Week 2 (09/01/2009) Summary – Joe Davis
Topic: The pre-Q Earth (setting the stage): 1. Snowball Earth and ice albedo feedbacks
2. Long-term Cretaceous and Cenozoic climate
evolution
Papers Discussed:
Group 1
Hoffman,P. F., Kaufman, A. J., Halverson, G. P., and D. P. Schrag, D. P., 1998..A neoproterozoic
snowball Earth. Science, v.. 281, pp. 1342-1346. Also Kerr ,1998, comments on the paper.
Hoffman, P. F., A snowball Earth poster.
Hyde et al., 2000. Neoproterozoic ‘snowball Earth’ simulations with a coupled climate/ice sheet model.
Nature, v.405, pp.425-429. Also, Runnegar, 2000, omments on the paper.
And, a 2002 Snowball Earth News piece
The snowball Earth hypothesis suggests that during the Neoproterozoic Era ice covered the
Earth’s entire surface. This coverage included oceans. Changes in 13C isotope concentrations
give evidence that support the hypothesis. During extreme glaciations periods, photosynthesis
would be reduced because ice cover would block out sunlight. If the 13C/12C ratio in sediment is
high, it means more photosynthesis was taking place. Thus, if the 13C/12C ratio in the sediment is
low, less photosynthesis was taking place and more precipitation of carbonates occurs.
Evidence of a Snowball earth include: (1) paleomagnetism - When sedimentary rocks form,
magnetic minerals within them tend to align themselves with the Earth's magnetic field. Through
the precise measurement of this palaeomagnetism, it is possible to estimate the latitude where the
rock matrix was deposited. (2) glacial deposits - Dropstones (stones dropped into marine
sediments), which can be deposited by glaciers. Varves (annual sediment layers in periglacial
lakes) form at higher temperatures. Glacial striations (formed when embedded rocks scrape
against bedrock). Diamictites (poorly sorted conglomerates). Originally described as glacial till,
most were in fact formed by debris flows. (3) Carbon Isotope ratios - biochemical processes, of
which photosynthesis is one, tend to preferentially incorporate the lighter 12C isotope. During the
proposed episode of Snowball Earth, there are rapid and extreme negative excursions in the ratio
of 13C to 12C. (4) Banded Iron formations - The banded iron formations are commonly very old
and their deposition is often related to the oxidation of the Earth's atmosphere during the
Paleoproterozoic era. Dissolved iron in the ocean came in contact with photosyntheticallyproduced oxygen and precipitated out as iron oxide. The bands were produced at the tipping
point between an anoxic and an oxygenated ocean. (5) Carbonate Cap rocks - Around the top of
Neoproterozoic glacial deposits there is commonly a sharp transition into a chemically
precipitated sedimentary limestone or dolostone. These cap carbonates sometimes occur in
sedimentary successions that have no other carbonate rocks, suggesting that their deposition is
result of a profound aberration in ocean chemistry. Melting of a Snowball Earth, would dissolve
the abundant CO2 from the atmosphere to form carbonic acid, which would fall as acid rain. This
would weather exposed silicate and carbonate rock (including readily-attacked glacial debris),
releasing large amounts of calcium, which when washed into the ocean would form distinctively
textured layers of carbonate sedimentary rock. Such abiotic "cap carbonate" sediment can be
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found on top of the glacial till that gave rise to the Snowball Earth hypothesis. (6) Climate
fluctuations - Using the ratio of mobile cations to those that remain in soils during chemical
weathering (the chemical index of alteration), it has been shown that chemical weathering varied
in a cyclic fashion within a glacial succession, increasing during interglacial periods and
decreasing during cold and arid glacial periods. Causes may include the earth’s albedo,
continental configuration, Milankovitch cycles, solar luminosity, and abrupt releases of CO2
from volcanoes.
Life may forms may have survived in warmer pools of melt-water on land surfaces or in areas
of warmer “open water” near the equator.
