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Dissolved Organic Carbon in Modeling Oceanic New Production

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Dissolved Organic Carbon in Modeling Oceanic New Production GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 5, NO. 1, PAGES '71-85, MARCH 1991 DISSOLVED ORGANIC CARBON IN MODELING OCEANIC NEW PRODUCTION R. Bacastow ScrippsInstitution of Oceanography La Jolla, California E. Maier-Reimer Max-Planck-Institut fur Meteorologie Hamburg, Germany Abstract. The flux of organic carbon associated with it not for this transport, the surfacedissolved inorganic new production has been modeled by advection of dis- carbon (DIC) concentrationwould be larger, and the solved organic carbon in addition to falling particulate atmosphericCO 2 concentrationwould be much larger. organic carbon, in a carbon cycle model that is based on Understandingthe mechanismsof this transport is impor- an oceanic general circulation model. Model predictions tant if we are to predict the effect of changein climate of chemical speciesinvolved in the carbon cycle are com- and resulting changein oceancirculation upon the atmos- pared with observations. Relative to a model in which pheric CO2 level. new production is carried only by falling particulate Until recently, it was generallythought that the flux of organic carbon, there is significantly better agreement new production was carried almost entirely by vertically between predicted and observed oceanic phosphate and falling particulate organic carbon (POC). However, oxygen concentrations if a large part of the new produc- measurementsof oceanicdissolved organic carbon (DOC) tion flux is carried by dissolvedorganic carbon. by a new technique[Sugimura and Suzuki,1988] has revealed an active pool that is severaltimes larger than INTRODUCTION indicated by previous measurements.The new measure- ments of DOC concentration show variations with depth A key element in an oceanic carbon cycle model is the which correlate with dissolved nitrate and oxygen concen- modeling of oceanic new production and the related tran- trations, suggestivethat DOC is actively involved in the sport of organic carbon and associatednutrients. Most carbon cycle, whereas older DOC measuredconcentra- primary production is recycled in the surface water near tions show little change with depth. Toggweiler where it was formed. A portion of primary production, [1988a,b],Williams and Druffel [1988], and Jackson called new production[Eppley and Peterson,1979], is [1988] have all discussedthese measurements and their transported to deeper water where it is remineralized and oceanographicimplications. The new measurementsof eventually returned to the surface by ocean circulation DOC are still controversial,but preliminary confirmation and mixing. The fraction of total primary production hasbeen reported (P.M. Williams,personal communica- that is new production varies from less than 10% in the tion, 1990;E. T. Peltzer,personal communication, 1990). central ocean gyres to over 80% in coastal upwelling Here we wish to compare models of oceanic chemical areas;the averageis about 25%. distributions, both with and without the assumption of New production is largely responsiblefor the gradient an active pool of DOC, with oceanographicdata. Is it in nutrients such as phosphorusand nitrogen between the possibleto get reasonableagreement with POC only, con- surface ocean, which is depleted, and deeper water. Were sidering uncertainties in the models, and if not, in what way do the models then differ from the data? How much Copyright 1991 better is agreement if transport of new production by by the American Geophysical Union. DOC is also included? (A preliminaryversion of this work has been submitted for publication in a volume of Paper number 91GB00015. papers presented at a conferencesponsored by the Elec- 0886-6236/91/91GB-00015510.00 tric Power Research Institute at Lake Arrowhead, 72 Bacastow and Maier-Reimer: Carbon in New Production Models October, 1988.) We employ an oceaniccarbon cycle The additional DOC measured by the new analysis model that is three dimensional and based on an oceanic method has been found to be of higher molecular weight generalcirculation model (GCM) developedat the Max than the DOC detectedby the oldermethods [Sugimura Planck Institute for Meteorology, Hamburg, Germany and Suzuki,1988] and must be moreresistant to ultra- [Maier-Reimer and Hasselmann,1987; Bacastowand violet and wet oxidation techniques but nevertheless Maier-Reimer,1990]. This type of modelis particularly more readily consumed in surface waters, possibly by appropriate for the questionsposed above becauseadvec- marine microbes. It must have a lifetime that is short tion of DOC has three-dimensionalconsequences. compared with the time water residesin the thermocline Toggweileret al. [1988b]and Sarmientoet al. [1988] because mostly