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 major  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

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 (NO 2 - ) 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 N 2 ).

(1) Presence of Nitrite (NO 2 - ) (cont.) 3. Sediment processes: coupled denitrification and nitrification. Large- scale effect is of denitrification. Existence of deep NO 2 - (in turbid bottom boundary layer): sediment-water interface is wide. Nitrification (oxidation) could occur in upper sediment interface, producing NO 2 -, using ammonium from decay of organic matter. Denitrification (reduction) farther down, net downward flux of NO 3 - 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. Depth N* Phosphate Nitrate

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 N* < 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 NO 2 - >  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 OxygenNitrite Nitrite far below the euphotic zone, together with lowered O m NO 2 ~ 0 High O 2 N* NO 2 > 0 NO 2 >> 0 Somewhat lower O 2 (205  mol/kg)

Profiles in the Ulleung Basin Bottom boundary layer: nitrite enhancement, O2 depletion, NO 3 depletion, N* suggesting denitrification, high turbidity (water full of sediment)

Japan Sea: Mid-depth oxygen minimum Ulleung Basin, Yamato Rise, Japan Basin Primorye, 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 m Bottom 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 water Bottom  <-1.0°C: dense shelf water Dense shelf water through brine rejection (NPIW formation) Spread of shelf waters southward in East Sakhalin Current 56 cm / s M2 tidal ellipses Large tidal currents on shelves - lots of mixing

Okhotsk Sea Nitrite and N* N* Nitrite  1000 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  m  1000 to 1500 m

N:P ratio in the NW Pacific marginal seas Phosphate Japan/East Sea and Tsushima Strait Okhotsk Sea Bering SeaNitrate Excursion left of line -> denitrification; right of line -> N fixation.

N* in the NW marginal seas Sigma 0 N* Denitrification in Japan/East Sea below the shallow pycnocline Depth 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* = 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/m 3 )(3000m)/50 yrs = 0.4 mol / m 2 yr Oxygen utilization rate from oxygen deficit compared with nitrate deficit in the bottom boundary layer: ~ 10 mol / m 2 yr