第六届青年地学论坛

1268 / 2019-08-08 15:20:30

Barium-isotope variations in the Ediacaran carbonates: a novel proxy for paleo-productivity?

14、古生物、古地理与古环境 > 专题8：金属稳定同位素示踪古环境演变

Barium (Ba) isotopes can be used as a potential proxy to trace variations in biological productivities in past oceans, but practical applications are scarce. The Ediacaran Period was marked as dramatic evolutions in the climate, atmospheric and oceanic redox condition, marine carbon cycle, and biosphere, which have been documented in the Ediacaran successions. In this study, we report first-order δ137Ba profiles of carbonates from the Ediacaran Doushantuo and Dengying formations in the Yangtze Gorges area (South China), in an attempt to test the potential of Ba isotopes in carbonates as a paleo-productivity proxy. Detrital and potentially diagenetic effects have been demonstrated to be not responsible for the δ137Ba variations through the whole section. The post-Marinoan carbonates record generally negative authigenic δ137Ba (δ137Baauth) value around -0.2 ‰, reflecting the existence of a replete, homogeneous, and isotopically lighter Ba reservoir in the deglacial ocean, which accumulated during the Marinoan glaciation or even before. Although documenting a prominent negative δ13Ccarb excursion, the Shuram/Wonoka correlative carbonates show minor δ137Baauth fluctuations between -0.01 and 0.26 ‰, possibly due to the local anoxia limiting the biogeochemical cycle of Ba isotopes. The late-Ediacaran carbonates are characterized by the co-elevation of δ137Baauth and Ce anomalies, suggesting the establishment of the modern-ocean-like cycle of Ba isotopes in shallow waters of the Yangtze Platform. This establishment likely resulted from the replacement of cyanobacteria by eukaryotic phytoplankton as the dominant primary producers. Based on these investigations, we suggest that Ba-isotope cycle in marine system is strongly controlled by local redox conditions, and thus Ba isotopes can be used to trace variations in paleo-productivities, especially in oxic marine sediments.