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Reply to comment by S. Peacock on “Glacial-interglacial circulation changes inferred from 231Pa/230Th.. Luo, Yiming 2012

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Click Here for Full Article Reply to comment by S. Peacock on “Glacial‐interglacial circulation changes inferred from 231Pa/230Th sedimentary record in the North Atlantic region” J.‐M. Gherardi,1 Y. Luo,2 R. Francois,2 J. F. McManus,3 S. E. Allen,2 and L. Labeyrie4 Received 8 October 2009; revised 24 February 2010; accepted 2 March 2010; published 18 May 2010. Citation: Gherardi, J.‐M., Y. Luo, R. Francois, J. F. McManus, S. E. Allen, and L. Labeyrie (2010), Reply to comment by S. Peacock on “Glacial‐interglacial circulation changes inferred from 231Pa/230Th sedimentary record in the North Atlantic region,” Paleoceanography, 25, PA2207, doi:10.1029/2009PA001867. 1. Introduction [1] Peacock [2010, P10 hereafter] takes issue with several aspects of our recent article [Gherardi et al., 2009, G09 hereafter]: (1) our description of the modern Atlantic meridional overturning circulation (AMOC), (2) apparent inconsistencies between modern circulation and our Holo- cene sedimentary 231Pa/230Th bathymetric profile, and (3) our use of cores for reconstructing AMOC from locations that do not lie beneath the Deep Western Boundary Current (DWBC). We welcome the comment as it affords us the opportunity to clarify important aspects of our approach to studying paleocirculation and to address any potential mis- understandings arising from the wording of our original text. 2. The 231Pa and 230Th Residence Times in the Water Column [2] The use of sedimentary 231Pa/230Th to reconstruct past changes in the rate and geometry of the AMOC relies on the fact that 231Pa has a longer residence time in the water column than 230Th [Anderson et al., 1983; Nozaki and Nakanishi, 1985]. The residence time of 230Th is short enough to limit severely the extent to which it can be lat- erally transported after its production by the radioactive decay of uranium dissolved in seawater. By contrast, as a result of its longer residence time, 231Pa is extensively redistributed by ocean circulation. Because the removal of the two radionuclides is controlled by reversible scavenging, their seawater concentrations in the absence of circulation and vertical mixing are very low at the sea surface and increase linearly with depth [Bacon and Anderson, 1982]. @ss X½ t=@t ¼   S @ Kss X½ t   =@Z ð1Þ where ss[X]t is the total activity of 230Th or 231Pa when the profile is at steady state with respect to scavenging, b is the production rate of the radioisotope (dpm m−3 yr−1), S is the sinking rate of the scavenging particles, K is the fraction of total X associated with particles (K = ss[X]p/ ss[X]t), and Z is the water depth. [3] In deep water, K is nearly constant. Therefore, at steady state: ss X½ t ¼  Z=SK ð2Þ [4] The lower ss[X]t in the upper water column result in shorter “steady state residence times” (tss): ss ¼ ss X½ t= ð3Þ [5] Because of these shorter residence times, it takes a stronger circulation to produce the same 231Pa deficit in shallower waters compared to deeper waters. This can be shown using the equation proposed by Rutgers van der Loeff and Berger [1993], which describes the lateral trans- port of 230Th or 231Pa. @ X½ t=@t ¼   S @ K X½ t   =@Z X½ t  i X½ t   =w ¼ 0 ð4Þ where i[X]t and [X]t are total 230Th (or 231Pa) concentration measured at two locations on the same isopycnal with i[X]t the concentration in the upstream source region and tw is the “transit time” of water between these two sites. In deep waters, K is nearly constant and to a first approximation can be removed from the derivative. Integrating this simplified equation thus gives X½ t  w þ i X½ t   1 eZ=wSK   ð5Þ 1Bjerknes Center for Climate Research, Institut for Geovitenskap, University of Bergen, Bergen, Norway. 2Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada. 3Lamont-Doherty Earth Observatory, Department of Earth and Environmental Sciences, Columbia University, Palisades, New York, USA. 4Laboratoire des Sciences du Climat et de l’Environnement, CNRS, Gif‐sur‐Yvette, France. Copyright 2010 by the American Geophysical Union. 