Yongbo Peng

A unifying model for Neoproterozoic–Palaeozoic exceptional fossil preservation through pyritization and carbonaceous compression

Soft-tissue fossils capture exquisite biological detail and provide our clearest views onto the rise of animals across the Ediacaran-Cambrian transition. The processes contributing to fossilization of soft tissues, however, have long been a subject of debate. The Ediacaran Gaojiashan biota displays soft-tissue preservational styles ranging from pervasive pyritization to carbonaceous compression, and thus provides an excellent opportunity to dissect the relationships between these taphonomic pathways. Here geochemical analyses of the Gaojiashan fossil Conotubus hemiannulatus show that pyrite precipitation was fuelled by the degradation of labile tissues through bacterial sulfate reduction (BSR). Pyritization initiated with nucleation on recalcitrant tube walls, proceeded centripetally, decelerated with exhaustion of labile tissues and possibly continued beneath the BSR zone. We propose that pyritization and kerogenization are regulated principally by placement and duration of the decaying organism in different microbial zones of the sediment column, which hinge on post-burial sedimentation rate and/or microbial zone thickness.

Timing the deposition of 17O-depleted barite at the aftermath of Nantuo glacial meltdown in South China

Barite in basal Ediacaran cap carbonates in South China shows distinct, non-mass-dependent depletion in 17O, a signal most likely reflecting an extremely high p CO2 atmosphere during Marinoan glacial meltdown. The precise geological context of the barite within the cap carbonate was not defined, however, and such information is crucial to an accurate interpretation of the anomalous 17O signal. Based on an extensive field survey of Marinoan cap carbonates in South China and detailed sedimentological, petrographic, and isotope data, we propose here a unified sequence of events that followed the deposition of the Nantuo diamictite: the cap dolostone was first deposited and later uplifted due to isostatic rebound, undergoing karstic dissolution in both shallow platform and transitional facies of the Yangtze Block; subsequent transgression initiated a consistent sequence of mineral deposition on karstic surfaces or within cavities. The 17O-depleted barite, either visible on outcrops or identifiable in thin sections, is among the first minerals precipitated when the transgression flooded the karstified cap dolostone. The calcite with extremely negative δ13C values occurs at the last stage of the cavity filling, well after the deposition of cap dolostone and the 17O-depleted barite, suggesting that the signal of methane hydrate was registered much later in the cap dolostone. A similar recognition of karstic dissolution and subsequent barite deposits in cap dolostones in northwest Africa and northwest Canada suggest that deposition of the 17O-depleted barites may be a global event, recording an atmospheric-biological-hydrological condition shortly (within 0–1.6 m.y.) after the initial meltdown of the Marinoan snowball Earth.

Environmental context for the terminal Ediacaran biomineralization of animals.

In terminal Ediacaran strata of South China, the onset of calcareous biomineralization is preserved in the paleontological transition from Conotubus to Cloudina in repetitious limestone facies of the Dengying Formation. Both fossils have similar size, funnel-in-funnel construction, and epibenthic lifestyle, but Cloudina is biomineralized, whereas Conotubus is not. To provide environmental context for this evolutionary milestone, we conducted a high-resolution elemental and stable isotope study of the richly fossiliferous Gaojiashan Member. Coincident with the first appearance of Cloudina is a significant positive carbonate carbon isotope excursion (up to +6‰) and an increase in the abundance and (34) S composition of pyrite. In contrast, δ(34) S values of carbonate-associated sulfate remain steady throughout the succession, resulting in anomalously large (>70‰) sulfur isotope fractionations in the lower half of the member. The fractionation trend likely relates to changes in microbial communities, with sulfur disproportionation involved in the lower interval, whereas microbial sulfate reduction was the principal metabolic pathway in the upper. We speculate that the coupled paleontological and biogeochemical anomalies may have coincided with an increase in terrestrial weathering fluxes of sulfate, alkalinity, and nutrients to the depositional basin, which stimulated primary productivity, the spread of an oxygen minimum zone, and the development of euxinic conditions in subtidal and basinal environments. Enhanced production and burial of organic matter is thus directly connected to the carbon isotope anomaly, and likely promoted pyritization as the main taphonomic pathway for Conotubus and other soft-bodied Ediacara biotas. Our studies suggest that the Ediacaran confluence of ecological pressures from predation and environmental pressures from an increase in seawater alkalinity set the stage for an unprecedented geobiological response: the evolutionary novelty of animal biomineralization.

