Read the short passage and focus on the requested support. [1] Researchers analyzed two isotope ratios in sediment cores taken north and south of an ancient ocean gateway. [2] Across the same dated interval, the northern ratio shifted toward a southern-water signature while the southern ratio shifted toward a northern-water signature, a paired pattern predicted by current reversal. [3] Shell abundance declined at both sites after the interval. [4] The gateway's width changed gradually over several million years. Which sentence supplies the reciprocal evidence used to infer that deep-water flow reversed direction?