238U234U230Th chronometry of FeMn crusts: Growth processes and recovery of thorium isotopic ratios of seawater (original) (raw)
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Persistence of deeply sourced iron in the Pacific Ocean
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.
Minerals, 2018
Two Fe–Mn crusts among 35 samples, from six seamounts in the Canary Island Seamount Province, were selected as representatives of the endpoint members of two distinct types of genetic processes, i.e., mixed diagenetic/hydrogenetic and purely hydrogenetic. High-resolution analyses pursued the main aim of distinguishing the critical elements and their association with mineral phases and genetic processes forming a long-lived Fe–Mn crust. The Fe–Mn crust collected on the Tropic Seamount is composed of dense laminations of Fe-vernadite (>90%) and goethite group minerals, reflecting the predominance of the hydrogenetic process during their formation. Based on high-resolution age calculation, this purely hydrogenetic crust yielded an age of 99 Ma. The Fe–Mn crust collected on the Paps Seamount shows a typical botryoidal surface yielding an age of 30 Ma. electron probe microanalyzer (EPMA) spot analyses show two main types of manganese oxides, indicating their origin: (i) hydrogenetic Fe-vernadite, the main Mn oxide, and (ii) laminations of interlayered buserite and asbolane. Additionally, the occurrence of calcite, authigenic carbonate fluor-apatite (CFA) and palygorskite suggests early diagenesis and pervasive phosphatization events. Sequential leaching analysis indicated that Co, Ni, Cu, Ba and Ce are linked to Mn minerals. Therefore, Mn-oxides are enriched in Ni and Cu by diagenetic processes or in Co and Ce by hydrogenetic processes. On the other hand, Fe-oxides concentrate V, Zn, As and Pb. Moreover, the evidence of HREE enrichment related to Fe-hydroxides is confirmed in the mixed hydrogenetic/diagenetic crust.
Study of Deep-Ocean Ferromanganese Crusts Ore Components
Iron Ores
A complex layer-by-layer morphology and phase analysis of a ferromanganese crust aged about 70 million years, extracted from the rise of the Magellan Mountains of the Pacific Ocean, was carried out using several physics methods: digital optical microscopy, scanning electron microscopy with high resolution, X-ray fluorescence and diffraction analysis and Mossbauer spectroscopy. This analysis showed that the crust is an association of several minerals with various dispersion and crystallization degree, between which fossilized bacterial mats with Fe- and Mn- oxides are located. These phenomena indicate the biogenic nature of the crust. Changes in the crusts phase composition from the lower layer to the upper layer indicate changes in the external environmental conditions during their formation.