Expedition 397 summary

Abstract

During International Ocean Discovery Program Expedition 397, we recovered a total of 6176.7 m of core (104.2%recovery) at four sites (U1586, U1587, U1385, and U1588) from the Promontório dos Príncipes de Avis, a plateau located on the Portuguese continental slope that is elevated above the Tagus Abyssal Plain and isolated from the influence of turbidites.The drill sites are arranged along a bathymetric transect (4692, 3479, 2591, and 1339 meters below sea level [mbsl], respectively) to intersect each of the major subsurface water masses of the eastern North Atlantic.Multiple holes were drilled at each site to ensure complete spliced composite sections, which will be further refined postcruise by a campaign of X-ray fluorescence core scanning.At Site U1586 (4692 mbsl), the deepest and farthest from shore, a 350 m sequence was recovered in four holes that extend as far back as the middle Miocene (14 Ma), which is nearly twice as old as initially predicted from seismic stratigraphy.Sedimentation rates are lower (averaging 5 cm/ky in the Quaternary) at Site U1586 than other Expedition 397 sites, and a few slumped intervals were encountered in the stratigraphic sequence.Despite these limitations, Site U1586 anchors the deep end-member of the bathymetric transect and provides an important reference section to study deepwater circulation, ventilation and carbon storage in the deep eastern North Atlantic.At Site U1587 (3479 mbsl), the second deepest site along the depth transect, we recovered a 567 m sequence of late Miocene to Holocene sediments that accumulated at rates between 6.5 and 11 cm/ky.The high sedimentation rates and long continuous record at this site will permit climate reconstruction at high temporal resolution (e.g., millennial) for the past 7.8 My.The Messinian Stage (7.25-5.33Ma) was recovered, which provides a valuable opportunity to study the Messinian Salinity Crisis in an open marine setting adjacent to the Mediterranean.Site U1385 (Shackleton site) was a reoccupation of a position previously drilled during Integrated Ocean Drilling Program Expedition 339.Expedition 339 Site U1385 has yielded a remarkable record of millennial-scale climate change for the past 1.45My (Marine Isotope Stage 47).During Expedition 397, we deepened the site from 156 to 400 meters below seafloor, extending the record to near the base of the Pliocene (5.3 Ma).Sedimentation rates remained high, averaging between 9 and 11 cm/ky throughout the sequence.The newly recovered cores at Expedition 397 Site U1385 will permit the study of millennial climate variability through the entire Quaternary and Pliocene, prior to the intensification of Northern Hemisphere glaciation.Site U1588 is the shallowest, closest to shore, and youngest site drilled during Expedition 397 and is also the one with the highest sedimentation rate (20 cm/ky).The base of the 412.5 m sequence is 2.2 Ma, providing an expanded Pleistocene sequence of sediment deposited under the influence Plain language summaryFrom 11 October to 11 December 2022, International Ocean Discovery Program Expedition took place off the coast of Portugal southwest of Lisbon.The main objective was to recover the exceptional sedimentary archive preserved beneath the seafloor on the Iberian margin to study past climate change at high temporal resolution.During the expedition, which carried 26 international scientists, four sites were drilled, recovering 6.2 km of marine sediments that accumulated rapidly, thereby providing a high-fidelity record of past climate change on timescales of hundreds to thousands of years and extending back millions of years ago.Climate signals contained in these marine sediment cores will be correlated precisely to polar ice cores from both hemispheres and with European pollen records, providing a rare opportunity to link oceanic, atmospheric, and terrestrial climate and environmental changes.The four drill sites are located at different water depths (1339, 2591, 3479, and 4692 m below sea level), permitting scientists to study how deepocean circulation and chemistry changed in the past, including its role in deep-sea carbon storage and atmospheric CO 2 changes.The sediment cores recovered during Expedition 397 will provide benchmark records of North Atlantic climate change at high temporal resolution from the late Miocene (about 8 million years ago) to present.This period includes the last 3 million years when changes in the Earth's orbit resulted in the growth and decay of large ice sheets in the Northern Hemisphere and a warmer world before this time when atmospheric CO 2 was similar to today.All cores recovered show strong changes in physical properties (such as color) that represent a response to known cyclic changes in Earth's orbit, which will aid in accurately dating the sediment.Many years of research will be needed to extract the detailed climatic signals from the kilometers of core recovered during Expedition 397, but the records to be produced will be vital for testing numerical climate models and understanding how the climate system evolved in the past and how it might change in the future.

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