaced soon after the opening of the Iapetus Ocean at about 615 Ma and may also be linked to mantle upwelling associated with continental rifting. The eastern Jurassic kimberlites record an age progression where magmatism youngs in a southeast direction from the ∼200 Ma Rankin Inlet kimberlites to the 155–126 Ma Timiskaming kimberlites. The location of several kimberlite fields and clusters in Ontario and Québec lie along a continental extension of the Great Meteor hotspot track and represents one of the best examples in the world of kimberlite magmatism triggered by mantle plumes. The central Cretaceous (103–94 Ma) corridor extends for more than 4000 km from Somerset Island in northern Canada through the Fort à la Corne field in Saskatchewan to the kimberlites in central USA. This is the first recognized corridor of kimberlite magmatism of this magnitude. The possible westward younging of Cretaceous to Eocene corridors of kimberlite magmatism could reflect major changes in plate geometry during subduction of the Kula–Farallon plate.

Biological identification of springtails (Hexapoda: Collembola) from the Canadian Arctic, using mitochondrial DNA barcodes

We evaluated sequence diversity in the mitochondrial cytochrome-c oxidase I (COI; EC 1.9.3.1) gene as a tool for resolving differences among species of Arctic springtails. The Collembola examined in this analysis were collected from Igloolik, Cornwallis, and Somerset islands and included representatives from all major families found in the Arctic. Members of 13 genera and 19 species were examined, including 4 species of the genus Folsomia and 3 species of the genus Hypogastrura. In all cases, species were successfully discriminated. Sequence divergences within species were generally less than 1%, whereas divergences between species were greater than 8% in all cases. Divergences among individuals of one species of Folsomia were much higher (up to 13%), but this likely represents the presence of an undescribed sibling species. We conclude that DNA barcoding is a powerful tool for identifying species of Collembola and should regularly be useful as a complement to traditional, morphological taxonomy.

Prehistoric Inuit whalers affected Arctic freshwater ecosystems.

It is commonly assumed that High Arctic lakes and ponds were not affected by direct local human activities before the arrival of Europeans, because most native peoples were primarily nomadic, maintained relatively low population densities, and practiced unintrusive hunting and gathering technologies. Our archeological and paleolimnological data show that this was not always the case. Thule Inuit whalers, whose winter settlements consisted of houses constructed from the bones of bowhead whales on Somerset Island between about anno Domini 1200 and 1600, markedly changed pond water quality and ecology. The arrival of whalers 8 centuries ago caused marked changes in water chemistry and the expansion of moss substrates. Although whalers abandoned the area >4 centuries ago, the legacy of these human disturbances is still evident in the pond's present-day limnology and is characterized by elevated nutrient concentrations and atypical biota. This is the earliest reported paleolimnological record of changes in aquatic ecology associated with local human activities in Canada or the United States, or for any circumpolar ecosystem.

A Processual Investigation of a Thule Whale Bone House, Somerset Island, Arctic Canada

In 1991, the authors excavated a Thule Inuit whale bone house at site PaJs-13 on southeast Somerset Island, Arctic Canada. Detailed analyses of house stratigraphy provide useful data for the study of the cultural and natural formation processes relating to the feature. Based on our analyses, and following McCartney (), six stages are recognized: 1) raw material procurement, 2) house construction, 3) house use and maintenance, 4) house abandonment, 5) dismantling of super-structure, and 6) post-occupation. Several characteristics of the house structure and the associated artifact assemblage indicate that this house probably functioned, at least during the later period of occupation, as a karigi [plural kariyit ], or ceremonial house.

A Re–Os isotope and PGE study of kimberlite-derived peridotite xenoliths from Somerset Island and a comparison to the Slave and Kaapvaal cratons

