Integration of ground-penetrating radar, high-resolution seismic and stratigraphic methods in limnogeology : holocene examples from

Thesis

Reference

Integration of ground-penetrating radar, high-resolution seismic and stratigraphic methods in limnogeology : holocene examples from

western Swiss lake deposits

FUCHS, Michael

Abstract

Cette étude porte sur l'enregistrement sédimentaire de quelques lacs de Suisse occidentale ainsi que sa relation avec les changements hydrologiques et climatiques durant l'Holocène.

Cette recherche intègre des informations géomorphologiques continues, du domaine terrestre au domaine lacustre profond, fournies par deux méthodes d'imageries géophysiques et une analyse "multiproxy" de carottes sédimentaires. Les données géophysiques ont été calibrées par des descriptions sédimentologiques détaillées et des mesures pétrophysiques haute résolution afin de déterminer les facteurs de production et de distribution des sédiments lacustres. De plus, les analyses minéralogiques, physico-chimiques et minéralogiques magnétiques permettent de déterminer les paramètres contrôlant les conditions paléoenvironnementales. Cette thèse comporte trois parties: 1) une phase de tests des méthodes géophysiques ; 2) une application de ces dernières sur le "site calibré" de la Baie de Genève (Léman) ainsi qu'une 3) étude intégrée complète (géophysique, sédimentologique et stratigraphique) du "site [...]

FUCHS, Michael. Integration of ground-penetrating radar, high-resolution seismic and stratigraphic methods in limnogeology : holocene examples from western Swiss lake deposits . Thèse de doctorat : Univ. Genève, 2008, no. Sc. 3997

URN : urn:nbn:ch:unige-22794

DOI : 10.13097/archive-ouverte/unige:2279

Available at:

http://archive-ouverte.unige.ch/unige:2279

Disclaimer: layout of this document may differ from the published version.

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UNIVERSIT ´E de GEN `EVE

D´epartement de G´eologie et Pal´eontologie

FACULT ´E DES SCIENCES Professeur Georges GORIN Professeur Walter WILDI Docteur Milan BERES

Integration of Ground-Penetrating Radar, High-Resolution Seismic and Stratigraphic

Methods in Limnogeology: Holocene

Examples from Western Swiss Lake Deposits

THESE

pr´ esent´ ee ` a la Facult´ e des sciences de l’Universit´ e de Gen` eve

pour obtenir le grade de Docteur ` es sciences, mention Sciences de la Terre

par

Micha¨ el FUCHS

de Hofen (SH)

Th` ese N

3997

GENEVE

Atelier de reproduction de la Section de physique

2008

Fuchs, M.:

Holocene examples from western Swiss lake deposits.

Terre & Environnement, vol. 77, xxii + 254 pp. (2008)

ISBN 2-940153-76-0

Section des Sciences de la Terre, Université de Genève, 13 rue des Maraîchers, CH-1205 Genève, Suisse Téléphone ++41-22-702.61.11 - Fax ++41-22-320.57.32

http://www.unige.ch/sciences/terre/

Integration of ground-penetrating radar, high-resolution seismic and stratigraphic methods in limnogeology:

Abstract

This study focuses on the Holocene sedimen- tary record from some lakes in western Switzerland (Swiss Plateau and Jura) and its relationship with hydrological and climatic changes. Continuous geo- morphological information from onshore to offshore lacustrine settings is provided by two geophysical methods and integrated with a ‘multiproxy’ anal- ysis (pooling of indirect data) of sediment cores.

The geophysical data were calibrated with detailed sedimentological descriptions and high-resolution petrophysical measurements in order to determine the main processes of lake sediment production and distribution. In this framework, parameters con- trolling the palaeoenvironmental conditions are de- termined through mineralogical, physico-chemical and magnetic mineral analyses.

Three main parts comprise this work: (1) geo- physical test surveys; (2) a novel integration of two geophysical methods to the ‘calibrated site’ (i.e.

numerous previous studies) of the Geneva Bay area (Lake Geneva) and (3) a complete limnogeologi- cal study (geophysics, sedimentology and stratig- raphy) of the ‘exploration site’ of Lakes Joux and Brenet (Joux Valley).

Geophysical survey tests

High-resolution reflection seismic (HRS) meth- ods are routinely used for determining relatively large lake-level fluctuations in lakes. However, sev- eral drawbacks limit its use in shallow-water and onshore settings. In the former, gas accumulation in sediments and multiple seismic reflections from the lake bottom are known to deteriorate the qual- ity of the data. Onshore high-resolution seismic methods are expensive and provide inadequate res- olution for Holocene sediment mapping.

Ground penetrating radar (GPR) methods are uncommon in lake-level studies but can be used in areas where HRS methods are less reliable. Thus, extensive GPR survey tests were conducted in nu- merous lakes and their shores (Geneva, Brenet, Joux, Murten, Biel and Taill`eres) with different an- tenna frequencies. These surveys allowed the eval-

uation of the usefulness and limitations of these methods, the identification and characterisation of typical radar facies/patterns in the littoral envi- ronments and the selection of two or three sites for conducting an integrated limnogeological study.

The main innovative aspect of this study is the use of GPR methods for continuous on- shore/offshore (i.e. amphibious) profiling. GPR survey tests show that despite the high water and fine-sediment contents of the investigation environ- ments and independent of the antenna frequency, sediments can be continuously imaged through the uppermost ca 3 m of both onshore and offshore settings. Below about 3 m water depth, the radar signal is attenuated quickly so that even the rela- tively strong lake-bottom reflection can no longer be seen. Reflections can be followed continuously beneath the lake-bottom multiple and from offshore to onshore. Four main radar facies, correspond- ing to distinct ‘sedimentary bodies’, are identified:

sheet-drape deposits, flat and lens-shaped beaches (‘t´enevi`ere’: natural and/or anthropic submersed shingle beach), beach ridges and associated com- plex, and prograding macrophyte-colonised littoral marl benches.

Shallow-water, nearly-coincident GPR and HRS (pinger 3.5-kHz) data allow the assessment of the comparative and complementary aspects of these two methods. Although these techniques record reflections generated by different impedance con- trasts (electrical vs acoustical), they both have ver- tical resolutions in the decimetre range, and the first metre of sublacustrine sediments displays sim- ilar reflection terminations at similar locations.

Contribution of HRS and GPR methods to sedimentological studies of the Geneva Bay area

GPR and HRS data acquired in the Geneva Bay area were calibrated with sedimentological and ar- chaeological data, as well as with the few existing publications of seismic studies. The acquisition of 14 kilometres of HRS data, 17 offshore GPR pro- iii

files, 65 onshore profiles, the extraction of 2 off- shore cores, the analysis of an archaeological ex- cavation as well as the consultation of 56 geotech- nical borehole data improve the understanding of the lacustrine sedimentary processes since the Late Pleistocene.

Offshore HRS investigations revealed sedimen- tary complexities and extended the seismic stratig- raphy of the Late Quaternary sediments towards the city of Geneva. The additional analysis of pub- lished seismic data in combination with offshore GPR data confirms the present-day 3D geometry and sedimentary dynamics of a previously mapped Late Holocene ooidal sand body.

The onshore radar stratigraphy is based on dense grids of GPR profiles acquired on opposite sides of the Bay and is calibrated with an archaeological excavation in Parc La Grange. This excavation re- veals littoral settlement remains from the Late Ne- olithic period. The archaeological layer is located at the base of a unit containing alternations of sand, gravel, pebbles and cobbles, which overlie the com- pact glaciolacustrine clay of the ‘Banc de Travers’.

These coarse-grained deposits are interpreted as a progradational beach ridge complex corresponding to the well-known ‘+ 3 m lacustrine terrace’, which was formed during several short-lived highstands since the Late Neolithic period. GPR data allow the accurate mapping of the landward extent of these terrace.

Finally, the interpretation of the complex ge- ometries revealed by the HRS and GPR profiles confirms that the ‘Bise’, the northeastern wind, is the major agent for erosion and deposition in the Geneva Bay area.

Holocene hydrological and climatic changes inferred from an integrated mul- tiproxy analysis of Lakes Joux and Brenet sedimentary records

This study is based on:

• 9 km of GPR data (more than 110 profiles);

• 27 km grids of HRS profiles;

• The analysis of 14 sediment cores (9 offshore and 5 onshore cores), including petrophysical properties, sedimentological desctiptions and mineralogical magnetic measurements;

• Ages obtained with different dating tech- niques: radiocaesium-137, radiocarbon, palaeomagnetism and tephrochronology.

The seismic stratigraphy of Lakes Joux and Brenet comprises three main units: U0, U1 and U2. Calibration with the core data reveals a lake- specific subdivision of the HRS facies. Unit U0 represents undifferentiated bedrock and moraine deposits and is characterised by high-amplitude chaotic reflections. Reflections from unit U1 (Late Glacial) change towards the northeast: from chaotic and semi-transparent reflections, inter- preted as glacial sediments, to medium- / high- amplitude parallel reflections of glacially deformed deposits, and finally low- / medium-amplitude, continuous and parallel reflections. This unit repre- sents sediments deposited in a subglacial lake (SW) and the associated proglacial lake sediments (NE).

