Extreme dry-season dynamics of trace element and isotope cycling in the Amazon-mangrove-shelf-ocean continuum: addressing seasonal biases in coastal metal fluxes

• Applicant: Dr. Antao Xu
• Subject Area: Oceanography ; Geology
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 574978370

Trace elements and isotopes (TEIs) in the ocean play pivotal roles in primary productivity, carbon cycling, and marine biodiversity, and serve as powerful tracers of oceanic processes. Therefore, accurately constraining their sources, cycling mechanisms, and fluxes is critical. The Amazon River-mangrove-shelf system, the Earth’s largest river-ocean continuum, is believed to substantially supply trace metals to the Atlantic. However, these inputs are poorly constrained due to existing research biases toward wet-season dynamics (e.g., April–June). While wet-season conditions are relatively well understood and possibly account for the majority of annual fluxes, dry-season dynamics (e.g., September–December), characterized by tripled sediment loads, intensified hydrodynamics, and distinct biogeochemistry remain largely unexplored. This research gap introduces considerable uncertainty in trace metal flux estimates. To resolve this, a coordinated field campaign was conducted in Amazon mangrove systems during the 2024 dry season (November) as part of the PROBRAL project, involving targeted sampling of river water, tidal coastal seawater, groundwater and mangrove sediment. This was followed by GEOTRACES-endorsed GApr21 cruise M206 (1–30 December 2024), across the Amazon and Pará River estuaries and continental shelf. Surface waters, near-bottom seawater, bottom water, surface sediments, and porewaters were sampled. Trace-metal-clean protocols ensured high-quality paired data for dissolved neodymium (Nd) and iron (Fe) concentrations and isotopic compositions (radiogenic Nd isotopes: εNd and stable Fe isotopes: δ56Fe). This enables to (1) quantify source contributions from the Amazon and Pará rivers, mangroves, and shelf sediments to the oceanic Nd and Fe budgets under low-discharge conditions; (2) investigate how seasonal hydrological shifts alter trace metal cycling in the Amazon continuum; and (3) resolve climate-sensitive metal fluxes from this key land–ocean interface. To achieve these goals, four working hypotheses will be tested: H1–seasonal hydrology modifies riverine Nd and Fe endmembers and enhances sediment–seawater exchange; H2–shelf sediments release rare earth elements and isotopically light Fe via reductive dissolution and porewater supply, influenced by particle interactions; H3–mangrove systems act as trace metal pumps, exporting rare earth elements and isotopically light Fe to the coast under tidal and hydrological forcing; and H4–integrated seasonal riverine and shelf fluxes help reconcile global Nd budget imbalances. Using high-resolution spatial sampling from surface to bottom water and porewaters, and process-specific isotope frameworks, the project will deliver the first mechanistic model for dry-season trace metal cycling in tropical estuaries and shelf, provide seasonally resolved Nd and Fe flux estimates to constrain global oceanic budgets, and offer policy-relevant insights for Amazonian coastal management under climate extremes.

• DFG Programme: Research Grants
• International Connection: Brazil, USA
• Co-Investigator: Professor Dr. Norbert Frank
• Cooperation Partners: Professor Dr. Nils Asp; Professor Dr. Tim M. Conway; Professor Dr. Carlos Eduardo de Rezende