A High-Resolution Coral-Based Δ14C Record of Surface Water Processes in the Western Mediterranean Sea

Abstract

ABSTRACT. The first high-resolution time series of pre-and post-bomb radiocarbon measurements is reported for surface waters in the western Mediterranean Sea. The  14 C record was obtained from the aragonite skeleton of Cladocora caespitosa using a 50-yr-old corallite collected in the Ligurian Sea in 1998. Laser-ablation ICP measurements of trace elements (Li/Mg and Sr/Ca) show a strong seasonal variability, enabling the chronology of the  14 C record to be determined at annual timescales. The mean  14 C of pre-bomb surface water is -56 ± 3‰, corresponding to a reservoir age of 262 ± 29 yr. The postbomb maximum occurs in 1972 with a  14 C value of 90‰, significantly lower than the peak of 150‰ observed in the North Atlantic. The dilution of the peak-amplitude of  14 C in western Mediterranean surface waters is attributed to mixing of North Atlantic Central Water inflow with relatively depleted underlying Intermediate Mediterranean and Levantine Intermediate waters. Intensification of this mixing is observed in 1963-1964, consistent with the change in atmospheric circulation from a positive to negative NAO phase (1960)(1961)(1962)(1963)(1964)(1965)(1966)(1967). The post-peak  14 C variability is relatively limited, reflecting mainly local vertical mixing forced by wind stress.

High resolution regional modeling of natural and anthropogenic radiocarbon in the Mediterranean Sea

“…These figures clearly show the surface inflow of Atlantic waters through the Strait of Gibraltar were progressively enriched during their spreading into the EMed, leading to a relatively higher 14 C level in the EMed surface water closer to -46 ‰. For both western and eastern surface water, the model simulates 14 C concentrations slightly higher than the in-situ observations (Broecker and Gerard, 1969;Siani et al, 2000;Reimer and McCormac, 2002;Tisnérat-Laborde et al, 2013). A careful comparison between model outputs and seawater observations (1959) reveals a more pronounced disagreement, especially in the EMed surface water where the model overestimates the 14 C values by more than 10 ‰ (Fig.4a).…”

“…To improve dense-water fluxes through the Cretan Arc during the EMT (1992EMT ( -1993 the ARPERA forcings were modified over the Aegean sub-basin (Beuvier et al, 2012a), by increasing mean values as done by Herrmann and Somot (2008) Figures 2d and 2e show the radiocarbon distribution over the whole water column in the Mediterranean along a longitudinal transect for both the eastern and western basins together with in-situ observations from Broecker and Gerard (1969). Figure 3 compares model results of reservoir ages and several marine reservoir 14 C age data available for the surface water of the Mediterranean; these data were obtained from pre-bomb calcareous marine shells between 1867 and 1948 and coral Cladocora caespitosa (Siani et al, 2000;Reimer and McCormac, 2002;Tisnérat-Laborde et al, 2013).…”

“…Finally, mollusc shells were also used to yield a more significant dataset for the Mediterranean Sea with a mean sea-surface reservoir 14 C age of 400 ± 16 yr (Reimer and McCormac, 2002). More recently, the first annually-resolved sea-surface 14 C record was obtained from a 50-year-old shallow-water coral (Cladocora caespitosa) from the western Mediterranean Sea, covering the pre-and post-bomb period (Tisnérat-Laborde et al, 2013). However, all these observations are discrete both in time and/or space, they cannot give a clear description of radiocarbon evolution between the past and the actual situation now at either the regional or the global scale.…”

“…Our model results are compared to available 14 C measurements of seawater and marine carbonates reported by Broecker and Gerard (1969), Stuiver et al (1983), Siani et al (2000) and Tanhua et al (2013), and to a 50-year high resolution 14 C record obtained from a shallow-water coral specimen (Tisnérat-Laborde et al, 2013).…”

Twenty-five-year longevity of European hake (Merluccius merluccius) from novel use of bomb radiocarbon dating in the Mediterranean Sea

“…Calculated fish age was based on the difference from collection year to the D 14 C reference alignment. The D 14 C reference was a recently validated time series from a 50-year-old coral (Cladocora caespitosa) from the Ligurian Sea (Tisnérat-Laborde et al 2013). This D 14 C record is appropriate for direct comparison with the juvenile portion of otoliths (core material) because each formed in the mixed layer of the central Mediterranean where ocean circulation patterns and mixed layer stratification is well documented (Tisnérat-Laborde et al 2013).…”