contact info

Dr. Olivier Rouxel

Associate Professor

University of Hawai‘i at Manoa

School of Ocean and Earth Science and Technology

Department of Oceanography

1000 Pope Road, MSB 510

Honolulu, HI 96822 USA

office: (808) 956-8333

email: orouxel(at)hawaii.edu

OVERVIEW

Variations in the isotopic ratios of light elements such as H, C, N, O and S have been widely studied over the last five decades and provided the foundation for the field of Stable Isotope Geochemistry.

These traditional stable isotope systems have been applied to a range of research fields such as planetary geology, oceanography, the origin and evolution of life, climate change and environmental studies. However, much less attention has been paid to the stable isotope variations of heavier elements, such as Fe, Cu, Zn, Mo, Se, or Sb mainly due to analytical challenges.

Our research involves the development and application of non-traditional stable isotope geochemistry to understand modern marine biogeochemical cycles, seafloor hydrothermal systems, geobiological interactions and Earth's Ocean-Atmosphere system history. Our interest focus primarily on the biogeochemistry of sulfur, iron and other metals in modern and ancient oceans, defining four major themes:

(1) Sources and biogeochemical cycling of metals in the upper ocean;

(2) Hydrothermal fluxes and their impact on ocean biogeochemistry;

(3) Geobiological interactions in hydrothermal and subsurface environments;

(4) Development and application novel geochemical proxies for Paleooceanography;

(5) Seafloor mineral deposits: formation, evolution and societal relevance.

All of our research projects require a combination of cutting-edge geochemical analyses, oceanographic cruise planning (from water column sampling to oceanic crust drilling), and interdisciplinary collaborations.

(1) Sources and biogeochemical cycling of metals in the upper ocean

Rationale: Over the last two decades, our understanding of the importance of trace elements in the nutrition and functioning of modern ocean ecosystems, and consequentially their effect on global carbon cycling, has radically changed. It is now widely recognized that Fe acts as a limiting nutrient for marine phytoplankton in large regions of world’s ocean; changes in the supply of Fe to the upper ocean may lead to climate change by altering rates of carbon sequestration though photosynthesis and nitrate reduction. Beyond Fe, the marine biogeochemistry of other metal micronutrients has stimulated analogous interest, in particular Cd, Co, Cu, Mn, Ni and Zn. The toxicity of certain metals and metalloids as well as their speciation and binding with marine organic-ligands and humic substances, also received increased attention through the 1990s, particularly in coastal settings proximal to anthropogenic sources.

Approach: The recent analytical development of metal isotope biogeochemistry may fill important gap in the current knowledge of oceanic metal cycling. Although initial attention focused on their potential use as biosignatures or paleoceanographic proxies, metal isotopes such as Fe, Zn, Cd and Cu are now emerging as powerful oceanographic tracers. Since the first priority was to understand metal isotope fractionation processes that occur as the elements enter the ocean, resulting in modification of the original source composition, I initially investigated the mechanisms of Fe isotope fractionation in coastal environments. For example, I conducted the first survey of Fe-isotopes in riverine and subterranean estuaries and coastal seawater to improve our understanding of how Fe mobilized from source regions, transported into the coastal ocean and recycled within the open ocean. This leads to the important finding that Fe flux in the upper ocean derived from benthic diagenesis has characteristic isotope signatures that are distinct from riverine and atmospheric iron sources.

Yet, important questions still remain poorly adressed:

What are the metal isotope compositions of major oceanic basins and water masses?

What is the relative importance of biological activity vs. abiotic processes in controlling metal isotope fractionation in the oceans?

What is the importance of sediment diagenesis under contrasted redox regime in controlling benthic metal fluxes ?

Addressing those questions is fundamentally important for our understanding of global biogeochemical cycles but also crucial for any investigation of paleo-oceanographic records of metal isotopes in seawater.

We're actively pursuing this research topic through the study of Fe isotopes and other metal isotope systems such as Ni, Cu and Ge isotopes. I'm currently investigating metal isotope systematics in key contrasted region of the world’s ocean, including (1) seawater column variably impacted by dust deposition and shelf input (e.g. Equatorial Atlantic region); (2) oxygen minimum zones and anoxic basins (Peru margin, Mediterranean brines). Ultimately, a source-to-sink approach will permit the establishment of a reliable mass balance of metals in seawater.

