Deconvolving the controls on the deep ocean's silicon stable isotope distribution
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 (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 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 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 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, 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 gradient, values also vary in deep waters at the global scale. Values of 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 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 values to the cumulative effect of the dissolution of low- opal along the deep limb of the MOC. A later study by Beucher et al. (2008), however, used observations in the deep Pacific and Southern Oceans to argue that opal dissolving in the deep Pacific bears a higher 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 gradient. The box models implemented by Reynolds (2009) did simulate systematic 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 values, implying a strong physical control on the 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 signal produced at the surface into the ocean interior. These studies revealed that high 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 (Si⁎Si–NO3; Sarmiento et al., 2004). Similarly, equatorial Pacific studies suggest that the Equatorial Undercurrent's 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 variations at the basin scale: de Souza et al. (2012a) demonstrated that the meridional 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 observations has thus provided powerful evidence for the largely conservative behavior of values in the ocean interior. The implied dominance of the physical circulation on the large-scale oceanic distribution of – 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 distribution. We deconvolve the physical and biogeochemical influences on the Si and 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 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 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 value of preformed Si sourced from different deep water formation regions thus leads to variations in deep ocean 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 data
The oceanic Si isotope systematics of open-ocean observations and our model simulation are compared in Fig. 1, which plots 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 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 distributions. We begin our discussion with a consideration of the most striking feature of the deep distribution, the meridional 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 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 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 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)
- et al.
Sources and biological fractionation of silicon isotopes in the Eastern Equatorial Pacific
Geochim. Cosmochim. Acta
(2008) - et al.
Mechanisms controlling silicon isotope distribution in the Eastern Equatorial Pacific
Geochim. Cosmochim. Acta
(2011) - et al.
Diatoms in the desert: plankton community response to a mesoscale eddy in the subtropical North Pacific
Deep-Sea Res., Part 2, Top. Stud. Oceanogr.
(2008) - et al.
Ratios of Si, C and N uptake by microplankton in the Southern Ocean
Deep-Sea Res., Part 2, Top. Stud. Oceanogr.
(2003) - et al.
Silicon isotopes in spring Southern Ocean diatoms: large zonal changes despite homogeneity among size fractions
Mar. Chem.
(2007) - et al.
Fractionation of silicon isotopes by marine diatoms during biogenic silica formation
Geochim. Cosmochim. Acta
(1997) - et al.
A first look at the distribution of the stable isotopes of silicon in natural waters
Geochim. Cosmochim. Acta
(2000) - et al.
The silicon isotopic composition of surface waters in the Atlantic and Indian sectors of the Southern Ocean
Geochim. Cosmochim. Acta
(2011) - et al.
Fractionation of silicon isotopes during biogenic silica dissolution
Geochim. Cosmochim. Acta
(2009) - et al.
Meridional asymmetry of source nutrients to the equatorial Pacific upwelling ecosystem and its potential impact on ocean-atmosphere CO2 flux; a data and modeling approach
Deep-Sea Res., Part 2, Top. Stud. Oceanogr.
(2002)
Diatom silicon isotopes as a proxy for silicic acid utilisation: a Southern Ocean core top calibration
Geochim. Cosmochim. Acta
Factors controlling the silicon isotope distribution in waters and surface sediments of the Peruvian coastal upwelling
Geochim. Cosmochim. Acta
Isotopic constraints on the Si-biogeochemical cycle of the Antarctic Zone in the Kerguelen area (KEOPS)
Mar. Chem.
The influence of water mass mixing on the dissolved Si isotope composition in the Eastern Equatorial Pacific
Earth Planet. Sci. Lett.
Evidence of deep- and bottom-water formation in the western Weddell Sea
Deep-Sea Res., Part 2, Top. Stud. Oceanogr.
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
Silicon isotope fractionation during nutrient utilization in the North Pacific
Earth Planet. Sci. Lett.
The Deep Western Boundary Current: tracers and velocities
Deep-Sea Res., A, Oceanogr. Res. Pap.
Dissolution and preservation of Antarctic diatoms and the effect on sediment thanatocoenoses
Quat. Res.
Species-dependent silicon isotope fractionation by marine diatoms
Geochim. Cosmochim. Acta
On the synoptic hydrography of intermediate and deep water masses in the Iceland Basin
Deep-Sea Res., A, Oceanogr. Res. Pap.
The hydrography of the mid-latitude northeast Atlantic Ocean I: the deep water masses
Deep-Sea Res., A, Oceanogr. Res. Pap.
The decoupling of production and particulate export in the surface ocean
Glob. Biogeochem. Cycles
Relevance of silicon isotopes to Si-nutrient utilization and Si-source assessment in Antarctic waters
Glob. Biogeochem. Cycles
Silicon uptake and supply during a Southern Ocean iron fertilization experiment (EIFEX) tracked by Si isotopes
Limnol. Oceanogr.
Atlas of Surface Marine Data 1994, vol. 1. National Oceanic and Atmospheric Administration
Southern Ocean control of silicon stable isotope distribution in the deep Atlantic Ocean
Glob. Biogeochem. Cycles
Silicon stable isotope distribution traces Southern Ocean export of Si to the eastern South Pacific thermocline
Biogeosciences
The production of North Atlantic Deep Water: Sources, rates, and pathways
J. Geophys. Res., Oceans
Sources and fates of silicon in the ocean: the role of diatoms in the climate and glacial cycles
Sci. Mar.
A synthesis of global particle export from the surface ocean and cycling through the ocean interior and on the seafloor
Glob. Biogeochem. Cycles
Silicon pool dynamics and biogenic silica export in the Southern Ocean, inferred from Si-isotopes
Ocean Sci.
Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography
Biogeosciences
World Ocean Atlas 2009, vol. 3: Dissolved Oxygen, Apparent Oxygen Utilization, and Oxygen Saturation
Cited by (37)
Deciphering the source of banded iron formations in the North China Craton
2024, Precambrian ResearchDeglacial Si remobilisation from the deep-ocean reveals biogeochemical and physical controls on glacial atmospheric CO<inf>2</inf> levels
2020, Earth and Planetary Science LettersCycling of zinc and its isotopes across multiple zones of the Southern Ocean: Insights from the Antarctic Circumnavigation Expedition
2020, Geochimica et Cosmochimica ActaBarium in deep-sea bamboo corals: Phase associations, barium stable isotopes, & prospects for paleoceanography
2019, Earth and Planetary Science LettersThe relationship between cadmium and phosphate in the Atlantic Ocean unravelled
2018, Earth and Planetary Science LettersCitation 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).