geoforumch: a platform for geosciences in switzerland · des paysage et de la nature qui nous...

About us

GEOforumCH is the platform for geosciences of theSwiss Academy of Sciences (scnat) that serves asan interface between the various disciplines of geo-sciences, as well as between research, practice,administrations, politics and the public.

Our Partners

GEOforumCH groups together all the societies andcommissions of the Swiss Academy of Sciences thatare active in the field of geosciences and works inclose collaboration with the other platforms of theSwiss Academy of Sciences, its supporting organi-sations and agencies as well as with the privatesector.

Our missions are

• To support researchers in endeavours that requirecoordination between research institutions• To build links between research and practice• To foresee developments in geosciences which

could be of societal relevance• To identify possible shortcomings in terms of re-

search• To represent, if need be, the interests of Swiss

geosciences in international organisations• To inform targeted audiences:

- Researchers about developments in neighbouringdisciplines

- Researchers about needs arising from society andthe practice

- Practitioners about developments in the field of re-search

- Politicians about thematic priorities- The public about themes related to geosciences

Our products

• The Website www.geoforum.ch contains newsand information pertaining to geosciences withspecial focus on Switzerland (geological paths,museums, geotopes, didactical material)

• «GEOforumCH Actuel»: The quarterly Swissgeosciences information bulletin

• «SwissGeoWeb»: The interactive database con-taining the contact information of all persons andinstitutions active in the field of geosciences inSwitzerland

• «Geoscience Switzerland»: The "telephone book"of Swiss geoscientists (updated yearly)

• «Swiss Geoscience Meeting»: Foreward planningand coordination of this meeting that assemblesSwiss geoscientists once a year in a different lo-cation.

• Visions: foreseeing the impact of societal and po-litical changes on the future of geosciences as adiscipline and elaboration of corresponding solu-tions.

• Working groups (WG):

- WG Swiss Geotopes / Geoparks: Protectionof natural sites of geoscientific interest anddissemination of information on the subject.

- WG for the coordination between Universi-ties: Application of the Bologna declaration togeosciences in the Swiss Universities

- Swiss stratigraphic Committee: Recommen-dations for the naming of stratigraphic unitsand publication of the Swiss stratigraphicglossary.

- WG Teaching: promotes the teaching of geo-sciences in Swiss secondary schools

GEOforumCH:

a Platform for Geosciences in Switzerland

GEOforumCH | Schwarztorstrasse 9 | CH-3007 Bern | T +41 31 310 40 99 | F +41 31 310 40 29 |

[email protected] | www.geoforum.ch

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3rd Swiss Geoscience Meeting

Geologie ist allgegenwärtig: Jeden Tag trinken wirsauberes Wasser, wohnen und arbeiten in zuver-lässig fundierten und errichteten Gebäuden, be-nutzen sichere Verkehrswege, verwenden Papieroder Zahnpasta und freuen uns an Landschaft undNatur. All dies ist erst möglich, dank des Fach-wissens der vielseitig ausgebildeten Geologen!

Im Rahmen des Öffentlichkeitsprojektes «ErlebnisGeologie» - gemeinsam lanciert vom SchweizerGeologen Verband CHGEOL und vom GEOforum-CH - vermitteln Geologen durch informative Geo-Events für Jung und Alt die Bedeutung der Geologiefür unsere Gesellschaft und unseren Lebens-standard. In unser aller Interesse sollen Öffentlich-keit, Medien und Politik für die Belange der Geologiesensibilisiert werden.

«Erlebnis Geologie» findet erstmals am 1.–2.06.2007 an zahlreichen Standorten in der ganzenSchweiz statt. Der Anlass soll alle 3 Jahre wiederholtwerden.

Alle Geologen von geologischen Organisationen,Hochschulen, privaten Geologiebüros, Museen so-wie der mit Steinen und Erden arbeitenden Industriesind aufgerufen, sich für die Anerkennung derGeologen und der Geologie einzusetzen.

Machen Sie mit, und vermitteln Sie das «ErlebnisGeologie»!

La géologie est partout autour de nous: chaque jour,nous buvons une eau pure, habitons et travaillonsdans des bâtiments aux fondations stables, utilisonsdu papier ou de la pâte dentifrice et pouvons profiterdes paysage et de la nature qui nous entourent. Toutceci n'est possible que grâce au savoir-fairequ'apportent les géologues!

«Géologie vivante» est une manifestation initiéeconjointement par CHGEOL et GEOforumCH. Sonbut est de permettre aux géologues de transmettre àun public jeune et moins jeune, leur enthousiasmepour les "pierres", ainsi que de sensibiliser le public,les médias et les politiciens sur l'importance querevêt la géologie pour notre société et dans notre viede tous les jours.

Cette manifestation se tiendra pour la premièrefois les 1er et 2 juin 2007 en de nombreux endroitsrépartis dans toute la Suisse et devrait être répétéetous les trois ans.

Toutes les personnes actives dans le domaine dela géologie sont invitées à s'impliquer dans cettemani-festation, que ce soit au travers d'organisationsgéo-logiques, des hautes écoles, des bureaux degéo-logues, des musées, des entreprises liées à lapierre et au (sous-) sol ou en tant que simplesindividus.

Soyez nombreux à participer et à rendre la géologievivante!

Erlebnis GeologieGéologie vivante

Geologia viva

1CHGEOL & 2GEOforumCH

1 Schweizer Geologen Verband2 Platform for Geosciences of the Swiss Academy of Sciences

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Figure 1. Geologie erleben – Vivre la géologie (Photokredit: GeoPark Sarganserland-Walensee-Glarnerland)

Contact

Erlebnis GeologieOrganisationsteamc/o Schweizer Geologen Verband CHGEOLDornacherstrasse 29PostfachCH-4501 Solothurn

Tel. +41 (0)32 625 75 75Fax +41 (0)32 625 75 79

[email protected]

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Plenary Session

4D - earth: views through space and time

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Extremes of heat have been seen in the recent pastto have a determining influence on water resourcesin the Alpine region; the 2003 heat wave, for exam-ple, saw a severe curtailing of discharge in many ofthe source regions of Europe’s major rivers such asthe Rhine and the Rhone, affecting not only the Alpsthemselves but also the populated regions down-stream of the Alps.

While heat waves are commonly associated withthe summer period, they can also occur at othertimes of the year in the form of strong and positivetemperature anomalies, particularly in winter. Warmand persistent “winter heat waves” tend to lead toearly snow melt and unusually-high discharge inmany rivers; there is thus the potential for flooding,

especially if early snow melt is associated with pre-cipitation in the form of rain. This occurred severaltimes in the past, and the Rhine floods of February1995, are one example of a critical flood situationthat affected Germany and the Netherlands.

In a warming climate, many regional model pro-jections suggest that heat waves will increase both insummer and in winter, with a double response of hy-drological systems, i.e., strongly-reduced dischargein the summer, and enhanced and possibly flood-generating discharge in the winter. Under such cir-cumstances, it is crucial to envisage possible adap-tation of current water-management strategies to theplausible shifts in water resources in the future.

