Here, we test new kill-mechanisms for iconic Devonian mass-extinctions. Our nascent hypothesis suggests that formation of exhalative (sedex) Zn-Pb-Au deposits resulted in global Ocean Anoxic Events. Our novel proxy measures the geochemistry of severely malformed organic-walled fossil plankton, coinciding with the extinctions. Tracing the heavy metal contamination and stratigraphy provides a new tool to pinpoint sedex and associated REE deposits.