Marine radiocarbon reservoir age of the coralline intertidal alga Lithophyllum byssoides in the Mediterranean

Coralline algae are the most significant framework builders outside the coral-reef belt. Among them, littoral rims built by the coralline rhodophyte Lithophyllum byssoides are a particularly precise archive of relative sea-level histories in the microtidal environment. Their potential has been underutilized due to the unresolved question of the size of the marine radiocarbon reservoir effect (MRE), which is needed for accurate calibration of dates obtained from marine-derived carbon samples. Because this intertidal alga is submerged during high tide and exposed to the atmosphere during low tide there are questions regarding the source of the carbon it uses and the level of MRE. Consequently, here we provide the first direct evidence regarding the L. byssoides MRE based on samples of known age from museum collections. A total of eight samples of pre-bomb L. byssoides from the Mediterranean revealed a mean reservoir age R(t) of 336 ± 47 ¹⁴ C yr and a ΔR value of −28 ± 41 ¹⁴ C yr for the carbonate fractions. The MRE values obtained from the alga revealed them to be more homogeneous and generally lower compared to the Mediterranean shell MREs for the same period. However, the mean values (mean Mediterranean shells MRE 437 ± 115 ¹⁴ C yr, ΔR 100 ± 99) are statistically indistinguishable as they overlap statistically at 1 sigma due to the great dispersion of mollusc shell values. In 6 out of 9 L. byssoides samples significant ¹⁴ C age differences between algal carbonate and organic fractions have been observed. The organic fractions revealed higher MRE values. The δ ¹³ C of organic matter in marine macroalgae L. byssoides collected from the natural environment ranged from −13.4‰ to −18.7‰ VPDB. The δ ¹³ C values of organic samples of L. byssoides from the Mediterranean and Atlantic showed homogeneous values within the species with a mean of −15.4‰ although the alga belongs to the phylum Rhodophyta where the variability is the highest. These δ ¹³ C values do not provide unequivocal indications on the source of the inorganic carbon (C _i ) used by the alga, but slightly lower reservoir ages (R(t)) of the L. byssoides carbonate fractions in relation to Mediterranean mean shells MRE could point to minor partial uptake of CO ₂ from the atmosphere during emersion (up to 20%). This could not be statistically confirmed. Homogeneous values of L. byssoides δ ¹³ C in the Mediterranean might be related to sharp environmental conditions in the intertidal zone in which the species grew.