Upper Silurian lithistid sponge reefs on Somerset Island, Arctic Canada

The Upper Ludlow Douro Formation contains the first reported Silurian sponge reefs. These relatively small (5–35 m diameter), mound-shaped structures contain, on average, 35% lithistid demosponges. Reefs are surrounded by irregular haloes of crinoid debris; abundance and diversity of all fossil groups decreases away from the reefs. Each reef is underlain by a lens of crinoid wackestone to grainstone rich in crinoid holdfasts; trepostomate bryozoans, solenoporacean algae and rhynchonellid brachiopods are locally common. The bulk of each reef consists of lime mudstone with abundant lithistid sponges. This is capped by a thin layer of wackestone with abundant tabulate and rugose corals and fewer lithistid sponges, calcareous algae, trepostomate bryozoans and stromatoporoids. This zonation, in which a sponge colonization community was replaced by a coral diversification community, is similar to that reported from some Middle Ordovician, Upper Jurassic and Holocene sponge reefs. The Douro sponge reefs were relatively low structures, with about 3 m maximum topographic relief. They grew on a broad carbonate platform, probably in warm, tranquil, turbid waters of normal or near-normal marine salinity. Periodic influxes of terrigenous mud adversely affected reef size, and caused biotic changes. Some of the reef lime mud was derived from non-reef sources, but significant quantities were also produced on the reefs. Reefs underwent synsedimentary lithification, bioerosion and minor storm erosion. Fabrics and compositions of sparry calcite in cavities record three generations of meteoric cementation. Originally siliceous spicules of the lithistid sponges were dissolved and the moulds later filled with sparry calcite. Early dissolution of siliceous spicules is common in reef environments, and may have caused fossil sponges to be under-represented in ancient reefs.     

Arctic plesiosaurs from the Lower Cretaceous of Melville Island,Nunavut, Canada

An expedition to Melville Island in Nunavut, Canada, recovered the fragmentary fossils of several plesiosaurs from non-marine deposits of the Hauterivian–Aptian Isachsen Formation. These plesiosaur fossils are some of the oldest Early Cretaceous records of the group in North America, and they likely predate the formation of a continuous Western Interior Seaway. The plesiosaurs from Melville Island appear to be primarily juveniles, and would have been living in a region that experienced at least seasonally cool temperatures. The presence of these fossils in a fluvial deposit support previous suggestions that juvenile plesiosaurs may have preferentially inhabited shallower waters rather than open marine environments. These fossils also show that polycotylid plesiosaurs were able to rapidly disperse and colonize high latitude coastal regions, as their occurrence in Arctic Canada only slightly postdates the first confirmed appearance of the group in Australia.     

Late Mesoproterozoic rifting in Arctic Canada during Rodinia assembly: impactogens,trans‐continental far‐field stress and zinc mineralisation

This paper challenges the conventional interpretation of a major, economically important Mesoproterozoic intracratonic rift system as a group of aulacogens, proposing instead that they are rifts that developed in response to far‐field stress caused by continent–continent collision (impactogens) during supercontinent assembly. The tectonostratigraphic evolution of the Bylot basins (Arctic Canada) records dramatic alternations between extensional and compressional stress regimes, precluding an aulacogen interpretation and favouring an impactogen interpretation. New geochronological data (U–Th–Pb whole‐rock depositional age; detrital zircon signatures) provide a record of impactogens that developed in Laurentia's interior as a result of Grenvillian (~1.1 Ga) far‐field stress during assembly of the supercontinent Rodinia. Formation of the world‐class Nanisivik zinc deposit in one of the rifts is temporally associated with the Grenvillian orogeny, and consequently represents both (i) a rare exception to the global pattern of few ore deposits forming during supercontinent episodes, and (ii) a hitherto unrecognised tectonic setting for carbonate‐hosted zinc deposits.     

