Between about 12.9 and 11.5 ka the progress of warming from the frigidity of the Last Glacial Maximum was rudely interrupted. For over a thousand years conditions returned to those of a mini ice age, with continental glaciers re-advancing on a large scale, an increase in aridity and a reversal of colonisation of high northern latitudes by both plants and humans. Pollen records become dominated by those of a diminutive alpine plant, the mountain avens (Dryas octopetala) from which the cold snap gets its name – the Younger Dryas. The pace at which cooling took place was dramatic, and glacial conditions swept in within a decade at most. The most likely scenario is failure of North Atlantic Deep Water to form, thereby shutting down the thermohaline circulation that draws the warming Gulf Stream into the Arctic Ocean off the northern cape of Norway. The reason for that was a massive and sudden freshening of surface water at high latitudes in the North Atlantic, but where the influx of fresh water came from is a puzzle. Wallace Broeker of the Lamont-Doherty Earth Observatory in New York State resurrected an earlier idea that a vast lake of meltwater in the region of the Great Lakes of North America burst down the St Lawrence Seaway, instead of quietly escaping to the Gulf of Mexico along the Missouri-Mississippi system. Broeker has recently reviewed this hypothesis (Broeker, W.S. 2006. Was the Younger Dryas triggered by a flood? Science, v. 312, p. 1146-1148).
Oxygen isotope records from sediments in the Gulf of Mexico had been recording massive influx there of water depleted in 18O; a sure sign that the Mississippi was carrying much of the water produced by melting of the Laurentian ice sheet. That signature stops abruptly at the outset of the Younger Dryas. The meltwater must have found another outlet, but so far its oxygen isotope signature has not been conclusively discovered. As well as the St Lawrence escape route there are three other possibilities: north-westwards along the MacKenzie River valley; beneath the great ice sheet and through Hudson Bay; and by massive break-up of the ice sheet to launch an ‘armada’ of icebergs that quickly melted to freshen northern Atlantic waters. One of the clearest signs that vast proglacial lakes suddenly emptied is that they carve immense channels resembling canyons, in which there is abundant evidence for extreme scouring. Examples are the ‘channelled scablands’ of the state of Washington, and the Minnesota River valley. The volume escaping at the start of the Younger Dryas would have been so immense that such overflow channels would be dominant features of northern North America’s terrain; but there are few that fit the bill, and those that do exist are poorly constrained by radiocarbon dating. The lack of accurate dates for sediments and channels associated with the demise of the Laurentian ice sheet is the main obstacle, and surely evidence for exactly how the sudden plunge into glacial conditions was triggered will emerge sooner rather than later. One thing seems certain, the Younger Dryas was a freak event. The new ice core from Antarctica (see Yet further back in the Antarctic ice in the December 2005 issue of EPN) penetrates the previous six glacial maxima and shows no sign of a similar event at their terminations.
Sedimentary evolution of the Arctic Ocean: a start is made
For the Northern Hemisphere, especially around the North Atlantic, what happens in the Arctic exerts a strong influence over climate. On the one hand, ice-cover increases the proportion of solar energy that is reflected back to space, giving a cooling effect. On the other, cooling and increasing salinity of high-latitude water at the ocean surface results in its sinking to draw in warmer waters from further south, to extend warming further north. The two are linked intricately, for sea-ice formation adds to surface waters’ salinity. How and when the delicate balances arose remained poorly known while thick sea ice prevented ships penetrating to the highest possible latitudes in the Arctic Ocean, because the key to climate evolution depends on access to long core through ocean-floor sediments. Ironically, the decrease in Arctic ice cover with global warming has created greater access by icebreakers and drilling vessels. A consortium of countries around the Arctic funded a major effort to resolve the gap in knowledge through such a marine drilling programme in 2004. Results from the polar expedition have just begun to emerge (Moran, K and 36 other 2006. The Cenozoic palaeoenvironment of the Arctic Ocean. Nature, v. 441, p. 601-605). The cores were taken almost at the North geographic pole on the Lomonosov Ridge, a sliver of continental crust separated from its connection with the northern Russian continental shelf when North Atlantic sea-floor spreading nosed into the Arctic about 57 Ma ago.
The core is from sediments deposited on the Lomonosov ridge since it became detached from Russia, and is over 400 m long. Analyses are not yet complete, and the report by the IODP Arctic Coring Expedition covers the simplest parameters to determine: sediment bulk density and lithology, and micro-organisms. Nonetheless, these preliminary results provide a major surprise. Previously it was believed that frigid conditions in northern polar regions became established long after the Antarctic developed an ice cap 43 Ma ago, which matches the Cenozoic fall in atmospheric CO2 and other evidence for lower mean global temperatures. The first glaciation in the Arctic was thought to be at 2-3 Ma, when pebbles dropped by icebergs first appear in the cores from the North Atlantic floor. In the Arctic Ocean core, such pebbles appear at much the same time as those around the Antarctic. They become widespread by 14 Ma. At the time of the Palaeocene-Eocene global warming, in response to massive methane emissions at 55 Ma, the Arctic waters were as warm as 18°C. The record is one of transition from a greenhouse world to an ice house. Surprisingly, considering the later influence of thermohaline processes that draw in warm water from lower latitudes, the earliest period is marked by fresh or at most slightly brackish waters. That was probably a result of isolation from the Atlantic and an excess of precipitation over evaporation. The early sediments record abundant carbon, then at around 14 Ma, the percentage of buried organic carbon drops dramatically to mark the start of increasing frigidity, when icebergs dropped significantly more debris in the Arctic Ocean.
HOW THE AMAZON FORMED
The world’s largest drainage system in the Amazon basin is so huge that it might seem to be an eternal feature of South America, at least since that continent formed when opening of the South Atlantic wrenched it from Africa in the Triassic. The upper Amazon takes much of its flow from rainfall in the eastern slopes of the Andes, but that range is still in the process of formation by tectonic and volcanic forces. A review of the Amazon’s evolution in a recent issue of Scientific American (Hoorn, C. 2006. The birth of the mighty Amazon. Scientific American, v. 294 May 2006, p. 40-47) shows that the river system is much younger than you might have expected. Lots of evidence points to the major eastward flow only beginning in the late Miocene, after 15 Ma ago. Before that drainage was northwards into the Caribbean, the reason being that the north-eastern Andes of Columbia and western Venezuela had not formed. When they did begin to rise, they hindered flow to create a huge wetland in what is now eastern Columbia. Eventually a northward drainage route was definitively blocked, so that flow took the easiest remaining route to the ocean; eastwards, to create the Amazon basin.