GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 27, 705–710, doi:10.1002/gbc.20067, 2013
Localized refractory dissolved organic carbon sinks
in the deep ocean
Dennis A. Hansell1 and Craig A. Carlson 2
Received 25 January 2013; revised 8 July 2013; accepted 13 July 2013; published 12 August 2013.
[1] The global ocean holds one of Earth’s major carbon reservoirs as dissolved organic
matter (662 ± 32 PgC). Most of this material (>95%) is termed refractory dissolved organic
carbon (RDOC) as Williams and Druffel (1987) found it to be old relative to the circulation
time of the ocean. While RDOC within the modern ocean is thus perceived as vast and only
slowly renewed, its mobilization has been implicated by Sexton et al. (2011) to explain
Earth’s transient warming events (i.e., hyperthermals) of the Paleocene and Eocene
epochs (65–34 million years ago). Assessing this proposed function of RDOC as a rapidly
(~5–10 kyr) exchangeable carbon reservoir is presently limited by insufficient knowledge of
the responsible processes. Here we investigate the dynamics of RDOC in the deep Pacific
Ocean, previously characterized by concentration gradients thought to be established by
slow but systematic RDOC removal with circulation and aging of the water masses. We
demonstrate that RDOC is instead conserved during much of its circulation, but that there
exist localized sinks in the deep, far North Pacific and at mid depth in the subtropical South
Pacific. Water mass mixing into these sink regions creates the observed RDOC gradients.
Together, the Pacific sinks remove 7–29% of the 43 Tg RDOC added to the deep global
ocean each year with overturning circulation, and point to an important but still unidentified
control on the RDOC inventory of deep marine systems.
Citation: Hansell, D. A., and C. A. Carlson (2013), Localized refractory dissolved organic carbon sinks in the deep ocean,
Global Biogeochem. Cycles, 27, 705–710, doi:10.1002/gbc.20067.
1.
Introduction
[2] Marine DOC exhibits a spectrum of reactivity, from
very fast turnover of the most bioavailable forms in the
surface ocean to long-lived materials entrained in abyssal
circulation [Hansell, 2013]. These disparate reactivities
differentiate DOC fractions with distinctive functions in the
carbon cycle, ranging from rapid (daily) turnover in support
of vast marine heterotrophic microbial populations, to decadal/centennial sequestration of carbon, to a hypothesized
major source/sink of atmospheric CO2 driving paleoclimate
variability [Sexton et al., 2011]. The DOC fractions that exhibit daily to decadal turnover sum to a relatively small mass
(20 ± 3 PgC) [Hansell, 2013; Hansell et al., 2012], so these
pools are important in terms of carbon flux and as substrates
to microbial processes [Carlson, 2002] but not as Earth’s major carbon sequestration reservoirs. The 642 ± 32 Pg pool of
refractory DOC (RDOC), in contrast, plays a relatively modest role in the carbon cycle of the modern ocean over decadal
1
Rosenstiel School of Marine and Atmospheric Science, University of
Miami, Miami, Florida, USA.
2
Department of Ecology, Evolution, and Marine Biology, University of
California, Santa Barbara, California, USA.
Corresponding author: D. A. Hansell, Rosenstiel School of Marine and
Atmospheric Science, University of Miami, 4600 Rickenbacker Cswy.,
Miami, FL 33149, USA. ([email protected])
©2013. American Geophysical Union. All Rights Reserved.
0886-6236/13/10.1002/gbc.20067
time scales, but when paleoclimatic variations require a large,
readily exchangeable reservoir of carbon, this pool is a plausible candidate. Ultimately, understanding the past and future
roles of RDOC in climate depends on illumination of its
dynamics in the modern ocean.
[3] RDOC is present at concentrations ranging from ~34 to
<45 μmol kg-1 [Hansell et al., 2012]. The pool is highly
resistant to microbial turnover [Barber, 1968], yet a concentration gradient of ~14 μmol kg-1 exists between the North
Atlantic and the North Pacific [Hansell and Carlson, 1998], indicating that sinks (i.e., removal processes) exist. When DOC
concentrations were correlated with radiocarbon ages of deep
Pacific waters, a removal rate of ~0.003 μmol kg-1 yr-1 was
determined, consistent with the modeled rate required to reproduce observed deep ocean DOC on the global scale [Hansell
et al., 2009]. However, inconsistencies between observed
and modeled DOC in some locales were noted, particularly
in deep equatorial waters where observed DOC concentrations
typically exceeded modeled concentrations. Resolving these
discrepancies led to the analysis reported here, with our finding
that the RDOC removal rate previously described depicts the
system incorrectly.
