Global Particulate Organic Carbon Flux to the Ocean Floor: The Importance Of Continental Margins

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Global Particulate Organic Carbon Flux to the Ocean Floor: The Importance Of Continental Margins
Frank E. Muller-Karger1, Ramon Varela2, Robert Thunell3, Haiying Zhang1, Chuanmin Hu1, and John J. Walsh1

1University of South Florida, College of Marine Science, St. Petersburg, FL 33701;

Phone: (727) 553-3335; (727) 553-1103 (fax);

2Fundacion La Salle de Ciencias Naturales, Estacion de Investigaciones Marinas de Margarita, Apartado 144 Porlamar, Isla de Margarita, Venezuela

3University of South Carolina, Department of Geological Sciences, Columbia, SC 29208

Submitted to: Science

Submitted date:
The amount of carbon sinking to the ocean floor in biogenic particles was estimated based on 9 km resolution global satellite data (1998 to 2001), the Vertically Generalized Production Model, a model of sinking flux based on field data (primary production and sediment traps) and bathymetry. Although net primary production (NPP) over continental margins, defined as areas 50-500 m deep, was ~5.22 Pg C y-1 (~11% of NPP for global ocean waters; 1 Pg = 1015 g), particulate organic carbon (POC) flux to the bottom was ~0.55 Pg C y-1 (~59% of the global flux to the bottom). Export production (the ratio of POC flux to NPP) to the bottom was ~0.8% at >500 m depth, while it was ~10% over margins. The results obtained at 9 km resolution demonstrate that present carbon fluxes of POC export from the continental margins may be comparable to or even larger than those from the high seas.


The marine biota have substantial impacts on the global carbon cycle over both very long time scales, as well as those of our own lifetimes. Over past geological time scales (104-106 years), geochemical imbalances at the Earth’s surface are tied closely to carbon sequestration by marine organisms. Over shorter time scales, their stocks and fluxes in the global oceans' carbon cycle effect [affect?] feedbacks of the climate system (IPCC, 2001). Quantifying these effects and their uncertainties is needed in order to guide policy regarding the fate of future anthropogenic carbon inputs to the environment.

We now know that approximately half of the annual global biospheric net primary production occurs in the oceans (Behrenfeld et al., 2001). However, where this carbon is stored, and whether or not the ocean mitigates the increase of anthropogenic greenhouse gases over short time scales (years to decades), remain some of the most pressing questions in ocean biogeochemistry.

Current models of the global biosphere and its carbon cycle are imperfect. Presently, computational power is a serious limitation to coupled physical-biological models, particularly to achieve the high-spatial resolution of 1-10 km now employed in regional grids (Walsh et al., 2004). Coarse biogeochemical global models use grids of 1°x1° spatial resolution, such that the continental margins (shelf, slope, and rise) and many oceanic processes are not well represented. Also, these models are at present constrained by averages of scarce ocean data, which were derived primarily from oceanographic work that focused on the ocean’s interior over the past 45 years (e.g. Takahashi et al., 1997). Imperfect parameterizations of physical, chemical, and biological processes yield additional uncertainties in stock and flux terms (Longhurst, 1991; Field et al., 1998; del Giorgio and Duarte, 2002; Lutz et al., 2002).

Due to these problems, boundary conditions of global carbon models are typically set at the shelf break, and the assumption is made that there is no exchange between the deep ocean and continental shelf or coastal waters. The impact of coastal eutrophication due to riverine inputs (Smith and Hollibaugh 1993; Walsh, 1991) on sequestration of anthropogenic CO2 is small (of the order of 0.2x1015 gC) because of respiration and denitrification losses (Walsh, 1991). However, because continental margins tend to be viewed as “coastal” environments in global carbon assessments, there has been a tendency to ignore new production along margins altogether.

