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Reply to comments by Curry et al. (2013) on “Atmospheric changes in North America during the last deglaciation from dune-wetland records in the Midwestern United States”
Quaternary Science Reviews 80 (2013) 200–203
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Correspondence
Reply to comments by Curry et al. (2013) on “Atmospheric changes in North America during the last deglaciation from dune-wetland records in the Midwestern United States” We welcome the scrutiny that Curry et al. (2013) have given to our paper published in Quaternary Science Reviews last year (Wang et al., 2012). We appreciate the opportunity to respond to their comments to clarify misunderstandings regarding our 14C age chronology and climate reconstructions in Illinois. In our paper, we did not intend to oversimplify interpretations made by Gonzales and Grimm (2009) and Gonzales et al. (2009); instead, we were attempting to emphasize the complexity of the Heinrich 1 (H1) records in Illinois. The physical evidence in dune and loess successions in our paper suggests that the H1 interval experienced changes from dry to wet conditions (i.e., Big Dry to Big Wet as described in Broecker and Putnam, 2012) and finally a very brief dry phase again. The sediments that would indicate this brief dry phase at the top of dune and loess successions are too thin to date accurately. From this view, our results appear to be consistent with the interpretation of the pollen record at Crystal Lake, Illinois, in which dry components from the very late phase could be mixed with the wet components in the middle phase. We are fully aware of the studies performed by Curry, Grimm, and Gonzales in northeastern Illinois. At Crystal Lake, Gonzales and Grimm (2009), Gonzales et al. (2009), and Curry and Filippelli (2010) used accelerator mass spectrometry (AMS) 14C dates of fossil plants to establish the chronology for the pollen, ostracode, and paleoenvironment studies. They reported that the timing of the Bølling/Allerød (B/A) warming, Younger Dryas (YD) cooling, and Holocene warming at Crystal Lake lagged behind other sites in the Northern Hemisphere by 400 years. However, the records from Nelson Lake (Schubert et al., 2004) and Brewster Creek (Curry et al., 2007), which are within a 30-km radius of Crystal Lake and are also based on 14C dates of fossil plants, suggested the B/A and YD events were synchronous with the Greenland ice record. The main concerns expressed by Curry et al. (2013) with our paper are that 1) dune building at our study sites did not occur during the entire H1 and YD intervals; therefore, the paleoclimate interpretation is misleading; 2) inaccurate 14C dates on bulk sediments and optically stimulated luminescence (OSL) dates on paleosol/ gyttja and dune sand yield an inaccurate climate model; and 3) the trace elements Zr and Rb are indicators of provenance but are not a climate proxy; thus, the climate interpretation is misleading. With regard to point 1), Curry et al. (2013) contended that the H1 and YD dunes we studied contain paleoclimate records that only partially reflect H1 and YD intervals. For the H1 dune sand unit, we differentiated three facies: laminated dune sand in the
DOI of original article: http://dx.doi.org/10.1016/j.quascirev.2013.04.001. 0277-3791/$ – see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.07.001
lower part, a weathered zone of dune sand in the middle part, and a less weathered zone at the top of the unit (Wang et al., 2012). Through a regional and continental comparison, we defined the lower unit as H1b, the middle unit as H1a, and the topmost unit as transition to B/A (t-B/A). We did not use 14C or OSL dates, but rather used the Greenland ice chronology to define the age of the boundaries of the major chronozones. Although OSL dating has limitations in accuracy and precision, it is currently the only direct dating technology used to study dune chronology. We acknowledge that our 14C and OSL chronology is not able to indicate whether the formation of dunes occurred continuously through the entire H1 and YD intervals, but our 14C and OSL chronology does not reject dune formation during the entire H1 and YD intervals, owing to relatively large dating uncertainties. We presently do not have any evidence to conclude that the YD was wet at our sites. On the other hand, the comparison of our YD dune building with four Ohio YD dunes at the same latitude (Campbell et al., 2011), one YD paleosol record in Missouri at a slightly lower latitude (Dorale et al., 2010), and more than 50 other records (see references in Wang et al., 2012) throughout the last deglaciation in the United States suggests that by approximately 18,000 calibrated years before present (cal BP), climate change in the Midwest was controlled by large-scale changes in atmospheric circulation rather than simply by the ice sheet geometry (Wang et al., 2012). We did not observe any overflow deposits from upland from the Lake Michigan basin through the Chicago Outlet during the Glenwood and Calumet phases, at approximately 15,800 and 13,300 cal BP. These events were not concurrent with dune formation in the YD. Regarding point 2), Grimm et al. (2009) compiled 14C chronologies of bulk lake sediments and fossil plants from four sites: Cottonwood Lake in South Dakota, Rice Lake in North Dakota, Devil’s Lake in Wisconsin, and Chatsworth Bog in Illinois. Generally, they showed that 14C dates of bulk sediments are systematically older than those of fossil plants. However, even with the age differences between the bulk sediments and fossil plants, the dates from these different materials are often systematically offset and may parallel each other in relation to sample depths. Grimm et al. (2009) convincingly showed that the spruce peak from Devil’s Lake, which they interpreted to be a regional pattern of the YD pollen assemblage, is consistent with the fossil plant chronology. They also showed that the 14C chronology of the bulk sediment samples at this site was about 1000 years too old compared with that of the fossil plants. However, in regard to the 14C chronology at Chatsworth Bog, Grimm et al. (2009) found that 1) during the deglacial period, the age of bulk sediments was older than that of fossil
