Fossil Leaves Reveal Effect of ‘Impact Winter’

Fossil Leaves Reveal Effect of ‘Impact Winter’

by Timothy Oleson Friday January 30th, 2015

In an analysis of fossil leaves from the Cretaceous-Paleogene boundary, when the Chicxulub impact occurred, scientists found an increase in vein density (A to B) and a decrease in leaf mass per area ( C to D). Credit: B. Blonder et al., 2014, PLOS Biol. 12(9):e1001949.

When the bolide of Chicxulub hit the Yucatán Peninsula at the end of the Cretaceous around 66 million years ago, widespread extinctions of land and sea animals resulted. However, the lasting effects of the blast on plants, which tend to be more resistant to impact-related fallout, have been less clear. Now, a new analysis of leaf fossils from the late Cretaceous offers some of the first quantitative evidence of a substantial shift in plant communities – towards more deciduous plants – following the impact.

The new study, published in PLOS Biology, supports a hypothesis first put forward in the 1980s by Jack Wolfe of the US Geological Survey and his colleagues, who examined fossils of North American plants and hypothesized that deciduous plants – which shed their leaves every year – were better suited. to the “post-impact winter” that lasted years after impacts like the Chicxulub explosion.

“After a meteorite impact, you should see a shift to more deciduous rather than evergreen plant species” due to the cooler temperatures and less sunlight that would have prevailed after the Light-reflecting dust and debris were thrown into the atmosphere by the impact, says lead author Ben Blonder, a plant ecologist and science coordinator at the University of Arizona Sky School. Such conditions, which are not conducive to photosynthesis and growth, should favor plants that do not need to invest energy to maintain their leaves all year round.

Deciduous “must have faster growth strategies than evergreens in order to absorb the same amount of carbon [through photosynthesis] throughout the year,” says Blonder. “What that means is that [deciduous leaves] should have functional traits…associated with faster growth.

In the study, Blonder and his colleagues analyzed hundreds of fossilized leaves ranging in age from 1.4 million years before to 0.8 million years after the Cretaceous-Paleogene (K-Pg) boundary. (The leaves were collected in southwestern North Dakota in the 1990s by co-author Kirk Johnson, now director of the Smithsonian’s National Museum of Natural History.) They focused on two functional traits in particular. : leaf mass per area (LMA) and vein density. (DV). Lower LMAs and higher VDs are thought to be characteristics of faster growth strategies in plants, as they represent lower carbon investments in leaves and greater capacity for rapid photosynthesis, respectively.

By measuring LMA and VD, the team quantified the “overall strategy of [each plant] whether fast or slow, and therefore whether it was deciduous or evergreen,” says Blonder. And as we cross the K-Pg boundary, “we’re seeing a move toward faster growth strategies.” By averaging the fossils before and after the boundary into two groups, the researchers reported that the LMA dropped by about 9%, while the VD rose by about 31%. The range of observed LMA values ​​also dropped in the post-impact group, largely due to the disappearance from the dataset of fossils with particularly high LMA values, which likely belonged to conifers.

The results support the hypothesis of a persistent to leafy transition just past the K-Pg boundary, he says, and suggest that as a result of this impact, there are very large changes in the functioning of terrestrial ecosystems. The exact nature of these changes and how they may have influenced the hydrological cycle or carbon storage, for example, are unknown, he says, but “this study [should] encourage others to find intelligent ways to reconstruct these changes.

Compared to earlier work by Wolfe and colleagues, which identified evergreen versus deciduous taxa based on overall plant characteristics, the new study’s approach was “in a sense, more strict because [the researchers used] quantifiable measurements,” explains Robert Spicer, a paleobotanist at the Open University in England who was not involved in the work. He says he agrees that the new observations confirm Wolfe’s idea regarding the region where the samples were found, but warns that the effect may have only been local, as there are few evidence of similar transitions in flora elsewhere.

The observed changes are also “much more subtle than I would have thought,” says Spicer. This subtlety could indicate that LMA and VD are not as strongly correlated with persistence and caducity as suggested, he says, or that the transition of plant taxa was more related to longer-term climate change that began before the K-Pg transition – cooling and lower atmospheric carbon dioxide in particular – only at the Chicxulub impact.

Spicer favors the latter explanation, noting a discrepancy he says is ignored in the new study: the re-emergence of ferns – whose spores can only survive a few years – in North America and beyond soon after the he Chicxulub explosion suggests that the post-impact winter was brief. But if the impact of winter had only lasted a few years, he notes, “most seeds of woody plants, including those of evergreen species, would have survived” because woody plants tend to produce large numbers of seeds that are known to survive for long periods of time. in the ground. Thus, whatever was driving the observed transition from evergreen to deciduous, it must have taken place “on a much longer time scale” of at least hundreds to thousands of years. years in order to “overcome the resilience of the soil seed bank” and kill as many species.

Blonder says the evidence from this study, which is “based on one of the best fossils [leaf] datasets in the world for this period”, still indicates an important role for the impact of Chicxulub in the modification of forests. But, he says, the “big limitation here” is that quality data – well-preserved sheets, around 66 million years old – from around the world are hard to come by. “We try to draw overall conclusions” using the available data, Blonder says, but “it’s very possible that better data from other sources…will provide a more nuanced view or potentially even challenge our conclusions.”