"Of the plausible scenarios for China's future, the possibility of a new constitutionalism has been taken seriously by only a few Western specialists. Yet the constitutionalist scenario gains credibility from the improbability of the alternatives", argued Andrew Nathan as early as 1995 under a title at that time quite avant-gardiste: China's Constitutionalist Option. 1 Ten years later, notwithstanding the ups and downs in the political and legal evolutions of China, that "option" seems even more credible. Indeed, constitutionalism rather than democracy looks like the rather consensual and efficient paths for China to go on. The PRC seems thus to have embarked on a path which is both traditional (democracy after the Rule of Law is the rule rather than the exception, and corresponds to the patterns of European history among others) and unusual (that of an evolving constitutionalism without a system of "State of Parties" (the German concept of "Parteienstaat") or political pluralism.

To begin with, the paper argues that one must consider the history of constitutional development in China in the long run. We hereby mean, firstly, the fact that constitutional legacy dates back to the end of the 19 th century but also that it remains hardly "workable"

or "usable" in the PRC as it still symbolizes both the dictatorial powers of Yuan Shikai and Jiang Jieshi and nowadays the democratization of Taiwan which was lead possible thanks to the 1946 constitution enacted on the mainland. Secondly, the recent historical transformations of socialist countries must also be considered when analyzing China's constitutional evolutions. Indeed, China has embarked in legal reforms 10 years before the rest of the socialist countries but remain today far behind them in terms of constitutional justice. Yet, comparatively, most of the Easter European countries had, before the Communism, an experience of constitutional democracy that China never fully experienced: for example, Czechoslovakia had a Constitutional Court after the First World War. At last, contemporary worldwide neo-constitutional movement is growingly based and intertwined with general global concepts such as good governance.

Considering the fact that current official discourse in China on Rule of law shares similar references with international discourses, the question is whether the PRC is truly engaged in a 18 th century type of constitutionalist movement or whether it is limited to a tool to prevent political and institutional reforms from its original socialist framework.

Leaving the historical perspective, the paper will also argue that two different kinds of constitutionalism exist in today China: an "official" one and one "popular" (minjian) or civil-society based. Popular constitutionalism embodies a true "rights movement" (locally labeled "defense of rights" movement-weiquan yundong, 维权运动) which progressively, cases after cases, evolves towards a movement for the protection of constitutional rights.

Nonetheless, and more interestingly so, both official and non-official constitutionalist movements are intertwined. A clear distinction can not easily be drawn between the two spheres (public and private or, more precisely, non official) as both types of constitutional activisms interact each other constantly. For instance, academic or professional based constitutional actions reflect both the Party-State and the civil society domain. Officially also, a constitutionalist movement exists in a sense that, besides the existence of the CCP Charter, the State Constitution becomes an important site of political and social debate and contestations. Thus and secondly, analyzing the most recent constitutional amendments of 1999 and 2004 on the light of constitutional law theory, we will argue that the State Constitution of the PRC seems to be growingly taken seriously.

The hypothesis we work on is that China does not seem to be an anomalous paradigm of transition to constitutionalism but rather a classic one. Its model and development better describe the 18 th century view of the relations between constitutional and political changes, according to which constitutionalism is a precondition to democracy and not vice versa. Therefore, we need not to focus on the nowadays standard relation between constitutionalism and democracy, but on the dynamics between constitutionalism and politico-institutional changes if not democratization.

As other countries of different waves of constitutionalism did previously, China needs to go through a Copernican revolutions in order to define, first, what is law and the sources of law and reconsider the institutional paradigm of parliament sovereignty ; second, to reject the traditional Chinese distrust of judges and, third, the distrust of non official intermediaries located between individuals and the State power. The traditional preference for direct interactions between political power and individuals (society) needs to evolve. For China, constitutionalism is challenging as its philosophy is to prevent excessive centralization of power. Nonetheless, this trend has proven to be reinforced in China constantly besides or thanks to the modernization and opening policies.

Recent constitutional activism: political and historical contexts

China benefits of a rather rich historical constitutional heritage. Indeed, the country has been involved in the worldwide second wave of constitutional movement that has influenced Europe, Americas and non-western systems such as Japan from the end of (3) the evolution from the notion of vassal, subordinated subjects (臣民) to the notion of "subject-citizen" (国民). Such trends are effective transformations inside the Chinese political system as well as for its nation-state building process. Yet, the early quest for a modern, opened, pluralistic and yet truly Chinese political system remains a battle that is only at its beginning. Therefore, China which has participated officially to the international second wave of constitutionalism in the first third of the 20 th century was never part of the third wave which acknowledged, after 1945 and again with the collapse of the communist block in 1989, the worldwide acceptance of democracy.

The characteristics of the Chinese constitutional movement are not mainly determined by so-called specific cultural dimensions but rather by specific historical features. Among them is the circumstance that in China each single element of a system based on rule of law has to be reformed or (re)invented ad hoc and at the same time the fact that each constitutional needed "block" to build a rule of law system must be in China reformed or or even contradictory objectives, their meanings tend to be blurred. Besides, there is not but one Rule of law system or one type of democracy. The formation of constitutionalism in Europe and in the US from the 18 th Century gave birth to two sisters traditions, sisters which are not twins! The American constitutional tradition has been inspired by the French political theorist Montesquieu, who praised himself English philosophers who had structured his own thought, and even more the English customary constitution.

In the last 25 years, the word "democracy" became mostly in the common (journalistic) language of the western societies a synonym of "good government", without any other conceptual specification. It seems, nonetheless, more rigorous to use that word to qualify a political system based on "repeated free elections", and as a mechanism to allocate peacefully governmental power among competing elites (essentially a Schumpeter's definition). We now tend to forget that such a mechanism, democracy, presupposes the existence of a plurality of organized political actors (parties) who have some reasons to

