Variations in Sr and Nd Isotopic Ratios of Mineral Particles in Cryoconite in Western Greenland

Naoko Nagatsuka,Sota Tanaka,Rigen Shimada,Jun Uetake,Nozomu Takeuchi,Yukihiko Onuma,Takanori Nakano

doi:10.3389/feart.2016.00093

Abstract

In order to better understand the source of minerals on the dark-colored ice, located in the Greenland ice sheet ablation zone, we analyzed the Sr and Nd isotopic ratios of minerals in cryoconite, which were collected from glaciers in northwest and southwest Greenland. We focused on the following: (i) comparison of the isotopes of minerals in cyroconite with those in sediments from local and distant areas, (ii) regional variations in western Greenland, and (iii) spatial variations across an individual a glacier. The mineral components of the cryoconite showed variable Sr and Nd isotopic ratios (87Sr/86Sr: 0.711335 to 0.742406, eNd (0): -33.1 to -22.9), which corresponded to those of the englacial dust and moraine on and around the glaciers but were significantly different from those of the distant deserts that have been considered to be primary sources of mineral dust on the Greenland Ice Sheet. This suggests that the minerals within the cryoconites were mainly derived from local sediments, rather than from distant areas. The Sr ratios in the northwestern region were significantly higher than those in the southwestern region. This is probably due to geological differences in the source areas, such as the surrounding glaciers in each region. The isotopic ratios further varied spatially within a glacier (Qaanaaq and Kangerlussuaq areas), indicating that the silicate minerals on the glaciers were derived not from a single source but from multiple sources, such as englacial dust and wind-blown minerals from the moraine surrounding the glaciers.

Highlights

Cryoconite is a dark colored surface dust, which is deposited on the surface of glacier ice and is commonly found on glaciers worldwide
The isotopic ratios were distinct between the sites in northwest (Qaanaaq) and southwest Greenland (Kangerlussuaq area)
Because a change in mineral particle size from coarser to finer due to eolian sorting increases the amount of clay minerals (Honda et al, 2004; Kanayama et al, 2005), the low clay mineral content of the cryoconites indicates that the minerals on the glaciers in Greenland are mainly derived from local sources, rather than from distant areas

Summary

Introduction

Cryoconite is a dark colored surface dust, which is deposited on the surface of glacier ice and is commonly found on glaciers worldwide. It is a mixture and/or aggregate of insoluble particles, such as black carbon, mineral dust, and organic matter. Mineral dust is transported by wind from local or distant arid terrestrial surfaces (e.g., Bøggild et al, 2010). Organic matter is produced by microbes, such as cyanobacteria and green algae, living on glaciers (e.g., Takeuchi et al, 2014). Cryoconite can act to reduce the surface albedo of the ice because of its dark coloration and accelerates glacier melting in many parts of the world. The influence of cyroconites on ice melt has been reported in the ablation area of the northwestern and southwestern Greenland Ice Sheet and has been studied since the 1950s (e.g., Gajda, 1958; Gerdel and Drouset, 1960; Gribbon, 1979)

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