Carbon Accumulation Rates.

We first calculated the rate at which carbon has been sequestered in different ecosystems during recent times. For sites with ¹⁴C dating, we took the core sample that was nearest to a century in age, calculated the total carbon in the sediment profile above that sample, and divided it by the age of the sample to obtain a mean carbon accumulation rate for the last century expressed in megagrams of carbon per hectare per year. In one site in La Encrucijada, where we did not perform radiocarbon dating, we used the layer of Santa Maria volcanic ash from the Chichonal eruption in October 1902 as a dating point. We calculated the total carbon in the sediment profile above the layer and divided it by 112.1 y, the time elapsed between the Chichonal volcanic explosion and our field sampling in 2014.

Combining the carbon–depth and the age–depth data, we created a carbon–age plot for the three sites in Baja California described above. To test how carbon accumulation scaled up with the age of the sediments, we fitted regression functions to different sections of the profile. Because both age and carbon measurements are subject to errors, we used major axis regression for this analysis.