Solar activity during the Holocene: the Hallstatt cycle and its consequence for grand minima and maxima

Abstract

Aims. Cosmogenic isotopes provide the only quantitative proxy for analyzing the long-term solar variability over a centennial timescale. While essential progress has been achieved in both measurements and modeling of the cosmogenic proxy, uncertainties still remain in the determination of the geomagnetic dipole moment evolution. Here we aim at improving the reconstruction of solar activity over the past nine millennia using a multi-proxy approach. Methods. We used records of the 14 C and 10 Be cosmogenic isotopes, current numerical models of the isotope production and transport in Earth's atmosphere, and available geomagnetic field reconstructions, including a new reconstruction relying on an updated archeo-and paleointensity database. The obtained series were analyzed using the singular spectrum analysis (SSA) method to study the millennial-scale trends. Results. A new reconstruction of the geomagnetic dipole field moment, referred to as GMAG.9k, is built for the last nine millennia. New reconstructions of solar activity covering the last nine millennia, quantified in terms of sunspot numbers, are presented and analyzed. A conservative list of grand minima and maxima is also provided. Conclusions. The primary components of the reconstructed solar activity, as determined using the SSA method, are different for the series that are based on 14 C and 10 Be. This shows that these primary components can only be ascribed to long-term changes in the terrestrial system and not to the Sun. These components have therefore been removed from the reconstructed series. In contrast, the secondary SSA components of the reconstructed solar activity are found to be dominated by a common ≈2400-year quasi-periodicity, the so-called Hallstatt cycle, in both the 14 C and 10 Be based series. This Hallstatt cycle thus appears to be related to solar activity. Finally, we show that the grand minima and maxima occurred intermittently over the studied period, with clustering near lows and highs of the Hallstatt cycle, respectively.

A history of solar activity over millennia

“…This conclusion has been confirmed (e.g., Usoskin et al 2003c;Muscheler et al 2007) in the sense that quantitative solar-activity reconstructions, based on 10 Be and 14 C data series for the last millennium, yield very similar results, which differ only in small details. However, a longer comparison over the entire Holocene timescale suggests that, while centennial variations of solar activity reconstructed from the two isotopes are very close to each other, there might be a discrepancy in the very long-term trend (Vonmoos et al 2006;Inceoglu et al 2015;Usoskin et al 2016a), whose nature is not clear (climate changes, geomagnetic effects or model uncertainties).…”

“…In this case the major source of errors in solar activity reconstructions is related to uncertainties in the paleomagnetic data . These errors are insignificant for the last several millennia (Licht et al 2013;Usoskin et al 2016a), but become increasingly important for earlier times.…”

“…Data available provides good global coverage for the last three millennia, allowing for a reliable paleomagnetic reconstruction of the true dipole moment (DM) or virtual axial dipole moment 2 (VADM) and its orientation (Licht et al 2013). Less precise, but still reliable reconstructions of the DM and its orientation are possible for the last seven millennia (Knudsen et al 2008;Usoskin et al 2016a). Directional paleomagnetic reconstruction are less reliable on a longer timescale, because of the spatial sparseness of the paleo/archeo-magnetic samples in the earlier part of the Holocene (Korte et al 2011).…”

“…Long-term sunspot-number reconstruction from 14 C data with error bars (after Usoskin et al 2016a). Group sunspot numbers (Hoyt and Schatten 1998) are shown after 1610 Vieira et al 2011).…”

“…They compared the expected 10 Be variations computed from 14 C-based reconstruction of cosmic ray intensity with the actually measured 10 Be abundance at the sites and found that: (1) There is good agreement between the 14 C and 10 Be data sets, on different timescales and at different locations, confirming the existence of a common solar signal in both isotope data; (2) The 10 Be data are driven by the solar signal on timescales from about centennial to millennial time scales; (3) The synchronization is lost on short (<100 years) timescales, either due to local climate or chronological uncertainties (Delaygue and Bard 2011) but the solar signal becomes important even at short scales during periods of Grand minima of solar activity, (4) There is an indication of a possible systematic uncertainty in the early Holocene (cf. Vonmoos et al 2006;Inceoglu et al 2015;Usoskin et al 2016a), likely due to a not-perfectly-stable thermohaline circulation. Overall, both 14 C-and 10 Be-based records are consistent with each other over a wide range of timescales and time intervals.…”