I think this is best done with a for loop, can't think of a function that could do so out of the box. The following should do what you want (if I understand you correctly).
current.sum <- 0
for (c in 1:nrow(caribou.sub)) {
current.sum <- current.sum + caribou.sub[c, "difference"]
carribou.sub[c, "difference_sum"] <- current.sum
if (current.sum >= 1470) {
caribou.sub[c, "keep"] <- 1
current.sum <- 0
}
}
Feel free to comment if it does not exactly what you want. But as pointed out by alexwhan, your description is not completely clear.