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Comment on "Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?".. Francois, Roger; Frank, Martin; van der Loeff, Michiel Rutgers; Bacon, Michael P.; Geibert, Walter; Kienast, Stephanie; Anderson, Robert F.; Bradtmiller, Louisa; Chase, Zanna; Henderson, Gideon; Marcantonio, Franco; Allen, Susan E. 2007-03-31

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Comment on ‘‘Do geochemical estimates of sediment focusing pass thesediment test in the equatorial Pacific?’’ by M. Lyle et al.Roger Francois,1Martin Frank,2Michiel Rutgers van der Loeff,3Michael P. Bacon,4Walter Geibert,3Stephanie Kienast,5Robert F. Anderson,6Louisa Bradtmiller,6Zanna Chase,7Gideon Henderson,8Franco Marcantonio,9and Susan E. Allen1Received 30 October 2005; revised 28 October 2006; accepted 15 November 2006; published 6 March 2007.Citation: Francois, R., et al. (2007), Comment on ‘‘Do geochemical estimates of sediment focusing pass the sediment test in theequatorial Pacific?’’ by M. Lyle et al., Paleoceanography, 22, PA1216, doi:10.1029/2005PA001235.1. Introduction[1] Accurately estimating the vertical flux of materialreaching the seafloor from the overlying surface waters isessential for the paleoceanographic reconstruction of a widevariety of oceanic processes. Two approaches are currentlybeing used. One consists of estimating mass accumulationrates (MAR) between dated horizons as the product of linearsedimentation rates, sediment dry bulk densities, and con-centrations. One pitfall with this approach is that sedimentscan be redistributed on the seafloor by bottom currents, andtheir accumulation may not necessarily reflect the truevertical rain rate originating from the overlying watercolumn. To address this problem, the method of230Thnormalization was developed [Bacon, 1984]. This methodis based on the assumption that the rapid scavenging of230Th produced in the water column by decay of dissolveduranium results in its flux to the seafloor always being closeto its known rate of production. To the extent that thisassumption is correct, scavenged230Th can be used as areference to estimate the settling flux of other sedimentaryconstituents and to correct for sediment redistribution on theseafloor [Henderson and Anderson, 2003; Francois et al.,2004].[2] MAR calculation and230Th normalization sometimesgive widely divergent results. This is particularly true in theequatorial Pacific, where MARs often indicate higher fluxesduring the last glacial period [Lyle et al., 2002], while230Thnormalization suggests unchanged or even lower glacialfluxes [Marcantonio et al., 2001; Loubere et al., 2004]. Theproponents of230Th normalization argue that changes inMAR mainly reflect changes in sediment focusing and donot record actual changes in vertical flux from the overlyingwater. In a recent paper, Lyle et al. [2005] dispute thisinterpretation and contend that230Th normalization grosslyoverestimates sediment focusing in the equatorial Pacific.They argue that lateral transport of230Th in the watercolumn is much larger than generally appreciated. Wedisagree with this view and in the following we point toshortcomings in their arguments.2. Correlation Between Surface Productivity andSediment Burial Rates of Biogenic Particles[3] One argument presented by Lyle et al. [2005] tosupport their interpretation of MAR is that ‘‘bulk sedimentburial correlates with surface productivity.’’ Yet burial ratesof biogenic particles in the eastern equatorial Pacific areclearly influenced by seafloor topography, as already dis-cussed at length by Loubere et al. [2004]. For instance,calcite MAR at the foot of Carnegie Ridge (Y69–71; 0.1C176N;86.5C176W; 2740 m) is ~ 2.5-fold higher [Lyle et al., 2002] thanat the top (V19–27; 0.5C176S; 82.1C176W; 1373 m), in spite of thefact that V19–27 lies well above and Y69–71 lies at thecalcite lysocline [Thunell et al., 1981] and the two cores arelocated under similar productivity regimes. There is thus amismatch between calcite MAR and productivity at thesetwo sites. This is by no means exceptional for this region. Acompilation of late Pleistocene-Holocene calcite MAR inPanama Basin [Lyle, 1992] clearly shows a general increasewith depth above the lysocline (C24 2800 m), where some ofthe calcite MARs reach values more than fivefold greaterthan the mean calcite MAR observed above 2000 m(Figure 1). It would be difficult to