Sedimentary cycling of zinc and nickel and their isotopes on an upwelling margin: Implications for oceanic budgets and paleoenvironment proxies

Zinc and Ni are essential micronutrients whose stable isotope systematics in marine sediments represent promising, but still developing, tracers of past ocean chemistry biology. Sediments from upwelling continental margins have been identified as important sinks for Zn Ni, driven by high productivity, incorporation the metals into cells organic carbon burial. At present, however, our understanding how composition reflects specific processes water column is incomplete. Here, we present coupled abundance data both solid phase porewater a series sediment cores collected on shelf-to-slope transect across an oxygen minimum zone (OMZ) southwest African margin off Namibia. Zinc/Al Ni/Al ratios Namibian elevated relative to lithogenic background, factors 1.4–2.4 1.7–5.7, respectively. Systematic relationships between concentrations accumulation corroborate view that authigenic mainly delivered these via export production photic zone. Corrections detrital sometimes substantial, δ60Ni values within uncertainty modern deep ocean, at 1.38 ± 0.18 ‰ (mean 1SD, n = 42). Authigenic δ66Zn more variable, −0.09 0.50 ‰, or below value average. This latter observation can be attributed fractionation during sequestration ZnS phases sediments, either quantitatively, partially. Porewater complex profiles do not necessarily reflect redox state sediment-porewater system time sampling, rather characteristics inherited reducing conditions temporally heterogeneous system. zinc (30–369 nM) much higher than increase with depth beneath sediment–water interface. Despite this prominent concentrations, constant lighter average, 0.06 33). These observations most consistent buffering small pool dissolution previously formed sulfide phase. closer mostly 5–15 nM. The lightest compositions (δ60Ni low 0.15 ‰) Mn-oxide source whereas heaviest (up 1.76 clearly uptake light sediment. Our data, largest site add sparse eastern Pacific, confirm bury close composition, generally lighter. thus importance such setting heavy oceanic Zn. kind hold significant potential construction records isotopes.