Biogeosciences, 5, 1057–1072, 2008 www.biogeosciences.net/5/1057/2008/ Biogeosciences © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License. Modeling the marine aragonite cycle: changes under rising carbon dioxide and its role in shallow water CaCO3 dissolution R. Gangstø1,2, M. Gehlen1, B. Schneider1,*, L. Bopp1, O. Aumont3, and F. Joos2,4 1LSCE/IPSL, Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS-UVSQ, Orme des Merisiers, Bat.ˆ 712, CEA/Saclay, 91198 Gif-sur-Yvette Cedex, France 2Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstr. 5, 3012 Bern, Switzerland 3LOCEAN/IPSL, Centre IRD de Bretagne, BP 70, 29280 Plouzane,´ France 4Oeschger Centre for Climate Change Research, University of Bern, Erlachstrasse 9a, 3012 Bern, Switzerland *now at: Institute of Geosciences, University of Kiel, Luedwig-Meyn-Str. 10, 24098 Kiel, Germany Received: 26 March 2008 – Published in Biogeosciences Discuss.: 16 April 2008 Revised: 4 July 2008 – Accepted: 7 July 2008 – Published: 28 July 2008 Abstract. The marine aragonite cycle has been included in 1 Introduction the global biogeochemical model PISCES to study the role of aragonite in shallow water CaCO dissolution. Aragonite 3 The ocean calcium carbonate (CaCO3) budget is a topic production is parameterized as a function of mesozooplank- of long standing interest in marine geochemical research ton biomass and aragonite saturation state of ambient waters. (e.g. Berger, 1978; Milliman, 1993; Milliman and Droxler, Observation-based estimates of marine carbonate production 1996; Milliman et al., 1999; Iglesias-Rodriguez et al., 2002; and dissolution are well reproduced by the model and about Berelson et al., 2007). The growing awareness of the impacts 60% of the combined CaCO water column dissolution from 3 of rising atmospheric CO2 concentrations on the chemistry aragonite and calcite is simulated above 2000 m. In contrast, of the ocean and related consequences on marine biota in a calcite-only version yields a much smaller fraction. This general and calcifying organisms in particular has recently suggests that the aragonite cycle should be included in mod- revived this area of study (e.g. Caldeira and Wickett, 2003; els for a realistic representation of CaCO3 dissolution and al- Feely et al., 2004; Orr et al., 2005; Royal Society, 2005, kalinity. For the SRES A2 CO scenario, production rates of 2 Kleypas et al., 2006). In a recent review of the marine CaCO3 aragonite are projected to notably decrease after 2050. By the cycle, Iglesias-Rodriguez et al. (2002) highlight the lack of end of this century, global aragonite production is reduced fundamental understanding of controls of CaCO3 formation, by 29% and total CaCO3 production by 19% relative to pre- dissolution and burial. This lack of fundamental understand- industrial. Geographically, the effect from increasing atmo- ing hampers our capability for assessing the present state of spheric CO2, and the subsequent reduction in saturation state, the carbonate system and forecasting its evolution under in- is largest in the subpolar and polar areas where the modeled creasing atmospheric CO2 and global warming. aragonite production is projected to decrease by 65% until The present study focuses on one striking and yet not well 2100. explained feature of past and recent estimates of the global ocean CaCO3 budget, namely the reported loss (60–80%) of pelagic CaCO3 production in the upper 500 to 1000 m of the water column (e.g. Milliman and Droxler, 1996; Berelson et al., 2007). This contradicts the general paradigm of the con- servative nature of pelagic CaCO3 at shallow depths (Sver- drup et al., 1942). The latter is directly derived from the evo- lution with depth of the saturation product for CaCO3 miner- als which increases with decreasing temperature and increas- ing pressure/depth (Zeebe and Wolf-Gladrow, 2001). As a Correspondence to: R. Gangstø consequence surface ocean waters are at present oversatu- ([email protected]) rated with respect to CaCO3 (e.g. Orr et al., 2005) precluding Published by Copernicus Publications on behalf of the European Geosciences Union. 1058 R. Gangstø et al.: Modeling the marine aragonite cycle dissolution from a thermodynamic point of view. Fabry and Deuser, 1991). A significant contribution of arag- Mechanisms proposed to explain the occurrence of CaCO3 onite to the reported shallow water dissolution can thus not dissolution above the saturation horizon invoke biological be ruled out a priori. Our study explores the potential con- mediation. It was proposed that CaCO3 particles would dis- tribution of aragonite to shallow water CaCO3 dissolution solve after ingestion by pelagic grazers during gut passage by means of a model study. As a follow-up of Gehlen et (Harris, 1994). Until recently, dissolution during gut passage al. (2007), we implemented aragonite production and disso- had, however, not been unequivocally demonstrated (Honjo lution to the global biogeochemical ocean model PISCES. In and Roman, 1978; Pond et al., 1995). Antia et al. (2008) have the first section of this paper, we