Delivery of terrigenous material to submarine fans: Biological evidence of local, staged, and full-canyon sediment transport down the Ascension-Monterey Canyon system

Abstract

Submarine canyons are prevalent in the world’s oceans and are instrumental in transporting sediment from coastal regions to deep-sea fans. Conventional sediment parameters such as mean grain size, sorting, and provenance have typically been used to characterize these deposits, but they provide little information on sediment source or the delivery processes involved. Fortunately, transported along with the mineral grains are the remains of organisms living within the sediment. Biological constituents have unique environmental signatures that are more precise proxies for source areas than are mineral grains alone. They may be used to identify a single biofacies deposit (SBD) due to local sediment transport or a displaced, multiple biofacies deposit (MBD) resulting from staged sediment transport or a full-canyon flushing event. This Ascension-Monterey Canyon system study demonstrates that by using the biological constituents in marine deposits, the source areas and transport mechanisms may be identified. The 19,000 year record of core S3-15G captured hemipelagic mud interspersed with individual turbidites of sand and silt transported to the core site at lower bathyal depths (3491 m). The relative abundance of displaced benthic foraminifera was found to correlate positively with grain size, with 75% of the fauna being displaced in the cross-bedded turbiditic sand (T c ) units, 39% in the laminated turbiditic sand (T d ) units, and 15% in the turbiditic mud (T et ) units. Nineteen samples were SBDs representing local canyon wall sloughing, bioerosion, or hemipelagic deposition at or near the core site. Sixty-five were MBDs, 31 of which were turbiditic sands originating in the estuarine to inner shelf, outer shelf, upper slope, or upper middle slope, and the remaining 34 were turbiditic muds with displacement initiated in the estuarine to inner shelf, outer shelf, and the upper slope. Commonly, the biological remains of several biofacies characterize these MBDs, reflecting staged sediment transport with storage occurring behind slumps that act as barriers to movement until finally released. Sediment bypassing, typically of the deeper biofacies, and full-canyon flushing, were also evident. Identifying and interpreting the distribution of allochthonous biological sediment constituents in marine deposits is a powerful tool in the investigation of sediment transport that can be applied to other submarine canyon systems.