Marine Reservoir Corrections for Moreton Bay, Australia

Article excerpt

Abstract

We present the first direct assessment of marine reservoir effects in the Moreton Bay region using radiocarbon dating of known-age, pre-AD 1950, shell samples from the east coast of Stradbroke Island and archaeological shell/charcoal pairs from Peel Island in Moreton Bay. The resulting [DELTA]R value of 9 [+ or -]19 [sup.14]C years for the open ocean conforms to regional values established for northeast Australia of 12 [+ or -] 10 [sup.14]C years. Negative [DELTA]R values of-65 [+ or -] 61 [sup.14]C years and -216 [+ or -] 94 [sup.14]C years for southern Moreton Bay highlight the potential for larger offsets over the last ~900 years. These may be linked to changing terrestrial inputs and local circulation patterns.

Keywords: [DELTA]R, marine reservoir effects, marine shell, radiocarbon dating, Moreton Bay

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Moreton Bay is a large, shallow, subtropical, semi-enclosed triangular embayment formed between the large sand islands of Stradbroke and Moreton Islands and the mainland coastline of Australia (Figure 1). The bay extends c.90 km north-south and c.30 km east-west and contains some 360 islands. Radiocarbon dating of marine samples from Moreton Bay forms the basis of archaeological and geomorphological chronologies used to model changes in Aboriginal occupation (McNiven 2006; Ulm and Hall 1996), sea-level change (Flood 1981, 1984; Lovell 1975), the development of fringing coral reef systems (Hekel et al. 1979: 17; Ward et al. 1977) and the establishment of intertidal and subtidal shellfish communities (Flood 1981: 21; Hekel et al. 1979: 9). However, despite a heavy reliance on radiocarbon marine shell ages to construct archaeological and geomorphological chronologies, there has been no systematic evaluation of the local applicability of the generalised marine reservoir value for ocean surface waters in the region.

Radiocarbon ages obtained on contemporaneous terrestrial and marine samples are not directly comparable. Shells and other organisms that have grown in marine environments exhibit older apparent radiocarbon ages caused by the uptake of carbon which has already undergone radioactive decay through long residence times in the deep ocean. On average, the ocean surface (<200 m) has an apparent [sup.14]C age around 400 years older than the atmosphere (Gillespie and Polach 1979; Stuiver et al. 1986). However, studies worldwide have shown that variation in [sup.14]C activity in near-shore marine and estuarine environments depends greatly on local and regional factors, such as hinterland geology, tidal flushing and terrestrial water input (e.g. Dye 1994; Southon et al. 2002; Stuiver and Braziunas 1993).

Regional differences in marine reservoir effect are most commonly determined through radiocarbon dating pre-AD 1950 known-age marine specimens (e.g. shell, coral, otoliths) (e.g. Bowman and Harvey 1983; Gillespie and Polach 1979; Southon et al. 2002) or dating shell and charcoal paired samples from contemporaneous archaeological contexts (e.g. Gillespie and Polach 1979; Ulm 2002). The marine reservoir effect is conventionally expressed as [DELTA]R, which is the difference between the conventional radiocarbon age of a sample of known-age from a specific locality and the equivalent age predicted by the global modelled marine calibration curve (Hughen et al. 2004; Stuiver et al. 1986).

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Marine and estuarine reservoir differences are a major issue in the investigation and dating of coastal archaeological and geomorphological deposits where these factors can result in calibration errors of up to several hundred years. For central Queensland a local open ocean [DELTA]R of 11 [+ or -] 10 [sup.14]C years has been established; but values for adjacent estuaries diverge significantly with values of up to [DELTA]R= -155 [+ or -] 55 [sup.14]C years documented (see Ulm 2002 for detailed discussion). In this case, the blanket application of the regional [DELTA]R value would produce calibrated ages approximately 200 years too young. …