Dongjie Tang

Organic carbon isotope gradient and ocean stratification across the late Ediacaran-Early Cambrian Yangtze Platform

Organic carbon isotope (δ13Corg) data from two well-preserved sections across a shallow-to-deep water transect of the late Ediacaran-Early Cambrian Yangtze Platform in South China show significant temporal and spatial variations. In the shallow-water Jiulongwan-Jijiapo section, δ13Corg values of the late Ediacaran Dengying Formation range from −29‰ to −24‰. In the deep-water Longbizui section, δ13Corg values from time-equivalent strata of the Dengying Formation are mostly between −35 and −32‰. These new data, in combination with δ13Corg data reported from other sections in South China, reveal a 6‰-8 shallow-to-deep water δ13Corg gradient. High δ13Corg values (>−30‰) occur mostly in shallow-water carbonate rocks, whereas low δ13Corg values (<−32‰) dominate the deep-water black shale and chert. The large temporal and spatial δ13Corg variations imply limited buffering effect from a large dissolved organic carbon (DOC) reservoir that was inferred to have existed in Ediacaran-Early Cambrian oceans. Instead, δ13Corg variations between platform and basin sections are more likely caused by differential microbial biomass contribution to total organic matter. High δ13Corg values (>−30‰) documented from shallow-water carbonates are within the range of typical Phanerozoic δ13Corg data and may record the isotope signature of organic matter from primary (photosynthetic) production. In contrast, low δ13Corg values (<−32‰) from deep-water sections may have resulted from higher chemoautotrophic or methanotrophic biomass contribution to bulk organic matter in anoxic environments. The δ13Corg data provide indirect evidence for ocean stratification and episodic chemocline fluctuations in the Ediacaran-Early Cambrian Yangtze Platform.

Microfabrics in Mesoproterozoic microdigitate stromatolites: Evidence of biogenicity and organomineralization at micron and nanometer scales

ABSTRACT Microdigitate stromatolites (MDS) are common in Neoarchean–Paleoproterozoic successions but declined and gradually disappeared in Meso- and Neoproterozoic carbonates. The abundance of well-preserved fibrous fabrics and the absence of identifiable microbial fossils in MDS have been taken as evidence of their abiotic origin in carbonate-supersaturated and anoxic Precambrian oceans. Micron- and nanometer-scale features of MDS from the Mesoproterozoic Wumishan Formation (ca. 1.45–1.5 Ga) of the North China platform composed of alternating submillimetric dark and light laminasets are morphologically similar to those reported from elsewhere in the geological record. The dark laminasets are micritic and contain abundant fuzzy-edged micropeloids and filaments. The micropeloids are commonly 10–70 µm in diameter and commonly surrounded by thin (<10 µm) rims composed of amorphous micrite or microsparite (some rims now replaced by silica). The filaments are morphologically similar to the bacterial filaments in modern microbialites and contain kerogenous components as indicated by Raman spectrometry analysis. All the laminasets are characterized by fibrous fabrics that are expressed by alternating brown fibers (<10–25 µm) and light microsparitic strips of approximately equal width in transverse direction. Filaments, relics of putative extracellular polymeric substances (EPS), micropeloids, and nanoglobules are closely associated with the brown fibers. These organomineralization-related features suggest a biogenic origin for the MDS of the Wumishan Formation and may have an implication to other MDS from the Neoarchean–Paleoproterozoic successions. Microbially induced micro- and ultrastructures including fibrous fabrics, filaments, micropeloids, and nanoglobules are best preserved in silicified MDS samples, implying that early silicification is critical for the preservation and recognition of organominerals.

Nitrogen Isotope Evidence for Redox Variations at the Ediacaran-Cambrian Transition in South China

Nitrogen isotope (δ15N) analyses of the early Cambrian (ca. 542–520 Ma) strata in the Yangtze Gorges area, south China, reveal a prominent positive δ15N excursion (≥+8%), concurrent with a negative shift in organic carbon isotopes (δ13Corg) at the early Nemakit-Daldynian or early Fortunian stage. This positive δ15N excursion, in combination with the previously reported paired δ13Ccarb-δ13Corg at this interval, can be interpreted as resulting from denitrification with large isotopic fractionation. Subsequent δ15N from early Nemakit-Daldynian to early Tommotian (stage 2) are dominated by stable values fluctuating between −2‰ and +2‰, close to that of atmosphere N2 (∼0‰) accompanied by the low and stable δ13Corg (∼−33‰) and decoupled δ13Ccarb-δ13Corg. We argue that the relatively low and invariant δ15N values from this interval likely resulted from enhanced N fixation with limited isotopic fractionation. Enhanced N fixation is generally associated with significant nutrient N loss mainly through denitrification and anammox, indicating the expansion of an anoxic environment. The δ15N records of the study sections show large difference from that of potential time-equivalent Zhujiaqing Formation in the Xiaotan section. This discrepancy can be plausibly attributed to different redox conditions in a stratified ocean inherited from the Ediacaran. Anoxic ocean conditions may have lasted for millions of years until the early Atdabanian (stage 3), when trilobites started to dominate the Cambrian ocean.

Ferruginous seawater facilitates the transformation of glauconite to chamosite: An example from the Mesoproterozoic Xiamaling Formation of North China

Abstract Berthierine and chamosite are iron-rich clay minerals that share similar chemical compositions. Berthierine forms at low temperature (25–45 °C) during early diagenesis and may transfer to chamosite at temperatures of ≥70 °C. Because the formation of berthierine and chamosite requires significant amount of Fe2+ supply, their presence in marine sediments is often used as a mineral proxy for ferruginous conditions in porewater. Recent studies reveal that the Precambrian oceans were characterized by pervasive ferruginous water-column conditions that may favor the formation of iron-rich clay minerals like berthierine and chamosite. To evaluate if ferruginous water-column conditions in the Precambrian ocean played a role on iron-rich clay mineral formation, we conducted an integrated petrographic, mineralogical, and geochemical study on the chamosite- and glauconite-bearing strata of the Mesoproterozoic Xiamaling Formation (~1.40–1.35 Ga) in North China. Petrographic, XRD, SEM, and EDS analyses show that the chamosites of the Xiamaling Formation was transferred from glauconite, with berthierine as an intermediate mineral phase during early diagenesis. Geochemical analyses indicate that a complete transformation from glauconite-dominated to chamosite-dominated end-members (samples) requires an addition of a large amount of Fe (16.9 wt%), Mg (2.4 wt%), and a small amount of Al (1.4 wt%), but a simultaneous release of Si (11.8 wt%) and K (6.0 wt%). Considering that the glauconite- and chamosite-bearing strata are devoid of iron-rich detrital minerals (e.g., biotite and iron oxides) and lack evidence of hydrothermal alteration, the required Fe2+ for glauconite-berthierine-chamosite transformation was most likely from Fe2+-rich (ferruginous) seawater, which may have promoted glauconite-berthierine transformation at the very early diagenetic stage when Fe2+ exchange between porewater and seawater was still available. This interpretation is consistent with the high FeHR/FeT (but low Fepy/FeHR), Fe/Al, and V/Al ratios from the hosting strata that support ferruginous depositional environments. Because most Precambrian strata have passed the oil window temperature (>50–150 °C), the preservation of berthierine would be rare and chamosite should be the representative iron-rich clay mineral. Thus, the abundance of chamosite in fine-grained, marine siliciclastic sediments may be used as a mineral indicator of ferruginous water-column conditions.