Organic carbon and carbonate fluxes: Links to climate change

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Deep-Sea Research II 54 (2007) 437–446 www.elsevier.com/locate/dsr2

Organic carbon and carbonate fluxes: Links to climate change Paul Louberea,, Samantha A. Siedleckib, Louisa I. Bradtmillerc a

Department of Geology and Environmental Geosciences, Davis Hall, Northern Illinois University, DeKalb, IL 60115, USA b Department of Geophysical Sciences, University of Chicago, Chicago, IL, USA c Lamont Doherty Earth Observatory of Columbia University, Palisades, NY, USA Available online 7 March 2007

Abstract This volume is a compendium of articles derived from a Chapman conference entitled ‘‘The Role of Marine Organic Carbon and Carbonate Fluxes in Driving Global Climate Change, Past and Future’’, which was held at Woods Hole Oceanographic Institution in July, 2005. The conference divided the topic into units as follows: concepts and models, production of particulate matter, fluxes through the water column, and sediment record of past fluxes. The volume follows this ‘vertically stratified’ approach, and we use the same units to organize the articles. The Chapman conference on which this volume is based was made possible by support from The American Geophysical Union (Chapman Conference Program), the National Science Foundation, The Ocean and Climate Change Institute at Woods Hole Oceanographic Institution, and the Analytical Center for Climate and Environmental Change at Northern Illinois University. We extend special thanks to Terry Joyce at the Ocean and Climate Change Institute for his administrative help. Also, we particularly appreciated the hard work of Andrew Daly at WHOI and Melissa Ficek at AGU who managed the conference details, making it a pleasant event. The articles in this volume benefited from evaluations given by a dedicated, and most helpful, group of reviewers. It was gratifying to reach out to the community and receive such a valuable contribution of thought and expertise. We gratefully acknowledge our reviewers. Finally, we acknowledge the help and advice of John Milliman, editor for Deep-Sea Research II, who helped us attain the high standards of publication with the journal. r 2007 Elsevier Ltd. All rights reserved.

1. Overview of the topic Since the early 1980s biogenic fluxes of organic matter and carbonate from the surface ocean to the deep sea have been implicated in regulation of atmospheric carbon dioxide content (and Greenhouse warming) (e.g., Knox and McElroy, 1984; Wenk and Siegenthaler, 1985; Dymond and Lyle, 1985; Boyle, 1986, 1988; Keir, 1988; Pedersen et al., Corresponding author.

E-mail addresses: [email protected] (P. Loubere), [email protected] (S.A. Siedlecki), [email protected] (L.I. Bradtmiller). 0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2007.02.001

1991). Considerable effort has been expended to investigate the role of biological production on modern ocean–atmosphere gas exchange, and the role of this production in global climate shifts, such as glacial–interglacial transitions (Sigman et al., 1998; Sigman and Boyle, 2000; Anderson et al., 2002; Moore et al., 2000; Matsumoto et al., 2002). General consensus exists that a key marine biotic climate variable is the ratio of organic carbon to carbonate fluxes from the surface to the deep ocean (Archer and Maier-Reimer, 1994). However, there is disagreement on the potential magnitude of effect that changing this ratio would have on climate, and there is disagreement on the degree to which the

