The geochemical conditions conducive to dolomite formation in
shallow evaporitic environments along the Triassic Tethyan margin are still
poorly understood. Large parts of the Triassic dolomites in the Austroalpine
and the southern Alpine realm are affected by late diagenetic or
hydrothermal overprinting, but recent studies from the Carnian Travenanzes
Formation (southern Alps) provide evidence of primary dolomite. Here a
petrographic and geochemical study of dolomites intercalated in a
100
The formation of dolomite (
In contrast, penecontemporaneous dolomite formation seems to have prevailed
in the Tethyan realm during the Triassic (Meister et al., 2013, and
references therein; Li et al., 2018), in an “aragonite sea”, while elsewhere
dolomite was not particularly abundant (see Given and Wilkinson, 1987). In
Norian shallow water dolomites of the Dolomia Principale, Iannace and Frisia
(1994) measured oxygen isotope values as positive as
The interpretation of primary dolomite in the Travenanzes Fm. needs further validation by nano- and atomic-scale analyses and further petrographic and geochemical investigations to establish the sedimentary and geochemical conditions in the depositional environment, an extended mud plain that occurred along the western Tethys margin during the Carnian. In particular, the origin of ionic solutions conducive to dolomite formation is still unclear. Comparison with modern environments shows that ionic solutions may either be seawater-derived, as shown for the sabkhas along the Persian Gulf coast, where several hydrological mechanisms have been discussed (Adams and Rhodes, 1960; Hsü and Siegenthaler, 1969; McKenzie et al., 1980, McKenzie, 1981; see Machel, 2004, for an overview; see also Teal et al., 2000), or derived from continental groundwater, as shown for the coastal ephemeral lakes of the Coorong area (Australia; Alderman and Skinner, 1957; Von der Borch et al., 1975; Rosen et al., 1989; Warren, 1990). While both types of fluid become concentrated during evaporation and are, perhaps, modified by the precipitation of carbonates and evaporites, it remains unclear which source prevailed during deposition of the Travenanzes Formation.
Dolomites occur in the Travenanzes Fm. as intercalated beds in a 100
Here we provide a detailed investigation of dolomites of the Travenanzes Fm.
to reconstruct the processes and factors conducive to dolomite formation. We
specifically searched for sedimentary structures indicating that the initial
authigenic dolomite (or a precursor carbonate phase) was unlithified, as
would be expected if it spontaneously precipitated from the shallow water
bodies of ephemeral lakes or tidal ponds. Radiogenic Sr-isotope ratios
(
The Dolomite Mountains (Southern Tyrol and Venetian Alps; Fig. 1a) are well
known for their characteristic peaks consisting of Triassic carbonate
platform limestones and dolomites. These platforms developed all along the
margins of the western Tethys ocean (Stampfli and Borel, 2002) and were
separated by deep basins in the middle Triassic and formed an extended
coastal plain during the Carnian and Norian. The Adriatic plate was rotated
almost 90
The Travenanzes Fm. lies unconformably above the Heiligkreuz Fm. and is
overlain by the Dolomia Principale (Hauptdolomit) along a transgressive
boundary (Fig. 1b). Large amounts of siliciclastic material were deposited
during the Carnian, presumably as a result of a change in climate and
increasingly humid episodes, and led to filling of basins that were more
than 100
Stratigraphic section at Rifugio Dibona:
The depositional environment of the siliciclastic facies of the Travenanzes Fm. has been interpreted as a dryland-river system by Breda and Preto (2011). Such a system occurs in arid environments if rivers drain into a coastal alluvial plain but do not reach the coast. Evaporation along the way may lead to the formation of playa lakes; on the seaward side of the system extended evaporative areas, i.e. coastal sabkhas, develop. Both types of environment are well known for giving rise to modern dolomite formation (see references above). As the southern Alps were located in tropical latitudes, a warm arid climate, perhaps influenced by a monsoon effect, developed (Muttoni et al., 2003). Rivers provided large amounts of clay, which were partially oxidized under subaerial conditions, leading to a typical red and green clay succession containing palaeosols. This facies association is widespread throughout the Alpine and Tethyan realm during the Carnian, but similar deposits are strongly deformed by Alpine tectonics in most Austroalpine units, forming a characteristic band of rauhwacke, the “Raibl beds” (e.g. Czurda and Nicklas, 1970). In the Travenanzes Fm. the entire sequence maintains its depositional architecture, providing a pristine archive to study the intercalated dolomites.
