The demise of a ‘salt giant’ driven by uplift and thermal dissolution

Abstract

The development of giant salt basins and eventual cessation of rapid salt deposition is founded on a delicate balance of salinity and heat fluxes within the water body governed by tectonic, climatic and eustatic change. The onset of salt deposition in such basins is widely accepted to be initiated by basin restriction. However, the processes that lead to the termination of salt deposition are comparatively unclear. Here we use an array of 2D and 3D seismic surveys to reveal that the truncation surface at the top of a thick salt sequence in the Eastern Mediterranean is far more extensive than previously thought. We show that uplift of the salt driven by deformation and thermal dissolution initiated the demise of the 'salt giant', even prior to the final dilution and emplacement of brackish Lago Mare and fluvial deposits. Progressive uplift of the salt through the thermocline and into the under-saturated epilimnion led to dissolution. We argue that dissolved salt was recycled and re-precipitation from the hypolimnion in the deepest sections of the basin contemporaneous with dissolution of halite from the shallower epilimnion. These findings explain how rapid basinwide salt deposition was brought to an end in the Eastern Mediterranean and present a novel process for sculpting the final architecture of a 'salt giant'.

Hydroclimatic Controls on Salt Fluxes and Halite Deposition in the Dead Sea and the Shaping of “Salt Giants”

“…The mechanisms forming these salt giants have been the focus of a longstanding debate (Hsu, 1972;Schmalz, 1969;Simon & Meijer, 2017). Evaporation from halite-saturated brine drives evaporitic deposition (Hsu, 1972;Lowenstein & Hardie, 1985;Schmalz, 1969); however, mass balance arguments alone cannot explain the observed spatial variations in thickness (Hsu, 1972;Meijer & Krijgsman, 2005), the required deposition rates (Hsu, 1972;Schmalz, 1969), and the basin architecture of these salt giants (Flecker et al, 2015;Kirkham et al, 2020;Meijer & Krijgsman, 2005). Evaporation at the surface of deep hypersaline basins sets in motion, intricately coupled mechanisms involving the diffusive and convective transport of heat and dissolved salt, driving halite deposition on the basin floor (Arnon et al, 2016;Sirota et al, 2016Sirota et al, , 2017.…”

“…Evaporation at the surface of deep hypersaline basins sets in motion, intricately coupled mechanisms involving the diffusive and convective transport of heat and dissolved salt, driving halite deposition on the basin floor (Arnon et al, 2016;Sirota et al, 2016Sirota et al, , 2017. Our data-rich observations were conducted for the Dead Sea (Figures 1a and 1b), the only modern analog of deep hypersaline basins (Arnon et al, 2016;Kirkham et al, 2020;Sirota et al, 2016Sirota et al, , 2017Sirota et al, , 2018. This deep (~290 m), perennial, hypersaline lake actively deposits thick halite units in response to negative water balance (Lensky et al, 2005;Sirota et al, 2017;Steinhorn, 1983;Stiller et al, 1997); the lake is Ca-Cl rich compared to evaporated ocean water, and its composition changes, for example, decrease of Na/Cl ratio, with halite deposition (Gavrieli et al, 2011).…”

Sedimentology and stratigraphy of a modern halite sequence formed under Dead Sea level fall

“…Thinning and facies change of halite units along a landward transect were observed in halite sequences related to the MSC (Garcia-Veigas et al, 1995;Bertoni & Cartwright, 2007;Gvirtzman et al, 2017;Kirkham et al, 2020). These large-scale examples emphasize the role of 'halite focusing' in the accretion of thick halite sequences (Sirota et al, 2018); rapid depocentre halite deposition at the expense of marginal thermally driven halite dissolution during the summers.…”

“…Similarly, the process of halite focusing also shapes the large-scale architecture of 'salt giants', i.e. enormously thick (ca kilometres) and wide (hundreds of kilometres) halite deposits (Kirkham et al, 2020).…”

“…The 'halite focusing' mechanism has a primary role in syn-depositionally shaping the original geometry of the marginal halite unit; thus, the marginal thinning of halite is not formed by post-depositional processes. This analysis becomes highly crucial because thinning of marginal halite sequences up to their complete absence were observed globally (Brownfield et al, 2009;Roveri et al, 2014;Gvirtzman et al, 2017;Demicco & Lowenstein, 2019;Kirkham et al, 2020).…”