Organic Burial Research Articles

Applying Statistical Analysis and Economics Models to Unscramble the Depositional Signals from Chemical Proxies in Black Shales

The complex controls on the accumulation of organic-rich rocks remain elusive, despite their economic importance as source rocks and unconventional reservoirs, partially due to the multitude of factors that may impact production and preservation of organic matter in sediments. The complexity of Earth systems is comparable to the intricacies of Economics, and application of statistical and econometrics methods and models to analyze geological data may assist interpretation of the processes controlling organic burial. Chemical indices calculated for mudrock datasets from modern sediments and the Woodford Formation were used as proxies for detrital input, primary productivity, redox conditions, and upwelling, and a series of statistical analyses were run to test whether these methods were useful to discriminate different depositional conditions and establish the controls on total organic carbon (TOC) in the sediments. Model results showed that chemical proxies reliably predict not only TOC but also indicate correlations between indices. Our results suggest that detrital input, primary productivity and bottom-water anoxia are relevant drivers of organic content in the sediments, but the first two appear to have a more significant role in organic burial, illustrating the usefulness of these methods to assess depositional parameters in organic-rich rocks.

Cu and Zn Isotopic Evidence for the Magnitude of Organic Burial in the Mesoproterozoic Ocean

Cu and Zn Isotopic Evidence for the Magnitude of Organic Burial in the Mesoproterozoic Ocean

Carbon cycle inverse modeling suggests large changes in fractional organic burial are consistent with the carbon isotope record and may have contributed to the rise of oxygen.

Abundant geologic evidence shows that atmospheric oxygen levels were negligible until the Great Oxidation Event (GOE) at 2.4–2.1 Ga. The burial of organic matter is balanced by the release of oxygen, and if the release rate exceeds efficient oxygen sinks, atmospheric oxygen can accumulate until limited by oxidative weathering. The organic burial rate relative to the total carbon burial rate can be inferred from the carbon isotope record in sedimentary carbonates and organic matter, which provides a proxy for the oxygen source flux through time. Because there are no large secular trends in the carbon isotope record over time, it is commonly assumed that the oxygen source flux changed only modestly. Therefore, declines in oxygen sinks have been used to explain the GOE. However, the average isotopic value of carbon fluxes into the atmosphere–ocean system can evolve due to changing proportions of weathering and outgassing inputs. If so, large secular changes in organic burial would be possible despite unchanging carbon isotope values in sedimentary rocks. Here, we present an inverse analysis using a self‐consistent carbon cycle model to determine the maximum change in organic burial since ~4 Ga allowed by the carbon isotope record and other geological proxies. We find that fractional organic burial may have increased by 2–5 times since the Archean. This happens because O2‐dependent continental weathering of 13C‐depleted organics changes carbon isotope inputs to the atmosphere–ocean system. This increase in relative organic burial is consistent with an anoxic‐to‐oxic atmospheric transition around 2.4 Ga without declining oxygen sinks, although these likely contributed. Moreover, our inverse analysis suggests that the Archean absolute organic burial flux was comparable to modern, implying high organic burial efficiency and ruling out very low Archean primary productivity.

A statistical analysis of the carbon isotope record from the Archean to Phanerozoic and implications for the rise of oxygen