The concentrations of platinum-group elements (PGE; Os, Ir, Ru, Pd and Pt) and Re, and the Os isotopic compositions were determined for 33 lithospheric mantle peridotite xenoliths from the Somerset Island kimberlite field. The Os isotopic compositions are exclusively less radiogenic than estimates of bulk-earth ( 187Os/ 188Os as low as 0.1084) and require a long-term evolution in a low Re–Os environment. Re depletion model ages ( T RD) indicate that the cratonic lithosphere of Somerset Island stabilised by at least 2.8 Ga, i.e. in the Neoarchean and survived into the Mesozoic to be sampled by Cretaceous kimberlite magmatism. An Archean origin also is supported by thermobarometry (Archean lithospheric keels are characterised by >150 km thick lithosphere), modal mineralogy and mineral chemistry observations. The oldest ages recorded in the lithospheric mantle beneath Somerset Island are younger than the Mesoarchean (>3 Ga) ages recorded in the Slave craton lithospheric mantle to the southwest [Irvine, G.J., et al., 1999. Age of the lithospheric mantle beneath and around the Slave craton: a Rhenium–Osmium isotopic study of peridotite xenoliths from the Jericho and Somerset Island kimberlites. Ninth Annual V.M. Goldschmidt Conf., LPI Cont., 971: 134–135; Irvine, G.J., et al., 2001. The age of two cratons: a PGE and Os-Isotopic study of peridotite xenoliths from the Jericho kimberlite (Slave craton) and the Somerset Island kimberlite field (Churchill Province). The Slave–Kaapvaal Workshop, Merrickville, Ontario, Canada]. Younger, Paleoproterozoic, T RD model ages for Somerset Island samples are generally interpreted as the result of open system behaviour during metasomatic and/or magmatic processes, with possibly the addition of new lithospheric material during tectono-thermal events related to the Taltson–Thelon orogen. PGE patterns highly depleted in Pt and Pd generally correspond to older Archean T RD model ages indicating closed system behaviour since the time of initial melt extraction. Younger Proterozoic T RD model ages generally correspond to more complex PGE patterns, indicating open system behaviour with possible sulfide or melt addition. There is no correlation between the age of the lithosphere and depth, at Somerset Island.

Major element composition of the lithospheric mantle under the North Atlantic craton: Evidence from peridotite xenoliths of the Sarfartoq area, southwestern Greenland

The composition and thermal state of the lithospheric mantle under the North Atlantic craton was investigated using a suite of peridotite xenoliths from the diamond-bearing Sarfartoq kimberlite dike swarm of southwestern Greenland. Elevated olivine and whole-rock Mg# (>0.9) attest to the refractory nature of the Sarfartoq mantle showing comparable degrees of depletion to other cratonic roots. Modal analyses indicate that the Sarfartoq mantle is not typified by the orthopyroxene enrichment observed in the Kaapvaal root, but shows more affinity with the Canadian Arctic (Somerset Island), Tanzania, and East Greenland (Wiedemann Fjord) peridotites. The Sarfartoq peridotites have equilibrated at temperatures and pressures ranging from 660 to 1,280 °C and from 2.2 to 6.3 GPa, and define a relatively low mantle heat flow of 13.2±1 mW/m2. In addition, the lithospheric mantle underneath the Sarfartoq area is compositionally layered as follows: (1) an internally stratified upper layer (70 to 180 km) consisting of coarse, un-deformed, refractory garnet-bearing and garnet-free peridotites and, (2) a lower layer (180 to 225 km) characterized by fertile, CPX-bearing, porphyroclastic garnet lherzolites. The stratification observed in the upper refractory harzburgite layer (70–180 km) is reflected by an increase in fertility (e.g., decrease in olivine abundance and forsterite content) with depth. The sharp nature of the boundary between the upper and lower layers may indicate multistage growth of the lithospheric mantle.

The migratory behaviour of narwhals (Monodon monoceros)

Sixteen female narwhals (Monodon monoceros) were tracked by satellite in 2000 and 2001 from their summering ground near Somerset Island in the Canadian High Arctic to their wintering ground in central Baffin Bay. The wintering ground location was spatially discrete from another narwhal wintering ground in southern Baffin Bay. Area extent of the summering ground was approximately 9464 km2and area extent of the wintering ground was 25 846 km2. Two of the narwhals were tracked for more than 12 consecutive months. These whales used three focal areas between their spring and autumn migration: a coastal area in the open-water season in August in the Canadian High Arctic, a wintering area from November through April in the consolidated pack ice of Baffin Bay, and an early summer area in front of the receding fast ice edge in Lancaster Sound. The whales showed remarkable site fidelity to summering grounds and had specific migratory routes that followed sea ice formation and recession.

