The marine inorganic carbon system along the Gulf of Mexico and Atlantic coasts of the United States: Insights from a transregional coastal carbon study
Zhaohui Aleck Wang*1 (zawang@whoi.edu), Rik Wanninkhof2, Wei-Jun Cai3, Robert H. Byrne4, Xinping Hu5, Tsung-Hung Peng2, and Wei-Jen Huang3
Chemistry and Geochemistry, Woods Hole Oceanographic Institution; 2 NOAA Atlantic Oceanographic and Meteorological Laboratory; 3 Department of Marine Sciences, University of Georgia; 4 College of Marine Science, University of South Florida; 5 Department of Physical and Environmental Sciences, Texas A&M University Corpus Christi
Abstract: Distributions of total alkalinity (TA), dissolved inorganic carbon (DIC), and other
TA (µmol kg-1)
2450
2400
2400
2350
2350
2300
2300
2250
TX
WFL
GA
NJ
NH
2200
2150
2100
2050
deep ocean
>1000 m
LA
EFL
NC
MA
2250
2200
2150
2100
2050
2000
TX
LA
WFL
32
34
36
2000
24
26
28
30
32
34
36
38
2430
24
C
28
30
38
D
2400
2410
2350
2390
2370
2300
2350
2250
2330
2200
2310
2150
2290
EFL
2270
2250
34.5
26
2450
2450
TA (µmol kg-1)
B
2100
GA
NC
NJ
MA
NH
35
36
2050
35.0
35.5
36.0
36.5
37.0
30
31
32
Salinity
Fig. 2. Total alkalinity (TA) and salinity (S)
33
34
Salinity
37
Shelf Mixing
 TA - S: Largely conservatively,
with two dominant mixing
regimes sharing a common
oceanic end-member (blue circle).
 The oceanic end-member: mean t
= 21.6 °C, S = 36.7, TA = 2404
μmol kg-1; the core of the Loop
1
Current–Florida Current–Gulf
Stream system
 The mixing of slope and deep
oceanic waters (green circle;
>1000m)
 Two distinct line segments on the
TA-S plot in the GoME and MAB
WFL
EFL
GA
NC
-1
2200
2300
2100
2200
2000
1900
DIC
TA
7
C
6
NH
5
A
1850
surface
1800
4
1.10
3
1.05
2
A
1
32
34
36
38
TX
35
25
15
GoME
subsurface
surface
0
26
28
0.90
0
38
30
36
25
34
20
32
15
30
10
28
t
TX
NH
S
LA WFL EFL
26
GA
NC
MA
NH
30
32
Salinity
34
36
38
2420
Fig. 5. Cross-sections of DIC (color) and potential
density (σ0, contour lines)
Fig. 4. DIC, nitrate and salinity
DIC Distribution
DIC – Salinity (Fig. 4)
• DIC and nitrate distributions show similarity, indicative of biological control
• Sharp contrast with TA distributions
• The lowest DIC concentrations (~1860 µmol kg-1) inside the MARS plume area
• At high salinities, DIC–S data fell along an arc defined by offshore surface, subsurface, and deep
waters
• Subsurface water in the Gulf of Maine (NH transect) had high DIC and low S, different from the
rest of the regions, reflecting the semi-enclosed nature of the GoME (accumulation of
remineralization products at depth)
Cross-sections of DIC (Fig. 5)
• Absence of an obvious boundary current signature
• DIC tended to increase with depth, but TA tended to decrease (except in the GoME), indicative of
net biological uptake of DIC dominated near the surface, while remineralization prevailed at depth.
• Low-DIC, low-S waters in the MARS plume, consistent with net biological uptake
• High-DIC upper-slope water associated with the upwelling at EFL
2140
A
2410
B
2120
-1
24
MA
DIC ( mol kg )
5
NC
200
Implications of regional differences
• Most of the difference in ΩA between the nGMx and the GoME was due to differences in chemical
composition; temperature difference only accounted for 15%
• Because of differences in buffering capacity and temperature (CO2 solubility), the US NE coastal
waters, especially the GoME, are more susceptible to acidification pressures and will reach critical
ecological thresholds (e.g., ΩA = 1) more quickly
20
10
GA
2400
2390
2380
2370
Central Sargasso Sea Water
2100
2080
2060
2040
2020
TA
2360
2000
300
10
250
DIC
Loop Current source water
D
C
8
Salinity-normalized DIC (enDIC; Friis et al., 2003):
enDIC = (DICspl – DICS=0)/S × Sref + DICS=0
To remove salinity (mixing) introduced DIC change
T able 1. D IC con centrations and salinities of northeastern U .S . shelf w aters. S alinitynorm alized D IC (enD IC ) w as calculated for M A B shelf-w ater sam ples according to E q.
