MidPleistocene Revolution By Robert Spellacy

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Mid-Pleistocene RevolutionByRobert Spellacy
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MPR

• Describes the transition between 41kyr and
  100kyr glacial interglacial cycles.
• Initiated between 900-650 kyr

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Eccentricity

• Eccentricity provides the pacing rather than the
  driving force.
• Shape of the earth orbit changes from near
  circular to an ellipse over a period of 100 kyr
  and 400 kyr

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Eccentricity
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Obliquity

• Tilt of the earth axis of rotation with respect
  to the plane of its orbit
• Varies from 21.8o to 24.4o
• 41,000 yrs cycle

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Obliquity
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Precession

• Two components of precession one relating to the
  elliptical orbit of the earth and the other
  relating to its axis of rotation
• Precessional cycles of 23 and 19 kyr.

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Precession
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Potential causes of the MPR

• Critical size of the Northern Hemisphere ice
  sheet
• Global cooling trend
• The global carbon cycle and atmospheric CO2
• Intermediate ocean circulation and gas hydrates
• Greenland-Scotland submarine ridge

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Critical size of the Northern Hemisphere ice
Sheets

• Ice sheets may have reach critical size during
  the MPR this allowed a non-linear response to
  eccentricity.
• Erosion of regolith which allowed the ice to rest
  on bedrock and build up.
• This allowed ice to survive longer than 41 kyr
  driving force.

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Global Cooling Trend

• Long term cooling through the Cenozoic large
  enough to ignore the 41 kyr orbited forcing.
• Cooling of deep ocean during Pleistocene
• Alters the relationship between atmospheric
  temperature and accumulation rates of snow on
  continental ice sheets.

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The global carbon cycle and atmospheric CO2

• Decline of Conc. of CO2 in atmosphere
• Allowing it to respond non-linearly to orbital
  forcing
• Atmospheric CO2 in Northern Hemisphere not ice
  volume is the primary driving force of the 100
  kyr glacial and interglacial cycles

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Carbon Dioxide and Ice Volume
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Intermediate Ocean circulation and gas hydrates

• MPR there was a re-organization of the ocean
  circulation at intermediate water depth
• Warmer intermediate water occurs only during
  periods deglaciations
• Warmer water would cause the destabilization of
  gas hydrates on continental shelves and slopes
  release of methane
• This caused an increase in global warming

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Greenland-Scotland submarine ridge

• MPR mechanism based on the uplift of the
  Greenland-Scotland Submarine ridge at about 950
  kyr
• Surge of tectonic activity along Iceland mantle
  plume
• Southward shift of the area of deep water
  production from arctic to the Nordic sea
• Increase effects oceanic circulation
• Making it much more difficult for thermohaline
  conveyor to re-set into an interglacial mode.

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Greenland-Scotland Submarine Ridge
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Changes in sedimentation patterns of the Nordic
seas region across the mid- Pleistocene Helmke,
Jan. et al. Marine Geology 215 (2005) 107-122.
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Introduction

• Strong glacial and interglacial climate cyclicity
  of the Northern Hemisphere as it is recognized
  for the past 500 kyr is not representative for
  the entire Pleistocene climate system.
• Nordic seas showed long periods of moderate
  glacial conditions and only episodic interglacial
  intervals

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It was after the so called MPR that glacial and
interglacial climates intensified leading to the
more pronounced contrasts of the high northern
climate system so typical for the late
Pleistocene Mid-Pleistocene climate
intensified . Mid-Pleistocene shift from a
dominant climate periodicity around 41 ka to a
dominant periodicity around 100 ka Earths
orbital eccentricity
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• To improve our current knowledge as well as our
  concepts about forcing factors and environmental
  consequences of the mid-Pleistocene climate
  shift. We need further high quality proxy
  records.
• Problem is that current proxy information about
  character and timing of the mid-Pleistocene
  paleoceanographic and paleoclimatic changes are
  rather limited. Especially where the MPR would be
  pronounced like the Nordic Sea.
• To gain more insights into the specific climate
  response of the high northern latitudes, high
  resolution sediment records were studied from
  the Norwegian Sea the sediments covered the past
  1.6Ma.

