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The Sequestration Efficiency of the Biological Pump

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The Sequestration Efficiency of the Biological Pump GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L13601, doi:10.1029/2012GL051963, 2012 The sequestration efficiency of the biological pump Tim DeVries,1 Francois Primeau,2 and Curtis Deutsch1 Received 9 April 2012; revised 29 May 2012; accepted 31 May 2012; published 3 July 2012. [1] The conversion of dissolved nutrients and carbon to shortcoming in our understanding of the global carbon cycle, organic matter by phytoplankton in the surface ocean, and our ability to link changes in ocean productivity and and its downward transport by sinking particles, produces atmospheric CO2. Indeed, it is often noted that global rates of a “biological pump” that reduces the concentration of organic matter export can increase even while the efficiency of atmospheric CO2. Global rates of organic matter export the biological pump decreases [Matsumoto, 2007; Marinov are a poor indicator of biological carbon storage however, et al., 2008a; Kwon et al., 2011]. This ambiguity stems because organic matter gets distributed across water masses from the fact that organic matter settling out of the euphotic with diverse pathways and timescales of return to the sur- zone may be stored for as little as months or as long as a mil- face. Here we show that organic matter export and carbon lennium before returning to the surface, depending on where storage can be related through a sequestration efficiency, the export occurs and the depth at which it is regenerated. which measures how long regenerated nutrients and carbon [4] Here we show that the strength of the biological pump will be stored in the interior ocean before being returned to can be related directly to the rate of organic matter export, the surface. For the first time, we derive global maps of the Fex, by considering the sequestration efficiency E (t|r)of sequestration efficiency of the biological pump at different regenerated nutrients. E (t|r) is equal to the proportion of residence time horizons. These maps reveal how regional nutrients regenerated from organic matter exported out patterns of organic matter export contribute to the biolog- of the euphotic zone below point r that remain sequestered ical pump, and how the biological pump responds to below the surface for a period t or longer (see methods changes in biological productivity driven by climate change. and Figure S1 in the auxiliary material).1 The inventory of Citation: DeVries, T., F. Primeau, and C. Deutsch (2012), The regenerated nutrients can be expressed as a convolution of sequestration efficiency of the biological pump, Geophys. Res. E with Fex, allowing the biological pump efficiency Ebio to Lett., 39, L13601, doi:10.1029/2012GL051963. be expressed as Z Z ∞ E ðÞ¼t 1 da dtEðjt ÞF ðÞ; t À t ; ð Þ 1. Introduction bio I ðÞt r ex r 1 tot a 0 [2] The ocean’s carbon reservoir exceeds chemical equi- librium with the atmosphere because the deep ocean is a where Itot is the ocean’s nutrient inventory, a is the ocean repository for CO2 and nutrients released during the surface area and t is calendar time. Ebio varies between 0 decomposition of organic matter falling from the surface (no biological pump) and 1 (perfectly efficient biological ocean. Diagnostic calculations [e.g., Ito and Follows, 2005] pump). Equation (1) is strictly valid for the case of steady reveal that while deep ocean nutrients are partly derived circulation but can be generalized to the case of non-stationary from regenerated organic matter, a comparable fraction is flow. The advantage of expressing Ebio, a globally integrated transported from surface waters where limitations on metric, using equation (1) is that it provides a direct con- plankton growth prevent complete nutrient consumption. nection to the time-dependent export production on regional These “preformed” nutrients do not contribute to biological scales. carbon storage, implying that the biological pump is not operating at maximum efficiency. Changes in the efficiency 2. Temporal and Spatial Dependence of the biological pump, as measured by the fraction of of the Sequestration Efficiency nutrients in the regenerated pool, therefore have the potential E to alter ocean carbon storage and atmospheric CO over time [5] We estimated using a data-constrained ocean circu- 2 lation and biogeochemistry model (see Appendix A). By [Sigman and Boyle, 2000; Ito and Follows, 2005; Marinov E t t et al., 2008a, 2008b]. construction, = 1 everywhere at =0.As increases, upwelling and eddy-diffusion return nutrients to the surface [3] The fact that no quantitative relationship has been where they are removed from the regenerated pool (see demonstrated between the rate of organic matter export and E → t → ∞ t the efficiency of the biological pump represents a significant Appendix B), so that 0as .At = 1 year, some regions have lost as much as 50% of the initial pool of regenerated nutrients (Figure 1). Such low sequestration 1Department of Atmospheric and Oceanic Sciences, University efficiencies are associated with regions of persistent California, Los Angeles, California, USA. upwelling or rapid vertical exchange, for example, along the 2 Earth System Science Department, University of California, Irvine, northern flank of the Gulf Stream, in the Peru and Benguela California, USA. upwelling systems, and in some parts of the Southern Ocean. Corresponding author: T. DeVries, Department of Atmospheric and By t = 10 years the highest sequestration efficiencies are Oceanic Sciences, University California, Los Angeles, CA 90095, USA. ([email protected]) ©2012. American Geophysical Union. All Rights Reserved. 