Long term solar-terrestrial records from sediments: carbon isotopes in planktonic foraminifera during the last millennium Original Research Article

We show the δ13C profile of Globigerinoides ruber measured in the GT90/3 shallow-water Ionian sea core. This core is dated with high accuracy (better than 1%) using radiometric and tephroanalysis methods, for the last 2000 years. The core, extracted from the Gallipoli platform, was sampled at contiguous steps of thickness 2.5 mm, corresponding to 3.87 years. The δ13C profile covers the period 1147–1975 AD. During the first seven centuries it appears fairly flat, while it shows a steep increase between 1760 and 1950 of ∼0.3‰. The analysis of the time series performed using different methods shows a dominant decadal periodicity throughout the record. The 11-year component is identified at high significance level by Monte Carlo singular spectrum analysis (MC-SSA); the SSA-reconstructed-11-year component is in phase with the sunspot solar cycle. The average amplitude of this component is A11y=0.04‰. It is commonly accepted that δ13C variations in symbiontic foraminifera mainly record the effects of symbiont density and of photosynthetic activity, varying with ambient light level. The δ13C peak-to-trough 11-year variation (0.08‰) can be related to solar irradiance variation of 0.1% on decadal time scale, like that measured in space in the last 20 years. By supposing that the observed δ13C modern increase of ∼0.3‰ is also produced by a solar irradiance variability on a longer time scale, through the same mechanisms, we estimate a secular increase of the total solar irradiance between 1760 and 1950 of (0.3/0.08)∗0.1% ≅ 0.37%. This value is at the higher limit of the long term variability estimates (0.25%–0.35%) that are currently proposed.