Observations on the Late Holocene Dynamics of the Boreal Geomagnetic Field From Ellesmere Island Lake Sediments
The geomagnetic field of the Arctic is complex, poorly understood and a key component of the global system. A prominent feature of the Boreal geomagnetic field is the accelerated motion of the North Magnetic Pole (NMP) away from North America having moved more than 1600 km in the last century. Here we present sediment paleomagnetic observations from Ellesmere Island lakes, calibrated by 400-year historic geomagnetic record, that allow us to constrain polar motion and begin to assess the geomagnetic dynamics that drive these changes over the last several thousand years. Our primary data sets are derived from two Ellesmere Island lakes (Sawtooth Lake, 79.21 N, 83.56 W and Lower Murray Lake, 81.34 N, 69.54 W) that preserve strong, stable, single component remanent magnetizations and possess independently derived varve based chronologies. U-channel paleomagnetic data obtained from multiple cores taken from each lake allow the development of composite directional secular variation and relative paleointensity records. These observations are supported by paleomagnetic data from other Canadian Arctic Archipelago lakes that lack either independent age control or replicate-coring. These data demonstrate that the Arctic geomagnetic field over the late Holocene is dominated by the same complex boundary conditions that influence the historic record including intensity variations of the high latitude flux patches and the westward drift of the polar geomagnetic vortex. The ongoing motion of the NMP appears to reflect the continuation of a millennial scale Late Holocene oscillation that is part of a significant geomagnetic change.
Sedimentary Record of the Last two Interglacials in the Terrestrial Canadian Arctic (Pingualuit Crater Lake, Nunavik)
The Pingualuit crater lake (Nunavik, Canada) resulted from a meteoritic impact that occurred ca. 1.4 million years ago. Due to its unique morphometry (depth and shape), the lake bottom may have escaped glacial erosion. Based on a punctual seismic profile acquired using a 12 kHz Knudsen echosounder and using both gravity and piston corers, we recovered the uppermost 8.5 m of sediments. High-resolution physical (CAT- Scan, Multi Sensor Core Logger, diffuse spectral reflectance), geochemical (ITRAX core scanner, carbon and nitrogen contents, δ13C of the organic matter) and magnetic (magnetic susceptibility, natural, anhysteretic, isothermal and saturation isothermal remanent magnetizations) analyses were performed. Two main lithofacies were clearly identified by the different measurements and likely represent successive interglacial/glacial cycles. Most of the sediment consists of light grey silts containing several angular rock fragments, that is characterized by very low organic carbon content, relatively high density and magnetic susceptibility values, suggesting a deposition during glacial conditions. Interbedded between this facies are at least two decimetre-thick, organic-rich and finely laminated intervals likely representing ice free periods. The presence of diatoms, chrysophytes and chironomid head capsules in smear and microscope slides from these two intervals supports this hypothesis. In addition, preliminary Infrared Stimulated Luminescence (IRSL) measurements indicate that the upper organic-rich layer has an age coeval with the last interglacial (Oxygen Isotope Stage 5), while the age of the lower organic-rich layer is consistent with an older interglacial, likely the Oxygen Isotope Stage 7. The sedimentary infill thus constitutes a unique long-term terrestrial record of environmental and climatic conditions in the Canadian Arctic. Furthermore, because these sediments escaped glacial erosion, it suggests the presence of a subglacial lake during the last two glacial periods. Micro- sedimentological analyses of thin sections are currently underway to define the depositional environments of each laminated interval.