Correspondence / Quaternary Science Reviews 80 (2013) 200–203
plants; 2) during the early Holocene period, the ages of bulk sediments and fossil plants were identical; 3) during the middle Holocene period, the age of bulk sediments was older than that of fossil plants; and 4) during the late Holocene period, they were identical. The pollen record from Chatsworth Bog published by Saunders et al. (2010) showed no YD spruce peak and almost half of the anticipated bell-shaped B/A black ash peak extending into the YD chronozone (as defined in that paper) when using the fossil plant chronology. The bell-shaped peak of black ash is also a regional marker that characterizes the Midwest B/A chronozone (Schubert et al., 2004; Gonzales and Grimm, 2009; Gonzales et al., 2009). Pollen records from sites in Ohio and Indiana showed consistent B/A black ash and YD spruce peaks in the bulk sediment (Shane and Anderson, 1993). The Two Creeks Forest dated between 14,000 and 13,000 cal BP could be interpreted as a delayed response between forestation and climate warming. Therefore, we believe it is tenuous to suggest using this record to support the time-lag hypothesis. Curry et al. (2013) also asserted that our use of the volatile fraction of bulk sediment for dating did not exclude older carbon contamination associated with aquatic plants. Our research has extensively investigated old and young carbon contamination in soils, loess, lake sediments, cave sediments, and weathered dune sand in Illinois, and the results suggest contaminants could be derived from older or younger sources (Wang et al., 2000, 2003, 2012). In this study, we used the pyrolysis-residue fraction to remove younger contaminants in the lake sediment (gyttja) and used the pyrolysis-volatile fraction to remove older contaminants from glaciogenic sources in the underlying paleosol (Wang et al., 2012). The pyrolysis-volatile fraction would also remove the old carbon contamination related to aquatic plants, which is expected to be a small portion of the overall old carbon pool. The goal of our dating was to identify climatic events, not to define the climate chronozone, by obtaining the mean residence time within sampling intervals of 10–20 cm. We commonly used 2–4 kg of sediment to partition the low- and highmolecular-weight organic compounds. Unlike the AMS 14C dates obtained on plant leaves, seeds, or wood fragments, our 14C dates for the less contaminated fractions in organic-rich sediments showed little or no age drift, even though they had lower resolution. Our 14C dates of selected high- or low-molecular-weight organic compounds and OSL dates are consistent for the H1, B/ A, YD, and Holocene units. Our comparison with the GISP2 (Greenland Ice Sheet Project 2) and other high-resolution records is typically part of the process of identifying climate events in the Midwest (e.g., Curry et al., 2007; Gonzales and Grimm, 2009; Gonzales et al., 2009). With regard to point 3), we used diffuse reflectance (i.e., L*a*b*) values and concentrations of the trace elements Zr and Rb to characterize the dune-wetland successions. These data are introduced as lithological proxies. In this study, the formation of dunes indicates dry conditions, wetland deposits indicate wet conditions, preservation of unweathered dune structures such as cross bedding and lamination indicates drier conditions, and weathering zones in dune sand with characteristics of bioturbation and leaching indicate relatively wetter conditions. These are direct observations, and there is no need to develop climate proxies. Because L*a*b* values and Zr and Rb concentrations differentiate these stratigraphic units very well, we simply borrowed them for comparison purposes. This concept is clearly described in Wang et al. (2012). On the other hand, Zr and Rb concentrations are excellent climate proxies, especially for indicating moisture changes in loess and studies of weathering profiles (Nesbitt and Markovics, 1997; Wang et al., 2008). Generally, Zr is enriched in the heavy minerals (e.g., zircon; as discussed by Fralick and Kronberg, 1997), whereas
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Rb displays inert behavior and is not associated with any specific Rb-bearing minerals but is present in K-feldspar, mica, and clay minerals (Heier and Billings, 1970). It is well known that in Illinois, deposits of aeolian and fluvial sand contain a high content of feldspar, commonly >20% (Hunter, 1965; Curry and Grimley, 2006) and up to 40% in many places, and are associated with an abundance of heavy minerals. Moreover, grain size and matrix color vary widely in different dune deposits. The YD dune sand is coarser and has a deeper pinkish hue, suggesting it contains more feldspar grains (Wang et al., 2012) than does H1 dune sand. Our grain size results (unpublished data) clearly indicate that the largest variation in grain size between the B/A wetland and H1 and YD dune sand is in the <0.6-mm size fraction. In addition, because the variations in Zr and Rb concentrations are closely related to the grain-size fraction sampled in fine-grained siliciclastic sediments (Dypvik and Harris, 2001), we chose Zr and Rb concentrations in the <1mm fraction as lithological parameters to define the stratigraphy. In their final statement, Curry et al. (2013) claimed that our interpretation of dry YD climatic conditions is misleading and that “An unpublished, well-dated pollen record from Chatsworth Bog. also supports a wet Younger Dryas” (p. 177). Actually, Saunders et al. (2010), with Grimm and Curry as co-authors, published the Chatsworth Bog pollen record but did not discuss the YD climate. To respond to their major concern about the YD climate, here we have chosen the Brewster Creek record to further discuss how Curry et al. (2007) interpreted the YD climate as wet. At the Brewster Creek site, the YD equivalent deposits are defined by three sedimentary units using three 14C dates. From top to bottom of the succession, they are: 1) a peat unit; 2) a yellowish marl unit with weak bedding; and 3) an olive marl unit with strong bedding (Fig. 1; Curry et al., 2007). The three dates used