From the Gobi to the bottom of the North Pacific.
Earth's crustal materials are recycled by water, wind, ice, and fire. In order for this recycling to occur, crust must be moved from one region to another. How does this movement happen?
* On many land surfaces, the crust has been elevated by mountain-building tectonic forces, while part of Earth's surface is still rising due to loss of the weight of glacial shields that covered a great deal of land during the last glacial period.
* Glaciers and sea ice carry loads of rock particles, from boulders to the finest dust, and deposit them on the polar-ocean floor.
* The crust is constantly eroded, adjusting the heights of mountains.
* Rocks are crushed to finer sizes as gravity pulls them down-slope; a conveyer belt, driven by gravitational force, moves crustal material steadily down-slope until it comes to rest in the deepest basin of all, the deep seafloor.
* Rock particles are transported from arid lands to mid-ocean regions by aeolian transport (the wind's lifting of fine particles from continents and transporting them to the oceans).
* Earth's thermonuclear engine transports deep-sea sediment to the subduction zones where it is sucked down into the mantle hundreds of millions of years later, perhaps to rise again as volcanoes or massive mountains, thus closing the grand geodynamic cycle.
How Much Crustal Material is Being Moved?
Balancing the "budgets" of materials involved in planetary-scale processes is as difficult as balancing a government's financial budget. Balancing the supply of rock particles that originate on land with their deposition in ocean basins is no exception, because there are enormous variabilities and uncertainties in time and space in this process. Recent estimates of the rate at which rivers move rock particles from land to ocean edges range from about 18.5 to 20 billion tons per year. Coastal erosion also supplies rock particles to the ocean. Assuming an annual sea-level rise rate of 1.5 millimeters, we estimate that worldwide coastal erosion contributes half a billion tons of rock particles to the ocean each year. The amount of ice-rafted rock particles is estimated to be about 1.3 to 1.5 billion tons per year. Alexander Lisitzin (University of Moscow) estimates that global aeolian transport of rock particles to the ocean is 1.6 billion tons per year. Thus the total supply of rock particles from land to the ocean adds up to 21 to 23 billion tons per year. After the particles are acted upon by a variety of physical and chemical processes, they end up as fine clay minerals that form a majority of chemically stable sediments, including fine grains of quartz, feldspar, and other rock-forming minerals.
One method for determining the current sedimentation rate in deep basins is to examine the sediment already deposited there. Study of rock particles in the thick Holocene sequence (the most recent 10,000 to 11,000 years) in the Atlantic is muddled by the presence of abundant tiny shells of organisms that once lived in the upper layers of the oceans. In contrast, the Holocene sediment in the middle of the North and South Pacific is estimated to be as thin as several centimeters where the water is very deep; here the majority of biologically produced particles disappear due to dissolution during descent. Rock particles are concentrated in the "deep sea red clay" discovered by the H.M.S. Challenger expedition in the 1870s. Applying a number of assumptions, the estimated rate of rock-particle deposition in the red-clay area is on the order of 1 milligram per square meter per year. Lisitzin estimated the average global flux of deep-sea sediment through the Holocene period was 1.7 billion tons per year, which closely coincides with the estimate of aeolian transport.
While these estimates are rudimentary, they clearly indicate that river-transported rock particles do not contribute significantly to deep-sea sedimentation, but rather the majority of the annual 20 billion tons of detritus that is eroded and transported via rivers and coastal erosion remains near the ocean edges. These studies lead us to hypothesize that rock particles are transported to the present-day central oceans primarily by aeolian transport.
Aeolian Transport: Riding the Winds
In the 1950s, scientists estimated that about 36 percent of Earth's land is sufficiently arid to allow the wind to transport airborne soil as dust. Probably the total of global desert or desertlike area has increased in recent years due to greatly increased land cultivation. The largest region of arid land lies south of the Mediterranean and extends from the Indian Ocean to the Atlantic Ocean. Aided by the tropical upwelling of air, great plumes of Sahara Desert sand are blown into the atmosphere. Some of the particles are transported across the Atlantic by the trade winds--dust from North Africa often falls over Miami, Florida.
