Mid-Pleistocene RevolutionBy

Robert Spellacy

MPR

Describes the transition between 41kyr and 100kyr glacial –interglacial cycles.

Initiated between 900-650 kyr

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

Eccentricity

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

Obliquity

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.

Precession

Potential causes of the MPR

Critical size of the Northern Hemisphere ice sheetGlobal cooling trendThe global carbon cycle and atmospheric CO2

Intermediate ocean circulation and gas hydratesGreenland-Scotland submarine ridge

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.

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.

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

Carbon Dioxide and Ice Volume

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

Greenland-Scotland submarine ridge

MPR mechanism based on the uplift of the Greenland-Scotland Submarine ridge at about 950 kyrSurge 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.

Greenland-Scotland Submarine Ridge

Changes in sedimentation patterns of the Nordic seas

region across the mid- Pleistocene

Helmke, Jan. et al. Marine Geology 215 (2005) 107-

122.

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

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 Earth’s orbital eccentricity

•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.

34 meter long piston core

MD992277 it was recovered from

the eastern slope of the Iceland

Plateau in the western Norwegian

Sea during 1999..

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.

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

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.

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.

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.

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.

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.

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.

The early Matuyama Diatom Maximum off SW Africa: a conceptual model

W.H. Berger, C.B. Lange, M.E. PerezMarine Geology 180 (2002) 105-116

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.

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.

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.

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.

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.

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.)

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 Ocean’s share in the global ocean silica sequestration drops.

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 + cwhere 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.

Fig. 4 Conceptual model of the record of the opal

deposition off SW Africa in the last 4 million years.

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.

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 < 0.01, but is comparatively poor in the last third of the record.

Fig 7 Relationship of opal record of Site 1084 to eccentricity of the Earth’s orbit.

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.

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.

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.