for the chronology are 10,555 40 14C year (yr) BP from the peat, 10,495 35 14C yr BP from the yellowish marl, and 10,980 35 14C yr BP from the olive marl, respectively. These dates were calibrated with 2s uncertainties (Reimer et al., 2009) to 12,400–12,740, 12,450–12,680, and 12,850–12,970 cal BP (Curry et al., 2007) and are indicated by the bracket in Fig. 1 (i.e., the fifth, sixth, and seventh dates from the top). We interpret the olive marl to be laminated, organicrich, fine-grained lacustrine sediment (Fig. 1). Given two units of organic-rich sediments, the peat on top and lacustrine sediments on the bottom, it is not surprising that the entire YD climate was interpreted as wet by Curry et al. (2007). However, the fifth date from the top for the upper YD boundary appears to be inconsistent with the sedimentation rate (Curry et al., 2007; Fig. 13 on p. 18). Regarding the geological context of these sediments, we interpreted the peat as being formed in an organic-rich swamp environment supporting dense vegetation and the yellowish marl as potentially reflecting a shallow water marshland environment supporting sparse vegetation. These interpretations are based on variations of the organic carbon content, indicated by the color changes in the density of the gray scale (Fig. 1). The contact between the two units is abrupt. We suggest that the upper boundary of the YD should be set at their contact. The seventh date falls at the boundary of the Allerød and YD at 12,900 cal BP as defined by the Greenland ice chronology. Specifically, the age range of 12,850–12,970 cal BP, with a probability of 0.96 (Reimer et al., 2009), can be used to assign the laminated lacustrine sediment to the YD or Allerød unit (Fig. 1). The laminated, organic-rich, fine-grained sediment represents a littoral lake facies, indicating a wave environment near the shoreline in a relatively large lake. Its geological contact with the thick yellowish marl unit is also sharp (Fig. 1). We propose using this date to indicate that the laminated lacustrine sediment (i.e., the olive marl described by Curry et al., 2007) belongs to the late phase of the
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Correspondence / Quaternary Science Reviews 80 (2013) 200–203
Fig. 1. Scanned image of the Brewster Creek-1 core, as well as AMS 14C dates (2s) and depths (modified from Curry et al., 2007). The bracket with the description for three sediment units indicates the YD chronozone interpreted by Curry et al. (2007), with which we disagree because it results in the YD containing Allerød and early Holocene signals. We propose the yellowish marl unit as the YD chronozone based on AMS 14C dates and stratigraphic contacts when the fifth date from the top is excluded. Lightness–darkness or gray-scale density (L*) values (green) and their standard errors (red) characterize the stratigraphic units well. The variation in gray-scale density indicates changes in organic and carbonate contents and can be used as a moisture index. Here, the H1b equivalent unit does not exist. Using H1a to interpret the entire H1 (Oldest Dryas) as wet (Curry et al., 2007) is misleading, which hinders the use of these data for regional correlations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Allerød. The lower boundary of the YD should be set at the contact between the yellowish marl unit and this unit (Fig. 1). Using image analysis software, we measured lightness–darkness (L*) variations with standard errors in the image of the Brewster Creek sediment core (Curry et al., 2007). The high and low L* values indicate brighter and darker colors, which characterize the stratigraphic units from the H1a, t-B/A, Bølling, Allerød, YD, and early Holocene periods very well when their fossil plant chronology is applied, if the fifth date from the top is excluded (Fig. 1). The standard errors of the L* values indicate the purity of the matrix color, reflecting homogeneous or heterogeneous properties in terms of proportions of organic and carbonate contents, and light and dark minerals (Fig. 1). These values show that Holocene peat differs from late Pleistocene deposits, which are more homogeneous in the matrix. The H1a lacustrine (Big Wet), the thin t-B/A marl bed, the B/A lacustrine, the thick YD marl, and the early Holocene peat (Fig. 1) indicate drastic changes from lake to marshland, to lake again, to marshland again, and finally to the swamp environment. The absence of the H1b (Big Dry) sedimentary unit at Brewster Creek (and Crystal Lake as well) further supports our model that an exceptionally cold, dry climate prevailed during the early H1 phase because the ice margin of the Lake Michigan lobe readvanced to these sites and kettle lake records could have been truncated. Without discussion of the H1b phase, the interpretation of the Oldest Dryas (Curry et al., 2007; Gonzales and Grimm,
2009; Gonzales et al., 2009) is incomplete and hinders the use of these data for regional correlations. The thin layer of marl sediments of the t-B/A phase, reflecting an abrupt change from lacustrine to marshland (Fig. 1) at the end of H1, suggests that the interpretation of the pollen record at Crystal Lake (Gonzales and Grimm, 2009; Gonzales et al., 2009) could indicate complicated climate conditions. Finally, in our paper we used climate records of more than 50 research sites in 13 geographic regions in the United States. These records disagree with the Crystal Lake record, the YD chronozone at Brewster Creek, and the Chatsworth Bog record, but they agree with the Nelson Lake record throughout the last deglaciation and with the Brewster Creek and Chatsworth Bog records, excluding the YD chronozone. Hence, the claim by Curry et al. (2013) that “our published data from sites also located in Illinois fail to support it [the Wang et al. (2012) model]” is not consistent with what Curry et al. have published. Acknowledgments We thank Thomas Lowell for advising us on this reply letter; Randall A. Locke, Steven E. Brown, and Richard C. Berg for their reviews and comments; and Susan Krusemark for commenting on and editing the manuscript. The loess study in this work was supported by National Science Foundation grant ATM-0001810 to H. Wang.
