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The Atlantic Meridional Overturning Circulation (AMOC) transports heat to high latitudes and carbon to the deep ocean. Paleoceanographic observations have led to the widely held view that the strength of the AMOC was significantly reduced at two intervals during the most recent glacial-to-interglacial transition, with global climate impacts. Climate models predict that the AMOC may decline in the future due to anthropogenic forcing, but the time periods for modern observations are too short to detect recent trends with high confidence. To understand the likelihood of future changes in the AMOC, it is important to understand the mechanisms that drove past changes in AMOC strength. In this paper we review (1) the paleoceanographic proxy data that have led to the widespread view that the AMOC sharply decreased for periods of several thousand years during the last deglaciation, (2) climate model simulations of the last deglaciation, with particular attention to their use of fresh water to alter the AMOC, (3) the physical mechanisms that could have driven past changes in the AMOC, and (4) how insights from past ocean change can inform our understanding of what may happen in the future. 

Abstract. Spatially distant sources of neodymium (Nd) to the ocean that carry different isotopic signatures (εNd) have been shown to trace out major water masses and have thus been extensively used to study large-scale features of the ocean circulation both past and current. While the global marine Nd cycle is qualitatively well understood, a complete quantitative determination of all its components and mechanisms, such as the magnitude of its sources and the paradoxical conservative behavior of εNd, remains elusive. To make sense of the increasing collection of observational Nd and εNd data, in this model description paper we present and describe the Global Neodymium Ocean Model (GNOM) v1.0, the first inverse model of the global marine biogeochemical cycle of Nd. The GNOM is embedded in a data-constrained steady-state circulation that affords spectacular computational efficiency, which we leverage to perform systematic objective optimization, allowing us to make preliminary estimates of biogeochemical parameters. Owing to its matrix representation, the GNOM model is additionally amenable to novel diagnostics that allow us to investigate open questions about the Nd cycle with unprecedented accuracy. This model is open-source and freely accessible, is written in Julia, and its code is easily understandable and modifiable for further community developments, refinements, and experiments. 

A compilation of radiocarbon measurements is used to characterize deep-sea overturning since the last ice age.