Dynamics of the Neoproterozoic carbon cycle

Abstract

The existence of unusually large fluctuations in the Neoproterozoic (1,000 -543 million years ago) carbon-isotopic record implies strong perturbations to the Earth's carbon cycle. To analyze these fluctuations, we examine records of both the isotopic content of carbonate carbon and the fractionation between carbonate and marine organic carbon. Together, these are inconsistent with conventional, steady-state models of the carbon cycle. The records can be well understood, however, as deriving from the nonsteady dynamics of two reactive pools of carbon. The lack of a steady state is traced to an unusually large oceanic reservoir of organic carbon. We suggest that the most significant of the Neoproterozoic negative carbon-isotopic excursions resulted from increased remineralization of this reservoir. The terminal event, at the ProterozoicCambrian boundary, signals the final diminution of the reservoir, a process that was likely initiated by evolutionary innovations that increased export of organic matter to the deep sea.T he coevolution of the biosphere and geosphere is reflected in large part by changes in the long-term carbon cycle (1). Past changes within the cycle are recorded in the isotopic content of carbonate and organic carbon buried in ancient sediments (2). Extraordinarily large fluctuations occur in the Neoproterozoic [1,000-543 million years ago (Ma)] carbon-isotopic record both immediately preceding the Cambrian diversification of complex animal life (3-5) and in the Ϸ200 million years before it (6). There is much interest in determining not only the cause of these isotopic events (3-9) but how, if at all, they are related to early animal evolution (10).Here we analyze a significant portion of the Neoproterozoic isotopic record by portraying it as a dynamical trajectory in a two-dimensional space indexed by the isotopic content of carbonate carbon and the fractionation between carbonate and marine organic carbon. This depiction, analogous to the construction of phase portraits in dynamical systems theory (11), provides specific predictions for a carbon cycle evolving quasistatically in a succession of steady states. We find that the Cenozoic portion (0-65 Ma) of the carbon-isotopic record satisfies these predictions; however, the Neoproterozoic record does not.The dynamics of a system with two sizeable and isotopically distinct pools of reactive carbon suffice to explain the Neoproterozoic records. Then as now one pool was oceanic and atmospheric CO 2 . Here we show that the other was probably oceanic organic carbon. The isotopic data indicate that this reservoir was large and that its average properties changed slowly. However, fluctuations in its size would have led to major variations in the isotopic record. Moreover, even if the size of the organic reservoir had been perfectly constant, changes in the fractionation associated with organic production would have led to isotopic fluctuations much greater than those predicted by steady-state theory.The fluctuations of greatest interest are those associ...

Dynamic redox conditions control late Ediacaran metazoan ecosystems in the Nama Group, Namibia

“…This has variously been interpreted as being due to oxidation of a substantial reservoir of organic carbon dissolved in the deep ocean (Rothman et al, 2003;Fike et al, 2006), to a large flux of methane released from clathrates (Bjerrum and Canfield, 2011), or to diagenetic phenomena (Derry, 2010). The models of Bristow and Kennedy (2008), however, suggest that there were not enough oxidants available for the model proposed by Fike et al (2006), and thus that the Shuram could not have represented a large scale oxidation event.…”

“…There is also evidence to suggest that mid-depth euxinia (free-sulphide in the water column) may have been a feature (but possibly temporally restricted) along some continental shelves at certain times in the Neoproterozoic, but these conditions were sparse compared to the preceding Mesoproterozoic (Canfield et al 2008;Johnston et al, 2010;Li et al, 2010;Sperling et al, 2013a). The evolution of large metazoans and skeletal hardparts during the Ediacaran period was therefore set within the framework of major perturbations to the C, Fe and S cycles (Des Marais et al 1992;Logan et al 1995;Rothman et al, 2003;Fike et al, 2006;Canfield et al, 2007) which are all potentially linked to rising oxygen.…”

“…There is also evidence to suggest that mid-depth euxinia (free-sulphide in the water column) may have been a feature (but possibly temporally restricted) along some continental shelves at certain times in the Neoproterozoic, but these conditions were sparse compared to the preceding Mesoproterozoic (Canfield et al 2008;Johnston et al, 2010;Li et al, 2010;Sperling et al, 2013a). The evolution of large metazoans and skeletal hardparts during the Ediacaran period was therefore set within the framework of major perturbations to the C, Fe and S cycles (Des Marais et al 1992;Logan et al 1995;Rothman et al, 2003;Fike et al, 2006;Canfield et al, 2007) which are all potentially linked to rising oxygen.Oxygen requirements in metazoans vary widely, and are determined by size, metabolism, mobility, and the presence or absence of a circulatory system (Vaquer-Sunyer and Duarte, 2008). As a result it has been proposed that metazoans may have been limited to small size (<3 mm), thin body plans, and low diversity communities with simple foodwebs by the relatively low levels of oxygen of the Proterozoic, with the explosion of larger and ecologically diverse organisms in the late Ediacaran and Cambrian related, in part, to increasing oxygen levels (e.g.…”

Evolution of Neoproterozoic Wonoka–Shuram Anomaly-aged carbonates: Evidence from clumped isotope paleothermometry

“…The origin of one of the most unusual carbon isotope features in the geologic record, the ∼580 million-year-old Wonoka-Shuram Anomaly, is debated as primary by some (Rothman et al, 2003;Fike et al, 2006;Kaufman et al, 2007) and diagenetic by others (Burns and Matter, 1993;Knauth and Kennedy, 2009;Derry, 2010). The association of the anomaly, the largest negative ␦ 13 C carb excursion in Earth's history , with the emergence and diversification of metazoan life makes it of broad interest.…”

“…If representative of ancient ocean chemistry, the carbon cycle must have been vastly different from today to allow such massive carbon isotope swings, and there is no shortage of hypotheses to explain them. These include interpretations such as volcanic carbon dioxide buildup linked to extensive glaciation (e.g., the "snowball Earth" hypothesis) (Hoffman et al, 1998), the release of methane (Kennedy et al, 2001), the partial oxidation of a large pool of dissolved organic carbon (Rothman et al, 2003), and the weathering of fossil organic matter with increasing oxygen in the atmosphere . In the absence of robust biostratigraphy and adequate radiometric dates, the Wonoka-Shuram and similar excursions form the backbone of carbon isotope chemostratigraphy, an approach that has been widely employed to correlate strata from place to place during this critical time in earth's history (Knoll, 2000).…”