No principal differences have been found between microfossils described from Cambrian and Phanerozoic and the 2000 Ma phosphorites. Numerous samples revealed diverse microbial microstructures interpreted as cyanobacterial mats consisting of filamentous (1-3 microns in diameter, 20 microns in length), coccoidal (0.8-1.0 microns) and ellipsoidal or rod-shaped microfossils (0.8 microns in diameter, around 2 microns in length) which morphologically resemble modern Microcoleus and Siphonophycus, Thiocapsa, and Rhabdoderma, respectively, reported from alkali ne or saline environment_ The sequence of the early Palaeoproterozoic events which point to a significant oxidation of the hydrosphere, including the formation of phosphorites and changes in the phosphorous cycle, mimics the sequence which was repeated at the Neoproterozoic-Cembrian transition, implying that oxidation of the terrestrial atmosphere-hydrosphere system experienced an irregular cyclic development

Astafieva, Marina M.

Hoover, Richard B.

Rozanov, Alexei Yu.

NASA Technical Reports Server

Early Proterozoic (2.04 GA) phoshorites of Pechenga
Greenstone Belt and their origin
Alexd Yu. Rozanova, Marina M. Astafieva *a, Richard B. Hoover b
aPaleontological Instituteof Rusaan Academy of ,Sciences,117997 Profsoyuenaya 123,
Moscow, Ru_a
bNASA/National Space Science and Technology Center, 320 Sparkman Dr.,
Huntsville, Alabama, 35802, USA
ABSTRACT
No principal differences have been found between microfossils described from Cambrian and
Phenerozoic and the 2000 Ma phosphorite_ Numerous samples revealed diverse microbial microstructures
interpreted as cyanobacterial mats consisting of filamentous (1-3 pm in diameter, 20 _m in length), coccoidal
(0.8-1.0 l_m) and ellipsoidal or rod-shaped microfossils (0.8 i_m in diameter, around 2 i_m in length) which
morphologically resemble modern Microcoleus and Siphonophycus, Thiocapsa, and Rhabdoderma,
respectively, reported from alkali ne or sal ine environment_ The sequence of the early Palaeoproterozoic events
which point to a significant oxidation of the hydrosphere, including the formation of phosphorites and changes
in the phosphorous cycle, mimics the sequencewhich was repeated at the Neoproterozoic-Cembrian transition,
implying that oxidation of the terrestrial atmosphere-hydrosphere _jstem experienced an irregular cyclic
development.
Keywords: Lower Proterozoic, phosphorites, bacteria, cyanobacteria, cocd, filaments, framboids, EukEyotes
Some biomorphic structures found in carbonaceous meteorites are practic_ly indistinguishable from
biomorphic structures encountered in Earth rocks, such as phosphorites That is why one of the most andent
(Early Proterozoic) phosphorites from Pechenga Greenstone belt was chosen for the investigation of fossil
bacteriaJrem_ n_
In the history of the development of the Earth, a few stages of phosphoritogenesis can be
distinguished. 1 Aravalli basin in India is considered to be one of the most ancient phosphorite beEing basin_
Phosphorites of Aravalli basin mainly are of a cyanobacterial nature and they are referred to as L_ztromatolitic
phosphoritesrJ Their age was estimated as 1000-1500 million yea's, which is considered to be Middle
Proterozoic 1'2.Later, it was suggested that Aravalli super group was of Early Proterozoic age (2.1 [] 1.9 Ga).
The age of Jham_kotra site (phosphatic dolomites) of theAravalli basin was determined to be 2.11 1_2.06 Ga.3
Phosphorites are still unknown in the Archaean Eon (2.5-3.8 Ga). Their absence is generally considered to be
connected to the absenceof an atmosphere enriched i n oxygen?