Heat Waves and Water in the Alps

Beniston, M. & *Zappa, M.

Department of Geosciences, University of Fribourg

* Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL), Switzerland

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Plenary Session

Studies in inter- and trans-disciplinary geoscienceare necessary to assess the long-term safety andsustainability of nuclear power production. Twocharacteristic examples are (a) paleo-seismologicalinvestigations relating to earthquake hazards for ex-isting power schemes and (b) climate-related paleo-glaciological and geomorphological assessmentswith respect to the long-term safety of nuclear-wastedeposits through coming ice ages. A brief overviewis provided of the corresponding research fields andof presently available results for conditions in Swit-zerland.

PALAEO-SEISMOLOGY PROVIDES INPUT FORNUCLEAR PLANT SAFETY IN SWITZERLAND

The safety of nuclear installations is paramount, andthe protection from earthquakes is a key element inthe Plant Safety Assessment, accounting for over80% of the total risk profile of Swiss nuclear powerplants. Switzerland is characterized by low crustaldeformation, resulting from the slow African-Europeconvergence as well as from the gravitational col-lapse of the Alpine chain. Moderate seismic activityis present along the whole Alpine domain. The as-sessment of seismic hazard in areas of moderateseismicity is inherently difficult, as the recorded his-tory of past earthquakes is not long enough to char-acterize the recurrence of large earthquakes, whichmay be occur every 10’000 yr and more on specificfaults. To help in the assessment of seismic hazard,we identify geological archives holding a continuousrecord of prehistorical activities and we mine them toilluminate the earthquake history of the past 10-15’000 years. The primary record in Switzerlandcomes from the sediments in the Swiss lakes, whichrecord both the seismically induced disturbances inthe otherwise regular sedimentation as well as wide-spread episodes of flank collapse and underwaterslamping produced by large shaking. Systematic in-

vestigations of the lakes in Central Switzerland al-lowed to identify 5 episodes in the last 12’000 years,of wich three were large enough to affect the wholeCentral Switzerland including the Zurich lake. Asecond important geological record is the identifica-tion of active faults: trenching and dating on theRheinach fault near Basel allowed to identify 3 pre-historical earthquakes of similar dimensions to thedestrictive 1356 earthquake. Finally, landslides androckfalls are often triggered by seismic shaking, andprovides additional information on past events. Pa-leo-seismological archives have produced a bountyof new information, allowing to reconstruct a consis-tent history of large earthquakes.

NUCLEAR WASTE DISPOSAL IN NORTHERNSWITZERLAND THROUGH COMING ICE AGES

The safety of the planned nuclear waste disposalsite in northern Switzerland (Benken) must be guar-anteed for one million years into the future. This timeinterval includes a number of coming ice ages withcorresponding surface and subsurface processesrelated to perennial ice. As a consequence, assess-ments must be made with respect to climatic condi-tions, glaciological characteristics and geomorpho-dynamic evolution during ice ages (Nagra 2002,2004).

Climatic conditions during stages of maximum iceextent in central Europe are quite well documentedand understood from paleoclimatic reconstructions(glaciers, permafrost, pollen etc.) and modernAOGCMs. They are primarily influenced by stronganticyclonic air circulation above the North Americanice sheet, a corresponding deviation of a jet-streambranch via the polar ocean and the freezing over ofthe Atlantic with winter pack ice reaching the latitudeof the Pyrenees. This causes the closing down of themain humidity source for, and an extreme cool-

Nuclear power, earthquakes and ice agesin Switzerland

*Giardini, G. & **Haeberli, W.

* Institute of Geophysics, ETH Zurich, Switzerland

** Department of Geography, University of Zurich, [email protected]

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ing/drying out of, central Europe. In comparison withmodern conditions, maximum regional depression ofmean annual air temperature is estimated at about15 to 20°C and precipitation is probably reduced byas much as 80%. In the future, the Earth may takeabout 50 - 70’000 years to “forget” anthropogenicallyenhanced greenhouse effects of the coming centu-ries and to enter another full ice age.

Under such conditions, large piedmont glacierscovering Alpine valleys and forelands have a poly-thermal structure with cold and temperate ice. Driv-ing stresses and thickness are low and, hence, iceflow and mass turnover strongly reduced. Continu-ous permafrost exists north of the Alps and extendsunderneath glacier margins. Thermal conditions andgroundwater flow are influenced to great depths be-low surface, can in places be dramatically differentfrom present-day conditions and must be consideredfor time periods of maximum ice extent in connectionwith deep burial of radioactive waste. While the initialbuild-up of ice-age ice must take place with higheratmospheric humidity and actively advancing gla-ciers, the terminal decay of glaciation in cold/drylateglacial environments is most likely a fast andlarge-scale downwasting or even collapse mecha-nism.

The most efficient processes of glacier erosionare related to subglacial water in temper-ate/maritime-type glaciers and are most likely to af-fect ground surfaces during advance periods of ice-age glaciers in the Molasse region and under humid

conditions. Subglacial excavation of deep burial forradioactive waste in marginal areas of polythermal tocold continental-type glaciers is less probable due toincreased cooling/drying and permafrost formationwithin the uppermost about 100 to 200 meters belowsurface. Even with ice-age glaciers advancing be-yond the maximum extent of the last (Würm-) glacia-tion, selective linear erosion of pre-existing valleysby relatively thick warm-based ice can be assumedto prevent excavation of completely new deep val-leys – which would threaten deep waste burial - onelevated terrain with cold-based ice.

The relative safety of such assessments aboutfuture long-term perspectives concerning abioticprocesses and landscape evolution at the plannedsite contrasts sharply with the extreme uncertaintyabout the knowledge, understanding and behaviourof humans and societies even in a very near future:the ice-age theory as a fundamental basis of modernscientific concepts for such assessments is not muchmore than a century old.

REFERENCES

NAGRA (2002): Technischer Bericht 99-08 „Geologische Ent-wicklung der Nordschweiz, Neotektonik und Langzeitszena-rien Zürcher Weinland“ von W.H. Müller, H. Naef und H.R.Graf

NAGRA (2004): “Eishaus + 106a - Zu Klima und Erdoberflächeim Zürcher Weinland während der kommenden Million Jah-re” von W. Haeberli

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Plenary Session

Hans Busch’s 1927 paper, which showed theoreti-cally that a coaxial magnetic or electric field couldfocus an electron beam and Louis de Broglie’s ear-lier hypothesis that electrons possess wave proper-ties, suggested to physicists that an electron micro-scope was not only possible but that it might produceresolution far superior to that of an optical micro-scope. The transmission electron microscope (TEM)was the first type of electron microscope to be de-veloped and is patterned exactly on the light trans-mission microscope except that a focused beam ofelectrons is used instead of light to "see through" thespecimen.