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ratio can change. Further, there are many questions concerning the processes that regulate both fluxes and their ratio. So, we are at a point where, for example, some modeling work indicates that changes in the marine organic carbon to carbonate flux ratio could account for nearly all the substantial change in atmospheric carbon dioxide content observed between the last full glacial and the present (Tre´guer and Pondaven, 2000; Harrison, 2000; Brzezinski et al., 2002; Matsumoto et al., 2002); while other work suggests that the flux ratio is relatively stable and therefore would not account for past natural variations in atmospheric carbon dioxide content (Klaas and Archer, 2002; Francois et al., 2002; Armstrong et al., 2002; Ridgwell, 2003). A key challenge is to understand the mechanisms by which organic carbon and CaCO3 are carried from the euphotic zone to the sea floor as these may have a large-scale impact on the pH of the ocean and hence the pCO2 of the surface ocean. Carbon export from the euphotic zone appears to be dominated by sinking diatoms with their shells of silica (Dugdale et al., 1995). In contrast, analysis of sediment trap data indicates that CaCO3 ballasts a majority of sinking organic matter in the deep ocean, because of its greater density and abundance (Klaas and Archer, 2002). The constancy of the rain ratio in sediment trap data might be explained by the CaCO3-ballast hypothesis, but sedimentary rain ratio indicators (Mekik et al., 2002) show a systematic dependence of rain ratio on sea surface environmental conditions such as productivity or temperature. Another conundrum is that organic carbon fluxes in the top kilometer of the water column exceed the capacity of the observed ballast fluxes to accommodate them (Armstrong et al., 2002). To date, modest attention has been given to the underlying mechanisms governing POC and carbonate remineralization/dissolution in the water column. As these mechanisms ultimately fix the deeper ocean fluxes of both organic carbon and calcium carbonate, a more quantitative treatment may shed light on additional important feedbacks within the global carbon cycle (Barker et al., 2003). The ocean’s biogenic fluxes are also critical in the response of climate to anthropogenic forcing. Future changes in the carbon cycle that may affect air-sea partitioning of CO2 are difficult to quantify, but a look into the past with paleoceanographic studies, combined with modern observations, can provide important evidence of what variations might occur (Barker et al., 2003). This is particularly true where

we face situations unprecedented in the contemporary ocean, such as the coming reduction of pH in the surface ocean which is projected to reach extremely low values in the coming century (Caldeira and Wickett, 2003). Surface ocean acidity can affect the production of CaCO3 (Zondervan et al., 2001), and thereby potentially the export and sedimentation fluxes of organic carbon through the ballasting mechanism. The degree to which this happens, and the response of oceanic biogeochemical cycles, will influence longer term changes in atmospheric pCO2 (e.g. Barker and Elderfield, 2002). 1.1. Theory and modeling of the organic carbon to carbonate flux ratio Our ability to model the oceanic carbon cycle has improved dramatically in the past decade (Archer, 1991; Archer et al., 2000; Ridgwell et al., 2002; Jansen et al., 2002; Zeebe and Westbroek, 2003). General circulation models that incorporate increasingly complex ecosystem models have provided global views of oceanic processes that could significantly impact planetary climate. Model outcomes, however, depend on our ability to parameterize complex, feedback controlled, biogeochemical processes. With respect to the organic carbon to carbonate flux ratio, modeling parameterization has been relatively crude; often depending on simple assumptions concerning the Si/N ratio of surface waters. Modeling of responses of calcification to pH changes is also fairly simple; as is model incorporation of plankton ecologic responses (Jin et al., 2004) and water column transformational processes. The sensitivity of atmospheric carbon dioxide in models to changing the rain ratio varies widely. Estimates range from 25% to 100% of the glacial to interglacial change in carbon dioxide (Ridgwell, 2003; Archer et al., 2000; Heinze et al., 1999; Archer and Maier-Reimer, 1994). Box models are relatively insensitive to changes in the rain ratio, while GCMs, like HAMOCC, are able to use changes in the rain ratio to achieve LGM atmospheric carbon dioxide values. The flux story begins with the export ratio. This is the quantity of organic carbon and carbonate that leaves the upper water column. Sarmiento et al. (2002) found an average molar export ratio of calcium carbonate to organic carbon of 0.0670.03. Biogeochemical models use global nutrient data to simulate production, remineralization of organic matter, dissolution of hard parts and the export