The result is that local processes of upwelling, mixing, and net photosynthesis on continental shelves and slopes now tend to be discounted. Since aggregation of data into 1°x1° resolution has been used in past models and historical ocean data analyses, the most recent assessments of surface ocean net production (productivity minus respiration) have also utilized a 1°x1° resolution and are restricted to oceanic depths greater than 200 m (Gregg et al., 2003). Therefore, a relevant question is: How good are global carbon (and nutrient) assessments if continental margins are omitted?

Annual new production (Dugdale and Goering, 1967) by phytoplankton, i.e. that amount of carbon dioxide fixed during photosynthesis that is available for export from the euphotic zone to ocean waters, in regions deeper than 1,000 m (31x107 km2) is estimated to be about 2.9 Pg C y-1 (1 petagram = 1 Pg = 1x1015 g), while over the significantly smaller area of the world's continental shelves and slopes (5.8x107 km2) the estimate is roughly 3.7 Pg C y-1 (Walsh, 1991; Jahnke, 1996; Liu et al., 2000). Furthermore, these estimates are based on carefully made but sparse measurements, providing only a static impression that does not account for either interannual variation or longer secular trends.

These previous estimates of new production provide an estimate of the particulate organic carbon available for export from the euphotic zone, over length scales ranging from a few to hundreds of kilometers (Deuser et al., 1990; del Giorgio and Duarte, 2002). For processes acting over geological time scales, and for the purpose of assessing how much carbon the ocean takes up over the short term relative to our injection of carbon into the atmosphere, it is critical to determine the fraction of particulate organic carbon (POC) that sinks to the ocean floor. Numerous studies have suggested an extremely rapid decrease in the flux of organic constituents with depth in the upper water column (Conte et al., 2001; Suess, 1980; Betzer et al., 1984; Pace et al., 1987; Francois et al., 2002; Lutz et al., 2002).

There are relatively few time series of vertical POC flux observations at ocean depths exceeding 4,000 m (see Lutz et al., 2002 and Francois et al., 2002 for reviews), and only a few sediment trap experiments have been carried out on continental margins (Honjo, 1982; Dymond and Lyle, 1994; Biscaye and Anderson, 1994; Jahnke, 1996; Jahnke et al., 1999; Thunell et al., 1996; Thunell et al., 2000; Pilskaln et al., 1996; Muller-Karger et al., 2003, others). Jahnke (1996) provided an assessment of organic carbon flux to the bottom, but considered only waters deeper than 1,000 m. He speculated that continental margins may export at least as much POC to the bottom as the deep ocean, and concluded that his assessment was an underestimate of the global ocean POC flux to the sediments. Thus, a second relevant question is: How much POC reaches the ocean bottom and is buried as sediment?

Here we use modern satellite data at 9 km resolution to estimate the POC flux to the sea floor over continental margins, relative to POC fluxes to the sea floor over the deep ocean. We specifically address the assumption that margins are simply a “dirty ring around the oceanic bath tub” that can be ignored in global carbon flux models.


Details of methods used are available as supporting material on Science Online. Briefly, we computed net primary production (NPP) at a 9x9 km2 spatial resolution using the Vertically Generalized Production Model (VGPM) using the parameterizations as presented in Behrenfeld et al. (2001). Sea surface temperature (SST) was derived from the Advanced Very High Resolution Radiometer (AVHRR) satellite sensor, and chlorophyll and Photosynthetically Active Radiation (PAR) data were derived from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Sinking POC flux was derived using an exponential decay model, with flux projected to the ocean bottom as defined by a global bathymetry database. Continental margins were defined as the regions between 50 and 500 m depth; for sensitivity analyses, computations were also made using an alternative definition, specifically of margins lying between 20 and 500 m.

Monthly mean global ocean net primary production and POC flux to the ocean's bottom were derived for 1998 through 2001. Figure 1 shows an example of the chlorophyll, SST, PAR, and photoperiod input parameters, as well as the global net primary production integrated over the euphotic zone for August 2001. Figure 1 also shows the global bathymetry used to compute POC flux to the ocean’s bottom and to define the continental margin mask.

