1

Minhan Dai (mdai@xmu.edu.cn)

Xiamen University, China

Coastal Ocean: Biogeochemistry & physical-biogeochemistry coupling II

Aug 4-15, 2009Aug 4-15, 2009

Outline

• Why coastal ocean• Basics of the coastal ocean• Coastal ocean in changing• Physical - biogeochemistry coupling: a case study

• Coastal ocean carbon cycling– Ocean margin in the global carbon cycle– Case in China Seas

• pCO2 and air-sea CO2– fluxes and variability

• 234Th and POC export fluxes• OC/IC fluxes

• Outlook

CO2 (ppm)

0°C

-8°C

280 ppm

200 ppm

5.8°C

1.4°C

960 ppm

550 ppm

Temperature (oC)

400,000 years

CO2 (ppm)

0°C

-8°C

280 ppm

200 ppm

5.8°C

1.4°C

960 ppm

550 ppm

Temperature (oC)

CO2 (ppm)

0°C

-8°C

280 ppm

200 ppm

5.8°C

1.4°C

960 ppm

550 ppm

Temperature (oC)

400,000 years

CO2 (ppm)

0°C

-8°C

280 ppm

200 ppm

5.8°C

1.4°C

960 ppm

550 ppm

Temperature (oC)

CO2 (ppm)

0°C

-8°C

280 ppm

200 ppm

Temperature (oC)

400,000 years

5.8°C

1.4°C

960 ppm

550 ppm

1850

JGOFS

2100

Vostok

global

Le Quéré, 2003

2000 - 2007: 2.0 ppm y-1

2007: 2.2 ppm y-1

1970 – 1979: 1.3 ppm y-1

1980 – 1989: 1.6 ppm y1

1990 – 1999: 1.5 ppm y-1

Year 2007Atmospheric CO2

concentration:383 ppm

37% above pre-industrial

Atmospheric CO2 Concentration

Data Source: Pieter Tans and Thomas Conway, NOAA/ESRL

Efficiency of Natural Sinks

Land Fraction

Ocean Fraction

Canadell et al. 2007, PNAS

• Part of the decline is attributed to up to a 30% decrease in the efficiency of the Southern Ocean sink over the last 20 years.

• This sink removes annually 0.7 Pg of anthropogenic carbon.

• The decline is attributed to the strengthening of the winds around Antarctica which enhances ventilation of natural carbon-rich deep waters.

• The strengthening of the winds is attributed to global warming and the ozone hole.

Causes of the Declined in the Efficiency of the Ocean Sink

Le Quéré et al. 2007, Science

Cred

it: N.

Metzl

, Aug

ust 2

000,

ocea

nogr

aphic

cruis

e OIS

O-5

2

The global carbon cycle

(Source, Sarmiento and Gruber, 2002)

The potential role of marginal seas in the global carbon cycle

Surface area~7%

Primary production ~28%

Sedimentary ~80%

Ocean margin

0.2-1.0 GTC/yr

~up to 50% open ocean uptake

Air-sea CO2 fluxes-current estimates：Global continental shelf—a big debate

Ducklow and McAllister 2005Ducklow and McAllister 2005

Ducklow and McAllister (2005) concluded that the global coastal ocean as a whole is autotrophic and potentially a strong sink for atmospheric CO2(~0.9 Pg CO2 yr-1).

Coastal Ocean CO2 Sink or Source?

~0.9 PgC

Mass balance of carbon in continental shelves (flows are in1012 moles C yr-1; modified from Chen, 2004)

Chen and Borges, 2009

0.33 to 0.36 Pg C yr-1

Cai, Dai, and Wang, 2006, GRL

SCSECS

3

Cai, Dai, and Wang, 2006, GRL

0.2 Pg C/yr

Current Estimates

• extrapolation from single shelf studies (e.g. Tsunogai & Watanabe, 1999; Thomas et al., 2004)

• area-weighted averaging of existing shelf fluxes (e.g. Duglow& McAllister, 2005; Borges et al., 2005)

• model simulations (?)

• Province-based (Cai et al., 2006)

– World margin is a heterogeneous system, dynamic exchanges – variability in time & space of both fluxes and controls

– Latitudinal trend might exist

Summary Coastal carbon Challenges !

