Interdisciplinary studies of CDOM in the global ocean

1
Interdisciplinary studies of CDOM in the global
ocean Norman B. Nelson1, Chantal M. Swan1, David
A. Siegel1, Craig A. Carlson1,2 1Institute for
Computational Earth System Science, 2Dept.
Ecology, Evolution and Marine Biology, University
of California, Santa Barbara
Overview
Global distribution and dynamics of CDOM in the
surface and deep ocean Norm Nelson, Dave Siegel,
Craig Carlson

• We are currently engaged in several research
  efforts concerned with the distribution,
  dynamics, and characterization of chromophoric
  dissolved organic matter (CDOM) in the global
  ocean. These include
• Ocean color algorithm development for retrieving
  CDOM absorption as well as chlorophyll and
  particulate backscatter in surface waters (with
  Stephane Maritorena). This has led us to new
  insight concerning the influence of CDOM on
  retrieval of chlorophyll as well as pointing
  toward research on the nature of CDOM cycling.
  
• Time-series study of apparent and inherent
  optical properties at the BATS site southeast of
  Bermuda in the subtropical North Atlantic. Our
  results have revealed a seasonal cycle in CDOM
  distribution that indicated photolysis was the
  main sink, and secondary production the main
  source.
• Photochemistry of CDOM Measuring quantum yields
  for bleaching and photoproduction of CDOM on
  samples collected from the field in low CDOM
  areas where these rates are difficult to measure.
• Our results are leading to new insights
  concerning the nature and cycling of CDOM in the
  global ocean. Ongoing and future efforts include
  characterization of CDOM along gradients of
  ventilation age, mixing, and photolysis using
  optical and chemical methods, and incorporation
  of CDOM terms into mixed layer and general
  circulation models.

