Global carbon sequestration in tidal, saline wetland soils

Abstract

1] Wetlands represent the largest component of the terrestrial biological carbon pool and thus play an important role in global carbon cycles. Most global carbon budgets, however, have focused on dry land ecosystems that extend over large areas and have not accounted for the many small, scattered carbon-storing ecosystems such as tidal saline wetlands. We compiled data for 154 sites in mangroves and salt marshes from the western and eastern Atlantic and Pacific coasts, as well as the Indian Ocean, Mediterranean Ocean, and Gulf of Mexico. The set of sites spans a latitudinal range from 22.4°S in the Indian Ocean to 55.5°N in the northeastern Atlantic. The average soil carbon density of mangrove swamps (0.055 ± 0.004 g cm À3) is significantly higher than the salt marsh average (0.039 ± 0.003 g cm À3). Soil carbon density in mangrove swamps and Spartina patens marshes declines with increasing average annual temperature, probably due to increased decay rates at higher temperatures. In contrast, carbon sequestration rates were not significantly different between mangrove swamps and salt marshes. Variability in sediment accumulation rates within marshes is a major control of carbon sequestration rates masking any relationship with climatic parameters. Globally, these combined wetlands store at least 44.6 Tg C yr À1 and probably more, as detailed areal inventories are not available for salt marshes in China and South America. Much attention has been given to the role of freshwater wetlands, particularly northern peatlands, as carbon sinks. In contrast to peatlands, salt marshes and mangroves release negligible amounts of greenhouse gases and store more carbon per unit area.

Full text

Global carbon sequestration in tidal, saline wetland soils
Gail L. Chmura
Department of Geography and Centre for Climate and Global Change Research, McGill University, Montreal,
Quebec, Canada
Shimon C. Anisfeld
Yale School of Forestry and Environmental Studies, New Haven, Connecticut, USA
Donald R. Cahoon
1
and James C. Lynch
1
National Wetlands Research Center, U.S. Geological Survey, Lafayette, Louisiana, USA
Received 16 April 2002; revised 4 June 2003; accepted 23 July 2003; published 10 December 2003.
[1] Wetlands represent the largest component of the terrestrial biological carbon pool and
thus play an important role in global carbon cycles. Most global carbon budgets,
however, have focused on dry land ecosystems that extend over large areas and have not
accounted for the many small, scattered carbon-storing ecosystems such as tidal saline
wetlands. We compiled data for 154 sites in mangroves and salt marshes from the western
and eastern Atlantic and Pacific coasts, as well as the Indian Ocean, Mediterranean
Ocean, and Gulf of Mexico. The set of sites spans a latitudinal range from 22.4°Sinthe
Indian Ocean to 55.5 °N in the northeastern Atlantic. The average soil carbon density of
mangrove swamps (0.055 ± 0.004 g cm
3
) is significantly higher than the salt marsh
average (0.039 ± 0.003 g cm
3
). Soil carbon density in mangrove swamps and Spartina
patens marshes declines with increasing average annual temperature, probably due to
increased decay rates at higher temperatures. In contrast, carbon sequestration rates were
not significantly different between mangrove swamps and salt marshes. Variability in
sediment accumulation rates within marshes is a major control of carbon sequestration
rates masking any relationship with climatic parameters. Globally, these combined
wetlands store at least 44.6 Tg C yr
1
and probably more, as detailed areal inventories are
not available for salt marshes in China and South America. Much attention has been given
to the role of freshwater wetlands, particularly northern peatlands, as carbon sinks. In
contrast to peatlands, salt marshes and mangroves release negligible amounts of
greenhouse gases and store more carbon per unit area.
INDEX TERMS: 1890 Hydrology:
Wetlands; 3020 Marine Geology and Geophysics: Littoral processes; 4235 Oceanography: General: Estuarine
processes; K
EYWORDS: salt marsh, mangroves, soil carbon density, greenhouse gas flux, wetland soil
Citation: Chmura, G. L., S. C. Anisfeld, D. R. Cahoon, and J. C. Lynch, Global carbon sequestration in tidal, saline wetland soils,
Global Biogeochem. Cycles, 17(4), 1111, doi:10.1029/2002GB001917, 2003.
1. Introduction
[2] Wetlands represent the largest component of the
terrestrial biological carbon pool [Dixo n and Krankina,
1995] and thus play an important role in global carbon
cycles [Sahagian and Melack, 1988]. Most global carbon
budgets, however, have focused on dry land ecosystems that
extend over large areas and have not accounted for the many
small, scattered carbon-storing ecosystems such as man-
grove swamps and salt marshes [Atjay et al., 1979; Olson et
al., 1983]. Syntheses that do include wetlands typically
exclude tidal saline wetlands (TSWs) because there have
been no empirically based estimates of their carbon storage
potential.
[
3] In this study we used published and our own unpub-
lished data to estimate the amount of carbon stored globally
in soils of salt marshes and mangrove swamps. We then
examine spatial patterns in carbon density and storage with
respect to climate parameters, as well as local variability, to
determine which are important controls.
[
4] Tidal saline wetlands, i.e., salt marshes and mangrove
swamps, are found on sheltered marine coastlines. The
former, dominated by herbaceous vegetation, exist in cli-
mates ranging from arctic to subtropical. Mangrove swamps
replace salt marshes in the subtropics, around 25°N and S
and are dominated by woody vegetation [Mitsch and
Gosselink, 2000]. Mangrove swamps and salt marshes are
intertidal ecosystems; in order to persist, their surface
elevations must increase with rising sea level.
GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 17, NO. 4, 1111, doi:10.1029/2002GB001917, 2003
1
Now at U.S. Geological Survey, Patuxent Wildlife Research Center,
Laurel, Maryland, USA.
Copyright 2003 by the American Geophysical Union.
