Geomorphometric Seabed Classification and Potential Megahabitat Distribution in the Amazon Continental Margin

Ana Carolina Lavagnino,Lais Silva Araujo,Alex Cardoso Bastos,Gilberto Menezes Amado Filho,Rodrigo Leão De Moura,Fernando Coreixas De Moraes

doi:10.3389/fmars.2020.00190

Abstract

The geomorphometry of the of the Amazon Continental Margin was analyzed using the Benthic Terrain Model, a spatial analysis technique that defines physical habitat classes based on seafloor relief heterogeneities. A compilation of available bathymetric and novel multibeam data was used to define the megahabitats, with emphasis on shelf-slope transitions and shelf-edge reefs. The analyses revealed a complex mosaic of benthic megahabitats associated to short and long-term geologic and sedimentary evolution of the margin. The continuous terrigenous sediment input is associated to a smooth muddy deposit along the inner and mid shelf (Regular Continental Shelf megahabitat). The portions of the shelf that are less influenced by riverine sediment accumulation are rougher and characterized by either sand (Irregular Sand Continental Shelf megahabitat) or carbonate-dominated bottom (Irregular Reef Continental Shelf megahabitat). The most notable difference in terms of morphometric analysis and megahabitats can be observed along the outer shelf and shelf-break. The Shelf-Slope Transition megahabitat is marked by ridges in the shelf break and by a more acute depth gradient that forms a distinct outer shelf edge. Three different alongshore sectors were explored in order describe the heterogeneous megahabitat mosaic in terms of slope and bottom morphology. The western-most sector (S3) is remarkable due to an indistinct separation between ridges and the outer shelf edge, as well as for presenting reefs with up to 20 m high, between 110 and 210 m water depths. The central sector (S2) presents no shelf-break and lacks ridges, a feature that is associated with the long-term sediment accumulation (Amazon Fan). The southern-most sector (S1) does not present an outer shelf edge, only ridges, and a number of shelf-incised channels, comprising a sediment bypass across the shelf, and carbonate sedimentation. The continental slope is divided into a Featured Slope megahabitat with numerous canyons and ravines, and areas that lack such features, including a Shallow Gentle Slope megahabitat, a Steep Slope megahabitat and a Deep Gentle Slope megahabitat. Our results confirm the usefulness of geomorphometric analyses to define benthic megahabitats and can be used as a starting point in a much-needed marine spatial planning process for the area.

Highlights

The Brazilian Equatorial Margin (BEM) is the widest portion of the Brazilian continental margin and comprises, among others, the Foz do Amazonas Basin, with approximately 360,000 km2 (Brandão and Feijó, 1994; Silva et al, 1999; Figueiredo et al, 2007). We refer to this part of the BEM as the Amazon Continental Margin (ACM) (Cruz et al, 2019)
The distinction between S1 and S3 is related to the depth of the continental shelf break, at 100- and 300-m depth, FIGURE 3 | Bathimetric grid (A), Benthic Terrain Model results (B–D) and Aspect Grid (E)
The geomorphometric analysis of a large bathymetric dataset from the ACM allowed for a novel classification of seabed classes and a mosaic of physical benthic megahabitats, that is, large features with dimensions ranging from kilometers to tens of kilometers (Greene et al, 1999)

Summary

Introduction

The Brazilian Equatorial Margin (BEM) is the widest portion of the Brazilian continental margin and comprises, among others, the Foz do Amazonas Basin, with approximately 360,000 km (Brandão and Feijó, 1994; Silva et al, 1999; Figueiredo et al, 2007). The modern set of this margin was established at 2.5 Ma (early-Pleistocene) and evolved based on the reshape of the Amazon River due to the Andean uplift event during the Miocene (∼10 Ma) (Hoorn et al, 1995; Campbell, 2005; Figueiredo et al, 2009; Gorini et al, 2014) This event, progressively, gave away the predominance of a mixed and carbonate platform (Neogene Amapá carbonates) to a siliciclastic-dominated shelf, contributing to the development of the Amazon Fan (Milliman et al, 1975; Brandão and Feijó, 1994; Gorini et al, 2014). Gravitational tectonics was responsible for mass wasting events, forming large megaslides, or mass-transport complexes that mobilized kilometer-thick deposits, extending for thousands of kilometers in the Amazon Fan (Reis et al, 2016)

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