Oxygen-isotope Variations in Post-glacial Lake Ontario

The role of glacial meltwater input to the Atlantic Ocean in triggering the Younger Dryas (YD) cooling event has been the subject of controversy in recent literature. Lake Ontario is ideally situated to test for possible meltwater passage from upstream glacial lakes and the Laurentide Ice Sheet (LIS) to the Atlantic Ocean via the lower Great Lakes. Here, we use the oxygen-isotope compositions of ostracode valves and clam shells from three Lake Ontario sediment cores to identify glacial meltwater contributions to ancient Lake Ontario since the retreat of the LIS (~16,500 cal [13,300 14C] BP). Differences in mineralogy and sediment grain size are also used to identify changes in the hydrologic regime. The average lakewater δ18O of –17.5 ‰ (determined from ostracode compositions) indicates a significant contribution from glacial meltwater. Upon LIS retreat from the St. Lawrence lowlands, ancient Lake Ontario (glacial Lake Iroquois) lakewater δ18O increased to –12 ‰ largely because of the loss of low-18O glacial meltwater input. A subsequent decrease in lakewater δ18O (from –12 to –14 ‰), accompanied by a median sediment grain size increase to 9 μm, indicates that ancient Lake Ontario received a final pulse of meltwater (~13,000-12,500 cal [11,100-10,500 14C] BP) before the onset of hydrologic closure. This meltwater pulse, which is also recorded in a previously reported brief freshening of the neighbouring Champlain Valley (Cronin et al., 2012), may have contributed to a weakening of thermohaline circulation in the Atlantic Ocean. After 12,900 cal [11,020 14C] BP, the meltwater presence in the Lake Ontario basin continued to inhibit entry of Champlain seawater into early Lake Ontario. Opening of the North Bay outlet diverted upper Great Lakes water from the lower Great Lakes causing a period (12,300-8,300 cal [10,400-7,500 14C] BP) of hydrologic closure in Lake Ontario (Anderson and Lewis, 2012). This change is demarcated by a shift to higher δ18Olakewater (~ –7 ‰), driven in part by strong evaporative conditions in the Ontario basin and in part by increasing δ18Oprecipitation at this time. The δ18Olakewater then fluctuated only slightly upon the eventual return of the upper Great Lakes water during the Nipissing phase at 5,800 cal [5,090 14C] cal BP (Anderson and Lewis, 2012), after which shelly fauna are no longer preserved in the sediment record

An Overview of Anorthosite-bearing Layered Intrusions in the Archaean Craton of Southern West Greenland and the Superior Province of Canada: Implications for Archaean Tectonics and the Origin of Megacrystic Plagioclase

Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (\u3eAn70) megacrystic (2–30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25–35%) in the hotter (1500–1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth

Nitrogen Isotopes Suggest a Change in Nitrogen Dynamics between the Late Pleistocene and Modern Time in Yukon, Canada

A magnificent repository of Late Pleistocene terrestrial megafauna fossils is contained in ice-rich loess deposits of Alaska and Yukon, collectively eastern Beringia. The stable carbon (δ13C) and nitrogen (δ15N) isotope compositions of bone collagen from these fossils are routinely used to determine paleodiet and reconstruct the paleoecosystem. This approach requires consideration of changes in C- and N-isotope dynamics over time and their effects on the terrestrial vegetation isotopic baseline. To test for such changes between the Late Pleistocene and modern time, we compared δ13C and δ15N for vegetation and bone collagen and structural carbonate of some modern, Yukon, arctic ground squirrels with vegetation and bones from Late Pleistocene fossil arctic ground squirrel nests preserved in Yukon loess deposits. The isotopic discrimination between arctic ground squirrel bone collagen and their diet was measured using modern samples, as were isotopic changes during plant decomposition; Over-wintering decomposition of typical vegetation following senescence resulted in a minor change (~0±1 ½) in δ13C of modern Yukon grasses. A major change (~2±10 ½) in δ15N was measured for decomposing Yukon grasses thinly covered by loess. As expected, the collagen-diet C-isotope discrimination measured for modern samples confirms that modern vegetation δ13C is a suitable proxy for the Late Pleistocene vegetation in Yukon Territory, after correction for the Suess effect. The N-isotope composition of vegetation from the fossil arctic ground squirrel nests, however, is determined to be ~2.8‰ higher than modern grasslands in the region, after correction for decomposition effects. This result suggests a change in N dynamics in this region between the Late Pleistocene and modern time

