The 4.2 ka event in the central Mediterranean: new data from a Corchia speleothem (Apuan Alps, central Italy)

Ilaria Isola,Jon Woodhead,Piero Lionello,Giovanni Zanchetta,John C Hellstrom,Russell N Drysdale,Monica Bini,Petra Bajo,Alan Greig,Eleonora Regattieri,Ilaria Baneschi

doi:10.5194/cp-15-135-2019

Abstract

Abstract. We present new data on the 4.2 ka event in the central Mediterranean from Corchia Cave (Tuscany, central Italy) stalagmite CC27. The stalagmite was analyzed for stable isotopes (δ13C and δ18O) and trace elements (Mg, U, P, Y), with all proxies showing a coherent phase of reduced cave recharge between ca. 4.5 and 4.1 ka BP. Based on the current climatological data on cyclogenesis, the reduction in cave recharge is considered to be associated with the weakening of the cyclone center located in the Gulf of Genoa in response to reduced advection of air masses from the Atlantic during winter. These conditions, which closely resemble a positive North Atlantic Oscillation (NAO) type of configuration, are associated with cooler and wetter summers with reduced sea warming, which reduced the western Mediterranean evaporation during autumn–early winter, further reducing precipitation.

Highlights

A major and much-discussed example of a potential global “megadrought” and cooling during the Holocene occurred between ca. 4.2 and 3.9 ka cal BP (Weiss, 2015, 2016)
Renella Cave is located in a narrow valley draining the western side of the Apuan Alps (Fig. 1), a mountain belt that receives its precipitation from air masses of North Atlantic origin (Reale and Lionello, 2013) that interact with the most important cyclogenesis center of the Mediterranean region, the Gulf of Genoa (Fig. 1)
5.1 Speleothem stable isotopes composition δ18O values of calcite (δ18Oc) in the Apuan speleothems have been mostly interpreted on the premise that the δ18O in precipitation (δ18Op) in the western Mediterranean is dominated by the “amount effect”; i.e., a positive shift in δ18Op values corresponds to a lower precipitation amount and vice versa, with a gradient of ca. −2 ‰ per 100 mm per month of precipitation (Bard et al, 2002)

Summary

Introduction

A major and much-discussed example of a potential global “megadrought” and cooling during the Holocene occurred between ca. 4.2 and 3.9 ka cal BP (the 4.2 ka event hereafter) (Weiss, 2015, 2016). Its location is determined by local topography, with the Alps (and the Apuan Alps and Apennines, which bound the eastern side of the Gulf of Genoa) playing a major role by trapping air masses moving eastward and triggering Genoa cyclones (Trigo et al, 2002). The combination of these factors produces high precipitation amounts, locally reaching values up to 3000 mm yr−1 over the Apuan Alps (Rapetti and Vittorini, 1994).

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Discussion

Conclusion