Two highstands during the last interglacial: insights from palaeoshorelines and marine terraced deposits of ionian coast of Apulia

Geomorphological studies in the Ionian Sea margin of Apulia region (southern Italy) enabled the identification of

1) three terraced deposits: TD1, TD2, and TD3, divided in several sub-units;

2) three related palaeoshorelines: PS1, PS2, PS3.

The higher PS2 (30±2 m a.p.s.l.) dates to the first highstand of MIS 5.5. The ages of 127.851±1.47 and 128.4±26.2 ky BP from the sub-units of TD2 marking the transgression, suggest that at ca. 128 ky BP the transgression toward the first highstand of MIS 5.5 was still occurring.

The lower PS3 (19±2 m a.p.s.l.) dates to a second highstand of MIS 5.5. The age of 122.49 ±1.25 ky BP from TD3 is the most reliable chronological constraint for this second highstand.

Also considering the above mentioned contraints, and since it was not possible to attribute an absolute age for the TD2 sub-units which mark the first MIS 5.5 highstand, we adopt, in this paper, for the first highstand of MIS 5.5, the age given by Kopp et al. (2013), namely between 125 and 123 ky BP.

In particular we considered two possible scenarios:

1) scenario 1, assuming the first highstand of MIS 5.5 at 125 ky BP

2) scenario 2, assuming the first highstand of MIS 5.5 at 123 ky BP

In both scenarios, we considered a mean sea level value during the first highstand of MIS 5.5 at + 7.5±1.5 m. We then calculated the uplift rate of PS2 for both scenarios, assuming a constant rate up to present.

With these assumptions, we tentatively calculate the position of PS3 at ⁓ 122 ky and of the dated layer of the lowest transgressive sub-unit of TD2 (today at +3.80 m, dated at ⁓ 128 ky BP), this latter assumed as deposited from 5 to 10 metres of water depth.

A mean uplift rate of ca. 0.18 mm/yr was quantified, with a minimum possible of 0.15 mm/yr and a maximum possible of 0.21 mm/yr, for both scenarios. This range of uplift rates fits well within the regional setting.

Accordingly, the mean elevation of PS3 at time of its formation (i.e., the sea level at ⁓122 ky BP, namely the second highstand of MIS 5.5) was -2.96 and -3.32 m for scenario 1 and 2, respectively. Taking into account all the uncertainties in PS2 and PS3 elevations, and in sea level at the first highstand of MIS 5.5, the whole range of the RSL values during the second highstand of MIS 5.5 is -8.38 to +2.45 m for scenario 1, and -8.79 to +2.15 m for scenario 2.

On the other hand, using the three possible values of uplift rate, we calculated also the position and then the sea level indicated by the dated layer at ⁓ 128 ky BP at Torre Castelluccia, which ranged from -5.5 to -18.08 m, with a mean value of -11.79 m.

Finally, we hypotesise that the observed alternating abundance of marine gastropods Bolma rugosa (Linneo) and Thetystrombus latus (Gmelin) across the recognised deposits can be interpreded as a palaeoclimatic indicator, although in a merely qualitative way and, for now, only in the study area.