The southwestward thining of the latter indicates glacial retreat, and the deformed sub-facies point to small-scale glacier readvances. Unit U2 (post- glacial) produces medium- / high-amplitude, par- allel and continuous reflections and is interpreted as Holocene lacustrine deposits. These are charac- terised by very high carbonate contents (magnesian calcite).

The radar stratigraphy of the littoral environ- ments of Lakes Joux and Brenet includes four main GPR facies (G-0, G-1, G-2 and G-3), which were calibrated with core data. Unit G-0 generates a chaotic facies with a very high-amplitude reflec- tion at the top. It corresponds to glacial deposits (G-0a = glacial diamicton and G-0b = glaciola- custrine). Unit G-1 is imaged by low- to high- amplitude parallel reflections inclined towards the lake. Unit G-2 produces low- to high-amplitude, sub-parallel sigmoidal reflections inclined towards the lake. These two units are interpreted as pro- grading macrophyte-colonised littoral marl bench deposits. Unit G-3 is observed in offshore pro- files within unit G-2 and creates relatively low- amplitude dome-shaped reflections interpreted as beach ridge deposits. Its presence constitutes the evidence of a low lake level at ca - 3 m. Radiocar- bon ages indicate several short-lived lowstand peri- ods during the Medieval Climatic Optimum. Units G-1 to G-3 corrrespond to Holocene lacustrine de- posits. The radiocarbon dating of the transition between units G-1 and G-2 provided an age of 5843

±103 cal yr BP, which coincides with the increas- ing frequency of lake-level fluctuations recorded in many lakes in the Jura and Northern Subalpine ranges.

Magnetic mineral data obtained from core BR- N1 allow the dating of certain important hydro-

v

logical and climatic changes. The S-ratio curve shows a positive correlation with δ¹⁸O data from the ’GISP2 ice core record’ and evidences the cold and dry event of the Younger Dryas as well as the Bølling - Allerød warmer period. Magnetic suscep- tibility (κ), SIRM and SIRM/κ show three dis- tinct positive peaks at 292.5 cm, 371.0 cm and 417.0 cm depth. Thin sections from these inter- vals indicate changes in the mineralogy (presence of quartz, plagioclase, amphibole and sanidine). Pub- lished limnogeological studies of marl lakes from the French Jura indicate that the positive mag- netic susceptibility peaks correspond to tephra lay- ers. Thus, on the basis of the ages provided by the S-ratio curve, it is proposed that the peak at 371 cm corresponds to the Laacher See Tephra (Eifel Volcanic Field, Germany) dated to 12’900±560 yr BP (Allerød - Younger Dryas transition). The up- per peak could represent the Ulmener Maar Tephra (Eifel Volcanic Field, Germany) dated to 9560¹⁴C yr BP (end of Preboreal). The lower peak can not

be attributed to a specific tephra.

Secular directional variations (declination, in- clination and intensity) of the geomagnetic field recorded in core BR-N1 correlates positively with the regional master curves, which allows the construction of a ‘magnetic’ age model. This model is enhanced by the addition of ¹³⁷Cs, ¹⁴C and tephrochronological ages. Thus, a detailed chronostratigraphical model extending back to ca.

18’000 cal yr BP is obtained.

Conclusions

In limnogeological studies, seismic stratigraphy and radar stratigraphy are complementary meth- ods, and when combined with the more ‘conven- tional’ sedimentary and magnetic analyses, allow a better understanding of the depositional processes since the last glaciation. This combination suc- ceded in confirming and reconstructing Lateglacial and Holocene hydrological and climatic conditions in Lakes Geneva, Joux and Brenet.

R´ esum´ e

Le pr´esent travail a pour but d’´etudier l’enregistrement s´edimentaire de quelques lacs de Suisse occidentale (Plateau et Jura) ainsi que sa relation avec les changements hydrologiques et cli- matiques au cours de l’Holoc`ene. Cette recherche a ´et´e effectu´ee par l’int´egration d’informations g´eomorphologiques continues du domaine terrestre au domaine lacustre profond fournies par deux m´ethodes d’imagerie g´eophysiques et une anal- yse ‘multiproxy’ (mise en commun de donn´ees indirectes) de carottes s´edimentaires pr´elev´ees en fonction des donn´ees g´eophysiques. Ces derni`eres ont ´et´e calibr´ees par des descrip- tions s´edimentologiques d´etaill´ees et des mesures p´etrophysiques haute r´esolution afin de d´eterminer les principaux facteurs de production et de distri- bution des s´ediments lacustres. Dans ce cadre, les analyses min´eralogiques, physico-chimiques et min´eralogiques magn´etiques permettent de d´eterminer les param`etres contrˆolant les conditions pal´eoenvironnementales.

Trois parties constituent l’ossature principale de cette ´etude : (1) une phase de tests des m´ethodes g´eophysiques ; (2) une application de ces derni`eres sur le ‘site calibr´e’ de la Baie de Gen`eve (Lac L´eman) ainsi qu’une (3) ´etude int´egr´ee compl`ete (g´eophysique, s´edimentologique et stratigraphique) du ‘site d’exploration’ des Lacs de Joux et Brenet (Vall´ee de Joux).

Tests g´eophysiques

La m´ethode de sismique r´eflexion haute r´esolution, ou HRS (High-Resolution Reflection Seismic) est commun´ement utilis´ee en milieu offshore, notamment dans l’´etude des fluctuations de niveaux lacustres. Cependant, de nombreux d´esavantages limitent son potentiel d’utilisation en milieu peu profond et en milieu terrestre.

Dans le premier milieu, la pr´esence de gaz dans les s´ediments (zones ‘sourdes’) ainsi que la faible profondeur du multiple du fond du lac d´et´eriorent fortement la qualit´e des donn´ees. En milieu terrestre, les techniques d’acquisition HRS sont

on´ereuses et de r´esolution verticale inad´equate.

Rarement appliqu´ees `a l’´etude des d´epˆots la- custres actuels, les m´ethodes g´eoradar, ou GPR (Ground-Penetrating Radar), peuvent combler les lacunes pr´ecit´ees de la sismique. Ainsi, de nom- breuses campagnes d’acquisition g´eoradar, ont

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et´e effectu´ees sur diff´erents lacs et leurs rivages (L´eman, Brenet, Joux, Morat, Bienne et Taill`eres) en utilisant diff´erentes fr´equences. Ces campagnes ont permis l’´evaluation du potentiel et des lim- ites de cette m´ethode g´eophysique, l’identification des principaux faci`es radars caract´eristiques de l’environnement littoral lacustre et la s´election de deux ou trois sites pour y mener une ´etude limnog´eologique int´egr´ee.

Le principal aspect innovateur de cette ´etude est l’utilisation du g´eoradar de mani`ere ‘amphibie’

pour fournir une image continue du domaine ter- restre au domaine lacustre. Les tests d’acquisition GPR montrent que, malgr´e la forte teneur en eau des milieux investigu´es et ind´ependamment de la fr´equence utilis´ee, les ondes ´electromagn´etiques p´en`etrent les trois premiers m`etres de s´ediments que ce soit en milieu terrestre ou lacustre. Sous une profondeur d’eau d’approximativement 3 m, l’att´enuation du signal devient importante `a tel point que mˆeme le r´eflecteur du fond du lac ne peut ˆ

etre identifi´e. Les r´eflexions peuvent souvent ˆetre point´ees de mani`ere continue `a travers le multiple du fond du lac ainsi qu’`a travers le rivage. Quatre principaux faci`es GPR correspondant `a des ‘corps s´edimentaires’ distincts ont ´et´e identifi´es : d´epˆots de drapages, t´enevi`eres (plages sous-lacustres de galets d’origine naturelle ou anthropique), rides de plages et leur complexe associ´e ainsi que des plate- formes littorales (beines) progradantes colonis´ees par des macrophytes.

L’´etude des performances et des r´eponses fournies par les donn´ees GPR et HRS (Pinger 3.5- kHz) co¨ıncidentes acquises sous une faible tranche d’eau permettent d’appr´ecier les aspects compara- tifs et compl´ementaires entre ces deux m´ethodes.

Bien que les ondes g´en´er´ees et re¸cues par ces deux vii

derni`eres soient diff´erentes (imp´edance ´electrique vs imp´edance acoustique), elles fournissent des donn´ees de r´esolution verticale de l’ordre du d´ecim`etre et les premiers m`etres de s´ediments sous- lacustres montrent des terminaisons de r´eflexions similaires.

La contribution des m´ethodes HRS et GPR dans l’´etude s´edimentologique de la Baie de Gen`eve

Les donn´ees acquises dans la r´egion de la Baie de Gen`eve ont pu ˆetre calibr´es avec l’aide de nombreuses donn´ees s´edimentologiques et arch´eologiques ainsi qu’avec les rares donn´ees sismiques existantes. L’am´elioration de la compr´ehension des processus s´edimentaires lacus- tres depuis le Pl´eistoc`ene sup´erieur a ´et´e rendue possible grˆace `a l’acquisition de 14 kilom`etres de profils HRS, de 17 profils GPR lacustres, de 65 pro- fils GPR terrestres, le pr´el`evement de deux carottes s´edimentaires lacustres, le lever de coupe d’une ex- cavation arch´eologique ainsi que la consultation de nombreux sondages g´eotechniques.

