Size fractionation, reproducibility and provenance of helium isotopes in north-equatorial pacific pelagic clays

Abstract

Helium isotope compositions were measured in late Quaternary pelagic clays from the North Equatorial Pacific, where terrestrial and extra-terrestrial end member components are identified and quantified in different grain size populations. The results provide important constraints on the nature of interplanetary dust particles (IDPs) in sedimentary environments and on their application as constant flux proxies in paleoceanographic studies.Approximately 92–95% of the extra-terrestrial 3He (3HeET) is concentrated in particles smaller than 20μm. Within this grain size group, ∼44% of 3HeET is concentrated in the 20-10 and 10-2μm fractions, while more than 50% of the total 3HeET is found in fine particles smaller than 2μm. The peak concentration of 3HeET in the very fine particle size population explains a consistently better reproducibility of 3HeET in sediments compared to model predictions that is related to the relatively homogenous distribution of clay-size particles in sediments compared to coarser particles, especially in deep-sea sediments and ice cores. The 3He/4He ratios are highest in the coarser than 20μm fractions, peaking between 63 and 38μm (1.78±1.13×10−4), compared to the bulk and the <20μm fractions (3.20±2.62×10−5 and 2.11±0.88×10−5 respectively). The isotopic composition of helium in each of the size fractions reflects a mixture between a pure extraterrestrial end member and a terrigenous component, which is itself comprised of varying mixtures of volcanic material and eolian material associated with the fine grain size fraction of loess deposits in South East Asia.Poor 3HeET reproducibility observed here and elsewhere, inhibiting its application as a constant flux proxy, stems from the presence of a small number of aliquots displaying significantly higher 3HeET contents compared to the average of their reference group. These Helium Rich Particles (HRPs), are probably characterized by a <20μm diameter and carry significant amounts of extraterrestrial 3He and 4He. Careful rejection of such aliquots improves the signal reproducibility by more than two fold.