Representative publications:

71. Chever, F., Rouxel, O., Croot, P.L., Ponzevera, E., Wuttig, K. and Auro, M. (2015) Total dissolvable and dissolved iron isotopes in the water column of the Peru upwelling regime. Geochimica et Cosmochimica Acta 162, 66–82 (PDF)

48. Boyle E.A., S. John, W. Abouchami, J.F. Adkins, Y. Echegoyen-Sanz, M. Ellwood, A.R. Flegal, K. Fornace, C. Gallon, S. Galer, M. Gault-Ringold, F. Lacan, A. Radic, M. Rehkamper, O. Rouxel, Y. Sohrin, C. Stirling, C. Thompson, D. Vance, Z. Xue, Y. Zhao (2012) GEOTRACES IC1 (BATS) contamination-prone trace element isotopes Cd, Fe, Pb, Zn, Cu, and Mo intercalibration. Limnol. Oceanogr.: Methods 10, 653–665 (PDF)

43. Roy, M., Rouxel, O., Martin, J.B., Cable, J.E. (2012) Iron isotope fractionation in a sulfide-bearing subterranean estuary and its potential influence on oceanic Fe isotope flux. Chemical Geology, 300: 133-142 (PDF)

30. Rouxel O. and Auro M. (2010) Iron isotope variations in coastal seawater determined by Multicollector ICP-MS. Geostandards and Geoanalytical Research, 34,135-144 (PDF)

21. Escoube R., Rouxel O., Sholkovitz E. and Donard O. (2009) Iron Isotope Systematics in Estuaries: The case of the North River, Massachusetts (USA). Geochim. Cosmochim. Acta., 73, 4045-4059 (PDF)

18. Rouxel O., Sholkovitz E., Charette M., Edwards K. (2008) Iron isotope fractionation in subterranean estuaries. Geochim. Cosmochim. Acta. 72, 3413-3430 (PDF)

 

(2) Hydrothermal fluxes and their impact on ocean biogeochemistry

Rationale: Seafloor hydrothermal activity at mid-ocean ridges (MOR) and ridge-flanks is one of the fundamental processes controlling the exchange of heat and chemical species between seawater and ocean crust. While it has long been established that hydrothermal vents represent an important source of many elements to the oceans, and constitute an important sink from the overlying water column for others, the far-field consequences of hydrothermal metal input in comparison to other marine sources is just starting to be recognized. In particular, mounting evidence suggests that organic compounds bind to and stabilize metals in hydrothermal fluids, increasing trace-metal flux to the global ocean. Despite its obvious importance, our current ability to quantify the impact of hydrothermal venting on global ocean metal reservoirs is far from satisfactory.

Approach: Guided by recent analytical advances in metal and metalloid isotope geochemistry, a major focus of our research is to define a critical path to characterizing the metal biogeochemistry of the ocean’s largest dissolved metal reservoir, the deep sea, within a spatial continuum spanning from high-temperature venting to far-field open seawater.

We're investigating metal isotope geochemistry in order to link isotope signatures, mineralogy and geochemistry of plume particles, metal speciation and microbial communities with the aim to address the following questions:

How do hydrothermal plume fluxes vary in response to volcanic perturbation and geodynamic settings?

What role do microbes play in plume biogeochemical cycles in general, and in Fe cycling in particular?

What it the importance of volcanic seamount in either global or local biogeochemical cycling of metals in seawater?

Among important results, we found that Fe isotope signatures and biogeochemical cycling of metal vary between sulfide-rich and sulfide-poor hydrothermal systems which bears important implications for tracing hydrothermal venting of metals in modern deep ocean and our ability to trace hydrothermal fluxes in the paleo-record. We are currently extending this research topic to other metal isotopes beyond Fe. On-going research projects continue to address the range of biogeochemical processes that occur within hydrothermal plumes and diffuse flow venting with the overreaching goal to unravel the global impact of seafloor hydrothermal venting on oceanic metal cycles.