The North Pacific is also subject to wind-transported dust, but unlike the Tropical Atlantic, where most dust is transported seaward by easterly (blowing from the east) trade winds, Pacific dust clouds ride the westerly jet stream at high altitude: Dust lifted to high altitudes from the Gobi Desert and other east Eurasian arid lands travels the great distance to the North Pacific. Geochemical studies on dust fallout collected from Pacific islands and deep-sea sediment attests to an extensive airborne rock-particle distribution, from arid, east Eurasian lands to the deep-sea sediment of the mid-latitude North Pacific. A classic example is that fine mica particles in Hawaiian soil were found to be as much as 180 million years old--the same age as Eurasian sand.
Volcanic eruptions often supply an enormous amount of ash to the open ocean. One of the largest and most violent incidents in human history was the eruption of Krakatau in 1883; scientists estimate that this eruption produced as much as 50 billion tons of tephra. Although it is not known how much of the erupted ash reached the stratosphere, a cloud encircled the earth in a few weeks and there was a full year of unusually red sunsets before it disappeared. These large volcanic eruptions significantly lower the atmospheric temperature by preventing part of the sun's radiation from reaching Earth. Airborne volcanic ash and any aeolian dust that has been lifted to stratospheric altitudes reflect heat, and contribute to the slowing of Earth's warming trend. Many scientists believe an asteroid--as heavy as a few trillion tons--hit Earth with tremendous impact about 65 million years ago. Clouds of particles produced by this impact were thousands of times thicker than the aeolian dust that normally falls into the ocean. This is an explanation of why almost all lineages of marine organisms, as well as dinosaurs on land, became extinct at the Cretaceous-Tertiary boundary.
Sledding Along with the Ice
The distribution of ice-rafted rock particles is an important indicator of ocean environmental change through the glacial and Holocene periods. Dust blown out of the vast exposures along the thousands of glacial walls in the Canadian Arctic, Greenland, and Spitsbergen coasts fall on arctic sea ice. Large arctic rivers like the Mackenzie, Ob', and Yenisey carry a large quantity of soil particles to the arctic basin. Sediments from rivers and estuaries are incorporated into sea ice, which is pushed far off shore by wind. During summer, the ice surface melts, freeing dust that is blown onto arctic sea ice. The abundance of dust in the arctic sea ice makes the ice dark in color, often bonded in many hues. Arctic ice is often quite far from the image of a snow-white, pure-ice world! In Antarctica, on the other hand, erosion is limited to that caused by glaciers that grind base rocks.
Millions of ice floes are carried by the Transpolar Drift, a sea-ice conveyer that runs across the arctic basin from the Laptev Sea to the Fram Strait. This drift is not only the largest body of heat transport on this planet, but also an efficient dust mover. Ice that travels with the Transpolar Drift melts along the east coast of Greenland and as it proceeds southward it annually dumps about a billion tons of rock particles onto the seafloor. Knowing this, one can understand why the annual flux of rock particles under the Transpolar Drift is as much as 3 to 4 grams per square meter, while the annual aeolian particle flux over the Greenland ice cap is only 50 to 100 milligrams per square meter. Great icebergs that separate mainly from the west coast of Greenland bring glacial boulders and sand very far south to the North Atlantic.
Directly Measuring Rock Particles in the Deep Ocean
A sediment trap (Oceanus, Spring 1992) can directly measure the amount of rock particles that arrive at great ocean depths far from their origins. A modern sediment trap, tethered to a strong mooring on the deep-ocean bottom, is programmed to open and close at regular intervals, trapping sediment particles that fall through the water column. The rate of particle arrival and deposition, known as the mass flux, can be calculated from the area of the trap's horizontal opening and the amount of time it is open to the water column. With these sediment-trap experiments at many ocean stations, scientists are beginning to understand the quality and fluxes of rock particles on the ocean floor during different seasons and years.
