Previous Paleoclimate Model Intercomparison Project (PMIP) simulations of the Last Glacial Maximum (LGM) Atlantic Meridional Overturning Circulation (AMOC) showed dissimilar results on transports and structure. Here we analyze the most recent PMIP3 models, which show a consistent increase (on average by 41 ± 26%) and deepening (663 ± 550 m) of the AMOC with respect to preindustrial simulations, in contrast to some reconstructions from proxy data. Simulations run with the University of Victoria (UVic) ocean circulation model suggest that this is caused by changes in the Northern Hemisphere wind stress, brought about by the presence of ice sheets over North America in the LGM. When forced with LGM wind stress anomalies from PMIP3 models, the UVic model responds with an increase of the northward salt transport in the North Atlantic, which strengthens North Atlantic Deep Water formation and the AMOC. These results improve our understanding of the LGM AMOC's driving forces and suggest that some ocean mechanisms may not be correctly represented in PMIP3 models or some proxy data may need reinterpretation.

The prevailing hypothesis for lower atmospheric carbon dioxide (CO2) concentrations during glacial periods is an increased efficiency of the ocean’s biological pump. However, tests of this and other hypotheses have been hampered by the difficulty to accurately quantify ocean carbon components. Here, we use an observationally constrained earth system model to precisely quantify these components and the role that different processes play in simulated glacial-interglacial CO2 variations. We find that air-sea disequilibrium greatly amplifies the effects of cooler temperatures and iron fertilization on glacial ocean carbon storage even as the efficiency of the soft-tissue biological pump decreases. These two processes, which have previously been regarded as minor, explain most of our simulated glacial CO2 drawdown, while ocean circulation and sea ice extent, hitherto considered dominant, emerge as relatively small contributors.

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Nitrogen is a key limiting nutrient that influences marine productivity and carbon sequestration in the ocean via the biological pump. In this study, we present the first estimates of nitrogen cycling in a coupled 3D ocean-biogeochemistry-isotope model forced with realistic boundary conditions from the Last Glacial Maximum (LGM) ∼21,000 years before present constrained by nitrogen isotopes. The model predicts a large decrease in nitrogen loss rates due to higher oxygen concentrations in the thermocline and sea level drop, and, as a response, reduced nitrogen fixation. Model experiments are performed to evaluate effects of hypothesized increases of atmospheric iron fluxes and oceanic phosphorus inventory relative to present-day conditions. Enhanced atmospheric iron deposition, which is required to reproduce observations, fuels export production in the Southern Ocean causing increased deep ocean nutrient storage. This reduces transport of preformed nutrients to the tropics via mode waters, thereby decreasing productivity, oxygen deficient zones, and water column N-loss there. A larger global phosphorus inventory up to 15% cannot be excluded from the currently available nitrogen isotope data. It stimulates additional nitrogen fixation that increases the global oceanic nitrogen inventory, productivity, and water column N-loss. Among our sensitivity simulations, the best agreements with nitrogen isotope data from LGM sediments indicate that water column and sedimentary N-loss were reduced by 17-62% and 35-69%, respectively, relative to preindustrial values. Our model demonstrates that multiple processes alter the nitrogen isotopic signal in most locations, which creates large uncertainties when quantitatively constraining individual nitrogen cycling processes. One key uncertainty is nitrogen fixation, which decreases by 25-65% in the model during the LGM mainly in response to reduced N-loss, due to the lack of observations in the open ocean most notably in the tropical and subtropical southern hemisphere. Nevertheless, the model estimated large increase to the global nitrate inventory of 6.5-22% suggests it may play an important role enhancing the biological carbon pump that contributes to lower atmospheric CO 2 during the LGM.

Abstract. Changes in the geometry of ocean meridional overturning circulation (MOC) are crucial in controlling past changes of climate and the carbon inventory of the atmosphere. However, the accurate timing and global correlation of short-term glacial-to-deglacial changes of MOC in different ocean basins still present a major challenge. The fine structure of jumps and plateaus in atmospheric and planktic radiocarbon (14C) concentration reflects changes in atmospheric 14C production, ocean–atmosphere 14C exchange, and ocean mixing. Plateau boundaries in the atmospheric 14C record of Lake Suigetsu, now tied to Hulu Cave U∕Th model ages instead of optical varve counts, provide a stratigraphic “rung ladder” of up to 30 age tie points from 29 to 10 cal ka for accurate dating of planktic oceanic 14C records. The age differences between contemporary planktic and atmospheric 14C plateaus record the global distribution of 14C reservoir ages for surface waters of the Last Glacial Maximum (LGM) and deglacial Heinrich Stadial 1 (HS-1), as documented in 19 and 20 planktic 14C records, respectively. Elevated and variable reservoir ages mark both upwelling regions and high-latitude sites covered by sea ice and/or meltwater. 14C ventilation ages of LGM deep waters reveal opposed geometries of Atlantic and Pacific MOC. Like today, Atlantic deep-water formation went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the northern North Pacific and an outflow of upper deep waters. During early HS-1, 14C ventilation ages suggest a reversed MOC and ∼1500-year flushing of the deep North Pacific up to the South China Sea, when estuarine circulation geometry marked the North Atlantic, gradually starting near 19 ka. High 14C ventilation ages of LGM deep waters reflect a major drawdown of carbon from the atmosphere. The subsequent major deglacial age drop reflects changes in MOC accompanied by massive carbon releases to the atmosphere as recorded in Antarctic ice cores. These new features of MOC and the carbon cycle provide detailed evidence in space and time to test and refine ocean models that, in part because of insufficient spatial model resolution and reference data, still poorly reproduce our data sets.