The Carnian and Norian deposits of the Keuper in the endorheic Germanic Basin contain a similar facies association as the Travenanzes Fm., but clearly represent continental playa lake deposits (Reinhardt and Ricken, 2000, and references therein). Here we consider dolomites from the Germanic Basin of confirmed continental origin for comparison of Sr-isotope compositions of continental and coastal environments.
A total of 39 hand specimens were collected from the stratigraphic section
at Rifugio Dibona, 5
For bulk mineralogical analysis, three dolomite samples were ground to a
fine powder with a disk mill. Clay mineralogy was determined on 40
Carbon and oxygen isotopes were measured on 28 samples, which where
microdrilled from thin section cuttings (see below). The samples were
analysed with a Delta V Plus mass spectrometer coupled to a GasBench II
(Thermo Fisher Scientific, Bremen, Germany) at ETH Zürich (Zurich,
Switzerland), following the procedure described in Breitenbach and
Bernasconi (2011). The precision was better than 0.1 ‰
for both isotopes. The oxygen isotope values were corrected for kinetic
fractionation during dissolution of dolomite in anhydrous phosphoric acid at
70
To ensure that Sr from the pure dolomite phase is extracted, specific areas
free of clay minerals were defined by SEM and identified using an Olympus
SZ61 microscope equipped with a MicroMill sampling system (Electro
Scientific Industries). Eleven samples were drilled over an area of 5–10
A sequential extraction was used to determine the mildest reagent that
efficiently extracts the pure dolomite phase without attacking other mineral
phases. The extractions were routinely performed in capped 2 or 15
Extraction efficiency was tested on bulk samples, clay samples, pure
celestine, and barite purchased from W. Niemetz (Servitengasse 12, 1090 Vienna, Austria); pure dolomite powder from Alfa Aesar (Thermo Fisher –
Kandel – GmbH, P.O. Box 11 07 65, 76057 Karlsruhe, Germany) and a fragment
of a single dolomite crystal (Montana; Geoprime Minerals and Earth Materials Co.) were analysed as controls. These samples were
crushed to a powder in an agate mortar and pestle. Dolomite, barite, and
celestine were mixed in a similar ratio as they occur in the dolomites of
the Travenanzes Fm. and run through the entire procedure as a control of
extraction efficiency; 14
Total element concentrations were measured in leachates of three dolomite
specimens previously analysed by XRD and the two claystones. Five millilitres of
each fraction was used for element concentration analysis (the rest was
further processed for Sr-isotope analysis; see below). The solutions were
evaporated on a heating plate and the residues were redissolved in 5
For Sr-isotope measurements, Sr was separated from interfering ions (e.g.
Fe, K, Rb, and Ca) using an ion exchange column packed with BIO RAD AG 50W-X8
resin (200–400 mesh, hydrogen form). Leachates were evaporated, dissolved in
6
The isotopic composition of Sr was measured with a Triton (Thermo Finnigan)
thermal ionization mass spectrometer at the University of Vienna. Sr
fractions were loaded (dissolved in 1
Hard cemented beds and nodules of dolomite are intercalated in a
100
Outcrop images of different types of dolomite intercalated with
red and grey clay of the Travenanzes Fm. at Rifugio Dibona:
Homogeneous dolomite beds are usually 10 to 50
Photomicrographs of thin sections of dolomites of the Travenanzes
Fm.:
The clay content in the homogeneous beds is generally low. A few beds (e.g.
