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If you wish to cite an abstract presented at AGU25, please cite as: Author(s) (2025), Title, Abstract (Final paper number, ex: AE14B-1234) presented at AGU25, 15-19 Dec. 

With the threat of rising temperatures, the Atlantic Meridional Overturning Circulation (AMOC) has been predicted to slow down or stop entirely, potentially exacerbating climate dysregulation in the Atlantic region. This project looks to the geologically recent past, to examine how much and in what way Atlantic ocean circulation has fluctuated over the last ~10,000 years. From IODP expedition 397, we processed 33 samples from site U1586, the sediment core at the greatest depth from the Iberian Margin. Stable isotope analysis of benthic foraminifera microfossils found in these sediment cores is a widely used technique for reconstructing past ocean circulation patterns; δ13C is a tracer for water masses, and δ18O is a proxy for sea temperature and land ice coverage. We searched specifically for Cibicidoides wuellerstorfi foraminifera and used mass spectrometry to find their values of δ13C and δ18O throughout the time-series. Our analyses of the stable isotopes generally indicate a warm climate and strong AMOC activity throughout the Holocene. Within the time interval 3.5-2.4 ka, stable oxygen isotope analysis shows a deep water temperature change from warmer to colder conditions. The lowest δ13C value occurs within that time interval; after δ18O values dropped at 3.5 ka, and gradually started increasing, the δ13C decreased significantly at 2.8 ka. The fact that the lowest δ13C value coincides with a 1,000 year period of deep water temperature change shown in the δ18O record suggests a link between climate change and AMOC activity in the past, and supports predictions for the impact that current climate change may have on AMOC in the future. 

The formation of North Atlantic Deep Water (NADW) is an important component of the Atlantic Meridional Overturning Circulation’s redistribution of solar heat to the northern latitudes, and is sensitive to salinity perturbations in its source locations. Fresh glacial meltwater lowers the density of seawater, potentially influencing affect the strength and position of the density-driven overturning circulation. During Heinrich Stadial 1, the Laurentide Ice Sheet retreated rapidly from its maximum southeastern extent during the Last Glacial Maximum, sending icebergs and meltwater into the western North Atlantic. The Bermuda Rise is located in the deep western subtropical North Atlantic, where Antarctic Bottom Water mixes with newly formed NADW. Sediment proxy records from the Bermuda Rise have demonstrated that the strength of the NADW varied with abrupt deglacial climate changes (e.g., McManus et al., 2004, Nature). This study seeks to explore the influence of meltwater pulses from the Laurentide and Greenland ice sheets on NADW during the last deglaciation through the lens of the transport and deposition of silt and clay by deep ocean currents. We present a detailed record of fine-grained sediment provenance across the deglaciation using K’/Ar ages from Bermuda Rise Core KNR191-CDH13 (33 41.2 N, 57 36.9 W, 4583 m). The K’/Ar ages are based on measured 40Ar* (radiogenic product of 40K) and an assumed K concentration of 2% (The 40Ar* measurements are rapid and simple, allowing the development of high resolution records. Initial results from deglacial sediments on Bermuda Rise display values within the range of K’/Ar ages documented by previous studies of iceberg deposition from the North Atlantic, from ~400 to 1,200 Ma (e.g., Jantschik and Huon,1992, Eclogae geol. Helv.; Hemming et al., 2002, Chem. Geol.). 