move from an open violent conflict to a battle fought with "paper stones" (an eloquent expression borrowed to A. Przeworski). Such a condition, the existence of a plurality of Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org politically organized forces, and as such "democracy" in the Schumpeterian sense doesn't seem for the time being a possible "option" for China. Therefore, the following question is to inquire what are the reasons, if any, to believe that "constitutionalism" is an option on the table. To better respond to this question, one needs to add something about democracy and elections. The fact that elections are a mechanism to assign political power to competing elites hasn't been in history the only reason to resort to them. In societies, like for instance France during the Revolution, which did not accept organized political pluralism, elections had an independent double function: 1) to attribute legitimacy to those who govern4 (establishing, by the same token, the obligation of the citizens to obey the elected officials as argued by Emmanuel Sieyes during the French Revolution), and 2) to introduce -vis-à-vis the monarchical or dictatorial regime -the practice of a limited government, given that those who govern have no "tenure", so that the government is limited over time, and those in power need to be reelected if they want to stay in office. 5 We here suggest that elections and representative government (institutions that we have named later on "democratic" ones) have a strong connection with constitutionalism -if we understand by it a "limited government"; more exactly they are an instrument to achieve that goal. If elections and democracy are a means and not an end, if accordingly we make these democratic institutions function of a superior goal, we suggest by the same token that there are possibly alternative paths or means to achieve the goal of an anti-despotic, or limited government. Constitutionalism, here in the sense of some form of separation of powers, seems to be a major alternative to achieve that goal. An alternative, which actually had been chosen by most of the European continent long before entering the path of a pluralistic "democracy". 6 By constitutionalism, one must also mean the existence of a set of norms (normally codified in a written text) that citizens expect political actors will respect in order to govern the country in a non-arbitrary way and that in fact the latter basically respect.

Independently from (and next to) free elections, the main instrument that allows constitutionalism (the same here, as we said, that limited, moderate government) 7 to exist and survive goes, since the 18 th century and the article 16 of the Declaration of Rights, under the name of "separation of powers". An elected, accountable Parliament was able to balance an hereditary king (or an elected president in the American republican version of the story) and, vice versa, the king (or the President), because of his prestige (see Benjamin Constant on "pouvoir neutre") and a veto power, was able to avoid the exorbitant power resulting from a government by a single assembly The complex structure of the new constitutional-democratic regimes produced a "new constitutionalism" characterized by the generalization of mechanisms of constitutional adjudication, presidentialization of executive power (American influence), rationalized parliamentary institutions and supranational courts (European influence). Nowadays, a fundamental shift in democratic practice is that democracy is no longer limited to competitive repeated elections, since constitutional adjudication has gained a normative legitimacy even in France, a country of a longstanding tradition of distrust of judges.

We now assume that elections alone cannot be considered as a sufficient guarantee for constitutional-democracy; providing a mechanism of constitutional adjudication is now a requirement even for countries where it was anathema or even taboo 8 . Indeed, 95% of the countries of the 3 rd wave of democratization and constitution making 9 , especially former communist countries have adopted between 1991 and 1996 a judicial review or constitutional adjudication mechanism, and for the majority the centralized system of a special court according to the European, Kelsenian, Continental model. In these cases, constitutional adjudication was basically the product of democratization.

The definitions of some theorists (Henkin and others 1992) according to which constitutionalism is equivalent to democracy has been echoed by the literature on political and democratic transition after the third and fourth wave democratization countries10 . Nonetheless, there is actually no definite sequence between democratization and constitutionalism, scholars have long ignored the interactions between constitutionalism and the paradigm of the democratization, i.e. the transition to democracy. Some particularities of the Chinese constitutionalist movements indicate a possibility of rule of law preceding democratization with a particular constitutionalist model. As such China better describes the 18 th century view of the relations between constitutional and political changes, by which we mean that constitutionalism might be a precondition for democracy rather than vice versa. As Juan Linz showed in a pioneer book in 1978, The Breakdown of Democratic Regimes, political life is not simply divided between the ones who are in favor of changes and democracy and the others who are in favor of conservatism and dictatorship. It is a more subtle process, a combination of official and "non official"

Popular and Official constitutionalism in

spheres. Indeed, one can observe in China since the mid 1990's the vitality of a truly constitutionalist movement embodied by different representatives and strata of the "non official spheres" society as well as by official channels.

As more new laws and regulations are being promulgated in China by governments at all levels, the amount of conflict and inconsistencies between local and national legislation have also increased considerably. Briefly, the main characteristics of this unofficial constitutionalism are as follow:

1-less and less isolated cases and, on the contrary, more sophisticated claim against

State institutional irrationality or brutality as well as increasing collective litigations 2-a growingly active public opinion, lobbying on related constitutional matters. Young legal professionals, media (radio, journals and TV shows specialized on law and broadcast trials) and ordinary citizens are the most involved (see for instance the TV' series " 走向共和" among others); even if the surveys tend to show that awareness and knowledge of the Constitution are virtually nil, both among citizens 12 (any comparative analysis, however, would show similar results, even in democratic regimes, with the probable exception of the United States) and among the legal professions or government officials. More precisely, if general knowledge about the law is increasing, the Constitution is still perceived as "a purely political document" and constitutional law remains marginal, almost irrelevant law. It is neither the noble material nor an essential resource of the know-how of leadership elites, as is the case in the West.

3-Both the legal activists, ordinary citizens who go to courts, and active judges' incentives are growingly sophisticated 4-to some extent it is possible to detect various forms of "legal dissent", by legal professionals (lawyers such as Xu Zhiyong, Li Subin or Li Fangping for examples 13 ), law students, activists in local people's congresses and judges, i.e. those who are in a position to address the core contradictions of the political system. The expression "judicialisation" (sifa hua) became recently a rather common word. It refers to the political uses of the law, the legal uses of the institutional reforms and the self-justification by the law of the authoritarian practices of the government but also the increasing influence of legal institutions in constitutional practice, as in the expression "judicialisation of the Constitution (xianfa de sifahua). The expression "judicial power-grabbing" (司法抢滩) Administrative Adjudication Law《行政诉讼法》(1990) pursuant to which these suits were filed, judicial review is limited to "concrete administrative action" (具体行政行为)

and does not reach "abstract administrative action" (抽象行政行为).

In brief, the 2 following changes are the most important:

-the appropriation by a specialized elite of judicial reforms and institutional changes -the progressive disconnection of the legal (and as such the "autonomization") reform It is true that this same rigidity is or maybe nullified by the fact that the CCP controls both the legislative majority and the supermajority needed to modify the constitution; but progressively this norm is being imposed on the CCP. In a "unilateral constitution"15 like the Chinese one, constitutional amendments are not de facto more difficult to enact than ordinary statutes. But they are passed as amendment in order to send a signal to the population, to the party bureaucracy and to the international community, to indicate that an important modification took place in the Party ideology and will take place in the society. This signaling mechanism seems innocent and notably without any legal and political consequences for the regime. Yet, the amendments, on one side, make visible a new equilibrium inside the CCP concerning very important questions both of ideology and policy-making. And -even more important -they confirm de facto the de jure distinction between constitutional norms and ordinary (statute) laws;

putting the first stones towards the recognition of the importance of hierarchy of norms which is the starting point of the modern constitutionalism, We need to add that the Constitution integrates ex post the legal transformations rather than giving ex ante impulsion to new laws or to the protection of rights. Even if the coherence of the system is nowadays largely a wishful thinking, it seems nonetheless inevitable that a consciousness of the hierarchy of the norms is emerging in China. One should not forget that it took two century in France to move from the idea of hierarchy of norms to the practice of constitutional control.

