Transcript Document

0
(N )
Using preformed nitrate
to infer recent changes in DOM remineralization in
the upper thermocline of the subtropical North Pacific
OS41B - 17
Jeffrey Abell & Steve Emerson, University of Washington, School of Oceanography, Box 355351
[email protected]
3. N0 Mass Balance
Ni
POCf
DOCi
40°N
Marathon
N0
Ni
DOCf
T, S
O2 < O2sat
AOU = O2sat - O2
N0 = Nm + AOU / R
intensity of the anomaly is proportional to the magnitude of DOC
remineralization and the DOC:DON remineralization ratio. From
N 0  Nm 
 Ni 
R NO3 :POCPOC
and/or the C:N ratio of DOM degradation has increased in the past
h
 R NO3 :DOCDOC 
R O 2 :POCPOC
R O 2 :NO3 h

b
R O 2:DOCDOC
R O 2:NO3
30°N
O cean Dat a View
35°N
STUD97 NO3
100
0.1
5
1
10
Depth
0.1
1
200
300
15°N
20°N
25°N
O cean Dat a View
30°N
35°N
0
0
0
100
0
0
25
100
80
60
40
20
0
0
0
c
200
DOC = 25 M
RNO3:DOC = 0.03
25.4 - 25.8 T
-34
-3.5
DOC = 15 M
RNO3:DOC = 0.10
-24
-4.0
DOC = 10 M
RNO3:DOC = 0.17
2
3
4
5
6
7
DON (M)
50
0
25
15°N
0
20°N
25°N
30°N
Ocean Data View
O cean Dat a View
100
0
20
10°N
35°N
Figure 2. a) N , b) NO3, and c) AOU distributions in the
eastern subtropical North Pacific along 152W during
November 1997. Cruistrack is depicted in Figure 5 (STUD97).
0
Negative N values are shaded white to green. The solid and
dotted white lines represent the 25.4 and 26.1 t isopycnals
respectively.
Table 4. For the STUD97 and GS2000 cruises, we compared the
observed preformed nitrate values with those calculated from the
0
AOU, DOC, and DON data. Observed N , as determined from the
equation in Fig 3a, was averaged over each isopycnal range
0
between 10N and 30N. Calculated N is the average preformed
nitrate as determined from the equation in Fig 3b. We assume
RO2:DOC = -1.3, RO2:NO3 = -10 and Ni is equivalent to the average
surface nitrate concentration in winter at the site of the outcrops
0
(Anderson and Sarmiento, 1994; Levitus and Boyer, 1994). N
was then calculated after substituting respective values for DOC
and RNO3:DOC. Standard deviation in parentheses is based on the
error in the DOC and RNO3:DOC terms. Observed and calculated
0
N agree well suggesting that DOM degradation at high C:N ratios
0
is primarily responsible for the negative N anomaly.
5
10
Depth
O cean Dat a View
Ocean Data View
5
1
Depth
Figure 6. Contours represent the deviation in N that would result
from a change in the magnitude of DOM remineralization given a
specific C:N ratio. Currently, 25 umol/kg of DOC is remineralized
at a C:N ratio of 30 along the isopycnals above 25.4 t. If the C:N
remained constant between 1980 and 1997, the magnitude of
DOM remineralization would have had to increase by 10umol/kg
0
to explain the –0.99 umol/kg deviation in N (solid arrow). This is
equivalent to a 60% increase in DOM remineralization during this
time period. If, instead the magnitude remained constant, the C:N
ratio would have had to increase from near Redfield values to 30.
5
7. Conclusion
400
25°N
30°N
Ocean Data View
O cean Dat a View
10
DOC = 15 M
RNO3:DOC = 0.07
the C:N remineralization ratio could explain the temporal
0
trends observed in N . At this point, it is difficult to separate
35°N
the relative importance of each of these factors. Evidence
25.4 - 26.1 T
WOCE-P16N Sep 1991
d
-22
-4.6
DOC = 10 M
RNO3:DOC = 0.20
2
3
4
5
6
DON (M)
station ALOHA have been used
1
-0.71
7
from shifts in the phytoplankton community structure at
0
to suggest that DOM
production and utilization has increased over the past few
100
DEPTH [M]
AOU (moles/kg)
100
80
60
40
20
0
Both an increase in the magnitude of DOM degradation or
-29
-2.0
decades (Karl, 1999; Karl et al., 2001). Our results would
200
support this conclusion. However, we cannot unequivocally
5
1
show that there was an increase in the utilization rate of
Figure 4. AOU, DOC and DON data from a) STUD97 and b)
GS2000 were used to determine the magnitude and
remineralization ratio of DOM along shallow isopycnal layers.