The purpose of this research is to investigate whether icebergs influenced the ocean’s circulation and contributed to significant climate changes during the last Ice Age. Previous studies have suggested that iceberg discharge from surrounding ice sheets introduced large volumes of freshwater into sensitive deep-water production locations in the North Atlantic Ocean, potentially altering ocean circulation and influencing regional and global climate. This research focused on the sequence of events approximately 40-50 thousand years ago in the central North Atlantic Ocean, utilizing a sediment core VM 30-100 PC recovered from the Mid- Atlantic Ridge. We quantified the abundance of ice-rafted debris (IRD) as an indicator of the presence of icebergs in the core sample from every cm at depths from 150-200 cm. In addition to IRD counting, we determined the relative abundance and stable oxygen isotope ratios (δ O) in the microfossil shells of the polar foraminifera species Neogloboquadrina pachyderma (N. pachy) as indicators of the surface ocean’s conditions during that time. By detecting an increase in δ O values over time it will indicate a decrease in ocean temperature, which we expect to correspond with a large abundance of N. pachy. Once IRD counting is completed, graphing the IRD concentration over depth will reveal periods of significant iceberg presence. By comparing the relative abundance of IRD and N. pachy in the samples, and by observing the δ O data, we aim to determine whether iceberg discharge preceded changes in ocean circulation or if sea-surface conditions shifted beforehand. Our hypothesis is that icebergs appeared first and disrupted ocean circulation, leading to subsequent changes in sea-surface conditions. This research will provide insight into the cause and sequence of natural variability in the climate system. We believe that there is a strong possibility that iceberg discharges played a crucial role in altering ocean circulation, thus driving significant climatic changes and contributing to the onset of the last Ice Age. 

Abundant proxy records suggest a profound reorganization of the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM, ~21,000 y ago), with the North Atlantic Deep Water (NADW) shoaling significantly relative to the present-day (PD) and forming Glacial North Atlantic Intermediate Water (GNAIW). However, almost all previous observational and modeling studies have focused on the zonal mean two-dimensional AMOC feature, while recent progress in the understanding of modern AMOC reveals a more complicated three-dimensional structure, with NADW penetrating from the subpolar North Atlantic to lower latitude through different pathways. Here, combining²³¹Pa/²³⁰Th reconstructions and model simulations, we uncover a significant change in the three-dimensional structure of the glacial AMOC. Specifically, the mid-latitude eastern pathway (EP), located east of the Mid-Atlantic Ridge and transporting about half of the PD NADW from the subpolar gyre to the subtropical gyre, experienced substantial intensification during the LGM. A greater portion of the GNAIW was transported in the eastern basin during the LGM compared to NADW at the PD, resulting in opposite²³¹Pa/²³⁰Th changes between eastern and western basins during the LGM. Furthermore, in contrast to the wind-steering mechanism of EP at PD, the intensified LGM EP was caused primarily by the rim current forced by the basin-scale open-ocean convection over the subpolar North Atlantic. Our results underscore the importance of accounting for three-dimensional oceanographic changes to achieve more accurate reconstructions of past AMOC. 

While the Atlantic Ocean is ventilated by high-latitude deep water formation and exhibits a pole-to-pole overturning circulation, the Pacific Ocean does not. This asymmetric global overturning pattern has persisted for the past 2–3 million years, with evidence for different ventilation modes in the deeper past. In the current climate, the Atlantic-Pacific asymmetry occurs because the Atlantic is more saline, enabling deep convection. To what extent the salinity contrast between the two basins is dominated by atmospheric processes (larger net evaporation over the Atlantic) or oceanic processes (salinity transport into the Atlantic) remains an outstanding question. Numerical simulations have provided support for both mechanisms; observations of the present climate support a strong role for atmospheric processes as well as some modulation by oceanic processes. A major avenue for future work is the quantification of the various processes at play to identify which mechanisms are primary in different climate states. 

Abstract While substantial changes in thermohaline circulation related to deglacial climate variability are well established, the role of this circulation in Holocene climate variability remains uncertain. Here we use two dynamical proxies,²³¹Pa/²³⁰Th ratios and mean sortable silt size (), to reconstruct Holocene bottom water circulation at the intermediate‐depth Carolina Slope. We find no substantial change in deep current speed or²³¹Pa export at this site during the Holocene, suggesting consistent²³¹Pa export via the Deep Western Boundary Current.shows increasing millennial‐scale variability in the middle‐late Holocene, which may reflect Labrador Sea Water contribution to current speed. We conclude that deepwater export from the North Atlantic has remained remarkably stable during the Holocene, decoupled from changing rates of specific water masses, while production of these water masses varied at millennial to centennial time scales. The persistence of the large‐scale overturning may reflect the ocean's stabilizing influence on Holocene climate.