Correspondence / Quaternary Science Reviews 80 (2013) 200–203
References Broecker, W., Putnam, A.E., 2012. How did the hydrologic cycle respond to the twophase mystery interval? Quat. Sci. Rev. 57, 17–25. Campbell, M.C., Fisher, T.G., Goble, R.J., 2011. Terrestrial sensitivity to abrupt cooling recorded by aeolian activity in northwest Ohio, USA. Quat. Res. 75, 411–416. Curry, B.B., Filippelli, G.M., 2010. Episodes of low dissolved oxygen indicated by ostracodes and sediment geochemistry at Crystal Lake, Illinois, USA. Limnol. Oceanogr. 55, 2403–2423. Curry, B.B., Grimley, D.A., 2006. Provenance, age, and environment of midWisconsin slackwater lake sediment in the St. Louis Metro East area, USA. Quat. Res. 65, 108–122. Curry, B.B., Grimm, E.C., Slate, J.E., Hansen, B.C., Konen, M.E., 2007. The Late Glacial and Early Holocene Geology, Paleoecology, and Paleohydrology of the Brewster Creek Site, a Proposed Wetland Restoration Site, Pratt’s Wayne Woods Forest Preserve and James “Pate” Philip State Park, Bartlett, Illinois. Illinois State Geological Survey, Champaign. Circular 571. Curry, B.B., Gonzales, L.M., Grimm, E.C., 2013. Correspondence regarding “Atmospheric changes in North America during the last deglaciation from dune wetland records in the Midwestern United States” by Wang, H., Stumpf, A.J., Miao, X., Lowell, T.V. (2012). Quat. Sci. Rev. 70, 176–178. Dorale, J.A., Wozniak, L.A., Bettis III, E.A., Carpenter, S.J., Mandel, R.D., Hajic, E.R., Lopinot, N.H., Ray, J.H., 2010. Isotopic evidence for Younger Dryas aridity in the North American midcontinent. Geology 38, 519–522. Dypvik, H., Harris, N.B., 2001. Geochemical facies analysis of fine-grained siliciclastics using Th/U, Zr/Rb and (Zr þ Rb)/Sr ratios. Chem. Geol. 181, 131–146. Fralick, P.W., Kronberg, B.I., 1997. Geochemical discrimination of clastic sedimentary rock sources. Sediment. Geol. 113, 111–124. Gonzales, L.M., Grimm, E.C., 2009. Synchronization of late-glacial vegetation changes at Crystal Lake, Illinois, USA with the North Atlantic Event Stratigraphy. Quat. Res. 72, 234–245. Gonzales, L.M., Williams, J.W., Grimm, E.C., 2009. Expanded response-surfaces: a new method to reconstruct paleoclimates from fossil pollen assemblages that lack modern analogues. Quat. Sci. Rev. 28, 3315–3332. Grimm, E.C., Maher Jr., L.J., Nelson, D.M., 2009. The magnitude of error in conventional bulk-sediment radiocarbon dates from central North America. Quat. Res. 72, 301–308. Heier, K.S., Billings, G.K., 1970. Rubidium. In: Wedepohle, K.H. (Ed.), 1970. Handbook of Geochemistry, vol. II/2. Springer-Verlag, Berlin, Germany. pp. 37B1–37K3. Hunter, R.E., 1965. Feldspar in Illinois Sands d a Further Study. In: Illinois State Geological Survey Circular, vol. 391, p. 19. Nesbitt, H.W., Markovics, G., 1997. Weathering of granodioritic crust, long-term storage of elements in weathering profiles, and petrogenesis of siliciclastic sediments. Geochim. Cosmochim. Acta 61, 1653–1670.
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Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0–50,000 years cal BP. Radiocarbon 51, 1111–1150. Saunders, J.J., Grimm, E.C., Widga, C.C., Campbell, G.D., Curry, B.B., Grimley, D.A., Hanson, P.R., McCullum, J.P., Oliver, J.S., Treworgy, J.D., 2010. Paradigms and proboscideans in the southern Great Lakes region, USA. Quat. Int. 217, 175–187. Schubert, B.W., Graham, R.W., McDonald, H.G., Grimm, E.C., Stafford Jr., T.W., 2004. Latest Pleistocene paleoecology of Jefferson’s ground sloth (Megalonyx jeffersonii) and elk-moose (Cervalces scotti) in northern Illinois. Quat. Res. 61, 231–240. Shane, L.C.K., Anderson, K.H., 1993. Intensity, gradients and reversals in late glacial environmental change in east-central North America. Quat. Sci. Rev. 12, 307– 320. Wang, H., Follmer, L.R., Liu, J.C.-L., 2000. Isotope evidence of paleo-El Niño-Southern Oscillation cycles in loess-paleosol record in the central United States. Geology 28, 771–774. Wang, H., Hackley, K.C., Panno, S.V., Coleman, D.D., Liu, J.C.-L., Brown, J., 2003. Pyrolysis combustion 14C dating of soil organic matter. Quat. Res. 60, 348–355. Wang, H., Stumpf, A.J., Miao, X., Lowell, T.V., 2012. Atmospheric changes in North America during the last deglaciation from dune-wetland records in the Midwestern United States. Quat. Sci. Rev. 58, 124–134. Wang, H.B., Liu, L.Y., Feng, Z.D., 2008. Spatiotemporal variations of Zr/Rb ratio in three last interglacial paleosol profiles across the Chinese Loess Plateau and its implications for climatic interpretation. Chin. Sci. Bull. 53, 1413– 1422.
