Question

In studying Holocene marine climate shifts, oceanographers have historically relied on shallow-water reef corals as proxies for sea-surface temperature variations. However, recent analyses of deep-sea scleractinian corals—specifically *Lophelia pertusa* harvested from bathyal depths along the North Atlantic margin—have challenged the assumption that deep-ocean thermal regimes remained stagnant throughout these epochal shifts. Unlike surface-dwelling corals, whose skeletal strontium-to-calcium ($Sr/Ca$) ratios correlate directly with ambient surface temperatures, *Lophelia pertusa* incorporates barium ($Ba/Ca$) into its aragonite skeleton in proportion to localized nutrient upwelling and sub-thermocline oceanic mixing rather than ambient temperature alone. Notably, during the 8.2-kiloyear cold event, while North Atlantic sea-surface temperatures registered marked cooling, deep-coral $Ba/Ca$ ratios exhibited a sharp transient reduction, whereas skeletal $Sr/Ca$ ratios in the same deep-sea specimens remained invariant. Researchers attribute this localized $Ba/Ca$ decline not to temperature fluctuations, but to the suppression of deep-water convective overturn caused by meltwater pulses flooding the subpolar gyre. Consequently, deep-sea coral skeletal geochemistry provides a precise record of vertical hydrographic reorganization independent of surface thermodynamic trends.

According to the passage, the temporary reduction in barium-to-calcium ($Ba/Ca$) ratios observed in *Lophelia pertusa* during the 8.2-kiloyear event was directly caused by which of the following?