Salt wedges and trapped brines of low-latitude endoreic saline lakes as potential modulators of GHG emission

Abstract

Large salt lakes are long-term witnesses to climatic conditions and land use in their basins. The majority are experiencing a drastic drop in water levels due to climate change and human impact. Endoreic Lake Urmia (NW Iran), the sixth largest salt lake worldwide, is a striking example of this decline. Quantification of the relative contributions of natural variability and human impact on the lake's water supply is therefore essential. Here we present isotopic and radiocarbon analyses of surface and groundwater from the Shahr Chay River catchment, entering Lake Urmia on its western shore, and radiocarbon dating of a sedimentary core. Lake Urmia behaves like a large saltwater wedge almost entirely fed by the river and shallow groundwater. This leads to trapping of residual brines and formation of CH₄ and secondary CO₂ greenhouse gases, impacting sediment geochemical records and corresponding time scales for paleoenvironmental reconstructions. We conclude that (1) salt lakes functioning like a saline wedge, allowing organic matter oxidation, could contribute to increasing methane sources or reducing carbon sinks globally, and (2) endoreic basins worldwide need to be monitored before aridification-related salinization leads to the establishment of a saline wedge precluding any possibility of return to an equilibrium state.

Figure 1

Location of Lake Urmia (Iran) (A) on the global map of the major endoreic systems at the global scale², (B) within the regional context of the Middle East region; (C) Lake Urmia basin with mention of showing the sub-basins separated by the causeway, regional aquifers, buffer zones, and the location of the 7 cored sedimentary sequences (G1 to G7) at the Shahr Chay River mouth; (D) Lake Urmia water evolution at coring site between 1984 and 2017; E. Photos of the lake surface water sampling site (close to G4 coring) during the two field missions in 2016 and in 2017.

Figure 2

Lithology (a) and ¹⁴C time-scale with (b) sedimentary rate, and (c) AMS ¹⁴C datings^, of the Composite core (C-core) reconstructed based on the 7 boreholes (numbered 1 to 7) drilled at the Sahar Chay River mouth during the 2016 and 2017 field works. The muddy waters from coring wells (d) are located within the corresponding empty sections.

Figure 3

(A) Stable isotope contents of surface and groundwater in the Shahr Chay River basin and of muddy water retrieved form Lake Urmia corings. The mean stable isotope compositions of rainfalls at the Diyarbakir (Turkey) and Tehran (Iran) stations are defined according to GNIP data (GNIP Database, IEAE/WMO, 2023; average weighted annual summer and winter means); (B) δ¹³C_TDIC content versus ¹⁴C activity diagram for surface and groundwater from the Shahr Chay River basin and for Lake Urmia surface water; (C) Diagram δ¹³C_PCO2 vs A¹⁴C and geochemical processes determined in the Lake Urmia basin.

Figure 4

Lithology of the Composite core (a), mean sedimentary rates (b), the ¹⁴C timescale (c), and evolution of geochemical and sedimentological parameters against the ¹⁴C timescale: (d) C/N ratio of total organic matter, (e) magnetic susceptibility, and (f) arsenic contents determined by XRF.

Figure 5

Schematic representation of the overall exchange between Lake Urmia, surface water and groundwater in the Shahr Chay River sub-basin, including the behavior of lake sediments that form a near-tight barrier to direct groundwater recharge.