Hong Wang*, Andrew J. Stumpf, Xiaodong Miao Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, IL 61820, USA * Corresponding author. Tel.: þ1 217 244 7692; fax: þ1 217 244 7004. E-mail address: [email protected] (H. Wang) Available online 19 July 2013
Contents lists available at SciVerse ScienceDirect
Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev
Correspondence
Reply to comments by Curry et al. (2013) on “Atmospheric changes in North America during the last deglaciation from dune-wetland records in the Midwestern United States” We welcome the scrutiny that Curry et al. (2013) have given to our paper published in Quaternary Science Reviews last year (Wang et al., 2012). We appreciate the opportunity to respond to their comments to clarify misunderstandings regarding our 14C age chronology and climate reconstructions in Illinois. In our paper, we did not intend to oversimplify interpretations made by Gonzales and Grimm (2009) and Gonzales et al. (2009); instead, we were attempting to emphasize the complexity of the Heinrich 1 (H1) records in Illinois. The physical evidence in dune and loess successions in our paper suggests that the H1 interval experienced changes from dry to wet conditions (i.e., Big Dry to Big Wet as described in Broecker and Putnam, 2012) and finally a very brief dry phase again. The sediments that would indicate this brief dry phase at the top of dune and loess successions are too thin to date accurately. From this view, our results appear to be consistent with the interpretation of the pollen record at Crystal Lake, Illinois, in which dry components from the very late phase could be mixed with the wet components in the middle phase. We are fully aware of the studies performed by Curry, Grimm, and Gonzales in northeastern Illinois. At Crystal Lake, Gonzales and Grimm (2009), Gonzales et al. (2009), and Curry and Filippelli (2010) used accelerator mass spectrometry (AMS) 14C dates of fossil plants to establish the chronology for the pollen, ostracode, and paleoenvironment studies. They reported that the timing of the Bølling/Allerød (B/A) warming, Younger Dryas (YD) cooling, and Holocene warming at Crystal Lake lagged behind other sites in the Northern Hemisphere by 400 years. However, the records from Nelson Lake (Schubert et al., 2004) and Brewster Creek (Curry et al., 2007), which are within a 30-km radius of Crystal Lake and are also based on 14C dates of fossil plants, suggested the B/A and YD events were synchronous with the Greenland ice record. The main concerns expressed by Curry et al. (2013) with our paper are that 1) dune building at our study sites did not occur during the entire H1 and YD intervals; therefore, the paleoclimate interpretation is misleading; 2) inaccurate 14C dates on bulk sediments and optically stimulated luminescence (OSL) dates on paleosol/ gyttja and dune sand yield an inaccurate climate model; and 3) the trace elements Zr and Rb are indicators of provenance but are not a climate proxy; thus, the climate interpretation is misleading. With regard to point 1), Curry et al. (2013) contended that the H1 and YD dunes we studied contain paleoclimate records that only partially reflect H1 and YD intervals. For the H1 dune sand unit, we differentiated three facies: laminated dune sand in the
DOI of original article: http://dx.doi.org/10.1016/j.quascirev.2013.04.001. 0277-3791/$ – see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.07.001
lower part, a weathered zone of dune sand in the middle part, and a less weathered zone at the top of the unit (Wang et al., 2012). Through a regional and continental comparison, we defined the lower unit as H1b, the middle unit as H1a, and the topmost unit as transition to B/A (t-B/A). We did not use 14C or OSL dates, but rather used the Greenland ice chronology to define the age of the boundaries of the major chronozones. Although OSL dating has limitations in accuracy and precision, it is currently the only direct dating technology used to study dune chronology. We acknowledge that our 14C and OSL chronology is not able to indicate whether the formation of dunes occurred continuously through the entire H1 and YD intervals, but our 14C and OSL chronology does not reject dune formation during the entire H1 and YD intervals, owing to relatively large dating uncertainties. We presently do not have any evidence to conclude that the YD was wet at our sites. On the other hand, the comparison of our YD dune building with four Ohio YD dunes at the same latitude (Campbell et al., 2011), one YD paleosol record in Missouri at a slightly lower latitude (Dorale et al., 2010), and more than 50 other records (see references in Wang et al., 2012) throughout the last deglaciation in the United States suggests that by approximately 18,000 calibrated years before present (cal BP), climate change in the Midwest was controlled by large-scale changes in atmospheric circulation rather than simply by the ice sheet geometry (Wang et al., 2012). We did not observe any overflow deposits from upland from the Lake Michigan basin through the Chicago Outlet during the Glenwood and Calumet phases, at approximately 15,800 and 13,300 cal BP. These events were not concurrent with dune formation in the YD. Regarding point 2), Grimm et al. (2009) compiled 14C chronologies of bulk lake sediments and fossil plants from four sites: Cottonwood Lake in South Dakota, Rice Lake in North Dakota, Devil’s Lake in Wisconsin, and Chatsworth Bog in Illinois. Generally, they showed that 14C dates of bulk sediments are systematically older than those of fossil plants. However, even with the age differences between the bulk sediments and fossil plants, the dates from these different materials are often systematically offset and may parallel each other in relation to sample depths. Grimm et al. (2009) convincingly showed that the spruce peak from Devil’s Lake, which they interpreted to be a regional pattern of the YD pollen assemblage, is consistent with the fossil plant chronology. They also showed that the 14C chronology of the bulk sediment samples at this site was about 1000 years too old compared with that of the fossil plants. However, in regard to the 14C chronology at Chatsworth Bog, Grimm et al. (2009) found that 1) during the deglacial period, the age of bulk sediments was older than that of fossil
