- Email: [email protected]
Questioning the age of the Moorhead Phase in the glacial Lake Agassiz basin
ARTICLE IN PRESS
Quaternary Science Reviews 25 (2006) 2688–2691
Rapid Communication
Questioning the age of the Moorhead Phase in the glacial Lake Agassiz basin Timothy G. Fishera,, Thomas V. Lowellb a
Department of Earth, Ecological, and Environmental Sciences, MS#604, 2801 W. Bancroft St., University of Toledo, Toledo, OH 43606 USA b Department of Geology, University of Cincinnati, Cincinnati, OH 45221, USA Received 17 February 2006; received in revised form 18 May 2006; accepted 25 May 2006
Abstract The stratigraphy of sites within the Lake Agassiz basin that constrain the timing of the onset of the Moorhead low-water Phase is reexamined. Stratigraphic interpretation of the oldest date (10,960 14C yr BP) from cross-bedded sand of the Ojata Beach is questioned, particularly in light of demonstrated long-distance transport and reworking of older organic material from elsewhere in the basin. A maximum onset age for the Moorhead Phase is suggested from a 10,675760 14C yr BP age from wood in gravel from the base of the southern outlet, whereas radiocarbon dates from in situ peat indicate a later minimum onset age of 10,3407100 14C BP. r 2006 Elsevier Ltd. All rights reserved.
1. Introduction The apparent temporal coincidence of the Younger Dryas cold period with the Moorhead low-water phase of Lake Agassiz has been used to establish a causal mechanism between Lake Agassiz outburst floods and rapid climate change (Broecker et al., 1989; Licciardi et al., 1999; Clark et al., 2001; Teller et al., 2002; Teller and Leverington, 2004). However, independent chronologic and physical evidence for meltwater routing from the Lake Agassiz basin is sparse, an eastern outlet at Younger Dryas time has not been documented, and current dating evidence suggests that deglaciation at the hypothesized eastern outlet was near the end of the Younger Dryas rather than the beginning (Lowell et al., 2005; Teller et al., 2005). Consequently, a reexamination of the data used to assign an 11,000 14C yr BP age for the beginning of the Moorhead Phase is in order.
2. Moorhead Phase The Moorhead Phase is the period in time when the southern outlet was abandoned and lake levels were low, Corresponding author. Tel.: +419 530 2883; fax: +419 530 4421.
E-mail address: [email protected] (T.G. Fisher). 0277-3791/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2006.05.007
and it ended when levels rose to the Upper Campbell Beach, reopening the southern outlet (Clayton, 1983); see Fisher (2005) for a recent review and discussion of the Agassiz chronology. There are widespread stratigraphic data for the Moorhead Phase, consisting of peat, littoral, and fluvial deposits interbedded with lacustrine sediments (e.g., Upham, 1895; Harris et al., 1974; Arndt, 1977; Fenton et al., 1983). These deposits are a minimum of 30 m elevation below the southern outlet. We review the relevant chronologic data for the beginning of the Moorhead Phase and hypothesize that it is younger than previously thought. For consistency with the previous literature, all ages are reported in radiocarbon years before present (BP). 3. Onset of the Moorhead phase 3.1. Rainy River lowland and Wampum Numerous Moorhead-aged dates have come from the Rainy River lowland (Fig. 1). By examining pollen, plant macrofossils, insect and mollusc assemblages, Bajc et al. (2000) determined that long-distance transport and sediment reworking affected all of their sample localities. For example, Bajc et al. (2000) suggested that the oldest wood at site F87-33 (10,8107240 BP, TO-1504) might be a
ARTICLE IN PRESS T.G. Fisher, T.V. Lowell / Quaternary Science Reviews 25 (2006) 2688–2691
2689
Fig. 1. Digital elevation model of the southern Lake Agassiz basin with locations noted in text. Radiocarbon ages discussed in the text are located on the map and plotted in chronological context as black arrows on the inset graph. W–Winnipeg, GF–Grand Forks, F–Fargo.
