OS35G15 The Coastal Gulf of Maine as an Atmospheric CO2 Sink With Seasonal Riverine Control Douglas

1
OS35G-15 The Coastal Gulf of Maine as an
Atmospheric CO2 Sink With Seasonal Riverine
Control Douglas Vandemark1, Joseph Salisbury1,
Christopher Hunt1, Wade McGillis2, Janet
Campbell1 and Fei Chai31University of New
Hampshire 2Columbia University
3University of Maine Contact
doug.vandemark_at_unh.edu [phone]
Net Annual CO2 Flux Estimates the Gulf as an
atmospheric sink One clear benefit of the
repeated cross-shore transects is its utility in
examining the seasonal and interannual cycles
across a coastal ecosystem. Results below
indicate this region acts as a net CO2 sink over
our observation period. Annual estimates of the
CO2 air-sea flux are readily computed using the
UNH cruise data and the nominal procedure for
mass flux given as flux (mmolC/m2/day) a
k(U) (?pCO2) (1) where a is the gas
solubility (a known quantity of T and S), k the
gas transfer coefficient - an unknown at sea and
empirically related to wind and surface
turbulence, and ?pCO2 pCO2airside
pCO2waterside, pCO2 being the gas partial
pressure at 1 atm. The transfer coefficient k
is estimated with several algorithms (e.g.
Wanninkhof, 1992 Wanninkhof and McGillis, 1999)
each in terms of an hourly or steady 10m
anemometer wind speed U. Atmospheric CO2
measurements are typically collected aboard the
Challenger several times within a given cruise
day, while waterside estimates are made
continuously with 80 s system response time (see
spatial survey figure 4). Hourly flux estimates
below are derived through spatial average of each
months data set over each predefined cross-shore
regime and subsequent hourly interpolation of the
terms in eq. 1 (aside from U).

• Objectives
• Goals of the U.S. Ocean Carbon and Climate Change
  (OCCC) program are to develop a robust
  understanding of where, when and how carbon is
  transferred between the earths reservoirs and
  then to monitor these exchanges. Carbon fluxes
  at the land-ocean boundary are one area of
  substantial uncertainty and our research project
  is focused on issues related to both optimal
  sampling and mechanistic understanding of CO2
  air-sea fluxes in the coastal zone.
• This work represents part of an ongoing study
  addressing several key land-ocean-atmosphere
  carbon transport questions by intensive
  observation of the coastal oceans surface layer
  carbon dioxide in space and time. These
  questions include
• Does the Gulf of Maine (temperate latitude,
  Northeastern U.S.) act as a net atmospheric
  carbon dioxide source or sink?
• Which controls dominate the Gulfs annual cycle
  of CO2 air-sea exchange?
• What is the magnitude of cross and along-shore
  variability in the surface layer carbon pool?
• Do large episodic terrestrial inputs of
  freshwater, nutrients, and carbonate species
  alter air-sea gas exchange in these coastal
  waters?
• What are the relevant temporal and spatial flux
  sampling scales for this region?

• Perturbations of CO2 flux associated with
  discharge the 2004 and 2005 freshets
• One empirical demonstration of surface water
  carbon flux perturbation in our region is seen
  across the 2004-2005 period. While 2004 was a
  year of average runoff, 2005 was a year of
  extremely high (sometimes 100 year) levels of
  freshwater flow to the W. Gulf (see below). A
  companion poster OS46K-8 looks at
  phys/bio/chemical controls on the surface carbon
  pool attributed to these events.
• In a CO2 flux context, key points are
• Annual flux estimates increase by gt 30
  depending solely on 5-7/2004 vs.5-7/2005 (Figs
  2,3,5)
• Large surface layer pCO2 drawdown occurs with
  the 2005 freshet, extending to the deep basin
• Preliminary analysis (see OS46K-8) suggests
  this drawdown is mostly productivity driven
• Monthly sampling is likely inadequate to
  quantify these episodic events

Figs. 5a,b - River discharge for W. Gulf
freshet periods (left) and cruise-observed
surface water CO2 (right) in Hovmuller format
from shore to the deep basin in X and from 4/2005
to 12/2005 in Y.
USGS G.Maine river discharge 5 river summation
2005 a record year
Fig. 2 Observed monthly air-sea CO2
disequilibrium and resulting CO2 flux versus
shelf regime in coastal Gulf of Maine - Seasonal
cycles in the upper panels show under saturation
in the spring and super saturation in early
winter with the largest variability nearest the
coast. Mass flux estimates and their annual net
value (lower panels above) follow ?pCO2 dynamics
in this monthly estimation method with the lowest
net flux occurring at the Wilkinson Basin end
member.
Flo-thru oxygen, IOPs and CTD

• Summary
• Annual cycles of ?pCO2 and air-sea flux show the
  shelf to be an atmospheric C sink with annual
  avg. flux magnitude -0.7 molC/m2 (-0.4)
  undersampling error not estimated
• Results along the cross-shore gradient (Fig 2 and
  3) indicate -
• similar seasonal pattern in ?pCO2 near shore out
  to deep basin
• elevated net atmospheric sink inshore, near
  balance in the deep basin
• higher disequilibrium magnitudes inshore in both
  source and sink periods
• Annual pCO2 cycle indicates drawdown with
  biological productivity (late spring) and a
  mixing or respiration-driven over saturation
  (fall-winter) resembling somewhat the data from
  North Seas southern coast (Thomas et al., 2004).
  This cycle does not follow neighboring east
  coast Mid- and South- Atlantic Bight findings
  (Degrandpre et al., 2002 Wang et al., 2005).
  Additionally, substantial perturbations are
  observed due to land-ocean boundary effects of
• freshwater flux driving carbonate and biological
  dynamics at monthly and interannual scales
• atmospheric pCO2 variations due to continental
  air shed advection
• Observations and analyses in spatial/temporal
  dynamics are ongoing to characterize, bound, and
  model the dominant processes and to address
  effective monitoring of metabolic dynamics and
  air-sea mass flux along the ocean margins.

