There and back again, a journey of many pathways: conceptualising the marine organic carbon cycle

Understanding and determining the pathways that organic carbon (OC) takes in the ocean is one of the pressing tasks of our time, as the fate of OC in the ocean is linked to the climate system and the functionality of marine ecosystems. The multitude and complexity of these pathways are typically investigated with sophisticated, mainly quantitative, methods focused on individual pathways in order to resolve their interactions and processes as realistically as possible. In addition to these approaches towards understanding and recreating complexity, there is a need to identify commonalities and differences between individual OC pathways and define their overarching structures. Such structures can provide a framework for the growing number of partly overlapping concepts, which conceptualise selected OC pathways, and promote more systematic comparisons and consistent communication, especially between different disciplines. In response, we propose a (visual) concept in which we define such higher-level "structures" by comparing and condensing marine OC pathways based on their sequences of processes and the layers of the marine system in which they operate. The resulting structures comprise "closed loops", three remineralisation and two recalcitrant dissolved organic carbon (rDOC) loops that close within marine systems, and "open loops", condensing pathways leaving the marine system for the atmosphere or deeper sediment layers. In addition, we provide a synthesis of embedded processes, OC pools, and process-performing organisms (agents) embedded in these loops. By translating a definition of the biological carbon pump (BCP) into our concept, we show how the application and discussion of our defined structures facilitate a consistent visualisation, a systematic comparison of differently resolved concepts and studies, and integration of these into the larger picture of the marine OC cycle. As a complement to quantitative studies and descriptions of individual pathways, our concept decomposes the complexity of OC pathways by defining new universal structures. These structures provide a skeleton that can be adapted to different systems and filled with life by the users.