at 33.5
One dolomite bed, located at 64
Laminated dolomites occur in the upper part of the clay-rich interval,
between 90 and 110
SEM images of dolomites in backscatter mode:
Under the SEM, laminated dolomites show an anhedral structure in the 1–5
SEM images of dolomites in backscatter mode showing different
types of crystal shape:
Nodular dolomites (Fig. 3b) often occur in beds of vertical peds linked to
palaeosols, as indicated by horizons of vertical cracks showing green
alteration fronts. Single nodules may also sporadically occur embedded
within metre-thick beds of red and green clay. Nodules are usually 5 to 10
A sample from the Weser Fm. (middle Lehrberg bed; clay pit Friedland, 12
A sample from the Norian Arnstadt Fm. (formerly termed
“Steinmergelkeuper”; middle grey series; locality of Krähenberg, 11
Bulk dolomite shows a position of the 104 peak at a mean
X-ray diffraction patterns:
Clay mineral analysis (Fig. 7b–d) reveals illite in samples TZ14-1 and TZ14-7 and an R3 ordered illite–smectite mixed-layer clay mineral in sample TZ14-9. In the ethylene-glycol-saturated state, the broad shoulder at 11.4 Å contains components of the illite 001 reflection and of the fourth order of a 47 Å superstructure peak whose unit cell consists of three 10 Å illite layers and one 17 Å smectite layer (Moore and Reynolds, 1997). This smectite component is not observed in samples TZ14-1 and TZ14-7.
Carbon isotope values range from
Sequentially extracted samples TZ14-1, TZ14-7, and TZ14-9 (Rieder et al., 2019) show Ca contents between 1.68 and 2.33
Element concentrations in sequentially extracted fractions of bulk
dolomite and clay samples of the Travenanzes Fm.:
Correlation of Sr contents to other elements did not show clear trends. In
particular, Sr content did not correlate with Mg or Ca. Sr correlates with K
(Fig. 9b), but at the same time, K is extremely low in all clay mineral
leachates. The Sr concentrations in bulk dolomite samples (Fig. 10a–c) are
in the range of 0.38 and 1.16
Sr-isotope ratios and Sr concentrations measured in sequential
and non-sequential extractions of dolomite and different control minerals.
Results of Sr-isotope measurements are available from Rieder et al. (2019).
Results of sequential and non-sequential leaching tests of bulk samples
TZ14-1, TZ14-7, and TZ14-9 are shown in Fig. 10a–c.
Sequential extractions of the clay samples TZ16-1 and TZ16-19B with the
lowest TIC of 0.02 wt % (Fig. 10d; Rieder et al., 2019) show a similar increase in the
Repeated extractions of chemically pure reference material (Fig. 10e, f)
dissolved in 0.1
In pure barite,
Eleven dolomite samples were microdrilled from areas where dolomite was
purest based on examination by SEM and dissolved in 0.1
Comparison of Sr isotopes in dolomites of the Travenanzes Fm.
with the Carnian seawater curve (Korte et al., 2003) in grey. The
Dolomites of Deep Springs Lake show strongly radiogenic values of
The homogeneous dolomite beds, which are mainly intercalated in the lower, clay-rich part of the Travenanzes Fm., consist of fine-grained dolomicrite (aphanotopic dolomite), with occasional intraclasts of the same aphanotopic dolomite. Soft sediment deformation and dolomicrite infill between mud clasts indicate that this sediment consisted of unlithified, albeit cohesive, carbonate mud. Based on the abundance of fine mud, water energy was probably not very high (Demicco and Hardie, 1994), although reworking and partial rounding of the mud clasts requires at least occasionally higher water energies. According to the standard microfacies (SMF) concept, homogeneous aphanotopic dolomite falls into SMF 23 (“non-laminated homogeneous micrite and microsparite without fossils”), indicating deposition in “saline and evaporative environments, e.g. in tidal ponds” (Flügel, 2010). In addition, SMF 24 (“lithoclastic floatstones, rudstones and breccias”) is observed in some of the beds where mud clasts are abundant. These facies types are consistent with supersaturation-driven precipitation of fine-grained authigenic carbonate in environments that were partially disconnected from open seawater and would match with a coastal sabkha environment and/or shallow ephemeral lake. Ephemeral lakes may have formed on extended coastal alluvial plains along the Tethyan margin during the Carnian, as suggested by Breda and Preto (2011). The fine mud may have been homogenized and redistributed due to minor wave action in the ponds (see Ginsburg, 1971), which is often observed in ephemeral lake settings, explaining the formation of homogeneous dolomite beds.
