Do metalliferous sediments record mid-ocean ridge hydrothermal activity?
High-temperature vents at mid-ocean ridges are an important source of heat and dissolved metals to the ocean interior. As discharging hydrothermal vent fluids mix with bottom waters, chemical reactions create metal-rich particles. These particles are carried up from the vents by warm (buoyant) hydrothermal plumes that spread into the surrounding seawater. The plumes carry the particles away from the vents, and the particles fall to the seafloor along ridge axes and ridge flanks. As a result, mid-ocean ridges are blanketed with metal-rich sediments, which reflect the combined influence of hydrothermal activity, particle formation, plume transport, and sedimentary processes. Recent studies show that metal fluxes to ridge crest sediments vary on glacial-interglacial timescales. This suggests that climate-linked changes in global sea level may modulate seafloor hydrothermal activity. However, the metal flux records may be biased by other processes. Such biases would complicate efforts to understand the impact of climate on seafloor hydrothermal activity. The project would evaluate: 1) the influence of iron oxidation rate on metal fluxes to mid-ocean ridge sediments, and 2) whether scavenging of thorium (Th) from seawater by hydrothermal particles precludes the use of the radioisotope 230Th as a constant flux proxy in ridge settings. The proposed work could transform understanding of how climate and sea level influence hydrothermal output from mid-ocean ridges. The education-related broader impacts of the project include the training and mentoring of a Ph.D. student and an undergraduate researcher.
Ridge-crest metal flux records from the Southern East Pacific Rise (SEPR), Mid-Atlantic Ridge, and Juan de Fuca Ridge indicate that hydrothermal sedimentation increased during the last deglaciation (~10-20 kyr BP). In addition, results from the SEPR also show that deglacial fluxes of 230Th to ridge crest sediments were 2-3x higher than the water column production rate. Although sea level-driven changes in hydrothermal activity are the simplest explanation of the results, variations in the oxidation rate of reduced iron may have modulated the metal flux signal. The proposed study will test the oxidation rate hypothesis using transects of metal flux records from the SEPR. If enhanced Fe(II) oxidation caused higher fluxes near the ridge axis, then corresponding deficits should be found at off-axis locations. Greater hydrothermal output, on the other hand, should yield greater fluxes on both the ridge crest and flanks. The study will also constrain oxidation rate changes using benthic foraminiferal B/Ca as a proxy for deep Pacific carbonate ion concentration. The B/Ca results will fill a key gap in existing records and improve our understanding of carbon cycling during the deglaciation. Finally, the study will use 3He-normalized 230Th fluxes to assess whether 230Th can be used to study sediment fluxes near mid-ocean ridges, a key region for paleoceanographic studies. The study will lay the necessary groundwork for evaluating how sea level influences hydrothermal output from mid-ocean ridges. The project will support the training of a Ph.D. student and an undergraduate researcher, including informal and formal guidance to develop teaching skills, professional networking, and grant proposal writing skills.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
