Paleobotany presentation 2.5.23

Author: Ethan Perry

On February 5, Brad Evraets, a Water Resources Science graduate student at UMD, gave us a presentation of his earlier research on fossil leaves and paleoclimate. Have you ever seen a leaf floating downstream and wondered where it will end up? Mr. Evraets can tell you exactly what happened to over 200 leaves that floated down a river 50-60 million years ago in what’s now Washington State, in ceded land of the Yakama Nation.

That’s because he found fossil imprints of many of those leaves by cracking open rocks. Leaves that settled on the riverbank ended up in coarse-grained sandstone, while those that floated onto the floodplain ended up between layers of finer sediments. The bedrock has been lifted and shifted since then by the movements of the earth’s crust, but where bare outcrops poke out of today’s forest, there are fossils for the finding. And by studying the fossils, Mr. Evraets discovered clues to what the climate was like in that time and place.

Decades of research has shown that broadleaf shapes and sizes tend to reflect the climate they grow in. Plants in warm climates often have larger leaves with simple shapes, while leaves in cooler climates are smaller with more lobes and teeth on the edges. Using equations describing the relationship between leaf shapes and climate, Mr. Evraets and colleagues estimated mean annual temperatures for this ancient river valley to be among the warmest recorded so far from the western U.S. during this time period. Which makes sense given that most estimates have come from inland and higher elevation sites.

Comparing the temperatures between two bedrock formations showed a climate warming from something like Atlanta to something like Tampa. The ancient leaves revealed a high diversity of trees in a warm, humid climate far different from the dry side of the Cascade Mountains we know today.

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