Small-scale Sr isotope investigation of clinopyroxenes from peridotite xenoliths by laser ablation MC-ICP-MS—implications for mantle metasomatism

Although numerous studies have shown that mantle xenoliths are isotopically heterogeneous on a hand specimen scale, experimental data imply that isotopic equilibrium should be attained within single mineral grains in the upper mantle. Until recently, this prediction has not been verified because of the difficulty of analyzing individual grains. We report in situ 87Sr/86Sr compositions of clinopyroxenes in garnet peridotite xenoliths from the Nikos kimberlite, Somerset Island (Arctic Canada) obtained by laser ablation MC-ICP-MS. Results for five different peridotites indicate the existence of large Sr isotope variations within individual xenolith samples, varying from 0.5‰ to as much as 1.1‰ (3 to 8 parts in 7000). This study is the first to document isotopic heterogeneity in peridotite xenoliths at the scale of individual grains. Multiple analyses of the same grain, however, indicate intra-grain Sr isotopic equilibrium.The Sr isotopic ratios for individual clinopyroxene grains correlate with major element abundances such as SiO2 and TiO2, and trend towards the composition of the host Nikos kimberlite. It thus appears likely that the Sr isotope heterogeneities recorded by the clinopyroxenes are the result of metasomatic interaction with the host kimberlite magma. The preservation of Sr isotopic variability in clinopyroxene separates from different peridotite samples and between clinopyroxene grains of individual xenoliths suggests that metasomatism occurred just prior to or during kimberlite transport. The Sr isotope heterogeneities documented here for a single peridotite phase suggest that isotopic compositions obtained on mineral separates from peridotite xenoliths likely represent weighted averages and could be less homogenous than previously assumed.

An estimate of the fraction of belugas (Delphinapterus leucas) in the Canadian high Arctic that winter in West Greenland

Five belugas, or white whales (Delphinapterus leucas), were tracked by satellite from Creswell Bay, Somerset Island, in the Canadian high Arctic towards West Greenland in autumn 2001. After 1 October, three of the whales stayed in the North Water polynya and the other two whales moved to West Greenland. One of the whales that moved to Greenland migrated south along the west coast, following a route and timing similar to another beluga tracked in 1996. The belugas that moved towards West Greenland from Canada did so before or near 1 October. The movements of both these whales followed a similar timing and assumed migratory route of belugas hunted in autumn in West Greenland. In Greenland, the hunt begins in September, where the first whales are taken in the northernmost community of Qaanaaq. Hunting takes place farther south in Upernavik in October, and finally in November and December, belugas are taken even farther south in Uummannaq and Disko Bay. The whales that remain in the North Water after 1 October most likely do not contribute to the harvest in West Greenland. Based on the total number of belugas satellite-tracked in Canada between 1995 and 2001 with tags that lasted beyond 1 October, approximately 0.15 (95% CI 0.06–0.35; n=26) of the summering stock of belugas in the Canadian high Arctic move to West Greenland for the winter. Genetic studies have indicated that belugas moving east through Lancaster Sound are significantly differentiated from belugas taken in the autumn hunt in West Greenland. These conflicting results suggest molecular genetics cannot be solely relied on to reveal the stock identity of these belugas.

Distribution and abundance of Canadian High Arctic belugas, 1974-1979

We conducted >236,000 km of aerial surveys and some supplementary studies of belugas (Delphinapterus leucas) in the central and eastern Canadian High Arctic in 974-79. Belugas that wintered in the “North Water” in Baffin Bay moved southwest into Lancaster Sound in April and early May. The main westward migration into Lancaster Sound occurred over a 2 to 3 week period during late June to late July. Estuaries along Somerset Island were occupied for <3 weeks from mid-July to mid-August. Little feeding occurred in estuaries. From mid-August until fall migration began in mid-September belugas occupied estuaries and offshore waters in Peel Sound. Fall migration eastward through Lancaster Sound was exclusively along the south coast of Devon Island, highly co-ordinated, and rapid; most of the population passed through the sound in <1 week. The whales then moved north along the east coast of Devon Island; some entered Jones Sound while others crossed directly to SE Ellesmere Island. Most calving occurred in July and early August; calving was not seen in estuaries and probably occurred offshore. Excluding calves, adults and yearlings formed 77% and 8.4%, respectively, of the population. The proportion of calves during mid-August was consistent with a triennial calving cycle. During late summer, belugas fed on coastal concentrations of polar cod (Boreogadus saida), under pan ice offshore (probably on cod), and in deep offshore waters. The size of the Canadian High Arctic population in the late 1970s was estimated to be at least 10,250 to 12,000 animals without allowing for animals that may have passed between surveys or that were below the surface at the time of the counts.