2.
Data Employed
[4] All data used in this analysis are publicly available: Key
et al. [2004] provided salinity at the 4000 m isopleth in the
Pacific Ocean, as shown in Figure 1. Feely et al. [2008] provided the data shown from the meridional CLIVAR Repeat
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HANSELL AND CARLSON: REFRACTORY DOC SINKS
Figure 1. Salinity observed at 4000 m in the Pacific Ocean;
arrows indicate northward flow of high-salinity Circumpolar
Deep Water. Lines indicate station locations; depths <4000 m
are masked in gray. Station locations for data shown in
Figures 2 and 3 are highlighted (open circles along ~150°W).
Hydrography section P16S/N, the location of which is
shown in Figure 1.
3.
Observational Context
[5] That RDOC sinks exist in the deep Pacific Ocean is
evidenced by concentration gradients [Hansell et al., 2009].
To evaluate those, salinity is employed here, as its distribution conservatively traces water mass circulation and mixing;
deviations between salinity and DOC distributions illuminate
the sinks. In the Pacific, net circulation of deep and bottom waters constitute a meridional cell, with high-salinity
Circumpolar Deep Water (CDW) of the Southern Ocean (with
a strong North Atlantic Deep Water component) flowing
northward along the bottom, thus ventilating the system as
a deep western boundary current east of New Zealand
(Figure 1). The water then moves north across the equator near
the dateline. During its flow into the far North Pacific, the water is warmed and freshened (note lowered salinity; Figure 1),
increasing buoyancy and rising (Figure 2a). Freshening occurs
by vertical mixing with the low-salinity, subarctic intermediate
waters (IW, located at <1500 m) that mix downward by tidally
induced turbulence over rough topography including the continental margins [Ledwell et al., 2000]. Mixing of IW and
CDW forms the intermediate-salinity, high-silicic-acid water
mass North Pacific Deep Water (NPDW; Figure 2a), which
moves southward at mid depths [Schmitz, 1996]. As the deep
basin of the Pacific Ocean does not experience exchanges with
basins other than the Southern Ocean, water mass mixing is
largely between those waters described above (i.e., CDW,
IW, and NPDW).
4.
Water Mass Mixing and RDOC Sinks
[6] Using the conservation of salinity, NPDW (salinity
~34.65) is estimated to consist of ~97% high-salinity CDW
(~34.71) and ~3% low-salinity IW (~34). Conservative mixing
of DOC held in these water masses (end-member values:
CDW DOC ~40 μmol kg-1; IW DOC ~45 μmol kg-1 DOC
[Hansell et al., 2002]) would result in NPDW with DOC
~40 μmol kg-1 (this product of conservative mixing, as
estimated from salinity, is here termed DOCsal). The
meridional distribution of DOCsal at >4000 m on P16
(Figure 3a) is invariably ~40 μmol kg-1 because CDW
dominates (>97%) those near-bottom waters. But observed DOC concentrations (DOCobs) in the far North
Pacific are ~37 μmol kg-1 (Figures 2b and 3a), indicating
a DOC deficit (i.e., deviation from conservation) of
~3-4 μmol kg-1 in that system (Figure 2c).
[7] The responsible DOC sink exists either within and
during the centennial-scale, basin-wide circulation and mixing
[Stuiver et al., 1983; Van Aken, 2007] of the near-bottom waters (such that the DOC is slowly removed with time [Hansell
et al., 2009]) or, alternatively, in more localized regions. The
sink dynamic can be isolated by comparing DOCobs to DOC
concentrations expected by conservative mixing between
CDW and, in this case, NPDW (the conserved product is again
termed DOCsal). A sink that exists within the basin-wide circulation will appear as a deviation between DOCobs and DOCsal,
as previously observed in Figure 3a. If on the other hand the
distributions of DOCobs and DOCsal are identical (within
analytical uncertainty), then the deep DOC gradient must
result from conservative mixing of DOC within the water mass
end-members (i.e., CDW and NPDW).
[8] In Figure 3b, DOCsal is indistinguishable from DOCobs
(see statistical analyses in section 7 below and in Figure 4),
indicating that conservative mixing between high-DOC
CDW and low-DOC NPDW creates the deep DOC gradient.
