Isotopic composition and origin of polar precipitation in present and glacial climate simulations

The Hamburg atmospheric general circulation model (AGCM) ECHAM‐4 is used to identify the main source regions of precipitation falling on Greenland and Antarctica. Both water isotopes H₂¹⁸O and HDO are explicitly built into the water cycle of the AGCM, and in addition the capability to trace water from different source regions was added to the model. Present and LGM climate simulations show that water from the most important source regions has an isotopic signature similar to the mean isotope values of the total precipitation amount. But water from other source regions (with very different isotopic signatures) contributes an additional, non‐negligible part of the total precipitation amount on both Greenland and Antarctica. Analyses of the temperature‐isotope‐relations for both polar regions reveal a solely bias of the glacial isotope signal on Greenland, which is caused by a strong change in the seasonal deposition of precipitation originating from nearby polar seas and the northern Atlantic. Although the performed simulations under LGM boundary conditions show a decrease of the δ ¹⁸O values in precipitation in agreement with ice core measurements, the AGCM fails to reproduce the observed simultaneous decrease of the deuterium excess signal.     

Theoretical post-glacial sedimentation rates for a semi-enclosed sea; example of the Kieler Bucht, Western Baltic

The theoretical rates of deposition for fine sediments over the last 10 ka have been deduced and plotted for the Kieler Bucht. Assumptions are that the bay has remained as a closed sedimentary system, and that the fine sediments deposit in water deeper than 10 m. A sharp peak of sedimentation activity is indicated between 7.5 and 8.5 ka B.P. with low rates prior to 9.5 ka and since 6 ka. Comparison of rates obtained from dated cores extracted from different parts of the Kieler Bucht with the theoretical curve shows general conformity, and confirms that peak sedimentation rates exceeding 3 mm/a, as averaged over 100–200 years, occurred between 8 and 9 ka, with the suggestion of a minor activity peak between 3 and 4 ka. The overall consistency supports the view that the bay has acted essentially as a closed sedimentary basin during the Holocene marine transgression and subsequently.     

A 12.5-kyr history of vegetation dynamics and mire development with evidence of Younger Dryas larch presence in the Verkhoyansk Mountains, East Siberia, Russia

Werner, K., Tarasov, P. E., Andreev, A. A., Müller, S., Kienast, F., Zech, M., Zech, W. & Diekmann, B. 2009: A 12.5‐kyr history of vegetation dynamics and mire development with evidence of Younger Dryas larch presence in the Verkhoyansk Mountains, East Siberia, Russia. Boreas, 10.1111/j.1502‐3885.2009.00116.x. ISSN 0300‐9483. A 415 cm thick permafrost peat section from the Verkhoyansk Mountains was radiocarbon‐dated and studied using palaeobotanical and sedimentological approaches. Accumulation of organic‐rich sediment commenced in a former oxbow lake, detached from a Dyanushka River meander during the Younger Dryas stadial, at ～12.5 kyr BP. Pollen data indicate that larch trees, shrub alder and dwarf birch were abundant in the vegetation at that time. Local presence of larch during the Younger Dryas is documented by well‐preserved and radiocarbon‐dated needles and cones. The early Holocene pollen assemblages reveal high percentages of Artemisia pollen, suggesting the presence of steppe‐like communities around the site, possibly in response to a relatively warm and dry climate ～11.4–11.2 kyr BP. Both pollen and plant macrofossil data demonstrate that larch woods were common in the river valley. Remains of charcoal and pollen of Epilobium indicate fire events and mark a hiatus ～11.0–8.7 kyr BP. Changes in peat properties, C31/C27 alkane ratios and radiocarbon dates suggest that two other hiatuses occurred ～8.2–6.9 and ～6.7–0.6 kyr BP. Prior to 0.6 kyr BP, a major fire destroyed the mire surface. The upper 60 cm of the studied section is composed of aeolian sands modified in the uppermost part by the modern soil formation. For the first time, local growth of larch during the Younger Dryas has been verified in the western foreland of the Verkhoyansk Mountains (～170 km south of the Arctic Circle), thus increasing our understanding of the quick reforestation of northern Eurasia by the early Holocene.     

A sand budget for the Alexandria coastal dunefield, South Africa

The sand in the Alexandria coastal dunefield is derived from the sandy beach which forms the seaward boundary of the dunefield. Sand is blown off the beach onto the dunefield by the high-energy onshore-directed dominant wind. The dunefield has been forming over the past 6500 years. Sand transport rates calculated from dune movement rates and wind data range from 15 to 30 m³ m ^-1 yr^-1 in an ENE direction. The sand transport rate decreases with increasing distance from the sea due to a reduction in wind speed resulting from the higher drag imposed upon the wind by the land surface. Aeolian sand movement rates of this order are typical of dunefields around the world. The total volume of sand blown into the dunefield is 375 000 m³ yr^-1. Sand is being lost to the sea by wave erosion along the eastern third of the dunefield at a rate of 45 000 m³ yr ^-1. The dunefield thus gains 330 000 m³ of sand per year. This results in dunefield growth by vertical accretion at about 1.5 mm yr^-1 and landward movement at about 0.25 m yr^-1. The dunefield is a significant sand sink in the coastal sand transport system. The rate of deposition in coastal dunefields can be 10 times as high as rates of deposition in continental sand seas. The higher rate of deposition may result from the abundant sand supply on sandy beaches, and the higher energy of coastal winds. Wind transport is slow and steady compared to fluvial or longshore drift transport of sediment, and catastrophic aeolian events do not seem to be significant in wind-laid deposits.