On NW European salt production and tradeThats an interesting point. I know you usually have some great sources, do you have anything on the expansion of salt production?
Basically, salt bricketing was important but regionally dispersed in Gaul and Britain even before the conquest, and expanded in Channel and Atlantic salted swamps, but Seille and Saale basins as well, trough salted springs exploitation.
Their commercialisation was local and regionally-focused, altough finds do point that it could go as far as Lower Danube. It's possible the production/commerce rivalty prevented a renew of salt exploitation north of Alps after the troubles of the end of First Iron Age, and they really were reused in Middle-Ages. The author gives the exemple of Reichnall's salt North of Danube compared to Salzburg's due to the limes construction and the market being cut for Noricans.
Now, for the maritime trade of salt and salt-ships, while they are big enough, they are flat-bottomed ships fit for coastal and lagunar navigation, with the author making a comparison, without saying it existed up to the IVth century (records and finds tend to disappear until the XIIth on this regard), with medieval trade of English salt in North Sea.
Let's remember that salt in Romania was often replaced by salty fish sauces, notably garum and that salt trade was under state supervision.