Collaborative Research: Quantifying the ocean's response to solar forcing:Multidecadal to century-scale variability of North Atlantic SSTs over recent millennia

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The purpose of this project is to develop the first detailed, calendrically-dated records of North Atlantic Sea Surface Temperature (SST) variability in order to investigate the ocean's response to natural solar forcing at multi-decadal to century time scales over the last 3000 years. Despite the availability of reliable SST proxies and of sampling sites with the necessary high deposition rates, there exist no reconstructions of the North Atlantic basin-scale SST field over past millennia based directly or substantially on ocean observations. This is due almost entirely to the lack of adequate age-dating control in marine sediments. This collaborative project, undertaken by a team from both Columbia University's Lamont-Doherty Earth Observatory and the University of Colorado, Boulder, develops a new marine application of the so-called radiocarbon "wiggle-match" dating method in which reliable calendar age estimates are obtained free of assumptions regarding the size and past variability of the local marine reservoir age, which is the principle source of dating uncertainty. Because the method objectively maps down-core marine sediment radiocarbon variations to the late Holocene radiocarbon calibration in tree-rings, which also contains a record of initial atmospheric radiocarbon activity controlled (at the timescale of interest here) largely by variations in the Sun's open magnetic flux, it provides for direct comparison of SST and solar proxy variability in the same sediments. The researchers makes use of eight high-accumulation rate sediment cores with a basin-wide distribution selected to capture the spatial expression of the leading modes of Atlantic temperature variability today (the NAO and AMO). Chronologies for each core are based on AMS radiocarbon dating (at a nominal resolution of 40 years) with calendar ages assigned objectively using an automated wiggle-match algorithm developed for this project. SST reconstructions will be based on foraminiferal oxygen isotope stratigraphy and Mg/Ca, which provide a reliable estimate of past calcification temperature. Together these data will provide the observations necessary to test for any possible basin-wide SST pattern response to solar forcing and whether this response has features in common with the North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation (AMO) or with other previously simulated responses. Broader Impacts: This work directly addresses the need to characterize natural climate variability as needed in order to separate ongoing responses to natural and anthropogenic forcing agents and to improve predictive models. Such characterization is presently lacking over the oceans although ocean regions are spatially and, in many instances, energetically dominant. The project will also support education and research training for a PhD student at Columbia University.