Organic and inorganic carbon isotope records reflect the burial of organic carbon over geological timescales. Permanent burial of organic carbon in the crust or mantle oxidizes the surface environment (atmosphere, ocean and biosphere) by removing reduced carbon. It has been claimed that both organic and inorganic carbon isotope ratios have remained approximately constant throughout Earth9s history, thereby implying that the flux of organic carbon burial relative to the total carbon input has remained fixed and cannot be invoked to explain the rise of atmospheric oxygen (Schidlowski, 1988; Catling and others, 2001; Holland, 2002; Holland, 2009; Kump and others, 2009; Rothman, 2015). However, the opposite conclusion has been drawn from the same carbon isotope record (Des Marais and others, 1992; Bjerrum and Canfield, 2004). To test these opposing claims, we compiled an updated carbon isotope database and applied both parametric and non-parametric statistical models to the data to quantify trends and mean-level changes in fractional organic carbon burial with associated uncertainties and confidence levels. We first consider a conventional mass-balance model where carbon input to surficial reservoirs is balanced by burial of sedimentary carbonates and organic carbon. For this model, statistical analysis implies fractional organic burial has increased over Earth history by a factor of 1.5 relative to organic burial at 3.6 Ga, with the 95 percent confidence interval ranging from factors of 1.2 to 2.0. An increase in organic burial by a factor of 1.2 cannot explain the rise of oxygen, whereas an increase by a factor of 2 could conceivably explain the rise of oxygen. There is, however, a highly significant and well constrained increase in organic burial from the Proterozoic to the Phanerozoic. We also analyze changes in the difference between carbonate and organic carbon isotopic ratios over Earth history. There is a statistically significant increase in this difference from the early to late Archean, possibly caused by increased biological fractionation due to methanotrophic recycling. This transition is consistent with the evolution of oxygenic photosynthesis at 2.8 Ga or earlier. Finally, we explore how these conclusions change if we modify the traditional mass balance model to include other carbon cycle fluxes, specifically ocean crust carbonatization and authigenic carbonates. Because the size of these fluxes has a large, poorly constrained range, our statistical analysis with this uncertainty implies that the carbon isotope record does not constrain the history of organic burial at all. However, it remains possible that the magnitude of these additional processes has been inconsequential throughout geologic time, in which case conclusions from the conventional model would be valid.

High-resolution stable isotope stratigraphy of the upper Cambrian and Ordovician in the Argentine Precordillera: Carbon isotope excursions and correlations

We report the occurrence of important carbon isotope excursions in early Paleozoic formations of the Eastern and Central Argentine Precordillera. The Steptoean positive isotope carbon excursion (SPICE) is known from North America, Kazakhstan, South China, Australia and South America, and the negative isotope carbon excursion (SNICE) has been described for the first time in South America. We report here the record of the SPICE and SNICE in a single section in the Eastern Precordillera. In the Central Precordillera, a minor middle Darriwilian positive carbon isotope excursion (MDICE) and a late Sandbian positive isotope carbon excursion, the GICE (~+3‰; C. bicornis biozone) are reported from two sections. One pre-GICE positive carbon-isotope excursion (Sandbian Sa1, N. gracilis biozone) in the Central Precordillera with a δ13C peak of ~+2‰ is, perhaps, equivalent to the positive Spechts Ferry δ13C excursion of North America. A positive δ13C excursion (~6‰; N. persculptus biozone) recorded at the base of the late Hirnantian La Chilca Formation probably corresponds to HICE.These carbon-isotope excursions resulted from more than one factor where paleoceanographic events probably played a major role: (a) sea-level fall and vigorous fluctuations in the Steptoean (SPICE), (b) sea-level rise in the Sunwaptan (SNICE), (c) important transgression in the Sandbian (pre-GICE and GICE), and (d) sea-level fall in the late Hirnantian (HICE). In the Darriwilian and Sandbian stages, organic burial has led to a large 12C sequestration in the deep anoxic ocean with saline bottom water, recorded by the graptoliferous black shales of the Gualcamayo and Los Azules formations in Central Precordillera, helped the building of the MDICE and GICE anomalies.δ18O values for the Upper Cambrian carbonates are likely near-primary isotope signals that point to progressive cooling from the SPICE to the SNICE, whereas for Sandbian carbonates they suggest strong temperature fluctuations. The δ13C peak of the GICE coincides with a cooler period with temperatures warming up towards the late Hirnantian to Rhuddanian.The Upper Cambrian to the Middle Ordovician of the Precordillera recorded a decrease of seawater 87Sr/86Sr ratios in accordance to the global picture. This decrease probably reflects the influence of widespread volcanic activity from arc terranes in low-latitude settings along eastern margins of Laurentia and in the Argentine Precordillera in the Early to Middle Ordovician. Five carbonate samples (from San Juan, Gualcamayo and Los Azules formations) yielded εNd values that along seven already published values seem to plot along the Nd isotopic evolution trend of the Iapetus Ocean.Together, the record of global Upper Cambrian and Ordovician carbon-isotope excursions in the Argentine Precordillera is a valuable proxy in refining Early Paleozoic stratigraphy, establishing of regional/global high-resolution correlations, and sea-level change history in South America.

