Distinct REY sources recorded in the Doushantuo phosphorite
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During the Ediacaran period, massive marine phosphorites were deposited worldwide, which could represent a global sink of rare earth elements and yttrium (REY) in oceans. In the Doushantuo Formation of South China, the REY source has been preliminarily constrained as the mixed sources of seawater and terrigenous matter in these phosphorites. However, the REY composition of both seawater and terrigenous endmembers is still unclear; therefore, it's difficult to identify the contributions of seawater and terrigenous sourced REY. In this work, REY and other trace elements of bulk phosphorites, in situ carbonates, and apatite grains were analyzed from a drill core (Fangmashan) of the Doushantuo Formation in South China. Combined with mineralogical observations, the REY composition of seawater and terrigenous endmembers is well-constrained. Along the investigated Fangmashan drill core, all in situ carbonates (calcites and dolomites) show a modern seawater-like REY pattern, indicating that the REY characteristics of the Doushantuo seawater could be similar to those of modern oxic seawater without significant changes. Alternatively, apatites from the lower and upper phosphorites showed distinct REY patterns. One type of apatite in the upper phosphorites showed a modern seawater-like REY pattern, indicating authigenic REE enrichment from the seawater column. However, another type of apatite in the upper phosphorites with an REE pattern that deviated from seawater showed high ΣREY (∼691.16 ± 309.78) and low Y/Ho (∼40.52 ± 4.05), probably reflecting mixed REY sources and/or the effects of early diagenesis. It is noted that most apatites in the lower phosphorites are obviously enriched in light REY (LREY: La - Nd) compared to heavy REY (HREY: Ho - Lu and Y), which may represent a significant contribution of the terrestrial REY source. This study reveals that the REY composition of paleo-seawater could be similar to modern seawater as early as the Neoproterozoic and supports a significant contribution to understanding the difference in the REY composition of phosphorites in geological history.Keywords:
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Abstract Measurements of natural cosmogenic 32 Si ( t 1/2 ~ 140 years) in tropical deltaic sediments demonstrate for the first time that most 32 Si is present in rapidly formed authigenic clays and not biogenic opaline silica (bSi). The burial of bSi in deltaic and continental margin sediments has likely been greatly underestimated because of diagenetic alteration of bSi to clay, little of which dissolves in the classically used operational bSi leach. Rapid reverse weathering reactions during early diagenesis must be considered as a significant pathway of reactive Si storage in deltaic deposits. Based on 32 Si, actual storage may be 2–3 times the best recent estimates extrapolated from diagenetic models or attempts to modify operational bSi methods to include authigenic clay (~900 µmol/g versus ~250 µmol/g). Measurements of natural 32 Si inventories in sediments and initial specific activities in biogenic silica provide a means to independently constrain the marine Si cycle.
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Mitridatite from lacustrine sediments in the Shungura Formation of southwestern Ethiopia occurs in small crystals (<2 pm) with hydroxy-apatite and carbonate-apatite. approximate structirral formula for the mitridatite is Ca(HrO)u[(FeBlrMnS.+e)O.(PO.)r] .3HzO, placing the mineral in the mitridatite-robertsite series. Mineral associations in the lacustrine sediments suggest that the mitridatite coprecipitated with hydroxy-apatite, following partial dissolution of carbonate-apatite fish scales and bones.
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No less than 82 species of heavy minerals have been identified from the very fine sand grains in the surface sediments from the Sansha Bay.The most minerals were terrigenous except for a few authigenic minerals.The contents of most minerals varied significantly.Among them,12 assemblages area of heavy minerals were formed under the control of lithological and tidal current.
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The effect of diagenesis upon natural remanent magnetization (NRM) has been studied at two sites within a single bed of dolomite from the Monterey Formation in Santa Barbara County, California. Paleomagnetic direction demonstrates that the NRM is primary at one site and remagnetized at the other site. Magnetic separates from samples carrying a primary NRM yielded detrital magnetite. Separates from samples carrying secondary NRM yielded an unusual form of authigenic magnetite. Evidence suggests that the formation of the authigenic magnetite accompanied late stage dolomite diagenesis. Correlating the remagnetization direction to the paleomagnetic history of the region gives a probable age of 0.72–2.47 m.y.b.p. for both of these diagenetic events.
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Paleoecological interactions among fossil microorganisms have garnered significant interest within the paleobotanical community; however, an understanding of the early diagenesis of associated plant material is of critical importance when assessing putative body fossils of fungi and bacteria.Structures preserved within permineralized petioles of the Carboniferous fern Botryopteris tridentata Felix (Scott) have been interpreted as the earliest remains of Actinobacteria found in association with vascular plants, but re-examination of the specimens indicates instead that these biomimetic structures (BMS) are authigenic carbonate minerals.Using spinning disk confocal microscopy, we generated monochromatic luminescence maps of BMS found within the phloem cells of Botryopteris.Luminescence was captured at wavelengths of 665 nm, consistent with an interpretation of these structures as disordered dolomites, an inference subsequently corroborated with energy-dispersive X-ray spectrometry (SEM-EDS).The presence of high-magnesium carbonates within Botryopteris is suggestive of an early anaerobic stage of plant tissue degradation characterized by metabolic activities of sulfate-reducing bacteria.Anaerobic biodegradation may also have been performed by chytridiomycetes, and we interpret larger (5-8 mm) unicells found within the specimens as fossils of chytrid zoosporangia.Understanding microbial contribution to the early diagenesis of plants preserved within calcium carbonate concretions (coal balls) is dependent upon both characterizing diversity of microbial communities within fossil plants, and elucidating the geomicrobiological parameters of mineralization.As such, this study underscores the necessity of integrating geomicrobiology with plant taphonomy in investigations of the microbial component of ancient ecosystems.
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