Unlocking the potential of IODP site U1534 to reveal the sensitivity of Atlantic Overturning Circulation to "cold water route" variability.

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The Atlantic meridional overturning circulation (AMOC) plays a fundamental role in regulating the Earth's climate. Past reductions in the strength of this overturning circulation have been associated with dramatic cooling in the northern hemisphere. A recent trend of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region is indicative of a weakening of the AMOC. General Circulation Models are typically biased toward a stable AMOC and unable to replicate the observed sensitivity of the AMOC to climate forcing. This model bias in AMOC stability suggests that certain parameters are inaccurately represented, compromising our ability to predict future AMOC change. One element of the AMOC that remains poorly understood is the role of Antarctic Intermediate Water (AAIW). AAIW is one of the primary water masses characterizing the northward-flowing upper limb of the AMOC. Counter-balancing the southward flow of North Atlantic Deep Water, AAIW currently penetrates as far as 20 deg N in the North Atlantic. Despite a recent surge in research effort a consensus on the role of AAIW in contributing to AMOC variability has not been achieved. Today, AAIW is primarily formed in the SE Pacific and SW Atlantic Oceans. AAIW and its precursor, Subantarctic Mode Water, are advected eastward through Drake Passage along the Subantarctic Front (SAF). This injection of cold, low salinity Pacific water into the South Atlantic Ocean is commonly referred to as the 'cold water route. Variability in the connectivity between the Pacific and Atlantic Oceans via this cold water route may modify the physical properties of AAIW forming in the SW Atlantic Ocean, in turn affecting the heat and salinity budget of the AMOC. It has been shown that during the last glacial period the SAF was likely in a more northerly position relative to today. Truncated by South America, the SAF would not have transported Pacific-sourced Subantarctic Mode Water into the South Atlantic effectively closing the cold water route. During the deglaciation the SAF migrated south and once established within Drake Passage the route was open and Pacific-sourced water was injected into the South Atlantic. The coincidence of this opening of the cold water route and reinvigoration of the AMOC demands further investigation to constrain the sensitivity of AMOC to Pacific-Atlantic connectivity through Drake Passage. Marine sediments recovered at IODP Site U1534 on Expedition 382 present the first opportunity to constrain variability in endmember physical and geochemical properties of AAIW as it enters the upper limb of the AMOC over multiple glacial-interglacial cycles. This proposal will generate a definitive age model for U1534 and will form the foundation for a future NERC grant to address the question of the cold water route influence on AMOC. In addition, the advances proposed here will benefit numerous associated projects with national and international collaborators and potential studentships and optimise the outputs of this UK-lead ocean drilling effort.