How Long did the Ediacaran NeoProterozoic Shuram Event Last?

Rock magnetic cyclostratigraphy of the Doushantuo Formation, South China and its implications for the duration of the Shuram carbon isotope excursion

Authors:

Gong et al

Abstract:

The Shuram excursion (SE), one of the largest-known negative carbon isotope anomalies, has been globally observed in Ediacaran rocks. Precisely determining the duration of the SE is pivotal to understanding its controversial origin. Here, we present a detailed paleomagnetic, rock magnetic, cyclostratigraphic and carbon isotopic study of the SE in the Doushantuo Formation at the Dongdahe section in eastern Yunnan Province, South China. Although paleomagnetic results likely show a late Mesozoic remagnetization, careful mineralogic analyses indicate that the rock magnetic cyclostratigraphy carried by detrital pseudo-single domain (SD) or small multidomain (MD) titanomagnetite grains faithfully records orbitally-forced climate cycles in the Ediacaran. Multi-taper method (MTM) spectral analysis of magnetic susceptibility (MS) and anhysteretic remanent magnetization (ARM) series reveals significant spectral peaks at similar frequencies. Based on the ratios of their frequencies, these spectral peaks are assigned to a suite of Milankovitch cycles (long eccentricity, short eccentricity, obliquity and precession), yielding a sediment accumulation rate of 1.0 cm/kyr for the Doushantuo Formation. A 9.1 ± 1.0 Myr duration is indicated for the entire SE in South China. This result is in good agreement with independent estimates from North America and South Australia, thus supporting a primary origin for the SE. In combination with published geochronologic data, we suggest that the onset of the SE occurred at ca. 560 Ma, which provides a chronostratigraphic framework for evaluating the relationship between the SE and the evolution of metazoans in Ediacaran time.

Is the Shuram Event Still Younger Than the 580 Million Years agoEdiacaran NeoProteorozoic Gaskiers Glaciation Though?

Silicified glendonites in the Ediacaran Doushantuo Formation (South China) and their potential paleoclimatic implications

Authors:

Wang et al

Abstract:

The Ediacaran Period is punctuated by the ca. 580 Ma Gaskiers glaciation in Newfoundland. However, paleoclimatic data are scarce in Ediacaran successions in South China, where abundant geochemical and paleobiological data are shaping current understanding of Ediacaran evolutionary and environmental history. Here, we report the occurrence of silicified glendonites in the Ediacaran Doushantuo Formation deposited in an inner-shelf environment on the South China block. Petrographic evidence suggests that these silicified glendonites are pseudomorphs after syndepositional or early authigenic ikaites formed at near-freezing temperatures. The glendonite-bearing stratigraphic interval is characterized by positive δ13C values. It predates both the negative δ13C excursion EN3 (widely believed to be an equivalent of the Shuram negative excursion) and excursion EN2. Although alternative interpretations may be possible, these glendonites may be related to and correlated with the Gaskiers glaciation. If confirmed, this correlation suggests that the Shuram event postdates the Gaskiers glaciation, thus having important implications for Ediacaran climate changes, carbon cycles, and biological evolution.

Could the mid Ediacaran Shuram Event be Significantly Older Than 551 Million Years Ago?

The stratigraphic complexity of the middle Ediacaran carbon isotopic record in the Yangtze Gorges area, South China, and its implications for the age and chemostratigraphic significance of the Shuram excursion

Authors:

Zhou et al

Abstract:

