Carla Taricco

Long-term solar activity reconstructions : direct test by cosmogenic 44Ti in meteorites

Aims. Long-term solar activity in the past is usually estimated from cosmogenic isotopes, 10 Be or 14 C, deposited in terrestrial archives such as ice cores and tree rings. A number of such reconstruction models have been proposed which differ from each other significantly. This approach suffers, however, from uncertainties due to the sensitivity of the data to several terrestrial processes. Here we propose a method to constrain these solar activity reconstructions using cosmogenic 44 Ti activity in meteorites which is not affected by terrestrial processes. Methods. We test the veracity of recent solar activity reconstructions using the data on the activity of cosmogenic isotope 44 Ti in meteorites which fell during the past 235 years, and provide an independent and direct measure of the cosmic ray flux near the Earth and allow decoupling of solar activity variations from terrestrial influences. Results. We demonstrate that the 44 Ti data can distinguish between various reconstructions of past solar activity based on cosmogenic isotope data in terrestrial archives, allowing unrealistic models to be ruled out. We also show that a model based on the sunspot number record is consistent with the data on 44 Ti activity in meteorites, thus confirming the validity of the method. In particular the 44 Ti data confirm significant secular variations of the solar magnetic flux during the last century.

Temperature and productivity influences on U37K′ and their possible relation to solar forcing of the Mediterranean winter

To assess the extent and nature of human-induced climate change, we need to understand natural climate variability. This heavily relies on proxy-based climate reconstructions, calling for excellent understanding of the conditions represented by the proxies. Here we report on an alkenone-based (U37K′) sea surface temperature (SST) proxy record originating from south Italian marine sediments, and covering 1305 A.D. to 1979 A.D. with a 3.87 year resolution. The shallow-water cores, extracted from the Gallipoli terrace in the Gulf of Taranto, were dated by the Torino group with high accuracy over the last two millennia, using radiometric and tephroanalysis methods. On the basis of comparison with historical and satellite-derived climate and productivity data, we propose that U37K′ reflects mainly SST of the cooler part of the year and is modified by variation in magnitude and timing of peak alkenone production on the seasonal temperature cycle. Comparison of the U37K′ record with the record of atmospheric Δ14C, a proxy for solar energy variability, shows a high correlation (r = −0.73) for the period between 1420 A.D. and before human interference with the Δ14C record by combustion of fossil carbon and the release of radioactive isotopes by nuclear tests. This suggests a centennial-scale solar forcing. We propose that wind-induced mixing resulting from Mediterranean climate dynamics plays a crucial role in translating solar activity to alkenone productivity and SST during the cooler part of the year.

Multispectral analysis of Northern Hemisphere temperature records over the last five millennia

AbstractnAiming to describe spatio-temporal climate variability on decadal-to-centennial time scales and longer, we analyzed a data set of 26 proxy records extending back 1,000–5,000 years; all records chosen were calibrated to yield temperatures. The seven irregularly sampled series in the data set were interpolated to a regular grid by optimized methods and then two advanced spectral methods—namely singular-spectrum analysis (SSA) and the continuous wavelet transform—were applied to individual series to separate significant oscillations from the high noise background. This univariate analysis identified several common periods across many of the 26 proxy records: a millennial trend, as well as oscillations of about 100 and 200 years, and a broad peak in the 40–70-year band. To study common NH oscillations, we then applied Multichannel SSA. Temperature variations on time scales longer than 600 years appear in our analysis as a dominant trend component, which shows climate features consistent with the Medieval Warm Period and the Little Ice Age. Statistically significant NH-wide peaks appear at 330, 250 and 110xa0years, as well as in a broad 50–80-year band. Strong variability centers in several bands are located around the North Atlantic basin and are in phase opposition between Greenland and Western Europe.

Marine sediments remotely unveil long-term climatic variability over Northern Italy

A deep understanding of natural decadal variability is pivotal to discuss recently observed climate trends. Paleoclimate proxies allow reconstructing natural variations before the instrumental period. Typically, regional-scale reconstructions depend on factors like dating, multi-proxy weighting and calibration, which may lead to non-robust reconstructions. Riverine records inherently integrate information about regional climate variability, partly overcoming the above mentioned limitation. The Po River provides major freshwater input to Eastern Mediterranean, as its catchment encompasses a large part of Northern Italy. Here, using historical discharge data and oceanographic measurements, we show that Po River discharge undergo robust decadal fluctuations that reach the Ionian Sea, ~1,000u2009km South of Po River delta, through propagating salinity anomalies. Based on this propagation, we use a high-resolution foraminiferal δ18O record from a sediment core in the Ionian Sea to reconstruct North Italian hydrological variability on millennial-scale for the first time. The reconstruction reveals highly significant decadal variability that persists over the last 2,000 years. Many reconstructed extremes correspond to documented catastrophic events. Our study provides the first millennial-scale reconstruction of the strength of decadal hydrological variability over Northern Italy. It paves the way to assess the persistence of large-scale circulation fingerprints on the North Italian climate.

Experimental set-up for gamma-activity measurements of astromaterials

The most important proxies of past solar activity are radionuclides produced by nuclear interactions of cosmic rays in the Earth’s atmosphere, in meteorites and in planetary surfaces. In fact, the production of these radioisotopes depends on the cosmic ray flux and energy spectrum and vary in the heliosphere because of solar modulation. Therefore the study of cosmogenic isotopes in terrestrial archives and in meteorites, which fell at different times, gives information on solar activity in the past. Because of its half life, 44Ti (T½ = 59.2 years) in meteorites has been proposed as an ideal index for the study of centennial scale solar activity variations. The main experimental difficulties are related to the very low (∼1 dpm/kg) 44Ti γ activity in meteorites and to the strong interference by 214Bi γ’s from the decay chain of the naturally occurring 238U. On the basis of the decay scheme of 44Ti, we have set up a low background spectrometer, whose high specificity derives from a selective coincidence between the principal HPGe detector and the active shield of NaI(Tl). Using this spectrometer we have reconstructed the solar activity over the past 235 years by measuring 44Ti and 26Al in meteorite falls; in particular we have shown that the intensity of cosmic rays has linearly decreased during this period, in agreement with some models proposed for the past solar activity. In order to improve the Ge-NaI coincidence spectrometer, we have recently developed a multiparametric acquisition system, allowing better selectivity in the choice of the coincidence windows, crucial for the 44Ti detection. A background measurement of the apparatus, taken for about a month, shows efficient elimination of the 214Bi γ signal in coincidence mode. Moreover the coincidence spectrum of Dhajala meteorite (fall, 1976) shows a good 44Ti γ peak.

Long-term evolution of the heliospheric magnetic field inferred from cosmogenic 44Ti activity in meteorites

Typical reconstructions of historic heliospheric magnetic field (HMF)

A foraminiferal δ18O record covering the last 2,200 years

B_{rm HMF}

Biogeochemical, Isotopic and Bacterial Distributions Trace Oceanic Abyssal Circulation.

are based on the analysis of the sunspot activity, geomagnetic data or on measurement of cosmogenic isotopes stored in terrestrial reservoirs like trees (

O VI 1032 Å intensity and Doppler shift oscillations above a coronal hole: Magnetosonic waves or quasi-periodic upflows?

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A LARGE CAVITY GAMMA-RAY SPECTROMETER FOR MEASUREMENT OF COSMOGENIC RADIONUCLIDES IN ASTROMATERIALS BY WHOLE ROCK COUNTING

C) and ice cores (