From: GREY, Kath
Date: Tues, Jan 2 2007 1:26 pm

Clive Calver and I want to comment on the paper by Fike et al., 2006 in
some detail because we want to set the record straight about some of the
Australian data the authors have used. We will also be submitting a
short letter to the editors of Nature, but feel the detailed corrections
need to be available and that the IGCP 512 site is probably the best
place for it. I've sent a copy of our comments directly to the authors
and have talked with Roger Summons about the problem, so they are
forewarned about our criticism.
I am currently trying to put together some comments on the use of
acritarchs for subdivisions of the Neoproterozoic and will post that
shortly.
<<Fike et al_2006_comments for IGCP 512.doc>> Kath Grey
Dr Kathleen Grey
Chief Palaeontologist
email: kath.g...@doir.wa.gov.au
Location: 37 Harris Street, Carlisle, 6101, WA
Mail address: Geological Survey of Western Australia, Department of
Industry and Resources, 100 Plain Street, East Perth WA 6004
Phone: +61 08 9470 0302
Fax: +61 08 9362 5694

From: Paul Hoffman - view profile
Date: Thurs, Dec 21 2006 12:37 pm

Hi all:

With respect to Philip Allen's description of the "original pillars"
of the snowball earth hypothesis as having "been discarded", it would
be useful to have a clear statement of what they are.

From my reading of the hypothesis (Kirschvink, 1992; and pers. comm.,
1989), the original pillars are the following. Each is more strongly
supported now than when the hypothesis was first proposed.

1. That Cryogenian glacigenic deposits occur on virtually every paleocontinent.

2. That Cryogenian glaciers flowed directly into the ocean close to
the paleoequator according to combined paleomagnetic and
sedimentological evidence.

3. That Neoproterozoic paleomagnetic data show that thick
carbonate-dominated successions were also deposited at paleolatitudes
less than 35 degrees, indicating a normal pole-to-equator temperature
gradient.

4. That the only regionally-extensive sedimentary Fe2O3 and MnO2 ores
in the past 1.9 Ga are intimately associated with Cryogenian glacial
marine deposits, implying exceptional perturbations in seawater
chemistry as expected if the oceans were ice covered for long periods.

5. That in low paleolatitudes, Cryogenian glacial strata begin and
end abruptly, consistent with a climatic instability due to
ice-albedo and other positive feedbacks, which resulted in sudden
glaciation and deglaciation in those regions.

Reference

Allen, P.A. (2006), Snowball Earth on trial. EOS, 87(45), pp. 495.
Kirschvink, J.L. (1992), Late Proterozoic low-latitude glaciation:
the snowball Earth, in The Proterozoic Biosphere: A Multidisciplinary
Study, edited by J.W. Schopf and C. Klein, pp. 51-52, Cambridge Univ.
Press, Cambridge.

-PAUL F. HOFFMAN, Harvard University

From: Yves Godderis
Date: Wed, Dec 20 2006 1:56 am

Hello,

we plan to simulate the response of continental weathering to
the expected super greenhouse effect at the end of the
snowball glaciation. The objective is to quantify the amount of
alkalinity
that can be provided by weathering and to compared it to the volume
of carbonates deposited.

As you probably know, modellers can only
do simple things. We thus plan to run a GCM at very high
CO2 for a Marinoan continental configuration and use the obtained
climatic fields (temperature
runoff) to force a model describing weathering through kinetic laws
that we published
earlier this year.

The question is: how does the continental surface at the end of the
glaciation
looks like ? I guess there should be two end members: the first one
being bare
rocks, and the second one some kind of loess. Is there any
sedimentary record
that can constrain this ? Grainsize, "soil" thickness ?

Thanks in advance.

Yves.

From: Paul Hoffman
Subject: Re: Rapitan and other BIFs

Frank:

If the earth was warmer, carbonates would not
necessarily be pushed to higher latitudes because it
is the relative, not absolute, temperature that
controls carbonate distribution. This is reflected in
Kiessling's Phanerozoic reef distribution data: the
width of the reef belt changes little between
greenhouse and icehouse periods.

