CONSTRAINING ACTIVITY IN THE NISYROS VOLCANIC COMPLEX: ESR DATING ON YALI AND PYRGOUSSA, GREECE

In the Hellenic Arc, the Aegean’s youngest large volcano began to erupt sealevel after the Kos Plateau Tuff (KPT) eruption at 161 ka under Nisyros and Yali. Lacking K, Nisyros’ calcalkaline rocks cannot be dated by ³⁹Ar/⁴⁰Ar. Dating terraces that separate the volcanic units on Yali and Pyrgoussa can constrain their ages and clarify the Nisyros eruptive history. On Yali, the Lower Pumice postdates the KPT at 161 ka and underlies the Yali marine terrace (YMT) which averages 0.5-1.0 m thick, now sitting up to 120 m above sealevel. This shallow marine sandy-silty limestone contains some pumice, as well as bivalves, molluscs, coralline algae, and numerous worm burrows deposited during a quiescent period. A reddish-brown terra rosa (YOP) averaging 40-50 cm thick contains shells, some reworked from the underlying YMT. Yali’s Upper Pumice (YUP) covers the terra rosa. On Pyrgoussa, a volcanic centre older than 161 ka, ash and pumice from the KPT underlies the 1-2-m-thick marine terrace (PMT), which now sits at ~ 30 m asl. Its well preserved molluscs, coralline algae, and many worm tubes suggest clear water, but little volcanic activity. Since ESR can date Quaternary molluscs from 0.5 ka to 2 Ma with 8-10% accuracy, fossils and associated sediment were collected from the YMT, YOP, and PMT. To calculate time-averaged dose rates, U, Th, and K concentrations were measured by NAA, and averaged weighted by their volume within each sample’s dose field. Time-averaged cosmic dose rates were modelled assuming the fossils’ water depths and geologic history. Molluscs from the PMT gave preliminary ages ranging from 114 ± 53 to 31 ± 11 ka, while the YMT gave a preliminary age of 52.2 ± 15.0 ka, hinting that the two may have grown penecontemporaneously. Preliminary ages for the YOP ranged from 50.4 ± 10.9 to 8.6 ± 2.4 ka. This long period of soil development is consistent with the area’s paleoclimatic history. More samples from the marine terrace and other units on Yali need to be dated to determine the full period for its growth.