Relationships between the content in total organic carbon (TOC) and the enrichment factors in copper (Cu-EF, panel A) or nickel (Ni-EF, panel B) for the 27 formations listed on the left-hand side of the figure. The formation names are listed in Table 1. Panel C shows the cross diagram opposing the enrichment factors in Cu and Ni for the same formations. Panel D illustrates the relationships between TOC and Cu-EF for the formations 29, 31, 45 and 52 with samples yielding relative excess in Cu compared to the TOC values (arrow). Panel E: same diagram for formations 43 and 50, showing a relative deficit in Cu compared to the TOC values (arrow). Panel F: same diagram for the samples of two modern euxinic basins, the Cariaco Basin (formations 22 and 35) and the Black Sea (formations 20 and 44).