wAbstract
@The causes of atmospheric oxygenation in the Paleoproterozoic
are multifaceted yet poorly understood. An increase in the level
of primary productivity however may have been an important factor
in producing large amounts of oxygen in the atmosphere. A likely
environmental consequence of atmospheric oxygenation is an increased
abundance of seawater nitrate, which would have resulted in a
fundamental change from the Archean nitrogen cycle. In this study,
we investigate these questions using the carbon and nitrogen isotope
compositions of carbonaceous shales from the Paleoproterozoic
Aravalli Supergroup, India. The shales occur stratigraphically
above one of the oldest phosphogenic events that took place around
2.0 Ga. Shales in the phosphate domain of the Jhamarkotra Formation
from the Lower Aravalli Group have distinct characteristics from
coeval shales in the non-phosphate domain. Carbonaceous shales
of the phosphate domain contain relatively high V and U/Th, typical
of oxygen-deficient environments. These shales also have a high
organic content of up to 14 wt, generally homogeneous Β13Corg values around -29ρ, C/NWR
up to 360 and a large range of Β15N values between
-5 and +27ρ. Their organic matter has low H/C and O/C and Raman
spectra characteristic of lower greenschist to lower amphibolite
facies metamorphism, which suggests a moderate alteration of geochemical
compositions. data from these shales also suggest high primary
productivity under eutrophic conditions, as well as intense recycling
of organic nitrogen through ammonium assimilation and denitrification
at the time of sedimentation.
@In contrast, carbonaceous shales from the non-phosphate domains
of the Jhamarkotra Formation contain trace element abundances
characteristic of more oxygenated conditions. They contain less
than 3 wt organic matter with Β13Corg
values often between -13 and -18ρ, wholerock Β15N values
between +5 and +12ρ and C/NWR below 56. Acid-insoluble
organic matter from these shales has high H/C and O/C, and Raman
spectral characteristics typical of lower greenschist facies metamorphism,
which suggests only minor alteration of isotope compositions.
Data gleaned from carbonaceous shales of the non-phosphate domain
are interpreted to indicate high rates of primary productivity
as well as the existence of a modern-like nitrogen cycle in this
open-marine environment at the mouth of the Udaipur Epicontinental
Sea. The interpretation of high primary productivity in carbonaceous
shales from both domains of the Jhamarkotra Formation is consistent
with stratigraphically underlying 13C-enriched carbonates
and with their associated stromatolitic phosphorites. Therefore,
increased fluxes of phosphorus in Paleoproterozoic seawater may
have played an important role in stimulating increased levels
of primary productivity and the rise of atmospheric oxygen.
Keywords: Paleoproterozoic; Nitrogen isotopes; Black shale; Phosphate;
Paleoproductivity; Denitrificationx
1. Introduction
2. Geological background
@2.1. Phosphate domain
@2.2. Non-phosphate domain
3. Analytical methods
@3.1. Sample preparation
@3.2. Analysis of trace elements
@3.3. Stable isotope geochemistry and light element abundances
@3.4. Electron microscopy
@3.5. Laser Raman spectroscopy
4. Results
5. Discussion
@5.1. Diagenetic imprints
@5.2. Assessment of paleoredox conditions
@@5.2.1. Thermal maturation of carbonaceous material
@@5.2.2. The carbon cycle during Lower Aravalli sedimentation
@@5.2.3. Thermal metamorphism and nitrogen
@@5.2.4. Nitrogen cycle
@5.3. Paleoenvironmental reconstruction
6. Summary and conclusions
Acknowledgements
References