MPT-ICE: Exploring the Mid-Pleistocene Transition using Antarctica's oldest ice core.

As atmospheric CO2 and sea levels continue to rise, understanding the interactions among ice sheets, oceanic processes, and the atmosphere becomes increasingly critical. While current climate change is driven by human activity, understanding natural climate cycles is essential for predicting future changes. MPT-ICE will investigate the processes driving glacial-interglacial cycles spanning the Mid-Pleistocene Transition (MPT), between 700-1500 thousand years (ky) before present (BP), a period when ice sheets were smaller than today. This will be achieved by generating the first continuous record of impurities (chemical elements, organic compounds, and insoluble particulate material) spanning the full MPT from the highly anticipated Beyond EPICA Oldest Ice core (hereafter Oldest Ice). Marine records hint at the climate before the MPT, revealing ~41-ky glacial cycles and smaller Northern Hemisphere ice sheets1. Post-MPT, the shift to longer ~100-ky glacial cycles and ice sheet expansion is intriguing, as it occurred without changes in orbital patterns affecting solar insolation2. Various theories exist to explain the MPT, but evidence increasingly points to the critical role of the Southern Ocean. We hypothesize that the processes driving CO2 exchange in the Southern Ocean shifted during the MPT. MPT-ICE will test this by reconstructing atmospheric circulation, sea ice, and marine primary productivity changes over the MPT from the oldest ice core ever drilled. The EU-funded Oldest Ice project, supported by NERC logistics and scientific leadership, is on-target to retrieve a 2756 m-long ice core in Antarctica (January 2025). Radar and ice flow modelling suggest this will surpass the iconic Dome C ice core, which provided unrivalled insight into glacial cycles spanning the past 800-ky. MPT-ICE is a standalone initiative that will go beyond both the research scope and funding of the Oldest Ice project, to deliver state-of-the-art impurities analysis. It will work in partnership with the international Oldest Ice consortium, enhancing the far-reaching scientific, socio-economic, and political impacts of the research. Based on our expertise, the Oldest Ice consortium have asked the MPT-ICE team to lead the impurities analysis of this highly valued ice core. Hosting the analysis at the NERC ice core laboratories will position UK scientists at the forefront of their field and create a legacy for NERC researchers. However, immediate investment in NERC’s analytical capabilities is crucial. Otherwise, this important suite of analyses will move to another European laboratory, diminishing the UK's pivotal role. The expected small ice sample volume, with an estimated time-depth resolution of just ~20 ky m-1 during the MPT, represents a considerable analytical challenge. MPT-ICE will surpass NERC's current impurities analysis capabilities by enhancing data acquisition and temporal resolution, crucial for testing our hypothesis and maintaining leadership in ice core and environmental research. MPT-ICE objectives: Optimise the capability of NERC’s ice core laboratories to measure a comprehensive and continuous suite of impurities at sub-centimetre depth-resolution. Analyse the impurities in the Oldest Ice across the MPT, from 700-1500 -ky BP, at sub-centennial temporal-resolution. Utilize the terrestrial impurities to reconstruct changes in Southern Ocean winds and the aerial deposition of mineral dust and bioavailable nutrients. Utilize a suite of impurities of marine origin to reconstruct changes in Southern Ocean marine primary productivity and sea ice extent. Characterise the relative changes in Southern Ocean winds, sea ice and marine productivity, over the MPT, to provide a mechanistic framework to test our hypothesis.