The CCP passed 4 sets of important constitutional amendments to the 1982 constitution.

Up to 2004, constitutional amendments were concerned almost exclusively with the change of orthodoxy in economic matters: the diversification of property rights (1988), the establishment of a "socialist market economy" (1993), the acceptance "within the limits defined by the law and on the basis of strong State control, of the individual and semi-private economy" (Article 11, 1999) and the recognition of private property rights (March 2004). By stressing the value and concept of human rights, the amendment adds a new definition to the provisions concerning citizens' rights in the current Constitution.

Although the concept of "citizens' fundamental rights," as described in the Constitution, is more definite in legal terms than that of human rights, regarding the subject and content, it has less influence in political terms. By adding the description that "the state respects and protects human rights" in the chapter of the Constitution concerning "the Fundamental Rights and Duties of Citizens", the amendment combines the two concepts in an organic way, upgrading the essential meaning and value of the concept of citizens' rights.

The inclusion of human rights in the Constitution ushered in a new era of using the In China, the lack of respect for the Constitution is explained by the lack of a clear conception of the position of the Constitution in the hierarchy of the normative system, despite the apparently unambiguous terms used to designate it: it is indeed defined as the "fundamental law" (genben dafa) or as the "mother" of all the laws (mufa) with equal the Constitution when it was revised in 1999, two years later. Up to now, however, no revision of the constitution has institutionalized these new rules, even though they are in direct opposition to the "fundamental law" of the PRC.

Interestingly, most of the Asian countries -with few exceptions, and most notable among them India -have the reputation of being countries where the constitution doesn't matter. Countries with constitutions but without "constitutionalism" (T. Groppi).

This document -a written constitution -which is nowadays one of the basic requirement to become a recognized member of the international community seems to be nothing more that a "parchment barrier" for political powers struggling with their transition to the promise land of democracy! China is a case in point because it seems to represent both the epitome of a political power without limits, and a country where the constitution can't be more than a mockery. It is this apparent evidence, suggesting arguments in favor of the slightly unconventional hypothesis that the CCP takes, since almost 20 years the constitution growingly seriously.

Concluding remarks

In former Eastern block, in Mongolia, in Taiwan, Leninist parties played a dominant role in constitutional design of incremental reforms. Therefore while Stalinist legacies may play a path dependence role, it cannot prevent on the long run inner institutional reforms in China. Remains the fact that the Western development of the Rule of Law was marked by two forms of control of power, two irreversible and decisive victories which are: the verification of the legality of administrative action (and thus of the government) and the constitutional review. Now, both of these questions lie at the heart of contemporary evolutions in both the political and legal systems in China. As Philippe Schmitter put it "what counts is when and how these collective decisions about metarules are made, debated, ratified and implemented more than what is in them", as the process of choosing the institution is more important than the institution itself.

The last century even more than the century of democracy has been the century of the universalisation of "rigid" constitutions. China is again a case in point since they have a rigid constitution but no democracy. The expression "rigid" constitution according to James Bryce and Hans Kelsen, refers to the legal quality of those norms that can't be modified by the majority of the legislative power, but need a larger agreement, let say the involvement of the opposition in order to be amended. Rigid constitutions are in a sense

(

  re)invented ad hoc and at a simultaneous pace. For example, in most European countries, including France, the codification of laws has been one of the main political tasks of the 19th century. At that time, legal professionals were trained in law and tradition of law faculties was very vivid. (even if the first chair of constitutional law was only created in France in 1831 by Guizot at the Collège de France for Pellegrino Rossi and did not last longtime). In China, legal agenda must concentrate all type of these different reforms at the same time: from the national unified examination for judges and lawyers (the first one occurred in March 2002) to the writing of a Civil Code (法典expected for 2010) and the formalization of a hierarchy of norms (法律等级) as well as a system of constitutional adjudication(违宪审查制度). Another difference with former communist countries lies in the fact that the American constitutionalist model seems to be the reference for Chinese changes where, in former communist Europe, the legal heritage being essentially the European one, the main debate was basically to choose between the French or the German model. In the case of China, the question doesn't seem to be how the political constitutional engineering affects the transition to democracy and its consolidation but how the current constitutionalist movement can affect a political regime, which refuses at this stage a democratic transition? Can a postauthoritarian system emerge from the conjunction of economic capitalist development and a thin form of rule of law? And whether such a post-or quasi-authoritarian regime emerges, how far can it go?Constitutionalism, Rule of law and good governance: interacting paradigmsTaken separately constitutionalism, rule of law and democracy are clear concepts, traditionally analyzed by political theory as well as legal philosophy. Yet, having complicated interactions (for instance, constitutional adjudication has long been suspects to democrats and it is hard to justify its necessity in terms of democratic theory)

(

  gouvernement d'assemblée). This elegant structure, which in different forms is the masterpiece of the American as well as of the first French constitution (1791), looses much, not to say all, of its grip in contemporary parliamentary regimes (or in the States under "united government"), which are moreover Parteinstaaten (political systems dominated by organized parties). The reason is pretty intuitive: since the executive lacks independence, because of the principle of political responsibility vis-à-vis the Parliament, the same political party or the same political majority are in control of the two major branches of the government: the legislative and the executive. This is indeed one of the main reasons why contemporary constitutionalism -the one established in Europe in successive waves after the Second World War -is based essentially on the crucial role of an independent judiciary and on judicial organs in charge of constitutional control of statute laws and (sometimes) of the acts of the administration.

  China: similar dynamics?On the aftermath of constitutional amendments adopted in March 2004 by China's 10th National People's Congress, various forums, Internet chats welcomed remarks from ordinary citizens on the event. The very official Xinhua Forum printed popular advocacy in favor of constitutionalism showing the high level of consciousness of educated urban (basically) as well as the official support of such claims. Numerous messages would indicate for instance that "the authority of the Constitution can only be maintained in a constitutional system" or "only when the provisions of the Constitution are object of judicial litigation can its provisions have any substance" etc 11 . A constitutionalist movement exists in a sense that, notwithstanding the existence of the CCP Charter, the State Constitution becomes an important locus of political and social contestations.

