Using triple oxygen measurements of lacustrine carbonates to constrain the Miocene topography of the Dinaric Alps

The Dinaric Alps formed as a result of the collision between the Adria microplate with Eurasia during ongoing closure of the Tethys Ocean. However, there remain a number of questions regarding the mechanisms that created and sustained the high topography (maximum modern elevation of ~2500 m) of this region. We take advantage of a series of lacustrine basins—known as the Dinaride Lake System (DLS)—that formed in the Early and Middle Miocene to constrain paleo-elevations of the Dinaric Alps using stable isotope paleoaltimetry. We collected authigenic lacustrine carbonate samples from six basins in Croatia and Bosnia and Herzegovina that span the range from sea-level to high-elevation (~1200 m) and measured these samples for δ18O. In addition, we also collected stream samples that span the range to constrain the modern change in δ18O across the Dinaric Alps. Stable-isotope paleoaltimetry is based on the concept that, as moist air parcels are forced upwards by orography, 18O is preferentially removed by the resulting precipitation, resulting in lower δ18O at higher-elevations and in the lee of ranges. Today, meteoric water δ18O is high (~ -6‰) at the coast and is ~5‰ lower at the crest of the range (~ -11‰). However, Middle Miocene lacustrine carbonate δ18O is high (~ -3‰) at the crest of the range and lower (~ -6‰) at the coast. Because lacustrine carbonate δ18O is frequently impacted by evaporation, we analyzed a subset of our samples for Δ17O, which is sensitive to the degree of evaporation. These carbonates have Δ17O values ranging from -68 to -150 per meg. Using our Δ17O data and a model of lake evaporation, we reconstruct the unevaporated meteoric water δ18O. Our preliminary results show a similar trend as in the modern, with higher δ18O values at the coast and lower δ18O at the crest of the range. Reconstructed unevaporated meteoric water δ18O at the crest is lower by 2-5‰ than modern water δ18O at the crest of the Dinaric Alps. That unevaporated meteoric water δ18O might have been lower than today at the crest of the range suggest that the Dinaric Alps were higher in the Middle Miocene that today, assuming that coastal meteoric water δ18O was similar to today. Thus, ongoing extension within the Dinaric Alps due to slab rollback may be responsible for lowering of topography.