Diving behaviour of narwhals (Monodon monoceros) at two coastal localities in the Canadian High Arctic

In August 1999 and 2000, four suction-cup-attached time–depth recorders (TDRs) were deployed and retrieved from narwhals (Monodon monoceros) in Tremblay Sound, Baffin Island, and Creswell Bay, Somerset Island, Nunavut, Canada. The TDRs remained on the whales for between 12 and 33 h and collected 64.5 h of dive data. Mean dive depths ranged from 20.8 m (SD = 14.8 m) to 50.8 m (SD = 43.8 m) and mean dive durations ranged from 3.4 min (SD = 1.6 min) to 4.9 min (SD = 4.5 min). There appeared to be individual differences in dive parameters both within a region and between regions. Three of the whales made short, shallow dives, while another whale made dives twice as deep and twice as long. One whale had maximum dive durations (>20 min) that exceeded predicted aerobic dive limits for narwhals. There was a strong relationship between maximum dive depth and duration for all whales (p < 0.0001). Narwhals spent between 30.3 and 52.9% of their time at depths <5 m and the range of correction factors for availability bias was 1.9–3.3. Satellite-linked TDRs were simultaneously deployed on the whales at both localities. Dive data collected using the two methods were compared and good agreement between the methods was obtained.

Probing Archean lithosphere using the Lu–Hf isotope systematics of peridotite xenoliths from Somerset Island kimberlites, Canada

A knowledge of the Hf isotopic composition of the subcontinental lithosphere beneath Archean cratons is essential to constrain the Hf isotope budget of the Earth’s mantle. Hf isotopic measurements were obtained by MC-ICP-MS for a suite of refractory peridotite xenoliths and constituent garnets from the Nikos kimberlite (100 Ma) on Somerset Island in order to constrain the isotopic composition and age of the lithosphere beneath the northern Canadian craton. The low-temperature Nikos peridotites (<1100°C), which represent the shallow Somerset lithosphere, are characterized by higher 176Lu/ 177Hf ratios (0.03–0.05) and Hf isotopic values ( 176Hf/ 177Hf (0.1Ga)=0.28296–0.28419) than the deep-seated high-temperature peridotites (>1100°C; 0.004–0.03, 0.28265–0.28333, respectively). These differences in Hf isotope signatures suggest that shallow and deep subcontinental lithosphere beneath Somerset Island represent isotopically distinct domains and do not share a common petrogenetic history. The Lu–Hf isotope systematics of the shallow low-temperature peridotites define a positively sloped line that plot along a 2.8 Ga reference isochron. A number of these peridotites are characterized by highly radiogenic Hf isotopic compositions suggestive of long-term radiogenic ingrowth (billions of years). These findings are consistent with an interpretation that the shallow Somerset lithosphere (to depths of ∼150 km) stabilized in the Archean. The majority of the high-temperature peridotites plot closer to the composition of the host kimberlite. Although the observed isotopic variation may be attributed in part to kimberlite-related Hf addition, it is possible that these deep-seated xenoliths represent younger mantle. The superchondritic 176Lu/ 177Hf ratios observed for a number of the shallow low-temperature peridotites indicate strong fractionation of Lu and Hf, suggesting mantle root formation in the garnet stability field (depths >80 km). The Hf isotope compositions for the Somerset low-temperature peridotites indicate that part of the mantle root beneath the North American craton is characterized by a more radiogenic Hf isotope signature than that estimated for a typical ‘depleted’ mantle.

Sir John Franklin's last arctic expedition: a medical disaster.

Sir John Franklin's last arctic expedition: a medical disaster.