Assessment on Redox Conditions and Organic Burial of Siliciferous Sediments at the Latest Permian Dalong Formation in Shangsi, Sichuan, South China

The redox sensitive elements, molybdenum (Mo) and uranium (U), in marine sediments from the latest Permian Dalong Formation at the Shangsi Section, Northeast Sichuan, South China, were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to determine their response to a range of redox conditions, and to estimate the organic carbon burial rate. On the basis of the correlation between authigenic Mo abundance and organic carbon content in modern oceans, the organic carbon burial rates were calculated for the rocks at Dalong Formation, ranging from 0.48–125.83 mmol/(m 2 ·d), which shows a larger range than the mineralization rate of organic carbon at the continental margins (1.6–4.23 mmol/(m 2 ·d)). The Zr-normalized Mo and U abundances show large fluctuations in the entire section. The maxima of Zr-normalized Mo abundance and thus the maxima of the organic carbon burial rates were observed at the interval between the 155th and 156th beds (404–407 m above the base of Middle Permian). A decrease (the minimum) in U/Mo ratios is present in this interval. It is speculated that the oxygen-limited conditions and ultimately anoxia or euxinia may develop within this depth interval. In contrast, an enhanced enrichment of Zr-normalized U abundance is found, in association with less enrichment in Zr-normalized Mo abundance in the interval from the 151st to 154th beds (395–404 m above the base of Middle Permian), inferring the dominance of a suboxic/anoxic depositional condition (denitrifying condition), or without free H 2 S. The presence of small quantities of dissolved oxygen may have caused the solubilization and loss of Mo from sediments. It is proposed that the multiple cycles of abrupt oxidation and reduction due to the upwelling at this interval lead to the enhanced accumulation of authigenic U, but less enrichment of Mo. A decrease in the contents of U, Mo, and TOC is found above the 157th bed (407 m above the base of Middle Permian), in association with the enhanced U/Mo ratio, suggesting the overall oxic conditions at the end of the Dalong Formation.

Organic fraction of the total carbon burial flux deduced from carbon isotopes across the Permo-Triassic boundary at Meishan, Zhejiang Province, China

By combining the carbon cycle model with the records of carbonate and organic (kerogen) carbon isotope, this paper presents the calculation of the fraction of organic carbon burial (forg) of beds 23–40 at the global boundary stratotype section and point (GSSP) of the Permian-Triassic boundary at Meishan, Zhejiang Province. The resulting calculation produces two episodes of forg maxima observed to occur at beds 23–24 and 27–29, which respectively corresponds to the two episodic anoxic events indicated by the flourish of green sulfur bacteria. Two episodic forg minima occurred at beds 25–26 and 32–34, generally coincident with the flourish of cyanobacteria (bed 26 and upper part of beds 29 to 34) as shown by the high value of 2-melthyhopnoanes. It appears that the forg is related to the redox conditions, with greater forg values observed under the reductive condition. The relationship between forg and the total organic carbon (TOC) content was complex. The forg value was low at some beds with a high TOC content (such as bed 26), while high observed at some beds with a low TOC content (e.g. bed 27). This association infers the important contribution of primary productivity to the TOC content. The original organic burial could be thus calculated through the configuration of the function of the primary productivity and forg, which can be used to correct the residual TOC measured today. This investigation indicates that compiling the organic-inorganic carbon isotopes with the carbon cycle model favors to understand the fraction of organic carbon burial, providing information for the reconstruction of the coupling among biota, environments and organic burial.

