Biogeosciences Discuss., 5, 1655–1687, 2008 Biogeosciences www.biogeosciences-discuss.net/5/1655/2008/ Discussions BGD © Author(s) 2008. This work is distributed under 5, 1655–1687, 2008 the Creative Commons Attribution 3.0 License. Biogeosciences Discussions is the access reviewed discussion forum of Biogeosciences Modeling the marine aragonite cycle Modeling the marine aragonite cycle: R. Gangstø et al. changes under rising carbon dioxide and Title Page its role in shallow water CaCO3 Abstract Introduction dissolution Conclusions References R. Gangstø1,2, M. Gehlen1, B. Schneider1,*, L. Bopp1, O. Aumont3, and F. Joos2,4 Tables Figures 1 LSCE/IPSL, Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS-UVSQ, J I Orme des Merisiers, Bat.ˆ 712, CEA/Saclay, 91198 Gif-sur-Yvette Cedex, France 2 Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstr. 5, 3012 J I Bern, Switzerland 3LOCEAN/IPSL, Centre IRD de Bretagne, BP 70, 29280 Plouzane,´ France Back Close 4 Oeschger Centre for Climate Change Research, University of Bern, Erlachstrasse 9a, 3012 Full Screen / Esc Bern, Switzerland *now at: Inst. of Geosciences, University of Kiel, Luedwig-Meyn-Str. 10, 24098 Kiel, Germany Printer-friendly Version Received: 26 March 2008 – Accepted: 26 March 2008 – Published: 16 April 2008 Interactive Discussion Correspondence to: R. Gangstø ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. 1655 Abstract BGD The marine aragonite cycle has been included in the global biogeochemical model 5, 1655–1687, 2008 PISCES to study the role of aragonite in shallow water CaCO3 dissolution. Aragonite production is parameterized as a function of mesozooplankton biomass and aragonite 5 saturation state of ambient waters. Observation-based estimates of marine carbon- Modeling the marine ate production and dissolution are well reproduced by the model and about 60% of aragonite cycle the combined CaCO3 water column dissolution from aragonite and calcite is simulated above 2000 m. In contrast, a calcite-only version yields a much smaller fraction. This R. Gangstø et al. suggests that the aragonite cycle should be included in models for a realistic represen- 10 tation of CaCO3 dissolution and alkalinity. For the SRES A2 CO2 scenario, production rates of aragonite are projected to notably decrease after 2050. By the end of this Title Page century, global aragonite production is reduced by almost one third and total CaCO3 Abstract Introduction production by 19% relative to pre-industrial. Geographically, the effect from increas- Conclusions References ing atmospheric CO2, and the subsequent reduction in saturation state, is largest in 15 the subpolar and polar areas where the modeled aragonite production is projected to Tables Figures decrease by 65% until 2100. J I 1 Introduction J I The ocean calcium carbonate (CaCO3) budget is a topic of long standing interest in Back Close marine geochemical research (e.g. Berger, 1978; Milliman, 1993; Milliman and Drox- 20 ler, 1996; Milliman et al., 1999; Iglesias-Rodriguez et al., 2002; Berelson et al., 2007). Full Screen / Esc The growing awareness of the impacts of rising atmospheric CO2 concentrations on the chemistry of the ocean and related consequences on marine biota in general and Printer-friendly Version calcifying organisms in particular has recently revived this area of study (e.g. Caldeira and Wickett, 2003; Feely et al., 2004; Orr et al., 2005; Royal Society, 2005, Kleypas Interactive Discussion 25 et al., 2006). In a recent review of the marine CaCO3 cycle, Iglesias-Rodriguez et al. (2002) highlight the lack of fundamental understanding of controls of CaCO3 for- 1656 mation, dissolution and burial. This lack of fundamental understanding hampers our capability for assessing the present state of the carbonate system and forecasting its BGD evolution under increasing atmospheric CO and global warming. 2 5, 1655–1687, 2008 The present study focuses on one striking and yet not well explained feature of past 5 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 Modeling the marine (e.g. Milliman and Droxler, 1996; Berelson et al., 2007). This contradicts the general aragonite cycle paradigm of the conservative nature of pelagic CaCO3 at shallow depths (Sverdrup et al., 1942). The latter is directly derived from the evolution with depth of the satu- R. Gangstø et al. 10 ration product for CaCO3 minerals which increases with decreasing temperature and increasing pressure/depth (Zeebe and Wolf-Gladrow, 2001). As a consequence sur- Title Page face ocean waters are at present oversaturated with respect to CaCO3 (e.g. Orr et al., 2005) precluding dissolution from a thermodynamic point of view. Abstract Introduction Mechanisms proposed to explain the occurrence of CaCO3 dissolution above the Conclusions References 15 saturation horizon invoke biological mediation. It was proposed that CaCO3 particles would dissolve after ingestion by pelagic grazers during gut passage (Harris, 1994). Tables Figures Until recently, dissolution