Seafloor characterization and backscatter variability of the Almería Margin (Alboran Sea, SW Mediterranean) based on high-resolution acoustic data

18 pages, 9 figures

An acoustic study of the Almería Margin (eastern Alboran Sea, SW Mediterranean) was carried out by means of an integrated dataset, comprising deep-towed TOBI sidescan sonar, swath-bathymetry, TOPAS high-resolution seismics and sediment gravity cores. The Almería Margin is a complex system dominated by volcanic and tectonic features shaped by recent sedimentary processes, in which a regional hemipelagic sedimentation is intercalated by sporadic gravitative events. The aim of this work is twofold. The first objective is to recognize the sedimentary and tectonic features that shape the Almeria Margin. The main morpho-sedimentary and morpho-structural elements observed are: the Almería canyon/channel Turbidite System, the Dalías Tributary Valley System, landslides, active faults and folds and volcanic banks. The second aim is to explore and quantify the surface and subsurface seafloor parameters that characterize the backscatter of the study area. Sedimentary, geomorphological and biological elements that play a role in the acoustic signature of the Almería Margin were detected. Nevertheless, it should be noted that seafloor acoustic backscatter is also affected by geometrical and instrumental factors. Despite the wide variability of the environments studied, a low average acoustic backscatter reflects a diffused draping of hemipelagic sedimentation in the area. Higher backscatter values were observed at the top of some volcanic banks, along the Almería canyon, and in the shallower sector of the Dalías tributary valley system. The influence of subseafloor properties in the acoustic signature of the area was revealed by backscatter-grain size correlations, which were carried out for different depth intervals in sediment cores collected in the Almería Turbidite System. A poor relationship was found between backscatter and superficial silty sediments of the area, whereas a higher correlation resulted in the upper 50 cm. The presence of subsurface turbidites in the cores associated with the higher backscatter values suggests volume interface scattering of these sandy layers as a controlling factor of the acoustic signature obtained in the Almería Margin. On the other hand, in rough settings such as the ones observed in the Dalías Tributary Valley System, topographic relief of up to 1–1.5 m strongly interacted with the TOBI acoustic pulse. This suggests that large-scale roughness is an additional parameter that characterizes the acoustic strength of the area. Extremely high reflective patches distributed along some of the volcanic banks (Chella and Pollux Banks) for depths ranging from 230 to 470 m coincide with areas where carbonate cold water coral mounds develop. In the TOBI images, coral facies reach the maximum value of acoustic backscatter, which is probably related to the rough morphology of coral ecosystems. The position of coral mounds in the banks suggests that the occurrence of strong bottom currents and reduced sedimentary inputs are environmental factors that favour their settling and development in the Almería Margin

The authors acknowledge the support of the MCYT Acción Especial HITS (REN2000-2150-E), European Commission European Access of Seafloor Survey Systems EASSS-III programme (HPRI-CT99-0047), MEC (Ministerio de Educación y Ciencia) post-doctoral funding (Ayudas para la movilidad de Investigadores extranjeros — SB2004-0082), Spanish national Projects IMPULS (REN2003-05996MAR), EVENT (CGL2006-12861-C02-02), SAGAS (CTM2005-08071-C03-02), WESTMED (01-ELC-EMA22F), and funding by Generalitat de Catalunya, research group SGR2001-0081

Peer reviewed