Deconvolving the controls on the deep ocean's silicon stable isotope distribution

https://doi.org/10.1016/j.epsl.2014.04.040Get rights and content

Highlights

  • General circulation model of oceanic cycle of Si and its isotopes.

  • Si parsed into its preformed and regenerated components.

  • Preformed Si exhibits strong δ30Si variability in the deep ocean.

  • Regenerated Si mainly added to the deep ocean in the Southern Ocean.

  • Southern Ocean control on preformed and regenerated Si explains δ30Si distribution.

Abstract

We trace the marine biogeochemical silicon (Si) cycle using the stable isotope composition of Si dissolved in seawater (expressed as δ30Si). Open ocean δ30Si observations indicate a surprisingly strong influence of the physical circulation on the large-scale marine Si distribution. Here, we present an ocean general circulation model simulation that deconvolves the physical and biogeochemical controls on the δ30Si distribution in the deep oceanic interior. By parsing dissolved Si into its preformed and regenerated components, we separate the influence of deep water formation and circulation from the effects of biogeochemical cycling related to opal dissolution at depth. We show that the systematic meridional δ30Si gradient observed in the deep Atlantic Ocean is primarily determined by the preformed component of Si, whose distribution in the interior is controlled solely by the circulation. We also demonstrate that the δ30Si value of the regenerated component of Si in the global deep ocean is dominantly set by oceanic regions where opal export fluxes to the deep ocean are large, i.e. primarily in the Southern Ocean's opal belt. The global importance of this regionally dynamic Si cycling helps explain the observed strong physical control on the oceanic δ30Si distribution, since most of the regenerated Si present within the deep Atlantic and Indo-Pacific Oceans is in fact transported into these basins by deep waters flowing northward from the Southern Ocean. Our results thus provide a mechanistic explanation for the observed δ30Si distribution that emphasizes the dominant importance of the Southern Ocean in the marine Si cycle.

Introduction

Diatoms are siliceous phytoplankton with an obligate requirement for silicon (Si) to form their opaline cell walls, or frustules. As a result of their large cell size and boom–bust ecological strategy, diatoms are important contributors to the ocean's biological carbon pump (Buesseler, 1998, Henson et al., 2012), and are the dominant driver of the oceanic Si cycle (Tréguer and De La Rocha, 2013). The resulting link between the oceanic cycles of silicon and carbon (e.g. Dugdale and Wilkerson, 2001, Ragueneau et al., 2006) has motivated much research into the controls on the biogeochemical Si cycle, both in the modern ocean and during past glacial–interglacial cycles (e.g. Dugdale et al., 2002, Brzezinski et al., 2003, Griffiths et al., 2013, Meckler et al., 2013). A promising tool for such research is the stable isotope composition of silicon, expressed as δ30Si (defined in permil units as (Rsample/Rstd − 1) × 1000, where Rsample is the 30Si/28Si isotope ratio in a sample and Rstd is this ratio in the standard NBS28).

Diatoms preferentially incorporate the lighter isotopes of Si into their frustules during silicification (De La Rocha et al., 1997, Milligan et al., 2004, Sutton et al., 2013). As a result, biological utilization of Si in surface waters enriches dissolved Si in its heavier isotopes, imparting an elevated δ30Si signature to surface waters that have experienced diatom Si uptake (Varela et al., 2004; Cavagna et al., 2011). Within the ocean interior, the dissolution of sinking diatom opal that has been exported from the surface also influences the δ30Si value of dissolved Si, an effect that might be enhanced by Si isotope fractionation during dissolution (Demarest et al., 2009). Since these two processes, diatom Si uptake and opal export, drive the largest Si fluxes in the ocean (Tréguer and De La Rocha, 2013), the oceanic δ30Si distribution bears information on the dominant pathways and processes by which Si is cycled within the ocean (e.g. Cardinal et al., 2005, Fripiat et al., 2011a, de Souza et al., 2012a).

