The behavior of trace elements in seawater
Trace elements enter the oceans from a variety of sources, including oceanic crust, mid-ocean ridges, seafloor sediments, hydrothermal vents, rivers, glacial meltwater, and eolian (wind-borne) particles (Martin et al., 1976; Martin and Whitfield, 1983; Duce et al., 1991; Metz and Trefry, 2000; Kim et al., 2015).
Trace elements that accumulate on the seafloor and become enriched in marine sediments originate from multiple pathways and processes. A significant portion is derived from sinking particulate material, both organic and inorganic, which transports trace elements from surface waters to the deep ocean. This “rain” of particles includes detritus from marine organisms, mineral grains, and atmospheric dust that settles into the water column. Additionally, trace elements are introduced directly from seawater through various geochemical and biological interactions.
In oxic (oxygen-rich) environments, trace elements are often removed from seawater by binding to oxide minerals, such as iron and manganese oxides, through adsorption and co-precipitation processes. This inorganic interaction plays a key role in scavenging dissolved trace elements from the water column, facilitating their deposition on the seafloor.
Biological cycling also contributes to trace element distribution, as marine organisms uptake and metabolize elements during growth. Upon their death, the resulting biomass, enriched with trace elements, sinks to the seafloor, further contributing to sediment enrichment. Some trace elements are incorporated into biogenic structures like shells and skeletons, which persist in sediments long after the organisms decay.
In anoxic (oxygen-depleted) environments, trace element behaviour shifts. The absence of oxygen allows for the precipitation of elements that might otherwise remain dissolved, promoting their accumulation in sediments. Anoxic conditions also foster the preservation of organic matter, which can bind and sequester trace elements over long timescales. This dynamic leads to localized enrichments of trace elements in regions with poor ventilation, such as oxygen minimum zones or deep basins.
“Overall, the cycling and deposition of trace elements in marine sediments reflect the interplay between biological activity, geochemical processes, and the physical transport of material through the ocean system.“
The primary source of many trace elements in marine sediments is seawater, and the rate at which these elements accumulate can be strongly influenced by oceanographic factors such as bottom water renewal rates and the residence times of trace elements in the water column. Bottom water renewal refers to the process by which deep ocean waters are replenished by the downward movement of surface waters, a mechanism driven by thermohaline circulation and other large-scale ocean currents. This circulation not only supplies oxygen and nutrients to the deep ocean but also regulates the transport and deposition of trace elements.
When bottom water renewal is slow or stagnant, trace elements that are introduced into the deep ocean, whether through hydrothermal activity, riverine input, or biological cycling, can remain in the water column for extended periods. This extended residence time allows for greater interaction between trace elements and suspended particles, promoting adsorption onto mineral surfaces or organic matter. As a result, trace elements are more likely to be scavenged from seawater and incorporated into sediments. Conversely, in regions where bottom waters are rapidly replenished, the continuous flushing of deep ocean basins can limit the accumulation of trace elements, reducing their overall concentration in sediments.
The residence time of trace elements in seawater, which can vary from days to millions of years depending on the element and its chemical properties, also plays a critical role in enrichment processes. Elements with long residence times, such as molybdenum and uranium, can become widely distributed throughout the oceans before being deposited in sediments, resulting in relatively uniform global patterns of enrichment. In contrast, elements with shorter residence times, like iron and manganese, tend to precipitate and accumulate closer to their source, often forming localized deposits around hydrothermal vents or within areas of high productivity and sedimentation.
In environments where bottom water renewal is limited, such as oxygen minimum zones (OMZs) or deep basins with restricted circulation, trace element accumulation is often enhanced by the development of reducing (anoxic) conditions. In the absence of oxygen, many trace elements are more soluble and remain in the dissolved phase until they encounter redox boundaries, where they precipitate and are sequestered in sediments. This process leads to the formation of trace element-enriched layers that serve as valuable records of past oceanographic conditions and redox fluctuations.