IOPs of suspended sediments in rivers and coastal

margins: Towards modeling turbid-water photochemistry

from space

Margaret L. EstapaUniversity of Maine

CDOM photodegradation

CDOM

Lower molecular weight DOM

Bleached DOM

Biological coupling

CO2

POC photodegradation

CDOM

Lower molecular weight DOM

Bleached DOM

Biological coupling

CO2

Resuspension and settling

POC

Atchafalaya R.Atchafalaya R.

Louisiana coastline, 4/7/2009.MODIS-Aqua, NASA

Keil et al 1997

SPM in large delta-forming rivers Sediments

buried @ deltas

Photodissolution of deltaic POC

Mayer et al 2006

Beyond the lab, how quickly does photodissolution unload organic carbon from river SPM? Could it account for low organic carbon in buried sediments on deltas?

Rate model for photochemical POC loss from sediments:

Model as a function of

in-water irradiance f(aCDOM, aaSPM, bb,SPM)

absorption by photodissolution-susceptible particles

efficiency of the reaction f(T)

PDpPD aE

dt

dPOC **0

ap [m-1] = ap* [m2 g-1] x SPM [g m-3]

SPM: 101 – 103 [g m-3] at surface, can be inverted from Rrs (D’Sa et

al, Miller & McKee, Walker et al)

ap*: ~10-1 [m2 g-1, blue ] (Bowers & Binding, Stramski et al)

Needs to be determined empirically.

Sample locations

-93 -92.5 -92 -91.5 -91 -90.5 -90 -89.5

29

29.5

30

30.5

31

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Bottom surficial sedimentsSuspended particulate matterIn situ optical dataIn situ filter stations

Various seasons, 2003-2008

March 13-15 2008

Absorption measurement methods

• Integrating sphere (Labsphere, 15 cm diameter, center-mounted 1-cm cuvette). – [SPM] known precisely, high resolution, UV

data, no scattering correction– Optical effects of isolation & storage?

• ac9 (WETLabs,10 cm path) in shipboard clean seawater system. – [SPM] determined from filters,

assumptions/corrections in ap* derivation– Particles measured near in situ

ap* () = ap () / [SPM]

Single marine sample in different media

300 350 400 450 500 550 600 650 700 750 8000

0.05

0.1

0.15

0.2

0.25

Wavelength [nm]

a p*

[m2 g

-1]

Freshwater samplesMarine samplesDifference

Mass-specific absorption of discrete samples in integrating sphere

300 400 500 600 700 8000

0.05

0.1

0.15

0.2

0.25

MilliQ water5.5 mM CaCl

2

Artificial seawater

(averages, 95% conf. int.)

][*

SPM

aaaa g

p

10 15 20 25 30

1

1.5

2

Salinity [psu]

a g [

m-1

]

ag:S

ag = -0.074 * Salinity + 2.86

Derivation of ap* from in situ optical measurements412 nm

5 10 15 200

500

1000

1500

SPM (gravimetric, mg L-1)

Eco

VS

F s

ign

al (

cou

nts

) Signal = 58.9 * SPM + 93.8

EcoVSF:SPM

ac9 total absorption:

- Temperature, salinity correction

- Spectral scattering correction

Pigment absorption, a, removed from ap* following Roesler L&O 1989

450 500 550 600 650 700

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Wavelength [nm]

a p*

[m2 g

-1]

Comparison of ap* spectra of isolated

seds/SPM (integrating sphere) and in situ SPM (ac9)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

ap* derived from field data (ac9, March '08)

ap* of isolated samples

(integrating sphere, collectedwithin 3 days of field samples)

5 10 15 20 25 30 35

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Organic Carbon [mg-OC g-sed-1]

a p*

(412

) [m

2 g-1

]

ap* (412):OC content

All samples: ap*(412) = 0.0044 * OC - 0.0094, R2 = 0.65, N = 32

Marine only: ap*(412) = 0.0049 * OC - 0.0203, R2 = 0.75, N = 19

Freshwater only (no fit)

Sample groupS (error) [nm-1 ]

= 412-715

Field, CDOM (0.2m filtered ac9) 0.016 (0.001)

Field, particles 0.010 (0.002)

Lab, freshwater particles 0.011 (0.001)

Lab, marine particles 0.010 (0.001)

)715(*)412()( )412( aeaa S

= 300-715

Lab, freshwater particles 0.010 (0.001)

Lab, marine particles 0.011 (0.001)

Spectral slope calculations

(Offset: eg, Bowers & Binding)

Sherman and Waite 1985

Structural features in SPM absorption spectra?

Atchafalaya SPM/sedsAtchafalaya SPM/sedsvarious Fevarious FexxOOyy minerals minerals

300 350 400 450 5000

0.05

0.1

0.15

0.2

0.25

Wavelength [nm]

a p*

[m2 g

-1]

300 350 400 450 500-0.015

-0.01

-0.005

0

0.005

0.01

0.015

Wavelength [nm]

a p*

resi

du

al f

rom

sin

gle

-exp

fit

[m

2 g-1

]

300 350 400 450 500 550 600 650 700 750 8000

0.05

0.1

0.15

0.2

0.25

Wavelength [nm]

a p*

[m2 g

-1]

Freshwater samplesMarine samplesDifference

• Agreement between field (ac9) and lab (integrating sphere) measurements of ap*() for mineral-associated POC .

• ap*() increases with mass fraction OC

• Spectral slopes for all mineral POC are ~0.010-0.011 nm-1. Spectral structure at UV-blue wavelengths differs for riverine/marine samples, possibly due to changes in Fe phases.

• ap*() ~ 0.05-0.1 [m2 g-1] at 412nm while SPM ~ 10-1000

mg/L. ap at the surface determined mostly by SPM concentration retrieve from Rrs

Final points

Acknowledgements• NASA Earth Systems Science Fellowship (project NNX08AU84H,

“Assessing Impacts on Carbon Transport from Land to Ocean: Photochemical Transformations of Particulate Organic Carbon”)

• NSF Chemical Oceanography• My advisors Emmanuel Boss and Larry Mayer, and committee

member Collin Roesler, for helpful advice and conversations• Mary Jane Perry and Mark Wells for use of lab equipment• Gail Kineke, John Trowbridge, ONR, and crew of the R/V Pelican

for ship time• Larry Mayer, Sam Bentley, and Mead Allison for collecting and

sharing archived sediment samples• Kathy Hardy and Linda Schick for archived sediment sample

processing and analysis