Differences in radiocarbon apparent ages between coexisting planktonic and benthic (B-P) foraminifers in the Santa Barbara Basin contain information about changes in the age (and thus the source) of water entering the basin. The Holocene sequence indicates a greater variability in circulation than previously detected. During the early Holocene, ca. 9 ka, B-P 14C apparent age differences fluctuated between 220 and 560 14C yr, similar to rapid variations seen during the Younger Dryas and the last glacial episode. During the relatively stable middle Holocene (8.5–2 ka), B-P 14C apparent age differences averaged 440 14C yr. After 2 ka, the average B-P 14C apparent age difference increased to 620 14C yr, which has persisted until the present. The shift in B-P 14C apparent age differences likely represents a change in the strength or source of North Pacific Intermediate Water production and is concurrent with changes in regional climate patterns as well as with changes in temperature and accumulation rates measured in the Greenland Ice Sheet Project 2 ice core, suggesting large-scale changes in atmospheric circulation patterns.

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