0886-6236/03/2002GB001917$12.00
22 - 1

Table 1. Soil Carbon Density, Soil Carbon Accumulation Rates, and Climate Normals at Salt Marsh Sites
a
Location: Site Name or Core Number, State/Province, Country Latitude Longitude Density, g cm
3
Rate, g m
2
yr
1
C Data Source
b
Average Annual Temperature, °C
Normals Period
c
Minimum Maximum Overall
Gulf of Mexico °N °W
Aransas, Tex. 28.4 96.8 0.040 178 1 17.2 25.3 21.2 1
Fina la Terre, La. 29.0 91.0 0.027 136 2 15.5 24.5 20.0 1
Fina la Terre, La. 29.0 91.0 0.018 18 2 15.5 24.5 20.0 1
San Bernard, Tex. 29.1 95.6 0.033 203 1 16.6 25.3 20.9 1
Old Oyster Bayou, La. 29.3 91.1 0.019 84 3 15.5 24.5 20.0 1
Bayou Chitigue, La. 29.3 90.6 0.016 516 3 15.4 25.3 20.4 1
Rockefeller Refuge, La. 29.5 92.7 0.028 309 2 15.2 25.2 20.2 1
Rockefeller Refuge, La. 29.5 92.7 0.033 27 2 15.2 25.2 20.2 1
Lafourche Parish, La. 29.5 90.3 0.019 186 4 15.4 25.3 20.4 1
Cameron Parish, La. 29.5 93.2 0.010 41 4 15.9 24.7 20.3 2
Cameron Parish, La. 29.5 93.2 0.010 115 4 15.9 24.7 20.3 2
Barataria Basin, La. 29.5 90.0 0.013 185 5 15.4 25.3 20.4 1
Barataria Basin, La. 29.5 90.0 0.012 71 5 15.4 25.3 20.4 1
Barataria Basin, La. 29.5 90.0 0.012 93 5 15.4 25.3 20.4 1
Unit 1, Marsh Island Refuge, La. 29.5 91.9 0.110 318 6 15.2 25.2 20.2 1
Unit 1, Marsh Island Refuge, La. 29.5 91.9 0.109 763 6 15.2 25.2 20.2 1
Unit 15, Rockefeller Wildlife Refuge, La. 29.6 92.7 0.120 349 6 15.2 25.2 20.2 1
Unit 15, Rockefeller Wildlife Refuge, La. 29.6 92.7 0.119 657 6 15.2 25.2 20.2 1
Three Bayous, La. 29.6 90.1 0.014 116 3 15.2 25.2 20.2 1
Rockefeller Wildlife Refuge unit 14, La. 29.7 92.7 0.116 337 6 15.2 25.2 20.2 1
Rockefeller Wildlife Refuge unit 14, La. 29.7 92.7 0.093 448 6 15.2 25.2 20.2 1
McFaddin National Wildlife Refuge, Tex. 29.7 94.1 0.012 95 3 15.1 25.6 20.4 1
Sabine National Wildlife Refuge unit 3, La. 29.9 93.5 0.190 1713 6 15.9 24.7 20.3 1
Sabine National Wildlife Refuge unit 3, La. 29.9 93.5 0.121 714 6 15.9 24.7 20.3 1
St. Bernard Parish, La. 30.0 89.9 0.028 140 7
St. Marks, Fla. 30.1 84.2 0.025 44 3 14.1 25.6 19.9 2
Biloxi Bay, Miss. 30.4 88.9 0.027 153 1 15.0 24.4 19.7 2
Northeastern Atlantic °N °E
St. Annaland, Netherlands 51.5 4.1 0.041 277 8
St. Annaland, Netherlands 51.5 4.1 0.041 139 8
Scheldt, Netherlands 51.5 4.1 0.029 587 9
Scheldt, Netherlands 51.5 4.1 0.020 650 9
Dengie Marsh, UK 51.7 0.9 0.041 187 8 7.2 12.9 10.1 2
Dengie Marsh, UK 51.7 0.9 0.041 139 8 7.2 12.9 10.1 2
Dengie Marsh, UK 51.7 0.9 0.041 159 8 7.2 12.9 10.1 2
Dengie Marsh, UK 51.7 0.9 0.041 110 8 7.2 12.9 10.1 2
Hut marsh, UK 53.0 0.7 0.027 165 10
Hut marsh, UK 53.0 0.7 0.027 77 10
Skallingen, Denmark 55.5 8.4 0.021 11
Skallingen, Denmark 55.5 8.4 0.027 11
Mediterranean °N °E
Rhone Delta, France 43.3 4.6 0.073 161 12
Northeastern Pacific °N °W
Tijuana Slough, Calif. 32.5 117.1 0.018 343 13 12.8 22.4 17.6 1
Tijuana Slough, Calif. 32.6 117.1 0.017 43 14 12.8 22.4 17.6 1
Tijuana Slough, Calif. 32.6 117.1 0.040 14 12.8 22.4 17.6 1
22 - 2 CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS

Table 1. (continued)
Location: Site Name or Core Number, State/Province, Country Latitude Longitude Density, g cm
3
Rate, g m
2
yr
1
C Data Source
b
Average Annual Temperature, °C
Normals Period
c
Minimum Maximum Overall
Alviso, San Francisco Bay, Calif. 37.5 122.0 0.009 385 15 10.4 20.6 15.5 1
Bird Island, San Francisco Bay, Calif. 37.6 122.2 0.014 54 15 10.4 20.6 15.5 1
Uculet, B. C. 48.9 125.5 0.017 16 5.5 12.7 9.1 4
Northwestern Atlantic °N °W
Cedar Island National Wildlife Refuge, N. C. 35.0 76.4 0.022 70 3 12.1 22.0 17.0 2
Oregon Inlet, N. C. 35.9 75.6 59 17 12.3 20.8 16.6 1
Oregon Inlet, N. C. 35.9 75.6 21 17 12.3 20.8 16.6 1
Jacob’s Creek, N. C. 35.3 76.8 146 17 12.3 20.8 16.6 1
Jacob’s Creek, N. C. 35.3 76.8 107 17 12.3 20.8 16.6 1
MC4, Chesapeake Bay, Md. 38.3 75.9 0.040 308 18 8.7 20.2 14.4 1
MCL8, Chesapeake Bay, Md. 38.3 75.9 0.027 213 18 8.7 20.2 14.4 1
MCL15, Chesapeake Bay, Md. 38.3 75.9 0.044 340 18 8.7 20.2 14.4 1
Sybil 1, Conn. 41.2 72.6 0.054 136 19 5.3 15.2 10.3 1