0883‐8305/10/2009PA001867 PALEOCEANOGRAPHY, VOL. 25, PA2207, doi:10.1029/2009PA001867, 2010 PA2207 1 of 5 [6] This equation predicts that, as one moves downstream (increasing tw) the radioisotope profiles relax back to line- arity slower as Z/SK increases. Since S and K change little with depth, the transit time required for relaxation of the profiles increases with depth; that is, linearity is regained closer to the source at shallower depth. [7] From equations (2) and (3): ss ¼ Z=SK ð6Þ [8] Therefore: X½ t  Thw þ i X½ t   1 ess=w   ð7Þ [9] This equation predicts that the radioisotope profiles relax back to linearity more slowly with increasing tss, which increases linearly with water depth (Figure 1). Profile linearity is thus regained closer to the source at shallower depths, and 230Th regains linearity faster than 231Pa because of its shorter steady state residence time in the water column. [10] Therefore, even though there is a significant south- ward flow above 2500 m in the North Atlantic today [P10; Lee et al., 1996; Smethie and Fine, 2001; Schott et al., 2004], it is not fast enough to produce a significant 231Pa export and, as a consequence, sediments deposited above that depth do not show a 231Pa deficit relative to the over- lying production. On the other hand, the same overturning rate at greater depth produces a measurable deficit in deep sediments because of the longer tss of 231Pa in deep water. [11] In consideration of the above, what is most remark- able in our data, is the large deficit found in the shallow cores during the last glacial maximum (LGM), and to a lesser extent during the deglaciation. This requires that the rate of the shallow overturning at that time must have been much faster than observed in the modern ocean, while the smaller deficit in the deeper core must indicate that the deep overturning rate was slower than in the modern circulation. Clearly, we cannot conclude from our data that the renewal rate of water above 2500 m is slow today in any absolute sense, but the rate of shallow overturning (depth < 2500 m) must be significantly slower today than it was during the LGM, while the rate of deep overturning (depth > 2500 m) is relatively faster today than it was during the LGM. [12] Sediment 231Pa/230Th integrates the meridional export of 231Pa over the entire overlying water column in a way that is weighted by the depth‐dependent tss of 231Pa [Luo et al., 2009] and our Holocene sediment 231Pa/230Th bathymetric profile is entirely consistent with the present‐day North Atlantic volume transport. The high sediment 231Pa/230Th measured above 2000 m reflects the shorter residence time of 231Pa at these shallower depths. Although substantial meridional volume transport occurs today above 2000 m, it is too weak to export a large fraction of the 231Pa from the upper overlying 2000 m of the water column. Between 2000 m and 3000 m, where we find the maximum lateral volume transport, we also find the sharpest decrease in sediment 231Pa/230Th (D231Pa/230Th 2000–3000 ∼ 0.03). This is because the combined effect of increasing tss and stronger lateral volume transport result in substantial lateral export of 231Pa between 2000 and 3000 m. Sediments deposited at 3000 m therefore record evidence that a higher proportion of 231Pa produced in the upper 3000 m is being laterally exported. Below 3000 m, lateral volume transport decreases but is still sufficient to effectively export a fraction of 231Pa in Figure 1. Relaxation of 231Pa activity at different depths as described by equation (7) with i[X]t = 0.14 dpm m−3, S = 500 m, and K = 0.04. After 50 years of transit from the upstream source region, [231Pa]t has reached 92.5% of its steady state value with respect to scavenging at 2000 m. It takes increasingly longer transit times to reach a similar level of concentration (relative to the steady state value) in deeper water (250 years at 3000 m and 850 years at 5000 m). GHERARDI ET AL.: COMMENTARY PA2207PA2207 2 of 5 these deeper waters. The fraction of laterally exported 231Pa integrated over the upper 4500 m therefore still increases (i. e., sediment 231Pa/230Th decreases). However, the decrease in sediment 231Pa/230Th between 3000 m and 4500 m is less (D231Pa/230Th 3000–4500 ∼ 0.01) than the decrease between 2000 m and 3000 m. In contrast, meridional volume trans- port in the upper 2000 m during the LGM was sufficiently vigorous to export a large fraction of the 231Pa produced in the upper water column. Below 2000 m, sediment 231Pa/230Th gradually increases indicating that very little of the 231Pa in deep water was exported. [13] This intuitive understanding of the relation between sediment 231Pa/230Th and lateral volume transport can be corroborated by a simple two dimensional scavenging model (Figure 