Sedimentary Regimes in the Northern Mendeleev Ridge During the Last Climatic Cycles
Due to low productivity and low sedimentation rates in the central Arctic Ocean, the setting of chronostratigraphy in deep sea-cores is a challenge. 14C stratigraphies are limited to the upper few centimeters, whereas oxygen isotopes in foraminifera are inconclusive due to both gaps in their records and the impact of isotopically light brines linked to sea-ice formation on their oxygen composition. To document further this issue, we use cores raise from two sites, with water depth of ∼1600 m (site 12) and ∼2500 m (site 11) respectively, in the northern Mendeleev Ridge area, during the 2005 HOTRAX cruise in the Arctic. Grain size, mineralogical (dolomite vs. calcite contents, clay minerals), geochemical (Corg, Cinorg, 13Corg, Corg/Ntotal ratios) and isotopic (210Pb, 238U, 234U, 232Th, 230Th, 231Pa, 137Cs) measurements were made on grounded bulk sediment, whereas when present, foraminifera were analyzed for their 13C and 18O contents. The first striking feature depicted is the remarkable similarity of sedimentological and geochemical profiles at both sites despite their bathymetric difference. 210Pb and 137Cs data indicate a mixed layer restricted to the top cm. Foraminifera from this layer yielded a 14C age of 8500 a (Libby's years) suggesting high fluxes during the early Holocene thermal optimum. Below this layer, scattered foraminiferal shells down to about 12 cm, yielded Libby's ages ranging 27-33 ka with reversals suggesting that this section likely incorporates reworked material. By combining all information available, two drastically distinct sedimentary regimes may be inferred in this area. One corresponds mostly to ice-rafted deposition (IRD), as indicated by high sand, but low carbon contents, and the absence of foraminifer shells. The second regime is characterized by high contents in foraminifer shells, clays, Corg. and Cinorg, as well as high excesses in 231Pa and 230Th linked to high scavenging rates in the overlying water column. From this information, a rough stratigraphic frame has been set in these cores. A discrete interglacial interval assigned to MIS (Marine Isotopic Stage) is found at about 16 to 19 cm in both multicores and a more pronounced MIS 7 interglacial is seen below, from 26 to about 37 cm. Both yielded high 13C-foraminiferal calcite expected under interglacial conditions. Sedimentation rates varying between appr. 1 to 3 mm/ka are proposed in this sector of the Mendeleev Ridge.
Arctic Amplification: Can the Past Constrain the Future?
Arctic climate is influenced by a suite of strong positive feedbacks, particularly changes in sea ice and snow cover (fast) and terrestrial ice sheets (slow). Similarly strong negative feedbacks that might stabilize Arctic climate are not known, so physical understanding indicates that climate changes should be amplified in the Arctic relative to lower latitudes. Arctic amplification in the contemporary world is nearly universal in climate model simulations forced by increasing concentrations of greenhouse gases, but its magnitude is poorly constrained. Past times when the Arctic was unusually warm or cold, due primarily to various combinations of insolation (Insol) and greenhouse gas (GHG) forcings, may inform the debate over the magnitude of Arctic amplification. Arctic summer temperature anomalies can be compared to the Northern Hemisphere average anomalies for the Holocene thermal maximum (HTM; Insol), last glacial maximum (LGM; Insol and GHG), and last interglacial (LIG; Insol and weak GHG) because of the similar forcing across the Northern Hemisphere, whereas the warmth of the Middle Pliocene (MP; mostly GHG) may be compared with hemispheric or global averages. HTM summer temperature anomalies have been summarized recently for much of the Arctic, indicating an Arctic warming of +1.7 +/- 0.8 C, compared to a Northern Hemisphere warming of +0.5 +/- 0.3 C. Similarly, the temperature anomaly for LGM Arctic cooling was -18 +/- 7 C, whereas Northern Hemisphere cooling was -5 +/- 2 C, LIG Arctic warmth was +5 +/- 1 C whereas Northern Hemisphere warmth was +1 +/- 1 C, and MP Arctic warmth was +12 +/- 3 C, whereas hemispheric and global warmth was 4 +/- 2 C. Within the stated uncertainties these paleoreconstructions are consistent with an Arctic amplification of ca. 3.5 times the hemispheric average anomaly, despite a wide range of forcings and variable contributions from slow and fast feedbacks. Because these feedbacks are still active, it is likely that future Arctic amplification will be of a similar magnitude.
