Influence of North Pacific Intermediate Water on biological productivity in the Eastern Equatorial Pacific under glacial conditions

Abstract:

The most important oceanic source of atmospheric CO₂ is the Eastern Equatorial Pacific (EEP), where carbon fixation by siliceous phytoplankton is limited nowadays by low silicic acid and iron availability. Several studies propose that the biological carbon pump in the EEP operated more efficiently under glacial conditions due to relaxed nutrient limitation, which has been explained by enhanced Southern Ocean nutrient delivery into the low latitudes. Recent studies, however, suggest a greater glacial “trapping” of nutrients in the Pacific Sector of the Southern Ocean. Thus, the reason for a relaxed nutrient limitation in the EEP remains enigmatic. Here we reconstruct the mid-depth ventilation history of the subarctic Pacific by using the stable carbon isotopic composition (δ¹³C) of epibenthic foraminifera Cibicides lobatulus from western Bering Sea core SO201-2-101KL (58°52.52’N, 170°41.45’E; 630 m water depth). We compare the reconstructed intermediate water characteristics of the subarctic Pacific to thermocline water masses upwelled in the EEP derived from δ¹³C of deep-dwelling planktic foraminifera Globorotaloides hexagonus at ODP Site 1240 (00°01.31’N, 82°27.76’W; 2921 m water depth) to trace glacial changes in thermocline nutrient availability and export productivity in the EEP. We found that foraminiferal δ¹³C proxy records from the subarctic Pacific and EEP show concomitant changes in nutrient- and ventilation dynamics under full glacial conditions. We propose that these changes are linked to Glacial North Pacific Intermediate Water. Enhanced injection of relatively nutrient-replete GNPIW into low-latitude thermocline waters coincides with times of relaxed nutrient limitation in the glacial EEP. We conclude that our results might help to explain a stimulated biological carbon pump in the EEP, which may have contributed to the glacial atmospheric CO₂ drawdown.