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Origin of Bermuda's Clay-Rich Quaternary Paleosols and Their Paleoclimatic Significance

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Origin of Bermuda's Clay-Rich Quaternary Paleosols and Their Paleoclimatic Significance University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Staff -- Published Research US Geological Survey 1996 Origin of Bermuda's clay-rich Quaternary paleosols and their paleoclimatic significance Stanley R. Herwitz Clark University Daniel R. Muhs U.S. Geological Survey, [email protected] Joseph M. Prospero University of Miami Shannon Mahan U.S. Geological Survey Bruce Vaughn U.S. Geological Survey Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub Part of the Earth Sciences Commons Herwitz, Stanley R.; Muhs, Daniel R.; Prospero, Joseph M.; Mahan, Shannon; and Vaughn, Bruce, "Origin of Bermuda's clay-rich Quaternary paleosols and their paleoclimatic significance" (1996). USGS Staff -- Published Research. 165. https://digitalcommons.unl.edu/usgsstaffpub/165 This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D18, PAGES 23,389-23,400, OCTOBER 27, 1996 Origin of Bermuda's clay-rich Quaternary paleosols and their paleoclimatic significance StanleyR. Herwitz,1 Daniel R. Muhs,2 JosephM. Prospero,3 ShannonMahan, 2 and Bruce Vaughn2 Abstract. Red clayeypaleosols that are chieflythe productof aerosolicdust deposition are interbeddedin the Quaternarycarbonate formations of the Bermuda oceanicisland system.These paleosolsprovide a basisfor reconstructingQuaternary atmospheric circulationpatterns in the northwesternAtlantic. Geochemicalanalyses were performed on representativepaleosol samples to identify their parent dust source.Fine-grained fractionswere analyzedby energy-dispersiveX ray fluorescenceto determine trace element (Zr, Y, La, Ti, and Nb) concentrationsand to derive geochemicalsignatures basedon immobile element ratios. These ratios were comparedwith geochemical signaturesdetermined for three possiblesources of airborne dust: (1) Great Plains loess, (2) MississippiRiver Valley loess,and (3) Saharandust. The Zr/Y and Zr/La ratios providedthe clearestdistinction between the hypothesizeddust sources.The low ratios in the paleosolB horizonsmost closely resemble Saharan dust in the <2-/•m size class fraction. Contributions from the two North American loessial source areas could not be clearlydetected. Thus Bermudapaleosols have a predominantlySaharan aerosolic dust signature.Saharan dust depositionon Bermuda during successiveQuaternary glacial periodsis consistentwith patternsof generalcirculation models, which indicatethat during glacial maxima the northeastsummer trade winds were strongerthan at present and reachedlatitudes higher than 30øN despitelower-than-present sea surface temperaturesin the North Atlantic. Introduction derived from biogenicbeach sedimentsthat constitute>90% of the total rock volume above sea level [Land et al., 1967; The usefulnessof reconstructingatmospheric circulation Vacher, 1973; Harmon et al., 1981; Vacherand Hearty, 1989]. during the glacial-interglacialcycles of the last 2 m.y. has be- These eolianites overlie a submergedvolcanic seamount. In- come increasinglyimportant due to growing concernsabout terbedded between the eolianites and some minor marine car- global climate change. General circulation modeling of the bonatesare clay-rich"terra rossa"paleosols [Ruhe et al., 1961; northernhemisphere suggests that the northeastsummer trade Rowe, 1990;Hearty and Vacher,1994; Herwitz and Muhs, 1995]. winds between 24ø and 36øN latitude were stronger during Paleosolsinterbedded between datable rocksprovide valuable Quaternary glacial periods than at present [Kutzbach,1987]. opportunitiesfor reconstructingpast conditions over definable Palynologicalanalyses of northwestAfrican marine sediments time periods. are consistentwith this interpretation [Hooghiemstraet al., The Bermuda paleosolsare reddish to reddish brown in 1987; Hooghiemstra,1988]. Kefferet al. [1988], on the other color and consistof high concentrationsof aluminosilicate hand, concluded that the reduction in North Atlantic sea sur- clays [Ruhe et al., 1961; Vacherand Hearty, 1989]. Reddish face temperaturesduring glacial maxima causeda significant brown, clayey soils that formed by different processeshave southward summer shift in the Gulf Stream-North Atlantic been studiedin many parts of the world [Pye, 1983]. In situ Current systemand the subtropical-subpolarfrontal system. weatheringof calcareousrock formationshas been suggested The geologicalrecord of Bermuda, located at 32øN latitude in as a possiblepedogenic mechanism in other areas [Prasad, the northwestAtlantic, providesa unique opportunityfor ad- 1983;Moresi and Mongelli, 1988;Pye, 