Representative publications:

82. Rouxel O., Toner B., Manganini S., German C.; Geochemistry and Iron Isotope Systematics in Hydrothermal Plume Fall-out at EPR 9°50’N. Chemical Geology doi: 10.1016/j.chemgeo.2016.08.027

72. Escoube, R., Rouxel, O., Edwards, K., Glazer, B. and Donard, O. (2015) Coupled Ge/Si and Ge isotope ratios as geochemical tracers of seafloor hydrothermal systems: case studies at Loihi Seamount and East Pacific Rise 9°50'N. Geochimica et Cosmochimica Acta 167, 93-112 (PDF)

35. Adams D.K., McGillicuddy D.J., Zamudio L., Thurnherr A., Liang X., Rouxel O., German C.R., Mullineaux L.S., (2011) Surface-generated mesoscale eddies transport deep-sea products from hydrothermal vents. Science, 332, 580-583 (PDF)

20. Toner B.M., Fakra S.C., Manganini S.J., Santelli C.M., Marcus M.A., Moffett J.W., Rouxel O., German C.R., and Edwards K.J. (2009). Preservation of Iron(II) at Hydrothermal Vents within Carbon-Rich Matrices. Nature Geosciences doi:10.1038/ngeo433 (PDF)

22. Bennett S. A., Rouxel O. J., Schmidt K., Garbe-Schönberg D., Statham P.J., German C.R. (2009) Iron isotope fractionation in a buoyant hydrothermal plume from the Mid-Atlantic Ridge at 5ºS. Geochim. Cosmochim. Acta, 73, 5619–5634 (PDF)

 

(3) Geobiological interactions in hydrothermal and subsurface environments

Rationale: Subseafloor environments host very diverse microbial communities that are mainly based on chemosynthesis involving CO2-fixing metabolism. Their metabolisms are based on the reduction-oxidation of locally available chemical species, such as hydrogen, sulfur and iron. Yet, the mechanisms by which deep-sea microbial communities use the chemical energy stored in sulfide minerals and rocks are however poorly constrained, prompting further studies of mineral/microbe interactions as well as sulfur and metal biogeochemical cycling.

Approach: Through integrated studies, our goal is to provide new insights into the role of microorganisms in sulfur, carbon and metal cycling in seafloor hydrothermal systems. In collaboration with microbiologists, we combined geochemical, isotopic and DNA/RNA analysis of field samples, hydrothermal sediment and porewater (e.g. Guaymas hydrothermal field) with laboratory culture under controlled conditions. We also investigated Fe biogeochemical cycling and formation mechanisms of massive Iron microbial mats at Loihi Seamount. Overall, this multidisciplinary approach allowed to study prokaryotes-biotope interactions in hydrothermal systems and to develop conceptual models for using metal isotope systems as tracers of life in extreme environments and its interaction with geological processes.

The study of an active biosphere in the basaltic ocean crust is still limited and lags behind our current understanding of subsurface life in deep-sea sediments or in seafloor hydrothermal systems. This is mainly due to technical difficulties involved in identifying and culturing indigenous microbes. Since subseafloor basaltic crust represents the largest habitable zone by volume on Earth, it is crucial to determine the abundances, activity, diversity and limit of the deep biosphere and its potential role in rock alteration processes. Since microbes may gain energy from the oxidation of reduced sulfur (S) and iron (Fe) in basalt, an important goal of our research has been to apply combined stable isotope composition such as S, C and Fe and fine-scale mineralogy of secondary minerals in altered basalts as potential biosignatures of the deep biosphere. We performed a pioneer study combining functional gene sequencing with S and C isotopic signatures to better constraints the key “players” in microbial ecosystems of deep subseafloor basalt. This provided the first comprehensive evidence of an active biosphere in deeply buried basalts on ridge flanks.

