Changes in hydrothermal plume iron speciation in the 1-100 km distance from vent source (original) (raw)

Biological carbon fixation is limited by the supply of Fe in vast regions of the global ocean. Dissolved Fe in seawater is primarily sourced from continental mineral dust, submarine hydrothermal-ism, and sediment dissolution along continental margins. However , the relative contributions of these three sources to the Fe budget of the open ocean remains contentious. By exploiting the Fe stable isotopic fingerprints of these sources, it is possible to trace distinct Fe pools through marine environments, and through time using sedimentary records. We present a reconstruction of deep-sea Fe isotopic compositions from a Pacific Fe−Mn crust spanning the past 76 My. We find that there have been large and systematic changes in the Fe isotopic composition of seawater over the Cenozoic that reflect the influence of several, distinct Fe sources to the central Pacific Ocean. Given that deeply sourced Fe from hydrothermalism and marginal sediment dissolution exhibit the largest Fe isotopic variations in modern oceanic settings, the record requires that these deep Fe sources have exerted a major control over the Fe inventory of the Pacific for the past 76 My. The persistence of deeply sourced Fe in the Pacific Ocean illustrates that multiple sources contribute to the total Fe budget of the ocean and highlights the importance of oceanic circulation in determining if deeply sourced Fe is ever ventilated at the surface. marine chemistry | micronutrient cycling | iron biogeochemistry | isotopic fingerprinting | ferromanganese oxides I ron (Fe) is the most abundant transition metal in marine phy-toplankton, reflecting its importance for a range of biochemical processes such as photosynthesis and nitrogen fixation (1). The high cellular requirements for Fe, coupled with its low solubility and concentrations in seawater, render Fe a limiting nutrient in vast regions of the global ocean (2). In turn, this makes the availability of dissolved Fe a potential controlling factor for changes in atmospheric pCO 2 and thereby major oscillations in Earth's climate. Global biogeochemical models show that more regions of the surface ocean are dominated by circulation-driven dissolved Fe fluxes from below than by surface aerosol fluxes (e.g., refs. 3 and 4). This upward flux of dissolved Fe is itself primarily sourced from three main pathways: dissolution of mineral dust (e.g., ref. 5), submarine hydrothermalism (e.g., refs. 6–8), and sediment dissolution along continental margins (e.g., refs. 9 and 10), with the main removal mechanism being scavenging onto sinking particles (e.g., ref. 11). However, the significance of deeply derived Fe sources—submarine sediment dissolution and hydro-thermalism—compared with surface Fe sources (dust dissolution), remains controversial (e.g., refs. 12 and 13). Given the key role of Fe in supporting oceanic primary production, quantifying the relative importance of the various Fe sources—both in the modern ocean and in the geological record—is critical to understanding how micronutrient cycles are related to Earth's climatic state. One promising way to trace Fe sources in the modern ocean is with measurements of stable Fe isotopic compositions, where δ 56=54 Fe = ð 56=54 Fe sample = 56=54 Fe IRMM − 14 − 1Þ × 1;000. Recent studies showed that the Fe isotopic composition of seawater is primarily controlled by the relative input of isotopically distinct Fe sources (14, 15), and that these source signatures can be transported and retained over thousands of kilometers within the ocean interior (14). The large range in Fe isotopic compositions observed between different Fe sources (≥4‰) and in seawater (>2‰) should therefore also be reflected in sedimentary archives that faithfully capture the Fe isotopic composition of seawater (14–17). Here, we report a record of δ 56=54 Fe from CD29-2, a miner-alogically uniform (18) Fe−Mn (ferromanganese) crust collected from the flank of the Karin Ridge at 16°42:4′ N, 168°14:2′ W in the central Pacific (ref. 19, Fig. 1). The present water depth of CD29-2 is ∼2,000 m, although the depth at the time when Fe−Mn crust formation commenced was likely ∼ 1;000 m [owing to thermal subsidence (SI Materials and Methods)]. Hydrogenetic Fe−Mn crusts are irregularly layered sedimentary deposits that form through chemical precipitation of Fe and Mn oxides from ambient seawater, forming the Fe oxyhydroxide mineral feroxyhyte (20). Their persistence on rock substrates away from sediment sources that might bury the crust (20) allows other metals to adsorb and become incorporated into Fe−Mn crusts via lattice replacement or coprecipitation with Fe or Mn oxides (21). Detailed elemental stratigraphy showed that CD29-2 is hydrogenetic—rather than hy-drothermal or diagenetic—in origin (18). This designation means that the Fe and other metals contained within CD29-2 were sourced from ambient seawater at the time of deposition, rather than dia-genetic remobilization of sedimentary metals, or through accretion of proximal hydrothermal vent-derived Fe and Mn oxides. Hydrogenetic Fe−Mn crusts are recorders of long-term changes in seawater trace element isotopic chemistry as they grow extremely slowly [1 − 10 mm·My −1 (20)]. Sample CD29-2 has an average growth rate of ∼1.4 mm·My −1 (22), with each discrete sample for δ 56=54 Fe (between 0.2 and 0.5 mm) integrating between 140 and 350 ky of Earth history. Since the residence time of dissolved Fe in the deep ocean [∼270 y (23)] is less than the mixing time of the oceans [∼1,000 y (24)], our record provides a local history of the central Pacific, rather than of global seawater Significance The vertical supply of dissolved Fe (iron) is insufficient compared with the physiological needs of marine phytoplankton in vast swathes of the open ocean. However, the relative importance of the main sources of " new " Fe to the ocean—con-tinental mineral dust, hydrothermal exhalations, and sediment dissolution—and their temporal evolution are poorly constrained. By analyzing the isotopic composition of Fe in marine sediments, we find that much of the dissolved Fe in the central Pacific Ocean originated from hydrothermal and sedimentary sources thousands of meters below the sea surface. As such, these data underscore the vital role of the oceans' physical mixing in determining if any deeply sourced Fe ever reaches the Fe-starved surface-dwelling biota.