To push carbonates to higher latitudes would require
an increase in the saturation state of the ocean wrt
carbonate. This might result from an increase in the
alkalinity flux due to carbonate and silicate
weathering.
The basal Ediacaran ("Marinoan") cap dolostones
apparently record such a perturbation, with carbonate
deposition simultaneously at almost all latitudes.
This is a very unusual occurrence. Slushball offers no
satisfactory explanation of cap dolostone. Snowball
just might, although we still have a long way to go.

My understanding of air-sea gas exchange in
ice-covered oceans comes mostly from discussions with
Jeff Severinghaus. On a million-year timescale, Jeff
thinks that the ocean and atmosphere would be in
equilibrium with respect to CO2 concentration even if
air-sea gas exchange was limited to cracks (which
continually form wherever landfast ice and sea-glacier
ice meet.

Oxygen is more complex because it will be consumed at
some rate in the ice-covered ocean, by respiration and
reaction with volcanic products. If the rate of gas
exchange is less than the consumption rate, anoxia
results.

As it stands, the deep oceans may well have been
anoxic before and after the Cryogenian glaciations
(e.g., Canfield et al's Science on-line paper this
week). In this case, the deposition of BIF during a
glaciation may be more a perturbation in Fe:S than in
degree of oxidation.

I'm sure we have lots to learn here too.

Paul

From: Frank Corsetti - view profile
Date: Thurs, Nov 23 2006 4:50 am

Hi Paul,
I was aware that the Red Sea iron deposits might not be a close
analogue for the Neoproterozoic IF's, as I noted in my initial comment.
Rather, I was suggesting the Red Sea example to show that rift basins
might not represent the world's ocean.

Since you bring up the uniqueness of the Rapitan IFs in volume and
extent versus the handful of other occurrences, perhaps another way
to approach the question is "Why is the Rapitan so unusual" rather
than trying to use the world's ocean to explain the Rapitan? Some
could focus on ice as the important factor, others could focus on
different avenues, and we hope that the data will lead us to converge
on a common solution.

No doubt, most of us have borrowed (!!) the picture of the dropstone
in the Rapitan IF from your website for use in talks and classes.
But, is this striking picture skewing our perception of the actual
abundance and significance of IFs? It's difficult to see that
picture and not think that IFs are important. On the other hand, as
you've noted, if the Rapitan is unusual, perhaps the emphasis is
misplaced? It's just a suggestion...I could be convinced either way
with additional observations.

Talk to you soon,

Frank

From: Graham Shields - view profile
Date: Sat, Nov 18 2006 9:55 pm

Hi Paul,

Some pointers to your reply below which I generally take on board and have little comment on. I am not convinced but also do not consider what you say to be unreasonable. Anoxia today is related to areas of high productivity, so much so that there can even be hydrogen sulphide events off the coast of Nambia. In a world of lower sulphate, I think that such upwelling would bring Fe-rich waters to the surface which would precipitate out above the redox boundary. In that case one could argue that the Canfield ocean scenario is unnecessary, but neither would a hard snowball?

Whoops, I have been rather hypocritically throwing around the word "Marinoan" haven't I? The problem is that we all know what I mean although it isn't strictly correct. I still prefer end-Cryogenian instead of Marinoan for this level - using direct correlation with the Nuccaleena cap dolostone (and presumed correlation of the Nantuo - Ghaub - Elatina glaciations) to determine what this means. The existence of a significant inter-glacial episode, approximately equivalent to the Sturtian-Marinoan interval still holds up. Phew......

Regarding the CAS data, I am highly skeptical about the meaning of those data. I could talk about this further but it suffices to say that the jury is still out as to what those data really mean. I was referring to the high d34S values of pyrites, which have been referred to low sulphate levels by Logan et al. (1995), Canfield (1998) etc. Cheers,

Graham

From: timothy.r...@yale.edu - view profile
Date: Sat, Nov 18 2006 9:19 am

Hi Graham and Joe et al., a few more thoughts on this email train.
The "Sturtian"/"Marinoan" BIF schism might cut both ways - if at all:

I agree, per Graham's email and also Joe's PPRG paper, that older Npz
glaciogenic BIFs certainly do occur sometimes closely interleaved with
plausibly anoxic strata - eg., Red Rock, Western Tasmania and lower
Tindir Group, among others.