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ratio (PIC:POC ratio of the fluxes out of the euphotic zone). Sarmiento et al. (2002) found maximum export ratios were located in the equatorial regions coinciding with the maximum of calcifying organisms. Net primary production (NPP) is one of the major controls on the export ratio. Another control is the allocation of functional groups: coccolithophores, bacteria, diatoms, and chlorophytes. Regionally, variation in the functional groups of phytoplankton in biogeochemical models is controlled by light, nutrient supply to the surface waters, rates of upwelling, and temperature. Attempts at modeling production by plankton ‘functional’ groups (e.g., coccolithophores) are only just beginning. The first section of this volume presents three articles that show what can be accomplished and what more needs to be done (Gregg and Casey; Balch et al.; Fujii and Chai). Using observations of light and nutrients in the ocean, models do a good job at capturing trends in regional distributions of functional groups. The dominance of siliceous versus calcifying functional groups is particularly important as each ballast organic matter, but affect the rain ratio differently. The ratio of Si:N in seawater varies between the Atlantic and the Pacific/Indian oceans, and with latitude. The differing dissolution depth scale of sinking particles drives the chemical fractionation of silica and nitrate in the ocean; silica sinks to greater depth before it dissolves (Jiang and Chai, 2004). Excess nitrate in the surface waters can be used by other phytoplankton. The nitrogen cycle provides a link between the carbon and silica cycles, but it is unclear what other links, if any, there may be. In this context, Fujii and Chai (2007) show how maximal carbonate production may occur with intermediate concentrations of silicate. They also show the sensitivity of export fluxes for carbon and silicate to subsurface variations driven by Kelvin waves and La Nina conditions in the tropical Pacific. This study’s results would suggest that the flux ratio is a balancing act driven by competition among the phytoplankton groups responding to variations in the biogeochemical background, particularly in the subsurface where carbonate production may be concentrated. 2. Field observations of organic carbon and carbonate production Considerable effort has been directed recently to quantifying carbonate production in the open

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ocean. For carbonate flux to the deeper ocean, the production is dominated by coccolithophores and Foraminifera. Zondervan (2007) reviews calcification and organic carbon production in coccolithophores and its relation to their abundance. The calcification rates will affect rain ratios, which could affect climate; and at the same time the calcification rates and organic carbon production are responsive to climate conditions. Poulton et al. (2007) measure calcification rates directly in the Atlantic Ocean and then compare these to observations in other oceanic areas. They find a strong relationship between photosynthetic and calcification rates. Yet, this relationship is regionally variable, indicating patchiness in coccolithophore distributions and non-uniform cell physiologies. Overall, coccolithophores appear to be responsible for less than 10% of the total inorganic and organic carbon fixation. But, this also is variable with values that may reach 20%. Working in the productive Bay of Bengal, and with sediment trap data, Stoll et al. (2007) find that coccolithophores account for 15–55% of carbonate flux in the upper ocean. In this case, production is dominated by one species adapted to the distinct halocline of the area (high nutrients, low light). The authors also find that the Sr/Ca ratio of coccoliths follows the seasonal production history of the species generating them. The story is not monolithic in that stable isotope data suggest that seasonal production records are species specific and that these are either locally differentiated or vertically stratified in the water column. Field studies on coccolithophore production indicate a complex system in which both organic and carbonate production are responsive to a number of factors. Production of foraminifera is apparently also complicated (see for example Ramaswamy and Gaye, 2006) and needs further examination, especially in regions of elevated fluxes. Schiebel et al. (2007) find that foraminifera contribute between 20% and 90% of the carbonate flux to the deep sea. 2.1. Settling particles: disjunctions among research communities The degree to which the ratio of the organic carbon to carbonate fluxes in the deep sea can change is a matter of controversy. If carbonate flux acts as the primary control on organic carbon flux to the deep sea, then ratio changes are limited. Nevertheless, ballasting of organic carbon fluxes is

