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  • 1. Birch, Heather S.
    et al.
    Coxall, Helen K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pearson, Paul N.
    Kroon, Dick
    Schmidt, Daniela N.
    Partial collapse of the marine carbon pump after the Cretaceous-Paleogene boundary2016In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 44, no 4, p. 287-290Article in journal (Refereed)
    Abstract [en]

    The impact of an asteroid at the end of the Cretaceous caused mass extinctions in the oceans. A rapid collapse in surface to deep-ocean carbon isotope gradients suggests that transfer of organic matter to the deep sea via the biological pump was severely perturbed. However, this view has been challenged by the survival of deep-sea benthic organisms dependent on surface-derived food and uncertainties regarding isotopic fractionation in planktic foraminifera used as tracers. Here we present new stable carbon (delta C-13) and oxygen (delta O-18) isotope data measured on carefully selected planktic and benthic foraminifera from an orbitally dated deep-sea sequence in the southeast Atlantic. Our approach uniquely combines delta O-18 evidence for habitat depth of foraminiferal tracer species with species-specific delta C-13 eco-adjustments, and compares isotopic patterns with corresponding benthic assemblage data. Our results show that changes in ocean circulation and foraminiferal vital effects contribute to but cannot explain all of the observed collapse in surface to deep-ocean foraminiferal delta C-13 gradient. We conclude that the biological pump was weakened as a consequence of marine extinctions, but less severely and for a shorter duration (maximum of 1.77 m.y.) than has previously been suggested.

  • 2. Büntgen, Ulf
    et al.
    Eggertsson, Ólafur
    Wacker, Lukas
    Sigl, Michael
    Charpentier Ljungqvist, Fredrik
    Stockholm University, Faculty of Humanities, Department of History.
    Di Cosmo, Nicola
    Plunkett, Gill
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Cambridge, UK.
    Newfield, Timothy P.
    Esper, Jan
    Lane, Christine
    Reinig, Frederick
    Oppenheimer, Clive
    Multi-proxy dating of Iceland's major pre-settlement Katla eruption to 822-823 CE2017In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 45, no 9, p. 783-786Article in journal (Refereed)
    Abstract [en]

    Investigations of the impacts of past volcanic eruptions on climate, environment, and society require accurate chronologies. However, eruptions that are not recorded in historical documents can seldom be dated exactly. Here we use annually resolved radiocarbon (C-14) measurements to isolate the 775 CE cosmogenic C-14 peak in a subfossil birch tree that was buried by a glacial outburst flood in southern Iceland. We employ this absolute time marker to date a subglacial eruption of Katla volcano at late 822 CE to early 823 CE. We argue for correlation between the 822-823 CE eruption and a conspicuous sulfur anomaly evident in Greenland ice cores, which follows in the wake of an even larger volcanic signal (ca. 818-820 CE) as yet not attributed to a known eruption. An abrupt summer cooling in 824 CE, evident in tree-ring reconstructions for Fennoscandia and the Northern Hemisphere, suggests a climatic response to the Katla eruption. Written historical sources from Europe and China corroborate our proposed tree ring-radiocarbon-ice core linkage but also point to combined effects of eruptions occurring during this period. Our study describes the oldest precisely dated, high-latitude eruption and reveals the impact of an extended phase of volcanic forcing in the early 9th century. It also provides insight into the existence of prehistoric woodland cover and the nature of volcanism several decades before Iceland's permanent settlement began.

  • 3. Cohen, Tim J.
    et al.
    Jansen, John D.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. University of Wollongong, Australia.
    Gliganic, Luke A.
    Larsen, Joshua R.
    Nanson, Gerald C.
    May, Jan-Hendrik
    Jones, Brian G.
    Price, David M.
    Hydrological transformation coincided with megafaunal extinction in central Australia2015In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 43, no 3, p. 195-198Article in journal (Refereed)
    Abstract [en]

    Central to the debate over the extinction of many of Australia's last surviving megafauna is the question: Was climate changing significantly when humans arrived and megafauna went extinct? Here we present a new perspective on variations in climate and water resources over the last glacial cycle in arid Australia based on the study of the continent's largest lake basin and its tributaries. By dating paleoshorelines and river deposits in the Lake Eyre basin, we show that major hydrological change caused previously overflowing megalakes to enter a final and catastrophic drying phase at 48 +/- 2 ka just as the giant bird, Genyornis newtoni, went extinct (50-45 ka). The disappearance of Genyornis and other megafauna has been previously attributed to ecosystem collapse coincident with the spread of fire-wielding humans. Our findings suggest a climate-driven hydrological transformation in the critical window of human arrival and megafaunal extinction, and the results call for a re-evaluation of a human-mediated cause for such extinctions in arid Australia.