explain such a trend interms of surface productivity and the most likely explana-tion is substantial sediment redistribution by down-slopetransport.The relative merit of230Th normalization andMAR calculation is particularly well illustrated when com-paring the results obtained from Y69–71 and nearby coreME0005-24JC (0.02C176N; 86.5C176W; 2941 m) in Panama Basin[Kienastetal., 2007]. Owing to their close proximity(~10 km apart), we can expect similar surface productivityPALEOCEANOGRAPHY, VOL. 22, PA1216, doi:10.1029/2005PA001235, 2007ClickHereforFullArticle1Earth and Ocean Sciences Department, University of British Columbia,Vancouver, British Columbia, Canada.2Leibniz Institute of Marine Sciences at University of Kiel (IFM-GEOMAR), Kiel, Germany.3Alfred Wegener Institute for Polar and Marine Research, Bremerhaven,Germany.4Woods Hole Oceanographic Institution, Woods Hole, Massachusetts,USA.5Department of Oceanography, Dalhousie University, Halifax, NovaScotia, Canada.6Lamont-Doherty Earth Observatory of Columbia University, Palisades,New York, USA.7Oregon State University, College of Oceanic and AtmosphericSciences, Corvallis, Oregon, USA.8Department of Earth Sciences, University of Oxford, Oxford, UK.9Department of Geology and Geophysics, Texas A&M University,College Station, Texas, USA.Copyright 2007 by the American Geophysical Union.0883-8305/07/2005PA001235$12.00PA1216 1of5and vertical particle rain rates at the two sites. While230Th-normalized fluxes are strikingly identical (Figure 2a), meet-ing this expectation, MARs are not (Figure 2b).3. High-Resolution Chirp Subbottom Profile andSediment Focusing[4] Lyle et al. [2005] argue against significant sedimentfocusing using a 15 nautical mile chirp profile just north ofCarnegie Ridge that intercepts the Y69–71 and ME0005-24JC coring sites. Continuous reflectors reveal that sedi-ment cover is about half on top of abyssal hills compared tothe troughs. Using this observation, they calculated afocusing factor of 1.1 (Y69–71) and 1.5 (ME0005-24JC)by dividing the sediment thickness at each site by the meansediment thickness for the entire section. In contrast, using230Th normalization, Kienast et al. [2007] calculate higherfocusing factors for the same cores (3.1 and 5.3, respec-tively, over the past 27 kyr). It is noteworthy that althoughmuch larger absolute focusing factors are estimated using230Th normalization, the ratio of the focusing factors esti-mated from230Th (3.1/5.3 = 0.6) is in good agreement withthe ratio of accumulation rate obtained from acoustic data(1.1/1.5 = 0.7), considering the disparity of timescales overwhich the two averages were calculated (27 kyr versus270 kyr). Clearly,230Th normalization captures the relativedegree of focusing between the two sites documented by thechirp profile. The crux of the problem is thus to establishwhether the accumulation of230Th observed in these cores,much larger than expected from production rate, is due tolateral redistribution of sediment on the seafloor [Loubere etal., 2004; Kienast et al., 2007] or lateral transport of230Thin the water column [Lyle et al., 2005].[5] Focusing factors calculated by Lyle et al. [2005] areonly valid if the small section of the seafloor acousticallysurveyed is a closed system with respect to sedimentation(i.e., that sediment redistribution only occurs within thesurveyed area of the ocean floor, without lateral sedimentinput from outside this area), which is difficult to justify.While the advocates of230Th normalization argue thatsediment (and adsorbed230Th) must be laterally transportedfrom farther afield, Lyle et al. [2005] dismiss that possibilitybased on their reckoning that if focusing factors as large asdeduced from230Th normalization in Y69–71 were extrap-olated over the entire region of the EEP where a glacialMAR spike has been identified (an area greater than 6C176 by6C176 [Lyle et al., 2002]), that would produce a 17C176 by 17C176 areaof sediment-barren seafloor, which is not observed. Instead,they propose that230Th is laterally transported in the watercolumn independently of sediment mass. Considering thatsediment focusing is controlled by both local and regionaltopography and is therefore highly variable and site-specific[e.g., Mangini and Kuhnel, 1987; Turnewitsch et al., 2004],we question the validity of extrapolating sediment focusingmeasured in one core to a 6C176 C2 6C176 area of the seafloor andfind this