Abstract. Since the first suggestion of 1500-year cycles in the advance and retreat of glaciers (Denton and Karlen, 1973), many studies have uncovered evidence of repeated climate oscillations of 2500, 1500, and 1000 years. During last glacial period, natural climate cycles of 1500 years appear to be persistent (Bond and Lotti, 1995) and remarkably regular (Mayewski et al., 1997; Rahmstorf, 2003), yet the origin of this pacing during the Holocene remains a mystery (Rahmstorf, 2003), making it one of the outstanding puzzles of climate variability. Solar variability is often considered likely to be responsible for such cyclicities, but the evidence for solar forcing is difficult to evaluate within available data series due to the shortcomings of conventional time-series analyses. However, the wavelets analysis method is appropriate when considering non-stationary variability. Here we show by the use of wavelets analysis that it is possible to distinguish solar forcing of 1000- and 2500- year oscillations from oceanic forcing of 1500-year cycles. Using this method, the relative contribution of solar-related and ocean-related climate influences can be distinguished throughout the 10 000 yr Holocene intervals since the last ice age. These results reveal that the 1500-year climate cycles are linked with the oceanic circulation and not with variations in solar output as previously argued (Bond et al., 2001). In this light, previously studied marine sediment (Bianchi and McCave, 1999; Chapman and Shackleton, 2000; Giraudeau et al., 2000), ice core (O'Brien et al., 1995; Vonmoos et al., 2006) and dust records (Jackson et al., 2005) can be seen to contain the evidence of combined forcing mechanisms, whose relative influences varied during the course of the Holocene. Circum-Atlantic climate records cannot be explained exclusively by solar forcing, but require changes in ocean circulation, as suggested previously (Broecker et al., 2001; McManus et al., 1999).
Abstract. Since the first suggestion of 1500-year cycles in the advance and retreat of glaciers (Denton and Karlen, 1973), many studies have uncovered evidence of repeated climate oscillations of 2500, 1500, and 1000 years. During last glacial period, natural climate cycles of 1500 years appear to be persistent (Bond and Lotti, 1995) and remarkably regular (Mayewski et al., 1997; Rahmstorf, 2003), yet the origin of this pacing during the Holocene remains a mystery (Rahmstorf, 2003), making it one of the outstanding puzzles of climate variability. Solar variability is often considered likely to be responsible for such cyclicities, but the evidence for solar forcing is difficult to evaluate within available data series due to the shortcomings of conventional time-series analyses. However, the wavelets analysis method is appropriate when considering non-stationary variability. Here we show by the use of wavelets analysis that it is possible to distinguish solar forcing of 1000- and 2500- year oscillations from oceanic forcing of 1500-year cycles. Using this method, the relative contribution of solar-related and ocean-related climate influences can be distinguished throughout the 10 000 Holocene intervals since the last ice age. These results reveal that the mysteriously regular 1,500-year climate cycles are linked with the oceanic circulation and not with variations in solar output as previously argued (Bond et al., 2001). In this light, previously studied marine sediment (Bianchi and McCave, 1999; Giraudeau et al., 2000), ice core (O'Brien et al., 1995) and dust records (Jackson et al., 2005) can be seen to contain the evidence of combined forcing mechanisms, whose relative influences varied during the course of the Holocene. Circum-Atlantic climate records cannot be explained by solar forcing, but require changes in ocean circulation, as suggested previously (Broecker et al., 2001; McManus et al., 1999).
A partir des thermogrammes issus de l’analyse Rock-Eval®, quels paramètres peuvent être obtenus et pour quels usages ? Ce chapitre décrit les principaux paramètres Rock-Eval® et les graphiques utilisés en géochimie pétrolière et pour l’analyse de la matière organique des sols.
Le remplissage sedimentaire des mares de Bangou-Bi et de Tankalawal (SW Niger) a fait l'objet d'une etude de son contenu organique qui fournit des informations sur l'origine et les conditions de depot du materiel sedimentaire. Deux carottes mesurant respectivement 140 cm (carotte BB2) et 60 cm (carotte TK1) ont ete etudiees. Les echantillons ont ete soumis a une pyrolyse Rock-Eval 6 renseignant sur la geochimie globale de la Matiere Organique (MO) et le signal S2 de la pyrolyse a ete exploite afin de mieux preciser la nature de ces MO. Le niveau argilo-limoneux de la carotte BB2 contient une MO aquatique et terrestre dont la composition en composes hydrocarbones se repartit entre les biopolymeres (F1+F2), et les geopolymeres matures (F3) et immatures (F4). Cette MO serait deposee dans un environnement calme. En revanche le niveau inferieur sableux renferme une MO terrestre tres alteree et enrichie en geopolymeres matures qui serait amenee par des pulses detritiques grossiers (colluvionnement) issus du bassin versant. Les valeurs des parametres geochimiques et leur evolution avec la profondeur indiquent que la MO de la carotte TK1 est aquatique et a enregistre une degradation en milieu anoxique. Cette MO contenant majoritairement des geopolymeres matures (F4) riches en lipides se revele etonnamment plus resistante face a l'alteration que celle presente dans la carotte BB2.