GS2000 data is from a longitudinal section conducted along 24N
between 110 and 158W in January 2000. Plots of AOU vs. DOC
and DON were used to determine the magnitude of DOC
degradation (DOC) and the remineralization ratio (RNO3:DOC)
along each isopycnal. To determine RNO3:DOC, the regression
slope of AOU vs. DOC was divided by AOU vs. DON for
each pair of graphs. On the upper isopycnals, in the region of the
negative preformed nitrate anomaly, RNO3:DOC is low and DOM
degradation increases AOU without returning a significant amount
of nitrate to the water column. On deeper isopycnals, where
preformed nitrate is positive, both DOC and DON contribute to
AOU and RNO3:DOC approaches the Redfield ratio (0.15).
25.8 - 26.1 T
DOC (M)
250
100
80
60
40
20
0
DOM along the isopycnal in response to an increase in
300
10
400
20°N
e
25°N
30°N
35°N
40°N

calculated from AOU, DOC, and DON
data agrees well with observed values
N0
 Negative anomaly primarily results from
DOM remineralization at high C:N ratio
Isopycnal
(t)
Stud97
24 .4 - 24.9
24.9 - 25.4
25.4 -25.8
25.8 - 26.1
GS2000
ML – 25.4
25.4 – 26.1
0
0
Ni
(mol/kg)
Calculated N
(mol/kg)
Observed N
(mol/kg)
0
0
1.80
4.60
-1.20 (0.21)
-1.20 (0.27)
1.56 (0.26)
5.00 (0.26)
-1.72
-0.83
1.69
5.56
0
3.2
-0.46 (0.20)
3.69 (0.38)
-0.41
3.83
supply at the outcrop regions. A reduction in the quality of
the DOM (as expressed in the C:N ratio) could have also
resulted from the community shift. Alternatively, a decrease
STUD97 Nov 1997
0
in the circulation rate could also lead to an increase in DOM
0
-0.94
0
0
remineralization without requiring any increase in supply or
0
100
1
0
-2
degradation rate or C:N ratio.
5
-1
-1
200
5
300
50
150
400
-60
-2.0
30 40 50 60 70 80
25
Depth
100
300
100
80
60
40
20
0
24.9 - 25.4 T
200
0
10
STUD97 AOU
100
80
60
40
20
0
DOC = 25 M
RN03:DOC = 0.03
30
10
10°N
100
1
400
Ocean Data View
10
15
0
-1
5
15
Marathon May 1984
0
AOU (moles/kg)
b
0
-50
-1.6
35°N
O cean Dat a View
25°N
c
30°N
Ocean Data View
20°N
25°N
10
400
10°N
15°N
20°N
0
25°N
60°N
50° N
40°N
30° N
20°N
10°N
EQ
120°E 150°E 180°E150°W120°W
30°N
Ocean Data View
15°N
400
300
DOC (M)
b
24.4 - 24.9 T
25
1
10°N
100
80
60
40
20
0
Ocean Data View
400
60° N
50°N
40°N
30° N
20° N
10° N
EQ
120°E150° E
180°E
150° W120°W
10
Depth
1
10
10
300
ML - 25.4 T
30 40 50 60 70 80
Stud 97
5
300
20
200
0
1
10
a
15
C:N
-1
-1
200
10
0
0
Depth
-1
5
1
5
1
0
0
0
0
-1
-0.5
GS2000
 Remineralization approaches Redfield
values on isopycnals where N0 is positive
0
-2
4. DOM Remineralization Data
 Little nitrate is remineralized from DOM
on isopycnals where N0 is negative.
1
1
0
0
0
10
5
-0.21
0
-1
5
0.05
Figure 3b. Integrating DOM remineralization into the N mass
balance. Both POC flux across the isopycnal (POC) and DOC
degradation along the isopycnal (DOC) contribute to AOU and
m
N . These values are dependent upon POC, DOC, the
0
residence time of water () and the isopycnal height (h). When N
is solved using these mass balance equations, the POC terms
cancel and only DOC, RO2:DOC, RO2:NO3, and RNO3:DOC remain. In
the case where no DOM is remineralized (i.e. DOC = 0), the
0
calculated N would equal the initial nitrate at the outcrop (Ni).
However, if DOC > 0 and RNO3:DOC is less than the Redfield ratio,
0
N will be less than Ni. In this way, DOC degradation may allow
for negative anomalies in the preformed nitrate distribution.
0
0
100
0
-1
a
0
-1
15
FIONA AUG 1980
0
Figure 3a. N mass balance in a “particulate-only” ocean. For a
water parcel advecting along an isopycnal, O2 is depleted as NO3
increases due to the biological degradation of sinking particulate
m
organic matter. The measured NO3 (N ) in the parcel at depth is
a combination of biological degradation and the NO3 originally
present at the surface (Ni). This latter contribution is the true
0
preformed nitrate value. We estimate the preformed nitrate as N
m
= N + AOU / R, where AOU is apparent oxygen utilization and R
is the stoichiometric relationship between oxygen depletion and
nitrate regeneration.
