Ikaite precipitation in a lacustrine environment – implications for palaeoclimatic studies using carbonates from Laguna Potrok Aike (Patagonia, Argentina)
Section snippets
Introduction and site description
Modern ikaite is a mineral frequently observed in certain marine anoxic sediments (e.g. Suess et al., 1982; Shearman and Smith, 1985; Stein and Smith, 1986; Jansen et al., 1987; Hacke et al., 1994; Schubert et al., 1997; Zabel and Schulz, 2001) and more currently also in Antarctic and Arctic sea ice (Dieckmann et al., 2008, 2010). To our knowledge, however, only four locations of modern occurrences of ikaite in fresh and brackish water systems have been reported. At all these locations ikaite
Material and methods
Lake water temperatures between March 2006 and October 2008 were recorded every 6 h by Minilog thermistors (M-08TR, Vemco ltd., Canada) in 0 and 90 m water depth attached to a mooring in the centre of the lake. Two sediment traps were also installed in 90 m water depth which had an active area of 55 cm2 (Ohlendorf et al., 2013 for more details). pH, electric conductivity (EC), salinity, and the alkalinity were measured with an Universal Pocket Meter (Multi 340i, WTW). The water chemistry
Evidence for modern ikaite and its pseudomorphs
The single crystal X-ray structure analysis of an exemplary carbonate crystal precipitated on aquatic moss and stored cool after sampling gave evidence of the mineral ikaite (Table 1). The structure was solved and refined in the space group C2/c with Z = 4 and the lattice parameters a = 8.7906 (13) Å, b = 8.2850 (9) Å, c = 10.9890 (17) Å, and β = 110.589 (17) °. The calculated density at the temperature of 253 K is 1.846 g cm−3. The final structure refinement including hydrogen positions
Discussion
The occurrence of pseudomorphs after ikaite is usually interpreted as an indicator of cold-water temperatures (e.g., Greinert and Derkachev, 2004). Our results demonstrate that during transformation from ikaite to calcite no effect on the oxygen and only minor effects on the carbon isotopic composition occurred. Hence the calcite pseudomorphs recorded the carbonate isotopic composition of the original ikaite which is in accordance with Greinert and Derkachev (2004). Stable isotope analyses on
Conclusions
During the PASADO drilling campaign transparent carbonate crystals were collected in Laguna Potrok Aike and classified as the mineral ikaite. These crystals, their pseudomorphs and the microscopic carbonate crystals in the lake sediment were studied with mineralogical, electron microscopic and isotopic methods resulting in the following conclusions:
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The lake water of Laguna Potrok Aike is supersaturated with respect to Ca-carbonates but the precipitation of anhydrous carbonates is inhibited by
Acknowledgements
We thank H. Wissel for the isotopic analysis of the lake water and E. Wessel for assistance with the SEM. This study was partially financed by the German Research Foundation (DFG MA 4235/4-1). This research would not have been possible without the fieldwork carried out in the framework of the "Potrok Aike Maar Lake Sediment Archive Drilling Project" (PASADO) supported by the International Continental Scientific Drilling Program (ICDP). Funding for PASADO was provided by the ICDP, the German
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Present address: Institute of Geography, University of Erlangen-Nürnberg, D-91054 Erlangen, Germany.