Metallic marine debris: Solid anthropogenic metal core concretions form in rocky intertidal crevices

Metallic debris is common in marine environments worldwide but information on corrosion and concretion is typically not provided by marine pollution studies. Therefore, this study examined solid anthropogenic metal core concretions (AMCCs) on rocky coasts in Spain, Japan and Germany. AMCCs consisted of common building materials, such as nails and screws, that had rusted with each other and were encased in mineral aggregates. Most AMCCs were firmly rusted with rocky intertidal crevices, contained anthropogenic debris (metal, sea glass) and natural materials (mollusk shells, pebbles, sediments) from their surroundings, and formed solid anthropogenic substrates. Crevices, which are important microhabitats and thermal refuges for benthic organisms, were partially to almost completely blocked by AMCCs. Due to their widespread distribution, AMCCs may equally function as novel pathways for debris into geological and archaeological records and as easily accessible baseline tools to track and understand these pathways in a marine pollution research framework.