Sunday, 17 November 2013

Climate Choke Point East of Greenland

The Arctic Ocean has an important role in Earth’s climate. The Arctic is part of the globe that is most sensitive to climate change. The choke point for the Arctic Ocean is the Fram Strait.  Currently the Arctic is shifting to a new normal; sea-ice is thinning, permafrost is thawing, and tundra is greening [1]. 

Arctic sea ice minimum 2012 compared to 30 average minimum [a]
The extend of its summertime sea-ice cover determines how much sun-light is reflected back into space. The Arctic receives important freshwater inflow from North American and Siberian rivers. Sea ice is export through the Fram Strait into the North Atlantic and is freshening the most salty ocean of the globe. The Frame Strait between Greenland and Svalbard is the only deep passage linking the Arctic Ocean to the global ocean. The main export vein of deep Arctic waters goes through this strait. Deep Arctic waters are forming in the interior of the Arctic ocean basin.

Freezing and cooling produce deep water in the Arctic ocean. Only if it is exported at depth out of the Arctic Ocean, then it is contributing to the global thermohaline circulation. The thermohaline circulation of the oceans is the slow vertical overturning of its water that brings heat and oxygen into the depth of the world ocean. One of the drivers of this circulation is the deep, cold and salty water that is flowing through the Fram Strait out in depth from the Arctic Ocean. To understand climate change processes it has to be assessed how the outflow of deep water from the Arctic Ocean varied during last several ten-thousand years.
Flow through Fram Strait - top/in & bottom/out [b]
Did the Arctic Ocean export waters through cold and warm climates, or did this export shut-off in warmer climates? Ratios of the radionuclides thorium-230 (230Th) and protactinium-231 (231Pa) in the sediments at the sea-bottom can be used to assess this question [2].

These radioactive tracers are produced in sea water by radioactive decay of natural uranium, which is transported by the rivers into the sea. Thorium and protactinium are not soluble in seawater and attach in a different time-depending manner to particles made of different minerals. These particles drop to the sea bottom and so remove the radioactive traces from the water column. This process is called “scavenging”. The “scavenging” of thorium and protactinium happens with a different speed. Much of the thorium will drop to the bottom even if much deep water is flowing out of the Arctic ocean. However an important part of the protactinium would be swept out through the Fram Strait if the outflow of Arctic waters is happening. Thus, the thorium and protactinium concentrations in sediments of the Arctic Ocean vary with the strength of the outflow of deep water through the Fram Strait. Whether that outflow varied during glacial, de-glacial and interglacial conditions can be studied in sediment cores taken from the bottom of the Arctic Ocean.

Thorium [c]
It has been found that the measured thorium burial is in balance with its production in Arctic seawater but that, for all time intervals, the burial of protactinium is in deficit. Thus, protactinium has been exported out of the Arctic Ocean all time throughout the past 35,000 years [2]. The outflow has to have been so strong that the replacement time for deep waters in the Arctic Ocean is many centuries since the most recent glaciation. Thus, Arctic waters are a persistent contribution to the global ocean circulation throughout the end of the last glacial periode and the following warmer Holocene.

[1] M. O. Jeffries, J.E: Overland and D.K. Perovich, 2013. The Arctic shifts to a new normal, Physics Today, Vol 66(10)
[2] Sharon S. Hoffmann, Jerry F. McManus, William B. Curry & L. Susan Brown-Leger, 2013, Persistent export of 231Pa from the deep central Arctic Ocean over the past 35,000 years
Nature 497, 603–606,

[a] from: 
[b] from:
[c] from: