Impact of end-member mixing on depth distributions of δ13C, cadmium and nutrients in the N. Atlantic Ocean

The PO4, δ13C and Cd depth distributions measured in the N. Atlantic Ocean during recent GEOTRACES and CLIVAR cruises are a result of biological and circulation processes, with in-situ uptake and remineralization of organic matter dominating in the upper ~300 m and mixing of water masses dominating in the thermocline and deep sea. The observed slopes of the NO3-PO4, preindustrial δ13C-PO4 and (by analogy) Cd-PO4 along isopycnals deviate significantly from that expected from Redfield in the thermocline and deep sea and in each case the observed slope is similar to the slopes of the end-member water masses being mixed. The depth increase in the Cd-PO4 slope resulting from end-member mixing explains the observed kink at ~1 μM PO4 in the Cd-PO4 relationship in the N. Atlantic. The observation that the PO4- preindustrial δ13C-Cd relationship in the modern N. Atlantic is heavily influenced by end-member mixing throughout most of the water column complicates the use of δ13C and Cd as proxies of the paleocean's nutrient distribution because changes in the PO4-δ13C-Cd composition of locally and remotely produced end-members water masses would change the PO4, δ13C and Cd depth distributions in the paleo N. Atlantic Ocean.