Representative publications:

69. Callac N., Rouxel O., Lesongeur F, Liorzou C, Bassoullet C, Pignet P, Cheron S, Fouquet Y, Rommevaux-Jestin C, Godfroy A. (2015) Insights into microbial-mineral interactions with continuous enrichment culture of hydrothermal chimneys using hydrothermal fluid as medium. Extromophiles. doi: 10.1007/s00792-015-0742-5 (PDF)

60. Orcutt, B.N., Wheat, C.G., Rouxel, O., Hulme, S., Edwards, K.J., Bach, W. (2013). Oxygen consumption rates in subseafloor basaltic crust derived from a reaction transport model. Nature Communications, 4. 10.1038/ncomms3539 (PDF)

51. Lever M.A., Rouxel O., Alt J., Shimizu N., Ono S., Coggon R.M., Shanks III W.C., Lapham L., Elvert M., Prieto-Mollar X., Heinrichs K.W., Inagaki F. and Teske A.P., (2013) Evidence for Microbial Carbon and Sulfur Cycling in Deeply Buried Ridge Flank Basalt. Science, 339, 1305-1308 (PDF)

36. Edwards, K.J., Glazer, B.T., Rouxel, O.J., Bach, W., Emerson, D., Davis, R.E., Toner, B.M., Chan, C.S., Tebo, B.M., Staudigel, H., Moyer, C.L. (2011) Ultra-diffuse Hydrothermal Venting and Biogenic Fe-Mn Umber Deposition at 5000m off Hawai`i. The ISME Journal, 5, 1748-1758, doi:10.1038/ismej.2011.48 (PDF)

15. Rouxel O., Ono S., Alt J., Rumble D. & J. Ludden (2008) Sulfur Isotope Evidence for Microbial Sulfate Reduction in Altered Oceanic Basalts at ODP Site 801. Earth Planet. Sci. Lett., 268, 110-123 (PDF)

 

(4) Development and application novel geochemical proxies for Paleooceanography

Rationale: A host of available geochemical and geological evidences are now indicating that changes in marine metal reservoirs were caused not only by geological events and the long-term cooling of the Earth’s mantle, but also by the rise and fall of specific microbial metabolisms over billions of years. This paradigm shift has profound implications for understanding biological and Earth system co-evolution, particularly with respect to ocean redox, carbon cycling, and the regulation of climate over geological time scales. Perhaps the time period in the Earth’s history that witnessed the most dramatic changes in ocean metal reservoirs is the Proterozoic (2500 to 543 million years ago), when the atmosphere and ocean first became oxygenated. Under these conditions, biologically important trace metals (nutrients) would have been scarce in most marine environments because many are characterized by low solubility in low-oxygen environments while others are highly reactive toward dissolved sulfide and iron sulfide minerals. Likewise, dramatic shifts in the availability of key metal micronutrients in seawater, such as Fe, Mo, Cu, Ni and Zn, likely shaped the evolution of life in marine environments.

Approach: Over the last 10 years, we developed and applied several isotope systems beyond Fe and S isotopes, including transition metals such as Cu, Zn, Cd, Ni, Mo, and metalloids such as Ge, Se, and Sb with the major aim of understanding modern and ancient ocean processes. We investigated how global metal and nutrient oceanic cycles (e.g. P, Cr, Ni, Mo, Fe, U) may have changed through the Precambrian and how such changes may be linked to the evolution of life and the redox state of the atmosphere/ocean system and vice-versa. For example, based on Fe isotope composition of sedimentary sulfides, we identified direct link between the rise of atmospheric oxygen by 2.3 Ga ago and changes in Fe ocean cycle that provide new insights into the past ocean redox state.

Despite those recent advances, our understanding of ancient metal biogeochemical cycling is limited – if not biased – by the available rock archive and our ability to decipher robust oceanographic information from the geochemical composition of ancient sedimentary rocks. Hence, a critical aspect of our on-going research involves to better “calibrate” these proxies by investigating modern analogues of ancient seafloor metalliferous deposits and organic matter-rich shales. This approach allows a particular focus on diagenetically induced metal isotope exchange and related biogeochemical and physical processes in sediments with a nonsteady state redox history.

