Biology-mediated temperature control on atmospheric pco 2 and ocean biogeochemistry

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L20605, doi: /2007gl031301, 2007 Biology-mediated temperature control on atmospheric pco 2 and ocean biogeochemistry Katsumi Matsumoto 1 Received 12 July 2007; revised 13 September 2007; accepted 4 October 2007; published 26 October [1] The remarkable correspondence between glacialinterglacial changes in atmospheric CO 2 levels and global climate over much of the Pleistocene suggests that CO 2 is also a key climate change driver. However, there is as yet no widely accepted explanation of the low glacial CO 2 levels. Here I use an intermediate-complexity climate model to show that glacial cooling, acting on the rates of organic carbon production and decay in the ocean, can explain a significant portion of the glacial CO 2 lowering. New model results show that cooling strengthens the vertical transport of organic carbon from the surface ocean to the deep ocean, reduces atmospheric pco 2, and shifts nutrients from the Atlantic basin to the Indo-Pacific basins. The overall vertical transport is increased because the cooling effect on reducing the degradation rate of sinking particulate organic carbon is greater than on reducing the export production. This net temperature effect on atmospheric pco 2 mediated by biology is comparable to the temperature effect on atmospheric pco 2 driven by solubility, which is almost always mentioned as a large factor in the glacial CO 2 levels. An implication for the future is that higher ocean temperatures will act as a positive feedback on atmospheric CO 2 by reducing the vertical transport of carbon to the deep ocean and thereby increasing CO 2 degassing from the ocean. Citation: Matsumoto, K. (2007), Biology-mediated temperature control on atmospheric pco 2 and ocean biogeochemistry, Geophys. Res. Lett., 34, L20605, doi: /2007gl Introduction [2] Particulate organic carbon (POC) sinks from the upper ocean to the deep ocean. This vertical transport of POC called the organic carbon (OC) pump reflects export production, a part of primary production that escapes respiratory consumption in the euphotic zone and is exported to the ocean interior. The pump depletes nutrients and dissolved inorganic carbon (DIC) in surface ocean waters. In the deep ocean, the continual supply of POC from above, combined with global meridional overturning circulation, creates the clear gradients in nutrient and DIC concentrations from the North Atlantic to the North Pacific along the path of the deep water flow. Because of its influence on DIC, the OC pump has been identified as key to controlling atmospheric CO 2 content [Volk and Hoffert, 1985]. Experiments with box models [Joos et al., 1991; Peng and Broecker, 1991] and general circulation 1 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, USA. Copyright 2007 by the American Geophysical Union /07/2007GL031301$05.00 models [Sarmiento and Orr, 1991] show that atmospheric pco 2 is drawn down by tens to over 100 ppm, when the pump is strengthened basin-wide under future CO 2 emissions scenarios. The pump is also called to explain the large and regular variability in atmospheric pco 2 over the last several glacial-interglacial cycles [Siegenthaler et al., 2005]. It is a prime candidate amongst many explanations of this yet unresolved, glacial CO 2 puzzle [Archer et al., 2000]. To trigger an enhanced OC pump, previous studies have called on increases in the availability of macronutrient phosphate [Broecker, 1982] or micronutrient iron [Martin, 1990]. The underlying idea is that these nutrients are limiting production in many parts of the world ocean today [Martin and Fitzwater, 1988; Wu et al., 2000], and excess supply of these would relieve the limitation and thereby boost primary and export production. [3] Here I propose that the strength of the OC pump can be changed instead by invoking the effects that temperature has on OC production and degradation. At the core of this proposal is the universal observation that metabolic rates increase with temperature. In a pioneering work, Eppley [1972] showed that marine phytoplankton growth rates are roughly doubled for a 10 C increase in temperature (i.e., Q 10 = 2.0 relation). In a significant step forward, Laws et al. [2000] showed that temperature is the primary determinant of how much of the primary production goes into export production. This is attributed to temperature-dependent remineralization; POC is more slowly degraded at lower temperatures, thus allowing more POC to be exported. There is a potential that a sizable fraction of the last glacial maximum (LGM) lowering of atmospheric CO 2 can be explained by temperature-driven POC degradation [Matsumoto et al., 2007]. However, there are a number of outstanding, unresolved issues at this time. First, it is unclear how temperature will affect the degradation rate and pool size of the dissolved organic carbon (DOC). Second, the effect of temperature on OC production vis-àvis remineralization in unknown, because they have opposite effects on the OC pump and atmospheric CO 2 (e.g., the same cooling that slows remineralization and strengthens the pump will also slow production and weaken the pump). Third, it is unclear how temperature-driven changes in production and remineralization will affect the global nutrient distribution, which in turn can affect the OC pump. 2. Model [4] In order to address these still unresolved issues, I use GENIE-1, a 3-D, intermediate-complexity, biogeochemical, climate model [Ridgwell et al., 2007]. Briefly, the physical model is based on the fast climate model of Edwards and Marsh [2005], which consists of a 3-D ocean circulation L of5