2) [Luo et al., 2009]. When a circulation scheme is imposed to reproduce the volume transport esti- mated byWunsch and Heimbach [2006], the model generates sediment 231Pa/230Th close to the production rate at 1500 m, even though lateral volume transport at this depth is relatively high and the minimum in sediment 231Pa/230Th is reached 1000 m below the depth of maximum lateral transport. [14] P10 calculates 231Pa residence time as the measured activity divided by the production rate using thewater column data reported by Luo et al. [2009]. She concludes that these residence times do not increase between 1710 m and 4550 m and are inconsistent with our explanation. From equation (7), it is clear that P10 is using the wrong “residence time” to reach her conclusion. Instead, she should have used the “steady state residence time” (tss) of 231Pa, which is clearly a linear function of depth (equation (6)). In contrast to its linear increase with depth, we can expect to find relatively little geographic variations in tss since it depends on the sinking rates of particles and K. This is consistent with the overlap for 230Th profiles in the upper 1000 m (Figure 2 by P10). The relaxation time of 230Th profiles in the upper 1000 m is fast enough that linearity is regained at all these stations. The similar slope for all the profiles indicates that SK must be similar at those different geographic locations. The wide divergence deeper in the profiles is clearly a reflection of circulation (why would scavenging affect deep water and not the upper 1000 m?), and may indeed help us better constrain abyssal circulation [Marchal et al., 2007]. Figure 2. (left) Pa/Th depth profiles: sediment 231Pa/230Th generated at 36°N by imbedding the revers- ible scavenging model of Bacon and Anderson [1982] in a simple 2‐D overturning circulation cell from Luo et al. [2009] that reproduces the volume transport estimated byWunsch and Heimbach [2006] (purple line) compared to G09 Holocene depth profile (red line and dots). Black and purple arrows highlight maximum lateral transport and minimum 231Pa/Th, respectively (see discussion). (right) Lateral volume transport (Sv) estimated by Wunsch and Heimbach [2006] used in the model to generate the 231Pa/230Th bathymetric profile. GHERARDI ET AL.: COMMENTARY PA2207PA2207 3 of 5 [15] P10 also uses a compilation of core top data (her Figure 3). She finds a general decrease in sedimentary 231Pa/230Th with increasing water depth, in general agree- ment with the G09 and the modeling results of Luo et al. [2009], but dismisses the significance of this finding because of the scatter in the data. It is clear that there is significant scatter in the compiled data, but sediment 231Pa/230Th does not just vary with depth but also with distance from the site of deep water formation [Luo et al., 2009], which must account for part of the scatter. The color coding used by P10 roughly supports this. We must also realize that these data have been collected over several decades, using different analytical techniques and without cross calibration between laboratories. This will definitely add substantial noise to the data. Moreover, some of the core tops may not be modern sediments, in which case 231Pa/230Th would be low because of decay. It is also clear that when opal is present 231Pa/230Th will be higher. Such scatter is therefore not surprising. 3. Reconstructing AMOC Changes From 231Pa/230Th Records Over the North Atlantic [16] The last point of contention is whether we must analyze cores taken directly under the DWBC to monitor past changes in the rate of AMOC. P10 argues that our cores are not taken from sites where the flow of water is repre- sentative of the zonally integrated circulation of the North Atlantic and therefore cannot monitor the strength of the AMOC. By her argument only cores taken from underneath the DWBC would have the potential to monitor the AMOC. In fact, sediment 231Pa/230Th just monitors a weighted integration of the residence time of water in the column overlying the coring sites and therefore average spatially (basin‐wide) and temporally (centennial to millennial scale, depending on core resolution) all the complexities of ocean circulation geometry arising from the formation of recircu- lation gyres, eddies, etc. Just like deep water, 231Pa is most likely to be exported from the North Atlantic through the DWBC. The strength of the DWBC largely dictates the residence time of deep water in the entire North Atlantic basin and a stronger