Download OS101 Marine Environment

OS101 Marine Environment --Winter 2007
Marine Sediments (Sverdrup Chapter 4)
I.
Definition
Sediments are particles accumulating on the seafloor—they are the
oceanographer’s history book, as they record the past biological, chemical, and
physical history of the ocean.
- They are important biogeochemical components of our planet, but they
also record manmade disturbances:
1) Burning fossil fuels
2) Increasing erosion
3) Increasing leaching of soils due to acid rain
4) Increasing dust and/or atmospheric pollution
- Therefore, we can look to the sediments for a history of both the
natural changes in the environment (glacial/interglacial, etc.) as well as
anthropogenic changes.
- There are no sediments older than about 175 million years in the
oceans, but there are much, much older sediments found in continental
mountain ranges
II.
Classification by Origin
- There are four major types of sediment:
1) Lithogenous (rock), including volcanic and glacial deposits
2) Biogenous (remains of “hard parts” of organisms)
3) Hydrogenous (reactions of compounds dissolved in seawater)
4) Cosmogenous (from outer space)
Lithogenous sediment
A. Lithogenous Sediments
- From continental land masses or volcanic activity in the open ocean
- Elements make up minerals, and there are about 2000 existing
minerals in earth’s crust—only about 24 are common, and are made up
of the elements O, Si, Al, Fe
-
Divide them into groups based on their mineralogy, texture, size, and
transport mechanism.
-
-
-
Mineralogy
- Ferromagnesian minerals contain ions of iron and magnesium and
negatively charged ion with one atom of Si and four atoms of
oxygen (silicate tetrahedron)
- Nonferromagnesian minerals are other common rock forming
minerals which don’t contain iron or magnesium
- Igneous rocks are rocks that solidify from a molten mass and are
classified according to texture and mineral composition
- extrusive rocks (basalt) fine-grained, cooled quickly
- intrusive rocks (granite) slow cooling at great depth
Grain Size
- We use the Wentworth Scale to classify by size
- Sorting describes the degree of selection by size—sediments can
be well sorted (beach sand) or poorly sorted (glacial deposits), or
anywhere in between
- Sediment Maturity refers to the combination of grain size, sorting,
and mineralogy—older sediments have:
- decreasing clay content
- increasing degree of size sorting
- increasing rounding of grains within a deposit
Transport
- Currents can erode, transport, or deposit sediment, based on the
grain size and current speed—clay turns out to be difficult to
erode, once it is suspended!
- Very small dust particles (<20 µm) can be carried great distances
by the wind, and serve as nuclei for snow and rain
B. Volcanogenic Particles
- ejected from volcanic eruptions, and are a special type of lithogenous
particle
- range in size from boulders to fine dust
C. Glacial Particles
- Carried by “rivers” of ice which pick up rocks and also scour rock
surfaces, forming “rock flour”
- They show the least amount of sorting
Biogenous Sediments
-
organically produced particles, typically made up of either CaCO3 (Calcium
Carbonate, or calcareous) or SiO2•nH20 (opal, or siliceous)
- Siliceous sediments come from:
- diatoms, silicoflagellates, radiolaria, sponges
- Calcareous sediments come from:
- foraminifera, coccolithophorids (calcite), pteropod snails (aragonite),
coral reefs (aragonite)
- Stromatolites are formed by cyanobacterial mats
- colonies of cyanobacteria form mats, which trap fine sediments in their
extruded mucous
- Form layered deposits, which become cemented by CaCO3 as the
cyanobacteria raise the pH (photosynthesis) and produce calcium as a
waste product
- 1-3 billion years ago, the earth was probably covered with
stromatolites!
- Biological Pump: 99% of the carbon dioxide added to the atmosphere
by volcanic activity over geological time has been removed from the
surface ocean and deposited as sediments forming CaCO3 and fossil
fuels (incomplete decomposititon)
- of the 5.3 Gt C/yr produced anthropogenically, ~3.2 Gt/C is
unaccounted for, but probably ends up in the ocean!