1314

  See Balme, 2005, Judicialisation of politics and politisation of the judiciary in China. Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org nascent. Thus, constitutionalism appears clearly for what it stands: not purely an academic subject but a key political and social issue.5-Medias are all entirely under official/party control but tend to express provocative opinion although amidst high constraints and pressures. Good law reports can be found in "21世纪经济报道", China Youth Daily《中国青年报》, The New Beijing Daily《新京报》, Internet chats. Yet, media coverage of legal cases can be, as elsewhere in the world, controversial.6-It is a true "rights movement" as its Chinese name indicated (rights protection movement), which evolve growingly towards a constitutional rights movement as the pervasive Chinese expression indicates it: "以宪法维权…". Today, the most invoked articles are the right to equity and equal protection (as stipulated in article 33, paragraphe2 of the 1982 Constitution: "all citizens of the PRC are equal before the law") and the protection of property rights. Moreover since the SARS crisis in the Spring of 2003, the demand of citizens that their fundamental right to information be respected is increasing.7-Administrative litigation plays the substitute role of the de facto inexistence of constitutional adjudication. Although a popular consensus exists and there is an elite based social demand for the instauration of a mechanism to protect constitutional rights vis-à-vis unconstitutional laws. As a result, more administrative adjudication litigation (行政诉讼案) are being brought by citizens seeking to reverse government actions based on local regulations that conflict with national laws. However, under China's

  from the others reforms, especially the economically oriented ones. Concretely, most of current reforms observed in China are the same as the ones observed in Eastern Europe from the 1980's to the 1990s : a process of constitutionalisation of the rights movement, judicial reforms, heated debates on the preservation of supposedly socialist characteristics, rebirth of constitutional law and professionalization of legal experts. Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org Official constitutionalism: "rigidity' of an unilateral constitution Constitutional rigidity is written in the PRC Constitution (art. 64) [Article 5 spells out moreover the principle of superiority: "No law or administrative or local regulation may contravene the Constitution"].

  frequency. There are endless examples of the liberties taken with the text of the Constitution. For example, Article 28 of the Penal Law of 1997 (revised) replaces the term "counter-revolutionary activities" with the expression "penal activities which threaten the security of the State": this modification, however, was only integrated into Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org

Stéphanie

  Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org the ultimate defeat of the English-British model of Parliamentary sovereignty and victory of the model invented (or consecrated) in the US by the Philadelphia Convention in 1787.The Rule of Law was forged over the course of the history of the countries of Europe. It is a body of "representations both of the State and of the Law", as Jacques Chevallier summarizes it. The theory of the Rule of Law is taking root in China in a soil which is very different not only from its founding historical context, but also from the political choices made since the end of the Cold War by the former regimes of the Communist bloc. In China today, as was to some extent the case in Europe in the past, and possibly particularly in France, the Rule of Law is a modern means of regulation, a tool in the service of the objective of rationalization of the State, here conceived by the single party in power. Thus, under the Ancien Regime, the philosophers of the Enlightenment sought, in the face of the theoreticians of Absolutism, to think through the political order rationally, from which there emerged the idea of the Social Contract. However the ideology which gave meaning to the Rule of Law stems from distrust of the State, whose power one always seeks to constrain and to supervise in order to protect the individual. Moreover, constitutional development has been based on a clear definition of the concept of law as defined as early as 1789 in the Declaration of the Rights of Man and the citizen approved by the National Assembly at that time: "Law is the expression of the general will. Every citizen has a right to participate personally or through his representative in its foundation (…) All citizens, being equal in the eyes of the law are equally eligible to all dignities and to all public positions and occupations, according to them and without distinction except that of their virtues and talents". Defining what is law means defining and deciding who has the power to draft, enact and vote laws. Such a power implies to delimit popular sovereignty, meaning also to take parliamentary sovereignty seriously.Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org

  Constitution to safeguard human rights and opened up a wide window for the all-round development of human rights in China. Most of Chinese legal scholars object that such amendments are essentially of political nature and that the constitution is or ought to be a legal document. What the distinction does or should mean in that context? If by political decision one means minor questions connected with policymaking, no one can think that introducing "property rights" in the Chinese constitution, as it happened in 2004, is a minor decision. More in general and concerning the question legal-political, C.

	Schmitt's theory argues that a "constitution" (see his Verfassungslehre, 1928) has
	necessarily two sides: it is at same time a legal structure and a political decision. The
	fact that many liberal-democratic constitutions do not present themselves also as
	political decisions is not a reason to think that they are not such. Many written
	constitution includes clear choices and not just technical provisions about the
	organization of the government [examples from the French and the Italian present
	Constitution are republic, a-confessional state, etc.]. It seems moreover strange that
	NPC, meaning the CCP, willing to disregard the constitution no matter what, did pass in
	2000 a Lifa Fa (a Law on Legislation), which "provide[s] a means by which individuals
	can suggest the Standing Committee of the National People's Congress (NPCSC) to
	check constitutionality and legality of some ordinary laws".

Journal of Democracy, 7.4 (1996) 43-57

Iyenaga, Toyokichi, 2004, The Constitutional Development of Japan 1863-1881 Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org

Zhang Xueren et Chen Ningsheng (dir.), Ershi shiji zhi zhongguo xianzheng, Le Constitutionnalisme de la Chine au XXème siècle, Wuhan daxue chubanshe,Wuhan, 2002,

497p. Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org

One has to be aware that a revolutionary ideology, for a while, and steep economic growth may be alternative mechanisms for political legitimacy of the government. It is interesting, moreover, to notice that "socialist" and some others authoritarian regimes prefer fake elections to no elections at all!

Which, needless to say, presupposes real and free elections.

The English case is slightly different, but also in England the early forms of "constitutionalism", in the Middle Age, predate the establishment of a regular two parties system[see Deborah Boucoyannis]

So the word designate at the same time the means (separation of powers) and the aim (limited government)! Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org

Democracy and the rule of law by Adam Przeworski, J M Maravall and Pasquino, 2003

Tom Ginsburg, 2005. Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org

Linz and Stepan, 1996.