However per text and references in Evans' 2000 AJS review, young
Neoproterozoic BIF could exist in West Africa's Bakoye Gp and
Oti-Pendjari Gp nappe-equivalents; and in central Brazil's Jacadigo Gp.
That paper recognizes abundant uncertainties with those age
assignments; even so, I bet we might all acknowledge general,
frequently-anomalous reddening toward the tops of many Marinoan and
younger Neoproterozoic glacial intervals. (Gaskiers stands out in my
mind.)

So if we were to accept a pan-Neoproterozoic glaciation/episodic
iron-oxidant relation, then perhaps the question of "stopping" versus
reducing sulfate delivery to the ocean seems second-order; either way
the itinerant change is glacial in origin and accompanies "each"
glaciation - Paul's email and Graham's response seemed to me to agree
on this point.

As Joe pointed out in Ascona, there are other, Snowball-specific
mechanisms that might do the same thing. Liang et al.'s in press
peroxide-UV pathway could be one; of course working the ferrous
iron-supply-side beneath ice cover rather than (or in addition to)
modulating sulfate delivery above it would be another.

That iron supply-side argument reminds me that other redox-sensitive
metals, like manganese, are also sometimes anomalously enriched, with
purported specific stratigraphic relation to BIF's, in a manner readily
explicable via ice-covered oceans. (Kirschvink et al., 2000 and Gaidos
et al., 1999 address the Ppz glaciation; I'm not familiar enough with
Urucum's Mn stratigraphy to know whether it is a good, young analog.)

To me, a final attraction to the supply-side glacial-BIF hypothesis
echoes Paul's iteration of Canfield: if short-lived dips in sulfate
delivery to an otherwise sulfidic ocean precipitate all BIF, wouldn't
even a few BIF's be expected between Geons 18 and 8. Glacial-specific
severe sulfate delivery inhibition or significant sub-ice ferrous iron
buildup permit that gap, but seem Snowball-handcuffed.

Turning the above arguments around, though: suppose the West African and
South American BIFs proved to be old after all, and we wished to
separate BIF from mere reddening. Then if "Sturtian" (and
Paleoproterozoic) glacial intervals were BIF-iferous but "Marinoan" and
younger glacials were not, wouldn't increasing oceanic sulfate still be
a nonunique explanation? Alternatively, the "Snowball"/non-Snowball
glacial mode transition could have begun post-Sturtian and included all
younger Proterozoic events.

Accepting that corner of possibility space might take care of BIF's, but
per previous emails to this list it still would not account for the
myriad other Marinoan and Ediacaran glacial peculiarities - to me, that
persistent inadequacy really is the heart of Snowball Earth's
attraction. As long as the spatial and stratigraphic response of
Earth's Precambrian glacial climate mode was essentially
nonuniformitarian (or even mixed the remarkable with the regular),
calling it "Snowball" seems effective communication.

Best,
Tim

From: Paul Hoffman
Sent: Friday, November 17, 2006 2:31 AM
Subject: Cryogenian BIFs

Graham:

The motivation for Don Canfield's (1998 Nature) model
was to explain the disappearance of large-scale BIFs
from the stratigraphic record after 1.89 Ga (excepting
the glacigenic Cryogenian BIFs) without permanent
oxidation of ocean deepwater at that time. If the
model is valid, the sulfur-iron flux ratio must have
changed to allow the formation of the Cryogenian BIFs.

This was clearly pointed out by Canfield & Raiswell
(1999, Am. J. Sci. 299, 697-723), to quote (pp. 711)
"We propose that reduced rates of sulfate reduction
during Neoproterozoic BIF formation could have
resulted from an overall collapse in surface-water
primary production due to extensive ice coverage, as
has been recently suggested (Hoffman et al., 1998).
Indeed, extensive glaciation of the land is also
indicated producing what has been termed a Snowball
Earth (Kirschvink, 1992; Hoffman et al., 1998).
Conceivably, attenuation of the hydrologic cycle
limited sulfate input into the oceans and allowed a
slow draw-down in sulfate concentration as sulfur was
continuously fixed in marine sediments as pyrite. A
return to low sulfate concentrations [i.e., return to
pre-1.89 Ga conditions] was perhaps a second factor
contributing to reduced rates of sulfate reduction."

