Side-scan sonar characteristics and manganese nodule abundance in the Clarion-Clipperton fracture zones, NE equatorial Pacific

Abstract

The deep ocean floor between the Clarion and Clipperton fracture zones (NEequatorial Pacific) has the highest known manganese nodule abundance in the world oceans. A detailed analysis of MR1 (Mapping Researcher 1, 11–12 kHz) sonar images and free-fall grab data in the Korean manganese nodule field areas reveals aclose relationship between side-scan sonar characteristics of the seafloor and manganese nodule abundance. Eight sonar facies are identified based on back-scattering intensity and distribution patterns. These sonar facies can be interpreted as (1) volcanic seamounts (facies I-1), (2) bounding faults of abyssal hills (facies I-2 and II-1), (3) lava flows or volcanoclastic mass-flow deposits around the volcanicseamounts (facies I-3 and II-2), (4) crests of abyssal hills (facies II-1), (5) abyssaltroughs between abyssal hills (facies III-1), (6) relatively flat areas (facies II-3 andIII-2). In the areas where facies II-1 (abyssal hill crests with thin sediment cover)and II-3 (relatively flat areas draped by thin sediments) are dominant, manganesenodules occur abundantly. In contrast, zones comprising facies III-1 (abyssaltroughs with thick sediment cover) and III-2 (relatively flat areas covered by thicksediments) are characterized by low abundance of manganese nodules. This relationship between distribution of sonar facies and manganese nodule abundanceimplies that (1) the qualitative difference in acoustic reflectivity of long-range sidescan sonar with some ground truth data is useful for regional assessment of manganese nodule occurrence over wide areas in a reasonable time, and (2) seafloor topography and sediment thickness are important controlling factors for regional occurrences of manganese nodules