Diagenetic manganese and iron cycling and implications for past redox conditions in sediments along the Emperor Seamount Chain, NW Pacific Ocean

Abstract

<p>The deep water of the North Pacific Ocean is enriched in CO<sub>2</sub> and nutrients as a result of organic matter degradation in the water column and surface sediments. Due to its large volume, the deep North Pacific may have played a fundamental role for the postulated glacial carbon sequestration leading to the observed drawdown of atmospheric CO<sub>2</sub>. As a consequence of increased CO<sub>2</sub> levels in the deep glacial ocean, bottom-water oxygen concentrations must have been correspondingly low compared to current oxygenated conditions (e.g., Anderson et al., 2019). Previous studies on sediments from the NW Pacific Ocean have provided evidence that glacial bottom‑water O<sub>2</sub> concentrations were significantly lower than today, which have induced suboxic surface sediment redox conditions (Jaccard et al., 2009) and have altered the primary sediment composition and properties of glacial deposits (e.g., magnetic susceptibility) due to diagenetic processes (Korff et al., 2016).</p><p>We have investigated seven 10- to 15-m-long sediment cores along a S-N transect at the Emperor Seamount Chain taken during RV SONNE cruise SO264 in order to (1) geochemically characterize the sediments and, (2) reconstruct past sediment redox conditions. The cores were retrieved from water depths between 3.5 and 5.7 km from organic-poor siliciclastic‑carbonaceous sediments in the South to more organic-rich siliciclastic‑siliceous sediments in the North with tephra layers found throughout all cores (Nürnberg et al., 2018).</p><p>Mn<sup>2+</sup> is released into the pore water at all study sites with increasing Mn<sup>2+</sup> concentrations below 20‑30 cm sediment depth. Pore-water Mn<sup>2+</sup> reraches up to 190 µM in siliciclastic‑siliceous sediments most likely associated with high rates of dissimilatory Mn(IV) reduction. The solid‑phase composition of a core taken from the Minnetonka Seamount (47°44’N, 168°40’E) at 4 km water depth shows Mn/Al ratios below 0.0003. These ratios are much lower than the average MORB Mn/Al value of 0.013 (Klein, 2004), which further indicates that Mn has been diagenetically lost from these sediments. As pore-water Fe<sup>2+</sup> concentrations are below detection limit at the Minnetonka Seamount and the depth distribution of solid-phase Fe/Al is mostly constant with ratios close to the average MORB Mn/Al value of 0.59 (Klein, 2004), Fe has probably not been diagenetically redistributed at this site. Pore‑water Fe<sup>2+</sup> concentrations of up to 20 µM are only found at two sites most likely as a result of dissimilatory Fe(III) reduction due to higher fluxes of organic material to the seafloor compared to the other sites.</p><p>References</p><p>Anderson, R.F., et al., 2019. Deep-sea oxygen depletion and ocean carbon sequestration during the last ice age. Global Biogeochem. Cycles 33, 301-317. </p><p>Jaccard, S.L., et al., 2009. Subarctic Pacific evidence for a glacial deepening of the oceanic respired carbon pool. Earth Planet. Sci. Lett. 277, 156‑165. </p><p>Klein, E.M., 2004. Geochemistry of the Igneous Oceanic Crust. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry, Vol.3. Elsevier, Amsterdam, pp. 433‑463.</p><p>Korff, L., et al., 2016. Cyclic magnetite dissolution in Pleistocene sediments of the abyssal northwest Pacific Ocean: evidence for glacial oxygen depletion and carbon trapping. Paleoceanography 31, 600‑624. </p><p>Nürnberg, D., 2018. RV SONNE Fahrtbericht /Cruise Report SO264, SONNE-EMPEROR, 30.6. – 24.8.2018. </p>