Feedbacks between oceanic hydrothermal systems and the Earth system

海洋热液系统与地球系统之间的反馈

• 批准号: RGPIN-2019-04147
• 负责人: Coogan, Laurence
• 金额: $ 2.62万
• 依托单位: University of Victoria
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=686635
• 关键词: Feedbacks; between; oceanic; hydrothermal; systems

The crust beneath the oceans forms along mid-ocean ridges, migrating slowly away from these ridges to eventually be subducted into the mantle. Seawater circulates through the oceanic crust at high-temperatures (>350C) near the ridge axis forming “black-smokers” and potentially economic sulphide deposits with unique chemosynthetic ecosystems. Seawater also circulates through the crust at low temperatures (<50C) over much of the seafloor in “off-axis” hydrothermal systems. Reactions between the crust and circulating seawater in both on- and off-axis hydrothermal systems lead to substantial changes in the composition of the crust and the fluid. Return of the modified fluids to the ocean has a profound effect on the chemistry of seawater, and alteration of the crust is significant for many solid earth geochemical cycles (e.g. adding volatiles to the mantle wedge and inducing mantle heterogeneity). My proposed research program builds on our recent work on chemical fluxes carried by on- and off-axis hydrothermal systems, aiming to determine the links and feedbacks between hydrothermal fluxes and the environment (e.g. bottom seawater temperature and composition).******To better understand the environmental controls on element fluxes between the crust and ocean associated with on-axis high-temperature hydrothermal systems we will use two approaches. First, we will use seagoing studies to investigate how high-temperature hydrothermal fluids interact with the overlying water column in so-called hydrothermal plumes. These are formed by extreme dilution (~10,000:1) of black smoker fluid and form ~200 m above the seafloor along mid-ocean ridges. Second, we will use the rock record of high-temperature fluid-rock reaction in the crust to test models that suggest that at times in the past, when the major ion chemistry (e.g. Ca, Mg, sulphate) of seawater was different, the composition of black smoker fluids was also different.******To better understand the feedbacks between low-temperature, off-axis, hydrothermal systems and environmental conditions we will use a three-pronged approach. First, we will undertake laboratory experiments to determine the dissolution rate of basalts in seawater at different temperatures and using different starting materials. Second, we will undertake field studies to determine if laboratory dissolution rates differ from natural dissolutions rates due to the very different timescales involved. Finally, we will study ancient ocean crust to determine whether there is a geological record of changing crustal alteration with changing environmental conditions.******Finally, all investigations of environmental controls on hydrothermal fluxes require that we understand the past temperature and composition of the deep ocean. We will use both rigorous data compilation approaches, and analysis of the composition of upper oceanic crust that was altered at different times during the Phanerozoic, to improve constraints on paleo-seawater compositions.**

海洋下方的地壳沿着洋中脊形成，慢慢地远离这些洋脊，最终俯冲到地幔中。海水在洋脊轴附近以高温（>350C）循环穿过洋壳，形成“黑烟囱”和具有独特化学合成生态系统的潜在经济硫化物矿床。海水还在“离轴”热液系统中的大部分海底以低温（<50℃）穿过地壳循环。在轴上和离轴热液系统中地壳和循环海水之间的反应导致地壳和流体的成分发生显着变化。改变后的流体返回海洋对海水的化学性质产生深远的影响，并且地壳的改变对于许多固体地球化学循环具有重要意义（例如，向地幔楔添加挥发物并诱发地幔异质性）。我提出的研究计划建立在我们最近对轴上和离轴热液系统携带的化学通量的研究基础上，旨在确定热液通量与环境之间的联系和反馈（例如底部海水温度和成分）。********为了更好地了解与轴上高温热液系统相关的地壳和海洋之间元素通量的环境控制，我们将使用两种方法。首先，我们将利用航海研究来研究高温热液流体如何与所谓热液羽流中的上覆水柱相互作用。它们是由黑烟熏液极度稀释（~10,000:1）形成的，沿着洋中脊在海底上方~200米处形成。其次，我们将利用地壳中高温流体-岩石反应的岩石记录来测试模型，这些模型表明，在过去，当海水的主要离子化学成分（例如钙、镁、硫酸盐）不同时，黑烟流体的成分也不同。******为了更好地理解低温、离轴、热液系统和环境条件之间的反馈，我们将使用三管齐下的方法。首先，我们将进行实验室实验，以确定不同温度和使用不同起始材料的玄武岩在海水中的溶解速率。其次，我们将进行实地研究，以确定实验室溶出率是否由于所涉及的时间尺度差异很大而与自然溶出率不同。最后，我们将研究古代海洋地壳，以确定是否存在随着环境条件变化而变化的地壳蚀变的地质记录。******最后，所有对热液通量的环境控制的研究都要求我们了解深海过去的温度和成分。我们将使用严格的数据汇编方法，并对显生宙不同时期变化的上洋地壳的成分进行分析，以改善对古海水成分的限制。 **