新华论坛, www.forum.xinhuanet.com Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org

We borrow this expression from Andrea Pozas Loyo. Stéphanie Balme and Pasquale Pasquino -Taking Constitution(alism) seriously? … -January 2006 http://www.ceri-sciences-po.org


Qihao He

Hubert Ranchon

Pascal Carrivain

Yannick Viero

Joris Lacroix

Charline Blatché

Emmanuelle Daran

Jean-Marc Victor

Aurélien Bancaud
email: abancaud@laas.fr

Conformational manipulation of DNA in nanochannels using hydrodynamics

Keywords: nanofluidics, hydrodynamics, DNA conformational manipulation, single molecule imaging, shear-induced depletion

The control over DNA elongation in nanofluidic devices holds great potential for large-scale genomic analysis. So far the manipulation of DNA in nanochannels has been mostly carried out with electrophoresis and seldom with hydrodynamics, although the physics of soft matter in nanoscale flows has raised considerable interest over the last decade. In this report the migration of DNA is studied in nanochannels of lateral dimension spanning 100 to 500 nm using both actuation principles. We show that the relaxation kinetics are 3-fold slowed down and the extension increases up to 3-folds using hydrodynamics. We propose a model to account for the onset in elongation with the flow, which assumes that DNA response is determined by the shear-driven lift forces mediated by the proximity of the channels' walls.

Overall we suggest that hydrodynamic actuation allows for an improved manipulation of DNA in nanochannels.

The developments in DNA sequencing technologies have been complemented by innovative methods aiming to obtain genomic maps with a low resolution larger than 10 3 bp, which can be sufficient to discriminate genomic imprints of individuals 1 . These methods, which have been vaunted for their high-throughput capabilities, generally involve whole chromosome manipulation and structural analysis by optical microscopy at the single molecule level. For instance DNA combing, which consists in spreading DNA molecules on hydrophobic surfaces by applying a receding meniscus, 2,3 was successfully applied to map the position of genomic sequences along linearized chromosomes, and it enabled to detect microdeletions in genes involved in tuberous sclerosis 4 . The technology of optical mapping allowed to produce whole genome restriction maps in bacteria 5 and more recently in human 6 and in goat 7 based on the sizing of chromosome fragments frozen in a linearly elongated state using an agarose matrix and shear flows. 8 The throughput of these technologies was further improved with the advent of microand nano-fluidics, which enable to generate tailored fluid flows that control the conformation of DNA. For instance elongational flows generated by funnels were shown to induce the transient spreading of DNA, allowing for the genomic mapping of target sequences in bacterial artificial chromosomes of 200 kbp at a throughput of 150 Mbp/s. 9,10 The conformation of chromosomes was controlled more precisely in nanofluidic channels: wall steric interactions trigger the steady elongation of DNA, and the degree of elongation is determined by the design of nanochannels. 11,12 This technology was shown to be compatible with DNA restriction mapping, 13 and positioning of target sequences in genomic DNA. 14,15 The controlled geometry of nanofluidic devices appeared to be an excellent technology to investigate the physics of DNA in confined environments, which had been thoroughly studied theoretically in the 80's. [START_REF] De Gennes | Scaling concepts in polymer physics[END_REF][START_REF] Odijk | [END_REF] For instance the degree of DNA elongation was enhanced by decreasing the lateral dimensions of nanochannels, 11 by decreasing the salt concentration, 18,19 or by adding neutral polymers. 20,21 In these experiments DNA manipulation was mostly performed using electrophoresis as actuation principle, because this method scales favorably to convey DNA in nanochannels in comparison to hydrodynamics, which requires the use of high pressure sources. 22 Nevertheless the fluid dynamics of polymer solutions in confined geometries raises conceptual challenges, which are essentially associated to the hydrodynamic interactions of polymers with channels' walls. 23 Furthermore the hydrodynamic migration of DNA in narrow capillaries opens new perspectives in analytical sciences, for the principle for matrix-free separation of DNA has recently been demonstrated. 24 In this report, we investigate the statics and the dynamics of DNA molecules of 50 kbp and ~1 Mbp inside nanochannels of lateral dimension spanning 100 to 500 nm in the presence of hydrodynamics or electrophoresis (Fig. 1A-D). Our study is divided in four sections, which consecutively describe the entry of DNA in nanochannels, its relaxation, and its steady conformation. We first show that DNA molecules nearly systematically fold in a linear conformation with hydrodynamics, whereas they frequently present loops at their leading edge with hydrodynamics. We then measure relaxation kinetics at the single molecule level, and uncover 3-fold slowed down dynamics with hydrodynamic actuation. We subsequently focus on DNA steady extension, and show that the shear constraints imposed by Poiseuille flows enable to control the level of spreading, which can be enhanced up to 3-fold in comparison to electrophoresis. We derive scaling predictions to account for this data, assuming that shear-driven lift forces mediated by the proximity of the channels' walls are responsible for DNA elongation. Overall our study shows that hydrodynamic actuation allows for an improved manipulation of DNA, and paves the way to the rational use of hydrodynamic actuation as an efficient solution for single molecule nanofluidic experiments.

