Modelling the Holocene marine transgression in the Gulf of Trieste (northern Adriatic Sea): The importance of paleotopography in paleoreconstructions

The global transgression following the Last Glacial Maximum (LGM) greatly impacted vast areas of previously subaerially exposed continental shelves. When modeling paleocoastlines related to the post-LGM transgression present-day bathymetry is most commonly used as input. This study demonstrates the considerable errors that result from such an approach and highlights the importance of using realistic paleobathymetry for modeling past land-sea extent. Additionally, it presents some new insights on the Holocene transgression in the Gulf of Trieste. The results of this study were recently published in Novak (2024).

Modeling was conducted in the Gulf of Trieste (northern Adriatic Sea) where the Late Quaternary stratigraphy and sea-level history are relatively well known. Two elevation datasets were used as input data: present-day bathymetry from the EMODNET Bathymetry consortium (2022) and pre-transgressional topography from Trobec et al. (2018). The paleocoastline was modelled at several water levels between 30 and 20 meters below present-day sea level. The two elevation datasets were flooded with the “Simulate Water Level Rise/Flooding” tool in the Global Mapper GIS software. Higher sea-levels during storms and high tides were accounted for with an added increase of 1 m to the water-levels. The sea-level curve from Kaniewski et al. (2021) was used to chronographically constrain the paleocoastlines.

The modeling results on both datasets demonstrate a striking disparity, especially at the initial phases of the transgression. The largest difference is evident for -25 m: the sea barely enters the gulf when flooding the present-day bathymetry, however roughly half of the gulf is already flooded then using the paleotopographic elevation model. In this case, the differences in the maximum extent of both coastlines amounts to more than 25 kilometers NE and approximately 10 kilometers NW. Such large discrepancies have significant implications for paleoreconstruction studies. As sea-level is increased the differences in the paleocoastline positions gradually reduce until they drop below 5 km.

Additional deeper sea levels were modelled on the paletopographic dataset in order to try to better understand the dynamics of the transgression. Results show that the sea started to enter the Gulf of Trieste after it rose above -29 m a.s.l. This is corroborated by so-far published sea-level data. After sea level reached this elevation approximately half of present-day gulf was abruptly flooded. The predominant transgression direction was towards the northeast and later oriented towards the northwest. The transgression was probably controlled by the regional paleotopography which resulted from the sedimentary activity of the Southernalpine megafan systems.  

 

REFERENCES:

EMODnet Bathymetry Consortium, 2022. EMODnet Digital Bathymetry (DTM 2022). https://doi.org/10.12770/ff3aff8a-cff1-44a3-a2c8-1910bf109f85

Kaniewski, D. et al., 2021. Coastal submersions in the north-eastern Adriatic during the last 5200 years. Global and Planetary Change 204, 103570. https://doi.org/10.1016/j.gloplacha.2021.103570

Novak, A., 2024. Paleocoastline modelling – What a difference a few meters of sediment make? Quaternary International 706, 49–59. https://doi.org/10.1016/j.quaint.2024.07.005

Trobec, A. et al., 2018. Thickness of marine Holocene sediment in the Gulf of Trieste (northern Adriatic Sea). Earth System Science Data 10, 1077–1092. https://doi.org/10.5194/essd-10-1077-2018