Award: OCE-1851007

Underwater hot-springs release heat from Earth’s interior as seawater flows through hot volcanic rock.  As downwelling seawater percolates deeper beneath the seafloor it gets hotter and reacts with the hot volcanic rock until, eventually, it becomes so hot and buoyant that it breaks its way back upward to the overlying ocean, having undergone significant chemical modification.  In particular, the hot submarine vent fluids that are released are enriched in a range of different metals, most especially iron (Fe) and manganese (Mn) both of which are about 1 million-fold higher in concentration than in the overlying ocean.  This is particularly important for Fe because the same element also acts as a biologically limiting micro-nutrient in most of Earth’s oceans.  In prior work we had made an important discovery that – prior to expectations for the preceding 20-30 years – all of the iron released from submarine vents did not end up in metal rich deposits on the seafloor in and around those vent-sites.  Rather, a significant proportion (more than double the typical concentration of deep-ocean dissolved Fe) could be detected thousands of kilometers from their source and appeared to travel so far that they could play an important role in photosynthesis-driven biological activity in the North Pacific and in the Southern Ocean where Fe is otherwise in short supply.  What that earlier work could not explain, however, is how the Fe could travel so far and why.

In this project we have taken a first step in a new direction in the study of trace metals in the deep ocean, using a free-swimming (autonomous) robot to follow the plume of effluent escaping from a set of well-studied seafloor vents in the NE Pacific off the shore of Canada.  By equipping that robot, called Sentry, with purpose-designed sampling equipment we have been able to trace the tortuous path of the “smoke” escaping from those vents for 10-20km downstream within the plume and collect samples at increasing distance to evaluate how much the plume has been diluted and how much of the dissolved and particulate Fe and Mn remained within the plume by this distance from the vents.  In our original study, all we knew was that whatever processes regulated the escape of Fe and Mn from venting to the ocean happened somewhere between the source and a point at 100km down-stream.  Here, we were able to fill in missing data points at 0.25km, 0.5km, 1km, 2km, 5km, 10km an 20km from the source.  We had hoped to fill in more gaps between 20 and 100km from the vents as well but bad weather during our field program cost us 8 days of 20 for research so we only had time to progress as far as we could.

What we found was surprising.  The pattern that the plume followed through the ocean was not a simple plume that flowed in a single direction but, instead, changed daily influenced by large-scale ocean weather patterns and the shape of the seafloor below: by the end of our project the plume was following a clockwise-turning spiral pattern through the ocean with a ~10km diameter, a few hundreds of meters above the seafloor.  Within that spiralling plume we found that, following an immediate 10% decrease within the first 1km or less, there was no further dilution of vent-supplied material over the next 10-20km down-stream.  Dissolved Mn showed the same kinds of behavior.  High concentrations of vent-sourced Mn were found directly above the vents and those same high concentrations persisted as far as we were able to follow the plume.  Intercomparison of Mn that was present in particles coarser than 0.2µm in the plume, in smaller colloidal particles and in truly dissolved form revealed that essentially all of this Mn was present in the most soluble component throughout. For Fe, the changes in behavior were quite different.  The iron that was in the dissolved (<0.2µm) fraction was predominantly present in colloidal (fine grained particulate) form and unlike Mn its abundance dropped off markedly within the first week of its dispersion through the ocean as it travelled 5-10km down-stream.  Over the same time and length scales, particulate Fe concentrations increased so that, by 15-20km downstream, particulate Fe concentrations were higher than dissolved Fe concentrations.  The changeover from dissolved Fe to particulate Fe was not the only change, however.  By 20km downstream the total amount of Fe present in the plume had dropped to no more than ~50% of what was present directly above the vents.  Mineralogical studies, by scanning electron microscope indicate that this removal might be driven by the formation of very fine grained (colloidal) Fe-rich particles close to the source which bind together to form ever larger aggregates, increasing numbers of which sink to the seafloor as they migrate downstream.

 

 

Last Modified: 09/22/2025
Modified by: Christopher R German