The wettability of mineral surfaces controls a range of phenomena in natural and industrial processes. In reservoirs, rock wettability determines the effectiveness of oil production; thus, modification of mineral surface properties can lead to enhanced oil recovery. Recent work reports that potential determining ions in seawater, Mg2+, Ca2+, and SO42–, are responsible for altering the wettability of calcite surfaces. In favorable conditions, e.g., elevated temperature, calcium at the calcite surface can be replaced by magnesium, making organic molecules bind more weakly and water molecules bind more strongly, rendering the surface more hydrophilic. We used atomic force microscopy in chemical force mapping mode to probe the adhesion forces between a hydrophobic CH3-terminated AFM tip and a freshly cleaved calcite {10.4} surface to investigate wettability change in the presence of Mg2+ and SO42– at 75 and 80 °C. We made submicrometer scale maps of adhesion force and contact angle and demonstrated that the adhesion force between the hydrophobic tip and calcite decreases when both Mg and SO4 are present. Surface analysis with X-ray photoelectron spectroscopy showed Mg associated with calcite even after rinsing with CaCO3-saturated deionized water, suggesting sorption on or in calcite. When the calcite-saturated solution of MgSO4 was replaced by calcite-saturated NaCl at the same ionic strength, adhesion force increased again, indicating that the effect is reversible and suggesting Mg replacement by Ca. Experiments with solutions of Na2SO4 and MgCl2 suggest that Mg2+ uptake is favored when SO42– is also present