minimum age for the Moorhead Phase if its origin was local. Overall, wood dates from cores and outcrops in the Rainy River lowland range between 10,8107240 BP (TO-1504) and 95307140 BP (WAT-1934). A possible in situ age comes from Mud Lake where a 6 cm thick bulk sample of fine organic detritus (peat?) yielded an age of 10,7007140 BP (WAT-1910). Otherwise, Bajc et al. (2000, p. 1338) conclude ‘‘Most Moorhead Phase organic remains are detrital and possibly reworked from older deposits.’’ Presently there is no way to distinguish between Moorhead-aged organic deposits originating in the basin from organic material transported from outside the basin. Similar evidence for reworked material is found at the Wampum site 200 km to the west (Fig. 1). Teller et al. (2000) proposed a similar interpretation for clasts of organic material (possible wood fragment, 12,240780 BP, TO-4870) 3000 years older than the Campbell Beach sediment at the Wampum site. Most dates from the Wampum site are younger than 10,100 BP, clustering around 10,000 BP. The youngest age in the upper Campbell Beach sediment is 9340790 BP (TO-4855). Below the Campbell Beach in the Rainy River lowlands, and in the Upper Campbell Beach at Wampum, Moorhead-aged sediment from a variety of depositional environments
contains reworked wood making an age assignment for the low-water phase problematic from these data. 3.2. Ojata Beach deposits The oldest wood age commonly cited for the beginning of the Moorhead Phase is 10,9607300 BP (W-723). This sample comes from cross-bedded sand over till, overlain by o1 m of pebbly, clayey silt and 1.3 m of massive sand from west of Grand Forks, ND (Fig. 1) on the Ojata strandline (Moran et al., 1973). This site lies about 35 m below the modern elevation of the Upper Campbell Beach at the southern outlet. Its elevation has been used to argue for an eastern outlet open at that time (Upham, 1895). The same sample was redated at 10,8207190 BP (TAM-1). Two other sites o1.6 km away, apparently on the same landform, reveal similar stratigraphy with a wood date in crossbedded sand between till and clay of 10,0807280 BP (W-900) and a wood date in sand of 10,3407340 BP (I5213). Moran et al. (1973) report a date of 10,0507300 BP (W-1005) on abraded wood within the top of a clay unit below 1.5 m of sand and gravel from the same Ojata strandline 23 km away (Fig. 1). Interpreting these ages to reflect the age of the beach requires 900 14C years for
ARTICLE IN PRESS 2690
T.G. Fisher, T.V. Lowell / Quaternary Science Reviews 25 (2006) 2688–2691
occupation of the Ojata Beach, which is unlikely in an isostatically rebounding glacial lake basin on a flat lake plain. An alternative interpretation of these radiocarbon ages might include transport and reworking of older wood from the west side of the lake basin. 3.3. Abandonment of the southern outlet Radiocarbon dates within gravel from the southern outlet give a maximum estimate for the beginning of the Moorhead Phase. Basal gravel overlain by mud recovered from a rotosonic core within the southern outlet contained wood fragments with ages of 10,675760 BP (AA-37029) and 10,8707170 BP (AA-37030), respectively (Fisher, 2003). The gravel and mud units record an active, then abandoned spillway. Two possible analyses of these data are: (1) average the ages because they are stratigraphically reversed, giving an estimate of 10,800 BP for abandonment of the southern outlet and commencement of the Moorhead Phase (Fisher, 2003); or alternatively, (2) because both samples are reworked, the youngest age sets the tightest bracket for falling lake levels. In the first analysis the Moorhead Phase started sometime after 10,800 BP and in the second analysis it started sometime after 10,675 BP. 3.4. Evidence from in situ organic material The oldest possible peat with a bulk date of 10,700 BP is from the Rainy River Lowland (Bajc et al., 2000). Two other sites are known with multiple in situ ages, which provide the most reliable, albeit minimum, ages for determining onset of the Moorhead Phase. The first site is at Wampum, Manitoba (Fig. 1) where sand, gravel, silt and gyttja overlay laminated lake sediment just below the Campbell Beach (Teller et al., 2000). Within this sequence is a 7 cm thick peat with a basal age of 10,3407100 BP (TO-4285) and an upper age of 9460790 BP (TO-4284). Assuming the peat is in situ, it would suggest lower water levels at Wampum at 10,3407100 BP. The second site is the Trollwood site at Fargo, ND (Fig. 1). This sequence is interpreted as a deltaic, wetland environment (Yansa and Ashworth, 2005) with fluvially transported and in situ macrofossils unconformably overlying lacustrine clay. The 2-m thick sequence has a basal date of 10,230780 BP (Beta-121851) and an upper age of 9920760 BP (AA-34343), both interpreted as being in situ (Yansa, personal communication, 2006). The Trollwood site is considerably thicker than the nearby Moorhead site (0.6 m) with a single age of 99307280 BP (W-388; Ruben and Alexander, 1958) and the Seminary site (0.15 m) with a single age of 99007400 BP (W-993; McAndrews, 1967). From the analysis of in situ organics by Yansa and Ashworth (2005), a minimum age for the beginning of the Moorhead Phase at the Trollwood site is 10,230780 BP.
