Introduction
Field estimates of particulate organic carbon (POC) flux have been made over
many decades in the interest of understanding the biological pump of carbon
to the deep ocean. While there have been a variety of new techniques to
quantify POC flux, sediment traps have been the most extensive temporally and
geographically, and 234Th has improved data resolution in the upper
500 m of the water column. POC flux depends largely on the biologically
mediated export of carbon from the surface ocean and its remineralization
with depth, thus capturing biological variables associated with POC flux are
essential to understand flux variability. Here we compile POC flux estimated
from sediment traps and 234Th from around the globe from public
repositories and directly in the literature. We then match the POC flux
observations with biological and physical parameters determined from
satellite imagery along with mixed layer depth (MLD) climatology. See Table 1 for a
list of products and units.
Geographical distribution of POC flux observations at 673
independent sites. The size of the circle indicates the length of the data
record at a given site (see legend). The color of the circles indicate the
depth of observation, where orange is ≤ 100 m, green is > 100 and
≤ 1000 m, and dark blue is > 1000 m. The location of sediment trap
is
indicated in black and the location of 234Th data is in red. Plus symbols (+) indicate which observations are during the
satellite era (i.e., September 1997–present). The diamonds highlight the
locations of time series sites; BATS/OFP (green, 14 %), CARIACO (orange,
10 %), K2 (dark blue, 2 %), OSP (purple, 7 %), and HOT (light
blue, 3 %) account for 36 % of the data record.
Understanding the impact of surface processes on the export of organic
carbon at depth has been an ongoing challenge in the oceanographic community
since the Joint Global Ocean Flux Study (JGOFS). Continued efforts with the
upcoming Export Processes in the Ocean from RemoTe Sensing (EXPORTS) program
along with the Pre-Aerosol, Clouds and ocean Ecosystem (PACE) satellite
mission seek to connect remotely sensed estimates of net primary production,
particle size distribution, phytoplankton carbon, biomass, and community
composition to water column carbon processes. To do this, existing data
sources capturing water column processes need to be compiled and
synthesized. Our data set provides researchers with access to a comprehensive
historical data set of POC flux throughout the global ocean along with
matched environmental parameters derived from remote sensing sources. The
community can use this resource to move further towards a mechanistic
understanding of the biological pump.
Data and methodology
Satellite products and mixed layer depth
We provide products derived from SeaWiFS (Sea-viewing Wide Field-of-view
Sensor) monthly global area coverage (level 3 mapped data, 9 km, 8-day resolution, version R2014)
imagery over the mission record (September 1997–December 2010) acquired from
NASA Ocean Biology Distributed Active Archive Center (OB.DAAC)
(http://oceancolor.gsfc.nasa.gov/). These include chlorophyll
concentration ([Chl]) (Maritorena et al., 2002), diffuse attenuation
coefficient at 490 nm (Kd(490)) (O'Reilly et al., 2000), and
photosynthetically available radiation (PAR) (Frouin et al., 2002). At the
time of writing, only 8 % of the publicly available POC observations were
measured beyond 2008, when the MODerate resolution Imaging Spectroradiometer
(MODIS) replaced the SeaWiFS record, and thus we focus our data compilation
here solely on SeaWiFS. Net primary production (NPP) estimates from the
Vertically Generalized Production Model (VGPM) (Behrenfeld and Falkowski,
1997) are obtained from
http://www.science.oregonstate.edu/ocean.productivity/ (9 km, 8-day
resolution). SeaWiFS data products and NPP are retrieved as the median of a
5 × 5 pixel box (2025 km2) centered on each POC flux location
(Bailey and Werdell, 2006). AVHRR Pathfinder Version 5 (4 km, 8-day
resolution) sea surface temperature (SST) imagery was acquired from the US
National Oceanographic Data Center and GHRSST
(http://pathfinder.nodc.noaa.gov) (Casey et al., 2010). To match the
spatial resolution of SeaWiFS as much as possible, SST was retrieved as the
median of an 11 × 11 pixel box (1936 km2) centered on each POC
flux location.