Representative publications:

79. Gueguen B., Rouxel O., Rouget M.-L., Bassoullet C., Ponzevera E., Germain Y., and Fouquet Y. Comparative geochemistry of four ferromanganese crusts from the Pacific Ocean and significance for the use of Ni isotopes as paleoceanographic tracers. Geochimica et Cosmochimica Acta. 189: 214-235.

64. Planavsky N.J., Asael, D., Hofmann, A., Reinhard, C.T., Lalonde, S.V., Wang, X., Knudsen, A., Ossa, F., Bekker, A., Lyons, T.W., and Rouxel, O.J. (2014) Evidence for Oxygenic Photosynthesis Half a Billion Years Before the Great Oxidation Event. Nature Geosciences DOI:10.1038/ngeo212 (PDF)

57. Canfield ,D.E., Ngombi Pemba L., Hammarlund E., Bengtson S., Chaussidon M., Gauthier-Lafaye F., Meunier A., Riboulleau A., Rollion Bard C., Rouxel O., Asael D., Pierson-Wickmann A.C. & El Albani A. (2013), Oxygen dynamics in the aftermath of the Great Oxidation of the Earth’s atmosphere. PNAS, doi: 10.1073/pnas.1315570110 (PDF)

55. Asael, D., Tissot, F., Reinhard, C., Rouxel, O., Dauphas, N., Lyons, T., Ponzevera, E., Liorzou, C., Cheron, S (2013) Coupled Molybdenum, Iron and Uranium stable isotopes as oceanic paleoredox proxies during the Paleoproterozoic Shunga event. Chemical Geology, 362, 193-210 (PDF)

46. Planavsky N.J., Rouxel O., Bekker A., Hofmann A., Little C., Lyons T.W. (2012) Iron isotope composition of some Archean and Paleoproterozoic iron formations. Geochimica et Cosmochimica Acta, 80, 158–169 (PDF)

40. Konhauser K.O., Lalonde S.V., Planavsky N., Pecoits E., Lyons T.W., Mojzsis S.J., Rouxel O.J., Barley M.E., Rosìere C., Bekker A. (2011). Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event. Nature, 478, 369-73 (PDF)

32. Planavsky N.J., Rouxel O., Bekker A., Reinhard C.T., Lalonde S., Konhauser K., Lyons T.W., (2010) The evolution of the marine phosphate reservoir. Nature, 467, 1088-1090 (PDF)

6. Rouxel O., Bekker A. and Edwards K. (2005) Iron isotope constraints on the Archean and Paleo-Proterozoic Ocean Redox State. Science, 307, 1088-1091 (PDF)

 

(5) Seafloor mineral deposits: formation, evolution and societal relevance

Rationale: While ferromanganese crusts and nodules have long been characterized for their concentrations in nickel, cobalt, and copper due to their very slow growth rates (e.g. few mm/Ma), the sources of metals and their enrichment patterns remain strikingly unclear. Past studies have also demonstrated the complexity and diversity of seafloor hydrothermal systems and have highlighted the importance of subsurface environments where a variety of chemical reactions between seawater, rocks and hydrothermal deposits is taking place over a wide range of temperature. However, despite these initial investigations, we have little understanding of the relative role of geodynamic context, source rock composition and subsurface fluid-rock-seawater interactions in producing specific enrichment of base and precious metals in sulfide deposits. The new application of metal and metalloid isotope geochemistry is expected to provide unique constraints on the source of metals in marine ferromanganese deposits and the importance of subsurface sulfide precipitation/remobilization in seafloor hydrothermal systems.

Approach: The roots of this research theme go back more than 10 years ago as part of Rouxel's PhD research topic. He performed the initial survey of Fe, Zn, Cu, Ge, Sb and Se isotope composition of hydrothermal sulfides (active and inactive deposits) and associated fluids, together with S isotopes to determine key parameters that control seafloor mineralization along mid-ocean ridges (e.g. EPR 9-10ºN, MAR) and back-arc hydrothermal systems (e.g. Manus basin). These initial studies opened the way for new approaches in the study of metal cycling in seafloor hydrothermal systems. For example, Cu isotopes are sensitive to hydrothermal diagenesis and sulfide alteration, Se isotopes may reflect interactions between the hydrothermal fluid and previously altered substratum, whereas Zn and Fe isotopes may trace, respectively, metal remobilization in subsurface environments and sulfide precipitation pathways.