during gut passage had, however, not been unequivocally demonstrated (Honjo and Roman, 1978; Pond et al., 1995). Antia et al. (2008) have J I shown that microzooplankton grazing may explain the reported amplitude of dissolu- 20 tion fluxes, while its exact quantitative contribution to shallow-water dissolution remains J I uncertain. Alternatively, the oxic remineralization of particulate organic C in aggregates Back Close could be at the origin of undersaturated microenvironments triggering the dissolution of CaCO3 particles associated to POC within the same aggregate (Milliman et al., 1999). Full Screen / Esc However, Jansen et al. (2002) concluded from a modeling study that CaCO3 dissolu- 25 tion in marine aggregates in response to CO2 production by POC remineralisation is Printer-friendly Version unlikely to be at the origin of observed shallow-water dissolution fluxes. Interactive Discussion In the absence of a clear identification of processes driving shallow water CaCO3 dissolution, we might address the topic from the side of methods used to infer the order of magnitude of dissolution fluxes. On one hand, we have the evolution of CaCO3 1657 fluxes recorded in sediment traps in comparison to upper ocean CaCO3 production estimates (e.g. Milliman et al., 1999; Feely et al., 2004; Wollast and Chou, 1998). BGD This approach has its own set of caveats (e.g. Milliman et al., 1999; Berelson et al., 5, 1655–1687, 2008 2007) the complete discussion of which is beyond the scope of this paper. On the other ∗ 5 hand, we have tracer based approaches, in particular the alkalinity based approach TA (Feely et al., 2002; Sabine et al., 2002; Chung et al., 2003). Conceptually, it relays on Modeling the marine the breakdown of total alkalinity (TA) into three components: the preformed alkalinity aragonite cycle TA0, the alkalinity of the water parcel as it leaves the surface ocean; the contribution AOU R. Gangstø et al. from organic matter remineralization at depth, TA , and the contribution from CaCO3 ∗ 10 dissolution at depth, TA . Friis et al. (2006) conclude from a model based assessment of the TA∗ method, that this method probably overestimates the magnitude of CaCO 3 Title Page dissolution at shallow water depth because this approach does not take into account the influence of water mass transport. Abstract Introduction In the pelagic realm CaCO3 is produced mainly as two polymorphs of differing sol- Conclusions References 15 ubility: calcite (coccolithophores, foraminifera) and aragonite (mostly pteropods), the latter being 50% more soluble than calcite. The contribution of aragonite to shallow Tables Figures water CaCO3 dissolution is mentioned by several authors (e.g. Milliman et al., 1999; Berelson et al., 2007), but judged to be insignificant on the basis of its supposed only J I minor contribution to total pelagic CaCO3 production (10% according to Fabry, 1990). 20 Estimates of aragonite production and fluxes in the modern ocean are however scarce. J I Moreover these estimates cover a wide range, spreading from 10 to 50% of the total Back Close global CaCO3 flux (Berner, 1977; Berger, 1978; Berner and Honjo, 1981; Fabry and Deuser, 1991). A significant contribution of aragonite to the reported shallow water Full Screen / Esc dissolution can thus not be ruled out a priori. Our study explores the potential con- 25 tribution of aragonite to shallow water CaCO3 dissolution by means of a model study. Printer-friendly Version As a follow-up of Gehlen et al. (2007), we implemented aragonite production and dis- solution to the global biogeochemical ocean model PISCES. In the first section of this Interactive Discussion paper, we present a global open ocean CaCO3 budget for calcite and for aragonite. Model improvements are discussed with reference to the study by Gehlen et al. (2007) 1658 which was also based on PISCES, but did only consider calcite. In the second section, we assess potential changes to the global CaCO3 budget in response to increasing BGD atmospheric CO levels following the IPCC SRES A2 scenario. 2 5, 1655–1687, 2008 2 The PISCES model Modeling the marine aragonite cycle 5 2.1 General model description R. Gangstø et al. The model used in the current study is the PISCES global ocean biogeochemical model (Aumont et al., 2003; Aumont and Bopp, 2006; Gehlen et al., 2006) which simulates the biogeochemical cycle of oxygen, carbon and the main nutrients controlling marine Title Page biological productivity: nitrate, ammonium, phosphate, silicate and iron. Biological 10 productivity is limited by the external availability of nutrients. The phosphorus and Abstract Introduction nitrogen cycles in the model are decoupled by nitrogen fixation and denitrification. Conclusions References The model distinguishes two phytoplankton size classes (nanophytoplankton and diatoms) and two zooplankton size classes (microzooplankton and mesozooplankton). Tables Figures The C/N/P ratios are assumed constant for all species and fixed to the Redfield ratios.