Abstract
The formation of cold-water coral (CWC) mounds is commonly seen as being the result of the sustained growth of framework-forming CWCs and the concurrent supply and deposition of terrigenous sediments under energetic hydrodynamic conditions. Yet only a limited number of studies investigated the complex interplay of the various hydrodynamic, sedimentological and biological processes involved in mound formation, which, however, focused on the environmental conditions promoting coral growth. Therefore, we are still lacking an in-depth understanding of the processes allowing the on-mound deposition of hemipelagic sediments, which contribute to two thirds of coral mound deposits. To investigate these processes over geological time and to evaluate their contribution to coral mound formation, we reconstructed changes in sediment transport and deposition by comparing sedimentological parameters (grain-size distribution, sediment composition, accumulation rates) of two sediment cores collected from a Mediterranean coral mound and the adjacent seafloor (off-mound). Our results showed that under a turbulent hydrodynamic regime promoting coral growth during the Early Holocene, the deposition of fine siliciclastic sediments shifted from the open seafloor to the coral mounds. This led to a high average mound aggradation rate of >130 cm kyr–1, while sedimentation rates in the adjacent off-mound area at the same time did not exceed 10 cm kyr–1. Thereby, the baffling of suspended sediments by the coral framework and their deposition within the ecological accommodation space provided by the corals seem to be key processes for mound formation. Although, it is commonly accepted that these processes play important roles in various sedimentary environments, our study provided for the first time, core-based empirical data proving the efficiency of these processes in coral mound environment. In addition, our approach to compare the grain-size distribution of the siliciclastic sediments deposited concurrently on a coral mound and on the adjacent seafloor allowed us to investigate the integrated influence of coral mound morphology and coral framework on the mound formation process. Based on these results, this study provides the first conceptual model for coral mound formation by applying sequence stratigraphic concepts, which highlights the interplay of the coral-framework baffling capacity, coral-derived ecological accommodation space and sediment supply.
Highlights
Cold-water coral (CWC) mounds are common and prominent sedimentary features along the upper and mid continental slopes (200–1,000 m water depths) in the Atlantic Ocean and the adjacent marginal seas (e.g., Roberts et al, 2006; Wheeler et al, 2007; Hebbeln and Samankassou, 2015; Wienberg and Titschack, 2017; Lo Iacono et al, 2018)
The hydrodynamic forcing appears to be the first order control on coral mound formation by providing food and suspended sediment, the complex interplay between the coral framework and sediment deposition has a major impact on the rates of mound aggradation, and mound formation
Coral framework alters the local hydrodynamic regime and its baffling capacity enables fine sediments to settle between the coral branches even under the influence of strong regional hydrodynamics
Summary
Cold-water coral (CWC) mounds are common and prominent sedimentary features along the upper and mid continental slopes (200–1,000 m water depths) in the Atlantic Ocean and the adjacent marginal seas (e.g., Roberts et al, 2006; Wheeler et al, 2007; Hebbeln and Samankassou, 2015; Wienberg and Titschack, 2017; Lo Iacono et al, 2018). Previous studies on coral mound formation mainly concentrated on the CWCs and the environmental factors supporting their proliferation, such as distinct physical and chemical properties of the ambient water masses (e.g., temperature, dissolved oxygen concentrations, pH, and water density; e.g., Freiwald, 2002; Davies et al, 2008; Davies and Guinotte, 2011; Flögel et al, 2014) and the supply of food by vertical fluxes triggered by surface ocean productivity and horizontal fluxes triggered by strong bottom-water hydrodynamics (e.g., Thiem et al, 2006; Mienis et al, 2007, 2012; Davies et al, 2009; Hebbeln et al, 2016; De Clippele et al, 2018). Detailed studies on the processes controlling sediment delivery and deposition on coral mounds and their impact on coral mound formation, notably over longer timescales, are still largely lacking
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