Read the following article and answer the questions: Article: BIOGENIC SEDIMENTS

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Read the following article and answer the questions:

Article:

BIOGENIC SEDIMENTS

Biogenic sediments, defined as containing at least 30% skeletal remains of marine organisms and

~70% terrigenous clay. Biogenic sediments cover approximately 62% of the deep ocean floor.

Clay minerals make up most of the non-biogenic constituents of these sediments. While a vast

array of plants and animals contribute to the organic matter that accumulates in marine

sediments, a relatively limited group of organisms contribute significantly to the production of

biogenic deep-sea sediments, which are either calcareous or siliceous oozes.

Distributions and accumulation rates of biogenic oozes in oceanic sediments depend on three

major factors:

• Rates of production of biogenic particles in the surface waters,

• Dissolution rates of those particles in the water column and after they reach the bottom,

• Rates of dilution by terrigenous sediments.

The abundances and distributions of the organisms that produce biogenic sediments depend

upon such environmental factors as nutrient supplies and temperature in the oceanic waters in

which the organisms live. Dissolution rates are dependent upon the chemistry of the deep ocean

waters through which the skeletal remains settle and of the bottom and interstitial waters in

contact with the remains as they accumulate and are buried. The chemistry of deep-sea waters,

is, in turn, influenced by the rate of supply of both skeletal and organic remains of organisms

from surface waters. It is also heavily dependent upon the rates of deep ocean circulation and

the length of time that the bottom water has been accumulating CO2 and other byproducts of

biotic activities.

Carbonate oozes

1. Production

Most carbonate or calcareous oozes are produced by the two different groups of organisms. The

major constituents of nanofossil or coccolith ooze are tiny (less than 10 microns) calcareous

plates produced by phytoplankton of the marine algal group,

the Coccolithophoridae. Foraminiferal ooze is dominated by the tests (shells) of planktic protists

belonging to the Foraminiferida (>61 mm in diameter).

Coccoliths and foraminiferal tests are all made of the mineral calcite. Carbonate oozes are the

most widespread shell deposits on earth. Nearly half the pelagic sediment in the world's oceans

is carbonate ooze. These organisms have been major producers of pelagic sediment for the past

200 million years. As a result, these are arguably among the most important and scientifically

useful organisms on Earth. Because their larger size makes them easier to identify and work with,

this is particularly true for the foraminifera.

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Figure 1. Organisms that secrete calcareous skeletons (also referred to as ‘tests’), which commonly form

carbonaceous sediments, known as carbonate oozes. Organisms are microscopic, and these images are all

taken using microscopy (colours are artificial, used to enhance images). A and B are examples of

Coccolithophores, C are Foraminifera. In B disarticulated disks are clumped in the left side of the image.

The distributions and abundances of living planktic foraminifera and coccolithophorids in the

upper few hundred meters of the ocean depends in large part on nutrient supply and

temperature. Coccolithophorids, because they are marine algae, require sunlight and inorganic

nutrients (fixed N, P, and trace nutrients) for growth. However, most coccolithophorid species

grow well with very limited supplies of nutrients and do not compete effectively with diatoms

and dinoflagellates when nutrients are plentiful. Furthermore, both high nutrient supplies and

cold temperatures inhibit calcium carbonate production to some degree. For these reasons,

diversities (number of different kinds) of coccolithophorids are high and production rates of

coccoliths are moderate even in the most nutrient-poor regions of the subtropical oceans, the

subtropical gyres. Production of coccoliths is higher in equatorial upwelling zones and often along

continental margins and in temperate latitudes where nutrient supplies are higher, though

diversities decline. In very high nutrient areas, such as upwelling zones in the eastern tropical

oceans (i.e., meridional upwelling), polar divergences and near river mouths, production of

coccoliths is minimal.

Even though planktic foraminifera are protozoans rather than algae, their distributions,

diversities, and carbonate productivity are quite similar to those of coccolithophorids. Many

planktic foraminifera, especially those that live in the upper 100 m of temperate to tropical

oceans, host dinoflagellate symbionts which aid the foraminifera by providing energy and

enhancing calcification. Having algal symbionts is highly advantageous in oceanic waters where

inorganic nutrients and food are scarce, so a diverse assemblage of planktic foraminifera thrives

along with the coccolithophorids in the nutrient-poor subtropical gyres. Greater abundances of

fewer species thrive in equatorial upwelling zones and along continental margins, so rates of

carbonate shell production are higher. And similar to coccolithophorids, few planktic

foraminifera live in very high nutrient areas, such as upwelling zones in the eastern tropical

oceans, polar divergences and near river mouths, so production of carbonate sediments is

minimal in these areas. Finally, planktic foraminifera require deep oceanic waters to complete

their life cycles, which they cannot do in neretic waters over continental shelves.