1992]. Bermuda'shigh- dressingthese contrastingviews of Quaternary paleocircula- purity eolianitescontain insutficient Si- and Al-bearingminer- tion. als to generate reddishbrown, clay-richsoil over periods of The oceanic island systemof Bermuda currently exposed <10s years[Herwitz and Muhs, 1995].The mineralogyand abovesea level is a lithified dune field composedof calcareous geochemistryof Bermuda's paleosolsindicate that the pa- leosols are not the insoluble residue of weathered eolianite. eolianites.The eolianitesare Quaternaryin age and are locally The generallyaccepted view is that most of the red clay frac- •Departmentsof Geographyand Biology, Clark University, Worces- tion of Bermudanpaleosols represents accumulations of fine- ter, Massachusetts. grained airborne dust originating from distant continental 2U.S.Geological Survey, Denver, Colorado. sourceareas [Bricker and Prospero,1969; Bricker and Macken- 3RosenstielSchool of Marineand Atmospheric Sciences, University zie, 1970; Herwitz and Muhs, 1995]. of Miami, Miami, Florida. Determination of the sourceof Bermuda'spaleosols would Copyright 1996 by the American GeophysicalUnion. providea basisfor testingmodels of Quaternaryatmospheric Paper number 96JD02333. circulationpatterns in the North Atlantic. Given the long res- 0148-0227/96/96JD-02333509.00 idencetime of atmosphericdust and the great distancesit can 23,389 23,390 HERWITZ ET AL.: BERMUDA PALEOSOLS AND PALEOCLIMATE travel [Rahn et al., 1979], any stable subaerialsurface is a Materials and Methods potential site of deposition. Long-term accumulationsat a Paleosol Collection given site may therefore representa mix of dust from widely Red clay soilsare distributedthroughout Bermuda. Plate la separated source areas. The objective of our study was to showsa subaeriallyexposed red clay soil surfacerepresenting identify the predominantsource area(s) that contributedto these accumulations on Bermuda. Immobile trace elements their presumedcondition before burial by calcareouseolian deposits.We studiedred clay paleosolsoverlain by carbonates that do not experiencedifferential chemical weathering, re- (Plate lb) at sitesdelineated on Vacheret al.'s [1989]geological lease,and removal have been used as a basisfor geochemical map of Bermuda.Soil samples(100-300 g) were collectedat fingerprintingbecause they maintainthe samerelative propor- siteswhere the differenteolianite formations were represented tionsover time [Muhset al., 1990].We identifiedthe geochemi- (Figure 1). Most of the samplesites were exposedat road or cal signaturesof Bermudanpaleosols and related them to the railway cuts or in the walls of abandonedquarries (Plate 2). signaturesof aerosolic dust from likely continental sources The numberof samplescollected at eachsite depended on soil usingimmobile element ratios. These data provideda basisfor thicknessand the numberof horizons;commonly, two to seven evaluatingmodels of Quaternary atmosphericcirculation pat- sampleswere collectedat each site. terns in the northwestern Atlantic. All sampleswere from paleosolprofiles with A and B hori- zons (Plate 3a), and, in somecases, distinguishable subhori- Quaternary History of Bermuda zons(e.g., Btl; Bt2). Paleosolsthat did not appearto have B The calcareous dunes that lithified to eolianite were first horizonswere not selected(Plate 3b) becauseof their high thoughtto have formed duringglacial low seastands when the carbonatecontent (> 50%) and their relativelylow proportions of red clay. Trace element concentrationsin soil A horizons Bermuda platform was more exposed,whereas the palcosols formed during intcrglacials [Sayles,1931; Livingston,1944; were so low that they could not be used for a meaningful definitionof a geochemicalsignature. Signatures described in O•cer et al., 1952].This interpretationwas challenged by Bretz this paper are mainly limited to samplescollected from B [1960],who arguedthat the dunesdeveloped during intergla- horizons.In preliminary studiesof Bermuda paleosols,our cial high seastands and the clay-richsoils formed duringglacial analyseswere limited to samplescollected from the interiorsof maxima. Submerged eolianite units, however, have been re- soil-filled solution pipes [Herwitz, 1993; Herwitz and Muhs, ported on the Bermudaplatform at depthsranging from 18 to 1995].Paleosol samples collected from the interiorsof solution 75 m below present-daysea level [Pirsson,1914; Stanleyand pipesat WhaleboneBay (Figure 1) were includedin this study Swift, 1967]. These submergedeolianites and other strati- becauseof their closeproximity to Whalebone Bay's Upper graphic evidenceindicate that the large lithified dune ridges Town Hill marine facies(Qtm), which contains the most con- that constitute
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