SHELL BIOMINERALIZATION OF GIANT CLAMS (T. GIGAS) FROM THE RAISED CORAL REEF TERRACES OF HUON PENINSULA, PAPUA NEW GUINEA: IMPLICATIONS FOR PALEOCLIMATE RECONSTRUCTIONS

The giant clam, Tridacna gigas, can live for several decades, biomineralizing massive and dense aragonite shells that are considered ideal paleoclimate bioarchives for the low latitude tropics that are otherwise poorly documented. Paleoclimate studies were performed on T. gigas under the assumptions that i) shells are secreted continuously, and ii) daily and seasonal growth increments are present. Using modern and fossil shells from raised coral reef terraces along the Huon Peninsula, Papua New Guinea, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), have been employed to study the microstructure and microtexture to test such assumptions. Analysis of modern T. gigas specimens allows a preliminary description of biomineralization processes involved in shell formation. Daily growth increments are common and are composed of long, organized aragonite needles that grow during the daytime; needles of the modern specimen have an average width of 0.7680 µm (σ=0.51851, n=92). Small crystals propagate at an angle from daily needles that likely represent periods of slower growth during nighttime. Seasonal growth increments can be identified, by an interval of small, disorganized crystals. Analysis of Holocene-age fossil samples shows a predominant microfabric of disorganized, shield-like crystals that have an average width of 1.5310 µm (σ=0.7356, n=29), which is indicative of change in the biomineralization mechanism. Furthermore, measuring the width of aragonitic needles shows that the width tends to change through geologic time which could be the result of external environmental influences. Finally, the combination of shell biomineralization observations with geochemistry will improve the use of paleoclimate data from these bivalve shells.