Denitrification in the well-oxygenated NW Pacific Denitrification in the well-oxygenated NW Pacific marginal seasmarginal 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 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

Data Sets (nitrate, nitrite, phosphate, oxygen)

WOCE Hydrographic Programme

1999 Scripps data collection in Japan and Okhotsk Seas

Standard autoanalyzer (colorimeter) method

(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).

(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.

(2) Use of N* to demonstrate denitrification

Denitrification can be measured using N* (Gruber and Sarmiento, 1997) (distance from Redfield ratio N:P line)

N* values greater than zero indicate nitrogen fixation.

N* values less than zero indicate denitrification.

-16 -14 -12 -10 -8 -6 -4 -2 0 2 4-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

N*

Depth

Japan/East Sea (blue), Okhotsk Sea (red), Bering Sea and N. Pac (green)

Dep

th

N*

0 5 10 15 20 25 30 35 40 450

0.5

1

1.5

2

2.5

3

3.5

Nitrate

Japan/East Sea (blue), Okhotsk Sea (red), Bering Sea and N. Pac (green), Tsushima St. (black)

Ph

osp

hat

eP

ho

s ph

a te

NitrateNitrate

Pacific distribution of N* (500 m)

500 m

NW Pacific (high oxygen)

Ross Sea (high oxygen)

Tropical Pacific (very low oxygen) Oxygen (< 5 only here)

Pacific distribution of N* (2000 m)

2000 m

Bering Sea (medium oxygen)

Tropical Pacific (medium oxygen) Oxygen (>50 everywhere)

Note well-oxygenated Japan Sea

N* (Gruber & Sarmiento)

Pacific WOCE

Nitrification N*>0

Denitrification Denitrification N* < 0N* < 0

Figure prepared by P. Robbins

NW Pacific distribution of N*

300 m

1000 m

2000 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

Presence of nitrite (all color dots) on bottom (JES, OS) or deep (Bering)

Measurable NO2 - > 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 -> bottom sediment process causing denitrification

Japan Sea Oxygen Nitrite

Nitrite far below the euphotic zone, together with lowered O2

1400 m

NONO2 2 ~ 0~ 0 High O2

N*

NO2 > 0

NONO2 2 >> 0>> 0

Somewhat lower O2

(205 mol/kg)

Profiles in the Ulleung Basin

Bottom boundary layer: nitrite enhancement, O2 depletion, NO3 depletion, N* suggesting denitrification, high turbidity (water full of sediment)

Japan Sea: Mid-depth oxygen minimum

Ulleung Basin, Yamato Rise, Japan Basin

Primorye, HokkaidoPrimorye, Hokkaido

Lower oxygen along margins

Is there a link to sedimentary processes? (There is no CFC minimum, and no CFC signature in the bottom boundary layers)

Japan Sea: bottom nitrite

1000 to 1500 m

> 1500 mBottom nitrite

At (a) depths below 300 m around all margins, and (b) > 1000 m in the southwest

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 > 2%

Okhotsk Sea August 1999

Silica at 26.9

Low silica - ventilated water

High silica - N. Pacific High silica - N. Pacific waterwater

Bottom

<-1.0°C: dense shelf water

Dense shelf water through brine rejection (NPIW formation) Spread of shelf waters southward in East Sakhalin Current

136oE 138oE 140oE 142oE 144oE

53oN

54oN

58oN

55oN

56oN

57oN

5656cmcm // ss

M2 tidal ellipses

Large tidal currents on shelves - lots of mixing

Okhotsk Sea Nitrite and N*

N*Nitrite1000 to 1500 m

> 1500 m

N*

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

Nitrite N*

1500 m

3800 m Aleutians

Bering Sea

400 m

500-1000 m

1000 to 1500 m

N:P ratio in the NW Pacific marginal seas

0 5 10 15 20 25 30 35 40 450

0.5

1

1.5

2

2.5

3

3.5

Nitrate

Japan/East Sea (blue), Okhotsk Sea (red), Bering Sea and N. Pac (green), Tsushima St. (black)

Ph

osp

hat

eP

ho

s ph

a te

Japan/East Sea and Tsushima Strait

Okhotsk Sea

Bering Sea

NitrateNitrate

Excursion left of line -> denitrification; right of line -> N fixation.

N* in the NW marginal seas

Sig

ma

0

N*

-16 -14 -12 -10 -8 -6 -4 -2 0 2 4-28

-27

-26

-25

-24

-23

-22

N*

Potential density sigma 0

Japan/East Sea (blue), Okhotsk Sea (red), Bering Sea and N. Pac (green), Tsushima St. (black)

Denitrification in Japan/East Sea below the shallow pycnocline

-16 -14 -12 -10 -8 -6 -4 -2 0 2 4-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

N*

Depth

Japan/East Sea (blue), Okhotsk Sea (red), Bering Sea and N. Pac (green)

Dep

th

N*

Denitrification in upper Okhotsk Sea is most pronounced.

Deeper denitrification most pronounced in Bering Sea: note split around Aleutians (lower N* inside Bering Sea)

Okhotsk Sea,

Bering Sea

Japan Sea

Tsushima Strait

Lower N* is more denitrified (zero is the Redfield ratio nitrate:phosphate line)

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 N:P ratio (North Pacific Intermediate Water - above 1000 m)

Bering Sea: impact on mid-depth N. Pacific N:P ratio

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