International Ocean Discovery Program
Expedition 395 Preliminary Report
Reykjanes Mantle Convection and Climate: Mantle Dynamics, Paleoceanography and Climate Evolution in the North Atlantic Ocean1
Ross Parnell-Turner, Anne Briais, Leah LeVay, and the Expedition 395 Scientists
1 Parnell-Turner, R., Briais, A., LeVay, L., and the Expedition 395 Scientists, 2024. Expedition 395 Preliminary Report: Reykjanes Mantle Convection and Climate. International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.395.2024
Abstract
The intersection between the Mid-Atlantic Ridge and Iceland hotspot provides a natural laboratory where the composition and dynamics of Earth's upper mantle can be observed. Plume-ridge interaction drives variations in the melting regime, which result in a range of crustal types, including a series of V-shaped ridges (VSRs) and V-shaped troughs (VSTs) located south of Iceland. Mantle upwelling beneath Iceland dynamically supports regional bathymetry and may lead to changes in the height of oceanic gateways, which in turn control the flow of deep water on geologic timescales. This expedition recovered basaltic samples from crust that is blanketed by thick sediments, that also contain climatic and oceanic records from modern to earliest Oligcene/late Eocene times. Major, trace, and isotope geochemistry of basalts from this expedition provide insight into spatial and temporal variations in mantle melting processes. These samples will enable testing of the hypothesis that the Iceland plume thermally pulses on two timescales (5–10 and ~30 Ma), leading to fundamental changes in crustal architecture. This idea will be tested against alternative hypotheses involving propagating rifts and buoyant mantle upwelling. Millennial-scale paleoclimate records are contained in the rapidly accumulated sediments of contourite drifts cored during Expedition 395. The accumulation rate of these sediments is a proxy for current strength, which is moderated by dynamic support of oceanic gateways such as the Greenland-Scotland Ridge. These sediments also provide constraints for climatic events including Miocene and Pliocene warmth, the intensification of Northern Hemisphere glaciation, and abrupt Late Pleistocene climate change. The integrated approach of Expedition 395 allows the relationships between deep Earth processes, ocean circulation, and climate to be explored. These objectives were addressed by recovering sediment and basement cores from four sites, plus an additional two sites which were completed during Expeditions 384 and 395C (U1555 and U1563). Two sites (U1554 and U1562) are located in Björn drift above a VSR/VST pair, and another site targeted the Holocene–Eocene sequence of sediments at Eirik drift, located on the eastern Greenland margin (U1602). The fourth site of Expedition 395 (U1564) is located on 32.4 My-old oceanic crust that is devoid of V-shaped features, and was chosen because it intersects the Holocene to Oligo–Miocene sedimentary sequence of Gardar drift. Considered together, the sediments, basalts and vast array of measurements collected during Expedition 395 will provide a major advance in our understanding of mantle dynamics and the linked nature of Earth's interior, oceans, and climate.
Plain language summary
In the North Atlantic Ocean, hot rocks are thought to rise up beneath Iceland from deep within Earth's interior, called the mantle, forming a giant mantle plume. This plume likely plays a key role in shaping the ocean crust around Iceland, including a pattern of distinctive crustal V-shaped ridges (highs) and V-shaped troughs (lows) that extend hundreds of kilometers on the seabed south of Iceland. Some think that these V-shaped ridges are generated by increases and decreases in plume activity, but their precise origin is hotly debated. Plume activity variations may also have contributed to changes in the height of oceanic gateways—passages that link Greenland, Iceland, and Scotland, thus controlling the amount of cold, deep water that has been flowing from the Norwegian Sea to the Atlantic Ocean over the past few million years. These deepwater currents deposit sediments on the seabed very quickly, recording the ambient ocean and climatic conditions along the way. As a result, the sediments contain a high-resolution record of the past, which we can measure in deep-sea cores. Expedition 395 cored through these sediments at four sites; at one of which, coring continued for 170 m into the ocean crust. Core, fluid, and microbial samples, along with measurements of the physical properties of both the core and the interior of the boreholes, will enable scientists to decipher processes taking place in the mantle, ocean, and climate over the past 32 million years.