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6.18 the Biological Pump in the Past D 6.18 The Biological Pump in the Past D. M. Sigman Princeton University, NJ, USA and G. H. Haug Geoforschungszentrum Potsdam, Germany 6.18.1 INTRODUCTION 491 6.18.2 CONCEPTS 496 6.18.2.1 Low- and Mid-latitude Ocean 499 6.18.2.2 High-latitude Ocean 503 6.18.3 TOOLS 509 6.18.3.1 Export Production 509 6.18.3.2 Nutrient Status 511 6.18.3.3 Integrative Constraints on the Biological Pump 512 6.18.3.3.1 Carbon isotope distribution of the ocean and atmosphere 512 6.18.3.3.2 Deep-ocean oxygen content 513 6.18.3.3.3 Phasing 513 6.18.4 OBSERVATIONS 514 6.18.4.1 Low- and Mid-latitude Ocean 514 6.18.4.2 High-latitude Ocean 517 6.18.5 SUMMARY AND CURRENT OPINION 520 ACKNOWLEDGMENTS 522 REFERENCES 522 6.18.1 INTRODUCTION pump is actually more specific: the sequestration It is easy to imagine that the terrestrial of carbon dioxide in the ocean interior by the biosphere sequesters atmospheric carbon dioxide; biogenic flux of organic matter out of surface the form and quantity of the sequestered carbon, waters and into the deep sea prior to decomposi- living or dead organic matter, are striking. In the tion of that organic matter back to carbon dioxide ocean, there are no aggregations of biomass (Volk and Hoffert, 1985)(Figure 1). The biologi- comparable to the forests on land. Yet biological cal pump extracts carbon from the “surface skin” productivity in the ocean plays a central role in the of the ocean that interacts with the atmosphere, sequestration of atmospheric carbon dioxide, presenting a lower partial pressure of carbon typically overshadowing the effects of terrestrial dioxide (CO2) to the atmosphere and thus low- biospheric carbon storage on timescales longer ering its CO2 content. than a few centuries. In an effort to communicate The place of the biological pump in the global the ocean’s role in the regulation of atmospheric carbon cycle is illustrated in Figure 2.The carbon dioxide, marine scientists frequently refer atmosphere exchanges carbon with essentially to the ocean’s biologically driven sequestration of three reservoirs: the ocean, the terrestrial bio- carbon as the “biological pump.” The original and sphere, and the geosphere. The ocean holds ,50 strict definition of the biological (or “soft-tissue”) times as much carbon as does the atmosphere, and 491 492 The Biological Pump in the Past (their photosynthesis less their respiration) and heterotrophic respiration by zooplankton and bacteria that oxidize most of the net primary production back to CO2. This cycle is by far the greatest in terms of the flux of carbon, but the reservoir of sequestered carbon that accumulates in surface waters (phytoplankton biomass, dead organic particles, and dissolved organic carbon) is small relative to the atmospheric reservoir of CO2, and it has a short residence time (less than a year). A small imbalance in this cycle, between net primary production and heterotrophic respiration, feeds the next cycle in the form of organic carbon that sinks (or is mixed) into the deep sea. This “export production” drives the biological pump (Figure 3). Figure 1 A schematic of the ocean’s “biological At the other extreme, a small fraction of the pump,” the sequestration of carbon and nutrients organic matter exported from the surface ocean (nitrogen and phosphorus) in the ocean interior (lower escapes remineralization in the water column and dark blue box) by the growth of phytoplankton (floating sediments and is thus buried in the sediments, unicellular algae) in the sunlit surface ocean (upper light removing carbon from the ocean/atmosphere blue box), the downward rain of organic matter out of system (Figure 3). On the timescale of geologic the surface ocean and into the deep ocean (green processes, this carbon removal is balanced by downward arrow), and the subsequent breakdown of the oxidation of the organic matter when it is this organic matter back to carbon dioxide (CO2), nitrate 2 32 exposed at the earth surface by uplift and weath- ðNO3 Þ, and phosphate ðPO4 Þ. The nutrients and CO2 are reintroduced to the surface ocean by mixing and ering, or when it is released by metamorphism upwelling (the circling arrows at left). The biological and volcanism. While the fluxes involved in this pump lowers the CO2 content of the atmosphere by cycle are small, the reservoir is large, so that its extracting it from the surface ocean (which exchanges importance increases with timescale, becoming CO2 with the atmosphere) and sequestering it in the clearly relevant on the timescale of millions isolated waters of the ocean interior. In most of the low- of years. and mid-latitude ocean, the surface is isolated from the The biological pump, in the strict sense, does deep sea by a temperature-driven