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Equatorial Atolls of Republic of Kiribati (Equatorial Pacific): Impact of Physical and Chemical Oceanographic Processes on Sedimentology and Geomorphology

Abstract

Isolated platforms represent the iconic carbonate macro-geomorphic form of warm, tropical and subtropical systems, and can host prolific reservoirs. Although the nature and general patterns of sediment accumulation are well constrained on platforms in oligotrophic settings, equatorial systems with elevated nutrients are less well understood. The purpose of this study is to describe processes, sediment, and geomorphic forms on five equatorial atolls (Abemama, Aranuka, Beru, Maiana, and Nonouti – Republic of Kiribati), highlighting their differences with low-nutrient settings. Each atoll lies within 200km of the equator, and thus all are subjected to equatorial upwelling and ENSO-related shifts in wind strength and direction. These upwelled waters can be moved onto shallow atolls by the large tides (~2 m), quotidian wave-generated currents, and currents driven by distal swell from passage of fronts further north. As equatorial systems, these atolls are not directly impacted by tropical depressions. Atolls are circular to ovoid, with sandy Recent or lithified Late Holocene islands most common on their east/northeast flanks. Annular reefs are best developed on the eastern and southern margins. Reef sand aprons extend up to 2 km inboard of the margin, passing fairly abruptly into patch reef-rich platform interiors up to 20 m deep. Sediment fines away from the margins; in some cases (Nonouti), coarse sand and gravel is ubiquitous across the atoll, in others (e.g., Abemama), platform interior can include considerable fines (in excess of 50% fine sand to mud) in the interior. Although corals and green algae are common near reefs, sediment away from the margins is dominated by foraminifera, bivalves, gastropods, and red algae. Ooids and peloids are absent. The results illustrate the influence of upwelled, nutrient rich waters on the types of sediment (foramol association) on tide-dominated, wave modified atolls. These examples provide actualistic conceptual models for heterogeneity on ancient isolated platforms.