Carbonate–organic decoupling during the first Neoproterozoic carbon isotope excursion

Abstract

The Neoproterozoic Era (1000–539 Ma) experienced repeated negative carbon isotope excursions, representing the largest negative excursions in Earth history and their origins remain enigmatic. One salient observation is that unlike younger Phanerozoic carbon isotope excursions that almost exclusively exhibit covariation in the carbon isotopes of carbonate and organic carbon, about half of Neoproterozoic excursions are expressed in carbonate carbon isotopes (δ13Ccarb) but their organic carbon isotopes (δ13Corg) lack the same anomaly. Documenting which excursions exhibit carbonate–organic coupling or decoupling may provide a possible clue as to the enigmatic dynamics of the anomalous Neoproterozoic carbon cycle. We report here that a newly recognized and the first Neoproterozoic carbon isotope anomaly also exhibits decoupling. Thus, the ca. 940 Ma Majiatun anomaly now represents the oldest-known decoupled Neoproterozoic excursion—300 m.y. earlier than previously thought. Ruling out altogether or mostly merely localized artifacts such as diagenesis, changes in water depth, and contamination of detrital organic matter, we interpret this new evidence of carbonate–organic δ13C decoupling in the context of other anomalies through time. Although the remineralization of a large pool of dissolved organic carbon (DOC) was proposed to explain late Neoproterozoic δ13C decoupled anomalies, our earlier evidence of decoupling suggests this pool was much more ancient. Our new data also require the temporal oscillation between decoupling and coupling of the Neoproterozoic carbon cycle, implying a dynamic diminution of the large Precambrian DOC pool reflective of the rising but equally variable oxygenation of the oceans and rise of animal life at this time.