First Steps Towards a New High Resolution Proxy for Paleomagnetic Field Instabilities:  Tritiogenic 3He Archived in Speleothems

GSTDTAP

项目编号	1723105
	First Steps Towards a New High Resolution Proxy for Paleomagnetic Field Instabilities: Tritiogenic 3He Archived in Speleothems
	Andrea Balbas
主持机构	California Institute of Technology
项目开始年	2017
	2017-08-01
项目结束日期	2018-09-30
资助机构	US-NSF
项目类别	Standard Grant
项目经费	265000(USD)
国家	美国
语种	英语
英文摘要	The terrestrial magnetic field is critical to life on Earth because it shields the planet from harmful cosmic radiation. It also protects technological infrastructure from damage from solar storms. The Earth's magnetic field is generated by organized motions of liquid iron in the Earth's outer core - the geodynamo - by processes that are not fully understood. Key observable features that can be used to inform models of the geodynamo include magnetic field strength and orientation, both of which change over geologic time. Existing records of the temporal variation of magnetic field strength are incomplete and potentially compromised by secondary processes. A potential new archive originates from the protective nature of the field. Cosmic rays continually strike the atmosphere, but their flux is modulated by the magnetic field. Stronger magnetic fields provide greater deflection of cosmic rays from Earth, and weaker fields provide less deflection. The flux of cosmic rays in turn controls the production rate of cosmogenic nuclides - rare isotopes produced when cosmic rays collide with gases in the atmosphere. Several cosmogenic nuclide archives have already been used to establish the history of magnetic field strength. This project will take exploratory steps towards development of a new high temporal resolution archive: the abundance of tritium - a cosmogenic nuclide with a half-life of 12 years - in fluid inclusions trapped in cave deposits of various ages. As an isotope of hydrogen, the post-production behavior of tritium will be controlled by the atmospheric water cycle, and will not suffer the same complications as previously measured cosmogenic nuclide archives of magnetic field strength. The decay product of tritium (3He) will be measured in water pockets that are trapped by stalagmites as they grow. The working hypothesis is that these water inclusions take in tritium in proportion to its production by cosmic rays in the atmosphere via the delivery of local rainwater into the cave system. After entrapment, tritium decays to 3He, which is thereafter retained in the water inclusion until analysis. Stalagmites often have annual bands, and can be dated accurately using U/Th geochronology. Taken together, these ideas suggest stalagmites offer a potential record of geomagnetic field changes with better temporal detail than those provided by alternative methods and with independent sources of potential error. This project will take the first steps for creating such a record by developing the necessary analytical methods and then by analyzing stalagmites that captured atmospheric tritium created by nuclear weapons testing in the latter half of the 20th century to confirm that the putative archive works as expected. Because the atmospheric tritium production from bomb testing is well-documented, the archive will be subjected to a very rigorous test. The two products sought by the end of the proposed work are a) a methodology that can successfully liberate and measure the tritiogenic 3He in speleothems, and b) a confirmation or refutation of the key hypothesis of this proposal: Tritiogenic 3He preserved in water inclusions in speleothems can be measured and will change as a function of the concentration of tritium in local precipitation.
文献类型	项目
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/71416
专题	环境与发展全球科技态势
推荐引用方式 GB/T 7714	Andrea Balbas.First Steps Towards a New High Resolution Proxy for Paleomagnetic Field Instabilities: Tritiogenic 3He Archived in Speleothems.2017.