Geoscience Reference
In-Depth Information
Marine food and mineral resources
HUMAN IMPACT
Oceans hide away an abundance of resources which humans have utilized at the margin since prehistoric times but
our appreciation of their true extent is obscured by ocean depths, expanse, remoteness and - until now in polar
regions - floating sea ice. Marine exploration does, of course, sample oceans and the sea floor through recovery of
water and sediment samples, and technology also equips us with remote sensing techniques such as sonar to explore
beyond our submarine capability. We explore here the essential character of the marine biosphere and its human
food and industrial resource potential.
Earth's early oceans and atmosphere formed between 4.5 Ga and 3.5 Ga and all planetary life has evolved from primitive
cyanobacteria, which may have originated around hot, geochemical submarine vents on MOR towards the end of that
time. Life forms remained poorly developed throughout the Cryptozoic aeon ('hidden life', spanning the Archaean and
Proterozoic aeons) until the 'explosion' of Phanerozoic ('visible') life 540 Ma ago. Although Earth's phyla are fully
represented in the oceans, marine and terrestrial biospheres differ in a number of key respects. Maximum terrestrial
primary productivity is closely correlated with radiation balance and optimal heat and moisture regimes, found in the
humid tropics. Rapid sub-surface light attenuation, far lower nutrient concentrations and more uniform global water
temperatures (except in near-surface water) drive quite different global ocean primary productivity patterns. Productivity
is greatest in nutrient-rich, cold upwelling currents - swept up from depths where high water pressure and low
temperature substantially raise their concentration (see
Figure 11.9
)
- and off terrestrial nutrient-effluent rivers.
Maximum open-ocean primary production of
600 gm C (carbon) m
-2
yr
-1
may be only some 25 per cent and 16
per cent respectively of maximum terrestrial productivity in grasslands and tropical moist forest but most of the latter
is undigestible by herbivores, attenuating the potential food chain. By contrast, marine secondary productivity is high
in protein, low in skeletal mass and enjoys lower energy costs in movement, since marine animals are supported by
water. Marine habitats are stratified by depth-related changes of light penetration, water pressure and nutrients, and
divided between
pelagic (open water) and benthic (sea bed) environments. Greatly simplified, planktonic primary
producers and consumers feed fish consumers in the former
(
Figure 11.19
);
and algae, sea grasses, seaweeds, worms,
shellfish, starfish, corals, etc., have complex internal feeding patterns and also support fish consumers in the latter.
Locally, benthic system productivity can reach 5,000 gm C m
-2
yr
-1
(coral reefs) and
f
1,000 gm C m
-2
yr
-1
and
f
3,000 gm C m
-2
yr
-1
respectively on rock shorelines and estuaries.
Humans use, under-use and abuse marine food resources! Fish and other marine organisms provide only 2 per cent
of human food consumption but - 20 per cent of animal protein. Global fish catches exceed 100 M t yr
-1
, with controlled
mariculture ('fish farming') adding another 5 M t yr
-1
, but this probably represents a substantial under-use of sustainable
marine food potential. However, we also abuse the system by overfishing stocks of a limited range of species towards
extinction, polluting coastal, nutrient-rich waters, discarding (and mostly killing) a high percentage of catches of young
(breeding stock) or 'unfashionable' fish and 'mining' shell beds and reefs.
The sea floor is also the tantalizing source and guardian of an array of mineral and hydrocarbon reserves, requiring
high expenditure and advanced technology to realize. We dredge sand and gravel for building aggregates and have
long known about manganese and other metalliferous nodules and sediments, concentrated by high water pressure,
proximity to MOR hot vents or offshore extensions of river discharge. Whilst their exploitation continues to grow,
there is renewed interest in energy resources. Since most onshore hydrocarbon reservoirs of coal, petroleum and
natural gas, accumulated in sedimentary basins, are known and substantially worked, extraction industries moved
into offshore continental shelf/slope basin extensions several decades ago. Future offshore hydrocarbon resourcing
will tread an extraordinarily fine balance between greenhouse and icehouse inheritances. We have discovered an
entirely novel gas hydrate or clathrate source, with methane and other hydrocarbon gases trapped as frozen 'mush'
in deep sea floor surface sediments by high water pressure ( 1 km water depth) and near-freezing temperatures.
Exploitation would carry the risk of uncontrolled greenhouse gas releases. It is also ironic that global warming is
rapidly melting Arctic sea ice, opening access to the Arctic sea floor and further hydrocarbon reservoirs - triggering
further carbon emissions! The extensive summer collapse of Arctic coastal sea ice during 2007 triggered a rush by
surrounding nations to claim, and prepare to defend, its sea floor for economic exploitation. The 370 km wide Economic
Exclusion Zone of several nations overlap here and is likely to test the 1982 Law of the Sea treaty and International
Seabed Authority.
