Anne Briais

Understanding the possible responses of Atlantic Meridional Overturning Circulation (AMOC) to climate warming is one of the major challenges of modern oceanography. Today, the lower (deeper) southward flowing limb of AMOC consists of the North Atlantic deep water (NADW), which is predominantly formed by deep water convection in the ocean basins to the north and south of Iceland. The southward transit of deep water formed in the northerly basins (Greenland-Iceland-Norwegian Seas) is constrained by gateway geometry to two major flow pathways to the east and west of Iceland. To the south of Iceland extensive deep-sea sediment archives, in the form of contourite drifts, are deposited by these currents and have provided critical information about AMOC and NADW dynamics through the Pleistocene. Here we make use of recently recovered cores from one of these sediment drifts (Gardar Drift, IODP Expedition 395, Site U1564,), that records the deep Iceland-Scotland Overflow water (ISOW) dynamics back to the warm climates states of the late Miocene to Pliocene. By combining sedimentological and X-Ray fluorescence derived elemental proxy evidence, we reconstructed deep ocean current activity and carbonate preservation between 5.0–6.2 million years ago (Ma). The record supports the periodic presence of deep currents since the latest Miocene, as well as a distinct ISOW weakening that coincided with the global warming trend, and the severe restriction on Mediterranean Outflow Water, just before the Miocene-Pliocene boundary. Low carbonate preservation hints to the presence of corrosive water masses in the North Atlantic following the termination of the Messinian salinity crisis.

Understanding the possible responses of Atlantic Meridional Overturning Circulation (AMOC) to climate warming is one of the major challenges of modern oceanography. Today, the lower (deeper) southward flowing limb of AMOC consists of the North Atlantic deep water (NADW), which is predominantly formed by deep water convection in the ocean basins to the north and south of Iceland. The southward transit of deep water formed in the northerly basins (Greenland-Iceland-Norwegian Seas) is constrained by gateway geometry to two major flow pathways to the east and west of Iceland. To the south of Iceland extensive deep-sea sediment archives, in the form of contourite drifts, are deposited by these currents and have provided critical information about AMOC and NADW dynamics through the Pleistocene. Here we make use of recently recovered cores from one of these sediment drifts (Gardar Drift, IODP Expedition 395, Site U1564,), that records the deep Iceland-Scotland Overflow water (ISOW) dynamics back to the warm climates states of the late Miocene to Pliocene. By combining sedimentological and X-Ray fluorescence derived elemental proxy evidence, we reconstructed deep ocean current activity and carbonate preservation between 5.0–6.2 million years ago (Ma). The record supports the periodic presence of deep currents since the latest Miocene, as well as a distinct ISOW weakening that coincided with the global warming trend, and the severe restriction on Mediterranean Outflow Water, just before the Miocene-Pliocene boundary. Low carbonate preservation hints to the presence of corrosive water masses in the North Atlantic following the termination of the Messinian salinity crisis.

Understanding the possible responses of Atlantic Meridional Overturning Circulation (AMOC) to climate warming is one of the major challenges of modern oceanography. Today, the lower (deeper) southward flowing limb of AMOC consists of the North Atlantic deep water (NADW), which is predominantly formed by deep water convection in the ocean basins to the north and south of Iceland. The southward transit of deep water formed in the northerly basins (Greenland-Iceland-Norwegian Seas) is constrained by gateway geometry to two major flow pathways to the east and west of Iceland. To the south of Iceland extensive deep-sea sediment archives, in the form of contourite drifts, are deposited by these currents and have provided critical information about AMOC and NADW dynamics through the Pleistocene. Here we make use of recently recovered cores from one of these sediment drifts (Gardar Drift, IODP Expedition 395, Site U1564,), that records the deep Iceland-Scotland Overflow water (ISOW) dynamics back to the warm climates states of the late Miocene to Pliocene. By combining sedimentological and X-Ray fluorescence derived elemental proxy evidence, we reconstructed deep ocean current activity and carbonate preservation between 5.0–6.2 million years ago (Ma). The record supports the periodic presence of deep currents since the latest Miocene, as well as a distinct ISOW weakening that coincided with the global warming trend, and the severe restriction on Mediterranean Outflow Water, just before the Miocene-Pliocene boundary. Low carbonate preservation hints to the presence of corrosive water masses in the North Atlantic following the termination of the Messinian salinity crisis.

Over geological time, the growth of the ocean floor involves magmatic and tectonic extension ¹ at mid-ocean ridges (MORs). Because seismogeodetic monitoring of these submarine plate boundaries remains challenging ^2–7 , little is known about how these systems operate on yearly timescales. Here we report the first, to our knowledge, in situ observation of a rifting event at a MOR segment that combines hydroacoustic, direct-path ranging and bottom-pressure measurements, with repeated seafloor mapping. This event started on 26 April 2024 at the axis of the Southeast Indian Ridge (SEIR) near 37° S, two months after instruments had been deployed across the ridge axis and nearby Amsterdam transform fault (TF). The event began as a rapidly migrating swarm of extensional seismicity along the axial valley. It caused 4 m of subsidence of the valley floor and more than a metre of horizontal extension across the valley. We interpret this as the deflation of a sill-like reservoir feeding propagating dykes along the ridge axis. The dykes eventually led to the outpouring of about 160 million m ³ of lava at the seafloor in about 16 days, while inducing both seismic and aseismic slip on valley-bounding normal faults and finally triggering seismic activity on the abutting TFs. Large-scale aseismic slip induced by magmatic processes could therefore be the primary mechanism by which MOR normal faults accrue their displacement, which would account for their well-documented seismic deficit ^8,9 .

