Another view of causes for the Younger Dryas cooling event

High latitudes in the North Atlantic, especially on its eastern side, are warmed today by the Gulf Stream.  That current, which defies the Coriolis effect, is pulled northwards by the sinking of cold dense sea water between Greenland, Iceland and Scandinavia to form North Atlantic Deep Water (NADW).  The thermohaline circulation here is driven by both cooling of salty surface water in the Gulf Stream and further salinisation as sea ice forms in this area each winter.  The Younger Dryas cold period between 13 and 11.5 ka is regarded by most oceanographers and climatologists to have resulted from sudden freshening of the North Atlantic at these critical high latitudes, so that surface water density became too low to sink.  Such a process had occurred several times during the last glacial period, each of which has been correlated with release of massive amounts of glacial ice as icebergs.  There melting caused the freshening. The Younger Dryas is a different kind of event, because it occurred well into the period of global warming that brought the Ice Age to an end.  A seemingly plausible explanation was suggested by Wallace Broecker in 1989, who looked to explosive release of meltwater trapped in glacial lakes roughly along the Canadian-US border along the present St Lawrence River Valley, effectively flooding the source of NADW with a surface layer of low-density, low-salinity water.

The problem with Broecker’s mechanism is that sea-level records through the Younger Dryas show no sudden rise, whereas at about 14 ka a meltwater pulse had resulted in a 20 m rise over about 500 years, with no sign of a climatic response to a shutdown of the Gulf Stream by the freshening that it caused.  A similar event occurred shortly after the waning of the Younger Dryas.  There is no doubt that throughout high northern latitudes the great ice sheets were melting since about 18 ka. A new approach to the Younger Dryas concentrates on where the meltwater formed in northern North America probably escaped to the sea (Tarasov, L & Peltier, W.R. 2005.  Arctic freshwater forcing of the Youner Dryas cold reversal.  Nature, v. 435, p. 62-665).  Through their analysis of the drainage chronology of the Canadian Shield Tarasov and Pelter conclude that at the time of the onset of the Younger Dryas most flow was roughly along the present MacKenzie River valley to the Arctic Ocean.  Freshening of the Arctic Ocean would escape through the narrow Fram Straits directly to the source region for NADW. It would not necessarily have been through currents, for escape of increased amounts of pack ice would have much the same effect.  Central to their hypothesis are new data that relate to extraordinarily thick continental ice in the Keewatin glacial dome, that formed just to the east of modern Great Slave Lake.

Acidification of the oceans

When gases such as CO2 and H2S permeate through ocean water they dissolve to form weak acids: carbonic and sulfurous acid respectively. So many organisms, plants as well as animals, incorporate carbonates into their hard parts that changes in acidity constitute an important kind of stress.  The acidity of water combines with increasing pressure as water deepens to create a zone (the lysocline) in which water is undersaturated in calcium carbonate.  Below the lysocline carbonate shells begin to dissolve.  Deeper still is a level (the carbonate compensation depth, or CCD) below which there is no free CaCO3 in the water column.  Falling shelly material dissolves completely, so that deep-ocean sediments contain few if any shells other than those of silica-secreting organisms.  At present the CCD is around 4 km deep.  Any shift in the pH of the oceans causes the CCD either to rise or fall.  The signatures of such shifts lie in the composition of ocean-floor sediments.  In the deepest parts, where silica and clays dominate, layers in which carbonate shells are preserved signify a decrease in acidity (increased pH) and descent of the CCD to below the elevation of the ocean floor.  On the other hand, the appearance of pure clay-silica oozes in otherwise shelly muds, where the sea floor has been well above the CCD for long periods, show that acidity increased (a drop in pH) over a period.  Such anomalous sediment layers are often easy to see in cores because their colour is different from the common sediments.

In cores from ocean depths between 2 and 4 km, the second kind of anomaly appears consistently at the level of the Palaeocene-Eocene boundary: it signifies a massive increase in acidity (Zachos, J.C. et al. 2005.  Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum.  Science, v. 308, p. 1611-1614). Carbon-isotope measurements from the same cores also show a marked shift.  The sediments are depleted in 13C, which has generally been taken to indicate a huge release of methane from storage as gas hydrate in sea-floor sediment at the time of the Palaeocene-Eocene boundary.  Most palaeoclimatologists consider the C-isotope “spike” to be a proxy for sudden, intense warming that resulted from methane – a more efficient ‘greenhouse’ gas than CO2 – and the carbon dioxide produced as it was oxidized.  The range of water depths where the carbonate-free layers occur enables marine geochemists to estimate the rate of acidification.  In around only 10 ka the CCD rose 1.3 to 2.0 km above its current level.  From the degree of acidification needed it seems that considerably more than 2 x 1012 t of carbon was released in the form of methane that eventually oxidized to CO2, and returned to the ocean.  The carbonate content of the ocean sediments rose gradually over the next 100 ka, by the end of which the former balance was restored. This information in turn gives a picture of the rate at which sudden ‘greenhouse’ events subside once their cause has stopped being produced, almost certainly by the drawdown of atmospheric CO2 by weathering of silicate minerals exposed on the continental surface.

At the end of the Palaeocene, the effect on organisms was mainly restricted to benthic foraminifera that live in moderately deep water, which show a selective extinction.  The eventual release by human activity of carbon contained in accessible fossil fuel reserves, will give a mass of carbon in ‘greenhouse’ gases of about twice that released at the Palaeocene-Eocene boundary over perhaps 300 years.  Such rapid release may result in acidity that is incompatible with carbonate-secreting organisms anywhere in the oceans: the CCD will effectively be at the sea surface

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