Abstract
The upwelling area off Peru is characterized by exceptionally high rates of primary productivity, mainly dominated by diatoms, which require dissolved silicic acid (dSi) to construct their frustules. The silicon isotope compositions of dissolved silicic acid (δ30SidSi) and biogenic silica (δ30SibSi) in the ocean carry information about dSi utilization, dissolution, and water mass mixing. Diatoms are preserved in the underlying sediments and can serve as archives for past nutrient conditions. However, the factors influencing the Si isotope fractionation between diatoms and seawater are not fully understood. More δ30SibSi data in today’s ocean are required to validate and improve the understanding of paleo records. Here, we present the first δ30SibSi data (together with δ30SidSi) from the water column in the Peruvian Upwelling region. Samples were taken under strong upwelling conditions and the bSi collected from seawater consisted of more than 98% diatoms. The δ30SidSi signatures in the surface waters were higher (+1.7‰ to +3.0‰) than δ30SibSi (+1.0‰ to +2‰) with offsets between diatoms and seawater (Δ30Si) ranging from −0.4‰ to −1.0‰. In contrast, δ30SidSi and δ30SibSi signatures were similar in the subsurface waters of the oxygen minimum zone (OMZ) as a consequence of a decrease in δ30SidSi. A strong relationship between δ30SibSi and [dSi] in surface water samples supports that dSi utilization of the available pool (70 and 98%) is the main driver controlling δ30SibSi. A comparison of δ30SibSi samples from the water column and from underlying core-top sediments (δ30SibSi_sed.) in the central upwelling region off Peru (10°S and 15°S) showed good agreement (δ30SibSi_sed. = +0.9‰ to +1.7‰), although we observed small differences in δ30SibSi depending on the diatom size fraction and diatom assemblage. A detailed analysis of the diatom assemblages highlights apparent variability in fractionation among taxa that has to be taken into account when using δ30SibSi data as a paleo proxy for the reconstruction of dSi utilization in the region.
Highlights
High rates of primary productivity characterize the upwelling area off Peru, as upwelling of silicic acid (Si(OH)4), hereafter referred to as dSi, orthophosphate (PO43−), nitrate (NO3−) and total dissolved iron stimulate phytoplankton growth (Bruland et al, 2005)
We present δ30SidSi and δ30SibSi data from the upper 350 m of the water column in the upwelling area off Peru together with the taxonomic composition of the diatom assemblages obtained from surface water samples to investigate the main controls on δ30SidSi and δ30SibSi
At stations 436 and 420 C. pelagica dominated the diatom assemblage with approximately 50%
Summary
High rates of primary productivity characterize the upwelling area off Peru, as upwelling of silicic acid (Si(OH)4), hereafter referred to as dSi, orthophosphate (PO43−), nitrate (NO3−) and total dissolved iron (dFe) stimulate phytoplankton growth (Bruland et al, 2005). The contribution of diatoms to global primary productivity links the marine silicon (Si) cycle to the carbon (C) cycle and exerts a strong control on C export from the atmosphere thereby directly impacting present and past climate (e.g., Tréguer et al, 2018). Dissolved silicon (Si) isotopes (δ30SidSi) are a powerful tool for understanding biogeochemical processes such as the utilization of dSi as well as intermediate and deep-water mass mixing in today’s ocean (e.g., de Souza et al, 2012; Grasse et al, 2013). The isotopic signal preserved in diatom frustules (δ30SibSi_sed.) in the underlying sediments can serve as a proxy to reconstruct the utilization of dSi in surface waters in the past (e.g., De La Rocha et al, 1998; Ehlert et al, 2013, 2015; Doering et al, 2019)
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