@article{TrauthFoersterJungingeretal.2018, author = {Trauth, Martin H. and Foerster, Verena and Junginger, Annett and Asrat, Asfawossen and Lamb, Henry F. and Sch{\"a}bitz, Frank}, title = {Abrupt or gradual?}, series = {Quaternary research : an interdisciplinary journal}, volume = {90}, journal = {Quaternary research : an interdisciplinary journal}, number = {2}, publisher = {Cambridge Univ. Press}, address = {New York}, issn = {0033-5894}, doi = {10.1017/qua.2018.30}, pages = {321 -- 330}, year = {2018}, abstract = {We used a change point analysis on a late Pleistocene-Holocene lake-sediment record from the Chew Bahir basin in the southern Ethiopian Rift to determine the amplitude and duration of past climate transitions. The most dramatic changes occurred over 240 yr (from similar to 15,700 to 15,460 yr) during the onset of the African Humid Period (AHP), and over 990 yr (from similar to 4875 to 3885 yr) during its protracted termination. The AHP was interrupted by a distinct dry period coinciding with the high-latitude Younger Dryas stadial, which had an abrupt onset (less than similar to 100 yr) at similar to 13,260 yr and lasted until similar to 11,730 yr. Wet-dry-wet transitions prior to the AHP may reflect the high-latitude Dansgaard-Oeschger cycles, as indicated by cross-correlation of the potassium record with the NorthGRIP ice core record between similar to 45-20 ka. These findings may contribute to the debates regarding the amplitude, and duration and mechanisms of past climate transitions, and their possible influence on the development of early modern human cultures.}, language = {en} } @article{StraussSchirrmeisterGrosseetal.2017, author = {Strauss, Jens and Schirrmeister, Lutz and Grosse, Guido and Fortier, Daniel and Hugelius, Gustaf and Knoblauch, Christian and Romanovsky, Vladimir E. and Schadel, Christina and von Deimling, Thomas Schneider and Schuur, Edward A. G. and Shmelev, Denis and Ulrich, Mathias and Veremeeva, Alexandra}, title = {Deep Yedoma permafrost: A synthesis of depositional characteristics and carbon vulnerability}, series = {Earth science reviews : the international geological journal bridging the gap between research articles and textbooks}, volume = {172}, journal = {Earth science reviews : the international geological journal bridging the gap between research articles and textbooks}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0012-8252}, doi = {10.1016/j.earscirev.2017.07.007}, pages = {75 -- 86}, year = {2017}, abstract = {Permafrost is a distinct feature of the terrestrial Arctic and is vulnerable to climate warming. Permafrost degrades in different ways, including deepening of a seasonally unfrozen surface and localized but rapid development of deep thaw features. Pleistocene ice-rich permafrost with syngenetic ice-wedges, termed Yedoma deposits, are widespread in Siberia, Alaska, and Yukon, Canada and may be especially prone to rapid-thaw processes. Freeze-locked organic matter in such deposits can be re-mobilized on short time-scales and contribute to a carbon-cycle climate feedback. Here we synthesize the characteristics and vulnerability of Yedoma deposits by synthesizing studies on the Yedoma origin and the associated organic carbon pool. We suggest that Yedoma deposits accumulated under periglacial weathering, transport, and deposition dynamics in non-glaciated regions during the late Pleistocene until the beginning of late glacial warming. The deposits formed due to a combination of aeolian, colluvial, nival, and alluvial deposition and simultaneous ground ice accumulation. We found up to 130 gigatons organic carbon in Yedoma, parts of which are well-preserved and available for fast decomposition after thaw. Based on incubation experiments, up to 10\% of the Yedoma carbon is considered especially decomposable and may be released upon thaw. The substantial amount of ground ice in Yedoma makes it highly vulnerable to disturbances such as thermokarst and thermo-erosion processes. Mobilization of permafrost carbon is expected to increase under future climate warming. Our synthesis results underline the need of accounting for Yedoma carbon stocks in next generation Earth-System-Models for a more complete representation of the permafrost-carbon feedback.}, language = {en} } @article{MischkeOpitzKalbeetal.2015, author = {Mischke, Steffen and Opitz, Stephan and Kalbe, Johannes and Ginat, Hanan and Al-Saqarat, Bety}, title = {Palaeoenvironmental inferences from late Quaternary sediments of the Al Jafr Basin, Jordan}, series = {Quaternary international : the journal of the International Union for Quaternary Research}, volume = {382}, journal = {Quaternary international : the journal of the International Union for Quaternary Research}, publisher = {Elsevier}, address = {Oxford}, issn = {1040-6182}, doi = {10.1016/j.quaint.2014.12.041}, pages = {154 -- 167}, year = {2015}, abstract = {Sedimentological, palaeontological and mineralogical analyses of sediments from the endorheic Al Jafr Basin were conducted to better understand the depositional and hydrological conditions on the southern Jordan Plateau in the late Quaternary. Surficially exposed carbonate-rich sediments in the western part of the basin contain ostracod (micro-crustacean) shells of Ilyocypris cf. bradyi, Candona neglecta, Heterocypris salina, Fabaeformiscandona fabaeformis, Pseudocandona sp. and Herpetocypris brevicaudata. The shells of these and other more rare species, and charophyte and mollusc remains indicate that the sediments were formed in a wetland setting of shallow freshwater to slightly oligohaline ponds, streams and swamps. The present more northern distribution of some of the recorded taxa implies that climate conditions were probably