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potentially an important feedback on future CO2 levels as these change due to anthropogenic forcing. If calcification rates are sensitive to pH (Bijma et al., 1999; Riebesell et al., 2000; Zondervan et al., 2001; Barker and Elderfield, 2002), then ballasting could be reduced as pH of surface waters decreases and this would alter organic carbon transfer to the deep sea (reducing the strength of the marine biotic organic carbon pump). This would serve as a positive feedback on rising CO2 levels (Barker et al., 2003). Just how tightly coupled the rain ratio of organic carbon and carbonate is to ballast materials is an important question that remains unresolved. Views of the organic carbon to carbonate flux ratio vary, depending on the research community. These are actually stratified by water depth. Those examining surface waters and plankton communities have proposed mechanisms by which significant changes of the flux ratio, and important changes in the marine carbon cycle, could be effected (Dugdale et al., 2002; Harrison, 2000; Brzezinski et al., 2002; Matsumoto et al., 2002; Passow, 2002). Marine biological research shows the existence of different ecosystems which have varying recycling efficiencies and flux generating potential (Wassmann, 1998; Kemp et al., 2000, 2006). This appears to be related to a variety of factors including regional circulation, source waters, upwelling, availability of trace nutrients and seasonal stability of production (Wassmann, 1998; Peinert et al., 1989). In many cases, diatoms are key to variations in the production ratio both in terms of generating organic matter, and in causing that organic matter to settle in the water column (Kemp et al., 2000). Also, diatoms generate noncalcareous ballast, but this, and its properties, depend on silicate availability which is linked to the larger oceanic silicate cycle (Ragueneau et al., 2000). The quite variable organic carbon and carbonate production examined by biologists feeds into the ocean’s larger carbon cycle. Feely et al. (2004) constructed a budget for the modern ocean carbonate budget based on high-resolution water column and sediment trap data. Rivers were found to bring in only 0.2 Gt CaCO3–C yr 1. Calcium carbonate formation in surface waters contributes 0.8–1.4 Gt CaCO3–C yr 1. Burial of carbonate sediments occurs in both the shallow and deep waters. Shallow burial is about 0.2 Gt CaCO3–C yr 1 while deep burial is about half that at 0.1 Gt CaCO3–C yr 1.

Only 0.4 Gt CaCO3–C yr 1 of the calcium carbonate that was formed at the surface makes it below 2000 m. Thus about 60% of the carbonate formed in the surface, or 0.5 Gt CaCO3–C yr 1, remineralizes in the upper 2000 m. Because Feely et al. (2004) used both sediment trap data and water-mass tracer results, they were able to differentiate between dissolution in the water column and dissolution fluxes from sediments. Tropical and subtropical regions sediment trap data had low particle dissolution rates, implying a greater role for sedimentary dissolution in those areas (Feely et al., 2004). Dissolution in the high latitudes, specifically the subarctic Pacific, was primarily due to carbonate particle dissolution with 20–80% of the total dissolution accounted for by the water mass approach. A source of alkalinity from the continental margins of 40 meq m 2 day 1 would be required to dissolve carbonates in the upper ocean (Berelson, pers. comm., 2005). For the open central Atlantic, Conte et al. (2001) present a long-term view of fluxes changing with depth. They observed a progressive decrease in heterogeneity of the bulk composition of the sinking flux. Destruction and reformation of aggregates in the mid-water column is a possible mechanism that could homogenize the sinking flux. Zooplankton and other grazers could be responsible for the destruction of aggregates, producing a standing stock of suspended particles. The reformation of aggregates would be influenced by the standing stock of suspended particles available for scavenging. The standing stock of particles also could be a potential source of alkalinity and the acidic guts of the grazers are a location for carbonate dissolution. This implies that ecosystem dynamics in the midwater column play a role in the dissolution of carbonates at depths shallower than the lysocline. Overlying these general observations are episodic pulsed flux ‘‘events’’ that deliver labile organic material and nutrients to the deep ocean (Conte, pers. comm., 2005). These ‘‘events’’ appear to be linked to upper-ocean forcing on, at most, monthly timescales. A sizeable fraction of the total flux is delivered by these ‘‘events’’. They may also control the relative magnitudes of carbonate and organic carbon fluxes to depth over annual time scales. The dissolution and re-packaging processes in the mid-water column filter the production of upper waters into fluxes at greater depth. Those studying sediment traps, especially deeper traps, find that flux ratios, once past the mid-water filter, are more

