The Silicon Cycle: Human Perturbations and Impacts on Aquatic Systems

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The Silicon Cycle

Human Perturbations and Impacts on Aquatic Systems

ISLAND PRESS

Introduction

Venugopalan Ittekkot, Daniela Unger, Christoph Humborg, and Nguyen Tac An

Silicon dioxide is the most abundant component of the earth's crust. It occurs as silicate minerals in association with igneous, metamorphic, and sedimentary rocks. They undergo physical and chemical weathering. The associated processes involving the release of weathered products, their transport and transformation, and their interaction with other nutrient elements form the basis of the global silicon cycle. Perturbation of this cycle by natural and anthropogenic factors and its impact on the environment and climate were at the core of the Land–Ocean Nutrient Fluxes: The Silicon Cycle project, conducted by the Scientific Committee on Problems of the Environment (SCOPE) of the International Council for Science.

Silicon rarely features in the study of nutrients at the land–sea continuum. Carbon is included when there is a need for a number to balance the carbon budget. However, most studies have concentrated on nitrogen and phosphorus because they represent the nutrient elements that are discharged by human activities. Their fluxes have dramatically gone up over the past few decades because of their increased discharges from domestic, agricultural, and industrial sources and deforestation. Accelerated algal growth in water bodies, known as eutrophication, is attributed to this increase, leading to a deterioration of water quality via oxygen depletion. Oxygen-deficient conditions in turn promote the production and emission to the atmosphere of climatically relevant gases such as nitrous oxide and methane. Toxic algal blooms also have been attributed to eutrophication, with devastating effects on fisheries and on biodiversity in general.

Not adequately taken into account is the role of silicon in all this. Diatoms form a major part of the aquatic food chain and are found to play a critical role in marine biogeochemical cycles, especially in the sequestration of carbon dioxide from the atmosphere via the biological pump. Diatoms need silicon, which is present in both cell contents and cell walls. Rock weathering is the natural source of dissolved silicate in aquatic systems. Land use changes and hydrological alterations have been changing the supply from this source to our rivers. There is evidence that the resulting changes in the ratios of nutrients (N, P, and Si) delivered to our coastal seas could effect species shifts from diatoms to nondiatoms, exacerbating the already declining quality and structure of aquatic ecosystems.

In three workshops within the project (1999, University of Linköping, Sweden; 2000, National Centre for Natural Science and Technology, Nha Trang, Vietnam; 2002, Center for Tropical Marine Ecology, Bremen, Germany), the SCOPE project brought together experts to review and synthesize the available information on the changing pattern of silicate transfer from land to sea and associated adverse impacts on the quality and structure of aquatic ecosystems. They reexamined the available data on land–ocean nutrient fluxes in light of new research on the role of silicates as nutrients and the changes in their fluxes caused by human activities. This appeared to be warranted in order to develop scientifically sound strategies to reduce the risk of ecosystem perturbation, especially in coastal waters, and to identify areas and needs for future research.

While summarizing the results of the project, the present volume also tracks the pathway of silicon from its source in the rocks and minerals on land to its burial in sediments at the sea floor. It discusses the biotic and abiotic modifications of silicon in transit and its cycling in the coastal seas. Natural geological processes in combination with atmospheric and hydrological processes are considered, as are human perturbations of the natural controls of the silicon cycle.

We hope that this book will be useful to a broad spectrum of readers, including students and researchers, coastal managers, lecturers, and national monitoring agencies, and that it will contribute to the ongoing efforts to understand the global cycling of silicon.

Silicate Weathering in South Asian Tropical River Basins

Vaidyanatha Subramanian,Venugopalan Ittekkot, Daniela Unger, and Natarajan Madhavan

Silicon dioxide occurs as silicate minerals in the earth's crust. Weathering of silicates can be represented as

2CO2 + 3H2O + CaSiO3 -> Ca++ + 2HCO3- + H4SiO4,

where Ca++ might also be substituted by Mg++.

Chemical weathering of silicates and the cycling of their products form the basis of silicon biogeochemistry and its interaction with elemental cycles such as carbon and nitrogen. Weathering of carbonates can be represented as

CO2 + H2O + CaCO3 -> Ca++ + 2HCO3-

or

2CO2 + 2H2O + CaMg(CO3)2 -> Ca++ + Mg++ + 4HCO3-

The CO2 consumed during these weathering reactions is supplied either directly from the atmosphere or from soils where it is produced during the degradation of plant biomass. However, in the case of silicate weathering all alkalinity produced is of atmospheric origin, whereas in the case of carbonate weathering only half of it is. Therefore, silicate weathering on land represents an important sink for atmospheric CO2 and is of special interest in the context of controlling the concentrations of CO2 in the atmosphere and the ocean over geological time scales (Berner et al. 1983; Wollast and Mackenzie 1983; Brady and Carrol 1994).

