1 Carbon - Motivation

Size: px

Start display at page:

Download "1 Carbon - Motivation"

Cory Marshall
5 years ago
Views:

1 1 Carbon - Motivation Figure 1: Atmospheric pco 2 over the past 400 thousand years as recorded in the ice core from Vostok, Antarctica (Petit et al., 1999). Figure 2: Air-sea flux of CO 2 (mol m 2 yr 1 ) estimated from a compilation of surface observations of pco 2 (Takahashi et al., 2002). 1

2 What Regulates the Distribution and Storage of Carbon in the Ocean? The vast majority of carbon in the oceans is in dissolved inorganic form. Observed meridional sections of DIC (Fig.

2 2 What Regulates the Distribution and Storage of Carbon in the Ocean? The vast majority of carbon in the oceans is in dissolved inorganic form. Observed meridional sections of DIC (Fig. 3) reveal a high background concentration, on the order of the saturation concentration at mean ocean temperature and alkalinity and pre-industrial atmospheric pco 2 ( 2000µmol kg 1 ). This suggests that at zero order, the carbon loading of the ocean may be regulated by changes in alkalinity (see Section XXX - MICK VERY LONG TERM CHANGES) and the temperature. There is, however, significant regional variation, up to 20% of the background value and the basin scale structure of the variations exhibits significant correspondence with passive tracers of water mass (e.g. salinity) indicating a significant first order contribution by transport of properties from sites of subduction and deep water formation. In addition, DIC concentrations in the thermocline and deep ocean are typically higher than the saturation concentration for the local temperature and alkalinity (compare Figs. MICK Csat-Alk AND Csat-T IN BASIC CARBON CHAPTER). This, along with the co-variation of interior ocean DIC with nutrients (compare Fig. 4 and Fig.??), indicates a contribution from interior sources related to biological cycling of nutrients and carbon. Figure 3: Observed meridional sections of dissolved inorganic carbon, DIC (µmol kg 1 ): (a) Atlantic, (b) Pacific. In this section we discuss how a combination of physical, chemical and biological processes set the observed distribution of carbon in the ocean. We describe a quantitative framework in which to quantify and discuss the rocesses which regulate the oceanic storage of carbon and its control on atmospheric pco 2 on annual to millenial timescales. What are the relative contributions of physical and biological processes to the storage of carbon in the ocean (Figs. 3 and 4)? How might ocean processes have contributed to the variations in pco 2 on glacial-interglacial timescales (Fig. 1)? How does the balance of physical and biological forcing regulate the pattern of air-sea exchange of CO 2 around the globe (Fig. 2)? 2.1 The Ocean s Carbon Pumps What is the balance of processes that regulates the oceans distribution and storage of carbon? The contributions of various physical, chemical and biological influences may be quantified using 2

3 Figure 4: Observed distribution of dissolved inorganic carbon, DIC, (µmol kg 1 ) on potential density surfaces (a) σθ = 26.5, (b) σθ = 27.0, (c) σθ = 27.5 ocean observations in the "carbon pumps" framework which we outline in the following sections. Developed with a focus on quantifying the uptake of anthropogenic contribution by the oceans (Brewer, 1978; Chen and Millero, 1979; Gruber et al., 1996) this framework is more broadly applicable. 2.2 Surface Ocean. In the mixed-layer of the ocean, where there may be active exchange of dissolved gases with the atmosphere, we may the total, dissolved inorganic carbon, C, as the sum of two components: Csat, the concentration the water parcel would have were it at equilibrium with the overlying pco2at at current temperature, salinity and alkalinity, and C, the disequilibrium contribution or the difference between actual and saturated concentrations. C = Csat + C (1) 2.3 Subsurface Ocean. As a biologically active tracers dissolved inorganic carbon in the oceans interior (see Box??) the dissolved inorganic carbon concentration, C, may be defined as the sum of two contributions: pre-formed carbon Cpre, the concentration that the water had at the time of subduction, and regenerated carbon, Cbio, the additional carbon accumulated since subduction due to the 3

