Global trends in nutrient dynamics during the Ediacaran / Cambiran period as revealed in nitrogen isotope signatures
Zusammenfassung der Projektergebnisse
Three important results can be extracted which in turn contribute directly to the better understanding of changes in the redox conditions in the global oceans and concomitant biotic evolutionary patterns: a.) Chemostratigraphic analyses document that the sections from South China and the Kazakh Kyrshabakty sections most likely span across the Pc-C boundary interval and thus serve as new representatives for global Pc-C boundary sections. Isotope records from both sedimentary basins show a prominent δ13Ccarb negative excursion down to ~-6‰ that correlates with the global negative δ13C excursion associated with the approximate beginning of the lower Cambrian. Bio- and chemostratigraphic analyses of the Gaojiashan section imply the necessity for stratigraphic reinvestigation in southern Shaanxi Province. In Kazakhstan, isotope results establish the first high-resolution global chemostratigraphic correlation based on three Ediacaran to Cambrian δ13Ccarb negative anomalies which demonstrate unambiguous similarities with isotopic records of South China and Oman. Furthermore, the obtained δ15N dataset is the first broad nitrogen record across the Ediacaran into the early Cambrian outside South China. b.) Both δ15N datasets point out that the Ediacaran nitrogen cycling did generally not differ from the modern nitrogen cycle. Elevated δ15N values >3‰ show that nitrate was present in widespread oceanic parts (except for oxygen minimum zones and anoxic water bodies) similar to today, thus suggesting at least mildly oxygenated surface and intermediate water conditions through most Ediacaran times. However, slightly fluctuating δ15N values during the cap carbonate interval (δ15N ~2‰) and across the Shuram-Wonoka event (δ15N from 1.6 to 3.7‰) at the Kazakh Kyrshabakty section assume frequent variations in the water redox conditions. Because NO3-availability is strongly controlled by the depth of the redox transition zone, the variable presence of oxygen likely facilitated N2-fixation to compensate for (weak) nitrate-limitation during the respective periods. In contrast, across the Pc-C boundary interval nitrogen cycling experienced significant changes indicated by a prominent δ15N negative interval. Isotope values suggest a predominantly anoxic to euxinic photic zone which forced the nitrogen cycle towards strong nitrate-limitation and weak denitrification rates. In this scenario for which the Black Sea serves as a modern analogue, NH 4+ assimilation and N2-fixation are predominantly responsible for the introduction of new nitrogen via oxygenic and anoxygenic photosynthetic bacteria. c.) The large-scale conformity of the δ15N trend from the Ediacaran until the lower Cambrian obtained from Gaojiashan, Kyrshabakty and Berkuty sections as well as Longbizui and Xiaofenghe sections with further datasets from South China implies that obtained δ15N data represent a global signal of perturbation in the past nitrogen cycle. Small-scale fluctuations within the Ediacaran δ15N record may result from regional environmental perturbations of the local nitrogen cycle causing temporal changes of the δ15N n the local nitrate pool or are simply attributed to the different resolution of the datasets. In either way, data imply that the ocean periodically contained enough dissolved oxygen since the early Ediacaran and probably already since the later Neoproterozoic Era to stabilize nitrate similar to the modern oceans.
Projektbezogene Publikationen (Auswahl)
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(2012) On the use of stable nitrogen isotopes in present an past anoxic environments. In: Altenbach, A. V., Bernard, J. M. and Seckbach, J. (eds.) Anoxia, Evidence for eukaryote survival and and paleontological strategies. Book series: Cellular origin, life in extreme habitats and astrobiology, 21, 497-513, Springer, Berlin
Struck, U.
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(2013) Marine biogeochemical cycling during the early Cambrian constrained by a nitrogen and organic carbon isotope study of the Xiaotan section, South China. Precambrian Research 225: 148- 165
Cremonese L., Shields-Zhou G., Struck U., Ling H.-F., Och L., Chen X., and Li D.
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(2014) Nitrogen and organic carbon isotope stratigraphy of the Yangtze Platform during the Ediacaran–Cambrian transition in South China. Palaeogeography, Palaeoclimatology, Palaeoecology 398, 165-186
Cremonese, L., GA Shields-Zhou, U Struck, HF Ling, LM Och
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(2015) Chemo- and biostratigraphy of the Gaojiashan section (northern Yangtze platform, South China): a new Pc-C boundary section. Foss. Rec., 18, 105–117
Gamper, A., U. Struck, F. Ohnemueller, C. Heubeck, and S. Hohl
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(2015) Fluctuations in the marine oxygenation and feedbacks as a control on biological innovation in the late Ediacaran - early Cambrian, South China: Evidence from the nitrogen isotopes. –Precambrian Research, 267: 209-226
Wang, Dan; Ulrich Struck, Hongfei Ling, Tianchen He, Ben Yang, Graham Shields-Zhou, Maoyan Zhu
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Palaeoceanographic controls on spatial redox distribution over the Yangtze Platform during the Ediacaran–Cambrian transition. Sedimentology
Och, Lawrence M; Lorenzo Cremonese, Graham A. Shields-Zhou, Simon W. Poulton, Ulrich Struck, Hongfei Ling, Da Li, Xi Chen, Christina Manning, Matthew Thirlwall, Harald Strauss and Maoyan Zhu
