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Sediment Transport Pathways Across a Holocene Reef Sand Apron: Distribution Patterns of Foraminifera, Aranuka Atoll, Kiribati

Abstract

A lasting paradigm in carbonate sedimentology is “carbonates are born, not made.” This concept recognizes that many carbonates are produced in situ, yet it remains unclear how much sediment is transported versus produced in situ. The purpose of this study is to explore sediment production and transport pathways by examining patterns of distribution of foraminifera across Holocene reef sand aprons of Aranuka Atoll, Kiribati. Surficial sediment from several transects across the ∼2 km-wide northern and southern aprons of the atoll was sampled, and stained with Rose Bengal, a dye that stains the live foraminifera bright pink and leaves the dead individuals unchanged. Results illustrate that the taxonomy, mortality, and abundance of foraminifera vary systematically across a reef sand apron. The most common families on the apron are Calcarinidae, Amphisteginidae, and Soritidae; more than 95% are deceased. Calcarinidae are most abundant near the margin (live and dead specimens make up >50% of foraminifera). Both live and dead Amphisteginidae are more abundant (40 – 50% of foraminifera) toward the middle of the sand aprons, and most Soritidae occur near the platform interior (all deceased). In general, foraminifera are more abraded away from the margin, towards the platform interior. These results are interpreted to reflect sedimentologic differentiation by variable production (Calcarinidae nearest the margin, Amphisteginidae 400 – 1100 m platformward), transport, winnowing by waves and currents across the reef sand apron, and abrasion. Collectively, these results illustrate the nature and causes of on-platform sedimentologic differentiation across these reef sand aprons, insights and trends that may be reflected in ancient analogs.