2006 RESEARCH NEWS
Evaporite basins, orbital tilt, and the geomagnetic axial-dipolar field
Evans, D.A.D., Proterozoic low orbital obliquity and axial-dipolar geomagnetic field from evaporite palaeolatitudes. Nature 444, 51-55 (2006); doi: 10.1038/nature05203
Two fundamental uncertainties have lingered over the problem of Proterozoic low-latitude glaciation. First, if Earth had a large (>54°) orbital obliquity (or tilt) at that time, the equator-to-pole temperature gradient would have been reversed, making tropical areas most susceptible to glaciation. Tropical glaciation would not indicate an extreme climate. Second, if the geomagnetic field had large non-dipolar (e.g., quadrupole, octupole) components, glacial deposits would have actually formed at higher paleolatitudes than inferred from paleomagnetic inclination data under the conventional assumption of a pure dipole field. One way to test both possibilities is through paleomagnetically-determined paleolatitudes of large-scale evaporite basins (e.g., basins containing deposits of salt, gypsum, potash salts, or their pseudomorphosed equivalents), which form in the subtropical arid zones under the descending limbs of the Hadley cells. (Exactly 50 years ago, the distribution of Phanerozoic evaporites was successfully used as an early test of continental drift. See E. Irving, Palaeomagnetic and palaeoclimatological aspects of polar wandering. Geofisica
Pura e Applicada 33 (1956) 23-41.) In a recent article in the journal Nature, David A. D. Evans of Yale University has compiled the record of paleomagnetically-constrained evaporite basins back to 2.25 Ga (early Paleoproterozoic), four times longer than any previous compilation.
Proterozoic evaporite basins have a statistically significant volume-weighted concentration in the paleo-subtropics, slightly (~6°) equatorward of the mean paleolatitude for the last 250 Ma. This falsifies the predicted equatorial disposition of evaporite basins if the Proterozoic Earth had had a large orbital obliquity. (Obliquity cannot switch back and forth between large and small angles because the latter are stabilized by the Moon's gravitational attraction on the Earth's equatorial bulge.) Therefore, low-latitude glacial episodes cannot be attributed to a reversed climatic gradient. The slightly-lower mean paleolatitude of Proterozoic evaporite basins, compared with the Mesozoic-Cenozoic, could reflect a broadening of the Hadley cells with time, due to slowing of the Earth's rotation rate. Alternatively, it could mean there were minor non-dipolar components (e.g., <15% octupole) in the Proterozoic geomagnetic field. Somewhat larger components (<20% octupole) are possible in the interval between 550 and 370 Ma (late Ediacaran through late Devonian), when the mean paleolatitude of evaporite basins was ~9° lower than in the Mesozoic-Cenozoic. The non-dipole components would have an equatorward biasing effect on the paleolatitudes of Proterozoic glaciation, but not enough to substantially alter the climatological problem of low-latitude glaciation.
A large isotope anomaly before the Sturtian glaciation
McClay, G.A., Prave, A.R., Alsop, G.I., and Fallick, A.E., 2006. Glacial trinity: Neoproterozoic Earth history within the British-Irish Caledonides. Geology 34, 909-912; doi: 10.1130/G22694A.1
A third glacigenic formation and associated post-glacial cap dolostone have
been found in the Dalradian Supergroup of the Irish-Scottish Caledonides
(McClay et al., 2006). The Stralinchy-Reelan formations, composed of diamictite
and ice-rafted debris, lie stratigraphically between the previously-known
Port Askaig Tillite and the Inishowen-Loch na Cille ice-rafted debris beds.
Carbon isotopes in the cap dolostone (basal Cranford Limestone) support
a terminal Cryogenian (635 Ma) age for the glaciation, consistent with existing
strontium isotope data indicating a "Sturtian" age for the older Port Askaig
Directly below the Port Askaig Tillite is the Islay Limestone, which hosts a remarkable isotope anomaly, first recognized by Brasier & Shields (2000). Carbonate δ13
C falls precipitously by ~10‰ to mantle-like values, and then rapidly recovers. Similar anomalies are found beneath "Sturtian" glacial deposits in the northern Canadian Cordillera (Coates Lake Group) and East Svalbard (Russøya Member), and also beneath "Marinoan" glacial deposits in many regions (the Trezona anomaly of Halverson et al., 2005). The occurrence of ~10‰ negative excusions in δ13
C before both the "Sturtian" and "Marinoan" glaciations (purported snowball events) makes their close association unlikely to be a matter of chance.
2005 RESEARCH NEWS
Extreme winds and waves in glacial aftermath (January, 2005)
Allen, P.A. and Hoffman, P.F., 2005. Extreme winds and waves in the aftermath
of a Neoproterozoic glaciation. Nature 433, 123-127.