En milieu lacustre offshore, les investigations sismiques ont pr´ecis´e la complexit´e et prolong´e l’interpr´etation des mod`eles sismo-stratigraphique pour le Quaternaire sup´erieur du ‘Petit Lac’. De plus, la combinaison entre l’analyse d´etaill´ee de donn´ees sismiques publi´ees et de profils GPR a mis en ´evidence la g´eom´etrie 3D actuelle ainsi que la dy- namique s´edimentaire d’un banc de sable `a oo¨ıdes d´ej`a ´etudi´e par d’autres auteurs.

En milieu terrestre, la stratigraphie radar des d´epˆots lacustres est bas´ee sur des grilles serr´ees de profils GPR acquises sur les deux rives du lac et cal- ibr´ees principalement par l’´etude d’une excavation arch´eologique au Parc La Grange (rive gauche).

Cette derni`ere a r´ev´el´e des vestiges d’un site littoral du N´eolithique final. Cet horizon anthropique est situ´e `a la base d’une succession de sables, graviers et galets lacustres reposant sur les argiles glacio- lacustres du ‘Banc de Travers’. Ces d´epˆots grossiers sont interpr´et´es comme un complexe de rides de plages constituant la terrasse lacustre dite ‘ter- rasse de 3 m`etres’ construite par plusieurs phases successives de hauts niveaux lacustres depuis le N´eolithique final. Les donn´ees GPR ont permis de cartographier pr´ecis´ement l’extension terrestre de cette terrasse.

Finalement, l’interpr´etation des faci`es sismiques et radars ainsi que leurs g´eom´etries plus ou moins

complexes montre que la ‘Bise’, vent du Nord- Est, est le principal facteur r´egissant les processus d’´erosion et de d´epˆot dans la Baie de Gen`eve.

Etude des changements hydrologiques et climatiques holoc`enes bas´ee sur une analyse multiproxy de l’enregistrement s´edimentaire des Lacs de Joux et Brenet

Cette ´etude est bas´ee sur :

• 9 Km de donn´ees GPR (plus de 110 profils),

• 27 Km de grilles de lignes sismiques,

• 14 carottes s´edimentaires, dont 9 lacustres et 5 terrestres, sur lesquelles ont ´et´e mesur´ees les donn´ees de propri´et´es p´etrophysiques, de s´edimentologie ainsi que la min´eralogie magn´etique,

• datations obtenues par diff´erentes m´ethodes incluant: le radioc´esium-137, le radiocarbone, le pal´eomagn´etisme et la t´ephrochronologie.

La sismostratigraphie des Lacs de Joux et Brenet comprend 3 unit´es sismiques principales : U0, U1 et U2. Celles-ci ont ´et´e calibr´ees avec les donn´ees des carottes et sont divis´ees en sous- unit´es propres `a chacun des lacs. L’unit´e U0 cor- respond au substratum rocheux et moraines in- diff´erenci´es, caract´eris´e par un faci`es chaotique de tr`es forte amplitude. L’unit´e U1 (tardi-glaciaire) passe, du Sud-Ouest au Nord-Est, d’un faci`es chaotique `a sourd interpr´et´e comme repr´esentant des s´ediments glaciaires, passant lat´eralement `a des r´eflexions parall`eles, d’amplitudes moyennes

`

a fortes et d´eform´ees par l’action du glacier puis

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a un faci`es parall`ele et continu de moyenne `a faible amplitude. Cette unit´e correspond `a des s´ediments d´epos´es sous un glacier flottant (au Sud- Ouest) et les d´epˆots de lac pro-glaciaire associ´es (au Nord-Est). L’amincissement de ces derniers en direction du Sud-Ouest traduit le retrait du front glaciaire. Le sous-faci`es d´eform´e est dˆu `a de petites r´eavanc´ees du glacier. L’unit´e U2 (post-glaciaire) montre un faci`es parall`ele et continu de moyenne `a forte amplitude correspondant aux d´epˆots lacustres holoc`enes. Ceux-ci sont caract´eris´es par une tr`es forte teneur en carbonates (calcite magn´esienne).

La stratigraphie radar des environnements lit- toraux des lacs de la Vall´ee de Joux comprend 4 unit´es principales (G-0, G-1, G-2 et G-3) cal- ibr´ees avec les donn´ees de carottes. L’unit´e G-0

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pr´esente un faci`es chaotique dont le sommet mar- que un tr`es fort contraste d’imp´edance avec l’unit´e sus-jacente. Cette unit´e correspond `a des d´epˆots glaciaires (G-0a = diamicte, glaciaire et G-0b = glaciolacustre). L’unit´e G-1 illustre des r´eflexions parall`eles de faibles `a fortes amplitudes inclin´ees en direction du lac. L’unit´e G-2 montre des r´eflexions sigmo¨ıdales, sub-parall`eles de faibles `a fortes ampli- tudes inclin´ees en direction du lac. Ces deux unit´es sont interpr´et´ees comme ´etant des d´epˆots de plate- formes littorales (beines) progradantes colonis´ees par des macrophytes. L’unit´e G-3 est observ´ee sur quelques profils lacustres `a l’int´erieur de l’unit´e G- 2 et pr´esente des r´eflexions de relativement faible amplitude en dˆome correspondant `a des rides de plages. La pr´esence de ces derni`eres indique un bas niveau lacustre `a environ - 3 m. Les ˆages radiocarbone obtenus dans les s´ediments de cette unit´e indiquent plusieurs phases successives de bas niveaux lacustres durant l’optimum climatique du M´edi´eval. Les unit´es G-1 `a G-3 repr´esentent des d´epˆots lacustres holoc`enes. La transition entre les unit´es G-1 et G-2 a ´et´e dat´ee par radiocarbone `a 5843 ± 103 cal yr BP. Cette date co¨ıncide avec l’augmentation de la fr´equence des fluctuations de niveaux lacustres enregistr´ee dans plusieurs lacs jurassiens et nord subalpins.

Les informations fournies par les donn´ees de min´eralogie magn´etique obtenues sur la carotte BR-N1 permettent la datation de certains

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ev`enements ainsi que leur relation avec des change- ments climatique et hydrologiques. Les donn´ees de S-ratio montrent une tr`es bonne corr´elation avec les donn´ees δ¹⁸O du ’GISP2 ice core record’ et mettent en ´evidence l’´episode froid et sec du Dryas r´ecent ainsi que les p´eriodes plus chaudes du Bølling et de l’Allerød. Les donn´ees com- bin´ees de susceptibilit´e magn´etique volumique (κ), SIRM et SIRM/κ mettent en ´evidence 3 pics de valeurs positives, `a 292.5 cm, 371.0 cm et 417.0 cm de profondeur. L’observation des lames minces faites `a partir de ces trois niveaux r´ev`ele

des changements min´eralogiques (notamment par la pr´esence de quartz, plagioclases, amphiboles et sanidines). De plus, par analogie avec d’autres

´

etudes limnog´eologiques men´ees sur les lacs car- bonat´es du Jura fran¸cais, les pics de suscepti- bilit´es magn´etiques correspondent `a des d´epˆots de t´ephras. Ainsi, sur la base de l’attribution de l’intervalle du Dryas r´ecent il est propos´e que le niveau situ´e `a 371 cm de profondeur corresponde aux retomb´ees du Laacher See Tephra (province de Eifel, Allemagne) dat´e `a 12’900±560 yr BP (tran- sition Allerød - Dryas r´ecent). Le niveau sup´erieur pourrait correspondre aux retomb´ees de l’Ulmener Maar Tephra (province de Eifel, Allemagne) dat´e `a 9560¹⁴C yr BP (fin du Pr´ebor´eal). Le peu d’indices r´ecolt´es dans le niveau inf´erieur ne permettent pas d’attribuer ce niveau `a un t´ephra sp´ecifique.

Sur la base des donn´ees de variations s´eculaires du champ magn´etique terrestre (inclinaison, d´eclinaison et intensit´e) enregistr´es dans les s´ediments lacustres de la carotte BR-N1 et leur comparaison avec les courbes de r´ef´erences r´egionales, il est possible d’obtenir des ˆages pr´ecis le long de la colonne s´edimentaire. De plus, en ad- ditionnant les datations obtenues par ¹³⁷Cs, ¹⁴C et t´ephrochronologie `a ces ˆages magn´etiques, un mod`ele chronostratigraphique d´etaill´e a ´et´e obtenu pour les derniers∼18’000 ans (ˆage calendaire cal- ibr´e).

