Download Significant contribution of authigenic carbonate to marine carbon

LETTERS
PUBLISHED ONLINE: 19 JANUARY 2014 | DOI: 10.1038/NGEO2070
Significant contribution of authigenic carbonate to
marine carbon burial
Xiaole Sun* and Alexandra V. Turchyn
Carbon is removed from the Earth’s surface through the
formation and burial of carbon-bearing rocks and minerals1,2 .
The formation of calcium carbonate and its burial in marine
sediments accounts for around 80% of the total carbon
removed from the Earth’s surface. However, the fraction of
calcium carbonate that precipitates in the oceans, versus
that which precipitates authigenically in marine sediments,
is unclear. Here, we compile measurements of the calcium
concentration of pore fluids collected at 672 seafloor sites
around the globe to calculate the global flux of calcium within
marine sediments. We use these data, combined with alkalinity
measurements of pore fluids, to quantify authigenic calcium
carbonate precipitation. We estimate that the net calcium
flux into marine sediments that can be ascribed to authigenic
carbonate precipitation amounts to around 1 × 1012 mol yr−1 .
As such, we estimate that authigenic carbonate precipitation
accounts for at least 10% of global carbonate accumulation. We
show that much of the precipitation occurs along the eastern
margins of ocean basins, where organic matter delivery to the
sea floor is likely to be high. We suggest that authigenic calcium
carbonate precipitation represents a non-negligible component
of the global carbon cycle.
The long-term carbon cycle refers to the exchange of carbon between Earth’s surface environment and the vastly larger subsurface
reservoir1,2 . Carbon is supplied to the surface through volcanism
and metamorphism, and is removed from the surface through the
formation, burial and subsequent lithification of carbon-bearing
rocks and minerals1,2 . Specifically, the burial of calcium carbonate
and organic carbon in marine sediments is the largest sink for
carbon from Earth’s surface environment3–5 . However, deposition
of calcium carbonate and organic carbon on the ocean floor does
not guarantee burial because of the highly dynamic environment
within marine sediments6,7 . Most organic carbon deposited on the
ocean floor is ultimately consumed through microbially mediated
oxidation, releasing a large excess of dissolved inorganic carbon into
marine sediments8,9 . This dissolved inorganic carbon may diffuse
back into the overlying ocean, or may drive in situ precipitation
of calcium carbonate, called authigenic carbonate10 . The precipitation of authigenic carbonate has recently been invoked as a
critical process in the carbon cycle over Earth history; however,
the amount of authigenic carbonate precipitation in the modern
ocean has not been quantified10,11 . This lack of quantification for
in situ carbonate production in the modern ocean generates a
major deficit in any budget for the global carbon cycle. Here we
carry out the first global scale calculation of the flux of authigenic
carbonate precipitation and deep carbonate dissolution to help fill
in the missing pieces in the subsurface carbon cycle. We carry out
this calculation using pore fluid calcium concentrations because
sample handling and subsequent analytical challenges compromise
interrogating aqueous carbon species directly. Calcium is the main
cation involved in the trapping of dissolved inorganic carbon as
authigenic carbonate, including dolomite, in marine sediments.
We employ the pore fluid database from various ocean drilling
programmes (Deep Sea Drilling Program, DSDP; Ocean Drilling
Program, ODP; Integrated Ocean Drilling Program, IODP) in
which we have found 672 sites that have high-quality aqueous
calcium concentration data, and cover the world’s ocean floor. The
rate of change in calcium concentrations in pore fluids with depth
below the sea floor reflects the rate of the processes in the sediments
that are producing or consuming calcium (Methods). We find that
the magnitude of calcium flux, both positive and negative, varies
across the oceans, showing strong regional heterogeneity (Fig. 1a).
We find 330 sites where calcium concentrations in the pore fluid
decrease with depth (Fig. 1b); at these sites there is a subsurface sink
for aqueous calcium. However, there are 342 sites where calcium
concentrations in the pore fluids increase with depth; at these sites
there is a source of calcium in the subsurface that diffuses back into
the overlying ocean (Fig. 1b).
By comparing the calcium pore fluid profiles with the initial
reports for each site we are able to ascribe the calcium flux
within marine sediments to various subsurface processes (Fig. 2).
At 309 sites, there is strong evidence for precipitation of authigenic
carbonate. Alkalinity (mainly in the form of the bicarbonate ion,
HCO3 − ) is produced through organic matter oxidation, coupled in
deep-sea sediments largely to bacterial sulphate reduction9,12 :
2CH2 O + SO4 2− → 2HCO3 − + H+ + HS−
Alkalinity is also produced during anaerobic methane oxidation13 :
CH4 + SO4 2− → HCO3 − + HS− + H2 O
The increase in subsurface alkalinity, coupled to the relatively
high pH during anaerobic organic carbon oxidation, leads to
carbonate supersaturation, driving carbonate precipitation:
Ca2+ + 2HCO3 − → CaCO3 + CO2 + H2 O
Precipitation of one mole of calcium carbonate consumes one
mole of aqueous calcium and two moles of carbonate alkalinity.