The dissolved products of weathering are transferred via the rivers to the oceans, where they are taken up biologically to form carbonate and biogenic silica in the tissues and skeletons of marine plankton.

Silicate weathering depends on temperature and precipitation (White and Blum 1995) and might be enhanced through global warming. Another factor affecting silicate weathering is the erosion rate, that is, the delivery of potentially weatherable material. However, estimates from the Lesser Himalayas indicate that because of the steep relief and the resulting abundant supply of eroded material, erosion is not a limiting factor in the case of Himalayan rivers (West et al. 2005). Both the dissolved silica (DSi) and alkalinity load have been shown to be related to land use changes and therefore are prone to anthropogenic alterations. DSi in pristine northern Swedish rivers was found to correlate with vegetation cover (Ittekkot et al. 2003). DSi concentration in small tropical rivers appears to decrease after the conversion of forested land into cropland when rice or crops from the Gramineae family are cultivated because these plants store silicon in phytoliths, reducing the export of Si from soil to rivers (Chapter 4, this volume). However, alkalinity increased with the proportion of cropland to forestland in the case of the Mississippi and its tributaries in the last decades (Raymond and Cole 2003). In small watersheds, as in southwestern India, extensive land use changes in the last three decades appear to have made irreversible changes in some of the major water quality parameters such as alkalinity, +DSi, Ca, and Mg (Sajeev and Subramanian 2003).

In the context of CO2-induced global warming, the role played in the carbon cycle by the possibly accelerated chemical weathering and the subsequent supply of solute components to the sea is of interest particularly in regions with enhanced anthropogenic alterations and pronounced intensity of weathering reactions.

Regional Background

The geology of the Indian subcontinent is diverse, representing Precambrian hard rocks (shield area) to the most recent alluvial terrain in the Indo-Gangetic belt (Table 2.1). The Himalayan region has a mixture of hard igneous and metamorphic rocks associated with Tertiary sediments, and their proportion varies in the drainage basins of rivers.

However, even in watersheds characterized by the predominance of silicate rocks it was found that water chemistry was determined to a large extent by the weathering of carbonates present in veins and thin layers (Blum et al. 1998; Quade et al. 2003). Silicate weathering appears to be much more intense in the huge Ganges Basin than in the high Himalayan region (West et al. 2002). The peninsular region is characterized by basaltic Deccan traps in the central and hard rocks in the southern parts. Ancient and recent sediments occur in the northern and coastal regions. Climatically, south Asia faces extreme conditions, from humid tropics in the southern part to cold arid temperatures in the Ladakh Himalayan region, which affect weathering intensity. The mountainous drainage basins of Himalayan rivers experience high chemical and physical erosion rates. The same is true for small west-flowing rivers on the Indian subcontinent, as in Kerala (Ittekkot et al. 1999). Dense populations on either side of the Himalayas in China and south Asia lead to severe land use alteration throughout the region. Thus south Asian rivers are particularly suited for study of the link between land use changes, weathering, alkalinity, DSi, and atmospheric CO2 drawdown.

Data and Discussion

For this study, pCO2 was estimated using the pH and HCO3-1 values for individual rivers. Alkalinity from silicate weathering (where all values are in milliequivalent units), following the method of Raymahasay (1986), is calculated as

Alkalinitycarbonate weathering = 0.74 Catotal + 0.4 Mgtotal

and

Alkalinity silicate weathering = Alkalinitytotal–Alkalinitycarbonate weathering.

Generally, the observed concentration of alkalinity and Ca and Mg in river water is the net balance from possible diverse sources (carbonate versus silicate rocks; Quade et al. 2003) and removal by precipitation of Ca–Mg–carbonate minerals in the river basin (Stallard and Edmond 1981).

According to Probst et al. (1998), weathering of both silicate and nonsilicate rocks contributes 70 percent of the river alkalinity, and the remaining 30 percent is accounted for by river-based biological processes caused by decomposition of organic matter. For large rivers in the subcontinent, the relative weathering contribution in the beginning of monsoon season is likely to be even larger because of the large discharge of various river systems flushing out weathered topsoil in the initial stages of the rain, when biochemical reactivity is inhibited by the rapid flow of water.

Figure 2.1 shows the plot of carbonate alkalinity against the total concentration of Ca and Mg for major rivers in south Asia as seasonally adjusted values.

