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The bivalve Tawera gayi, a potential archive of southern South America Holocene climate variability

The venerid Tawera gayi could be a suitable Holocene bioarchive for Southern South-America given that it is found in a wide distribution range from the Beagle Channel (54o 50´ S) to 33o S along the Pacific coast, and to the North Patagonia (36°S) in the South Atlantic. In the Beagle Channel, both extant T. gayi populations and shell beds of mid-Holocene origin can be found. On the other hand there is reliable life history information in modern populations (Lomovasky et al. 2005 J. Appl. Ichthyol. 21, 64-69), i.e. shell growth patterns and the confirmation of the annual periodicity of the translucent bands. Finally, the shells provide geochemical proxies, e.g., d18O for temperature reconstruction. In order to investigate climate variability in the Beagle Channel, the individual age, growth increments and isotopes analyses of modern and fossil shells of T. gayi were used. 
The shell cuts of T. gayi showed a pattern of alternating broad opaque and narrow translucent bands, which were confirmed by acetate peels too. In general, the translucent bands showed a pink to purple colour, similar to internal part of the shell. Both modern and fossil populations showed a maximum age of 13 years old. Radiocarbon dating revealed ages ranging between ca 3800 to 4400 years b.p. in the fossil shells corresponding to the Holocene Climate Optimum. The δ18O values obtained in fossil shells ranged from 1.316‰ to –0.064‰ We correlated the most positive δ18O values with winter forming the translucent bands and the most negative δ18O with summer. The comparison of the von Bertalanffy growth curve showed no difference in the H∞ between modern (32.50 mm; Confidence Interval (CI)=31.07, 33.94) and fossil (33.23 mm; CI=31.94, 34.51) populations, but higher values (p < 0.05) were observed in the growth rate k and t0 in the modern (0.37 (0.31, 0.42) and 1.12 (0.98, 1.25) respectively) than fossil shells (0.24 (0.21, 0.27) and 0.57 (0.44, 0.69) respectively). 
This study demonstrated that this species clearly exhibited annual cycles showing seasonality patterns from the mid-Holocene to the present with translucent bands corresponding to slow or halted growth formed in fall/winter; the growth rate was lower during the past warm epochs than the present possible related to a different productivity in the Channel and/or a lower metabolic rate of the clams exposed to a higher temperature.