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

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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  1. 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

  2. Outline • About the project • Distribution and hydrography • Global dynamics of CDOM • CDOM and DOM diagenesis • Ongoing and future activities

  3. 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)

  4. Global Surface CDOM Distribution(From SeaWiFS) Siegel et al. [2005] JGR

  5. 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

  6. Global CDOM Project Sections EUCFe 2006 AMMA 2006 EqBOX 2005 2006

  7. 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

  8. 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

  9. UltraPath Example CDOM Profiles

  10. 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

  11. Selected CDOM sections acdom (443 nm, m-1) (Global CDOM map from SeaWiFS/GSM, mission mean)

  12. GS STMW AAIW Deep Caribbean NADW GS STMW AAIW Deep Caribbean NADW Atlantic A22 CDOM / AOU (Apparent Oxygen Utilization)

  13. Pacific P16 CDOM / AOU

  14. Indian I8S/I9N CDOM / AOU

  15. 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

  16. Atlantic A22 CFC-12 Age STMW AAIW Deep Caribbean NADW Age calculations by Bill Smethie & Samar Khatiwala [LDEO]

  17. Pacific P16 CFC-12 AAIW Very Old Water AABW

  18. 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

  19. 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

  20. CDOM Atlantic / Pacific sections Top: (A16N, A20, AMMA, A16S) Bottom: P16N/S

  21. 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

  22. 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

  23. 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

  24. 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)

  25. 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

  26. 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

  27. 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)

  28. 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

  29. Spectral Slope to Age? Handwaving age estimate: Snlf of ≈ 0.014 nm-1 … >50 years mean ventilation age

  30. 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.

  31. 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

  32. 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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