In contrast to X-ray diffraction and light micros-copy, mineralogists were not frontrunners in the de-velopment and use of electron microscopy. Early on,natural minerals such as micas were occasionallyused by physicists as model samples, because ofthe ease to prepare thin samples. Probably the firstsignificant application of TEM in mineralogy was thestudy exsolution textures in moonstone by Fleet andRibbe (1963). Feldspars and quartz remained in thefocus of conventional TEM investigations by a fewmineralogists for the remaining of the decade. Thebreakthrough in sample preparation with the adventof ion-milling, the return of lunar material and thenear atomic resolution possible with the generationof new microscopes available in the beginning of theseventies gave a big boost to the application ofelectron microscopy in Earth Sciences. Most of thework was devoted to investigate mineral microstruc-tures such as exsolution phenomena and crystaldefects. A great testimony of this first "golden age" ofTEM in Earth Sciences is the book Electron Micros-copy in Mineralogy by Wenk (1976). In the eightiesand nineties, mineral microstructures were not onlyinvestigated per se, but the TEM observations weremore and more used to extract informations aboutthe temperature-pressure-deformation history of thestudied samples (Buseck, 1992 and references

therein). In the last 10 years, geomicrobiology, lowtemperature processes such as diagenesis andweathering, environmental and health issues involv-ing minerals and rocks, shocked rocks and meteor-ites and high pressure/high temperature mineralphysics are in the forefront of the TEM scene inEarth Sciences.

I have chosen a few examples to show the ana-lytical potential and versatility of transmission elec-tron microscopy and associated spectroscopicmethods such as energy dispersive spectroscopy(EDS), electron energy loss spectroscopy (EELS)and energy filtered TEM (EFTEM).

Very fine grained minerals and rocks are naturaltargets for electron microscopy. Serpentinites arerocks in which individual grains can hardly be re-solved by light microscopy. The only analytical tech-nique to get textural, chemical and crystallographicinformation for such submicronic minerals is electronmicroscopy. The serpentine mineral group containsthree polymorphs, lizardite, antigorite and chrysotile.The structures of the latter two have been exten-sively studied by Electron Diffraction (ED) andHRTEM (f. ex. Grobéty, 2003). The misfit betweenoctahedral and tetrahedral sheets is relieved bybending of the layers, leading in the case ofchrysotile to a cylinder. Detailed electron diffractionanalyses revealed for the first time a natural struc-ture with 5-fold symmetry.

In antigorite the layers are also bent but the cur-vature changes periodically leading to a corrugated,wavy structure. The details of the structure are stillcontroversial (Grobéty, 2003). The wavelength inantigorite is not fixed, but varies with equilibrationtemperature. Wavelength distributions determined byelectron diffraction were, therefore, used to recon-struct the thermal history of contact metamorphicserpentinites.

Minerals at the atomic scale: Transmission electron microscopy

Grobéty, B.

Department of Geosciences, University of Fribourg

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The investigation of exsolution phenomena andalteration in amphiboles and pyroxenes (biopyri-boles) are classic examples of the use of transmis-sion electron microscopy to elucidate the real struc-ture of minerals. High-resolution TEM (HRTEM)investigations of anthophyllite and orthopyroxenesrevealed such predicted wide chain silicates (Veblen,1991, and references therein). It turned out that theformation of wide chains were an essential reactionstep in the transformation of amphiboles (Figure 1)and pyroxenes to layered silicates (Grobéty, 1997).

Figure 1. Lattice image of a "anthophyllite" crystal from AlpeBena, Ticino. The image is taken at Scherzer defocus with thebeam parallel to the c-axis. The bright spots are centered in thespace between two adjacent I-beam along the a - direction (A-site position in amphiboles). The positions of some I-beamsare schematically indicated in the image. In the left part of theimage is a double chain slab, whereas in the center and rightpart the sequence is disordered containing triple quadruple andquintuple chains. Slightly left from the center of the image, achange in chain width along the a - direction is visible (43 ->52). This is interpreted as frozen in reaction front (Grobéty,1997).

TEM observations were also responsible for thecontroversial claims of an ultra-deep origin of garnetlherzolites found in the Lepontine Alps e.g. at Cimadi Gagnone, Alpe Arami, and Monte Duria (Risold et

al, 2000). Although the electron diffraction resultsclaiming the presence of high pressure phases withilmenite compositions in these rocks could not beconfirmed, new TEM results on exsolution lamellaein diopside point to an origin of > 250 km.

The examples cited above put forward thestrength of transmission electron microscopy: it isthe possibility to obtain simultaneously structural,chemical and morphological information from thesame area of the sample at ultrahigh resolution. Theadvent of field emission microscopes and the devel-opment of energy filtering allow today not onlystructural resolution down to the atomic level butalso to push the resolution of chemical analysis tothe nanometer level.

REFERENCES

Fleet, S.G. & Ribbe, P.H. (1963): An electron-microscope in-vestigation of a moonstone. Philosophical Magazine 8:1179-1187.

Buseck (ed) (1992): Minerals and reactions at the atomic scale:Transmission electron microscopy. Reviews in MineralogyVol. 27, Mineralogical Society of America.

Grobéty, B. (1997): The replacement of anthophyllite by jim-thompsonite: a model for hydration reactions in biopyri-boles. Contributions in Mineralogy and Petrology 110: 237-247.

Grobéty, B. (2003): Polytypes and higher-order structures ofantigorite: A TEM study. American Mineralogist 88: 27–36.

Risold, A.-C., Trommsdorff, V. & Grobéty, B. (2000): Genesis ofilmenite rods and palisades along humite-type defects inolivine from Alpe Arami. Contributions in Mineralogy andPetrology 140: 619-628.

Veblen, D.R. (1991): Polysomatism and polysomatic series: Areview and applications. American Mineralogist 76: 801-826.

Wenk, H.-R. (1976): Electron microscopy in mineralogy.Springer Verlag, Berlin.

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Plenary Session

Assessment of current and prediction of future eco-system functioning, and change (trajectories, ampli-tudes and rates) will depend on how well we under-stand the ecosystems’ responses to forcing, bothnatural and man-made. Holocene paleoenviron-mental records offer unique and most informativehigh-resolution, multi-proxy, (potentially) worldwideinsight into a variety of forcing parameters and re-lated changes imposed on a broad range of eco-systems. Most critical are biological systems and thewater cycle. Despite the fact that future changes arelikely to be without analogs, Holocene records, inparticular the last 1000 years, provide quantitativeinformation on ecosystem transformation processes,which then can be compared and tested with nu-merical models to study ecosystem sensitivity, criti-cal thresholds and adaptation. This information isfundamental to assess ecosystem services and lifesupport systems under a changing regime, and toevaluate related risks. The Holocene matters!

