Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Biogeosciences Discuss., 8, 5881–5906, 2011 Biogeosciences www.biogeosciences-discuss.net/8/5881/2011/ Discussions BGD doi:10.5194/bgd-8-5881-2011 8, 5881–5906, 2011 © Author(s) 2011. CC Attribution 3.0 License. Biomineralization of This discussion paper is/has been under review for the journal Biogeosciences (BG). dolomite by coralline Please refer to the corresponding final paper in BG if available. algae M. C. Nash et al. Biomineralization of dolomite and Title Page Abstract Introduction magnesite discovered in tropical coralline Conclusions References algae: a biological solution to the Tables Figures geological dolomite problem J I 1 1 1 2 3 M. C. Nash , U. Troitzsch , B. N. Opdyke , J. M. Trafford , B. D. Russell , and J I D. I. Kline4,5 Back Close 1Research School of Earth Sciences, The Australian National University, Acton, ACT, 0200, Full Screen / Esc Australia 2Geoscience Australia, GPO Box 378 Canberra ACT 2601 Australia 3Southern Seas Ecology Laboratories, School of Earth & Environmental Sciences, University Printer-friendly Version of Adelaide, Adelaide, South Australia, 5005, Australia 4Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, Interactive Discussion 92093, USA 5881 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | BGD 5 Coral Reef Ecosystems Laboratory, School of Biological Sciences, University of Queensland, 8, 5881–5906, 2011 Brisbane, QLD, 4072, Australia Received: 24 May 2011 – Accepted: 31 May 2011 – Published: 27 June 2011 Biomineralization of dolomite by coralline Correspondence to: M. C. Nash ([email protected]) algae Published by Copernicus Publications on behalf of the European Geosciences Union. M. C. Nash et al. Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 5882 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract BGD Dolomite is a magnesium-rich carbonate mineral abundant in fossil carbonate reef plat- forms but surprisingly rare in modern sedimentary environments, a conundrum known 8, 5881–5906, 2011 as the “Dolomite Problem”. Marine sedimentary dolomite has been interpreted to form 5 by an unconfirmed, post-depositional diagenetic process, despite minimal experimen- Biomineralization of tal success at replicating this. Here we show that dolomite, accompanied by magnesite, dolomite by coralline forms within living crustose coralline alga, Hydrolithon onkodes, a prolific global tropical algae reef species. Chemical micro-analysis of the coralline skeleton reveals that not only are the cell walls calcitised, but that cell spaces are typically filled with magnesite, rimmed M. C. Nash et al. 10 by dolomite, or both. Mineralogy was confirmed by X-ray diffraction. Thus there are at least three mineral phases present (magnesium calcite, dolomite and magnesite) rather than one or two (magnesium calcite and brucite) as previously thought. Our results Title Page are consistent with dolomite occurrences in coralline algae rich environments in fos- Abstract Introduction sil reefs. Instead of a theory of post-depositional dolomitisation, we present evidence 15 revealing biomineralization that can account for the massive formations seen in the Conclusions References geologic record. Additionally, our findings imply that previously unrecognized dolomite Tables Figures and magnesite have formed throughout the Holocene. This discovery together with the scale of coralline algae dominance in past shallow carbonate environments raises the possibility that environmental factors driving this biological dolomitisation process have J I 20 influenced the global marine magnesium/calcium cycle. Perhaps, most importantly, we J I reveal that what has been considered a geological process can be a biological process, having many implications for both disciplines. Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion 5883 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 Introduction BGD 1.1 Background on the “Dolomite Problem” 8, 5881–5906, 2011 The “Dolomite Problem” has been of interest to geologists and carbonate chemists for more than a century and relates to the mystery surrounding the abundant presence Biomineralization of 5 of the mineral dolomite (Ca0.5Mg0.5CO3) in fossil reefs (e.g., Daito-jima) and carbon- dolomite by coralline ate platform sediments (e.g. the Dolomites) and its apparent absence from equivalent algae modern reef environments (e.g., Ohde and Kitano, 1981; McKenzie and Vasconcelos, 2009; Budd, 1997). Sedimentary dolomite in the geological record is commonly found M. C. Nash et al. in formerly warm, shallow, high energy, platform or atoll margin environments (McKen- 10 zie and Vasconcelos, 2009; Ohde and Kitano, 1981; Budd, 1997; Schlanger, 1957) and is typically associated with coralline algae (Ohde and Kitano, 1981; Budd, 1997; Title Page Schlanger, 1957). In recent coral-algal reefs the primary marine carbonate minerals Abstract Introduction are low magnesium calcite (rhombohedral CaCO3 with small amounts (< 5 %) of mag- nesium substituting for calcium) and aragonite (orthorhombic CaCO3). Yet in a Pleis- Conclusions References 15 tocene fossil reef around the reef crest the dolomite content was found to be more than 70 % (Ohde and Kitano, 1981). It has been considered that the inclusion of dolomite Tables Figures into sedimentary systems must be a post-depositional, diagenetic process, where mag- nesium replaces calcium in the existing carbonate crystal structures (“dolomitisation”) J I (e.g., Budd, 1997). This paradigm has been adhered to for at least half a century J I 20 despite experimental studies failing to replicate the process (McKenzie and Vasconce- los, 2009). Many geochemical models and environmental reconstructions (e.g., Griffith Back Close et al., 2008; Bao et al., 2009) incorporate dolomitisation as a parameter even though Full Screen / Esc the exact process has not been identified. While the discovery of dolomite forming in anoxic microbial environments points towards a microbial mediation (Vasconcelos and Printer-friendly Version 25 McKenzie, 1997), it does not explain the abundant dolomite found in relict coral-algal reefs. Interactive Discussion 5884 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1.2 Background on coralline algae BGD Coralline algae are calcifying red algae and are major reef builders, occurring globally (Adey and Macintyre, 1973). While modern corallines have only been confirmed back 8, 5881–5906, 2011 to the Cretaceous (Aguirre et al., 2000), calcifying red algae have a long history in the 5 geologic record back through the Paleozoic (Brooke and Riding, 1998; Aguirre et al., Biomineralization of 2000) and possibly even the Neoproterozoic (Xiao et al., 2004) and Mesoproterozoic dolomite by coralline (Butterfield, 2000). They have a high magnesium calcite (Mg-calcite) skeleton, typically algae ranging from 10–20 mol % MgCO3 (Moberly, 1970; Chave, 1952, 1954; Milliman et al., M. C. Nash et al. 1971) (mol % MgCO3 is % of magnesium substituting for calcium), although there is 10 some degree of uncertainty around these measurements as results vary depending on the method used (Milliman et al., 1971; Chave, 1954). This high magnesium calcite is meta-stable and prone to dissolution as pH declines (Morse et al., 2006). While Title Page modern coralline algae are composed of Mg-calcite, it has been shown that the incor- Abstract Introduction poration of magnesium, as measured in the cell wall, decreases with declining magne- 15 sium/calcium (Mg/Ca) ratio of the ambient seawater (Ries, 2006; Stanley et al., 2002) Conclusions References and in low Mg/Ca ratios equalling calcite seas (1:1), coralline algae are able to con- Tables Figures tinue growing albeit with a low Mg-calcite skeleton. Researchers in the late 19th and early 20th century proposed coralline algae contained dolomite as part of the magne- sium enriched calcite skeleton, but were unable to prove this hypothesis (Chave, 1954). J I 20 Later research identified magnesium enriched skeletal portions approaching dolomite J I composition, however the presence of dolomite was not confirmed (Moberly, 1970). Back Close Here we present evidence that dolomite and magnesite (MgCO3) form inside living coralline algae and thus must be influenced, if not caused, by biological processes. The Full Screen / Esc intimate association between dolomite and coralline algae suggested by our findings is 25 consistent with the predominant occurrence of sedimentary dolomite in fossil carbonate Printer-friendly Version reef environments (McKenzie and Vasconcelos, 2009; Budd, 1997; Ohde and Kitano, 1981), typically in coralline algae facies (Saller, 1984; Schlanger, 1957; Budd, 1997; Interactive Discussion McKenzie and Vasconcelos, 2009; Ohde and Kitano, 1981). 5885 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Note that we limit our discussion to marine sedimentary dolomite, excluding that of metamorphic origin. While the stoichiometric composition of dolomite BGD is Ca Mg CO , sedimentary dolomite typically deviates from this (38–50 mol % 0.5 0.5 3 8, 5881–5906, 2011 MgCO3, Budd, 1997), is imperfectly ordered, and is commonly referred to as 5 “protodolomite” (Ohde and Kitano, 1981). We use the term protodolomite to reflect the sedimentary nature