Jerolmack, D.J. and Mohrig, D., 2005. Formation of Precambrian sediment
ripples: Arising from P.A. Allen & P.F. Hoffman Nature 433, 123-127
(2005). Nature, 10.1038/nature04025.
Allen, P.A. and Hoffman, P.F., 2005. Formation of Precambrian sediment
ripples: Reply to Jerolmack, D.J. & Mohrig, D. Nature, 10.1038/nature04025.
Sedimentologists from ETH-Zürich and Harvard University interpret “tepee” structures
in younger Cryogenian cap dolostones as giant wave ripples, formed by extreme
winds and waves in a pan-glacial aftermath. Hindcasting from gravitational
wave theory, they suggest that the bedforms were produced at water depths
200-400 meters by waves with maximum wave periods (21-30 seconds) significantly
longer than those prevailing in today’s oceans. The reconstructed
wave conditions could only have been generated by sustained wind speeds >20
meters per second (~3 times faster than present trade-wind speeds) in fetch-unlimited
ocean basins. In a discussion published on-line, Jerolmack & Mohrig
(MIT) suggest that the ripples were formed by hurricanes, not by prevailing
winds. In response, the original authors note that multiple generations
of ripples have similar azimuthal orientations, whereas hurricanes make
landfall at different places and produce variable wind and wave orientations
at any given location.
Iridium spikes suggest long-lived glaciations
Bodiselitsch, B., Koeberl, C., Master, S., and Reimold, W.U., 2005. Estimating
duration and intensity of Neoproterozoic snowball glaciations from Ir anomalies.
Science 308. 239-242.
High concentrations (~1 part per billion) of the platinum-group-element
iridium (Ir) were discovered at the base of both the older and younger
Cryogenian cap carbonates in four drill-cores from the Katangan succession
of Zambia and southern Congo. The discoverers, from the University of Vienna
(Austria) and Witwatersrand (South Africa), interpret the Ir as extraterrestrial
(most terrestrial Ir is segregated in the iron core, leaving the silicate
Earth depleted in Ir). Based on the average Cenozoic extraterrestrial Ir
flux, they estimate that the Ir accumulated in glacial ice for ~12 million
years and was then released when the glaciers melted. Using the Ordovician
flux (considered anomalous), the estimated time-scale for ice entrainment
is ~4 million years; using the Holocene flux (from Greenland ice), the
time-scale is ~24 million years. If the discovery is replicated elsewhere
and the Ir is truly extraterrestrial (testable with complete platinum-group-element
ratios, osmium isotopes and spinel compositions), it will strongly support
the snowball earth hypothesis.
Precise age for snowball termination (April,
Condon, D., Zhu, M., Bowring, S.A., Wang, W., Yang, A., and Jin, Y., 2005.
U-Pb ages from the Neoproterozoic Doushantuo Formation, China. Science
Hoffmann, K.-H., Condon, D.J., Bowring, S.A., and Crowley, J.L., 2004.
U-Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: constraints
on Marinoan glaciation. Geology 32, 817-820, doi:10.1130/G20519.l
A team led by Dan Condon from MIT (Massachusetts Institute of Technology)
succeeded in precisely dating the termination of the the younger Cryogenian
glaciation. They determined (using isotope-dilution thermal-ionization
mass spectrometry, or IDTIMS) that zircons from a volcanic ash layer within
in the cap dolostone on the Nantuo glacials in South China have a concordia
age of 635.2 ± 0.6 Ma. They previously obtained a statistically-indistinguishable
age of 635.5 ± 0.5 Ma for correlative glacials in Namibia using
the same method. The glacials and cap dolostones in both areas were
correlated with the Marinoan succession in South Australia, where the base
of the formal Ediacaran Period is defined at the base of the cap dolostone.
If the correlations are correct, and assuming the deglaciation was rapid
at low paleolatitudes (where all three areas existed), the termination
of the Marinoan snowball earth and the start of the Ediacaran Period occurred
at 635 Ma.
Microbial origin for vertical tubular structures in cap dolostones
Corsetti, F.A. and Grotzinger, J.P., 2005. Origin and significance of tube
structures in Neoproterozoic post-glacial cap carbonates: example from
Noonday Dolomite, Death Valley, United States. Palaios 20, 348-363.
Tubular structures filled by sediment and/or cement occur profusely in
younger Cryogenian (Marinoan) cap dolostones in different areas (California,
NW Canada, western Brazil and throughout Namibia). They were previously
interpreted as gas or fluid escape pathways on account of their consistent
paleovertical orientation, irrespective of the tilt of host layering. They
occur exclusively within layered, non-skeletal, organo-sedimentary buildups
known as stromatolites. Frank Corsetti (University of Southern California)
and John Grotzinger (California Institute of Technology) reexamined
the classic examples in the Noonday cap dolostone of the Death Valley area,
Cloud and Lauren Wright. Corsetti and Grotzinger's painstaking analysis
lends support for a microbial-ecological explanation for the tubular structures,
strikingly similar to those in correlative cap dolostones on other paleocontinents.
spiny explosion in Australia
Grey, K., 2005. Ediacaran palynology of Australia. Association of Australian
Paleontologists, Memoir 31, Canberra, 439 p.
