Electron microscope picture of an interplanetary dust particle. The size of the particles carrying the extraterrestrial 3 He that are accumulated in marine sediments typically is between 3 and 35 μ m (e.g., Farley et al. 1997) [photo- graph courtesy of Scott Messenger, ]. 

Context in source publication

Context 1

... values of 1.3±0.5 × 10 6 and 3.1±1.2 × 10 6 atoms cm -2 s -1 , respectively. These values are comparable to the fluxes estimated for the continental crust (2.7±1 × 10 6 atoms cm -2 s -1 normalized to the continental area; e.g., Torgersen 1989). This shows that on a local/regional basis there are significant radiogenic 4 He fluxes from the ocean sediment into the water column. Such features are not unexpected because at certain locations the seafloor is closer to the composition of continental crust than oceanic crust. However, averaged over the entire ocean basins, the flux of crustal 4 He is small compared to that from the continental crust. Well et al. (2001) evaluated high-quality data from the Pacific Ocean that became available during the World Ocean Circulation Experiment (WOCE). Separating the crustal 4 He component from the mantle component, they revealed widespread occurrence of a crustal radiogenic 4 He flux out of deep-sea sediments and the oceanic crust into the water column (Fig. 9). Although the flux of 4 He is small in these regions, Well et al. (2001) were able to estimate a flux of roughly 1±0.4 × 10 5 atoms cm -2 s -1 . Further evaluation of the WOCE helium isotope and neon data will hopefully improve our knowledge on the distribution of helium sources and their relative flux values in the global ocean. After the discovery of mantle-derived helium in the oceans (Clarke et al. 1969), the first estimate of the 3 He flux yielded a value of about 2 atoms cm -2 s -1 (Clarke et al. 1969). This estimate was based on simple advection/diffusion or straight advection models that utilize the mean concentration of excess 3 He in the deep waters of the ocean ( ∆ 3 He) and mean residence time of the deep waters. The flux of this mantle-derived helium is concentrated in areas of active mid-ocean ridge spreading or ocean islands (e.g., Craig and Lupton 1981). Later estimates by Craig et al. (1975) were based on a larger, although still sparse, data set. In their new assessment of the 3 He flux from the ocean, these authors use two methods: one based on the average mean upwelling rate and the mean 3 He excess in the deep ocean, the other based on the 4 He flux and the 3 He/ 4 He ratio of the excess helium calculated from profiles located on the East Pacific Rise. They obtain values of 3.3 atoms cm -2 s -1 and 4.8 atoms cm -2 s -1 , respectively (average value given by Craig et al. 1975: 4±1 atoms cm -2 s -1 . Since these estimates of 3 He fluxes in the mid-1970s, little work has been done that would have resulted in significant refinements of these fluxes. Simulations of the mantle helium distribution using a global general ocean circulation model (Farley et al. 1995) used a simple parameterization of the 3 He flux from mid-ocean ridges and determined the distribution of the He emanated from these sources throughout the world ocean. These simulations showed that with the specific source parameterization used in their study, Farley et al. (1995) could reproduce the global oceanic balance of 3 He reasonably well. However, at the same time the simulations clearly revealed that there is still much work to be done to understand the details of the simulated fields. Comparison with high-density surveys of 3 He in the ocean that are now becoming available (e.g., Lupton 1998, Rüth et al 2000) will enable us to reach a deeper level of understanding of the 3 He patterns observed in the ocean. Such progress should make new efforts to refine the estimate of the oceanic 3 He flux a worthwhile exercise. Four decades ago, He of apparent extraterrestrial origin was identified in ocean sediments. Merrihue (1964) observed 3 He/ 4 He ratios in marine sediments roughly 2 orders of magnitude higher than those observed in atmospheric helium and attributed it to the presence of cosmic material. Systematic studies of noble gases in marine sediments were started in the early 1980s by Ozima et al. (1984). Shortly thereafter, Takayanagi and Ozima (1987) recognized the potential of 3 He as a proxy