ASCII actually had up-arrow instead of caret before 1967/1968, although that was slightly before the birth of Pascal (published in 1970).
In the case of Smalltalk, it was Digitalk's "Methods" (the precursor of Smalltalk/V) that adopted := for assignment in place of the left arrow (though the IBM PC character set actually had one, just not with the same code as ASCII 1963) since all of its tutorials were aimed at Pascal programmers.