The biogeochemistry of trace metals and their isotopes in the Mediterranean and Black Seas

Abstract

The marine biogeochemistry of trace metals and their isotopes is a burgeoning field in Earth sciences. The primary interest in trace metals stems from the fact that many metals are utilized in biochemical functions of marine phytoplankton which form the base of the marine food web. Low concentrations of bioessential elements may limit primary productivity in large swaths of the global ocean, while elevated concentrations around anthropogenic sources can prove toxic to marine phytoplankton. Thus, it is important to understand the sources and sinks of metals to the ocean and the biogeochemical processes that influence their distributions. Furthermore, sedimentary records of trace metals provide important constraints on environmental change, such as the redox evolution of the ocean-atmosphere system throughout Earth’s history.

In this study, the biogeochemistry of trace elements and their isotopes was investigated as part of the 2013 GEOTRACES expeditions in the Mediterranean and Black Seas. The unique oceanographic properties of these marine basins were exploited to better understand how specific processes influence the distributions of trace metals and their isotopes. The Mediterranean Sea receives the largest flux of atmospheric deposition of any modern marine basin, is strongly impacted by anthropogenic activity, and experiences overturning circulation analogous to the global ocean. The distribution of dissolved aluminum displayed high concentrations (up to 175 nM) in the Mediterranean Sea that are likely a result of the intense dust deposition. Strong correlations between dissolved aluminum and silica with salinity indicate that the general distribution of dissolved aluminum is controlled by conservative mixing, including the long recognized covariation between aluminum and silica. The distributions of dissolved iron, zinc, cadmium and lead in the Mediterranean Sea were also investigated, which reveal striking concentration gradients between the eastern and western Mediterranean basins. Atmospheric deposition of lithogenic material results in elevated concentrations of dissolved iron in Mediterranean surface waters, while anthropogenic sources of zinc, cadmium and lead within the western Mediterranean basin may support the observed concentration gradients. The Black Sea is the world’s largest anoxic marine basin and is an ideal natural laboratory for investigating the behavior of trace metals and their isotopes under variable redox conditions. The distributions of a suite of dissolved trace metals including aluminum, titanium, manganese, iron, copper, zinc, gallium, yttrium, zirconium, cadmium, lanthanum, and uranium in the Black Sea water column are reported. The distributions of trace metals in the Black Sea are primarily controlled by redox reactions that induce changes in solubility of the metals, the formation of insoluble metal-sulfides in the deep sulfidic basin, and the scavenging/regeneration of metals associated with the remineralization of sinking particles. Additionally, the iron and uranium isotope systems are investigated in the water column and sediments of the Black Sea. Iron isotope fractionation factors associated with pyrite formation, +2.75 ± 0.22‰ (2SE), and oxidation reactions, -0.19 ± 0.07‰ to -0.40 ± 0.12‰ (2SE), are calculated from the isotopic composition and concentration of iron in the water column. Similarly, the isotopic composition and concentration of U in the water column is used to calculate the uranium isotope fractionation factor associated with reduction of U(VI) to U(IV), -0.61 ± 0.08‰ to -0.84 ± 0.11‰ (2SE). Implications for the application of the iron and uranium isotope paleoredox proxies to the sedimentary record are discussed.