  • 4.
    Cohen, Timothy
    et al.
    Department of Environment and Geography, Macquarie University, Sydney, NSW 2109, Australia.
    Nanson, Gerald
    Jansen, John
    School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK.
    Jones, Brian
    Jacobs, Zenobia
    Treble, Pauline
    Price, David
    May, Jan-Hendrick
    Smith, Andrew
    Ayliffe, Linda
    Hellstrom, John
    Continental aridification and the vanishing of Australia's mega-lakes2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, p. 167-170Article in journal (Refereed)
    Abstract [en]

    The nature of the Australian climate at about the time of rapid megafaunal extinctions and humans arriving in Australia is poorly understood and is an important element in the contentious debate as to whether humans or climate caused the extinctions. Here we present a new paleoshoreline chronology that extends over the past 100 k.y. for Lake Mega-Frome, the coalescence of Lakes Frome, Blanche, Callabonna and Gregory, in the southern latitudes of central Australia. We show that Lake Mega-Frome was connected for the last time to adjacent Lake Eyre at 50–47 ka, forming the largest remaining interconnected system of paleolakes on the Australian continent. The fi nal disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridifi cation that coincided with the arrival of humans and the demise of the megafauna. The supply of moisture to the Australian continent at various times in the Quaternary has commonly been ascribed to an enhanced monsoon. This study, in combination with other paleoclimate data, provides reliable evidence for peri-ods of enhanced tropical and enhanced Southern Ocean sources of water fi lling these lakes at different times during the last full glacial cycle.

  • 5. Gaines, Robert R.
    et al.
    Droser, Mary L.
    Orr, Patrick J.
    Garson, Daniel
    Hammarlund, Emma
    Naturhistoriska riksmuseet, Enheten för paleozoologi.
    Qi, Changshi
    Canfield, Donald E.
    Burgess Shale-type biotas were not entirely burrowed away2012In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 40, no 3, p. 283-286Article in journal (Refereed)
    Abstract [en]

    Burgess Shale–type biotas occur globally in the Cambrian record and offer unparalleled insight into the Cambrian explosion, the initial Phanerozoic radiation of the Metazoa. Deposits bearing exceptionally preserved soft-bodied fossils are unusually common in Cambrian strata; more than 40 are now known. The well-documented decline of soft-bodied preservation following the Middle Cambrian represents the closure of a taphonomic window that was only intermittently open in marine environments thereafter. The prevailing hypothesis for this secular shift in taphonomic conditions of outer shelf environments is that soft-bodied biotas were literally burrowed away from the fossil record by increasing infaunal activity in muddy substrate environments; this would have affected geochemical gradients and increased the efficiency of organic matter recycling in sediments. New and recently published data, however, suggest a more complex scenario. Ichnologic and microstratigraphic data from Burgess Shale–type deposits indicate that (1) bioturbation exerts a limiting effect on soft-bodied preservation; (2) the observed increase in the depth and extent of bioturbation following the Middle Cambrian would have restricted preservation of Burgess Shale?type biotas in a number of settings; but (3) increasing depth and extent of bioturbation would not have affected preservation in many other settings, including the most richly fossiliferous portions of the Chengjiang (China) deposit and the Greater Phyllopod Bed of the Burgess Shale (Canada). Therefore, increasing bioturbation cannot account for the apparent loss of this pathway from the fossil record, and requires that other circumstances, including, but not limited to, widespread benthic anoxia, facilitated widespread exceptional preservation in the Cambrian.