Whatever the ultimate explanation, the fundamental
observation is that no large-scale BIFs were deposited
in the past 1.89 Ga, except for those deposited around
marine ice grounding-zones during one or more of the
Cryogenian glaciations.

Is there an explanation of this fact other than that
proposed by Canfield and Raiswell (1999)?

Cheers,
Paul

Hi Philip:

Since when was "clustering of continental plates" a
"pillar" of the snowball hypothesis?

Let's see what the three papers you like to cite as
defining the hypothesis actually say about the
continental paleogeography.

(1) Kirschvink (1992)

"...large portions of the continental land masses
probably were within middle to low latitudes during
the late Precambrian glacial episodes, a situation
that has not been encountered at any subsequen time in
earth history."

He then goes on to explain how this preponderance of
continents in middle to low latitudes would raise the
planetary albedo, and also the albedo feedback
associated with glacioeustatic change.

Nothing about continents being clustered (the tropics
are large).

(2) Hoffman et al. (1998)

(p. 1345) "Fragmentation of the Rodinia supercontinent
may have contributed to the CO2 drawdown [required for
glaciation] by creating many new continental margins,
which are major repositories for organic carbon in the
modern ocean, consistent with the high d13C values
observed before the glaciation. This is also
consistent with the observation that Sturtian and
Varangian glaciations accompanied the opening of the
Pacific and Iapetus oceans, respectively, and might
explain why the only known older examples of similar
carbon isotope excursions and low-latitude glaciations
accompanied the fragmentation of a late Archean
megacontinent."

(3) Hoffman & Schrag (2002)

(p. 148) "The Neoproterozoic snowball era was a period
of continental dispersal, involving the breakup of
supercontinent Rodinia and the aggregation of
megacontinent Gondwana."

If you must put the snowball hypothesis "on trial", is
it too much to ask that the trial be fair?

Better, why don't you line up different theories for
low-latitude Proterozoic glaciation, and ask which
best satisfies the agreed-upon observations?

This is what was done at the famous Maui conference on
the origin of the moon. Going into the conference, the
giant impact hypothesis would surely have failed an
up-or-down vote (a "trial"). But when forced to choose
between the known theories in a straw poll, giant
impact emerged less fatally wounded than the other
options.

I regret that no such poll was taken at Ascona. I have
no idea how the participants would have voted. But I
would not have been afraid to find out.

Yours sincerely,
Paul

From: Graham Shields - view profile
Date: Sat, Nov 18 2006 12:35 am

Dear all,

we are trying to put together the annual report for IGCP 512. As part of that report, we will reference the IGCP 512 website hosted by Galen Halverson (now in Adelaide) so we need to make it as up to date as possible. Could you please send Galen any new publication references of yours that may be relevant to IGCP 512 participants? This way he can add them to his already very extensive list:

galen.halverson@adelaide.edu.au

You can view existing references at:

http://www.igcp512.com/

and there is an up-to-date and historical list at:

www.snowballearth.org

We are aware that the membership list on the website is out-of-date - we are endeavouring to bring that also bang up-to-date in the coming few weeks.

Cheers,

Graham

From: Graham Shields - view profile
Date: Fri, Nov 17 2006 7:54 am

Hi y'all again,

Moderation has now been switched off by Breandan from an internet cafe in the Sahara! So if you wish to contribute simply send your email to:

IGCP-512@googlegroups.com

Remember to ensure that replies to messages go to the person/group intended. You can read previous mail at the googlegroups archive.

Emmanuelle and I will try to get the IGCP 512 annual report ready as soon as possible for circulation before official submission so that stuff can be added, polished, etc.

