The age and correlation of shorelines around the Mediterranean basin have been addressed by analyzing the extent of isoleucine epimerization (aIle/Ile ratio) in protein preserved in molluscan fossils collected from raised marine deposits. The taxodont genera Glycymeris and Arca were selected as the primary taxa for this study because of their simple shell structure, reproducible aIle/Ile ratios, and ubiquitous occurrence. Direct comparison of aIle/Ile ratios in associated mollusks allows correlation of disjunct marine deposits and relative dating of sequential marine units in nearby areas that have similar thermal histories. The thermal gradient across the Mediterranean basin is, however, sufficiently high that shells from isochronous shorelines have significantly higher ratios at warmer than at cooler localities. Absolute dating, primarily U-series dates on corals directly associated with molluscan samples, provides an independent calibration of the amino acid data and compensates for dissimilar thermal histories.
AIle/Ile ratios in shells from 46 marine units cluster into 5 discrete groups (C, E, F, G, and K) that are related to positive sea-level events (interglacials/interstadials) associated with odd-numbered deep-sea isotopic stages. The most complete sequences are in southern Italy, where group C ratios in Glycymeris that average 0.30 are associated with the Neotyrrhenian, a post–last-interglacial (late stage 5), high–sea-level event. Group E ratios (0.38) are associated with classical Eutyrrhenian deposits from which four U-series coral dates (126 ± 7 ka) substantiate the correlation to isotope substage 5e. Group F ratios (0.50) are associated with U-series coral and mollusk dates between 200 and 300 ka; the deposits are tentatively correlated with stage 7. Deposits that have group G ratios (0.58) are correlated with stage 9. Shells from lower Pleistocene marine deposits that have ratios between 1.0 and 1.2 (group K) constrain the ages of the younger groups. An exponential decrease in the epimerization rate inhibits resolution of the older events. AIle/Ile ratios in last-interglacial deposits are similar in the northern and central region of the study area but increase sharply in southern Sicily, North Africa, and Crete, similar to the modern thermal gradient.
A lacuna between group G and group K is similar to gaps identified in California and Alaska, suggesting generally lower interglacial sea levels between stage 11 and sometime before the Brunhes/Matuyama boundary.