Role for tropical weather in last glacial epoch

The Cariaco Basin off Venezuela lies in an area that is sensitive to climate change and has been for at least the last 90 thousand years.  As the trade winds change with the seasonal migration of the Intertropical Convergence Zone (ITCZ), cold, nutrient-rich waters well up along the coast of northern Venezuela.  Biological productivity waxes and wanes on an annual rhythm, as too do sediments transported into the basin by the great rivers of this part of South America – shifts of the ITCZ also impose annual wet and dry seasons over land.  This cyclicity seems to have functioned since at least 90 ka ago, and drill cores from the Cariaco trench are dateable at the annual level because of the colour banding of seasonal sediments (Peterson, L.C. et al.,. 2000.  Rapid changes in the hydrological cycle of the tropical Atlantic during the last glacial.  Science, v. 290, p. 1947-1951).

Matching the varying thicknesses of colour bands beneath the Cariaco Basin to the high-latitude climate record preserved in Greenlandic ice-cores shows a remarkable correlation.  Warming (interstadials) over Greenland correspond to periods of increased rainfall and ocean bio-productivity (the layers are thicker) off Venezuela.  Peterson and his co-workers believe that this could signify periods of greater transport of water vapour from the Atlantic to the Pacific.  That would increase the salinity of the Atlantic.  Working through to high latitudes, saltier surface water would more easily become dense cold brine once sea ice had been frozen from it.  That would enhance the thermohaline deep circulation of the North Atlantic, so that warm, tropical waters might be dragged further to the north during interstadials, in the manner of today’s Gulf Stream.  It is hard to see how just melting ice sheets during interstadials could do that; in fact that would encourage a further reduction of deep circulation.  So, a tropical connection seems plausible.  However, interstadials stopped extremely rapidly, repeatedly plunging high latitudes into full glacial, or stadial conditions.  That may well have been an outcome of all the fresh water from melting glaciers acting to dilute surface waters’ saltiness, and thereby shutting down thermohaline processes.

The annual precision of sediment cores from the Cariaco Basin carries a bonus, by helping better to calibrate 14C dating.  Radiocarbon dates have long been known not to correspond predictably to calendar years.  For instance, dates from around 11 ka ago, the time of the last major glacial advance (the Younger Dryas) show a mismatch of about a thousand years between dates based on counting tree rings and annual ice layers (exact calendar years), and those provided by 14C dating of carbon-rich samples.  The reason for this is partly fluctuations in the production of 14C by bombardment of nitrogen atoms in the stratosphere by cosmic radiation and the solar wind.  The Cariaco Basin layering extends calendar dating at least 5 000 years further back, into the period when deglaciation accelerated as the Earth’s climate emerged from the last glacial maximum (Hughen, K.A. et al., 2000.  Synchronous radiocarbon and climate shifts during the last glaciation. Science, v. 290, p. 1951-1954).  That helps to evaluate shifting rates of 14C production over this part of the core (maybe related to varying solar output because they match shifts in 10Be, also produced by upper atmosphere processes), and to add meaning to radiocarbon dates from it.  However, not all the shifts in 14C can be due to solar fluctuations, and it is clear that the largest, during the Younger Dryas event, stemmed from increased carbon preservation on the ocean floor, that removes all isotopes of such carbon from the atmosphere and upper ocean.  This supports the notion that the Younger Dryas, and perhaps all the stadial-interstadial events of the last 90 ka stem from changes in ocean processes.


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