cooler during the wetland formation. Radiocarbon age data for biogenic carbonate from two locations suggest that the wetland setting existed during the second half of Marine Isotope Stage (MIS) 3 or possibly earlier. A significantly higher water table must have existed in the basin during wetland formation; and wetter climate conditions are inferred for the catchment or at least for its highest and most humid westernmost part. Deflation and local sediment accumulation by wind and occasional sheet-wash events apparently prevailed in the region since MIS 2. Our newly presented data and inferences do not support the reconstruction of a previously reported large and relatively deep Pleistocene lake in the Al Jafr Basin. However, more extensive studies are certainly required for a detailed assessment of the Quaternary hydrological conditions in southern Jordan. (C) 2014 Elsevier Ltd and INQUA. All rights reserved.}, language = {en} } @article{MischkeLaiZhang2014, author = {Mischke, Steffen and Lai, Zhongping and Zhang, Chengjun}, title = {Re-assessment of the paleoclimate implications of the Shell Bar in the Qaidam Basin, China}, series = {Journal of paleolimnolog}, volume = {51}, journal = {Journal of paleolimnolog}, number = {2}, publisher = {Springer}, address = {Dordrecht}, issn = {0921-2728}, doi = {10.1007/s10933-012-9674-6}, pages = {179 -- 195}, year = {2014}, abstract = {The Shell Bar in the Qaidam Basin, China, is a prominent geological feature composed of millions of densely packed Corbicula shells. Since the mid 1980s, it has been regarded as evidence for existence of a large lake during Marine Isotope Stage (MIS) 3 in the presently hyper-arid Qaidam Basin. Early studies suggested the bivalve shells accumulated at the shore of a large lake, whereas more recent work led to the conclusion that the Shell Bar was formed within a deeper water body. Based on our re-assessment of sediments and fossils from the Shell Bar, investigation of exposed fluvio-lacustrine sections upstream of the Shell Bar and study of nearby modern streams, we infer that the Shell Bar represents a stream deposit. Corbicula is a typical stream-dweller around the world. Preservation of Corbicula shells of different sizes, as well as occurrence of many articulated shells, provide evidence against post-mortem transport and accumulation along a lake shore. Additionally, the SE-NW alignment of the Shell Bar is similar to modern intermittent stream beds in its vicinity and corresponds to the present-day slope towards the basin centre further NW, and furthermore, the predominantly sandy sediments also indicate that the Shell Bar was formed in a stream. Abundant ostracod shells in the Shell Bar sediments originated from stream-dwelling species that are abundant in modern streams in the vicinity of the Shell Bar, or in part from fluvio-lacustrine sediments exposed upstream of the Shell Bar, as a result of erosion and re-deposition. Deflation of alluvial fine-grained sediments in the Shell Bar region and protection of the stream deposits by the large and thick-walled Corbicula shells reversed the former channel relief and yielded the modern exposure, which is a prominent morphological feature. Occurrence of Corbicula shells in the Qaidam Basin indicates climate was apparently warmer than present during the formation of the Shell Bar because Corbicula does not live at similar or higher altitudes in the region today. Because the Shell Bar is no longer considered a deposit formed within a lake, its presence does not indicate paleoclimate conditions wetter than today.}, language = {en} } @article{LaiMischkeMadsen2014, author = {Lai, ZhongPing and Mischke, Steffen and Madsen, David}, title = {Paleoenvironmental implications of new OSL dates on the formation of the "Shell Bar" in the Qaidam Basin, northeastern Qinghai-Tibetan Plateau}, series = {Journal of paleolimnolog}, volume = {51}, journal = {Journal of paleolimnolog}, number = {2}, publisher = {Springer}, address = {Dordrecht}, issn = {0921-2728}, doi = {10.1007/s10933-013-9710-1}, pages = {197 -- 210}, year = {2014}, abstract = {A geological feature in the Qaidam Basin known as the "Shell Bar" contains millions of freshwater clam shells buried in situ. Since the 1980s, this feature in the now hyper-arid basin has been interpreted to be lake deposits that provide evidence for a warmer and more humid climate than present during late marine isotope stage 3 (MIS 3). Global climate during late MIS 3 and the last glacial maximum, however, was cold and dry, with much lower sea levels. We re-investigated the feature geomorphologically and sedimentologically, and employed optically stimulated luminescence (OSL) dating to verify the chronology of the sediments. We interpret the Shell Bar to be a remnant of a river channel formed by a stream that ran across an exposed lake bed during a regressive lake phase. Deflation of the surrounding older, fine-grained lacustrine deposits has left the fluvial channel sediments topographically inverted, indicating the erosive nature of the landscape. Luminescence ages place the formation of the Shell Bar in MIS 5 (similar to 113-99 ka), much older than previous radiocarbon ages of < 40 ka BP, but place the paleoclimatic inferences more in accord with other regional and global climate proxy records. We present a brief review of the age differences derived from C-14 and OSL dating of some critical sections that were thought to represent a warmer and more humid climate than present during late MIS 3. We attribute the differences to underestimation of C-14 ages. We suggest that C-14 ages older than similar to 25 ka BP may require re-investigation, especially dates on samples from arid regions.}, language = {en} }