Una recente aminocronologia dei depositi marini pleistocenici dell'area di Montalto di Castro e Tarquinia (Viterbo)

Abstract

Numerous samples of the mollusk genera Glycymeris from 16 raised marine deposits of Pleistocene age in the Montalto di Castro and Tarquinia areas (Latium, Central Italy) have been analysed using the D-alloisoleucine/ L-isoleucine (aile/ile) method. Sampled sites are: 1) Prati di Santa Lucia; 2) Hotel Vulci area; 3) Casale Carelli; 4) Statai Route 1, km 104; 5) Podere San Pietro; 6) Lestra dell'Ospedale, 7) Bandita San Pantaleo; 8) Casale San Vincenzo; 9) Casale il Giglio; 10) Sant'Agostino Nuovo; 11) Statai Route 1, km 110; 12) Fosso dei Due Ponti; 13) Fosso Pian d'Arcione, 14) Mandra Grande; 15) Il Fossaccio and 16) Casale Torrone. Sites 1 to 10 have been the object of previous studies; sites 11 to 16 have been examined for the first time. On the basis of the age model developed (see Table 1), three marine horizons have been identified, dating to 120, 200 and 300 Ka, about. These are attributed to stages 5, 7 and 9 of Shackleton and Opdyke's palaeoclimatic curve. The oldest horizons (300 Ka) are those in the localities km 104 of the Statai Route 1, Lestra dell'Ospedale and Bandita SanPantaleo. These deposits have been attributed to stage 9. The 200 Ka old marine horizons (stage 7) are found at Prati di Santa Lucia, in the area of Hotel Vulci, along the Statai Route 1 at km 110, and in the localities Casale Carelli, Sant'Agostino Nuovo, Fosso Pian d'Arcione, Podere San Pietro (at elevations of 18 and 25 m a. s. I.), Mandra Grande, Casale Torrone, Lestra dell'Ospedale, Bandita San Pantaleo and Casale il Giglio. The youngest horizons (120 ka) are located in the localities Casale San Vincenzo, II Fossaccio, Fosso dei Due Ponti, Podere San Pietro (at the elevation of 25 m a. s. I.), and Sant'Agostino Nuovo. Dates obtained are in good agreement with those obtained for the same sediments using other techniques, ²³⁰Th/²³⁴U and ESR (Electron Spin Resonance), and, by other Authors, Amino Acid Racemization-Epimerization Reaction (AAR). Finally, in order to identify the chronology of "mixed age" deposits, the paper uses two different applications of AAR, a technique commonly used to date and correlate marine units of Pleistocene age, to mollusk shells: a) aile/ile epimerization ratios; b) the use of a kinetic analysis which involves the utilization of three components: the stable amino acid leucine and the products of 2 diagenetic amino acid reactions, D-alloisoleucine from the L-isoleucine epimerization and a-amino-N-butyric acid from the dehydration of L-threonine. For bivalve mollusk shells from raised marine deposits from temperate environments, the three component analysis yields a linear relationship. Although the shells come from widely dispersed localities and thus their average exposure temperatures may be different, as are environmental parameters (e. g. soil pH, humidity), these factors apparently do not greatly the position of similar age samples on the plot. Based on the estimated ages of the samples, we have made a preliminary age assignment of the zones on the diagram. The age zones indicate that the three component technique can be used to directly estimate the ages of Holocene to Pleistocene mollusks.