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

含金属沉积物记录了洋中脊热液活动吗？大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。当排出的热液喷口流体与底层沃茨混合时，化学反应产生了富含金属的颗粒。这些微粒被热液羽流从火山口带出，扩散到周围的海水中。羽流将微粒带离喷口，微粒沿沿着脊轴和脊侧落到海底。因此，大洋中脊覆盖着富含金属的沉积物，这反映了热液活动、颗粒形成、羽流迁移和沉积过程的综合影响。最近的研究表明，金属通量的山脊顶部沉积物变化的冰川间冰期的时间尺度。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，金属通量记录可能被其他过程偏置。这种偏见将使理解气候对海底热液活动影响的努力复杂化。该项目将评价：1）铁氧化速率对大洋中脊沉积物中金属通量的影响; 2）热液颗粒对海水中钍（Th）的清除是否排除了使用放射性同位素230 Th作为大洋中脊环境中的恒定通量代用指标。拟议的工作可以改变对气候和海平面如何影响大洋中脊热液输出的理解。该项目在教育方面的广泛影响包括培训和指导一名博士。东南太平洋海隆（SEPR）、大西洋中脊和胡安·德富卡海岭的脊顶金属通量记录表明，在末次冰消期（约10-20 kyr BP），热液沉积增加。此外，从SEPR的结果还表明，冰消通量的230 Th的山脊顶部沉积物是2- 3倍高于水柱生产率。虽然海平面驱动的热液活动的变化是最简单的解释的结果，在还原铁的氧化速率的变化可能调制的金属通量信号。拟议的研究将使用SEPR的金属通量记录的断面来检验氧化率假设。如果增强的Fe（II）氧化引起更高的通量附近的脊轴，那么相应的赤字应被发现在离轴位置。另一方面，更大的热液输出应在脊顶和侧翼产生更大的通量。这项研究还将利用底栖有孔虫B/Ca作为深太平洋碳酸盐离子浓度的代用指标，限制氧化率的变化。B/Ca结果将填补现有记录中的一个关键空白，并提高我们对冰消期间碳循环的了解。最后，本研究将使用3 He标准化的230 Th通量来评估230 Th是否可用于研究古海洋学研究的关键区域--大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大洋中脊的热液输出奠定必要的基础。该项目将支持培养一名博士。该奖项反映了NSF的法定使命，并通过使用基金会的智力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.