Changes in the ocean iron cycle could help explain the low atmospheric CO2 during the Last Glacial Maximum (LGM). Previous modeling studies have mostly considered changes in aeolian iron fluxes, although it is known that sedimentary and hydrothermal fluxes are important iron sources for today's ocean. Here we explore effects of preindustrial‐to‐LGM changes in atmospheric dust, sedimentary, and hydrothermal fluxes on the ocean's iron and carbon cycles in a global coupled biogeochemical‐circulation model. Considering variable atmospheric iron solubility decreases LGM surface soluble iron fluxes compared with assuming constant solubility. This limits potential increases in productivity and export production due to surface iron fertilization, lowering atmospheric CO2 by only 4 ppm. The effect is countered by a decrease in sedimentary flux due to lower sea level, which increases CO2 by 15 ppm. Assuming a 10 times higher iron dust solubility in the Southern Ocean, combined with changes in sedimentary flux, we obtain an atmospheric CO2 reduction of 13 ppm. The high uncertainty in the iron fluxes does not allow us to determine the net direction and magnitude of variations in atmospheric CO2 due to changes in the iron cycle. Our model does not account for changes to iron‐binding ligand concentrations that could modify the results. We conclude that when evaluating glacial‐interglacial changes in the ocean iron cycle, not only surface but also seafloor fluxes must be taken into account.

Plateaus and jumps in the atmospheric radiocarbon record -Potential origin and value as 1 global age markers for glacial-to-deglacial paleoceanography, a synthesis 2 3 4 ABSTRACT 27 Changes in the geometry of ocean Meridional Overturning Circulation (MOC) are crucial in 28 controlling changes of climate and the carbon inventory of the atmosphere. However, the precise 29 timing and global correlation of short-term glacial-to-deglacial changes of MOC in different ocean 30 basins still present a major challenge. A possible solution is offered by the fine structure of jumps 31 and plateaus in the record of radiocarbon ( 14 C) concentration of the atmosphere and surface ocean 32 that reflects changes in atmospheric 14 C production as well as in the 14 C exchange between air 33 and sea and within the ocean. Boundaries of atmospheric 14 C plateaus in the 14 C record of Lake 34 Suigetsu, now tied to Hulu U/Th model-ages instead of optical varve counts, provide a 35 stratigraphic 'rung ladder' of ~30 age tie points from 29 to 10 ka for correlation with and dating of 36 planktic oceanic 14 C records. The age difference between contemporary planktic and atmospheric 37 14 C plateaus gives an estimate of the global distribution of 14 C reservoir ages for surface waters of 38 the Last Glacial Maximum (LGM) and deglacial Heinrich Stadial 1 (HS-1), as shown by 19 planktic39 14 C records. Clearly elevated and variable reservoir ages mark both high-latitude sites covered by 40 sea ice and/or meltwater and upwelling regions. 14 C ventilation ages of LGM deep waters reveal 41 opposed geometries of Atlantic and Pacific MOC. Similar to today, Atlantic deep-water formation 42 went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the 43 northern North Pacific and an outflow of upper deep waters. Vice versa, 14 C ventilation ages 44 suggest a reversed MOC during early HS-1 and a ~1500 year long flushing of the deep North 45 Pacific up to the South China Sea, when estuarine circulation geometry marked the North Atlantic, 46 gradually starting near 19 ka. Elevated 14 C ventilation ages of LGM deep waters reflect a major 47 drawdown of carbon from the atmosphere. Inversely, the subsequent massive age drop and 48 change in MOC induced two major events of carbon release to the atmosphere as recorded in 49 Antarctic ice cores, shifts that highlight the significance of ocean MOC for atmospheric CO 2 and its 50 14 C inventory. These new features of MOC and the carbon cycle offer a challenge to model 51 simulations that, in part because of insufficient spatial model resolution and reference data for 52 testing the model results, still poorly reproduce them.

Global marine iron model tests varying levels of atmospheric deposition, sedimentary release, ligand distributions and scavenging rates • Simulations that best reproduce observations include variable ligands and high rates of atmospheric deposition and sedimentary release • Simulations with high iron sources require high scavenging rates resulting in short residence times Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as