Stefan Bengtson
Dr Rafat J. Azmi
Wadia Inst. Himalayan Geology
33 GMS Road
Dehra Dun 248001
India
Department of Palaeozoology
Swedish Museum of Natural History
Box 50007
SE-104 05 Stockholm
Sweden
tel.
e-mail
+46-8 5195 4220 (work)
+46-8 732 5218 (home)
+46-8 5195 4184 (fax)
stefan.bengtson@nrm.se
Stockholm, 2007-05-11
Dear Dr Azmi,
I am writing to give you a brief report of the first results of our investigations
based on our joint field work in the Lower Vindhyan in November last year. I
want to stress that the results are preliminary and should not be made public
until they have been properly worked out and published in a peer-reviewed
journal. However, you are welcome to discuss them with your colleagues
within and outside the Wadia Institute of Himalayan Geology. You may also
show my letter to whomever it may concern within the Wadia Institute. For
that purpose I also include information that is already well known to you.
Sampling and processing
We (you, me and Christina Franzén) visited and collected the Jankikund
section (the Tirohan Dolomite) in the Chitrakoot area as well as a number of
key localities along the southern flank of the Vindhyan Plateau. The sampling
spots were documented through photographs and GPS co-ordinates. Our
microfossil samples were split in two parts, one to go to the Wadia Institue,
and one to be sent to the Swedish Museum of Natural History in Stockholm.
Because of the suspicions that have been expressed about the provenance of
the fossils you have reported from the Lower Vindhyans, I point out here that
we sent the Stockholm samples directly from Varanasi to Stockholm; these
samples have thus not been in your laboratory. Through this dual processing
we can thus determine that whatever fossils we find in both sets of samples
are not contaminations from other samples. The only possible remaining
source of contamination for the dual samples is from their common
environment in the field (see discussion of the Chitrakoot samples below).
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Part of the samples were cut for preparation of ordinary petrographic thin
sections, and the carbonate samples were dissolved in buffered 10% acetic
acid for recovery of microfossils. The Chitrakoot samples were initially
immersed in chlorine (sodium hypochlorite) to remove potential organic
contaminants on the surfaces.
After HAc digestion, the samples were sieved using stainless-steel sieves. No
heavy-liquid or other separation techniques were employed, and care was all
the time taken to avoid possible contamination from other samples. The
sample residues were thereafter manually picked for microfossils under a
stereomicroscope. In this work I have had valuable help from Veneta
Belivanova and Therese Huldtgren in our department.
Microfossils recovered
At present, we have microfossils only from the Chitrakoot samples. These are
of four major types:
1. Filamentous and coccoid assemblages resembling the types commonly
referred to Renalcis and Girvanella. These are abundant in the Chitrakoot
samples, and occur as phosphatic clasts reworked into the carbonates. The
filaments are about 10–20 µm in diameter, although the original diameter was
presumably smaller before the phosphatic encrustation.
These structures are clearly biogenic, and they are likely to represent calcified
cyanobacteria. As they are large, abundant, and occur as rounded clasts
within the carbonates, there is no possibility that they could be
contaminations. Although the stratigraphic value of cyanobacterial
morphologies is practically nil and cyanobacteria are thought to have arisen
already before the beginning of the Palaeoproterozoic, it is noteworthy that
this type of fossils has so far not been reported from rocks older than the
Neoproterozoic.
2. Spheroidal, usually hollow phosphatic structures ranging from tens of
micrometers to millimeter size. They commonly form aggregates. Some
spheroids are perfectly smooth; others have a granulated surface or show a
polygonal pattern. They are particularly common in the phosphatic clasts in
sample Ind06110804, but they frequently occur isolated from the clasts.
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The polygonal patterns are not so common, and where they can be observed
in specimens within the phosphatic clasts it appears that they relate to
spherulitic structures or smaller spheroids that occur also in the phosphatic
ground mass:
The spheroids have a complex diagenetic history, however, as can be seen by
evidence of radial wrinkling, possibly caused by vertical displacement of the
content of the spheroids in relation to the ground mass:
Although the specimens with polygonal surface structures are very similar to
cleavage embryos of the kind that we commonly find in Neoproterozoic–
Cambrian phosphoritic deposits, the considerable variability in size and the
bubble-like aggregation of the spheroids suggest other origins. A possible
interpretation is that the spheroids form as bubbles from gas (O2?) release in
the cyanobacterial biomats. It is clear, however, that some sort of membrane
was formed that was resilient enough to preserve the spheroids also outside
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the biomats and to become wrinkled when the spheroids were deflated or
distorted by vertical movements. In brief, these are interesting and unusual
structures that need to be explained within a context of biomat development.