  • 6.
    Hall, Adrian
    et al.
    University of St Andrews, Scotland, UK.
    Ebert, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kleman, Johan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Nesje, Atle
    Ottesen, Dag
    Selective glacial erosion on the Norwegian passive margin2013In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 41, no 12, p. 1203-1206Article in journal (Refereed)
    Abstract [en]

    Glaciated passive margins display dramatic fjord coasts, but also commonly retain plateau fragments inland. It has been proposed recently that such elevated, low-relief surfaces on the Norwegian margin are products of highly efficient and extensive glacial and periglacial erosion (the glacial buzzsaw) operating at equilibrium line altitudes (ELAs). We demonstrate here that glacial erosion has acted instead to dissect plateaus in western Norway. Low-relief surfaces are not generally spatially associated with cirques, and do not correlate regionally with modern and Last Glacial Maximum ELAs. Glacier dynamics require instead that glacial erosion is selective, with low-relief surfaces representing islands of limited Pleistocene erosion. Deep glacial erosion of the coast and inner shelf has provided huge volumes of sediment (70,000 km3), largely resolving apparent mismatches (65–100,000 km3) between fjord and valley volumes and Pliocene–Pleistocene sediment wedges offshore. Nonetheless, as Pleistocene glacial valleys and cirques are cut into preexisting mountain relief, tectonics rather than isostatic compensation for glacial erosion have been the main driver for late Cenozoic uplift on the Norwegian passive margin.

  • 7.
    Hansman, Reuben J.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Albert, Richard
    Gerdes, Axel
    Ring, Uwe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite2018In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 46, no 3, p. 207-210Article in journal (Refereed)
    Abstract [en]

    Direct dating of brittle structures is challenging, especially absolute dating of diagenesis followed by a series of superimposed brittle deformation events. We report 22 calcite U-Pb ages from tectonites and carbonate host rocks that date 3 diagenetic and 6 brittle deformation events. Results show that U-Pb dating of calcite fibers from these structures is compatible with overprinting relationships. Ages indicate that diagenesis occurred between 147 +/- 6 Ma and 103 +/- 34 Ma, and was followed by top-to-the-south, layer-parallel shearing due to ophiolite obduction at 84 +/- 5 Ma (2 sigma errors). Sheared top-to-the- northeast, layer-parallel veins were dated as 64 +/- 4 Ma and are interpreted to have developed during postobduction exhumation. After this event, a series of strike-slip structures, which crosscut and reactivated older faults due to northwest-southeast horizontal shortening, were dated as 55 +/- 22 Ma and 43 +/- 6 Ma. Eight ages from strike-slip faults and thrusts resulting from northeast-southwest shortening range from 40.6 +/- 0.5 Ma to 16.1 +/- 0.2 Ma. The youngest ages are from minor overprinting fibers ranging in age between 7.5 +/- 0.9 Ma and 1.6 +/- 0.6 Ma. Our results show that U-Pb dating of calcite fibers can be successfully used to constrain a complicated succession of brittle deformation structures that encompasses two orogenies and an intervening extension period.

  • 8.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Anderson, John B.
    Nitsche, Frank O.
    Dowdeswell, Julian A.
    Gyllencreutz, Richard
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Mohammad, Rezwan
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Alley, Richard B.
    Anandakrishnan, Sridhar
    Eriksson, Björn
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kirshner, Alexandra
    Fernandez, Rodrigo
    Stolldorf, Travis
    Minzoni, Rebecca
    Majewski, Wojciech
    Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 7, p. 691-694Article in journal (Refereed)
    Abstract [en]

    The catastrophic break-ups of the floating Larsen A and B ice shelves (Antarctica) in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? Here we describe features from the seafloor in Pine Island Bay, West Antarctica, which we interpret as having been formed during a massive ice shelf break-up and associated grounding line retreat. This evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega-icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line. The break-up occurred prior to ca. 12 ka and was likely a response to rapid sea-level rise or ocean warming at that time.

