North Pacific deglacial hypoxic events linked to abrupt ocean warming

Abstract

Marine sediments from the North Pacific document two episodes of expansion and strengthening of the subsurface oxygen minimum zone (OMZ) accompanied by seafloor hypoxia during the last deglacial transition. The mechanisms driving this hypoxia remain under debate. We present a new high-resolution alkenone palaeotemperature reconstruction from the Gulf of Alaska that reveals two abrupt warming events of 4-5 degrees Celsius at the onset of the Bølling and Holocene intervals that coincide with sudden shifts to hypoxia at intermediate depths. The presence of diatomaceous laminations and hypoxia-tolerant benthic foraminiferal species, peaks in redox-sensitive trace metals, and enhanced (15)N/(14)N ratio of organic matter, collectively suggest association with high export production. A decrease in (18)O/(16)O values of benthic foraminifera accompanying the most severe deoxygenation event indicates subsurface warming of up to about 2 degrees Celsius. We infer that abrupt warming triggered expansion of the North Pacific OMZ through reduced oxygen solubility and increased marine productivity via physiological effects; following initiation of hypoxia, remobilization of iron from hypoxic sediments could have provided a positive feedback on ocean deoxygenation through increased nutrient utilization and carbon export. Such a biogeochemical amplification process implies high sensitivity of OMZ expansion to warming.

Major impacts of climate change on deep-sea benthic ecosystems

“…The deep sea has experienced dramatic changes in physical and chemical variables in the geological past. For example, major expansion and strengthening of oxygen minimum zones (OMZs; O 2 < 0.5 mL L -1 ) are known to have occurred during abrupt, decadal-centennial-scale warming events during the last deglaciation (Moffitt et al, 2015b;Praetorius et al, 2015). Similar fluctuations in OMZ intensity have occurred during the Dansgaard-Oeschger (D-O) events (millennial-scale abrupt climate oscillations) during the last glacial period Schmittner et al, 2007).…”

Climate engineering by mimicking natural dust climate control: the iron salt aerosol method

“…During the glacial maxima with cold temperatures, dustiness and low greenhouse gas levels, the ocean basins had well and vertically mixed water columns with highest carbonate C burial and lowest organic C burial. ing the glacial climate warming events, enormous meltwater volumes were generated and induced stratification effects in ocean basins by placing a meltwater blanket on the saline ocean water surface (Praetorius et al, 2015). The transport of CO 2 and O 2 into the basin bottoms became interrupted.…”

“…According to Praetorius et al (2015), climate warming events during the last deglacial transition induced subsurface oxygen minimum zones accompanied by sea floor anoxia in the northern Pacific. This meltwater-induced stratification was accompanied by meltwater iron-induced phytoplankton blooms.…”

Hazards of decreasing marine oxygen: the near-term and millennial-scale benefits of meeting the Paris climate targets

“…For instance, atmospheric CO 2 and abyssal Southern Ocean oxygenation co-varied throughout the past 80,000 years reflecting enhanced deep 30 ocean ventilation and less iron fertilization (Jaccard et al, 2016). Also, abrupt warming events in the past have been shown to coincide with sudden appearance of hypoxia at intermediate depths in the North Pacific (Praetorius et al, 2015) raising concern for impacts in the future. Across the last deglaciation, the deep ocean became better oxygenated and low oxygen water in the upper ocean expanded as the Earth transitioned to a warm interglacial state, opposite the general expectation from a pure solubility point of view (Jaccard and Galbraith, 2012;Jaccard et al, 2014).…”