We would expect the largest amount of authigenic carbonate
precipitation in locations with the largest microbial respiration
rates. Indeed, we find the largest downward flux of calcium within
sediments in coastal areas, specifically the eastern margins of the
ocean basins where primary production is particularly high and
there is high delivery of organic carbon to marine sediments6,14 .
Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. *e-mail: [email protected]
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201
NATURE GEOSCIENCE DOI: 10.1038/NGEO2070
LETTERS
s
e nt
di m
Lower Ca flux into se
30° N
nto s
Depth (mbsf)
60° N
edim
en
ts
b
Higher Ca flux
i
a 90° N
0°
0
5
30° S
1.0
2.0
150° E
180°
Depth (mbsf)
120° E
5.0
mmol m¬2 yr¬1
0
20
e n ts
0.5
90° E
e dim
0.2
60° E
of s
0.0
30° E
o f s e di m e nts
¬5.0 ¬2.0 ¬1.0 ¬0.5 ¬0.2
0°
o ut
30° W
ux
60° W
ut
Ca flux o
150° W 120° W 90° W
a fl
rC
he
Hig
L ower
60° S
180°
10
Calcium concentration
(mmol l¬1)
40
Figure 1 | The net flux of authigenic carbonate precipitation and dissolution in marine sediments. a, Global distribution of the calcium flux within marine
sediments; dots are the locations of studied sites; positive values (yellow areas) represent areas that have a net precipitation of authigenic carbonate;
negative values (grey areas) represent areas where calcium concentrations increase in the subsurface and calcium diffuses out of sediments. b, Schematic
examples of pore fluid calcium concentration profiles in marine sediments. The slope of the change in calcium concentrations with depth (mbsf, metres
below the sea floor) is related to the magnitude of the flux of calcium within marine sediments.
180
Number of sites
150
120
Carbonates
Alterations of basement
and volcanic ash
Non-carbonates
Ion exchange
Unable to interpret
90
60
30
0
¬5.0 ¬2.0 ¬1.0 ¬0.5 ¬0.2 0.0 0.2 0.5 1.0 2.0 5.0
Higher calcium flux out of sediments Higher calcium flux into sediments
Flux (mmol m¬2 yr¬1)
Figure 2 | Frequency distribution of the calcium flux within marine
sediments. The sources and sinks of calcium within marine sediments
include carbonate dissolution and precipitation (black), alteration of
basement and volcanic ash (dark grey), dissolution of other
calcium-bearing minerals (mainly gypsum, anhydrite, apatite, brine and so
on; horizontal lines) and ion exchange (vertical lines). Calcium profiles at a
few sites show little variation along depth and are labelled as ‘unable to
interpret’ (light grey).
We also calculate the alkalinity flux within marine sediments to
compare with the calculated calcium flux. If every mole of alkalinity
produced within marine sediments was consumed by molar
equivalent carbonate precipitation, we would expect no change
in alkalinity with depth but a decrease in calcium concentrations
with depth. Instead, we find a linear correlation between the
flux of authigenic calcium carbonate precipitation within marine
sediments and the alkalinity flux out of marine sediments (focusing
only on the flux of those sites that are dominated by precipitation of
calcium carbonate, Fig. 3). This illustrates that in the modern ocean
202
there is an excess of alkalinity produced to carbonate precipitated
in marine sediments. We cannot parse the alkalinity flux into the
relative microbial process, which may be a combination of bacterial
sulphate reduction and anaerobic methane oxidation, or other
processes. The alkalinity flux at the deeper sites is, on average, lower
than the coastal regions.
Many regions of the world are not characterized by precipitation
of authigenic carbonate within sediments, but rather a diffusive
calcium flux out of marine sediments; this is particularly the case
in the pelagic ocean. The source of aqueous calcium within these
sediments is partly attributed to the dissolution of carbonate or
other calcium-bearing minerals deeper within the sediment pile15
and alteration of basement and volcanic ash. The increase in
pore fluid calcium concentrations does not preclude authigenic
carbonate precipitation within these sediments, but suggests the
subsurface processes are dominated by dissolution or other release
of calcium into the pore fluid.
