Humid climate during deposition of sapropel 1 in the Mediterranean Sea: Assessing the influence on the Alps

Cave and lake isotope records from the circum-Mediterranean realm show anomalously low O isotope values suggesting high rainfall intensity during the time of sapropel 1 deposition (9.5 to 6.5 ka; all ages are given before the year AD 2000, i.e. b2k), coincident with an interval of conspicuously low sea-surface salinities in the entire Mediterranean Sea. Speleothem data from Corchia Cave (Tuscany) currently provide the most precise terrestrial chronology and constrain the wettest interval to ca. 8.2 to 7.3 ka. We have traced this isotopic signal to the north and observe a synchronous isotopic change in stalagmites from southalpine and eastalpine caves, but in opposite direction. We attribute this to a shift in the local moisture balance, i.e. to a higher proportion of moisture advected from the Mediterranean Sea relative to the otherwise dominant northwesterly air masses in the Alps. This isotopic source effect can be traced up to the northern rim of the Alps, albeit with decreasing amplitude. Forest density at the treeline in the Central Alps decreased during this time interval indicating short vegetation periods consistent with rainy summers. The glaciers in the Eastern Alps, which did not show far-reaching advances during the preceding 8.2 ka event, responded strongly (positively) to this humid phase. Additionally, two of the largest alluvial fans in the Eastern Alps showed a massive accumulation peak radiocarbon dated to between ca. 8.3 to 7.4 ka and thus providing one of the strongest pieces of evidence for anomalously high rainfall intensities coeval with ʹpluvial’ conditions in the Mediterranean region.