Wind regime change from mis 5.5 aeolian dunes in the Apulia region. A key for the reconstruction of a Meditterranean pressure pattern during the last interglacial.

We studied two geo/morphological elements located southern the town of Gallipoli (Apulia region, southern Italy): the Il Carmine-Li Foggi paleo littoral ridge (CLlr) and the fossil dunes of Il Campo locality, both dating back to MIS 5.5.

A detailed study was conducted on the aeolian units present within CLlr (AU1 and AU2) and at Il Campo locality (unit AU3), in order to reconstruct the paleo-directions of the winds andthe atmospheric pressure patterns. AU1 is the first aeolian unit to settle, it is concentrated at the northern and southern extremities of CLlr and shows wavy to cross-bedding co-sets, dipping in various directions. The accumulation of AU1 at the northern and southern edges of CLlr indicates an effective aeolian transport both northward (due to winds from the southern quadrants) and southward (due to winds from the northern quadrants); the same conclusion can be drawn from the internal structure of AU1. The AU1 unit therefore allows us to recognise a “first aeolian phase”. It involved an autumn-winter pressure pattern characterised by an Atlantic footprint similar to the current one: the Atlantic lows entering the Mediterranean caused, as happens today, the onset of winds from the S and NW, respectively before and after the passage of the low-pressure minima over Apulia. Even in spring-summer, the pressure fields were likely very similar to the current one, with the Azores high on the central-western Mediterranean and the low pressure on the eastern Mediterranean: consequently, the prevailing winds showed a marked unimodality from NW but their speed was generally weak, and therefore these winds were not able, just as it happens today, to significantly impact wind transport; this summer regime leaved the prevailing imprint of the Atlantic winter regime, therefore NW-S bimodality, as recorded by AU1.

AU2 and AU3 settled later than AU1, in a “second aeolian phase” of MIS 5.5. AU2 is characterised by south-eastward-dipping foresets. AU3 consists of domal-linear dunes, the latter oriented in the NW-SE direction. We interpret AU2 and AU3 as coastal dunes originating in a prevailing NW wind regime.

Based on the principle of actualism and on literature data, we conclude that AU2 and AU3 settled under a pressure pattern similar to the current one in fall-winter, that is, characterised by Atlantic cyclones transiting over the Mediterranean, and consequent NW-S bimodality of the winds; in the spring-summer, instead, the regime was characterised by a pressure gradient from west (high) to east (low) much stronger than the current one, causing NW winds stronger than today. Such strong summer winds from NW, effective for wind transport, coupled with the NW-S bimodality in fall-winter, caused the overall prevalence of NW winds during the “second aeolian phase”. This pressure gradient on the Mediterranean greater than today was caused by a summer strengthening of the east Mediterranean low-pressure, in turn caused by the northward shift of the ITCZ, widely documented during the Sapropel S5 event and verified during the MIS 5.5 after sea level had risen to the highstand.