MICROSTRUCTURAL CHANGES IN FOSSIL GIANT CLAM SHELLS THROUGH 200 KA: INITIAL RESULTS

Well preserved aragonitic shells of the giant clam (Tridacna gigas) are used in paleoclimate studies because their internal layers offer robust bioarchives of the understudied tropics. T. gigas live in coral reef environments for up to several decades recording local environmental and climate changes within their massive shells. Modern and fossil specimens from the uplifted coral reef terraces of the Huon Peninsula, Papua New Guinea, display daily growth increments at the microscale that are composed of pairs of aragonitic needles (daytime) and small oblong crystals (nighttime). The symbiotic relationship with photosynthetic zooxanthellae yields higher growth during the daytime, promoting the deposition of aragonitic needles, and slower growth during the nighttime. Here we test the hypothesis that a relationship between solar radiation variability and aragonitic needle width (ANW) may be unraveled in fossil T. gigas specimens spanning the last 200 ka over a full glacial cycle. A correspondence between interannual solar radiation variability and the elemental chemistry of the daily growth increments has previously been established. Utilizing SEM, needle width measurements (n = 20/sample) were taken from the first season of growth within 1-2 mm of the umbo. Our results show that ANW of fossil specimens (n = 12) co-vary with changes in solar modulation over the past 200 ka. The cyclic nature is thought to be associated with a change in biomineralization associated with solar variations. The promising results of this initial study, suggesting co-variation between ANW of fossil T. gigas and solar modulation advances the potential of T. gigas to assist in the reconstruction of past solar activity at high resolution. A population size with greater geologic age distribution is required to confirm our present findings.