2 production and remineralization, as well as solubility. Temperatures below freezing are not allowed. The nonlinearity in the model is such that the sum of the three separate responses in terms of atmospheric pco 2 is typically within a few ppm of the model response that includes all three. There is no CO 2 radiative feedback in this study. Figure 1. Transient response of atmospheric pco 2 to temperature forcings of (a) +5 C and (b) 5 C. Forcings are applied one by one to remineralization (solid lines), production (dashed), and solubility (dotted). Horizontal lines represent the control experiment. model, a 2-D energy-moisture balance model of the atmosphere, and a dynamic-thermodynamic sea ice model. They have objectively optimized key physical model parameters using atmospheric and oceanic observations as constraints. Biogeochemical tracers in the ocean include phosphate (PO 4 ), DIC, DOC, alkalinity (ALK), and oxygen. Biogeochemical model is akin to the international OCMIP formulation [Najjar and Orr, 1999], except that PO 4 uptake is prognostic and based on Michaelis-Menton kinetics. Ridgwell et al. [2007] have objectively optimized key bioparameters. For this study, I modified the original formulation of nutrient uptake to include temperature dependence following Maier-Reimer [1993]. Also, I modified the formulation of POC remineralization, based originally on a fixed profile like that of Martin et al. [1989], with an explicit POC sinking velocity (125 m d 1 ) and temperature dependent remineralization according to Yamanaka et al. [2004]. The remineralization temperature dependence is the same for DOC as POC. These dependence are consistent with the rule-of-thumb Q 10 = 2.0 relation, whose variations are commonly used in even more complex ecosystem models [Moore et al., 2002]. [5] In order to simply elucidate the effects of temperature, I present a set of experiments where the effects of whole ocean temperature change are examined separately for 3. Results and Discussion [6] Figure 1 shows the typical transient response of atmospheric pco 2 to ±5 C perturbations. Solubility effect is the fastest and largest. A 5 C warming (Figure 1a) cause an instantaneous reduction in CO 2 solubility and net increase in CO 2 outgassing. Atmospheric pco 2 response by way of OC pump is slower, because a change in the preformed nutrient concentrations is required. The steady state response to a 5 C warming via remineralization is +35 ppm, of which less than 2 ppm is due to DOC and the rest due to POC. The warming effect via production is only 5 ppm, which opposes the other two effects, since warming enhances production and CO 2 drawdown from the atmosphere. The relatively small effect of temperature on production is not surprising, given that production is limited by many other factors, notably nutrients. There is no such limitation on remineralization. [7] The effects in a 5 C cooling case (Figure 1b) are opposite the warming case. The magnitudes are not equal, because the temperature dependence is nonlinear. Also, temperature cannot reach below the seawater freezing point, so a full 5 C cooling is not realized in some parts of the model domain. [8] The steady state responses of atmospheric pco 2 to smaller perturbations all show that the remineralization effect is mostly due to POC and much larger than the opposing production effect (Figure 2). The net biology and solubility effects have the same sense of change and add up. [9] Perhaps a surprising result from these experiments is the systematic, global redistribution of nutrients between the deep Atlantic and the deep Indian and Pacific (Figure 3). When the pump is strengthened, due to either the warming effect on production or the cooling effect on remineralization, more POC reaches deeper waters and thus approaching what Boyle [1988] referred to as a bottom heavy nutrient Figure 2. Steady state atmospheric pco 2 response to temperature forcings after 1000 years of integration. See Figure 1 for legend. 2of5

3 Figure 3. (b) 5 C. Steady state PO 4 concentration anomaly at 3000 m in response to temperature forcings of (a) +5 C and redistribution. The North Atlantic Deep Water will sweep the more deeply remineralized nutrients out of the Atlantic into the Indian and Pacific basins (Figure 3b). [10] The opposite case, where the OC pump is weakened, causes a net nutrient transport to the Atlantic basins (Figure 3a). The ±5 C forcings acting on remineralization change the deep PO 4 concentration by as much as 0.3 mmol kg 1 or roughly 10% of the typical deep values. The same forcings acting on production is an order or magnitude smaller. [11] Finally, model results indicate that export production and the OC pump can be decoupled under certain circumstances. Figure 4 shows that global export production increases with temperature, regardless of whether the temperature forcing is applied to production or remineralization. The former case is expected, since the formulation of production contains direct, positive temperature dependence (dashed line, Figure 4). The latter case illustrates how temperature-dependent remineralization controls nutrient redistribution, which in turn controls export production (solid line, Figure 4). At higher temperatures, production becomes higher, because OC is remineralized and nutrients are recycled in shallower waters; essentially nutrients are being trapped in the upper water column. Mean surface PO 4 3of5