4. Discussion and conclusions Past workers in the Agassiz basin have used radiocarbon ages to reflect the age of the encasing sediment, resulting in complex lake-level histories (e.g., Arndt, 1977). We contend that interpretation issues arise because wood in fluvial or littoral sediment may be reworked from elsewhere, providing only maximum ages for the sediment. The interpretation of the site with the oldest Moorhead Phase age (10,9607300 BP; W-723) in cross-bedded sand is further questioned because the Ojata Beach has always been shown as chronostratigraphically younger than beaches that lie above it in elevation (e.g., Upham, 1895; Elson, 1967; Teller and Thorleifson, 1983; Teller and Leverington, 2004). In light of the potential for reworking older organic material, the adjacent site with the same stratigraphy and younger age (10,0807280 BP) questions the validity of using the W-723 date for the beginning of the Moorhead Phase. Any progress in such settings requires many dates from a single stratigraphic unit to find the best (youngest) maximum age. However, by using in situ organic material with dates throughout its sequence (e.g., Yansa and Ashworth, 2005), better chronologic assignments are possible. Throughout the history of Lake Agassiz studies, the honoring and vetting of radiocarbon ages has found lake levels, lake phases, and deglaciation to be too old (e.g., Elson, 1967; Clayton and Moran, 1982; Teller, 1989; Lowell et al., 2005; Teller et al., 2005). Here, we reexamined evidence for an old Moorhead Phase thought to be synchronous with the Younger Dryas cold period and find that the starting age of 10,960 BP is problematic. Instead, we propose that the maximum age of 10,675760 BP from the southern outlet be combined with a minimum age of 10,3407100 BP from Wampum to provide a refined bracket on the beginning of the low-water Moorhead Phase (Fig. 1). This bracket falls well within the Younger Dryas event, opening the question of cause and effect of Lake Agassiz and Younger Dryas cooling. Acknowledgments Some of this material is based upon work supported by the National Science Foundation under Grant No. 9972998 to TGF, and also supported by the Comer Science and Education Foundation to TGF and TVL. Henry Loope and two anonymous reviewers provided clarifications to the text. References Arndt, B.M., 1977. Stratigraphy of offshore sediment of Lake Agassiz. North Dakota Geological Survey Report of Investigations 60, 1–58. Bajc, A.F., Schwert, D.P., Warner, B.G., Williams, N.E., 2000. A reconstruction of Moorhead and Emerson phase environments along the eastern margin of glacial Lake Agassiz, Rainy River basin, northwestern Ontario. Canadian Journal of Earth Sciences 37, 1335–1353.