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constant (Armstrong et al., 2002; Klaas and Archer, 2002; Francois et al., 2002). The dominant ballasting agent and the amount of organic carbon it is carrying are major controls over the remineralization depth. The depth at which organic carbon remineralizes determines to what degree the carbon dioxide returned to the water column has the ability to communicate with the atmosphere. In the modern ocean, most is remineralized between 500 and 1000 m. Shifting the remineralization to greater depth could decrease atmospheric carbon dioxide concentrations. This can be achieved by changing the dissolution or production rate of the hard parts, altering the initial composition of the aggregates, shifting the size spectrum of the aggregates, or changing the temperature. The majority of the global organic carbon flux that reaches the sea floor is thought to be ballasted by carbonate, and the amount of carbonate available to ballast organic carbon is dependent on rates of calcification and dissolution. The carbonate ballast is composed primarily of coccoliths and foraminifera. The link between coccoliths and organic carbon flux seems clearest. For example, Ziveri et al. (2007) used sediment trap data to examine the relationship between coccolith carbonate and organic carbon fluxes. This was done at a number of moorings across the global ocean where data on organic carbon, coccoliths and sediment fine fraction were available. They found that even if quantification of coccolith carbonate fluxes can be improved, generally the flux was a function of coccolith abundance and species present. Species can have significantly different coccolith masses. An illustration of this point is that in spite of having equivalent overall production, the western equatorial Pacific appears to have a coccolithophore carbonate flux which is half that of the North Atlantic. The difference is due to the species present in each basin. Highest fluxes occur in the carbonatedominated N. Atlantic thanks to ‘heavier’ taxa such as Calcidiscus leptoporus. For this study, the coccolith carbonate to organic carbon flux ratio appears fairly uniform. Foraminifera are perhaps a key unknown in the problem of varying rain ratio in the oceans since they provide a carbonate flux that is not directly tied to organic carbon flux. The foraminifera have their own flux story because they have an unusual means of reproduction. Adult foraminifera subdivide their protoplasm into tens of thousands of gametes that swim off, abandoning the shell that then settles

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rapidly to the ocean floor. Thus, we have carbonate ‘bombs’ delivered to the depths containing relatively little organic carbon. The gametes develop into little foraminifera, most of which die before maturity with their shells sinking slowly as debris, or being incorporated in some way with other fine material. Much of this gets dissolved in the upper 700 m of the water column. Large foraminiferal shells could dominate the carbonate flux to the deep sea but might have nothing to do with the organic carbon flux. In this way, the two fluxes may become decoupled. The likelihood and importance of such decoupling is presently unknown, although some data indicates that the decoupling can occur (Ramaswamy and Gaye, 2006). If so, then there may be processes which we have not fully explored that can cause the deeper ocean rain ratio to vary more than open ocean, lower productivity, sediment traps might indicate. The carbonate ballasting process clearly depends not only on supply of particles, but also on dissolution. The latter is difficult to model because mid-water column processes are not yet well understood. Contrary to thermodynamic predictions, dissolution of carbonates occurs in supersaturated waters above the saturation horizon. Several explanations have surfaced involving mid-water column processes. One such process is grazing by zooplankton of carbonate producing organisms, which would result in the dissolution of carbonate in their acidic guts. Another is that dissolution occurs within aggregates in micro-environments resulting from bacterial action. Until the relative importance of these processes is assessed, we will not be able to effectively model dissolution of carbonates. Schiebel et al. (2007) provide field evidence on dissolution for the foraminiferal component of the carbonate flux to deeper water. They examine the weight to size ratio of settling foraminifera and compare this to the carbonate ion concentration of the ambient water. Overall, they find about a 20% decrease in weight in the 100–1000 m depth interval. This corresponds to the zone where carbonate ion concentration decreases most rapidly, presumably driving dissolution. From 1000 to 2500 m there is no weight loss and further dissolution must occur at the seabed. A final issue in the constancy of the rain ratio to the deep sea is the nature of ecosystems and their flux export (Wassman, 1998). Lam and Bishop (2007) compare size-fractionated suspended

