mineralogical and chemical variability of fluvial 2010 · mineralogical and chemical
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Earth and Planetary Science Letters 299 (2010) 368381
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Earth and Planetary Science Letters
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Mineralogical and chemical variability of fluvial sediments1. Bedload sand (GangaBrahmaputra, Bangladesh)
Eduardo Garzanti a,, Sergio And a,1, Christian France-Lanord b,2, Giovanni Vezzoli a,1, Paolo Censi c,3,Valier Galy b,4, Yani Najman d,5
a Laboratorio di Petrografia del Sedimentario, Dipartimento di Scienze Geologiche e Geotecnologie, Universit di Milano-Bicocca, 20126 Milano, Italyb Centre de Recherches Ptrographiques et Gochimiques, BP 20, 54501 Vandoeuvre-ls-Nancy, Francec Dipartimento CFTA, Universit di Palermo, 90123 Palermo, Italyd Department of Environmental Science, Lancaster University, LA1 4YQ Lancaster, UK
Corresponding author. Tel.: +39 02 64482088; fax:E-mail addresses: email@example.com (E. Ga
(S. And), firstname.lastname@example.org (C. France-Lanord), gio(G. Vezzoli), email@example.com (P. Censi), firstname.lastname@example.org@lancaster.ac.uk (Y. Najman).
1 Tel.: +39 02 64482097, +39 02 64482069; fax: +2 Tel.: +33 383 594220; fax: +33 383 511798.3 Also at IAMC-CNR UOS di Capo Granitola, 91026 Ca
Italy. Tel.: +39 091 23861638; fax: +39 091 347990.4 Now atWoods Hole Oceanographic Institution, 360W
MA 02543, USA.5 Tel.: +44 1524 593898; fax: +44 1524 593985.
0012-821X/$ see front matter 2010 Elsevier B.V. Adoi:10.1016/j.epsl.2010.09.017
a b s t r a c t
a r t i c l e i n f o
Article history:Received 19 April 2010Received in revised form 15 September 2010Accepted 16 September 2010Available online 15 October 2010
Editor: M.L. Delaney
Keywords:sedimentary geochemistrysedimentary petrologysettling equivalenceselective entrainmentplacer sandsopaque mineralsREE-bearing mineralsEu anomalyHimalaya
This study investigates the natural processes that control concentration of detrital minerals and consequentlychemical elements in river sand. The novelty of our approach consists in the systematic integration of detailedtextural, petrographical, mineralogical and chemical data, and in the quantitative description andmodeling ofrelationships among mineralogical and chemical variables for each sample and each grain-size class in eachsample. Bed sediment in transit in the largest sedimentary system on Earth chiefly consists of fine-grainedlithofeldspathoquartzose sand including rich amphiboleepidotegarnet suites, mixed with minor very-fine-grained-sand to silt subpopulations containing less heavy minerals and representing intermittent suspension.Mineralogical and particularly chemical differences between Ganga and Brahmaputra bedload are orders ofmagnitude less than both intersample variability associated with selective-entrainment effects andintrasample variability associated with settling-equivalence effects. Any provenance interpretation ofmineralogical, chemical, or detrital-geochronology datasets therefore requires quantitative understanding ofhydraulically controlled compositional variability. Mineralogical and chemical, intrasample and intersamplevariability can be deduced with simple equations and numerical solutions. The underlying assumptions on thechemical composition of detrital minerals, as well as the possible pitfalls, uncertainties and approximationsinvolved are discussed. Principal results include calibration of rare REE-bearing ultradense minerals, ill-determined by optical analyses but crucial in both detrital-geochronology and settling-equivalence studies,and assessment of progressively changing concentration for any detrital component with increasing intensityof selective-entrainment effects. Contributions by each mineral group to the chemical budget were inferredwith sufficient precision and accuracy. Although complex because of diverse controlling factors includingprovenance, weathering and anthropogenic pollution, mineralogical and consequently chemical variability offluvial sediments can be quantitatively predicted. This path, difficult because of insufficient information butfar from hopeless, shall eventually lead to more accurate calculation of sediment fluxes and chemical budgets,as well as to a deeper understanding of sedimentary geochemistry and fluvial sedimentology.
+39 02 64482073.rzanti), email@example.com@unimib.it(V. Galy),
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2010 Elsevier B.V. All rights reserved.
