Cdom in the deep sea distribution and dynamics from trans ocean sections
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CDOM in the Deep Sea: Distribution and Dynamics from Trans-ocean Sections. Norm Nelson, Dave Siegel, Craig Carlson Chantal Swan, Stu Goldberg UC Santa Barbara Special thanks to : Bill Smethie and Samar Khatiwala, LDEO Dennis Hansell, University of Miami. Ocean Sciences Meeting 2008.

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CDOM in the Deep Sea: Distribution and Dynamics from Trans-ocean Sections

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CDOM in the Deep Sea: Distribution and Dynamics from Trans-ocean Sections

Norm Nelson, Dave Siegel, Craig CarlsonChantal Swan, Stu GoldbergUC Santa Barbara

Special thanks to: Bill Smethie and Samar Khatiwala, LDEO

Dennis Hansell, University of Miami

Ocean Sciences Meeting 2008


Outline

  • About the project

  • Distribution and hydrography

  • Global dynamics of CDOM

  • CDOM and DOM diagenesis

  • Ongoing and future activities


What we already know (Bermuda)

  • CDOM is produced and destroyed in the top 250m on an annual basis

  • Sources include microbes and zooplankton

  • Sinks include solar bleaching and possibly consumption by microbes

  • Lab experiments show microbes and zooplankton can produce CDOM faster than observed rates of change in water samples

  • Estimated turnover time scales ~100 days. (we can’t measure these rates very well in the lab)


Global Surface CDOM Distribution(From SeaWiFS)

Siegel et al. [2005] JGR


UCSB Global CDOM Project Goals

  • Quantify global distribution of CDOM Surface, intermediate, and deep water

  • Determine physical and biological factorscontrolling CDOM distribution

  • Apply knowledge gained to problems of ocean circulation and DOM characterization and cycling

  • Collect calibration and validation data for ocean color models


Global CDOM Project Sections

EUCFe 2006

AMMA 2006

EqBOX

2005 2006


UCSB Global CDOM Project Measurements & MethodsCDOM Analysis At Sea

  • 200 cm Liquid Waveguide Absorption Cell (UltraPath, WPI Inc)

  • Single-beam spectrophotometer with D2 & Tungsten-halogen light sources, diode-array spectrometer detector

  • Fast, low sample volume (2 min/sample, 30-60 ml)

  • Issues with blanks(refractive index correction)

Nelson et al. [2007] DSR-I


UltraPathPrecision

  • Duplicate sampleanalysis (same Niskin)

  • RMS differenceat 325 nm:0.0034 m-1

  • This is ~4% of mean

  • RMS/Mean is between 5 and 10%between 300 and 400 nm

  • Longer wavelengths are not as good

  • Overall project: precision not as good, ca. 0.01 m-1

Nelson et al. [2007] DSR-I


UltraPath Example CDOM Profiles


CDOM Dynamics and Hydrography

  • Distribution of CDOM in the ocean basins

    • Are there spatial gradients in the deep sea?

  • Relationship with AOU and age tracers

    • Is CDOM produced/consumed by microbes at depth?

  • Atlantic vs. Pacific& Indian


Selected CDOM sections

acdom (443 nm, m-1)

(Global CDOM map from SeaWiFS/GSM, mission mean)


GS

STMW

AAIW

Deep

Caribbean

NADW

GS

STMW

AAIW

Deep

Caribbean

NADW

Atlantic A22 CDOM / AOU (Apparent Oxygen Utilization)


Pacific P16 CDOM / AOU


Indian I8S/I9N CDOM / AOU


Atlantic vs. Pacific/Indian: what’s different?

  • Atlantic: Productivity high but meridional overturning time scales much shorter

  • North Pacific / Indian: Most distant part of the global conveyor, longest time since ventilation, considerable remineralization

  • Southern Ocean / S. Pacific: Massive ventilation and deep water formation, productivity limited (iron?)

