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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jökulhlaups
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Past climate changes
I Milankovitch and the Astronomical or
Orbital theory of the ice ages.
II Successes and potential failures of
Milankovitch.
III Abrupt climate change.
IV External mechanisms of rapid climate
change Non-linear orbital forcing. Variable
solar luminosity. Aliased seasonal cycle.
V Internal mechanisms of rapid climate
change Binge-purge ice sheet
instability. Salt oscillator ocean
circulation. Tropical ocean-atmospheric
dynamics.
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Long-term decline into glaciation
Oxygen isotopes in benthic foraminifera from
deep-sea sediments reveal a cooling trend on
Earth over tens of millions of years.
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MARINE ISOTOPE STAGES (MIS)
Warm (odd)
?18Oforams
Cold (even)
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Milutin Milankovitch
Serbian mathematician who painstakingly
calculated the insolation effects of changes in
the Earths orbital parameters.
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Eccentricity
Variations in the shape of the Earths orbit
around the sun, from circular to more elliptical,
occur over 100,000 years.
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Obliquity
Variations in the tilt of the Earths axis of
rotation, driving contrasts in seasonality, occur
over 40,000 years.
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Precession
Variations in the orientation of the Earths
tilted axis of rotation, moving seasons along the
eccentric orbit, occur over 20,000 years.
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Orbital variations in received insolation
Each orbital influence has its own distinctive
pattern of influence through time. They combine
to vary sunlight on Earth.
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Specmap
Study of isotope and other paleo-climate records
from deep-sea sediments around the world reveals
Milankovitch signal.
Period (1/frequency) in thousands of years.
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Devils Hole
Well dated speleothem record not consistent with
Milankovitch? Should it? Remains an unsolved
puzzle.
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Abrupt climate change
Evidence from ice cores and deep-sea sediments
reveals climate changes that seem too abrupt to
be explained orbitally.
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Dansgaard-Oeschger Cycles discovered in Greenland
ice
warm
cold
d18Oice
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N. Atlantic SSTs track Greenland Air Temperatures
warmer
cooler
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THE YOUNGER DRYAS IN THE SANTA BARBARA BASIN

• Increased bioturbation suggesting higher O2
  levels (D-O interstadials show the opposite with
  reduced bioturbation)
• Increased ?18Obenthic
• Increased Pacific ventilation?

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GLOBAL SIGNATURE OF D/O EVENTS CARIACO BASIN
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ANTI-PHASING BETWEEN HEMISPHERES
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THE DISCOVERY OF HEINRICH EVENTS.
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. IN MARINE SEDIMENT ARCHIVES
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Transport of ice-rafted debris (IRD)
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Site withoutice-rafteddebris
Site withice-rafteddebris
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MECHANISMS BEHIND MILLENNIAL VARIABILITY

• EXTERNAL FORCING
• Suborbital (Combination tones)
• Solar Variability
• INTERNAL FORCING
• Ice sheets (e.g., Binge-Purge theory)
• Thermohaline circulation (e.g., Broeckers Salt
  Oscillator)
• Tropical Dynamics (e.g., Eastern tropical
  Pacific - ENSO dynamics)
• ALTERNATIVE
• Aliasing

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SUBORBITAL FORCING MILANKOVITCH COMBINATION
TONES
Tropical Atlantic coccolith abundance records
Variations in tropical winds/upwelling cause ice
rafting events?
F. profunda (upwelling indicator) maxima coeval
with Heinrich events

• Spectral Analysis
• spectral power at 8.4 ky (7.6 ky 14C), a
  combination tone of precession and eccentricity

The mechanism Easterly winds diminish, warm
water propagates northward via the Gulf Stream,
delivery of heat causes ice sheet melting and
subsequent iceberg discharge
McIntyre and Molfino (1996)
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SOLAR VARIABILITY IRD and Cosmogenic Nuclides in
the Holocene
10Be
14C
Similar pacing of ice rafting events during last
interglacial (MIS-5)
Bond et al. (2001)
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ICE SHEET STABILITY THE BINGE-PURGE THEORY
MacAyeals explanation for Heinrich Events
BINGE
PURGE
Laurentide Ice Sheet (LIS) Model Binge (Growth)
phase - basal sediment frozen, ice sheet
immobile slow growth of LIS Purge phase -
geothermal flux gradually increases basal
temperature, thawing basal sediments rapid
discharge of icebergs (meltwater input reduces
THC, cool North Atlantic, allowing ice growth
again)
BINGE
PURGE
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THERMOHALINE CIRCULATION THE SALT OSCILLATOR
Conveyor On Northward heat transport initiates
ice melting, which reduces density of upper
ocean, reducing deep water formation Conveyor
Off Cooling of North Atlantic (less melt water
input) and reduced salt export gradually increase
density (and thus deep water formation) return
to Conveyor On mode
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Three Modes of North Atlantic Deep Ocean
Circulation
Oc/Atm
Atm
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TROPICAL DYNAMICS EASTERN TROPICAL PACIFIC
The Idea El Niño/Southern Oscillation (ENSO)
system in the eastern tropical Pacific affects
tropical SST distribution, which controls
convection/atmospheric circulation patterns -
very strong teleconnections, which could explain
near-global synchroneity of millennial
events Experiment Coupled ocean-atmosphere
model of the tropical Pacific is run for 150,000
years, both with and without orbital
(precession) forcing Result Millennial
increases or decreases in the frequency of ENSO
warm or cold events and changes in their
amplitudes
EVENT FREQUENCY
EVENT AMPLITUDE
Clement et al. (1999)
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THE PROBLEM OF ALIASING
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D-O events show up in major ion chemistry of
Greenland ice
Polar Circulation Index (PCI)

• Record developed from the analysis of GISP2
  major ion chemistry
• Proxy for relative measure of the average size
  and intensity of polar atmospheric circulation
• Generally increase (e.g., more continental dusts
  and marine contributions) during stadials and
  decrease during interstadials

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SPECTRAL ANALYSIS OF THE PCI
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GLOBAL SIGNATURE OF D/O EVENTS ARABIAN SEA
Greenland d18Oice
Total organic carbon (TOC) records from two
marine sediment cores off Pakistan showing
millennial-scale variability of monsoonal surface
water productivity and bottom-water oxygen
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D/O temperature oscillations also occur in
Antarctica (Vostok), but with generally smaller
amplitudes
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HEINRICH EVENTS

• Helmut Heinrich discovered a cyclic pattern of
  ice-rafted debris over the last 130 ky in a
  series of sediment records from the North
  Atlantic
• Broecker and Bond later identified six of these
  events between 70 and 14 ky B.P., and named them
  Heinrich events (H1-H6)
• Heinrich events occur every 7-10 ky, and are
  associated with pronounced cooling in the
  Greenland ice core records and North Atlantic
  region
• Bond et al. (1993) noted that Heinrich events
  appear to occur towards the end of increasingly
  colder D-O cycles

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SEDIMENTARY CHARACTERISTICS OF A HEINRICH LAYER
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OTHER TYPES OF IRD FOUND IN THE NORTH ATLANTIC
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HEINRICH EVENT CHRONOLOGY
Bond et al. (1992)
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BOND IDENTIFIES MORE IRD PEAKS IN N. ATLANTIC
SEDIMENTS
COLDER
COLDER
Bond and Lotti (1995)