Although calcium carbonate deposition on marine sediments
has been well studied, understanding volumetrically how much is
precipitated globally within marine sediments has not previously
been addressed. We calculate that the net calcium flux into marine
sediments is ≈1 × 1012 mol yr−1 , of which more than 90% is
attributed to authigenic precipitation of calcium carbonate. This
is about 7–20% of the calcium carbonate accumulation in deep
marine sediments, excluding the shelf area (5–15 × 1012 mol yr−1 );
the uncertainty associated with calcium carbonate accumulation
rates can be 50% (ref. 3).
We also note that our calculation of authigenic carbonate
precipitation must be a minimum estimate for three reasons. First,
carbonate diagenesis studied in shallowest marine and marginal
settings, for example Florida Bay16 and the Bahama Banks17 , could
be accelerated over deeper marine settings; the ODP/IODP/DSDP
programmes do not sample these shallowest settings. Second, even
for the sampled areas, we do not have data for the upper 1.5 m
of sediments; the ODP/IODP/DSDP programmes did not sample
the uppermost sediments. The processes within this boundary
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2070
Coastal areas
Deep oceans
Linear (coastal areas)
Linear (deep oceans)
5
4
Increasing
3
y = 0.3x + 0.1
R2 = 0.63
y = 0.2x + 0.1
R2 = 0.87
ity
2
1
0
0
2
4
of authigenic carbonate (note that if this sulphide is reoxidized
back to sulphate then there is no impact on alkalinity). However,
protons are consumed during precipitation of sulphide minerals
(for example, pyrite)12 :
3HS− + 3H+ + 2FeOOH → 2FeS + 4H2 O + S0
amo
unt
of
ex
ce
ss
n
ali
alk
Net flux of authigenic carbonate precipitation
into sediments (mmol m¬2 yr¬1)
6
LETTERS
6
8
10
12
14
Alkalinity flux out of sediments (mmol m¬2 yr¬1)
Figure 3 | The calcium flux at sites dominated by carbonate precipitation
versus the corresponding alkalinity flux. Data points are divided into two
groups, sites from coastal areas (diamonds) and from deep oceans
(crosses) with their linear regression lines (the dashed line and the solid
line, respectively).
layer could be different from the diffusive fluxes below; it has
been suggested that oxic respiration should drive local carbonate
dissolution within the boundary layer not sampled by our pore fluid
database18,19 . Thus the total calcium flux to deeper sediments could
be much larger than we measure. Third, in many sites where calcium
concentrations decrease in the top tens of metres of pore fluids,
there is a subsequent increase in calcium concentrations deeper
in the sediment, suggesting two fluxes to the zone of carbonate
precipitation, one from the overlying ocean and one from deeper
within the sediment pile; our calculation deals only with the flux
down from the surface sediments.
We calculate the calcium flux diffusing back to the overlying
ocean from within marine sediments at ≈0.8 × 1012 mol yr−1 , of
which 20–40% is derived from carbonate dissolution. This estimate
is 5–10% of the global annual input of calcium from rivers3,5 .
Again, this is a minimum estimate for carbonate dissolution,
because it does not include carbonate dissolution at and near the
sediment–water interface.
We also calculate the global excess alkalinity flux out of marine
sediments at ≈5 × 1012 mol yr−1 , which is ≈15% of the global
riverine alkalinity flux (30 × 1012 mol yr−1 ; ref. 20). The global
alkalinity flux out of marine sediments is approximately five times
higher than the net flux of authigenic carbonate precipitation
within sediments. In situ carbonate formation is controlled by the
carbonate saturation state, which is linked to alkalinity released
during organic carbon oxidation10 . Although excess alkalinity is
produced in modern marine sediments, calcium concentrations in
the pore fluids are never observed to decrease to zero; calcium
concentrations tend to decrease by 70–90% and remain at these low
levels (2–3 mmol l−1 ). Pore fluid pH may play an important role, as
the effect of pH has an order-of-magnitude effect on the carbonate
ion concentration and thus dramatically impacts the carbonate
saturation state within marine sediments.
Organic carbon oxidation can increase or decrease pH, depending on the electron acceptor12,21 . Modern organic carbon
oxidation occurs primarily through bacterial sulphate reduction,
during which pH is poised at 6.7 (ref. 22). The lowering of pH
from seawater values (8.1) to 6.7 decreases the carbonate ion
concentration by more than two orders of magnitude, leading to
conditions that are undersaturated with respect to carbonate. It
could be, therefore, that sulphate reduction and accumulation of
pore fluid sulphide leads to conditions that limit the formation
This would help mitigate the pH decrease in sediments and
promote authigenic carbonate precipitation. Thus we speculate
that in marine environments characterized by iron limitation
for pyrite formation, the decrease in pore fluid pH may limit
the formation of authigenic carbonate22 . Below the sulphate
minimum zone, anaerobic methane oxidation, converges to a
pH of 7.9 (ref. 21), which would promote authigenic carbonate
precipitation as recently shown by carbon isotopes preserved
in authigenic carbonate in Cascadia23 . It could be that the
coincident decrease in sulphate and calcium concentrations often
seen in marine pore fluids is due to sulphate consumed during
anaerobic methane oxidation and calcium bound into authigenic
carbonate at the same depth.