This result requires that the DOC sink be regionally localized in the far North Pacific, perhaps associated with the
formation of NPDW. It is this sink (and subsequent
conservative mixing of NPDW with CDW) that creates
Figure 2. Distributions of (a) salinity and (b) DOC observed
(μmol kg-1) and (c) the DOC deficit (μmol kg-1; DOCsal minus
DOCobs, where DOCsal is the conserved mixing product of IW
and CDW) along a meridional section in the central Pacific
Ocean (location in Figure 1). Arrows indicate northward flow
of CDW near bottom, with formation and southward flow of
NPDW following freshening with IW. Isopycnals employed
in Figure 3c are shown in the upper plot. CDW salinity is off
scale (>34.7).
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HANSELL AND CARLSON: REFRACTORY DOC SINKS
DOC (µmol kg-1)
42
40
38
36
34
a
DOC (µmol kg-1)
Latitude (oN)
42
40
38
36
34
b
Latitude (oN)
DOC (µmol kg-1)
42
40
38
36
c
34
Latitude (oN)
Figure 3. DOC observed (filled) and predicted if conserved (open) at (a) >4000 m, mixing of IW from the north
with CDW from the south; (b) >4000 m, mixing of NPDW
from the north with CDW from the south; and (c) in the density interval σ3 41.43–41.455, mixing of the water mass
end-members at the northern and southern termini of the
meridional section.
the DOC concentration gradient observed in the deep waters
(Figures 2b and 3b).
[9] A DOC sink is evident in the midwater column
(~1500–3500 m) of the subtropical South Pacific as well,
where concentrations reach <35 μmol kg-1 (Figure 2b). The
dynamics of DOC there are similarly tested using salinity
as the conservative tracer. Following formation, NPDW is
transported to the South Pacific (Figure 2a), isopycnally
mixing with waters originating in the southern hemisphere.
DOCsal is estimated from two end-member mixing between
those two water masses (end-members: northern component
NPDW salinity ~34.65 and DOC ~37 μmol kg-1; southern
component salinity 34.73 and DOC ~ 40 μmol kg-1); mixing
is evaluated on the isopycnal surface 41.43 < σ3 < 41.445
(~3000 m; Figure 2a) as it lies near the core of NPDW.
There is consistency between DOCobs and DOCsal north of
the equator (Figures 3c and 4b), but DOCsal at 10–46°S is
~38 μmol kg-1 while DOCobs reaches <35 μmol kg-1, with
their trends against latitude unequal (Figure 4c). The DOC
deficit in this zone reaches as high as ~4 μmol kg-1
(Figure 2c), or an ~10% reduction in DOC relative to its
CDW source. Between 40°S and 60°S, DOCobs and DOCsal
are similar (Figure 3c), indicating conservative downward
mixing of the DOC-enriched upper-layer waters.
Figure 4. (a) Bivariate plot of DOCobs (red) and DOCsal
(green) versus latitude for data >4000 m. (b) Bivariate plot
of DOCobs (red) and DOCsal (green) versus latitude for all
data within the isopycnals σ3 41.43–41.455 in the North
Pacific. (c) Bivariate plot of DOCobs (red) and DOCsal
(green) versus latitude for all data within the isopycnals σ3
41.43–41.455 between 10°S and 46°S. Shaded areas around
regression lines represent the 95% CI.
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HANSELL AND CARLSON: REFRACTORY DOC SINKS
5.
RDOC Removal Rates
[10] The rate of RDOC removal in these two sinks can be
estimated from water mass transport rates and DOC concentration changes. Estimates for the net northward transport of bottom water into the North Pacific, supporting the formation of
NPDW, range from 1.5 to 5 Sv (1 Sv = 106 m3 s-1) [Schmitz,
1996; Ganachaud and Wunsch, 2000; Macdonald, 1998].
DOC removal associated with this formation (at ~3 μmol kg-1)
lies between 1.7 and 5.7 TgC yr-1. In the South Pacific,
NPDW is transported at 4–9 Sv [Schmitz, 1996] and experiences an additional ~1–2 μmol kg-1 loss of DOC (1.5–
6.8 TgC yr-1). Summing these sinks (3.2–12.5 Tg yr-1) indicates
that 7–29% of the 43 Tg yr-1 of DOC introduced to the deep
ocean each year to maintain its global inventory [Hansell
et al., 2012] is removed in these two systems.
