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3042578242
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13C from North Atlantic benthic foraminifera indicates that NADW formation was absent or greatly reduced during the Younger Dryas (47).
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3042573462
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note
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Although the possibility of a time lag in wetlands formation or organic-matter production necessary for a methane increase following the Bølling climate shift is acknowledged by Severinghaus and Brook (4), an immediate response is invoked for at least some of the earliest methane rise, thus linking the timing of initial methane rise and tropical climate change. Any comparison of the timing of high- and low-latitude abrupt shifts between Greenland and Cariaco data relies on the assumption that the tropical vegetation changes reconstructed at both sites were synchronous.
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20
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3042539015
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note
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n+1)], where X = abundance, i = 25 and n = 33 for n-alkanes, and i = 24 and n = 32 for n-alkanoic acids and n-alcohols.
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3042527044
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Materials and methods are available as supporting material on Science Online.
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34
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3042527045
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3042578138
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32 n-alkanoic acids shows no significant correlation (slope = 0.11 and correlation coefficient r = 0.14).
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36
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3042617266
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i, where X = abundance and i = 24, 26, and 28 carbon atoms.
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37
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3042619715
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4 plants are well known, comparable measurements for ACL indices have not been made.
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38
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3042573463
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(Academic Press, San Diego, CA), chap. 9
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3042626848
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note
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13C-ACL relationships seen in African leaf waxes (27). The simplest explanation, supported by local pollen data (13, 14), is that Cariaco leaf wax data represent changes in local vegetation during deglacial climate changes.
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43
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3042524639
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14C was used at its original 10- to 15-year resolution.
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44
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3042534318
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The lag calculation is dependent on cross-correlations using all data points within the interval of overlap, rather than the timing of the initial change recorded between data points (42). Therefore, although biomarker sampling resolution is ∼40 years during the Younger Dryas onset, the transition itself lasted -200 years, encompassing five or six data points, and the cross-correlation can be evaluated uniquely for each annual time step. Similarly, the biomarker sample resolution during the Glacial/Bølling transition is ∼20 years, providing two or three data points within the duration of the shift. However, the biomarker resolution across the Younger Dryas termination is low (∼40 years) relative to the climate shift, and the transition appears to occur between data points; thus, the calculated vegetation lag of 25 ± 15 years must be considered a minimum estimate. The robustness of these analyses relies on the assumption that the lower-resolution proxy data are not systematically offset above or below the true trend (32).
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45
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3042585422
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note
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It is possible that some leaf waxes may be transported to the Cariaco basin by rivers, and thus the apparent lag could in part reflect the residence time of leaf waxes during riverine transport. However, leaf waxes are predominantly ablated and transported via eolian processes (22, 26-28), and thus if the vegetation change were synchronous with local climate, there should be an immediate response in leaf waxes carried by eollan transport following an abrupt climate shift, with a slower rise due to fluvial input of leaf waxes following the climate change. However, the timing of rapid change in leaf waxes distinctly lags the rapid changes in climate, especially during the Younger Dryas onset (Fig. 3), in agreement with an overall lag of ∼50 years.
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49
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3042585423
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We thank N. Ohkouchi and P. Huybers for technical expertise and three anonymous reviewers for helpful discussions that improved this paper. Funding for this work was provided by the Frank and Lisina Hoch Endowed Fund and by the Woods Hole Oceanographic Institution (WHOI) Director's General Discretionary Fund. This is WHOI contribution no. 11112.
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