  • We can look at this more closely using age tracers -- CFC invasion


Atlantic A22 CFC-12 Age

STMW

AAIW

Deep

Caribbean

NADW

Age calculations by Bill Smethie & Samar Khatiwala [LDEO]


Pacific P16 CFC-12

AAIW

Very Old Water

AABW


T ~ 50y

T ~ 10y

P < 0.025

P < 0.025

T > 200 y

P < 0.025

P < 0.025

Age vs. CDOM

Nelson et al. [2007] DSR-I


CDOM Dynamics

  • Pacific / Indian: Overall correlation with AOU, wide CDOM range

  • Atlantic: Correlation with age & AOU in the main thermocline, subtropical mode water, and upper AAIW, narrow CDOM range

  • Advection obscures CDOM production signal in the Atlantic


CDOM Atlantic / Pacific sections

Top: (A16N, A20, AMMA, A16S) Bottom: P16N/S


CDOM Dynamics: Atlantic

Subtropics

EQ

Subtropics

North Atlantic

South Atlantic

Mode Water

Mode Water

Rapid meridional overturning allows little CDOM accumulation

Advection + bleaching balances net production


CDOM Dynamics: Pacific / Indian

South Pacific

Southern O.

Subtropics

EQ

Subtropics

North Pacific

Mode Water

North: Long residence time allows CDOM accumulation

South: Production limited (iron?) Low surface signal carried to depth by advection / water mass formation


CDOM Dynamics

  • Surface: Rapid turnover, production, consumption, and bleaching balanced, upwelling a minor contributor.

  • Mode waters: Ventilation carries surface signature across wide areas

  • Intermediate + Deep waters: CDOM abundance controlled by advection/net production balance


Transformations of CDOM & DOM in the ocean

  • What chemical transformations of CDOM occur in the ocean?

    • We don’t have many handles to turn on this at the moment, but we have:

  • Changes in the CDOM/DOC relationship(a*cdom)

  • DOM quality indexes(Neutral sugar and carbohydrate content)

  • Changes in the CDOM spectrum(Spectral slope parameter)


a*cdom(325)

a*cdom = CDOM / DOC(units m2g-1)

Upper layers bleaching & production signals

a*cdom increases w/ depth & age

CDOM “abundance” changes less than the DOC decline -- CDOM is refractory DOM

New

Bleaching

Aging

Nelson et al. [2007] DSR-I


DOM Quality: Carbohydrates and DOC, A20

STMW

LTCL

uAAIW

Sugars decrease

as CDOM increases

Neutral sugar content of

DOC also decreases

AOU increases

STMW

LTCL


Spectral Slope Parameter

  • S (nm-1), 280-400 nm, non linear fit

  • Typical Coastal: 0.015 nm-1

  • Typical Sargasso Surface: > 0.025 nm-1

  • Newly Produced Sargasso: ~ 0.022 nm-1

    (Nelson et al. Mar. Chem 2004)


Trends in CDOM spectral characteristics - N. Atl.

P < 0.025

P < 0.025

P < 0.025

P < 0.025

P < 0.025

P < 0.025

P < 0.025

Nelson et al. [2007] DSR-I


Spectral Slope to Age?

Handwaving age estimate:

Snlf of ≈ 0.014 nm-1 … >50 years mean ventilation age


Summary / Conclusions

  • CDOM dynamics worldwide reflect a balance between production and bleaching, moderated by the rate of advection.

  • CDOM is also produced (slowly) at depth as a byproduct of remineralization.

  • The CDOM optical signature is more refractory than the bulk DOC pool.

  • DOM undergoes chemical transformations with age that are reflected in the carbohydrate composition and optical properties.


Ongoing and future work

  • What is the nature of CDOM in the deep ocean and what transformations occur?

  • We’re tackling this with fluorescence spectroscopy and hopefully more advanced techniques to try and identify key chromophore groups and how they change over time and space


Acknowledgments

  • NASA Ocean Biology and Biogeochemistry

  • NSF Chemical Oceanography

  • U.S. CLIVAR/CO2 Repeat Hydrography Project(Jim Swift, Lynne Talley, Dick Feely, Rik Wanninkhof, Rana Fine)

  • UCSB Field Teams: Dave Menzies, Jon Klamberg, Meredith Meyers, Ellie Wallner, Meg Murphy, Natasha McDonald

  • Hansell Group: Charlie Farmer, Wenhao Chen

  • Bill Landing (FSU) and Chris Measures (UHI) (Water samples)

  • Ru Morrison & Mike Lesser, UNH (MAA analysis)

  • Wilf Gardner and Team, TAMU (C-Star transmissometer)

  • Mike Behrenfeld and Team, OSU (Equatorial BOX project)

  • Erica Key and Team, U Miami (AMMA-RB 2006)

  • Jim Murray and Team, UW (EUCFe 2006)

  • R/Vs Brown, Knorr, Revelle, Melville, Thompson, Ka’I, Kilo Moana


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