25th International Meeting on Organic Geochemistry IMOG 2011

P-475
Stable isotopes (C, S) and hydrocarbon biomarkers in
Neoproterozoic sediments of the Sierras Bayas Group,
Argentina
Mariluz Bagnoud-Velasquez, Jorge E. Spangenberg
Institute of Mineralogy and Geochemistry, University of Lausanne, Lausanne, Switzerland (corresponding
author:Jorge.Spangenberg@unil.ch)
The late Neoproterozoic or Ediacaran period, (635
to ~543 Ma) is a primordial time in the Earth history
corresponding to the beginning of animal life and the
most extreme ice ages on Earth. This contribution is
part of a long-standing project whose aim is to
reconstruct the palaeoenvironmental conditions for
Ediacaran, post-Gaskiers (last major Neoproterozoic
glaciation, ~580 Ma) shelf deposits in SW-Gondwana
and evaluate their changes according to the diversity
of organisms. To address the question of interactions
between increase of biodiversity and environmental
change a detailed elemental and isotopic geochemical
study of sedimentary rocks and associated
organic matter in conjunction with biomarker
investigations were performed on sedimentary
sequences from a large basin extended from the
Paraguay belt to the Rio de la Plata craton. The
studied Neoproterozoic to Cambrian sections include
the Corumbá Group (CG) in SW-Brazil, the Arroyo del
Soldado Group (ASG) in Uruguay and the Sierras
Bayas Group (SBG) in Argentina [1]. These
sedimentary successions have been investigated by
means of stable isotopes from carbonates (� 13 Ccar
and � 18 O), their associated organic carbon (� 13 Cker,
� 15 Nker), Rock-Eval and biomarker analyses, and
concentrations of major, trace and rare earth
elements (REE).
In all studied sections, the � 13 Ccarb excursions, the
strong enrichment of authigenic trace-elements, the
occurrence of longer chain n-alkanes, gammacerane
and low Pr/Ph and Ph/n-C18 ratios, combined with the
previous sedimentological and paleontological
observations indicate that the chemistry of the ocean
was strongly controlled by the oxygen availability;
waters being moderately oxic at the surface and
anoxic at depth for much of the Neoproterozoic. This
water column stratification was favourable to the
storage of large amounts of nutrients in the deep
ocean. During upwelling periods, the export of
nutrient-rich waters may have triggered an important
bioproductivity in surface waters. Drops in ∆ 13 Ccarb-ker
and positive � 13 Ccarb excursions record the increase in
primary productivity. Preservation of organic carbon
was ensured by reducing conditions at the bottom.
The ∆ 13 Ccarb-ker excursions could also reflect changes
in the primary biomass. Here we present new
geochemical data including sulfur isotope composition
of pyrite and structurally substituted carbonateassociated
sulfate (� 34 Spy and � 34 SCAS) from the
central region of the Tandilia orographic belt, Sierras
Bayas Group, Argentina.
The occurrence of 34 S enriched sulfides, with � 34 S
values (32.5‰) locally exceeding coeval � 34 SCAS
values (24.2 and 28.9‰) is viewed as a combined
product of globally low seawater sulfate, high rates of
bacterial sulfate reduction, and sulfate limitations
under a stratified water column inherited from
glaciations, particularly the lower water layer. The 34 Senriched
pyrites were plausibly derived from the
anoxic bottom waters while the relatively 34 S-depleted
CAS may have come from surface waters or be
produced in the redox chemocline by direct oxidation
of sulfides The sulfate concentration from a
weathering input should be higher in surface waters
than in the deep ocean and therefore, hardly
consumed due to its low abundance in the seawater.
Alternatively, direct oxidation of deep water sulfide
could also take place in the redox chemocline
producing 34 S-depleted sulfate. Diverse geochemical
proxies in the red limestones of Loma Negra
Formation are pointed to reducing conditions during
its deposition and/or early diagenesis (e.g., higher
concentrations of Fe, Mo, Zn and REE, the
occurrence of authigenic isotopically-enriched pyrite
and a molecular-inferred microbial ecosystem most
probably including green non-sulfur bacteria).
Reference
[1] Bagnoud, M. (2010) PhD thesis, University of
Lausanne, 200 pp.
600