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particulate matter in the subantarctic and the Antarctic within the twilight zone (100–1000 m). The latter is significantly more productive than the former, but shows similar particle fluxes in deeper water. This does not appear to be related to differences in water column viscosity or ballasting mechanisms between the two regions. Rather, recycling of particulate organic carbon seems especially efficient in the upper water column of the Antarctic. The addition of iron to the subantarctic water column results in transition to Antarctic like conditions (HBLE). The question now is how stable the high biomass low export regime in the Southern Ocean can be, and what would stimulate high export events. Related to the ecosystem issue is that of the exact nature of the ballasting mechanism. Passow and De La Rocha (2006) examine the role of the biological pump in export production. They observe that variable biogenic silica to POC production across ocean basins leads to lower sediment trap correlation between these forms of particulate matter. By contrast, higher concentrations of suspended coccoliths lead to a higher correlation between carbonate and organic carbon fluxes. Foraminiferal and POC fluxes are less well related. The authors propose that organic matter scavenges available mineral matter, which leads to a reduction in porosity and a marked increase in settling velocities. If so, then ballasting by carbonate is a function of its availability, not a special ballasting quality that it might have. The importance of carbonate ballasting in much of the open ocean seems clear. The question now is whether this also holds for the more dynamic, variable and productive regions of the oceans where upwelling and cycling of carbon dioxide are prominent. It has generally been accepted that higher flux ratios occur in the Southern Ocean in and south of the Polar Front region, where diatom production dominates. But a certain amount of work with sediments in tropical upwelling settings indicates that higher flux ratios can occur in these areas as well (Mekik et al., 2002). Also, sediment trap work in the Arabian Sea indicates that the rain ratio to deeper waters below productive areas can be elevated, and that organic carbon and carbonate fluxes need not be closely tied to one another (Ramaswamy and Gaye, 2006). Determining the possibilities for rain ratio change requires that we be able to confidently model the processes that ultimately control the organic carbon and carbonate fluxes from the surface ocean to the

deep sea. A key to doing this will be a better understanding of flux-altering processes in the water column. For instance, many diatoms produce socalled transparent exopolymer particles (TEP) during the bloom and the crash of the bloom. TEP have high C to N and P ratios and play an important role in aggregate formation because of their stickiness (Passow, 2002). Hence, TEP may play an important role in controlling the rain ratio (Passow and De La Rocha, 2006). Other environmental conditions may also effect TEP production. For instance, at higher pCO2, the production of TEP is increased (Engel, 2002). Clearly, the impact of TEP on the rain ratio needs our attention. 2.2. Reconstruction and prediction of flux ratio changes and relation to climate Our ultimate goal is a model that is capable of predicting flux ratio changes and the climate effects of these. To do this we need not only model improvements based on our latest understanding of biogeochemical processes, but we also need scenarios against which model performance can be tested. For tests based on past climates, we need methods for reconstructing biogenic fluxes in which we can have confidence. The ability to reconstruct changes in burial fluxes of important carbon species (both organic and inorganic) and the ratio between these species is fundamental to our understanding of the role of the oceans in climate change. The work presented in this section of the conference falls broadly into three categories: modeling of the impact of rain ratio changes on sediment properties, estimates of changes in the rain rate ratio through time, and the establishment of proxies using modern environments. While much of the research presented was consistent, there were also some fundamental discrepancies, indicating that the broader area of temporal variations in fluxes and ratios merits continued study. Those conference presentations concerning proxy development were a good indicator that further work in this area is needed to better understand proxies as they are applied to the paleo record. Modeling can be used to test climate scenarios with predictions of sediment impacts that these should produce. An example of this is the article by Munhoven (2007), which presents a challenge to how rain ratio changes might have occurred on the glacial timescale, and implies a more complex picture than simple whole ocean alterations. This

