Elsevier

Geochimica et Cosmochimica Acta

Volume 151, 15 February 2015, Pages 172-191
Geochimica et Cosmochimica Acta

Dissolved silicon and its isotopes in the water column of the Bay of Bengal: Internal cycling versus lateral transport

https://doi.org/10.1016/j.gca.2014.12.019Get rights and content

Abstract

The concentration of dissolved Si and its isotope composition are measured in the Bay of Bengal (BoB) region of the northern Indian Ocean; the isotope data are the first data set from the northern Indian Ocean. The measurements are made in eight depth profiles closely along the 87°E transect (GIO1 section of the international GEOTRACES program) and in a few samples from the northern shelf of the bay. Dissolved Si in the water column varies from ∼0.6 to ∼152.5 μmol/kg, whereas the δ30Si data cover a range +1.2‰ to +3.6‰. The depth profiles of dissolved Si show generally lower values in the surface increasing with depth, whereas the pattern reverses in the case of δ30Si. These vertical distribution patterns of Si and δ30Si are similar to those reported in other oceanic regions and suggestive of the significant role of biological processes in governing Si biogeochemistry in the upper layers (top ∼1500 m). In contrast, dissolved Si in near surface waters of the northern shelf and the southernmost station is exceptionally high. These results indicate a continental supply of dissolved Si from the Ganga–Brahmaputra river system (G–B) and submarine groundwater discharge (SGD) to the shelf region, and an intrusion of high salinity waters from the Arabian Sea in the southern bay. The δ30Si values of ∼1.34 ± 0.10‰ for deep/bottom waters of the BoB (depth >1500 m) are similar to those reported for the deep Southern Ocean and indicate the dominant control of water mass mixing. The dissolved Si concentrations in the bottom waters of the BoB are generally higher than those of the water mass endmembers, which suggest the need for an additional source of Si; in situ particle dissolution and/or benthic release in the central bay seem to be the potential candidate.

The annual Si budget in the top ∼100 m of the BoB seems to suggest that meso-scale eddies frequently occurring during non-monsoon periods can supply at the most ∼2.6 g Si/m2/year, which is about 33% of the Si requirement to support new production in the bay. The supply of dissolved Si (∼1.3 ± 0.5 × 1011 mol/year) from the G–B river system and SGD has been calculated based on the distributions of dissolved Si concentration and δ30Si in the northern shelf waters. A comparison of this supply with the reported Si flux upstream of the estuarine zone indicates about 40% removal of dissolved Si in the G–B estuary. The mass balance of Si isotopes in the deep waters indicates that the dissolution of diatoms is the main cause of excess Si in the bay.

Introduction

Studies on marine biogeochemistry of Si and its isotopes have attracted attention in recent years because of its role in regulating the export flux of carbon from surface to deeper reservoirs of the global oceans (De La Rocha, 2003, Tréguer and De La Rocha, 2013, and references therein). Earlier studies on this topic were focused on the marine budget of dissolved Si, its cycling, and application as a water mass tracer (DeMaster, 1981, Nelson et al., 1995, Tréguer et al., 1995, Dileep Kumar and Li, 1996, You, 2000). Investigations on the budget of Si require knowledge of its sources to, and sinks within the oceans; in this context there have been several studies on its behavior in estuaries and shelf regions (Borole et al., 1977, DeMaster et al., 1991, DeMaster et al., 1996, Eyre and Balls, 1999, Somayajulu et al., 2002, Michalopoulos and Aller, 2004). During the last decade, studies on Si marine biogeochemistry took a major leap with the advent of Si isotope measurements in conjunction with its dissolved concentrations; this provided better insights into its various sources, sinks and internal cycling in the open ocean environments (Varela et al., 2004, Cardinal et al., 2005, Reynolds et al., 2006, Beucher et al., 2011, Cavagna et al., 2011, De La Rocha et al., 2011, Fripiat et al., 2011, de Souza et al., 2012a, de Souza et al., 2012b, Grasse et al., 2013). Ocean margin basins, being very productive due to nutrients supply via coastal upwelling and continental alluvial/fluvial inputs, are known to play an important role in the marine budget of Si. However studies of the Si cycle in these margin basins are still limited (Brzezinski et al., 2003, Cao et al., 2012, Ehlert et al., 2012).