Social and ritual determinants of whale bone transport at a classic Thule winter site in the Canadian Arctic

AbstractRecent research has demonstrated that patterning related to harvesting selectivity and architectural bone utilization persists in the surface distribution of bowhead whale (Balaena mysticetus) bones at classic Thule sites in the Canadian central Arctic, despite scavenging by later non‐whaling Inuit groups. Since skeletal riders with minimal architectural or artifactual utility are associated with carcass portions that were socially and ritually prized ethnographically in North Alaska, this surface record may also preserve spatial structure relating to loci of high status settlement and/or ceremonial activity within sites. Close to 3400 bowhead bones were mapped at the major Thule winter village of Qariaraqyuk, southeast Somerset Island. The results of a principal components analysis of the element distributions are consistent with expectations for special treatment of bowhead flippers, tails, and tongues. These results are supported by excavation data which reveal that flipper and tongue bones were preferentially discarded in the vicinity of wealthy whaling households and a major ceremonial structure. Contrary to the longstanding belief that Thule whale bone assemblages are hopelessly compromised by prehistoric and historic bone transport, these assemblages hold great promise for investigating Thule social and ritual practices. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Sr-Nd-Pb isotope systematics of mantle xenoliths from Somerset Island kimberlites: Evidence for lithosphere stratification beneath Arctic Canada

Sr, Nd, and Pb isotopic compositions were determined for a suite of Archean garnet peridotite and garnet pyroxenite xenoliths and their host Nikos kimberlite (100 Ma) from Somerset Island to constrain the isotopic character of the mantle root beneath the northern Canadian craton. The Nikos peridotites are enriched in highly incompatible trace elements (La/SmN = 4–6), and show 143Nd/144Nd(t) (0.51249–0.51276) and a large range in 87Sr/86Sr(t) (0.7047–0.7085) and Pb (206Pb/204Pb(t) = 17.18 to 19.03) isotope ratios that are distinct from those estimated for “depleted mantle” compositions at the time of kimberlite emplacement. The Nd isotopic compositions of the peridotites overlap those of the Nikos kimberlite, suggesting that the xenoliths were contaminated with kimberlite or a kimberlite-related accessory phase (i.e., apatite). The highly variable Sr and Pb isotopic compositions of the peridotites, however, indicate that kimberlite contribution was restricted to very small amounts (∼1 wt % or less).The high-temperature peridotites (>1100°C) that sample the deep Somerset lithosphere trend toward more radiogenic 87Sr/86Sr(t) (0.7085) and unradiogenic 206Pb/204Pb(t) (17.18) isotopic ratios than those of the low-temperature peridotites (<1100°C). This is in agreement with Sr isotopic compositions of clinopyroxene from the low-temperature peridotites (87Sr/86Sr(t) = 0.7038–0.7046) that are significantly less radiogenic than those of clinopyroxene from the high-temperature peridotites (87Sr/86Sr(t) = 0.7052–0.7091). The depth correlation of Sr isotopes for clinopyroxene and Sr and Pb isotopic compositions for the Nikos whole-rocks indicate that the deep Somerset lithosphere (>160 km) is isotopically distinct from the shallow lithospheric mantle. The isotopic stratification with depth suggests that the lower lithosphere is probably younger and may have been added to the existing Archean shallow mantle in a Phanerozoic magmatic event. The radiogenic Sr isotope ratios of the high-temperature peridotites and their clinopyroxenes suggest that the underplated deep lithosphere contained recycled (altered oceanic crust and sedimentary component?) material introduced during earlier subduction.

Lepidoptera collected in the Canadian Arctic during the Tundra Northwest 99 expedition

During the Swedish-Canadian icebreaker-based expedition to the Nearctic in summer 1999 (July and early August), moths and butterflies (Lepidoptera) were collected during short, helicopter-aided visits to eight sites in the Canadian archipelago, three sites on the Canadian mainland near the arctic coast and also at Thule Air Force Base in north Greenland. Some of the visited sites (notably those on Melville Peninsula, Somerset Island and King William Island) have not previously been investigated with respect to their insect faunas. Several records thus represent species range extensions. They generally corroborate an earlier hypothesis, based on a floristic zonation scheme, which has been used as a framework for understanding the distribution of Arctic butterflies.

