Size Modes in Biogenic Carbonate Sediment, Southeastern Alaska

Abstract

ABSTRACT Living communities of intertidal encrusting barnacles and submarine infaunal pelecypods supply skeletons to presently accumulating carbonate sediment in Sitka Sound, Alaska. The barnacle community contains an average of 1.95 105 gms/m2 of shell. Waves erode shells from the rocks; and transport through boulder beds produces shell fragmentation. Clam-bombing of rocks by seagulls and boring sponges and algae (?) or fungi (?) contribute to shell disintegration. Chemical interaction between shells and beach porewater is not active. Size-frequency distribution and point count data show that shell fragmentation of these barnacles and mollusks produces a continuous spectrum of particles between 12 and 0.1 mm. Size modes are weak and diffuse in the zone of grinding, but size modes are sharp and well developed in accumulated carbonate sediment of inner bays and beaches. Matching size-frequency distributions for biogenic carbonate and detrital silicate fractions of the same sample suggest size modes are the result of selective sorting during transport and do not reflect the size-frequency distribution of the source. Beach and inner bay carbonate accumulations are sandy gravels to gravelly sands and contain an average of 66 wt. percent CaCO3. Average mineralogy of the carbonate is 15-30 wt. percent aragonite and 85-70 percent calcite with calcites containing less than 1 mol. % Mg. Average constituent gram composition for 24 samples from Pirate Cove is barnacles 74, mollusks 12, echinoids 10, forams 2, and bryozoans 2 percent by volume. Radiocarbon ages for shells (1783 ± 160, 1698 ± 162 years) from trenches dug in beach deposits show these carbonates to he of post-Pleistocene age.