Abstracts / Quaternary International 279-280 (2012) 9–12080
millennial scale. In particular, data evidence that a new heat flux equilib-rium between the two hemispheres is obtained for extraordinary long coldGreenland Stadial. However, such a thermal bipolar seesaw pattern is notat play during periods associated with large ice sheet (MIS2 and the end ofMIS4). Our results highlight a close interplay between millennial-scaleclimate variability and the background climate.
MEGAFLOOD SEDIMENTATION IN THE ALTAI MOUNTAINS OF SIBERIA:WHAT CAN WE LEARN ABOUT GENERIC PROCESSES?
Paul Carling. University of Southampton, School of Geography, UnitedKingdomE-mail address: [email protected]
Sedimentary valley fill within the Chuja and Katun valleys of the AltaiMountains of south-central Siberia consists primarily of valley-marginalgiant bar deposits and valley-floor terraced gravels. The sedimentologyand stratigraphy may be best explained as owing to Quaternary megaflooddeposition within the valley network modified by Holocene braidplainevolution and dissection. Largely, or in part, the fill originally may havebeen complete across the valley floor with the giant bars now representingmarginal remnants of this fill trenched by later river action. Alternatively,gigantic gravel-bars may have developed across the entrances to valleystributary to the Chuja and Katun rivers with sedimentation occurring inthe main valley floors but at a lower elevation in comparison with theheight of the bars. Presently, there is insufficient evidence to choosebetween these two styles of sedimentation. Sedimentological evidenceindicates that the stratigraphy of the bars may be attributed to a series ofindividual floods that over-topped pre-existing flood-bar deposits. Eachflood entering the tributary valleys caused the deposition of an individualsequence of sedimentation that might be termed a rhythmite. Sedimen-tological evidence indicates that there is no obvious hiatus in depositionbetween rhythms of sedimentation, although the dating control wouldsuggest several thousand years separate some flood units. A simpleconceptual model is proposed to link environmental conditions and floodhydraulics with the observed sediment characteristics. Although there areimportant differences, the timing, scale and the basic style of bar-deposi-tion in the Altai Mountains of Asia are strikingly similar to that describedfor the Missoula-flood bars in North America, and the sedimentology issimilar to that of Icelandic jökulhlaup deposits. These observations raisethe possibility of developing a general model for styles of megaflooddeposition both modern and ancient.
RESPONSE OF THE SOUTHERN GREENLAND ICE SHEET TO EARLYHOLOCENE AND LAST INTERGLACIAL CLIMATE
Anders Carlson. University of Wisconsin-Madison, United StatesE-mail address: [email protected]
The early Holocene (11.5-6 ka) and the last interglacial (129-116 ka)represent the twomost recent periods when boreal summer temperatureswere significantly warmer than the pre-industrial from elevated NorthernHemisphere summer insolation. How the Greenland Ice Sheet (GIS)responded to these periods of enhanced radiative forcing provides insightinto its future stability as climate warms through this century. In the earlyHolocene, a combination of new and existing radiocarbon and 10-Becosmogenic dates indicate that the southwest GIS margin reached itspresent extent by 8.5-7 ka with subsequent retreat within its presentmargin. We compare this timing with fully coupled atmosphere-oceangeneral circulation model (GCM) simulations spanning the Holocene.GCM-predicted GIS melt anomalies of w6 mm/day for 7 ka are similar toGCM-predicted anomalies for 2030 to 2050 C.E. depending on the carbonemission scenarios. During the last interglacial, sea level wasw4 m higherthan present indicating a significant reduction in the GIS and/or AntarcticIce Sheet. To determine which southern GIS terranes contained significantice during the last interglacial, we use radiogenic provenance tracers ofGreenland sediment discharged to the ocean during the last two deglaci-ations and subsequent interglacials. Our results show that all southernGreenland terranes discharged sediment through the last interglacialsuggesting no single terrane was completely deglaciated similar to ourrecord for the last deglaciation and early Holocene. GIS models with ice on
all southern Greenland geologic terranes predict a sea-level contributionof w2.2 m, requiring a significant contribution from Antarctica to the w4m last interglacial sea-level high stand. The paleo-record of the southernGIS thus implies that the GIS significantly retreated in the past in responseto radiative forcings only slightly higher than current levels.