simple regional sediment mass balance calculationunwarranted. We submit that it is nearly impossible togather the comprehensive data set necessary to convincinglybalance zones of sediment winnowing and focusing over theentire equatorial Pacific. On the other hand, testing thevalidity of230Th normalization is more readily amenable toFigure 1. Changes in carbonate mass accumulation ratesas a function of water depth in Panama Basin [Lyle, 1992]and estimated depth of the lysocline (C242800 m [Thunell etal., 1981]).Figure 2. The (a)230Th-normalized fluxes and (b) massaccumulation rates in two adjacent cores (Y69-71 andME0005-24JC) subjected to different degree of sedimentfocusing [Kienast et al., 2007].PA1216 FRANCOIS ET AL.: COMMENTARY2of5PA1216deductions from more fundamental principles, since it isbased on our understanding of the geochemical behavior of230Th in the water column.4. Are the Mechanisms Proposed by Lyle et al.[2005] to Explain Lateral Transport of230Th in theWater Column Consistent With Measurements of230Th in the Ocean?[6] The most basic argument against the possibility ofextensive lateral transport of230Th in the water column restson its residence time, which can be readily calculated fromthe disequilibrium between230Th and234U[Anderson et al.,1983].230Th activities range from< 0.1 dpm mC03in surfacewaters to C24 1.5 dpm mC03in the deep Pacific Ocean[Francois, 2007], corresponding to residence times from afew months in surface waters to ~50 years in Pacific bottomwaters. Using accepted estimates for lateral eddy diffusioncoefficient, models of various complexity [Bacon, 1988;Anderson et al., 1990; Henderson et al., 1999; Marchal etal., 2000] invariably show that lateral transport of230Th inseawater must be limited, resulting in vertical230Th fluxwithin 30% of the production rate over most of the ocean.To argue for a more extensive lateral transport of dissolved230Th, the onus is thus on Lyle et al. to demonstrate that230Th activity in seawater can be much higher than thus farmeasured. Profiles of230Th have been measured in mostocean basins and it is unlikely that such high concentrationswould have remained unnoticed.[7] Lyle et al. [2005] suggest two mechanisms to laterallytransport230Th in the water column: (1) with small sus-pended particles and (2) as a result of leakage of230Th fromslowly accumulating sediments.4.1. Lateral Transport With Small SuspendedParticles[8] In this scenario, most of the sediment mass reachingthe seafloor would be carried by large particles originatingfrom the surface and sinking rapidly with relatively littledirect uptake of dissolved230Th from seawater or exchangewith the suspended particle pool. Instead,230Th would bemainly adsorbed on small suspended particles and laterallytransported toward regions of high particle flux to beincorporated into the settling flux. This would add a lateralflux of230Th with little additional mass of particles; that is,high accumulation of230Th in sediment would not neces-sarily indicate an equivalent lateral transport of sedimentmass.[9] There are several lines of evidence that argue stronglyagainst this idea:[10] 1. Vertical profiles of230Th associated with sus-pended particles indicate that their average sinking velocityis typically 500–1000 m yrC01[e.g., Marchal et al., 2000].Therefore the mean residence time of suspended particles(and adsorbed230Th) in the water column is C245–10 years,i.e., shorter than for dissolved230Th in deep water(C2440 years). Lateral transport of230Th with particles musttherefore be even more limited than lateral transport in itsdissolved form. Again, to convincingly argue in favor ofthis mechanism, Lyle and coworkers would have to provideevidence for sinking rates of particulate230Th much slowerthan found to date.[11] 2. In order to have a net lateral transport of suspendedparticles by turbulent diffusion from low- to high-produc-tivity regions, lateral particle concentration gradients wouldbe needed, with lower suspended loads in the latter. Theopposite is observed [Beardsley et al., 1970; Biscaye andEittreim, 1977]. Therefore such lateral transport, if signifi-cant at all, would occur in the opposite direction required toexplain sediment focusing in the eastern equatorial Pacific.