Hoadley 1, Conn. 41.2 72.0 0.037 154 19 5.3 15.2 10.3 1
Hoadley 2, Conn. 41.2 72.0 0.040 169 19 5.3 15.2 10.3 1
Hoadley 3, Conn. 41.2 72.0 0.035 114 19 5.3 15.2 10.3 1
East River 1, Conn. 41.2 72.7 0.030 134 19 5.3 15.2 10.3 1
East River 2, Conn. 41.2 72.7 0.060 204 19 5.3 15.2 10.3 1
Sluice 1, Conn. 41.2 72.7 0.026 99 19 5.3 15.2 10.3 1
Sluice Core 2, Conn. 41.2 72.7 0.045 85 19 5.3 15.2 10.3 1
Leetes 1, Conn. 41.2 72.7 0.039 153 19 5.3 15.2 10.3 1
Leetes 2, Conn. 41.2 72.7 0.030 93 19 5.3 15.2 10.3 1
Sybil 2, Conn. 41.2 72.6 0.029 72 19 5.3 15.2 10.3 1
Sybil 3, Conn. 41.2 72.6 0.046 116 19 5.3 15.2 10.3 1
Branford River 1, Conn. 41.2 72.6 0.029 182 19 5.3 15.2 10.3 1
Branford River 2, Conn. 41.2 72.6 0.026 181 19 5.3 15.2 10.3 1
Farm River, Conn. 41.2 72.9 0.025 70 20 5.3 15.2 10.3 1
Bloom’s Point, Little Narragansett Bay, Conn. 41.3 71.9 0.036 62 21 5.3 15.2 10.3 1
Inlet 1, Nauset Bay, Mass. 41.5 70.0 0.028 105 22 5.7 14.0 9.8 1
Nauset Bay, Mass. 41.5 70.0 0.041 155 22 5.7 14.0 9.8 1
Wells National Estuarine Research Reserve, Maine 43.3 70.5 0.020 16 1.5 12.5 7.0 1
Dipper a, Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.048 85 23 0.2 9.8 4.8 5
Dipper d, Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.033 63 23 0.2 9.8 4.8 5
Little Lepreau, Bay of Fundy, N. B. 45.1 66.5 0.059 80 23 0.2 9.8 4.8 5
Chance Harbour, Bay of Fundy, N. B. 45.1 66.3 0.038 72 23 0.2 9.8 4.8 5
DH SA 3, Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.035 23 0.2 9.8 4.8 5
DH SA 2, Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.034 23 0.2 9.8 4.8 5
DH SA 1, Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.036 23 0.2 9.8 4.8 5
DH Sp3, Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.047 23 0.2 9.8 4.8 5
DH Sp2, Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.036 23 0.2 9.8 4.8 5
DH Sp1, Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.043 23 0.2 9.8 4.8 5
Bocabec River, Bay of Fundy, N. B. 45.1 67.0 0.034 456 16 0.2 9.8 4.8 5
Bocabec River, Bay of Fundy, N. B. 45.1 67.0 0.046 113 16 0.2 9.8 4.8 5
Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.030 445 16 0.2 9.8 4.8 5
Dipper Harbour, Bay of Fundy, N. B. 45.1 66.4 0.033 94 16 0.2 9.8 4.8 5
Cape Enrage, Bay of Fundy, N. B. 45.6 64.8 0.018 582 16 0.2 9.8 4.8 5
Cape Enrage, Bay of Fundy, N. B. 45.6 64.8 0.023 186 16 0.2 9.8 4.8 5
Lorneville, Bay of Fundy, N. B. 45.2 66.2 0.028 277 16 0.2 9.8 4.8 5
Lorneville, Bay of Fundy, N. B. 45.2 66.2 0.033 330 16 0.2 9.8 4.8 5
CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS 22 - 3

Location: Site Name or Core Number, State/Province, Country Latitude Longitude Density, g cm
3
Rate, g m
2
yr
1
C Data Source
b
Average Annual Temperature, °C
Normals Period
c
Minimum Maximum Overall
St. Martins, Bay of Fundy, N. B. 45.3 65.5 0.027 265 16 0.2 9.8 4.8 5
St. Martins, Bay of Fundy, N. B. 45.9 65.5 0.024 928 16 0.2 9.8 4.8 5
Wood Point, Bay of Fundy, N. B. 45.8 64.4 0.026 264 16 0.2 9.8 4.8 5
Wood Point, Bay of Fundy, N. B. 45.8 64.4 0.025 253 16 0.2 9.8 4.8 5
Kouchigouguacis Lagoon, Gulf of St. Lawrence, N. B. 46.7 64.9 0.031 102 24 0.6 10.0 5.3 6
Bay St-Louis, Gulf of St. Lawrence, N. B. 46.8 64.9 0.032 93 24 0.6 10.0 5.3 6
Tabusintac Bay, Gulf of St. Lawrence, N. B. 47.4 65.0 0.033 66 24 0.6 10.0 5.3 6
Malpeque Bay, Gulf of St. Lawrence, Prince Edward Island 46.5 63.7 0.029 71 24 0.9 3
Brackley Bay Gulf of St. Lawrence, Prince Edward Island 46.4 63.2 0.035 89 24 0.9 3
Pubnico Harbour, Gulf of Maine, N. S. 43.6 65.3 0.041 113 24 2.8 10.7 6.8 7
Cheboque Harbour, Gulf of Maine, N. S. 43.8 66.1 0.045 75 24 2.8 10.7 6.8 7
Little River Harbour, Gulf of Maine, N. S. 43.7 66.1 0.078 304 24 2.8 10.7 6.8 7
Cole Harbour, N. S. 44.7 63.4 0.042 161 24 0.4 11.6 6.0 1
Lawrencetown Lake, N. S. 44.7 63.4 0.024 60 24 0.4 11.6 6.0 1
Chezzetcook Inlet, N. S. 44.7 63.4 0.038 106 24 0.4 11.6 6.0 1
Rustico Bay, Prince Edward Island 46.4 63.2 0.034 125 24 0.9 3
a
Carbon values are calculated according to the formula of Craft et al. [1991].
