12'710 Intoduction to Marine Geology and Geophysics

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12.710 Intoduction to Marine Geology and
Geophysics 11/1 Mid Term Sediments,
Processes, and the Sedimentary Record 11/6
(McManus) Deep-sea sediments composition,
distribution 11/8 (McManus) Biological,
chemical, and physical abyssal processes 11/13
(McManus) Dating methods and the sedimentary
record 11/15 (McManus) Paleo-environmental
proxies 11/20 (McManus) Deep water chemistry and
atmospheric p(CO2) 11/22 Thanksgiving 11/27
(Hoffmann) Paleothermometry 11/29
(Thompson) Pleistocene ice-age cycles 12/4
(McManus) Sedimentary records of abrupt climate
change 12/6 Final Exam
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black body
A theoretical object that absorbs all incoming
electromagnetic radiation. It emits radiation as
a function of temperature.
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Atmospheric CO2
I Greenhouse gases and the temperature of the
Earth
II Ice core evidence that glacial pCO2 was 80
ppm lower.
III Could it be due to terrestrial biosphere
change? No!
IV How did the ocean do it? Physical -
chemical property changes (T, S) Physical
pumps Biological pumps (nutrients, Corg,
alkalinity)
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Blackbody temperature The sun radiates primarily
in the visible. Earth radiates in the IR. Earth
has a blackbody Temperature below the freezing
point of water.
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Greenhouse effect Atmosphere allows visible
light to pass, but greenhouse gases (H20, CO2,
CH4) trap outgoing infrared and warm the Earth.
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Phase relationships (relative timing) These
allow us to rule out several possible
mechanisms for driving the CO2 changes.
Vostok ice core (Antarctica) CO2 and ?D are
nearly in phase. Both lead ?18O atm O2 (ice
volume). Both lead Greenland ?D and ?18O ice
(Northern hemisphere temperature).
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Epica Dome C ice core (Antarctica) CO2
(greenhouse gas) and ?D (temperature) vary
repeatedly through multiple glacial cycles.
The variations are approximately 90 ppmv.
CO2 (ppmv)
?D () Temperature
Siegenthaler et al. (2005)
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Vostok ice core (Antarctica) CO2 and ?D are
nearly in phase. Both lead ?18O atm O2 (ice
volume). Both lead Greenland ?D and ?18O ice
(Northern hemisphere temperature).
Time
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Carbon reservoirs Most carbon is in solid
earth. Ocean has most of the rest
(601) Atmosphere small reservoir, but important!
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Isotopic fractionation Stable isotope
fractionation may be diagnostic tool.
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Carbon reservoirs Different carbon pools are
isotopically distinct.
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Ocean carbon shift Mean ocean isotopic ratio
changed during ice age.
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Climate and Land Climate changes themselves
cannot account for the observed changes in CO2.
Changes in the terrestrial biosphere, evident
in carbon isotopes in the ocean, are too small to
explain CO2 change, and they indicate a shift in
the wrong direction!
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Biological pumps Productivity near the sea
surface changes the chemistry of both the surface
and deep ocean.
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CO2 in the ocean The inorganic as well as organic
chemistry of CO2 in the ocean plays an important
role.
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CO2 cycling Organic carbon is produced by
photosynthesis and then recycled through
respiration in the deep ocean. Both processes
influence the total dissolved carbon and the
dissolved carbonate ion concentration, but the
effect may not be intuitive.
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Alkalinity Excess positive charge of dissolved
ions to be balanced. In the case of carbonate
alkalinity, the net positive charge is balanced
by a combination of CO3 and HCO3_ .
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Coral reef hypothesis Growth of coral reefs on
flooded margins as sea level rises would have the
effect of increasing atmospheric pCO2, but the
timing is wrong.
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Phosphate burial hypothesis Burial of phosphate
and other nutrients on shelves as sea level
falls, and then release during sea level rise
would have the right effect on atmospheric pCO2,
but again, the timing is wrong.
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Influence of pumps Various pumps would have
differing effects on atmospheric pCO2, and also
on the isotopic composition of dissolved
inorganic carbon (DIC).
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Productivity Biological activity results in
systematic changes in concentration and
isotopic ratio of bio-limiting and
bio-intermediate elements.
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Carbon isotopes Photosynthetic fractionation of
organic carbon leaves seawater enriched in
heavier carbon-13. The resulting Isotopic ratio
in seawater is then incorporated in
CaCO3, providing a nutrient-tracer.
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Isotopic influences Photosynthetic fractionation
results in a strong negative correlation between
nutrients and carbon isotopes. Gas exchange,
local productivity, and global reservoir shifts
can also influence ?13C. So ?13C can be used as
a tracer for water masses (circulation), although
gradients are more reliable than the absolute
values.
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The meridional overturning circulation (MOC)
produces North Atlantic Deep Water (NADW).
NADW
GEOSECS
Evident in salinity and many other properties
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The meridional overturning circulation (MOC)
produces North Atlantic Deep Water (NADW).
NADW
Kroopnick (1985)
Also evident in carbon isotopes (?13C).
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LGM meridional section, western basin
Curry and Oppo (2005)
Paleocean circulation The configuration was
different, but not the rate of circulation?
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Cadmium as a tracer Dissolved cadmium
is strongly correlated with phosphate and
nitrate. Cd a nutrient tracer High Cd high
nutrients
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Cadmium as a tracer Dissolved cadmium in bottom
water is reflected in benthic foraminifera shells.
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Ocean circulation Biological pump and
conveyor belt combine to distribute nutrients
and nutrient proxies in ocean.
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Combined proxies Both carbon isotope and cadmium
tracers support repeated glacial to interglacial
changes in ocean circulation combine to
distribute nutrients and nutrient proxies in
ocean.
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Nutrient changes and pCO2. Changes in the
inventory or whole ocean distribution of
nutrients could explain the observed shifts in
pCO2. But there is no evidence for change in
whole ocean inventory, and no evidence for
widespread oxygen depletion in the deep ocean.
Signal is most evident in Atlantic, and is most
likely circulation.
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Nutrient shift Carbon isotopes suggest a
wholesale shift to lower values. Cadmium harder
to discern. If anything, there is a small change
to lower values in glacial, opposite required
shift.
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Ocean carbon shift Mean ocean isotopic ratio
changed during ice age.
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Ocean carbon shift Most likely caused by decrease
in terrestrial biosphere. Too small and with the
wrong sense to drive changes in pCO2, but enough
to change ocean reservoir.
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Carbon shift hypothesis A change in the ratio of
inorganic to organic carbon produced in the
surface ocean and exported to depth might help
explain the changes in atmospheric in pCO2.
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Modeled carbon shift A geochemical modeling
experiment failed to show that the change in
carbon ratio could shift the lysocline enough to
have the required change in atmospheric pCO2.
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Focus on Southern Ocean It is the main region
where deep ocean and in atmosphere are in nearly
direct contact.
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Southern Ocean Several models reveal strong
connection to pCO2.
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Southern Ocean Nutrient utilization (dust?),
stratification (sea ice?), and/or circulation
combined with carbonate compensation Remain the
leading explanations for CO2 change.
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Southern Ocean Nutrient and sea ice connection to
pCO2.
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Southern Ocean Nutrient, dust, and sea ice
connection to pCO2. Remains the
leading explanation.
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Isotopic influences Photosynthesis and
temperature equilibration are competing
influences on seawater ?13C.