Due to isotope fractionation by diatom Si uptake at the surface, δ30Si values of dissolved Si reach their maximum at the surface, and exhibit lower values in the subsurface water column. In addition to this intuitive vertical δ30Si gradient, δ30Si values also vary in deep waters at the global scale. Values of δ30Si exhibit a systematic inter-basin gradient along the deep limb of the ocean's meridional overturning circulation (MOC), decreasing from high values in the deep Atlantic, through intermediate values in the Southern Ocean, to low values in the deep North Pacific (De La Rocha et al., 2000, Cardinal et al., 2005, Reynolds et al., 2006). The exact controls on this large-scale deep water δ30Si gradient – and thus the nature of oceanic Si cycling reflected by this feature – remain incompletely understood. Early observational and modeling studies invoked a variety of processes to explain its origin. For instance, De La Rocha et al. (2000) attributed the difference between Atlantic and Pacific deep water δ30Si values to the cumulative effect of the dissolution of low-δ30Si opal along the deep limb of the MOC. A later study by Beucher et al. (2008), however, used δ30Si observations in the deep Pacific and Southern Oceans to argue that opal dissolving in the deep Pacific bears a higher δ30Si value than deep waters. Wischmeyer et al.'s (2003) early general circulation model simulation, using the Large-Scale Geostrophic model of Maier-Reimer et al. (1993), was unable to shed light on this issue, since the model did not produce the inter-basin deep water δ30Si gradient. The box models implemented by Reynolds (2009) did simulate systematic δ30Si variation along the deep limb of the MOC, which he attributed to a combination of physical Si transport by the MOC and high Southern Ocean opal export fluxes.

Recently, a number of observational studies have documented strong regional- to basin-scale coherence in δ30Si values, implying a strong physical control on the δ30Si distribution at a range of depths in the water column. Fripiat et al. (2011b) and de Souza et al. (2012b) demonstrated the importance of physical processes in communicating the elevated δ30Si signal produced at the surface into the ocean interior. These studies revealed that high δ30Si values produced during summer are retained in the deep winter mixed layers of the Southern Ocean, and that this utilization signal is transported into the subtropical interior by Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW), consistent with the Si distribution (Sidouble bondSi–NO3; Sarmiento et al., 2004). Similarly, equatorial Pacific studies suggest that the Equatorial Undercurrent's δ30Si signature may be traced across 60 degrees of longitude (Beucher et al., 2008, Beucher et al., 2011; Ehlert et al., 2012, Grasse et al., 2013). Furthermore, a dominant control of the circulation has been observed on deep water δ30Si variations at the basin scale: de Souza et al. (2012a) demonstrated that the meridional δ30Si gradient within the deep Atlantic Ocean is related not to opal dissolution, but rather to the quasi-conservative mixing of Si between North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW).

The recent increase in the volume and spatial resolution of oceanic δ30Si observations has thus provided powerful evidence for the largely conservative behavior of δ30Si values in the ocean interior. The implied dominance of the physical circulation on the large-scale oceanic distribution of δ30Si – and hence that of Si – is somewhat surprising for a biogeochemically-cycled nutrient. In this study, we aim to identify the processes that determine this behavior, with a focus on the global deep water δ30Si distribution. We deconvolve the physical and biogeochemical influences on the Si and δ30Si distributions in an ocean general circulation model (OGCM) simulation that explicitly traces the preformed and regenerated components of Si.

Our conceptual distinction between preformed and regenerated Si is motivated by the differing controls on their distributions in the ocean interior. At the ocean's surface, the balance between Si supply by the circulation on the one hand, and Si uptake & export by biology on the other, determines Si concentrations in the mixed layer. The properties of the winter mixed layer, including its Si concentration and δ30Si value, are communicated to the ocean interior during subduction or deep water formation (Stommel, 1979, de Souza et al., 2012b), giving rise to the preformed component of the interior ocean Si inventory. Thus, whilst the concentration of preformed Si and its δ30Si value reflect the physical–biogeochemical Si balance in the mixed layer at the sites of deep water formation, this component enters the ocean interior in dissolved form, without being cycled by biology. Once within the ocean interior, preformed Si behaves conservatively, such that its interior concentration and isotopic distributions are influenced only by the physical circulation. Differences in the δ30Si value of preformed Si sourced from different deep water formation regions thus leads to variations in deep ocean δ30Si values that are purely related to the physical transport of these conservative source signals. The regenerated component, on the other hand, is added to the ocean interior by the dissolution of sinking opal that has been exported from the euphotic zone, representing the non-conservative component of the interior Si inventory that has been cycled biogeochemically. The distribution of total dissolved Si in the ocean interior is governed by the combination of these two components. Our approach of separating preformed and regenerated Si in the context of a model simulation, and tracing their isotopic compositions, thus allows assessment of the relative importance of physical versus biogeochemical controls on the distribution of Si and its isotopes in the deep sea.

