Supernova versus cosmic ray origin for exotic nuclides in geomaterials: A test using 3He with 60Fe in marine sediments

Abstract

We report 3He and 4He concentrations in 57 sediment samples from the southeast Indian Ocean where 60Fe excesses were previously identified in a subset of the same samples (Wallner et al., 2016). The coupled 60Fe-3He data allow further evaluation of two competing hypotheses: (1) a nearby supernova (SN) showered Earth with exotic radionuclides such as 60Fe during the last 3 million years, or (2) 60Fe in terrestrial archives was generated by reactions of galactic cosmic rays (GCRs) on micrometeorite grains that were irradiated for hundreds of millions of years in the interstellar medium, where 3He production by GCRs is larger than the solar wind 3He flux.Piston core ELT49-53 sediments show no correlation between 3He and 60Fe, and sedimentary 3He appears to be dominated by the presence of interplanetary dust particles (IDPs). Because 3He is not supplied in significant amounts by SN ejecta, the absence of a 3He-60Fe correlation provides additional, although circumstantial evidence for the supernova hypothesis. Large uncertainties in the relatively small number of sediment 60Fe measurements currently limit a firmer conclusion.The extraterrestrial 3He accumulation rate in ELT49-53 from 3.2 to 1.7 Ma was 0.88 ± 0.26 × 10−12 cm3 STP/g/kyr, similar to IDP 3He flux estimates from previous sedimentary and ice core records that span both shorter and longer time scales. 4He and 60Fe accumulation rates during this time interval were 0.11 ± 0.04 × 10−6 cm3 STP/cm2/kyr and 1.9 ± 0.5 × 104 atoms/cm2/kyr. Bulk sediment [4He] is strongly anti-correlated with sediment CaCO3 content, evidence for modulation of the terrigenous and cosmic sedimentary fractions primarily by changes in biogenic carbonate deposition. Although the dominant terrigenous source has not been uniquely identified at the Indian Ocean deposition site, it resembles eolian material from the continental interior of Australia, and shows a narrow range of 3He/4He (from 2 to 4 × 10−8, 0.015–0.030 RA) over the last ∼3 Myrs.