Natural atmospheric deposition of molybdenum: a global model and implications for tropical forests

Molybdenum (Mo) is an essential trace metal that plays a central role in biological nitrogen fixation (BNF) as the cofactor in the conventional form of the nitrogenase enzyme. The low availability of Mo in soils often constrains BNF in many terrestrial ecosystems. Atmospheric sources may supply a critical source of exogenous Mo to regions with highly weathered soils likely low in Mo, particularly in tropical forests where BNF is thought to be high. Here, we present results of a global model of Mo deposition that considers the principal natural sources of atmospheric Mo—windborne mineral dust, sea-salt aerosols, and volcanic sources—which operate over geologic time. The largest source of mineral dust globally is from North Africa. We quantified Mo concentrations in dust and sediments from the Bodele Depression, a large source within North Africa, to constrain our model. Because the Mo concentration of seawater is relatively high for a trace element, we also hypothesized that sea-salt aerosols would contribute atmospheric Mo. Our model predicts higher Mo deposition to terrestrial ecosystems along coasts downstream in trade winds, near active volcanoes, and in areas that receive dust deposition from North Africa, such as the northern Amazon Basin, the Caribbean, and Central America. Regions with higher Mo deposition tend to be areas where BNF has previously been measured. The lowest Mo deposition occurs in the high latitudes, northern parts of North America, Western Australia, Southern Africa, and much of central South America. Atmospheric transport of Mo likely plays an important role in supplying Mo to ecosystems across geologic time, particularly in regions with highly weathered soils.