Sr-isotope values (
Episodic flooding of the alluvial plain by the dryland-river system may have
supplied water to temporary evaporating ponds. Alternatively, the alluvial
plain may have been sporadically flooded by seawater, explaining the
intercalations of authigenic dolomite layers with alluvial clays (Breda and
Preto, 2011). Homogeneous dolomites show a positive carbon isotope signature
between 0.7 ‰ and 4 ‰ VPDB (except one outlier), which is
consistent with formation from unaltered marine carbon in evaporative brine,
with no significant contribution of
A bed of dolomitic ooid grainstone that is devoid of matrix occurs at 64
Laminated dolomites reminiscent of loferites (Fischer, 1964) occur in the upper part of the clay-rich interval. The change from more homogeneous to laminated dolomite intercalations correlates with the change from red to dark grey clay. The laminations consist of millimetre-scale dolomite–clay interlayers, suggesting alternating deposition of clay and fine dolomite. This microfacies falls into SMF 25 (“laminated evaporite-carbonate mudstone facies”), indicating an “upper intertidal to supratidal sabkha facies in arid and semiarid coastal plains and evaporitic lacustrine basins” (Flügel, 2010). Laminae showing soft sediment deformation cannot be attributed to stromatolitic bindstone facies (SMF 19 to 21). Only some layers that show a coarser fabric with interstitial dolosparite or dolomicrosparite containing putative peloids have been interpreted as microbial laminites (Preto et al., 2015). Graded bedding mostly indicates a direct sedimentation process rather than in situ precipitation of the primary carbonate within a microbial mat (Vasconcelos et al., 2006; Bouton et al., 2016; Court et al., 2017; Perri et al., 2018). A detrital origin of the clay in the dolomites is confirmed by a well-ordered illite–smectite mixed-layer composition, which is atypical for authigenic clay minerals. Frequent subaerial exposure and desiccation may explain why the sediment was not homogenized and the lamination is preserved. This is supported by the occurrence of pseudo-teepee structures as remnants of desiccation cracks. Rip-up clasts were formed during subsequent flooding, when angular flat pebbles formed as the sediment was desiccated or partially lithified. However, laminae commonly exhibit plastic deformation (e.g. in Fig. 3g) where the mud was still unlithified.
Some uncertainty exists as to whether this facies was peritidal or represents an ephemeral lake, as suggested for the homogeneous dolomites above. Episodic high water energy, as indicated by the rip-up clasts, combined with frequent desiccation, could point to evaporative tidal conditions that occur in a sabkha. What is atypical for a modern sabkha is the large amount of clay input. This is attributed to seasonally wet conditions during the Carnian, and the sediments can be considered to be a mixed facies of alluvial plain and coastal sabkha: a “dirty” sabkha (see discussion below). Under such conditions, large amounts of evaporites, in particular gypsum, could have been dissolved. Why the occurrence of laminated dolomites coincides with the transition from red to grey clays is not clear, but it may be related to more permanently water-saturated conditions in the subsurface, while the surface was exposed to periodic desiccation. These conditions would also be consistent with a sabkha environment.
During intervals of arid conditions, the clay beds were subject to strong
evaporation and vadose diagenesis, causing oxidation and the red colour.
