Characterizing organic ligands in hydrothermal plumes along the Mid-Atlantic Ridge

Hydrothermal vents are emerging as a potentially significant source of iron (Fe) to the ocean and one that is insensitive to changes in climate. Globally, hydrothermal vents are estimated to transport as much Fe into the ocean as rivers (2.3-19 x 10¹⁰ mol/yr) but were hypothesized to have little impact on global geochemical cycles. Yet, within the last decade, multiple geochemical studies have demonstrated basin-scale transport of hydrothermal Fe. Organic ligands are hypothesized to be one of the key mechanisms to aid in the transport and protection of hydrothermal Fe into the ocean interior. However, very little is known about the sources and transformation of organic ligands in hydrothermal plumes. To identify and characterize organic ligands associated with hydrothermal Fe, samples for trace metal and organic ligand analyses were collected in proximity to 11 known venting locations along the Mid Atlantic Ridge (MAR), as part of the FRidge 2018 cruise. These vents span a range of trace metal concentrations (~0.4 nM-192 nM) and vent types. The binding strength and concentrations of organic Fe-binding ligands were quantified using competitive ligand exchange cathodic stripping voltammetry. Different types of venting along the MAR were observed to have distinct total excess ligand concentrations, including high concentrations of excess weaker (L₃) ligands (log K < 11) associated with black smokers. L₁ (log K > 12) and L₂ (log K=11-12) ligand concentrations varied depending on venting type and plume depth, with elevated L₁ ligands observed at Lost City. The identity of these ligands was probed further using liquid chromatography coupled to inductively coupled plasma and electron spray ionization mass spectrometry to characterize and identify the potential sources of organic ligands associated with Fe near the vent sites. This study showed organic complexation of hydrothermal Fe varied depending on vent type and proximity to the vent. Understanding these changes are crucial to estimating the global impact of hydrothermal Fe.