-1
2. D IC and enD IC concentrations are in µ m ol kg .
N C T ransect
M A T ransect
N H T ransect
M ean shelf-w ater salinity
35.2
33.2
32.2
F reshw ater end -m em ber D IC *
671
809
933
M ean shelf-w ater D IC
2053
2057
2047
M ean shelf-w ater enD IC
2047
2108
2133
C hange in enD IC betw een N H and N C transects
–86
Analysis Assumption: Most water
transport in the MAB is from (2)
north to
south; Steady state
• Along-shore (NH to NC) mixing
introduced change in transect mean
DIC and S : ΔDIC +120 µmol kg-1,
ΔS = +3
• Biogeochemical processing: ΔDIC 86 µmol kg-1.
• The downstream effects of the two
processes roughly the same
magnitude but opposing sign.
Shelf–boundary current–ocean interactions
along the southern U.S. coasts
Rationale: Tracking chemical changes along the
flow path of Gulf Stream (a ‘conveyer belt’) to
assess shelf-current-ocean interactions
Gulf of Mexico
• The Loop Current core water experienced no
4
appreciable TA change but a DIC increase ~65
100
-1 from the Yucatan entrance to the West
2
50
µmol
kg
Nitrate
Depth of Gulf Stream Water
0
0
Florida Shelf
LA WFLEFL GA NC MA
100
0.5
F
E
• Mixing with surrounding water likely contributes
80
0.4
60
~65% of this DIC increase, equivalent to a DIC
0.3
40
9 mol C d-1;
influx
(to
Loop
Current)
of
~9.1
×
10
0.2
20
respiration accounted for the rest ~35%; other
0.1
0
AOU
Phosphate
processes were likely minor
-20
0.0
LA WFL EFL GA NC MA
LA WFL EFL GA NC MA
Transects
• Entrainment of shelf water into the Loop
Transects
Fig. 8. Boundary current (Gulf stream) core-water
Current is a plausible mechanism (Fig. 1), and
means at the surveyed transects along its flow path.
best studied in the nGMx and WFLS regions.
200
150
6
Southeastern Coast
• North Atlantic subtropical recirculation
contributes to the Florida Current intensification,
with the transport volume increasing ~3 fold
• As a result, incorporation of oligotrophic
Sargasso Sea water can explain >90% of the
decreases in DIC, TA, nutrients, and AOU of
the boundary current core water; other processes
are likely minor
Alongshore Mixing and Biogeochemical Processing along the Northeastern Coast
*F rom C ai et al. 2010
400
0.95
Fig. 7. Comparisons of transect means for shelf
samples. Vertical bars, one standard deviation.
deep ocean
>1000 m
B
30
LA WFL EFL
pHT(20)
TA:DIC
fCO2(20)
5
Nitrate ( mol kg-1)
30
TA ( mol kg )
28
Depth (m)
26
Phosphate ( mol kg-1)
24
0.95
600
1.00
D
1.00
TA:DIC
8.0
35
1.20
1.15
1900
1.05
Comparison of Shelf Water Means
• Mean shelf TA is high in the southern
shelves, and decreases from GA northward
• Mean shelf DIC varied little
• Mean shelf TA:DIC (thus buffering capacity)
generally high for the southern shelves; it fell
steadily north of GA; lowest in the GoME
• Shelf mean pHT(20) followed the shelf mean
TA:DIC ratio
• fCO2(20) followed an inverse relationship
with TA:DIC, making high TA:DIC water
more favorable for CO2 uptake
AOU ( mol kg-1)
1950
MA
800
1.10
7.7
Temperature (t, °C)
NJ
1.15
8.1
7.8
2000
2000
8.2
7.9
2100
2050
B
8.3
pHT(20)
2100
LA
DIC ( mol kg )
DIC (µmol kg-1)
TX
2400
1000
fCO2(20) ( atm)
A
2450
2500
ocean endmember
2150
8.4
A
2300
1.20
Salinity (S)
2500
A
2200
2500
TA:DIC
Fig. 1. U.S. Gulf of Mexico and Atlantic Coasts
deep ocean
>1000 m
GoME
subsurface
Cross-sections of TA:DIC ratios and ΩA
• TA:DIC ratios greatest in the MARS plume (~1.24,
most buffered) – riverine input and biological uptake
• Ratios lowest at the bottom of the GoME (~1.04,
least buffered), reflecting the accumulation of
remineralization products at depth.
