Characterization of hydrothermal sources of iron in the oceans - Constraints from iron stable isotopes

Abstract

In recent years, iron (Fe) has been recognized as one of the key elements in Earth’s biogeochemical cycles, acting as an important micronutrient for photosynthetic organisms, and a limiting factor in regulating primary productivity. Consequently, the oceanic biogeochemical cycle of Fe is tightly linked to the global carbon cycle and hence global climate processes. This perception has triggered intense scientific interest in understanding the marine biogeochemistry of Fe and quantifying its spatial distribution and transport in the oceans, as well as the technological advancements necessary to facilitate high-precision analyses of Fe concentrations and Fe isotopic compositions of marine samples, including seawater. Indentifying the sinks, sources and transformations of Fe species in the global ocean, as well as lateral and vertical modes of transportation and ocean mixing, supplying dissolved Fe to areas away from Fe sources, are the main aims of current research campaigns. Multiple Fe sources, such as aerosol dust, contribute to the marine dissolved Fe budget and novel tools utilizing isotopic characterization of these sources and mass balance calculations are anticipated to facilitate resolution of the complex interplay of the various Fe fluxes. Hydrothermal discharge has received growing attention as a major source of dissolved Fe to the deep ocean, particularly in areas with little dust deposition. However, studies of Fe isotope fractionation in hydrothermal plumes have resulted in controversial findings with implications for constraining the isotopic signature/s of hydrothermally derived Fe and the role of hydrothermal Fe in the global oceanic Fe inventory. In order to constrain Fe isotope fractionation within a submarine hydrothermal vent field and hydrothermal plume, this study aided in the development of new seawater preconcentration and separation protocols. These new methods facilitate direct Fe isotope analysis of dissolved Fe in plume samples, for the first time, utilizing double spiking techniques and multiple-collector ICPMS (MC-ICPMS) instrument settings specifically adapted to meet the requirements for analysis of low concentration samples. In order to provide a better understanding the Fe isotope systematics in submarine hydrothermal systems, the present study investigates the Fe isotopic compositions of hydrothermal fluids and precipitates from the newly discovered Nifonea vent field in Vanuatu. Located in a young back-arc basin, Nifonea provides a unique opportunity to (1) study processes affecting Fe isotope fractionation, represented by the 56Fe/54Fe signature (reformulated as δ56Fe), in environments characterized by short-lived heat pulses and relatively low water depths, and (2) better constrain Fe isotope effects resulting from subsurface sulfide precipitation and phase separation. The results indicate the complex interplay of sulfide formation and phase separation producing large spatial variability of fluid Fe isotopic compositions with generally low δ56Fe. High temperatures from recent volcanic events are interpreted to facilitate slow precipitation of chalcopyrite with systematically higher δ56Fe compared to hydrothermal fluids causing considerable Fe isotope effects. In addition, phase separation at relatively low pressure conditions produces low-Cl vapor phases and appears to strongly partition the Fe isotopes into vapor and liquid phases. For the first time, we demonstrate fractionation of Fe isotopes during phase separation with similar isotope effects as suggested from recent experimental studies (Syverson et al., 2014). The processes controlling Fe isotope fractionation and Fe speciation during plume formation above Nifonea are approached by coupled Fe isotope analysis of dissolved and particulate Fe and the chemical composition (major and trace elements) of plume particles. Plume processes largely regulate the transport of dissolved Fe to the open ocean through mineral precipitation and redox processes. Removal and transformation of hydrothermally derived Fe from, or within, the plume also strongly fractionate the original Fe isotopic composition of dissolved hydrothermal Fe towards heavier isotopic compositions, resulting in δ56Fe as low as -0.74‰ and up to -0.19‰ in more distal parts of the plume, as suggested in the presented study. These results further support predictions from previous research (Bennett et al., 2009) however, they also reveal the very complex interplay of various processes affecting the Fe isotope systematics in hydrothermal plumes and preclude a generalized isotopic signature for hydrothermally derived Fe for input into mass balance models. Results of this thesis emphasize the need for further research investigating transformation processes of Fe within hydrothermal plumes, from near-field to far-field, thereby integrating dissolved and particulate Fe isotopic composition, and speciation, as well as particle geochemistry.