ARTICLE IN PRESS T.G. Fisher, T.V. Lowell / Quaternary Science Reviews 25 (2006) 2688–2691 Broecker, W.S., Kennett, J., Flower, B., Teller, J., Trumbore, S., Bonani, G., Wolfli, W., 1989. Routing of meltwater from the Laurentide Ice Sheet during the Younger Dryas cold episode. Nature 341, 318–321. Clark, P.U., Marshall, S.J., Clarke, G.K.C., Hostetler, S.W., Licciardi, J.M., Teller, J.T., 2001. Freshwater forcing of abrupt climate change during the last glaciation. Science 293, 283–287. Clayton, L., 1983. Chronology of Lake Agassiz drainage to Lake Superior. In: Teller, J.T., Clayton, L. (Eds.), Glacial Lake Agassiz. Geological Association of Canada, St. John’s, Newfoundland, Special Paper 26, pp. 291–307. Clayton, L., Moran, S.R., 1982. Chronology of Late Wisconsinan glaciation in middle North America. Quaternary Science Reviews 1, 55–82. Elson, J.A., 1967. Geology of glacial Lake Agassiz. In: Mayer-Oakes, W.J. (Ed.), Life, Land and Water. University of Manitoba Press, Winnipeg, pp. 37–96. Fenton, M.M., Moran, S.R., Teller, J.T., Clayton, L., 1983. Quaternary stratigraphy and history in the southern part of the Lake Agassiz Basin. In: Teller, J.T., Clayton, L. (Eds.), Glacial Lake Agassiz. Geological Association of Canada, St. John’s, Newfoundland, Special Paper 26, pp. 49–74. Fisher, T.G., 2003. Chronology of glacial Lake Agassiz meltwater routed to the Gulf of Mexico. Quaternary Research 59, 271–276. Fisher, T.G., 2005. Strandline analysis in the southern basin of glacial Lake Agassiz, Minnesota and North and South Dakota, USA. Geological Society of America Bulletin 117, 1481–1496. Harris, K.L., Moran, S.R., Clayton, L., 1974. Late Quaternary stratigraphic nomenclature, Red River Valley, North Dakota and Minnesota. North Dakota Geological Survey Miscellaneous Series 52, 47p. Licciardi, J.M., Teller, J.T., Clark, P.U., 1999. Freshwater routing by the Laurentide Ice Sheet during the last deglaciation. In: Clark, P., Webb, R.S., Keigwin, L.D. (Eds.), Mechanisms of Global Climate Change at Millennial Time Scales. American Geophysical Union, pp. 177–201. Lowell, T.V., Fisher, T.G., Comer, G.C., Hajdas, I., Waterson, N., Glover, K., Loope, H.M., Schaefer, J.M., Rinterknecht, V., Broecker,
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W.S., Denton, G.H., Teller, J.T., 2005. Testing the Lake Agassiz Meltwater Trigger for the Younger Dryas. EOS Transactions 86, 365–372. McAndrews, J.H., 1967. Paleoecology of the Seminary and Mirror Pool peat deposits. In: Elson, J.A. (Ed.), Life, Land and Water. University of Manitoba Press, Winnipeg, pp. 253–269. Moran, S.R., Clayton, L., Scott, M.W., Brophy, J.A., 1973. Catalog of North Dakota radiocarbon dates. North Dakota Geological Survey Miscellaneous Series 53, 51pp. Ruben, M., Alexander, C., 1958. U.S. Geological Survey radiocarbon dates IV. Science 127, 1476–1487. Teller, J.T., 1989. Importance of the Rossendale site in establishing a deglacial chronology along the southwestern margin of the Laurentide Ice Sheet. Quaternary Research 32, 12–23. Teller, J.T., Leverington, D.W., 2004. Glacial Lake Agassiz: a 5000 yr history of change and its relationship to the q18O record of Greenland. Geological Society of America Bulletin 116, 729–742. Teller, J.T., Thorleifson, L.H., 1983. The Lake Agassiz-Lake Superior connection. In: Teller, J.T., Clayton, L. (Eds.), Glacial Lake Agassiz. The Geological Association of Canada, Special Paper 26, pp. 261–290. Teller, J.T., Risberg, J., Matile, G., Zoltai, S., 2000. Postglacial history and paleoecology of Wampum, Manitoba, a former lagoon in the Lake Agassiz basin. Geological Society of America Bulletin 112, 943–958. Teller, J.T., Leverington, D.W., Mann, J.D., 2002. Freshwater outbursts to the oceans from glacial Lake Agassiz and their role in climate change during the last deglaciation. Quaternary Science Reviews 21, 879–888. Teller, J.T., Boyd, M., Yang, Z., Kor, P.S.G., Mokhtari Fard, A., 2005. Alternative routing of Lake Agassiz overflow during the Younger Dryas: new dates, paleotopography, and a reevaluation. Quaternary Science Reviews 24, 1890–1905. Upham, W., 1895. The Glacial Lake Agassiz. United States Geological Survey Monograph 25, 685pp. Yansa, C.H., Ashworth, A.C., 2005. Late Pleistocene palaeoenvironment of the southern Lake Agassiz Basin, USA. Journal of Quaternary Science 20, 255–267.