4 remineralization of sinking particles or dissolved organic matter in the water parcel. C = C pre + C bio (2) Since the preformed component represents the concentration at the time of subduction we redefine C pre following (1): C pre = C sat + C. (3) Since pco2 at has been changing rapidly over the past century or so, due to anthropogenic emissions (MICK-REF-ATMOSpCO2-HISTORY-FIGURE) water parcels which have been in the surface mixed layer during that time have experienced significantly different atmospheric partial pressures, pco2 at. When considering the present day distribution of carbon, we describe this as a timevarying component of the saturation carbon concentration, Csat ant (t), relative to a fixed, pre-industrial reference value, C pre sat, which is defined for pcoat,pre 2 = 278 ppmv. C sat = C pre sat + Cant sat (4) C bio represents the accumulation of dissolved inorganic carbon in the water parcel since the time of subduction with contributions from the remineralization of particulate or dissolved organic matter, C soft (the soft tissue pump) and the dissolution of calcium carbonate, C carb (the carbonate pump). C bio = C soft + C carb (5) A comprehensive definition of the contributions to dissolved inorganic carbon concentrations in the deep ocean may thus be written: C = C pre sat + Cant sat + C + C soft + C carb. (6) This is a statement of conservation of carbon in a water parcel and does not involve any approximation. If the water parcel is a mixture each component represents the linear combination of concentrations from the various source waters. With some assumptions and approximations most of these components may be quantified using appropriate hydrographic observations. Evaluating the Saturation Carbon Concentration C sat. For a given pco2 at we may use our knowledge of the carbonate system and the T,S dependence of the thermodynamic equilibrium coefficients to evaluate C sat (Section BASIC-CARBONATE- CHEM). There are two significant sources of uncertainty: Firstly, the assumption of fixed stoichiometry in all transformations to and from organic matter. Secondly, an observation in the ocean samples a water parcel which is a mixture from several sources. If the saturation concentration of a gas, O 2,sat, is not a strictly linear function of temperature and salinity (Fig. CsatvsT) an error is introduced by determining O 2,sat from local T and S. 1 Evaluating the Soft Tissue Pump, C soft. If we may assume that transitions of carbon and phosphorus to and from the organic form occur 1 Over the observed oceanic range, C sat is linear with T to quite a good approximation (Fig. CsatvsT). A rough estimate suggests this assumption might lead to an error of XXXX. 4

5 in fixed ratios, R CP, R OP, then we may relate the soft tissue component of carbon, C soft, to regenerated phosphate, PO 4,bio, and, (following??), apparent oxygen utilization: C soft = R CP PO 4,bio R CP R PO (O 2 O 2,sat (T,S)) (7) There are three sources of uncertainty: Firstly, the assumption of fixed stoichiometry in all transformations to and from organic matter. Secondly, an observation in the ocean samples a water parcel which is a mixture from several sources. If the saturation concentration of a gas, O 2,sat, is not a strictly linear function of temperature and salinity (Fig. XXXX for oxygen; Fig. XXXX for dissolved inorganic carbon) an error is introduced by determining O 2,sat from local T and S. Thirdly, and most significant, we assume that water parcels are at equilibrium with the atmospheric partial pressure of oxygen at the time of subduction (see Section??). Observational evidence (MICK- CITE-KORTZINGER) and numerical models indicate that oxygen concentrations do significantly deviate from equilibrium at the time of formation of oceanic deep waters indicating an uncertainty in the estimation of C soft of up to 40 µmol kg 1 in the ocean s deep and bottom waters. Evaluating the Carbonate Pump, C carb. Following Box??, since we may consider alkalinity as a biologically active tracer A T = A T,pre + A T,bio. (8) The change in A T since the time of subduction may be related to the total biological contributions from the dissolution of calcium carbonate, δ[ca 2+ ], and the regeneration of nitrate from organic matter, δ[no 3 ]: A T,bio = A T A T,pre = 2δ[Ca 2+ ] δ[no 3 ]. (9) Since each mole of calcium released from calcium carbonate is associated with a mole of carbon into the dissolved inorganic pool, Ca 2+ + [CO 2 3 ] CaCO 3, (10) we may estimate the cumulative effect of the dissolution of calcium carbonate since subduction from A T,bio corrected for the accumulation of regenerated nitrate (Brewer, 1978); C carb = A T,bio = ((A T A T,pre ) + δ[no3 ])/2. (11) Assuming fixed stoichiometry (R NP = 16, Redfield et al.., MICK-REF), δ[no 3 ])/2 = R NPPO 4,bio and can be estimated from observations of oxygen, temperature and salinity (see Section XXX). Since A T is closely correlated with salinity in the surface ocean because of the significant control by freshwater fluxes (MICK-FIGURE SOMEWHERE?), A T,pre may be estimated using a regression of A T and S and other parameters. Hence, with some assumptions, it is possible to estimate the carbonate pump contribution, C carb from local observations of T, S, A T and O 2. (MICK - ERRORS?) Residual: Disequilibrium and Anthropogenic Carbon. The sub-surface dissolved inorganic carbon concentration may thus be defined as the sum of four contributions C = C pre sat + C soft + C carb + C res (12) 5

