UC Santa Cruz

ABSTRACT

Anne Brooks Warner

RECENT SEDIMENTS OF MIDWAY ATOLL

Midway Atoll lies at the far northwestern end of the volcanic Hawaiian island chain and is one of few relatively pristine coral reef ecosystems remaining worldwide. Midway’s unusual morphology, high-latitude location (29°N), and history of anthropogenic changes make it a unique setting, which is likely recorded in sediment deposits. Surface sediment samples (N=356) were collected between July 2008 and September 2011 and analyzed for grain size and composition. Sediment derived from corals, coralline algae, and foraminifera comprises ca. 75% of the surface sediments across Midway, while the actively accreting reef only makes up 6.3 –13.9% of the atoll’s total area. The abundance of coral and foraminifera is greatest in the central lagoon (33 – 52% and 23 – 39%, respectively) and lowest (<27% and <11%, respectively) across the northwest platform. In contrast, coralline algal abundance is greatest in the northwest (34 – 48%) and lower in the central lagoon (<18%). Living Halimeda is locally abundant but is scarce in sediment deposits atoll-wide (<10%). High-energy winter storms and swells out of the northwest are the primary forces affecting the movement and distribution of sediments at Midway. A ca. 5 km wide submerged margin along the northwest rim allows wave action to scour the substrate and carry particles lagoonward, creating a coarse-to-fine gradient in mean grain size (MGS). Sediment input from the outer reef and hardgrounds is redistributed within the lagoon by wave action and currents, masking local sediment production and patterns of deposition. While MGS is relatively coarse across the atoll, finer sediments (<0.36 mm MGS) characterize the central lagoon, coarse sand (0.74 – 0.87 mm MGS) covers broad areas in the northeast and south central atoll, and very coarse sand (1.05 – 1.80 mm MGS) is found on the northwest platform. Sorting of surface sediments is poor atoll-wide (>0.80 ϕ). Sedimentation at Midway resembles those other coral reefs in the Pacific that are under ‘reef control’ and differs from the commonly ‘lagoon controlled’ platforms of the Caribbean. Patterns described at Midway may be applicable regionally and/or globally and contribute to understanding of the evolutionary trajectories of high-latitude coral reefs.