Quantifying competing carbon pathways in mesoscale upwelling filaments off NW Africa

Nick Hardman-Mountford (CSIRO), Carol Robinson (UEA), Ricardo Torres, Tim Smyth, Ian Brown, Vasilis Kitidis, P. Nightingale, C. Widdicombe (PML)

(or the pitfalls of seawater CO2 inversions)

What is relative contribution of different CO2 pathways: air-sea flux vs. export production?

CoolHigh NHigh CO2

Warms

CO2 flux

Phytoplankton production

Respiration

Carbon export

NCP = E

Lagrangian study: plume tracking with SF6 and drifters

• 3 patches seeded• P1 & P3 filaments tracked• P2 subducted

SOLAS-ICON+ (D338)

+The impact of coastal upwelling on the air-sea exchange of climatically important gases

Rees et al. 2011

Sampling

Underway:T, S, fCO2, O2, Fl

Surface drifters:T, S, fCO2

Physics:CTD, MVP, ADCP, micro-turbulence, wirewalker, optics

Rosette bottle samples

Deck incubations

Spatial structure – satellite view

Patch 1: freshly upwelled, followed for 9 days

Patch 3: ~10 days old, followed for 8 days

Spatial structure – in situ

Temporal variability

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O2

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ol

l-1)

fCO

2(μ

atm

)Patch 1 fCO2 O2

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l-1)

fCO

2(μ

atm

)

Patch 3 fCO2 O2

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Sum of BIOMASS

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Phytoplankton community and primary productionPatch 1

Patch 3

ncptrsptc

x

tc

JJDICDICt

h

hxxDIC

K

hz

DICKzhh

F

t

DIC

111

Daily DIC change

Sea-air Flux

Vertical diffusion flux

Horizontal diffusion flux

Vertical entrainment (ventilation)

Horizontal advection

NCP

• Assume advection/diffusion terms negligible because lagrangian expt, i.e. tracking water patch.

• Supported by lack of relationship between salinity and DIC within patch

• Salinity normalise DIC to make sure

2110.0

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Patch 1

Patch 3

?Controls on CO2 dynamics

Shadwick et al. 2010

• Focus on NCP, F and V?

y = 49.986x + 564.18R² = 0.8356

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TA (µ

mol

kg-1

)

Salinity

DIC calculations• Need continuous DIC

• Use discrete TA / S relationship to calculate continuous TAs

• Calculate DIC from TAs and measured underway fCO2 in CO2SYS

• Salinity normalise calculated DIC = nDIC

intint

SS

DICnDIC

Daily δnDIC calculation

δnDIC day

δnDIC night

δnDIC day+night

nDIC

Time

depth integrated NCPt = Zeut (max DICt- max DICt-1) – Ft (– Vt)

A. Daily nDIC change

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l-1 dnDIC_day

dnDIC_night

Patch 1

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dnDIC_day

dnDIC_night

Patch 3

Daily DIC reduction

Night time DIC increase

production/respiration signal

Patch 1 has larger signals and is more variable than Patch 3

B. Sea-air CO2 fluxes

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Patch 3Patch 3

Calculated using Nightingale et al. (2000)

Winds 6-14 m s-1 P1, 8-14 m s-1 P3

ΔpCO2 20-100 µatm P1, 60-110 µatm P3

Patch 1 sea-air flux starts high and reduces as seawater pCO2 reduces

Increase on 25-26/4 from ventilation?

Patch 3 sea-air flux higher on average, more gradual decline, driven by seawater pCO2 decline

C. Depth Integrated NCP* vs. sea-air flux

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Patch 3

y = -4.9816x + 921.34R² = 0.7242

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CO

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%O2 Sat

y = -4.7253x + 949.31R² = 0.8301

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Np

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Louicades et al. 2011

Patch 1

C. Depth Integrated NCP* vs. sea-air flux

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Patch 3

Patch 1 is net autotrophic and NCP* dominates over sea-air flux

Patch 3 shifts from autotrophic to heterotrophic between days

In ~trophic balance over all

NCP* dominates the signal but overall sea-air flux is greater

mmol C m-2 Patch 1 Patch 3

NCP* 1285 29

Sea-air flux 86 124

NACW>50%Max(80%,75m)

SACW>50%Max(95%,300m)

SACW<50%Max(40%,150m)(NACW or BDA shelf water)

SACW>50%(Max 100%)

Patch 1

Patch 3

Water masses

D. NCP vs. entrainment/ventilation vs. sea-air flux

-400-300-200-100

0100200300400500600

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ol C

m-2

d-1

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- NCP_M

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F_M N'00

Use change in nutricline depth and DIC gradient over nutricline

NCP (residual) has to increase with ventilation

Accounting for ventilation increases estimate of autotrophy - Is it real?

mmol C m-2 Patch 1 Patch 3

NCP-V2823 715

Vent1537 687

Sea-air flux 86 124

Preliminary conclusions

1. Biogeochemistry different between filaments:– phytoplankton, CO2 dynamics, [nutrients]

– Water masses or age?

2. Variable influence of NCP vs Sea-Air Flux– Patch 1: net autotrophic, NCP dominates; sea-air CO2 flux has minor

influence

– Patch 3: trophic status looks neutral but depends on external sources of DIC; sea-air CO2 flux may be dominant over time

3. Method– Ventilation calculation critical for determining NCP?

– Method needs testing / refining for a lagrangian /sub-mesoscale framework

Next steps

• Consider sub-mesoscale physics to calculate ventilation fluxes

• Compare results with DOC, C14 PP, O18 R, N-flux estimates

• Look at heterotrophic dynamics (diurnal variability in grazing?)

Acknowledgements: UK-SOLAS ICON team, National Marine Facilities staff, Captain and crew of RRS Discovery.

Funding: UK Natural Environment Research Council (NERC). Satellite images provided by NEODAAS, UK.

Thank you!

B. Sea-air CO2 fluxes

Units on time plots legend!!!

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Nutrients