Multi‐Proxy Evidence for Atlantic Meridional Overturning Circulation (AMOC) Weakening During Deglaciations of the Past 150,000 Years.

Despite decades of research, the cause of deglaciations is not fully understood, leaving a critical gap in our understanding of Earth's climate system. During the most recent deglaciation (Termination I (T I)), abrupt declines in the stable carbon isotope ratio (δ13C) of benthic foraminifera occurred throughout the mid‐depth (1,500–2,500 m) Atlantic. The spatial pattern in δ13C anomalies was likely due to Atlantic Meridional Overturning Circulation (AMOC) weakening and the accumulation of respired carbon, which also yields negative excursions in carbonate ion concentration (CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$). To investigate whether a similar pattern occurred during prior deglaciations, we developed δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ records from 1,800 and 2,300 m water depth in the Southwest Atlantic spanning the last 150 ka. The new records reveal negative δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ anomalies during Termination II (TII) and the smaller deglaciations of Marine Isotope Stages (MIS) 4/3, 5b/a, and 5d/c, suggesting AMOC weakening is a common feature of deglaciation. The anomalies are more pronounced in the shallower core following MIS 2, 4, and 6 and in the deeper core following MIS 5b and 5d. The depth‐dependent pattern is most likely due to shoaling of Northern Source Water during glacial maxima and deepening during interglacial intervals. Comparison of CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ records from TI and TII suggests similar levels of carbon accumulation in the mid‐depth Atlantic. The Brazil Margin δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ results indicate the AMOC plays a key role in the series of events causing deglaciation, regardless of differences in orbital configuration, ice volume, and mean global temperature. Key Points: Brazil Margin δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ records suggest the Atlantic Meridional Overturning Circulation weakened during each deglaciation of the last 150,000 yearsMovement of northern source water yields larger δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ anomalies at 1,800 m during MIS 2, 4, and 6 and at 2,300 m during MIS 5The dynamic nature of Atlantic watermass structure complicates the diagnosis of overturning patterns on glacial‐interglacial timescales [ABSTRACT FROM AUTHOR]