Correspondence / Quaternary Science Reviews 80 (2013) 200–203
plants; 2) during the early Holocene period, the ages of bulk sediments and fossil plants were identical; 3) during the middle Holocene period, the age of bulk sediments was older than that of fossil plants; and 4) during the late Holocene period, they were identical. The pollen record from Chatsworth Bog published by Saunders et al. (2010) showed no YD spruce peak and almost half of the anticipated bell-shaped B/A black ash peak extending into the YD chronozone (as defined in that paper) when using the fossil plant chronology. The bell-shaped peak of black ash is also a regional marker that characterizes the Midwest B/A chronozone (Schubert et al., 2004; Gonzales and Grimm, 2009; Gonzales et al., 2009). Pollen records from sites in Ohio and Indiana showed consistent B/A black ash and YD spruce peaks in the bulk sediment (Shane and Anderson, 1993). The Two Creeks Forest dated between 14,000 and 13,000 cal BP could be interpreted as a delayed response between forestation and climate warming. Therefore, we believe it is tenuous to suggest using this record to support the time-lag hypothesis. Curry et al. (2013) also asserted that our use of the volatile fraction of bulk sediment for dating did not exclude older carbon contamination associated with aquatic plants. Our research has extensively investigated old and young carbon contamination in soils, loess, lake sediments, cave sediments, and weathered dune sand in Illinois, and the results suggest contaminants could be derived from older or younger sources (Wang et al., 2000, 2003, 2012). In this study, we used the pyrolysis-residue fraction to remove younger contaminants in the lake sediment (gyttja) and used the pyrolysis-volatile fraction to remove older contaminants from glaciogenic sources in the underlying paleosol (Wang et al., 2012). The pyrolysis-volatile fraction would also remove the old carbon contamination related to aquatic plants, which is expected to be a small portion of the overall old carbon pool. The goal of our dating was to identify climatic events, not to define the climate chronozone, by obtaining the mean residence time within sampling intervals of 10–20 cm. We commonly used 2–4 kg of sediment to partition the low- and highmolecular-weight organic compounds. Unlike the AMS 14C dates obtained on plant leaves, seeds, or wood fragments, our 14C dates for the less contaminated fractions in organic-rich sediments showed little or no age drift, even though they had lower resolution. Our 14C dates of selected high- or low-molecular-weight organic compounds and OSL dates are consistent for the H1, B/ A, YD, and Holocene units. Our comparison with the GISP2 (Greenland Ice Sheet Project 2) and other high-resolution records is typically part of the process of identifying climate events in the Midwest (e.g., Curry et al., 2007; Gonzales and Grimm, 2009; Gonzales et al., 2009). With regard to point 3), we used diffuse reflectance (i.e., L*a*b*) values and concentrations of the trace elements Zr and Rb to characterize the dune-wetland successions. These data are introduced as lithological proxies. In this study, the formation of dunes indicates dry conditions, wetland deposits indicate wet conditions, preservation of unweathered dune structures such as cross bedding and lamination indicates drier conditions, and weathering zones in dune sand with characteristics of bioturbation and leaching indicate relatively wetter conditions. These are direct observations, and there is no need to develop climate proxies. Because L*a*b* values and Zr and Rb concentrations differentiate these stratigraphic units very well, we simply borrowed them for comparison purposes. This concept is clearly described in Wang et al. (2012). On the other hand, Zr and Rb concentrations are excellent climate proxies, especially for indicating moisture changes in loess and studies of weathering profiles (Nesbitt and Markovics, 1997; Wang et al., 2008). Generally, Zr is enriched in the heavy minerals (e.g., zircon; as discussed by Fralick and Kronberg, 1997), whereas
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Rb displays inert behavior and is not associated with any specific Rb-bearing minerals but is present in K-feldspar, mica, and clay minerals (Heier and Billings, 1970). It is well known that in Illinois, deposits of aeolian and fluvial sand contain a high content of feldspar, commonly >20% (Hunter, 1965; Curry and Grimley, 2006) and up to 40% in many places, and are associated with an abundance of heavy minerals. Moreover, grain size and matrix color vary widely in different dune deposits. The YD dune sand is coarser and has a deeper pinkish hue, suggesting it contains more feldspar grains (Wang et al., 2012) than does H1 dune sand. Our grain size results (unpublished data) clearly indicate that the largest variation in grain size between the B/A wetland and H1 and YD dune sand is in the <0.6-mm size fraction. In addition, because the variations in Zr and Rb concentrations are closely related to the grain-size fraction sampled in fine-grained siliciclastic sediments (Dypvik and Harris, 2001), we chose Zr and Rb concentrations in the <1mm fraction as lithological parameters to define the stratigraphy. In their final statement, Curry et al. (2013) claimed that our interpretation of dry YD climatic conditions is misleading and that “An unpublished, well-dated pollen record from Chatsworth Bog. also supports a wet Younger Dryas” (p. 177). Actually, Saunders et al. (2010), with Grimm and Curry as co-authors, published the Chatsworth Bog pollen record but did not discuss the YD climate. To respond to their major concern about the YD climate, here we have chosen the Brewster Creek record to further discuss how Curry et al. (2007) interpreted the YD climate as wet. At the Brewster Creek site, the YD equivalent deposits are defined by three sedimentary units using three 14C dates. From top to bottom of the succession, they are: 1) a peat unit; 2) a yellowish marl unit with weak bedding; and 3) an olive marl unit with strong