High sensitivity to cosmogenic isotope production calculations

Ce chapitre s’intéresse à l’utilisation du 10Be pour prédire les variations de 14C atmosphérique durant l’excursion de Laschamp il y a ∼41 ka, ceci à l’aide d’un modèle océanique en boîtes simulant le cycle du carbone. L’objectif initial de cette étude était de d’estimer l’influence de cet événement sur le rapport 14C/C ∗ dans l’atmosphère entre 37,5 et 45,5 ka, déterminé à l’aide de mesures dans divers archives (spéléothèmes, coraux, sédiments marins). De plus, il était intéressant d’utiliser des données de 10Be à haute résolution afin d’avoir accès au variations rapides du ∆14C atmosphérique dû à la hausse de sensibilité de la production d’isotopes cosmogé- niques à l’activité solaire durant l’excursion de Laschamp. Plusieurs étapes sont nécessaires pour obtenir les variations de ∆14C à partir d’un enregistrement de 10Be (voir section 2.2.2 et articles de Beer et al. [1988]; Bard et al. [1997]; Muscheler et al. [2004]; Nilsson et al. [2011]). En effet, après avoir corrigé la différence de sensibi- lité estimée entre la déposition polaire et globale de 10Be, les données de 10Be sont converties en 14C à l’aide de calculs de production, puis entrées dans un modèle du cycle du carbone. Les calculs de production utilisés pour la conversion 10Be – 14C sont ceux de Masarik and Beer [2009] et la combinaison de ceux de Kovaltsov and Usoskin [2010] pour le 10Be et de Kovaltsov et al. [2012] pour le 14C. En comparant les amplitudes résultantes de ∆14C atmosphérique avec ces deux calculs, nous avons finalement montré la forte sensibilité de cette méthode aux incertitudes liées aux cal- culs de production des isotopes cosmogéniques utilisés lors de la conversion du 10Be en 14C, en particulier durant les périodes de faible intensité du champ magnétique (telles que l’excursion de Laschamp).

Cosmogenic isotopes like 14C and 10Be are produced in the Earth’s atmosphere mainly by interaction of Galactic Cosmic Rays (GCR) with nitrogen of the upper atmosphere. Since the GCR flux is modulated by the geomagnetic and heliomagnetic fields, records of 14C and 10Be provide useful information about variations in solar activity and geomagnetic field intensity in the past [Lal and Peters, 1967]. As a consequence, the higher the solar or geomagnetic field, the more primary cosmic ray particles are deflected, which leads to a decrease of cosmogenic isotope production. 14C and 10Be have been studied in natural archives for several decades. 14C measurements were performed to establish 14C calibration records because the ra- tio 14C/12C in the atmosphere has changed during the past due to variations of production (geomagnetic field intensity and solar activity) and modifications of the carbon cycle. Many such studies have been done in sediments [Hughen et al., 2004, 2006; Bronk Ramsey et al., 2012], speleothems [Beck et al., 2001; Hoffmann et al., 2010], corals [Fairbanks et al., 2005], and tree rings [Muscheler et al., 2008; Turney et al., 2010]. Calibration curves, regrouping all 14C measurements, as IntCal04 and IntCal09 [Reimer et al., 2004, 2009] have been constructed for the conversion of ra- diocarbon ages to calibrated ages. 10Be has been studied in ice cores from Antarctica [Yiou et al., 1985; Raisbeck et al., 1990, 1992; Horiuchi et al., 2008; Baroni et al., 2011] and Greenland [Beer et al., 1990; Finkel and Nishiizumi, 1997; Yiou et al., 1997; Wagner et al., 2001; Muscheler et al., 2004, 2005], as well as in sediments [Raisbeck et al., 1985; Robinson et al., 1995; Frank et al., 1997; Ménabréaz et al., 2011; Nilsson et al., 2011]. One advantage of ice cores is that they offer a relatively simple way to calculate 10Be fluxes (from the measured concentration 10Be and the estimated accumulation rate). Moreover, their higher resolution can be helpful for the study of shorter events due to solar activity for example.

Although 14C and 10Be are both produced by cosmic rays, their behaviors differin the atmosphere. Indeed, 10Be atoms become fixed to aerosols and are deposited [1981a]) whereas the 14C atom is oxidized to CO2 and enters in the global carbon cycle in which it is homogenized with stable carbon. As a consequence, 14C con- centration variations in different reservoirs are smoothed and delayed with respect to 14C production variations. Masarik and Beer [1999] found that the stratosphere contributes 56% of the global production of 10Be and Heikkilä et al. [2009] deter- mined with their model that the stratospheric fraction of the total production is 65%. While most 10Be produced in the troposphere is deposited near the latitude band in which it is formed, even the dominant proportion coming from the strato-sphere probably does not have the time to be completely well-mixed because of its relatively short residence time compared to the mixing time of the air in the strato- sphere. According to Field et al. [2006], the polar flux is about 20% less sensitive to variations of geomagnetic field intensity (and 20% more sensitive to variations of solar activity) than the global production. This fact will be taken into account for the 10Be – 14C conversion (see section 3.2.2).