Holocene variations of sea-ice cover and sea-surface temperature in the western Arctic, from the Fram Strait to the Chukchi Sea
The Northern Hemisphere sea ice record of the last decades shows large amplitude changes, which can be associated to the global warming. However, the anomalies also show regional patterns, which illustrate the importance of atmosphere and ocean dynamics in the distribution of sea ice. Beyond satellite observations, proxy data such as those provided by dinocyst assemblages permit to extent the record of sea ice back in time. Sea ice reconstructions spanning the last millennia are now available at several sites of the Arctic Ocean and subarctic seas (Fram Strait, Baffin Bay, Canadian Arctic Archipelago channels, Beaufort Sea and Chukchi Sea). The data illustrate large amplitude temporal trends but more importantly, opposite trends between the eastern and western Arctic. In particular, the results show a clear mid-Holocene to recent cooling accompanied with sea ice increase in the northern Baffin Bay and eastern Fram Strait. This contrasts with the changes recorded in the western part of the Canadian Arctic and Chukchi Sea, where a warming trend towards the late Holocene is accompanied with a decrease in the sea ice cover. In addition, when suitable resolution is achieved, the results indicate large amplitude centennial to millennial scale oscillations of sea-surface conditions and sea- ice cover, pointing to complex atmosphere/ocean interactions, at regional scale, in the Arctic.
Evolution Of Late Holocene Sea-Surface Parameters In The Beaufort Sea Area (Mackenzie Slope, Canadian Arctic): Insights From CASES 2004-804-803 Cores
This study aims at reconstructing late Holocene sea-surface parameters in the Beaufort Sea area (Western Canadian Arctic) on the basis of sedimentary cores collected over the Mackenzie Slope. Piston, trigger and box cores were sampled at station 803 in 2004 aboard the CCGS Amundsen (CASES) at 218 m water depth. Sedimentation at this particular location is influenced by both the Beaufort gyre and the Mackenzie River, whose sedimentary discharge is by far the largest among all other Arctic rivers. The chronology of the piston core is constrained by 4 AMS-14C dates, as the sedimentation rate in the box core is assessed from 210Pb data. We obtain a continuous composite sequence covering the last 4600 years, with a sedimentation rate of ∼140 cm/kyr. Palynological data reveal that dinocyst assemblages are dominated by Operculodinium centrocarpum (mean of 43.3%), with the accompanying taxa Brigantedinium spp. (19.6%), Islandinium minutum (15.6%) and cysts of Pentapharsodinium dalei (13.7%). Four zones have been established on the basis of dinocyst relative abundances, with the following key taxa. Zone I (4600-4000 cal BP): high relative abundances of cysts of P. dalei (mean of 14.9%) and I. minutum var. cezare (up to 8.9%); zone II (4000-2600 cal BP): high abundances of Spiniferites frigidus/elongatus (up to 7.3%); zone III (2600-1600 cal BP): well- represented heterotrophic taxa Brigantedinium spp. (30.8%) and cysts of Polykrikos sp. arctic/quadratus (up to 4.2%); zone IV (1600 cal BP - present): high relative abundances of I. minutum (mean of 19.8%) and cysts of P. dalei (17.3%). Quantitative reconstructions of sea-surface parameters indicate relatively stable conditions from 4600 to 1600 cal BP. Conversely, a trend of increasing summer (August) sea-surface temperature (from ∼5° C to ∼6° C; actual value = 6° C), increasing salinity (from ∼20 to ∼26 psu; modern value = 19 psu) and decreasing sea-ice cover (from ∼9 to 8 month/yr; actual value = 10) is observed over the last 1600 yrs. These data are consistent with similar studies held in adjacent areas, describing the warming of Western Canadian Arctic, in comparison with a cooling Eastern Arctic. Prevailing hydroclimatic cycles in the Beaufort Sea area are also investigated on the basis of freshwater input indicators concentrations, using spectral and wavelet analysis. Potential links with the Arctic Oscillation and the Pacific Decadal Oscillation are discussed.