The Mouw and Yoder (2010) approach is used for satellite retrieval of
phytoplankton size classes from SeaWiFS imagery (9 km, monthly resolution).
The imagery files were obtained from: 10.1594/PANGAEA.860474. The
method estimates the percentage of microplankton (Sfm) from
satellite imagery of remote sensing reflectance (Rrs(λ)).
This is an absorption-based approach where the chlorophyll-specific
absorption spectra for phytoplankton size class extremes, pico-
(0.2–2 µm) and microplankton (> 20 µm), are weighted
by Sfm (Ciotti et al., 2002; Ciotti and Bricaud, 2006). Briefly,
Sfm is estimated from a look-up table containing simulated
chlorophyll [Chl], absorption due to dissolved and detrital material at
443 nm (acdm(443)), Rrs(λ), and Sfm.
For a given pixel, satellite-estimated [Chl] and acdm(443)
(Maritorena et al., 2002) are used to narrow the search space within the
look-up table. Of the remaining options, the closest simulated
Rrs(λ) to the satellite-observed Rrs(λ) is
selected and the associated Sfm is assigned. Sfm is
retrieved on a monthly timescale as the median of a 5 × 5 pixel box
(2025 km2) centered on each POC flux location.
Export depth is often chosen as either the base of the euphotic zone or MLD (Lutz et al., 2007; Lam et al., 2011); thus both are compiled
here. The depth of the euphotic zone was determined from Kd(490)
(O'Reilly et al., 2000) as 4.6/Kd(490) (Morel and Berthon, 1989)
from 8-day SeaWiFS data products. MLD estimates are
obtained from the IFREMER/LOS Mixed Layer Depth Climatology group
(http://www.ifremer.fr/cerweb/deboyer/mld) from density profiles using
a variable density threshold equivalent to 0.2 ∘C, which accounts
for both changes in temperature and salinity (level 3, monthly climatology,
1∘ resolution; de Boyer Montégut et al., 2004, 2007; Mignot et
al., 2007). We retrieve monthly MLD climatology for each pixel containing a
POC flux location (1∘ resolution).
Summary of data sources for POC flux from sediment traps and
234Th, the latter indicated in the description when applicable. Date
ranges are from first deployment to last retrieval for a given data set, but
do not necessarily indicate a continuous time series. Sources are listed in
order of first deployment.
Latitude/longitude range
Date rangeyyyy-mm-dd
Description/ project
Reference
78.9∘ N–76.5∘ S All
1976-07-04 to 2005-05-09
Global collection
Lutz et al. (2007) and references therein
81.1∘ N–71.1∘ S 138.9∘ E–74.2∘ W
1982-06-07 to 2007-06-04
Atlantic Ocean Data Compilation
Torres Valdés et al. (2014)
50∘ N 145∘ W
1987-09-23 to 2006-06-04
Ocean Station Papa
Timothy et al. (2013)
60.3∘ N–67.8∘ S All
1987-06-06 to 2009-08-08
Global collection of 234Th
Henson et al. (2011) and references therein
22.8∘ N 158∘ W
1988-12-01 to 2010-10-05
HOT, station ALOHA
Church and Karl (2013)
32.7–30.6∘ N 63.1–65.3∘ W
1988-12-16 to 2011-12-10
BATS
http://bats.bios.edu, last access:27 September 2013
48–34∘ N 21∘ W
1989-04-03 to 1990-04-02