Quaternary Science Reviews 25 (2006) 2688–2691
Rapid Communication
Questioning the age of the Moorhead Phase in the glacial Lake Agassiz basin Timothy G. Fishera,, Thomas V. Lowellb a
Department of Earth, Ecological, and Environmental Sciences, MS#604, 2801 W. Bancroft St., University of Toledo, Toledo, OH 43606 USA b Department of Geology, University of Cincinnati, Cincinnati, OH 45221, USA Received 17 February 2006; received in revised form 18 May 2006; accepted 25 May 2006
Abstract The stratigraphy of sites within the Lake Agassiz basin that constrain the timing of the onset of the Moorhead low-water Phase is reexamined. Stratigraphic interpretation of the oldest date (10,960 14C yr BP) from cross-bedded sand of the Ojata Beach is questioned, particularly in light of demonstrated long-distance transport and reworking of older organic material from elsewhere in the basin. A maximum onset age for the Moorhead Phase is suggested from a 10,675760 14C yr BP age from wood in gravel from the base of the southern outlet, whereas radiocarbon dates from in situ peat indicate a later minimum onset age of 10,3407100 14C BP. r 2006 Elsevier Ltd. All rights reserved.
1. Introduction The apparent temporal coincidence of the Younger Dryas cold period with the Moorhead low-water phase of Lake Agassiz has been used to establish a causal mechanism between Lake Agassiz outburst floods and rapid climate change (Broecker et al., 1989; Licciardi et al., 1999; Clark et al., 2001; Teller et al., 2002; Teller and Leverington, 2004). However, independent chronologic and physical evidence for meltwater routing from the Lake Agassiz basin is sparse, an eastern outlet at Younger Dryas time has not been documented, and current dating evidence suggests that deglaciation at the hypothesized eastern outlet was near the end of the Younger Dryas rather than the beginning (Lowell et al., 2005; Teller et al., 2005). Consequently, a reexamination of the data used to assign an 11,000 14C yr BP age for the beginning of the Moorhead Phase is in order.
2. Moorhead Phase The Moorhead Phase is the period in time when the southern outlet was abandoned and lake levels were low, Corresponding author. Tel.: +419 530 2883; fax: +419 530 4421.
E-mail address: [email protected] (T.G. Fisher). 0277-3791/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2006.05.007
and it ended when levels rose to the Upper Campbell Beach, reopening the southern outlet (Clayton, 1983); see Fisher (2005) for a recent review and discussion of the Agassiz chronology. There are widespread stratigraphic data for the Moorhead Phase, consisting of peat, littoral, and fluvial deposits interbedded with lacustrine sediments (e.g., Upham, 1895; Harris et al., 1974; Arndt, 1977; Fenton et al., 1983). These deposits are a minimum of 30 m elevation below the southern outlet. We review the relevant chronologic data for the beginning of the Moorhead Phase and hypothesize that it is younger than previously thought. For consistency with the previous literature, all ages are reported in radiocarbon years before present (BP). 3. Onset of the Moorhead phase 3.1. Rainy River lowland and Wampum Numerous Moorhead-aged dates have come from the Rainy River lowland (Fig. 1). By examining pollen, plant macrofossils, insect and mollusc assemblages, Bajc et al. (2000) determined that long-distance transport and sediment reworking affected all of their sample localities. For example, Bajc et al. (2000) suggested that the oldest wood at site F87-33 (10,8107240 BP, TO-1504) might be a
ARTICLE IN PRESS T.G. Fisher, T.V. Lowell / Quaternary Science Reviews 25 (2006) 2688–2691
2689
Fig. 1. Digital elevation model of the southern Lake Agassiz basin with locations noted in text. Radiocarbon ages discussed in the text are located on the map and plotted in chronological context as black arrows on the inset graph. W–Winnipeg, GF–Grand Forks, F–Fargo.