On the geobiological evaluation of hydrocarbon source rocks

Hydrocarbon source rocks are characterized by the hydrocarbon discharge, and the alteration and variation in organic compositions and organic content due to the enhanced thermal maturation. These variations throw constraints on the application of the conventional inversion evaluation of hydrocarbon potential by assessing the residual organic matter left in source rocks. Geobiology, probing the interaction between the life system and the earth system, provides new principles in deciphering the whole dynamic processes related to the organic evolution history from living biomass to organic burial. Geobiological subdisciplines, including molecular geobiology, geomicrobiology, geoecology and biogeochemistry, offer new methodology and techniques to estimate the paleoproductivity, depositional organics and organic burial capacity and their components. Geobiofacies, newly proposed herein, is terminologized to define the geobiological dynamic processes through the combination of biofacies with organic facies and sedimentary facies, and expressed by the biohabitat types, paleoproductivity, depositional and preserved organics. Geobiofacies is identified as a useful means to create the geobiological evaluation system, which in turn rectifies the conventional evaluation system for the marine source rocks.

Calci-microbialite as a potential source rock and its geomicrobiological processes

The calci-mircobialite is a special carbonate buildup, which is formed due to the activities of different kinds of microbes. Abundant microfossils preserved in the microbialite show the high-level productivity during deposition, while characteristic sedimentary minerals and geochemical compositions suggest an anoxic marine environment for organic burial. The high-level productivity and anoxic sedimentary environment favor the efficient preservation of organic matter and thus the formation of source rocks. On these points, microbialites could be one of the potential hydrocarbon source rocks, awaiting further geobiological investigation and exploration. Precambrian and some of the great transitional stages in Phanerozoic are critical periods when microbialites were well developed. Widespread microbialites have been found in North and South China. Bitumen observed in many outcrops of Precambrian and late Devonian microbialites further raises the possibility of the calci-microbialite as a potential hydrocarbon source rock.

Parallel, high-resolution carbon and sulfur isotope records of the evolving Paleozoic marine sulfur reservoir

Links between the biogeochemical cycles of carbon and sulfur are expressed in the evolving stable isotope composition of the ocean. Carbonate rocks record the inorganic carbon isotope composition of the oceanic reservoir through geological time, along with the sulfate sulfur isotope composition preserved as carbonate-associated sulfate (CAS). An inverse relationship exists between the first-order carbon and sulfur records of the Paleozoic; however, the isotope curves also reveal higher-frequency variations superimposed on the first-order (10 7–10 8 yr) trend. Because of links between carbon and sulfur cycling, high-resolution, CAS-based S isotope data have the potential to shed essential light on the mechanisms behind carbon isotope excursions observed in the geologic record. Results from the Late Cambrian SPICE event at Shingle Pass, Nevada, show parallel positive C (5‰) and S (25‰) isotope excursions, likely recording a large-scale marine organic carbon burial event with sympathetic pyrite burial. Carbon and sulfur records from other events throughout the Paleozoic reveal different and evolving relationships. A Silurian–Devonian boundary section at Strait Creek, West Virginia, reveals a positive C excursion of 6‰ but with an enigmatic, apparently negative S excursion (15‰) antithetic to the C shift. By contrast, a Kinderhookian–Osagean (Early Mississippian) section in the Confusion Range, Utah, shows invariant S isotope ratios across a C isotope excursion of 6‰. The coupling of the high-resolution C–S isotope records may have weakened in the mid-to-late Paleozoic with decreasing sensitivity of the seawater S isotope reservoir to flux changes as the marine sulfate concentrations increased through the Paleozoic. The progressively decreasing isotopic variability of CAS across three positive carbon excursions suggests that the Paleozoic was transitional between a Proterozoic ocean with rapid isotopic variability and thus low sulfate concentration and the comparatively more stable isotopic properties of a relatively more sulfate-rich Mesozoic and Cenozoic ocean. Furthermore, the emergence of the terrestrial ecosystems during the late middle and late Paleozoic yielded new loci of organic burial that also likely played a central role in the decoupling carbon and sulfur isotope records. Organic carbon burial in these sulfate-limited environments occurred in the absence of significant pyrite burial.

