Diatom diversity vs. silica dissolution during the last glacial-interglacial cycle in Laguna Potrok...

Abstracts / Quaternary International 279-280 (2012) 346–461 395

To understand climate system dynamics, it is essential to have a closer lookat global climate change signals and their terrestrial documents. Here wefocus on the timing of the advance and retreat of glaciers in the past.During the Last Glacial Maximum (LGM), huge piedmont lobes coveredlarge areas of the northern Alpine foreland. This is evidenced by thedistribution of erratic boulders at different terminal positions of thepaleoglaciers. A timing of the maximum extent and the retreating phasesof the LGM can be established by exposure dating of these erratic boulderswith cosmogenic nuclides.For this purpose three boulders in the lower Reuss Valley were sampledfor surface exposure dating with cosmogenic 10Be. Two boulders arelocated on moraines of the LGM maximum extent. The third boulder islocated between two retreating stadial positions. Radiocarbon ages froma site located only several kilometers eastward of our study area yielda time span between the maximum extent and the recession of tworetreating stadials between approximately 32 kcal. BP and 23 kcal. BP.Our surface exposure ages for the maximal extent and the retreatingphases are younger. The oldest age is around 22 ka, and the boulder rep-resenting the local deglaciation is around 18 ka old. The exposure ages fitwell with the global LGM around 21 ka. Former studies suggest a two-cyclemodel for the maximal extension of the last glaciation in the Swissnorthern Alpine foreland. Consequently, further boulders have to be datedto verify this argument founded on field observations. It is important toensure that the detected time shift is not only caused by different datingmethods.

DIATOM DIVERSITY VS. SILICA DISSOLUTION DURING THE LASTGLACIAL-INTERGLACIAL CYCLE IN LAGUNA POTROK AIKE (PATAGONIA)

Cristina Recasens. University of Geneva, SwitzerlandE-mail address: cristina.recasens@unige.ch

Laguna Potrok Aike is a maar lake located in southernmost Patagonia and,as one of the few permanent lakes in the area, provides an exceptionalcontinuous sedimentary record. Diatom assemblages were analyzed athigh resolution on the composite core 5022-2CP, obtained within theframework of the ICDP-sponsored project PASADO (Potrok Aike Maar LakeSediment Archive Drilling Program). More than 216 species, varieties andforms have been identified so far, including endemic species and somenew to science. The quantitative analysis reveals diatom abundancesranging from nearly none to 460 million valves per gram of dry sediment,with substantial fluctuations along the record. During Glacial times,floristic assemblages are relatively stable with plankton being dominatedby Discostella stelligera. Remarkable diatom concentration peaks atdifferent depths in the coremost probably reflect variations in the nutrientavailability in the system. The causes for these changes in nutrients supplywill be inferred from the analyses of other proxies and a more detailed agemodel. The Late Glacial to Holocene transition is defined by a diversifica-tion of the plankton, with the occurrence of Cyclostephanos patagonicus atapproximately 15’550 cal yrs BP and its decline and stepwise replacementby indicators of more brackish conditions, Thalassiosira patagonica andCyclotella agassizensis, from approximately 11’500 cal yrs BP to present day.Additionally, a high degree of valve dissolution can be noticed in the top 10m of the record, corresponding to very low values of diatom concentrationin the sediment. Likewise, samples at the sediment-water interface,present equally dissolved specimens of Cyclotella agassizensis. The mech-anisms behind this dissolution, together with variations in the diatomabundance and species distribution will shed some new light on thebehavior of this lake, its internal chemistry and nutrient supply as well aslake level variations and periods of ice-cover.