ICE SHEET CONTRIBUTIONS TO SEA-LEVEL RISE AND FRESHWATERDISCHARGE DURING THE LAST DEGLACIATION
Anders Carlson. University of Wisconsin-Madison, United StatesE-mail address: [email protected]
During the last deglaciation, global sea level rose 125-130 m with vari-ability arising from changes in ice-sheet melting or calving rates. Ice-sheetmargin retreat also rerouted continental runoff, changing the location offreshwater flux relative to sites of deepwater formation without neces-sarily changing sea level. The LGM was terminated by an abrupt 10-15 msea-level rise at w19 ka, likely sourced from retreat of Northern Hemi-sphere ice sheets in response to high-northern-latitude insolation forcing.Subsequent sea-level rise of w15 m between 19-14.5 ka is attributable toLaurentide (LIS) and Scandinavian (SIS) ice sheet retreat, with an addi-tional freshwater forcing delivered by Heinrich event 1 at w17 ka. Thesource of the abrupt 15-25 m sea-level rise in <700 years w14.5 ka,meltwater pulse 1a, remains widely debated. Geochemical, ice margin andfar-field sea-level records point to a small Northern Hemisphere ice-sheetcontribution. West Antarctic Ice Sheet land-records indicate retreat onsetat 14.0-15.0 ka, supported by geophysical modeling of far-field sea-levelrecords, suggesting a significant contribution to this meltwater pulse. Thecause of the subsequent Younger Dryas cold event is also debated. Theoriginal hypothesis, routing Lake Agassiz runoff from the Mississippi Riverto the St. Lawrence River, is questioned based on St. Lawrence Estuaryrunoff records, terrestrial outlet chronologies and ice-sheet modeling of anArctic meltwater forcing. However, there is a clear signal of eastwardfreshwater routing in the St. Lawrence Estuary and outlet chronologies arenot in conflict with the routing hypothesis. The modeled LIS discharge tothe Arctic is also too small to have caused the Younger Dryas and there isno evidence for an Arctic freshwater forcing. Much of the early Holocenesea-level rise can be explained by LIS and SIS retreat, with the LIS collapsew8.4 ka and routing of Lake Agassiz runoff to the Labrador Sea causing the8.2 ka cold event.
MARKERS OF MIS 5 SEA LEVEL FLUCTUATIONS IN THE BERGEGGIMARINE CAVE (NW MEDITERRANEAN SEA)
Luigi Carobene. Università degli Studi di Genova, ItalyE-mail address: [email protected]
A common method to define the tectonic setting of coastal areas is thecoupled use of geomorphological markers of past sea levels and radio-metric dating or chronostratigraphic cross-correlations. Marine cavesrepresent an important environment for this kind of studies, because theygenerally better preserve sea level markers with respect to coastal sectorsexposed to weathering.This study was carried out in the Bergeggi Cave, located in the westernLigurian coast (NWMediterranean Sea). The site is characterized by cliffs ofdolomitic limestones displaying several morphological indicators offormer sea levels. The detailed surveys carried out inside the marine caveallowed to identify both erosional and depositional markers of past sealevels up to 5.20 m asl. A particular attention was paid to the “marine wallgroove”, a marker already described in other Mediterranean areas, such asthe Orosei marine caves in Sardinia. These structures are the results ofa bio-erosion process, as testified by the large presence of L. Lithophagaboreholes, and are typical of the caves with low hydrodynamic regime. Adetailed mapping of the cave allowed to identify at least 3 ancientshoreline levels at 5.20 m; 4.35 m and 3.45. These shorelines wereattributed to M.I.S. 5.5 displaying that the last interglacial was character-ized by diverse marine highstands that may have created the evidence ofthe paleo-shorelines recorded inside the cave. Radiometric dating (U/Th),coupled with stratigraphic analysis of littoral deposits partially validatedthis chronologic hypothesis. These results are found in accordance withthe neotectonic setting already identified in contiguous areas, and add an
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