Giant Beaver Palaeoecology Inferred from Stable Isotopes

This is a multi-individual (n = 11), stable carbon and nitrogen isotope study of bone collagen (δ13Ccol and δ15Ncol) from the giant beaver (genus Castoroides). The now-extinct giant beaver was once one of the most widespread Pleistocene megafauna in North America. We confirm that Castoroides consumed a diet of predominantly submerged aquatic macrophytes. These dietary preferences rendered the giant beaver highly dependent on wetland habitat for survival. Castoroides’ δ13Ccol and δ15Ncol do not support the hypothesis that the giant beaver consumed trees or woody plants, which suggests that it did not share the same behaviours as Castor (i.e., tree-cutting and harvesting). The onset of warmer, more arid conditions likely contributed to the extinction of Castoroides. Six new radiocarbon dates help establish the chronology of the northward dispersal of the giant beaver in Beringia, indicating a correlation with ice sheet retreat

Stable Isotopes of Clay Minerals from Autoclave Tests of Oil Sands: Implications for Clay Formation during Steaming of Alberta Clearwater Oil Sands

In an effort to evaluate mineral-water isotopic exchange during cyclic steam stimulation (CSS), solutions and\u3c2 μm berthierine-dominated solids from the Clearwater Formation oil sands of Alberta, Canada were analyzed for stable isotope compositions before and after reaction in autoclaves for 1008 h at 250 °C. There was no significant change in solution δ18O and δ2H, which is consistent with the high water/mineral ratio used in the experiments. The solids showed a marked decrease in both δ18O and δ2H following the experiments. Pre-run solids have δ18O of +9.5 to +12.9‰and δ2H of −114 to −113‰, whereas post-run solids have δ18O of −4.7 to +2.1‰ and δ2H of −147 to −128‰. Neither oxygen- nor hydrogen-isotope equilibrium was established between the solids and the solutions. Calculation suggests that oxygen-isotope exchange (44–58%) was greater between the solids and the solutions than was the case for hydrogen isotopes (23–50%). We propose that this behaviour resulted from partial inheritance of the pre-run berthierine structure during formation of the post-run smectite, chlorite-smectite and chlorite. This process confounds the use of clay mineral stable isotope compositions as a temperature indicator of in situ steam/steam condensate interaction with oil-sands reservoirs. The results also suggest an additional mechanism by which new clay minerals can be formed during CSS-related, artificial diagenesis

Stable isotope investigation of the migratory behavior of silverhaired bats (Lasionycteris noctivagans) in eastern North America

We investigated the migratory movements of silver-haired bats (Lasionycteris noctivagans) across the eastern extent of the species’ range. We conducted stable hydrogen isotope analysis of fur samples (δ2Hfur) from museum specimens collected across latitudes and at all times of the year. We first used these data to estimate the timing of fur replacement and to develop a model associating δ2Hfur with that of local precipitation (δ2Hprecip) at the location where fur replacement occurred. We then used this model to 1) identify individuals that had migrated across latitudes, and 2) investigate the presence of continental-scale patterns in the estimated distance traveled. Bats were at their location of fur replacement between 20 June and 26 August, and there was a strong linear relationship between δ2Hfur and δ2Hprecip in bats collected during this time. There was substantial variation in the migratory movements of this species. Twenty-four of 38 females and 14 of 30 males showed isotopic evidence of leaving the area where fur replacement occurred (i.e., migrating across latitudes), whereas the remaining bats were either sedentary or moved at a small spatial scale. Males appeared to migrate consistently, regardless of latitude of origin, whereas there was a partial leapfrog pattern in migratory movements of females. To our knowledge, this is the first evidence of leapfrog migration in bat

Maize Provisioning of Ontario Late Woodland Turkeys: Isotopic Evidence of Seasonal, Cultural, Spatial and Temporal Variation

The isotopic composition (δ13C, δ15N) of bone collagen from Ontario Late Woodland archaeological turkeys was compared with that of modern Ontario wild turkeys, and archaeological turkeys from American Southwestern, Mexican and other Woodland sites to determine whether Late Woodland Ontario peoples managed wild turkeys by provisioning them with maize, the only isotopically distinct horticultural plant at that time. Despite the fact that humans from Late Woodland Western Basin and Iroquoian traditions consumed equal amounts of maize, wild turkeys utilized by the two groups exhibit different diets. Western Basin turkeys reflect a C3-only diet, whereas Iroquoian turkeys were consuming significant quantities of maize (a C4 plant). Both groups of archaeological turkey consumed less maize than modern wild turkeys with access to waste left in fields by mechanized agriculture, but because ancient crop yields were much lower, we suggest that Iroquoian turkeys must have been provisioned, probably to create a reliable and nearby hunting niche (Linares 1976). Archaeological and isotopic evidence supports ethnohistoric accounts that turkeys were hunted after the fall harvest. Iroquoian archaeological turkey diets, in general, reflect the seasonal consumption of maize that would have been created by cold weather maize provisioning, with the major exception of one turkey from an Attawandaron (Neutral) site that appears to have been fed maize year round. Motivations for provisioning by Middle Ontario Iroquoian people likely included climate change and ritual/ceremonial activity as well as a reliable food supply. Because Iroquoian women controlled the harvest, it is likely that they were instrumental in altering this human/animal interaction, creating a position on the wild/domesticated continuum that is unique in the North American archaeological literature