The goal of this work is to better understand and characterize the processes regulating the distribution of dissolved Si in the Bay of Bengal (BoB) through a systematic study on its concentration and stable isotope composition (δ30Si). The BoB is characterized by moderately high primary productivity (∼80 to 120 g C/m2/year in open ocean waters) largely dominated by diatoms (Madhupratap et al., 2003, Madhu et al., 2006, Prasanna Kumar et al., 2010, and references therein), that consume dissolved Si to make their frustules and export dissolved Si from surface waters to ocean interior. The dominance of diatoms (>85%) in the phytoplankton community of the BoB requires adequate supply of dissolved Si to surface waters; in this context there have been enquiries about the sources of Si and their relative significance. The Ganga–Brahmaputra (G–B) river system is the dominant source of freshwater and other fluvial materials including dissolved Si to the BoB. The available limited data on dissolved Si in these rivers and ground waters of Bangladesh yield an annual flux of ∼2.3 × 1011 mol/year (Georg et al., 2009a), which is ∼4% of the global riverine Si supply (Laruelle et al., 2009, Dürr et al., 2011, Tréguer and De La Rocha, 2013). Earlier studies have suggested that much of the nutrients including Si supplied by various rivers to the BoB are consumed in estuaries and therefore do not reach the open bay (Qasim, 1977, Prasanna Kumar et al., 2002). Another important source of Si and nutrients to surface waters of the BoB is meso-scale eddies (Kumar et al., 2004, Prasanna Kumar et al., 2004, Prasanna Kumar et al., 2007, Nuncio and Prasanna Kumar, 2012, Chen et al., 2013, Vidya and Prasanna Kumar, 2013), the relative significance of these two modes of nutrient supply to support the biological production in the open BoB however, is a topic of debate.

The GEOSECS program provided high depth resolution data on dissolved Si in two profiles from the BoB (stations 445 and 446, Fig. 1a). The data show that bottom waters of the BoB along with those of the Arabian Sea have higher dissolved Si concentrations compared to the rest of the Indian Ocean; this has been attributed to dissolution of biogenic silica at the sediment–water interface (Broecker et al., 1980, Dileep Kumar and Li, 1996, Gordon et al., 2002). More recently, some of the studies have underscored the importance of release of Si and other elements (e.g., REEs) from lithogenic detritus deposited on the ocean margins (Lacan and Jeandel, 2005, Arsouze et al., 2009, Jeandel et al., 2011, Singh et al., 2012). Thus, the distribution of dissolved Si in oceans is dictated by multiple processes; its supply from continents and hydrothermal sources, redistribution by mixing of water masses, particle–water interactions in the water column and pore waters, and export from the water column through biogenic debris. Si isotopes serve as a tool to assess the importance of various sources to the Si budget as their isotope compositions are distinct, further their isotope fractionation provides insight into particle–water interactions (De La Rocha et al., 1997, Cardinal et al., 2005, Reynolds et al., 2006, Beucher et al., 2008, Demarest et al., 2009, Georg et al., 2009a, Fripiat et al., 2011, de Souza et al., 2012a, de Souza et al., 2012b).

In this paper, we present the first data set on the isotope composition of dissolved Si in the northern Indian Ocean, specifically from the BoB and their use to infer the factors influencing the abundance and distribution of Si in the water column. The results also provide (i) an estimate of the fraction of dissolved Si supplied by the G–B and SGD that escapes the estuarine removal and reach the shelf and open ocean regions of the BoB, and (ii) clues to the source of excess Si in bottom waters, which are based on the mass balance modeling of dissolved Si and δ30Si.

The Bay of Bengal receives ∼1015 L of freshwater and ∼1015 g of fluvial sediments annually; one of ocean basins receiving very high flux of continental materials (Milliman and Mead, 1983, Sarin et al., 1989, Galy and France-Lanord, 1999, Galy and France-Lanord, 2001, Basu et al., 2001). The upper water column of the BoB is stratified due to freshwater input; this restricts the supply of nutrients to the euphotic layer by wind driven vertical mixing (Shetye et al., 1996, Gopalakrishna et al., 2002, Prasanna Kumar et al., 2002, Madhupratap et al., 2003). The South–West monsoonal winds are however capable of inducing coastal upwelling along the southeastern Indian coast (Shetye et al., 1991), whereas upwelling in the open bay is facilitated by cyclones and meso-scale eddies occurring predominantly during the inter-monsoon seasons (Gomes et al., 2000, Vinayachandran and Mathew, 2003, Nuncio and Prasanna Kumar, 2012, Chen et al., 2013).

Inverse modeling of the hydrographical properties and distributions of tracers such as Nd and εNd in the bay (Singh et al., 2012) have led to better understanding of the water mass structure of the BoB. The Nd and εNd results show that surface waters (depth ∼0 to 100 m) of the open bay are composed of the Indonesian surface water (IW), Arabian Sea High Salinity water (ASHS) and the local less saline surface water (GB–BBLS). The intermediate waters (depth ∼100 to 1500 m) are predominantly a mixture of the Bay of Bengal subsurface water (BBSS), Indonesian Intermediate water (IIW), North Indian Intermediate water (NIIW), North Indian Deep water (NIDW), and the modified North Atlantic Deep water (MNADW). The Antarctic Bottom water (AABW) is the dominant water mass below a depth of ∼1500 m and makes up the bottom waters of the bay.