The Holocene (11,500 cal yr B.P. to the present)is unique in several aspects: (i) it offers the study ofdifferent forcings at different time scales (Mi-lankovitch to interannual) and ecosystem responsesin a world with relatively stable interglacial boundaryconditions (atmospheric composition, sea ice andland cover) that are similar to those of today andmay, therefore provide ‘best’ analogues, (ii) it pro-vides insight into the amplitudes, rates of changeand spatial patterns of natural variability, which is akey for anthropogenic signal detection and attribu-tion, and which will always underlie the trajectories ofany human induced change, and (iii) discovers theincreasing impact of the human fingerprint, from thetime when early cultures started to engineer, trans-form and modify selected components of the EarthSystem from local to nowadays global scales. Holo-cene records provide insight into the functioning of

the Earth System at a degree of detail that that isunique and novel, but at the same time discloses in-creasingly significant inconsistencies and limitationsof the methods, techniques and interpretations cur-rently available.

In this keynote we will briefly review the globalboundary conditions and natural forcings at differenttemporal scales (orbital, solar, GHG, land cover)over the last 10,000 years, and summarize the eco-system responses in mid-/high- and low-latitude ar-eas. It appears that mid- and high latitude archivesrecorded mostly (summer)-temperature changes,while large-scale and high-amplitude changes in thehydrological cycle were most pronounced and fun-damental for the (sub)-tropical areas. While at multi-millennial time scales, orbital forcing (mainly summerinsolation) was the main driver for summer tem-perature changes in the NH mid-latitudes (“HoloceneThermal Optimum”) or summer precipitation in theNH (sub)tropics (“Early Holocene paleomonsoon”),solar forcing is believed to be the key forcing atcentennial to multi-decadal scales, and volcanicforcing (mostly on the northern hemisphere) at thesub-decadal scale. High-frequency solar and vol-canic forcing seem to be less important on thesouthern hemisphere, which, in theory offers thusbest opportunities for the detection of recentchanges in the greenhouse gas forcing. The lower-frequency changes are often modulated by oscilla-tors at (sub)decadal variability (such as e.g., ENSOand AO/NAO), which may be stochastic in nature, orbe locked in one phase over longer periods of time(climate regimes). Observations and models showthat, for instance, a strong NH summer monsoon(10-6 kyr B.P.) suppresses the frequency and inten-sity of ENSO, which in turn has strong impacts onecosystems in large parts of the world.

As a case study, the “Holocene Thermal Opti-mum” in the Alpine region will be highlighted and

Climate and ecosystem variability:from the Holocene to the Anthropocene

Grosjean, M. & *Tinner W.

NCCR Climate and Institute of Geography, University of Bern, Switzerland

* Institute of Plant Science, University of Bern, Switzerland

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compared with NH high latitudes, showing that earlyand mid-Holocene warm pools (mainly summer TT)were not stationary despite the orbitally driven sum-mer insolation forcing. It is on purpose, why this key-note focuses on the forcings and the reasons thatare thought to stand behind the observed Holoceneecosystem transformations. Although still limited, thisinformation is absolute key if Holocene ecosystemchanges (such as e.g. the extent of Alpine glaciers)are compared with current changes, were the ob-served result may be similar in nature but for verydifferent underlying reasons.

The Holocene experienced also several RapidClimatic Changes (RCCs) and “swings” with relatedhigh impacts on terrestrial ecosystems (often “coldpoles - dry tropics RCCs”, for the Little Ice Age “coldpoles – humid in some parts of the tropics”,Mayewski et al. 2004). During the most prominentRCC, the 8.2 event, many NH tropical lakes showmulti-centennial low-stands, while mid- and high-latitude areas reveal a cooling as indicated by bio-logical proxy in aquatic systems or pollen. It hasbeen argued that the RCCs are cyclic. While EarlyHolocene RCCs are attributed to major freshwaterreorganizations in the N-Atlantic, mid- and late Holo-cene RCCs are believed to be related to solar min-ima (such as e.g. the 2800 yr BP-event).

The (NH, extratropical) climate of the last 1000years are well documented and suitable to study thetransition from the Holocene to the Anthropocene.Data on the SH are still extremely scarce. New datasets emphasize the importance of seasonality andthe pivotal significance of the regional nature of natu-ral climate variability and change (Luterbacher et al.2004), and the related ecosystem responses. Whilethe structure of NH changes over the last 1000 yearsis largely consistent, a major controversy existsabout the amplitude of pre-industrial climate variabil-ity (Esper et al. 2005). This is a key to scale the sen-sitivity of the climate system to radiative forcing, andto assess the impact of future changes.

Recent developments in the field (higher resolution,new sites, new proxies) suggest the following currentshort-comings and future challenges for research:

- There is growing evidence that Transfer Functionsare not stable in time and the sensitivity of a givenproxy may change during the calibration period(Esper et al. 2005). The intrinsic problem is thatthe calibration period (20th Century) undergoesunusually strong trends. An up-date of data setsand independent reconstructions (documentarydata!) to extend the calibration period are needed.

There is growing evidence that the spatial structureof large-scale atmospheric patterns (teleconnectivity)is not persistent (Schmutz et al. 2000), which makesthe reconstruction of climate fields and indices, andthe comparison of ecosystem impacts from pointsources very difficult. The error can usually not beassessed.

- The transformation of a given signal (anthropo-genic or climate) into proxy information is mostlypoorly understood (seasonality, parameters andprocesses involved, diagenesis, response lagsand memory effects), and calibration periods areshort. More process studies are needed and new(statistical) analysis tools have to be employed(holds also for the curricula!).

- Natural archives are biased towards the warmgrowing (summer) season. The winter season ismostly not seen. The cold season is, at least forTT changes in Europe during the last 500 yearsmuch more important than the summer season.

- A strong development of new analytical techniquesand proxies (focus on corals, speleothems, lakesediments, organic geochemistry), and model-based interpretations (paleo-meteorology) takesplace. There is a lack of adequate data sets, par-ticularly for the cold and transitional seasons andlow-latitudes and the SH (Grosjean and Villalba,2005).

- In practice, the attribution of human impact andclimate/environment variability is often very difficultor impossible. Novel statistical approaches mayoffer perspectives.

REFERENCES

Bradley, R., et al. (2002): The Climate of the Last Millennium.IN : Alverson K. et al. (Eds) “Paleoclimate, global changeand the future”, pp. 105-149. Springer.

Esper, J., Frank, D.C., Wilson, R.J.S. & Briffa, K.R. (2005): Ef-fect of scaling and regression on reconstructed temperatureamplitude for the past millennium. GRL 32.

Grosjean, M. & Villalba, R. (2005): Regional Multiproxy ClimateReconstruction for Southern South America: A new Re-search Initiative. PAGES News 13(2), 5.

Luterbacher, J., Dietrich, D., Xoplaki, E., Grosjean, M., WannerH. (2004): European seasonal and annual temperaturevariability, trends and extremes since 1500. Science 303,1499-1503.

Mayewski, P.-A. and 16 co-authors (2004): Holocene climatevariability. Quaternary Research 62, 243-255.