present a global open ocean shown that microzooplankton grazing may explain the re- CaCO3 budget for calcite and for aragonite. Model improve- ported amplitude of dissolution fluxes, while its exact quanti- ments are discussed with reference to the study by Gehlen tative contribution to shallow-water dissolution remains un- et al. (2007) which was also based on PISCES, but did only certain. Alternatively, the oxic remineralization of particu- consider calcite. In the second section, we assess potential late organic C in aggregates could be at the origin of un- changes to the global CaCO3 budget in response to increas- dersaturated microenvironments triggering the dissolution of ing atmospheric CO2 levels following the IPCC SRES A2 CaCO3 particles associated to POC within the same aggre- scenario. gate (Milliman et al., 1999). However, Jansen et al. (2002) concluded from a modeling study that CaCO3 dissolution in 2 The PISCES model marine aggregates in response to CO2 production by POC remineralisation is unlikely to be at the origin of observed 2.1 General model description shallow-water dissolution fluxes. In the absence of a clear identification of processes driving The model used in the current study is the PISCES global shallow water CaCO3 dissolution, we might address the topic ocean biogeochemical model (Aumont et al., 2003; Aumont from the side of methods used to infer the order of magnitude and Bopp, 2006; Gehlen et al., 2006) which simulates the of dissolution fluxes. On one hand, we have the evolution of biogeochemical cycle of oxygen, carbon and the main nutri- CaCO3 fluxes recorded in sediment traps in comparison to ents controlling marine biological productivity: nitrate, am- upper ocean CaCO3 production estimates (e.g. Milliman et monium, phosphate, silicate and iron. Biological productiv- al., 1999; Feely et al., 2004; Wollast and Chou, 1998). This ity is limited by the external availability of nutrients. The approach has its own set of caveats (e.g. Milliman et al., phosphorus and nitrogen cycles in the model are decoupled 1999; Berelson et al., 2007) the complete discussion of which by nitrogen fixation and denitrification. is beyond the scope of this paper. On the other hand, we have The model distinguishes two phytoplankton size classes tracer based approaches, in particular the alkalinity based ap- ∗ (nanophytoplankton and diatoms) and two zooplankton size proach TA (Feely et al., 2002; Sabine et al., 2002; Chung et classes (microzooplankton and mesozooplankton). The al., 2003). Conceptually, it relays on the breakdown of total C/N/P ratios are assumed constant for all species and fixed to alkalinity (TA) into three components: the preformed alka- the Redfield ratios. For phytoplankton, the prognostic vari- 0 linity TA , the alkalinity of the water parcel as it leaves the ables are total biomass, iron, chlorophyll and silicon con- surface ocean; the contribution from organic matter reminer- tents. The internal ratios of Fe/C, Chl/C and Si/C are pre- AOU alization at depth, TA , and the contribution from CaCO3 dicted by the model. The only prognostic variable for zoo- dissolution at depth, TA*. Friis et al. (2006) conclude from a plankton is total biomass. The bacterial pool is not modeled model based assessment of the TA* method, that this method explicitly. probably overestimates the magnitude of CaCO3 dissolution There are three non-living compartments for organic car- at shallow water depth because this approach does not take bon in PISCES: small particulate organic carbon (POCs), big into account the influence of water mass transport. particulate organic carbon (POCb) and semi-labile dissolved In the pelagic realm CaCO3 is produced mainly as organic carbon (DOC). The C/N/P composition of dissolved two polymorphs of differing solubility: calcite (coccol- and particulate matter is also coupled to Redfield stochiom- ithophores, foraminifera) and aragonite (mostly pteropods), etry. However, the iron, silicon and calcite pools of the par- the latter being 50% more soluble than calcite. The contribu- ticles are fully simulated and their ratios relative to organic tion of aragonite to shallow water CaCO3 dissolution is men- carbon are allowed to vary. The production of calcite is as- tioned by several authors (e.g. Milliman et al., 1999; Berel- signed to nanophytoplankton as a function of temperature, son et al., 2007), but judged to be insignificant on the ba- saturation state, nutrient and light availability. The particu- sis of its supposed only minor contribution to total pelagic late detrital pools (POCs, POCb, biogenic silica and calcite) CaCO3 production (10% according to Fabry, 1990). Esti- are fuelled by mortality, aggregation from nanophytoplank- mates of aragonite production and fluxes in the modern ocean ton and diatoms, fecal pellet production and grazing. are however scarce. Moreover these estimates cover a wide range, spreading from 10 to 50% of the total global CaCO3 flux (Berner, 1977; Berger, 1978; Berner and Honjo, 1981; Biogeosciences, 5, 1057–1072, 2008 www.biogeosciences.net/5/1057/2008/ R.