DWBC result in shorter residence times and a more effective export of the 231Pa produced in this basin. A key point here is that the strength of this export can therefore be monitored from anywhere in the basin. This spatial integration is not unique to 231Pa/230Th but is also clearly documented for all the other geochemical parameters that are sensitive to the residence time of water in ocean basins (e.g., O2, nutrients, D 14C, d13C, etc. [Broecker and Peng, 1982; Broecker et al., 1991; Schlitzer, 2000]). We can expect subtle spatial variations in the 231Pa/230Th signal within the North Atlantic (just as we find small variations in seawater nutrient concentrations, D14C, O2 etc. between the eastern and western basins of the North Atlantic) and we have begun to document these in the work by Gherardi et al. [2005], but irrespective of the spatial distribution of our cores, the 231Pa/230Th signals retrieved from them monitor the broad features of the AMOC. This is not to say that we should not strive to increase our sediment 231Pa/230Th database. Contrasting sediment 231Pa/230Th within the North Atlantic may provide additional insights into past changes in the path of the DWBC and sites of deep water formation, but first‐order qualitative conclusions on changes in the strength and geometry of the AMOC, as done by G09, can already be reached with some confidence. The goal of developing sediment 231Pa/230Th as a quantitative proxy for paleocirculation will require a partnership between data generation involving coring programs specifically tar- geting this problem, and modeling. 4. Conclusion [17] Sedimentary 231Pa/230Th in the North Atlantic con- tinues to be a promising tool to track past changes in the mean export rate of water masses, despite evident limita- tions related to particle flux and composition that must be evaluated carefully before interpretation of the records and any attempt to quantify the results. The temporal evolution and depth dependence of 231Pa/230Th as we present it in the work by G09 can best and most simply be explained by changes in the water depth distribution of integrated export of Pa, which must be related to the export of water masses. [18] Acknowledgments. We thank the editor for his positive com- ments. R. Francois acknowledges financial support from NSERC and CFCAS. This is publication A282 from the Bjerknes Centre for Climate Research. References Anderson, R. F., et al. (1983), Removal of 230Th and 231Pa at ocean margin, Earth Planet. Sci. Lett., 66, 73–90, doi:10.1016/0012-821X(83) 90127-9. Bacon, M. P., and R. F. 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(2010), Comment on “Glacial‐ interglacial circulation changes inferred from 231Pa/230Th sedimentary record in the North GHERARDI ET AL.: COMMENTARY PA2207PA2207 4 of 5 Atlantic region” by J.‐M.Gherardi et al.,Paleo- ceanography, 25, PA2206, doi:10.1029/ 2009PA001835. Rutgers van der Loeff, M. M., and G. W. Berger (1993), Scavenging of 230Th and 231Pa near the Atlantic Polar Front in the South Atlantic, Deep Sea Res., Part I, 40, 339–357. Schlitzer, R. (2000), Electronic atlas of WOCE hydrographic and tracer data now available, Eos Trans. AGU, 81(5), 45. Schott, F. A., et al. (2004), Circulation and deep‐ water export at the western exit of the subpolar North Atlantic, J. Phys. Oceanogr., 34, 817– 843, doi:10.1175/1520-0485(2004)034<0817: CADEAT>2.0.CO;2. Smethie, W. M., Jr., and R. A. Fine (2001), Rates of North Atlantic Deep Water formation calculated from chlorofluorocarbon invento- ries, Deep Sea Res., Part I, 48, 189–215, doi:10.1016/S0967-0637(00)00048-0. Wunsch, C., and P. Heimbach (2006), Estimated decadal changes in the North Atlantic meridi- onal overturning circulation and heat flux 1993–2004, J. Phys. Oceanogr., 36, 2012– 2024, doi:10.1175/JPO2957.1. S . E. Al len , R. Francois , and Y. Luo, Department of Earth and Ocean Sciences, University of British Columbia, 6270 University Blvd., Vancouver, BC V6T 1Z4, Canada. J.‐M. Gherardi, Bjerknes Center for Climate Research, Institut for Geovitenskap, University of Bergen, Allégaten 41, N‐5007 Bergen, Norway. (jeanne.scao@bjerknes.uib.no) L. Labeyrie, Laboratoire des Sciences du Climat et de l’Environnement, CNRS, F‐91190 Gif‐sur‐Yvette, France. J. F. McManus, Lamont‐Doherty Earth Obse rva to ry , Depa r tmen t o f Ea r th and Environmental Sciences, Columbia University, Palisades, NY 10964‐8000, USA. GHERARDI ET AL.: COMMENTARY PA2207PA2207 5 of 5


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