The sands of the river Ganga are violently trampled on by fishes,tortoises, porpoises, crocodiles, buffaloes, lions, elephants, but the sandsare not troubled, have no ill feelings, nor are they unconscious of beingtrampled on; they are without imagination, beautifully clear and devoidof impurities. The sands of the river Gangaflow along the stream and arepermanent, and so is the essence of Buddahood.MahayanaLankavataraSutra, The parable of the sands of the Ganga
The physical characteristics of turbulent river-flows vary rapidly,irregularly, and by orders of magnitude in space and time. Moreover,data from a big river in flood, when the bulk of sediment transport
369E. Garzanti et al. / Earth and Planetary Science Letters 299 (2010) 368381
takes place, are not easily obtained. Our knowledge of detrital fluxes,distribution of sediments through the water column, and hydrauliccontrol on their composition is consequently modest at best. Bedloadfluxes are generally unknown, and we often rely on unprovenassumptions such as that they represent some 10% of suspended ortotal load (Hay, 1998).
Mineralogical and chemical composition of sediments offer us anopportunity to investigate hydrodynamic processes during transportand deposition, but our knowledge on how detrital minerals aresegregated by sizedensity and sizeshape sorting into distinct sizeclasses by different transport modes is still limited (Komar, 2007).Because diverse chemical elements are preferentially contained inminerals with different density or shape (e.g., Rb, Cs in slow-settlingplaty mica, Zr, Hf in fast-settling dense zircon), hydrodynamicprocesses that control mineralogical variability control chemicalvariability as well (McLennan et al., 1993; Ohta, 2004). By quantifyingsizedensityshape relationships among detrital minerals entrainedon the channel bed, we can assess hydrodynamic impact onmineralogical and thus chemical composition of bedload sand. Butin order to do this we must know how various chemical elements arehosted in different detrital minerals. A deeper understanding of suchaspects is needed to correctly calculate relative contributions frommultiple sediment sources, and thus to make accurate estimates ofdetrital fluxes, sediment budgets, and erosion rates based onmineralogical or chemical composition of sediments.
The present article quantitatively analyses the physical processesthat control concentration of detrital minerals and consequentlychemical elements in bedload sand. It represents the first part of adetailed investigation on fluvial sediment in transit, the second partbeing dedicated to mud carried in suspension (Garzanti et al., inpress). As a giant natural laboratory, we used the GangaBrahmaputrafluviodeltaic system in Bangladesh. This is the largest entry point ofdetritus in the world oceans, which has fed throughout the Neogenethe huge turbidite fan expanding on Bengal Sea floors (France-Lanordet al., 1993; Goodbred and Kuehl, 2000). Our work is based on acontinuing research project carried out by CNRS-CRPG researchersduring the full monsoon season through the last decade (Galy andFrance-Lanord, 1999; Galy et al., 2007; Singh and France-Lanord,2002). The novelty of our approach consists in the systematicintegration of textural, petrographical, mineralogical and chemicaldatabases, and in the quantitative description and modeling ofrelationships among mineralogical and chemical variables for eachsample and each grain-size class in each sample.
Mineralogical and chemical scrutiny of an active fluviodeltaicsystem allows us to constrain how and to what extent sedimentcomposition is modified physically and chemically during transferfrom detrital sources to depositional sinks, a fundamental requisite tomake accurate quantitative provenance diagnoses. Unraveling min-eralogical fractionation in deltaic environments is specifically neededto compare the composition of fluvial and turbidite deposits, and thusto unambiguously trace detrital signatures from mountain belts toocean floors (Ingersoll et al., 2003). The same knowledge is crucial forprovenance analysis of ancient sandstone wedges, and may eventu-ally enable us to unravel the full record of paleotectonic andpaleoclimatic events stored in sedimentary archives. Practical appli-cations of economic interest include a better understanding of detritalfluxes, soil erosion, lifetime of reservoirs, mechanisms of placerformation, and prospecting of sedimentary ore deposits.
2. The GangaBrahmaputra sedimentary system
The huge drainage basins of Rivers Ganga and Brahmaputra(1,060,000 km2 and 630,000 km2, respectively) include large tracts ofthe Himalayan belt as well as Precambrian basement of the Indiancraton (Gansser, 1964). The two rivers join in central Bangladesh toform the PadmaMeghna estuary, strongly influenced by 4.56 m
tides and delivering annually to the Bay of Bengal