A – Coccolilthophore B – Coccolithophores C – Foraminifera

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Cool temperatures work together with higher nutrient supplies to reduce diversities of

coccolithophorids and planktic foraminifera, and ultimately to shift the ecological community to

organisms that do not produce carbonate sediments. A 10o C drop in temperature is

physiologically similar to doubling nutrient supply, which is why the pelagic community in an

equatorial upwelling zone resembles that of a temperate oceanic region, while the pelagic

community of an intensive meridional upwelling zone resembles subpolar to polar communities.

If surface production was the only factor controlling accumulation rates of carbonate oozes, deepsea

sediment patterns would be quite simple. Carbonate oozes would cover the seafloor

everywhere except:

• beneath intensive meridional upwelling zones,

• beneath polar seas, and

• where they are overwhelmed by terrigenous sedimentation.

Rates of accumulation would be on the order of 3-5 cm/1000 years in the open ocean and 10-20

cm/year beneath equatorial upwelling zones and along most continental margins.

2. Dissolution

Over much of the ocean floor, carbonate accumulation rates are controlled more by dissolution

in bottom waters than by production in surface waters. Dissolution of calcium carbonate in

seawater is influenced by three major factors: temperature, pressure and partial pressure of

carbon dioxide (CO2). The easiest way to understand calcium carbonate (CaCO3) dissolution is to

recognize that it is controlled, in large part, by the solubility of CO2:

CaCO3 + H20 + CO2<====> Ca++ + 2HCO3

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The more CO2 that can be held in solution, the more CaCO3 that will dissolve. Since more CO2 can

be held in solution at higher pressures and cooler temperatures, CaCO3 is more soluble in the

deep ocean than in surface waters. Finally, as CO2 is added to the water, more CaCO3 can dissolve.

The result is that, as more CO2 is added to deep ocean water by the respiration of organisms, the

more corrosive the bottom water becomes to calcareous shells.

The rain of organic matter from surface waters through time increases the partial pressure of

CO2 in bottom water, so the longer the bottom water has been out of contact with the surface,

the higher its partial pressure of CO2. Beneath high-nutrient surface waters, primary production

exceeds what is utilized in the surface mixed layer. Excess organic matter falling through the

water column accumulates on the bottom, where organisms feed upon it and oxidize it to CO2.

The depth at which surface production of CaCO3 equals the amount of CaCO3 dissolution is

called the calcium carbonate compensation depth (CCD). Above this depth, carbonate oozes can

accumulate, below the CCD only terrigenous sediments, oceanic clays, or siliceous oozes can

accumulate. The calcium carbonate compensation depth beneath the temperate and tropical

Atlantic is ~5,000 m deep, while in the Pacific, it is shallower at ~4,500 m, except beneath the

equatorial upwelling zone, where the CCD is ~5,000 m. The CCD in the Indian Ocean is

intermediate between the Atlantic and the Pacific. The CCD is relatively shallow in high latitudes.

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Surface waters of the ocean tend to be saturated with respect to CaCO3; low latitude surface

waters are usually supersaturated. At shallow to intermediate seafloor depths (less than 3000

m), foraminiferal tests and coccolith plates tend to be well preserved in bottom sediments.

However, at depths approaching the CCD, preservation declines as smaller and more fragile

foraminiferal tests show signs of dissolution. The boundary zone where gradual dissolution ends

and a notable increase in rapid dissolution takes place is known as the lysocline. Pelagic

sediments from relatively shallow depths in low latitudes are often dominated by calcareous

oozes above the CCD, and by red clays below the CCD.

Regional changes in the depths of the lysocline and CCD result, in part, from changes in

CO2 content of bottom waters as they "age". In modern oceans, deep ocean circulation is driven

by formation of bottom waters during the freezing of sea ice. Seawater, due to its salt content,

can cool below -1o C before ice begins to form. When sea ice forms, the salt is excluded and is

left behind in the seawater. Water in the vicinity of the freezing sea ice becomes more saline and

therefore more dense. As a result, large-scale sea ice formation creates very dense water masses

that sink to the bottom of the ocean to form deep bottom water. During the Antarctic winter,

the freezing of sea ice in the Weddell Sea produces Antarctic Bottom Water (AABW), which sinks

to the sea bottom and spreads northward into the South Atlantic. During the Arctic winter, sea

ice formation in the Norwegian and Greenland Seas produce North Atlantic Deep Water (NADW),

which sinks to the bottom of the North Atlantic and flows southward. AABW is slightly more

dense than NADW, so when they meet, AABW flows beneath NADW. As the NADW and AABW

spread eastward into the Indian and Pacific Oceans, they mix to become Deep Pacific Common

Water (DPCW). The "youngest" bottom waters are in the Atlantic, the "oldest" are in the North

Pacific.