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34 meter long piston core MD992277 it was
recovered from the eastern slope of the Iceland
Plateau in the western Norwegian Sea during 1999..
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The bulk carbonate content wt was measured
every 5 cm in the same core as IRD. The Ca ,Fe,
and Ti measured 2 cm using X-ray fluorescence
core scanner Glacial and interglacial marine
isotope stages (MIS) older than MIS 10 were
identified in core MD992277 using the planktic
delta 18 O, the Ca- counts, as well as the
carbonate and the IRD records. Interglacial
periods are characterizes by low delta 18 O, high
Ca-counts and high values of carbonate content as
well as low IRD.
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The Ti and Fe are in generally in good agreement
. Both show many changes that are recognized in
the MS record over the 1.6 Ma.Differences between
XRF and MS data 1) minimum in Fe and Ti during
full interglacial MIS 11at about 400 ka is not
accompanied by a large MS minimum.2) MS maximum
in early MIS 15 at about 600 ka is represented
neither in the Ti nor the Fe values 3) High Fe
and Ti input is noted during peak interglacial
substage 21.1 at about 820 ka 4) XRF and MS
records is a clear mid-Pleistocene shift in mean
values. Fig 3
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Systematic shift varies between the three records
and occurs between 700 and 550 ka Early and
Middle Pleistocene times the mean values of the
magnetic components and the mean MS values were
significantly higher than during Late Pleistocene
times. Prominent minimum in the XRF records
during MIS 11these constantly lowered Late
Pleistocene mean values are most obvious between
about 550 and 150 ka. At 150 ka another less
pronounced decrease of mean values can be
observed for both the records of XRF and MS IRD
records covers the time between 1.6 ma and 350 ka
shows alternating periods of massive terrigenous
input that can be related to glacial and stadial
intervals.
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The bandpass- filter IRD record from site
MD992277 reveals a clear change from a dominant
variance around 41 ka during Early and early
Middle Pleistocene times to a dominant 100 ka
cyclicity during the late Middle Pleistocene a
pattern also recognizable in the MS
signal. Amplitude minimum of the 100 ka filter
from both IRD and MS as well as the amplitude
maximum of the 41 ka filter from IRD are
observed at 1 Ma. Maximum in the amplitude of
the 41 ka filter MS signal noted at 1.2 Ma Both
filter signals are most pronounced between 1 Ma
and 500 ka. The first shift in the MS and XRF
values the amplitude of the 100-ka cycle begins
to increase at about 700 ka.
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During MIS 39 the fine fraction component and Ti
both have a maximum time coeval with low IRD
implying that an increased bottom water flow
caused enhanced accumulation of fine-grained
magnetic particles at the site. MIS 37 is
characterized by comparatively low Ti values and
thus would not support the idea of an increased
input of magnetic particles by increased strength
of interglacial bottom currents. Nordic seas MIS
15 and 11 are the two most pronounced warm
intervals within the entire Middle Pleistocene
period. These interglaciations should be
characterized by a vigorous deep-water flow.
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Ti concentration shows certain minima during
glacial times with massive IRD during MIS 12
enhanced Ti values occur before as well as after
the MPR, usually during times of notable IRD
deposition It seems as if the proposed
mid-Pleistocene shift in the pattern of deep
water circulation of the Nordic seas has some
influence on the sedimentological records of the
site.
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Role of sea ice and iceberg drift

• MPR related shift in the sea ice export from the
  Artic Ocean into the Nordic seas may contribute
  to the terrigenous changes observed at the site.
  
• Arctic sea ice can carry large amounts of
  terrigenous silt and clay into the Nordic sea and
  release it after melting.
• Most of the coarse grained terrigenous material
  in the Nordic seas is probably derived from
  icebergs.
• A continuous input of terrigenous material by ice
  is observed for the past 1.6 Ma.

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Conclusions

• Sediment record suggests that the Norwegian Sea
  has under gone a systematic gradual shift in its
  environmental conditions during the course of the
  MPR.
• Major shift can be observed in the evolutionary
  frequency analysis of the IRD and MS which was
  around 1.0 Ma, when a variance around 100 ka
  emerges from a 41 ka cyclicity.
• The sediment records document that next to the
  current system the ice-drift pattern in the
  Nordic seas has to be considered as important
  mechanism in order to explain the mid-Pleistocene
  climate change.

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The early Matuyama Diatom Maximum off SW Africa
a conceptual modelW.H. Berger, C.B. Lange, M.E.
PerezMarine Geology 180 (2002) 105-116
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Introduction

• The prolonged maximum is centered around 2.6-2.0
  Ma. and follows a rapid increase of diatom
  deposition near 3.1 Ma.
• Ocean Drilling Program Leg 175 occupied five
  sites off SW Africa in order to retrieve the
  record of the Namibia upwelling system
• The paradox of the Matuyama Diatom Maximum (MDM)
  is that increased coastal upwelling in the
  Pleistocene is accompanied by an apparent
  decrease in total diatom deposition.

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Introduction

• Thus as the glacial component of the climate
  becomes increasingly dominant, after 2 Ma, we
  should expect an overall increase in coastal
  upwelling, as a general trend within the
  Quaternary.
• Possible explanation is silicate content of
  thermocline waters.
• They search for the simplest possible description
  of the phenomenon to be explained.

