THE FATE OF CARBON AND NUTRIENTSEXPORTED OUT OF THE SOUTHERN OCEAN

Judith HauckAndrew Lenton, Clothilde Langlais, Richard Matear

„Southern Ocean nutrient supplyaccounts for about three-quarters ofbiological production north of 30°S“

(Sarmiento et al., 2004)

„Southern Ocean nutrient supply accounts for aboutthree-quarters of biological production north of 30°S“

„Furthermore, this region acts also as the keygatekeeper controlling the supply of nutrients to the lowlatitude oceans, and thus the magnitude of low latitudeproductivity in the past, present and future (Sarmientoet al. 2004; Matsumoto et al. 2002; Moore et al. 2018)."

„models suggest that vertical exchange within theSouthern Ocean is responsible for supplyingnutrients that fertilize three-quarters of the biologicalproduction in the global ocean north of 30°S (Sarmiento et al., 2004, Marinov et al., 2006)“

„… providing nutrients that support low- latitudeproductivity […] Sarmiento et al., 2004 […] „

Previous studies„Southern Ocean nutrient supply accounts for about three quarters of biological productionnorth of 30°S“

Nutrient supply

Carbon sequestration

Preindustrial results atequilibrium!

Marinov et al., 2006

Sarmiento et al., 2004

Southern Ocean –decoupling of anthropogenic CO2 uptake and storage

Khat

iwal

aet

al.

(200

9)

>50% of Southern Ocean Cant in SAMW and AAIW (Langlais et al., Sci. Rep., 2017)

Aims of this talk

Provide a similar concept for carbon fluxes

Spoiler: Southern Ocean nutrient supply to low-latitudes is overestimated

Transient response – large decadal variability / future change

Panassa et al (2018): Significantincrease in nitrate concentrationin AAIW based on Polarstern data

Large decadal variability of Southern Ocean CO2 uptakeLandschützer et al., 2015

Iida et al., 2013; Ayers & Strutton, 2013; Hoppema et al., 2015; Pardo et al., 2017

Transient response – large decadal variability / future change

Land

schü

tzer

et a

l., 2

015

Pana

ssa

et a

l (20

18)

Moore et al (2018): Increased SO nutrient trapping leads to declineof global primary production after 2100.

Regulated Ecosystem Model (REcoM2)

Based on Schartau et al., BG, 2007

Non-Redfield model

Explicit sinking

Hauck et al., 2013, 2016Hauck et al., GBC, 2013

Model Experiments

1. Control2. No bio in Southern Ocean

x

Hauck et al., GBC, 2018

Model Experiments

1. Control2. No bio in Southern Ocean3. No bio + no gas-ex in SOx xx

Hauck et al., GBC, 2018

Model Experiments

Southern Ocean south of 40°S

x xx1. Control2. No bio in Southern Ocean3. No bio + no gas-ex in SO

Hauck et al., GBC, 2018

Model ExperimentsControl

Atm

CO2

No bio + no gas-exNo bio

biological pump

physical pump

MITgcm-REcoM2 (Hauck et al., 2013; 2015; 2016)

JRA55 reanalysis 1958-2016

Spin-up: two cycles of JRA forcing, third cycle until 2005Experiment: keep atmospheric forcing and CO2 concentration constant at 2005 levels

Positive: Southern Ocean primary production increase in DINNegative: Southern Ocean primary production decrease in DIN

Negative DIN anomaly in SO surface and north of 40°S surface and to ~3500 m.Positive DIN anomaly in SO subsurface to bottom

Atlantic Pacific

Results – Perturbations of nutrient fields, year 196-200

Positive: Southern Ocean primary production increase in DINNegative: Southern Ocean primary production decrease in DIN

DIN anomaly spreads all over the Atlantic in 200 years

Atlantic Pacific

Results – Perturbations of nutrient fields, year 196-200

Pacific shadow zone

De Lavergne et al., 2017

Pacific shadow zone, little overturning

Vertically-Integrated Net Primary Production

No nutrient utilization in Southern Ocean:

7.0 PgC/y loss south of 40°S

3.0 PgC/y gain north of 40°S locally 10% increase

32.8 PgC/y

29.8 PgC/y

43% compensation on 200-year time-scale

39% for export

7.0 PgC/y

Vertically-Integrated Net Primary Production

4.5 PgC/y

+3.9 PgC/y

2.5 PgC/y -1.0 PgC/y

No nutrient utilizationin Southern Ocean:

Locally 32% increase ofdiatom NPP (87% compensation)

Reinforcement ofnano-NPP rather thancompensation

‚silicate leakage‘

Change in mean Southern Ocean surface nutrients

DIN: 15 mmol m-3 26 mmol m-3 (+75%)DSi: 14 mmol m-3 50 mmol m-3 (+255%)DFe: 0.15 µmol m-3 0.64 µmol m-3 (+340%)

100% change in DIN would leadto 13% in NPP north of 40°S

„Southern Ocean nutrient supply accounts for about threequarters of biological production north of 30°S“

Sarmiento et al., 2004

??

