Biogeosciences, 11, 2295–2308, 2014 Open Access www.biogeosciences.net/11/2295/2014/ doi:10.5194/bg-11-2295-2014 Biogeosciences © Author(s) 2014. CC Attribution 3.0 License. Environmental controls on the Emiliania huxleyi calcite mass M. T. Horigome1, P. Ziveri1,2, M. Grelaud1, K.-H. Baumann3, G. Marino1,*, and P. G. Mortyn1,4 1Institute of Environmental Science and Technology, Universitat Autònoma de Barcelona, Bellaterra 08193, Spain 2Earth & Climate Cluster, Department of Earth Sciences, FALW, Vrije Universiteit Amsterdam, FALW, HV1081 Amsterdam, the Netherlands 3Fachbereich Geowissenschaften, Universität Bremen, Postfach 330440, 28334 Bremen, Germany 4Department of Geography, Universitat Autònoma de Barcelona, Bellaterra 08193, Spain *now at: Research School of Earth Sciences, The Australian National University, Canberra 0200, Australia Correspondence to: P. Ziveri ([email protected]) Received: 28 March 2013 – Published in Biogeosciences Discuss.: 10 June 2013 Revised: 3 March 2014 – Accepted: 5 March 2014 – Published: 24 April 2014 Abstract. Although ocean acidification is expected to im- 1 Introduction pact (bio) calcification by decreasing the seawater carbon- 2− ate ion concentration, [CO3 ], there is evidence of nonuni- form response of marine calcifying plankton to low seawa- Coccolithophores are an abundant marine phytoplankton 2− group that plays a significant role in both the marine food ter [CO3 ]. This raises questions about the role of environ- mental factors other than acidification and about the com- web and the carbon cycle (Young, 1994), comprising an im- plex physiological responses behind calcification. Here we portant sedimentary carbon reservoir (Berger, 1976; Ridg- investigate the synergistic effect of multiple environmental well and Zeebe, 2005). They are responsible for the photo- parameters, including seawater temperature, nutrient (nitrate synthetic fixation of inorganic carbon, regulating the particu- and phosphate) availability, and carbonate chemistry on the late inorganic : organic carbon ratio and a large portion of the coccolith calcite mass of the cosmopolitan coccolithophore calcium carbonate (CaCO3) production (Raven et al., 2005). Emiliania huxleyi, the most abundant species in the world The relative strength of photosynthesis and calcification at ocean. We use a suite of surface (late Holocene) sediment the surface ocean determines the biologically mediated ex- samples from the South Atlantic and southwestern Indian change of carbon dioxide (CO2) between the oceanic and at- Ocean taken from depths lying above the modern lysocline mospheric carbon reservoirs (Sigman et al., 2010), making (with the exception of eight samples that are located at or quantification of these two processes central to our under- below the lysocline). The coccolith calcite mass in our re- standing of the dynamics of the global carbon cycle. The sults presents a latitudinal distribution pattern that mimics the export of carbon and CaCO3 to the seafloor enhances the main oceanographic features, thereby pointing to the poten- ocean’s capability to buffer the rise of atmospheric CO2 con- tial importance of seawater nutrient availability (phosphate centrations (Van Cappellen, 2003; Ploug et al., 2008; Doney et al., 2009). The coccolithophore calcite plates (coccoliths) and nitrate) and carbonate chemistry (pH and pCO2) in de- termining coccolith mass by affecting primary calcification are in fact a major source of calcite to the calcareous deep- and/or the geographic distribution of E. huxleyi morphotypes. sea oozes that cover almost half of the global oceanic floor Our study highlights the importance of evaluating the com- (Berger, 1976; Ridgwell and Zeebe, 2005). Despite the role bined effect of several environmental stressors on calcify- of coccolithophores in the marine carbon cycle, the envi- ing organisms to project their physiological response(s) in a ronmental factors modulating their calcification remain de- high-CO world and improve interpretation of paleorecords. bated. In order to investigate the controlling factors of coc- 2 colithophore calcification, research has centered on their variability in mass and size (Beaufort and Heussner, 1999; Young and Ziveri, 2000) in different types of experimen- tal and field observational settings. Several environmental Published by Copernicus Publications on behalf of the European Geosciences Union. 