Seasonal carbon remineralization in the Mediterranean Sea: Results from a paired-radioisotopic approach

Vertical particle transport rates in the upper ocean euphotic zone are an important component of the marine carbon cycle and the ocean’s ability to sequester atmospheric carbon dioxide. An important pathway for the loss of CO₂ from the surface ocean is via the “biological pump”, driven by the production of particulate organic carbon (POC) and its subsequent export to the deeper ocean via vertical settling. The ocean is a major reservoir of carbon which is potentially sequestered in the deep ocean for millennia. During the vertical settling of POC, certain fractions are remineralized via microbial activity, and the bulk of this remineralization occurs at depths of 100-1200 m in the water column. Global climate and ocean biogeochemical models suggest that the remineralization depth is a highly sensitive parameter for predicting the ocean’s capacity to store carbon. Hence identifying the remineralization depth and its potential variability with a changing climate is important to advance our understanding of the marine carbon cycle and to guide models for delivering more precise predictions under future conditions. Here we assess the seasonal carbon remineralization depth at the DYFAMED time-series station in the north-western Mediterranean Sea using multiple paired radiotracers. The vertical POC flux is estimated using ²³⁴Th:²³⁸U and ²¹⁰Po:²¹⁰Pb isotopic ratios from samples collected in 2018 and 2019. From the calculated POC flux profiles we estimate the remineralization depth and its seasonal variability.