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kind of modeling can help point to the combination of factors required to account for carbon system changes that have occurred in concert with climate change. At a more extreme end, models of events like the Paleocene–Eocene Thermal Maximum (PETM) can help elucidate the meaning of sediment records for which we have no modern analog. Modeling within the framework of modern processes can also help reveal flux ratio changes which are preserved in recent sediments. An example of this is shown in Mekik et al. (2007) who present a novel method for estimating carbonate flux to the seabed, and use this in conjunction with satellite derived organic carbon production to evaluate potential changes in the rain ratio for the equatorial Pacific, a region with a strong productivity gradient. They find similar results whether ballasting is incorporated or not in the calculation of organic carbon fluxes. The proxies, and the pattern of carbonate preservation at the seabed indicate that the rain ratio must change significantly across the eastern equatorial Pacific. This is evidence from the seabed that the deep ocean rain ratio can be variable, and is probably only partially buffered by the ballasting mechanism. Proxies for individual biogenic fluxes can be based on either relative abundance methods (Loubere and Fariduddin, 1999) or on accumulation rates of sediment components. An example of the latter is in Paytan and Griffith (2007) who review estimation of changing organic carbon export using bio-barium accumulation rates from deep-sea sediments. They explain the processes behind this tracer and the assumptions involved in its application. A fundamental part of such proxies is the calculation of sediment accumulation rates. This may be done by dividing sediment thickness by time difference between sedimentary horizons, or by normalization to a constant accumulation rate tracer (such as 230Th, see review in Francois et al., 2004). Differences arise between these two approaches, leading to conflicts of interpretation (Lyle et al., 2005; Francois et al., 2007) but are partially resolved by corrections for age dating models (Loubere and Richaud, 2007). Relative abundance proxies offer a means of assessing production changes without depending on accumulation rate estimates. As an example of this in the context of carbon system changes on the glacial–interglacial timescale, Loubere et al. (2007) search for evidence that nutrient abundances and ratios changed in the tropical ocean with the glaciations. If they did, such

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changes could have driven the plankton community variations necessary for alteration of deep ocean rain ratios. They find important changes, and differences among ocean basins. These could have resulted in an average ocean increase in the rain ratio (org C/carbonate). The results indicate that the rain ratio needs to be examined on a regional scale in the context of local planktonic systems. Some work has been done on directly estimating changes in the rain ratio by comparing the organic carbon and calcite paleo-fluxes, or by examining opal accumulation rates, which are taken as correlated to shifts in the rain ratio. In the tropical Pacific, opal analysis by Kienast et al. (2006) and Bradtmiller et al. (2006), and ratio reconstruction by Richaud et al. (2007) indicate significant glacial– interglacial changes. Previous hypotheses suggest that iron fertilization may have allowed Si to leak into the equatorial Pacific during the last glacial maximum (LGM). Since the modern EEP is limited by Fe and Si, this would have allowed diatoms in particular to increase productivity (since Si allows them to compete for other nutrients with carbonate producers), resulting in an opal pulse to the equatorial Pacific during the LGM (expected to be strongest in the EEP). An increase in opal production at the expense of carbonate production would cause a change in the rain ratio, and might account for lower glacial CO2 levels. To test this hypothesis, these authors examined downcore records for an LGM opal peak across the equatorial Pacific. All results were 230Th-normalized, and Bradtmiller et al.’s were also compared to 231Pa/230Th ratios. Nonetheless, there were discrepancies between the separate studies. Neither Bradtmiller et al. nor Kienast et al. found any evidence for increased opal burial during the LGM in any cores, although both datasets contain an opal pulse during the deglaciation. Richaud et al. found increased opal burial during the LGM in cores closest to the Peru upwelling, some of which were geographically quite close to cores used in the former studies. They found a decrease in burial at sites further from the Peru upwelling. The extent to which rain ratio changes have been driven by competition and replacement between carbonate and opal producing plankton in response to biogeochemical changes in the ocean can only be fully evaluated when we can determine how opal fluxes have varied in time. This is difficult because dissolution strongly imprints the record of opal that is preserved in sediments. Warnock et al. (2007)