Petrology and Geochemistry of the Tasse Mantle Xenoliths of the Canadian Cordillera: A Record of Archean to Quaternary Mantle Growth, Metasomatism, Removal, and Melting

Mantle xenoliths hosted by the Quaternary Tasse alkaline basalts in the Canadian Cordillera, southeastern British Columbia, are mostly spinel lherzolite originating from subcontinental lithospheric mantle. The xenoliths contain abundant feldspar veins, melt pockets and spongy clinopyroxene, recording extensive alkaline metasomatism and partial melting. Feldspar occurs as veins and interstitial crystal in melt pockets. Melt pockets occur mainly at triple junctions, along grain boundaries, and consist mainly of olivine, cpx, opx and spinel surrounded by interstitial feldspar. The Nd, Sr and Pb isotopic compositions of the xenoliths indicate that their sources are characterized by variable mixtures of depleted MORB mantle and EM1 and EM2 mantle components. Large variations in εNd values (−8.2 to +9.6) and Nd depleted mantle model ages (TDM=66 to 3380 Ma) are consistent with multiple sources and melt extraction events, and long-term (\u3e 3300 Ma) isolation of some source regions from the convecting mantle. Samples with Archean and Paleoproterozoic Nd model ages are interpreted as either have been derived from relict Laurentian mantle pieces beneath the Cordillera or have been eroded from the root of the Laurentian craton to the east and transported to the base of the Cordilleran lithosphere by edge-driven convection currents. The oxygen isotope compositions of the xenoliths (average δ18O=+5.1 ± 0.5‰) are similar to those of depleted mantle. The average δ18O values of olivine (+5.0 ± 0.2‰), opx (+5.9 ± 0.6‰), cpx (+6.0 ± 0.6‰) and spinel (+4.5 ± 0.2‰) are similar to mantle values. Large fractionations for olivine-opx, olivine-cpx and opx-cpx pairs, however, reflect disequilibrium stemming from metasomatism and partial melting. Whole-rock trace element, Nd, Sr, Pb and O isotope compositions of the xenoliths and host alkaline basalts indicate different mantle sources for these two suites of rocks. The xenoliths were derived from shallow lithospheric sources, whereas the alkaline basalts originated from a deeper asthenospheric mantle source

Agriculture causes nitrate fertilization of remote alpine lakes

Humans have altered Earth’s nitrogen cycle so dramatically that reactive nitrogen (Nr) has doubled. This has increased Nr in aquatic ecosystems, which can lead to reduced water quality and ecosystem health. Apportioning sources of Nr to specific ecosystems, however, continues to be challenging, despite this knowledge being critical for mitigation and protection of water resources. Here we use D17O, d18O and d15N from Uinta Mountain (Utah, USA) snow, inflow and lake nitrate in combination with a Bayesian-based stable isotope mixing model, to show that at least 70% of nitrates in aquatic systems are anthropogenic and arrive via the atmosphere. Moreover, agricultural activities, specifically nitrate- and ammonium-based fertilizer use, are contributing most (B60%) Nr, and data from other North American alpine lakes suggest this is a widespread phenomenon. Our findings offer a pathway towards more effective mitigation, but point to challenges in balancing food production with protection of important water resources

Limiting the impact of destructive analytical techniques through sequential microspatial sampling of the enamel from single teeth

A fundamental research concern within contemporary bioarchaeology is the sensitive balance between the preservation of human remains and the use of destructive techniques to collect information. Here we describe one example of how multiple microspatial destructive/semi-destructive techniques may be carried out in sequence using only the enamel of a single tooth. With careful planning of both sample preparation strategies and sequencing of sampling methods, it is possible to produce multiple datasets, and yet to retain material for future analyses. In this case, enamel from the teeth of 27 individuals who lived during the early medieval period (AD 1170-1198) in Bergen, Norway, were subjected to histological, trace element (LA-ICP-MS), diagenetic (FTIR), and isotopic analyses (δ18O and δ13C, via micromill/multiprep/IRMS)