Section snippets

Model description

The physical ocean model used in this study is the Modular Ocean Model 3.0 (MOM3; Pacanowski and Griffies, 1999). MOM3 is a z-level primitive-equation ocean general circulation model, run here with 3.75° × 4.5° (longitude–latitude) horizontal resolution and 24 vertical levels. Our study uses the P2A configuration of MOM3 (Gnanadesikan et al., 2004). This configuration conforms to the observational constraints of low diapycnal diffusivity in the low-latitude thermocline (e.g. Ledwell et al., 1993

Model validation: comparison to δ30Si data

The oceanic Si isotope systematics of open-ocean observations and our model simulation are compared in Fig. 1, which plots δ30Si values against Si concentrations. The model reproduces the observed non-linear relationship between Si concentration and its isotopic composition in the global ocean (Fig. 1a). Furthermore, the model captures a key feature of the regional variability in oceanic δ30Si systematics (Fig. 1b): the simulated isotope systematics of the Southern Ocean south of 50°S

Discussion

The deconvolution of deep ocean Si into its physically-controlled preformed component and its biogeochemically-cycled regenerated component enables us to take a detailed look at the manner in which these two controls interact to determine the oceanic Si and δ30Si distributions. We begin our discussion with a consideration of the most striking feature of the deep δ30Si distribution, the meridional δ30Si gradient in the deep Atlantic Ocean.

Conclusions

By parsing dissolved Si into its preformed and regenerated components in an OGCM simulation, we have separated the influence of physical and biogeochemical processes on the deep ocean's δ30Si distribution. The results indicate a strong control of the preformed component of Si, whose interior distribution is determined solely by the circulation, on the systematic meridional δ30Si gradient in the deep Atlantic Ocean. Furthermore, we have shown that, due to the pronounced regional differences in

Acknowledgments

Fruitful discussions with Irina Marinov, Mark Brzezinski, Ben Reynolds and Robbie Toggweiler are gratefully acknowledged. Feedback from Florian Wetzel helped considerably improve an earlier version of this manuscript. The authors thank the observational δ30Si community for generously sharing data, Damien Cardinal and an anonymous reviewer for their constructive reviews, and Gideon Henderson for editorial handling. This work was supported by Swiss National Science Foundation postdoctoral

References (73)

  • K.E. Egan et al.

    Diatom silicon isotopes as a proxy for silicic acid utilisation: a Southern Ocean core top calibration

    Geochim. Cosmochim. Acta

    (2012)
  • C. Ehlert et al.

    Factors controlling the silicon isotope distribution in waters and surface sediments of the Peruvian coastal upwelling

    Geochim. Cosmochim. Acta

    (2012)
  • F. Fripiat et al.

    Isotopic constraints on the Si-biogeochemical cycle of the Antarctic Zone in the Kerguelen area (KEOPS)

    Mar. Chem.

    (2011)
  • P. Grasse et al.

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

    Earth Planet. Sci. Lett.

    (2013)
  • O. Huhn et al.

    Evidence of deep- and bottom-water formation in the western Weddell Sea

    Deep-Sea Res., Part 2, Top. Stud. Oceanogr.

    (2008)
  • O. Ragueneau et al.

    A review of the Si cycle in the modern ocean: recent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy

    Glob. Planet. Change

    (2000)
  • B.C. Reynolds et al.

    Silicon isotope fractionation during nutrient utilization in the North Pacific

    Earth Planet. Sci. Lett.