JGOFS North Atlantic Bloom Experiment
Honjo and Manganini (1995)
31.8∘ N 64.2∘ W
1989-06-09 to 2010-11-09
Ocean Flux Program
M. Conte (personal communication, 2015)
12∘ N–12∘ S 140∘ W
1992-01-18 to 1993-02-04
JGOFS Equatorial Pacific
Collier and Dymond (1994a, b);Honjo and Dymond (1994); Newton and Murray (1995a, b)
12∘ N–12∘ S 135–140∘ W
1992-02-04 to 1992-09-13
JGOFS Equatorial Pacific, 234Th
Murray et al. (1996)
77–80∘ N 8–16∘ W
1992-07-19 to 1993-08-11
Northeast Water Polyna, Greenland, 234Th
Cochran et al. (1995)
43.2∘ N 5.2∘ W
1993-10-16 to 2006-01-15
Mediterranean Sea
Rigual-Hernández et al. (2013)
55–88∘ N 34∘ E–176∘ W
1994-08-01 to 1999-07-01
Arctic Ocean, 234Th
Wassmann et al. (2003)
2∘ N–2∘ S 175∘ E–177∘ W
1994-10-06 to 1996-05-01
FLUPAC and Zonal Flux Study, Western Equatorial Pacific, 234Th
Dunne et al. (2000)
17.7–10.0∘ N 57.8–65.0∘ E
1994-11-11 to 1995-12-24
JGOFS Arabian Sea
Honjo (1999)
10.3∘ N 64.4∘ W
1995-11-08 to 2012-12-10
CARIACO
Thurnell (2013)
61.5–22.0∘ N 160∘ E–170∘ W
1996-05-15 to 2005-08-15
Review of 234Th measurements
Buesseler and Boyd (2009) and references therein
73.6–76.5∘ S 176.9∘ E–178.0∘ W
1996-06-12 to 1999-07-25
Ross Sea
Collier et al. (2000)
73.6–76.5∘ S 177∘ E–178.0∘ W
1996-10-18 to 1997-04-30
Ross Sea, 234Th
Cochran et al. (2000)
53.0–76.5∘ S Circumpolar
1996-11-28 to 1998-01-27
JGOFS Southern Ocean
Honjo and Dymond (2002)
53.0–70∘ S Circumpolar
1997-10-23 to 1998-03-13
JGOFS Southern Ocean, 234Th
Buesseler et al. (2003)
39–25∘ N 147–137∘ E
1997-11-19 to 1999-08-12
Kuroshio Extension, Pacific
Mohiuddin et al. (2002)
36.7–36.0∘ N 147–154.9∘ E
1998-08-29 to 2000-08-29
Kuroshio Extension, Pacific
Mohiuddin et al. (2004)
44∘ N 155.1∘ E
1998-11-02 to 1999-05-26
North Pacific
Honda et al. (2002)
62.6∘ S 178.1∘ W
1999-02-12 to 2001-09-17
Antarctic Polar Front
Tesi et al. (2012)
Continued.
Latitude/longitude range
Date rangeyyyy-mm-dd
Description/ project
Reference
77.0–77.8∘ S 172.5–180∘ W
2001-12-22 to 2006-02-03
Ross Sea, Antarctica
Smith Jr. et al. (2011)
51–39∘ N 155–165∘ E
2002-10-16 to 2005-03-06
NW Pacific, 234Th
Kawakami and Honda (2007)
43.3∘ N 7.7∘ E
2003-03-06 to 2005-04-28
MedFlux, Mediterranean Sea
Lee (2011)
33.6∘ N 118.4∘ W
2004-01-07 to 2008-06-19
Southern California Bight
Collins et al. (2011)
34.9–29.6∘ N 58.2–67.2∘ W
2004-02-22 to 2005-03-13
New Production During Winter Convective Mixing Events
Lomas et al. (2009)
47.0–22.8∘ N 161∘ E–158∘ W
2004-06-22 to 2005-08-10
VERTIGO, Pacific
Lamborg et al. (2008)
47–30∘ N 145–160∘ E
2005-03-21 to 2011-07-24
OceanSITES, K2 and S1, NW Pacific
Honda (2012)
44.6∘ N 2.8∘ W
2006-06-22 to 2006-06-26
Bay of Biscay
Kuhnt et al. (2013)
10.3∘ N 64.4∘ W
2007-02-28 to 2008-12-31
CARIACO
Montes et al. (2012)
62.3–55.3∘ N 167.9–176.8∘ W
2008-03-30 to 2008-07-03
Bering Sea
Moran et al. (2012)
61.1∘ N 26.5∘ W
2008-05-05 to 2008-05-19
North Atlantic Spring Bloom
Martin et al. (2011)
Latitudinal distribution of POC flux observations.