Schmutz, C., Luterbacher, J., Gyalistras, D., Xoplaki, e: &Wanner, H. (2000): Can we trust proxy-based NAO indexreconstructions? Geophysical Research Letters, 27(8),1135.

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Plenary Session

Although the Andes and the Alps are extremely di-verse in terns of size, internal architecture, comple-tion of a Wilson cycle, etc. the Alps may have shareda common early pre-collisional stage of evolutionduring the Cretaceous with that of the Cenozoic An-des. In both cases, consumption of oceanic crustwas reflected by various modes of upper plate de-formation. Although the latter has been obscured inthe Alps owing to subsequent collision, its traces arewell preserved in the Eastern and Southern Alps.Yet, direct comparison between both orogens indi-cates a substantial difference: Alpine deformation,including also subsequent collision was essentiallyfocused on the forearc domain of the Adriatic plate(although no real arc is present with exception of theColli Euganei). The Alps have virtually grown bycontinuous material addition through mainly basalaccretion to the former forearc system. In contrast,nearly all of the Andean deformation seems to beconfined to the backarc domain. Only minor defor-mation has affected the South American forearcduring Cenozoic plateau building with very diversestyles from southern Peru to southern Chile. Themechanisms responsible for these features aregradually emerging from various ongoing researchinitiatives and may provide clues for understandingthe early evolution of the Cretaceous Alpine orogen.

In the case of the Alps initial stacking of the Aus-troalpine nappes during subduction of the PenninicOcean was succeeded by extensional collapse andcrustal thinning of the internally shortened forearcduring the Late Cretaceous Gosau stage with sub-stantial subsidence of the depositional surface (e.g.Schmid et al., 1996). This evolution has been inter-preted to reflect a stage of subduction erosion uponcollision of the active margin with the approachingoceanic spreading system (Wagreich, 1995). Otherauthors have argued that the kinematic evolution ofthe former upper plate may have been related toslab rollback (e.g. Froitzheim et al., 1997). However,

these processes are extremely difficult to reconstructfor fossil systems. In contrast, systematic analysis offorearc kinematics at the Circum-Pacific convergentmargins by Heuret and Lallemand (2005) has re-vealed the fundamental role of upper plate motionversus motion of the oceanic hinge line for deforma-tion.

A detailed analysis of kinematics in the SouthAmerican margin system reveals two main kinematicdomains (see Hoffman-Rothe et al., 2005, for de-tails). In southern Peru and northern Chile nearly theentire forearc is under extension, collapsing towardsthe trench with significant mass wasting at its base(Adam and Reuther 2000). The slope is steep withno accretionary prism existent and a trench fill ofless than 500m. The offshore forearc has subsidedby up to several km since the Middle Miocene whilea coastal Cordillera is uplifting at a very slow rate.Supported by the observed landward migration ofthe volcanic arc since the Jurassic (some 200 km)this set of observations has been interpreted to re-flect long-term basal erosion of the upper plate by arough sediment-starved oceanic plate (Rutland,1971).

In contrast, the southern Chilean margin reveals anarrow accretionary wedge, thick trench fill (> 2 km)steep slope only at the upper plate tip, a slowly up-lifting coastal Cordillera and dominantly shorteningkinematics within the wedge. This kinematic mode istypical of slow accretion with most of the sedimentsubducted towards greater depth. This kinematic re-gime has only started in the Pliocene with some ac-celeration of rates since. From a study of erosion ofthe Main Cordillera it becomes obvious that trench fillwith significant rates has also only started during thatperiod and was controlled by Southern hemisphereglaciation starting around 7 Ma. A series of analogueexperiments clearly demonstrates that the differencein behaviour of these two contrasting styles is in ef-

Forearc deformation at convergent margins – does the Peru-Chilesystem throw light on the early Alpine evolution?

Oncken, O. and SFB267/TIPTEQ research groups

GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

37


fect dominantly controlled by the trench fill thickness.Hence, we argue that the Cenozoic climatic evolu-tion of the Andean margin and its meridional trendthrough various climate zones was instrumental indetermining the various mass flux modes at themargin and its kinematic response.

On a short-time scale, accumulation of this kine-matic mode can be seen to be partly related to thestyle of seismicity. The extent and degree of seismiccoupling play a major role in force transmission aswell as in the generation mechanism of great inter-plate earthquakes with nearly all interplatemegathrust earthquakes (magnitudes >8) occurringin the seismogenic coupling zone between the con-verging plates. Despite the key role of the couplingzone for plate tectonics, the processes that shape itand its relation to surface deformation are poorly un-derstood. Project TIPTEQ (From the incoming plateto megathrust earthquakes) is currently analyzingthe seismotectonic deformation and its control insouthern Chile, the site of the 1960 Chile earth-quake. The vision of our integrated study is a quan-titative understanding of megathrust earthquakeseismicity in subduction zones and its relation toprocesses at depth and at the surface, and betweenincoming plate and epicentre. We have started witha series of experiments that are designed to imagethe processes operating at the seismogenic plateinterface and their effect for surface deformation.

First results clearly show that upper plate defor-mation is governed by various transients. Besidesthe well known observation of an elastic strain cycleobserved by GPS during the seismic cycle, we notethat permanent deformation is probably accumulatedduring key parts of this cycle as well as during tran-sients with longer periods. Local upper plate faultingappears to occur either during the postseismic re-laxation stage (Northern Chile) or during the in-terseismic accumulation stage (Southern Chile) andonly rarely during the coseismic events. Along mostof the Chilean coast discontinuous uplift is recordedby terraces and uplifted strandlines. Only restrictedareas occurring above segments preferentially slip-ping with very large events may show local subsi-dence. In addition, activation of faults in the forearcmay encompass the entire domain until the arc, atdistances of the faults of more than 100 km from theseismogenic coupling zone.

Based on these first observations, we concludethat the forearc system tends to be close to self-organized criticality, reacting in complex mode withhighly complex kinematics from surface to depth atdifferent time scales. This observation does not allowstandard separation of kinematic regimes or their re-

gional correlation, a consequence that may also bevalid for other tectonic settings. These kinematicvariations and the transients related are poorly un-derstood but probably a future key to understandingstrain accumulation in the brittle crust. Hence, in thepresent day Andean forearc as well as in the Creta-ceous Alpine system we may be facing a complexsystem of coupled processes responsible for defor-mation that primarily include the interaction of theclimatically controlled trench fill evolution, the upperplate structural heterogeneity and various transientsrelated to the seismic cycle as well as to changes inaccretion mode. These may – in conjunction withother aspects – ultimately be the cause for makingthe Alps a forearc orogen as opposed to the Andeanbackarc orogen.

REFERENCES

Adam, J., and C.-D. Reuther, Crustal dynamics and active faultmechanics during subduction erosion. Application of fric-tional wedge analysis to the North Chilean Forearc., Tec-tonophysics, 321, 297-325, 2000

Froitzheim, N., Conti, P., van-Daalen, M. 1997. Late Creta-ceous, synorogenic, low-angle normal faulting along theSchlinig Fault (Switzerland, Italy, Austria) and its signifi-cance for the tectonics of the Eastern Alps. Tectonophysics280: 267-293.

