Chapter 7 : Extraterrestrial He in sediments : From recorder of asteroid collisions to timekeeper of global environmental changes

by D. McGee and S. Mukhopadhyay

Most 3He in deep-sea sediments is derived from fine-grained extraterrestrial matter known as interplanetary dust particles (IDPs). These particles, typically <50 μm in diameter, are sufficiently small to retain solar wind-implanted He with high 3He/4He ratios during atmospheric entry heating. This extraterrestrial 3He (3HeET) is retained in sediments for geologically long durations, having been detected in sedimentary rocks as old as 480 Ma. As a tracer of finegrained extraterrestrial material, 3HeET offers unique insights into solar system events associated with increased IDP fluxes, including asteroid break-up events and comet showers. Studies have used 3HeET to identify IDP flux changes associated with a Miocene asteroid break-up event and a likely comet shower in the Eocene. During much of the Cenozoic, 3HeET fluxes have remained relatively constant over million-year timescales, enabling 3HeET to be used as a constant flux proxy for calculating sedimentary mass accumulation rates and constraining sedimentary age models. We review studies employing 3HeET-based accumulation rates to estimate the duration of carbonate dissolution events associated with the K/Pg boundary and Paleocene-Eocene Thermal Maximum. Additionally, 3HeET has been used to quantify sub-orbital variability in fluxes of paleoproductivity proxies and windblown dust. In order to better interpret existing records and guide the application of 3HeET in novel settings, future work requires constraining the carrier phase(s) of 3HeET responsible for long-term retention in sediments, better characterizing the He 2 isotopic composition of the terrigenous end-member, and understanding why observed extraterrestrial 3He fluxes do not match the predicted variability of IDP accretion rate over orbital timescales.

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Chapter 7

11 juin 2012
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1.7 Mo