(a) Temporal distribution showing observations prior to (shaded
area) and during (right panel) the satellite era. The length of each grey bar
represents a sediment trap deployment (darker bars indicate observations
coincident with satellite NPP and Sfm); note some bars may overlap.
Time series locations are denoted by color as in Fig. 1. 234Th data are
differentiated in all subplots (red). (b) The percentage of total
observations binned by every 10 ∘ of latitude.
(c) Observations prior to the continuous satellite era (before
September 1997). (d) Observations collected during the continuous
satellite era (beginning September 1997). (e) Observations with
coincident satellite imagery within the same month of collection.
Spatial distribution of coincident satellite and POC flux
observations. The size of the circle represents the
number of coincident observations
(see legend).
POC flux data
POC sediment trap data are acquired from public repositories and published
literature (Table 2; Fig. 1). Estimates from 234Th measurements are also
acquired to improve the resolution of observations in the upper 500 m of the
water column (Dunne et al., 2005; Henson et al., 2012; Guidi et al., 2015).
These represent 4 % of the total data set. Collected field estimates of
POC flux derived from 234Th maintain the original authors' analysis,
where POC flux is retrieved based on 234Th activity in the water column
accounting for the ratio of POC to 234Th concentration (Buesseler and
Boyd, 2009). Both sediment traps and 234Th methodologies have documented
challenges associated with accurately retrieving POC flux and characterizing
uncertainty. Sediment traps have possible bias associated with the
interaction of hydrodynamics with trap design, the capture of zooplankton
(“swimmers”), and incomplete preservation of material. 234Th-based
measurements have associated biases accounting for local advection,
quantifying particulate adsorption and with variability in the ratio of
POC : 234Th. See the discussions of Buesseler (1991), Buesseler et
al. (2000), Lee et al. (1992), Murray et al. (1996), Quay (1997), and van der
Loeff et al. (2006), for in-depth analyses of these issues.
A significant number of studies occurred prior to the launch of SeaWiFS in
September 1997 (see Honjo et al., 2008, and references therein). While we
compiled observations across all available time frames, greater focus is
placed on collecting data concurrent with the satellite record to allow
corresponding imagery-based environmental parameters to be matched. Overall,
the data set comprises a total of 15 792 individual measurements at 673
unique locations with 6842 (43 %) collected during the satellite record.
In the interest of matching the timescale of POC flux to satellite-derived
products to the greatest degree possible, we focused on collecting short-term
sediment trap deployments with individual cup intervals of 30 days or less.
The majority of the data set (14 555 measurements or 92 %) fell into this
category with a median cup interval of 14 days and a standard deviation of
6 days. Data are skewed towards shorter deployments with 59 % of
qualified measurements deployed 14 days or less and 93 % deployed 20 days
or less.
Global POC flux variability with depth. POC flux observations are
binned every 100 m in the upper 1000 m and every 500 m throughout the rest
of the water column. Box edges enclose the 25th and 75th percentiles of data
within each bin with the median shown as a vertical line. Error bars extend
to the 5th and 95th percentiles and remaining outliers are indicated with
+. There are eight data points not represented on the plot, as they were
significantly higher than the majority of the data set. These values were
observed < 225 m and are 620, 660, 677, 694, 830, 852, 950, and
1238 mg C m-2 d-1.
Depth distribution of POC flux observations. The percentage of total
observations was binned every 100 m in the upper 1000 m and every 500 m
throughout the rest of the water column. Coloration is the same as in Fig. 2.