They have no anticipated stratigraphic usefulness.
3. Thin, flexible, annulated tubes with a diameter of more than 100 µm. In
contrast to the previous types of fossils, these are exceedingly rare: We have
so far found only two specimens, in sample Ind06110804. They are preserved
as phosphatic internal moulds, but it is clear from the way they are distorted
mechanically that they represent thin-walled, flexible tubes.
In view of their rarity and small size, the possibility that they are
contaminations must be considered seriously. I would rule out the possibility
of laboratory contamination, because you have previously reported (in Azmi
et al. 2006) very similar tubes from your own Chitrakoot samples, and you
have also reported to me that you found such forms in the part of sample
Ind06110804 that was brought to Dehra Dun. However, the sample was taken
in a stream section, and the tubes might conceivably be derived from the
extant biota now living in the river. Against this speaks, however, that the
tubes in our sample are preserved as phosphatic internal moulds, and even
when there appears to be a thin layer covering the mould, this layer also
consists of minute apatite crystals:
The phosphatic composition of these materials has been confirmed by EDS
analyses. The tubes would thus appear to be genuine fossils. We cannot yet
entirely rule out the possibility of Recent contamination, however, because
phosphate may conceivably have been remobilized and redeposited in the
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tubes in Recent times. We are therefore preparing to analyze the REE (rare
earth element) spectrum by means of the ion microprobe available here in the
NordSIM laboratory, in co-operation with Martin Whitehouse. The idea is to
compare the REE spectra in the phosphorite pebbles with that of the
phosphatic mould of a tube. If the former were Proterozoic or Cambrian and
the latter recently formed in a freshwater fluviatile environment, the REE
spectra should be quite different from each other.
Assuming that these tubes are indigenous to the Tirohan Dolomite, what
would they represent? As individual trichomes or trichome sheaths, they are
far too wide for cyanobacteria, and if the annulation would represent cell
boundaries the volume would exceed that of the largest known
cyanobacterial cells by orders of magnitude. If they represent sheaths of
trichome bundles rather than individual trichomes, the regular annulations
would be unexplained.
4. Elongated, usually irregularly curving, structures, coarsely crystalline,
sometimes with a central filament or canal:
These are moderately common. They are probably of different origins, but
many seem to be have been formed by crystallization, or possibly
agglutination, around an organic core. In many cases they may be derived
from the filamentous cyanobacterial(?) structures described above.
Preliminary conclusions
The fossils we find in the Stockholm sample parts appear to be the same that
you and your co-workers have reported from the Tirohan Dolomite. I’m thus
confident that we are not dealing with laboratory contaminants; furthermore,
since the cyanobacterial(?) filaments and the spheroids occur as parts of the
phosphatic clasts in the carbonate rock, contamination can be excluded also in
these cases. With regard to the most distinct – but also the most rare – fossils,
the annulated tubes, contamination from the recent biota seems a remote
possibility, but the planned REE analysis should be able to give a conclusive
answer about their provenance.
Regarding the contentious issue of the age of the Lower Vindhyans, this is
still unresolved by our data. Nothing in the Chitrakoot samples that I have
seen is definitely indicative of a Neoproterozoic or Cambrian age, but the
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presence of calcified cyanobacteria(?) and annulated tubes of the type seen
here is at least consistent with a such an age, whereas no such fossils are
known from the Palaeoproterozoic. In order to resolve the age issue we need
either age-diagnostic fossils or reliable radiometric dates of the rocks in which
the fossils occur. We keep searching for the former in the collected samples;
the latter we try to obtain with the help of isotope geochronometric dating of
diagenetic minerals at the laboratories of the Swedish Museum of Natural
History (in co-operation with Martin Whitehouse) and the University of
Western Australia (in co-operation with Birger Rasmussen). I will of course
inform you as soon as we have any results in this direction. We are also
processing samples from the more southern sections and will investigate
these in the near future.
I hope you will find these first results encouraging. We can confirm that the
fossils you have reported from the Tirohan Dolomite are indeed there, and
also that in terms of stratigraphy they are more similar to what we see in the
Cambrian than to anything that has ever been reported from the Palaeoproterozoic. This is not sufficient to for a conclusive age determination, but
whatever age we finally determine for the Lower Vindhyan rocks, it is clear
that you and your colleagues have made important discoveries that deserve
to be widely publicized.
Sincerely,
Stefan Bengtson
Professor, Head of the Department of Palaeozoology
Member of the Royal Swedish Academy of Sciences