Although red beds may also form in humid environments if drainage is rapid
(Sheldon, 2005), drainage was certainly slow due to the large amounts of
poorly permeable clay in the Travenanzes Fm., and the climate was clearly
seasonally arid (Breda and Preto, 2011). Dolomite nodules that occur
sporadically within certain intervals show internal brecciation, which must
have occurred after sedimentation. Internal brecciation is a typical feature
of present-day calcretes in arid environments (e.g. Mather et al., 2018).
Slightly negative
Overall, the dolomites in the Travenanzes Fm. show a facies association that matches a variety of potential depositional environments. They have similarities to the Germanic Keuper succession, and it is not entirely clear if a marine influence occurred, except where indicated by marine fossils, as in the tempestite beds. Sr isotopes were analysed in order to better trace the origins of ionic solutions to the environments that were conducive to dolomite formation.
Radiogenic
Comparison with the seawater curve shows that the dolomites of the
Travenanzes Fm. have largely marine
The marine signature shown by the Sr isotopes does not support the classical
Coorong model for dolomite formation, where alkalinity is largely derived
from continental groundwater. The Coorong lakes in South Australia are
ephemeral lakes largely supplied by groundwater (Von der Borch et al.,
1975). Strangely, though, the
We conclude that the
Despite precautions to prevent contamination by other mineral phases by
microdrilling and using mild reagents, some scatter occurs in the
Sr-isotope data. Higher
In the NaCl fraction, only minimal amounts of dolomite are dissolved. The
slightly more radiogenic
Clay minerals leached in 6
The question is whether the Sr-isotope ratio reflects the conditions of dolomite formation or whether it is inherited from a precursor phase. Baker and Burns (1985) and Vahrenkamp and Swart (1990) documented very small distribution coefficients between aqueous and solid solutions, and high Sr contents measured in Abu Dhabi Sabkha dolomites (Müller et al., 1990a) may be derived from precursor aragonite. However, if dolomite in the Travenanzes Fm. is largely primary (Preto et al., 2015) and, thus, not formed from an aragonite precursor, the Sr content should derive from the dolomite phase. Although some Sr may have been released due to replacement of the dolomite and excess Sr can explain the occurrence of celestine and barite inclusions, nanocrystal structures imply that primary dolomite is partially preserved. Indeed, Sánchez-Román et al. (2011) demonstrate a protodolomite forming in culture experiments that contains Sr in the range of several thousand parts per million. The incorporation mechanism of Sr is still not entirely clear, since Sr is a large ion that should occupy the sites of Ca in the crystal lattice. However, in Sánchez-Román et al. (2011), Sr appears to correlate with the Mg content, and another incorporation mechanism may occur, such as surface entrapment. Also the correlation of Sr contents with K contents could be explained by such a mechanism of Sr incorporation. Even if only protodolomite formed in microbial culture experiments (Gregg et al., 2015), natural modern dolomites are often rich in Sr (e.g. Meister et al., 2007). The Sr could occur in disordered nano-structural domains that are not picked up in the bulk XRD signal. Non-classical nucleation and growth pathways, e.g. by nanoparticle attachment, could play a role in the abnormal partitioning of Sr in the dolomite lattice. Thus, a high Sr content in the Travenanzes Fm. or in Abu Dhabi Sabkha dolomites is likely a true signature of primary dolomites.
Several results support a largely primary origin of dolomite in the
Travenanzes Formation. Formation temperatures reconstructed from oxygen
isotopes (assuming Triassic seawater composition of
While most of the dolomite may have been primary, micron-scale interstices
between the dolomicrite grains must have been cemented after deposition.
This cementation resulted in rims visible under SEM and results in near-hexagonal compromise boundaries. The cement may have contributed
While dolomite formation under Earth surface temperatures has been suggested to be catalysed by microbes, perhaps via a secreted extracellular polymeric substance (EPS; see Bontognali et al., 2013), this mechanism is currently under debate (see Gregg et al., 2015). The present study neither supports nor rules out such a mechanism. We can raise the question whether microbial EPS is enriched in the surface waters, where it may affect precipitation of fine dolomite mud.