DNA is preferentially linearly elongated with hydrodynamics

Using our nanofluidic set-up to perform single molecule experiments described in ref. 25 and our custom-made imaging system (see methods section), the entry dynamics of single λ-DNA molecules has been studied in 200 nm square nanochannels. A set point triggering the uptake of DNA has been detected at 20+/-5 bar/cm and 700+/-100 V/cm, and DNA migration velocity 4+/-1 and 20+/-4 µm/s for hydrodynamic and electrophoretic actuation, respectively (dashed line in Fig. 2A). This threshold is consistent with the idea of an entropic barrier that has to be crossed to enter nanochannel, [START_REF] De Gennes | Flexible polymers in nanopores[END_REF] and the solvent flux to trigger the passage of DNA is k B T/η ~8.10 -19 m 3 /s given that the viscosity is η~5 cP, as inferred from Couette rheometry (not shown; k B is the Boltzmann constant and T the temperature of ~300 K). For a square nanochannel of 200 nm, the mean fluid velocity is thus expected to be 20 µm/s, in excellent agreement with electrophoretic measurements. Above this threshold the velocity of λ-DNA increases approximately linearly with the pressure drop or the electric field (Fig. 2A), enabling us to evaluate the mobility of 3.2 10 -4 cm 2 /bar.s and 0.2 10 -4 cm 2 /V.s for hydrodynamics and electrophoresis, respectively. Note that the threshold field strength to force the passage of molecules is consistent with earlier studies, [START_REF] Menard | [END_REF] but the electrophoretic mobility is slow, and we recently showed that this response was specific to PDMS nanochannels. 25 DNA hydrodynamic mobility compares well with the mobility of a particle travelling at the mean flow velocity in square nanochannels of 200 nm, which is 2.8 10 -4 cm 2 /bar.s using the characteristics of Poiseuille flows defined in e.g. ref. [START_REF] Stone | Introduction to Fluid Dynamics for Microfluidic Flows[END_REF] . Thus the principles driving DNA uptake and migration can be recapitulated with simple physics models. The entropic barrier at the interface between micro-and nano-structures causes the trapping of molecules at the entry of nanochannels, [START_REF] Cao | [END_REF] and favors the formation of hairpins at the DNA leading edge. 30 Indeed the forces acting on DNA are much greater in nano-than in micro-channels due to the conservation of the electric or hydrodynamic fluxes, so whenever one DNA segment faces the entrance of nanochannels, it drags the molecule, which frequently adopts a looped conformation. We thus set out to evaluate the proportion of hairpins by monitoring the fluorescence intensity along the molecule, given that the leading edge is twofold brighter for looped DNA (Fig. 2B). The proportion of hairpins is ~85% (n=88) using electrophoresis, and it is nearly constant with the electric field (red dataset in Fig. 2C). This conformation is energetically unfavorable due to the bending and the repulsion of the DNA strands in the loop, but the relaxation towards a linearly elongated state is slow, occurring in tens of seconds (not shown), as was also observed in earlier studies. 14,30,31 When hydrodynamic pressure is applied to force DNA uptake, hairpins are also detected, but they rapidly unfold, leading the formation of DNA molecules linearly elongated conformation (Fig. 2B, upper panel). For slow migration velocities of ~50 µm/s, the vast majority of molecules is linear (hairpin proportion of 5%, blue dataset in Fig. 2C), and the proportion of hairpins increases to 60% at 200 µm/s, a value yet much lower than with electrophoresis. Thus the use of hydrodynamics allows for a better control on the conformation of DNA at the entry of nanochannels. This statement is strongly reinforced by the demonstration that chromosomes fragments of hundreds to thousands of kbp in length can be manipulated in nanochannels with a low proportion of hairpins of 46% (n=52) at a migration velocity of 200 µm/s using hydrodynamics (Fig. 2D).

In conclusion we demonstrate that hydrodynamic actuation allows for a better control over DNA conformation during the uptake in nanochannels, and it can be employed to complement the strategies described in the literature to avoid the formation of hairpins. For instance it has been suggested to wait for the complete relaxation of the molecules in the linear state. 14 This relaxation however occurs in ~50 s for DNA of 150 kbp, and the time scale is expected to increase with the size of DNA, thus leading to excessively long time periods for chromosomal DNA. Nanoposts arrays have also been fabricated ahead of nanochannels in order to disentangle molecules before their uptake, [START_REF] Cao | [END_REF] and hence favor the linear state.

Interestingly we recently showed that DNA / nanoposts hooking events were different using hydrodynamics vs. electrophoretics actuation due to the presence of shear in Poiseuille flows. 32 DNA is indeed more elongated and its conformational space is more restricted, hence better controlled, with hydrodynamics, suggesting that nanofluidic experiments combining hydrodynamics and nanoposts arrays should be conducted in the future to achieve optimal configurational manipulations.

DNA relaxation is slowed down with hydrodynamics

The behavior of single λ-DNAs has then been studied in the course of their migration, only focusing on linearly elongated molecules (stacked time series represented in Fig. 3A).

The degree of elongation is maximal right after the uptake due to the transient intramolecular tension between the leading edge, which is dragged in the nanochannel by the force field, and the lagging end that is located ahead of the entropic barrier, and resists the progression in the channel. The initial longitudinal spreading is enhanced from 11.9 to 19.9 µm with hydrodynamics in comparison to electrophoresis at 100 µm/s (Fig. 3B). This data in turn shows that the end-to-end distance transiently reaches ~90% of the contour length with hydrodynamics given that YOYO-1 stained λ-DNA measures L~22 µm. 33 A relaxation is then detected (Fig. 3C), and its kinetics is characterized by measuring the end-to-end length of the molecule. In square nanochannels of 200 nm and using electrophoretic actuation, the relaxation time decreases from 0.4 to 0.1 s for migration velocities spanning 30 to 100 µm/s, respectively (Fig. 3C). Notably the retraction kinetics is faster than the characteristic time of length fluctuations at steady state, which is ~1 s in 200 nm square channels. 11 The relaxation time is proportional to the ratio of the friction to the stiffness, 11 which scale as 6 (ref. 34 ) and (1 -/) -3 , according to the worm like chain response, 35 respectively, with l the molecule length and L its contour length. We therefore expect the retraction kinetics to be accelerated as the elongation of the molecule increases, in agreement with our experiments. DNA relaxation has then been examined with hydrodynamics, showing that the kinetics is three-fold slowed down in comparison to electrophoresis (Fig. 3C). This difference in relaxation dynamics is likely associated to the shear constraints induced by Poiseuille flows, which are characterized by the shear rate ̇ of 2/ℎ ~10 3 s -1 for a flow velocity v of 100 µm/s in square nanochannels of h~200 nm. It has indeed been observed in bulk shear flows that the fluctuations of DNA end-to-end distance are slowed down as the shear rate increases. 33 The same trend has been reported for end-tethered DNA chains in shear flows, 36 and this response was shown to arise from cyclic dynamics that occur due to the coupling between the chain velocity in the flow direction to fluctuations in the shear-gradient direction.

Interestingly the observation of slowed down relaxations may account for the larger maximal elongation with hydrodynamics through a shear-enhanced "deformability" of DNA. Indeed the threshold flow to trigger the deformation of an end-tethered polymer is determined by the ratio of its gyration radius to its relaxation time, 37 so slowly relaxing polymers can be extended with low flow rates, or equivalently reach large extensions for a given flow rate.

Similarly one may speculate that the linearization of molecules with hairpins is accelerated by the shearing that accelerates the relaxation the stable conformation (Fig. 2B). Overall we observe that the kinetics of DNA relaxation is slowed down with hydrodynamics, and we propose that the conformational properties of DNA, which are characterized by a higher extension and a lower proportion of hairpins with hydrodynamics, are determined by this change in dynamics.

