Condrona, A., and P. Winsor, 2012: “Meltwater routing and the Younger Dryas.” Proceedings of the National Academy of Sciences, v. 109, 19,928-19,933, doi: 10.1073/pnas.1207381109.
The Younger Dryas—the last major cold episode on Earth—is generally considered to have been triggered by a meltwater flood into the North Atlantic. The prevailing hypothesis, proposed by Broecker et al. [1989 Nature 341:318-321] more than two decades ago, suggests that an abrupt rerouting of Lake Agassiz overflow through the Great Lakes and St. Lawrence Valley inhibited deep water formation in the subpolar North Atlantic and weakened the strength of the Atlantic Meridional Overturning Circulation (AMOC). More recently, Tarasov and Peltier [2005 Nature 435:662-665] showed that meltwater could have discharged into the Arctic Ocean via the Mackenzie Valley ~4,000 km northwest of the St. Lawrence outlet. Here we use a sophisticated, high-resolution, ocean sea-ice model to study the delivery of meltwater from the two drainage outlets to the deep water formation regions in the North Atlantic. Unlike the hypothesis of Broecker et al., freshwater from the St. Lawrence Valley advects into the subtropical gyre ~3,000 km south of the North Atlantic deep water formation regions and weakens the AMOC by <15%. In contrast, narrow coastal boundary currents efficiently deliver meltwater from the Mackenzie Valley to the deep water formation regions of the subpolar North Atlantic and weaken the AMOC by >30%. We conclude that meltwater discharge from the Arctic, rather than the St. Lawrence Valley, was more likely to have triggered the Younger Dryas cooling.
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Not, C., and C. Hillaire-Marcel, 2012: “Enhanced sea-ice export from the Arctic during the Younger Dryas.” Nature Communications, v. 3, article no. 647, doi: 10.1038/ncomms1658.
The Younger Dryas cold spell of the last deglaciation and related slowing of the Atlantic meridional overturning circulation have been linked to a large array of processes, notably an influx of fresh water into the North Atlantic related to partial drainage of glacial Lake Agassiz. Here we observe a major drainage event, in marine sediment cores raised from the Lomonosov Ridge, in the central Arctic Ocean marked by a pulse in detrital dolomitic-limestones. This points to an Arctic-Canadian sediment source area with about fivefold higher Younger Dryas ice-rafting deposition rate, in comparison with the Holocene. Our findings thus support the hypothesis of a glacial drainage event in the Canadian Arctic area, at the onset of the Younger Dryas, enhancing sea-ice production and drifting through the Arctic, then export through Fram Strait, towards Atlantic meridional overturning circulation sites of the northern North Atlantic.
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Ghoneim, E., M. Benedetti, and F. El-Baz, 2012: “An integrated remote sensing and GIS analysis of the Kufrah Paleoriver, Eastern Sahara.” Geomorphology, v. 139–140, pp. 242-257, doi: 10.1016/j.geomorph.2011.10.025.
A combined remote sensing (optical and radar imagery) and GIS (hydrologic network delineation) analysis allows mapping of the Kufrah Paleoriver of Libya and sheds light on its geomorphic evolution during the Neogene. The Kufrah system, which is now largely buried beneath the windblown sands of the Eastern Sahara, drained an area of about 236,000 sq km in central and southern Libya. The river discharged across a large inland delta to the Al-Jaghbub depression in northern Libya, and ultimately through the Sirt Basin to the Mediterranean Sea. Radar imagery reveals buried features of the landscape including drainage divides, locations of possible stream capture, deeply-incised valleys, and the distal margins of the inland delta. Previous studies have shown that the Kufrah Paleoriver is the successor of the Sahabi River, which drained most of central Libya during the late Tertiary. Satellite imagery supports the concept of large-scale drainage rearrangement in the Quaternary, driven by tectonic subsidence that diverted streamflow and sediment discharge away from the Sahabi basin toward the inland delta of the lower Kufrah basin. Paleochannels crossing the delta suggest that at various times during the Quaternary, the Kufrah Paleoriver either drained externally through the deeply-incised Sahabi Paleochannel to the Mediterranean Sea, or drained internally to paleolakes in the Al-Jaghbub depression. Thick alluvial deposits on the delta and lake margins likely provided a major sediment source to build the Great Sand Sea, which covers the region today. The southwestern branch of the Kufrah drainage is aligned with an elongated trough that connects to the Amatinga River system in Chad. Thus the Kufrah watershed may have served as an outlet from Megalake Chad to the Mediterranean Sea during humid phases of the Neogene. If so, the combined Amatinga/Kufrah system may have served as one of the proposed natural corridors used by human and animal populations to cross the Sahara during the Pleistocene. These findings hold promise for modeling past lake levels and paleoclimates, locating groundwater sources in the region, and exploring for reservoirs of oil and natural gas in the region.
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Shuman, B., 2012: “Patterns, processes, and impacts of abrupt climate change in a warm world: the past 11,700 years.” Wiley Interdisciplinary Reviews: Climate Change, doi: 10.1002/wcc.152.
Abrupt environmental changes punctuated the warm Holocene epoch (the past ~11,700 years), and studies of these episodes can provide insight into the dynamics that produce rapid climate changes, as well as their ecologic, hydrologic, and geomorphic impacts. This review considers the processes that generated warm world abrupt changes and their landscape and resource effects, including nonlinear climate system interactions, as well as the possibility that large climate variability can linearly produce apparent ‘state shifts.’ Representative examples of Holocene changes illustrate processes that could produce future changes, including (1) rapid changes in ice sheets, such as by ca 8200 years before AD 1950, (2) shifts in the behavior of the El Nino-Southern Oscillation (e.g., at ca 5600 years before AD 1950) and Atlantic Meridional Overturning Circulation (e.g., at ca 2700 years before AD 1950), and (3) land–atmosphere feedbacks, such as were possible in North Africa in the mid-Holocene. These case examples, drawn primarily from the Northern Hemisphere, also reveal the dynamics that generate the types of climate change impacts that would be particularly evident to individuals and societies, such as rapid tree species declines (observed to have taken place within as little time as 6–40 years) and persistent shifts in the regional availability of water. Holocene changes also demonstrate that even progressive climate change can produce important abrupt impacts; that increased rates of background climate forcing may increase the frequency of abrupt responses; and that impacts may well depend upon the particular sequence of changes.
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Farmer, J.R., T.M. Cronin, A. de Vernal, G.S. Dwyer, L.D. Keigwin, and R.C. Thunell, 2011: “Western Arctic Ocean temperature variability during the last 8000 years.”Geophysical Research Letters, v. 38, paper no. L24602, doi: 10.1029/2011GL049714.
We reconstructed subsurface (∼200–400 m) ocean temperature and sea-ice cover in the Canada Basin, western Arctic Ocean from foraminiferal δ18O, ostracode Mg/Ca ratios, and dinocyst assemblages from two sediment core records covering the last 8000 years. Results show mean temperature varied from −1 to 0.5°C and −0.5 to 1.5°C at 203 and 369 m water depths, respectively. Centennial-scale warm periods in subsurface temperature records correspond to reductions in summer sea-ice cover inferred from dinocyst assemblages around 6.5 ka, 3.5 ka, 1.8 ka and during the 15th century Common Era. These changes may reflect centennial changes in the temperature and/or strength of inflowing Atlantic Layer water originating in the eastern Arctic Ocean. By comparison, the 0.5 to 0.7°C warm temperature anomaly identified in oceanographic records from the Atlantic Layer of the Canada Basin exceeded reconstructed Atlantic Layer temperatures for the last 1200 years by about 0.5°C.
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