Previous studies„Southern Ocean nutrient supplyaccounts for about three quarters ofbiological production north of 30°S“

Sarmiento et al., 2004

33 – 75% of low latitude productivityis sustained by nutrient export fromSAMW

Palter et al., 2010

27% of midlatitude and subpolar North Atlantic fuelled by nutrientslast utilized in the Southern Ocean, with even lower numberselsewhere Holzer and Primeau, 2013 Holzer and Primeau, 2013

Previous studies

Primeau et al., 2013

unperturbed

Increase in SO production Decrease

Signal dependson direction ofperturbation

Non-linear state-dependency

Previous studies

Primeau et al., 2013

unperturbed

Signal dependson direction ofperturbationNon-linear state-dependency

NPP frac sustained by SO nutrients

Sarmiento et al

This study

Increase in SO production Decrease

Carbon

Results – Perturbations of DIC fields, year 196-200Bi

ol. p

ump

Phys

. pum

p

Atlantic Pacific

The fate of carbon

-0.52 PgC/yr

Uptake low, as atm. CO2 is constant

-1.14 PgC/yr (uptake)

+0.62PgC/yr (outgassing)

0.2 PgC/yr(outgassing)

0.57 PgC/yr(tendency to outgas)

-0.55 PgC/yr(tendency to uptake)

The fate of carbon

Bio and physeffects nearlybalance, but

Changes in SO phys, incompletelycompensated by bioLeQuéré et et al. (2007), Hauck et al. (2013)

Changes in SO bio, uncompensated by SO physicsMoore et al. (2018)

The fate of carbon

Const. CO2

Atm

CO2

Atm CO2 increasing with rate of increase(2000-2010) = 1.98 ppm/year

388

773

The fate of carbon

Larger uptake due to higherand increasing atm CO2

SO turns fromoutgassing to uptakeafter ~50 years

Smaller uptake

Southern Ocean carbon pumpslead to outgassing of 0.47 PgC/yr

Implications for carbon uptake

Compensation = - CO2 flux north of 40°SCO2 flux south of 40°S

0: SO C-pumps do not lead to changenorth of 40°S

1: complete compensation

Implications for carbon uptake

0: SO C-pumps do not lead to changenorth of 40°S

1: complete compensation

Compensation = - CO2 flux north of 40°SCO2 flux south of 40°S

Biological pump: 50% compensation

Physical pump: 90% compensation

Implications for carbon uptake

Compensation = - CO2 flux north of 40°SCO2 flux south of 40°S

Biological pump stores CO2 for longerthan physical pump

Incomplete compensation of NPP Sinking of particles to deeper

depths

Feedbacks in the ocean carbon sink due to potential future changes in• physically-driven CO2 uptakeoccur faster and are nearly balancedafter 200 years• biological C-pumpoccur slower but last longer

Summary

Hauck et al., 2018, GBC

Physical pump: CO2 outgassing, 90% reventilated after 200 yearsBiological pump: CO2 uptake, 50% reventilated after 200 yearsNet Primary Production perturbation: 40% compensation on 200 year time-scale

% S

AMW

reac

hed

Equa

toria

lUnd

ercu

rren

t Tmin 28 years

50% ~ 100 years

70% ~ 200 years

Rodgers et al., GRL, 2003

Timescales of subduction and reventilation

Results – timescale of reventilation

5°S to5°N

Emergence of signal in equatorial

Atlantic: ~15 years

Pacific: ~30 years

Somewhat shorter time-scalethan Rodgers et al, because:• Rodgers neglect stochastic terms

in Lagrangian models to accountfor transport by eddies (Shah et al., 2017)

• Includes perturbation of surface• Rodgers stricter definition of eq.

Pac.

Si remineralizationdeeper than for DIN

Fe stark contrast deepAtl and Pac

SO NPP accumulation of Fe in surface Atl

Atlantic Pacific

Results – Perturbations of nutrient fields, year 196-200

The fate of carbonDIC CO2 flux

The fate of carbon

SO active biology negative NPP anomaly positive DIC anomalypositive CO2 flux anomaly (more outgassing or less uptake)

The fate of carbon

SO active biology negative NPP anomaly positive DIC anomalypositive CO2 flux anomaly (more outgassing or less uptake) SO active biology upwelling of DIC depleted water negative DIC anomaly negative CO2 flux anomaly (less outgassing or more uptake)

Previous studies

Holzer and Primeau, 2013

27% North Atlantic

Blue: fraction ofNPP sustained bySO nutrients

Previous studies„Southern Ocean nutrient supply accounts for about three quarters of biological productionnorth of 30°S“ Sarmiento et al., 2004

33 – 75% of low latitude productivity is sustained by nutrient export from SAMWPalter et al., 2010

27% of midlatitude and subpolar North Atlantic fuelled by nutrients last utilized in theSouthern Ocean, with even lower numbers elsewhere

Holzer and Primeau, 2013

Increasing nutrient utilization in the Southern Ocean resulted in a larger fraction of nutrientslast utilized in the Southern Ocean (up to 45%), but decreaded the overall productivity.

Holzer and Primeau, 2013Non-linear response of productivity outside the Southern Ocean to perturbations within theSouthern Ocean (increase/decrease of nutrient utilization) Primeau et al., 2013

Southern Ocean carbon cycle

Transport of Cant into the ocean interior through:Northward-Ekman transport, and formation of Subantartcic Mode Water (SAMW) andAntarctic Intermediate Water (AAIW)>50% of Southern Ocean Cant in SAMW and AAIW (Langlais et al., 2017)