The elongation of DNA is controlled by the flow

The elongation of λ-DNA at the end of the relaxation has finally been measured (Fig. 4A-B). The steady extension using electrophoresis is 5.3 µm, hence ~24% of the total length, in square nanochannels of 200 nm, in agreement with earlier studies carried out in the absence of force fields. 11,25 Thus the electrophoretic actuation has marginal effects on DNA conformation, as expected from the fact that DNA behaves as a free-draining polymer during electrophoresis. 38 Conversely DNA end-to-end distance is more than doubled to ~55% for a migration velocity of 100 µm/s using hydrodynamics (Fig. 4B). This result is reminiscent of the onset in elongation of ~30% in bulk shear flows characterized by ̇5 s -1 , 33 but the extension exhibited large fluctuations due to an end-over-end tumbling of DNA, which limited the use of this principle for biomolecule analysis (see discussion in e.g. ref. 39 ).

Tumbling is impeded in nanochannels due to the level of confinement, accounting for the steady elongation in Fig. 4B. In addition the degree of elongation can be monitored from 24% to 65% by tuning the migration velocity in the range 0-200 µm/s (Fig. 4C), allowing us to reproduce the performances of confined nanochannels of ~80 nm in cross-section without changing fabrication technology. 11 We thus argue that the control of the pressure source provides an efficient macroscopic control parameter for the manipulation of single DNA molecules in nanofluidic channels.

Shear-driven lift forces are responsible for DNA elongation

We set out to investigate the physics driving the flow-driven elongation of DNA. The end-to-end length of single λ-DNA molecules was monitored in nanochannels of different geometries as specified in Fig. 1D, and for different migration velocities (Fig. 5A), showing a variety of responses that we wished to reconcile with a model. We first considered that DNA behaved as a free-draining polymer, i.e. neglecting hydrodynamic interactions between monomers, as was suggested to model critical flow rates for dragging linear polymer in nanochannels. 40,41 We performed Langevin dynamics simulations of freely-jointed chains confined in nanochannels of 200 nm, and measured the elongation of the molecule in plug or Poiseuille flows. Given the computational cost limitations of simulations, we focused on DNA chains of 1 and 4 µm in contour length, which adopt globular conformations of ~300 and ~600 nm in solution according to our simulations (not shown). The end-to end distance remained roughly constant to ~20% of the contour length for every flow spanning 0-200 µm/s (Supplementary Fig. S2), thus strongly suggesting that this model is inadequate to reproduce the spreading forced by the flow. We then considered that the response of the molecule was determined by its elasticity, as well as monomer/monomer and monomer/wall hydrodynamic interactions. The effect of hydrodynamic interactions has been investigated in molecular dynamics simulations, showing that DNA migrates toward the centerline of the channel where it is stretched. 42 This transverse migration is associated to the development of a depletion layer near the walls, which has been studied numerically, analytically, 43,44 and experimentally in microchannels. 45 The thickness of the depletion layer is unfortunately not measurable in nanochannels, so we developed a model focused on the response of polymer to shear stress. We consider that the de Gennes-Pincus blob picture [START_REF] De Gennes | Scaling concepts in polymer physics[END_REF] can be applied to study the response of polymers in hydrodynamic flows. According to this model the chain consists of a series of self-avoiding blobs of diameter D, each of them behaving as a chain in good solvent, and its degree of elongation is:

~� 2 � 1/3 (1)
with w and P the polymer diameter and Kuhn length, respectively. The long-distance hydrodynamic coupling between monomers inside each blob should lead to coils behaving as massive objects impermeable to the flow, as observed in e.g. analytical centrifugation (Fig. 5B). [START_REF] De Gennes | Scaling concepts in polymer physics[END_REF] We may thus model each blob as a globule, and consider that hydrodynamic interactions with the walls can be treated as in the physics of globules in shear flows.

Deformable objects, such as globules, conveyed by flows are indeed subjected to lift forces at the proximity of surfaces, which are expressed at the scaling level as 3 / with y the distance of the object to the surface. 46 The cumulative effect of each channel wall leads to a net centripetal lift force that tends to elongate the molecule at the expense of an entropic cost.

The onset in elongation associated to the flow ∆l can then be derived (see details of the calculation in supplementary materials):

∆ ℎ -8/15 ~() 1/3 � 8 � 2/5 ̇2 /5 (2)
The numerical prefactor in the right term is 43 m -8/15 s -2/5 taking 2 and 100 nm for the DNA radius and Kuhn length, respectively, showing good correspondence of our model with the data (solid curve in Fig. 5C). Altogether we propose that DNA spreading is forced by the shear, and we argue that our model is readily suited to define operating conditions according to specifications on the degree of spreading required for single molecule genomic analysis.

Conclusion

We have investigated the migration of DNA in nanochannels using hydrodynamics, and report that relaxation dynamics are slowed down and the steady elongation is enhanced in comparison to electrophoresis. We propose that these changes are associated to the shear constraints imposed by Poiseuille flows and to the existence of hydrodynamic interactions of the polymer with channels' walls. We also derive an analytical model that reproduces the variations of the elongation of the molecule with the flow velocity. Altogether we believe that this report can serve as a guideline to design single molecule manipulation experiments in nanochannels. Future work is also needed to study the response of DNA in nanochannels with lateral dimension narrower than 100 nm, because the physics of the molecule is described by the Odijk regime, 11,[START_REF] Odijk | [END_REF] which readily departs from the de Gennes response that is relevant to our experimental settings. In another direction molecular dynamics simulations of polymers flowing in micro-and nano-channels have been mostly focused on the thickness of the depletion layer near the walls. The depletion layer is expected to force the spreading of DNA, but the degree of elongation is poorly reproduced by the scaling analysis derived from this model (Supplementary Fig. S3). Further clarifications to reconcile the mechanics of DNA and its depletion from the walls can be obtained from simulations to improve our description of the fluid dynamics of DNA in confined geometries. PDMS chips were obtained by the sequential baking of one 40 µm layer hard PDMS covered by a layer of conventional PDMS of ~1 cm for 45 min and 3 hours at 75°C, respectively (see details in reference 25 on the mold fabrication process). Note that PDMS is deformable, but the deformation δ for square nanochannels of lateral dimension h scales as δ/h~∆P/E 47 with E the Young modulus of hard PDMS of ~5 MPa 48 , implying that δ is small in our experiments of ~2 nm for an actuation of 0.5 bar. Silicon nanochannels were obtained by electron beam lithography, followed by reactive ion etching using an Alcatel system (etching rate=22 nm/s). Microfluidic channels were produced by conventional photolithography, and access holes, which served as fluidic inlet and outlet, were drilled through silicon by sand blasting. Note that the comparison of electrophoretic vs.