Kath Grey’s long-awaited monograph on the Ediacaran palynomorphs
(organic-walled microfossils separated after masceration) of Australia
has arrived. Based on extensive deep drill coring throughout central Australia,
she documents the explosive increase in the size, diversity, ornamentation
and turnover rate among eukaryotic marine plankton near the stratigraphic
horizon (lower Bunyeroo Formation and equivalents) marked by ejecta from
the Acraman impact crater on the Gawler craton of South Australia. A broadly
contemporaneous (early Ediacaran) explosion of acanthomorphic (spiny) palynomorphs
is observed in Europe and South China, and has been interpreted as a response
to the evolution of microscopic multicellular animals capable of eukaryote
predation (Peterson & Butterfield, PNAS, 102, 9547-9552, 2005).
carbon isotope record for Neoproterozoic seawater (Sept/Oct 2005)
Halverson, G.P., Hoffman, P.F., Schrag, D.P., Maloof, A.C., and Rice, A.H.N.,
2005. Toward a Neoproterozoic composite carbon-isotope record. Geological
Society of America Bulletin 117, 1181-1207, 10.1130/B25630.1
Galen Halverson and colleagues compiled high-resolution carbon isotope
records from carbonate-dominated successions in Australia, Canada, Svalbard
and Namibia into a composite δ13
C record for Neoproterozoic
seawater. The most striking feature of the new record is that for 170 My
prior to the
younger Cryogenian (Marinoan) glaciation, seawater was enriched in 13
on average by ~5‰ compared with Proterozoic and Phanerozoic norms.
This could indicate that the organic carbon burial flux, as a fraction
of the total (organic plus carbonate) carbon burial flux, was twice the
normal value, or possibly that a huge reservoir of dissolved organic carbon
existed in the ocean at that time (Rothman et al., 2003, PNAS 100, 8124-8129).
Lynchpins of the new compilation are negative isotopic excursions ~800
Ma (Bitter Springs stage) and ~650 Ma, preceding the Marinoan glaciation
CO2-rich glacial aftermath inferred from pH-proxy record
Kasemann, S.A., Hawkesworth, C.J., Prave, A.R., Fallick, A.E., and Pearson,
P.N., 2005. Boron and calcium isotope composition in Neoproterozoic carbonate
rocks from Namibia: evidence for extreme environmental change. Earth and
Planetary Science Letters 231, 73-86.
Geochemists and geologists based in the U.K. report that the boron isotope
B in a marine carbonate section in Namibia drops
significantly across the younger Cryogenian (Marinoan) glaciation. The
through the post-glacial cap dolostone. As continental weathering tends
to lower the δ11
B of seawater and hydrothermal exchange
tends to raise it, the observed change is unlikely to reflect a change
in the composition
of seawater. Rather, the authors suggest that the change in δ11
was caused by a drop in pH associated with CO2
calcium isotope data from the same section to estimate the relative change
in the continental
weathering flux, they suggest that CO2
levels may have reached
0.1 bar (100,000 ppm, or ~333 times pre-industrial concentration) in the
level and the BIF-glacial connection
Kump, L.R. and Seyfried, W.E., Jr., 2005. Hydrothermal Fe fluxes during
the Precambrian: effect of low oceanic sulfate concentrations and low hydrostatic
pressure on the composition of black smokers. Earth and Planetary Science
Letters 235, 654-662.
The association of banded iron formation (BIF) and glacial marine deposits
was long-ago attributed by Henno Martin (1965) to “oxygen deficiency
in stagnating bottom waters caused by an ice cover”. However, the
steady-state iron-to-sulfur ratio (Fe:S) must exceed 0.5 or all iron will
be removed as pyrite (FeS2
), not BIF (Fe2
). Lee Kump (Pennsylvania State
University) and Wilfred Seyfried (SUNY Buffalo) show that a large fall
in sea level caused by a snowball earth would boost the Fe:S in hydrothermal
vent fluids because of lower water pressure. The spotty distribution of
glacial-hosted BIF may reflect regional variation in Fe:S in a poorly-mixed,
Shining a light on snowball earth
Olcott, A.N., Sessions, A.L., Corsetti, F.A., Kaufman, A.J., and de Oliveira,
T.F., 2005. Biomarker evidence for photosynthesis during Neoproterozoic
glaciation. Science 310, 471-474.