b
1, Callaway et al. [1997]; 2, Cahoon [1994]; 3, D. R. Cahoon and J. C. Lynch, unpublished data, 1993; 4, Cahoon and Turner [1989]; 5, Hatton [1981]; 6, Bryant and Chabreck [1998]; 7, Markewich et al.
[1998]; 8, Callaway et al. [1996]; 9, Oenema and Delaune [1988]; 10, French and Spencer [1993]; 11, Morris and Jensen [1998]; 12, Hensel et al. [1999]; 13, Cahoon et al. [1996]; 14, D. R. Cahoon,
unpublished data, 1993; 15, Patrick and DeLaune [1990]; 16, G. L. Chmura, unpublished data, 1997; 17, Craft et al. [1993]; 18, Kearney and Stevenson [1991]; 19, Anisfeld et al. [1999] and S. C. Anisfeld,
unpublished data, 1995; 20, McCaffrey and Thomson [1980]; 21, Orson et al. [1998]; 22, Roman et al. [1997]; 23, Connor et al. [2001]; 24, Chmura and Hung [2003].
c
Climate normals were calculated over different periods: 1, 1961 – 1990; 2, 1971 – 2000; 3, over history of station (93 years); 4, 1957/1959 –1990; 5, 1946 – 1990; 6, 1965 – 1990; 7, 1940 – 1990; and 8, 1951 –
1980.
Table 1. (continued)
22 - 4 CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS

Table 2. Soil Carbon Density, Soil Carbon Accumulation Rates, and Climate Normals at Mangrove Sites
Location: Site Name or Core Number, State/Province, Country Latitude Longitude Density, g cm
3
Rate, g m
2
yr
1
C Data Source
a
Average Annual Temperature, °C
Normals Period
b
Minimum Maximum Overall
Gulf of Mexico °N °W
CAR, Columbia 11.0 74.2 0.071 4
RIN, Columbia 11.0 74.2 0.061 4
HON, Columbia 11.0 74.2 0.058 4
Terminos Lagoon, Boca Chica, Mexico 18.7 91.5 0.047 308 5 21.3 31.1 26.2 3
Terminos Lagoon, Estero Pargo, Mexico 18.7 91.5 0.052 194 5 21.3 31.1 26.2 3
Terminos Lagoon, Estero Pargo, Mexico 18.7 91.5 0.058 146 5 21.3 31.1 26.2 3
Terminos Lagoon, Boca Chica, Mexico 18.7 91.5 0.051 654 5 21.3 31.1 26.2 3
FL keys: Lignumvitae, to Key Largo 25.0 80.6 0.036 143 1 21.7 28.7 25.2 2
FL keys: Lignumvitae, to Key Largo 25.0 80.6 0.037 100 1 21.7 28.7 25.2 2
S1, Shark River Estuary, Fla. 25.0 80.8 0.051 6 18.9 28.3 23.6 2
S3, Shark River Estuary, Fla. 25.0 81.1 0.039 6 18.9 28.3 23.6 2
S4, Shark River Estuary, Fla. 25.0 81.1 0.046 6 18.9 28.3 23.6 2
S6, Shark River Estuary, Fla. 25.0 81.1 0.050 6 18.9 28.3 23.6 2
Rookery Bay, Fla. (Fringe) 26.0 81.7 0.036 265 7 17.
6
29.4 23.5 2
Rookery Bay, Fla. (Basin) 26.0 81.7 0.066 381 7 17.
6
29.4 23.5 2
Rookery Bay, Fla. (Exposed Island) 26.0 81.7 0.052 338 7 17.
6
29.4 23.5 2
Rookery Bay, Fla. (Sheltered Island) 26.0 81.8 0.049 222 7 17.