When seawater is at the surface, it equilibrates with the atmosphere with respect to O2 and CO2.

From the time a water mass sinks from the surface until it comes back to the surface, respiration

by organisms in the water column and on the bottom use up O2 and add CO2. As a result, the

longer bottom water is away from the surface, the more corrosive it is to CaCO3.

3. Carbonate Sedimentation Worldwide

The depth of the CCD and the pattern of carbonate sedimentation in any part of the world's

ocean reflects the influences of surface production of organic matter, surface production of

carbonates, and the corrosiveness of the bottom water to CaCO3.

Because coccolithophorids and planktic foraminifera thrive in temperate to subtropical oceans

where surface nutrient supplies are very limited, these organisms produce a continual rain of

CaCO3 to the sea floor. In equatorial upwelling zones, organic productivity is elevated enough to

stimulate higher rates of production of calcareous and siliceous skeletal remains, but not enough

to export excess organic matter to the deep ocean where its respiration would increase

corrosiveness of bottom waters to CaCO3.

In more intensive upwelling zones, especially in the eastern tropical Pacific and the Antarctic

divergence, and off major river deltas, high nutrient supplies stimulate high rates of organic

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productivity by diatoms and dinoflagellates, often to the exclusion of coccolithophorids and

planktic foraminifera, which reduces CaCO3 production. At the same time, the rain of organic

matter to the ocean floor supports abundant deep-sea life whose respiration adds significantly

to the CO2 in bottom waters. The result is substantial shoaling of the lysocline and CCD in these

regions.

Pelagic sediments in the Atlantic and Indian Oceans are predominantly calcareous oozes. In the

Pacific Ocean, where the CCD is deeper, red clays dominate, especially in the North Pacific.

Carbonate oozes delineate shallower regions in the south Pacific, including the East Pacific Rise

and the complex topography to the southwest. Mid-oceanic ridges are typically at an elevation

above the CCD, calcareous oozes accumulate on the ridge slopes. As sea-floor spreading occurs

and the plates move away in opposite directions, abyssal clays or siliceous oozes are deposited

on top of the calcareous ooze and the plates eventually subside below the CCD. The abyssal

clay/siliceous ooze layer thus protects the calcareous ooze from being exposed to the cold, acidic

sea water and dissolved.

Question:

2. Define the term ‘calcium carbonate compensation depth’. What is the average depth of the CCD, in meters & in feet in the

Pacific ocean? (1 m = 3.28 ft)

3. Describe the different conditions that cause CaCO3 to dissolve. Include the chemical reaction for the dissolution of calcium

carbonate.

4. As the calcareous tests sink, does the dissolution occur gradually with depth or suddenly once the CCD is reached? Explain.

5. Define ‘calcareous ooze’. List the two main types of organisms that contribute to this biogenous sediment.

6. Calcareous oozes can be found in sediment cores drilled from locations in waters deeper than the CCD.

Explain how this occurs with regards to plate tectonics and deposition of sediment.

7. Where can a high percentage of calcareous ooze be found in modern ocean sediments around the globe?

Explain why this occurs.

8. If you were vacationing on a cruise ship and the captain informed you that your current position was over the Juan de Fuca

Ridge at 46o N latitude, would you expect calcareous ooze to be accumulating? Why or why not?

Explanation / Answer

2. The depth at which production of CaCO3 and dissolution of CaCO3 is equal is known as the "calcium carbonate compensation depth".

For Pacific the compensation depth is,

= 4500 meters

= 4500*3.28 = 14,760 ft

3. The dissolution of CaCO3 is dependent on temperature, pressure and parital pressure of CO2

CaCO3 + H20 + CO2<====> Ca++ + 2HCO3

This means the more there will be CO2 in the water the more the dissolution of the CaCO3 is possible. Since the deep ocean have high pressure and cooler temperature the CO2 holding capacity is larger and hence dissolution capacity of CaCO3 is higher than surface.

Please ask the rest of the questions in separate questionnaire,

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