The classical sabkha model involves dolomite formation under
intertidal to supratidal conditions, the concentration of brines through either
seepage reflux (Adams and Rhodes, 1960) or evaporative pumping (Hsü and
Siegenthaler, 1969; Hsü and Schneider, 1973; McKenzie et al., 1980;
McKenzie, 1981), and precipitation of dolomite as
The sabkha model is thus a reasonable model for the uppermost parts of the Travenanzes section, which contain laminated dolomites, marine Sr-isotope values, and indications of frequent desiccation and flooding in a peritidal setting. Yet, the conditions differed from the modern sabkhas along the Persian Gulf due to the large amount of alluvial clay (dirty sabkha), as opposed to aeolian sand. Most of the fine laminations may therefore result from periodically varying conditions, perhaps with clay deposition during episodes of fluvial discharge and carbonate deposition during evaporative conditions.
The playa lake model was originally suggested by Eugster and Surdam (1973) for dolomite of the Green River Formation (Wyoming), but the primary formation of fine dolomite mud is observed in many alkaline playa lakes, such as Deep Springs Lake (Peterson et al., 1963; Clayton et al., 1968; Meister et al., 2011), Lake Acıgöl (Turkey; Balci et al., 2016), Neusiedler See (Austria; see Neuhuber et al., 2015), and Lake Van (Turkey; McCormack et al., 2018). For an overview see Eugster and Hardie (1978) and Last (1990). This type of setting has also been suggested for the Germanic Keuper deposits during the late Carnian and Norian, when the Germanic Basin was entirely disconnected from Panthalassa and was continental (Reinhardt and Ricken, 2000). The Travenanzes Fm., with its homogeneous dolomite intercalations in red and green clays, is strikingly similar to playa lake Keuper facies in the Germanic Basin. There, dolomite formed following evaporation and concentration of the continental brines under a semiarid climate.
Sr-isotope data, however, support a dominantly marine origin of ionic solutions to the Travenanzes Fm., whereas Sr isotopes are strongly radiogenic in the Germanic Keuper dolomites (or in Deep Springs Lake; Fig. 12). The two settings are thus fundamentally different. Even dolomite nodules, showing somewhat more radiogenic values than seawater in the Travenanzes Fm., still indicate a predominantly marine influence. The slightly more radiogenic influence could be due to clay minerals present in the nodules that were difficult to entirely separate from the carbonate. Also, dolomite nodules may have formed in relation to palaeosols, during somewhat more humid times and, thus, may have been slightly influenced by continental water input from rivers.
The Coorong model was proposed by Von der Borch et al. (1975), Von der Borch (1976), and Rosen et al. (1989) (see Warren, 2000, for detailed information) to explain the formation of primary and uncemented dolomite in the Coorong lakes of South Australia. The Sr-isotope values (Fig. 12) show that the contribution of ionic solutions, and hence alkalinity, of continental origin to the dolomitizing fluids was minimal, and that the dolomites are seawater derived. This may be distinct from the typical Coorong model, where alkalinity is provided from an inland karst system. But other coastal ephemeral lakes exist, including along the Brazilian coast, north of Rio de Janeiro. Partially unlithified dolomite occurs in Brejo do Espinho (Sánchez-Román et al., 2009), which is largely similar to the Coorong lakes, but ionic solutions are mostly derived from seawater.
A coastal ephemeral lake model would probably be most suitable to explain the homogeneous dolomite beds of the Travenanzes Fm., where hypersaline ponds may have formed in a dryland-river system. However, unlike recent ephemeral lakes (such as Lagoa Vermelha, Brejo do Espinho, and the Coorong lakes) the clay-rich sediment must have inhibited groundwater flow. Hence, while modern coastal ephemeral lakes receive their water largely through seawater percolating through porous dune sand, episodic flooding with seawater must have provided ionic solutions for dolomite formation on a coastal plain.
