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Conventional wisdom and climate history

Steven M. Colman^*

Large Lakes Observatory and Department of Geological Sciences, University of Minnesota Duluth, Duluth, MN 55812

The Younger Dryas interval, a cold snap that chilled many parts of the world for 1,500 years or so in the midst of the last deglaciation (13,000–11,500 years ago), is perhaps the best known and most studied paleoclimate event of the last 2 million years. Only a few years ago, it was well accepted that a change in the drainage routing of the huge proglacial lake that fronted the North American ice sheet occurred at just about the same time as the beginning of the Younger Dryas cold period. This and other coincidences in timing, as well as considerations of the effects this event might have had on ocean circulation, led many to believe that the meltwater rerouting caused, or triggered, the Younger Dryas cold interval. Over the last few years, all of this conventional wisdom was thrown into turmoil by a few new observations and age determinations. Now, in this issue of PNAS, Carlson et al. (1) provide a new set of data about meltwater discharge at the start of the Younger Dryas, as well as new detail regarding events within this period. They also suggest that the conventional wisdom about the inception of the Younger Dryas may not be as flawed as has been suggested recently.

The Younger Dryas began and ended abruptly, at least as indicated in ice core records from Greenland, where temperature initially may have fallen by 15°C, with transitions no longer than a few decades (2); most of the final (warming) transition may have occurred in just a few years (3). At just about the time of the inception of the Younger Dryas, a major change in the routing of meltwater from the Laurentide Ice Sheet in North America seems to have occurred. The outlet of glacial Lake Agassiz, which fronted the Laurentide Ice Sheet across a vast section of the continental interior, appeared to have switched from Mississippi River drainage (and thence to the Gulf of Mexico) eastward to the Laurentian Great Lakes (and thence to the St. Lawrence River and the North Atlantic Ocean). The switch in outlets was accompanied by a major (>40 m) initial drawdown of Lake Agassiz during its Moorhead Phase, perhaps eventually reaching as much as 150 m below the southern outlet (4). The switch was also accompanied (within the uncertainties of radiocarbon dating) by an abrupt change in oxygen isotopes in the Gulf of Mexico (documented in many studies, most recently in ref. 5), interpreted as resulting from an increase in seawater salinity that accompanied the removal of Agassiz drainage down the Mississippi. Indeed, together with a plausible route for the eastern outflow, the evidence for an eastward switch in drainage seemed compelling. Ocean climate modeling studies (e.g., see references in ref. 6) suggested that the estimated increase in freshwater input to the North Atlantic would be sufficient to suppress Atlantic meridional overturning circulation, distinctly cooling the region where the Younger Dryas is best documented, as well as other areas. Oceanic proxy evidence for a slowdown or cessation of

The Younger Dryas began and ended abruptly with transitions no longer than a few decades.

Atlantic meridional overturning circulation (in the past loosely called "thermohaline circulation" or "the conveyor belt") during the Younger Dryas is relatively strong (1, 7).

Nevertheless, the inference that eastward routing of Lake Agassiz discharge was the cause of the Younger Dryas has not been without its problems. Isotopic evidence for a freshening of the North Atlantic off the mouth of the St. Lawrence has been equivocal at best (8, 9). In addition, the effect of Lake Agassiz inflow on the sediments of the Laurentian Great Lakes has not been clear; sedimentological and isotopic evidence from Lake Michigan was interpreted as a signal of Lake Agassiz inflow (10), whereas similar kinds of data from the Huron basin were interpreted differently (11).

Despite these difficulties, eastward routing of Lake Agassiz discharge as the trigger for the Younger Dryas was widely accepted. Until recently, the main issues were whether Younger Dryas meltwater rerouting provided an analog for mechanisms to explain earlier abrupt climate changes when the ice sheets were of intermediate size (12) and whether the changes in ocean circulation during the Younger Dryas were mostly due to the long-term increase in base discharge (12), the postulated initial catastrophic flood (13), or a combination of the two (6).

Troubles with the conventional wisdom began with recent expeditions to the remote terrain postulated as the route of the catastrophic eastward drainage of Lake Agassiz at the beginning of the Younger Dryas. These observations failed to find geomorphic evidence of major flood channels or depositional features (14). In addition, new radiocarbon and cosmogenic radionuclide dating on glacial features south of the proposed outlet suggested that the outlet area was not deglaciated until after the start of the Younger Dryas (14). Finally, new ages related to the Moorhead Phase of Lake Agassiz have been interpreted to mean that this phase occurred significantly after the start of the Younger Dryas (15). On the other hand, complexities exist in the geomorphic interpretations, and virtually all of the ages on glacial features are minimum ages or have large uncertainties (16). The differences in interpretation have wide-ranging climatic and oceanographic implications (7).