Radiogenic isotope signatures of sediment sources vs. water masses in Late Holocene sediments from Fram Strait
Exchanges of water-masses between the Arctic and the North Atlantic Ocean occur through Fram Strait and the Barents Sea. Here, we investigate geochemical signatures in the Fram Strait area based on radiogenic isotope measurements of i) surface sediments recovered along an E-W transect, and ii) two late Holocene sequences recording sediments supplied by the incoming North Atlantic waters (multicore MC-04) and the outflowing Arctic waters (MC-16), respectively located in the east and west of the Strait. Sediments <100μm were leached in order to separate their "authigenic" fractions, which carry a water-mass signature scavenged mostly by Fe-Mn oxyhydroxides, from the "inherited" detrital fractions that reflect sediment sources. Authigenic leachates and initial bulk sediments were analysed for their major and trace element concentrations as well as for their Pb and Nd isotopic compositions. The mass budget of the leaching procedure is approximately balanced, both in content and isotopic composition, within ±15% uncertainty (2σ). The fraction of each element removed by leaching varies between elements. Particle-reactive elements such as Pb, Nd and Th display relatively similar behaviour (although Pb is 3 times more easily leached than Nd and Th), in comparison with that of Fe and Mn. Isotopic compositions of surface sediments show three clusters representative of, respectively, the western, central and eastern Fram Strait areas. In the east, MC-04 displays nearly constant detrital isotopic compositions during the last 2.5 ka, not unlike those reported in the Yermak Plateau by Tutken et al. (2002). These isotopic compositions suggest variable mixing between two major end-members, the Atlantic Crust (Abouchami et al. 1999) and the European PanAfrican Crust (Fagel et al. 2004). Pb isotopes in the western core, MC-16, as well as in surface sediments from the central area of the Strait, suggest mixing between an Arctic source (cf. Alpha-Ridge; Winter et al. 1997) and the North Atlantic component, suggesting either some water-mass mixing in the central Fram Strait, and/or the superimposition of an Arctic imprint provided by the gyre of the North Atlantic Water mass in this basin.
Revisited Inventory of Glaciers on Axel Heiberg Island, Nunavut
As documented in the IPCC's Climate Change 2007 report, the high latitude regions of the Northern Hemisphere are experiencing the highest rates of warming. Given that 35% of the global glacial ice exists within the Arctic Archipelago, this region provides an excellent laboratory for monitoring the anticipated degree of glacial recession . Evidence of arctic warming through negative mass balance trends has been detected in several studies already [e.g., 2]. Here, we show the importance and value of historical records in the task of monitoring glacial retreat. A highly detailed inventory developed by S. Ommanney in 1969 , has been revisited and transformed into digital format for the purposes of integration with modern inventories. The Ommanney inventory covers the entirety of Axel Heiberg Island , NU, and includes details often lacking in present day inventories, including orientations (accumulation and ablation zones), elevations (highest, lowest, elevation of the snowline, and the mean elevations of both the accumulation and ablation areas), length (of the ablation area, exposed ice, and of the total glacier including debris cover), area (of the ablation area, exposed ice, and of the total glacier), accumulation area ratio (AAR), depth, volume, and a six digit code which gives qualitative details on glacier attributes. This report is one of the most thorough and comprehensive glacier inventory report ever published in Canada. More recent inventories used for comparison include the glacier extents created by the National Topographic System based on photography from 1980-1987, as well as extents developed by Dr. Luke Copland for the Global Land Ice Measurements from Space (GLIMS) database using 1999-2000 satellite imagery. Our preliminary results show that approximately 90% of ice bodies under 0.2kmē on Axel Heiberg Island have disappeared entirely in the 40 year period of interest. The issue of glacier definition will be discussed as a possible cause of these drastic changes in the status of small, remnant glaciers. Recession trends will also be discussed with respect to glacier characteristics and regional distribution.  Barry, R. G., Progress in Physical Geography, 2006. 30(3): p. 285-306;  Dowdeswell, J. A., et al., Quaternary Research, 1997. 40: p. 1-14;  Ommanney, C. S. L., McGill Subarctic Research Paper, 1969. 40: p. 5-67.