4 in particular on OC pump have been underappreciated. This study indicates that biology-mediated temperature control is significant for the OC pump strength and therefore atmospheric pco 2 and nutrient redistribution, even in the absence of any change in ocean circulation. The importance of temperature-dependent remineralization is also indicated by its ability to decouple export production and the OC pump through nutrient redistribution. [15] Acknowledgments. A. Ridgwell and the GENIE team generously made GENIE available to the larger community. K. Tokos and M. O. Chikamoto helped code and run the model. This research was supported by the Office of Science (BER), U.S. Department of Energy (grant DE-FG02-06ER64216) and the University of Minnesota. Figure 4. Steady state export production response to temperature forcings applied to remineralization (solid lines) and production (dashed). concentration is 0.31 mmol kg 1 higher in the 5 C warming experiment than the control. At lower temperatures, export production becomes smaller, as the deeply sinking POC and increasing DOC pool remove nutrients from the surface. In the 5 C cooling experiment, mean surface PO 4 concentration is reduced by 0.2 mmol kg 1 compared to the control. This experiment clearly illustrates a decoupling between export production, which was reduced by almost 0.8 Pg-C yr 1 (Figure 4), and OC pump, which was increased as evidenced from lower surface PO 4 concentration and lower atmospheric pco 2 (Figure 2). [12] An implication for the glacial CO 2 puzzle is that all else being equal, the glacial ocean would have experienced a smaller export production but a stronger OC pump and lower atmospheric pco 2. The LGM deep ocean temperature was colder than today by perhaps 4 C [Adkins et al., 2002; Stouffer and Manabe, 2003], which in the model draws down 50 ppm in total (Figure 2; 27 ppm and +8 ppm from temperature effects on remineralization and production respectively, 31 ppm from solubility). This is roughly half of the glacial-interglacial CO 2 change. The remineralization effect alone is comparable to the solubility effect, which is almost always mentioned as a very large factor. Also, the predicted increase of d 13 C in the upper Atlantic Ocean due to a 4 C cooling (not shown) is consistent with glacial benthic foraminifera measurements [Duplessy et al., 1988; Matsumoto and Lynch-Stieglitz, 1999]. [13] There are implications for warmer climates as well. A coupled ocean-atmosphere model run to equilibrium indicates a 3 C ocean warming for doubling atmospheric pco 2 [Stouffer and Manabe, 2003], which is well within the range of IPCC predictions in the coming century. Dee sea sediments suggest 8 12 C warmer than present temperatures during the Cretaceous and Eocene [Zachos et al., 2001]. A 3 C warming in GENIE-1 causes changes of +21 ppm by remineralization, 4 ppm by production, and +25 ppm by solubility (Figure 2) for a total of 42 ppm. This would amount to a significant, positive radiative feedback. 4. Conclusions [14] Until now, the OC pump was nearly synonymous with export production. The effects of POC remineralization References Adkins, J. F., K. McIntyre, and D. P. Schrag (2002), The salinity, temperature, and delta O-18 of the glacial deep ocean, Science, 298(5599), Archer, D., A. Winguth, D. Lea, and N. Mahowald (2000), What caused the glacial/interglacial atmospheric pco 2 cycles?, Rev. Geophys., 38(2), Boyle, E. A. (1988), The role of vertical chemical fractionation in controlling late quaternary atmospheric carbon dioxide, J. Geophys. Res., 93(C12), 15,701 15,714. Broecker, W. S. (1982), Ocean chemistry during glacial time, Geochim. 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5 Stouffer, R. J., and S. Manabe (2003), Equilibrium response of thermohaline circulation to large changes in atmospheric CO 2 concentration, Clim. Dyn., 20(7 8), Volk, T., and M. I. Hoffert (1985), Ocean carbon pumps: Analysis of a relative strengths and efficiencies in ocean-driven atmospheric CO 2 changes, in The Carbon Cycle and Atmospheric CO 2 : Natural Variations Archean to Present, Geophys. Monogr. Ser., vol. 32, edited by E. T. Sundquist and W. S. Broecker, pp , AGU, Washington, D. C. Wu, J., W. G. Sunda, E. Boyle, and D. Karl (2000), Phosphate depletion in the western north Atlantic Ocean, Science, 289(5480), Yamanaka, Y., N. Yoshie, M. Fujii, M. N. Aita, and M. J. Kishi (2004), An ecosystem model coupled with nitrogen-silicon-carbon cycles applied to station A7 in the northwestern Pacific, J. Oceanogr., 60(2), Zachos, J. C., M. Pagani, L. C. Sloan, E. Thomas, and K. Billups (2001), Trends, rhythms, and aberrations in global climate 65 Ma to present, Science, 292(5517), K. Matsumoto, Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, USA. (katsumi@umn.edu) 5of5

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