[M-IS12] Paleoclimatology and paleoceanography

[MIS12-13] The reactive phase of sediments as an archive for the magnesium isotope composition of seawater

*Adam David Sproson¹, Toshihiro Yoshimura¹, Shigeyuki Wakaki², Tsuyoshi Ishikawa³, Naohiko Ohkouchi¹ (1.Biogeochemistry Research Center, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan, 2.National Museum of Japanese History, 117 Jonai-cho, Sakura City, Chiba Prefecture 285-8502, Japan, 3.Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology 200 Monobe Otsu, Nankoku City, Kochi 783-8502, Japan)

Keywords:Magnesium isotopes, Cenozoic, Seawater chemistry, Ferromanganese oxides

Magnesium isotopes (δ²⁶Mg) are fractionated during mineral formation, whereby Mg-carbonates (e.g., calcite and dolomite) become depleted relative to the precipitating fluid (by ~1-4 ‰) and Mg-clays become enriched in the heavier ²⁶Mg [1-3]. Because Mg-carbonates and Mg-clays represent the two major sources and sinks of Mg to the ocean, Mg isotope records can be used to quantify the relative contribution of carbonate and silicate fluxes to seawater major ion chemistry and its link to the carbon cycle on geological timescales [4, 5]. The δ²⁶Mg composition of bulk foraminifera, bulk limestones, and corals provide an archive of the Mg isotope composition of seawater during the Cenozoic (last 65 million years) [4, 6]. However, the potential for the reactive phase (ferromanganese coatings) of sediments to record the δ²⁶Mg of seawater has not been explored, despite promising signs from Mg isotope studies of ferromanganese nodules [7, 8].

Here, we present the Mg isotope composition of the carbonate and reactive phase of sediments collected from the Atlantic, Indian, and Pacific Ocean covering the last 70 million years. The δ²⁶Mg of the carbonate phase is identical to Cenozoic corals [6] representing a ~0.9 ‰ fractionation from seawater. The δ²⁶Mg of the reactive phase displays limited fractionation from seawater and is identical to previous estimates from corrected carbonates [4, 6]. This study suggests the sequential leaching of the carbonate and reactive phase from marine sediments can provide a new archive for Mg isotopes in seawater that can be used to understand carbonate weathering, dolomite formation, Mg-silicate weathering, and Mg uptake in marine silicates over geological timescales.

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