Post-Glacial climatic change on Boothia Peninsula, Nunavut, Canada

A high temporal resolution pollen diagram from a lake in the mid-Arctic region of the Boothia Peninsula, Nunavut, Canada, documents the history of the regional vegetation and climate for the past 7200 yr. Major tundra pollen taxa in the core include Cyperaceae and Salix, with Cyperaceae comprising over 50% of the pollen in the early and late Holocene. Tree pollen, transported from far to the south, comprised a large percentage of the pollen sum, with Pinus accounting for 30% of the pollen in some levels of the core. Pollen percentages and concentrations of taxa typical of the mid-Arctic were highest in the mid-Holocene, corresponding to warm conditions. Decreasing pollen concentrations indicate cooling temperatures, with more rapid decreases occurring around 4200, 3800-3400, and 2500 cal yr BP. Pollen percentages of Salix, Cyperaceae, and Artemisia increased in the past 35 yr in response to global warming. Reconstructions of July temperature using the modern analog technique showed that the mid-Holocene (5800-2800 cal yr BP) was approximately 1 °C higher than during the past 1000 yr.     

Apparent radiocarbon ages of recent marine shells from Norway,Spitsbergen, and Arctic Canada

The mean apparent radiocarbon ages of marine shells, colleted alive before the initiation of atomic bomb testing, and also before the main input of dead carbon derived from fossil fuels, are found to be 440 yr for the coast of Norway, 510 yr for Spitsbergen, and 750 yr for Ellesmere Island, Arctic Canada. The relationship between these apparent ages and the oceanic circulation pattern, is discussed. Also possible variations of the apparent ages back in time are discussed.     

Holocene alpine glaciation inferred from lacustrine sediments on northeastern Baffin Island,Arctic Canada

With accelerated melting of alpine glaciers, understanding the future state of the cryosphere is critical. Because the observational record of glacier response to climate change is short, palaeo‐records of glacier change are needed. Using proglacial lake sediments, which contain continuous and datable records of past glacier activity, we investigate Holocene glacier fluctuations on northeastern Baffin Island. Basal radiocarbon ages from three lakes constrain Laurentide Ice Sheet retreat by ca. 10.5 ka. High sedimentation rates (0.03 cm a^?1) and continuous minerogenic sedimentation throughout the Holocene in proglacial lakes, in contrast to organic‐rich sediments and low sedimentation rates (0.005 cm a^?1) in neighbouring non‐glacial lakes, suggest that glaciers may have persisted in proglacial lake catchments since regional deglaciation. The presence of varves and relatively high magnetic susceptibility from 10 to 6 ka and since 2 ka in one proglacial lake suggest minimum Holocene glacier extent ca. 6–2 ka. Moraine evidence and proglacial and threshold lake sediments indicate that the maximum Holocene glacier extent occurred during the Little Ice Age. The finding that glaciers likely persisted through the Holocene is surprising, given that regional proxy records reveal summer temperatures several degrees warmer than today, and may be due to shorter ablation seasons and greater accumulation‐season precipitation. Copyright © 2009 John Wiley & Sons, Ltd.     

Postglacial chironomid assemblage succession in northernmost Ungava Peninsula,Canada

Sediment cores spanning the postglacial period were recovered from two coastal freshwater basins located 300 km apart in northernmost Ungava (Nunavik, Canada). A basal date from the sequence recovered near Wakeham Bay suggests that the region was ice‐free by as early as 9600 cal. a BP. The initial development of both investigated lacustrine ecosystems was characterised by chironomid assemblages strongly dominated by the Subtribe Tanytarsina. These early postglacial assemblages showed greatly reduced diversity but much greater abundance than during subsequent periods of lake evolution. An abrupt and distinct turnover in sedimentary chironomid assemblage composition and productivity, marked by a switch to a dominance of Orthocladiinae, occurred around 6 ka BP. This turnover likely corresponds with a significant disturbance in the regional environment, which could be linked to the final disappearance of the last remnants of the Laurentide Ice Sheet. These findings, based on the first palaeolimnological investigations undertaken in northernmost Ungava, provide new insights into lateglacial and postglacial faunal recolonisation patterns in aquatic ecosystems of the eastern Canadian Arctic. Copyright © 2009 John Wiley & Sons, Ltd.     

