A Mid-Holocene Shift and Millennial-Scale Variations in North Pacific Intermediate Water Oxygenation and Upper Ocean Hydrography

Abstract:

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.