Paleobiology of the late Mesoproterozoic (ca. 1200 Ma) Hunting Formation, Somerset Island, arctic Canada

Silicified shallow water carbonates of the late Mesoproterozoic (ca. 1200 Ma) Hunting Formation of Somerset Island, arctic Canada, preserve four distinct fossil assemblages. Three of these are dominated by prokaryotic fossils, whereas the fourth includes abundant populations of the red alga Bangiomorpha pubescens, as well as two problematic forms interpretable as possible multicellular eukaryotes. A few of the spheroidal microfossils show features suggestive of eukaryotes, but the majority are small simple, probable cyanobacteria; filamentous and stalk-forming cyanobacteria are also present. The different assemblages document distinct environments within a warm, intertidal to supratidal carbonate facies. Physical attributes, such as relative exposure, salinity, and substrate were clearly the determining factors for the majority of these environments and their respective communities. In the Bangiomorpha assemblage, however, it was the organisms themselves that controlled community structure. Vertically oriented populations of Bangiomorpha appear to have successfully excluded benthic microbial mats and, in the process, produced an environmentally heterogeneous environment populated by a high diversity, low dominance community. As the oldest recorded instance of competitive superiority by a multicellular eukaryote over prokaryotes, this occurrence also has implications for understanding ecosystem evolution and secular change in carbonate sedimentation. In particular, it provides key evidence for the hypothesis that the rise of multicellular algae contributed to major environmental shifts through the Mesoproterozoic–Neoproterozoic transition. Two new taxa are described, Bicamera stigmata n. gen., n. sp. and Salome nunavutensis n. sp.

Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada

Peridotites that sample Archean mantle roots are frequently in- INTRODUCTION compatible trace element enriched despite their refractory major The mantle underlying many Archean cratons has anelement compositions. To constrain the trace element budget of the omalously high seismic velocities to depths of 350–400 km lithosphere beneath the Canadian craton, trace element and rare ( Jordan, 1988; Grand, 1994), indicating the presence of earth element (REE) abundances were determined for a suite of cold refractory roots, depleted in the fusible major elegarnet peridotites and garnet pyroxenites from the Nikos kimberlite ments compared with fertile mantle (Boyd & Mertzman, pipe on Somerset Island, Canadian Arctic, their constituent garnet 1987; McDonough, 1990). These deep residual peridotite and clinopyroxene, and the host kimberlite. These refractory mantle roots probably contribute to the stability of Archean xenoliths are depleted in fusible major elements, but enriched in continental lithosphere because of their lower density and incompatible trace elements, such as large ion lithophile elements higher viscosity compared with that of the surrounding (LILE), Th, U and light rare earth elements (LREE). Mass asthenospheric mantle (Boyd & McCallister, 1976; Jorbalance calculations based on modal abundances of clinopyroxene dan, 1979; Pollack, 1986). Mantle xenoliths that are and garnet and their respective REE contents yield discrepancies hosted by kimberlites and alkaline basalts are our only between calculated and analyzed REE contents for the Nikos bulk window into the subcontinental lithosphere. They provide rocks that amount to LREE deficiencies of 70‐99%, suggesting essential evidence on the chemical composition and evoluthe presence of small amounts of interstitial kimberlite liquid tion of the upper mantle to depths of >200 km. Studies (0·4‐2 wt %) to account for the excess LREE abundances. These of these mantle xenoliths enable us to characterize the results indicate that the peridotites had in fact depleted or flat abundance and distribution of major, minor and trace LREE patterns before contamination by their host kimberlite. LREE elements in peridotites and between their constituent and Sr enrichment in clinopyroxene and low Zr and Sr abundances minerals. Most subcratonic peridotite samples have in garnet in low-temperature peridotites (800‐1100°C) compared undergone a complex history of melt extraction that has with high-temperature peridotites (1200‐1400°C) suggest that the changed their chemical composition and resulted in shallow lithosphere is geochemically distinct from the deep lithosphere depletion of the residual mantle in fusible major elements beneath the northern margin of the Canadian craton. The Somerset such as Fe, Al and Ca (e.g. Nixon, 1987; Herzberg, 1993; mantle root appears to be characterized by a depth zonation that Boyd et al., 1997). In contrast to their depletion in may date from the time of its stabilization in the Archean. incompatible major elements, however, many peridotite