Conclusions

La stratigraphie sismique et la stratigraphie radar sont des m´ethodes compl´ementaires qui, as- soci´ees aux m´ethodes stratigraphiques plus ‘clas- siques’ s´edimentaires et magn´etiques, permettent de mieux comprendre les processus d´epositionnels qui se sont succ´ed´es `a partir du dernier retrait glaciaire `a aujourd’hui. Leur combinaison a per- mis de confirmer et de reconstruire les condi- tions hydrologiques et climatiques au cours du Tardiglaciaire et de l’Holoc`ene dans les lacs L´eman, de Joux et Brenet.

Remerciements

Je souhaite `a travers ces quelques lignes exprimer toute ma reconnaissance et ma gratitude aux diff´erentes personnes qui, chacune `a leur fa¸con, de pr`es ou de loin, ont contribu´e au bon d´eroulement de ce travail de th`ese.

Cette recherche a ´et´e soutenue financi`erement par le Fonds National Suisse de la Recherche Scientifique (projets n^◦ 21-67082.01 et n^◦ 200020-107546, Dr. Milan Beres).

Je tiens `a remercier le Dr. Milan Beres, le Prof. Georges Gorin et le Prof. Walter Wildi, directeurs de cette th`ese, pour m’avoir propos´e ce sujet de recherche passionnant, pour la confiance ainsi que la grande libert´e d’action qu’ils m’ont accord´es. Leurs connaissances et disponibilit´es ont grandement contribu´e `a la qualit´e de ce travail et `a l’´edition du pr´esent manuscrit. Je tiens particulierement `a t´emoigner ma profonde reconnaissance au Dr. Milan Beres qui est `a l’origine de ce projet. Il m’a initi´e aux secrets de la g´eophysique et m’a accompagn´e avec son expertise lors de toutes les campagnes de carottage et d’acquisition de donn´ees g´eoradar et sismiques et lors du traitement de ces derni`eres.

Je remercie le Dr. Flavio Anselmetti (Swiss Federal Institute of Aquatic Science and Technology), membre du jury, pour avoir guid´e les premi`eres campagnes d’acquisition sismique et de carottage sur les lacs de Joux et Brenet, pour m’avoir accueilli dans le LimnoLab de L’ETH Z¨urich, pour ses conseils d’expert en limnog´eologie et pour son enthousiasme. Je souhaite ´egalement lui adresser mes chaleureux remerciements pour avoir accept´e d’´evaluer le pr´esent manuscrit.

Merci au Prof. Fran¸cois Marillier (Universit´e de Lausanne) qui m’a prˆet´e le syst`eme g´eoradar indispensable

`

a cette ´etude.

Le Dr. Daniel Ariztegui m’a fait partager avec enthousiasme ses connaissances en limnog´eologie lors de nombreuses discussions et je souhaite aussi lui t´emoigner mes sinc`eres remerciements.

Je tiens ´egalement a remercier le Dr. St´ephanie Girardclos pour avoir guid´e les campagnes d’acquisition sismiques sur le lac L´eman, pour les nombreuses discussions que nous avons ´echang´e ainsi que pour son enthousiasme stimulant.

Nicolas Waldmann, Daniel Whittle et Gr´egory Fr´ebourg ont apport´e leur bonne humeur, leur force et leur courage lors des campagnes de carottages sur les lacs de Joux et Brenet. Je tiens `a les remercier vivement pour cette aide pr´ecieuse et indispensable sur le terrain.

Merci ´egalement aux arch´eologues Pierre Corboud et Christiane Pugin (D´epartement d’Anthropologie et d’Ecologie, UNIGE) pour les nombreux ´echanges de discussions constructives.

Je remercie le Dr. David Williamson (CEREGE, Aix-en-Provence, France) pour m’avoir accueilli dans son laboratoire et initi´e au pal´eomagnetisme. Ses connaissances m’ont ´et´e d’une aide tr`es pr´ecieuse dans l’acquisition, le traitement et l’interpr´etation de mes resultats, et je le remercie pour ses nombreux conseils.

Mes remerciements vont `a Catherine Ginibre pour les analyses microsonde `a la recherche de tephras, le traitement des donn´ees et les discussion ´echang´ees. Merci `a Fran¸cois Gishig pour sa gentillesse, son efficacit´e, sa rapidit´e et sa disponibilit´e au laboratoire. Merci au Dr. Rossana Martini pour sa bonne humeur et sa grande disponibilit´e pour les analyses au MEB. Je souhaite remercier ´egalement Jacqueline Fellmann pour la gestion des tˆaches administratives ainsi que Sandra Levai pour son aide dans les recherches bibliograhiques.

xi

Ma reconnaissance et mes remerciements s’adressent ´egalement `a tous les autres membres du corps enseignant ainsi qu’aux doctorants et post-doctorants du D´epartement de G´eologie et Pal´eontologie de l’Universit´e de Gen`eve. Parmi les enseignants, je souhaite remercier particuli`erement le Prof. Eric Davaud qui m’a transmis sa passion pour la s´edimentologie et le travail de terrain durant mon diplˆome et qui m’a encourag´e `a entreprendre cette th`ese. Parmi les doctorants et post-doctorants, je tiens a remercier S´ebastien Biass, J´erˆome Chablais, Corinne Clerc, Yann Floris, Yannick Fuchey, Fabienne Godefroid, Fer- nando Guarin, Claude-Alain Hasler, Stephan Jorry, Olivier Kaufmann, Rapha¨el Klaus, Pierre Le Guern, Paola Mu˜noz, Mapath´e Ndiaye, Matar Ndiaye, Ralf Neuwerth, Lina Ospina, Sabrina Paolacci, Caroline Pellaton, Andr´e Piuz, Karine Pl´ee, Sylvain Rigaud, Nicolas Roduit, Fiore Suter et Chadia Volery pour avoir contribu´e `a une excellente ambiance de travail.

Un immense merci a Marina D´efago qui m’a ´enorm´ement soutenu durant la phase finale de r´edaction du pr´esent manuscrit.

J’adresse aussi un merci tout particulier `a Cyril Ruchonnet, Julien Fiore et Lucia Carcione (Fiore) pour leur amiti´e.

Finalement, je ne saurais oublier d’exprimer mes remerciements du fond du coeur `a mes chers parents et

`

a mon fr`ere pour leur affection, leur confiance et leur soutien constant tout au long de ces ´etudes. Je leur d´edie ce travail.

Contents

Abstract iii

R´esum´e vii

Remerciements xi

List of Figures xvii

List of Tables xxi

1 Introduction 1

1.1 Lake sediments as climatic archives . . . 1

1.2 Coastal studies: advantages & drawbacks . . . 3

1.2.1 Lake system . . . 3

1.2.2 Deep basinal realm . . . 5

1.2.3 Coastal/marginal realm . . . 6

1.3 Purpose of the study . . . 7

1.4 Structure of this Thesis . . . 9

2 Methodology 11 2.1 Geophysics . . . 11

2.1.1 Ground-penetrating radar (GPR) . . . 11

2.1.2 High-resolution reflection seismic (HRS) . . . 15

2.2 Sediment coring . . . 16

2.3 Sediment logging techniques . . . 18

2.4 Sedimentological analysis . . . 19

2.5 Dating methods . . . 20

2.5.1 Radiocaesium-137 . . . 20

2.5.2 Radiocarbon . . . 20

2.5.3 Palaeomagnetism . . . 21

2.5.4 Tephrochronology . . . 26

3 Geophysical survey tests 29 3.1 GPR results . . . 29

3.1.1 Offshore GPR results . . . 31

3.1.2 Offshore GPR synthesis . . . 37

3.1.3 Onshore GPR results . . . 37

3.1.4 Onshore GPR synthesis . . . 43

3.1.5 Amphibious GPR results . . . 43

3.1.6 GPR discussion . . . 46

3.2 HRS results . . . 48 xiii

3.3 GPR and HRS surveys conclusions . . . 52

4 Contribution of HRS and GPR methods to sedimentological studies of the Geneva Bay area 57 4.1 Lake Geneva setting . . . 57

4.2 Previous studies of the ‘Petit Lac’ . . . 57

4.3 Methods . . . 60

4.4 Offshore results . . . 61

4.4.1 Upper Pleistocene and Holocene seismic stratigraphy . . . 61

4.4.2 Late Holocene ooidal sands . . . 67

4.5 Onshore results . . . 70

4.5.1 Related archaeological studies . . . 70

4.5.2 GPR and sedimentological investigations . . . 70

4.6 Late Holocene lale-level fluctuations . . . 74

4.7 Conclusions . . . 75

5 Holocene hydrological and climatic changes inferred from an integrated multiproxy analysis of Lakes Joux and Brenet sedimentary records 77 5.1 Introduction . . . 77