Humanity is in desperate need of discovering new natural resources due to global population growth and strong economic demand of emerging countries. Hence, due to their high economic potential, deep-sea metallifereous deposits, such as seafloor massive sulfides, polymetallic nodules and ferromanganese crusts have attracted considerable interest from commercial mining companies and research institutions worldwide. Only in the past few decades has it been recognized that sulfide deposits are variably enriched in base and precious metals such as copper, silver and gold and other “strategic” metals that have increasing interest for high-level technologies. Hence, scientific challenges in this field of research are not anymore restricted to the academic study of metal sources and formation processes of seafloor mineral deposits, but also to their economic potential and sustainable exploitation, both of which are now becoming a central interest for Pacific Islands Countries. In particular, addressing the problem related to the environmental impact of deep-sea mineral resources exploitation is particularly timely and relevant since a growing number of countries and national or private organizations are claiming large areas of the seafloor for mineral exploration, both in EEZ or international waters. In this context, I'm convinced that the field of metal isotope biogeochemistry will have an increasing role in future impact studies, such as those related to (i) evaluation of baseline environmental conditions; (ii) development of protocols for environmental data collection during mining and restoration; (iii) evaluation and discrimination of far-field dispersal of metals due to natural vs. mining-induced processes and their transfer into the ecosystems and potentially food-web.

Representative publications:

80. Dekov, V.M., Rouxel, O., Kouzmanov, K., Bindi, L., Asael, D., Fouquet, Y., Etoubleau, J., Burgaud, G. and Wälle, M. (2016) Enargite-luzonite hydrothermal vents in Manus Back-Arc Basin: submarine analogues of high-sulfidation epithermal mineralization. Chem. Geol. 438, 36-57

79. Gueguen B., Rouxel O., Rouget M.-L., Bassoullet C., Ponzevera E., Germain Y., and Fouquet Y. Comparative geochemistry of four ferromanganese crusts from the Pacific Ocean and significance for the use of Ni isotopes as paleoceanographic tracers. Geochimica et Cosmochimica Acta. 189: 214-235.

78. Toner B.M., Rouxel O., Santelli C.M., Bach W., Edwards K.J., Iron oxidation pathways and redox micro-environments in seafloor sulfide-mineral deposits: spatially resolved Fe XAS and d57/54Fe observations. Frontiers in Microbiology, section Extreme Microbiology 7 10.3382/finicb.2016.00648 (PDF)

74. Marcus, M.A., Edwards, K.J., Gueguen, B., Fakra, S.C., Horn, G., Jelinski, N.A., Rouxel, O., Sorensen, J., Toner, B.M., 2015. Iron mineral structure, reactivity, and isotopic composition in a South Pacific Gyre ferromanganese nodule over 4 Ma. Geochimica Et Cosmochimica Acta 171, 61-79 (PDF)

17. Rouxel O., Shanks W.C., Bach W. and Edwards K. (2008) Integrated Fe and S isotope study of seafloor hydrothermal vents at East Pacific Rise 9-10°N. Chem. Geol., 252, 214-227 (PDF)

16. John S.G., Rouxel O.J., Craddock P.R., Engwall A.M., and Boyle E.A. (2008) Zinc isotope composition and fractionation in hydrothermal vent fluids and chimneys. Earth Planet. Sci. Lett., 269,17-28 (PDF)

5. Rouxel O., Fouquet Y. and Ludden J. (2004) Subsurface Processes at the Lucky Strike Hydrothermal Field, Mid-Atlantic Ridge: Evidence from Sulfur, Selenium and Iron Isotopes. Geochim. Cosmochim. Acta, 68, 2295-2311 (PDF)

4. Rouxel O., Fouquet Y. and Ludden J. (2004) Copper Isotope Systematics of the Lucky Strike, Rainbow and Logatchev Seafloor Hydrothermal Fields on the Mid Atlantic Ridge. Economic Geology, 99, 585-600 (PDF)