This is an interesting piece of research as it has been suggested the chalk formations have a connection with planktonic blooms – but what might have caused such huge blooms. A study at Stanford University shows how hydrothermal vents fuel massive phytoplankton blooms – see for example https://wattsupwiththat.com/2019/06/05/how-deep-ocean-vents-fuel-massive… … It is published in Nature Communications (June 2019) and suggests hydrothermal vents, or openings on the sea floor that gush scorching hot streams of mineral rich fluidic materials from the belly of the earth into surface waters, stimulate explosive growth in microscopic ocean algae. The question then might be – what would an impact such as the K/T asteroid do (as the chalk formation immediately precedes that boundary event). Indeed, chalk is mostly composed of the shells of planktonic algae, the very thing that blooms as a result of an influx of iron oxides into the oceans. One should also bear in mind chalk is very very white in its pristine condition. Not much chance of that after millions of years as sea bottom sediment slowly building up.
Phytoplankton need iron to thrive and that limits their abundance in most of the oceans where nutrients are low. However, where conditions are right phytoplankton numbers can grow explosiviely, blooming across thousands of miles – in a matter of days. This is what was discovered recently by a fleet of floating robots outfitted with optical sensors in the Southern Ocean. Something to chew on.