Sediment-trap studies show that annual rock-particle fluxes in the Pacific basin vary from 0.4 grams per square meter at a station located east of Hawaii (where there is probably one of the lowest rock-particle sedimentation rates in the world's oceans) to 8 grams per square meter at a station west of Panama. At the Panama station, annual rock particle flux ranged from 0.6 grams to 2.0 grams per square meter, but flux should progressively increase closer to the area where Saharan dust plumes pass; indeed, at a station to the east of Barbados, the annual flux of rock dust was about 6 grams per square meter. Under the antarctic Weddell Sea mixed-ice zone, lithogenic particle flux was only a trace (less than 1 milligram per square meter per year, the smallest rate ever recorded). But at an arctic counterpart station west of Spitsbergen, fluxes were as large as 14 grams per square meter per year because of the phenomenon called "winter outburst," which is the flushing of rock particles from the fjords to the deep basin by heavy brine sinking during midwinter. The largest annual lithogenic particle flux recorded in an open ocean was about 28 grams per square meter from an open-sea station in the Bay of Bengal in 1988.
In the central Arabian Sea, annual rock-particle flux was measured at 4.4 grams per square meter in 1988, but it was only 1.0 gram per square meter during previous years. Rock-particle deposition rates are, then, highly variable in some areas, reflecting oceanographic changes from one year to another. It is important to continue flux measurements for many years so that we may understand these annual changes.
Rock Particles Help to Remove Atmospheric Carbon Dioxide
The term "rock particles" sounds very inorganic; it does not seem to indicate a major role in Earth's carbon cycle (including the planet's ability to deal with fossil-fuel-produced carbon dioxide), where all actors on stage appear to be chemical and biological processes. Sediment-trap experiments show that the flux of rock particles into the deep ocean is clearly correlated with the flux of organically produced carbon. Rock particles interact with the global carbon cycle in two ways. One is as a phytoplankton growth stimulator. Enhanced primary productivity (that is, when more organisms take up carbon) increases carbon dioxide removal from the atmosphere, just as tropical-forest trees help to keep Earth's carbon dioxide level low. The other form of interaction, of particular interest to oceanographers, is rock particles' role as ballast for removing organic matter and its load of atmospherically derived carbon dioxide to the deep-ocean interior. Unless organic carbon produced in the shallow ocean is removed quickly, oxidation by microbial processes returns the carbon dioxide to the air.
There are instances of aeolian dust enhancing productivity in the open sea. Shizuo Tsunogai (Hokkaido University), for example, found that the plankton Trichodesmium blooms in the Okinawa area and the Philippine Sea on the heels of "Kosa," dust clouds blown up from the Loess Plateau in northern China, which also create blood-red sunsets in far-eastern countries.
In the open ocean, far from land, iron is a critical element in chlorophyll synthesis (an integral part of primary production). The only fresh source of iron to the open ocean is the fallout of iron-rich aerosol. Therefore, John Martin (Moss Landing Marine Laboratory) and other scientists hypothesized that rock particles transported directly from arid lands may control the ocean's fertility on a global scale. The Southern Ocean, for example, is far less fertile, compared to its arctic counterpart, than it should be considering its abundance of major nutrients such as nitrate and phosphate. By studying ice cores recovered at Vostok, a Russian antarctic scientific base--the most remote from the coast of this ice-covered continent--Russian and French scientists revealed that the iron concentration in the dust found in the ice cores was sharply elevated during the last glacial maximum, 18,000 years ago, and that the carbon dioxide content of air trapped in ice bubbles of the same ice cores decreased during the periods when more dust accumulated.
