Evidence from wavelet analysis for a mid-Holocene transition in global climate forcing
Introduction
Holocene climate variability is different than that of the last glacial period and is the subject of intense research aimed at characterising forcing factors of the observed climate oscillations evidenced in different regions of the world. The Atlantic sector is a key area to improve our knowledge of the climatic system. Indeed its physiography and inter-hemispheric extent provide a connection between the two poles. Located at a key region, the North Atlantic is sensitive to climate variation because it is the starting point of water and salt export. Moreover this area is surrounded by a continent where ice caps (Greenland and Laurentian ice caps) develop during glacial periods and melt to generate meltwater pulses during the deglaciation and early warm periods. These characteristics are responsible for the dynamic behaviour of the North Atlantic in the transfer, amplification and/or modulation of climatic variation in the global thermohaline circulation mode.
For the last glacial period, a link has been established between the amplitude of fast climatic events and their duration in Antarctica (EPICA community, 2006). However, for the Holocene, the only prominent event that could be seen in both hemispheres, although damped in the Southern Hemisphere, occurred 8200 years ago (Alley et al., 1997). A different approach is therefore necessary.
Debret et al. (2007) have shown that wavelet analysis can be used to identify common spectral signatures for a wide range of paleoclimatic records: marine sediments (Bianchi and McCave, 1999; Chapman and Shackleton, 2000, Giraudeau et al., 2000), ice cores (O'Brien et al., 1995, Vonmoos et al., 2006) and dust records (Jackson et al., 2005). In their initial study, limited to the North Atlantic area, they showed that Holocene paleoclimate records do not exhibit continuous 1500 year cycles as indicated by previous studies (e.g. Bianchi and McCave, 1999, Bond et al., 2001). Indeed, the use of wavelets to detect non-stationarity has made it possible to demonstrate that several Holocene paleoclimatic records include a continuous 2500-year cycle over the whole period and a 1000-year cycle during the Early Holocene, sometimes associated with a 1600-year cycle. Identification of the same spectral signature in 14C and 10Be records made it possible to assign these frequencies to external forcing (i.e. solar activity). Despite these evidences, direct evidence between solar activity and weather and climate remains unclear (Van Geel et al., 1999, Beer et al., 2000). In addition, some paleoclimatic records also show a cyclic period close to 1600 years with an increasing intensity during the Late Holocene. This second spectral signature, not present in solar activity records, has been attributed to internal forcing, likely oceanic-driven as suggested by Broecker et al. (2001) and McManus et al. (1999).
Here we propose to explore the Holocene variability over the North to South Atlantic Ocean and in the circum-Antarctic region in order to characterise Holocene frequency patterns using wavelet analyses. South America is of particular interest because it is connected to water masses coming from the Atlantic and Pacific sectors. We will also propose a possible explanation for methane evolution during the Holocene, which is still under debate (Ruddiman et al., 2008) and of particular interest with regard to global warming.
Section snippets
Wavelet transforms
Wavelet analysis (WA) presents the advantage of describing non-stationarities, i.e. discontinuities and changes in frequency or magnitude (Torrence and Compo, 1998). Redundancy of the continuous wavelet transform is used to produce a time/frequency or time/scale mapping of a power distribution, called the local wavelet spectrum (or scalogram). With respect to classical Fourier analysis, the local wavelet spectrum provides a direct visualisation of the changing statistical properties in
Data sets and results
Main criteria to select the times series used in this study were (1) high resolution data sets, (2) good chronologies to minimize the incertitudes about marine reservoir corrections and/or drifting ages and (3) proxies of fast reacting parameters to reduce dampening effect of the forcing by the internal climate system.
Spectral imprint of solar origin – Early Holocene
This characteristic spectral signature (continuous cyclic period of 2500 years and a period of 1000 years during the Early Holocene) was recognised in several sedimentary records of the North Atlantic area (Debret et al., 2007), notably in the IRD record (Bond et al., 2001). The solar origin of this imprint is shown by the 10Be record from Greenland ice core (Vonmoos et al., 2006) (Fig. 6), 14C production rates (Reimer in Bond et al., 2001) and wavelet analysis. Moreover, sunspot numbers (
Worldwide pattern?
The results presented reveal two distinct spectral imprints in various paleoclimatic records throughout the world ocean. They also make it possible to attribute several of these imprints (1000 and 2500 years) to direct solar forcing. On the other hand, the second imprint (around 1600 years) is missing in records related to cosmogenic isotopes. It has been assigned to internal forcing with an oceanic and/or atmospheric influence that could not be determined because this imprint is present in
Holocene variability: ocean, atmosphere, solar interaction
The structure of Holocene paleoclimatic records is comparable in Antarctica/South Atlantic, in the Tropics and in the North Atlantic area. The millennial variability of the last 10 000 years can be divided into three periods, all with different forcing (Fig. 8).
Conclusion
Wavelet analysis of numerous records throughout the Atlantic in both hemispheres not only helped to assess Holocene climate variability by comparing distinct paleoclimatic records (i.e. marine, terrestrial or ice cores) around the world, but also to propose a worldwide pattern for the Holocene millennial modes of variability. The first part of the Holocene was characterised by frequencies typical of high solar activity (1000 years and 2500 years continuous throughout the Holocene). However, it
Acknowledgements
We acknowledge Basile Deflorian, John Andrews, Matthias Moros, Bjørg Risebrobakken, Martine DeAngelis, Jerôme Chappellaz, Laetitia Loulergue, Jean-Marc Barnola, Dominique Raynaud, Emmanuel LeMeur, Valérie Masson-Delmotte for their valuable insight, availability and/or for having shared their data. This work is supported by the ANR project: “Intégration des contraintes Paléoclimatiques pour réduire les Incertitudes sur l'évolution du Climat pendant les périodes Chaudes” – PICC
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