  • 9.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Björck, Svante
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Flodén, Tom
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Greenwood, Sarah L.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Swärd, Henrik
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Lif, Arne
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Ampel, Linda
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Koyi, Hemin
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Major earthquake at the Pleistocene-Holocene transition in Lake Vattern, southern Sweden2014In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 42, no 5, p. 379-382Article in journal (Refereed)
    Abstract [en]

    Lake Vattern, Sweden, is within a graben that formed through rifting along the boundary between two Precambrian terrains. Geophysical mapping and geological coring show that substantial tectonic movements along the Lake Vattern graben occurred at the very onset of the Holocene. This is evident from deformation structures in the soft sediment accumulated on the lake floor. Our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a >80-km-long fault system. If these large displacements are from one tectonic event, Lake Vattern must have had an earthquake with seismic moment magnitudes to 7.5. In addition, our geophysical mapping shows large landslides along sections of the steep lake shores. Pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the Younger Dryas-Preboreal transition, ca. 11.5 ka. We suggest that this event is mainly related to the rapid release of ice-sheet load following the deglaciation. This paleoseismic event in Lake Vattern ranks among the larger known intraplate tectonic events in Scandinavia and attests to the significance of glacio-isostatic unloading.

  • 10.
    Jansen, John D.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Fabel, Derek
    Bishop, Paul
    Xu, Sheng
    Schnabel, Christoph
    Codilean, Alexandru T.
    Does decreasing paraglacial sediment supply slow knickpoint retreat?2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 6, p. 543-546Article in journal (Refereed)
    Abstract [en]

    In four rivers in western Scotland for which there is a well-constrained record of relative base-level fall, the rate of postglacial bedrock erosion is quantified by measuring the concentration of in situ cosmogenic (10)Be on strath terraces downstream of headward-retreating knickpoints. Along-channel gradients in (10)Be exposure age show two distinct trends: upstream younging and constant age, which we interpret as diagnostic of knickpoint retreat and diffusive transport-limited incision, respectively. We show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation (ca. 11.5 ka), and that knickpoint retreat rates peaked in the early to mid-Holocene. Erosion rates have since decreased by two orders of magnitude, converging in the late Holocene to low rates independent of stream power per unit channel area. We infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the Holocene, leading to a deficiency in tools for bedrock erosion. Our results imply that episodes of major fluvial erosion may be in tune with glacial cycles, and that sediment depletion following glacial-interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes.

  • 11. Jilbert, Tom
    et al.
    Conley, Daniel J.
    Gustafsson, Bo G.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre, Baltic Nest Institute.
    Funkey, Carolina P.
    Slomp, Caroline P.
    Glacio-isostatic control on hypoxia in a high-latitude shelf basin2015In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 43, no 5, p. 427-430Article in journal (Refereed)
    Abstract [en]

    In high-latitude continental shelf environments, late Pleistocene glacial overdeepening and early Holocene eustatic sea-level rise combined to create restricted marine basins with a high vulnerability to oxygen depletion. Here we show that ongoing glacio-isostatic rebound during the Holocene may have played an important role in determining the distribution of past hypoxia in these environments by controlling the physical exchange of water masses and the distribution of large-scale phosphorus (P) sinks. We focus on the Baltic Sea, where sediment records from a large, presently oxic sub-basin show evidence for intense hypoxia and cyanobacteria blooms during the Holocene Thermal Maximum. Using paleobathymetric modeling, we show that this period was characterized by enhanced deep-water exchange, allowing widespread phosphorus regeneration. Intra-basin sills then shoaled over a period of several thousand years, enhancing P burial in one of the sub-basins. Together with climate forcing, this may have caused the termination of hypoxia throughout the Baltic Sea. Similar rearrangements of physical and chemical processes likely occurred in response to glacio-isostatic rebound in other high-latitude shelf basins during the Holocene.

  • 12. Kelly, Meredith A.
    et al.
    Lowell, Thomas V.
    Applegate, Patrick J.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Smith, Colby A.
    Phillips, Fred M.
    Hudson, Adam M.
    Late glacial fluctuations of Quelccaya Ice Cap, southeastern Peru2012In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 40, no 11, p. 991-994Article in journal (Refereed)
    Abstract [en]