bedding (Fig. 1; Curry et al., 2007). The three dates used for the chronology are 10,555 40 14C year (yr) BP from the peat, 10,495 35 14C yr BP from the yellowish marl, and 10,980 35 14C yr BP from the olive marl, respectively. These dates were calibrated with 2s uncertainties (Reimer et al., 2009) to 12,400–12,740, 12,450–12,680, and 12,850–12,970 cal BP (Curry et al., 2007) and are indicated by the bracket in Fig. 1 (i.e., the fifth, sixth, and seventh dates from the top). We interpret the olive marl to be laminated, organicrich, fine-grained lacustrine sediment (Fig. 1). Given two units of organic-rich sediments, the peat on top and lacustrine sediments on the bottom, it is not surprising that the entire YD climate was interpreted as wet by Curry et al. (2007). However, the fifth date from the top for the upper YD boundary appears to be inconsistent with the sedimentation rate (Curry et al., 2007; Fig. 13 on p. 18). Regarding the geological context of these sediments, we interpreted the peat as being formed in an organic-rich swamp environment supporting dense vegetation and the yellowish marl as potentially reflecting a shallow water marshland environment supporting sparse vegetation. These interpretations are based on variations of the organic carbon content, indicated by the color changes in the density of the gray scale (Fig. 1). The contact between the two units is abrupt. We suggest that the upper boundary of the YD should be set at their contact. The seventh date falls at the boundary of the Allerød and YD at 12,900 cal BP as defined by the Greenland ice chronology. Specifically, the age range of 12,850–12,970 cal BP, with a probability of 0.96 (Reimer et al., 2009), can be used to assign the laminated lacustrine sediment to the YD or Allerød unit (Fig. 1). The laminated, organic-rich, fine-grained sediment represents a littoral lake facies, indicating a wave environment near the shoreline in a relatively large lake. Its geological contact with the thick yellowish marl unit is also sharp (Fig. 1). We propose using this date to indicate that the laminated lacustrine sediment (i.e., the olive marl described by Curry et al., 2007) belongs to the late phase of the
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Fig. 1. Scanned image of the Brewster Creek-1 core, as well as AMS 14C dates (2s) and depths (modified from Curry et al., 2007). The bracket with the description for three sediment units indicates the YD chronozone interpreted by Curry et al. (2007), with which we disagree because it results in the YD containing Allerød and early Holocene signals. We propose the yellowish marl unit as the YD chronozone based on AMS 14C dates and stratigraphic contacts when the fifth date from the top is excluded. Lightness–darkness or gray-scale density (L*) values (green) and their standard errors (red) characterize the stratigraphic units well. The variation in gray-scale density indicates changes in organic and carbonate contents and can be used as a moisture index. Here, the H1b equivalent unit does not exist. Using H1a to interpret the entire H1 (Oldest Dryas) as wet (Curry et al., 2007) is misleading, which hinders the use of these data for regional correlations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Allerød. The lower boundary of the YD should be set at the contact between the yellowish marl unit and this unit (Fig. 1). Using image analysis software, we measured lightness–darkness (L*) variations with standard errors in the image of the Brewster Creek sediment core (Curry et al., 2007). The high and low L* values indicate brighter and darker colors, which characterize the stratigraphic units from the H1a, t-B/A, Bølling, Allerød, YD, and early Holocene periods very well when their fossil plant chronology is applied, if the fifth date from the top is excluded (Fig. 1). The standard errors of the L* values indicate the purity of the matrix color, reflecting homogeneous or heterogeneous properties in terms of proportions of organic and carbonate contents, and light and dark minerals (Fig. 1). These values show that Holocene peat differs from late Pleistocene deposits, which are more homogeneous in the matrix. The H1a lacustrine (Big Wet), the thin t-B/A marl bed, the B/A lacustrine, the thick YD marl, and the early Holocene peat (Fig. 1) indicate drastic changes from lake to marshland, to lake again, to marshland again, and finally to the swamp environment. The absence of the H1b (Big Dry) sedimentary unit at Brewster Creek (and Crystal Lake as well) further supports our model that an exceptionally cold, dry climate prevailed during the early H1 phase because the ice margin of the Lake Michigan lobe readvanced to these sites and kettle lake records could have been truncated. Without discussion of the H1b phase, the interpretation of the Oldest Dryas (Curry et al., 2007; Gonzales and Grimm,
2009; Gonzales et al., 2009) is incomplete and hinders the use of these data for regional correlations. The thin layer of marl sediments of the t-B/A phase, reflecting an abrupt change from lacustrine to marshland (Fig. 1) at the end of H1, suggests that the interpretation of the pollen record at Crystal Lake (Gonzales and Grimm, 2009; Gonzales et al., 2009) could indicate complicated climate conditions. Finally, in our paper we used climate records of more than 50 research sites in 13 geographic regions in the United States. These records disagree with the Crystal Lake record, the YD chronozone at Brewster Creek, and the Chatsworth Bog record, but they agree with the Nelson Lake record throughout the last deglaciation and with the Brewster Creek and Chatsworth Bog records, excluding the YD chronozone. Hence, the claim by Curry et al. (2013) that “our published data from sites also located in Illinois fail to support it [the Wang et al. (2012) model]” is not consistent with what Curry et al. have published. Acknowledgments We thank Thomas Lowell for advising us on this reply letter; Randall A. Locke, Steven E. Brown, and Richard C. Berg for their reviews and comments; and Susan Krusemark for commenting on and editing the manuscript. The loess study in this work was supported by National Science Foundation grant ATM-0001810 to H. Wang.