CO2 Analyzer
Fig. 3 Net annual cross-shelf CO2 flux across
shelf estimated using the WM92 algorithm Note
the substantial difference in magnitude compared
to Fig. 2 comes from use of the 2005 freshet
season in place of the 2004 season.
Fig. 4 Coastal atmos. CO2 variability 10 km
offshore (red) and 15 km inland (gray) it is
clearly important to measure CO2 in both sea and
air surface layers to obtain accurate fluxes.
Data (courtesy UNH/AIRMAP) below show diurnal and
episodic signals out to sea that differ
dramatically from higher-altitude flask data.

• Annual flux estimates, uncertainty, and
  perturbations of CO2 flux in the coastal zone
• Annually averaged CO2 fluxes for the regimes
  above are
• Analyses indicate that at least two land-ocean
  factors strongly influence our net annual flux
  estimates -
• interannual variability associated with episodic
  freshwater discharge
• atmospheric marine layer CO2 variability
  associated with land-to-sea airmass advection
  (see Fig. 4)
• Uncertainty also comes from the more
  well-recognized issues of gas transfer rate
  models and unresolved (undersampled) spatial and
  temporal variability

• Study Location and data collection
• The Gulf of Maine is a biologically productive
  temperate latitude inland sea that is home to a
  diverse set of coastal and deepwater provinces.
  This study uses cross-shore cruise data collected
  across the Western Gulf of Maine (left box above,
  Wilkinson Basin Transect) by the University of
  New Hampshires Coastal Ocean Center. Sampling
  enfolds our near shore shelf, a ledge
  (Jeffreys), and then a deepwater end member to
  the east. This western Gulf coastal biome is
  subject to significant freshwater inputs each
  year during the spring freshet and then again
  with fall-winter storms. The combined freshwater
  flux into the Gulf from New England and Canadian
  watersheds rivals that of any Eastern U.S. river
  system and much of this water passes along this
  coastline as part of the Western Maine Coastal
  Current a persistent flow enhanced during the
  freshet (Geyer et al. 2004). Monthly cruises
  have been conducted from May 2004 to present.
  Data collection includes flow through, profiler,
  and discrete sampling for a suite of biological,
  chemical, bio-optical and physical oceanographic
  variables (see http//www.cooa.unh/edu ).
• The subset of measurements used here are
• Continuous along-track surface layer carbon
  dioxide partial pressure pCO2 (via fast-rate flow
  through system photos above) including surface
  barometric pressure. Intake depth 0.7 m.
• Surface layer sea surface temperature and
  salinity
• Atmospheric pCO2 via the flow through system and
  other stations (http//www.airmap.unh.edu)
• Wind speed from the NDBC Isles of Shoals station
  along the transect (10 km offshore)
• discrete sampling of TA and pH with depth
• ancillary physical fields from the GoMPOM model,
  USGS river discharge data
• To see more data and analyses on the carbonate
  system dynamics for these cruises see the
  associated poster of Salisbury et al. OS46K-08.

References DeGrandpre, M.D., G.J. Olbu, C.M.
Beatty, and T.R. Hammar, Air-sea CO2 fluxes on
the U.S. Middle Atlantic Bight, Deep-Sea Research
Part II-Topical Studies in Oceanography, 49,
4355-4367, 2002. Thomas, H., Y. Bozec, K.
Elkalay, and H.J.W. de Baar, Enhanced open ocean
storage of CO2 from shelf sea pumping, Science,
304 (5673), 1005-1008, 2004. Wang, Z.A., W.J.
Cai, Y.C. Wang, and H.W. Ji, The southeastern
continental shelf of the United States as an
atmospheric CO2 source and an exporter of
inorganic carbon to the ocean, Continental Shelf
Research, 25 (16), 1917-1941, 2005. Wanninkhof,
R., Relationship Between Wind-Speed and
Gas-Exchange Over the Ocean, Journal of
Geophysical Research-Oceans, 97 (C5), 7373-7382,
1992. Wanninkhof, R., and W.R. McGillis, A cubic
relationship between air-sea CO2 exchange and
wind speed, Geophysical Research Letters, 26
(13), 1889-1892, 1999. Acknowledgments Thanks
to all members of the R/V Challenger data
collection team and to UNH/AIRMAP for atmospheric
time series data. This work was supported in
part by the NOAA Coastal Services Center through
an award to the UNH Center for Coastal Ocean
Observation and Analysis (COOA) NOAA award
NA16OC2740, and by NASAs Earth Science Division
- Physical Oceanography Office.
2006 Ocean Sciences Meeting Honolulu Hawaii