5.1.1 Geographical setting . . . 77

5.1.2 Geological setting . . . 78

5.1.3 Glacial history . . . 79

5.1.4 Climate . . . 80

5.1.5 Hydrography . . . 81

5.1.6 Limnology . . . 81

5.1.7 Previous studies . . . 84

5.2 Geophysical investigations . . . 87

5.2.1 GPR stratigraphy . . . 87

5.2.2 HRS stratigraphy . . . 98

5.3 Sedimentological investigations . . . 105

5.3.1 Sedimentary facies . . . 106

5.3.2 Description of onshore cores . . . 110

5.3.3 Description of offshore cores . . . 112

5.3.4 Discussion of sedimentary stratigraphy . . . 126

5.4 Chronological framework . . . 130

5.4.1 Radiocaesium-137 . . . 131

5.4.2 Radiocarbon-14 . . . 131

5.4.3 Palaeomagnetism . . . 131

5.4.4 Discussion of the chronological framework . . . 141

6 General conclusions and perspectives 145 Bibliography 149 A HRS data information 163 B Core and borehole data 165 B.1 Core photographs . . . 171

B.2 Core description . . . 176

B.2.1 Core BR-V1 . . . 176

B.2.2 Core BR-V2 . . . 176

B.2.3 Core BR-V3 . . . 177

B.2.4 Core BR-V4 . . . 177

CONTENTS xv

B.2.5 Core BR-V5 . . . 178

B.2.6 Core BR-N1 . . . 179

B.2.7 Core BR-N4 . . . 179

B.2.8 Core BR-N5 . . . 180

B.2.9 Core JO-N3 . . . 180

B.2.10 Core JO-N4 . . . 181

B.3 MSCL data . . . 183

B.4 Water Content and Loss On Ignition data. . . 229

B.5 Geotechnical boreholes . . . 242

C Palaeomagnetic data 245

List of Figures

1.1 Lake hydrodynamic response to various forms of physical input. . . 4

1.2 Schematic illustration of major sedimentological and bottom dynamical processes in lakes. . . 5

1.3 Response of sediments to waves and currents. . . 6

1.4 Schematic facies model for lacustrine carbonates. . . 7

1.5 Scheme of field data acquisition by geophysical profiling. . . 8

2.1 Schematic diagram of the GPR system. . . 11

2.2 Schematic illustration of the relfection and CMP mode of GPR operation. . . 13

2.3 Typical GPR data processing flow chart. . . 14

2.4 CMP data from the shore of Lake Brenet and its corresponding velocity model. . . 14

2.5 Typical HRS data processing flow chart. . . 15

2.6 Schematic diagrams of the (A) gravity corer, (B) piston corer and (C) vibracorer used in this study. . . 16

2.7 General operation of offshore coring by using successively the gravity and the piston coring systems. . . 17

2.8 Typical ranges of room-temperature magnetic susceptibility values for environmental mate- rials and minerals (modified after Dearing, 1999a). . . 20

2.9 Example of a magnetic hysteresis loop/cycle and initial magnetisation curve for a ferrimag- netic mineral. . . 26

3.1 Panoramic map of Switzerland showing all surveyed lakes for amphibious GPR feasibility tests. 30 3.2 200-MHz offshore GPR profile BIE-11 from Lake Biel. . . 33

3.3 200-MHz offshore GPR profile MOR-3 from Lake Murten. . . 33

3.4 200-MHz offshore GPR profile TAI-12 from Lake Taill`eres. . . 34

3.5 200-MHz offshore GPR profile GIR-1A from Lake Geneva. . . 34

3.6 100-MHz offshore GPR profile BAY-9 from Lake Geneva. . . 35

3.7 200-MHz offshore GPR profile TOL-9 from Lake Geneva. . . 35

3.8 200-MHz offshore GPR profile JOU-1 from Lake Joux. . . 36

3.9 100-MHz offshore GPR profile BRE-66 from Lake Brenet. . . 36

3.10 200-MHz onshore GPR profile BIE-10 from Lake Biel. . . 39

3.11 200-MHz onshore GPR profile MOR-6 from Lake Murten. . . 39

3.12 250-MHz GR-15 (A) and 100-MHz GR-42 (B) onshore GPR profiles from Parc La Grange (Lake Geneva). . . 40

3.13 200-MHz onshore GPR profile VID-1G from Vidy (Lake Geneva). . . 41

3.14 200-MHz onshore GPR profile ROC-5 from Lake Geneva. . . 41

3.15 100-MHz onshore GPR profiles from Lake Brenet. . . 42

3.16 200-MHz amphibious GPR profile BRE-11+12 from Lake Brenet. . . 44

3.17 Frequency spectra of reflected radar signals for 100-, 200- and 250-MHz antennas. . . 45

3.18 Maps of Lakes Geneva, Brenet and Joux with locations of HRS and GPR tracklines. . . 50

3.19 Nearly coincident GPR profile PL-26 and HRS profile GE-06/p1 from the Geneva Bay area. . 51 xvii

3.20 Nearly coincident GPR profile BRE-70 and HRS profile BRE-p1 from the western part of

Lake Brenet. . . 52

3.21 Lake-crossing HRS profile BRE-p2 from the central part of Lake Brenet. . . 53

3.22 Schematic representation summarising the potential usefulness and limitations of GPR and HRS methods in profiling lacustrine environments. . . 54

4.1 Map of Lake Geneva showing large-scale bathymetry, major tributaries and simplified regional geology. . . 58

4.2 Map of Geneva Bay area showing the geophysical tracklines (HRS and GPR), gravity cores, geotechnical boreholes and archaeological excavation used in this study. . . 62

4.3 HRS profile Gen`eve04-1-4 illustrating the complex and discontinuous seismostratigraphical information of Geneva Bay. . . 63

4.4 HRS profile Gen`eve04-1-4 (see Figure 4.3) of Geneva Bay interpreted on the basis of the seismic stratigraphy proposed by (Fiore, 2007). . . 64

4.5 Detailed HRS portion (profile Gen`eve04-1-4) of the boxed area in Figures 4.3 and 4.4. . . 65

4.6 HRS profile Gen`eve04-1-6 showing detailed seismic stratigraphy of unit E. . . 66

4.7 Simplified description of the two short gravity cores extracted in Geneva Bay. . . 67

4.8 Map of the main current velocities at 2 m water depth (modified after B´etant & Perrenoud, 1932). . . 68

4.9 Map of Geneva Bay showing detailed bathymetry obtained with a 200-kHz HRS system and a trackline spacing of∼50 m, and two portions of offshore GPR profiles. . . 69

4.10 Dataset collected from the archaeological excavation of Parc La Grange. . . 71

4.11 Onshore GPR profiles PL-21 and GR-15 from, respectively, (A) Perle du Lac Park and (B) Parc La Grange showing the beach ridge deposits of the +3 m lacustrine terrace. . . 72

4.12 Onshore longitudinal GPR profile GR-18 acquired in Parc La Grange indicating the SW direction of ‘Bise’-driven sediment transport which was responsible for constructing the beach ridge complex of the +3 m lacustrine terrace. . . 73

4.13 Detailed maps of the west and east shore GPR sites displaying the landward termination of GPR facies of the +3 m lacustrine terrace. . . 74

4.14 Lake Geneva water-level fluctuations between 4000 BC and 2000 AD based on archaeological data (modified after Corboud, in press). . . 75

5.1 Map of Switzerland showing location of the Joux Valley. . . 77

5.2 Geological map of the Joux Valley region showing the surface watershed area. . . 79

5.3 Quaternary glaciers of the Joux Valley (after Aubert, 1938) . . . 80

5.4 Hydrographical map of lakes Joux and Brenet. . . 82

5.5 Lake-level fluctuation curves for the period 2002-2005 (MeteoSuisse and SESA data). . . 82

5.6 Temperature vs depth curve for Lake Joux, 1995-2004 mean values (after Fiaux et al., 2006). 83 5.7 Lacustrine zonation inferred from the distribution of macrophyte vegetation (after Hutchin- son, 1967; Lods-Crozet et al., 1995; Fiaux et al., 2006). . . 84

5.8 Maps of Lakes Brenet (a) and Joux (b) showing the geophysical tracklines (HRS and GPR) and sediment cores location. . . 88

5.9 Enlarged map of the southwestern part of Lake Brenet showing the locations of the presented GPR profiles. . . 89

5.10 Radar stratigraphy of the littoral zones of Lakes Joux and Brenet. . . 90

5.11 200-MHz amphibious GPR profile BRE-11+12 from Lake Brenet showing the location of cores BR-N2, BR-N3, BR-V2, BR-V3 and BR-V5. . . 92

5.12 200-MHz transversal offshore GPR profile JOU-5 from Lake Joux showing the location of core JO-N4. . . 93

5.13 200-MHz transversal offshore GPR profile JOU-3 from Lake Joux showing the location of core JO-N3. . . 93

5.14 200-MHz longitudinal offshore GPR profile BRE-9 from Lake Brenet. . . 94

LIST OF FIGURES xix

5.15 250-MHz transversal offshore GPR profile BRE-50 from Lake Brenet. . . 94

5.16 200-MHz transversal offshore GPR profile BRE-4 from Lake Brenet showing the location of core BR-N4. . . 95

5.17 100-MHz transversal offshore GPR profile BRE-63 from Lake Brenet showing the location of core BR-N5. . . 95