Heuret, A., Lallemand, S., 2005. Plate motions, slab dynamicsand back-arc deformation. Physics of the Earth and Plane-tary Interiors, 149, 31-51.

Hoffmann-Rothe, A., N. Kukowski, G. Dresen, H. Echtler, O.Oncken, J. Klotz, E. Scheuber, A. Kellner. Oblique conver-gence along the Chilean margin: Partitioning, margin-parallel faulting and force interaction at the plate interface.In: Deformation of the Andes from top to bottom (eds:O.Oncken, G. Franz, H.J. Götze, M. Strecker, P. Wigger, P.Giese, V. Ramos, G. Chong), Frontiers in Earth Sciences,Springer, (submitted)

Rutland, R.W.R., Andean orogeny and ocean floor spreading,Nature, 233, 252-255, 1971.Schmid, S., Pfiffner, O., Froitz-heim, N., Schoenborn, G. Kissling, E. 1996. Geophysical-geological transect and tectonic evolution of the Swiss-Italian Alps. Tectonics 15: 1036-1064.

Wagreich, M. 1995. Subduction tectonic erosion and Late Cre-taceous subsidence along the northern Austroalpine margin(Eastern Alps, Austria). Tectonophysics 242: 63-78.

38

Plenary Session

The planet Earth in accelerated change is a commonresearch focus at the Department of Geography Uni-versity of Zurich (Müller-Böker et al 2003). Soil andwildland fire may change along with globalchange.From a global perspective fire is a frequentfactor in forest ecosystems. Fire influences size andcomposition of the soil organic carbon (SOC) pool bycontrolling aboveground biomass and by altering theamount and composition of SOC via oxidation toCO2 + H20 + heat or production of pyrogenic organiccarbon during the combustion process (simplifiedafter Pyne et al. 1998).

In the global carbon cycle, fire is a major short-term C source to the atmosphere (0.137 to 0.194 PgC yr-1 in Europe or overall Russia, respectively). Onthe long run it may provide a small long-term C sink(globally 0.05 to 0.27 Pg C yr-) via the production ofdegradation-resistant black carbon (BC). QuantifyingBC, and studying how fire alters the molecular SOCstructure and thus its degradation resistance, maybe a key in understanding C dynamics in forest soils.(Czimzik et al. 2005). The fate and turnover rates ofblack carbon from biomass burning, however, hasnot yet been understood (Figure 1, Schmidt 2004).

For soil chemical properties, few studies existon the conversion to BC of aboveground biomassfrom (sub-)tropical and temperate ecosystems.Some studies investigated fire effects on C stocks ormolecular SOC structure. However, little is knownabout how fire affects the amount and composition offorest SOC, especially in the boreal region.

Since the early human history the deliberate useof fire as a management tool has left traces in the ar-chaeological record in soil. Evidence from CentralEuropean Loess region suggests that fire may be aso far overlooked factor for the formation of Cher-nozems.

Figure 1. Wildland fires form black carbon in ecosystems suchas savannah grasslands and high-latitude coniferous forests. a,Immediately after a fire, the local area is covered by largepieces and dust-sized particles of black carbon. b, After 60years, a few charred tree stumps remain among a new genera-tion of trees, and only small quantities of black carbon can bedetected in the soil. c, Its fate could be export by rivers into theoceans (the Yenessei River, which runs into the Arctic Ocean,is shown here). Recent results, however, indicate that some ofthe carbon must be degraded at an earlier stage or storedelsewhere (from Schmidt 2004).

Soil and wildland fire – from Boreal to Alpine fires

1Schmidt, M. W. I. & 2Allgöwer, B.

1 Soil Science and Biogeography, Department of Geography, University of Zurich, Switzerland2 GIS Division, Department of Geography, University of Zurich, Switzerland

39


Only little is known on how fire affected the evolu-tion of forest ecosystems in the Alps. It is not wellunderstood whether or not a natural fire regime ex-isted or if fire occurrence was mainly linked to hu-man settlement activities. For centuries, land-usepractices have altered ecosystem dynamics way be-yond their natural potential. Nevertheless, paleo-ecological studies in the Swiss National Park andsurroundings (Stähli et al. accepted, Allgöwer et al.in press) show that fire must have been a far morefrequent (natural) disturbance factor in the CentralAlps than assumed so far. The analysis of charcoal,pollen and plant macrofossils revealed species de-pendent fire return intervals from 250 to 600 yearsbefore the onset of human settlement. In the light ofdecreasing forestry activities and changing climaticconditions this is most significant as it reveals thepotential for natural fire occurrence and hence theeventual ‘return’ of a natural fire regime to the Alps.If and how society is willing or ready to deal with thisphenomenon, remains another question though.

REFERENCES

Allgöwer B., Stähli M., Bur M., Morsdorf F., Kötz B., FinsingerW., Tinner W. (in press): Mehr Wald – mehr Waldbrände?

Erkenntnisse aus 8000 Jahren Feuergeschichte und derheutigen Brandgutsituation im Schweizerischen National-park. Bündnerwald 6(2005), in press.

Czimczik C. I., Schmidt M.W.I., Schulze E.-D. (2005): Effects offire frequency on black carbon, soil organic matter stocks inPodzols Siberian Scots pine forests. European Journal ofSoil Science, 56 (1) Online publication date: 13-Oct-2004doi: 10.1111/j.1365-2389.2004.00665.x.

Gerlach R., Baumewerd-Schmidt H., van den Borg K., Eck-meier. E., Schmidt M.W.I. (submitted): Prehistoric alterationof soil in the Lower Rhine Basin, Northwest Germany – ar-chaeological, 14C and geochemical evidence. Geoderma,submitted.

Müller-Böker U., Haeberli W., Elsasser H. Brassel K., Itten K.,Schmidt M. W. I., Weibel, R. (2003): Earth in acceleratedchange: Habitats in the 21. century. Geographica Helvetica58: 184-196.

Pyne, S.J., Andrews, P.L. and Laven, R.D. (1996): Introductionto wildland fire, second edition. New York: John Wiley, pp.769

Schmidt M. W. I. (2004):. Carbon budget in the black. Nature.427: 305-306

Schmidt M. W. I., Noack A. G. (2000): Black carbon in soils andsediments: Analysis, distribution, implications, and currentchallenges. Global Biogeochemical Cycles 14: 777-794.