Experimental Fabrication

hydrodynamic actuation required the growth of an insulating layer of silicon dioxide of 100 nm. PMDS channels were eventually sealed after plasma activation of the chip and one glass coverslip, and the sealing of silicon nanochannels was obtained using a thin layer of PDMS spin-coated on glass coverslips (see reference 25 for details of the protocol), that was also activated by oxygen plasma. The bonding strength was enhanced by curing the resulting chips at 100°C during 20 minutes, enabling us to apply pressures up to 2 bars.

DNA preparation, manipulation, and imaging

DNA was fluorescently labeled with YOYO-1 (Molecular Probes) at a staining ratio of cells. Genomic DNA purification consisted in harvesting 10 6 cells and resuspending the cells in 2 mL of a solution containing 1% low-melt agarose. This mix was then dispensed in 10 scaffolds of 10x10x2 mm 3 until agarose reticulated, and these agarose blocks used as matrices to preserve genomic DNA from mechanical constraints during purification, which was performed using 1% SDS and 250 µg/mL proteinase K. Agarose blocks were eventually stored in TE (Tris-HCl 10 mM and EDTA 1 mM) at 4°C. They were melted at 42°C with 1unit/mL β-agarase before experiments.

Imaging was performed with a Zeiss epifluorescence microscope equipped with the 38HE filter set (Zeiss), and with a Lumencor Light Engine emitting at 475 nm with a 28 nm bandwidth and a power of 20 mW. An ANDOR iXon-885 camera was used to observe single DNAs using a binning of 4*4, and a pixel size of 0.33 µm. Exposure times varied from 20 to 40 ms depending on the requirements of the experiment (see Supplementary Video 1 to assess the quality of videos). DNA manipulation was performed using electrophoresis or pressure actuation that was monitored by a Fluigent pressure manager in the range 10 to 1000 mBar.

The pressure was applied uniformly in the two inlet and outlet channels (Fig. 1A). Note that this technology provides a stable and pulseless flow with a rapid response in comparison to syringe pumps, which deliver hysteretic flows with long equilibration times for nanoscale volumes.

Videos sequences were eventually analyzed using custom macros implemented in ImageJ. Molecules were automatically segmented by Otsu thresholding in order to retrieve its center of mass and length at every time step. the molecule is ~2.7 s, so we estimate that its length is ~550 µm, or equivalently ~2 Mb, given that λ-DNA elongation is ~15 µm (see Fig.

He

  et al. (2012) 8 / 28

Figure 1 :

 1 Figure 1: Nanofluidic devices for hydrodynamic and electrophoretic DNA manipulation.

Figure 2 :

 2 Figure 2: Uptake of DNA in nanochannels using hydrodynamics and electrophoresis. (A) The velocity of λ-DNA molecules is plotted as a function of the pressure drop or electric field (left and right panel, respectively), showing the existence of a threshold (dashed lines) above

  4 below).

Figure 3 :

 3 Figure 3: λ-DNA relaxation dynamics. (A) The stacked time series of micrographs shows the migration of one λ-DNA in a square nanochannel of 200 nm at a velocity of 30 µm/s using electrophoresis. The upper image is recorded as the molecule has just completely

Figure 4 :

 4 Figure 4: Flow-driven DNA elongation in nanofluidic channels. (A) The upper and lower panels are stacked time series of single λ-DNA molecules in the course of their migration at 100 µm/s using electrophoresis or hydrodynamics. Note that the lagging edges of the

Figure 5 :

 5 Figure 5: Scaling analysis of flow-driven elongation. (A) The steady extension of single λ-DNA molecules was recorded as a function of their migration velocity for different

He

  et al. (2012)

Acknowledgements:

We are grateful to F. Brochard for enlightening discussions on the manuscript. This work and the fellowship of Q. He were supported by the ANR program JC08_341867. This He et al. (2012) 15 / 28 work was partly supported by the LAAS-CNRS technology platform, a member of the French RENATECH network. Y. Viero and J. Lacroix thank the Defense Procurement Agency (DGA) and the RITC foundation for PhD fellowship funding, respectively.


Philippe Négrel

Pauline Humez

Vincent Lagneau

Julie Lions

Interaction eau-roche-CO 2 en contexte de fuitecontrôlée de CO 2 : apport du monitoring géochimique et isotopique lors d'un cas réel d'injection de CO 2

Cette étude montre, dans un cas réel d'injection de CO 2 , comment une approche multiisotopique (B, Li, S, O, Sr) combinée aux données chimiques permet (i) de tracer indirectement la réactivité et présence du CO 2 , (ii) de contraindre et comprendre précisément les interactions eau-roche-CO 2 et les réponses isotopiques.

L'originalité de ce travail consiste à utiliser des outils isotopiques développés dans les géosciences pour les appliquer à ce contexte particulier. L'idée majeure est d'utiliser ces outils comme traceurs des interactions eau-roche-CO 2 afin de détecter toute anomalie de fuites de CO 2 non décelables par les autres moyens de monitoring existants.

La mise en oeuvre des outils isotopiques a permis de faire avancer les méthodologies de monitoring géochimique nécessaire pour la surveillance des sites de stockage géologique de CO 2 et le traçage des fuites de potentielles de CO 2 . Cette étude porte sur l'impact et le traçage des fuites précoces de CO 2 provenant d'un réservoir géologique de stockage de CO2 sur les aquifères d'eau douce.

Une application grandeur nature, dans le cadre du projet CO2FieldLab sur le site de Svelvik en Norvège, a consisté en une injection de CO 2 à 20 m de profondeur dans un aquifère stratifié(1.67 tonnes de CO 2 ont été injectées sur une période de six jours). Ce cas d'étudea permis de mettre en place le programme de monitoring isotopique et le suivi de l'évolution d'unhydro-système naturel d'eau souterraine. Cette étude a fourni une approche complémentaire des processus qui ne peuvent être étudiés ou ne sont pas nécessairement pris en compte dans les modélisations numériques ou à l'échelle du laboratoire, par exemple, lesintrusions salines. Des changements importants dans les signatures isotopiques et chimiques de l'eau pendant la durée de l'expérience ont indiqué trois processus : 1) de mélange entre l'eau de mer et l'eau de pluie suite à une intrusion saline, 2) de processus d'échange ioniqueà l'interface eau-douce /eau saléeet 3) d'interactions eau-roche-CO 2 (e.g. processus de dissolution/précipitation, etc.) engendrées par les conditions acides du fait de la dissolution du CO 2 . L'efficacité de cette approche globale géochimique et isotopique permet de distinguer les processus et phénomènes naturels et les processus reliés à l'injection de CO 2 .