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？

• 批准号: 2030343
• 负责人: David Lund
• 金额: $ 44.28万
• 依托单位: University of Connecticut
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2030343&HistoricalAwards=false
• 关键词: Do; metalliferous; sediments; record; mid

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.

含金属沉积物是否记录了大洋中脊的热液活动？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海底混合时，化学反应会产生富含金属的颗粒。这些颗粒被温暖的(浮力)热液羽流从喷口带到周围的海水中。羽流将颗粒带离喷口，颗粒沿着山脊轴线和山脊侧翼落到海底。因此，大洋中脊被富含金属的沉积物覆盖，反映了热液活动、颗粒形成、羽流输送和沉积过程的综合影响。最近的研究表明，到山脊沉积物的金属通量在冰期-间冰期的时间尺度上是不同的。这表明，全球海平面与气候有关的变化可能会调节海底的热液活动。然而，金属通量记录可能会受到其他工艺的影响。这种偏见将使了解气候对海底热液活动的影响的努力复杂化。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)热液颗粒对海水中钍(Th)的清除是否排除了在海脊环境中使用放射性同位素230作为恒定通量替代品的可能性。拟议的工作可能会改变人们对气候和海平面如何影响大洋中脊的热液产出的理解。该项目与教育相关的更广泛的影响包括对一名博士生和一名本科生的培训和指导。来自东南东太平洋隆起(SEPR)、中大西洋海脊和胡安德富卡海脊的海脊-峰顶金属通量记录表明，在最后一次冰川消融期间(约10-20 KYR BP)，热液沉积增加。此外，9月的结果还表明，230°至山脊沉积物的去冰川通量是水柱产生率的2-3倍。尽管海平面驱动的热液活动变化是对这一结果的最简单解释，但还原铁的氧化速率的变化可能已经调制了金属熔剂信号。这项拟议的研究将使用9月以来金属通量记录的横断面来检验氧化速率假说。如果增强的Fe(II)氧化导致脊轴附近有更高的通量，那么在离轴处应该发现相应的缺陷。另一方面，更大的热液输出应该会在脊顶和侧翼产生更大的通量。这项研究还将限制氧化速率的变化，使用底栖有孔虫B/Ca作为反映太平洋深层碳酸盐离子浓度的指标。B/Ca的结果将填补现有记录中的一个关键空白，并提高我们对冰川消融期间碳循环的理解。最后，这项研究将使用3He归一化的230th通量来评估230th是否可以用于研究大洋中脊附近的沉积物通量，这是古海洋学研究的关键区域。这项研究将为评估海平面如何影响大洋中脊的热液产出奠定必要的基础。该项目将支持一名博士生和一名本科生研究人员的培训，包括发展教学技能、专业网络和拨款提案撰写技能的非正式和正式指导。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。