Description: Prominent maxima of biological productivity are recorded in both the Northwest and Northeast Pacific during the deglacial, interstadial Bølling-Allerød. These have been linked to a suite of differing causes and mechanisms, such as preservation effects, iron fertilization, riverine fluxes, upper ocean stratification and coastal upwelling. There is also widespread evidence for shifts in the subarctic Pacific ocean circulation during the deglaciation. However, while the dynamics of nutrient provision and limitation within the photic zone are certainly of high significance, the important role of physical circulation changes in the subsurface to deep ocean in replenishing nutrient supplies to the upper ocean, and of upper ocean temperature changes in fostering productivity peaks, remain largely unconstrained over the course of the last deglaciation. Here, using an Earth System Model COSMOS, we conducted a simulation representing the climate transition from the Last Glacial Maximum to the Bølling-Allerød. In association with marine proxy evidence, we will discuss the deglacial evolution of the surface to deep ocean circulation and mixing in the North Pacific, and examine their respective roles in determining the upwelling of nutrients from deeper layers, along with the formation of the North Pacific Intermediate water.

Description: The subarctic North Pacific and its marginal seas constitute a key area in which rapid environmental changes over the past decades have been observed in instrumental records, like sea ice decreases, or alterations of nutrient inventories and oxygenation of mid-depth water masses. However, knowledge about the past climatic and oceanographic variability beyond instrumental time series in the subarctic North Pacific and its marginal seas is limited. Few temporally and spatially well-resolved high-resolution and spatially well datasets exist, with spatial and temporal coverage being insufficient to gain a detailed picture of past variations. Our proxydata-based study focuses on a collection of sediment records from the Okhotsk Sea as major source area for well ventilated North Pacific Intermediate Water (NPIW) that cover the last ca. 12,000 years with high temporal and adequate spatial resolution. We decipher rapid changes in NPIW ventilation patterns on centennial to millennial time scales and show that the current ventilation of the mid-depth North Pacific has likely only been prevalent for the last 2 ka. We further provide evidence for a Mid-Holocene shift in mid-depth NPIW ventilation characteristics. Additionally, changes in North Atlantic Deep Water flow speed and patterns are reflected in our records of North Pacific mid-depth water mass dynamics, thus indicating a hemispheric-wide connection between the Atlantic and Pacific regions during the Holocene. Planktic oxygen isotope data suggest a high variability in the stratification of local surface water masses and the formation of sea ice, influencing the formation of new, well ventilated water masses near to our core sites. We compare the main Holocene baseline changes evidenced in our proxy reconstructions to Early Holocene and Pre-Industrial time slice results from the fully-coupled MPI-ESM (COSMOS) Earth System Model, with a focus on the Pacific Ocean to better understand NPIW and upper ocean dynamic changes.

Description: A compilation of the published literature on dust content in terrestrial and marine sediment cores was synchronized with pollen data and speleothem growth phases on the Greenland Ice Core Chronology 2005 (GICC05) time axis. Aridity patterns for eight key areas of the global climate system have been reconstructed for the last 60 000 years. These records have different time resolutions and different dating methods, i.e. different types of stratigraphy. Nevertheless, all regions analysed in this study show humid conditions during early Marine Isotope Stage 3 (MIS3) and the early Holocene or deglaciation, but not always at the same time. Such discrepancies have been interpreted as regional effects, although stratigraphic uncertainties may affect some of the proposed interpretations. In comparison, most of the MIS2 interval becomes arid in all of the Northern Hemisphere records, but the peak arid conditions of the Last Glacial Maximum (LGM) and Heinrich event 1 differ in duration and intensity among regions. In addition, we also compare the aridity synthesis with modelling results using a global climate model (GCM). Indeed, geological archives and GCMs show agreement on the aridity pattern for the Holocene or deglaciation, for the LGM and for late MIS3.