The Nickel isotope composition of the authigenic sink and the diagenetic flux in modern oceans

We investigated Ni isotope composition in a stratigraphic sequence of pelagic clays collected during ODP core leg 185 on site 1149 in Western Pacific Ocean near the Izu-Bonin subduction trench in order to determine the Ni isotope composition of the authigenic Mn-oxides sink and evaluate the Ni isotope composition of the diagenetic input flux. This predominant oxic sink likely controls the Ni isotope budget in modern oceans. The sequence presented here is a 170 m-thick sequence of pelagic sediments deposited on some of the oldest oceanic crust of the seafloor, and the base was dated at 104 Ma. Nickel isotope values (δ60/58Ni relative to NIST SRM 986) vary in the range of 0.04 ± 0.04 to 1.03 ± 0.03‰. There is a trend between depth in the stratigraphic sequence and increasing Mn/Fe, Mn/Al, higher Ni concentration and heavier Ni isotope values. This trend is accounted for by authigenic Mn-oxyhdroxides precipitation in the sediment and scavenging of dissolved metals in the sediment porewaters. This indicates that authigenic oxide minerals in deep-sea pelagic clays are a relevant sink for Ni in modern oceans. Results enable us to determine the authigenic oxic output flux of 3.58 × 108 mol/yr of Ni with a Ni isotope composition of 1.2‰, this oxic output is ~0.4‰ lighter than the oxic output flux in FeMn crusts and nodules. This study shows that Ni isotope variations along the stratigraphic sequence could be the result of mixing between a pure pelagic clays end-member at ~0.1‰ and a pure Mn-oxyhydroxides end-member at ~1.2‰. We suggest that to keep the system at steady-state conditions the oxic output flux is compensated for by a diagenetic input flux of 3.7 × 108 mol/yr which is characterized by a highly fractionated Ni isotope composition of ~2.9‰