1Auxiliary materials are available in the HTML. doi:10.1029/ 0094-8276/12/2012GL051963 2012GL051963. L13601 1of5 L13601 DEVRIES ET AL.: BIOLOGICAL PUMP SEQUESTRATION EFFICIENCY L13601 Figure 1. The sequestration efficiency of exported organic matter, EðÞtjr , is shown for (a) t = 1 year, (b) t = 10 years, (c) t = 100 years, and (d) t = 1000 years. Here and in Figure 2 all fields have been smoothed with a 200 km Gaussian averaging filter before plotting, to suppress grid-scale noise and to focus on large-scale patterns. found in subtropical gyre regions where remineralized Atlantic and Southern Ocean. Mean sequestration times gen- nutrients are downwelled by Ekman convergence. At t = erally decrease away from these regions. In the North Atlantic, 100 years, the areas which feed the deep ocean circulation there is a gradual transition from regions of high sequestration increasingly stand out, with E exceeding 50% in areas where efficiency in the polar regions to regions of low sequestration North Atlantic Deep Water and Antarctic Bottom Water are efficiency in the tropics (except near the Gulf Stream region produced. Sequestration efficiencies over most of the rest of where the transition is abrupt) (Figure 2b). By contrast, the the ocean have dropped to around 10–30% after 100 years. transition between regions of high- and low-sequestration By t = 1000 years, only small amounts of regenerated times in the Southern Ocean is well-defined and abrupt. nutrients remain, mainly originating from regions of deep Waters along the Antarctic coast in the Weddell Sea convection in the Labrador Sea and Greenland-Iceland- (Figure 2c), and to a lesser extent the Ross Sea, have very high Norwegian seas in the North Atlantic, and from the Weddell Sea in the Southern Ocean. A small proportion of the regenerated nutrients derived from organic matter exported from these regions is transported into the deep limb of the global overturning circulation, where it is sequestered for more than a thousand years. 3. Mean Sequestration Time and Regional Biological Pump Efficiencies [6] It is widely thought that variations in biological pump efficiency are important drivers of glacial-interglacial var- iations in atmospheric CO2 [Broecker, 1982; Sigenthaler and Wenk, 1984; Sarmiento and Toggweiler, 1984; Knox and McElroy, 1984; Sigman and Boyle, 2000; Sigman et al., 2010]. For persistent quasi-stationary patterns of organic matter export associated with the 100,000 year periods of glacial-interglacial cycles, the export rate Fex(r, t)canbe approximated by its time average, Fex(r), so that equation (1) simplifies to Figure 2. On long timescales, the sequestration efficiency Z of the biological pump can be characterized by the mean E ≈ 1 RðÞF ðÞda; ð Þ bio I r ex r 2 sequestration time of regenerated organic matter. (a) Spa- tot a tial variability in this sequestration time results from the R ∞ global pattern of ocean circulation, with high sequestration where RðÞ¼r EðtjrÞdt is the mean sequestration time of o times in areas where deep and bottom waters form, and regenerated nutrients originating from organic matter sinking RðÞ low sequestration times in areas of persistent upwelling. out of the euphotic zone below r. Figure 2 shows r esti- Subplots (marked by dashed red lines in Figure 2a) high- mated using our data-constrained model, revealing a structure light the divides between regions of high and low sequestra- similar to that shown in the longer timescales of Figure 1. tion efficiency in (b) the North Atlantic and (c) the Weddell Maximum mean sequestration times of 400+ years are found Sea region of the Southern Ocean. Timescale is in years. in the high-latitude deep-water formation regions of the North 2of5 L13601 DEVRIES ET AL.: BIOLOGICAL PUMP SEQUESTRATION EFFICIENCY L13601 [8] The efficiency of the biological pump for a particular oceanic region can be expressed as the ratio of the total amount of regenerated nutrients supplied to the interior ocean by organic matter export within that region, to the sum of the regenerated and preformed nutrients delivered to the interior ocean from that region. This calculation reveals that the biological pump is inefficient at high latitudes, with efficiencies of 0.1–0.4 in polar regions, and that efficiencies increase to near 1 in the tropics (Figure 3b).
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