    The last glacial-interglacial transition (ca. 18-11 ka) was interrupted by abrupt climate events that differed in each hemisphere. During the Antarctic Cold Reversal (ca. 14.5-12.9 ka), the Southern Hemisphere high and mid latitudes cooled, while the Northern Hemisphere warmed. The pattern of change then reversed during the Younger Dryas (ca. 12.9-11.7 ka), which was characterized by cold conditions in much of the Northern Hemisphere. Well-dated paleoclimate records serve to reveal the possible mechanisms for these events. Here we present a reconstruction of the late glacial fluctuations of Quelccaya Ice Cap, located in the southern tropics, based on 38 new radiocarbon ages. Quelccaya was retreating from its Last Glacial Maximum (ca. 21 ka) extent by ca. 17.2 ka, and was located upvalley from its late glacial moraines by 13.6-12.8 ka. Quelccaya experienced a significant readvance that culminated at 12.5-12.4 ka, and then receded several kilometers to near, or within, its late Holocene extent by ca. 11.6 ka. This record provides the most detailed evidence yet of glacier fluctuations in the southern tropics during late glacial time.

  • 13.
    O'Regan, Matt
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Forwick, Matthias
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Moran, Kathryn
    Mosher, David
    Seafloor cratering and sediment remolding at sites of fluid escape2015In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 43, no 10, p. 895-898Article in journal (Refereed)
    Abstract [en]

    Episodic fluid escape from marine sediments results from overpressure development and pressure release, and can occur slowly through geologic time or catastrophically. Morphological features in regions of fluid seepage include doming, mud volcanism, cratering, and pockmark formation. Vertical sediment mobilization and surface erosion are considered the principal mechanisms for these topographic changes. However, the impact of mobilization on the geotechnical properties of sediments has not been explicitly considered. Here we develop a one-dimensional numerical subsidence model that incorporates the well-established behavior of remolded fine-grained cohesive sediments. We use this to show that if subsurface overpressure results in the mobilization of sediments, large settlements (20%-35% reduction in volume) can occur when overpressure dissipates. This presents a novel mechanism to explain changes in seafloor and subsurface topography in areas of fluid escape, while highlighting an important interplay between subsurface fluid flow and the geotechnical properties of fine-grained cohesive sediments.

  • 14. Parnell, John
    et al.
    Boyce, Adrian
    Thackrey, Scott
    Muirhead, David
    Lindgren, Paula
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Mason, Charles
    Taylor, Colin
    Still, John
    Bowden, Stephen
    Osinski, Gordon R.
    Lee, Pascal
    Sulfur isotope signatures for rapid colonization of an impact crater by thermophilic microbes2010In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 38, no 3, p. 271-274Article in journal (Refereed)
    Abstract [en]

    In the 23-km-diameter Haughton impact structure, Canadian High Arctic, in sulfate-rich bedrock, widespread hydrothermal sulfide mineralization occurred in breccias formed during the impact. The sulfides exhibit extreme sulfur isotopic fractionation relative to the original sulfate, requiring microbial sulfate reduction by thermophiles throughout the crater. This evidence of widespread microbial activity demonstrates that colonization could occur within the lifetime of a moderately sized, impact-induced hydrothermal system. The pyrite was subsequently oxidized to jarosite, which may also have been microbially mediated. The successful detection of evidence for microbial life suggests that it would be a valuable technique to deploy in sulfate-rich impact terrain on Mars.

  • 15.
    Preusser, Frank
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Rufer, Daniel
    Schreurs, Guido
    Direct dating of Quaternary phreatic maar eruptions by luminescence methods2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 12, p. 1135-1138Article in journal (Refereed)
    Abstract [en]

    The suitability of quartz optically stimulated luminescence (OSL) and feldspar infrared stimulated luminescence (IRSL) for the direct dating of phreatic eruptions was tested on examples from the Eifel Volcanic Field, Germany. The mean IRSL age of 11.6 +/- 0.5 ka for Ulmener Maar Tephra is in excellent agreement with the independent age control (11 +/- 0.1 ka), but the mean OSL age of 14.2 +/- 0.6 ka overestimates the known age by 3 ka. For Meerfelder Maar Tephra, consistent IRSL (mean 74.9 +/- 5.0 ka) and OSL ages (74.9 +/- 5.5 ka) have been observed. The consistent results from Meerfelder Maar imply that the overestimation observed for Ulmener Maar quartz OSL might not be relative to the eruption age, but rather represents a small absolute offset. In samples taken from deposits of the eruption of Laacher See Volcano, no resetting of the OSL signal and highly scattered IRSL estimates were found. This implies that phreatomagmatic eruptions are less well suited for this dating approach compared to pure phreatic maar eruptions, where the effect of high-pressure shock waves probably dominates the process of resetting the luminescence signal.