minimum age for the Moorhead Phase if its origin was local. Overall, wood dates from cores and outcrops in the Rainy River lowland range between 10,8107240 BP (TO-1504) and 95307140 BP (WAT-1934). A possible in situ age comes from Mud Lake where a 6 cm thick bulk sample of fine organic detritus (peat?) yielded an age of 10,7007140 BP (WAT-1910). Otherwise, Bajc et al. (2000, p. 1338) conclude ‘‘Most Moorhead Phase organic remains are detrital and possibly reworked from older deposits.’’ Presently there is no way to distinguish between Moorhead-aged organic deposits originating in the basin from organic material transported from outside the basin. Similar evidence for reworked material is found at the Wampum site 200 km to the west (Fig. 1). Teller et al. (2000) proposed a similar interpretation for clasts of organic material (possible wood fragment, 12,240780 BP, TO-4870) 3000 years older than the Campbell Beach sediment at the Wampum site. Most dates from the Wampum site are younger than 10,100 BP, clustering around 10,000 BP. The youngest age in the upper Campbell Beach sediment is 9340790 BP (TO-4855). Below the Campbell Beach in the Rainy River lowlands, and in the Upper Campbell Beach at Wampum, Moorhead-aged sediment from a variety of depositional environments
contains reworked wood making an age assignment for the low-water phase problematic from these data. 3.2. Ojata Beach deposits The oldest wood age commonly cited for the beginning of the Moorhead Phase is 10,9607300 BP (W-723). This sample comes from cross-bedded sand over till, overlain by o1 m of pebbly, clayey silt and 1.3 m of massive sand from west of Grand Forks, ND (Fig. 1) on the Ojata strandline (Moran et al., 1973). This site lies about 35 m below the modern elevation of the Upper Campbell Beach at the southern outlet. Its elevation has been used to argue for an eastern outlet open at that time (Upham, 1895). The same sample was redated at 10,8207190 BP (TAM-1). Two other sites o1.6 km away, apparently on the same landform, reveal similar stratigraphy with a wood date in crossbedded sand between till and clay of 10,0807280 BP (W-900) and a wood date in sand of 10,3407340 BP (I5213). Moran et al. (1973) report a date of 10,0507300 BP (W-1005) on abraded wood within the top of a clay unit below 1.5 m of sand and gravel from the same Ojata strandline 23 km away (Fig. 1). Interpreting these ages to reflect the age of the beach requires 900 14C years for
ARTICLE IN PRESS 2690
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occupation of the Ojata Beach, which is unlikely in an isostatically rebounding glacial lake basin on a flat lake plain. An alternative interpretation of these radiocarbon ages might include transport and reworking of older wood from the west side of the lake basin. 3.3. Abandonment of the southern outlet Radiocarbon dates within gravel from the southern outlet give a maximum estimate for the beginning of the Moorhead Phase. Basal gravel overlain by mud recovered from a rotosonic core within the southern outlet contained wood fragments with ages of 10,675760 BP (AA-37029) and 10,8707170 BP (AA-37030), respectively (Fisher, 2003). The gravel and mud units record an active, then abandoned spillway. Two possible analyses of these data are: (1) average the ages because they are stratigraphically reversed, giving an estimate of 10,800 BP for abandonment of the southern outlet and commencement of the Moorhead Phase (Fisher, 2003); or alternatively, (2) because both samples are reworked, the youngest age sets the tightest bracket for falling lake levels. In the first analysis the Moorhead Phase started sometime after 10,800 BP and in the second analysis it started sometime after 10,675 BP. 3.4. Evidence from in situ organic material The oldest possible peat with a bulk date of 10,700 BP is from the Rainy River Lowland (Bajc et al., 2000). Two other sites are known with multiple in situ ages, which provide the most reliable, albeit minimum, ages for determining onset of the Moorhead Phase. The first site is at Wampum, Manitoba (Fig. 1) where sand, gravel, silt and gyttja overlay laminated lake sediment just below the Campbell Beach (Teller et al., 2000). Within this sequence is a 7 cm thick peat with a basal age of 10,3407100 BP (TO-4285) and an upper age of 9460790 BP (TO-4284). Assuming the peat is in situ, it would suggest lower water levels at Wampum at 10,3407100 BP. The second site is the Trollwood site at Fargo, ND (Fig. 1). This sequence is interpreted as a deltaic, wetland environment (Yansa and Ashworth, 2005) with fluvially transported and in situ macrofossils unconformably overlying lacustrine clay. The 2-m thick sequence has a basal date of 10,230780 BP (Beta-121851) and an upper age of 9920760 BP (AA-34343), both interpreted as being in situ (Yansa, personal communication, 2006). The Trollwood site is considerably thicker than the nearby Moorhead site (0.6 m) with a single age of 99307280 BP (W-388; Ruben and Alexander, 1958) and the Seminary site (0.15 m) with a single age of 99007400 BP (W-993; McAndrews, 1967). From the analysis of in situ organics by Yansa and Ashworth (2005), a minimum age for the beginning of the Moorhead Phase at the Trollwood site is 10,230780 BP.
