What Caused the Intensification of the Northern Hemisphere Glaciation

1
What Caused the Intensification of the Northern
Hemisphere Glaciation?

• 1. Closure of Isthmus of Panama

2
Background to Glaciation

• The Northern Hemisphere Glaciation was Preceded
  by the Pleiocene Warm Period, which ended around
  3.2 Ma
• Two periods of glaciations, indicated by
  Ice-Rafted Debris (IRD) followed, at 2.9Ma and
  2.7Ma
• These represent the intensification of Northern
  Hemisphere Glaciations into the present Icehouse
  World

3
IRD as an indicator of Glaciation
Ice-Rafted Debris (IRD) is represented by this
thin line
Bartoli et al 2005
4
Panama Isthmus

• The Panama Isthmus is the narrow strip of land
  joining North and South America
• This was created over a period from 16Ma to
  around 3Ma by tectonic movements of the American
  continents closing the Central American Seaway
  (CAS)
• The actual timing of closure depends on the
  definition of closure!

5
The Closure of the Panama Isthmus
Haug et al 2004
6
Timing of Closure for the Isthmus of Panama

• CAS Closure for Deep Ocean waters
• 12.9Ma - Pacific and Caribbean benthic fossils
  homogenous (Duque-Caro 1990)
• 11.8Ma - Abrupt end to homogeneity in benthic
  fossils (Duque-Caro 1990)
• CAS Closure for Intermediate Ocean Waters
• 5.0Ma - Delta 18O values for foraminifera from
  Atlantic and Pacific begin to diverge (Keigwin
  1982, Haug et al 2001)
• 4.2Ma - Delta 18O values of foraminifera from
  Atlantic and Pacific show definite divergence
  (Keigwin 1982, Haug et al 2001)
• Complete CAS Closure
• 3.0Ma - Great American Faunal Interchange. Land
  bridge complete between the American continents
  (Marshall 1988)

7
Separation of Ocean Basins

• Closure of the CAS leads to separation of the
  Atlantic and Pacific Oceans
• Evaporation from the Atlantic-Caribbean leads to
  the Atlantic becoming 1ppt more saline
• The Closure of the CAS results in a redirection
  of the Western Boundary Current to form the Gulf
  Stream

Haug et al 2004
8
Initiatition of Thermohaline Circulation (THC)

• The dense, warm waters of the Gulf Stream reach
  high latitudes, warming the North Atlantic and
  increasing evaporation in the region
• The density of the current is high enough for the
  waters to sink as they cool in the Labrador sea,
  initiating THC and the Global Ocean Conveyor

Driscoll and Haug 1998
9
Effects of the CAS Closure
DWT Deep Water Temperature
Bartoli et al 2005
10
Effects of the CAS Closure
DW Deep Water
Bartoli et al 2005
11
Short Circuit of THC

• The prevailing winds for the North Atlantic blow
  the moisture evaporated to Siberia, where
  freshwater discharge is to the Arctic Ocean
• This fresh water is more easily frozen, allowing
  a build-up of ice, with consequent positive
  feedback due to albedo
• The return of fresh water to the North Atlantic
  via the Greenland Current would inhibit THC,
  short circuiting the system and reducing the
  heat transport to the North - entrenching glacial
  conditions in the Northern Hemisphere

Driscoll and Haug 2001
12
Short Circuit of THC
Driscoll and Haug 2001
13
CAS Closure as Preconditioning for the Initiation
of Northern Hemisphere Glaciation

• Driscoll and Haug (1998) note that although this
  system could have acted as soon as CAS closure
  had fully started Thermohaline Convection in the
  North Atlantic, it could only have caused
  ice-build up if polar Summer temperatures were
  low enough
• The period of low Orbital obliquity at 3.1Ma,
  causing low seasonality, is proposed as the
  tipping point for initiation of ice-build up
  (leading to Glaciation) after the preconditions
  of moisture transport had been met by the THC
  system

14
References

• Bartoli, G., Sarnthein, M., Weinelt, H.,
  Erienkeuser, H., Garbe-Schönberg, D. Lea, D.W.
  1995. Final Closure of Panama and the onset of
  northern hemisphere glaciation. Earth and
  Planetary Science Letters, 237, 3344.
• Driscoll, N.W. Haug, G.H. 1998. A Short Circuit
  in Thermohaline Circulation A Cause for Northern
  Hemisphere Glaciation? Science, 282, 436438.
• Duque-Caro, H. 1990. Neogene stratigraphy,
  paleoceanography and paleobiogeography in
  northwest South America and the evloution of the
  Panama Seaway. Palaeogeography, Palaeoclimatolgy,
  Palaeoecology, 77, 203-234.
• Haug et al 2004. How the Isthmus of Panama Put
  Ice in the Arctic. Oceanus http//www.whoi.edu/oce
  anus/viewArticle.do?id2508