Balancing supply and demand: controls on carbon isotope fractionation in the Cariaco Basin (Venezuela) Younger Dryas to present

Carbon isotope fractionation associated with photosynthetic carbon fixation ( p) is known to be controlled by many factors. Among these factors are the availability and carbon isotopic composition of dissolved inorganic carbon (DIC), the form of DIC that is taken up (CO2 vs. HCO3−), whether DIC is taken up via passive diffusion or active uptake, specific growth rates, nutrient availability (e.g. NO3, PO3, trace nutrients), cell size and geometry, and irradiance. An understanding of the oceanographic controls on these factors and how they influence the final p signature preserved in organic matter is critical to accurate reconstructions of past environments based on carbon isotope data, including the potential to reconstruct paleo-pCO2 values. We have investigated the sedimentary record of carbon isotope fractionation in the Cariaco Basin, spanning the past 12.6 14C ky utilizing both bulk and molecular carbon isotope analyses. Our results indicate that the carbon isotope composition of CaCO3 is controlled predominantly by the supply of DIC, regardless of variations in primary production. Furthermore, p remains low and constant throughout the interval of study, despite major variability in primary production and planktonic ecosystem structure, and is in fact lower than would be predicted based on empirical relationships between p, [CO2]aq, and [PO4] developed for haptophyte algae. These results are consistent with active uptake of bicarbonate, rather than [CO2]aq by haptophyte algae, followed by intracellular carbonic anhydrase-catalyzed conversion of HCO3− to CO2 for cellular utilization.