This relationship is explained as follows: During the glacial period, winds were far stronger than at present, and there were more arid regions. As a result, rock particles, which always contain iron-rich minerals, spread to a much larger area of open ocean than they do now, resulting in higher primary production, and more carbon dioxide being fixed to organic carbon. More organic carbon was being exported to the ocean interior and seafloor, thus reducing carbon content in the air. Today, the air parcel around the Southern Ocean and Antarctica is isolated by its own westerly wind system, which allows virtually no dust to reach deep into the continent, despite the fact that Antarctica is surrounded by many southern hemisphere arid lands. But 18,000 years ago, during the most recent glacial maximum, the wind was strong enough to break this air current system, bring dust storms as far as the center of the Antarctic continent, and perhaps enhance the primary productivity of the Southern Ocean as well as other global open oceans. At present, however, not enough paleoceanographic evidence has been gathered to support this hypothesis.
Fine Rock and Clay Particles Sink Surprising Rapidly
A discovery that went totally against scientific intuition is that tiny rock particles settle down through ocean layers at a speed a few hundred times faster than Stokes's law predicts. A piece of clay, for example, is typically a few micrometers in diameter. It settles less than half a meter in a day. In other words, it takes thousands of years to reach the deep-ocean bottom. However, surprisingly, the bulk sinking speed of fine particles (even in the 1-micrometer range) measured in the deep ocean is 120 to 250 meters per day. We make these measurements by deploying two or more sediment traps at both shallow and deep layers, then dividing the length of time it takes a particle species to arrive at the deeper trap (residence time) by the distance between the two traps. This gives us the settling speed. The settling-speed resolution is limited by how frequently a trap is opened and closed. A rapidly sinking particle does not move laterally while sinking, except in areas with fast currents, such as the Gulf Stream or the Kuroshio, and such currents are only strong in the upper several-hundred meters, a distance a particle can traverse in several days.
The pathway to the seafloor may, however, be complex. A network of surface ecosystem processes often controls the sedimentation rate. For example, many filter-feeding animal plankton ingest fine particles regardless of their nutrient value. Rock particles may pass through an organism's digestive system unchanged physically and chemically, but they render the organism's feces heavier than its original food. Thus they settle toward the seafloor in fecal pellets. The ocean's surface layer is also rich in mucus and other sticky stuff produced by plants and animal plankton. Free particles are entrapped by these materials into aggregates typically half a millimeter in size. These aggregates, though not compacted and rather fragile, drastically decrease each participating particle's drag coefficient because the surface area of an individual particle is irrelevant when it is part of a larger aggregate.
The critical role of rock particles is to add weight and therefore a faster sinking speed to the host aggregate traveling from the surface layers. An aggregate disintegrates from its own shear speed as well as by being eaten up by bacteria and other single-celled organisms. Participating particles constantly fall off the aggregate host and become suspended, and are then picked up by other passing aggregates. Vertical transport of rock particles as well as other fine particles in the ocean is accomplished by many repetitions of this process, with the particles cycling between aggregation and suspension.
Monsoons, Arabian Dust, Himalayan Rivers, and Ocean Productivity
The Arabian Sea and the Bay of Bengal are two of the most productive areas in the world's oceans. Rivers transport large quantities of rock particles from the Himalayas to these areas, and twice a year strong monsoon winds bring abundant dust from the Arabian and African dry lands. Recent studies reveal the fascinating oceanographic roles played by rock particles in biogeochemical cycles.
Ravindrathan Nair (National Institute of Oceanography, Goa, India) and his international colleagues report that the variability of organic carbon flux and rock particles measured in the middle of the Arabian Sea and the variability of the southwest monsoon wind velocity were almost identical, corresponding in every detail. The monsoon wind causes vigorous upwelling of nutrient-laden bottom water, which increases primary production in shallow layers. The fallout of desert dust from arid African lands depends upon wind velocity and increased availability of ballast to force light organic matter to settle on already-abundant organic matter in the shallow layers.