[11] Other RDOC sinks must exist elsewhere in the deep
global ocean, with anomalously low DOC concentrations
indicating those locations. The deep Mediterranean Sea,
for example, holds DOC concentrations of <38 μmol kg-1
[Santinelli et al., 2012]. As those deep waters are formed
locally with surface waters of North Atlantic origin (with
elevated DOC concentrations), and as the concentrations at
<38 μmol kg-1 are lower by 2–3 μmol kg-1 than found in
the adjacent deep North Atlantic [Carlson et al., 2010], an
RDOC sink is apparent in that system.
6.
Sinks of Unknown Mechanisms
[12] UV oxidation of the RDOC pool has been described as
an important sink in the modern ocean [Mopper and Kieber,
2002], but it is a process limited to the surface layer and so
not involved with the deep sinks described here. The mechanisms for deep RDOC removal observed here are unknown.
Both Pacific sink regions are bordered by ocean ridges and/
or margins (Figure 1), and both are within gyre circulations
[Reid, 1997]. In the deep North Pacific, the lowest DOC
concentrations coexist with the lowest salinity (Figures 2a
and 2b), with this low-salinity feature emanating from the
continental margin of the NE North Pacific (Figure 1).
Downward mixing of low salinity IW is likely enhanced at
the margins by tide-induced turbulence [Ledwell et al.,
2000]. In the subtropical South Pacific, the low-DOC waters
circulate within an anticyclonic gyre [see Reid, 1997], guided
southward along the ocean ridge located near the dateline
and then eastward by a ridge (the Chatham Rise) at 45°S
(Figure 1). Turbulence associated with these circulations
likely suspends seafloor particles, which may serve as surfaces for DOC adsorption [Druffel et al., 1998].
[13] A strong sinking particle flux from highly productive
surface oceans, applicable to the subarctic North but not the
subtropical South Pacific, offers three mechanisms for
removing RDOC. The release of exoenzymes by particleattached microbial assemblages [Karner and Herndl,
1992; Arnosti, 2011] could lead to the breakdown and
subsequent uptake of recalcitrant compounds [Williams,
2000]. Alternatively, solubilization of sinking particles would
add bioavailable DOC to the deep-water column [Kiørboe
and Jackson, 2001; Azam and Long, 2001; Nagata et al.,
2010] that could support cometabolism (priming). It has been
hypothesized that labile solubilized substrates provide energy
to the free-living or particle-attached heterotrophic prokaryotes
to produce hydrolytic exoenzymes for the cometabolism of
RDOC [Madigan et al., 1997]. This priming effect is observed
in soils and aquatic systems, where increased remineralization
of recalcitrant organic matter occurs after input of biologically
labile materials [Carlson et al., 2002; Bianchi, 2011].
However, incorporation of RDOC into microbial biomass is
not discernable in the deep North Pacific [Ingalls et al., 2006;
Hansman et al., 2009], indicating that if cometabolism is
occurring, then little is assimilated into microbial biomass. A
third mechanism is that sinking particles scavenge RDOC as
they sink, but there is a much larger and deeper flux of particles
in the subarctic than the subtropical Pacific [Buesseler et al.,
2007], so exported particles should not play equivalent roles
in the two systems.
[14] Hydrothermal systems expelling water that has been
altered within Earth’s crust should influence RDOC distributions in the deep ocean, although the extent of influence has
not been established. Hydrothermal fluids circulate through
the upper oceanic crust, especially on ridge flanks [Johnson
and Pruis, 2003] where fluids flow through porous basalts.
Outflows on the Juan de Fuca Ridge have low DOC concentrations (~10–15 μM) [Lang et al., 2006], indicating RDOC
removal during the fluid’s 10 kyr crustal residence time
[Mottl, 2003]. Alternatively, as hydrothermal plumes release
chemically reduced fluids into an oxygenated water column,
redox disequilibria may provide energy for free-living
chemoautotrophs [Lam et al., 2004; McCarthy et al., 2011]
whose production could prime the removal of RDOC.
[15] Finally, self-assembling organic gels that contribute to particle formation may constitute an RDOC sink.
Biopolymers such as DOC, gels, and transparent exopolymers
[Wells, 1998; Carlson, 2002; Passow and Alldredge, 2004]
can move organic molecules up the particle size spectrum to
sizes capable of sinking through the water column [Verdugo
et al., 2004; Engel et al., 2004] and/or of being mineralized
by the resident microbes.
7.
