Processes driving seagrass soils composition along the western Mediterranean: The case of the southeast Iberian Peninsula.

Seagrasses are distributed all along the coast of the Mediterranean Sea being Posidonia oceanica and Cymodocea nodosa the most common species. They promote sedimentation, leading to the formation of well-structured soils. Over the last decade, a growing attention has been paid to their role as CO sinks in the form of organic carbon (C) and to their use as environmental archives. However, most of the knowledge about pedogenetic processes in these soils refer to the rhizosphere. This study aims to understand seagrass soils biogeochemistry in the rhizosphere and below, which in turn can help to understand their long term formation processes. Fifteen cores were strategically sampled along a 350 km stretch of the Southeast Iberian coast, and analyzed for elemental composition (XRF core-scanning), magnetic susceptibility, C content and gran size distribution. The cores were dated by Pb and C-AMS techniques to estimate soil accretion. Principal component analysis was used to explore the main geochemical processes linked to soil formation. The results showed that terrestrial runoff plays a key role in meadow soil composition. Furthermore, C accumulation did not follow any general depth trend in our soil records, suggesting that temporal variation in C inputs is an important factor in determining carbon depth distribution within the soil. We obtained evidence that the establishment of well-developed, stable C. nodosa meadows in the Mediterranean Sea may be promoted by adverse environmental conditions to P. oceanica settlement. Metal's behavior within the meadow deposit and their interaction with organic matter and carbonates is unclear. The results presented in this paper highlight the importance of the influence of land-based inputs in the characteristics of seagrass meadow deposits, highly determining their C content, as well as the need for further studies on metal behavior, to understand their full potential as environmental records.