Question

Passage:

In the mid-twentieth century, paleoclimatology underwent a paradigm shift following Cesare Emiliani’s pioneering analysis of oxygen isotope ratios ($\delta^{18}\text{O}$) in fossilized planktonic foraminifera retrieved from deep-sea sediment cores. Emiliani posited that variations in the ratio of $^{18}\text{O}$ to $^{16}\text{O}$ preserved in calcium carbonate shells primarily reflected past fluctuations in sea-surface temperatures, thereby offering a continuous quantitative record of Pleistocene glacial-interglacial oscillations. However, this temperature-centric interpretation was subsequently challenged by geophysicist Nicholas Shackleton. Utilizing benthic foraminifera from bottom waters—where temperature changes were negligible—Shackleton demonstrated that the preferential evaporation and continental sequestration of lighter $^{16}\text{O}$ during glacial advances fundamentally altered the isotopic composition of the ocean itself. Consequently, Shackleton established that marine $\delta^{18}\text{O}$ variations predominantly record shifts in global continental ice volume rather than localized sea-surface thermal changes. Although Emiliani’s original hypothesis catalyzed interest in oceanic paleoclimate proxies, Shackleton’s isotopic recalibration proved essential for synchronizing deep-sea records with orbital climate theory, ultimately reshaping twentieth-century climatological methodology.

Which of the following statements accurately reflect the primary purpose or central thesis of the passage? Select all that apply.