Cheers,

Graham

From: Graham Shields - view profile
Date: Fri, Nov 17 2006 1:05 am

Hi there,

Just to remind you that the IGCP-512 mailing list is moderated which means that emails sent to the group are not auotmatically released until vetted by Breandan - this sometimes proves necessary as personal emails may be inadvertently sent to the group, which can be embarassing. However, considering the moderate volume of email traffic this is probably unnecessary and now that Breandan is away in Morrocco on fieldwork until mid-December it can cause delays. Until we speed up the moderation process I suggest therefore that responses to the previous two emails and future discussions be forwarded also to me as so that I can more speedily get them to the rest of the group and onto the IGCP-512@googlegroups.com website. Cheers,

Graham

Please find below message from Joe Kirschvink

Hi Phillip,
I have some quick comments on your meeting report concerning this summary paragraph:

re: " What is clear is that the original pillars that supported the Snowball Earth hypothesis—such as the presence of iron formations, a low-latitude clustering of continental plates, and the characteristic carbon isotopic composition of marine carbonates deposited during deglaciation--have been discarded. "

I beg to differ, both with your list of the original pillars and whether they have been discarded.

If you go back and read my original Snowball paper (Kirschvink, 1992, pdf at:http://www.gps.caltech.edu/~jkirschvink/pdfs/firstsnowball.pdf ), and my paleogeography paper in the same volume (from which the little snowball note was excised by Bill Shopf, pdf at: http://www.gps.caltech.edu/~jkirschvink/pdfs/pprg.pdf ) you will see that the "original pillars that supported the snowball hypothesis" were:

1. The peculiar abundance of carbonate clasts in the Neoproterozoic diamictites compared to Gondwanan or Pleistocene deposits.

2. Dropstones associated with sediments typical of low-latitudes (carbonates and evaporates).

3, Paleomagnetic data showing that at least one clear glacial unit (the Elatina) was at Sea level on, or very near, the Equator, and the lack of any clear data for polar continents.

4. The unique association of banded iron stones with the glacials.

Certainly none of these observations have changed, and even the paleogeography in the PPRG chapter is not too bad (although certainly things like TPW were not considered then). Clustering of plates in the Tropics was NOT one of the pillars of this hypothesis – the mere fact that ONE continent on the Equator had bullet-proof data for widespread, sea-level glaciation was the important point that argued strongly for an ice-albedo runaway.

In the same paper, I made some suggestions on how to test this idea beyond the straightforward paleomagnetic work. These were:

A. Global Glaciations should be synchronous (as noted much earlier by Harland and others).

B. Sedimentary sections on widely scattered localities should have striking similar variations in lithology, due to the global-scale nature of the climatic signals and fluctuations thereof.

C. The deep oceans should go anoxic, allowing metal-rich waters to build up and eventually deposit the Neoproterozoic BIF deposits towards the end of the glaciations.

Well, "A" is still being debated heavily, but certainly the Chinese/Namibian termination is supporting it, as you point out.

Item "B" is still running strong, with the recognition of the Cap Carbonates themselves (which, by the way, were not appreciated when I wrote the 1992 paper back in 1988!), as well as the tubers, crystal fans, BIFs, etc.

Item "C" is still in the running as well, despite your statement to the contrary. Do you remember when I stood up in the discussion session at the meeting and asked point blank if anyone present could explain how to get BIFs with the hydrothermal REE fingerprints without sealing off the oceans with ice? NO ONE ANSWERED. Saying that it is simply local rifting (e.g., Williams) does not work, as we have had 630 myr of rifting since then without similar production of BIFs, much less BIFs associated with glacial deposits at low latitudes. Remember Paul made the comment that to get a BIF it is also necessary to stop the input of continental sulfate to the oceans, as the sulfate reducers turn this into sulfide that knocks out the iron before it could ever form a BIF. A Phanerozoic-style high-latitude-only glaciation will not do that because the glacial runoff is loaded with sulfates oxidized from the glacial flour. In contrast, a snowball does this nicely. The statement in your article does not reflect that component of the meeting at all.

Hence, may I suggest that the original pillars of the Snowball Hypothesis are still firmly in place?

Cheers,

Joe