Localized refractory dissolved organic carbon sinks in the deep ocean
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 34.7).
706
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
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 34.7).
706
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