Genesis of Disseminated Sulphide Assemblages in Niagara Peninsula,Canada

In the Niagara Peninsula of Southern Ontario, Canada, subeconomic occurrences of Mississippi Valley-type base metal assemblages are found in the petroliferous carbonate units of the Middle Silurian sedimentary succession. These units, which were deposited on the northwestern margin of the Appalachian Basin, were buried by about 2.4 km of Paleozoic sediments up to the time of the Appalachian orogeny. The sulphide minerals form 13 different assemblages and occur in veins, veinlets, vugs and stylolite seams. Distribution of the gangue minerals is irregular but the sulphide minerals display an overlapping vertical zonation. Although dolomite is more abundant in the Lockport Formation, the amount of dolomitization is not directly related to the degree of lead-zinc mineralization. The origin of the assemblages can be explained by a model that relates mineralization to the evolution of the Appalachian Basin. Minor quantities of sphalerite (in chert nodules) and pyrite (as framboids) precipitated during the early diagenetic stage. During later diagenesis, “hot” petroleum-bearing connate waters were expelled from sediments by compaction, and possibly later by the compressive forces of the Appalachian orogeny. As these waters moved outward and upward to zones of lower pressure, they became more saline and separated into liquid petroleum and connate brine phases. Petroleum, due to its lower density, preceded the general outward and upward movement of the connate brines to the basin margins. Here, the Middle Silurian carbonates of the Niagara Peninsula impeded the previous, relatively rapid flow, through the underlying clastics, and were locally enriched in petroleum as it moved into the immediately overlying Upper Silurian reservoirs. This petroleum provided source of reduced sulphur. During their passage, the connate brines probably leached metals from a variety of sediments of differing provenance. These metals precipitated as sulphides on reaching the zone of reduced sulphure. After uplift of the basin, the late Paleozoic strata were eroded and downward-moving meteoric groundwater leached metals, sulphates and carbonates from the host rocks. On reaching the reducing conditions of the water table, sulphides precipitated where the concentration of reduced sulphur was sufficiently high. This process, which is still continuing enables us to propose that many carbonate-hosted Pb-Zn deposits may have been enriched during periods of uplift.     

Slope hummock development, Fosheim Peninsula, Ellesmere Island, Nunavut, Canada

Slope hummocks, a type of nonsorted patterned ground, are composed of stratified, organic, silty sand, and develop through the interaction of niveo-eolian deposition, solifluction, slopewash, and vegetation growth. Fields of hummocks show consistent patterns: forms on convex slopes increase in height downslope until the channel is reached, whereas those on convexo-concave slopes increase on the upper convexity but are buried by niveo-eolian deposition downslope of the snowbank remnant. These trends can be reproduced using a simple numerical model based on measured slope and snow depth profiles, sediment concentrations in the snow and solifluction rates. The model indicates that hummocks transit slopes of 20-40 m in about 2-4 ka, a time-frame that is plausible given site emergence, measured rates of solifluction, and published dates for organic horizons within hummocks on northern Ellesmere Island. Sensitivity analyses show that long-term effect of climate warming on hummock heights may differ depending on whether it is accompanied by precipitation increase or decrease. The required combination of two-sided freezing to promote plug-like movement, incomplete vegetation cover and thin snow that enable eolian erosion during winter and spring, and vegetation growth in snow-bed sites to stabilize niveo-eolian deposits may explain why these forms are important regionally but apparently are not present throughout the Arctic.     