•Complex physical forcing in various time scales•Complex physical-biogeochemical domains

•River-margin-ocean connections•Mesoscale processes and their interactions-possibility of non-linear •Diverse ecosystems

•Modulation of air-sea CO2 exchange complex

Challenges in coastal carbon studies:

variability in time and space – fluxes and controls

150

250

350

450

550

650

0 50 100 150 200 250 300

Spring Summer Autumn Winter

Distance from the PRE (km)

pCO

2(μ

atm

)

Shelf SlopeCoast

Zhai, Dai et al., Mar Chem 2005Dai et al., Cont Shelf Res 2008

river input

river input

upper waterupper water

deepwaterdeepwater

thermoclinethermocline

atmospheric atmospheric COCO22

microbialmicrobial

zooplanktonzooplankton

phytoplanktonphytoplankton

resuspensionresuspension

export and burial

exchange at the interfacesexchange at the interfacesinternal cycleinternal cyclecarbon reservoirscarbon reservoirsbiological pump componentsbiological pump components

DOCDOCterrestrial organic carbonterrestrial organic carbon

openopenoceanocean

exchange with exchange with open ocean

open oceanPOCPOC

resuspendedresuspended organic carbonorganic carbon

sedimentsediment

CHOICE-C, 2009

4

Time scale

• Diurnal (!?)• Seasonal (!)• Inter-annual (?)• Decadal (?)• Longer time scale

Open SCS

N SCS-pCO2-temp diurnal variation range: ~10 μatm

SST(oC)29.2 29.4 29.6 29.8

pCO

2(μat

m)

362

372

382

RegressionSST vs in situ pCO2

08:00 16:00 00:00 08:00

pCO

2(μat

m)

372

377

382

TpC

O2(μ

atm

)

-8

-4

0

4

8NpCO2

in situ pCO2

TpCO2

(a)

(b)

Dai et al., L&O, 2009

pCO2 diurnal variation range: ~30 μatm

Aug., 2004

Taiwan Strait (shelf)-tidal/current

07:00 15:00 23:00 07:00

Tem

pera

ture

(o C)

24

25

26

27

Salin

ity

33.3

33.6

33.9

34.2SSTSalinity

pCO

2(μat

m)

360

375

390

405

in situ pCO2

NpCO2

(a)

(b)

pCO2 (Sal, T) = 27.6 (Salinity) – 2.6 (Temperature) – 482.6

Dai et al., L&O, 2009

6-17 6-18 6-19 6-20 6-21 6-22 6-23 6-24

Sal

inity

31

32

33

34

35

Tem

pera

ture

(oC

)

25

26

27

28SalinitySST

pCO

2(μ a

tm)

300

450

600

Tide

Hei

ght (

cm)

0

300

600

900

pCO2 Tide Height

Shenhu Bay Tidal mixing control

Measured by a fiber optical chemical CO2 sensor mounted on a mooring.

Dai et al., L&O, 2009

06-2-1 06-2-2 06-2-3 06-2-4 06-2-5

T(o C

)/Sal

22

24

26

28

34

Tide

hei

ght(c

m)

0

40

80

120

160

200

Tide_Height(predicted)Temp( ) ℃SalinityTide_Height(measured)

Date(yy-m-d)

DO

(mg/

L)

4

6

8

10

12

14

16

pCO

2(μa

tm)

200

300

400

500

600

DO(mg/L) pCO2(water)pCO2(air)

pCO2 diurnal variation range: 200 ~ 600 μatm

Coral reef -- Xisha Islands

Salinity

Temp

pCO2

DO

Dai et al., L&O, 2009

4.14 (Borges et al., 2005)

-1.9 (Cai et al., 2006)

-3.0 (Chen & Borges 2008)

Flux mmol C m-2 d-1

±9.61-28.82200~600 μatm2.8×105Coral reefs

±1.7640 μatm2.60×107Shelf(ie TS)

~±0.48-0.77~10-16 μatm3.36×108Open areas of coastal ocean

Flux variationmmol C m-2 d-1

pCO2 dielvariation

Areakm2

Significant uncertainties may be derived solely by ΔpCO2 potentially caused at different sampling time (e.g., in an underway observation), especially in coastal seas.

5

Kienast et al., 2001

Longer Time Scale: SCS as a source during the past 200 ky

Hu et al, 2002

万年

104 yr

Spatial domains

• Upwelling• Bloom• Eddies• Coral reef

Summary

• Time scale matters in the constraint of source and sink terms

• Diurnal variation critical for sampling strategy

• Heterogeneous in space-key domain must be considered for regional extrapolation

Modulation of pCO2 in marginal seas

• Physical/solubility• land carbon input-IC/OC mass balance• Biological pump

– Primary productivity– Export

• OC• IC

• Exchange with open ocean

Th-234 based export production

• Thorium-234 is a particle-reactive nuclide with a half-life of 24.1 days.