Widespread contamination of carbonate-associated sulfate by present-day secondary atmospheric sulfate: Evidence from triple oxygen isotopes

The isotope composition of seawater sulfate is an important tracer of sulfur, carbon, and oxygen cycles in Earth’s deep past. Carbonate-associated sulfate (CAS) extracted by acid digestion is widely used as a proxy for sulfate in paleo-seawater from which the carbonate minerals precipitated. Early and late diagenesis, weathering, and laboratory processing can in some cases compromise original seawater sulfate signals. Here, we report that extracted CAS can also be severely contaminated by recent atmospheric sulfate, especially when the sampled carbonates are from outcrops in arid to semi-arid climates or in heavily polluted regions. Our evidence comes from triple oxygen isotope compositions of sequentially extracted water-leachable sulfate and acid-leachable sulfate from carbonates of diverse ages from northwestern and north-central China and southwestern North America. Independent of the age of the rocks, almost all the water-leachable sulfates and half of the acid-leachable sulfates bear positive 17O anomalies, clearly distinguishable from those of typical seawater sulfate. Because secondary atmospheric sulfate (SAS) is the only source of sulfate known to bear positive 17O anomalies, we conclude that sulfate extracted from carbonate outcrops in these regions has a significant component of SAS. Because SAS generally has a much lower δ34S value than paleo-seawater sulfate, it could shift the δ34S of the extracted CAS to lower values and in some cases even lower than that of the co-occurring pyrite, i.e., the “super-heavy pyrite” enigma reported in geological records. Our findings call for a re-evaluation of many published, outcrop-based CAS data and conclusions.

A carbonate-based proxy for sulfate-driven anaerobic oxidation of methane

Sulfate-driven anaerobic oxidation of methane (SD-AOM) supports chemosynthesis-based communities and limits the release of methane from marine sediments. Formation of authigenic carbonates at active methane seeps is promoted by SD-AOM stoichiometry. While distinctively small δ 18 O/δ 34 S slopes of pore fluid sulfate have been shown to typify modern methane-rich environments, identification of such environments has been difficult for the geological past due to the lack of sedimentary pore fluids. However, if the isotopic composition of sulfate were archived in authigenic carbonate during early diagenesis, carbonate-associated sulfate (CAS) should display the characteristic δ 18 O-δ 34 S pattern. To test this hypothesis, we investigated the δ 18 O CAS , δ 34 S CAS , and 87 Sr/ 86 Sr signatures of authigenic carbonate minerals from three modern and two ancient methane-seep provinces. The data obtained demonstrate that all deposits regardless of age or location display consistently small δ 18 O CAS /δ 34 S CAS slopes (∼0.3) and CAS does not represent ambient seawater but pore-water sulfate. This finding confirms the utility of CAS as a recorder of SD-AOM in methane-rich environments. In addition, we report that aragonites bear higher CAS contents, 87 Sr/ 86 Sr ratios closer to that of contemporary seawater, and a larger δ 18 O CAS /δ 34 S CAS slope than calcites, reflecting the shallower formation depth of aragonite where pore-water has a composition close to that of seawater with high concentrations of sulfate. The new proxy can be used to constrain the record of SD-AOM through most of Earth history by measuring the δ 18 O and δ 34 S values of CAS of methane-derived diagenetic carbonates including but not limited to seep carbonates.

Integrated carbon, sulfur, and nitrogen isotope chemostratigraphy of the Ediacaran Lantian Formation in South China: Spatial gradient, ocean redox oscillation, and fossil distribution.

The Ediacaran Doushantuo Formation in South China is a prime target for geobiological investigation because it offers opportunities to integrate chemostratigraphic and paleobiological data. Previous studies were mostly focused on successions in shallow-water shelf facies, but data from deep-water successions are needed to fully understand basinal redox structures. Here, we report δ13 Ccarb , δ13 Corg , δ34 Spyr , δ34 SCAS , and δ15 Nsed data from a drill core of the fossiliferous Lantian Formation, which is a deep-water equivalent of the Doushantuo Formation. Our data confirm a large (>10‰) spatial gradient in δ13 Ccarb in the lower Doushantuo/Lantian formations, but this gradient is probably due to the greater sensitivity of carbonate-poor deep-water sediments to isotopic mixing with 13 C-depleted carbonate cements. A pronounced negative δ13 Ccarb excursion (EN3) in the upper Doushantuo/Lantian formations, however, is spatially consistent and may be an equivalent of the Shuram excursion. δ34 Spyr is more negative in deeper-water facies than in shallow-water facies, particularly in the lower Doushantuo/Lantian formations, and this spatial pattern is interpreted as evidence for ocean redox stratification: Pyrite precipitated in euxinic deep waters has lower δ34 Spyr than that formed within shallow-water sediments. The Lantian Formation was probably deposited in oscillating oxic and euxinic conditions. Euxinic black shales have higher TOC and TN contents, but lower δ34 Spyr and δ15 Nsed values. In euxinic environments, pyrite was predominantly formed in the water column and organic nitrogen was predominantly derived from nitrogen fixation or NH4+ assimilation because of quantitative denitrification, resulting in lower δ34 Spyr and δ15 Nsed values. Benthic macroalgae and putative animals occur exclusively in euxinic black shales. If preserved in situ, these organisms must have lived in brief oxic episodes punctuating largely euxinic intervals, only to be decimated and preserved when the local environment switched back to euxinia again. Thus, taphonomy and ecology were the primary factors controlling the stratigraphic distribution of macrofossils in the Lantian Formation.

Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans.

Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (∼2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end.

Oxygen isotope composition of meltwater from a Neoproterozoic glaciation in South China

The water cycle is an integral part of Earth surface dynamics, and water’s oxygen-isotope composition retains information about the forcing and response of Earth’s local and global climate. Water isotope signals of the recent geological past can be directly obtained from archives such as ice cores, groundwater, or pore fluid. For the more distant past, mineral proxies have to be used. Multiple episodes of global glaciation may have occurred in the Neoproterozoic Era, of which the record of oxygen-isotope composition of glacial meltwater is sparse; the few records that are derived from carbonate minerals are prone to late-burial and metamorphic alteration, and therefore subject to alternative explanations. Here we present a case in which meltwater δ 18 O is retrieved from barite (BaSO 4 ) and malachite (Cu 2 CO 3 (OH) 2 )–associated sulfate (MAS) in a diamictite in Kaiyang, Guizhou, South China. The core of our argument is based on the lowest-ever-published sulfate δ 18 O values found in the barite and MAS, reaching as low as –20.3‰ (Vienna standard mean ocean water, VSMOW). These data suggest that the water involved in the oxidative weathering of these chalcocite clasts had a δ 18 O value of –34‰ ± 10‰, similar to that of polar glaciers today. Excluding the possibility of glacier meltwater alteration during the past 600 m.y. after the deposition of the diamictite, the sulfate mineral assemblage reported here provides an important constraint on the nature of the Neoproterozoic glaciation that the Kaiyang diamictite represents.

Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

Significance At least two extreme glaciations (dubbed as snowball Earth events), each lasting millions of years and affecting the entire globe, occurred during the Cryogenian Period (∼720–635 Ma). These glaciations are recorded by globally distributed glacial deposits overlain by cap carbonates. According to the snowball Earth hypothesis, cap carbonate deposition was driven by intense continental weathering during deglaciation, but geochemical evidence is lacking. Using Mg isotopes as a proxy, we reconstruct the history of chemical weathering during and following the terminal Cryogenian Marinoan glaciation in South China. Our data confirm an episode of strong chemical weathering during the termination of the 635-Ma Marinoan glaciation, but its climax occurred before cap carbonate deposition. Cryogenian (∼720–635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth’s history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)—indicative of an episode of intense chemical weathering—occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (<+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2 concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition.

Marine Carbon-Sulfur Biogeochemical Cycles during the Steptoean Positive Carbon Isotope Excursion (SPICE) in the Jiangnan Basin, South China

Global occurrences of Steptoean Positive Carbon Isotope Excursion (SPICE) during Late Cambrian recorded a significant perturbation in marine carbon cycle, and might have had profound impacts on the biological evolution. In previous studies, SPICE has been reported from the Jiangnan slope belt in South China. To evaluate the bathymetric extent of SPICE, we investigate the limestone samples from the upper Qingxi Formation in the Shaijiang Section in the Jiangnan Basin. Our results show the positive excursions for both carbonate carbon (δ13C) and organic carbon (δ13Corg) isotopes, as well as the concurrent positive shifts in sulfur isotopes of carbonate associated sulfate (CAS, δ34SCAS) and pyrite (δ34Spyrite), unequivocally indicating the presence of SPICE in the Jiangnan Basin. A 4‰ increase in δ13Ccarb of the Qingxi limestone implies the increase of the relative flux of organic carbon burial by a factor of two. Concurrent positive excursions in δ34SCAS and δ34Spyrite have been attributed to the enhanced pyrite burial in oceans with extremely low concentration and spatially heterogeneous isotopic composition of seawater sulfate. Here, we propose that the seawater sulfur isotopic heterogeneity can be generated by volatile organic sulfur compound (VOSC, such as methanethiol and dimethyl sulfide) formation in sulfidic continental margins that were widespread during SPICE. Emission of 32S-enriched VOSC in atmosphere, followed by lateral transportation and aerobic oxidation in atmosphere, and precipitation in open oceans result in a net flux of 32S from continental margins to open oceans, elevating δ34S of seawater sulfate in continental margins. A simple box model indicates that about 35% to 75% of seawater sulfate in continental margins needs to be transported to open oceans via VOSC formation.