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A robust marine radiocarbon (14C) reservoir correction (ΔR) is essential for calibrating 14C dates of marine mollusks and fish bones routinely found in archaeological sites as discarded food remains and bones of terrestrial animals (including humans) with an appreciable marine diet. New ΔR values are reported for the atoll archipelago of the Marshall Islands, eastern Micronesia. Atolls consist of biogenetic material—mostly coral and foraminifera—that can be directly dated for establishing sequences of atoll emergence and islet development. After sectioning and examination using scanning electron microscopy (SEM) to screen for sample diagenesis, 6 pristine branch coral samples were selected from the modern oceanside beach, 3 archaeological sites, and islet developmental facies from Ebon Atoll (4º34′N, 168º41′E). Each sample was analyzed by U-series and accelerator mass spectrometry (AMS) 14C showing no substantial temporal ΔR variations and yielding a weighted mean ΔR of 41±42 yr1 spanning ~500 yr before earliest human colonization (the period when islets first became habitable) through the entire 2000-yr occupation sequence. Reliable published ΔR values for Micronesia and Δ14C data for Palmyra Island, together with our results for Ebon Atoll, indicate that the Pacific North Equatorial Counter Current is almost stable for the past 2500 yr.
The response of platform reefs to sea-level stabilization over the past 6 ka is well established for the Great Barrier Reef (GBR), with reefs typically accreting laterally from windward to leeward. However, these observations are based on few cores spread across reef zones and may not accurately reflect a reef's true accretional response to the Holocene stillstand. We present a new record of reef accretion based on 49 U/Th ages from Heron and One Tree reefs in conjunction with re-analyzed data from 14 reefs across the GBR. We demonstrate that hydrodynamic energy is the main driver of accretional direction; exposed reefs accreted primarily lagoon-ward while protected reefs accreted seawards, contrary to the traditional growth model in the GBR. Lateral accretion rates varied from 86.3 m/ka–42.4 m/ka on the exposed One Tree windward reef and 68.35 m/ka–15.7 m/ka on the protected leeward Heron reef, suggesting that wind/wave energy is not a dominant control on lateral accretion rates. This represents the most comprehensive statement of lateral accretion direction and rates from the mid-outer platform reefs of the GBR, confirming great variability in reef flat growth both within and between reef margins over the last 6 ka, and highlighting the need for closely-spaced transects.
Radiocarbon dating of marine samples requires a local marine reservoir correction, or ΔR value, for accurate age calibrations. For the Samoan Archipelago in the central Pacific, ΔR values have been proposed previously, but, unlike some Polynesian archipelagoes, ΔR values seem not to vary spatially and temporally. Here, we demonstrate such variability by reporting a ΔR of –101±72 ΔR for the Manu‘a Group—the eastern-most islands in the archipelago—for the colonization period. This value is based on accelerator mass spectrometry (AMS) 14C and uranium-thorium (U-Th) series dating of individual coral branches from pre-2300 cal BP archaeological contexts. This figure differs from the previously proposed modern ΔR of 28±26 yr derived from dated historic, pre-1950, shell samples from the western islands of Samoa. Consequently, we recommend using the ΔR of –101±72 yr for the 1st millennium BC in Manu‘a, and 28±26 yr for calibrating dates within the 2nd millennium AD in the western islands (Savai‘i to Tutuila). Until more data from across the archipelago and from throughout the entire culture-historical sequence document ΔR variability, we recommend that researchers use both of these ΔR values to evaluate how the dates of marine-derived samples compare with AMS dates on identified, short-lived wood charcoal.
High-precision analysis using accelerator mass spectrometry (AMS) was performed upon known-age Holocene and modern, pre-bomb coral samples to generate a marine reservoir age correction value (ΔR) for the Houtman-Abrolhos Archipelago (28.7°S, 113.8°E) off the Western Australian coast. The mean ΔR value calculated for the Abrolhos Islands, 54 ± 30 yr (1 σ) agrees well with regional ΔR values for Leeuwin Current source waters (N-NW Australia-Java) of 60 ± 38 yr. The Abrolhos Islands show little variation with ΔR values of the northwestern and north Australian coast, underlining the dominance of the more equilibrated western Pacific-derived waters of the Leeuwin Current over local upwelling. The Abrolhos Islands ΔR values have remained stable over the last 2896 cal yr BP, being also attributed to the Leeuwin Current and the El Niño Southern Oscillation (ENSO) signal during this period. Expected future trends will be a strengthening of the teleconnection of the Abrolhos Islands to the climatic patterns of the equatorial Pacific via enhanced ENSO and global warming activity strengthening the Leeuwin Current. The possible effect upon the trend of future ΔR values may be to maintain similar values and an increase in stability. However, warming trends of global climate change may cause increasing dissimilarity of ΔR values due to the effects of increasing heat stress upon lower-latitude coral communities.
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