2296 M. T. Horigome et al.: Environmental controls on the Emiliania huxleyi calcite mass parameters have been examined (Broerse et al., 2000; Beau- 20˚W 10˚W 0˚ 10˚E 20˚E 30˚E 40˚E Depth (m) fort et al., 2008; Henderiks et al., 2012), such as light EQ 50 (Paasche, 2001), nutrient availability (Winter et al., 1994; 100 Båtvik et al., 1997; Paasche, 1998; Müller et al., 2012), cal- cification temperature (Bollmann et al., 2002; Ziveri et al., 10˚S 500 South 2004; Boeckel et al., 2006), salinity (Bollmann and Herrle, Equatorial 1000 Current 2007; Bollmann et al., 2009; Fielding et al., 2009), and car- 20˚S South Atlantic bonate chemistry (Iglesias-Rodriguez et al., 2008; Langer et Gyre al., 2009; de Bodt et al., 2010; Müller et al., 2010; Barcelos e 2000 Agulhas 30˚S Ramos et al., 2010; Beaufort et al., 2011; Bach et al., 2012). Current 3000 leakage Ongoing ocean acidification (due to the oceanic uptake Agulhas Current of the anthropogenic carbon from the atmosphere) is ex- Subtropical Front 4000 40˚S pected to impact marine calcifying organisms, such as coc- Subantarctic Front 5000 colithophores (Van Cappellen, 2003; Feely et al., 2004; Polar Front Delille et al., 2005; Fabry et al., 2008). Increasing partial 50˚S 6000 pressures of CO2 in the ocean (pCO2) leads to a decrease 2− of [CO3 ] and to a decline of the calcite saturation state of seawater (Zeebe and Wolf-Gladrow, 2001; Raven et al., 1.5 2 2.5 3 3.5 4 4.5 5 Mean E. huxleyi coccolith mass (pg) 2005; Fabry et al., 2008), which has been proposed as an im- portant factor in the reduction of coccolith mass (Riebesell Fig. 1. Distribution map of the studied sites (circles). The color of et al., 2000; Delille et al., 2005; Langer et al., 2009; Beau- the symbols (scale at the bottom) refers to the averaged mass of E. fort et al., 2011). However, complementary evidence points huxleyi coccoliths measured at the different sites. The bathymetry to a nonuniform response of calcification to high CO2 (cf. is given by the scale on the right side. The surface hydrography is Langer et al., 2006; Iglesias-Rodriguez et al., 2008; Riebe- depicted by the black arrows and the main fronts by the dotted lines. sell et al., 2008; Doney et al., 2009), casting doubts on the 2− notion that [CO3 ] is the prime (and sole) controlling fac- tor of (bio)calcification. In order to advance our understand- mass of the most common living, blooming coccolithophore ing of the role played by different physicochemical proper- species Emiliania huxleyi. The majority of studies employ ties of seawater on coccolithophore calcification, we exam- culture experiments to test the response on the calcite mass ined a widely distributed suite of surface sediment samples of E. huxleyi to changing environmental parameters (Langer taken along oceanic transects characterized by steep surface et al., 2006; Iglesias-Rodriguez et al., 2008; Riebesell et al., 2008; Bach et al., 2012; Müller et al., 2012). Although sur- ocean environmental gradients, such as the South Atlantic Figure 1 and southwestern Indian oceans, the Agulhas System, and the face sediments may constrain variations in the individual en- subantarctic sector of the Southern Ocean. Most of the sam- vironmental parameters less precisely than culture studies ples were selected from coring sites lying above the depth they allow evaluating the combined effect of a full suite of of the modern lysocline (Boeckel and Baumann, 2008). This environmental property gradients on the (E. huxleyi) coc- reduces (or even precludes) the post-depositional effects (dis- colith mass variations. In addition, surface sediment studies solution) on the coccolith calcite preservation, thereby allow- apply methodological protocols (and assumptions) that are ing recognition of the surface ocean environmental factors identical to down-core studies, thereby providing an ideal influencing the coccolith mass. Significant calcium carbon- format from which to interpret past coccolith mass changes, ate dissolution is expected to begin firstly below 5000 m in e.g., across glacial–interglacial changes in atmospheric CO2 the deep Guinea and Angola basins and below 4400 m in the concentrations (e.g., Monnin et al., 2001; Lüthi et al., 2008) Cape Basin (Volbers and Henrich, 2002), although ultrastruc- and seawater carbonate chemistry (e.g., Hönisch and Hem- tural breakdown of foraminifera shells already begins at shal- ming, 2005; Foster, 2008). lower depths. However, only eight of the studied samples are from a depth close to or slightly below 4400 m and only two 1.1 Oceanographic setting of them are from > 4500 m (Table 1). Therefore, the preser- vation of the selected samples is mostly good and has been The South Atlantic, the Agulhas System, and the Southern documented by scanning electron microscope (SEM) in ear- Ocean are characterized by strong gradients in surface water lier work (e.g., Boeckel et al., 2006: Boeckel and Baumann, properties, such as temperature, salinity, and nutrient con- 2008). centration (Mizuki et al., 1994; Lutjeharms, 2006). This re- Hence, the new surface sediment data set presented here gion is marked by the strongest physicochemical gradients has the potential to elucidate the influence of multiple en- in the entire global ocean, with temperature changes of ap- vironmental parameters at the ocean surface (temperature, proximately 13 ◦C within 12◦ of latitude. The surface circula- 2− salinity, nutrients, pH, [CO3 ], and pCO2) on the coccolith tion is driven by the atmospheric pressure gradients (winds), Biogeosciences, 11, 2295–2308, 2014 www.biogeosciences.net/11/2295/2014/ M. T. Horigome et al.: Environmental controls on the Emiliania huxleyi calcite mass 2297 Table 1. Sample information. The sedimentation rates or the maximum age of samples were extracted from (1) Mollenhauer et al. (2004), (2) Jonkers et al.