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

Abstract

Seasonally resolved records of carbon and oxygen isotopic composition (δ18O) in fossil shells of giant clams (Tridacnidae) recovered from the Kogachibaru Shell Mound (26°26′N, 127°49′E) and the Second Aragusuku‐Shichabaru Ruin (26°18′N, 127°46′E) in Okinawa‐jima, southwestern Japan, were investigated to reconstruct subtropical coral reef environments of the past. The samples, mainly composed of aragonite shells with limited amounts of diagenetically‐altered calcite cements, were selected for geochemical analyses. The radiocarbon dating of the samples for stable isotope analyses yields about 4000 yr cal. BP for the Kogachibaru Shell Mound and about 1900–1700 yr cal. BP for the Second Aragusuku‐Shichabaru Ruin, respectively, that correspond to the early and middle Shell Mound periods in Okinawa‐jima. These show good agreement with the respective ages inferred from excavation. The shell δ18O values roughly show seasonal variations, coincident with the occurrence of annual growth bands. Applying corrections for the effect of diagenetic alteration, the averages of annual, summer, and winter δ18O values of fossil shells were significantly lower (by 0.1–0.7‰) than those of aragonite theoretically precipitated in present‐day coral reef water of Okinawa‐jima. These results demonstrate that annual mean seawater temperature and salinity at the sites where the Tridacnida spp. lived were about 1–3°C higher and/or about 1–2 lower than at present. This variation cannot be fully explained by global climate change, sea level change, and topographic growth of coral reef. Therefore, it is likely that the giant clams lived in extremely small and/or closed coral‐reef lagoons with less water circulation where seawater is highly susceptible to insolation‐induced temperature increase and input of fresh water; the effect could be enhanced by the fisheries lifestyle that stonewalling would be constructed at shallow waters through the use of tidal variation during the early and middle Shell Mound period in Okinawa‐jima.

Carbon and Oxygen Isotope Records from Tridacna derasa Shells: Toward Establishing a Reliable Proxy for Sea Surface Environments

“…The δ 13 C shell values show no statistically significant ontogenetic trend. Previous studies have shown the same results in the inner shell layers of other tridacnids [ 32 , 35 , 38 , 55 ]. In contrast, distinct ontogenetic decreases in δ 13 C values have been detected in some bivalves [ 58 – 60 ].…”

“…This is true for the inner shell layers of not only T . derasa but also other tridacnids [ 35 , 38 , 53 – 55 ]. These indicates that such a link is unlikely because only minor amounts of metabolic carbon are incorporated into the bivalve shells as shown in non-zooxanthellate bivalves [ 56 , 57 ].…”

“…However, no such negative peaks have been found in the δ 13 C profiles of T . derasa or other tridacnid shells [ 32 , 35 , 38 , 55 ].…”

“…The isotopic data collected previously indicate that tridacnids precipitate their shells in oxygen isotopie equilibrium with seawater [ 32 , 33 ]. Patzöld et al (1991) [ 34 ] showed that the biogenic (daily growth banding) and geochemical (δ 18 O values) records in the inner shell layer are more suitable for paleoenvironmental reconstruction than those of the outer shell layers or hinge, so the former have been used for paleoenvironmental studies [ 2 , 30 , 35 – 38 ]. Therefore, we also studied the isotopic records in the inner shell layer.…”

Carbonate sedimentation on Pacific coral reefs

“…Asami et al . () present seasonally resolved records of carbon and oxygen isotopic compositions (δ 18 O) in the fossil shells of giant clams recovered from archeological sites in Okinawa, Japan. Results demonstrate that the annual mean seawater temperature and salinity at Okinawa were approximately 1–3°C higher and 1–2 lower, respectively, at 4000 yr cal.…”