Extensive primary production promoted the recovery of the Ediacaran Shuram excursion

Member IV of the Ediacaran Doushantuo Formation records the recovery from the most negative carbon isotope excursion in Earth history. However, the main biogeochemical controls that ultimately drove this recovery have yet to be elucidated. Here, we report new carbon and nitrogen isotope and concentration data from the Nanhua Basin (South China), where δ13C values of carbonates (δ13Ccarb) rise from − 7‰ to −1‰ and δ15N values decrease from +5.4‰ to +2.3‰. These trends are proposed to arise from a new equilibrium in the C and N cycles where primary production overcomes secondary production as the main source of organic matter in sediments. The enhanced primary production is supported by the coexisting Raman spectral data, which reveal a systematic difference in kerogen structure between depositional environments. Our new observations point to the variable dominance of distinct microbial communities in the late Ediacaran ecosystems, and suggest that blooms of oxygenic phototrophs modulated the recovery from the most negative δ13Ccarb excursion in Earth history.

Early Eocene Palynology from Mead Stream, New Zealand

<p>This study documents the first detailed palynological analysis of early Eocene strata from the Lower Marl unit at Mead Stream, southern Marlborough, New Zealand. Examination of marine palynomorph assemblages and palynofacies analysis were used to improve biostratigraphic resolution and investigate paleoclimate across the Early Eocene Climatic Optimum (EECO; ~53–49 Ma)—a period characterised by the highest temperatures of the Paleogene. Early Eocene New Zealand dinocyst zones from NZE2–NZE4, and the Charlesdowniea coleothrypta zone are established across the Lower Marl for the first time in this study. Marine palynomorph assemblages originating in outer-neritic conditions and deposited on the upper slope represent low energy transport along the margin of a terrestrial discharge plume. Palynomorph assemblages do not provide insight into sea surface temperature (SST) trends. Palynomorph assemblages may reflect extremely low surface productivity. Cycles in organic matter between marine-dominant limestones to terrestrial-dominant marls, controlled by changes in temperature and seasonal precipitation, may represent orbital forcing by way of precession cycles; however the limited range of high resolution data from this study cannot statistically confirm this. An overall increase of allochthonous organic matter across the Lower Marl is likely representative of long-term background warming, culminating in peak EECO temperatures. The base of a hyperthermal, represented by carbon isotope excursion (CIE) previously identified at Mead Stream has been redefined in this study. High abundances of warm water, extreme salinity taxa coincide with the onset of this warming event.</p>

Early Eocene Palynology from Mead Stream, New Zealand

<p>This study documents the first detailed palynological analysis of early Eocene strata from the Lower Marl unit at Mead Stream, southern Marlborough, New Zealand. Examination of marine palynomorph assemblages and palynofacies analysis were used to improve biostratigraphic resolution and investigate paleoclimate across the Early Eocene Climatic Optimum (EECO; ~53–49 Ma)—a period characterised by the highest temperatures of the Paleogene. Early Eocene New Zealand dinocyst zones from NZE2–NZE4, and the Charlesdowniea coleothrypta zone are established across the Lower Marl for the first time in this study. Marine palynomorph assemblages originating in outer-neritic conditions and deposited on the upper slope represent low energy transport along the margin of a terrestrial discharge plume. Palynomorph assemblages do not provide insight into sea surface temperature (SST) trends. Palynomorph assemblages may reflect extremely low surface productivity. Cycles in organic matter between marine-dominant limestones to terrestrial-dominant marls, controlled by changes in temperature and seasonal precipitation, may represent orbital forcing by way of precession cycles; however the limited range of high resolution data from this study cannot statistically confirm this. An overall increase of allochthonous organic matter across the Lower Marl is likely representative of long-term background warming, culminating in peak EECO temperatures. The base of a hyperthermal, represented by carbon isotope excursion (CIE) previously identified at Mead Stream has been redefined in this study. High abundances of warm water, extreme salinity taxa coincide with the onset of this warming event.</p>

New Chronostratigraphic Constraints on the Lower Jurassic Pliensbachian–Toarcian Boundary at Chacay Melehue (Neuquén Basin, Argentina)

The Pliensbachian–Toarcian boundary interval is characterized by a ~3‰ negative carbon-isotope excursion (CIE) in organic and inorganic marine and terrestrial archives from sections in Europe, such as Peniche (Portugal) and Hawsker Bottoms, Yorkshire (UK). A new high-resolution organic-carbon isotope record, illustrating the same chemostratigraphic feature, is presented from the Southern Hemisphere Arroyo Chacay Melehue section, Chos Malal, Argentina, corroborating the global significance of this disturbance to the carbon cycle. The negative carbon-isotope excursion, mercury and organic-matter enrichment is accompanied by high-resolution ammonite and nannofossil biostratigraphy together with U-Pb CA-ID-TIMS geochronology derived from intercalated volcanic ash beds. A new age of ~183.71 ± 0.40/-0.51 Ma for the Pliensbachian–Toarcian boundary, and 182.77 +0.11/-0.21 for the tenuicostatum–serpentinum zonal boundary, is assigned based on high-precision U-Pb zircon geochronology and a Bayesian Markov chain Monte Carlo (MCMC) stratigraphic age model.

