Open Ocean CDOM Production and Flux Norm Nelson, Dave Siegel, Stéphane Maritorena Chantal Swan, Craig Carlson UC Santa Barbara ACE Ocean Productivity and Carbon Cycle (OPCC) Workshop UCSB, June 2011

Outline What is CDOM and why should we care Remote sensing of CDOM CDOM Dynamics in the global ocean

CDOM What and Why CDOM is dissolved (passes 0.2 m filter) organic matter that absorbs light. CDOM has a major impact upon ocean color -- influences retrieval of chlorophyll, penetration of PAR and UV to depth. This enables remote sensing of surface CDOM. CDOM is produced by all kinds of heterotrophic activity and is destroyed primarily by solar radiation. CDOM is found in measurable (if you’re careful) quantities at all depths everywhere in the ocean. CDOM is not correlated to DOC abundance in the open sea.

Zooplankton CDOM Production Example spectra for controls vs. plankton

Solar Bleaching of CDOM daCDOM/dt (measured) E0*ācdom Time Time Time Chantal Swan, UCSB E0*ācdom*(o=325nm) E0*ācdom*(o=325nm) Time Time

CDOM What and Why (2) CDOM is also an primary sensitizer of photochemical reactions involving climate-relevant trace gases (CO2, CO, OCS, DMS) CDOM is an indicator of terrestrial runoff and riverine input to the ocean Alas: In the open ocean we can’t ascribe carbon content to CDOM (yet?) nor do we know much about the identities of the chromophores

Photochemical CO production from space

CDOM from Ocean Color

CDOM Optics and Remote Sensing CDOM absorption spectrum is distinct from phytoplankton absorption Ocean color reflectance spectra can be inverted to retrieve absorption by CDOM and particles and particulate backscattering CDOM Particles

Mean Global Surface CDOM Distribution From SeaWiFS as acdm(443 nm, m-1) Garver-Siegel-Maritorena model acdom (443 nm, m-1) Siegel et al. [2005] JGR -- Nelson et al. [2010] GRL

CDOM from Ocean Color Matchup with NOMAD data (IOCCG IOP report; Lee et al. 2006) Model-data fits are pretty good – though not excellent GSM01 is optimized for all 3 retrievals (CHL, CDM, BBP)

Global Dynamics of CDOM

Example CDOM Profiles

CDOM and AOU Distribution Atlantic Pacific Indian CDOM and AOU Distribution [Nelson et al. 2010]

CDOM / AOU Correlation

CDOM Dynamics Summarized CDOM is produced primarily as a function of remineralization (terrestrial inputs are local, only a few taxa of autotrophs produce CDOM) CDOM is destroyed primarily by photolysis (we don’t observe labile DOM as it’s consumed too rapidly by microbes) Time scales for CDOM production / destruction are comparable to time scales for ocean circulation (otherwise the ocean would be yellow) Observed CDOM distribution results from a balance between source/sink processes and circulation

CDOM Dynamics - N. Pacific / Indian Weak ventilation in northern basin Particle flux leads to CDOM accumulation Eq Biological Physical Bleaching N

CDOM Dynamics - N/S Atlantic Strong ventilation in subarctic basin Higher surface CDOM signal transmitted to deep Eq Biological Physical Bleaching N or S

CDOM Dynamics - S Pacific/Indian Strong ventilation in Southern Ocean Lower surface CDOM signal transmitted to deep Eq Biological Physical Bleaching S

Summary / Conclusions CDOM is a remotely sensible semiconservative tracer, produced by heterotrophs and destroyed by photolysis The relationship between CDOM and oxygen (as AOU) in the deep sea is modulated by circulation processes CDOM assessment is important to do ocean color right, and is useful in its own right

Extra slides

CDOM Optics and Remote Sensing Remote sensing reflectance spectrum can be inverted to retrieve inherent optical properties (absorption and backscattering spectra) of the surface water (mixed layer to ~ 60m). Absorption spectra can be deconvolved into particle absorption and CDOM+detritus absorption spectra (which we call CDM) given some assumptions about the shape of the component spectra. Garver-Siegel-Maritorena model (GSM) uses shape functions determined using a global optimization of available global open ocean field data.

Seasonal CDOM Cycle CDM Seasonal changes at most latitudes Lower in summer Reduced in tropics Higher towards poles Hemispheric asymmetry %CDM

Surface CDOM & SeaWiFS r2 = 0.65; N = 111 slope = 1.16 Siegel et al. [2005] JGR A20 A22 Overall, there was good correspondence between the GSM01 estimated CDM and the in situ CDOM absorption coefficient at 443 nm (r2 = 0.65; N = 111). The linear regression slope was 1.16, with an intercept of 0.003 m-1, indicating approximately a 15% overestimation of CDOM absorption by the GSM algorithm. A16N

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

Regressions between age and CDOM T ~ 10y T ~ 50y T > 200 y P < 0.025 Nelson et al. [2007] DSR-I

Trends in CDOM spectral characteristics - N. Atl. Nelson et al. [2007] DSR-I