• All shelf and upper slope waters were supersaturated
with respect to aragonite (ΩA > 1)
• ΩA higher in the south than the north
Fig. 6. Cross-sections of TA:DIC ratios (color) • ΩA highest in the MARS plume, lowest at the bottom
and aragonite saturation states (ΩA, contour lines) of the GoME
TA:DIC
Transects, stations, and the
oceanographic setting
 Six coastal regions, 9 survey
transects (by state names)
 Western boundary margin: Loop
Current – Florida Current – Gulf
Stream
 Labrador Coast Current in the NE
coastal waters
 The high production, plume water
of the Mississippi-Atchafalaya river
system (MARS)
 A powerful Loop Current eddy with
water entrainment and southward
transport of highly productive
nGMx water
2250
TA : DIC Ratios and Carbonate Chemistry
• The TA:DIC ratio is an indicator of the relative
abundance of carbonate species (e.g., HCO3- and
CO32-)
• The ratio is an indicator of buffering capacity
(intensity): the buffering capacity attains a minimum
when TA:DIC ~ 1
• The ratio is closely correlated with pH and ΩA.
-1
Study Focuses:
 The summertime distributions of TA and DIC across geographic regions characterized by a wide
range of oceanographic and biogeochemical conditions
 Regional differences in CO2 species and properties (e.g., aragonite saturation state, buffer
intensity)
 Alongshore mixing and biogeochemical processing in the Middle Atlantic Bight (MAB)
 Shelf–boundary current–ocean interaction (Gulf of Mexico and southeastern shelves).
Fig. 3. Cross-sections of TA (color) and salinity
(contour lines)
Regional differences in carbonate chemistry
TA ( mol kg )
Background: In support of the North America Carbon Program (NACP) two summer coastal
carbon cruises were conducted in 2007 that covered both the U.S. east and west coasts. The
objectives were to assess carbon distributions and fluxes across geographic boundaries and to
construct climate-relevant carbon inventories and budgets for evaluation of future changes in
the coastal waters of the North America. Results from the west coast cruise have been reported
by Feely et al. (2008). Herein we present results from the east coast cruise, the Gulf of Mexico
and East Coast Carbon (GOMECC) cruise (10 Jul – 04 Aug, 2007, Galveston, TX – Boston,
MA, R/V Ronald H. Brown).
Cross-shelf Gradients
Nitrate (µmol kg-1)
parameters relevant to the marine inorganic carbon system were investigated in shelf and adjacent
ocean waters during a U.S. Gulf of Mexico and East Coast Carbon cruise in July–August 2007. TA
exhibited near-conservative behavior with respect to salinity. Shelf concentrations were generally
high in southern waters (Gulf of Mexico and East Florida) and decreased northward from Georgia
to the Gulf of Maine. DIC was less variable geographically and exhibited strongly nonconservative
behavior. As a result, the ratio of TA to DIC generally decreased northward. The spatial patterns of
other CO2 system parameters closely followed those of the TA:DIC ratio. All sampled shelf waters
were supersaturated with respect to aragonite (saturation state ΩA > 1). The most intensely
buffered and supersaturated waters (ΩA > 5.0) were in northern Gulf of Mexico river-plume
waters; the least intensely buffered and least supersaturated waters (ΩA < 1.3) were in the
deep Gulf of Maine. Due to their relatively low pH, VA, and buffer intensity, waters of the
northeastern U.S. shelves may be more susceptible to acidification pressures than are their
southern counterparts. In the Mid-Atlantic Bight, alongshore mixing tended to increase DIC
concentrations southward, but this effect was largely offset by the opposing effects of
biogeochemical processing. In the Gulf of Mexico, downstream increases in Loop Current DIC
suggested significant contributions from shelf and gulf waters, estimated at 9.1 × 109 mol C d-1.
Off the southeastern U.S., along-flow chemical changes in the Florida Current were dominated by
mixing associated with North Atlantic subtropical recirculation.
Cross-section TA and S
• The offshore high-S, high-TA signature of the
boundary current waters (S > 36.5, TA ~ 2400
µmol kg-1)
• Between the WFL and GA-NC transects, the
apparent cross-sectional area of this boundary
current water increased
• Westward advection of the MARS plume from
LA to TX: the plume water is more offshore at
TX
• Shelf water was well stratified in most areas
• Upwelling event seaward of the shelf break at
EFL (low t, low TA, and high nutrients at the
bottom)
• North-south difference in the MAB: lower TA
and stronger cross-shelf gradient at MA vs. NC
• The shelfbreak front was clearly evident at MA
• In the GoME, water column had lowest TA and
was stratified, except over the Georges Bank
-1
1 Marine
Fig. 9. Total transport (Sv) streamlines for the North
Atlantic Circulation. Adopted from Worthington
(1976) and Schmitz (1996).
Acknowledgement: We thank the officers and crew of the National Oceanic
and Atmospheric Administration (NOAA) ship Ronald H. Brown and the
participants of the 2007 Gulf of Mexico and East Coast Carbon (GOMECC)
cruise (http://www.aoml.noaa.gov/ocd/gcc/GOMECC/participants.html).
The study was supported by the NOAA Global Carbon Cycle Program,
proposal GC05-208. We thank T. Clayton for insightful editorial assistance.
We also express gratitude to Burke Hales and an anonymous reviewer for
their substantial and constructive reviews.