of sediment accumulation rates. Most studies of noble gases in marine sediments (including this chapter) focus on helium isotopes, however, neon (Nier and Schlutter 1990, 1993) and argon isotopes (Tilles 1966, 1967; Amari and Ozima 1988) have also been studied. In the early 1990s, extraterrestrial 3 He in ocean sediments received considerable attention when Anderson (1993) questioned the understanding of mantle geochemistry and proposed that the mantle 3 He might not represent volatiles trapped in the earth in early earth history but rather is derived from subducted interplanetary dust particles (IDPs). However, this hypothesis could be refuted based on mass flux considerations (Allègre et al. 1993) and experimental evidence showing that helium would not be retained during subduction due to sufficiently high diffusion coefficients (Hiyagon 1994). Extraterrestrial He is delivered to the earth surface by interplanetary dust particles (IDP, Fig. 10). IDPs are derived from asteroid collisions as well as cometary debris (Dohnanyi 1976) and are thought to acquire their characteristic helium signature from implantation of solar wind and solar flare gases (e.g., Nier and Schlutter 1990). Approximately 40,000 tons of IDPs are deposited annually on the earth’s surface (Love and Brownlee 1993). However, the major fraction of the IDPs is heated to temperatures >800°C during entry into the earth’s atmosphere and looses its helium signal (Farley et al. 1997). Only IDPs with diameters ≤ 35 microns, corresponding to 0.5% of the total IDP mass flux, transit the atmosphere at temperatures of 500-800°C or below (Fraundorf et al. 1982) and retain their extraterrestrial helium signature. After being removed from the troposphere mainly by wet deposition and rapid settling through the oceanic water column, the IDPs continuously accumulate in marine sediments (e.g., Takayanagi and Ozima 1987) or on ice shields (Brook et al. 2000). The helium isotope characteristics of IDPs are well constrained from analysis of individual particles collected from the stratosphere. They have a 3 He/ 4 He ratio of 2.4 × 10 -4 , as well as a fairly constant 3 He concentration of 1.9 × 10 -5 cm 3 STP g -1 (Nier and Schlutter 1992). As the cosmic dust is enriched in 3 He by ~8 orders of magnitude compared to terrigenous matter it can be readily detected. However, in spite of various noble gas studies of single IDP grains, both from the stratosphere and ocean sediments (e.g., Fukumoto et al. 1986; Nier et al. 1990; Nier and Schlutter 1992) the carrier phase that actually hosts the extraterrestrial helium has not been unambiguously identified. Amari and Ozima (1985) found that the helium resides mainly in the magnetic fraction of sediments and therefore identified magnetite to be the main carrier of the extraterrestrial 3 He signal. The magnetite is thought to be produced by heating of the IDPs during atmospheric entry (Amari and Ozima 1985). In a subsequent study, Fukumoto et al. (1986) found significant helium contributions from a non-magnetic fraction and suggested extraterrestrial silicates to be another likely host mineral. The presence of a non-magnetic carrier phase was later confirmed by Patterson et al. (1998) and Farley (2001). Future work is needed to identify the relative importance of different carrier phases and whether they are associated to magnetic and/or non-magnetic sediment fractions. The extraterrestrial helium signal is extremely well preserved in marine and terrestrial sedimentary archives over geological time scales. Various studies have shown that the extraterrestrial helium carried to the seafloor by IDPs can be retained for periods of at least 65 million years (Ma) against diffusive and/or diagenetic loss (Farley 1995; Farley et al. 1998). Recently, Patterson et al. (1998) identified extraterrestrial 3 He in 480- Ma old sedimentary rocks. Helium contained in ocean sediments can be interpreted as a mixture of helium from two sources, extraterrestrial and terrigeneous helium (Takayanagi and Ozima 1987; Marcantonio et al. 1995). Contributions from an atmospheric helium component have been shown to be negligible (e.g., Farley and Patterson 1995). Assuming that the isotopic compositions of both mixing end-members are known, one can easily calculate the amount of extraterrestrial 3 He, using the following ...