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 δ13Ccarb, δ13Corg, δ34Spyr, δ34
Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (
- PAR ID:
- 10033966
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Geobiology
- Volume:
- 15
- Issue:
- 2
- ISSN:
- 1472-4677
- Page Range / eLocation ID:
- p. 211-224
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract SCAS , and δ15Nseddata 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 δ13Ccarbin the lower Doushantuo/Lantian formations, but this gradient is probably due to the greater sensitivity of carbonate‐poor deep‐water sediments to isotopic mixing with13C‐depleted carbonate cements. A pronounced negative δ13Ccarbexcursion (EN 3) in the upper Doushantuo/Lantian formations, however, is spatially consistent and may be an equivalent of the Shuram excursion. δ34Spyris 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 δ34Spyrthan that formed within shallow‐water sediments. The Lantian Formation was probably deposited in oscillating oxic and euxinic conditions. Euxinic black shales have higherTOC andTN contents, but lower δ34Spyrand δ15Nsedvalues. In euxinic environments, pyrite was predominantly formed in the water column and organic nitrogen was predominantly derived from nitrogen fixation orNH 4+assimilation because of quantitative denitrification, resulting in lower δ34Spyrand δ15Nsedvalues. 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. -
Abstract Compound‐specific stable isotope analysis (
CSIA ) of amino acids (AA ) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns inAA isotope fractionation in birds. We conducted a controlledCSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua ), to examine patterns in individualAA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ13CC‐Dand Δ15NC‐D, respectively). We found that essentialAA δ 13C values and sourceAA δ 15N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessentialAA Δ13CC‐Dvalues reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDF Glu‐Phe= 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ15NC ‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDFG lu‐Pheequation with the avian‐specificTDFG lu‐Phevalue from our experiment provided estimates that were more ecologically realistic than estimates using a singleTDFG lu‐Pheof 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use ofCSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers. -
Rationale It is imperative to understand how chemical preservation alters tissue isotopic compositions before using historical samples in ecological studies. Specifically, although compound‐specific isotope analysis of amino acids (CSIA‐AA) is becoming a widely used tool, there is little information on how preservation techniques affect amino acid
δ 15N values.Methods We evaluated the effects of chemical preservatives on bulk tissue
δ 13C andδ 15N and amino acidδ 15N values, measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS), of (a) tuna ( ) and squid (Thunnus albacares ) muscle tissues that were fixed in formaldehyde and stored in ethanol for 2 years and (b) two copepod species,Dosidicus gigas andCalanus pacificus , which were preserved in formaldehyde for 24–25 years.Eucalanus californicus Results Tissues in formaldehyde‐ethanol had higher bulk
δ 15N values (+1.4, ; +1.6‰,D. gigas ), higherT. albacares δ 13C values for (+0.5‰), and lowerD. gigas δ 13C values for (−0.8‰) than frozen samples. The bulkT. albacares δ 15N values from copepods were not different those from frozen samples, although theδ 13C values from both species were lower (−1.0‰ for and −2.2‰ forE. californicus ) than those from frozen samples. The mean amino acidC. pacificus δ 15N values from chemically preserved tissues were largely within 1‰ of those of frozen tissues, but the phenylalanineδ 15N values were altered to a larger extent (range: 0.5–4.5‰).Conclusions The effects of preservation on bulk
δ 13C values were variable, where the direction and magnitude of change varied among taxa. The changes in bulkδ 15N values associated with chemical preservation were mostly minimal, suggesting that storage in formaldehyde or ethanol will not affect the interpretation ofδ 15N values used in ecological studies. The preservation effects on amino acidδ 15N values were also mostly minimal, mirroring bulkδ 15N trends, which is promising for future CSIA‐AA studies of archived specimens. However, there were substantial differences in phenylalanine and valineδ 15N values, which we speculate resulted from interference in the chromatographic resolution of unknown compounds rather than alteration of tissue isotopic composition due to chemical preservation. -
Summary Traditionally, leaves were thought to be supplied with
CO 2for photosynthesis by the atmosphere and respiration. Recent studies, however, have shown that the xylem also transports a significant amount of inorganic carbon into leaves through the bulk flow of water. However, little is known about the dynamics and proportion of xylem‐transportedCO 2that is assimilated, vs simply lost to transpiration.Cut leaves of
Populus deltoides andBrassica napus were placed in eitherKC l or one of three [NaH13CO 3] solutions dissolved in water to simultaneously measure the assimilation and the efflux of xylem‐transportedCO 2exiting the leaf across light andCO 2response curves in real‐time using a tunable diode laser absorption spectroscope.The rates of assimilation and efflux of xylem‐transported
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Abstract The Smithian–Spathian boundary interval is characterised by a positive carbon isotopic excursion in both δ13Ccarband δ13Corg, concurrent with a major marine ecosystem reorganisation and the resurgence of microbialite facies. While these δ13C records have been traditionally interpreted as capturing global carbon cycle behaviour, recent studies have suggested that at least some excursions in early Triassic δ13C values may incorporate influences from authigenic or early diagenetic processes. To examine the mechanistic drivers of Smithian–Spathian boundary geochemistry, the carbonate geochemistry of a core from Georgetown, Idaho (USA), was analysed using a coupled δ44/40Ca, δ26Mg and trace‐metal framework. While the δ13C record in the Georgetown core is broadly similar to other Smithian–Spathian boundary sections, portions of the record coincide with substantial shifts in δ44/40Ca, δ26Mg and trace‐metal compositions that cannot feasibly be interpreted as primary. Furthermore, these geochemical variations correspond with lithology: The δ13C record is modulated by variations in the extent of dolomitisation, and the diagenetic styles recognised here coincide with individual lithostratigraphic units. A primary shift in local sea water δ13C values is inferred from the most geochemically unaltered strata, from
ca 3‰ in the middle Smithian toca 5‰ in the early Spathian, although the timing and pathway through which this occurs cannot be readily identified nor extrapolated globally. Therefore, the Georgetown core may not directly record exogenic carbon cycle evolution, showing that there is a need for the careful reconsideration of the Smithian–Spathian boundary—and more broadly, Early Triassic—geochemical records to examine potential local and diagenetic influences on sedimentary geochemistry.