Transcript Document

Palaeoclimate Change SOES 3015
Lecture 4:
Tools and Insights-4: The global facies map
& introduction to the CCD (PAW)
Lecture outline:
• Deep ocean sediments & their origins
-
Biogenic siliceous sediments
Non-biogenic sediments components
• Pelagic sediment distribution & ocean cycling
-
Pelagic siliceous sediments
Pelagic carbonate sediments (introduction to the CCD)
www.oceanography.ac.uk
(1) Deep ocean sediments & their origins
•
•
Biogenic sediment components
Non-biogenic sediments
components
(i) Biogenic sediment components
Siliceous: SiO2(H2O) opal
a) Diatoms- Class of unicellular algae,
usually singular but sometimes colonial
ranging in size from ~5-2000 µm. Cell
wall or “frustule” is impregnated w.
silica, consists of two valves, one of
which overlaps other like a lid on a box
and delicately ornamented.
Jurassic to Recent
Credit: Randolph Femmer /life.nbii.gov
b)
Radiolaria- Large group of
marine unicellular protist
zooplankton (~50-500mm).
Shell has a perforated
membraneous capsule
containing endoplasm &
siliceous or celestite (SrSO4)
skeleton consisting of a
lattice of variable
morphology made up of
spicules, bars and spines.
Courtesy: Jane K. Dolven/
radiolaria.org
From: Ernst Haeckel: Kunstformen der Natur (1904).
Image located in wikipedia (Public domain)
Cambrian to Recent … go
wild in Eoc …
Courtesy: Richard N. Benson
/radiolaria.org
Courtesy: Hannes Grobe/AWI/
Radiolaria.org
With exception of Ernst Haeckel’s drawings, all other images
on this page are licensed under Creative Commons 3.0
1
Calcareous: CaCO3 almost always
calcite (contrast neretic- aragonite
& Mg-calcite)
Courtesy: Jeremy Young. EOL
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a) Coccolithophophorids- A family
of planktonic unicellular marine
calcareous algae that secrete tiny
calcite platelets called “coccoliths”
(~1-5µm) arranged about their
single cell in a coccosphere.
Courtesy: Jeremy Young. EOL
Together with other calcareous
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nannofossils they are often a
dominant component of sediment.
Triassic to Recent
2
Courtesy: Jeremy Young. EOL
4
Courtesy: Hannes Grobe/AWI
6
Courtesy: Hannes Grobe/AWI
Courtesy: Hannes Grobe/AWI
All images on this page are licensed under Creative Commons 3.0
Paul Pearson, Cardiff University
b)
Foraminifera- Subclass of unicellular
protozoan zooplankton which secrete
a calcareous skeleton or “test”
(normally ~50-500µm). They may
be planktonic or benthic but the
planktonic ones are usually the
dominant type found in calcareous
ooze.
Jurassic to Recent
Paul Pearson, Cardiff University
Creative commons 3.0
Hannes Grobe/AWI
Foraminifera: Halkyardia minima
Ian McMillan, Cardiff University.
Creative commons 3.0
Michael Hesemann
(ii) Non-biogenic sediment components
a)
Deep-sea “red clays”- Clay is the most widely distributed terrigenous
component in pelagic sediments. Clays often dominant on land but in ocean
are normally masked by other components. Major exception = deep ocean
away from turbidite input and below CCD- “red clay” facies so named on
Challenger expedition. Colour (in fact brown) caused by amorphous Fe2O3
oxide coatings. Windblown so composition varies according to proximity to
continent. sed rates <1 mm/ka
Redrawn and modified based on: Berger, W.H.,
(1974), Deep-Sea Sediments In. The Geology
of Continental Margins. p. 213-241. (eds) Burk
, C.A., Drake, C.D. Springer Verlag. New York.
b)
Terrigenous sediments- derived from continents transported by gravity flows
(eg. turbidity currents, slides, slumps etc) proximity to canyons and deltas.
c)
Glacial sediments- Ice-rafted debris (IRD) poorly sorted gravels, sands
w/ high component of rock flour. Delivered by iceberg calving thus
distribution controlled by 0°C isotherm.
(2) Pelagic sediment distribution &
ocean cycling
• Pelagic siliceous sediments
• Pelagic carbonate sediments
(i) Pelagic siliceous sediments
Question: Why do siliceous oozes
dominate Southern Ocean &
Eq. Pacific?
Answer: These are sites of upwelling
of nutrient–rich waters, high
productivity & flux of silica to sea
floor.
Abundance pattern opal in deep-sea sediments is closely related to the pattern
of productivity of diatoms & “rads” in overlying surface ocean:
• perimeter of Antarctic
• Eastern Eq. Pacific
• West African coast Atlantic
Redrawn and modified based on: Berger, W.H.,
(1974), Deep-Sea Sediments In. The Geology
of Continental Margins. p. 213-241. (eds) Burk
, C.A., Drake, C.D. Springer Verlag. New York.
These are all regions of high surface-ocean dissolved silicate & nutrient
concentrations.
Why? Because physical oceanography says so- they are regions where
nutrient-rich waters from deep within ocean are brought to surface.
Why? Because of surface divergence
Paul Wilson, University of Southampton
However, the ocean is everywhere under-saturated wrt opal. Thus, once formed,
hard parts of diatoms are immediately subject to dissolution.
Question: So how come we see
siliceous sediments on the
sea floor at all?
Answer: dissolution doesn’t really
start until particles hit the sea
floor- (‘cos they are packaged
by faecal pellets etc.)
& in zones of high rates of
sedimentation, the flux
exceeds rates of sea floor
dissolution.
Courtesy of Broeker, W., Peng, T.H., (1982) Tracers in the Sea. Eldigio Press, LDEO, 690p
Rate of sea floor dissolution minimised (ie. preservation maximised) where:
• Rain rate of opal is high enough such that pore water saturation is
achieved- cf chert (SiO2).
• Rain rate of non-opaline material is high such that opal grains are
buried rapidly.
(ii) Pelagic carbonate sediments
Question: Where are calcareous
oozes in the modern oceans?
Answer:
• CO3s cover around 50% of
sea floor
• but are absent from the
deepest regions (ie below the
calcite compensation depth, or
CCD (mean depth 4500 m)
• CCD shows strong inter-ocean variability (Atlantic >Indian >Pacific)
CCD = “the local level at which carbonate input from the surface is balanced
by dissolution”
Redrawn and modified based on: Berger, W.H.,
(1974), Deep-Sea Sediments In. The Geology
of Continental Margins. p. 213-241. (eds) Burk
, C.A., Drake, C.D. Springer Verlag. New York.
in practice, CCD is mapped as level where CaCO3 goes
to zero so seen on sea floor as a “snow line”
Taken by Mitchell Lyle, Texas A&M University
.
CO3-rich seds
lysocline
CCD
CO3-free seds
The foothills above Boise, Idaho after a snowstorm on Dec 2001.
lysocline = “the level at which dissolution begins”
. ..
seen in sed. column as contact between good and poor preservation.
University of Southampton
An aid to remember your geological periods:
“Camels often sit down carefully.
Perhaps their joints creak?
Prophylactic early oiling might
possibly prevent rheumatism”
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