Coccoliths from recent sediments of the central Portuguese margin: Taphonomical and ecological inferences

Abstract

In this study we describe recent coccolith assemblages from surface sediments of the central Portuguese continental margin. By investigating variation in coccolith concentrations and relative abundances along north–south and onshore–offshore gradients and between submarine canyons and open shelf and slope areas, and by correlation of coccolith distribution with sediment characteristics (sediment bulk composition and particle-size, Corg/Ntot and sediment accumulation rate), we assess to what extent (paleo)ecological information can be distinguished from taphonomical effects (dissolution and mechanical destruction, lateral transport and dilution with terrigenous material). The main finding of this study is that original distribution patterns reported for living coccolithophores from the Portuguese margin surface waters are reflected by patterns in relative abundances of coccoliths in the underlying sediment, despite the vigorous current dynamics and active sediment transport on the shelf and in the canyons. Like in the surface water assemblages, Gephyrocapsa oceanica, Coronosphaera mediterranea, Helicosphaera carteri and Coccolithus pelagicus are more prevalent in sediments of the continental shelf and upper canyon reaches, whereas Gephyrocapsa muellerae, Calcidiscus leptoporus and the group comprising Umbilicosphaera sibogae, Umbellosphaera irregularis and Rhabdosphaera spp. have higher relative abundances in sediments of the open slope. Greater abundance of the coastal assemblage in sediments of the upper Nazaré Canyon appears associated with persistent high productivity driven by amplification of coastal upwelling and internal tidal pumping in the canyon head and the shelf area south of it. Enrichment of oceanic species in the upper reaches of both the Nazaré and Lisbon–Setúbal Canyon, compared with adjacent shelf areas, suggests displacement of oceanic coccolithophores in oceanic surface water masses advected coastwards along the canyon axes. While coccolith relative abundances in the sediment thus seem to reflect original distribution patterns, precluding major post-mortem redistribution of coccoliths, we found a distinct inverse relationship between coccolith concentration and sediment particle size, suggesting that sedimentary processes control the accumulation of coccoliths in the sediment. We propose a taphonomy-driven scenario, in which coccolith preservation is favored by rapid incorporation in the sediment near their area of origin, while coccoliths reworked by currents and transported away from their area of origin are subject to non-selective destructive processes, including dissolution and mechanical abrasion. This scenario would explain the observed apparent paradox of the preservation of the primary ecological signal in an environment known for its dynamic sedimentary processes. This study provides important insights on the processes governing the preservation of the ecological signature in the sedimentary record and testifies on the enormous potential of coccoliths in paleoapplications.