Do metalliferous sediments record mid-ocean ridge hydrothermal activity? Constraining the roles of iron oxidation rate and 230Th scavenging

含金属沉积物是否记录了洋中脊热液活动？洋中脊的高温喷口是海洋内部热量和溶解金属的重要来源。当喷出的热液与海底水混合时，化学反应产生了富含金属的颗粒。这些颗粒被温暖的（浮力的）热液柱从喷口带上来，扩散到周围的海水中。烟羽带着颗粒离开喷口，颗粒沿着脊轴和脊翼落到海底。因此，洋中脊被富含金属的沉积物覆盖，这反映了热液活动、颗粒形成、羽流运输和沉积过程的综合影响。最近的研究表明，山脊沉积物的金属通量在冰期-间冰期时间尺度上是不同的。这表明，与气候有关的全球海平面变化可能会调节海底热液活动。然而，金属通量记录可能会受到其他过程的影响。这种偏见会使了解气候对海底热液活动影响的努力复杂化。该项目将评估：1)铁氧化速率对大洋中脊沉积物金属通量的影响，以及2)热液颗粒从海水中清除钍（Th）是否排除了在海脊环境中使用放射性同位素230Th作为恒定通量代理。这项提议的工作可能会改变人们对气候和海平面如何影响洋中脊热液输出的理解。该项目与教育相关的更广泛影响包括培训和指导一名博士生和一名本科生研究员。东南东太平洋隆起、大西洋中脊和Juan de Fuca脊的脊脊-脊脊金属通量记录表明，末次消冰期（~10-20 kyr BP）热液沉积增加。此外，SEPR的结果还表明，2330s对脊顶沉积物的消冰通量比水柱生成速率高2-3倍。虽然海平面驱动的热液活动变化是对结果的最简单解释，但还原铁氧化速率的变化可能调节了金属通量信号。拟议的研究将使用SEPR的金属通量记录的横断面来测试氧化速率假设。如果增强的Fe（II）氧化引起了山脊轴附近较高的通量，那么在离轴位置应该发现相应的缺陷。另一方面，更大的热液输出应该在脊顶和侧翼产生更大的通量。该研究还将限制氧化速率的变化，使用底栖有孔虫B/Ca作为深太平洋碳酸盐离子浓度的代理。B/Ca结果将填补现有记录的一个关键空白，并提高我们对冰川消融期间碳循环的理解。最后，本研究将利用3he归一化的230Th通量来评估230Th是否可以用于研究古海洋学研究的关键区域洋中脊附近的沉积物通量。这项研究将为评估海平面如何影响洋中脊的热液输出奠定必要的基础。该项目将支持一名博士生和一名本科生研究员的培训，包括非正式和正式指导，以发展教学技能、专业网络和拨款申请写作技能。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.