Understanding the possible responses of Atlantic Meridional Overturning Circulation (AMOC) to climate warming is one of the major challenges of modern oceanography. Today, the lower (deeper) southward flowing limb of AMOC consists of the North Atlantic deep water (NADW), which is predominantly formed by deep water convection in the ocean basins to the north and south of Iceland. The southward transit of deep water formed in the northerly basins (Greenland-Iceland-Norwegian Seas) is constrained by gateway geometry to two major flow pathways to the east and west of Iceland. To the south of Iceland extensive deep-sea sediment archives, in the form of contourite drifts, are deposited by these currents and have provided critical information about AMOC and NADW dynamics through the Pleistocene. Here we make use of recently recovered cores from one of these sediment drifts (Gardar Drift, IODP Expedition 395, Site U1564,), that records the deep Iceland-Scotland Overflow water (ISOW) dynamics back to the warm climates states of the late Miocene to Pliocene. By combining sedimentological and X-Ray fluorescence derived elemental proxy evidence, we reconstructed deep ocean current activity and carbonate preservation between 5.0–6.2 million years ago (Ma). The record supports the periodic presence of deep currents since the latest Miocene, as well as a distinct ISOW weakening that coincided with the global warming trend, and the severe restriction on Mediterranean Outflow Water, just before the Miocene-Pliocene boundary. Low carbonate preservation hints to the presence of corrosive water masses in the North Atlantic following the termination of the Messinian salinity crisis.

International Ocean Discovery Program Expeditions 384, 395C and 395 investigated ocean crust formation at the Reykjanes Ridge, the variable influence of the nearby Iceland plume, the origin of V-shape ridges and troughs marking the flanks of the ocean ridge, and the alteration of basaltic crust with time. These Expeditions collected cores from a transect of five drill sites along a plate-spreading flowline spanning seafloor ages from 2.8 to 32 Ma. Combined, over 400 m of oceanic basalt core was recovered, and downhole logging collected physical property measurements in the crust, and resistivity and ultrasound images of the boreholes. These datasets provide a unique record of volcanic and tectonic characteristics of the uppermost basaltic crust, and of the progressive basalt alteration. Here we use downhole logging images along with observations from the recovered cores to characterize the lava morphology and quantify flow types in basement holes, and to investigate the fracturing and alteration of the basalts. This analysis complements the observations from the cores especially where basalt recovery was low. Data from the different sites along the flowline allows us to analyze how these physical characteristics vary with age, and to compare the flows emplaced at V-shaped ridges with those emplaced in the troughs. We estimate the fluid circulation paths from downhole images and compare with the basalt alteration observed from the cores.

Drift sediments in the North Atlantic accumulated rapidly (≥10 cm/kyr) over the past ~3.6 Ma, enabling high-resolution reconstructions of geomagnetic field behavior. Previous scientific drilling expeditions (e.g., ODP Leg 162, IODP Exp. 306) from the North Atlantic refined the Quaternary Geomagnetic Instability Time Scale (GITS; 0–2.58 Ma) by revealing short-lived geomagnetic instabilities like the Iceland Basin Excursion (~188 ka). Here, we aim at further refining the GITS by studying newly collected cores in the same area.Between 2020 and 2023, the International Ocean Discovery Program (IODP) Expeditions 384, 395C, and 395 targeted six sites: five along a transect east of the Mid-Atlantic Ridge (20–30°W, ~60°N) and one west of Greenland. These expeditions targeted the volcanic basement that forms the V-shaped ridges/troughs on the Reykjanes Ridge and the overlying sediments that record deepwater current evolution. Shipboard age models are based on paleomagnetic and microfossil data, but a robust and reliable paleomagnetic record extends up to 11 Ma.We present results from Expeditions 384/395C/395 sediments that capture most GITS magnetic events (excursions and reversals) within the Matuyama chron. These records hold potential to extend the GITS backward in time, bolstering magnetic instabilities as geochronologic tools and refining models of Earth's ancient geomagnetic variations.

IODP Expeditions 395 and 395C drilled six sites on and adjacent to three major sediment drift bodies—the Gardar, Bjørn, and Erik Drifts—in the North Atlantic Ocean along an east–west transect at ~60°N, south of Iceland (Parnell-Turner et al., 2025). The development of high-resolution age models is essential for robust palaeoenvironmental reconstructions from these sedimentary archives. This study reports progress in constructing astronomically-tuned Pleistocene age models for sedimentary sequences recovered during IODP Expedition 395 east of the Reykjanes Ridge.As an initial step, magnetic susceptibility records from the newly drilled sites were compared with reference records from ODP Leg 162, with particular emphasis on sites within the Bjørn (ODP Site 984) and Gardar (ODP Site 983) Drifts. Individual glacial–interglacial cycles were identified and correlated across sites, providing a stratigraphic framework for age-model development.Pleistocene sedimentation rates derived from the new age models for the Bjørn and Gardar Drift records reveal distinct spatial patterns. ODP Site 984 and IODP Site U1554, both located on the Bjørn Drift, exhibit remarkably similar sedimentation rates. In contrast, ODP Site 983 and IODP Site U1564 on the Gardar Drift show substantial divergence. This difference may suggest more spatially homogeneous sedimentation on the Bjørn Drift and greater regional variability on the Gardar Drift, potentially reflecting contrasting ocean current dynamics.Despite these differences, both drift systems exhibit common large-scale features, including a pronounced decrease in sedimentation rates between ~850-900 ka. Such shared signals possibly reflect basin-wide oceanographic or climatic processes and provide insight into large-scale changes in sediment transport and circulation across the North Atlantic. Parnell-Turner, R.E., Briais, A., LeVay, L.J., and the Expedition 395 Scientists, 2025. Reykjanes Mantle Convection and Climate. Proceedings of the International Ocean Discovery Program, 395: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.395.2025