Section snippets

Sampling

This study is a part of the GEOTRACES program of our group. Seawater samples from the BoB for this study were collected onboard RV Sagar Sampada during cruise SS259 in November, 2008 (Fig. 1a). The samples were collected from eight depth profiles along 87°E transect (∼6°N to ∼20°N) and a few shallow depth profiles near the mouth of the G–B river system. The location and water column depth of the stations are given in Table 1; the details of sampling have been discussed in our earlier

Results

Dissolved Si and δ30Si in the water column of the BoB vary from ∼0.6 to ∼152.5 μmol/kg and +1.2‰ to +3.6‰, respectively (Table 1). The depth profiles of dissolved Si and δ30Si in the coastal stations (0814–0820) and open ocean stations (0806–0813) are shown in Fig. 2; this figure also includes an expanded version of the Si concentration–depth profiles of the top ∼500 m of the open ocean stations. The depth profiles of dissolved Si show an increase with depth generally from very low concentrations

Spatial distributions of dissolved Si and δ30Si: sources of Si

The uptake of Si during the production of siliceous plankton and ensuing Si isotope fractionation (De La Rocha et al., 1997), have significant impact on the distributions of dissolved Si and δ30Si. The isotope fractionation models (Varela et al., 2004, Beucher et al., 2008, Beucher et al., 2011, Fripiat et al., 2011, Cao et al., 2012, Ehlert et al., 2012) serve as a tool to obtain insights into the Si cycling in these waters. Two fractionation models are frequently used, (i) the Steady State

Conclusions

The δ30Si values and their vertical distribution patterns in the water column of the Bay of Bengal are similar to those reported in other oceanic regions, which hints at the internal cycling of biogenic silica in the upper waters (top ∼1500 m). The distribution of δ30Si in surface waters shows relatively high values in the central BoB compared to the northern and southern parts of the bay. High δ30Si and depleted Si in the central BoB indicate a proportionally higher utilization of dissolved Si

Acknowledgments

We thank the Ministry of Earth Sciences, India for providing financial support for the project “GEOTRACES” and ship time onboard FORV Sagar Sampada. The help provided by the Master and crew of Sagar Sampada during sampling is thankfully acknowledged. We also thank J.P. Bhavsar, B. Srinivas, V. Goswami and J. Chatterjee for their help during the onboard sample collection and K.R. Chandana for a few standard/sample preparations. We also acknowledge the Associate Editor Silke Severmann, two

References (101)

  • X. Chen et al.

    Episodic phytoplankton bloom events in the Bay of Bengal triggered by multiple forcings

    Deep Sea Res. Part I

    (2013)
  • C.L. De La Rocha et al.

    Fractionation of silicon isotopes by marine diatoms during biogenic silica formation

    Geochim. Cosmochim. Acta

    (1997)
  • C.L. De La Rocha et al.

    The silicon isotopic composition of surface waters in the Atlantic and Indian sectors of the Southern Ocean

    Geochim. Cosmochim. Acta

    (2011)
  • G.F. de Souza et al.

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

    Earth Planet. Sci. Lett.

    (2014)
  • S. Delstanche et al.

    Silicon isotopic fractionation during adsorption of aqueous monosilicic acid onto iron oxide

    Geochim. Cosmochim. Acta

    (2009)
  • M.S. Demarest et al.

    Fractionation of silicon isotopes during biogenic silica dissolution

    Geochim. Cosmochim. Acta

    (2009)
  • D.J. DeMaster

    The supply and accumulation of silica in the marine environment

    Geochim. Cosmochim. Acta

    (1981)
  • D.J. DeMaster et al.

    Biogeochemical processes in Amazon shelf waters: chemical distributions and uptake rates of silicon, carbon and nitrogen

    Cont. Shelf Res.

    (1996)
  • M. Dileep Kumar et al.

    Spreading of water masses and regeneration of silica and 226Ra in the Indian Ocean

    Deep Sea Res. Part II

    (1996)
  • C.B. Douthitt

    The geochemistry of the stable isotopes of silicon

    Geochim. Cosmochim. Acta

    (1982)
  • C.B. Dowling et al.

    The groundwater geochemistry of the Bengal Basin: weathering, chemsorption, and trace metal flux to the oceans

    Geochim. Cosmochim. Acta

    (2003)
  • 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)
  • A. Galy et al.

    Weathering processes in the Ganges–Brahmaputra basin and the riverine alkalinity budget

    Chem. Geol.

    (1999)
  • S. Geilert et al.