6 where C res = C ant sat + C (13) The first three components (C pre sat, C soft and C carb ) may be evaluated using observed data as described above. The remaining contributions to deep ocean carbon, the disequilibrium carbon concentration, C, and the increase in saturation carbon concentration relative to the pre-industrial, Csat ant, are more difficult to evaluate. Here we consider the sum of these contributions,c res, which we term the residual carbon concentration since it can be quantified by subtracting the evaluated terms from C. We note that C res also includes a significant contribution from the cumulated errors in the estimation each of the other components. 2.4 What are the Contributions of the Ocean s Carbon Pumps in the Major Water Masses? Sections through the Atlantic (Fig. 5) and Pacific basins (Fig. 6) along with isopycnal maps in the thermocline (σ theta = 26.5; Fig. 7) reveal the relative contributions of the carbon pumps in the major water masses. The high background concentration throughout the ocean largely reflects the pre-industrial saturation concentration which is principally set by the alkalinity (abundance of salts), ocean temperature and the pre-industrial pco2 at (see Chapter XXXX - BASIC CARBON). Since the ocean s isopycnal structure is, to a large extent, determined by temperature, the distribution of C pre sat largely follows the isopyncnals. C pre sat is uniform along isopyncals though the warmer, lighter, thermocline waters have smaller C pre sat than the deep waters. Over most of the ocean the concentration of C exceeds C pre sat due to the biological pumps of carbon. The soft tissue pump C soft is the next major component highlighting the older, deep water masses of the Pacific which have accumulated regenerated carbon. In the North Pacific C soft can exceed 200µmol kg 1 )with a maximum contribution at around 1200m. This maximum reflects the accumulation of regenerated carbon in upwelling waters and the reduced sinking particle flux with depth (see Chapter XXXX - BASIC BIO). The circumpolar deep waters have elevated concentrations due to local regeneration and the influence of the deep Pacific waters. In the Atlantic basin, young North Atlantic deep waters have a much smaller minor contribution from C soft, while the Antarctic Intermediate waters carry a signature from the Circumpolar waters northwards which becomes enhanced in the tropics where upwelling in the Eastern basin drives productivity and focuses nutrients and C soft onto the isopycnal surfaces in the thermocline (see Chapter XXXX - ATLANTIC NUTRIENT PATHS). This is also evident in the Pacific basin on lighter isopycnals (Fig. 7) C carb measures the direct effect on ocean carbon storage due to the carbonate pump (i.e. not accounting for the impact on surface alkalinity which is convoluted into C pre sat ). The carbonate pump contribution in the Atlantic basin is small (on the order of the error, hence some negative regions) since the saturation horizon is deep (Fig. 5: MICK-PLOT SAT HORIZON ON FIGS) and most calcium carbonate exported from the euphotic zone is buried (see Chapter XXXX CARBONATE SECTION). In contrast the saturation horizon is much shallower in the Pacific basin (Fig. 6) and dissolution occurs. The greatest accumulation of C carb also occurs in the older waters of the deep 6