• In the open ocean, its parent nuclide 238U (half-life = 4.5×109a) has conservative behaviors:

238U (dpm/Kg)= 0.0686×Salinity (Chen et al. 1986)In open oceans, the 238U concentration is in the vicinity of 2.5 dpm/L

Thorium-234/Uranium-238 disequilibria:

6

ThoriumThorium--234 approach for estimating particle export234 approach for estimating particle export

depthdepth(m)(m)

234Th

238U

∗∗

∗ ∗

At Steady State:(238U - 234Th) λ *Z = 234Th Export (dpm m-2 d-1)

Measure C/234Th on sinking particles --> Carbon Export

∗ ∗ ∗∗ ∗ ∗ ∗

Courtesy of Buesseler

Carbon flux = 234Th flux • [C/234Th]sinking particlesCarbon flux = 234Th flux • [C/234Th]sinking particles

• Empirical approach

• Must use site and depth appropriate ratio

POC/234Th

From Buesseler (2004)From Buesseler (2004)

The quantification of 234Th flux

• Fe(OH)3 precipitation followed by ion exchange separationand beta counting (Bhat et al., 1969; Anderson and Fleer, 1982)– labour intensive/limited sampling resolution

• MnO2-impregnated cartridge technique (Mann et al., 1984; Livingston and Cochran, 1987)– overestimation of Thcollection efficiency (Cai et al., G3, 2006)

• MnO2 ppt+Small-volume techniques+on-site beta counting (RvdL & Moore 1999; Buesseler et al., 2001; Benitez-Nelson et al. 2001; Cai et al. 2006) –high resolution sampling

See FATE SI at Mar. Chem., 2006 Buesseler et al. (2006)

Plus:

the decay effect of 234Th (Cai et al., GRL, 2006)

Issues on the POC/234Th ratio changes with particle size

-Empirical C/Th ratios and the controls

Dependence of 234Th/228Th ratio with particle size:

At any given depth, 234Th/228Th ratio consistently decreases with particle size. This is due to the decay of 234Th (T1/2=24.1d).

RP=Rd×EXP[-(λ234-λ228)t]

As λ234>>λ228 , we have

RP=Rd×EXP[-λ234t]

where RP and Rd are 234Th/228Th ratio in particulate and dissolved phases.

Cai et al., GRL, 2006228Th (T1/2=1.91 yr),

The application of high-resolution sampling using small-volume technique

• Southern SCS (Cai et al., JGR-Ocean, 2008)

• Northern SCS (Chen et al., in preparation)

7

Sampling (S-SCS)

During the South China cruise in spring 2004, 36 stations were occupied and samples were collected for 234Th and POC analyses.

2 L of seawater was sampled for the analysis of total 234Th, and 6-10 L of seawater was used to measure particulate 234Th and POC.

111 112 113 114 115 116

Total Th-234 (dpm/l)

-100

-80

-60

-40

-20

0

Dep

th (m

)

A4 01 11 09 07 45 47 48

111 112 113 114 115 116

Particulate Th-234 (dpm/l)

-100

-80

-60

-40

-20

0A4 01 11 09 07 45 47 48

109 110 111 112 113Longitude (E)

-100

-80

-60

-40

-20

0

Dep

th (m

)

22 20 18 33 35 37

109 110 111 112 113Longitude (E)

-100

-80

-60

-40

-20

022 20 18 33 35 37

7 8 9 10 11 12-100

-80

-60

-40

-20

0

Dep

th (m

)

605916182729

7 8 9 10 11 12-100

-80

-60

-40

-20

0605916182729

6 7 8 9 10 11 12

Latitude (N)

-100

-80

-60

-40

-20

0

Dep

th (m

)

30 32 33 43 45 55 57 49

6 7 8 9 10 11 12

Latitude (N)

-100

-80

-60

-40

-20

030 32 33 43 45 55 57 49

1.6

1.8

22.2

2.4

2.6

2.8

33.2

00.1

0.2

0.3

0.4

0.5

• Spatial variability substantial

• Depth of maximum scavenging of 234Th shoals from west to east

• Maximum scavenging of 234Th occurs roughly at the same depth as florescence maximum.

