During the Mid-Pleistocene Transition (MPT), occurring approximately 1.2 to 0.7 million ye… — GMAT

During the Mid-Pleistocene Transition (MPT), occurring approximately $1.2$ to $0.7$ million years ago, Earth's glacial-interglacial cycles shifted from a $41,000$ -year to a $100,000$ -year periodicity. Paleoceanographers long debated whether this transition was driven primarily by internal ice-sheet dynamics or by atmospheric carbon dioxide fluctuations. Recent reconstructions utilizing boron-to-calcium ( $B/Ca$ ) ratios in fossilized shells of the benthic foraminifer *Epistominella exigua* have provided crucial empirical constraints on deep-ocean carbonate chemistry.

Boron incorporates into calcite shells predominantly as the charged borate ion, $B(OH)_4^-$ , whose concentration relative to bicarbonate is directly proportional to seawater $pH$ and carbonate ion concentration ( $[CO_3^{2-}]$ ). By analyzing $B/Ca$ ratios isolated from North Atlantic sediment cores, researchers led by Dr. Elena Rostova established that deep-water $[CO_3^{2-}]$ experienced a marked drop of roughly $15\text{ }\mu\text{mol/kg}$ during marine isotope stage 22 (MIS 22). Crucially, Rostova’s team noted that this drawdown of deep-ocean carbon storage preceded the expansion of continental ice sheets by nearly $20,000$ years.

Previous hypotheses by paleoclimatologist Marcus Thorne had posited that deep-water sequestration of carbon dioxide was merely a secondary consequence of reduced oceanic overturn caused by initial ice-sheet growth. However, Rostova's direct measurements of $B/Ca$ ratios demonstrate that deep-ocean reorganization and carbon storage preceded continental glaciation rather than following it. Moreover, while earlier proxies such as carbon isotope ratios ( $\delta^{13}C$ ) were frequently confounded by regional terrestrial vegetation shifts, the $B/Ca$ proxy depends strictly on the thermodynamic equilibrium of seawater borate, thereby isolating marine carbon dynamics from terrestrial biosphere noise.

According to the passage, Rostova's research team established which of the following regarding the drawdown of deep-ocean carbon storage during marine isotope stage 22?

It occurred prior to the expansion of continental ice sheets rather than as a subsequent outcome of that expansion.Answer
B
It was caused primarily by regional terrestrial vegetation shifts that altered seawater thermodynamic equilibrium.
C
It triggered an immediate shift from a 100,000-year cycle to a 41,000-year cycle in glacial-interglacial periodicity.
D
It occurred approximately 20,000 years after reduced oceanic overturn initiated initial continental glaciation.
E
It proved conclusively that atmospheric carbon dioxide fluctuations were the sole driver of the Mid-Pleistocene Transition.

Answer

Rostova's research team established that the drawdown of deep-ocean carbon storage occurred prior to the expansion of continental ice sheets rather than as a subsequent outcome of that expansion.

The passage explicitly states in the second paragraph that Rostova's team determined the drawdown of deep-ocean carbon storage preceded continental ice-sheet expansion by nearly 20,000 years, and reinforces in the third paragraph that this reorganization preceded glaciation rather than following it as a secondary consequence.

Step-by-Step Solution

Locate the specific detail in the text regarding Rostova's findings during marine isotope stage 22 (MIS 22).

The second paragraph explicitly states: 'Rostova’s team noted that this drawdown of deep-ocean carbon storage preceded the expansion of continental ice sheets by nearly 20,000 years.'

Direct factual retrieval requires identifying the exact sentence stating the empirical finding.

Cross-reference Rostova's finding with the contrast presented regarding Thorne's hypothesis.

The third paragraph confirms that while Thorne posited carbon sequestration was a secondary consequence following ice growth, Rostova showed ocean reorganization 'preceded continental glaciation rather than following it.'

Evaluating how the finding is framed clarifies the correct semantic paraphrase.

Select the option that accurately paraphrases this explicit factual statement without introducing unwarranted claims.

The statement expressing that the drawdown occurred prior to ice-sheet expansion rather than as a subsequent outcome accurately reflects the text.

Semantic equivalence confirms the option directly answers the prompt based on explicit passage evidence.

Key Concept

Direct Factual Retrieval from Complex Academic Text

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