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Fig 1 Location of the Leg 175 sites off
southwestern Africa that show the MDM centered
between 2.6 and 2.0.Heavy shading, coastal
upwelling zone light shading, zone influenced by
eddies and filaments from the upwelling zone.
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The strongest representation of the MDM is in
site 1084 water depth 1992 m. The overall trends
are reflected in greatly smoothed versions of the
diatom and opal abundance series. (Fig. 2b). At
the same time, overall diatom deposition
decreases. The excellent correlation between the
two indices is noteworthy. The disagreements
between the two indices stem from the fact that
different samples were analyzed along a rather
variable record.
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Fig. 2 A) summary of raw data B) Greatly smoothed
versions of the series in the upper panel.DAI and
opi as above. Frontal zone marks the position of
the MDM. The relative importance of coastal
upwelling increases toward the present, as
documented by Chaetoceros resting spores. At the
same time, overall diatom deposition decreases.
Note the relative insensitivity of the visual
index (DAI) compared to opal at low values of
opal. C) Residual values of DAI and opi, after
removal of smoothed series from original series.
Note the increase of variability in the late
Quaternary.
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Fig 3 Hypothesis of early Pleistocene opal
maximum in the Southern Ocean, based on the
concept of a link to an optimum in NADW
production ( at the critical level of cooling.)
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Hypothesis Concept of optimum

• Ramp-up beginning between 4 and 3 Ma a maximum
  centered near 2.3 and 2.2 Ma and a subsequent
  decline (Fig 2B).
• The overall trend is reminiscent of the
  proposition that the share of opal deposition
  around Antarctica moves through an optimum as the
  planet cools.
• The reason given is that an overall increase in
  the production of NADW, due to cooling in the
  late Pliocene, will move silicate into the
  Southern Ocean, increasing diatom production
  there. The Fire-hose effect
• At some point additional Cooling interferes with
  NADW production negatively impacting the
  fire-hose effect.
• At that point diatom production drops off in the
  Southern Ocean and the Antarctic Oceans share in
  the global ocean silica sequestration drops.

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Model Overall cooling and distance from optimum

• Two driving factors global system state and
  distance from optimum condition.
• The algorithm that translates system state and
  distance from optimum into an estimate of diatom
  deposition has the form of a linear regression
• Dx aX f (dist) bX delta 18O c
• where f(dist) is the inverse of the difference of
  the given state (x) to the nearest pt. on the
  optimum (fz) augmented by 0.5 to avoid dividing
  by zero f(dist) 1/ (I (x fz) I 0.5) The
  coefficients a and b and the constant c are
  adjusted for best fit.

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Fig. 4 Conceptual model of the record of the opal
deposition off SW Africa in the last 4 million
years.
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Fig 5 Performance of the algorithm (eq.1) for
modeling short-term fluctuations of the Opal
deposition. Input is unsmoothed original delta 18
O series of Site 849 benthic foraminifers. The
results are disappointing peaks and valleys do
not match the observed ones. The mismatch on the
scale of 100 kyr could be due to dating problems.
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Fig 6 The link between the Namibia opal record
and global ocean deepwater nutrient chemistry, as
seen in the relationships between opal abundance
(opx) and carbon isotope composition of Pacific
deep water. The correlation over the entire 2 Ma
is significant at P lt 0.01, but is comparatively
poor in the last third of the record.
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Fig 7 Relationship of opal record of Site 1084 to
eccentricity of the Earths orbit.
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Fig 8 Evolutionary spectrum of opal record (opx
for last 3Ma, DAI only before that) Fourier
expansion of auto correlation series is used to
determine amplitude, which was modified by
dividing each entry by the log period squared.
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Conclusion

• Thus if deepwater chemistry is important in opal
  record off SW Africa and if tied to NADW
  production, we should expect a correlation
  between the Pacific water delta 13 C such
  correlation does exist.
• Glacial-interglacial cycles are less important
  than deep circulation in the early two thirds of
  post MDM time , but gain important for the last
  third, when they attain a dominant role in
  climate change.
• Site 1084 shows affinity to eccentricity on the
  400-kyr scale. No such relationship exists for
  the 100 kyr scale. The correlation between
  eccentricity and opal abundance is zero.

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Conclusion

• The fact that the 400-kyr cycle is represented in
  the opal record, while the 100-kyr cycle is not
  would seem to pt. toward processes that have to
  do with long-term cyclic geologic processes such
  as intensity of weathering of silicate minerals
  on land, with associated changes in supply of
  silica by rivers.
• The lack of consistency in periodicity is perhaps
  the most striking property of the spectral
  landscape of the opal record of site 1084.