Question

In paleoceanographic investigations of the Younger Dryas stadial—an abrupt cooling event approximately 12,900 years ago—researchers long hypothesized that a catastrophic discharge of glacial meltwater from Lake Agassiz into the North Atlantic disrupted the Atlantic Meridional Overturning Circulation (AMOC) by lowering surface salinity. Initial corroboration relied almost exclusively on oxygen isotope ratios ($\delta^{18}\text{O}$) preserved in the calcite shells of planktonic foraminifera. Because glacial meltwater is heavily depleted in $^{18}\text{O}$, localized negative $\delta^{18}\text{O}$ anomalies were interpreted as unambiguous evidence of meltwater routing through the St. Lawrence River.

However, subsequent spatial mapping of benthic $\delta^{18}\text{O}$ records revealed that temperature-induced fractionation during calcite precipitation can mimic isotopic meltwater signals, obscuring whether salinity variations or localized thermal shifts drove the planktonic anomalies. To decouple these variables, recent studies have integrated neodymium isotopic composition ($\epsilon_{\text{Nd}}$) extracted from deep-sea sediment coatings. Unlike $\delta^{18}\text{O}$, $\epsilon_{\text{Nd}}$ is unaffected by temperature, evaporation, or biological fractionation; instead, it reflects the distinct continental age and lithology of the drainage basin through which weathering runoff enters the ocean. Radiogenic $\epsilon_{\text{Nd}}$ signatures indicative of runoff derived from the Canadian Shield were detected along the Mackenzie River corridor discharging into the Arctic Ocean, while contemporaneous St. Lawrence sediment cores exhibited non-radiogenic signatures typical of baseline North Atlantic deep water. This geochemical divergence suggests that while freshwater forcing did decelerate the AMOC, the primary discharge pathway was rerouted northward rather than eastward, thereby invalidating earlier models that treated the St. Lawrence outlet as the exclusive catalyst for the circulation shutdown.

Based on the passage, which of the following statements can be logically inferred regarding the paleoceanographic evidence from the Younger Dryas? Select all that apply.