it is confined to the upper 400-500 m. have described the effect of transport of the new produc- The DOC found by older methods is only a little higher tion flux by DOC, in addition to POC, in a in concentration in surface water and shows little struc- three-dimensional, flat bottomed sector model of a single ture in deeper water, so it must be resistant to consump- ocean basin. Their chemical model included only the tion by marinemicrobes. Its 14Cage is about 1300years nutrient phosphate. The predicted phosphateconcentra- in surfacewater and about6000 years in deepwater [Wil- tion was adjusted to be close to the observed concentra- liams and Druffel, 1987]. Consequently,the DOC found tion, and the required adjustment was then attributed to by older methods cannot be important for the transport new production. They found that with transport only by of organic carbon and associatednutrients in the ocean. POC, subsurfacevalues of phosphatein upwelling regions We are thus led to the simplifiedpicture (Figure 1) were much too large. We use a model with realistic where there are two DOC pools, "old" DOC, which is geometry, within the limitations of the grid, and include inactive, long-lived, and probably not important to the in our chemical model, in addition to phosphate, dis- transport of organic carbon, and "ew"DOC, a pool of solvedinorganic carbon (DIC), alkalinity, and oxygen. about 150-200 /•mol kg-1 at the surfacethat penetrates We model new production directly, and "spin-up" the into the oceanonly about 500 m. We have includedonly model until unchanging, equilibrium concentrations the new DOC pool in the carbon cycle model. result. About 1800 model years is required, which takes Constant Redfield ratios of O2:C:N:P = 135:105:15:1 23 minutes of Cray Y-MP time. This time is short have been assumed to hold for both POC and new DOC. enough that the effect of variation of parameters in the The new DON measurements[Sugimura and Suzuki, model can be studied. 1988] are roughlyconsistent with this nitrogenRedfield ratio and the size of the new DOC pool discussedabove. NEW PRODUCTION These DON measurementshave not yet been confirmed, but preliminary confirmation of the DOC measurements New production has been defined by Dugdale and is supportive. Correspondingmeasurements of dissolved Goering[1967] to be that portionof primaryproduction organicphosphate (DOP) have not yet beenreported, which incorporates nitrogen from the uptake of dissolved but are eagerly awaited. nitrate. It can be measured by isotope techniques. Des- truction of soft tissue ordinarily releasesreduced forms of nitrogen, such as ammonia and urea, which phytoplank- CARBON CYCLE MODEL ton prefer to the more oxidized nitrite and nitrate. In surface water, reduced nitrogen is quickly converted to The carbon cycle model is as described by Bacastow organic matter through photosynthesis,but in deep water and Maier-Reimer[1990] (hereinafterreferred to as it becomes oxidized to nitrate before being returned to BMR), exceptfor a few changes,which will be listed the surface. Consequently, at steady state, new produc- below. Chemical speciesimportant to the carbon cycle tion as defined by Dugdale and Goering must equal the are advected by the current field of the oceanic GCM. portion of primary production that is transported to The ocean is representedby 27443 boxes arranged in 10 deeper water. levels. Mixing occurs through numerical diffusivity (relatedto finite box size),a very small explicithorizon- NEWLY DISCOVERED DOC tal diffusivity, and a convective adjustment. An atmos- pheric box exchangesCO 2 with the surface ocean boxes. Ocean water apparently contains a large pool of DOC There is no land biota provided in this version of the that has not been previously recognized. Ocean chemists model. The oceanic GCM is seasonal, but the carbon in Japan, with a new method of analysis, have reported cycle model is nonseasonal and employs a seasonally finding much more DOC and dissolved organic nitrogen averaged ocean circulation. All photosynthesisis limited (DON) in the oceanthan waspreviously found [Suzuki et to the surface boxes, 112.5 m deep. Dissolved oxygen al., 1985; Sugimuraand Suzuki, 1988]. They use a surfaceconcentrations are set to 3% supersaturation. high-temperature catalytic oxidation method; older New production,PRODC/•mol kg-1 yr-1 of C in sur- methods are based on oxidation by ultraviolet light, per- faceboxes (or equivalently,g C m-2 yr-•), is determined sulfuric acid, and other "wet"oxidizing agents. In by a Michaelis-Mentonkinetics equation [Dugdale, 1967], near-surfacewater, the difference between the new meas- dependent on nutrient concentration, represented by the urements and older measurements is several times the phosphate concentration, PO4, multiplied by a latitudi- amount found through the older methods. nally varying incident light
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