In Jules Verne’s 20,000 Leagues Under the Sea*, the iconic Captain Nemo announced that “in the depths of the ocean, there are mines of zinc, iron, silver and gold that would be quite easy to exploit” while predicting that the abundance of marine resources could satisfy human need. If the pace of development for deep-sea mining projects and the estimated value of deep-sea ores is any indicator, it seems as though our misanthropic mariner was wrong on both counts.

In The abundance of seafloor massive sulfide deposits, an international team of geologists attempts to quantify the total available copper and zinc contained in deep-sea massive sulfide mounds. Seafloor massive sulfide mounds are a byproduct of the processes that create deep-sea hydrothermal vents. As super-heated sea water emerges from the vent, it deposits heavy metals and other elements and minerals along the walls of the vent. Over thousands of years, an active vent field can build up a huge mound of metal and mineral rich ore – a massive sulfide mound. In addition to copper and zinc, these mounds can contain gold and silver. Generally, the ore is of much higher quality than its terrestrial counterpart. Over the last few decades, many exploration companies were eyeing these deposits, but it’s only recently that technological developments and economic incentives have aligned to permit potentially profitable deep-sea mining.

Not all hydrothermal vent systems produce massive sulfide mounds, and not all massive sulfide mounds are rich in heavy metals and valuable ore. To determine how much ore really is available in the deep sea, Hannington and his team examined 32 control sites of approximately equal size that broadly represent the geologic conditions of the global seafloor. They discovered 106 ore deposits great than 100 square meters, with many concentrated around neo-volcanic regions (areas of volcanic activity where most hydrothermal vents are found). Based on these samples, they estimated that there are approximately 900 neo-volcanic massive sulfide deposits, but that number could be as low as 500 or as high as 5000.

Determining how many deposits exists is the easy part, figuring how much ore they actually contain is much harder. Using drilling data from the most well-studied of these deposits, they attempted to estimate the tonnage of copper and zinc. From 62 well-studied sulfide deposits, they extrapolated out to total available copper and zinc contained within neo-volcanic seafloor massive sulfides. Obviously, the total tonnage is an estimate with wide error bars, biased by, among other issues, the fact that the most well-studied sulfide deposits also tend to be the largest. As a baseline, they provide a solid preliminary estimate of the amount of available ore locked up in massive sulfide mounds.

From the assumption that there are approximately 1000 deposits greater than 100 tons, and based on previously determined size distributions, it appears as though the total amount of ore in neo-volcanic massive sulfides is on the order of 600,000,000 tons, with 400,000,000 contained in the largest 10% of deposits. From there, they estimate that the total copper and zinc contained in that ore is approximately 30,000,000 tons.

Thirty million tons sounds like a lot, and it is, but to put things in perspective, older massive sulfide deposits found on land contain 850,000,000 tons of copper and zinc. Terrestrial copper production was approximately 15,000,000 tons while terrestrial zinc was 11,000,000 tons in 2009. This means that the total available copper and zinc in all neo-volcanic massive sulfide mounds is roughly equivalent to one year of current global annual production.

While the potential payouts for deep sea mines is still large, it is not nearly as enormous as previously predicted. This is good news for the creatures that live at hydrothermal vents, as it means that the oft-predicted gold rush of deep-sea mining companies racing to exploit the ocean will be more of a trickle, with fewer companies likely targeting only the largest sulfide deposits. The fact that a third of the mineral wealth is contained in only 10% of deposits means that mining efforts will be more spread out, limiting the impact on any one region.

This is not entirely good news though. While I loath the idea of deep-sea mining, it is impossible not to draw comparisons between deep sea and terrestrial mines. In terms of human impacts and environmental destruction, terrestrial mining has one of the worst track records of any industry (see Amy’s article here). In contrast, deep-sea mining, as an emergent industry, has the potential to act as a responsible environmental steward, setting a high bar for monitoring, mitigation, and conservation (see Rumors from the Abyss here). The dynamic nature of hydrothermal vent ecosystems means that they are much more resilient to environmental disturbance than their terrestrial counterparts. The relatively low value of massive sulfide deposits means that we are stuck with terrestrial mining, and its history of exploitation and environmental degradation, for a long time.

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Hannington, M., Jamieson, J., Monecke, T., Petersen, S., & Beaulieu, S. (2011). The abundance of seafloor massive sulfide deposits Geology, 39 (12), 1155-1158 DOI: 10.1130/G32468.1

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*Though it varies, a league is commonly define as approximately 3 miles, thus the 20,000 leagues referenced in the title refers to the distance the Nautilus traveled while under the sea, and not the depth it dove.