Solar radiation variability in the last 1400 years recorded in the carbon isotope ratio of a mediterranean sea core Original Research Article

We present measurements and data analysis of the carbon stable isotopes (δ13C) in the planktonic Globigerinoides ruber extracted from the GT90/3 shallow water Ionian sea core, dated with high precision. It is commonly accepted that δ13C variations in symbiontic foraminifera mainly record the effects of productivity and of photosynthetic activity, varying with the ambient light level. Therefore from this time series we can deduce information on the sea surface illumination at the time of the planktonic foraminifera growth. The profile (359 points) covers the period 590–1979 AD, with a resolution of 3.87 years and it is an extension of the time series (215 points) previously published in this journal. The spectral analysis of the longer time series confirms the presence of the 11 y signal, with amplitude ∼0.08‰ (peak-to-trough), found in the shorter time series in phase with the sunspot solar cycle; furthermore it shows the presence of two centennial cycles of 100 and 200 years, with amplitude 0.08‰ and 0.02‰ respectively. These components are identified at high significance level by Monte Carlo singular spectrum analysis (MC-SSA). A comparison between the δ13C profile and the historical aurorae series (600–1500 AD) shows that the long-term δ13C variations are at least partially generated by the solar activity modulation and in phase with the solar output, as represented by the solar wind interaction with the magnetosphere.