A team led by Alison Olcott (University of Southern California) discovered
biomarkers (molecular fossils) diagnostic of prokaryotic photosynthesis
in drill-cores of organic-rich black shale from within the older Cryogenian
(Sturtian) glacial succession of the Sao Francisco platform (Bambuí Basin)
of southeastern Brazil. The existence of organic matter in the original
sediment is strengthened by the presence of glendonites, calcite pseudomorphs
after the morphologically distinctive mineral ikaite (CaCO3
which forms in organic-rich sediments only, where diagenetic organic remineralization
produces alkalinity and orthophosphate, the latter known to inhibit calcite
formation in favor of ikaite. So does the existence of photosynthetic biomarkers
in strata sandwiched between glacial marine diamictites repudiate tropical
sea-glaciers in favor of tropical sea ice that is thin or absent? This
depends on which part of the glacial cycle the organic-rich strata represent.
If the glacial maximum, the question can be answered in the affirmative.
But glacial deposits on Quaternary marine shelves and platforms commonly
date from the glacial retreat, because lowered sea-level and expanded ice-sheets
made them areas of erosion during glacial maxima. On the West Antarctic
Peninsula, ice-shelf collapse appears to trigger glacier surge. In these
scenarios, it would be a mistake to take evidence for photosynthesis in
the glacial deposits as a constraint on the presence or thickness of marine
ice during the glacial maximum.
Astronomical origin for snowball earths?
Pavlov, A.A., Toon, O.B., Pavlov, A.K., Bally, J., and Pollard, D.,
2005. Passing through a giant molecular cloud: “Snowball” glaciations
produced by interstellar dust. Geophysical Research Letters 32, L03705,
Alexander Pavlov (University of Colorado) and others proposed an astronomical
theory for snowball earth episodes. They note that the solar system encounters
a dense (>2000 H atoms cm-3
) giant molecular cloud (GMC)
every 109 years on average (and GMC’s of average density every 108
years), with greatest probability every ~140 My when the solar system crosses
the galactic spiral
arms where GMC’s are concentrated. They estimate that tropospheric
dust loading during dense GMC encounters would lower radiative forcing
by >8 W/m2 for ~200,000 years. This is close to the forcing required
to convert the present climate to a snowball earth. The change in forcing
upon entering a GMC is rapid enough that it cannot be compensated by silicate-weathering
feedback. The authors conclude that in its 4.5 Gyr of existence, the solar
system encountered ~4 high-density GMC’s, which could have triggered
snowball earths, and ~15 lower-density (~1000 H atoms cm-3
capable of causing moderate ice ages. They propose that GMC-triggered snowball
earths might leave an isotopic signal, elevated 235
ratio, at the base of post-glacial cap carbonates.
First big-game hunters
Peterson, K.J. and Butterfield, N.J., 2005. Origin of the Eumetazoa: testing
ecological predictions of molecular clocks against the Proterozoic fossil
record. Proceedings of the U.S. National Academy of Sciences 102, 9547-9552.
Kevin Peterson (Dartmouth College) and Nick Butterfield (Cambridge University)
postulate that a sudden increase in size, diversity, ornamentation and
turnover among organic-walled microplankton, which occurred early in the
Ediacaran Period, was a response to the evolution of eumetazoa, the first
predators of eukaryotes. The timing is broadly consistent with the age
of the last common ancestor of eumetazoa and calcisponges, as estimated
from extant descendents using an invertebrate-calibrated molecular “clock”.
As there is no early Ediacaran (635-600 Ma) fossil record of eumetazoa,
it must be assumed that they were themselves microscopic and free-floating
Thin equatorial sea-ice solution for snowball earth with
dynamic sea glaciers
Pollard, D. and Kasting, J.F., 2005. Snowball Earth: a thin-ice solution
with flowing glaciers. Journal of Geophysical Research 110, C07010, 10.1029/2004JC002525
Dave Pollard and Jim Kasting (Pennsylvania State University) simulated
a snowball earth in which thin (<2 m) equatorial sea-ice can coexist
with sea-glacier dynamics, provided that the exposed equatorial ice is
free of bubbles (i.e., mature marine ice). If bubble-rich ice (i.e., consolidated
snow) is substituted, the equatorial ice thickens (>100 m) as in previous
models. In the new simulation, the equatorial zone of thin ice is ~2200
km wide and separated from km-thick sea-glaciers by a narrow transition
zone lying directly equatorward of the snow line. “Healthy” rates
of photosynthesis could occur beneath the thin ice and the threshold CO2
radiative forcing required for glacial termination is only 8% as high as
that originally calculated for a snowball earth. The authors include an
excellent review of the status of sea-glacier dynamics in snowball earth