Lower Cambrian patch reefs and associated sediments: southern Labrador, Canada

The complex pattern of biological accretion, internal sedimentation, early lithification, and biological destruction, that characterizes modern reefs and many fossil reefs has been recognized in archaeocyathid-rich patch reefs of Lower Cambrian age in the Forteau Formation, southern Labrador. Patch reefs occur as isolated masses or complex associations of many discrete masses of archaeocyathid-rich limestone and skeletal lime sands, surrounded by well-bedded skeletal limestones and shales. Each reef is composed of many loafshaped mounds stacked on top of one another. The limestone of each mound comprises archaeocyathids and Renalcis or Renalcis-like structures in a matrix of argillaceous lime mud rich in sponge spicules, trilobite and salterellid skeletons. Numerous growth cavities roofed by pendant Renalcis-like organisms and Renalcis are partially to completely filled with geopetal sediment indicating that much of the matrix was deposited as internal sediment. Two stages of diagenetic alteration are recognized: (1) syn-depositional, which affected only the reefs, and (2) post-depositional, which affected both reefs and inter-reef sediments. On the sea floor reef sediments were pervasively cemented and fibrous carbonate was precipitated in intraskeletal and growth cavities. These limestones and cements as well as archaeocyathid skeletons, were subsequently bored by endolithic organisms. Later post-depositional subaerial diagenesis resulted first in dissolution of certain skeletons and precipitation of calcite cement above the water table, followed by extensive precipitation of pore-filling calcite below the water table. These carbonate reefs are similar in structure to the basal pioneer accumulations of much younger lower and middle Palaeozoic reefs. They did not develop into massive ‘ecologic’ reefs because archaeocyathids never developed the necessary large, massive, hemispherical skeletons. This occurrence indicates that reefs developed more or less coincident with, and not long after, the appearance of skeletal metazoans in the Lower Cambrian.     

Paleoecology of a >90,000-year lacustrine sequence from Fog Lake, Baffin Island, Arctic Canada

A 137 cm sediment core from Fog Lake, a small upland basin that was not glaciated during the Late Wisconsinan, reveals the following stratigraphic succession: (I) basal diamicton, (II) laminated silts, (III) compacted dewatered gyttja, (IV) stratified minerogenic sediments, and (V) overlying gyttja. Units I–III, and most of unit IV, were deposited prior to the Holocene; unit V represents approximately the last 8000 ¹⁴C years. Alternate luminescence dating techniques (TL and IRSL) place unit III in latter oxygen isotope stage (OIS) 5, >74,000 years BP. A suite of 20 AMS ¹⁴C dates suggests that much of the post-OIS 5 paleoenvironmental record is characterized by either extremely low continuous sediment accumulation, or by one or more episodes of more rapid sedimentation. Diverse pollen and spore assemblages have concentrations that are unexpectedly high throughout the pre-Holocene sediments (>10,000 grains cm⁻³), by comparison to the Holocene (5000 grains cm⁻³). Exceedingly low sediment accumulation rates appear to have resulted in the concentration of atmospherically derived pollen and spores. Diatom assemblages record the evolution of the lake's chemistry and past algal production. Several abrupt changes occurred synchronously in both the terrestrial and aquatic components of the system, and these appear to represent ecological responses to pre-Holocene climatic variability. The sediments of Fog Lake provide the first integrated paleoecological record older than Holocene for a vast region of the Canadian Arctic, thereby refining our understanding of long-term arctic ecosystem ontogeny.     