Early Proterozoic (2.04 Ga) phosphorites were collected by V. Melezhik and A. Lepland in 2005. The
period of 2500-2000 Ma is heraJdedby several hallmark events such aE the onset and decline of the greatest
positive excursion of (3_3Cc_D(Lomagundi-Jatuli Paradox); the development of a significant seawater sulphate
reservoir; the abundant deposition of anomalously organic matter 4(OM)-rich sediments; the oldest known
significant petroleum deposits (Shunga Event); and Melezhik et al. reported he appearance of first known
marine phosphorites at 2000 Ma.
*astafieva@paleo.ru; +7 rJ495 7]339-7911; fax +7 U495 L_339- 1266
https://ntrs.nasa.gov/search.jsp?R=20070037447 2019-08-30T02:04:11+00:00Z
ThephosphodtasunderdiscussionwerediscoveredinthePechengaGreenstoneBelt.They occur as
numerous rounded, soft-deformed, dasts in fi ne-pabble intra-formational conglomerates, forming two separate
(c. 200 m-thick) turbidite fans within the 1000 m-thick OM- and sulphide-rich turbiditic greywackas of the
Pilguj_rvi Formation in the Pechenga Greenstrone Belt, Kola Peninsula (NW Rus_a). Carbonate-fluorapatite is
the main mineral in the phosphorite dasts OM framboidal and micronodular pyrite, as well as inclusions of
qua't.z and chlorite, are additional components 4 Many das[s show microlayering with a variable degreeof soft-
deformation, implying that they were derived from non-lithified, bedded phosphorite_
The origin of phosphorites is well documented and ancient microfossils in phosphorites have been
compared with microorganis'ns of recent cyanobacterial mats _-15The peculiarity of the study of recent
phosphorites was that: 1) they were under investigation using Scanning Electron Microscopes with Microprobe
analysis in considerable (mass) quantity; 2) all samples were treated with adds for 30-60 seconds, and then
washed and dried. This was done i n order to receive a surface relief of the samples that appearsdue to the great
difference in the solubility of the carbonates, phosphates and siliceous components as well as the organic-
mineral compl exes.
The phosphorite pebbles in the samples under investigation were found to contain numerous and
diverse remains of fossil microorganisms
They can be divided into three group_ 1) microorganisms of the same age as host rocks; 2) endoliths
(i .e., forms that inhabited rocks that formed e_'lier); and, finally 3) subsequent contamination. This paper is
devoted to structures of the first group which is comprised of fossil biomorphic structures that were organically
i nterlaced within (and represmtati ve of ) the host rocks. The biomorphi c structures that were encountered exhi bit
rather diverse morphologies There ere filaments and thread, coccoidal, oval and rod-shaped forms, etc. and
framboidal structures are aJso of interest.
I. Figure ].a & b shows almost circular forms It is rather difficult to judge their external form, but
their diameter exceeds 100 pm.
ao
bo
Figure la. Ring-shaped, sack-shaped, or spherical biomorphic microstructure with b. detail of eage of ring-shaped
biomorphic microstructur&
II. Their chemical composition differs slightly from that of the host rocks Although sulfur is
practically absent in the rock matrix (Fig lc), these well of these microstructures have sulfur content that is
comparable to their phosphorus content (Fig. ld). It is considered probable that their formation was connected
with sulfur cycle, and is associated with the vital activity of sulfur bacteria or sulfate-reducing bacteria in
particular. Thisconformsto supposed enrichment of marinewaters by sulfates 4 during the2.0-2.5 Ga period. It
is considered probable that we have some rather highly organized forms with complicated (complex) extern_
structureswhich may represent a new genus
I_.al
iD.al
inside of the "circle"
e
C° d.
Figure le. Spectrum of interior rock inside spherical biomorphic structure and b. wall of the microstructures showing
significant increase in sulfur content.
III. The diameters of the filament or thread-like forms are typically - 1.0-1.51Jm,although they can
be as large as 3 IJm (fig. 2a) and their length may exceed 201Jm. Sometimes it seems that they gathered in
bunches, surrounded by sheath or external enveiop_
al
Ca
e
b
Figure 2.a. Scanning electron microscope image of filaments and b. the spectrum.