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examine a method for evaluating opal dissolution and how this has changed in time. Variations in preservation appear to be more linked to ecological and upper water-column processes than to conditions at the seabed. Broader study in settings with measured opal fluxes is needed to relate quantitative measures of dissolution with fraction of opal flux lost from the sediments. 3. Summary The story begins with the production ratio of organic carbon to carbonate in the plankton community. This depends on plankton species ecology interacting with mixed layer and thermocline biogeochemistry. There are strong regional differences in how this works out because of chemical (e.g. trace elements) and physical (e.g., water temperature, circulation, latitude-light availability) factors. There is also feedback since climatic consequences of the production ratio can alter species biochemical actions, as when carbonate producers secrete less in response to pH lowered by additional carbon dioxide in the atmosphere–upper ocean system. All this makes the production ratio strongly variable on the regional scale, and perhaps a complex function of changing climate. We are only beginning to develop the modeling tools to explore the feedbacks. The production ratio is rapidly modified in the upper water column as both organic carbon and carbonate are consumed, but at different rates and by different processes. The fluxes of the two are also decoupled because foraminifera, which are major carbonate producers, deliver shells to the deeper water column relatively free of organic matter due to their reproductive cycle. The organic matter remains in the upper water column system while the shells head rapidly for the deeps. The recycling and packaging mechanisms at work in the twilight zone are complex. They have been studied in only a few locations, and their sensitivity to climate variation is not well understood. It is clear that the twilight zone ‘factory’ is regionally variable however. So, we can observe high biomass low-export systems in the Southern Ocean and relatively low chlorophyll but high sedimentation regimes in the eastern equatorial Pacific. Why we have these differences and what maintains them seems a mystery. At greater depth, the organic carbon to carbonate flux ratio may stabilize and even become invariant

due to ballasting of the organic material by carbonate. This would be especially true if carbonate particles were much superior at weighting settling organic matter. There are two aspects to this issue. These are the efficiency of particle transport to the deep sea that determines the total quantity of matter that escapes the upper ocean, and the flux ratio of that material. If carbonate ballast leads to markedly higher efficiency and even has an affinity for organic matter, then the transport to the deeps will be dominated by oceanic areas with higher carbonate productivity and the flux ratio will remain fixed by the organic matter loading that carbonate can accommodate. If, on the other hand, ballasting is relatively independent of particle composition, and is more a function of what is available in the water column, then the flux ratio becomes a function of whatever controls the particle types in the local setting. This would be especially true if biological processes dictated particle formation and ultimate particle size and density. Which of these scenarios applies for the ocean’s most productive areas is not entirely clear. Most sediment trap work on the ballasting topic has been conducted in lower productivity, open ocean settings. The record we find in the sediments indicates that the flux ratio reaching the deep ocean is variable, and that this variation can be tracked back to the plankton ecosystem. That means a surface to deep ocean connection in fluxes remains, even through all the transformational and ballasting processes of the twilight zone. Variations in the flux ratio are regional, and it remains to be seen how this plays into climate changes on a global scale. References Anderson, R., Chase, Z., Fleisher, M., Sachs, J., 2002. The Southern Ocean’s biological pump during the Last Glacial Maximum. Deep-Sea Research II 49, 1909–1938. Archer, D., 1991. Modeling the calcite lysocline. Journal of Geophysical Research 96, 17037–17050. Archer, D., Maier-Reimer, E., 1994. Effect of deep-sea sedimentary calcite preservation on atmospheric CO2 concentration. Nature 367, 260–263. Archer, D., Winguth, A., Lea, D., Mahowald, N., 2000. What caused the glacial/interglacial atmospheric pCO2 cycles. Review Geophysics 38, 159–189. Armstrong, R.A., Lee, C., Hedges, J.I., Honjo, S., Wakeham, S.G., 2002. A new, mechanistic model for organic carbon fluxes in the ocean: based on the quantitative association of POC with ballast minerals. Deep-Sea Research II 49, 219–236.

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