The cruise M176/2 is part of the International GEOTRACES Programme as a process study. The overall aim of cruise M176/2 is to conduct a detailed geochemical sampling of the hydrothermal plume at the Rainbow vent field located at 36°13.80 N, 33°54.14 W on the Mid-Atlantic Ridge (MAR). The deep ocean work will be complimented by biological surface ocean investigations of productivity and diazotrophy.

The Rainbow field itself is well studied and time series data for the fluid chemistry indicates a very stable system over time, making it an ideal study site for our detailed plume study. The project will obtain a mechanistic and quantitative understanding of processes that set hydrothermal fluxes of trace elements and their isotopes (TEIs) to the deep ocean interior at MAR. This work will quantify TEIs fluxes from the ocean crust and determine their biogeochemical cycling and the plume spread in relationship to large scale ocean circulation in the N Atlantic in the vicinity of MAR. We will conduct detailed water column surveys in close vicinity to the vent field and up to 60 km distance to determine the fate of hydrothermally derived TEIs with increasing distance from the source, including an assessment of changes in physical and chemical speciation. We will assess to what extent TEIs are controlled by vent supply, particle removal processes and advection. Sampling of Ra, Th and He isotopes will help to quantify hydrothermal fluxes of TEI, assess removal strength, and fingerprint sources to elucidate TEI distributions. Atmospheric and deep water (micro)nutrient supply to the surface ocean will be assessed to investigate controls on microbial ecosystem functioning, including productivity and diazotrophy. In addition, the role of dissolved organic nitrogen (DON) as a N source to phytoplankton will be assessed. The work will use GEOTRACES trace metal clean sampling and analysis approaches.