(a) Temporal distribution indicates observations prior to (shaded
area) and during (right panel) the satellite era. (b) The percentage
of depth dinned total observations. (c) Observations prior to the
continuous satellite era (before September 1997). (d) Observations
collected during the continuous satellite era (beginning September 1997).
(e) Observations with coincident satellite imagery within the same
month of observation.
Time series sites
Six long-term oceanographic time series locations are included in the
compilation, providing detailed temporal resolution of POC flux export and
remineralization. These were the Carbon Retention In A Colored Ocean
(CARIACO) project site in the Cariaco Basin (10.5∘ N,
64.7∘ W), K2 in the northwest Pacific (47∘ N,
160∘ E), Ocean Station Papa (50∘ N, 145∘ W), the
Bermuda Atlantic Time Series (BATS) study site in the Sargasso Sea
(31.7∘ N, 64.2∘ W), the Ocean Flux Program (OFP;
31.8∘ N, 64.2∘ W), and the Hawaii Ocean Timeseries (HOT;
22.8∘ N, 158.0∘ W). Data from BATS and OFP could be
combined to create a complete water column profile with BATS sediment traps
deployed ≤ 300 m and OFP traps deployed ≥ 500 m. Also, with the
exception of the first deployment year, HOT only reports POC flux at a single
depth.
Fluxes of other constituents, uncertainty estimates, and metadata
Where readily available, we collect concurrent flux estimates of other
organic and inorganic components in addition to POC flux including
particulate inorganic carbon, particulate nitrogen and phosphorus, calcium
carbonate, biogenic silica, trace metals, and phytoplankton pigments
(Table 1). These data are included to explore relationships between POC
export and remineralization and ballasting materials. Where reported by the
original authors, we include uncertainty estimates for measured fluxes in the
compilation. We also collect and include metadata as reported by the original
authors. At a minimum, we require each observation be associated with
latitude and longitude, deployment date, and depth to be included in the
data set. Other information, such as sediment trap type and trap funnel area,
is included where available. The majority of measurements (58 %) were not
associated with a reported total water depth. Bathymetry was retrieved for
POC flux locations from the ETOPO1 1 arcmin Global Relief Model (Amante and
Eakins, 2009) from the single pixel containing the measurement location.
Locations close to shore were sometimes classified as being on land by
ETOPO1; bathymetry is excluded in these cases.
Results
The deployment, retrieval, and analysis of sediment trap and 234Th
samples represents a significant expenditure of both effort and resources and
projects are often funded on a short-term local/regional basis (Honjo et al.,
2008). This is reflected in the patchy distribution of observations across
the globe in multiple dimensions: space, time, and vertical resolution
(Fig. 1). Collection efforts are more prevalent in the Northern Hemisphere,
with 63 % of unique station locations comprising 85 % of total
observations falling north of the Equator (Fig. 2a and b). Long-term
oceanographic time series locations at BATS/OFP, CARIACO, K2, OSP, and HOT
(all in the Northern Hemisphere) collectively account for 36 % of the
total data set. If time series locations are removed, 77 % of remaining
observations still concentrate north of the Equator. The most sampled regions
in the Northern Hemisphere are at midlatitudes, with a quarter of the
data set (discounting time series locations) falling between 30 and
40∘ N (Fig. 2b). In the Southern Hemisphere, data are concentrated
at higher latitudes, with a little over half of collected measurements
derived from the Southern Ocean at ≥ 60∘ S. In both
hemispheres, the second-most sampled latitudes are near the Equator
(10∘ N–10∘ S).
The data set spans 4 decades from 1976 to 2012 with the majority of efforts
(62 %) deployed between 1990 and 2000 (Fig. 2, Table 2). In addition,
43 % of the measurements were collected after September 1997, when the
SeaWiFS mission was launched. Prior to SeaWiFS, 79 % of observations are
in the Northern Hemisphere (Fig. 2c). After September 1997, the latitudinal
distribution becomes even more skewed with 93 % of the observations in
the Northern Hemisphere concurrent with the satellite record (Fig. 2d).
