There can be very little doubt, given the identity of the explanations and the similarity of language, that Ovid and Macrobius were drawing on a common source. The same source is reflected at Gellius XVII viii 8-16. Here Taurus the philosopher asks Gellius why oil often congeals, but wine does not. Gellius answers that wine is fiery by nature, which is why Homer called it [Greek: aithopa oinon]. Taurus responds that wine is indeed known to have fire in it, for it warms the body when drunk; yet vinegar, in spite of its cooling effects, never freezes; perhaps things which are light and smooth are more prone to freezing. It is also worth asking why fresh water freezes, but seawater does not. 'tametsi Herodotus ... historiae scriptor contra omnium ferme qui haec quaesiuerunt opinionem scribit mare Bosporicum, quod Cimmerium appellatur, earumque partium mare omne quod Scythicum dicitur, gelu stringi et consistere'. No explanation for the freezing-over is given.[22]
[Footnote 22: Macrobius does include the explanation for the freezing-over. In view of his fuller account, I believe that Macrobius drew his material from Gellius' source and not from Gellius. It is of course possible enough that Macrobius conflated Gellius with another source.]
Ammianus Marcellinus XXII 8 48 gives the same two explanations for the Euxine's freezing as Ovid: 'quicquid autem eiusdem Pontici sinus Aquilone caeditur et pruinis, ita perstringitur gelu ut nec amnium cursus subteruolui credantur, nec per infidum et labile solum gressus hominis possit uel iumenti firmari, quod uitium numquam mare sincerum, sed permixtum aquis amnicis temptat'. At XXII 8 46 he once again mentions the sweetness of the Euxine's waters.
Lucan describes the freezing of the Euxine (V 436-41), but gives no explanation of the cause.
=39. PLAVSTRI PRAEBENTIA FORMAM ... SIDERA.= The Great Bear. Other mentions of the constellation at _Met_ X 446-47 'inter ... triones / flexerat obliquo plaustrum temone Bootes', _Tr_ III iv b 1-2 (47-48), III x 3-4 & V iii 7-8, and _EP_ I v 73-74. Compare as well Germanicus _Aratea_ 24-26 'axem Cretaeae dextra laeuaque tuentur / siue Arctoe seu Romani cognominis Vrsae / Plaustraue [_Grotius_:-que _codd_], quae facie [_scripsi (datiuum)_[23]: facies _codd_] stellarum proxima uerae [_Barth_: uera _uel_ uero _codd_]', _Her_ XVIII 152, Sen _Ag_ 66-68, and Lucan V 23 'Hyperboreae plaustrum glaciale sub Vrsae'.