HOLOCENE MICRO-CHARCOAL RECORD FROM LAKE IFRAH (MIDDLEATLAS – MOROCCO): CLIMATIC IMPLICATIONS

Hanane Redda. UAE. Faculty of Sciences and Techniques-Tangier, MoroccoE-mail address: reddad_hanane@yahoo.fr

Paleo-fire activity in North Africa and its connections with past climaticchanges still remains poorly documented. A high resolution multiproxy

analysis (using organic and inorganic matter, major and trace elements,magnetic susceptibility, charcoal and pollen data) from Lake Ifrah (MiddleAtlas –Morocco) provides new insights for better understanding paleo-fireoccurrence during the Late-Pleistocene and Holocene periods. Thecomparison between size-classes micro-charcoal distribution and lacus-trine reconstructed environmental variables further highlights the vari-ability of paleo-fire activity that is interpreted in term of climate-drivenchanges.Increases in micro-charcoal abundance associated to changes in arborealpollen composition likely testify regional emissions from forest fires andatmospheric transport. Such biomass burning events were associated toprolonged periods of drought, as inferred by synchronous abrupt decreasein major and trace elements contents (e.g., titanium which is a proxy ofsurface runoff input) and increase in carbonate concentration. In fact, thereconstructed fire activity not only follows the climatic variability thatoccurred during the Late-Pleistocene and Holocene (known by millennialphases of pronounced aridity), but also strikingly synchronizes with themajor phases of aridity previously highlighted by Damnati (2009), Chad-dadi et al. (1998), Benkaddour et al. (2005), and Rhoujjati et al. (2010).

CHIRONOMID-INFERRED PLEISTOCENE AND HOLOCENE TEMPERATURERECORDS FROM TASMANIA, AUSTRALIA

Andrew B.H. Rees. University of New Brunswick, CanadaE-mail address: a.rees@unb.ca

Tasmania is ideally situated to study past climate because it is positionedbetween two important controls of synoptic-scale climate: Antarctica tothe south and the Western Pacific Warm Pool to the north. Consequently,Tasmania has the potential to respond in concert with hemisphericclimate change. However, few well-dated, quantitative, paleoclimaterecords exist for the region so that important questions regarding thetiming and magnitude of climate events, such as the Antarctic ColdReversal (ACR) and Holocene Thermal Maximum (HTM), remain unan-swered. We present a consensus reconstruction for Mt. Field NationalPark, south-central Tasmania, based on three chironomid-inferredtemperature (TWARM) records spanning the lateglacial and Holoceneperiods. Preliminary results from Lake Selina, a site in western Tasmaniathat dates back to the previous interglacial, are also presented. Based onthe consensus reconstruction, TWARM reached modern values byapproximately 13 200 cal a BP and remained high until 12 500 cal a BP.After this early warm period, temperatures began to steadily cool,reaching a minimum around 8500 cal a BP. Subsequently, temperaturesincrease to culminate in a HTM around 4500 cal a BP. There is no evidencefor any major temperature reversal during deglaciation based onchironomid temperature inferences.

OZ-INTIMATE: TOWARD A CLIMATE EVENT STRATIGRAPHY FOR THEAUSTRALIAN REGION, 0-35 KA

Jessica Reeves. RMIT University, AustraliaE-mail address: Jessica.reeves@rmit.edu.au

INTIMATE (INTegration of Ice-core, MArine and TEerrestrial records) isa core programme of the INQUA (International Union for QuaternaryResearch) Palaeoclimate Commission (PALCOMM). The purpose of INTI-MATE is to gain a better understanding of 'geologically recent' climatechanges, particularly during the Last Ice Age (Glaciation) and the world-wide transition from ice age climates to the present 'interglacial' climate.In 2003, an Australasian project (AUS-INTIMATE) commenced, with formalrecognition from INQUA (Project Number 0806), and comprising twocentres of activity, one in Australia (OZ-INTIMATE) and one in New Zealand(NZ-INTIMATE). After an initial successful workshop in 2004, the Austra-lian chapter became somewhat dormant, and was reinvigorated in 2009,with a commitment made to the development of an Australian ClimateEvent Stratigraphy (CES) covering the last thirty thousand years. Toapproach the >60 degrees latitude, the Australia region was initiallydivided into four broad climatic zones: tropical, temperate, arid interior,Southern Ocean. From this, a minimum dataset to describe the majorclimatic events across the continent has been determined. A key time