Process zone specific alteration of rare earth element (REE) patterns across the land–ocean transition of the southern North Sea

Abstract

Rare earth elements (REE) are powerful tracers in oceanography for tracking boundary exchange and internal biogeochemical processes. While extensive research has focused on REE cycling in marine environments, studies regarding the coastal ocean are limited. Coastal ecosystems are biogeochemically complex, because they are subject to intense organic matter cycling and shifts in redox gradients, driven by both in situ processes and benthic-pelagic interactions. The impact of these dynamics on REE cycling remains largely unclear, raising concern as certain potentially harmful REE, such as samarium (Sm) and gadolinium (Gd), increasingly enter coastal environments due to anthropogenic activities. To identify key drivers of REE cycling in these dynamic environments, we analyzed dissolved REE concentrations across the land–ocean continuum in the German Bight (southern North Sea). We assessed REE concentrations and respective shale-normalized patterns in relation to inorganic and organic parameters such as nutrients, manganese, suspended particulate matter, particulate organic matter, and dissolved organic matter (DOM), characterized both as bulk dissolved organic carbon (DOC) and at the molecular level using ultrahigh-resolution mass spectrometry. Natural and anthropogenic REE primarily originated from riverine input, with significant enrichments at riverine and estuarine stations. The magnitude of anthropogenic Sm and Gd anomalies and heavy-to-light ratio in shale-normalized REE patterns varied among the Elbe, Weser and Ems rivers, likely reflecting the characteristics of their respective catchment areas. The coastal interface was identified as a critical zone where interactions with the benthic environment through deposition and porewater flux led to significant alterations of REE patterns. Offshore, benthic-pelagic coupling diminished, allowing in situ processes to dominate. Changes in heavy-to-light ratio in shale-normalized REE patterns across the land–ocean continuum were attributed to shifts in complexing partners and changes in scavenger pool composition from terrestrial to more stable microbial-derived highly unsaturated DOM nearshore, followed by less stable marine metal-DOM complexes offshore. We highlight the crucial role of coastal interfaces for the cycling of anthropogenic Sm and Gd, acting as important alteration interfaces and reservoirs for anthropogenic REE, affecting their reactivity, environmental fate and affecting marine REE budgets.