If the absence of polar ice shuts down the ocean's conveyor currents and fouls the ocean, what about..?

...during the age of dinosaurs the ocean was what, in a perpetual state of toxic algae bloom? There has been no glaciation for much of Earth's history. I'm not dismissive of what I heard about this periodic extinction event that causes oceans to foul, but a healthy ocean doesn't absolutely need to have polar ice, does it? Perhaps thawed ice plus more heating besides.