It is interesting that the forms shown in Fig. 2.a. have morphological characteristics of multiseriate
trichomes such as are known in recent cyanobacteria of the genus Microcoleus that is common in modern
halophilic mats and the fossil form Siphonophycus.(Not_ Images of those forms are shown in Figs 2a, 3a, 4a,
16a of References 10, 13 and 15.)
Figure 3 shows images and the associated chernicaJ composition for a mat-like network of interlacing
filamanta The chemical composition of ancient, presumably bacterial, forms and host rocks (phosphorites) is
identi ca], i ndi cating that the bacterial fossi l izati on occurred simultaneou_ly with sedi mentati on
S_P Ca T i
_a
$ Ca Fe
Fig. 3. Mat-like network of interlacing filament microstructures shown alongsidetheir chemical compositions
III. Elongated-oval or rod-shaped forms (Figure 4) could be comparable to polymorphic cells of
recent _anobacteria Bhabdoderma from alkaline mats or purple bacteria such as Ectothiorhodospira (See
References 10; 13; 15, Fig. 6; 16, Fig.14-1]. Attention should be paid to the resemblance with the picture of
microorganisms from the upper ayer of microbial mat of Bolsherechensk spring (North Baikalian region of
Buryati a L-I:IBaikal rift zone), developi ng under the tsmperature about 54°C, pH 9.7 and sulfi de content 2.8 mg/I
(these characteristics correspond to alkal i ne hydrotherms). This Iayer i s represented by oval cel Is of Synecoccus
elongates(representatives of this genus could be co-dominants under increased temperature) and by threads of
Chloroflexus aurantiacus and Phormidium sp. The upper sub-zone of the layer is occupied by cyenobacteria
Synecoccuselongates,and in the deeper layer mats of Phormidium sp. predominate. Filamentous green bacteria
Chloroflexus aurantiacus can be found all over the mat in an almost equal quantity [16, fig. 11a].
Ca
P
. _ Ca
Figure4. Rod-shaped,oval andfilament microstructuresandtheir spectrum.
IV The size and morphology of the rounded, coccoidal forms with diameters of ~ 1.5 pm (Figure. 5) are
similar to the purple bacteria Thiocapsafrom recent halophi lic mats [See Reference 10; 13; and 15, fig. 7a].
Figure 5. Coccoidalmicrostructuresandtheir spectrum.
P F_
m
V. It is necessary to note the rare framboids (Fig. 6) that presumably consist of pyrite. The framboi d diameter is
about 15 pm. It has rounded form and consists of crystallites with smoothed angle_ Framboids are widely
spread in different types sedimentary rocks from many different age_ They are also found in modern marsh
sedimerits, peat bogs, and in marine and oceanic silts. There i s l itUe doubt that framboids are shaped by the way
that the crystallization of amorphous iron sulfides developed during the early stages of diagenesis in the local
centersofhydrogensulphidecontamination.Theformationofframboidalstructuresisconnectedwithbacterial
processingfollowedbythemineralizationoforganicmatter.At least,inmarineconditionstheprecipitationof
framboidalstructuresisconnectedwithvitalactivityofsulfatereducingbacteria(SRB)[17[]22].Theycan
serveasmineralbiomarkersp ovidingindirectevidenceofthevitalactivityofsulfateredudngbacteriaduring
theformationofthinlayersofthecyanobacterialm tcomponentsencounteredintherocksjnderinvestigation
Figure 6. Framboidal(?)microstructureandits spectrum.
