UC San Diego

We present radiocarbon results from a geographically constrained depth transect of sediment cores from water depths spanning 400-3500 meters in the southeastern Atlantic. The distribution of radiocarbon is a tracer of the ventilation history of mid-depth water and can delineate water of North Atlantic versus Southern sources. Coexisting benthic and planktonic foraminifera pairs in the sedimentary record have been used to reconstruct past changes in ventilation, yet the fidelity of the method has been called in to question due to potential problems and biases. Thus, before interpretation of past changes to the ocean’s interior can be made, it is important to assess the validity of using benthic/planktonic pairs to distinguish the essential features of modern seawater radiocarbon distribution. Here we present radiocarbon and stable isotope results from twenty-seven high sedimentation rate modern samples from the southeastern Atlantic, an area where the seafloor intersects all the principal water masses involved in the thermohaline circulation of the Atlantic. We use two cosmopolitan planktonic species, from different habitats, and with a 14C age of less than 2000 years and find several consistent trends; it is these consistent trends that provide quantitative insight to the benthic-planktonic radiocarbon method. In all cases, the G. bulloides 14C ages are older than O. universa. However, this age offset between O. universa and G. bulloides is not consistent at all water depths – the ages diverge significantly in the 2000-1500 water depth range, in parallel with the shifts in sedimentary organic carbon %. This inter-species variability is large enough to overwhelm any possible radiocarbon differences in deep water masses (benthic foraminifera). Clearly, even among the most abundant taxa, planktonic foraminifera do not provide an interchangeable chronological reference for the benthic foraminiferal radiocarbon. Restricting the analysis to planktonic foraminifera that are most likely to be “surface dwelling”—in our case, O. universa—results in a benthic-planktonic radiocarbon depth profile that bears the same structure as modern seawater nutrient profiles (especially dissolved silica) in the region. However, benthic-planktonic radiocarbon depth profile does not reproduce the features of the closest dissolved inorganic carbon 14C (modern seawater) profile. In particular, with respect to modern DIC 14C measurements, the benthic-planktonic depth profile gives the impression of significantly “older” water bathing mid depth sediments (600-800 meters) and abyssal depth sediments (greater than 2500 meters). The consistency of the trends with water depth provides a measure of the fidelity of the method for reconstructions over the interval since the last glacial maximum.