Radiocarbon ages of microcrystalline authigenic carbonate in Lake Neusiedl (Austria) suggest millennial‐scale growth of Mg‐calcite and protodolomite

Abstract

Authigenic Mg‐calcite and dolomite are frequently observed in restricted, evaporative environments, such as lagoon or lake systems, but their formation is difficult to capture due to slow growth rates. Lake Neusiedl, an alkaline and subhaline lake with a mean water depth of 0.7 m in Austria, offers a natural system to study the precipitation of Ca‐Mg‐carbonate phases, which occur as fine‐grained, unconsolidated and largely homogenized mud. To elucidate the timing and formation mechanisms of these authigenic carbonate phases, the mineralogical and isotopic composition and radiocarbon age of different sediment grain‐size fractions from <0.2 to >3.0 μm were analysed. X‐ray diffraction analyses show two broad peaks of Mg‐calcite and protodolomite (lacking ordering peaks), suggesting that the carbonates are authigenic rather than detrital in origin. Calibrated carbon‐14 ages range between 200 cal yr BP and 3700 cal yr BP. The linear correlation of age and grain size suggests a very slow growth rate of single crystals of 0.23 to 0.60 μm/ka. These rates suggest an extremely slow sedimentation rate in a shallow lake that existed during most of the Holocene. The higher abundance of protodolomite in older grain fractions, in contrast to the presence of high‐Mg calcite in the youngest fractions, suggests a growth succession where high‐Mg calcite develops first and subsequently transforms into protodolomite. Much higher ages of 6 cal ka BP to 15 cal ka BP are measured in carbonates of lake deposits exposed on land, in a section northwest of the recent lake, suggesting a growth rate of those carbonate minerals of 0.13 μm/ka. These time constraints further suggest that some carbonate grains could already have nucleated from lake water before or during the last glacial maximum, although under slightly different hydrochemical conditions.

Co‐precipitation of primary dolomite and Mg‐rich clays in Deep Springs Lake, California

“…After accounting for the minimal <1 wt.% detrital Precambrian dolomite (Peterson et al ., 1963; Clayton & Jones, 1968), the carbonate sedimentation rate for Deep Springs Lake is approximately 0.01 to 0.50 mm/year. Recent radiocarbon work has shown that Lake Neusiedl has a carbonate sedimentation rate that ranges from 0.08 to 0.30 mm/year (Neuhuber et al ., 2023). Holocene carbonate sedimentation rates have been reported in literature to be 0.30 to 3.00 mm/year in Florida Bay, the Bahamas and Bermuda (Strasser & Samankassou, 2003), 0.29 mm/year off the coast of Abu Dhabi (Lokier & Steuber, 2009), 0 to 1.20 mm/year in northern Belize (Yang et al ., 2004) and 0.16 mm/year in stromatolites from San Salvador, Bahamas (Paull et al ., 1992).…”

“…Newly precipitated dolomite nanocrystals formed through epitaxial growth as they follow crystallographic orientations of bulk crystals demonstrated by SAED and TEM images (Figs 6, 10 and S12). Similar nucleation and growth of nanocrystal dolomites is also observed in modern sediments such as the Great Salt Lake with dissolved silica as a catalyst (Fang et al ., 2023a), Lake Neusiedl (Meister et al ., 2023; Neuhuber et al ., 2023), the Coorong region (Raudsepp et al ., 2022) and Abu Dhabi sabkha (Wenk et al ., 1993), as well as in older rock such as Dolomia Principale (Meister & Frisia, 2019), dolomite crust associated with deep sea modern methane seeps (Xu, 2010) and the eastern Dolomites, Northern Italy (Preto et al ., 2015). The small misoriented nanocrystals of dolomite indicate that they formed directly and nucleated on bulk crystals implying a primary precipitation origin as opposed to replacement products.…”

“…Attachment of Mg‐bearing clays on carbonate surfaces were also observed in laboratory synthesis of disordered dolomite with dissolved silica present (Fang & Xu, 2022). A similar texture of dolomite rhombs in a Mg‐silicate matrix was observed in modern dolomite in the Great Salt Lake where poorly crystalline Mg‐rich clays are co‐precipitated and attached to the dolomite {104} faces (Fang et al ., 2023a) as well as in Lake Neusiedl (Meister et al ., 2023; Neuhuber et al ., 2023). This texture was also observed in late Palaeozoic alkaline lake deposits in the Junggar Basin (Guo et al ., 2023) and early Cretaceous lacustrine carbonates in the Pre‐salt Barra Velha Formation in the northern Campos Basin, offshore Brazil (Wright & Barnett, 2015).…”

“…Testing these dolomite growth mechanisms in the field presents a challenge because few modern‐day sites exist with primary dolomite precipitating. These sites are primarily limited to playa lake settings such as the Great Salt Lake in Utah, USA (Eardley, 1938; Fang et al ., 2023a), Coorong Lakes in Australia (Jones et al ., 1967), Lake Neusiedl in Austria (Meister et al ., 2023; Neuhuber et al ., 2023), Lake Albert in East Africa (Beuning et al ., 1997), Lake Van in Eastern Anatolia (Reimer et al ., 2009), Amboseli Basin in Kenya and Tanzania (Stoessell & Hay, 1978) and Deep Springs Lake in California, USA (Jones, 1965). Deep Springs Lake is of particular interest to this study due to its well‐documented primary dolomite formation (Jones, 1965; Clayton & Jones, 1968; Meister et al ., 2011; Zhang et al ., 2015).…”