6
29.4 23.5 2
Southeast Everglades, Fla. 25.3 80.6 0.040 8 17.9 28.5 23.2 2
Southeast Everglades, Fla. 25.3 80.6 0.033 8 17.9 28.5 23.2 2
Southeast Everglades, Fla. 25.3 80.6 0.027 8 17.9 28.5 23.2 2
Rookery Bay, Fla. 26.0 81.7 0.043 142 5 17.6 29.4 23.5 2
Rookery Bay, Fla. 26.0 81.7 0.050 154 5 17.6 29.4 23.5 2
Rookery Bay, Fla. 26.0 81.7 0.044 154 5 17.6 29.4 23.5 2
Rookery Bay, Fla. 26.0 81.7 0.067 170 5 17.6 29.4 23.5 2
Rookery Bay, Fla. 26.0 81.7 0.024 20 2 17.6 29.4 23.5 2
Rookery Bay, Fla. 26.0 81.7 0.033 39 2 17.6 29.4 23.5 2
Pacific and Indian Ocean °N °E
Kosrae 5.3 163.0 0.023 3 22.7 31.2 26.9 2
Kosrae 5.3 163.0 0.040 3 22.7 31.2 26.9 1
Kosrae 5.3 163.0 0.031 3 22.7 31.2 26.9 1
°S °E
HM 2, Hinchinbrook Channel, Australia 18.5 146.3 67 9 18.8 28.8 23.8 1
HMF 3, Hinchinbrook Channel, Australia 18.5 146.3 48 9 18.8 28.8 23.8 1
HMF 4, Hinchinbrook Channel, Australia 18.5 146.3 336 9 18.8 28.8 23.8 1
Core 576, Herbert River region, Australia 18.5 146.3 26 10 18.8 28.8 23.8 1
Core 577, Herbert River region, Australia 18.5 146.3 168 10 18.8 28.8 23.8 1
Core 582, Herbert River region, Australia 18.5 146.3 84 10 18.8 28.8 23.8 1
Core 583, Herbert River region, Australia 18.5 146.3 336 10 18.8 28.8 23.8 1
Core 584, Herbert River region, Australia 18.5 146.3 300 10 18.8 28.8 23.8 1
Core 585, Herbert River region, Australia 18.5 146.3 100 10 18.8 28.8 23.8 1
Core 586, Herbert River region, Australia 18.5 146.3 71 10 18.8 28.8 23.8 1
Core 587, Herbert River region, Australia 18.5 146.3 97 10 18.8 28.8 23.8 1
Umengi estuary, Durban, South Africa 22.4 31.0 0.107 11 17.0 25.0 21.0 1
Umengi estuary, Durban, South Africa 22.4 31.0 0.105 11 17.0 25.0 21.0 1
Umengi estuary, Durban, South Africa 22.4 31.0 0.115 11 17.0 25.0 21.0 1
Umengi estuary, Durban, South Africa 22.4 31.0 0.109 11 17.0 25.0 21.0 1
CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS 22 - 5

[5] Both types of wetlands are noted for exceptional rates
of production, rivaling that of productive agricultural lands
[Odum, 1959]. Root to shoot ratios of salt marsh plants
range from 1.4 to 50 [see review in the work of Smith et al.,
1979], thus a large portion of the primary production is
found in belowground biomass that contributes to vertically
extensive deposits, as great as 8 m deep [e.g., Scott and
Greenberg, 1983]. Mangrove deposits can attain compara-
ble depths [e.g., Woodroffe et al., 1993]. In mangr ove
swamps, peat formation primarily occurs through deposition
and slow turnover of mangrove roots as aboveground
tissues rapidly decay or are transported from the system
[Middleton and McKee, 2001].
[
6] The global importance of wetlands as carbon sinks
is widely recognized [Adams et al., 1990; Watson et al.,
2000]. Because of their great expanse, the role of peat-
lands as carbon sinks has received the greatest attention
by researchers [Roulet, 2000], who report rates of soil
carbonsequestrationfrom20to30gCm
2
yr
1
.
However, decomposition of peatland soils results in high
rates of CH
4
flux [Bartlett and Harriss, 1993], reducing
their value as a means to moderate greenhouse warming.
The soil chemistry and carbon accumulation patterns of
TSWs differ in several respects from those of peatlands
or other freshwater wetlands. For one thing, carbon
concentrations in TSWs are often lower than in peatlands,
since tidal wetlands can receive significant inputs of fine-
grained minerals (through tidal exchange with adjacent
coastal waters), which dilute the inputs of organic matter
from above- and belowground production. On the other
hand, rates of soil accumulation in tidal wetlands tend to
be higher than in peatlands, so net carbon sequestration is
potentially substantial. Perhaps most important is that the
presence of abundant sulfate in TSW soils hinders CH
4
production, so these ecosystems are considered to be
negligible sources of CH
4
, if not CH
4
sinks [Bartlett
and Harris, 1993; Magenheimer et al., 1996; Giani et al.,
1996]. Studies of gas fluxes in TSWs suggest that
emissions of the greenhouse gas N
2
O are also negligible
[Smith et al., 1983; DeLaune et al., 1990].
2. Methods
[7] We found 26 studies (Tables 1 and 2) that reported soil
carbon densities or parameters necessary for calculation of
soil carbon densities (soil bulk density and percent of soil
organic matter or percent of soil carbon) in TSWs. From
these studies (and our unpublished data) we compiled data
for 154 sites in TSWs from the western and eastern Atlantic
and Pacific coasts, as well as the Indian Ocean, Mediterra-
nean Ocean, and Gulf of Mexico (Tables 1 and 2). The set
of sites spans a latitudinal range from 22.4°S in the Indian
Ocean [Naidoo, 1980] to 55.5°N in the northeastern Atlan-
tic [ Morris and Jensen, 1998]. Most of the data found
(75%) were from salt marshes (Table 1). Some of the
measurements came from the same estuary or a contiguous
wetland area, allowing us to compare local variability to
large-scale variability.
[
8] In most cases the carbon densities reported were
derived from measurements of loss on ignition (LOI), but
Table 2. (continued)
Location: Site Name or Core Number, State/Province, Country Latitude Longitude Density, g cm
3
Rate, g m
2
yr
1
C Data Source
a
Average Annual Temperature, °C
Normals Period
b
Minimum Maximum Overall
Umengi estuary, Durban, South Africa 22.4 31.0 0.097 11 17.0 25.0 21.0 1
Umengi estuary, Durban, South Africa 22.4 31.0 0.106 11 17.0 25.0 21.0 1
a
1, Callaway et al. [1997]; 2, D. R. Cahoon and J. C. Lynch, unpublished data, 1994; 3, D. R. Cahoon, unpublished data, 1997; 4, Cardona and Botero [1998]; 5, Lynch [1989]; 6, Chen and Twilley [1999];
7, Cahoon and Lynch [1997]; 8, Ross et al. [2000]; 9, Alongi et al. [1999]; 10, Brunskill et al. [2002]; 11, Naidoo [1980].
b
Climate normals were calculated over different periods: 1, 1961 – 1990; 2, 1971 – 2000; and 3, 1951 – 1980.
