Abstract
Member IV of the Ediacaran Doushantuo Formation records the recovery from the most negative carbon isotope excursion in Earth history. However, the main biogeochemical controls that ultimately drove this recovery have yet to be elucidated. Here, we report new carbon and nitrogen isotope and concentration data from the Nanhua Basin (South China), where δ13C values of carbonates (δ13Ccarb) rise from − 7‰ to −1‰ and δ15N values decrease from +5.4‰ to +2.3‰. These trends are proposed to arise from a new equilibrium in the C and N cycles where primary production overcomes secondary production as the main source of organic matter in sediments. The enhanced primary production is supported by the coexisting Raman spectral data, which reveal a systematic difference in kerogen structure between depositional environments. Our new observations point to the variable dominance of distinct microbial communities in the late Ediacaran ecosystems, and suggest that blooms of oxygenic phototrophs modulated the recovery from the most negative δ13Ccarb excursion in Earth history.
Highlights
Member IV of the Ediacaran Doushantuo Formation records the recovery from the most negative carbon isotope excursion in Earth history
There are various hypotheses proposed to explain the origin of the Shuram excursion, and these can be summarised in two principal groups
The second group, suggests that the excursion is a primary depositional carbon isotope signature related to the oxidation of massive 13C−depleted dissolved organic matter (DOC)[12] or other types of organic carbon pools[13,14] during a globally synchronous ocean oxygenation event[3,5]
Summary
The carbon isotope composition of carbonates and organic matter (δ13Ccarb and δ13Corg) from the proximal sections preserve the most similar signal to the Shuram excursion in Oman[3]. In this environment, values of δ13Ccarb from the uppermost part of Member III to the Member IV-Dengying Fm. boundary exhibit a trend that gradually shifts from −7.1‰ to −0.2‰ (Fig. 1a). Values of δ13Corg of the same interval present a shift from ∼−28‰ to −38.6‰ and recover again to ∼−28‰ at the Member IV-Dengying Fm. boundary (Fig. 1b) These characteristic trends are consistent with previously reported data from nearby outcrop samples[2,5,26,29]. Vertical dashed lines from left to right show Peedee belemnite, isotopic fractionation imparted to biomass by primary producers typically around −27‰, photosynthetic range 22–32‰ based on compilations of Δ13Ccarb-org throughout the Phanerozoic[70], and lastly δ15N for atmospheric N2
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