The middle Ediacaran Shuram excursion represents the most pronounced negative carbon isotopic shift in Earth history, and has been considered as evidence for a profound disturbance to the global carbon cycle and proposed as a key chemostratigraphic marker for Ediacaran stratigraphic subdivision and global correlation. Previous study has revealed a pronounced negative δ13C shift (EN3) in the upper Doushantuo Formation of South China, which has been interpreted as an equivalent of the Shuram excursion. Detailed δ13C investigation of multiple sections of the Ediacaran Doushantuo Formation around the Huangling Anticline, western Hubei Province, South China, indicates that the δ13C variation in the upper Doushantuo Formation is more complex than previously reported. In the western region of the Huangling anticline, a moderately positive δ13C excursion (here termed the Diaoyapo positive excursion) bisects two strongly negative δ13C excursions (here termed the Jiuqunao negative excursion and the younger Miaohe negative excursion), with a ∼551 Ma ash bed capping the Miaohe negative excursion. In the central region, there is only one negative δ13C excursion, whereas in the eastern region, the negative δ13C excursion is absent. The variable stratigraphic expression of these δ13C excursions is most parsimoniously interpreted as reflecting local facies variation, diagenesis, and/or a cryptic unconformity. The more complete but more complex δ13C chemostratigraphic record in the western region of Huangling Anticline implies that, depending on how this record is correlated with the Shuram excursion, the latter may be significantly older than 551 Ma if it does represent a global chemostratigraphic marker.

The Strangeness of Organic Matter Cycling During the Ediacaran NeoProterozoic Shuram Event

Marine organic matter cycling during the Ediacaran Shuram excursion

Authors:

Lee et al

Abstract:

Ediacaran (ca. 635–541 Ma) marine carbonates capture a global δ13C carbon isotope excursion to extremely negative values (~–12‰)—known as the Shuram excursion (SE)—that cannot be explained by conventional mass balance scenarios. Furthermore, the carbon isotopic variation of bulk organic matter (OM) does not mirror that of carbonate through the excursion, suggesting that the OM reflects a mixture of different sources. To evaluate this hypothesis, we investigated thermally immature marine sedimentary rocks that record the SE from the Sultanate of Oman. Compound-specific carbon isotopic analyses of the extractable hydrocarbons reveal low δ13C values of long-chain (greater than C20) n-alkanes and mid-chain monomethyl alkanes as low as –40‰. Such light signatures are rare in marine rocks of any age and provide evidence that the SE reflects a primary carbon cycle perturbation. The magnitude of the SE recorded in these organic phases is smaller than observed in carbonate and implies that the primary perturbation to dissolved inorganic carbon (DIC) was at least 5‰–7‰, and more likely 7‰–12‰, in magnitude when correcting for end-member source mixing. Due to isotopic differences in stratigraphic patterns of the different organic compounds, we propose that bulk organic carbon (both bitumen and kerogen) reflects source mixing between two distinct pools that previously masked the excursion in bulk δ13Corg measurements. OM sources were derived both from autotrophs fixing 13C-depleted DIC and from a less 13C-depleted heterotrophic microbial biomass feeding on a marine OM pool sustained by petroleum expelled from older sedimentary OM. Expulsion of these sedimentary fluids also helps explain both the duration and magnitude of the SE.

Did an Impact Cause the Shuram Event in the Ediacaran by Shifting the Earth's Axis?

Evolution of Earth's climatic system: Evidence from ice ages, isotopes, and impacts

Author:

Grant M. Young

Abstract:

Multiple glaciations took place near the beginning and end of the Proterozoic Eon. Neoproterozoic (Cryogenian) glacial deposits are more widespread than those of older (Paleoproterozoic) glacial episodes. Paleomagnetic results suggest that most Proterozoic glaciogenic rocks were deposited at low paleolatitudes. Some contain enigmatic evidence of strong seasonal temperature variations, and many formed at sea level. These attributes inspired both the snowball Earth hypothesis and the high obliquity theory, but only the latter explains strong seasonality at low latitudes. The Proterozoic glaciations may have been triggered by drawdown of atmospheric CO2 during enhanced weathering of elevated supercontinents. Multiple glaciations resulted from a negative feedback loop in the weathering system that ended when the supercontinent broke apart. A radical reorganization of the climatic system took place in the Ediacaran Period. In contrast to previous glaciations, these ice sheets developed in high latitudes and many follow mountain building episodes. During the Ediacaran Period, Earth’s climatic zonation and controls appear to have undergone a radical change that persisted throughout the Phanerozoic Eon. The change may coincide with the world’s greatest negative δ13C excursion, the Shuram event, here interpreted as the result of a very large marine impact that decreased the obliquity of the ecliptic, causing the Earth’s climatic system to adopt its present configuration. Attendant unprecedented environmental reorganization may have played a crucial role in the emergence of complex life forms.