This plot shows that HCO3 nearly balances Ca + Mg, which is indicative of mainly carbonate weathering and was observed earlier by Sarin et al. (1989) for highland tributaries of the rivers Ganges and Brahmaputra. But at higher alkalinities, a deficit of Ca + Mg occurs, which would have to be balanced by alkalis (i.e., Na, Ka) from silicate weathering.

Because of the monsoon-related precipitation pattern, all the rivers in the subcontinent show extreme seasonality in their flows. Whereas the rivers in the southern region transport 95 percent of their annual runoff in three monsoon months (June to August), the Himalayan rivers—Ganges, Indus, and Brahmaputra—carry up to 50 percent of their annual runoff during the four-month monsoon period (June to September). Seasonal variability of river water composition as a result of weathering is well documented for a number of river systems in the world; see Raymond and Cole (2003) for the Mississippi River, Stallard and Edmond (1981) and Gibbs (1970) for the Amazon, and Galy and France-Lanord (1999) and Subramanian (2000) for several rivers in the Indian subcontinent. Major south Asian rivers listed in Table 2.2 reveal strong seasonality in their solute transport, reflecting variable export of DSi, Ca, Mg, and alkalinity in response to seasonal changes in the weathering zone (i.e., precipitation and temperature). In the downstream region of the Indus River in Pakistan, for example, the winter values for alkalinity, Ca, and Mg are generally higher than the corresponding summer values (data from Karim and Veizer 2000). Similar variation was observed for the Indus from the Ladakh Himalayas and for the Ganges and the Brahmaputra. The Cauvery, which is one of the peninsular Indian rivers draining hard Precambrian shield areas, shows low HCO3-1/SiO2 ratios compared with the Indus, reflecting the relevance of silicate weathering (Quade et al. 2003). This implies that the source of excess Mg and Ca in Cauvery waters are the silicate minerals of the shield rocks. Data presented in Table 2.2 also show that the amount of DSi in the lower Ganges–Brahmaputra system is generally larger than reported by many earlier workers because the data here are based longterm observation.

Because silicate and carbonate weathering differ in their significance for the carbon cycle, it is essential to separate the components of alkalinity derived from silicate and nonsilicate weathering. The use of 87Sr/86Sr led to the conclusion that accelerated silicate weathering especially in the Himalayas contributed significantly to reduced atmospheric CO2 levels during the Cenozoic (Raymo and Ruddiman 1992). However, recent work has shown that weathering of Himalayan carbonates might well produce similarly high isotopic ratios (Blum et al. 1998; Quade et al. 2003).

In Figure 2.2 we plot alkalinity from silicate weathering against total alkalinity in order to estimate the relative contribution of silicate weathering to alkalinity.

Dessert et al. (2001) calculated the alkalinity from silicate weathering in the basaltic terrain in central India and found it to be higher than in other regions such as the Himalayan watershed. Accordingly, the p CO2 consumed during weathering to generate the alkalinity, shown in Figure 2.3, exhibits great variability because of the diverse lithology in the subcontinent.

The lower part of the Indus in Pakistan (data calculated from Karim and Veizer 2000) and the combined Ganges–Brahmaputra–Megna system in Bangladesh (data calculated from Datta and Subramanian 1997) show consumption of higher levels of p CO2 than in the upper reaches of these rivers in the Himalayas. This observation is in line with the results of West et al. (2002), who found silicate weathering to be much more intense in the huge Ganges Basin than in the high Himalayan region. Additionally, tributaries might also affect the solute load composition of the Ganges in its lower reaches, as pointed out by Galy and France-Lanord (1999) and Sarin et al. (1989).

Ca/Na and Mg/Na observed by Quade et al. (2003) result in seasonally variable fractions of CO2 consumed by silicate weathering. Their calculations for the rivers Ganges, Brahmaputra, and Indus result in a silicate weathering contribution of 15–19 percent and 36 percent during the rainy and nonrainy seasons, respectively, an estimate slightly higher than that reported by Galy and France-Lanord (1999).

Based on the data in Table 2.3, the export of silicate alkalinity to the adjoining oceans is estimated to be 2.94 × 1013 g C/year. It is interesting to note that for the Mississippi drainage basin, Raymond and Cole (2003) report an increase of total bicarbonate export from 1.1 to 1.75 × 1013 g C/year during the second half of the last century, of which 60 percent is estimated to originate from atmospheric CO2. The comparison highlights the importance of south Asian rivers with respect to alkalinity supply to the ocean. Further research is needed to assess the variation and changes resulting from anthropogenic impact and climate change.

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Excerpted from The Silicon Cycle by Venugopalan Ittekkot, Daniela Unger, Christoph Humborg, Nguyen Tac An. Copyright © 2006 Scientific Committee on Problems of the Environment (SCOPE). Excerpted by permission of ISLAND PRESS.
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