How variability in late Neoproterozoic shallow water carbonate δ13Ccarb values and modern fluvial δ13CDIC values may impact fundemental assumptions in carbon isotope stratigraphy




Wren, Olivia

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The carbon isotope composition of ancient shallow water carbonates (δ13Ccarb) serves as the primary archive of ocean chemistry for a majority of Earth history and provides insight into the dynamics of global carbon cycling in deep time. The use of shallow water carbonates in reconstructing ancient ocean chemistry requires that δ13Ccarb values reflect the global mean open-ocean δ13CDIC value (Kump and Arthur, 1999). However, in practice, this assumption is difficult to constrain due to several factors, such as the potential for diagenetic alteration to primary δ13Ccarb values (Ahm et al., 2018; Higgins and Schrag, 2003; Allan and Matthews, 1982), and the possibility that δ13Ccarb values reflect a local, rather than global, marine δ13CDIC signal (Geyman and Maloof, 2019; Patterson and Walter, 1994a; Swart and Eberli, 2005; Swart et al., 2009). It is understood that freshwater input can strongly influence local marine δ13CDIC values and potentially shallow water carbonate δ13Ccarb values in marginal marine environments (Patterson and Walter, 1994a,b), but a global dataset of δ13C values for river DIC has not yet been analyzed. Here, I explore the relationship between fluvial DIC, marginal marine DIC, and shallow water carbonate δ13Ccarb values by analyzing: (1) δ13Ccarb measurements for several stratigraphic sections from a late Neoproterozoic shallow water carbonate platform (Mount Fitzwilliam) and (2) a global compilation of fluvial δ13Ccarb values and associated river chemistry data. Through this analysis, I explore whether ancient shallow water carbonate δ13Ccarb values reflect open-ocean DIC or local processes, what dominant process controls modern fluvial δ13CDIC values in a global synthesis, and whether it is possible to determine the fluvial isotopic influence on ancient shallow water carbonate δ13Ccarb values. The shallow water carbonate δ13Ccarb values measured from Mount Fitzwilliam reveal both facies dependence within single stratigraphic sections and significant lateral variability across stratigraphic sections of putatively similar age. This finding is contrary to the expectation that δ13Ccarb values will be reproducible across laterally displaced sections when values reflect open-ocean DIC. While multiple models are discussed to explain this variability, both field observations and isotope data point to freshwater influence on δ13Ccarb values. The observed variability and facies dependence further complicate the reliability of shallow water carbonate δ13Ccarb values in global carbon cycle reconstructions and stratigraphic correlations. Furthermore, these results emphasize the importance of testing for diagenetic and local signals through the measurement of multiple, parallel sections of ancient shallow water carbonates. The interpretation for a fluvial influence on Mount Fitzwilliam δ13Ccarb values emphasizes the need better understand the relationship between freshwater input and shallow water carbonate δ13Ccarb values. In order to accomplish this, it is first necessary to identify the factors that govern fluvial δ13CDIC values. Analysis of the global fluvial δ13CDIC dataset, which captures 4,730 δ13CDIC measurements from 612 rivers across all seven continents, offers the opportunity to explore the dominant processes that control fluvial DIC and δ13CDIC values. Values of fluvial δ13CDIC in the global synthesis are highly variable, ranging from −27.5‰ to 5.1‰. The mean value from this dataset (−9.3‰) is more negative than the assumed river input value used in classic carbon cycling models (−5‰; Kump and Arthur, 1999). Explanations for the high variability and mean value are explored by comparing fluvial δ13CDIC values with estimated isotopic ranges that reflect the dominant sources of fluvial DIC. Through this analysis, I found that 67% of fluvial δ13CDIC values fit into the biotic carbonate weathering δ13CDIC range (−12.5‰ to −5.5‰). These results show the clear effect of soil respiration in the terrestrial biosphere. The strong role that biotic weathering plays in controlling fluvial δ13CDIC values, and the wide range of δ13CDIC values observed, has serious implications for the interpretation of δ13Ccarb values from shallow water carbonates deposited in near-shore, marginal marine environments, and emphasizes the need to strongly consider the potential for freshwater influence on shallow water δ13Ccarb values in both modern and ancient studies.



Carbon isotopes, δ13CDIC, δ13Ccarb, carbonate, Late Neoproterozoic, Ediacaran, Carbon cycle, long term carbon cycle, paleoclimate, river chemistry, fluvial δ13CDIC, carbon, shallow water carbonate, Earth history, lithostratigraphy, biostratigraphy, Upper Miette Group, biotic weathering, DIC, fluvial DIC, carbon isotope stratigraphy