A 60-year isotopic record from a mid-Holocene fossil giant clam (Tridacna gigas) in the Ryukyu Islands: physiological and paleoclimatic implications

Abstract

We have constructed a 60-year stable isotope record from a 14 C-dated fossil giant clam, Tridacna gigas (6216 years BP), at its northernmost latitudinal limit in the geological past, on Kume Island, Central Ryukyu Archipelago, Japan. Stable oxygen (δ 18 O) and carbon (δ 13 C) isotopic analyses are combined with observations of growth lines seen on the inner shell layer.Sixty pairs of summer/winter growth lines which preserve daily growth increments were observed in the inner shell layer. Two growth phases, characterized by a growth curve and isotopic profiles, are clearly recognized throughout the growth history of this specimen. No significant shifts in average values of the two isotopic ratios were detected during its growth history, although the growth rate varied widely from 1 mm to 15 mm per year over 60 years, including after the onset of sexual maturity. Spectral analysis of the fossil Tridacna δ 18 O 21 time-series implies that decadal variability observed in the North Pacific Ocean during the past hundred years also existed 6000 years ago. Our study implies that fossil giant clams are one of the best means of inferring isotopic records of annual to decadal climate variations. Giant clams have the advantages of a dense shell, high growth rate, long lifespan, and geographically and geologically broad distributions.

Subdaily‐Scale Chemical Variability in a Torreites Sanchezi Rudist Shell: Implications for Rudist Paleobiology and the Cretaceous Day‐Night Cycle

“…In tridacnid bivalves, subdaily resolved Mg/Ca and Sr/Ca variations were previously observed (Sano et al, ; Warter et al, ). While daily variability in Sr/Ca in T. sanchezi is similar to that measured in tridacnids (~1 mmol/mol; Figure ; Watanabe et al, ; Elliot et al, ), daily fluctuations in Mg/Ca far exceed those in diurnal Tridacna profiles (20 versus 0.4–1.4 mmol/mol in Tridacna ). However, strong variability in Mg/Ca ratios between tridacnids (3–50 mmol/mol) may reflect vital effects in these species (Elliot et al, ; Gillikin et al, ; Watanabe et al, ).…”

“…While daily variability in Sr/Ca in T. sanchezi is similar to that measured in tridacnids (~1 mmol/mol; Figure ; Watanabe et al, ; Elliot et al, ), daily fluctuations in Mg/Ca far exceed those in diurnal Tridacna profiles (20 versus 0.4–1.4 mmol/mol in Tridacna ). However, strong variability in Mg/Ca ratios between tridacnids (3–50 mmol/mol) may reflect vital effects in these species (Elliot et al, ; Gillikin et al, ; Watanabe et al, ). The difference in mineralogy between aragonitic tridacnid shells and the LMC shell of T. sanchezi might also explain the difference in Mg content between these taxa, as the partition constant of Mg (and other trace elements) is different for aragonite and calcite (e.g., Hasiuk & Lohmann, ; Lear et al, ; Onuma et al, ; Rimstidt et al, ).…”

“…Accurately dated and well‐preserved hard tissues (e.g., fossil mollusk shells, corals, and bioapatites) are valuable archives of subannual to multidecadal paleoenvironmental change (Butler et al, ; Fricke et al, ; Gothmann et al, ; Steuber, ). Bivalve shells have proven to be especially useful for reconstructing paleoseasonality and decadal climate variability (e.g., Schöne et al, ; Steuber et al, ; Watanabe et al, ). Indeed, shells of many bivalve species record daily growth increments, especially during the first years of growth (e.g., Pannella, ; Schöne, Fiebig, et al, ; Warter & Müller, ).…”

Late Holocene coral reef environment recorded in Tridacnidae shells from archaeological sites in Okinawa‐jima, subtropical southwestern Japan

“…; Watanabe & Oba ; Watanabe et al . ). The oxygen isotope composition (δ 18 O) of Tridacnidae shell, which is precipitated in isotopic equilibrium with seawater at their living sites (Aharon & Chappell ; Jones et al .…”

“…; Aharon ; Watanabe & Oba ; Watanabe et al . ). The δ 18 O sw is related to salinity associated with the evaporation–precipitation balance, upwelling of deeper water, and/or river water input (Craig & Gordon ; Fairbanks et al .…”