Trace metal in the ocean: less is more

Metals have been utilised by human beings since the Bronze Age, and 4000 years later they seem to be even more associated with the modern life. Mining (from land and controversially from seafloor) is the way we acquire metals, which include iron, chromium, nickel, copper, zinc, and so on. These are called trace elements as they differ from the major elements (carbon, nitrogen, sodium, etc.) in terms of concentration or other measure of amount in the earth’s material. Despite the relatively high abundances in the solid planet, trace elements are usually scarce in the oceans.

One billion litres of seawater would be required to gather only 25 grams of iron. However, this element is essential to every form of life on the planet. ‘Give me half a tanker of iron and I’ll give you the next ice age’ – this is what the iron hypothesis (Martin, 1990) would expect. The adding of micronutrient iron to some parts of the oceans (see the figure below) can stimulate the growth of phytoplankton, and may cure the global warming. Although the consequence of iron fertilization is not yet clear, we indeed realised the significance of trace metals as many of them are critical for marine life and therefore influence the functioning of ocean ecosystems and the global carbon cycle.

Global distribution of sea-surface chlorophyll levels. As a proxy for phytoplankton mass, chlorophyll is relatively low in North Pacific, Equatorial Pacific, and Southern Ocean, yet nutrients are available in these three regions. Image credit: Wikipedia

 

The next question is, in the oceans, where do the trace metals come from and where are they going? The weathering process initially provides both dissolved and particulate forms of the metals through riverine input. Dust deposition contributes to the inventory, so does sediment input from the other way round – see how important the ocean boundaries are. When I was an undergraduate student 5 years ago, the text book told me hydrothermal iron delivered to the oceans could be neglected, but this idea has now been challenged, and the deep leaky vents have become the hotspots.

The simplified oceanic iron cycle from a review paper. The major external source is dust, with the iron supplied from continental margins and hydrothermal activity on mid-ocean ridges. However, this paper does emphasise that this schematic is not up-to date now because of our improved understanding about the sources and internal cycling of trace elements in the ocean. Image credit: Tagliabue et al. (2017)

 

If you compare iron data from 1960s onwards and you find a decrease in concentrations through time, it may not reflect the real situation as such element is contamination sensitive considering its scarce amount in seawater. Moreover, to measure trace elements at large time and space scale is clearly required for us better knowing the distributions of these elements and the processes behind. I went onto a GEOTRACES expedition to the North Atlantic, and GEOTRACES is a global collaboration of oceanographers seeking to find the chemistry clues. The clues are about, trace metal and isotopes, and during the past 15 years contributed by 35 nations, more than 100 research cruises have been completed under the GEOTRACES scheme. We look forward to hearing more stories about the ocean chemistry, especially about the role of trace elements in the past and at present, and the impact on the future.

In the map, yellow lines represent completed cruises, red lines are planned expeditions and black are cruises completed as part of a collaboration with other research programs. Image credit: GEOTRACES

 

References

Martin, J.H., 1990. Glacial-interglacial CO2 change: the iron hypothesis. Paleoceanography, 5, 1-13.

Tagliabue, A., Bowie, A. R., Boyd, P. W., Buck, K. N., Johnson, K. S., Saito, M. A., 2017. The integral role of iron in ocean biogeochemistry. Nature, 543(7643), 51.