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Applied Geochemistry 26 (2011) S289–S291
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Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
Human perturbations on the global biogeochemical cycles of coupled Si–C and
responses of terrestrial processes and the coastal ocean
Darcy Dan Li a,⇑, Abraham Lerman a, Fred T. Mackenzie b
a
b
Dept. of Earth & Planetary Sciences, Northwestern University, Evanston, IL, USA
Oceanography, SOEST, University of Hawaii, Honolulu, HI, USA
a r t i c l e
i n f o
Article history:
Available online 12 April 2011
a b s t r a c t
We present a model of the global biogeochemical cycle of silicon (Si) that emphasizes its linkages to the
carbon cycle and temperature. The Si cycle is a crucial part of global nutrient biogeochemistry regulating
long-term atmospheric CO2 concentrations due to silicate mineral weathering reactions involving the
uptake of atmospheric CO2 and production of riverine dissolved silica, cations and bicarbonate. In addition and importantly, the Si cycle is strongly coupled to the other nutrient cycles of N, P, and Fe; hence
siliceous organisms represent a significant fraction of global primary productivity and biomass. Human
perturbations involving land-use changes, burning of fossil fuel, and inorganic N and P fertilization have
greatly altered the terrestrial Si cycle, changing the river discharge of Si and consequently impacting marine primary productivity primarily in coastal ocean waters.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The importance of the Si in global biogeochemical cycling of the
elements is demonstrated by its ubiquitous abundance in the land
and aquatic biomass, where the Redfield Si/C atomic ratio is 0.02 in
land plants and 0.15 in marine organisms (Conley, 2002; Brzezinski,
1985), and by its coupling with carbon and the nutrient elements
of N, P, and Fe. Estimates show that Si-bioproduction accounts
for 1.5% of primary production on land, and 4.5% in the coastal
and open waters. On long-term geological timescales, the C cycle
is regulated by processes related to the Si cycle, foremost the
chemical weathering of silicate minerals, that removes CO2 from
the atmosphere and releases dissolved Si and other elements to
river waters. On the centurial or decadal timescale, the Si and C
cycles are coupled via primary production and remineralization
of organic matter as the Redfield ratios are conserved in different
biological reservoirs on land and in the ocean. Conventional
wisdom is that Si is not always an essential element or nutrient because it is a component of only some organismal groups. However,
Si does alleviate environmental stresses from biotic factors, such as
diseases and pests, and provides structural support against gravity
and relief from metal toxicities (Epstein, 2009). These functions are
accomplished either via polymerization of silicic acid to form solid
amorphous, hydrated silica, or by assisting in creation of organic
defence compounds through alteration of gene expressions. The
essentiality of biogenic Si is especially intriguing today because
⇑ Corresponding author.
E-mail address: [email protected] (D.D. Li).
0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apgeochem.2011.03.084
the world is facing food shortages with crop yields undergoing
manifold stresses, such as limited land area, water shortages, extreme weathering conditions, and increasing pest hazards. Plant
phytoliths in fallen plant litter undergo diagenesis and regeneration in reactive humus on a time scale of only several years to tens
of year (Gerard et al., 2008). However, in recent times, human landuse activities have substantially changed regional patterns of vegetation distribution, soil conditions, and nutrient fluxes via runoff
to the coastal ocean. Studies have shown that the flux of sediments
reaching the world’s coasts has been reduced due to retention
within water reservoirs on land. Dams create an efficient trap for
terrestrial Si delivery to coastal marine systems, giving rise in some
instances to carbonate-secreting coccolithophores rather than siliceous diatoms, as well as a shift in the alkalinity and pH of coastal
seawater (Syvitski et al., 2005). In addition, human practices, such
as river-flow manipulation, land-use changes, eutrophication, and
damming exert a negative control on the fluvial filtering of Si on
the continents. Anthropogenic chemical fertilization on land has
caused a significant increase in fluvial N and P transport, whereas
land-use and vegetation mass changes have caused a continuous
drop in the riverine Si input to the coastal ocean, all eventually
affecting the cycling of nutrients in the marine environment. Moreover, insufficient Si supply and production of diatoms can make a
coastal marine environment, e.g. the proximal North Sea, vulnerable to toxic blooms of N-fixing dinoflagellates (Humborg et al.,
1997). Therefore, given the fact that there are about 36,000 dams
in operation, and more under construction at an appreciable rate,
dams will greatly alter the water cycle and Si biogeochemical cycling on land and subsequently cause changes in ocean chemistry
and biology. Thus, it is essential to investigate and quantify the
S290
D.D. Li et al. / Applied Geochemistry 26 (2011) S289–S291
Fig. 1. Global biogeochemical cycle of Si in the Anthropocene, with detailed biogenic Si reservoirs in the ocean.
Table 1
Global biogenic Si and C mass size in units of 1012 mol of each organismal reservoir.
Reservoir
(1012 mol)
Si
C
Organic C from
TOTEM II
Terrestrial
domain
Land plants
Freshwater diatoms
1071
2
53,550
15.4
49,800
Coastal domain
Coastal diatoms
Coastal Siflagellates
Coastal Sponges
9.3
1
71.5
7.7
88.7
682
Open domain
Ocean diatoms
Ocean Radiolaria
Ocean Si-flagellates
115.4
5
5.5
884.6
38.5
42.3
367
3820
global biogeochemical consequences of river manipulation of the Si
cycle and budget (Goudie, 1994).
A global biogeochemical model of the Si cycle was developed
(Fig. 1) featuring major inorganic and biogeochemical reservoirs,
as well as the processes controlling the Si transfers among them.