5.18 100-MHz longitudinal onshore GPR profile BRE-59 from Lake Brenet. . . 96

5.19 250-MHz longitudinal onshore GPR profile BRE-28 from Lake Brenet. . . 96

5.20 100-MHz transversal onshore GPR profile BRE-55 from Lake Brenet showing the location of core BR-V4. . . 97

5.21 100-MHz transversal onshore GPR profile BRE-58 from Lake Brenet showing the location of core BR-V1. . . 97

5.22 250-MHz transversal onshore GPR profile BRE-89 from Lake Brenet. . . 98

5.23 Seismic stratigraphy of Lakes Joux and Brenet. . . 99

5.24 Deep offshore longitudinal profile HRS JOUX-1 / p1 from the central part of Lake Joux. . . . 100

5.24 Interpretation of profile HRS JOUX-1 / p1 . . . 101

5.25 Detail of glacially-deformed glacio-lacustrine deposits of the seismic unit JH-1. . . 102

5.26 Deep offshore longitudinal profile HRS JOUX-1 / p2 from the central southwestern part of Lake Joux showing the location of core JO-N2. . . 103

5.27 Lake-crossing profile HRS BRENET-p2 acquired in the central part of Lake Brenet showing the location of core BR-N1. . . 104

5.28 Legend for all described cores from Lakes Brenet and Joux. . . 106

5.29 Description of the surfaces and structures observed in the onshore and offshore cores from Lakes Brenet and Joux. . . 107

5.30 Main macroscopic components found in the onshore and offshore sedimentary cores from Lakes Brenet and Joux. . . 108

5.31 Atlas of sedimentary facies and associated GPR and HRS facies of the subsurface beneath Lakes Joux and Brenet. . . 109

5.32 Core BR-V1: Sedimentology, MSCL, WC, LOI data and corresponding GPR facies. . . 111

5.33 Core BR-V2: Sedimentology, MSCL, WC and LOI data and corresponding GPR facies. . . . 113

5.34 Core BR-V3: Sedimentology, MSCL, WC, LOI data and corresponding GPR facies. . . 114

5.35 Core BR-V4: Sedimentology, MSCL, WC, LOI data and corresponding GPR facies. . . 115

5.36 Core BR-V5: Sedimentology, MSCL, WC, LOI data and corresponding GPR facies. . . 116

5.37 Core BR-N1: Sedimentology, MSCL, WC, LOI, calcimetry data and corresponding HRS facies.117 5.38 Very fine laminations (rhythmites or varves) observed in the 480 - 580 cm depth interval (HRS facies BH-1b**) of core BR-N1. . . 118

5.39 Core BR-N2: Sedimentology, MSCL, WC, LOI data and corresponding GPR facies. . . 119

5.40 Core BR-N3: Sedimentology, MSCL, WC, LOI data and corresponding GPR facies. . . 120

5.41 Core BR-N4: Sedimentology, MSCL, WC, LOI data and corresponding GPR facies. . . 122

5.42 Core BR-N5: Sedimentology, MSCL, WC, LOI data and corresponding GPR facies. . . 123

5.43 Core JO-N1: Sedimentology, MSCL, WC, LOI₅₆₀data and corresponding HRS facies. . . 124

5.44 Core JO-N2: Sedimentology, MSCL and WC data, and corresponding HRS facies. . . 125

5.45 Core JO-N3: Sedimentology, MSCL and WC data, and corresponding GPR facies. . . 127

5.46 Core JO-N4: Sedimentology, MSCL and WC data, and corresponding GPR facies. . . 128

5.47 Correlation of the radar and sedimentary stratigraphy. . . 129

5.48 Correlation of the seismic and sedimentary stratigraphy. . . 130

5.49 ¹³⁷Cs activity in cores (a) BR-N1 from Lake Brenet and (b) JO-N1 from Lake Joux. . . 131

5.50 Radiocarbon-based age model and estimated sedimentation rates for Lake Brenet cores BR- V2, BR-V5, BR-N2 and BR-N3. . . 134

5.50 Radiocarbon-based age model and estimated sedimentation rates for Lake Brenet cores BR- N4, BR-N5 and BR-N1. . . 135

5.51 AF demagnetisation pattern of the NRM of pilot U-channels B2 (70 cm depth) and B3 (30 cm depth). . . 136

5.52 The Day et al. (1977) biplot of Mrs/Ms versus Hcr/Hc of the samples from Lakes Joux and

Brenet as well as common magnetic minerals (after Peters & Dekkers (2003)). . . 137

5.53 Comparison of the mineral-magnetic record from cores JO-N2 and BR-N1 to the Lake Joux record from core n^◦ 3 of Creer et al. (1980). . . 138

5.54 Lithology and variations in mineral magnetic parameters from cores BR-N1 and JO-N2, and comparison of S-ratio data from core BR-N1 with the oxygen isotope record from the Greenland Ice Sheet Project 2 (GISP2). . . 139

5.55 Comparison of the directional palaeomagnetic oscillations record from cores JO-N2 and BR- N1 with the Lake Joux core n^◦ 3 record from Creer et al. (1980). . . 140

5.56 Comparison of the Lake Brenet (core BR-N1) directional palaeomagnetic record with the composite ‘European record’ proposed by Thouveny & Williamson (1987). . . 142

5.57 Age model based on palaeomagnetic (A) declination adn (B) inclination features recorded in core BR-N1 . . . 143

A.1 Map of affected and fully usable HRS profiles in Lake Geneva. . . 163

A.2 Map of affected and fully usable HRS profiles in Lakes Brenet and Joux. . . 164

B.1 Example illustrating the core labelling code used in this study. . . 165

B.2 Photographs of offshore sediment cores LE-1 and LE-2 (Lake Geneva). . . 171

B.3 Photographs of Lake Brenet offshore sediment core BR-N1. . . 172

B.4 Photographs of Lake Brenet offshore sediment cores BR-N2 to N5. . . 173

B.5 Photographs of Lake Brenet onshore sediment cores BR-V1 to V5. . . 174

B.6 Photographs of Lake Joux offshore sediment cores JO-N1 to N4. . . 175

List of Tables

2.1 Typical electrical properties of common geological materials. . . 12 2.2 Magnetic hysteresis parameters of single-domain (SD) and mutidomain (MD) grains of mag-

netite, hematite, titanomagnetite, maghemite, goethite, pyrrhotite and greigite in SI units (sources Thompson & Oldfield, 1986 and Peters & Dekkers, 2003). . . 24 2.3 Magnetic minerals in lake sediments: major types and sources (after Thompson & Oldfield,

1986). . . 25 3.1 Main hydrological characteristics of the investigated lakes with geographical location, name

and figure number of illustrated offshore and onshore GPR profiles. . . 31 3.2 Tabulation of nominal centre frequency of antenna, sediment location and calculated vertical

resolution. . . 46 4.1 Hydrographical characteristics of Lake Geneva, ‘Petit Lac’ and ‘Grand Lac’. Source: CIPEL

(1999). . . 58 4.2 Litho-/seismo-stratigraphical units of the Geneva Bay area. Sources: Moscariello (1996),

Moscariello et al. (1998a). . . 59 4.3 ‘Bise’ wave characteristics and corresponding wave base level in the ‘Petit Lac’ (after Bruschin

& Schneiter, 1978; Girardclos, 1993). . . 67 5.1 Hydrographical characteristics of Lakes Joux and Brenet (after Bosset, 1961 and De Heer,

1981). . . 81 5.2 Littoral zone characteristics of Lakes Joux and Brenet (after Lods-Crozet et al., 1995). . . 83 5.3 Correlation between the seismic units from this work and those proposed by Bruder (2003). . 105 5.4 Correlation of the sedimentary (coastal and basinal), GPR and HRS stratigraphy. . . 126 5.5 Correlation of the profundal and coastal sedimentary facies of Lakes Joux and Brenet with

typical fjord-type sedimentary infill. . . 130 5.6 Radiocarbon dates of the onshore cores from Lake Brenet. . . 132 5.7 Radiocarbon dates of the offshore cores from Lake Brenet. . . 133 5.8 Correspondance among the two labeling systems (alphabetic and alphanumeric), conventional

(uncalibrated) radiocarbon ages and depths of the main declination peaks. . . 140 5.9 Correspondance among the two labeling systems (Greek and alphanumeric), conventional