density gradient, or not include the burial of organic matter in marine “thermocline,” keeping nutrient supply low and leading 2 sediments. There are several related reasons for to essentially complete consumption of NO3 and 32 this exclusion. First, as described below, the PO4 by phytoplankton at the surface. At the higher latitudes, where there is no permanent thermocline, variations in atmospheric carbon dioxide that are more rapid communication with the deep sea leads to correlated with the waxing and waning of ice ages 2 32 incomplete consumption of NO3 and PO4 . have driven much of the thinking about the role of organic carbon production on atmospheric com- position. Only a very small fraction of the organic ,20 times as much as the terrestrial biosphere. matter exported out of surface waters is preserved Thus, on timescales that are adequately long to and buried in the underlying sediment, so that this allow the deep sea to communicate with the process cannot sequester a significant amount of surface ocean and atmosphere (*500 yr), carbon carbon on the timescale of millennia and thus is storage in the ocean all but sets the concentration not a candidate process for the major carbon of CO2 in the atmosphere. The effect of the dioxide variations over glacial cycles. Second, on biological pump is not permanent on this time- the timescale for which organic matter burial is scale of ocean circulation; the downward transport relevant, i.e., over millions of years, it is thought of carbon is balanced by the net upward flux from to be only one of several mechanisms by which the CO2-rich waters of the deep sea, which, in the atmospheric CO2 is regulated. Most hypotheses absence of the biological pump, would work to regarding the history of CO2 over geological time homogenize the carbon chemistry of the ocean, involve weathering of rocks on land and the raising atmospheric CO2 in the process. precipitation of carbonates in the ocean, due to The dynamics of marine organic carbon can be both the larger fluxes and larger reservoirs described as a set of three nested cycles in which involved in the geological trapping of CO2 as the biological pump is the cycle with flux and carbonate as opposed to organic carbon (Figure 2). reservoir of intermediate magnitude (Figure 3). Finally, the importance of biological productivity The cycle with the shortest timescale, which in determining the burial rate of organic carbon is operates within the surface ocean, is composed not at all clear. While some examples of high of net primary production by phytoplankton organic carbon burial appear to be due to high Introduction 493 Figure 2 A simplified view of the Holocene (pre-industrial) carbon cycle (largely based on Holmen (1992)). Units of carbon are petagrams (Pg, 1015 g). The fluxes are colored according to the residence time of carbon in the reservoirs they involve, from shortest to longest, as follows: red, orange, purple, blue. Exchanges of the atmosphere with the surface ocean and terrestrial biosphere are relatively rapid, such that changes in the fluxes alter atmospheric CO2 on the timescale of years and decades. Exchange between the surface ocean and the deep ocean is such that centuries to millennia are required for a change to yield a new steady state. Because the deep-ocean carbon reservoir is large relative to the surface ocean, terrestrial biosphere, and atmosphere, its interactions with the surface ocean, given thousands of years, determine the total amount of carbon to be partitioned among those three reservoirs. In a lifeless ocean, the only cause for gradients in CO2 between the deep ocean and the surface ocean would be temperature (i.e., the “solubility pump” (Volk and Hoffert, 1985))—deep waters are formed from especially cold surface waters, and CO2 is more soluble in cold water. The biological pump, represented by the right downward arrow, greatly enhances the surface–deep CO2 gradient, through the rain of organic matter (“Corg”) out of the surface ocean. The “carbonate pump,” represented by the left downward arrow, is the downward rain of calcium carbonate microfossils out of the surface ocean. Its effect is to actually raise the pCO2 of the surface ocean; this involves the alkalinity of seawater and is discussed in Chapters 6.04, 6.10, 6.19, and 6.20. Almost all of the organic matter raining out of the surface ocean is degraded back to CO2 and inorganic nutrients as it rains to the seafloor or once it is incorporated into the shallow sediments; only less than 1% (,0.05 Pg out of ,10 Pg) is removed from the ocean/ atmosphere system by burial (the downward blue arrow). This is in contrast to the calcium carbonate rain out of the surface (the downward purple arrow), ,25% of which is buried.
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