[12] 3. The mechanism proposed for transporting230Thlaterally would require that the230Th concentration per unitmass of suspended particles be many times higher than230Th concentration in sinking particles. Comparing230Thconcentration in suspended and settling particles in thePanama Basin, Anderson et al. [1983] show that suspendedparticles have230Th concentrations only 1.5 to 3 timesgreater than sinking particles. Therefore lateral transport ofparticulate230Th would automatically be accompanied bysignificant lateral addition of particle mass.[13] We thus conclude that the230Th concentration differ-ences between sinking and suspended particles are too smalland lateral transport of suspended particles too limited (andgoing in the wrong direction) to allow significant lateraltransport of particulate230Th independently of particle massfrom the central gyres toward the margins or other regionsof high particle flux.4.2. Leakage of230Th From Slowly AccumulatingSediments[14] Lyle et al. [2005] also argue that230Th scavenged incentral gyre regions could diffuse back from the sediment tothe water column as settling particles are remineralized atthe sediment water interface.[15] A significant seafloor release of230Th would create aclear deviation from the linear dissolved230Th profilespredicted by the reversible scavenging model of Nozaki etal. [1981] and Bacon and Anderson [1982]. This is illus-trated by adding to the model a diffusive flux originatingfrom the sediment (Figure 3). The dissolved and particulate230Th profiles obtained for a range of vertical eddy diffusioncoefficient (Kz) and230Th remobilization from the seafloor(F) display sharp increases in230Th concentration towardthe bottom, which is not observed in profiles measured incentral gyre regions [Nozaki and Nakanishi, 1985; Roy-Barman et al., 1996; Francois, 2007].5. The230Th Fluxes Measured With SedimentTraps[16] Sediment traps have been used to further assess theextent to which the vertical flux of230Th can deviate fromits rate of production [Yu et al., 2001; Scholten et al., 2005].Lyle et al. [2005, paragraph 41] misrepresent the work of Yuet al. [2001], who did not simply assess trapping efficiencyfrom the intercepted flux of230Th by assuming a constantvertical supply rate and neglecting lateral transport. On thecontrary, the bulk of the paper is devoted to assessing theextent of this lateral transport by combining the interceptedflux of230Th and231Pa and taking advantage of thePA1216 FRANCOIS ET AL.: COMMENTARY3of5PA1216difference in residence time between these two nuclides.The conclusion that the mean annual vertical flux of230This within 30–50% of its production rate over most of theocean is further corroborated by new sediment trap data.Scholten et al. [2005] found similar230Th fluxes, close tothe rate of production, in the eastern and western ArabianSea, even though mass fluxes were four times larger at thelatter site. Annually averaged fluxes of230Th in the equa-torial Pacific at 140C176W are also nearly equal to productionrates (data available athttp://usjgofs.whoi.edu/jg/dir/jgofs/eqpac/). Lyle et al. [2005] rightly note that sediment trapdata often display a strong dependency between particleflux and230Th flux, but they fail to point out that thesevariations happen on short timescales (subannual) andreflect short-term imbalances between production and scav-enging, which are largely averaged out when integratingover a full year or more [Bacon et al., 1985].6. Increased Sediment Focusing During the LastGlacial Period[17] Compilation of existing230Th-normalized flux dataindicates a general increase in sediment focusing during thelast glacial period, particularly in the eastern equatorialPacific region [Kienast et al., 2007]. This is an intriguingobservation, which has also been used to argue against230Th normalization. If sediment focusing is such a variableand site specific process, how could we explain such asystematic increase? Instead, this spatial coherence seems topoint to the classical interpretation of MARs as reflectingchanges in particle export flux. Recent findings provide analternative explanation, however, which is compatible withincreased sediment focusing. Wunsch [2003] argues thattidal dissipation, which occurs today mainly on the conti-nental shelves, must have migrated to the deep sea duringperiods of low sea level stand. Stronger tidal currents in thedeep ocean during glacial periods could thus be responsiblefor the higher focusing factors observed during MIS2[Kienast et al., 2007]. Coring is preferentially done in areasof sediment ponding, which would have a tendency toreceive more laterally transported sediment during periodsof stronger tidal currents (this sampling bias also explainswhy focusing is commonly found in core collections). Twobathymetric profiles of230Th-normalized flux from theequatorial Atlantic also show evidence for increaseddown-slope transport during MIS2 [Francois et al., 1990],while Mangini and Kuhnel [1987] report enhanced accu-mulation of siliceous clays in sediment focusing pockets ofthe Clarion-Clipperton zone, suggesting that this may be aglobal phenomenon, as would be expected if controlled bytidal dissipation.