Open-Ocean Minima in δ13C Values of Particulate Organic Carbon in the Lower Euphotic Zone

Extensive studies in the 1980s-1990s led to the characterization of latitudinal variations in sea surface delta C-13 values of particulate organic carbon (delta C-13(POC)), and relationships were found with CO2 concentrations, temperature, growth rates, and cell geometries. Surprisingly, no large-scale efforts have been made to describe variations in delta C-13(POC) values over depth in the water column. Here we compile published examples demonstrating a widespread isotopic pattern in particulate organic carbon (POC) of the upper water column. In 51 vertical profiles, delta C-13(POC) values in the lower euphotic zone on average are 1.4 parts per thousand lower than delta C-13(POC) values in the upper euphotic zone of open ocean settings. In a majority of locations this vertical decrease in delta C-13(POC) values is >2 parts per thousand and up to 5 parts per thousand, larger than the commonly recognized vertical delta C-13 variation in dissolved inorganic carbon over the same depths. We briefly review hypotheses and supporting evidence offered by previous studies of individual water columns: The observed patterns could result from vertical differences in photosynthetic growth rates or community composition, biochemical composition of organic matter due to degradation, isotopic disequilibrium within the dissolved inorganic carbon pool, particle dynamics, or seasonal vertical mixing. Coordinated isotopic, biological, and seawater chemistry data are sparse, and consistent drivers of this widespread isotopic pattern are currently elusive. Further work is needed to adequately characterize the environmental conditions coinciding with this pattern, to test its origins, and to determine if the magnitude of upper water column delta C-13(POC) variations could be a useful marker of upper ocean carbon cycle dynamics.