Statistical Analyses of Correlations
[16] The slopes and intercepts of the linear regression
of DOC observed (DOCobs) and DOC modeled from salinity (DOCsal) against latitude were compared. For data
from >4000 m, Figure 4a shows that both linear regression models were significant at p < 0.0001. The estimated
slope of the linear regression of DOCsal versus latitude
(DOC = 38.4 0.03*Latitude, n = 381) lies within the
95% confidence interval ( 0.026 to 0.034) of the slope
of the linear regression of DOCobs versus latitude
(DOC = 38.6 0.03*Latitude, n = 382). The intercepts of
the two regression lines are within 0.2 μmol kg-1 of each
other. The agreement between these two linear regression
models indicates that the observed DOC distributions are
consistent with basin-wide conservative mixing of RDOC
held within CDW and NPDW. The observed values have
more scatter than those predicted, reflecting less analytical precision for DOC (coefficient of variation (CV) of
2–3%) than for salinity (CV 0.003%).
[17] The same analysis was conducted within the
isopycnals that define the core of the NPDW. We examined how the models compared for the stations north
of the equator (0–56°N) and from 10°S to 46°S. In the North
Pacific, the linear regression model of the observed DOC
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HANSELL AND CARLSON: REFRACTORY DOC SINKS
(DOCobs = 37.7 0.015*Latitude; n = 71, p = 0.0035) and
salinity-modeled DOC (DOCsal = DOC = 37.9 0.017*Latitude;
n = 70, p < 0.0001) were both significant (Figure 4b). The slopes
of the modeled DOC concentrations were within the 95% CI
( 0.023 to 0.005) of the observed DOC versus latitude, with
the intercepts being within 0.2 μmol kg-1 of each other.
Agreement between these two linear regression models indicates
that the observed DOC distributions are consistent with conservation of RDOC in NPDW north of the equator.
[18] In the South Pacific between 10°S and 46°S, the linear models of observed and predicted DOC concentration
versus latitude were not consistent with each other
(Figure 4c). The linear model between DOCsal versus
latitude (DOC = 37.51281 0.0149233*Latitude [°N],
n = 23, p = 0.0007) was significant while the DOCobs
versus latitude (DOC = 36.390234 0.0130115*Latitude
[°N], n = 24, p = 0.485) was not; there was no overlap
of the 95% CI. The lack of agreement between these
two linear models indicates that the observed DOC did
not behave conservatively in this latitudinal range in the
South Pacific.
8.
Conclusions
[19] Elevated DOC concentrations were found associated
with the high-salinity waters of CDW in the Southern
Ocean. The elevated salts in CDW ultimately derive from
saline North Atlantic Deep Water (NADW) [Reid, 2005],
so it is likely that the elevated DOC in CDW has the same
source. Much of the RDOC concentration gradient in the
global deep ocean may exist by conservative mixing between
the RDOC-enriched North Atlantic Deep Water and globally
distributed but localized sites of RDOC removal. Further
analyses, including compositional and isotopic analytical
techniques, need to be conducted on the global scale to determine if RDOC exported with NADW is essentially conserved until it reaches localized sinks.
[20] The discrepancy between observed and modeled DOC
in the deep ocean noted above stems from erroneous model
assumptions used by Hansell et al. [2009]. The correlation
between DOC and water mass ages determined in that work
assumed single end-member mixing; it appeared from that
result that RDOC was removed slowly (0.003 μmol kg-1 yr-1)
but steadily, likely everywhere in the deep ocean. The removal
rate was then applied in the model, leading to the discrepancies. But the results reported here inform us that, in fact,
RDOC is conserved over great distances of the deep
Pacific and that mixing into localized sinks creates the
concentration gradients.
[21] The relevance of these RDOC sinks to the carbon that
forced variability in paleoclimate remains unknown until two
important determinations are made. First, the controls on the
sinks need to be identified: what changes in the ocean system
would cause acceleration of RDOC loss required by the
hyperthermals of the Paleocene and Eocene, where atmospheric
CO2 concentrations changed rapidly (over ~5–10 kyr), and
what changes would reverse this loss in order to rebuild a large
RDOC inventory (over ~30 kyr), setting the stage for the next
massive release and associated climate hyperthermal. Second,
the fate of the DOC removed from the water column must be
identified. If the fate is mineralization, then RDOC is indeed
an exchangeable pool of CO2 for the atmosphere. However,
if particle scavenging of RDOC is the primary sink, such that
carbon is sequestered in the sediments, then the carbon may
remain buried over geologic timescales.
[22] Acknowledgments. Statistical analyses benefited from discussion
with C. Nelson. This work was supported by the U.S. National Science
Foundation grants OCE-OCE0752972 and OCE-1153930.
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