ALBERT

Description: Paleo-sea-ice history in the Arctic Ocean was reconstructed using the sea-ice dwelling ostracode Acetabulastoma arcticum from late Quaternary sediments from the Mendeleyev, Lomonosov, and Gakkel Ridges, the Morris Jesup Rise and the Yermak Plateau. Results suggest intermittently high levels of perennial sea ice in the central Arctic Ocean during Marine Isotope Stage (MIS) 3 (25-45 ka), minimal sea ice during the last deglacial (16-11 ka) and early Holocene thermal maximum (11-5 ka) and increasing sea ice during the mid-to-late Holocene (5-0 ka). Sediment core records from the Iceland and Rockall Plateaus show that perennial sea ice existed in these regions only during glacial intervals MIS 2, 4, and 6. These results show that sea ice exhibits complex temporal and spatial variability during different climatic regimes and that the development of modern perennial sea ice may be a relatively recent phenomenon.

Description: In the Arctic Ocean, the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier Atlantic Layer by a halocline. Ongoing sea ice loss and warming in the Arctic Ocean have demonstrated the instability of the halocline, with implications for further sea ice loss. The stability of the halocline through past climate variations is unclear. Here we estimate intermediate water temperatures over the past 50,000 years from the Mg/Ca and Sr/Ca values of ostracods from 31 Arctic sediment cores. From about 50 to 11kyr ago, the central Arctic Basin from 1,000 to 2,500m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1-2°C warmer than modern Arctic Intermediate Water, with temperatures peaking during or just before millennial-scale Heinrich cold events and the Younger Dryas cold interval. We use numerical modelling to show that the intermediate depth warming could result from the expected decrease in the flux of fresh water to the Arctic Ocean during glacial conditions, which would cause the halocline to deepen and push the warm Atlantic Layer into intermediate depths. Although not modelled, the reduced formation of cold, deep waters due to the exposure of the Arctic continental shelf could also contribute to the intermediate depth warming. © 2012 Macmillan Publishers Limited. All rights reserved.

Description: Stable isotopic values on planktonic foraminifera in a suite of cores from basins across the SE Baffin Shelf are used to extract a record of meltwater events during Termination I deglaciation. Resolution and Hatton basins lie on the SE Baffin Shelf at water depths 〉 500 m, seaward of major conduits for ice drainage from the eastern sector of the Laurentide Ice Sheet (LIS). Accelerator mass spectrometry 14C dates are used to constrain our chronology of events in ten cores. In Resolution Basin, three cores have 14C AMS dates on foraminifera of 〉 20 ka at their bases; whereas Hatton Basin cores terminate in sediments 〈 13 kyr. Sedimentation rates varied between 0.1 to 4.5 m/ka. Stable oxygen and carbon isotopic ratios were obtained on 146 samples of the planktonic foraminifera Neogloboquadrina pachyderma (Ehrenberg) sinistral, from seven of the ten cores. No evidence was found to indicate that test morphology or size affected ∂18O. Between 7 and 13.5 ka the surface water on the shelf was on average 1 ‰ lower than the open ocean signal. Significant temporal variations were found in both ∂18O and ∂13C. Evidence for significant low ∂18O events occurred between 13 and 8 ka. The ∂13C record from the planktonic foraminifera suggests a threefold division of events between 13 and 7 ka, with positive values between 10.8 and 13.0 ka, negative values between 9 and 10.8 ka, and positive values from 7 to 9 ka. The ∂18O data suggest the presence of meltwater on the shelf some 3,000 years prior to the first late glacial dates on terrestrial deglaciation (at circa 10.4 ka). “Hudson Strait must be the real key to the importance of the calving process during deglaciation, because it is potentially the largest marine outlet for the Laurentide Ice Sheet and because it leads into the very center of the ice sheet.....the rates of calving through Hudson Strait during the period of initial ∂18O rise unfortunately are unknown.”

Description: In the Arctic Ocean, the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier Atlantic Layer by a halocline. Ongoing sea ice loss and warming in the Arctic Ocean1, 2, 3, 4, 5, 6, 7 have demonstrated the instability of the halocline, with implications for further sea ice loss. The stability of the halocline through past climate variations8, 9, 10 is unclear. Here we estimate intermediate water temperatures over the past 50,000 years from the Mg/Ca and Sr/Ca values of ostracods from 31 Arctic sediment cores. From about 50 to 11 kyr ago, the central Arctic Basin from 1,000 to 2,500 m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1–2 °C warmer than modern Arctic Intermediate Water, with temperatures peaking during or just before millennial-scale Heinrich cold events and the Younger Dryas cold interval. We use numerical modelling to show that the intermediate depth warming could result from the expected decrease in the flux of fresh water to the Arctic Ocean during glacial conditions, which would cause the halocline to deepen and push the warm Atlantic Layer into intermediate depths. Although not modelled, the reduced formation of cold, deep waters due to the exposure of the Arctic continental shelf could also contribute to the intermediate depth warming.

Description: Proxy records from Arctic Ocean sediment cores show that major paleogeographic changes occurred during the last glacial-interglacial cycle, but there is minimal data on Arctic Ocean temperature history. Mg/Ca ratios in the calcitic shells of Krithe, a benthic marine ostracode characteristic of deep-sea and Arctic continental shelf environments, have been used to reconstruct bottom water temperature (BWT) in the North Atlantic (Dwyer et al. 1995, Cronin et al. 1996). We analyzed Mg/Ca and Sr/Ca ratios in more than 500 specimens of K. glacialis and K. minima from 114 coretops in the Arctic Ocean and Nordic Seas to improve the Mg/Ca–temperature calibration and to evaluate the influence of other factors on Mg/Ca and Sr/Ca ratios (e.g. vital effects, carbonate ion concentration). Mg/Ca concentrations range from 6 to 13 mmol/mol and exhibit a positive correlation to temperature from -1.5 to 0.5ºC (r 2=0.4) with a sensitivity of 0.471 mmol/mol/ºC. Temperature, or temperature-related factors affecting physiology, molting and/or calcification processes, appear to be an influence on Mg/Ca variability. Carbonate ion shows no apparent relationship to Mg/Ca at ∆[CO3-2] values from -20 to 70 μmol/kg, however Sr/Ca ratios are positively correlated to ∆[CO3-2] (r2=0.5). We applied Mg/Ca paleothermometry for K. glacialis and K. minima to 32 sediment cores from the central Arctic Ocean (Lomonosov, Mendeleyev, Gakkel Ridges) and the Iceland Plateau. Marine Isotope Stage 3 (MIS3, 60-25 ka) Mg/Ca ratios at mid-depth sites (1000-2600 m water depth) average 2 to 8 mmol/mol higher than those in the late Holocene suggesting MIS3 BWTs were 1-3 ̊C warmer. In contrast, at core sites below 3000 meters, Mg/Ca ratios indicate little or no BWT change during MIS 3. Warmer mid-depth MIS 3 BWTs are consistent with oxygen isotope evidence for glacial-age elevated BWTs in the Iceland Sea (Bauch t al. 2001). Mid-depth Arctic Ocean warming most likely involves changes in the depth, circulation or temperature of the warm Atlantic Layer (AL). Possible mechanisms include AL depth suppression due to ice cover (Jakobsson et al. 2010) and/or higher AL temperatures due to enhanced Atlantic Meridional Overturning Circulation. Hypothesized elevated Arctic and Nordic Sea MIS3 BWTs can be tested against other proxies, with better radiocarbon chronology to determine if BWT warming occurred during interstadials or stadials, and in comparison to extra-Arctic paleoclimate records.