Once you go down 3,000 feet, the ocean becomes pitch black, a harsh environment for only the toughest marine creatures. At the end of the Cretaceous period, though, the asteroid that killed the dinosaurs tried to wreak havoc at this depth near modern-day Haiti with unconventional weapons: land grasses and trees.

According to a new article in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, evidence of land plants decaying on the ocean floor is present in sediments formed just after the asteroid impact that killed the dinosaurs some 65.5 million years ago. As blooming bacterial populations feasted on the woody material, oxygen levels plummeted in the nearby waters, partially contributing to the extinction of 95 percent of marine plankton, one of the oceans’ crucial food sources.

By carefully analyzing the contents of marine sediments formed just after the asteroid struck the earth, the article’s authors—a trio of researchers from Tohoku University in Sendai, Japan—were able to identify a spike in the amount of molecules from terrestrial plant resins, waxes, and essential oils. The team also found fossils of benthic foraminifera, microscopic, shelled organisms that live on the sea floor in the bathypelagic zone, the lightless swath of ocean between 3,000 and 13,000 feet deep also known as the “midnight zone.” In other words, massive amounts of land plant material suddenly settled into deep water just after the asteroid impact, which only would have happened after catastrophic tsunamis and soil erosion.

When combined with previous work, the study “shows those changes [to land plants] are global,” says paleobiologist Kunio Kaiho, one of the authors, noting that his team and others have found similar evidence of plant remains in Spain and Japan. However, finding direct evidence of “low dissolved oxygen conditions in the intermediate water [is] new.”

For the first time, researchers uncovered a massive spike of dibenzothiophenes, plant derivatives that only form in low-oxygen environments, alongside the other plant materials and plankton fossils present just after the impact. Confirming that plant decay lowered deep-sea oxygen levels illustrates how much more difficult it would have been for marine life at these depths, says Kaiho, noting that 10 percent of the Cretaceous’ benthic foraminifera species went extinct.

However, Kaiho stresses that the bulk of plankton die-offs occurred in surface waters, which were much more disrupted by the asteroid’s devastating effects.