Abstract
Iron (Fe) limits or co-limits primary productivity and nitrogen fixation in large regions of the world’s oceans. Hydrothermal supply of Fe to the deep ocean is extensive; however most of this work has focused on examining high temperature, focused flow on ridge axes that create “black smoker” plumes. The contribution of other types of venting to the global ocean Fe cycle has received little attention. To thoroughly understand hydrothermal Fe sources to the ocean, different types of vent site must be compared. To examine the role of more diffuse, higher pH sources of venting, a hydrothermal plume above the Von Damm vent field (VDVF) was sampled for Total dissolvable Fe (unfiltered, TDFe) dissolved Fe (<0.2 µm, dFe) and soluble Fe (<0.02 µm, sFe). Plume particles sampled in-situ were characterised using scanning electron microscopy and soft x-ray spectromicroscopy. The VDVF vents emit visibly clear fluids with particulate Fe (TDFe-dFe) concentrations up to 196 nmol kg-1 comparable to concentrations measured in black smoker plumes on the Mid-Atlantic Ridge. Colloidal Fe (cFe) and sFe increased as a fraction of TDFe with decreasing TDFe concentration. This increase in the percentage of sFe and cFe within the plume cannot be explained by settling of particulates or mixing with background seawater. The creation of new cFe and sFe within the plume from the breakdown of pFe is required to close the Fe budget. We suggest that the proportional increase in cFe and sFe reflects the entrainment, breakdown and recycling of Fe bearing organic particulates near the vents. Fe plume profiles from the VDVF differ significantly from previous studies of “black smoker” vents where formation of new pFe in the plume decreases the amount of cFe. Formation and removal of Fe-rich colloids and particles will control the amount and physico-chemical composition of dFe supplied to the deep ocean from hydrothermal systems. This study highlights the differences in the stabilization of hydrothermal Fe from an off-axis diffuse source compared to black smokers. Off-axis diffuse venting represent a potentially significant and previously overlooked Fe source to the ocean due to the difficulties in detecting and locating such sites.
Highlights
At present there are several known types of hydrothermal system
Despite the Fe concentration in the hydrothermal vent fluids being at the lower end of the global range (Table 1), the Von Damm vent field (VDVF) plume has a similar range of TDFe concentrations (VDVF = 4 to 196 nmol kg−1) comparable to plumes directly over ridge axes sampled within a similar distance from the vent source (James and Elderfield, 1996; Hawkes et al, 2013, 2014)
Given the VDVF plume has a similar range of TDFe concentrations compared to black smoker plumes but the relative amounts of sFe and Colloidal Fe (cFe) increase as pFe decreases it is clear from the plume profile that the processes of Fe(II) oxidation, Fe oxyhydroxide formation and colloid formation/aggregation are not expressed in the same way
Summary
At present there are several known types of hydrothermal system. The most widely discovered type are black smokers that are located at ridge axes and emit acidic pH (2–5), high temperature (up to 400◦C) metal rich fluids. The Von Damm vent field (VDVF) is a new type of hydrothermal system located away from the ridge axis emitting clear, moderately low pH (6–7) fluids with temperatures up to 215◦C (Hodgkinson et al, 2015). This style of venting is chemically similar to diffuse venting that seeps from the seafloor in areas where there is hydrothermal activity. Seawater percolates through fissures in the seafloor mixing with hydrothermal fluids in the sub-surface resulting in lower temperature, lower metal content, less acidic pH fluids compared to the focused flow from chimneys. The flux of trace metals from these systems is poorly constrained as a result of uncertainties in the volume flux and no previous investigations into the behavior of metals as diffuse fluids mix with seawater (Tagliabue and Resing, 2016)
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