Evidence for changes in carbon isotopic fractionation by phytoplankton between 1960 and 2010

Rising CO₂ is expected to drive a myriad of environmental changes in the surface ocean. Deciphering the phytoplankton response to this complex change is difficult. Here we determine whether a trend in the biological fractionation of stable carbon isotopes (ε_p) has occurred over the past 50 years. ε_p is primarily controlled by the acquisition and intracellular transport of inorganic carbon and the rate of carbon fixation. In turn, these processes are sensitive to phytoplankton physiology, community composition, and notably inorganic carbon availability. ε_p may therefore carry a signal of biological response to climate change. Temporal and spatial records of ε_p can be deciphered from the difference between the stable carbon isotopic composition of particulate organic matter (δ¹³C_POC) and that of the ambient inorganic carbon pool (δ¹³C_CO2). Here we establish a global record of ε_p extending from the 1960s to today, extracted from a newly compiled data set of global measured δ¹³C_POC and part measured/part climatology δ¹³C_CO2. We find that ε_p has changed significantly since the 1960s in the low- to mid-latitude surface ocean. The increase is most pronounced in the subtropics, where it is estimated at > 0.015‰ per year. Our findings of such rates of change are further supported by a high resolution temporal record from a single sediment trap near Bermuda. Our results are consistent with the idea that ε_p is affected by increased inorganic carbon availability driven by the rise in atmospheric CO₂.