On the other hand, Venugoplan Ittekkot (University of Hamburg) and others report that organic carbon flux in the Bay of Bengal also responded positively to the strength of the southwest monsoon, though the process was quite different from that in the Arabian Sea. Again, rock particles were a vital factor in the sedimentation processes. At a station in the northern Bay of Bengal, organic carbon flux was correlated to the variability of the discharge from two great Himalayan rivers, the Ganges and Brahmaputra, which peaked during the summer southwest monsoon. At a station farther south in the bay, east of Madras, the organic carbon flux peak overlapped with the arrival of dust from the Indian continent during the winter northeast monsoon, though input of the Ganges-Brahmaputra rivers from the Indian continent was quite obvious in summer. At the southernmost station, about the latitude of Sri Lanka, the variability of organic carbon was no longer correlated with river discharge but, similar to the Arabian Sea stations, was clearly related to airborne supply of particles associated with the southwest monsoons.
Lateral Transport of Rock Particles
One other striking observation regarding the behavior of rock particles in the ocean is that their flux increases linearly with depth. It has been found that such a linear increase now occurs at almost all stations studied in the world's oceans. As of this writing, we do not have information from an Arctic Basin station established this past year, but we anticipate that it also will show this increase. The closer to the continental slope, the greater the rate of flux increase with depth. In the stations closer to the continental slope, rock-particle fluxes increased five times or even more. This increase is probably not directly related to the nepheloid layer, a "ground-fog-like" concentration of suspended particles extending from the ocean floor to a height of a few hundred meters, as a steady increase of lithogenic particle flux was observed in the Panama Basin water column, which lacks a nepheloid layer. Because rock particle flux increases linearly with depth, the gradient of this increase can be calculated by measuring the fluxes at two different depths; the gradient R (the rate of increase of rock particle flux) ranged from 17 micrograms per meter per day at the Panama Basin to a trace, about 0.13 micrograms per meter per day, at the mid-Pacific station east of Hawaii.
When rock particle fluxes are plotted against depth, the intersection of the flux with the ocean surface is the flux that enters the ocean from the air. (Fluxes of particles at near-surface layers are technically very difficult. They are disturbed by too much biological activity and waves.) The discrete aeolian flux can thus be estimated at a station where rock particle fluxes are measured at a minimum of two depths simultaneously. For example, an annual flux of rock particles from the air at a station in the Demerara Abyssal Plain was 4.5 grams per square meter per year, and 1.5 grams per square meter per day was added to the original aeolian flux. At two stations in the Atlantic where the measurement was made more precisely, about 1.2 grams of laterally transported rock particles were added to the aeolian fluxes.
We are looking for effective geochemical tracers as indicators for the origins of rock particles. The titanium and aluminum ratio, neodynium isotopic variations, produced by the decay of the long-lived radio isotope samarium-147 to neodynium-143, for example, seem to be useful to distinguish the origin of rock particles that settle in the ocean. But this is still an area of oceanography that science has just begun to explore.
Arriving at a more accurate and detailed budget of rock particles formed by land erosion and determining their rates of deposition along the coasts and basins of the ocean are critical to understanding the geodynamics of Earth's present and past. Recently ocean science has revealed that the processes of oceanic rock-particle sedimentation are strongly coupled with atmospheric carbon-dioxide removal. On the other hand, understanding the processes of transportation and provenance of rock particles that are constantly redistributed in the air and in the ocean provides practical knowledge of how to protect our ocean environment from industrial pollutants and waste disposal, from the coasts to the deep ocean basins. Such research will yield basic information to direct us in future uses of the ocean. A sediment trap left in the middle of the Black Sea caught radioactive dust almost instantly after the Chernobyl nuclear disaster. As Jim Lovelock says with his Gaia theory, Earth processes are all interconnected, as if the Earth is a living body. The Earth's mechanisms for transporting rock particles in the air and in water are certainly an important function of Gaia.
Marine geologist Susumu Honjo is a Senior Scientist and holds the Columbus O. lselin Chair for Excellence in Oceanography at Woods Hole Oceanographic Institution. He does not hesitate to go anywhere in the world to bring back a part of the ocean to his laboratory, where he and his colleagues work hard, argue loudly, and ultimately enjoy research related to the biogeochemical cycles of matter in the world's oceans.
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|Title Annotation:||how rock particles travel from high mountains to the oceanic abyss|
|Date:||Dec 22, 1992|
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