AmericanChemicalSociety.com

The discovery of three levels of bituminous shale and carbonate deposited before, during, and after glacial deposits of the Vazante Group offers a unique opportunity to understand the distribution of biomarkers in the context of extreme climatic and environmental change. Twelve exploration cores spanning over 150 km – including carbonate, shale, and glacial diamictite from proximal and distal environments – were sampled in June 2007 for this study.  These cores were chosen to evaluate the spatial extent of the diamictite and associated syn-glacial shale, and to test observations and interpretations of two earlier focused biomarker studies based on three closely-spaced cores.  GC-MS analyses of organic extracts from the syn-glacial Serra do Poço Verde Formation revealed the presence of specific hopanes suggesting active photosynthesis during the ice age, an important constraint of the controversial ‘Snowball Earth' hypothesis.  Similarly, the presence of aryl isoprenoids suggested the presence of a euxinic photic zone, which is supported by new iron-speciation data through the interval. On the other hand, biomarker distributions in the post-glacial Lapa Formation shale reveals evidence for mixing of bitumen from various sources, consistent with the presence of pyro-bitumen in basal Lapa Formation lithologies.  The general absence of C29 steranes in the Vazante extracts, however, indicates that the admixed components were not likely of Phanerozoic origin. To better understand biomarker distributions in these Proterozoic source rocks, we proposed a basin wide study that will address the issue of syngeneity and environmental context of organic constituents, and whether remobilization of these reflects local or regional events.

For this study, sixty-one biomarker samples were collected.  An estimate of extractible organic material was made by rock-eval pyrolysis; total organic carbon (TOC), % S, and d³⁴S were also measured.   In the first year, samples from the Serra do Poço Verde Formation were analyzed.  Nine were extracted with an accelerated solvent extractor at the Virginia Institute of Marine Science.  The remaining 12 were extracted by sonication in dichloromethane.  Organic extracts were separated into saturated, aromatic, and polar fractions using column chromatography.  An internal standard, 3-methylheneicosane, was added to the saturated fraction.  Structural analyses were performed at the Carnegie Institution for Science using a Hewlett Packard 6890N gas chromatograph coupled to a Hewlett Packard 5973 Mass Selective detector.  Samples were run in full scan and selected ion monitoring (SIM) modes.

N-alkanes, pristane, phytane, hopanes and steranes were observed in 17 samples.  Abundances of n-alkanes, pristane and phytane were calculated from the 191, 217, and 85 SIM ion chromatograms, respectively.  Peak areas of each compound were manually integrated using Hewlett Packard MS ChemStation software.  The absolute abundances of n-alkanes, pristane and phytane were calculated by comparing compound peak areas with the peak area of the 3-methylheneicosane internal standard.  The n -alkane distributions differ between cores and range from unimodal low molecular weight to slightly bimodal or higher molecular weight distributions.  The abundance of low molecular weight alkanes may be related to high levels of thermal maturity; however, n -alkane distribution may also be affected by source inputs.

Pristane and phytane, the C₁₉ and C₂₀ isoprenoid products of phytol, were present in all samples, indicating active photosynthesis.  During diagenesis, the phytyl side chain of chlorophyll a and bacteriochlorophyll a and b is cleaved resulting in phytol.  Phytol is then either reduced to phytane or oxidized to form pristane.  Therefore, the ratio of pristane/phytane is used to estimate the redox conditions present during deposition.  The pristane/phytane ratios range from 0.43-1.4, indicating sub-oxic conditions and no input from terrigenous matter. 

Hopanes, generally attributed to prokaryotic input, were detected in all samples.  The C₂₉ and C₃₀ hopanes are the most abundant and the ratio of C₂₉/C₃₀ hopanes ranges from 0.66 to 1.3.  The C₂₉/C₃₀ ratio is suggestive of the source rock lithology; values <0.8 are typical of anoxic carbonate or marl whereas values between 0.3 and 0.7 are typical of a shale source rock.  Both the R and S conformations of the C₃₁-C₃₄ homohopanes  and the R and S C₃₅ homohopanes were detected.  Under reducing conditions, the amount of the C₃₅ homohopanes relative to the total C₃₁-C₃₅ homohopanes is >5%.  The C₃₅ % total of the samples in which the C₃₅ homohopanes were detected ranges from 3.7 to 7.4 %.  Most of the samples however are near or greater than 5% which may indicate reducing conditions.  Additionally, the ratio of C₃₁22R/C₃₀ hopanes is near or greater than 0.25 (0.23-0.36) for most samples indicating a marine source.

C₂₇ through C₂₉ regular steranes and diasteranes, generally attributed to eukaryotic input, were identified in all samples.  Groupings of the samples on a ternary sterane diagram indicating a similar source.

The low abundance of hopanes and steranes coupled with high abundances of low molecular weight n-alkanes may indicate high thermal maturity.  Hopane and sterane biomarker ratios indicate that the samples are mature, between the early and peak oil generating stages.  Tm (C₂₇ Trisnorhopane) is less stable than Ts (C₂₇ Trisnorneohopane) during catagenesis, so as temperature increases Tm decreases but Ts remains about the same.  The ratio of Ts/(Ts+Tm) ranges from 0.28 to 0.61.  This ratio is strongly dependent on source lithology, so low values could alternatively be the result of a carbonaceous source rock.  The ratio of C₃₀ ba/(ba+ab) decreases with increasing maturity to values <0.15 in mature rocks.  The samples are all <0.15 (0.08-0.14) indicating that they are mature.  Furthermore, the hopane isomerization ratio C₃₂ 22S/(22S+22R) reaches equilibrium at ~0.58 in the early oil generation window.  The C₃₂ 22S/(22S+22R) ratio for samples range from 0.56 to 0.65 suggesting they have past through the early oil generation window.

The ACS PRF award has funded a Research Assistantship for Ph.D. candidate Kristen E. Miller and covered the cost of extractions as well as structural and isotopic characterization of samples.  PRF funds were used to support undergraduate laboratory assistant, Jessica Little (directed by Miller in a biomarker study of samples from the Bambuí Group in Brazil), and faculty research assistant, Eugenia Gold, who prepared Vazante Group samples for bulk geochemical measurements.