The deglaciation of Clyde Inlet,northeastern Baffin Island,Arctic Canada

The behaviour of ice sheets as they retreated from their Last Glacial Maximum (LGM) positions provides insights into Lateglacial and early Holocene ice‐sheet dynamics and climate change. The pattern of deglaciation of the Laurentide Ice Sheet (LIS) in arctic fiord landscapes can now be well dated using cosmogenic exposure dating. We use cosmogenic exposure and radiocarbon ages to constrain the deglaciation history of Clyde Inlet, a 120 km long fiord on northeastern Baffin Island. The LIS reached the continental shelf during the LGM, retreated from the coastal lowlands by 12.5 ± 0.7 ka (n = 3), and from the fiord mouth by 11.7 ± 2.2 ka (n = 4). Rapid retreat from the outer fiord occurred 10.3 ± 1.3 ka (n = 6), with the terminus reaching the inner fiord shortly after 9.4 ka (n = 2), where several moraine systems were deposited between ca. 9.4 and ca. 8.4 ka. These moraines represent fluctuations of the LIS during the warmest summers since the last interglaciation, and this suggests that the ice sheet was responding to increased snowfall. Before retreating from the head of Clyde Inlet, the LIS margin fluctuated at least twice between ca. 7.9 and ca. 8.5 ka, possibly in response to the 8.2 ka cold event. Copyright © 2006 John Wiley & Sons, Ltd.     

First Data on the Age of Metamorphic Schists from the Taz Peninsula (Arctic,Western Siberia)

Doklady Earth Sciences - The 40Ar/39Ar dating of metamorphic schists from the basement of the West Siberian Platform exposed by Lenzitskaya well no. 77 on the Taz Peninsula was performed. The...     

Anatomy of a cyclically packaged Mesoproterozoic carbonate ramp in northern Canada

Carbonates in the upper member of the Mesoproterozoic Victor Bay Formation are dominated by lime mud and packaged in cycles of 20–50 m. These thicknesses exceed those of classic shallowing-upward cycles by almost a factor of 10. Stratigraphic and sedimentological evidence suggests high-amplitude, high-frequency glacio-eustatic cyclicity, and thus a cool global climate ca. 1.2 Ga.The Victor Bay ramp is one of several late Proterozoic carbonate platforms where the proportions of lime mud, carbonate grains, and microbialites are more typical of younger Phanerozoic successions which followed the global waning of stromatolites. Facies distribution in the study area is compatible with deposition on a low-energy, microtidal, distally steepened ramp. Outer-ramp facies are hemipelagic lime mudstone, shale, carbonaceous rhythmite, and debrites. Mid-ramp facies are molar-tooth limestone tempestite with microspar-intraclast lags. In a marine environment where stromatolitic and oolitic facies were otherwise rare, large stromatolitic reefs developed at the mid-ramp, coeval with inner-ramp facies of microspar grainstone, intertidal dolomitic microbial laminite, and supratidal evaporitic red shale.Deep-subtidal, outer-ramp cycles occur in the southwestern part of the study area. Black dolomitic shale at the base is overlain by ribbon, nodular, and carbonaceous carbonate facies, all of which exhibit signs of synsedimentary disruption. Cycles in the northeast are shallow-subtidal and peritidal in character. Shallow-subtidal cycles consist of basal deep-water facies, and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies. Peritidal cycles are identical to shallow-subtidal cycles except that they contain a cap of dolomitic tidal-flat microbial laminite, and rarely of red shale sabkha facies or of sandy polymictic conglomerate. A transect along the wall of a valley extending 8.5 km perpendicular to depositional strike reveals progradation of inner-ramp tidal flats over outer- and mid-ramp facies during shoaling. The maximum basinward progradation of peritidal facies coincides with a zone of slope failure that may have promoted the development of the stromatolitic reefs.The sea-level history of the Victor Bay Formation is represented by three hectometre-scale sequences. An initial flooding event resulted in deposition of the lower Victor Bay shale member. Upper-member carbonate cycles were then deposited during highstand. Mid-ramp slumping was followed by late-highstand reef development. The second sequence began with development of an inner-ramp lowstand unconformity and a thick mid-ramp lowstand wedge. A second transgression promoted a more modest phase of reef development at the mid-ramp and shallow-water deposition continued inboard. A third and final transgressive episode eventually led to flooding of the backstepping ramp.Overall consistent cycle thickness and absence of truncated cycles, as well as the high rate and amount of creation of accommodation space, suggest that the periodicity and amplitude of sea-level fluctuation were relatively uniform, and point to a eustatic rather than tectonic mechanism of relative sea-level change. High-amplitude, high-frequency eustatic sea-level change is characteristic of icehouse worlds in which short-term, large-scale sea-level fluctuations accompany rapidly changing ice volumes affected by Milankovitch orbital forcing. Packaging of cyclic Upper Victor Bay carbonates therefore supports the hypothesis of a late Mesoproterozoic glacial period, as proposed by previous workers.     