CONCLUSIONS
Thus it is probable that we have all of the components of a cyano-basterial mat. It is known that
cyano-bacterial mats represents prokaryotic benthic communities united in a physical structure by the slime
(glycocalyx) as a forming tool which transforms the community into a morphologically unified unit. In these
communities phototrophic bacteria dominate, and among them are the cyanobacteria are the main producers of
organic matter. They are primarily responsible for the mat structure 2° As a rule, oscillatorilaean cyanobacteria
are the dominant forms representing motile filament organisms In modern thermophilic mats there is the
h_ophile and alkalophile - Microco/eus chthonoplastes, which forms multiseriate filaments with multiple
trichomes within a common external envelope of sheath. Phototrophic bacteria (&g., green Cloroflexus
aurantiacus) in thermophilic mats, and purple bacteria (e.g., Thiocapsa, Ectothiarhodospira) in halophilic and
alkalophilic mats provide anoxygenic photosynthesis under extreme conditions for cyanobacterial growth.
Cyanobacteria may grow under favourable conditions by organotrophic growth by oxidizing products of vital
functions of cyanobacteria or by oxidizing of hydrogen sulphide and, in such a way, they take part in sulphur
cyde. These microorganisms occupy the middle regi me. On one hand, they delay photosynthetic oxygen that is
received from the cyanobacteria, and on the other hand, they protect the cyanobacteria from the hydrogen
sulphide that is produced by the SRB in the lower layers of mat. It is considered quite possible that the
formation of the above mentioned spherical formswith a high content of sulphur and calcium is connected with
this proces_ Sulfide-genesi s and methane-genesis are i n concurrent dependence, the predominance of one or the
other of these processesis determi ned by the sulfate content23
It is considered logical to assume,that we are dealing with a cyano-bacterial mat, probably allkalophilic
or halophilic. Moreover, the presenceof framboidal iron sulphide suggests the presenceof the sulfate-ion in the
water basin or in the silt. This is confirmed by the presenceof sulphur in practically in every chernic_ analysis
carried out on these forms, becausethe layer of filamentous sulphur bacteria (which is typically situated under
the layer of the oxygenic phototrophs) is connected the deposition of gypsum (CaSO4). 20.
f all of the components of the supposed mat were found separately and the laminated mat structure
was not locked in, there is the possibility that all of the described components of a cyanobacterial mat could
have developed independently. Representatives of Rhabdoderma are widely spread in the plankton and the
planktonic mode of life is also possible for other cyanobacteria and the purple bacteria For this reason, it is
impossible to exclude that all these bacteria were planktonic and precipitated on the bottom only after
exti ncti on.
Thus, it is seen, that the Lower Proterozoic phosphorites, as well as younger: Cambrian Khubsugul,
Early Cambrian and Sinian phosphorites of Yantzy Region of China and many other phosphorites have biogenic
odgin,thatiseitherbacterialorisconnectedwithcyanobacterialm t&24Therearenoprincipledifferencesin
natureofthesephosphorite&
It isextrerndy importanttounderlinethatalmostallknownmineralcompoundsofphosphorusinthe
crustoftheEartharesaltsof orthophosphoricacid(H3PO4)end that the presence of oxygens a necessary
condition for formation of this add.
Thus it is quite d ear that the studied phosphorites were formed in the conditions of a warm _hallow-
water basin with oxygen-enriched waters
ACKNOWLEDGMENTS
The authors are grateful to V. M etezhik and A. Lepland for vested materiel; L.M. Gerasimenko,
T.N. German, G.T. Ushatinskaya, E.A. Zhegallo for valued advice during discussion of the material;
A.V. Kravtse_ and L.T. Protasevich for assistancewith the CamScan-4 Scanning Electron Microscop&
The study was executed within the framework of the complex program of basic research of the
Presidium of the RussianAcademy of Sciences LOrig_n and Evolution of Biosphere[](subprogram II) and
was supported by the Ru_an Foundation for Basic Research, projects 05-04-48008 end SS-974.2003.5.
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Early Proterozoic (2.04 GA) Phoshorites of Pechenga Greenstone Belt and Their Origin

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