Group 2
Zachos, J., Pagani, M., Sloan, L., Thomas, and Billups, K., 2001. Trends, Rhythms, and Aberrations in
global Climate 65 Ma to Present. Science, v. 292, pp. 686-693.
Shevenell, A. E., Kennett, H. P., and Lea D. W., 2004, Middle Miocene Southern Ocean Cooling and
Antarctic Cryosphere Expansion. Science, v. 305, pp. 1766-1770.
The news related blurb – “Why The Ice?”
Zachos, C., Dickens, G. R., and Zeebe, R. E., 2008. An early Cenozoic perspective on greenhouse
warming and carbon-cycle dynamics. Nature, v. 451, pp. 279-283.
Magnesium/calcium data from Southern Ocean planktonic foraminifera demonstrate that
high-latitude (55°S) southwest Pacific sea surface temperatures cooled 6° to 7°C during the
middle Miocene climate transition (14.2 to 13.8 million years ago). Stepwise surface cooling is
paced by eccentricity forcing and precedes Antarctic cryosphere expansion by 60 thousand years,
suggesting the involvement of additional feedbacks during this interval of inferred lowatmospheric partial pressure of CO2 Comparing SSTs and global carbon cycling proxies
challenges the notion that episodic pCO2 drawdown drove this major Cenozoic climate
transition. SST, salinity, and ice-volume trends suggest instead that orbitally paced ocean
circulation changes altered meridional heat/vapor transport, triggering ice growth and global
cooling.
The Mg/Ca content of foraminifera increases exponentially with temperature (9 ± 1% per
1°C) and is relatively insensitive to salinity and ice-volume fluctuations. Low-resolution paired
benthic foraminifer Mg/Ca and 18O studies designed to constrain the timing and magnitude of
pre-Quaternary ice-volume fluctuations suggest substantial Antarctic ice growth (0.85) and a
concomitant deep ocean cooling (2°C to 3°C) during the MMCT. The magnitude of Antarctic ice
growth and rapidity of this climate transition [<0.5 million years (My)] suggests that Earth's
climate system was highly sensitive to oceanic, atmospheric, and cryospheric feedbacks.
SSTs cooled 6°C to 7°C between 14.2 and 13.8 Ma, revealing that the Southern Ocean was a
dynamic component of the MMCT (Middle Miocene climate transition). Eccentricity-paced
Southern Ocean surface cooling and freshening suggest that atmospheric/oceanic circumpolar
circulation intensified in response to orbital forcing, increasingly thermally isolating Antarctica
during the MMCT (14.2 to13.8 Ma). Middle Miocene intensification of the ACC may have
played a major role in Cenozoic climate evolution, both directly through changes in meridional
heat transport and indirectly through changes in vapor transport. We speculate that sensitivity to
eccentricity forcing increased at 14.2 Ma, immediately following peak warmth of the MCO as a
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result both of low atmospheric pCO2 and of a fundamental reorganization of the climate system,
specifically a tectonically mediated reduction in meridional heat/vapor transport related to the
constriction of the Eastern Tethys at 15 Ma.
Southern Ocean surface-water cooling and Antarctic ice growth occurred between CM5 and
CM6 during an interval of increasing pCO2. This sequence of change challenges the notion that
episodic pCO2 drawdown was the primary forcing that triggered the MMCT, again implying that
other feedbacks (e.g., ice/albedo) played a more significant role in this climate transition.
Records also indicate that CM6, one of the largest CM events, corresponds with maximum
Antarctic ice volume and a slight warming of the high southern latitudes. This relationship raises
the possibility that feedbacks related to Antarctic cryosphere expansion may have exerted control
over the global carbon cycle through enhanced ventilation of intermediate and deep ocean waters
and falling sea levels. The long-term trend observed in the global 13C record after the rapid
expansion of Antarctic ice volume supports this interpretation.
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