Inferring sediment transport using Large Benthic Foraminifera: composition, taphonomy and geochemical proxies

Coral reef geomorphological processes are responsible for landform evolution, habitat change and sediment dynamics. Understanding reef sediment transport allows insight into antecedent processes and future landform responses. Large Benthic Foraminifera (LBF) live in the algal rim and deposit tests (shells) which are transported by hydrodynamics (eg. waves, currents). Due to well-defined source areas, LBF tests may act as a ‘tracer’ to infer sediment transport pathways. We analysed surficial (n=74) and down-core (n=97) samples from two sand aprons at One Tree Reef, in the southern Great Barrier Reef (Australia). LBF tests (Baculogypsina sphaerulata and Calcarina spp.) exhibited increased abrasion (rs=0.60, p<0.001) with cross-reef lagoonward transport. Transport trends were consistent with wave direction and refraction. Down-core and surficial sediments were spatially consistent suggesting transport conditions have prevailed for the last 3 ka. While the effectiveness of LBF as tracers has been proved in this and other studies, low abundance remains a limitation. We propose elemental ratios using LBF tests as a novel proxy for determining sediment transport pathways. LBF test analysis (n=40) revealed decreased Mg/Ca and increased Sr/Ca with lagoonward transport. Mg/Ca (R2=0.83, p<0.001) and Sr/Ca (R2=0.32, p<0.001) ratios were significantly correlated against abrasion, suggesting a link with test alteration. This novel proxy is rapid, requires minimal materials and provides quantitative measures for LBF test alteration.