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Routing of Meltwater from the Laurentide Ice Sheet During The LETTERS TO NATURE very high sulphate concentrations (Fig. 1). Thus, differences in P release has yet to prove the mechanism behind this relation­ P cycling between fresh waters and salt waters may also influence ship. If sediment P release were controlled largely by sulphur, the switch in nutrient limitation. our view of the lakes that are being affected by atmospheric A further implication of our findings is a possible effect of S pollution could be altered. It is believed generally that anthropogenic S pollution on P cycling in lakes. Our data lakes with well-buffered watersheds are insensitive to the effects indicate that aquatic systems with low sulphate concentrations of atmospheric S pollution. However, because changing have low RPR under either oxic or anoxic conditions; systems atmospheric S inputs can alter the sulfate concentration in with only slightly elevated sulphate concentrations have sig­ surface waters22 independent of acid neutralization in the water­ nificantly elevated RPR, particularly under anoxic conditions shed, the P cycle of even so-called 'insensitive' lakes may be (Fig. 1). Work on the relationship between sulphate loading and affected. D Received 22 February; accepted 15 August 1987. 17. Nurnberg. G. Can. 1 Fish. aquat. Sci. 43, 574-560 (1985). 18. Curtis, P. J. Nature 337, 156-156 (1989). 1. Bostrom, B .. Jansson. M. & Forsberg, G. Arch. Hydrobiol. Beih. Ergebn. Limno/. 18, 5-59 (1982). 19. Carignan, R. & Tessier, A. Geochim. cosmochim. Acta 52, 1179-1188 (1988). 2. Mortimer. C. H. 1 Ecol. 29, 280-329 (1941). 20. Howarth, R. W. & Cole, J. J. Science 229, 653-655 (1985). 3. Hecky, R. E. & Kilham. P. Limnol. Oceanogr. 33, 776-795 (1988). 21. Seitzinger, S. P., Nixon, S. W. & Pilson, M. E. Q. Limno/. Oceanogr. 29, 73-83 (1984). 4. Howarth, R. W. A. Rev. Eco/. Syst. 19, 191-198 (1988). 22. Likens. G. E. Hydrobiologia 176/177, 331- 348 (1989). 5. Richards, F. A. in Chemical Oceanography (eds Riley, J. P. & Sl<irow, G.) 601-645 (Academic. 23. Brunskill, G. J.. Povoledo. D., Graham, B. W. & Stainton, M.1 Fish. Res. Bd Can. 28, 277-294 (1971). London, 1965). 24. Heaney, S. 1., Smyly, w. J.P. & Tailing, J. F. Int. Rev. ges. Hydrobiol. 71, 441-494 (1986). 6. Richards, F. A .. Cline, J. D.. Broenl<ow, W. W. & Atkinson, L. P. Umnol. Oceanogr., Supp/. 10, 25. Gorham, E. & Swaine, D. J. Limnol. Oceanogr. 10, 268-279 (1965). R185-R202 (1965). 26. Schelske, C. L. in Biological Effects in the Hydrobiological Cycle 59-81 (Purdue University. 1971). 7. SenGupta. R. thesis, Univ. Goteborg, (1973). 27. Williams. J. D. H.. Jaquet, J. M. & Thomas. R. L. 1 Fish. Res. Bd Can. 33, 413-429 (1976). 8. Burns. N. M. & Ross. C. Proc 14th Conf. Great Lakes Research 7 49-760 (1971). 28. Menzel. D. W. & Corwin, N. Limnol. Oceanogr. 10, 280-282 (1965). 9. Uehlinger, U. & Bloesch, J. Freshwater Biology 17, 99-108 (1987). 29. Stainton, M. P. 1 Fish. Res. Bd Can. SO, 1441-1445 (1973). 10. Honjo, s .. Manganini, S. J. & Cole, J. J. Deep Sea Res. 29, 609-625 (1982). 30. Capone. D. G. & Liene, R. P. Umno/. Oceanogr. 33, 725-7 49 (1988) 11. Levine, S. N., Stainton, M. P. & Schindler, D. W. Can. 1 Fish. aquat. Sci. 43, 366-378 (1986). 12. Hawl<e, D .. Carpenter, P. D. & Hunter. K. A. Envir. Sci. Techno/. 23, 187-191 (1989). 13. Sugawara, K .. Koyama. T. & Kamala, E. 1 Earth Sci., Nagoya Univ. 5, 60-67 (1957). ACKNOWLEDGEMENTS. We thank S. Nolan, M. Pace, K. Porter, A. H. Puccoon and D. W. Schindler for 14. Stauffer. R. E. Umnol. Oceanogr. 30, 123-146 (1985). advice and assistance. This project was supported by the NSF. the Andrew W. Mellon Foundation. 15. Gachter, R. & Mares, A. Umno/. Oceanogr. 30, 364-371 (1985). and by the Mary Flagler Cary Charitable Trust, and is a contribution to the program of the Institute 16. Lean. D. R. s .. McQueen. D. J. & Story, V. A. Arch. Hydrobiol. 108, 269-280 (1986). of Ecosystem Studies. Evidence for these diversions of meltwater comes from Routing of meltwater from the changes in the level of proglacial Lake Agassiz, which acted as a 'clearinghouse' for runoff from a 2 x 106 km2 area along the Laurentide Ice Sheet during southwestern side of the Laurentide Ice Sheee. Before 11,000 yr BP, the meltwater reaching Lake Agassiz overflowed the Younger Dryas cold episode to the Gulf of Mexico through the Mississippi River drainage system (Fig. la). By -11,000 yr BP the Laurentide Ice Sheet Wallace S. Broecker*, James P. Kennettt, had retreated far enough to open a series of channels leading Benjamin P. Flo wert, James T. Teller:j:, to the Lake Superior basin, creating a diversion of the Agassiz Sue Trumbore§, Georges Bonani§ & Willy Wolfli§ *Lamont-Doherty Geological Observatory, Palisades, New York 10964, TABLE 1 Key radiocarbon dates of wood USA t Marine Institute and Department of Geology, University of California, Moorhead low-water phase of Lake Agassiz* Santa Barbara. California 93106, USA 10,960±300 (W-723) 10,340 ± 170 0-5213) t Department of Geological Sciences, University of Manitoba, Winnipeg, 10,820 ± 190 (T AM-1) 10,200 ± 80 (GSC-1909) Manitoba, Canada, R3T2N2 10,680 ± 190 (GSC-677) 10,080 ± 280 (W-900) § ETH Honggerberg, CH-8093 Zurich, Switzerland 10,550 ± 200 (Y -411) 10,050 ± 300 (W-1005) Marquette glacial advance into Superior basint ROOTH 1 proposed that the Younger Dryas cold episode, which 10,250 ± 250 (W-1541) 10,100 ± 100 (WIS-409) chilled the North Atlantic region from 11,000 to 10,000 yr BP, was 10,230 ± 300 (W-3896) 9,850 ± 300 (W-3866) initiated by a diversion of meltwater from the Mississippi drainage 10,230 ± 500 (W-1414) 9,780 ± 250 (W-3904) to the St Lawrence drainage system. The link between these events 10,220 ± 215 (DAL-338) 9,730 ± 140 0-5082) is postulated to be a turnoff, during the Younger Dryas cold 10,200 ± 500 (M-359) 9,545 ± 225 (DAL-340) episode, of the North Atlantic's conveyor-belt circulation system 10,100 ± 250 (WIS-409) which currently supplies an enormous amount of heat to the Emerson high-water phase of Lake Agassizt 2 atmosphere over the North Atlantic region • This turnoff is 10,000 ± 280 (GSC-1428) 9,890 ± 300 0-4853) attributed to a reduction in surface-water salinity, and hence also 10,000 ± 150 (GSC-870) 9,880 ± 225 (GX-3696) in density, of the waters in the region where North Atlantic Deep 9,990 ± 160 (GSC-391) 9,820 ± 300 (W-1361) Water (NADW) now forms. Here we present oxygen isotope and 9,940 ± 160 0-3880) 9,810 ± 300 (W-1360) accelerator radiocarbon measurements on planktonic foraminifera 9,930±280 (W-388) 9,700 ± 140 (GAC-797) from Orca Basin core EN32-PC4 which reveal a significant reduc­ 9,900 ± 400 (W-993) tion in meltwater flow through the Mississippi River to the Gulf These dates (yr BP) were used to establish the chronology for the routing of Mexico from about 11,200 to 10,000 radiocarbon years ago. of meltwater from Lake Agassiz drainage basin during the Younger Dryas This finding is consistent with the record for Lake Agassiz which cold episode. The sample numbers are given in brackets. indicates that the meltwater from the southwestern margin of the * Low-water phase during which meltwater flowed east to the northern Laurentide Ice Sheet was diverted to the northern Atlantic Ocean Atlantic Ocean. through the St Lawrence valley during the interval from -11,000 t Glacial advance that dammed eastern overflow from Lake Agassiz. to 10,000 years before present (yr BP). t High-water phase during which meltwater flowed south to Gulf of Mexico. 318 NATURE · VOL 341 · 28 SEPTEMBER 1989 © 1989 Nature Publishing Group LETTERS TO NATURE FIG. 1 a, Map showing Laurentide Ice Sheet and routing of overflow from the Lake Agassiz basin (dashed outline) to the Gulf of Mexico just before the 22 23 Younger Dryas · ; b, Routing of over­ Area enlarged In b flow from Lake Agassiz through the Great Lakes to the St Lawrence and northern Atlantic during the Younger b 24 Dryas . 0 500 ---KILO M ETRES water eastward through the Great Lakes and the St Lawrence 9,545 yr and average 10,020 yr. Eleven radiocarbon ages for the valley to the northern Atlantic (Fig. 1 b). Thus, during the Emerson high-water phase of Lake Agassiz range from 10,000 Younger Dryas, meltwater and precipitation from the Lake to 9,700 yr. Agassiz basin, the discharge of which is estimated4 to have been An appropriate independent test for these reconstructions of 3 1 -30,000 m s - , was diverted to the northern Atlantic Ocean drainage history is to date meltwater influx episodes to the Gulf from its previous route to the Gulf of Mexico. This diversion of Mexico, recorded in the sedimentary record as negative values 18 12 13 caused the level of Lake Agassiz to drop by ?40 m, creating in the 8 0 of planktonic foraminifera • . Because of bioturba­ 5 the Moorhead low-water phase . Radiocarbon dates of material tion effects however, necessary resolution is not attainable for from the portions of the floor of Lake Agassiz exposed during sediments that accumulate at rates of 8 em per 103 yr. The Orca the Moorhead low-water phase range from 10,960 to 10,050 Basin, located on the continental rise off Louisiana, not only radiocarbon years (Table 1 ). offers high sedimentation rates (50 em per 103 yr) but also anoxic 14 17 14 15 Drainage from Lake Agassiz was routed back to the Mis­ conditions and hence a lack of sediment mixing - • There is • sissippi River and Gulf of Mexico beginning -10,000 years ago, a major negative 8 180 spike in Orca Basin core EN32-PC6.
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