(uncalibrated) radiocarbon ages and depths of the main inclination peaks. . . 141 B.1 List of offshore and onshore sediment cores extracted from Lakes Geneva, Brenet and Joux. . 166 B.2 List of all retrieved offshore and onshore cores (from Lakes Geneva, Brenet and Joux) showing

the corresponding sections, samples and systematic laboratory measurements (MSCL, WC and LOI). . . 167 B.3 Core sediment samples and corresponding section depths. . . 170 B.4 MSCL data of core BR-N1. . . 194 B.5 MSCL data of core BR-N2. . . 198 B.6 MSCL data of core BR-N3. . . 201

xxi

B.7 MSCL data of core BR-N4. . . 205 B.8 MSCL data of core BR-N5. . . 209 B.9 MSCL data of core BR-V1. . . 211 B.10 MSCL data of core BR-V2. . . 213 B.11 MSCL data of core BR-V3. . . 214 B.12 MSCL data of core BR-V4. . . 215 B.13 MSCL data of core BR-V5. . . 217 B.14 MSCL data of core JO-N1. . . 218 B.15 MSCL data of core JO-N2. . . 222 B.16 MSCL data of core JO-N3. . . 225 B.17 MSCL data of core JO-N4. . . 228 B.18 WC data of core LE-1. . . 229 B.19 WC data of core LE-2. . . 229 B.20 WC, LOI and calcimetry data of core BR-N1. . . 232 B.21 WC and LOI data of core BR-N2. . . 233 B.22 WC and LOI data of core BR-N3. . . 234 B.23 WC and LOI data of core BR-N4. . . 235 B.24 WC and LOI data of core BR-N5. . . 236 B.25 WC and LOI data of core BR-V1. . . 237 B.26 WC and LOI data of core BR-V2. . . 238 B.27 WC and LOI data of core BR-V3. . . 238 B.28 WC and LOI data of core BR-V4. . . 239 B.29 WC and LOI data of core BR-V5. . . 239 B.30 WC and LOI₅₆₀ data of core JO-N1. . . 240 B.31 WC data of core JO-N2. . . 240 B.32 WC data of core JO-N3. . . 241 B.33 WC data of core JO-N4. . . 241 B.34 Table showing the list of the geotechnical boreholes used to calibrate the geophysical data

(source: SCG, Gen`eve). . . 242 B.35 List of the geotechnical boreholes crossing the +3 m lacustrine terrace deposits and showing

thickness and vertical extent (source: SCG, Gen`eve). . . 243 C.1 Units and relationships for common quantities of magnetism. . . 245 C.2 Magnetic hysteresis data from pellet-samples of cores BR-N1 and JO-N2. . . 246 C.3 List of core sections and corresponding U-channels used for NRM, ARM and IRM measure-

ments. . . 246 C.4 Palaeomagnetic data of core BR-N1. . . 252 C.5 Palaeomagnetic data of core JO-N2. . . 254

Chapter 1

Introduction

Limnogeology is the study of ancient and recent environments of continental water bodies (lacus- trine systems). In addition to hydrocarbon and mineral resource investigations, palaeoenvironmen- tal sciences constitute one of the major disciplines of limnogeology. Environmental systems are gov- erned by changes in the interaction among the at- mosphere, oceans, land, cryosphere and biosphere (i.e. climate dynamics). Since the end of the 20^th century, the global warming debate is a major con- cern that focuses on the anthropogenic influences on climate. In order to test and improve models of the very complex climate systems, numerous high- resolution climate data (local, regional and global scales) are needed. Moreover, these data are re- quired for separating natural and human factors in studying the natural forces of climate change (e.g. ocean currents, solar activity, volcanic erup- tions) and evaluating the potential effect of human activity on climate change (e.g. O’Brien et al., 1995). Thus, understanding palaeoclimate dynam- ics is critical if we want to reasonably predict future climate changes.

In this context, the Holocene period, covering the last 10,000 years, is well-suited for palaeoen- vironmental and palaeoclimatic investigations (e.g.

St¨otter et al., 1999; Mayewski et al., 2004; Rousse et al., 2006). Also known as the Age of Man, this period hosted the emergence and development of human civilisation. The Holocene is commonly recognised as a relatively warm and stable inter- glacial (weaker climatic fluctuations than in the last glacial cycle). Yet many Holocene climate records indicate abrupt shifts (e.g. the Little Ice Age and the cooling at about 8.2 cal kyr BP), reflecting a more unstable climate than often as- sumed (e.g. O’Brien et al., 1995; Bond et al., 1997;

Mayewski et al., 2004). Based on Holocene glacial expansion and contraction evidences, the pioneer-

ing work of Denton & Karl´en (1973) pointed out short-term palaeoclimatic changes of ca. 1500 years and their correlation with atmospheric ¹⁴C varia- tions, which is linked to changes in the solar out- put. Thereby Holocene climatic fluctuations are driven by centennial- to millennial-scale climate cy- cles, which are far too frequent to be a linear re- sponse to the slower changes in the earth’s orbital configuration (Milankovitch forcing), and thus op- erate independently of the glacial-interglacial cli- mate state (Oppo, 1997). Changes in the ocean- atmosphere circulation (e.g. the North Atlantic Oscillation (NAO); O’Brien et al., 1995; Mayewski et al., 1997) as well as in the solar activity (Magny, 1993; Bond et al., 2001; Hu et al., 2003) were im- portant triggers of Holocene cyclic climate varia- tions, but still more investigations are needed to- wards deciphering the mechanisms of the global cli- matic pattern.

1.1 Lake sediments as climatic archives

In continental areas, lacustrine sediments con- stitute very sensitive records of past climate and environmental conditions (e.g. Oldfield, 1977;

Zolitschka et al., 2003). According to Scholz (2001), lake levels, which largely control the forma- tion of depositional sequences, are tightly coupled through regional climate and hydrology. Thereby, Holocene lake-level fluctuations recorded in the sedimentary archives can be used to reconstruct and interpret palaeohydrological variability and thus palaeoclimatic changes (e.g. Dearing & Fos- ter, 1986).

Lake and marine systems have various similar aspects, meaning that many of the investigation techniques involved in limnogeology are identically 1

applied to marine science (e.g. Last & Smol, 2001;

Glew et al., 2001). Despite the size of a few lakes (e.g. Lake Victoria, South Africa and Lake Baikal, Siberia), most lakes can be considered as rela- tively small and closed or nearly closed environ- ments. This size factor significantly limits the gen- eration of long-period wind-waves, geostrophic cur- rents (induced by gravitational forces and Coriolis effect) and tides, therefore implying substantially lower energy levels than in marine systems (Sly, 1978). The energies reaching ocean coasts derive from several distant sources so that the response to changes in a specific source is very muted. In the case of lakes, the energy processes are tightly controlled by fetch, waves, wind-driven water cir- culations and river inflows (Carter, 1988). Ad- ditionally, Sly (1978) indicates that the ratio be- tween land watershed and lake area is generally high, leading to much higher sediment loadings and sedimentation rates than in marine environ- ments. Such conditions illustrate the lake’s sensiv- ity to external forcing and favor the high-resolution recording/archiving of high-frequency cyclical en- vironmental changes (Scholz, 2001). According to Tucker & Wright (1990), while lakes have much less diverse biota than oceans or seas, the biological and chemical processes are much more intimately linked, and the influence of the biota on lacustrine carbonate sedimentation is more important. They also point out that the pronounced sensitivity of lakes to environmental gradients leads to major and abrupt facies pattern changes (both vertically and laterally). Thus, lacustrine realms are much less stable and constitute more complex sedimentary systems than marine ones.

Despite these differences, most lacustrine coastal (shorezone) landforms and processes have an equiv- alent marine counterpart (Sly, 1978; Carter, 1988;

Sack, 2001). For example, the studies of Adams

& Wesnousky (1998) and Gilli et al. (2005) use constructional beach-ridge shoreline features as di- rect indicators of the level of a lake. In addi- tion, the concept of seismic sequence stratigraphy (Vail, 1987) can also be applied to lacustrine sys- tems (Lezzar et al., 1996; Ariztegui et al., 2001;

Scholz, 2001; Anselmetti et al., 2006). Moreover, the study of Scholz et al. (1998) shows the differ- ent responses of lacustrine rift basins to changes in the main controls on sedimentary depositional sequences (eustasy, subsidence, tectonics, climate- hydrology and sediment supply) and the compari- son with their marine passive margin counterparts.

Lakeward or landward shifts of the sedimentary units are caused by lowering or rising of the lake level, respectively. The response of the level of a lake (rise or fall) to changes in the hydrological input-to-output ratio (e.g. precipitation and evap- oration) depends on the lake basin configuration (size of the lake and its watershed area) and the presence of an outflowing stream (Street-Perrot &

Harrison, 1985). There is an inverse relationship between the size of the water body and its sensivity to changes in the energy input (Carter, 1988). Nar- row and closed lake systems are sensitive to small perturbations and thus record local climatic fluctu- ations. Inversely, large open lake systems are more complex and react less drastically to environmen- tal changes (buffer effect), providing less detailed palaeoclimatic information.

Preserved relict/abandoned shorelines (land- water interface) are direct and precise indications of the extent and dimension of a palaeolake, and are crucial for reconstructing the hydrographical changes through time (e.g. Sack, 2001). Thus, past lake-level changes may be recorded from geo- morphological features such as raised shore ridges, wave-cut cliffs, beaches and barriers, but strati- graphical evidence is more abundant (Richardson, 1969). Stratigraphical records of lake-level fluc- tuations can be diverse as fluctuations may af- fect the sedimentological, limnological and biolog- ical processes in lakes (Digerfeldt, 1986). More- over, these processes are triggered by many inter- nal and external mechanisms: trophic state of the lake, autochtonous production, heat transfer, pre- cipitation rate, water table level, inflow discharge, turbidity, waves and wind-driven currents, expo- sure of the shore, subaqueous mass movements, etc. Although the majority of these mechanisms are climate-induced, some can also be explained by other environmental changes such as natural eu- trophication, neotectonic movements and human activity.