CDOM Cycling Box Models
Remote sensing of CDOM distributions in surface
waters over the global ocean highlight a
superficial correlation with chlorophyll and
productivity, but with some significant
differences. Significant CDOM in upwelling zones
raised the question of whether CDOM was present
in the ocean interior, if so what controlled its
abundance in the deep sea, and how are the
surface and interior coupled. We are in the
process of conducting a global field survey of
CDOM distribution and characteristics relative to
hydrography, optics, and selected biological
parameters, as an ancillary project on the U.S.
CO2/CLIVAR Repeat Hydrography surveys. Since 2003
we have collected data on meridional sections
covering the full range of surface CDOM in every
open ocean basin. Our results have highlighted
the importance of thermohaline circulation and
remineralization in determining the abundance of
CDOM in the deep ocean. In the Atlantic, rapid
meridional overturning mixes CDOM more
homogenously, rapidly transmitting surface CDOM
concentrations to the interior. In the North
Pacific and Indian Ocean, slow mixing allows
accumulation of CDOM formed as a result of
remineralization. In the Southern Ocean, low
production at the surface and rapid ventilation
transmit low CDOM signals to the interior,
creating the interhemispheric imbalance reflected
in remotely sensed data.
In these flow charts, the straight yellow arrows
represent advective fluxes of CDOM (including
horizontal transport, upwelling, and
downwelling), curled arrows represent local
production of CDOM, and the red arrows represent
photobleaching. The top row of boxes represent
surface waters, and the second row represents the
main thermocline down to 1 km. The color
corresponds to CDOM absorption coefficient (as in
the section plot to the left).
Completed (full measurement set including CDOM,
microbes, optics)
Completed (limited measurement set, CDOM and
hydrography)
Future (in planning)
In surface waters, the distribution of CDOM is
easily explained by a balance between production
and photolysis. In subtropical waters persistent
stratification and net downwelling leads to low
CDOM concentrations. Formation of subtropical
mode water in regions with seasonal mixed layers
carries low CDOM water to the ocean interior,
where a low CDOM signature is easily observed in
the mode waters of the North and South Atlantic.
Meridional sections across the Equator clearly
shows the transport of CDOM to the surface where
it is bleached. An outstanding question is the
high CDOM observed in the North Atlantic a
residual of terrestrial CDOM from the Arctic? We
hope to resolve this with ongoing research and a
repeat of the 2003 North Atlantic sections in
2011-2012.
Bermuda Bio-Optics Project Decadal scale
observations Norm Nelson, Dave Siegel
The contribution of CDOM to ocean color
variability not related to chlorophyll abundance
was first suggested by Siegel and coworkers
studying the first year (1992) of in situ
radiometry data from the Bermuda Bio-Optics
Project, a time-series study of optical
properties in the water column piggybacking on
the successful Bermuda Atlantic Time-series Study
cruises. Since 1994 the real contribution of
CDOM to ocean color has been assessed using
spectrophotometric measurements of CDOM
absorption spectra, as part of an integrated
study of component absorption. Sustained
observations of CDOM are being considered in the
context of climate-related changes in the factors
controlling CDOM abundance.
CDOM Dynamics Pacific / Indian
acdom (325 nm, m-1)
Quantum yield (?) of CDOM photolysis -- the major
sink of CDOM Chantal Swan, NASA Earth System
Science Fellowship
Photobleaching / photoproduction quantum yields
At BATS, seasonal mixing homogenizes the CDOM
profile from a characteristic summer pattern that
includes a surface minimum, a local maximum in
the 60-150m range, and a local minimum in the
mode water. But this mean pattern summarizes much
variability. We are examining interannual
patterns in CDOM distribution in the upper water
column to assess the relative contribution of
local effects (production, mixing) versus remote
effects (mode water formation, overall irradiance
leading to photolysis). An intriguing hint of
CDOM-climate teleconnections can be seen in a
correlation between the North Atlantic
Oscillation and CDOM abundance at 160 m.
Sample Site Latitude Region Depth (m) T (C) Sal. (ppt) Chl-a (ug/l) Initial aCDOM (m-1) at 325nm Initial aCDOM (m-1) at 440nm ?(325nm,325nm) (m2 mol photons-1) ?(440nm,440nm) (m2 mol photons-1) Total irradiation time (days)
PB189S5 34N Coastal Pacific (So. Cal. Bight) 0 16 33.1 0.600 0.20 0.02 -0.08 -0.004 2.02
BATS 32N Subtropical N. Atlantic 80 23 36.6 0.200 0.06 0.001 -0.05 -0.001 3.02
P16NS51 29N Subtropical N. Pacific 140 19 35.2 0.078 0.07 0.003 -0.09 -0.002 2.97
P16NS51 29N Subtropical N. Pacific 40 20 35.3 0.096 0.05 0.001 -0.05 0 3.06
P16NS76 55N Subarctic Pacific 200 4 33.9 0.004 0.17 0.02 -0.07 0.004 2.00
P16NS76 55N Subarctic Pacific 80 3 33.0 0.019 0.14 0.01 -0.03 0.009 2.00
P16NS19 0.5N Equatorial Pacific 100 22 35.6 0.060 0.06 0.005 -0.05 0.015 2.95
P16SS61 46S Subantarctic Pac. Frontal Zone 80 10 34.4 0.242 0.07 0.01 0 0.012 2.06
P16SS61 46S Subantarctic Pac. Frontal Zone 0 13 34.4 0.132 0.05 0.01 0 0.008 3.01

• Photolysis moderates global surface distribution
  of CDOM
• ? (and photolysis rate) can be used in concert
  with upper ocean vertical mixing rate to deduce
  microbial production rate of CDOM (a term
  otherwise hard to measure)
• Constraining a mixed-layer budget of CDOM will
  permit its use as the first remotely-sensed
  tracer of upper ocean circulation

Key Findings

• Loss of CDOM absorption in the UV due to
  full-spectrum solar irradiation occurs in nearly
  all regions sampled. We can constrain
  environmental range of open ocean ?photolysis
• 50 of regions sampled show a very unexpected
  concurrent phenomenon at longer wavebands
  formation of CDOM due to irradiation

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 Dennis
Hansell, Charlie Farmer, Wenhao Chen Bill Landing
(FSU) and Chris Measures (UHI) (Water
samples) Bill Smethie Samar Khatiwala, LDEO
(CFC ages) 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, KaI, Kilo Moana
Table Summary of associated hydrographic data
and calculated ? values (325nm and 440nm, shaded
blue). Sites and values marked in red denote
where ?(440nm440nm) is positive, corresponding
to observed photoproduction of CDOM during
irradiation.
Schematic of inversion terms using Subtropical N.
Atlantic (BATS) example