I mean which carbon pools are contributing, which pools aren't, have there been any dramatic changes, and which pools might be implicated in those dramatic changes. Many reservoirs of carbon in the environment are composed of a unique mixture of carbon isotopes. Jahren's lab, for example, has found evidence, now in review for publication, suggesting a release of carbon into the environment 115 million years ago from clathrates, deposits of methane on the bottom of the oceans. It's a more subtle question than how much CO2 is in the atmosphere, but it's closer to the heart of what we want to know about how carbon inhabits chemical spheres of the earth, and how that affects climate. Data on the plants and their environments revealed a number of factors that could disrupt connections between carbon dioxide levels in the atmosphere and in the plant.
Water is a good example.
But that's also how water gets out of the plant. When there's low humidity in the atmosphere, water at the stomata naturally evaporates up and out, and plants will close their stomata to prevent this, reducing carbon intake. That finding wasn't too disappointing, Jahren notes, because arid environments normally do not yield plant fossils. But other phenomena more common to fossil-forming environments also change carbon intake, including levels of salt in the soil, light levels, temperature extremes, and humidity. Although there may be an individual species that shows a less variable intake of atmospheric carbon, the results strongly suggest that plant composition as a whole typically doesn't reflect the level of carbon in the atmosphere.