Table 1 summarizes the results of the study. Over the global ocean (>50 m deep), annual net primary production averaged 48.2 Pg in 1998-2001, varying less than ±2% from year to year over the four-year period (the average for waters >20 m was 50.8 Pg). We estimated an average of 5.2 Pg y-1 net primary production over continental margins (or 7.8 Pg for 20-500 m), with interannual variability < ±1%.

Previous global ocean estimates derived with the VGPM using the Coastal Zone Color Scanner (CZCS), SeaWiFS, and AVHRR satellite data are similar to ours within about 20% (see Morel and Antoine, 2002; Gregg et al., 2003). The data used here were released by NASA in 2003 (version 4 reprocessing), and included corrections to the original calibration, atmospheric correction, and bio-optical algorithms. Behrenfeld et al. (2001) obtained global net primary production values of about 54 to 59 Pg C y-1 using a version of the SeaWiFS data released by NASA in 2000. Gregg et al. (2003) estimated net production for areas >200 m to be around 42.5 Pg C y-1 based on SeaWiFS (version 3 reprocessing) for the period September 1997-June 2002, and 45.3 Pg C y-1 based on data from CZCS for the period January 1979 through June 1986. They suggested that these differences between the CZCS and SeaWiFS estimates were real and represented a change in marine ecosystems. We attribute differences to variation in the way data were processed.

The annual POC flux to the ocean bottom for 1998, 1999, 2000, and 2001 was estimated using the Pace et al. (1987) model (Table 1; Figure 2). The four-year average estimate of POC flux to the bottom of the global ocean was 0.94 PgC (or 1.6 PgC for >20 m), with ±1% variation between 1998 and 2001. Over this period, continental margins exported 0.55 PgC (1.25 Pg for 20-500 m), with ±1% interannual variation. The Pace et al. (1987) relationship may not be applicable globally (Lampitt and Antia, 1997; Lutz et al., 2002) and factors such as ballasting influence the efficiency of organic carbon flux to the sea floor (Armstrong et al., 2002; Francois et al., 2002), but these results do provide a first-order approximation.

Discussion and Conclusion

Most past studies have attempted to assess the impact of the biological pump (Volk and Liu, 1988) by estimating global new production (the fraction of Net PP exported to below the euphotic depth), since any CO2 released below 200 m would be sequestered for hundreds if not thousands of years. It would be possible to estimate new production as an intermediate step in the computations done for this study, i.e. by using the depth of the euphotic zone obtained from the VGPM with one of various export flux models (Lutz et al., 2002), but this was not the primary objective of the study. Rather, our goal was to obtain an estimate of how much POC is exported directly to the bottom of the ocean, where it may be sequestered longer in sediments.

We estimate that the total amount of particulate organic carbon deposited on the bottom of the global oceans deeper than 500 m amounts to about 0.34 Pg C y-1, with very little interannual variation (Table 1). Most of the deep sea (> 3,000 m) received very small amounts of POC (i.e. << 1 gC m-2 y-1). However, there were some small differences in the spatial patterns of POC flux to the ocean bottom from year to year (Figure 2). The largest values of POC flux (i.e. > 2 gC m-2 y-1) were observed under major divergences (equatorial upwelling zones and upwelling areas along eastern ocean margins), under the South Atlantic and Southern Indian Ocean Currents, under the north Pacific convergence and on the slopes and shelves of the world. Seamounts and substantial portions of the mid-ocean ridges were apparent in the bottom POC flux images; these topographic highs received over twice the amount of POC derived from surface production as the surrounding sea floor, because they intercepted sinking POC. Particularly high flux values were calculated for the mid-ocean ridges, and especially south of Iceland (> 3 gC m-2 y-1). The POC flux to the bottom of continental margins (values > 4 g C m-2 y-1) exceeded anything seen in the deep ocean. Margins showed both high production and high deposition of POC.