Assuming R does not vary significantly
0
along the path of the isopycnal, N should be constant along
isopycnals.
STUD97 PreNO3
90°W
100
may have increased since the late 1970’s due to changes in the
 Within anomaly, AOU varies from 0-50
umol/kg and NO3 varies from 0.1-5
umol/kg
120°W
AOU
R O 2 :NO3
two decades. This supports recent findings that DOM production
 Negative anomaly of 0-2.5 umol/kg
between mixed layer and 25.4 t
150°W
-0.5
N0
eastern subtropical North Pacific, we infer that the magnitude
180°E
DOCi
0
temporal trends in the N distribution along similar transects in the
150°E
-1.5
-0.5
results agree with the observed N distribution and show that the
EQ
120°E
20
0
N0
0
2. N0 Distribution
-2
WOCE-P16N
20°N
1
 R NO 3 :DOCDOC
Stud97
Calculated
phytoplankton community structure (Karl et al., 2001).
25
Fiona
0.5
h
a
2
R NO 3 :POCPOC
60°N
1.5
N  Ni 
m
POCi
O cean Dat a View
to calculate N using AOU, DOC and DON data.
Nm
DOCf
preformed nitrate. We develop a simple isopycnal mass balance
0
 C:N may have increased from 7 to 30
1
with a high C:N ratio is primarily responsible for negative
 R O 2 :DOCDOC
80°N
Nm
O2
h
DOC (umol/kg)
in the eastern subtropical North Pacific that degradation of DOM
R O 2 :POCPOC
Ocean Data View
1995). Here we present quantitative evidence from two transects
Nm
T, S
O2 = O2sat
AOU = 0
N0 = Nm
a
DOCi
0
nitrogen-poor dissolved organic matter (Emerson & Hayward,
POCf
AOU  O i  Of  
feature is created during nitrate uptake by vertically migrating
diatom mats, nitrate uptake by respiring bacteria or degradation of
DOCf
1
It has been suggested that this
 Integrating DOM into the
mass
balance allows for negative anomalies in N0
Oi
1
from Redfield stoichiometry.
b
N0
POCi
0
remineralization in the upper thermocline deviates significantly
Of
 DOM remineralization may have
increased by 60% in response to an
increase in DOM export or a slowing of
circulation
1
Its presence indicates that nitrogen
N0 decreases by about 1umol/kg between
1980 and 1997
10
mixed layer and 25.4 t.
O2
5
Pacific is characterized by a negative anomaly between the
 Ideally N0 is constant along isopycnals
and reflects the initial N03 at the outcrop
6. Reasons for N0 Decrease
10
0
The preformed nitrate (N ) distribution in the subtropical North
5. Temporal Trends in N0
-1
1
1. Introduction
8. References
35°N
Figure 5. A comparison of N distributions along similar cruise
transects. Cruises in (a) were conducted along 152-158W
between 1980 and 1997. Contours, shading, and isopycnals are
similar to Fig. 2a for b) Fiona, c) Marathon, d) Woce-P16N, and e)
0
Stud97. To compare the N distribution between cruises, we
calculate the weighted average between 100m and 25.4 t along
20N to 30N. These averages are listed to the left of each section.
No effort is made to correct for calibration differences among
cruises for nitrate, oxygen, and oxygen saturation determinations.
0
A trend to more negative N is evident between 1980 and 1997.
This suggests that the magnitude and/or C:N ratio of DOM
remineralization increased during this time.
Abell, et al. 2000. Distributions of TOP, TON, and TOC in the North Pacific subtropical
gyre: implications for nutrient supply in the surface ocean and remineralization in the
upper thermocline. Journal of Marine Research, 58(N): 1-21.
Anderson, L.A. and Sarmiento, J.L., 1994. Redfield ratios of remineralization determined by
nutrient data analysis. Global Biogeochemical Cycles, 8: 65-80.
Emerson, S. and Hayward, T.L., 1995. Chemical tracers of biological processes in shallow
waters of the North Pacific: Preformed nitrate distributions. Journal of Marine
Research, 53: 499-513.
Garcia, H., and L. Gordon. Oxygen solubility in seawater: Better fitting equations.
Limnology and Oceanography, 37, 1307-1312, 1992.
Levitus, S., and T.P. Boyer. World Ocean Atlas 1994. US Dept of Commerce,
Washington, D.C. 1994
Karl, D.M., et al., 2001. Long-term changes in plankton community structure and
productivity in the North Pacific Subtropical Gyre: The domain shift hypothesis, DeepSea Research II, 48, 1449-1470.
Karl, D.M., 1999. A sea of change: Biogeochemical variability in the North Pacific
Subtropical Gyre. Ecosystems, 2: 181-214.
Schlitzer, R. Ocean Data View. http://www.awi-bremerhaven.de/GEO/ODV, 2001
WOCE P16N. http://whpo.ucsd.edu/data/tables/onetime/1tim_pac.htm