Abstract
AbstractAnnual radiocarbon from a massive Porites lutea coral collected from Hon Tre Island, Vietnam, South China Sea (SCS) was analyzed over a ~100-yr-long period from AD 1900 to 1986. The pre-bomb results from 1900–1953 show a steady Δ14C value of –54.4±1.8‰ (n=60). These values are similar to coral records located in the central and southern SCS and from Indonesian waters, but are lower than those from Japan. Following the input of anthropogenic bomb 14C, our results show a sharp increase in Δ14C from 1960, reaching a peak value of 155.3‰ in 1973. The Hon Tre Island post-bomb Δ14C values are lower than those of other corals located in the SCS and Japan, but higher compared to those in the Indonesian Seas. This study infers a seasonal input of upwelled water depleted in 14C from the deeper SCS basin that originates from the tropical Pacific via the Luzon Strait. The bifurcation of the North Equatorial Current feeds the surface and intermediate currents in the SCS and Makassar Strait region. However, unlike the Makassar site, this study’s coral Δ14C does not receive lower 14C water from the South Pacific Equatorial Current. The Vietnam record therefore represents a unique oceanographic position, reflecting the seasonal influence of older, deeper SCS waters that upwell periodically in this area and have modified the surface waters locally in this region over the last 100 yr.
Highlights
The calcium carbonate skeletons of marine calcifiers such as foraminifera, bivalves, and corals contain a useful array of geochemical proxies that have been used to improve our understanding of past climate, ocean circulation, and atmospheric to surface ocean processes (Smith et al 1979; Beck et al 1992; Gagan et al 1998; Elderfield and Ganssen 2000; Schöne et al 2004; Lynch-Stieglitz 2006)
Over the last 50 yr, the atmosphere and surface ocean Δ14C increased by nearly 100% and 20%, respectively, due to nuclear weapons testing in the late 1950s and early 1960s. 14C records in marine carbonates in surface waters lag those in the atmosphere, due to long equilibration times (~10 yr) and regional oceanographic features caused by the global thermohaline circulation, local upwelling, stratification, and freshwater impact (Southon et al 2002; Hua et al 2004)
Because the Δ14C values are corrected for known age of formation, the decrease is assumed to be due to environmental changes including the Suess effect
Summary
The calcium carbonate skeletons of marine calcifiers such as foraminifera, bivalves, and corals contain a useful array of geochemical proxies that have been used to improve our understanding of past climate, ocean circulation, and atmospheric to surface ocean processes (Smith et al 1979; Beck et al 1992; Gagan et al 1998; Elderfield and Ganssen 2000; Schöne et al 2004; Lynch-Stieglitz 2006). 14C records in marine carbonates in surface waters lag those in the atmosphere, due to long equilibration times (~10 yr) and regional oceanographic features caused by the global thermohaline circulation, local upwelling, stratification, and freshwater impact (Southon et al 2002; Hua et al 2004). The differences between Δ14C records of marine carbonates allow the investigation of local and regional ocean circulation and can be used for testing and improving ocean circulation models (Reimer et al 2013), adjustment of fossil marine sample ages, and allowing for the investigation of oceanographic processes in specific regions.
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