4. Discussion and conclusions Past workers in the Agassiz basin have used radiocarbon ages to reflect the age of the encasing sediment, resulting in complex lake-level histories (e.g., Arndt, 1977). We contend that interpretation issues arise because wood in fluvial or littoral sediment may be reworked from elsewhere, providing only maximum ages for the sediment. The interpretation of the site with the oldest Moorhead Phase age (10,9607300 BP; W-723) in cross-bedded sand is further questioned because the Ojata Beach has always been shown as chronostratigraphically younger than beaches that lie above it in elevation (e.g., Upham, 1895; Elson, 1967; Teller and Thorleifson, 1983; Teller and Leverington, 2004). In light of the potential for reworking older organic material, the adjacent site with the same stratigraphy and younger age (10,0807280 BP) questions the validity of using the W-723 date for the beginning of the Moorhead Phase. Any progress in such settings requires many dates from a single stratigraphic unit to find the best (youngest) maximum age. However, by using in situ organic material with dates throughout its sequence (e.g., Yansa and Ashworth, 2005), better chronologic assignments are possible. Throughout the history of Lake Agassiz studies, the honoring and vetting of radiocarbon ages has found lake levels, lake phases, and deglaciation to be too old (e.g., Elson, 1967; Clayton and Moran, 1982; Teller, 1989; Lowell et al., 2005; Teller et al., 2005). Here, we reexamined evidence for an old Moorhead Phase thought to be synchronous with the Younger Dryas cold period and find that the starting age of 10,960 BP is problematic. Instead, we propose that the maximum age of 10,675760 BP from the southern outlet be combined with a minimum age of 10,3407100 BP from Wampum to provide a refined bracket on the beginning of the low-water Moorhead Phase (Fig. 1). This bracket falls well within the Younger Dryas event, opening the question of cause and effect of Lake Agassiz and Younger Dryas cooling. Acknowledgments Some of this material is based upon work supported by the National Science Foundation under Grant No. 9972998 to TGF, and also supported by the Comer Science and Education Foundation to TGF and TVL. Henry Loope and two anonymous reviewers provided clarifications to the text. References Arndt, B.M., 1977. Stratigraphy of offshore sediment of Lake Agassiz. North Dakota Geological Survey Report of Investigations 60, 1–58. Bajc, A.F., Schwert, D.P., Warner, B.G., Williams, N.E., 2000. A reconstruction of Moorhead and Emerson phase environments along the eastern margin of glacial Lake Agassiz, Rainy River basin, northwestern Ontario. Canadian Journal of Earth Sciences 37, 1335–1353.
