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What evidence showed that nobelium can form dipositive ions in water?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transuranium element Modern version of the periodic table of the elements (printable).

Radiochemists have demonstrated that nobelium exists in aqueous solution in both the +2 and +3 oxidation states using traces of the isotope nobelium-259.1

The evidence for the formation of dipositive ions in water was established through the following experimental methods:

Cation-exchange chromatography: This technique was used to separate and identify the oxidation states of the element.12
Coprecipitation experiments: These studies provided conclusive evidence that the +2 state is more stable than the +3 state for nobelium.1

This stability of the dipositive state is more pronounced in nobelium than in its homologous lanthanoid element, ytterbium, and is attributed to the stability of the filled $5 f^{14}$ electron shell. Consequently, the $N o^{2 +}$ ion exhibits chemical properties similar to those of the alkaline-earth elements, such as calcium, strontium, and barium.12