I don't think anyone is going around spraying calcium carbonate on the ocean from tugboats for no discernable reason

There could be reasons, really - changing albedo of ocean to lower temperature of surface layers and preventing sunlight to go under surface effectively killing sea biomes. First could be an attempt to create hurricanes on temperature gradient in open sea and second could lead to seafood shortages.

Also, there could be an accidents with cargo barges with limestone or chalk, both are pretty common resource used in chemical industry and often moved by sea.

That could be checked with AIS tracking, such activity should leave some traces in ship movements.