  • 16. Rosenberg, T. M.
    et al.
    Preusser, Frank
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Fleitmann, D.
    Schwalb, A.
    Penkman, K.
    Schmid, T. W.
    Al-Shanti, M. A.
    Kadi, K.
    Matter, A.
    Humid periods in southern Arabia: Windows of opportunity for modern human dispersal2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 12, p. 1115-1118Article in journal (Refereed)
    Abstract [en]

    Arabia is a key area for the dispersal of anatomically modern humans (AMH, Homo sapiens) out of Africa. Given its modern hostile environment, the question of the timing of dispersal is also a question of climatic conditions. Fresh water and food were crucial factors facilitating AMH expansions into Arabia. By dating relict lake deposits, four periods of lake formation were identified: one during the early Holocene and three during the late Pleistocene centered ca. 80, ca. 100, and ca. 125 ka. Favorable environmental conditions during these periods allowed AMH to migrate across southern Arabia. Between ca. 75 and 10.5 ka, arid conditions prevailed and turned southern Arabia into a natural barrier for human dispersal. Thus, expansion of AMH through the southern corridor into Asia must have taken place before 75 ka, possibly in multiple dispersals.

  • 17. Scharf, Taryn E.
    et al.
    Codilean, Alexandru T.
    de Wit, Maarten
    Jansen, John D.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kubik, Peter W.
    Strong rocks sustain ancient postorogenic topography in southern Africa2013In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 41, no 3, p. 331-334Article in journal (Refereed)
    Abstract [en]

    The Cape Mountains of southern Africa exhibit an alpine-like topography in conjunction with some of the lowest denudation rates in the world. This presents an exception to the often-cited coupling of topography and denudation rates and suggests that steep slopes alone are not sufficient to incite the high denudation rates with which they are commonly associated. Within the Cape Mountains, slope angles are often in excess of 30 degrees and relief frequently exceeds 1 km, yet Be-10-based catchment-averaged denudation rates vary between 2.32 +/- 0.29 m/m.y. and 7.95 +/- 0.90 m/m.y. We attribute the maintenance of rugged topography and suppression of denudation rates primarily to the presence of physically robust and chemically inert quartzites that constitute the backbone of the mountains. Be-10-based bedrock denudation rates on the interfluves of the mountains vary between 1.98 +/- 0.23 m/m.y. and 4.61 +/- 0.53 m/m.y. The close agreement between the rates of catchment-averaged and interfluve denudation indicates topography in steady state. These low denudation rates, in conjunction with the suggestion of geomorphic stability, are in agreement with the low denudation rates (<20 m/m.y.) estimated for southern Africa during the late Cenozoic by means of cosmogenic nuclide, thermochronology, and offshore sedimentation analyses. Accumulatively, these data suggest that the coastal hinterland of the subcontinent may have experienced relative tectonic stability throughout the Cenozoic.

  • 18.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Flux rate for water and carbon during greenschist facies metamorphism2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 1, p. 43-46Article in journal (Refereed)
    Abstract [en]

    The time-averaged flux rate for a CO2-bearing hydrous fluid during greenschist facies regional metamorphism was estimated to be 10–10.2 ± 0.4 m3 m−2 s−1 by combining (1) Peclet numbers obtained by chromatographic analysis of the propagation of reaction fronts in 33 metamorphosed basaltic sills in the southwest Scottish Highlands (UK), (2) empirical diffusion rates for CO2 in water, and (3) calculated time-averaged metamorphic porosities. The latter were calculated using an expression obtained by combining estimated Peclet numbers with empirical porosity-permeability relationships and Darcy's law. This approach yielded a time-averaged metamorphic porosity of 10–2.6 ± 0.2 for greenschist facies conditions. The corresponding time scale for metamorphic fluid flow was 103.6 ± 0.1 yr. By using mineral assemblages to constrain fluid compositions, a time-averaged annual flux rate for carbon of 0.5–7 mol C m−2 yr−1 was calculated. This matches measured emission rates for metamorphic CO2 from orogenic hot springs and exceeds estimated rates of CO2 drawdown by orogenic silicate weathering, suggesting that orogenesis is a source rather than a sink of atmospheric CO2.

