"salinity differences" refers to the variations or disparities in the amount of salt content found in different bodies of water or substances. It represents the contrast in the levels of saltiness between various areas or elements. Full definition
> Quote by Andrew Dodds / If I recall correctly, the time frame from 70 to 50 million years ago accomodates the opening of large chunks of the atlantic and finally the Artic ocean, allowing an ocean circulation regieme dominated by temperature difference instead of salinity differences. (realclimate.org)
After it closed, approximately 3 million years ago, ocean currents changed considerably, with for instance an increasing salinity difference between the Pacific and Atlantic Ocean. (bitsofscience.org)
This is illustrated well in salinity differences shown for the Atlantic (1985 — 1999 minus 1955 — 1969) and Pacific (1980s minus 1960s) in Figure 5.6. (ipcc.ch)