Browsing by Author "Mortimer, GE"

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    Mid-holocene variability in the marine C-14 reservoir age for northern coastal Papua New Guinea
    (Elsevier, 2008-08) McGregor, HV; Gagan, MK; McCulloch, MT; Hodge, E; Mortimer, GE
    Changes in oceanic radiocarbon (C-14) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Nino events. Reservoir ages were calculated using paired U-Th and conventional C-14 dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS C-14 and MC-ICPMS U-Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U-Th and conventional C-14 ages. The combined results show average reservoir ages of 185 +/- 30 C-14 yr (n = 4) for 7220-5850 yr BP compared to 420 C-14 yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air-sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Nino activity around this time would have served to enhance the intrusion of C-14-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Nino-Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region. © 2008, Elsevier Ltd.
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    Uplift rates defined by U-series and C-14 ages of serpulid-encrusted speleothems from submerged caves near Siracusa, Sicily (Italy)
    (Elsevier, 2009-02) Dutton, A; Scicchitano, G; Monaco, C; Desmarchelier, JM; Antonioli, F; Lambeck, K; Esat, TM; Fifield, LK; McCulloch, MT; Mortimer, GE
    We have established a plausible rate of uplift near Siracusa in southeastern Sicily (Italy) over the last glacial-interglacial cycle using U-series ages of submerged speleothem calcite and C-14 ages of calcite serpulid layers that encrust the speleothems during cave submergence. The precisely determined ages of these sea level benchmarks were compared with expected relative sea level position based on glaciohydro-isostatic modeling to assess the rate of uplift in this region. When combined with the age of various late Holocene archaeological sites that have been recently described and characterized in terms of their functional position relative to sea level these data collectively define a rate of uplift <= 0.4 mm a(-1) along this portion of the Sicilian coastline. These results are consistent with an age assignment of marine isotope stage (MIS) 5.3 or 5.5 for the Akradina terrace. which in turn places temporal constraints on paleoshorelines above and below this level. © 2008, Elsevier Ltd.