Reconstructing conditions during dolomite formation on a Carnian coastal sabkha/alluvial plain using 87Sr/86Sr isotopes - Travenanzes Formation, northern Italy

The study of large amounts of dolomite that formed in the Triassic Tethyan realm is hampered by late diagenetic
or hydrothermal overprint. These dolomites are difficult to link to past environmental and early diagenetic conditions,
and their correlation to models for dolomite formation in modern environments is problematic. Preto et al.
(2015) suggested, based on evidence from nano-scale structure analysis by transmission electron microscopy and
petrographic observations, that dolomites in the Carnian Travenanzes Formation of the Southern Alps (Dolomites
area) represent a preserved primary phase. The Travenanzes Formation was deposited in an extended alluvial plain
or coastal sabkha environment subject to a semi-arid climate. Beds and nodules of nearly stoichiometric dolomite
are embedded in large amounts of clay, which shielded early formed dolomite from diagenetic fluids.
This finding of penecontemporaneous dolomite provides an ideal model case for reconstructing past environmental
conditions at the time of dolomite precipitation. While Preto et al. (2015) argued that dolomite formation
was mediated by extracellular polymeric substances produced by sulphate-reducing bacteria, it remains unclear
whether precipitation occurred from evaporating seawater or mainly from brine derived from evaporating continental
groundwater. Both cases exist in modern environments of dolomite formation. In the coastal sabkhas of Abu
Dhabi and Qatar, dolomite precipitates from concentrated brine derived from seawater, either through seepage and
reflux or through evaporative pumping (the sabkha model). In the coastal ephemeral lakes of the Coorong Lagoon
system (South Australia) dolomite precipitation occurs from evaporating groundwater.
The goal of this study is to distinguish marine from continental influence during formation of Carnian dolomite
using 87Sr/86Sr isotope ratios. Sr isotopes could reveal different origins of ionic solutions for dolomite precipitation,
which is not indicated by oxygen isotopes. The marine 87Sr/86Sr values have been reconstructed for most
of the Phanerozoic and are nearly constant in the Carnian (McArthur et al., 2012), while the age of the dolomite
beds of the Travenanzes Formation is constrained by their stratigraphic position in the measured section (Dibona
Section; Preto et al., 2015). The continental Sr isotope signal is governed by weathering rates, especially during
silicate weathering of the source rock in the catchment area (McArthur et al., 2012). Through 87Sr/86Sr isotope
investigation of primary dolomite in beds and nodules of the coastal sabkha or alluvial plain environment, the
influence of marine or continental conditions can be determined. The finding of celestine SrSO4 and Sr-rich barite
BaSO4 within the cemented dolomite by SEM indicates enrichment of Sr, possibly during strong evaporative conditions.
Hence, the generation of phase-specific Sr-isotope data will allow for a more precise reconstruction of the
conditions that led to dolomite formation in the Triassic shallow coastal sabkha/alluvial plain environment.