Between approximately 17,500 and 15,000 years ago, the Atlantic meridional overturning circulation weakened substantially in response to meltwater discharges from disintegrating Northern Hemispheric glacial ice sheets. The global effects of this reorganization of poleward heat flow in the North Atlantic extended to Antarctica and the North Pacific. Here we present evidence from North Pacific paleo surface proxy data, a compilation of marine radiocarbon age ventilation records, and global climate model simulations to suggest that during the early stages of the Last Glacial Termination, deep water extending to a depth of approximately 2500 to 3000 meters was formed in the North Pacific. A switch of deepwater formation between the North Atlantic and the North Pacific played a key role in regulating poleward oceanic heat transport during the Last Glacial Termination.

The future conditions of Arctic sea ice and marine ecosystems are of interest not only to climate scientists, but also to economic and governmental bodies. However, the lack of widespread, year-long biogeochemical observations remains an obstacle to understanding the complicated variability of the Arctic marine biological pump. Here we show an early winter maximum of sinking biogenic flux in the western Arctic Ocean and illustrate the importance of shelf-break eddies to biological pumping from wide shelves to adjacent deep basins using a combination of year-long mooring observations and three-dimensional numerical modelling. The sinking flux trapped in the present study included considerable fresh organic material with soft tissues and was an order of magnitude larger than previous estimates. We predict that further reductions in sea ice will promote the entry of Pacific-origin biological species into the Arctic basin and accelerate biogeochemical cycles connecting the Arctic and subarctic oceans.

The Drake Passage (DP) is the major geographic constriction for the Antarctic Circumpolar Current (ACC) and exerts a strong control on the exchange of physical, chemical, and biological properties between the Atlantic, Pacific, and Indian Ocean basins. Resolving changes in the flow of circumpolar water masses through this gateway is, therefore, crucial for advancing our understanding of the Southern Ocean’s role in global ocean and climate variability. Here, we reconstruct changes in DP throughflow dynamics over the past 65,000 y based on grain size and geochemical properties of sediment records from the southernmost continental margin of South America. Combined with published sediment records from the Scotia Sea, we argue for a considerable total reduction of DP transport and reveal an up to ∼40% decrease in flow speed along the northernmost ACC pathway entering the DP during glacial times. Superimposed on this long-term decrease are high-amplitude, millennial-scale variations, which parallel Southern Ocean and Antarctic temperature patterns. The glacial intervals of strong weakening of the ACC entering the DP imply an enhanced export of northern ACC surface and intermediate waters into the South Pacific Gyre and reduced Pacific–Atlantic exchange through the DP (“cold water route”). We conclude that changes in DP throughflow play a critical role for the global meridional overturning circulation and interbasin exchange in the Southern Ocean, most likely regulated by variations in the westerly wind field and changes in Antarctic sea ice extent.

[1] Carbon system parameters measured during several expeditions along the coast of Chile (23°S-56°S) have been used to show the main spatial and temporal trends of air-sea CO 2 fluxes in the coastal waters of the eastern South Pacific. Chilean coastal waters are characterized by strong pCO 2 gradients between the atmosphere and the surface water, with high spatial and temporal variability. On average, the direction of the carbon flux changes from CO 2 outgassing at the coastal upwelling region to CO 2 sequestering at the nonupwelling fjord region in Chilean Patagonia. Estimations of surface water pCO 2 along the Patagonian fjord region showed that, while minimum pCO 2 levels (strong CO 2 undersaturation) occurs during the spring and summer period, maximum levels (including CO 2 supersaturation) occur during the austral winter. CO 2 uptake in the Patagonia fjord region during spring-summer is within the order of −5 mol C m −2 yr −1 , indicating a significant regional sink of atmospheric CO 2 during that season. We suggest that the CO 2 sink at Patagonia most probably exceeds the CO 2 source exerted by the coastal upwelling system off central northern Chile.

Millennial‐scale variability in the behavior of North Pacific Intermediate Water during the last glacial and deglacial period, and its association with Dansgaard‐Oeschger (D‐O) cycles and Heinrich events, are examined based on benthic foraminiferal oxygen and carbon isotopes (δ18Obf and δ13Cbf) and %CaCO3 using a sediment core recovered from the northeastern slope of the Bering Sea. A suite of positive δ18Obf excursions at intermediate depths of the Bering Sea, which seem at least in part associated with increases in the δ18Obf gradients between the Bering and Okhotsk Seas, suggest the Bering Sea as a proximate source of intermediate water during several severe stadial episodes in the last glacial and deglacial period. Absence of such δ18Obf gradients during periods of high surface productivity in the Bering and Okhotsk Seas, which we correlate to D‐O interstadials, suggests a reduction in intermediate water production in the Bering Sea and subsequent introduction of nutrient‐rich deep waters from the North Pacific into intermediate depths of the Bering Sea. We argue that a reorganization of atmospheric circulation in the high‐latitude North Pacific during severe cold episodes in the last glacial and deglacial period created favorable conditions for brine rejection in the northeastern Bering Sea. The resulting salinity increase in the cold surface waters could have initiated intermediate (and deep) water formation that spread out to the North Pacific.

No abstract

Marine nitrogen fixation occurs not only in subtropical and tropical regions but also in colder regions. However, the distribution of diazotrophs, nitrogen fixation rate, and its contribution to the nitrogen cycle in the Arctic Ocean remain poorly understood. We examined the diazotroph community structure and activity in the shelf and off‐shelf regions of the Chukchi Sea, western Arctic Ocean, during late summer 2015. The nitrate and ammonium assimilation rates were determined simultaneously to gain insights into the role of nitrogen fixation in the nitrogen cycle of the region. The diazotroph community determined by Illumina sequencing was mainly composed of Cluster III nifH phylotypes (putative anaerobes), accounting for > 80% of the total sequences examined, except for one surface sample. This result is strikingly different from previous findings in other oceanic regions. The nifH sequences other than those from Cluster III were mostly affiliated with UCYN‐A2 (symbiotic cyanobacteria), which accounted for < 15% of the total sequences. UCYN‐A2 tended to be abundant (maximum 2.9 × 103 copies L−1) in the high‐temperature low‐salinity water mass that is characteristic of Pacific‐originating water. Nitrogen fixation rate was detectable at most stations, with a range of 0.08–3.60 nmol L−1 d−1, displaying no clear relationship with depth (light intensity) or nitrate or ammonium concentration. Nitrogen fixation locally exceeded nitrate assimilation, but accounted for 1.00% at most in the total nitrogen assimilation in the euphotic zone.

No abstract