Metalliferous sediments from the H.M.S. Challenger voyage (1872-1876)

The legendary cruise of H.M.S. Challenger (1872-1876) around the globe must always occupy an eminent place in the annals of oceanography, as being the first systematic attempt made on a global scale to explore the ocean. This expedition made fundamental discoveries in biology and geology which have not been surpassed by any later scientific cruise. Sediment with high content of metals (later called “metalliferous”) was among the enigmatic findings taken onboard. Although the nature of metalliferous sediments is well known today, the very first sampled sediments of this type have not been studied to date. Motivated by the historical value of Challenger’s metalliferous sediment collection we undertook an investigation addressing two questions: (1) the composition of sediments from seafloor for which we have very limited data; (2) Sr-Nd-Pb-Fe-Zn-isotope signature of these sediments collected before the substantial human impact on the ocean during the 20th century.The SE Pacific metalliferous sediments sampled by the Challenger’s explorers are of 2 types: (1) metalliferous oozes blanketing ridge crests and flanks down to the calcite compensation depth (CCD); and (2) stripped of CaCO₃ metalliferous sediments located beneath the CCD in the deeps near the mid-ocean ridges. The abiogenic part of these sediments is composed mainly of poorly-crystalline to X-ray amorphous Fe-Mn-oxyhydroxides, and an amorphous silicate phase. These sediments have geochemical features similar to those of all the other metalliferous sediments: very high Fe and Mn content (on abiogenic basis), very low Al/(Al + Fe + Mn), and high content (on abiogenic basis) of As, Ba, Be, Bi, Cd, Co, Cu, Mo, Ni, Pb, Sb, Th, Tl, U, V, W, Y, Zn and Zr. Their REE distribution patterns are similar to that of deep seawater and show weak signs of hydrothermal imprint (weak positive or no Eu anomaly).Seawater and/or terrigenous input from South America control the Sr-Nd-Pb-isotope signature of the Challenger metalliferous sediments and have almost completely obliterated any original MORB-derived hydrothermal signal. Zn isotopes are mainly contributed from seawater although other Zn sources (hydrothermal fluid and detrital aluminosilicates, barite and volcanic glass) are necessary to fully explain Zn-isotope ratios. Fe isotopes indicate relatively slow Fe²⁺ to Fe³⁺ oxidation in the non-buoyant plume, thus producing relatively lighter Fe-isotope signature of the FeOOH particles that formed the studied metalliferous sediments.