Denitrification in the well-oxygenated NW Pacific marginal seas

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Denitrification in the well-oxygenated NW Pacific
marginal seas
Lynne D. Talley, Pavel Tishchenko Scripps
Institution of Oceanography, La Jolla,
CA Pacific Oceanological Institute, Vladivostok

• Nitrate/phosphate ratios (N) indicating
  denitrification on nearly the same scale as in
  the tropical denitrification zones
• Presence of measurable deep nitrite in turbid
  bottom layers, relation to extremes of N
• Japan Sea - interesting, but isolated source of
  denitrification
• Okhotsk Sea - major source to upper/intermediate
  Pacific
• Bering Sea - source to intermediate/deep Pacific
• Sedimentary process, impact on oxygen consumption

Gruber and Sarmiento (1997) N Jahnke and
Jackson (1987), Christensen et al (1987), Janhke
et al (1990) sediment effects on large-scale
oxygen and carbon distribution Anderson and
Sarmiento (1994) sedimentary denitrification
rates
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Data Sets (nitrate, nitrite, phosphate, oxygen)
WOCE Hydrographic Programme 1999 Scripps data
collection in Japan and Okhotsk Seas Standard
autoanalyzer (colorimeter) method
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(1) Presence of Nitrite (NO2 -)
1. Euphotic zone, mixed layer base primary
nitrite maximum.
2. Nearly anoxic regions, mainly in the eastern
tropical regions, such as in the Pacific.
Created by reduction of nitrate by denitrifying
bacteria, part of process to complete
denitrification (to N2).
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(1) Presence of Nitrite (NO2 -) (cont.)
3. Sediment processes coupled denitrification
and nitrification. Large-scale effect is of
denitrification. Existence of deep NO2- (in
turbid bottom boundary layer) sediment-water
interface is wide. Nitrification (oxidation)
could occur in upper sediment interface,
producing NO2- , using ammonium from decay of
organic matter. Denitrification (reduction)
farther down, net downward flux of NO3- out of
water column and into sediments.
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(2) Use of N to demonstrate denitrification
Denitrification can be measured using N (Gruber
and Sarmiento, 1997) (distance from Redfield
ratio NP line) N values greater than zero
indicate nitrogen fixation. N values less than
zero indicate denitrification.
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Pacific distribution of N (500 m)
500 m
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Pacific distribution of N (2000 m)
2000 m
Oxygen (gt50 everywhere)
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N (Gruber Sarmiento) Pacific WOCE
Nitrification Ngt0
Denitrification N lt 0
Figure prepared by P. Robbins
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NW Pacific distribution of N
1000 m
300 m
High values south of subarctic front. Spread of
low values into NW corner. Japan Sea high in
newer waters in center, lower around
edges Okhotsk Sea low off northern shelves
2000 m
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Presence of nitrite (all color dots) on bottom
(JES, OS) or deep (Bering)
Measurable NO2 - gt 0.004 ?mol/kg Standard
autoanalyzer, colorimetric procedure (SIO/ODF
WOCE quality). (Much smaller quantities are
measured with other specialized
techniques.) These are well-oxygenated basins -gt
bottom sediment process causing denitrification
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Japan Sea
Nitrite far below the euphotic zone, together
with lowered O2
Oxygen
Nitrite
N
NO2 gtgt 0
High O2
NO2 0
NO2 gt 0
1400 m
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Profiles in the Ulleung Basin
Bottom boundary layer nitrite enhancement, O2
depletion, NO3 depletion, N suggesting
denitrification, high turbidity (water full of
sediment)
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Japan Sea Mid-depth oxygen minimum
Is there a link to sedimentary processes? (There
is no CFC minimum, and no CFC signature in the
bottom boundary layers)
Lower oxygen along margins
Ulleung Basin, Yamato Rise, Japan Basin
Primorye, Hokkaido
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Japan Sea bottom nitrite
At (a) depths below 300 m around all margins, and
(b) gt 1000 m in the southwest

• 1000 to 1500 m
• gt 1500 m

Bottom nitrite
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Sediment composition
High percentage of organic carbon in the
sediments in the Ulleung Basin and around the
margins (Likht et al.) - coinciding with
occurrence of deep nitrite. High percentages are
similar to (greater than) percentages found in
Washington coast sediment measurements that
showed denitrification (Devol, 1991)
Organic carbon gt 2
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Okhotsk Sea August 1999
Dense shelf water through brine rejection (NPIW
formation) Spread of shelf waters southward in
East Sakhalin Current
Silica at 26.9?? Low silica - ventilated
water High silica - N. Pacific water
Bottom ? lt-1.0C dense shelf water
Large tidal currents on shelves - lots of mixing
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Okhotsk Sea Nitrite and N

• 1000 to 1500 m
• gt 1500 m

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Bering Sea Nitrite and N
We had a more limited data set WOCE section only
shown here. Nitrite occurs to 400 m. Deep nitrite
only at Aleutians Low N throughout, very low at
bottom. Greater impact than Okhotsk on Pacific
distribution at greater depths

• 500-1000 m
• 1000 to 1500 m

400 m
1500 m
Bering Sea
Aleutians
3800 m
Nitrite
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NP ratio in the NW Pacific marginal seas
Excursion left of line -gt denitrification right
of line -gt N fixation.
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Lower N is more denitrified (zero is the
Redfield ratio nitratephosphate line)
N in the NW marginal seas
Denitrification in upper Okhotsk Sea is most
pronounced.
Deeper denitrification most pronounced in Bering
Sea note split around Aleutians (lower N inside
Bering Sea)
Denitrification in Japan/East Sea below the
shallow pycnocline
Okhotsk Sea, Bering Sea Japan Sea Tsushima Strait
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Conclusions

• Signature of sedimentary denitrification is clear
  in the Okhotsk, Bering and Japan Seas

• Presence of deep nitrite is shown in Japan and
  Okhotsk Seas active turbid sediment/water
  interface (eddies, tides). Suggestion of deep
  nitrite in Bering - need more nutrient data

• Japan Sea useful for rate calculations because
  of isolation
• Coincidence of oxygen minimum, nitrite presence,
  low N, and unperturbed CFCs suggests role of
  sediments in producing oxygen minimum layer.
  Could perhaps generalize this to the N. Pacific.

• Okhotsk Sea major impact on N. Pacific NP ratio
  (North Pacific Intermediate Water - above 1000 m)
• Bering Sea impact on mid-depth N. Pacific NP
  ratio

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Japan Sea denitrification rate (VERY ROUGH)

• Inflow at Tsushima Strait N 1.5
• Nitrate surplus of Tsushima Strait 2 ?mol/kg
• Waters below pycnocline N -3.2
• Nitrate deficit for subpycnocline waters - 4
  ?mol/kg
• 6 ?mol/kg denitrification loss
• Turnover time 20 to 50 years
• Denitrification rate
• (6 ?mol/kg )(1027 kg/m3)(3000m)/50 yrs
  0.4 mol / m2 yr
• Oxygen utilization rate from oxygen deficit
  compared with nitrate deficit in the bottom
  boundary layer 10 mol / m2 yr