Iron partitioning between the dissolved and particulate phase within a rising hydrothermal plume: Beebe Vents, Piccard Hydrothermal Field, Mid-Cayman Rise.

Particle processes place first order controls on the transfer of high temperature hydrothermal metals from the lithosphere to the oceans. Within buoyant rising hydrothermal plumes, settling attenuates metal concentrations, and settling is controlled by particle precipitation and growth. To improve our understanding of these processes we collected samples from rising hydrothermal plumes in the Piccard field of the Mid-Cayman Rise in 2012 and 2013. We used a combination of remotely operated vehicle and water-rosette sampling techniques. From these samples, we measured dissolved and particulate Fe and optical proxies of particle size distribution in vertical plume profiles spanning >150 m of the initial plume rise. The measurements from 2013 show that more than 50% of the >1 mmol per kg vent flux partitions to the particulate phase within the first meter of plume rise. The plume is rapidly diluted over 1000 times in the first 16 meters but during this stage the partitioning ratio between dissolved and particulate Fe remains relatively constant. Above 16 m, plume dilution continues more slowly but the Fe partitioning ratio shifts towards the dissolved until 90% of the total Fe concentration, 620 nmol per kg plume fluid, is dissolved. It is not clear to what extent this shift is due to settling or oxidative particle dissolution and both are likely. However, particle size distribution measurements from 2013 show the plume is dominated by small particles, and that particles >100 μm are rare. Thus in this setting, despite rapid precipitation, particle growth and settling are surprisingly slow and a relatively stronger role for particle dissolution is suggested.