7 North Pacific, though significantly deeper than the contribution from the soft tissue pump. The remaining contribution, C res, is on the order of a few tens of µmol kg 1 ) and is negative throughout the deep and bottom waters with a stronger signature in the Atlantic basin. In the deep waters, which are uncontaminated by anthropogenic CO 2, C res should reflect C, the surface disequilibrium at the time when the waters were subducted from the mixed-layer. We must view any interpretation of the deep C res (or C) with caution since the magnitude is on the same order as the uncertainties in the analysis. However, we may speculate that the North Atlantic deep waters may be expected to have a negative C since they are formed in a region which is strongly cooling, taking up atmospheric carbon and probably far from equilibrium with the atmosphere. In contrast, the deep and bottom waters of the Pacific have a significant contribution from Southern Hemisphere source waters. The surface waters of the high latitude Southern Ocean have are significantly affected by the upwelling of circumpolar deep waters, rich in regenerated carbon (high C soft ). This increases C in the surface, compensating for the decrease associated with cooling (see Section XXXX - MICK SURFACE C PUMPS SECTION - SCHEMATIC). This suggests that deep waters of Southern origin might have a positive, or less negative, C. C res increases, with positive values in the thermocline and younger water masses reaching 100 driving an additional uptake of carbon by the ocean. Despite the significant uncertainties, the framework laid out here has been extended, and is most often applied, in efforts to separate out the contribution of anthropogenic carbon (Brewer, 1978; Chen and Millero, 1979; Gruber et al., 1996). The ventilation of anthropogenic carbon by the gyre circulations is clear in the upper thermocline (Fig. 7). C res is positive over most of the the Atlantic basin on this isopycnal. The same surface in the Pacific shows a significant contribution in the subtropical gyres but also a clear signature of the upwelling of deep waters little affected by anthropogenic carbon in the tropics. µmol kg 1 ) or so, due to the increasing atmospheric pco at Surface Processes: What Regulates C res? While C and C ant sat are problematic to evaluate in the thermocline and deep ocean due to large uncertainties in the estimated regenerated components, they are set at the surface and are components of the preformed carbon, C pre = C. C = C pre sat + Cant sat + C = C pre sat + C res (14) Hence these terms are more readily evaluated at the surface and we may best understand what controls them in the interior by considering what regulates them in regions of subduction and ventilation. 2.6 Background reading Carbon pump diagnostics: Brewer (1978), Chen and Millero (1979), Gruber et al. (1996), Sabine et al Anthro CO2 Nutrient Utilization: Sigman and Boyle, Ito and Follows Glossary 7

8 Figure 5: Meridional diagnostics of Atlantic carbon pumps: (a) Observed dissolved inorganic carbon, C(µmol kg 1 ); (b) saturation carbon concentration, C pre sat (µmol kg 1 ), with respect to the pre-industrial atmosphere (pco 2 = 278ppmv); (c) soft tissue pump of carbon, C soft (µmol kg 1 ); (d) carbonate pump, C carb (µmol kg 1 ); (e) residual carbon concentration, C res (µmol kg 1 ) which includes contributions from anthropogenic carbon, the disequilibrium component of preformed carbon, and errors introduced through approximations made in the estimation of other terms. 8

9 Figure 6: Meridional diagnostics of Pacific carbon pumps: (a) Observed dissolved inorganic carbon, C(µmol kg 1 ); (b) saturation carbon concentration, C pre sat (µmol kg 1 ), with respect to the pre-industrial atmosphere (pco 2 = 278ppmv); (c) soft tissue pump of carbon, C soft (µmol kg 1 ); (d) carbonate pump, C carb (µmol kg 1 ); (e) residual carbon concentration, C res (µmol kg 1 ) which includes contributions from anthropogenic carbon, the disequilibrium component of preformed carbon, and errors introduced through approximations made in the estimation of other terms. 9

10 Figure 7: Carbon pumps, diagnosed from observations, in the main thermocline on potential density surface σ θ = 26.5 (a) Observed dissolved inorganic carbon, C(µmol kg 1 ); (b) saturation carbon concentration, C pre sat (µmol kg 1 ), with respect to the pre-industrial atmosphere (pco 2 = 278ppmv); (c) soft tissue pump of carbon, C soft (µmol kg 1 ); (d) carbonate pump, C carb (µmol kg 1 ); (e) residual carbon concentration, C res (µmol kg 1 ) which includes contributions from anthropogenic carbon, the disequilibrium component of preformed carbon, and errors introduced through approximations made in the estimation of other terms. 10

11 Figure 8: Surface carbon pumps, diagnosed from observations: (a) Observed dissolved inorganic carbon pre (1990s), C(µmol kg 1 ); (b) saturation carbon concentration, Csat (µmol kg 1 ), with respect to the pre1990 industrial atmosphere (pco2 = 278ppmv); (c) saturation carbon concentration, Csat (µmol kg 1 ), with respect to the atmosphere in 1990 (pco2 = 353ppmv); (d) Disequilibrium carbon concentration (1990), C 1990 (µmol kg 1 ): reflects "present day" disequilibrium which drives air-sea flux. (e) Disequilibrium carbon concentration with respect to pre-industrial atmosphere, C pre (µmol kg 1 ): Includes present day disequilibrium and accumulated anthropogenic carbon. 11

THE OCEAN CARBON CYCLE

THE OCEAN CARBON CYCLE 21st February 2018 1 Box-model of the global ocean phosphorus, alkalinity, carbon 2 Pre-industrial model 3 Evolution during the industrial period 4 13 C isotopic evolution BOX-MODEL