SEM EVIDENCE OF THE EARLY DIAGENESIS ASSOCIATED WITH ARAGONITIC SHELLS THAT SURVIVE 100S-1000S YEARS OF TIME-AVERAGED RESIDENCE IN BIOTURBATED SEABEDS

Age-dating of shells from tropical and temperate shelves consistently finds that ≤1% of all shells produced survive post-mortem processes in the bioturbated mixed layer. However, this surviving ~1% persists for 100s-1000s years, implying luck (e.g., finding refuge in super-saturated pockets) and/or diagenetic modification that reduces shell reactivity, albeit with perils for proxy data (e.g., crystallite coarsening, precipitation of crusts, change in mineralogy). Aragonite diagenesis likely varies with latitude given ‘algal micritization’ reported in low latitudes, chemical etching in high-latitudes, and microbial maceration (disintegration via loss of organic matrix) expected everywhere. Analyzing shells of known post-mortem age provides a means of quantifying trends and rates of diagenesis. Bivalve shells from polar Alaska, warm-temperate Southern California, and tropical Red Sea shelves show that 10x magnification suffices to categorize shell interior surfaces by luster (original, chalky, ‘secondarily shiny’), opacity, delamination, and color changes. SEM of tropical lucinid bivalves shows that macroscopic and microstructural features are correlated (AAR-dating is underway). Transparent ‘pristine’ crossed-lamellar aragonite presents 0.2-0.4µm wide laths with rounded ends and straight sides; laths form inclined sheets in first-order domains with commarginal dip. With increasing shell opacity, laths acquire wavy edges and ‘ruffled’ ends (stage 2) and are then transformed into a non-dipping sheet of amoeboid plates that retain the widths and elongation of laths but have increasingly ‘jigsawed’ edges (stage 3), suggesting dissolution-recrystallization. Porcellaneous luster develops as plates coalesce (stage 4). Stages 1-4 can conserve the 3^rd-order lamellar components of laths, indicating only surface remodeling; despite considerable microboring, micritization is not yet evident. Syntaxial calcite appears only on shells with stage 3 plates, but can overgrow any surface, including both remnant patches of pristine laths and delaminated areas. Similar stage 3 plates develop within a few decades on aragonitic bivalves in temperate seabeds, but have not yet been detected in polar ones where we suspect shorter residence times, suggesting a critical step in aragonite persistence.