Lower Cambrian Carbon Isotope Evidence of Photosynthesis from Krol-Tal Carbonates of the Lesser Himalaya and Global Stratification in Early Cambrian Oceans

Extensive development of cyanobacterial mats, digitate, crenulated and stratified microbialites, oncolites, conical-colunmar stromatolitic build-ups, microbial balls and calcimicrobe (Eiphyton, Renalcis) in Upper Kxol Carbonates of the Lesser Himalaya indicate presence of benthic microbial communities (BMNC) and photosynthesis in Precambrian-Cambrian boundary-Lower Cambrian sea. The carbonate-phosphate transitional facies of the Lower Cambrian Tal Formation reflects a major change in chemical sedimentation. The Tal Formation also indicates the evidence of bacterial activity, since small phosphatised stromatolites and oncolites are preserved in Chert-Phosphorite Member. Carbon isotope (813 C carb., ~13C org.) and rare earth element (REE) distribution in Krol-Tal carbonates as evidence of organic productivity and global stratification in early Cambrian oceans has been discuess across a well-studied profile in Mussoorie syncline. Isotopically heavy carbonates (+6.6% PDB) were deposited in Upper Krol D stromatolitic facies representing enhanced organic burial. This strong positive isotopic shift is followed by a decrease in 813C from +2% to near zero in Krol E carbonates (topmost unit). There is a negative shift in 813C (-2.2%~ just below the Lower Cambrian or PC-C Boundary. These 813C records reflect global isotopic variations in Neoproterozoic-early Cambrian oceans of the world. Carbon isotopic shifts may indicate stratification of oceans due to photosynthesis in tidal flat areas (photic zone). Oxydising depositional environment is supported by the Ce anomaly of the Krol carbonates.

Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism.

The carbon isotope geochemistry of carbonates and organic carbon in the late Proterozoic Damara Supergroup of Namibia, including the Nama, Witvlei, and Gariep groups on the Kalahari Craton and the Mulden and Otavi groups on the Congo Craton, has been investigated as an extension of previous studies of secular variations in the isotopic composition of late Proterozoic seawater. Subsamples of microspar and dolomicrospar were determined, through petrographic and cathodoluminescence examination, to represent the "least-altered" portions of the rock. Carbon-isotopic abundances in these phases are nearly equal to those in total carbonate, suggesting that 13C abundances of late Proterozoic fine-grained carbonates have not been significantly altered by meteoric diagenesis, although 18O abundances often differ significantly. Reduced and variable carbon-isotopic differences between carbonates and organic carbon in these sediments indicate that isotopic compositions of organic carbon have been altered significantly by thermal and deformational processes, likely associated with the Pan-African Orogeny. Distinctive stratigraphic patterns of secular variation, similar to those noted in other, widely separated late Proterozoic basins, are found in carbon-isotopic compositions of carbonates from the Nama and Otavi groups. For example, in Nama Group carbonates delta 13C values rise dramatically from -4 to +5% within a short stratigraphic interval. This excursion suggests correlation with similar excursions noted in Ediacaran-aged successions of Siberia, India, and China. Enrichment of 13C (delta 13C> +5%) in Otavi Group carbonates reflects those in Upper Riphean successions of the Akademikerbreen Group, Svalbard, its correlatives in East Greenland, and the Shaler Group, northwest Canada. The widespread distribution of successions with comparable isotopic signatures supports hypotheses that variations in delta 13C reflect global changes in the isotopic composition of late Proterozoic seawater. Within the Damara basin, carbon-isotopic compositions of carbonates provide a potentially useful tool for the correlation of units between the Kalahari and Congo cratons. Carbonates depleted in 13C were deposited during and immediately following three separate glacial episodes in Namibia. The correspondence between ice ages and negative delta 13C excursions may reflect the effects of lowered sea levels; enhanced circulation of deep, cold, O2-rich seawater; and/or the upwelling of 13C-depleted deep water. Iron-formation is additionally associated with one of the glacial horizons, the Chuos tillite. Carbon-13 enriched isotopic abundances in immediately pre-glacial carbonates suggest that oceanographic conditions favored high rates of organic burial. It is likely that marine waters were stratified, with deep waters anoxic. A prolonged period of ocean stratification would permit the build-up of ferrous iron, probably from hydrothermal sources. At the onset of glaciation, upwelling would have brought 13C-depleted and iron-rich deep water onto shallow shelves where contact with cold, oxygenated surface waters led to the precipitation of ferric iron.