ICTJA PhD Presentation Award 2019 – Hanneke Heida (Oral Presentation)

Numerical modeling of flexural isostasy in the Western Mediterranean during the Messinian Salinity Crisis: implications for sea level and connectivity between sub-basins.

The Mediterranean as we know it today has only been around relatively recently, forming as the Neotethys ocean separating the European and African continents closed, causing mountain building, extension of new basins, and isolation and separation of the Mediterranean from its surrounding seas. Around six million years ago (5.96-5.33 Ma), these events culminated in  a period of restricted water exchange with the Atlantic Ocean, evidenced by massive deposits of salt (evaporites) and deep incised river canyons below the sea floor. Ever scince the discovery of these features on ocean drilling expeditions in the 1970’s and subsequent seismic investigations there has been an intense scientific debate about what caused this ‘Messinian Salinity Crisis’ (MSC), if and how much the sea level fell, and how it has been possible to accumulate such a large amount of salt (~5% of the global oceans worth) in a restricted setting. This research is part of the European Research Network 'Saltgiant' aimed at interdisciplinary scientific collaboration to help resolve some of these questions.  

Figure 1: Most recent estimate of Messinian evaporite thicknesses in the Mediterranean, by Haq et al., (2019)

Understanding how this crisis progressed requires an clear picture of the shape and depth of the basin at the time, as this determines the amount of water required for connectivity between the eastern and western parts of the sea, the distribution of evaporites, and erosional patterns. We apply planform (pseudo-3D) modelling of flexural isostasy, determining the vertical sinking effect of the accumulating evaporites and sediments since the MSC to reconstruct the depth of the basin before and at key moments during the crisis. The work is currently focussed on the basins surrounding the Balearic Promontory as their particular topography and evaporite distribution provides good constraints on the progression of the MSC, and on the Alborán sea, where incision due to a catastrophic refolding at the end of the Messinian are most apparent and which is a key area to understand the drivers leading to dis- and reconnection of the Mediterranean and Atlantic.
Reconstructing of the erosional features seen in the geological record allows us constrain the amount of sea level drop required to expose the Mediterranean shoreline to such erosion. Finding a consistent value and timing for a drawdown phase during the crisis is a crucial step towards understanding the type of sea the evaporites formed in, the climatic effects of the crisis and allows us to imagine the Mediterranean landscape looking very different from today.

Figure 2: TISC workflow, showing step-by-step removal of sediments and water, calculating deflection and decompaction 

 Our models are sensitive to a wide variety of features. The strength of the crust carrying the load of the sediments determines the magnitude of the response to an applied load on the margins of the basin, and the nature of the sediments deposited is crucial for the calculation of their thickness from the speed of the seismic waves, as well as the amount compaction since deposition.
Another complicating factor is tectonic deformation or volcanic activity after the crisis, which might have caused vertical motions that are not due to the sediment loading and needs to be corrected for.
We use lithosphere scale modeling of the thermal an rheological features and test various sea level drop scenarios to find a best fit between our model results and the geological data provided by seismic stratigraphy and boreholes penetrating to the Messinian succession.

Figure 3: Top: Model results for the Western Mediterranean at the end of the Upper Unit deposition and at a sea level of -1400 meters. Shown is the topography, with modeled paleeoshorelines (red) and the onlap of Mobile Unit (yellow dashed) and Upper Unit (green dashed). Center: Cross section at location indicated in top panel in current day. Bottom: Same cross section after removal of Pliocene and Quaternary sediments and sea level drop.

Preliminary results indicate that a kilometric scale sea level drop is required to allow for the top of the Upper Unit evaporites to be exposed to erosion at the end if the crisis. At such a sea level the central part of the Balearic Promontory is completely exposed, requiring a higher sea level for evaporite deposition in the Central Mallorca Depression, one of the most intriguing features of the Messinian succession in the area.

 In two years we hope to be able to compare results across different parts of the Mediterranean and have an improved constraint on the timing, duration and magnitude of sea level fall. Combining this with geochemical and hydrological modeling as well as insights from field and experimental studies will hopefully help resolve some of the mysteries surrounding this enigmatic period in the history of the Mediterranean.