Question

Passage:

The Neoproterozoic era, spanning roughly 1,000 to 541 million years ago, contains geological evidence of the most severe glaciations in Earth's history, inspiring the "Snowball Earth" hypothesis. This model posits that runaway ice-albedo feedback loops resulted in global ice cover reaching equatorial latitudes. A critical pillar of empirical support for the Snowball Earth hypothesis rests upon the ubiquitous presence of "cap carbonates"—distinctive sequences of dolostone and limestone that directly overlay Neoproterozoic glacial deposits worldwide. The abrupt transition from glacial diamictites to warm-water carbonate precipitates suggests a violent and rapid greenhouse climate reversal, theoretically driven by the accumulation of volcanic carbon dioxide ($CO_2$) in an atmosphere decoupled from the silicate weathering cycle by planetary ice cover.

Beyond their stratigraphic position, cap carbonates exhibit profound geochemical anomalies, most notably a sharp negative excursion in carbon-13 ($\delta^{13}C$) isotopes. In standard marine environments, the burial of organic carbon—which preferentially sequesters the lighter carbon-12 isotope—leaves the dissolved inorganic carbon pool of the ocean enriched in carbon-13. Consequently, typical marine carbonates display positive $\delta^{13}C$ values. The precipitous drop to negative $\delta^{13}C$ values in cap carbonates was initially interpreted by proponents of the Snowball Earth hypothesis as a reflection of a lifeless ocean, where the cessation of photosynthetic biological productivity during the global freeze halted organic carbon burial, causing the oceanic reservoir to equilibrate with the mantle's baseline isotopic signature.

However, subsequent sedimentological analyses have introduced significant complications to this primary isotopic signal interpretation. Critics of the hard Snowball model argue that the negative $\delta^{13}C$ excursion may not solely record the primary chemistry of the Neoproterozoic ocean, but rather reflects secondary diagenetic processes. Specifically, meteoric fluid flow—the percolation of fresh groundwater through the porous carbonate platforms during episodes of sea-level drawdown—can introduce isotopically light carbon derived from terrestrial organic matter into the rock matrix long after its initial deposition. Furthermore, the localized presence of highly depleted carbon signatures in methane-derived authigenic carbonates (seep deposits) within some cap sequences suggests that the destabilization of marine gas hydrates during post-glacial warming could have injected massive volumes of isotopically light carbon into the ocean, generating a localized rather than global excursion.

To distinguish between a primary global oceanic signal and secondary local alterations, geochemists increasingly rely on the analysis of trace elements, such as strontium and rare earth elements, which are less susceptible to post-depositional isotopic exchange than carbon. Where negative carbon isotope excursions are demonstrably coupled with primary marine strontium signatures, the case for a genuine whole-ocean isotopic shift is strengthened. Conversely, where carbon isotope values covary significantly with indicators of fluid alteration, such as elevated manganese-to-strontium ratios, the primary nature of the $\delta^{13}C$ excursion is profoundly undermined, suggesting that the dramatic geochemical signature often cited as proof of a lifeless Snowball ocean may, in some strata, be a diagenetic artifact.

According to the passage, which of the following provides evidence that the negative $\delta^{13}C$ values found in some cap carbonates might be a result of localized phenomena rather than a global characteristic of the Neoproterozoic ocean?