With respect to the first question posed for this study, we find that while net primary production on continental margins (50-500 m) contributes ~11% to total global ocean net production (~15% if 20-500 m), POC flux to the bottom on margins accounts for >59% of total global POC flux reaching the sea floor (76% if 20-500 m). Our results suggest that ~0.8% of the overlying net primary production reaches the sea floor in the high seas (>500 m depth), in agreement with previous studies (Lutz et al., 2002). Even with the 2.8 Pg C decade-1 decadal-scale decrease in high seas NPP estimated by Gregg et al. (2003) between the 1980's and the late early 2000's, this would lead to no more than a decrease of 0.002 Pg C sequestered at the bottom every year - or a total decrease of 0.02 Pg C over 10 years. By comparison, nearly 10% of the NPP over continental margins, or over 0.5 Pg C y-1, reach the sea floor. In this sense, the biological pump is more significant here than in the ocean's interior.

Along margins, natural and anthropogenic perturbations may also have an amplified effect on nutrient supply rates because of vigorous exchanges of energy and matter between land, atmosphere, and the ocean (Smith and Hollibaugh, 1993; Walsh, 1991). Also, traps may undersample flux in the mesopelagic zone by about 40% (Lutz et al., 2002, Francois et el., 2002). Either way, ignoring the role of margins in the ocean's carbon cycle is a serious shortcoming of many carbon cycle models.

The answer to our second question, namely how much carbon is actually buried in marine sediments, is still difficult to resolve. Most recently, it has been estimated that 0.13-0.16 Pg of carbon are buried in the oceans each year, with 80-85% of this occurring along continental shelves and deltas (Berner, 1992; Hedges and Keil, 1995). If an average 30 to 50% of the POC that is deposited on continental margins is preserved and buried in the sediments (Jahnke, 1996; Dymond and Lyle, 1994), then our estimates of flux to the sea floor suggest that this may be an underestimate by as much as a factor of two. The point again is that our high-resolution results suggest that margins indeed play an important role in the carbon cycle.

We expect that the emerging integrated ocean observing system (IOOS) that includes the coastal ocean and encompasses global scales will include mechanisms to assess carbon flux across horizontal, vertical, and time dimensions. Combined with robust models that incorporate processes on the globe’s continental margins, this emerging observing system will help gain more than the rudimentary understanding possible today using the limited tools presented here.


This work was supported by the National Science Foundation (NSF Grants OCE-9216626, OCE-9729284, OCE-9401537, OCE-9729697, OCE-9415790, OCE-0118566, OCE-0118349, and OCE-9711318), the National Aeronautics and Space Administration (NASA Grants NAG5-6448, NAS5-97128, NAG5-11253 and NAG5-10738), and the Consejo Nacional de Investigaciones Cientificas y Tecnologicas (CONICIT, Venezuela, Grant 96280221). We thank the US SeaWiFS Project (Code 970.2), the Distributed Active Archive Center (NASA GSFC Code 902), and OrbImage for the production and distribution of the satellite data.


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Table 1. Net primary production (Net PP) and bottom particulate organic carbon (POC) flux over the global ocean (defined as waters deeper than either 20 or 50 m) and over continental margins (defined as either 50-500 m or as 20-500 m depth). Two depth ranges were used in a sensitivity analysis of computations to depth. Net PP was derived using the VGPM (Behrenfeld and Falkowski, 1997). Flux was derived using the model of Pace et al. (1987).



[50 m- max]


[20 m- max]


[50-500 m]


[20-500 m]

%Margins NPP and Flux

[>50 m]

%Margins NPP and Flux

[>20 m]

Net PP [Pg]








(1 Pg=1015 g)






















Flux [Pg]





























Figure 1. The chlorophyll, sea surface temperature, photosynthetically active radiation, and photoperiod input parameters for the VGPM, as well as global net primary production integrated over the euphotic zone (lower right hand corner) for August 2001. Also included is a graphical representation of the global bathymetry database used to define the 500 m continental margin mask.