Three major domains are land, shallow coastal ocean, and open
ocean. The terrestrial domain includes processes occurring on land
and in freshwater environments, such as mineral weathering, dissolution, precipitation, erosion, and primary production of land
plants as well as of freshwater diatoms. The coastal and openocean domains consist of several Si-secreting organism taxa that
are considered important biological reservoirs with respect of Si
besides ubiquitous diatoms and silicoflagellates, sponge producing
silica spicules are considered a significant taxa and primary producer in coastal waters while radiolaria exist mainly in the open
ocean. The coastal ocean domain has a relatively high primary productivity per unit volume coastal bioproduction is 1030% of total
oceanic primary production due to nutrient availability via river
discharge and upwelling, chemical and physical parameters that
provide crucial linkages between the land and open-ocean domains (see Table 1).
In order to further investigate the interactions between C and Si
in a global biogeochemical cycling model, the C cycle was linked to
the current Si cycle infrastructure. The Si-coupled global C cycle is
a simplified version of the precursor TOTEM (Terrestrial Ocean
aTmosphere Ecosystem Model, Ver et al., 1999) with separate Si
biological reservoirs and detailed processes enabling the focus of
attention to be on the behavior of the global Si cycle. In particular,
in the ocean domains, only the organic C occurring in the Si-bioproduction reservoirs is considered with estimations of the Si/C ratio in different organisms. These C transformations are closely
limited by Si primary production, dissolution rate and subsequently sedimentation and burial. The interaction between the Si
and C cycles under human perturbation can further refine future
atmospheric CO2 predictions, especially under the more drastic
scenarios of land-use changes worldwide.
So far, results have been extrapolated from the coupled to the
global C-N-P cycle model, TOTEM II (a revision of the original TOTEM) as forcings driving the simulation of the coupled Si–C cycles
between the year 1700 and 2050. In the model analysis from year
1700, taken as the start of the Anthropocene, to 2050, (Fig. 2) the
bioproduction of Si on land and in the ocean is coupled to the bioproduction of C, perturbed by the atmospheric CO2 rise, land-use
changes, and chemical N and P fertilization. Also, temperature rise
affects Si cycling on land through bioproduction rates, terrestrial
organic matter remineralization, and weathering, thereby affecting
its delivery flux to the coastal zone. Damming of major rivers plays
an important role in the Si–C cycle in that it extends the Si residence time on land. With the current global capacity of major dams
worldwide, 2.5% of instantaneous river water volume is retained
behind dams. Retention enhances terrestrial Si biological cycling,
leading to a decline in dissolved Si as well as HCO3 concentrations
in rivers and estuaries due to bio-uptake by plants and aqueous
autotrophic organisms on land. This results in strengthening of Si
nutrient limitation in coastal marine waters and decrease in marine primary production, especially growth of siliceous organisms.
Regional scarcity of Si and eutrophication due to nutrient excesses
of nutrient N, and P can lead to serious ecological hazards, including toxic dinoflagellate blooms. Results show that biological uptake
and subsequent release of Si on land strongly affect the Si river flux
to the coastal ocean. During the 350-a period of model simulation,
Si river discharge increased by 10% until 1940, decreasing since
then to below its 1700 value and continuing to drop, under the current IPCC IS92 projections of CO2, temperature and other forcings.
D.D. Li et al. / Applied Geochemistry 26 (2011) S289–S291
S291
Fig. 2. Preliminary results from the simulation of TOTEM-forced Si cycle from year 1700 to 2050. (A) Changes in dissolved Si (DSi) in terrestrial freshwater reservoir and
corresponding enhanced diatom biological uptake due to anthropogenic land-use changes and temperature. (B) Changes in coastal ocean dissolved Si and coastal diatom
biological uptake.
From 1700 to 1950, land-use changes, associated with slash and
burn of large areas of high-productivity land, caused a decrease
of global land vegetation. Dissolution of the phytolith-Si in soil humus and weathering of silicate minerals are the main dissolved Si
sources for rivers and groundwater. The decrease in Si uptake by
land biomass made more Si available for river discharge, causing
an increase in the Si river input until an increase in the land primary production reversed the process. Around 1950, the use of N
and P fertilizer on land, and subsequent leaching into aquatic systems, increased dramatically, driving the growth of coastal marine
primary producers, including such Siliceous organisms as diatoms,
silicoflagellates, and sponge spicules, and thus causing a decrease
of dissolved Si in the coastal surface ocean. The percentage decrease of coastal dissolved Si due to increased primary production
is greater than that of the surface open ocean due to the shorter
residence time of Si in global coastal water (2.7 years) compared
to that of the surface open ocean (10 years). The combination of
the relatively small size and location of the coastal ocean at the
junction of the land, atmosphere and open ocean make it highly
susceptible to changes in water chemistry, in situ biological production, and sedimentary storage induced by human activities on
land. Its buffer effect and fast response to perturbations are also
shown in the results of this preliminary study of the coupled Si
cycles.
In summary, model analysis has shown that Si delivery to the
ocean is controlled by its cycling between soil and land plants
and its interaction with the C cycle on land. The coastal ocean is
very sensitive to changes in water chemistry, in situ biological production, and sedimentary storage, because of its location and short
water and element residence time. Its fast response to perturbations is demonstrated in this study of the coupled biogeochemical
cycles of C and Si. Human perturbations on land have greatly altered the Si river delivery flux to the coastal ocean and altered
its Si content and bioproductivity. Further studies involving the
kinetics of terrestrial Si cycling, in estuaries and behind dams,
should help to quantify the effect of major river damming on the
river and groundwater discharges of Si to the coastal ocean and
its implications for the marine Si cycle.
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