The influence of water mass mixing on the dissolved Si isotope composition in the Eastern Equatorial Pacific

Abstract:

Silicon isotopes are a powerful tool to investigate the cycling of dissolved silicon (Si). Here we present the stable Si isotope composition of dissolved silicic acid (δ³⁰Si(OH)₄) in the water column of the Eastern Equatorial Pacific (EEP) where one of the globally largest Oxygen Minimum Zones (OMZs) is located. The samples were obtained during cruises with R/V Meteor in 2008/2009 and 2012/2013. Surface waters show a large range in δ³⁰Si(OH)₄ (+1.7‰ to +4.4‰). The samples with the highest δ³⁰Si(OH)₄ also revealed the most depleted Si(OH)₄ concentrations (0.2 μmol/kg), which is a function of the high degree of Si utilization by diatoms and admixture of waters from highly productive areas. Samples within the OMZ at oxygen concentrations below 10 μmol/kg are characterized by a large range in δ³⁰Si(OH)₄, which mainly reflects advection and mixing of different water masses, even though the highly dynamic hydrographic system of the upwelling area off Peru does not allow the identification of unambiguous Si isotope signals of distinct water masses. Thus we cannot rule out that dissolution processes also influenced the δ³⁰Si(OH)₄ signature in the subsurface water column. Deep water masses (> 2000 m) in the study area show a mean δ³⁰Si(OH)₄ of +1.2 ± 0.2‰, which is in agreement with previous studies from the Eastern and Central Pacific. Comparison of the new deep water data of this study and previously published data from the Central Pacific and Southern Ocean reveal substantially higher δ³⁰Si(OH)₄ values than deep water signatures from the North Pacific. As there is no clear correlation between δ³⁰Si(OH)₄ and Si(OH)₄ concentrations in the entire Pacific the distribution of δ³⁰Si(OH)₄ signatures in deep waters of the Pacific is considered to be a consequence of the mixing of several end member water masses with distinct Si isotope and Si concentration signatures.