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References Broecker, W., Putnam, A.E., 2012. How did the hydrologic cycle respond to the twophase mystery interval? Quat. Sci. Rev. 57, 17–25. Campbell, M.C., Fisher, T.G., Goble, R.J., 2011. Terrestrial sensitivity to abrupt cooling recorded by aeolian activity in northwest Ohio, USA. Quat. Res. 75, 411–416. Curry, B.B., Filippelli, G.M., 2010. Episodes of low dissolved oxygen indicated by ostracodes and sediment geochemistry at Crystal Lake, Illinois, USA. Limnol. Oceanogr. 55, 2403–2423. Curry, B.B., Grimley, D.A., 2006. Provenance, age, and environment of midWisconsin slackwater lake sediment in the St. Louis Metro East area, USA. Quat. Res. 65, 108–122. Curry, B.B., Grimm, E.C., Slate, J.E., Hansen, B.C., Konen, M.E., 2007. The Late Glacial and Early Holocene Geology, Paleoecology, and Paleohydrology of the Brewster Creek Site, a Proposed Wetland Restoration Site, Pratt’s Wayne Woods Forest Preserve and James “Pate” Philip State Park, Bartlett, Illinois. Illinois State Geological Survey, Champaign. Circular 571. Curry, B.B., Gonzales, L.M., Grimm, E.C., 2013. Correspondence regarding “Atmospheric changes in North America during the last deglaciation from dune wetland records in the Midwestern United States” by Wang, H., Stumpf, A.J., Miao, X., Lowell, T.V. (2012). Quat. Sci. Rev. 70, 176–178. Dorale, J.A., Wozniak, L.A., Bettis III, E.A., Carpenter, S.J., Mandel, R.D., Hajic, E.R., Lopinot, N.H., Ray, J.H., 2010. Isotopic evidence for Younger Dryas aridity in the North American midcontinent. Geology 38, 519–522. Dypvik, H., Harris, N.B., 2001. Geochemical facies analysis of fine-grained siliciclastics using Th/U, Zr/Rb and (Zr þ Rb)/Sr ratios. Chem. Geol. 181, 131–146. Fralick, P.W., Kronberg, B.I., 1997. Geochemical discrimination of clastic sedimentary rock sources. Sediment. Geol. 113, 111–124. Gonzales, L.M., Grimm, E.C., 2009. Synchronization of late-glacial vegetation changes at Crystal Lake, Illinois, USA with the North Atlantic Event Stratigraphy. Quat. Res. 72, 234–245. Gonzales, L.M., Williams, J.W., Grimm, E.C., 2009. Expanded response-surfaces: a new method to reconstruct paleoclimates from fossil pollen assemblages that lack modern analogues. Quat. Sci. Rev. 28, 3315–3332. Grimm, E.C., Maher Jr., L.J., Nelson, D.M., 2009. The magnitude of error in conventional bulk-sediment radiocarbon dates from central North America. Quat. Res. 72, 301–308. Heier, K.S., Billings, G.K., 1970. Rubidium. In: Wedepohle, K.H. (Ed.), 1970. Handbook of Geochemistry, vol. II/2. Springer-Verlag, Berlin, Germany. pp. 37B1–37K3. Hunter, R.E., 1965. Feldspar in Illinois Sands d a Further Study. In: Illinois State Geological Survey Circular, vol. 391, p. 19. Nesbitt, H.W., Markovics, G., 1997. Weathering of granodioritic crust, long-term storage of elements in weathering profiles, and petrogenesis of siliciclastic sediments. Geochim. Cosmochim. Acta 61, 1653–1670.
203
Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0–50,000 years cal BP. Radiocarbon 51, 1111–1150. Saunders, J.J., Grimm, E.C., Widga, C.C., Campbell, G.D., Curry, B.B., Grimley, D.A., Hanson, P.R., McCullum, J.P., Oliver, J.S., Treworgy, J.D., 2010. Paradigms and proboscideans in the southern Great Lakes region, USA. Quat. Int. 217, 175–187. Schubert, B.W., Graham, R.W., McDonald, H.G., Grimm, E.C., Stafford Jr., T.W., 2004. Latest Pleistocene paleoecology of Jefferson’s ground sloth (Megalonyx jeffersonii) and elk-moose (Cervalces scotti) in northern Illinois. Quat. Res. 61, 231–240. Shane, L.C.K., Anderson, K.H., 1993. Intensity, gradients and reversals in late glacial environmental change in east-central North America. Quat. Sci. Rev. 12, 307– 320. Wang, H., Follmer, L.R., Liu, J.C.-L., 2000. Isotope evidence of paleo-El Niño-Southern Oscillation cycles in loess-paleosol record in the central United States. Geology 28, 771–774. Wang, H., Hackley, K.C., Panno, S.V., Coleman, D.D., Liu, J.C.-L., Brown, J., 2003. Pyrolysis combustion 14C dating of soil organic matter. Quat. Res. 60, 348–355. Wang, H., Stumpf, A.J., Miao, X., Lowell, T.V., 2012. Atmospheric changes in North America during the last deglaciation from dune-wetland records in the Midwestern United States. Quat. Sci. Rev. 58, 124–134. Wang, H.B., Liu, L.Y., Feng, Z.D., 2008. Spatiotemporal variations of Zr/Rb ratio in three last interglacial paleosol profiles across the Chinese Loess Plateau and its implications for climatic interpretation. Chin. Sci. Bull. 53, 1413– 1422.
Hong Wang*, Andrew J. Stumpf, Xiaodong Miao Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, IL 61820, USA * Corresponding author. Tel.: þ1 217 244 7692; fax: þ1 217 244 7004. E-mail address: [email protected] (H. Wang) Available online 19 July 2013