    Silicon isotope fractionation during abiotic silica precipitation at low temperatures: inferences from flow-through experiments

    Geochim. Cosmochim. Acta

    (2014)
  • R.B. Georg et al.

    New sample preparation techniques for the determination of Si isotopic compositions using MC-ICPMS

    Chem. Geol.

    (2006)
  • R.B. Georg et al.

    Silicon fluxes and isotope composition of direct groundwater discharge into the Bay of Bengal and the effect on the global ocean silicon isotope budget

    Earth Planet. Sci. Lett.

    (2009)
  • R.B. Georg et al.

    Stable silicon isotopes of groundwater, feldspars, and clay coatings in the Navajo Sandstone aquifer, Black Mesa, Arizona, USA

    Geochim. Cosmochim. Acta

    (2009)
  • H.R. Gomes et al.

    Influence of physical processes and freshwater discharge on the seasonality of phytoplankton regime in the Bay of Bengal

    Cont. Shelf Res.

    (2000)
  • V.V. Gopalakrishna et al.

    Upper ocean stratification and circulation in the northern Bay of Bengal during southwest monsoon of 1991

    Cont. Shelf Res.

    (2002)
  • A.L. Gordon et al.

    Bay of Bengal nutrient-rich benthic layer

    Deep Sea Res. Part II

    (2002)
  • V. Goswami et al.

    Impact of water mass mixing and dust deposition on Nd concentration and εNd of the Arabian Sea water column

    Geochim. Cosmochim. Acta

    (2014)
  • 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)
  • F. Lacan et al.

    Neodymium isotopes as a new tool for quantifying exchange fluxes at the continent–ocean interface

    Earth Planet. Sci. Lett.

    (2005)
  • N.V. Madhu et al.

    Lack of seasonality in phytoplankton standing stock (chlorophyll a) and production in the western Bay of Bengal

    Cont. Shelf Res.

    (2006)
  • M. Madhupratap et al.

    Biogeochemistry of the Bay of Bengal: physical, chemical and primary productivity characteristics of the central and western Bay of Bengal during summer monsoon 2001

    Deep Sea Res. Part II

    (2003)
  • J.H. Martin et al.

    VERTEX: carbon cycling in the northeast Pacific

    Deep Sea Res. Part A. Oceanogr. Res. Pap.

    (1987)
  • P. Michalopoulos et al.

    Early diagenesis of biogenic silica in the Amazon delta: alteration, authigenic clay formation, and storage

    Geochim. Cosmochim. Acta

    (2004)
  • W.S. Moore

    High fluxes of radium and barium from the mouth of the Ganges–Brahmaputra River during low river discharge suggest a large groundwater source

    Earth Planet. Sci. Lett.

    (1997)
  • Y. Nozaki et al.

    Importance of vertical geochemical processes in controlling the oceanic profiles of dissolved rare earth elements in the northeastern Indian Ocean

    Earth Planet. Sci. Lett.

    (2003)
  • M. Nuncio et al.

    Life cycle of eddies along the western boundary of the Bay of Bengal and their implications

    J. Mar. Syst.

    (2012)
  • M. Oelze et al.

    Si stable isotope fractionation during adsorption and the competition between kinetic and equilibrium isotope fractionation: implications for weathering systems

    Chem. Geol.

    (2014)
  • S. Opfergelt et al.

    Impact of soil weathering degree on silicon isotopic fractionation during adsorption onto iron oxides in basaltic ash soils, Cameroon

    Geochim. Cosmochim. Acta

    (2009)
  • S. Opfergelt et al.

    Variations of δ30Si and Ge/Si with weathering and biogenic input in tropical basaltic ash soils under monoculture

    Geochim. Cosmochim. Acta

    (2010)
  • S. Opfergelt et al.

    Silicon isotopes and the tracing of desilication in volcanic soil weathering sequences, Guadeloupe

    Chem. Geol.

    (2012)
  • S. Prasanna Kumar et al.

    Eddy-mediated biological productivity in the Bay of Bengal during fall and spring intermonsoons

    Deep Sea Res. Part I

    (2007)
  • V. Ramaswamy et al.

    Regional variations in the fluxes of foraminifera carbonate, coccolithophorid carbonate and biogenic opal in the northern Indian Ocean

    Deep Sea Res. Part I

    (2006)
  • S.B. Ray et al.

    Suspended matter, major cations and dissolved silicon in the estuarine waters of the Mahanadi River, India

    J. Hydrol.

    (1984)
  • B.C. Reynolds et al.

    Silicon isotope fractionation during nutrient utilization in the North Pacific

    Earth Planet. Sci. Lett.

    (2006)
  • M.M. Sarin et al.

    Major ion chemistry of the Ganga–Brahmaputra river system: weathering processes and fluxes to the Bay of Bengal

    Geochim. Cosmochim. Acta

    (1989)
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