• Tight coupling between 234Th scavenging and physical structure

Total Th-234 (dpm/l)

6

8

10

12

14

Latit

ude

(N)

Particulate Th-234 (dpm/l)

6

8

10

12

14

6

8

10

12

14

Latit

ude

(N)

6

8

10

12

14

108 110 112 114 116 118

Longitude (E)

6

8

10

12

14

Latit

ude

(N)

108 110 112 114 116 118

Longitude (E)

6

8

10

12

14

1.4

1.6

1.8

22.2

2.4

2.6

2.8

33.2

3.4 00.1

0.2

0.3

0.4

0.5

0.6

0 m 0 m

25 m 25 m

50 m 50 m

• Enhanced surface scavenging of 234Th in the western part of southern SCS.

• Enhanced sub-surface scavenging of 234Th in the eastern part of southern SCS.

• Intense shallow remineralization in the euphotic zone.

6

8

10

12

14

Latit

ude

(N)

6

8

10

12

14

6

8

10

12

14

Latit

ude

(N)

6

8

10

12

14

6

8

10

12

14

Latit

ude

(N)

6

8

10

12

14

6

8

10

12

14

Latit

ude

(N)

6

8

10

12

14

108 110 112 114 116 118Longitude (E)

6

8

10

12

14

Latit

ude

(N)

108 110 112 114 116 118Longitude (E)

6

8

10

12

14

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

0.00

0.10

0.20

0.30

0.40

0.50

0.60

• Enhanced surface scavenging of 234Th in the western part of southern SCS.

• Enhanced sub-surface scavenging of 234Th in the eastern part of southern SCS.

• Intense shallow remineralization in the euphotic zone.

0 m

25 m

50 m

75 m

100 m

Spatial Variability of POC export fluxes

108 110 112 114 116 118Longitude (E)

6

8

10

12

14

Latit

ude

(N)

-1000

-700

-400

-100

200

500

800

1100

1400

1700

108 110 112 114 116 118Longitude (E)

6

8

10

12

14

1

3

5

7

9

11

108 110 112 114 116 118Longitude (E)

6

8

10

12

14

-11

-8

-5

-2

1

4

7

10

13a b c

234Th flux (dpm m-2 d-1) POC/234Th (μmol dpm-1) POC flux (mmolC m-2 d-1)

•Negative flux of POC export is the result of lateral input of particulate matters•Extensive zones of 234Th excess, possibly due to intense remineralization•Overall low export (3.8±4.0 mmolC/m2/d)

8

Summary• Method:

– 234Th relatively easy to measure yet constraint of POC export takesgreater effort

– High-resolution sampling helps in defining Th fluxes as well as providing insights of particle dynamics

– 228Th helps in defining the decay of 234Th when using C/Th ratio

• South China Sea:– High spatial variation, consistent with hydrodynamics– Shallow Th-234 excess in S SCS (re-mineralization)– Significant seasonal variation (enhanced in winter)– Overall low (higher in N SCS)

Export production vs community structure

• Classic view: large phyto exports more• Richardson & Jackson (2007): small

phytoplankton equally important• Case in the SCS

Proportional contributions of varying phytoplankton groups or size classes (picoplankton, diatoms, pelagophytes, and prymnesiophytes for the EqPac study and pico-, nano-, and microphytoplankton for the Arabian Sea) to NPP versus their proportional contributions to export as detritus (A) or through consumption of mesozooplankton (B). Proportional contributions were calculated as NPP or export due to the size class/total NPP or export flux. (Richardson & Jackson, Science, 2007)

Summary

•Coastal ocean carbon deserves more studies

•South China Sea is good site for carbon dynamic and process studies - a mini oceanwith large river input & exchange with open ocean

•South China Sea is a weak source of atmospheric CO2

•Overall low export production

Challenges!

•Complex physical forcing in various time scales•Inter-annual?•Intra-seasonal?

•Complex physical-biogeochemical domains•River-margin-ocean connections•Mesoscale processes and their interactions-possibility of non-linear •Diverse ecosystems

•Modulation of air-sea CO2 exchange complex•Longer time scale? Twilight zone?•DIC vs DOC export to Pacific?

Concluding remarks:

• Oceanography is to understand the variability of the ocean in time and space. Such variability is particularly large in coastal ocean where atmosphere, land and ocean interplay with prominent anthropogenic forcing:– Considering different domains (upwelling, plume, eddy, fronts etc.)

is a must– Time-scale matters

• A multidisciplinary and multi-time-scale approach must be taken to study coastal oceanography, which needs– Physical-biogeochemical coupling– Time-series observation– Observation-model integration

• Adoption of approaches/concepts established for open ocean should be justified