    (2006)
  • M. Rhein

    The Deep Western Boundary Current: tracers and velocities

    Deep-Sea Res., A, Oceanogr. Res. Pap.

    (1994)
  • A. Shemesh et al.

    Dissolution and preservation of Antarctic diatoms and the effect on sediment thanatocoenoses

    Quat. Res.

    (1989)
  • J.N. Sutton et al.

    Species-dependent silicon isotope fractionation by marine diatoms

    Geochim. Cosmochim. Acta

    (2013)
  • H.M. van Aken et al.

    On the synoptic hydrography of intermediate and deep water masses in the Iceland Basin

    Deep-Sea Res., A, Oceanogr. Res. Pap.

    (1995)
  • H.M. van Aken

    The hydrography of the mid-latitude northeast Atlantic Ocean I: the deep water masses

    Deep-Sea Res., A, Oceanogr. Res. Pap.

    (2000)
  • K.O. Buesseler

    The decoupling of production and particulate export in the surface ocean

    Glob. Biogeochem. Cycles

    (1998)
  • D. Cardinal et al.

    Relevance of silicon isotopes to Si-nutrient utilization and Si-source assessment in Antarctic waters

    Glob. Biogeochem. Cycles

    (2005)
  • A.-J. Cavagna et al.

    Silicon uptake and supply during a Southern Ocean iron fertilization experiment (EIFEX) tracked by Si isotopes

    Limnol. Oceanogr.

    (2011)
  • A. da Silva et al.

    Atlas of Surface Marine Data 1994, vol. 1. National Oceanic and Atmospheric Administration

    (1994)
  • G.F. de Souza et al.

    Southern Ocean control of silicon stable isotope distribution in the deep Atlantic Ocean

    Glob. Biogeochem. Cycles

    (2012)
  • G.F. de Souza et al.

    Silicon stable isotope distribution traces Southern Ocean export of Si to the eastern South Pacific thermocline

    Biogeosciences

    (2012)
  • R.R. Dickson et al.

    The production of North Atlantic Deep Water: Sources, rates, and pathways

    J. Geophys. Res., Oceans

    (1994)
  • R.C. Dugdale et al.

    Sources and fates of silicon in the ocean: the role of diatoms in the climate and glacial cycles

    Sci. Mar.

    (2001)
  • J.P. Dunne et al.

    A synthesis of global particle export from the surface ocean and cycling through the ocean interior and on the seafloor

    Glob. Biogeochem. Cycles

    (2007)
  • F. Fripiat et al.

    Silicon pool dynamics and biogenic silica export in the Southern Ocean, inferred from Si-isotopes

    Ocean Sci.

    (2011)
  • F. Fripiat et al.

    Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography

    Biogeosciences

    (2012)
  • Gao, S., Voelker, C., Wolf-Gladrow, D., 2013. Fractionation during biogenic silicon dissolution: consequences for...
  • H.E. Garcia et al.

    World Ocean Atlas 2009, vol. 3: Dissolved Oxygen, Apparent Oxygen Utilization, and Oxygen Saturation

    (2010)
  • Garcia, H.E., Locarnini, R.A., Boyer, T.P., Antonov, J.I., 2010b. World Ocean Atlas, 2009, vol. 4: Nutrients...
  • Cited by (37)

    • The relationship between cadmium and phosphate in the Atlantic Ocean unravelled

      2018, Earth and Planetary Science Letters
      Citation Excerpt :

      The Cd/PO4 ratio just below the thermocline was largely set during water mass formation and remineralisation of biogenic material in the northern and southern high latitudes. The driving role of the Southern Ocean is not surprising as this has been established for trace elements as well as isotopes (e.g. de Souza et al., 2014; Xie et al., 2017), but the role of the northern high latitude oceans as driver of trace element distributions has received less attention. For the deep Cd–PO4 relationship, it is the difference between the Cd/PO4 ratios in the northern and southern deep water endmembers that drives the relationship between the two elements (see section 3.2 and 3.3).

    View all citing articles on Scopus
    View full text