Overall, the depositional environment reconstructed for the Travenanzes Fm. shows similarities to modern systems where dolomite forms. Among all the modern scenarios, a coastal ephemeral lake model would be most similar to the conditions conducive to homogeneous dolomites, lacking signs of frequent desiccation, while a coastal sabkha model may explain the laminated intervals near the top of the studied succession. In contrast to modern systems, the clay-rich sediments of the Travenanzes Fm. preclude any input of groundwater, which plays a role for ionic transport in both the modern-day ephemeral lake model and the different versions of sabkha models. Although modern systems provide valid analogues for the mechanism of dolomite formation in the past, and probably throughout Earth history, none of them is a faithful environmental analogue. The Carnian coastal plains that covered an enormous area along the Tethys margin (Garzanti et al., 1995) represent a system without a single modern analogue in terms of their sedimentary, hydrological, and climatic boundary conditions. Also, the geochemistry of Tethys seawater may have been different from modern seawater, an issue that requires further investigation (see Burns et al., 2000; Li et al., 2018). These aspects need to be taken into account if we intend to understand the conditions that led to dolomite formation through Earth history.
In the light of the possibility of spontaneous precipitation of fine dolomite mud in the water column, perhaps via formation and aggregation of nanoparticles, further discussion of a nucleation and growth pathway of dolomite is necessary. While several modifiers may also play a role in the water column, such as dissolved organic matter (Frisia et al., 2018), microbial EPS (Bontognali et al., 2013), or suspended clay particles (Liu et al., 2019), fluctuating conditions inducing spontaneous nucleation and growth of dolomite, in agreement with Ostwald's step rule (Deelman, 1999), require further consideration as a factor favourable for dolomite formation on a seasonally variable platform (Meister and Frisia, 2019).
The main finding of this study is that most of the dolomite in the
Dolomite beds intercalated in a 100
Sr isotopes clearly show a marine signature, indicating seawater as the main source of ions. The depositional environment is most similar to coastal ephemeral lakes resulting in the deposition of homogeneous dolomite beds through most of the sequence, changing into a dirty sabkha near the top of the sequence, where fine dolomite–clay interlayers suggest alternating deposition of extremely fine authigenic dolomite from evaporating water and clay.
Overall, Sr isotopes and petrographic observations provide insight into a system without modern analogue, including elements of both coastal ephemeral lake systems and sabkhas as the environment of primary dolomite formation. Considering the precipitation of primary dolomite from coastal lakes or ponds may help explain other dolomite deposits with preserved primary sedimentary features from throughout geologic history.
All data mentioned in the text are in the article or available through the
Pangaea data repository (
The supplement related to this article is available online at:
Fieldwork and sampling were performed by GA, AB, PM, SK, NP, and MR. MH, MR, and WW measured the Sr isotopes; SMB provided the C and O isotope analyses; SG, MR, and SK provided the XRD analyses; and PM, MR, and SK performed the petrographic analysis. The concept and idea of the study were developed by PM, NP, and UK. PM and MR primarily wrote the paper. All authors provided comments and corrections for the paper. PM supervised the project.
The authors declare that they have no conflict of interest.
We thank Claudia Beybel, Ilka Wünsche, and Leo Slawek for preparing high-quality petrographic thin sections. Thanks also to Wolfgang Obermaier for analysing element concentrations by ICP-OES and Petra Körner for support during TOC measurements. Simon Niebergall provided some of the petrographic images. We furthermore thank Sebastian Viehmann for help during sampling and supervision of the students in the field and Beatrix Bethke for her strong support in the laboratory. Thanks also to Matevz Lorencak for help during the sampling of dolomite from the Coorong Lagoon. We thank Silvia Frisia for input and constructive criticism. We thank Wolfgang Blendinger, Fulvio Franchi, Stephen Lokier, Hans Machel, Chris Romanek, an anonymous reviewer, and the editor Elias Samankassou for constructive comments.
This research has been supported by the European Commission, FP7 People: Marie-Curie Actions (project TRIADOL; grant no. 626025) and the University of Vienna.
This paper was edited by Elias Samankassou and reviewed by Chris Romanek and one anonymous referee.