Despite the abundance of stratigraphical evi- dence, it is usually difficult to unequivocally inter- pret them in terms of lake-level changes. Also, the stratigraphical record may give ambiguous infor- mation about the magnitude of lake-level changes.

According to Digerfeldt (1986), convincing links between the sediment record and lake-level fluctu- ations necessitate support through a combination of multiple lines of stratigraphical evidence. This introduces the concept of ‘proxy’; a proxy being a measurable discrete variable from which a vari-

1.2. COASTAL STUDIES: ADVANTAGES & DRAWBACKS 3

able of interest (not directly measurable) can be inferred. In this sense, a multiproxy approach in- cludes the gathering (aggregation) of various prox- ies (source variables) in order to infer the variable of interest (target). These proxies mainly com- prise changes in the distribution of lake vegetation, the sediment composition, the sediment distribu- tion, the sediment limit (highest deposition limit of organic-rich sediment) and the presence of sedi- mentary hiatuses (Digerfeldt, 1986). Moreover, in- dividual records (single sediment cores) are suscep- tible to alternative interpretations. Thus, the com- bination of stratigraphical evidence analysed along core transects can lead to reliable and thorough re- construction of lake-level fluctuations.

Since the analysis of lake sediments to deter- mine specific, relatively local climatic factors of the past remains a formidable challenge, Street- Perrot & Harrison (1985) argue that a combina- tion of stratigraphical and geomorphological ap- proaches is necessary. While stratigraphical ap- proaches provide superior time resolution, geomor- phological approaches can yield more quantitative lake-level curves. In a similar way, concerning ma- rine studies, Vail & Mitchum Jr. (1977) pointed out the importance of the “interdependance of seis- mic data with all other forms of geological infor- mation”. A number of authors have been success- ful in applying this combination, in which survey- ing archaeological excavations (Gaillard & Moulin, 1989), hand-coring and diving (Moscariello, 1997) and offshore high-resolution seismic reflection (Ab- bott et al., 2000) or onshore ground penetrating radar methods (Dott & Mickelson, 1995) were the geomorphological approaches.

Reliable palaeoclimatic models can be achieved using geomorphological and sediment core infor- mation from all lake depths. Geomorphological and stratigraphical evidences of past lake-level fluc- tuations are typically deduced from onshore to shallow-water (a few metres depth) investigations (Dearing, 1997). But the littoral platform record may be incomplete because of hydrodynamic dis- turbances causing sediment erosion and reworking (Digerfeldt, 1988). On the other hand, continu- ous sedimentary records can be found in deep-water environments, where hydrodynamic conditions are minimal.

The climatic significance of lake-level fluctua- tions and stratigraphical archives should always be based on a reliable comparison among the numer- ous individual records of the studied region. Addi-

tionally, the use of different chronological references (e.g., radiocarbon dating, dendrochronology, pollen analysis, palaeomagnetic dating and archaeology) combined with external proxies, such as glacial re- currences and timberline oscillations, further con- strain Holocene climate models.

Palaeoenvironmental research on peri-alpine lakes in Switzerland and France have seen a surge of interest and application over the past decades.

Holocene litho- and biostratigraphies, inferred from Swiss Plateau and Alpine Foreland lakes, reveal changes in hydrological conditions linked with cli- matic changes (Lake Geneva: Moscariello, 1996;

Lake Neuchˆatel: Schwalb et al., 1998; Magny et al., 2005; Lake Constance: Wessels, 1998; Lake Biel:

Wohlfarth & Schneider, 1991a,b; Lake Seedorf:

Magny & Richoz, 1998; Lake Annecy (France):

Brauer & Casanova, 2001). A synthesis of lake-level fluctuations synchronously recorded in many lakes from the Jura Mountains, the French Pre-Alps and the Swiss Plateau suggest a climatic control of the fluctuations (Magny, 1992, 2004). Stable isotope curves from Lake Geneva reflect major Holocene climate variability (Anad´on et al., 2006). In the Alpine Foreland, the Holocene environmental and climate history has been reconstructed from com- bined seismic reflection and sediment stratigraphy (Lake Geneva: Baster et al., 2003; Girardclos et al., 2005; Lake Bourget (France): Chapron, 1999).

However, disagreements in portions of lake-level curves, general uncertainties in the results and the lack of complete onshore/offshore records demon- strate the need for more investigations.

1.2 Coastal studies:

advantages & drawbacks

1.2.1 Lake system

The hydrodynamics of a lake is governed by the interactions between many physical processes, the most important of which are wind, river in- flow and atmospheric heating (Sly, 1978; Figure 1.1). All these physical processes influence the formation, composition and behaviour (redistribu- tion) of lacustrine sediments. A detailed and syn- thetic discussion on the control of the energy in- puts on sedimentation is given in Dearing (1997).

The relative importance of these physical inputs are mainly ruled by the morphometry, the orien- tation and the size of the basin (influencing the

Figure 1.1: Lake hydrodynamic response to various forms of physical input (after Sly, 1978).

fetch), by the relief of the surroundings and by climatic conditions (Sly, 1978). Limnic sediments can be separated into two genetic groups, which consist of (1) allochthonous/terrigenous sediments transported by rivers, glaciers or wind and (2) autochthonous/endogenic sediments resulting from processes taking place within the water mass (i.e., connected to organisms and physico-chemical prop- erties of water). Redistribution of the settled ma- terial is principally governed by four processes that were defined by Hilton (1985). These include:

• Sliding and Slumping (SS) on slopes with an- gles of 4% - 14% (H˚akanson, 1977), which are the main effects of gravity processes;

• Intermittent Complete Mixing (ICM) involves the resuspension of material from the entire lakebed during the short overturn period (at- mospheric heating process);

• Peripheral Wave Action (PWA) consists in the resuspension and redeposition of coastal sedi- ments in deeper waters driven by the action of breaking waves (wind process);

• Random Redistribution of sediment (RR) principally occurs in shallow lakes where wave energy directly acts on the lakebed and redis- tributes sediments across the whole bed (wind process).

Also, according to H˚akanson & Jansson (1983), ar- eas dominated by river action and wind/wave ac- tion distinguish the two main lacustrine contexts in terms of sedimentological and bottom dynami- cal processes (Figure 1.2).

As the present study focuses on non-deltaic coasts, the notion of ‘wave base’ is of major in- terest. The wave base corresponds to the depth at which wave-induced shear stress may exceed the critical shear stress of bed materials. This creates a hydrodynamic division of lake basins into two zones, above and below wave base. Above the wave base, wave action leads to sediment erosion, trans- portation, winnowing and deposition, while below the wave base, sedimentary processes are mainly dominated by the settling of fines, generally rich in organic particles (Carter, 1988). A subdivision above the wave base zone is also applied. As indi-

1.2. COASTAL STUDIES: ADVANTAGES & DRAWBACKS 5

Figure 1.2: Schematic illustration of major sedimentological and bottom dynamical processes in lakes (modified after H˚akanson, 1982). 0, 1, 2 and 3 refer to successive time lines.

cated by Sly (1978), a double interaction between bottom sediments and surface waves occurs in the nearshore area: (1) in the breaker zone where the waves become unstable and break, and (2) in the zone beyond, which is only affected by the orbital velocities of vortex water motion. In the former, the erosional processes prevail.

The influence of wind-induced waves and cur- rents on sediment response is modified by the basin configuration, the bathymetrical context (beach, nearshore, transitional zone and offshore region) and the particulate fraction (grain size, cohesive strength and bed form), as depicted in figures 1.2 and 1.3. From these figures, it is evident that ar- eas of main hydrodynamic activity are mainly lo- cated in shallow-water nearshore areas, and that both grain size and sedimentation rate decrease with depth, in response to the decrease of physi- cal energy in the system.

1.2.2 Deep basinal realm

The deep-water basinal sedimentation is gen- erally characterised by decantation/settling pro- cesses (nearly motionless water masses), low de- position rates, organic-rich muddy to clayey facies, conformable bedding and continuity of deposition (e.g. H˚akanson & Jansson, 1983; Carter, 1988;

Tucker & Wright, 1990) and gravity/turbidity cur- rent deposits. According to Lister et al. (1991), deep-water sediments provide a high potential for preserving the biological (faunal and floral assem- blages), mineral (e.g. authigenic precipitates) and chemical (trace elements and stable isotopes) trac- ers. Some studies involve such tracer records (e.g.

δ¹³C, δ¹⁸O, organic matter, biogenic silica and Rock-Eval analyses) from deep-water cores in or- der to correlate value shifts with Holocene climate variability (Filippi et al., 1999; Meyers & Lallier- Verg`es, 1999 and Brenner et al., 2003. However, deep lacustrine sediments are usually the last to register the changing levels of lakes, as they are not directly controlled by the latter, and thus consti-