Late Ediacaran occurrences of the organic-walled microfossils Granomarginata and flask-shaped Lagoenaforma collaris gen. et sp. nov.

New occurrences of flask-shaped and envelope-bearing microfossils, including the predominantly Cambrian taxon Granomarginata, are reported from new localities, as well as from earlier in time (Ediacaran) than previously known. The stratigraphic range of Granomarginata extends into the Cambrian System, where it had a cosmopolitan distribution. This newly reported Ediacaran record includes areas from Norway (Baltica), Newfoundland (Avalonia) and Namibia (adjacent to the Kalahari Craton), and puts the oldest global occurrence of Granomarginata in the Indreelva Member (< 563 Ma) of the Stáhpogieddi Formation on the Digermulen Peninsula, Arctic Norway. Although Granomarginata is rare within the assemblage, these new occurrences together with previously reported occurrences from India and Poland, suggest a potentially widespread palaeogeographic distribution of Granomarginata through the middle–late Ediacaran interval. A new flask-shaped microfossil Lagoenaforma collaris gen. et sp. nov. is also reported in horizons containing Granomarginata from the Stáhpogieddi Formation in Norway and the Dabis Formation in Namibia, and flask-shaped fossils are also found in the Gibbett Hill Formation in Newfoundland. The Granomarginata–Lagoenaforma association, in addition to a low-diversity organic-walled microfossil assemblage, occurs in the strata postdating the Shuram carbon isotope excursion, and may eventually be of use in terminal Ediacaran biostratigraphy. These older occurrences of Granomarginata add to a growing record of body fossil taxa spanning the Ediacaran–Cambrian boundary.

A prolonged, two-step oxygenation of Earth’s early atmosphere: Support from confidence intervals

The Great Oxidation Event (GOE), among Earth’s most transformative events, marked the sustained presence of oxygen above 10–5 times the present atmospheric level. Estimates of the onset of the GOE span 2501–2225 Ma and are based primarily on the loss of mass-independent fractionation of sulfur isotopes (MIF-S) in pyrite. To better constrain the timing of the GOE, we apply probabilistic techniques to estimate the confidence intervals of four proxies: MIF-S, redox-sensitive detrital minerals, "red beds," and I/(Ca + Mg). These GOE proxies are drawn from a highly fragmentary geologic record, and consequently, estimates of the 95% confidence intervals span tens to hundreds of millions of years—orders of magnitude larger than suggested by radiometric constraints on individual successions. Confidence interval results suggest that red beds and nonzero I/(Ca + Mg) values may have appeared earlier than 2480 Ma and 2460 Ma, respectively, whereas redox-sensitive detrital minerals and MIF-S may have disappeared after 2210 Ma and 2190 Ma, respectively. These data suggest a delay of potentially &gt;300 m.y. between initial and permanent oxygenation of the atmosphere and a delay of tens of millions of years between onset of the Lomagundi-Jatuli carbon isotope excursion and permanent oxygenation of the atmosphere.

A rapid sedimentary response to the Paleocene-Eocene Thermal Maximum hydrological change: new data from alluvial units of the Tremp-Graus Basin (Spanish Pyrenees)

A massive emission of light carbon about 56 Ma ago, recorded in marine and terrestrial sediments by a negative carbon isotope excursion (CIE), caused a short-lived (~170 kyr) global warming event known as the Paleocene–Eocene Thermal Maximum (PETM). The core of this event is represented in the south Pyrenean Tremp-Graus Basin by two successive alluvial units, the Claret Conglomerate (CC) and the Yellowish Soils, which represent laterally juxtaposed depositional environments. It is generally agreed that these units record a dramatic increase in seasonal rain and an increased intra-annual humidity gradient during the PETM, but the timing of the sedimentary response to the hydrological change is a matter of debate. Some authors maintain that the CC was developed during the early, most intense phase of the carbon emission, others that its formation lagged by 16.5 ± 7.5 kyr behind the onset of the PETM. The latter claim was mainly based on the assumption that in two sections of this basin, Claret and Tendrui, the onset of the CIE occurs 3 and 8 m below the base of the CC, respectively. Here we show that in the zone between these two sections the CC is missing and the Yellowish Soil unit rests directly and conformably on the underlying deposits. New d13Corg data from this zone provide sound evidence that the onset of the CIE is situated just ~1 m below the Yellowish Soils. The CC erosional base cuts down deeper than this figure, rendering it highly unlikely the preservation of the CIE onset below it. A tentative estimate based on sedimentation rates indicates that ~3.8 kyr, or less, may have elapsed from the onset of the CIE to the arrival of PETM alluvium into the Claret-Tendrui study area, about a third of the lowest estimate of previous authors. Since the study area was situated about 15 km from the source area, our new estimate supports a rapid response of the sedimentary system to the hydrological change at the onset of the PETM.