Our calculations strongly suggest that authigenic carbonate
precipitation should be included in all calculations involving
calcium carbonate accumulation in marine sediments. Subseafloor
sediments are both a carbon and calcium reactor and here we
demonstrate that the global ocean can be differentiated into
areas where the sediments source calcium back to the ocean and
areas that consume aqueous calcium into the sediments. This
precipitation-induced flux of calcium and dissolved inorganic
carbon is a non-negligible part of both the global biogeochemical
calcium and carbon cycles. Shallow carbonate platforms, boundary
layer processes and marginal marine environment, which are not
represented here, could have even larger precipitation-induced
fluxes of calcium; our flux calculations can be considered as
a minimum estimate. These processes were necessarily different
in the geological past when the oxic/anoxic boundary varied
in the ocean and sediments and the aerial extent of shallow
platforms was considerably larger than it is in the present.
In theory, extrapolation of our global fluxes onto continental
shelf environments and further reconstruction of these shelf
environments in the past could yield valuable insights into the role
of this process in regulating the global calcium and carbon cycles
over geological time.
Methods
To calculate the calcium flux within marine sediments, we compile all of the
available calcium concentration profiles of pore fluids acquired through the DSDP,
ODP and IODP Janus database. This compilation is limited to sites with good data
quality and calcium profiles starting from the uppermost sediments. The flux of
aqueous calcium (mmol m−2 yr−1 ) at each site is calculated by Fick’s first law and
the measured porosity from the initial reports for each site:
dC
F = −ϕ×DCa ×
dz
where ϕ is the porosity of the sediments, which is calculated by averaging the
porosity of each selected site. dC/dz is obtained from the first derivative of a best fit
line based on the pore fluid gradient in the upper part of the sediments, that is, before
the calcium concentration profiles start to inflect (see Supplementary Information
for error evaluation associating with this linear regression). A positive flux means
aqueous calcium diffuses into the sediments and a negative flux means aqueous
calcium diffuses out of the sediments. DCa is the diffusion coefficient of calcium in
pore fluids, which is corrected for sediment tortuosity using the equation24 :
DCa = DSW /(1 − ln(ϕ 2 ))
where DSW = 4.41 × 10−6 cm2 s−1 , the diffusion coefficient of calcium in sea water
assuming average temperature of 4 ◦ C in surface sediments. Therefore, each site has
a specific diffusion coefficient (DCa ) based on the porosity. We vary the exponent
in the tortuosity equation from 0.5 to 3 but the calculated flux varies by only up
to 50%, therefore, the exponent of ≈2 is chosen here because it has been shown
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2070
LETTERS
to work well for most marine sediments25 . The flux of alkalinity for each site is
calculated in the same way as the calcium flux calculations. The diffusion coefficient
of alkalinity is 6 × 10−6 cm2 s−1 for bicarbonate ions in sea water assuming an
average temperature of 4 ◦ C (ref. 26).
Our calculation of the site-specific calcium fluxes allows us to extrapolate to the
total global rates of carbonate precipitation and dissolution by making a reasonable
estimate of the area covered by certain fluxes. Interpolation of the calcium flux
in the entire world’s oceans is carried out using an inverse distance-weighted
technique, which is a deterministic spatial interpolation method based on the
assumption that the interpolating surface should be influenced most by the nearby
points and less by the more distant points. This allows us to control the impact
of known flux on the interpolated areas based on their distance from the sampled
sites, that is, the flux of each non-sampled area is mapped using distance-weighted
combination of a set of sampled sites around it. Finally the global distribution of
the calcium flux is visualized by ESRI ArcMap 10.0. Afterwards, the map is divided
into areas by polygons based on the magnitude of the calcium flux. The total global
calcium flux is the sum of those areas multiplying the average calcium flux in
each corresponding area.
Received 23 July 2013; accepted 17 December 2013;
published online 19 January 2014
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Acknowledgements
This work was supported by an ERC Starting Investigator Grant (307582) to A.V.T.
J. A. A. Dickson read this paper before submission and his comments greatly
improved the manuscript.
Author contributions
A.V.T. conceived this project. X.S. conducted the calculations and data analysis.
Both authors discussed the results and implications and commented on the
manuscript at all stages.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints. Correspondence
and requests for materials should be addressed to X.S.
Competing financial interests
The authors declare no competing financial interests.
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