Stähli, M., Finsinger, W., Tinner, W., Allgöwer B. (accepted):Wildland fire history and fire ecology of the Swiss NationalPark (Central Alps): New evidence from charcoal, pollen

40

Plenary Session

The interior, shell-like structure of our planet isclearly constrained by geophysical measurements, inparticular through seismic studies. Recent develop-ments in seismology, most prominently in tomogra-phy, provide a more detailed structure that goes be-yond the fundamental separation in crust, mantleand core and clearly implies that the earth mantle isnot homogenous in vertical and lateral extent. Themost striking features are cold subduction zoneswhere material locally descents from the earth sur-face down to the core-mantle boundary and plumestructures where hot (or wet?) material ascents in 1-dimensional pipe-like structures from the core-mantle boundary towards the earth surface wheretheir existence is evidenced by extensive volcanismthat is unrelated to plate tectonics. Seismic studiesunequivocally demonstrate that the earth’s mantle issubdivided into several different layers, separated bysharp discontinuities in the seismic wave propaga-tion velocities at 400, 670 and 2700km (just 150 kmabove the core-mantle boundary), separating theupper mantle, from the transition zone, the lowermantle and the so-called d’’-layer (‘d-double-prime’).The high-density, magnetic core can be subdividedinto a liquid outer core and a solid inner core; recentspeculations point towards a further innermost corethat could be in liquid-state.

Seismology provides the basic data such as p-and s-wave propagation velocity profiles and therelative increase of these velocities at the discrete,sharp boundaries within the earth’s mantle. Seismicvelocities are, at first order, dependant on the com-pressibility (p-waves) and shear modulus (s-waves)of the material they travel through. These parame-ters, in turn, depend on the physical and thermody-namic properties of the constituent minerals formingthe bulk of the mantle and core rocks. Petrology,geochemistry and mineral physics can and do con-tribute the fundamental data required to interpret

seismic velocity profiles across our planet. The finalgoals of these efforts are to constrain the physicalstate, chemical and mineralogical composition in-cluding potential volatile contents and temperaturedistribution of the deep interior of our planet. How-ever, modern petrology and geochemistry goes con-siderably beyond feeding necessary parameters intoseismologic models. Trace element and isotope ratioconstraints allow assessment of processes operatingearly in the earth history related to accretion anddifferentiation of our planet, as well as distinction ofvarious long-lived ‘reservoirs’ located deeply insidethe planet.

Direct investigations and observations of materialoriginating from the earth interior are limited to thetop 250 km through exposed sections of mantlerocks (e.g. ultramafic massifs in the Alps and else-where, Ivrea-zone, Saas-Zermatt Zone, eclogite-facies ultramafics from the central Alps such as AlpeArami) providing direct access of the top 100 km.Deeper parts of the upper mantle (up to 250 km) areonly accessible through isolated decimetre-sizedpieces of rocks or single crystals brought to the sur-face as mantle xenoliths by alkaline and kimberlitemagmas. These direct witnesses that can be studiedby petrological and geochemical tools indicate thatthe earth’s uppermost mantle is dominantly lherzoli-tic, i.e. composed of olivine and pyroxenes with ad-ditional spinel or garnet depending on the depths.Further information is gained through igneous pe-trology investigating major and trace element com-positions as well as isotope ratios of primary mag-mas generated at various depths within the earthupper mantle

Information on the mineralogy and associatedphysical and chemical properties of deeper parts ofthe earth’s mantle or even its core is only accessibleby experimental petrology and mineral physics (in-cluding both experimental and computational meth-

Deep Earth – A petrologist’s view of the planet’s interior

Ulmer, P.

Institute of Mineralogy and Petrology, Department of Earth Sciences, ETH Zurich, Switzerland

41


ods) Experimental methods that have been devel-oped over the last 40 years, starting in 1961 with theintroduction of the solid-media high-pressure appa-ratus, opened a completely new field to simulate andinvestigate the deep interior of the earth, comple-menting geophysical methods. Equipment developedover the last 10-15 years enables petrological stud-ies to conditions of 40 GPa and 2800K, correspond-ing to conditions prevailing at 1000 km depth. Min-eral physics studies, employing externally and laser-heated diamond anvil cells, cover the entire pres-sure-temperature space of the earth, i.e. up 350 GPaand 6000K. In addition, powerful computer codesusing either molecular dynamics or ab initio simula-tions of crystal structures provide a complementarysource of information that is combined with experi-mental studies to constrain the properties of materi-als that build up the deep parts of the planet.

The most important outcomes of research in thesedisciplines regarding the earth deep interior are:

(1) Discrete changes in the seismic velocitystructure of the mantle observed at 400, 670and 2700km depth can directly be liked tostructural/mineralogical changes occurring inan isochemical mantle represented by a peri-dotitic bulk composition. A phase change inthe dominant mineral olivine is responsible forthe 400km discontinuity and a reaction of anolivine-type mineral to a perovskite-structuredmineral plus ferro-periclase is responsible forthe 670 km discontinuity. Very recent numeri-cal and experimental constraints indicate thatalso the seismic discontinuity close to thecore-mantle boundary separating the d’’-layerfrom the lower mantle is most probably asso-

ciated with a phase change from MgSi-perovskite to a ‘post-perovskite’ phase;

(2) Trace element and isotope geochemistry ofigneous rocks originating from various depthsranges and/or tectonic settings indicate thatthe mantle is chemically inhomogeneous withlarge-scale chemical heterogeneities thathave been maintained over very long periods,possibly originating from the accretion anddifferentiation of the planet more than 4 byago. (3) Gravimetric, seismologic and geo-chemical data require that the earth core iscomposed predominantly of Fe-Ni alloys butthat an additional ‘light’ element must be pre-sent. Oxygen, sulphur, hydrogen, carbon orsilicon or a mixture of these are consideredas likely candidates. Experimental studies onthese systems are currently under way; thesubject is vigorously debated and far from agenerally accepted solution.

Current and future research in petrology / mineralphysics on the deep interior of the earth focus on:

(1) Areas of extensive mass transfer betweendifferent ‘layers’ of the system such as sub-duction zones, mantle plumes and the core-mantle boundary;

(2) The processes that might have operatedduring the early history of the planet, accre-tion and differentiation, with particular em-phasis on the fractionation of elements be-tween various reservoirs (crust, mantle,core); and (3) The potential existence of amagma ocean deeply inside the planet dur-ing incipient stages of planetary differentia-tion.

42

Plenary Session

The sun and the planets formed ~4.6 billion yearsago from a collapsing fragment of a giant molecularcloud, a huge mass of dust and gas. At the sametime, many other stars formed in the same molecularcloud, and probably a sizeable fraction of them be-came accompanied by planets too. Star and planetformation is a major topic in present-day astrophys-ics. The Hubble telescope is providing exciting pic-tures of star forming clouds and protoplanetary diskswithin them but also of dying stars. These are re-turning processed matter into the cloud, from whereit may get incorporated into freshly born stars andplanets.

Meteorites are our best witnesses of the birth andearly evolution of our own solar and planetary sys-tem. Many of the same processes which can bestudied remotely in young stellar systems left tracesin meteoritic matter, and meteorites also allow us toshed light on processes which cannot (yet) be stud-ied remotely. As samples from left-over buildingblocks of planets, meteorites are also crucial for theunderstanding of the Earth and the other planets.

