Is Microstructural Variability in Giant Clams (T. gigas) Controlled by Periodicity in Solar Irradiance?
Abstract
Well-preserved aragonitic shells of the giant clam (Tridacna gigas) offer, in their inner layers, robust bioarchives of their coral reef habitats. T. gigas has a symbiotic relationship with photosynthetic dinoflagellate algae (zooxanthellae) from which they receive elevated nutrition during the daytime. Macroscopically, annual growth increments can be identified in the transverse cross section of modern and fossil shells. Daily growth increments are identified microscopically by the coupling of light and dark increments in the inner shell layers. Previous studies have established a correlation between daily growth increments, Sr/Ca oscillations and solar irradiance for a modern giant clam, and the method was applied to reconstruct insolation for the mid-Holocene. Under a scanning electron microscope, we observed that daily growth is composed of aragonitic laths during the daytime (light increments) and oblique crystals during the nighttime (dark increments). Utilizing modern and well-preserved fossil specimens from the first flight of uplifted coral-reef terraces on the Huon Peninsula, Papua New Guinea (modern to Reefs VIIb and VIIa), we tested the hypothesis that aragonitic needle width (ANW) is controlled by the coeval solar modulation spanning 140 kyr. ANW measurements (n = 20) in 13 samples were taken from the area closest to the umbo (within 1 to 2 mm) during the first season of growth. Our results show that ANW of fossil T. gigas covary with solar modulation obeying a highly significant linear relationship (R = 0.68; P = 0.011). The covariance is attributed to enhanced daylight biomineralization assisted by zooxanthellae. The results of our study suggest that fossil giant clams may offer a useful archive of past solar activity at high resolution. A larger population size of giant clams over a longer timescale is required to confirm our initial findings.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP23G1577G
- Keywords:
-
- 1105 Quaternary geochronology;
- GEOCHRONOLOGYDE: 1120 Isotopic disequilibrium dating;
- GEOCHRONOLOGYDE: 4916 Corals;
- PALEOCEANOGRAPHYDE: 4946 Milankovitch theory;
- PALEOCEANOGRAPHY