Evidence of Early Metalworking in Arctic Canada

This paper examines new evidence related to an early (pre‐Columbian) European presence in Arctic Canada. Artifacts from archaeological sites that had been assumed to relate to pre‐Inuit indigenous occupations of the region in the centuries around A.D. 1000 have recently been recognized as having been manufactured using European technologies. We report here on the SEM‐EDS analysis of a small stone vessel recovered from a site on Baffin Island. The interior of the vessel contains abundant traces of copper–tin alloy (bronze) as well as glass spherules similar to those associated with high‐temperature processes. These results indicate that it had been used as a crucible. This artifact may represent the earliest evidence of high‐temperature nonferrous metalworking in the New World north of Mesoamerica.     

Radiocarbon Geochemistry of Modern and Ancient Arctic Lake Systems, Baffin Island, Canada

The accuracy of Arctic lake chronologies has been assessed by measuring the¹⁴C activities of modern carbon sources and applying these isotopic mass balances to dating fossil lake materials. Small (<1 km²) shallow (<25 m) Arctic lakes with watersheds <12 km²have soil and peat stratigraphic sections with¹⁴C activities ranging from 98 to 51% Modern. The¹⁴C activity of particulate organic carbon, dissolved organic carbon, and dissolved inorganic carbon from lake and stream waters ranges from 121 to 95% Modern. The sediment–water interface of the studied lakes shows consistent¹⁴C ages of ∼1000¹⁴C yr, although the¹⁴C activity of living aquatic vegetation is 115% Modern. Radiocarbon measurements of components of the lacustrine carbon pool imply that the ∼1000¹⁴C yr age of the sediment–water interface results from deposition of¹⁴C-depleted organic matter derived from the watershed.     

Late Pleistocene and Holocene vegetation and climate on the northern Taymyr Peninsula, Arctic Russia

Pollen data from a Levinson-Lessing Lake sediment core (74°28'N, 98°38'E) and Cape Sabler, Taymyr Lake permafrost sequences (74°33'N, 100°32'E) reveal substantial environmental changes on the northern Taymyr Peninsula during the last c. 32 000 [Formula: See Text]C years. The continuous records confirm that a scarce steppe-like vegetation with Poaceae, Artemisia and Cyperaceae dominated c. 32 000-10 300 [Formula: See Text]C yr BP, while tundra-like vegetation with Oxyria, Ranunculaceae and Caryophyllaceae grew in wetter areas. The coldest interval occurred c. 18 000 yr BP. Lateglacial pollen data show several warming events followed by a climate deterioration c. 10 500 [Formula: See Text]C yr BP, which may correspond with the Younger Dryas. The Late Pleistocene/Holocene transition, c. 10 300-10 000 [Formula: See Text]C yr BP, is characterized by a change from the herb-dominated vegetation to shrubby tundra with Betula sect. Nanae and Salix. Alnus fruticosa arrived locally c. 9000-8500 [Formula: See Text]C yr BP and disappeared c. 4000-3500 [Formula: See Text]C yr BP. Communities of Betula sect. Nanae, broadly distributed at c. 10 000-3500 [Formula: See Text]C yr BP, almost disappeared when vegetation became similar to the modern herb tundra after 3500-3000 [Formula: See Text]C yr BP. Quantitative climate reconstructions show Last Glacial Maximum summer temperature about 4°C below the present and Preboreal (c. 10 000 [Formula: See Text]C yr BP) temperature 2-4°C above the present. Maximum summer temperature occurred between 10 000 and 5500 [Formula: See Text]C yr BP; later summers were similar to present or slightly warmer.