Given the well documented, if poorly dated, drawdown of Lake Agassiz during its Moorhead Phase, the massive amounts of water involved had to go somewhere. Some researchers have suggested that the water discharged through the northwestern Clearwater Outlet to the Arctic Ocean, although there are chronological problems with this scenario (7, 16). The new data presented by Carlson et al. (1) strongly suggest that the water did indeed discharge eastward, eventually through the St. Lawrence estuary. They use three different geochemical signatures of ambient water, preserved as trace elements in the shells of foraminifera eventually buried in dated sediment. Each of the geochemical tracers indicates a source of water from the Canadian interior plains, i.e., the drainage basin of Lake Agassiz. The authors also attempt deconvolve complex stable-isotope proxies in a way that is consistent with the geochemical tracers. The isotope exercise is less convincing because the isotope ratios depend in a complex way on temperature and salinity, as well as on the isotopic composition of river and ocean water. The interpretation of each tracer or proxy has its complications, but taken together, they make a strong case for major Lake Agassiz discharge through the St. Lawrence. The chronology for their sediment cores is good enough for Carlton et al. to argue convincingly that a surge in Lake Agassiz discharge through the St. Lawrence estuary coincided with the start of the Younger Dryas. Moreover, the authors were able to use mixing models with the geochemical tracer data to quantitatively estimate the amount of the Lake Agassiz discharge and how it varied with time (see Fig. 1). They infer substantial variations in discharge within the Younger Dryas interval. Although other studies have suggested variation in climate, vegetation, and ocean processes during the Younger Dryas (e.g., ref. 17, as well as the references cited by Carlson et al.), this is the first estimation, independent of changes in the level of Lake Agassiz itself, of quantitative variation in the purported driver of climate change at the time: discharge of fresh water to the North Atlantic.

View larger version (39K): [in this window] [in a new window] Fig. 1. Comparison of reconstructed discharges down the St. Lawrence River around the time of the Younger Dryas (YD) cold interval. Items in blue are related to the article by Carlson et al. (1) discussed here. The thin blue line with symbols is one (U/Ca) of the five proxies used in their study. The thick blue line is their reconstructed discharge, calculated from an average of the different geochemical proxies and a water mixing model. Light blue shading indicates the time interval they consider as showing evidence for Lake Agassiz eastward discharge, and dark blue shading indicates their "intra-Younger Dryas routing event." The red line is a previous estimate of Lake Agassiz discharge down the St. Lawrence (12), based on reconstructions of lake levels and drainage areas for Lake Agassiz (19). The spike at 12.9 ka indicates possible catastrophic flood discharge (off-scale) of as much as 0.45 Sv (6, 13). The arrow at the top indicates the duration of the Younger Dryas (12.9–11.6 ka) in Greenland's GISP2 ice core (2). The arrow at lower left indicates the modern average annual discharge of the St. Lawrence River.

Many issues and questions remain. As always, chronological uncertainties obscure interpretations of history. Some of the dating conflicts that exist may arise from using minimum ages for glacial events, even if the ages are closely limiting minima. In addition, the Younger Dryas is a challenging interval for radiocarbon dating because it coincides with a change in the content of radiocarbon in the atmosphere. This change, itself partly the result of changes in oceanic circulation related to the Younger Dryas (18), makes age resolution of less than a few hundred years very difficult at this time.

The question of the route of the discharge from Lake Agassiz to Lake Superior at the beginning of the Younger Dryas is still problematic. Seismic stratigraphic studies beneath Lake Superior are underway to help address this issue. The classical story of the switch from southward to eastward drainage of Lake Agassiz involved an initial massive and catastrophic flood (13). Carlson et al. (1) make no mention of this flood and instead explain their record entirely on the basis of increases and decreases in base flow down the St. Lawrence. Therein may lie an explanation for the lack of spectacular flood canyons on the drainage route to the east, such as those that are related to a younger, post-Younger Dryas drainage event. If the drawdown during the Moorhead Phase took place gradually (over decades to a couple of centuries, not resolvable in the records of Carlson et al.), the discharge may not have formed pronounced geomorphic features (16). If the discharge occurred near the ice margin but under (or over) the ice (7), bedrock erosion and large-scale depositional forms would also be minimized.

Whatever the issues that remain, the article by Carlson et al. (1) is a major contribution to the controversy over the Younger Dryas cold interval, and it is sure to stimulate a great deal of follow-up research. As for the recent differences of opinion that have arisen from reexamining the accepted history of the Younger Dryas, they have generated much new research, including the article by Carlson et al. Other creative theories, including the possibility of an extraterrestrial impact at this time, are looming on the horizon. There is something to be said for challenging conventional wisdom.

Footnotes

Author contributions: S.M.C. wrote the paper.

The author declares no conflict of interest.

See companion article on page 6556. Click on the page number to go to the original article by Carlson et al.