ARTICLE IN PRESS T.G. Fisher, T.V. Lowell / Quaternary Science Reviews 25 (2006) 2688–2691 Broecker, W.S., Kennett, J., Flower, B., Teller, J., Trumbore, S., Bonani, G., Wolfli, W., 1989. Routing of meltwater from the Laurentide Ice Sheet during the Younger Dryas cold episode. Nature 341, 318–321. Clark, P.U., Marshall, S.J., Clarke, G.K.C., Hostetler, S.W., Licciardi, J.M., Teller, J.T., 2001. Freshwater forcing of abrupt climate change during the last glaciation. Science 293, 283–287. Clayton, L., 1983. Chronology of Lake Agassiz drainage to Lake Superior. In: Teller, J.T., Clayton, L. (Eds.), Glacial Lake Agassiz. Geological Association of Canada, St. John’s, Newfoundland, Special Paper 26, pp. 291–307. Clayton, L., Moran, S.R., 1982. Chronology of Late Wisconsinan glaciation in middle North America. Quaternary Science Reviews 1, 55–82. Elson, J.A., 1967. Geology of glacial Lake Agassiz. In: Mayer-Oakes, W.J. (Ed.), Life, Land and Water. University of Manitoba Press, Winnipeg, pp. 37–96. Fenton, M.M., Moran, S.R., Teller, J.T., Clayton, L., 1983. Quaternary stratigraphy and history in the southern part of the Lake Agassiz Basin. In: Teller, J.T., Clayton, L. (Eds.), Glacial Lake Agassiz. Geological Association of Canada, St. John’s, Newfoundland, Special Paper 26, pp. 49–74. Fisher, T.G., 2003. Chronology of glacial Lake Agassiz meltwater routed to the Gulf of Mexico. Quaternary Research 59, 271–276. Fisher, T.G., 2005. Strandline analysis in the southern basin of glacial Lake Agassiz, Minnesota and North and South Dakota, USA. Geological Society of America Bulletin 117, 1481–1496. Harris, K.L., Moran, S.R., Clayton, L., 1974. Late Quaternary stratigraphic nomenclature, Red River Valley, North Dakota and Minnesota. North Dakota Geological Survey Miscellaneous Series 52, 47p. Licciardi, J.M., Teller, J.T., Clark, P.U., 1999. Freshwater routing by the Laurentide Ice Sheet during the last deglaciation. In: Clark, P., Webb, R.S., Keigwin, L.D. (Eds.), Mechanisms of Global Climate Change at Millennial Time Scales. American Geophysical Union, pp. 177–201. Lowell, T.V., Fisher, T.G., Comer, G.C., Hajdas, I., Waterson, N., Glover, K., Loope, H.M., Schaefer, J.M., Rinterknecht, V., Broecker,
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W.S., Denton, G.H., Teller, J.T., 2005. Testing the Lake Agassiz Meltwater Trigger for the Younger Dryas. EOS Transactions 86, 365–372. McAndrews, J.H., 1967. Paleoecology of the Seminary and Mirror Pool peat deposits. In: Elson, J.A. (Ed.), Life, Land and Water. University of Manitoba Press, Winnipeg, pp. 253–269. Moran, S.R., Clayton, L., Scott, M.W., Brophy, J.A., 1973. Catalog of North Dakota radiocarbon dates. North Dakota Geological Survey Miscellaneous Series 53, 51pp. Ruben, M., Alexander, C., 1958. U.S. Geological Survey radiocarbon dates IV. Science 127, 1476–1487. Teller, J.T., 1989. Importance of the Rossendale site in establishing a deglacial chronology along the southwestern margin of the Laurentide Ice Sheet. Quaternary Research 32, 12–23. Teller, J.T., Leverington, D.W., 2004. Glacial Lake Agassiz: a 5000 yr history of change and its relationship to the q18O record of Greenland. Geological Society of America Bulletin 116, 729–742. Teller, J.T., Thorleifson, L.H., 1983. The Lake Agassiz-Lake Superior connection. In: Teller, J.T., Clayton, L. (Eds.), Glacial Lake Agassiz. The Geological Association of Canada, Special Paper 26, pp. 261–290. Teller, J.T., Risberg, J., Matile, G., Zoltai, S., 2000. Postglacial history and paleoecology of Wampum, Manitoba, a former lagoon in the Lake Agassiz basin. Geological Society of America Bulletin 112, 943–958. Teller, J.T., Leverington, D.W., Mann, J.D., 2002. Freshwater outbursts to the oceans from glacial Lake Agassiz and their role in climate change during the last deglaciation. Quaternary Science Reviews 21, 879–888. Teller, J.T., Boyd, M., Yang, Z., Kor, P.S.G., Mokhtari Fard, A., 2005. Alternative routing of Lake Agassiz overflow during the Younger Dryas: new dates, paleotopography, and a reevaluation. Quaternary Science Reviews 24, 1890–1905. Upham, W., 1895. The Glacial Lake Agassiz. United States Geological Survey Monograph 25, 685pp. Yansa, C.H., Ashworth, A.C., 2005. Late Pleistocene palaeoenvironment of the southern Lake Agassiz Basin, USA. Journal of Quaternary Science 20, 255–267.