Figure 2. Annual particulate organic carbon (POC) flux deposited on the ocean bottom based on net primary production model (VGPM), the Pace et al. (1987) export flux model, ocean color and infrared satellite data, and global bathymetry.

Supporting Online Material

Satellite data

Sea surface temperature (SST, oC), surface chlorophyll-a concentration (chl, mg m-3), and photosynthetically available radiation (PAR, mol photons m-2 day-1) were derived from global satellite observations. Monthly averages of global SST (9x9-km2 spatial resolution), derived from the Advanced Very High Resolution Radiometer (AVHRR), were obtained from NASA’s Jet Propulsion Laboratory Physical Oceanography Distributed Active Archive Center (JPL PODAAC; Vazquez et al., 1998). Chlorophyll and PAR data were derived from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) at a resolution of 9x9 km2. We used the monthly mean SeaWiFS Level-3 products obtained from NASA's Goddard Space Flight Center (GSFC DAAC; Version-4 chl and PAR; McClain et al., 1998; Frouin et al., 2001). SeaWiFS and AVHRR data between 1998 and 2001 were used in this study.
Empirical models: net productivity and vertical POC flux

Two models are used to relate surface chlorophyll to bottom POC flux. First we applied the Vertically Generalized Production Model (VGPM; Behrenfeld et al., 2001). This model estimates depth-integrated net primary production (gC m-2 day-1) based on surface chlorophyll (mg m-3), surface PAR, and SST. In our case, estimates of these parameters were all obtained from satellites. The accuracy of the VGPM model has been tested by Campbell et al. (2002) using select field data, and it has been used to study global primary production patterns over short (Behrenfeld et al., 2001) and decadal-scales (Gregg et al. 2003). The model was used successfully by Muller-Karger et al. (2003) in mimicking multi-year time series observations taken in a tropical continental margin setting, after modified to reflect higher Pbopt at temperatures above 21oC (Pbopt is the maximum value of primary production per unit chlorophyll in the water column). We did not include the VGPM modifications to derive the results presented here to facilitate comparison of results with contemporary studies. We believe that the effect is that the global NPP likely remains underestimated until such modifications are included.

The second model relates primary production to the depth-dependent POC flux. In the open ocean, the export ratio (the proportion of POC flux to integrated euphotic zone primary production) tends to decrease exponentially with depth, due to decomposition and consumption of the POC. Several empirical models have been derived, including those of Suess (1980), Betzer et al. (1984), and Pace et al. (1987), and more recent revisions by Lutz et al. (2002) and Francois et al. (2002). We used the Pace et. Al. (1987) model {flux(Z) = 3.523* PP*Z-0.734 where PP is the integrated net primary production (g C m-2 d-1) and Z is the water depth in meters (Z > 50m?)}, which yielded the best fit to primary production and sediment trap data from the Cariaco Basin (Muller-Karger et al., 2003). We assume that this vertical model can be applied to shelves, since only a relatively small fraction of the total POC produced on margins moves laterally (Peña et al., 1999; Etcheber, 1996). The statistical funnel (Deuser et al. 1990; Siegel and Deuser, 1997) was very shallow compared to that of the deep ocean. For example, particles settling at 100 m d-1 on the continental shelf would intercept the bottom in about 2 days. Assuming horizontal velocities of about 20 cm s-1 throughout the water column, these particles would travel laterally less than 18 km. This distance is less than two pixels in our images.
Practical Definition of Margins

To derive a first-order estimate of the POC flux we use a simple approach, namely to define a series of masks using a global digital bathymetry database obtained from NOAA's National Geophysical Data Center, NGDC (see Smith and Sandwell, 1997). The spatial resolution of the database is 2-minutes (5-minutes in the Arctic Ocean). Margins are defined as the regions between either 50 and 500 m depth, or as 20 and 500 m depth as an alternative to test the sensitivity to a narrow area (20-50 m depth) with high pigment concentration.

Additional Methods References

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  3. Frouin, R., B. Franz, and M. Wang (2001). Algorithm to estimate PAR from SeaWiFS data.

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