Fig. 1 shows the carbonaceous chondrite Allende.Chondrites are "cosmic conglomerates", as theirconstituents accreted from the "solar nebula", theflattened gas and dust disk around the young sun.All chondrites have subsequently been altered withintheir asteroid-sized parent bodies, but this aqueousor thermal metamorphism was weak enough to leavethe original structure more or less intact, allowing usto largely reconstruct the accretionary history. Thefigure shows the major constituents of chondrites:chondrules, calcium-aluminium-rich inclusions (CAIs)and "matrix" (metallic iron-nickel is hardly discerniblein this picture). It is still enigmatic how chondrulesformed, but they must have experienced probablymultiple melting events. CAIs exclusively consist ofminerals with very high melting points such as co-rundum and spinel. CAIs are thus thought to repre-

sent very early condensates from a cooling gaseous"nebula" of solar composition. Matrix partly consistsof volatile-rich low-temperature components.

Figure 1 (left). Sub-millimeter sized spheroidal chondrules andirregularly-shaped light calcium-aluminium-rich Inclusions(CAIs) of the Allende meteorite. Chondrules and CAIs are em-bedded in very fine-grained "matrix". Metallic iron-nickel grainsare rare in this meteorite type and hardly discernible here.Width of picture ~2cm.Figure 2 (right). A large (~5 µm) presolar graphite grain ex-tracted from the carbonaceous chondrite Murchison.

It has been known since many years that chon-drites are the oldest material available on Earth (inmacroscopic amounts) and the best samples to de-termine the age of sun and planets, as well as thetiming of a variety of subsequent processes in theearly solar system, such as formation and metamor-phic history of planetesimals. CAIs and chondrulescan now be dated by the Pb-Pb system with a preci-sion of better than 1 Ma. Somewhat surprisingly,CAIs are systematically older than chondrules by 2-3Ma (e. g. 4567.2±0.6 Ma vs. 4564.7±0.6 Ma). Thisage difference seems to be real, as it is confirmed byisotope systems involving "short-lived" nuclides (e. g.26Al, T1/2 = 0.7 Ma; 53Mn, T1/2 = 3.7 Ma). Short-livednuclides do not yield absolute ages, because the ra-dioactive parents are extinct, but the respectivedaughter nuclides (e. g. 26Mg and 53Cr) allow, in

The beginning: from dust to planets

Wieler, R.

ETH Zürich, Institute for Isotope Geology and Mineral Resources, Department of Earth Sciences, NO C61, 8092 Zürich

43


principle, to measure very precise age differencesbetween different early objects. It is not easy to ex-plain how CAIs should have been stored in the neb-ula for ~2-3 Ma before having been incorporated intolarger objects, because cm-sized objects in the neb-ula are expected to spiral into the sun in muchshorter times. Perhaps the nebula was very turbulentor CAIs once were parts of larger objects.

Extinct nuclides are also very useful to date laterevents, e. g. differentiation of planetesimals, coreformation of Earth and Mars, the giant Moon-formingimpact or the early degassing of Earth. A very pow-erful nuclide pair to date iron-silicate fractionation is182Hf-182W (8.9 Ma), because Hf does not entermetal. The mean age of core formation of the Earthis in the range of 30-50 Ma after formation of CAIs,the exact value depending on how well the cores ofimpacting planetesimals were remixed with the sili-cate portion of proto-Earth. Planetesimals them-selves, represented by certain differentiated meteor-ites, had separated into core and mantle muchearlier, often within a few Ma after CAI formation.

Whether or to what extent ages based on extinctnuclides are meaningful depends on how homoge-neous a nuclide was distributed in the solar nebulaor portions thereof. This issue is controversial and isclosely related to another hotly debated topic: theorigin of the short-lived nuclides present in the earlysolar system. The two contenders are nucleosynthe-sis in stars having formed in the same molecularcloud as, but earlier than the sun, and production bybreak-up of heavier nuclei through interaction withenergetic elementary particles, either from the gal-axy or the very active early sun itself. It has recentlybecome clear that both processes must have oper-ated, because 10Be (1.5 Ma) cannot have been pro-duced in stars, while 60Fe (1.5 Ma) is too neutron-rich to possibly have been produced by cosmic-rays.For other nuclides the origin remains unclear, but itappears now likely that at least some of them, e.g.26Al and 129I (15.7 Ma) have been homogeneouslydistributed in the nebula (at least in the parts wheremeteorite parent bodies formed), and hence providemeaningful age information. 26Al and 60Fe probablywere also potent early heat sources, responsible forthe melting and differentiation of the largerplanetesimals.

As noted above, CAIs are the oldest matter avail-able on Earth in macroscopic amounts. However,primitive meteorites contain tiny "presolar" grainsthat formed in outflows of stars near the end of theirlives before the sun was born. Fig. 2 shows a rela-tively large presolar graphite grain. SiC (also µm-sized) and nanometer-sized diamonds (each con-

sisting of roughly 1000 atoms only!) are other com-mon types of presolar grains. They are extracted bydissolving all other minerals by strong acids(HF/HCl). Much recent progress is being made to-wards more benign methods of extracting presolargrains, or analysing them in-situ. This led to the de-tection of less-refractory grain types, includingpresolar oxides and silicates.

Surprisingly, presolar grains carry a major fractionof the "primordial" noble gases of bulk meteorites,and today noble gases can even be measured insingle grains. The most important characteristics ofthese grains are extreme variations in isotopic com-positions of structural as well as trace elements.While isotope geochemists often deal with isotopicdifferences on the order of permil or less, diagramsdisplaying presolar grain data often require logarith-mic scales on the axes. For example, 12C/13C ratiosin presolar SiC range between about 1 and 10'000.This fact is compelling proof that these grains arepresolar, i. e. carry the nucleosynthetic fingerprints ofa single star (or a region thereof). They entered thesolar system and were subsequently incorporated inplanetesimals as intact stellar condensates. In con-trast, the extremely homogeneous isotopic composi-tion of almost all elements in all other terrestrial andextraterrestrial matter demonstrates that the solarnebula has been almost perfectly mixed otherwise.

Presolar grains thus allow us to test and refinetheories of the formation of the chemical elements instars as well as to gauge the galactic chemical evo-lution, i. e. the change in time and space of abun-dances of chemical elements in the galaxy. Hence,the study of presolar grains in meteorites is "Astro-physics in the laboratory", perfectly complementingclassical astrophysics.

REFERENCES

Hutchison, R. (2004). Meteorites – a petrologic, chemical andisotopic synthesis. Cambridge Univ. Press: 506pp.

Lauretta, D. S., Leshin, L. A. and McSween H. Y. (eds.) (2005).Meteorites and the early solar system II. Univ. ArizonaPress, Tucson AZ: in press.

McSween, H. Y. (1999). Meteorites and their parent planets,2nd ed. Cambridge Univ. Press: 310pp

44

Plenary Session

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