  • 19.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Flux rates for water and carbon during greenschist facies metamorphism2011In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 39, no 1, p. 43-46Article in journal (Refereed)
    Abstract [en]

    The time-averaged flux rate for a CO2-bearing hydrous fluid during greenschist facies regional metamorphism was estimated to be 10–10.2 ± 0.4 m3 m−2 s−1 by combining (1) Peclet numbers obtained by chromatographic analysis of the propagation of reaction fronts in 33 metamorphosed basaltic sills in the southwest Scottish Highlands (UK), (2) empirical diffusion rates for CO2 in water, and (3) calculated time-averaged metamorphic porosities. The latter were calculated using an expression obtained by combining estimated Peclet numbers with empirical porosity-permeability relationships and Darcy's law. This approach yielded a time-averaged metamorphic porosity of 10–2.6 ± 0.2 for greenschist facies conditions. The corresponding time scale for metamorphic fluid flow was 103.6 ± 0.1 yr. By using mineral assemblages to constrain fluid compositions, a time-averaged annual flux rate for carbon of 0.5–7 mol C m−2 yr−1 was calculated. This matches measured emission rates for metamorphic CO2 from orogenic hot springs and exceeds estimated rates of CO2 drawdown by orogenic silicate weathering, suggesting that orogenesis is a source rather than a sink of atmospheric CO2.

  • 20.
    Steinthorsdottir, Margret
    et al.
    School of Biology and Environmental Science, University College Dublin.
    Ian Woodward, F.
    Surlyk, F.
    McElwain, J. C.
    Deep-time evidence of a link between elevated CO 2 concentrations and perturbations in the hydrological cycle via drop in plant transpiration2012In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 40, no 9, p. 815-818Article in journal (Refereed)
    Abstract [en]

    The physiological effects of high CO 2 concentrations, i.e., [CO 2], on plant stomatal responses may be of major importance in understanding the consequences of climate change, by causing increases in runoff through suppression of plant transpiration. Radiative forcing by high [CO 2] has been the main consideration in models of global change to the exclusion of plant physiological forcing, but this potentially underestimates the effects on the hydrological cycle, and the consequences for ecosystems. We tested the physiological responses of fossil plants from the Triassic-Jurassic boundary transition (Tr-J) succession of East Greenland. This interval marks a major high CO 2-driven environmental upheaval, with faunal mass extinctions and significant floral turnover. Our results show that both stomatal size (expressed in fossil material as SL, the length of the stomatal complex opening) and stomatal density (SD, the number of stomata per mm 2) decreased significantly during the Tr-J. We estimate, using a leaf gas-exchange model, that the decreases in SD and SL resulted in a 50%-60% drop in stomatal and canopy transpiration at the Tr-J. We also present new field evidence indicating simultaneous increases in runoff and erosion rates. We propose that the consequences of sto- matal responses to elevated [CO 2] may lead to locally increased runoff and erosion, and may link terrestrial and marine biodiversity loss via the hydrological cycle. © 2012 Geological Society of America.

  • 21. Webber, A. P.
    et al.
    Roberts, S.
    Taylor, R. N.
    Pitcairn, Iain K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Golden plumes: Substantial gold enrichment of oceanic crust during ridge-plume interaction2013In: Geology, ISSN 0091-7613, E-ISSN 1943-2682, Vol. 41, no 1, p. 87-90Article in journal (Refereed)
    Abstract [en]

    Mantle plume events are increasingly implicated as the source of gold (Au) in regions of the Earth that show a high Au endowment. However, the process of enriching oceanic crust in Au by plume activity is poorly understood and unconstrained. We present the first systematic study of Au concentrations in oceanic basalts as a function of distance from a plume center. We show that the influence of the Iceland plume on the Mid-Atlantic Ridge progressively enriches the oceanic crust in Au along the Reykjanes Ridge by as much as 13 times normal levels, over a distance of similar to 600 km, and that the enrichment can be attributed to specific plume components. This Au enrichment by the Iceland plume implies a genetic relationship between deep mantle upwelling and major gold mineralization.

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