22 - 6 CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS

the Walkley Black and dichromate digestion methods were
also used, as well as percent of carbon determined from
carbon analyzers. LOI measurements of mangrove soils
were transformed to organic carbon by dividing by a factor
of 1.724 [Allen, 1974], but for salt marsh soils we applied
the quadratic relationship specific to salt marshes reported
by Craft et al. [1991]:
percent of organic carbon ¼ð0:04ÞLOI þð0:0025ÞLOI
2
:
If data were reported as percent of carbon using other
methods we transformed values to be consistent with our
data set.
[
9] Many investigators also reported rates of vertical soil
accumulation, allowing us to compile carbon accumula tion
rates for 124 sites. (This includes the Australian mangroves
for which only rates of carbon accumulation were reported
[Brunskill et al., 2002].) Vertical soil accumulation rates
represent averages over variable periods, from 1 to
100 years. The depth of the maximum concentration of
137
Cs (associated with peak fallout in 1963 [DeLaune et al.,
1978]) or the pattern of unsupported
210
Pb with depth was
employed to determine long-term average rates of vertical
accretion. Where reports made available both rates and
carbon densities, we averaged carbon density over the dated
depths.
[
10] At 13 s ites (Florida mangroves, Louisiana salt
marshes, and Bay of Fundy salt marshes), we employed
clay-marker horizons [Cahoon and Turner, 1989; Chmura
et al., 2001] to determine soil accumulation rates. Measure-
ment of the thickness of soil accumulate d over these
horizons in a given period provides a short-term accretion
rate. Although a few samples were lost because of erosion,
we used only positive accretion values in our calculations.
To estimate rates of carbon accumulation we multiplied
rates by carbon density of the surface 2 cm of paired soil
samples.
[
11] A global inventory of mangrove area was compiled
by Spaulding et al. [1997], who estimated 181,000 km
2
of
mangrove swamps. No single global inventory of tidal salt
marshes has been published. Regional or national salt marsh
inventories are available for Canada [Letourneau and Jean,
1996; Hanson and Calkins, 1996], Europe [Dijkema, 1987],
the United States [Field et al., 1991], and South Africa
[O’Callaghan, 1990]. Together these regions hold approx-
imately 22,000 km
2
of salt marsh (Table 3). We found no
data on the extent of salt marshes on the temperate coasts of
Asia, South America, and Australia, but we expect these to
be substantial.
[
12] For most sites we were able to locate nearby
meteorological stations for which climate normals, average
monthly minima and maxima computed over at least three
decades, were available. Where possible we located sta-
tions at low elevations and avoided large urb an areas.
Because data were compiled by various agencies such as the
U.S. Weather Service (http://ggweather.com, http://cirrus.
dnr.state.sc.us), the Meteorological Service of Canada
(www.msc-smc.ec.gc.ca), Mexico’s Servicio Meteorolo´gico
Nacional (http://smn.cna.gob.mx), the Australian Common-
wealth Bureau of Meteorology (www.bom.gov.au), the
South African Weather Service (www.weathersa.co.za),
and the Met Office of the UK (www.metoffice.com), the
period over which the averages were calculated varies
(Tables 1 and 2).
3. Results and Discussion
3.1. Climatic Controls
[
13] The average soil carbon density of all sites is 0.043 ±
0.002 g cm
3
.At test for difference of means (P < 0.05)
shows that the average soil carbon density of mangroves
Table 3. Area of Salt Marsh Reported
Region Square Kilometers Sources
United States 19,265 Field et al. [1991]
Europe and
Scandinavia
2,302 Dijkema [1987]
Canada 328 Letourneau and Jean [1996]
Hanson and Calkins [1996]
Wetlands International Inventory
Tunisia 59 Wetlands International Inventory
Morrocco 34 Wetlands International Inventory
South Africa 170 O’Callaghan [1990]
Total 21,988
Table 4. Results of Simple Linear Regression of Soil Carbon Density and Rate of Sequestration to Average Annual Temperatures at Salt
Marshes and Mangrove Swamp Sites (R, Coefficient of Correlation; P, Probability; N, Sample Number)
a
Average Annual Temperature Annual Maximum Annual Minimum
RPNRPNRPN
Carbon Density
All sites 0.23 * 122 0.21 * 122 0.25 ** 122
Mangroves 0.70 *** 33 0.80 *** 33 0.49 *** 33
Salt marshes 0.19 ns 90 0.18 ns 90 0.20 ns 93
S. alterniflora marshes 0.21 ns 20 0.21 ns 20 0.20 ns 20
S. patens marshes 0.50 *** 21 0.54 *** 32 0.54 *** 35
Rate of Carbon Sequestration
All sites 0.05 * 108 0.06 ns 113 0.08 ns 113
Mangroves 0.33 ns 28 0.43 * 28 0.21 ns 28
Salt marshes 0.14 ns 85 0.14 ns 85 0.15 ns 88
S. alterniflora marshes 0.45 ns 19 0.44 ns 19 0.47 * 20
S. patens marshes 0.13 ns 28 0.13 ns 28 0.11 ns 31
a
*P < 0.05, **P < 0.01, ***P < 0.005, ns P > 0.05. Regressions run using SPSS 11.0.
CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS 22 - 7

swamps is significantly higher than the salt marsh average,
0.055 ± 0.004 versus 0.039 ± 0.003 g cm
3
. This difference
could be due simply to higher productivity in warmer
climates [Turner, 1976], yet average annual temperature
explains only a small amount (<25%) of the variability in
soil carbon density in the entire data set (Table 4).
[
14] Production and decay rates could vary with plant
species; so w e tested average annual temperature as a
predictor for carbon density in soils of salt marshes dom-
inated by Spartina patens and those dominated by Spartina
alterniflora, all on the northwest Atlantic and Gulf of
Mexico (Figure 1). Rather than increase with temperature,
soil carbon density in both vegetation types decreases with
increasing average annual temperature, as well as annual
maximum and minimum averages (Table 4). Only in the S.
patens marshes do climate parameters explain a significant
portion of the variability in soil carbon density. This
relationship may be driven by a cluster of sites on the Gulf
of Mexico that have low C density and high average annual
temperature, but it is accepted that soil carbon decreases
with average annual temperatures in terrestrial soils, pre-
sumably due to stimulated microbial decay [Schimel et al.,
1994].
[
15] Climate parameters explain more of the variability in
mangrove soils (Figure 2). Here carbon density also
decreases with increasing temperatures (Table 4).
[
16] Globally, rates of carbon sequestration average 210 ±
20 g m
2
yr
1
.At test (P < 0.05) shows no significant
difference between average rates of carbon sequestration in
mangrove and salt marsh systems (Figure 3). Average
annual temperature explains only 5% of the variability in
rates of carbon sequestration. Climatic parameters have
limited explanatory power when by wetland type (Table 4).
The exception is soil of S. alterniflora salt marshes for
which C accumulation rates decline with increasing average
annual minimum temperature. Thus therm al controls on
decomposition rates may be a factor in C accumulation
rates, but regional or local factors must be the dominant
controls on rates of carbon sequestration in TSW soils.
3.2. Local Controls
[
17] What is most noticeable about the data is the high
variability within a given region, such as the 14 salt marsh
sites on the Connecticut coast of Long Island Sound
(41°N) and the 22 salt marsh sites on the Bay of Fundy
(45.1° –45.9°N), as well as the 25 salt marsh (29 ° –30°N)
and 20 mangrove (25° –26°N) sites on the northern Gulf of
Mexico (Figure 3). Much of this variability can be
explained by differences in suspended se diment supply
and tidal water flooding.
[
18] The range in carbon densities of individual surface
samples (0 – 2 cm) from single wetlands is broad with
respect to the global range (Figure 4). Th ere are also
significant differences (t test, P < 0.05) in carbon density
Figure 1. (opposite) Relationship of soil carbon density to
annual average temperature in soils of (a) all salt marshes,
(b) Spartina patens marshes, and (c) Spartina alterniflora
marshes.
22 - 8 CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS

or carbon accumulation rates within distinct zones of single
mangrove wetlands or salt marshes.
[
19] At Shark River, Florida fringe mangroves (those
adjacent to coastal waters) have lower soil carbon density
than soils in basin mangroves located more distant from
open waters. This relationship, however, is reversed for
rates of soil carbon accumulation, as rates in fringe man-
groves are significantly higher.
[
20] The pattern seen in mangroves is repeated in salt
marsh soils. In the Bay of Fundy, soil carbon densities can be
significantly greater in the S. patens zone, where elevations
are higher, tidal flooding is less frequent, and suspended
sediment supply is lower with respect to the S. alterniflora
zone [Chmura et al., 2001]. Because sediment deposition is
more rapid at lower marsh elevations, soil and soil carbon
accumulation rates are significantly higher there. It is likely
that sediment deposition enhances carbon sequestration by
trapping organic matter from both macrophytes and micro-
flora growing on the soil surfaces [Connor et al., 2001]. Our
Louisiana example actually comes from two sites on the
Mississippi Delta that are 105 km apart. The average soil
carbon density of the Louisiana S. alterniflora marsh is
significantly higher than the S. patens marsh, but greater
accretion rates (1.6 times greater in the S. patens) are enough
to balance carbon accumulation rates in the two marshes.
3.3. Global Stocks and Rates
[
21] Because there is no significant difference in carbon
sequestration rates by ecosystem type (mangrove swamp or
salt marsh) or climatic regime, we calculate an overall
average rate of carbon sequestration per unit area: 210 g
CO
2
m
2
yr
1
. This is an order of magnitude greater than
C sequestration by peatlands (20–30 g CO
2
m
2
yr
1
)
[Roulet, 2000]. Using the documented value of 203 
10
3
km
2
of global wetland area (which is an underestimate,
as discussed above), this means that at least 42.6 ± 4.0 Tg C
are sequestered by TSWs each year. Using the TSW area
estimate from the U.S. wetland inventory [Field et al.,
1991] we can assess the importance of the TSW carbon
sink with respect to the total carbon sink estimated for the
conterminous U.S. [Pacala et al., 2001]. At a magnitude of
5TgCyr
1
, the TSWs would make up roughly 1 – 2% of
the carbon sink (300 – 580 Tg C yr
1
) previously estimated
for the conterminous U.S.
[
22] Assessment of the potential value of TSWs as an
enhanced carbon sink in the future must include consider-
ation of methane as well, with a global warming potential
of 23 times that of CO
2
(over a 100-year time horizon
[Ramaswamy et al., 2001]). Methane flux in TSWs has not
been studied to the same degree as in peatlands, where a
Figure 2. Relationship of soil carbon density to annual
average temperature in soils of mangrove swamps of the
Gulf of Mexico and Indian/South Pacific Ocean.
Figure 3. Relationship of soil carbon accumulation rates to annual average temperature in soils of all
tidal saline wetlands.
CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS 22 - 9

range of 9.6 –13.0 g CH
4
m
2
yr
1
has been reported
[Bartlett and Harris, 1993]. The pre sence in TSWs of
abundant SO
4
2
for organic matter decomposition through
sulfate reduction means that CH
4
production is expected to
be considerably smaller than in peatlands. Measurements
indicate that methane flux in TSWs appears to decrease
with increasing sal inity and increase with t emperature
[Bartlett et al., 1987; Magenheimer et al., 1996]. Although
fluxes as high as 22 g CH
4
m
2
yr
1
have been measured
in some TSW sites [e.g., Bartlett et al., 1987], other TSWs
have been reported to be methane sinks [e.g., Giani et al.,
1996]. The combination of greater C burial and possibly
lower CH
4
emissions means that TSWs could be more
valuable as C sinks per unit area than peatlands if anthro-
pogenic activity or natural processes were to increase
ecosystem CO
2
assimilation and burial [Whiting and
Chanton, 2001]. This could o ccur, for example, as a
response to an increase in the rate of sea level rise [Morris
et al., 2002], nitrogen fertilization, or global area.
[
23] As depths of TSW soil deposits are variable, we
estimate the carbon stored in only the surface 0.5-m of soil.
Salt marsh surface deposits store 430 ± 30 Tg C, while
mangrove deposits store another 5000 ± 400 Tg C.
Although adequate inventories have not been made, it is
likely that average soil depths are closer to 1 m, and the
magnitude of carbon storage is probably 10,000 Tg C.
Figure 4. Local variability in carbon storage in tidal saline wetland soils. (a) Carbon density in surface
soils (0–2 cm depth); (b) carbon accumulation rate in surface soils (0 –2 cm depth). Vertical bars
represent range of values, solid circles represent average, and averages from same wetland are connected.
Averages within wetlands are significantly different if labeled with different letters; 1 and 2, fringe and
basin mangroves, Shark River, Fl.; 3, S. alterniflora marsh, Old Oyster Bayou, La.; 4, S. patens marsh,
Three Bayous, La.; 5 and 6, low- and high-elevation S. alterniflora zone, Bocabec River, Bay of Fundy,
N. B.; 7, S. patens zone, Bocabec River; 8 and 9, low- and high-elevation S. alterniflora zone, Dipper
Harbour, Bay of Fundy, N. B.; 10, S. patens zone, Dipper Harbour; 11 and 12, low- and high-elevation
S. alterniflora zone, Cape Enrage, Bay of Fundy, N. B.; 13, S. patens zone, Cape Enrage.
22 - 10 CHMURA ET AL.: GLOBAL CARBON STORAGE IN TIDAL SALINE WETLAND SOILS

[24] Projected climate changes caused by greenhouse
warming are expected to alter processes related to carbon
storage in wetlands. Higher temperatures should increase
primary production but also increase decomposition rates in
wetland soils. In TSWs, the net effect is expected to be
minor in light of the limited relationship between annual
average temperatures and soil carbon sequestration rate.
Regional increases in aridity will result in lower water
tables in inland peatlands and freshwater wetlands, increas-
ing decomposition and release of CO
2
and CH
4
[Gorham,
1991]. This effect is not expected in tidal wetlands, as
water tables are controlled by tidal flooding regimes, but
increases in aridity great enough to cause shifts in TSWs
from vegetated systems to salt flats would result in local
losses of this carbon sink. However, salt marshes exist in
areas with high evapotranspiration, such as the Tijuana
Estuary on the Mexico/USA border [Cahoon et al., 1996]
and the Rhone Delta [Hensel et al., 1999], where soil
carbon accumulation rates are 343 and 161 g m
2
yr
1
,
respectively.
[
25] Greenhouse warming is likely to have the greatest
impact on TSWs through an acceleration in the rates of sea
level rise. Since TSWs vertically accumulate soil roughly
in equilibrium with sea level rise [Church et al., 2001],
rates of soil carbon sequestration and the magnitude of the
soil carbon pool also will increase. In addition, TSWs can
expand inland over terrestrial soils that have a lower carbon
storage capacity. However, there is a limit to the rates at
which TSWs can vertically accrete, and submerged salt
marsh peats found on the inner Scotian shelf [Shaw and
Forbes, 1990] provide striking evidence that rapid sea level
rise exceeded the rate of marsh elevation increase during
the early Holocene. Where there is an accretion deficit, soil
surfaces become submerged and edges of the remaining
wetland are subject to lateral erosion, releasing carbon
stored from their deposits. Wetland loss is expected to
be particularly prevalent where coastal development limits
the landward migration of the wetland [Working Group on
Sea Level Rise and Wetland Systems, 1997] or where
disturbances to hydrol ogic or sedimentological regimes
prevent the wetland from adjusting to sea level rise [e.g.,
Templet and Meyer-Arendt, 1988; Kearney and Stevenson,
1991].
[26] Acknowledgments. The comments of J. Adams and two anon-
ymous reviewers helped to improve this manuscript. Support for this
research came from Quebec’s FCAR, the Natural Sciences and Engineering
Research Council of Canada, the Climate Change Action Fund of Canada,
the United States Global Change Research Program, and the Long Island
Sound Research Fund of the Connecticut Department of Environmental
Protection (grant CWF319-R).
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S. C. Anisfeld, Yale School of Forestry and Environmental Studies, 370
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D. R. Cahoon and J. C. Lynch, U.S. Geological Survey, Patuxent Wildlife
Research Center, 11510 American Holly Drive, Laurel, MD 20708, USA.
G. L. Chmura, Department of Geography and Centre for Climate and
Global Change Research, McGill University, 805 Sherbrooke Street West,
Montreal, Quebec, Canada H3A 2K6. (chmura@geog.mcgill.ca)
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