Collaborative Research: constraining the role of benthic nepheloid layers in altering benthic fluxes of trace elements in the Labrador Sea

Benthic nepheloid layers (BNLs) are persistent layers of enhanced particle concentrations near the seafloor. Intense BNLs of a few hundred meters thick have been observed globally and may significantly influence the cycling and overall budget of sediment-sourced trace elements and isotopes (TEIs). BNLs can act as elemental sources or sinks, potentially enhancing or suppressing elemental fluxes across the sediment-water interface. However, sampling of BNLs has been limited and very few chemical measurements have been made. In conjunction with a previously-funded research expedition in the Labrador Sea, this project will conduct high-resolution sampling of particle composition and isotopes in BNL particles and surface sediments. The overall goal of the project is a deeper understanding of the role of BNLs in regulating deep-ocean chemistry. Beyond the scientific contributions, the project will train graduate and undergraduate students, engage the public in collaboration with a science media specialist, incorporate scientific and outreach content into undergraduate teaching curricula, and foster international collaboration. The overarching goal of the proposed research is to generate process-level understanding of how BNLs change the net flux of TEIs into the overlying water column. To achieve this, the team aims to address three key questions: (1) What is the benthic flux of TEIs from sediments in the Labrador Sea? (2) How do BNLs modify net benthic fluxes of TEIs? (3) What characteristics of the BNL are the most important controls on TEI concentrations? The investigators hypothesize that BNL regions will exhibit a higher benthic flux of TEIs at the sediment-water interface, as determined by radium-thorium disequilibrium, but that the net benthic flux of particle-reactive TEIs will be lower in BNLs with more manganese oxides due to their greater scavenging capacity. This project will evaluate different scavenging intensities by estimating partition coefficients using particle composition data and investigate the particulate Mn mineral phases responsible for scavenging using synchrotron techniques. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.