[18] Another possible explanation may be that there areactually no real (or much smaller) increases in sedimentfocusing (and MAR) during the last glacial period. Chro-nology-based MARs and focusing factors are very sensitiveto chronological errors [Francois et al., 2004; Kienast et al.,2007; Loubere and Richaud, 2007]. Core chronologies usedto estimate focusing and MARs are often based on d18Ostratigraphy. A recent study comparing isotopic stratigraphyin radiocarbon dated cores from the North Atlantic andeastern equatorial Pacific has revealed that the decreasein benthic d18O linked to the last deglaciation occurs~ 4000 years later in the Pacific, because of a late rise in deepwater temperature [Skinner and Shackleton, 2005]. FocusingFigure 3. Expected dissolved and particulate230Th concentration profiles that would result if230Thwere released from bottom sediments: (a) no release (F = 0) or 50% of production released (F = 0.5) and(b) 50% release with two different vertical eddy diffusion coefficients (Kz).PA1216 FRANCOIS ET AL.: COMMENTARY4of5PA1216factors and MAR calculated on the assumption that the shiftin benthic d18O represents a purely glacioeustatic signal (i.e.,placing the event atC2415 ka instead of 11 ka) would attributetoo much material accumulating during the glacial period,thereby overestimating both glacial MARs and focusing.[19] On the basis of these considerations, we thus standby our position that230Th normalization largely corrects themost serious errors associated with earlier interpretations ofsediment mass accumulation rates and should be systemat-ically used to retrieve information on particle flux from theLate Quaternary sedimentary record.PA1216 FRANCOIS ET AL.: COMMENTARY5of5PA1216ReferencesAnderson, R. 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J. Shackleton (2005), AnAtlantic lead over Pacific deep-water changeacross termination I: Implications for the ap-plication of the marine isotope stage stratigra-phy, Quat. Sci. Rev., 24, 571–580.Thunell, R. C., R. S. Keir, and S. Honjo (1981),Calcite dissolution: An in situ study in thePanama Basin, Science, 212, 659–661.Turnewitsch, R., J.-L. Reyss, D. C. Chapman,J. Thomson, and R. S. Lampitt (2004), Evi-dence for a sedimentary fingerprint of an asym-metric flow field surrounding a short seamount,Earth Planet. Sci. Lett., 222, 1023–1036.Wunsch, C. (2003), Determining paleoceano-graphic circulations, with emphasis on the LastGlacial Maximum, Quat. Sci. Rev., 22, 371–385.Yu, E.-F., R. Francois, M. P. Bacon, and A. P.Fleer (2001), Fluxes of230Th and231Pa to thedeep sea: Implications for the interpretation ofexcess230Th and231Pa/230Th profiles in sedi-ments, Earth Planet. Sci. Lett., 91, 29–230.C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0C0S. E. Allen and R. Francois, Earth and OceanSciences Department, University of BritishColumbia, 6270 University Blvd., Vancouver,BC, Canada V6t 1Z4. (rfrancois@eos. ubc.ca)R. F. Anderson and L. Bradtmiller, Lamont-Doherty Earth Observatory of Columbia Uni-versity, P.O. Box 1000, Palisades, NY 10964,USA.M. P. Bacon, Woods Hole OceanographicInstitution, Woods Hole, MA 02543-1050, USA.Z. Chase, College of Oceanic and AtmosphericSciences, Oregon State University, Corvallis, OR97331-5503, USA.M. Frank, IFM-GEOMAR, Wischhofstrasse 1-3, D-24148, Kiel, Germany.W. Geibert and M. Rutgers van der Loeff,Alfred Wegener Institute for Polar and MarineResearch, Am Handelshafen 12, D-27570 Bre-merhaven, Germany.G. Henderson, Department of Earth Sciences,University of Oxford, Oxford OX1 3PR, UK.S. Kienast, Department of Oceanography,Dalhousie University, Halifax, NS, CanadaB3H 4J1.F. Marcantonio, Department of Geology andGeophysics, Texas A&M University, CollegeStation, TX 77843, USA.


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