『Abstract
Sedimentological observations and palaeomagnetic data for Cryogenian
glacial deposits present the climatic paradox of grounded glaciers
and in situ cold climate near sea-level, glaciomarine deposition,
and accompanying large (up to 40℃) seasonal changes of temperature,
all in low to near-equatorial (<10゜) palaeolatitudes (equated
with geographic latitudes). Neither the “snowball Earth” nor the
“slushball Earth” hypothesis can account for such strong seasonality
near the palaeoequator, which together with findings from sedimentology,
chemostratigraphy, biogeochemistry, micropalaeontology, geochronology
and climate modelling argue against those scenarios. An alternative
explanation of glaciation and strong seasonality in low palaelatitudes
is offered by a high (>54゜) obliquity of the ecliptic, which would
render the equator cooler than the poles, on average, and amplify
global seasonality. A high obliquity per se would not have been
a primary trigger for glaciation, but would have strongly influenced
the latitudinal distribution of glaciers. The principle of low-latitude
glaciation on a terrestrial planet with high obliquity is validated
by theoretical studies and observations of Mars. A high obliquity
for the early Earth is a likely outcome of a single giant impact
at 4.5 Ga, the widely favoured mechanism for lunar origin. This
implies that a high obliquity could have prevailed during most
of the Precambrian, controlling the low palaeolatitude of glaciations
in the early and late Palaeoproterozoic and Cryogenian. It is
postulated that the obliquity changed to <54゜ between the termination
of the last Cryogenian low-palaeolatitude glaciation at ≦635 Ma
and the initiation of Late Ordovician-Early Silurian circum-polar
glaciation at 445 Ma.
The High Obliquity, Low-latitude Ice, STrong seasonality (HOLIST)
hypothesis for pre-Ediacaran glaciation emerges favourably from
numerous glacial and non-glacial tests. The hypothesis is in accord
with such established or implied features of Cryogenian glaciogenic
successions as extensive and long-lived open seas, an active hydrological
cycle, aridity and palaeowesterly (reversed zonal) winds in low
palaeolatitudes, and the apparent diachronism or non-correlation
of some low-palaeolatitude glaciations. A pre-Ediacaran high obliquity
also offers a viable solution of the faint young Sun paradox of
a warm Archaean Earth. Furthermore, reduction of obliquity during
the Ediacaran-early Palaeozoic would have yielded a more habitable
globe with much reduced seasonal stresses and may have been an
important factor influencing the unique evolutionary events of
the Ediacaran and Cambrian. The palaeolatitudinal distribution
of evaporites cannot discriminate unambiguously between high-
and low-obliquity states for the pre-Ediacaran Earth. Intervals
of true polar wander such as postulated by others for the Ediacaran
and Early Cambrian imply major mass-redistributions within the
Earth at those times, which may provide a potential mechanism
for reducing the obliquity during the Ediacaran-early Palaeozoic.
Keywords: Proterozoic; Ediacaran; glaciation; palaeoclimate; palaeomagnetism;
obliquity of the ecliptic』
1. Introduction
1.1. Global glaciation?
1.2. A pre-Ediacaran high obliquity?
2. The Proterozoic (pre-Ediacaran ) glacial environment
2.1. Late Cryogenian glacial settings in South Australia
2.2. In situ cold climate near sea-level and strong seasonality
2.3. Low palaeolatitude of pre-Ediacaran glaciations
2.3.1. Cryogenian glaciations
2.3.2. Plaeoproterozoic glaciations
2.3.3. Reality of low-latitude glaciation
3. The HOLIST hypothesis
3.1. The Proterozoic climatic paradox
3.2. High-obliquity Earth
4. High obliquity and low-latitude glaciation of Mars
5. Correcting misunderstandings about the HOLIST hypothesis
5.1. Not just a model for ice distribution
5.2. Ediacaran glaciation excluded
5.3. High obliquity not a trigger for glaciation
5.4. Synchronism of glaciations not required
5.5. Climate cycles and glaciation-carbonate association permitted
6. Testing the HOLIST hypothesis
6.1. Glacial tests
6.1.1. Periglacial and glacial aridity in low palaeolatitudes
6.1.2. Palaeowind direction in low palaeolatitudes
6.1.3. Long-lived and extensive open seas
6.1.4. Diachronous or non-correlative Cryogenian glaciations
6.2. Non-glacial tests
6.2.1. Viable solution of the faint young Sun paradox
6.2.2. Cryogenian palaeotidal regime
6.2.3. No heliotropism of Proterozoic stromatolites
6.2.4. Evolutionary events of the Ediacaran-Cambrian
7. Evaporite palaeolatitudes and obliquity
8. Obliquity acquisition and change
8.1. Early acquisition of a high obliquity
8.2. Subsequent reduction of obliquity
9. Discussion
9.1. The Proterozoic climatic dilemma
9.2. Future directions and tests
10. Conclusions
Acknowledgements
References