While 43 % of the data were observed during the continuous satellite era,
not all observations had coincidental imagery. Here we define coincident as
retrieved satellite observations within the same month as sediment trap
deployment or 234Th measurement for a given POC flux location. We
consider only the Sfm and NPP imagery for this purpose as they are
representative of phytoplankton surface processes and the NPP product already
requires SST and [Chl] imagery as inputs. This reduces the total satellite
era observations from 6842 to 3722, a drop in total contribution from 43 to
24 %. These are spread over 121 unique locations (Fig. 3). Of the
coincident observations, 95 % are in the Northern Hemisphere primarily
between 10 and 50∘ N, with the majority found between 30 and
40∘ N (Fig. 2e). Data sets in some regions of the ocean (e.g., the
equatorial Pacific and the Arabian Sea in Fig. 1) have no satellite overlap
(Fig. 3).
The depth resolution of the observations is important for investigators
interested in fitting export flux relationships (Martin et al., 1987; Lima et
al., 2014). The greatest variability in POC flux is found in the first 500 m
of the water column (Lam et al., 2011, Fig. 4). Considering all POC
observations together, median POC flux rapidly diminishes from
160 mg C m-2 d-1 in the upper 100 m to
30 mg C m-2 d-1 at 500 m and 6 mg C m-2 d-1 at
1000 m. Below 1000 m, the average POC flux is 3 mg C m-2 d-1
(Fig. 4).
Overall, 70 % of the compiled data set is measured at ≥ 500 m
(Fig. 5). Thus, the upper water column close to the depth of export is
relatively underrepresented. To increase depth resolution, we consider
234Th and sediment traps together (Dunne et al., 2005; Guidi et al.,
2015). Guidi et al. (2015) also merged data from the underwater vision
profiler (UVP), which is not included in this compilation as it has not yet
been released into a public archive. Shallow observations are critical for
capturing the impact of phytoplankton on POC export flux as these data are
most connected to surface processes. By adding 234Th measurements to the
data set, 249 locations gain depths in the upper water column < 500 m.
234Th data contribute 32 % of all POC flux estimates resolved at
depths between 100 and 200 m (Fig. 5a). Overall, the most common deployment
depths are between 1000 and 1500 m (14 %) followed by 200 to 300 m
(11 %) and then 3000 to 3500 m (9 %) (Fig. 5b). The dominance of the
1000 to 1500 m observation depth is weighted to the pre-satellite era
(Fig. 5c). During the satellite era, 200 to 300 m (6 %) became the most
sampled depth, largely due to persistent time series observations at BATS and
OSP, followed closely by the 1000 to 1500 and 3000 to 3500 m bins (5 %
each) again the result of time series observations at CARIACO and OFP
(Fig. 5d). Reasonable depth resolution is found in the observations
coincident with satellite matchups (Fig. 5d).
Data availability
The data set contains 15 792 individual POC flux estimates at 674 unique
locations collected between 1976 and 2012. Where available, the flux of other
minerals is also reported. 43 % (6842) of POC flux measurements overlap
with the SeaWiFS satellite record (September 1997 to December 2010).
Satellite parameters in this compilation include: chlorophyll concentration,
net primary production, sea surface temperature, diffuse attenuation
coefficient, euphotic depth, photosynthetically active radiation, and
microplankton fraction. Estimated mixed layer depths and bathymetry are also
provided. Parameters associated with observation sites are extracted as the
median of a 5 × 5 (chlorophyll concentration, NPP,
Kd(490), PAR and Sfm), 11 × 11 (SST), or
1 × 1 (MLD, bathymetry) pixel box. The compiled data are available
on PANGAEA (https://www.pangaea.de/): 10.1594/PANGAEA.855600
(Mouw et al., 2016).