Hydrogenous Sediments
- form slowly by chemical reactions on the ocean floor—not very well
understood.
- Manganese Nodules: polymetallic nodules of potential economic
importance. Generally about 30% MnO2, 20% Fe2O3, also Cu, Co, Ni
- It’s not known how/why they form, but 3 hypotheses to explain the
presence of the minerals. Probably mediated by bacteria:
1) Volcanic weathering
2) Hydrothermal vents
3) Runoff from continental land masses
- Phosphorites: sediments with a high concentration of phosphate, about 1000
m (ca. 30% P by weight!) They are found in upwelling areas where the
oxygen minimum zone intersects the ocean floor…incomplete decomposition
of biomass
- Evaporites: minerals that precipitate as sea water evaporates.
-
Zeolites and Clays: Chemical alteration of volcanic material on the
seafloor—zeolites make clay red, especially in the deep Pacific
Cosmogenous Sediments
- particles that originate in outer space. There is significant evidence for meteor
impacts on earth’s surface, and we use the presence of Iridium (Ir) as evidence of
past impacts.
III.
Classification by Pattern of Sediment Deposits
A. Neritic Sediment deposits are found near the continents and show a wide
range of sizes—they are mostly lithogenous sediment entering at the shore,
and are distributed across the continental shelf and down the slope by waves,
currents, and turbidity currents. They make up 75% of all sediments.
-
-
-
Relict Sediments: 3000-7000 years old, cover about 70% of the
world’s continental shelf, especially where sediment is trapped in
estuaries.
- 80% of the neritic sediments come from Asia as river sediments!
- they are strongly latitude-dependent…near the equator, lots of
coral. near the poles, lots of boulders and gravel, dropped by
glacial action
Turbidites are coarse sediments deposited at the base of continental
slope by turbidity currents—coarser material at the base, finer at the
top
Glacial Deposits are poorly-sorted deposits at high latitudes
Stromatolites—found in ancient oceans but also Australia, the
Bahamas
B. Deep-Ocean Sediments: fine grained sediments which collect slowly on the
deep sea floor. Although lithogenous sediments dominate, some areas show
biogenous or hydrogenous deposits because of the slow accumulation rate.
-
-
Abyssal Clay: covers most of the deep ocean floor. Accumulates very
slowly, 1 mm per 1000 years! Mostly clay-sized particles from the
continents, transported by winds and currents
Oozes: slightly shallower depths. Biogenous deposits…if more than
30% by weight, forms an ooze.
IV.
Controls on Oceanic Sediment Accumulation
-
-
V.
RATE of accumulation of biogenous material depends on:
- sedimentation rate (production)
- degree of preservation (destruction)
- amount of other materials deposited (dilution)
Production
- Oceanic sediment distributions rely upon and closely reflect the
particle composition of the surface water directly above them.
- Production of plankton in coastal waters and equatorial
divergences mean there’s more accumulation there
- Destruction
- occurs mainly through dissolution of particles in seawater, which is
a very reactive liquid!
- Silica is ALWAYS undersaturated in the oceans, so 80% of
siliceous material dissolves before being buried
- CaCO3 solubility increases with increasing CO2 concentration,
(low temperature, high pressure), so that below about 4500 m
CaCO3 dissolves easily
- Carbonate Compensation Depth (CCD) is where
sedimentation and dissolution are equal (3500-6000 m)
- Dilution occurs where biogenous sediment is deposited at rates of
1-15 mm/1000 years and will not form oozes on continental
margins or other locations where lithogenous sedimentation rate is
very high
- Mn nodules poorly understood: grow 1-200 mm every million
years, and appears to correlate with overlying productivity
Distribution of Sediments
- Combining all of the above, we expect to see calcareous ooze decrease with
depth, as we approach the CCD…siliceous oozes are less abundant, because they
ALWAYS dissolve…abyssal clay is found where there’s low productivity, and
below the CCD
-