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JOURNALOF GEOPHYSICALRESEARCH,VOL. 99, NO. B7, PAGES13,871-13,883,ULY 10, 1994

Natural controlsof fluvial denudation ates in major world

drainage basins

M. A. Summerfield and N.J. HuRon

Macrogeomorphologyesearchroup, epartmentf Geography,niversityf Edinburgh,dinburgh

United Kingdom

Abstract. We present new compilation f estimates f modem atesof mechanical nd

chemical enudationor externally rained asins xceeding x 105 m2 n area. These

estimates re basedon sediment nd solute oad data selected n order o representnatural atesas

far aspossible.Chemical enudationateshavebeen alculatedy deductinghe

nondenudationalomponent f solute oad. Mechanical enudationates ange rom 1 mm kyr

for the St. Lawrence ndDneprbasins o 670 mm kyr x or the Brahmaputra asin. Chemical

denudationatesvary rom 1 mm kyr (Kolyma,Niger, Nile andRio Grandebasins) O27 mm

kyr4 (ChiangJiangbasin). The Kolymabasinhas he owest 4 mm kyr ), and the Bmhmaputra

basin he highest, verall ateof denudation688 mm kyr4 ). Relationshipsetweendenudation

ratesanda rangeof morphometric, ydrologic, ndclimaticvariables re nvestigatedhrough

correlation ndregression nalysis. Morphometric ariables,suchas mean ocal relief, are

accurately alculatedor largebasins or the first time by using he NationalGeophysical ata

Center 0-minuteopographicatabase. ariables xpressingasin eliefcharacteristicsnd

runoff are found o be moststrongly ssociated ith both mechanical ndchemicaldenudation

rates,with more than60% of the variance n total denudation eing accounted or by basin elief

ratioand unoff. Basinarea, unoffvariability,andmean emperature, owever, re only weakly

associated ith ratesof denudation.Althoughdirectcomparisonsannotbe made, t appearshat

ratesof basindenudation erived rom present-day ass lux estimatesre not,overall,

significantly ifferent rom estimates f long-term atesbasedon sediment olumeand

thermochronologicata. It therefore ppearshat he key factors dentifiedas controlling

denudationateshereare alsoapplicable o the geological ime spans elevant o the interaction

between ectonic nd denudational rocesses.

Introduction

A knowledgeof rates of denudation nd an understanding f

the factors that control them are important for a number of

reasons. Quantitativemodels of landscapeevolution [Ahnert,

1987] dependon estimates f the ratesat which andscape hange

occurs,while those nterested n the interactionbetween tectonic

and subaerialprocesses, oth n orogenic Molnar and England,

1990; Beaumont et al., 1992] and cratonic [Gilchrist and

SummerfieM,1990; Bishop and Brown, 1992] terrains need to

apply ealisticdenudationates n the calibration f their models.

Estimates f long-termdenudationatesare a vital component f

both geochemical Berner, 1991] and sediment fLeeder, 1991]

massbalancestudies,while the importance f understandinghe

factors hat controlspatial and temporalvariations n sediment

supply to sedimentarybasins is now becomingmore widely

appreciated Cross, 1990; Sinclair and Allen, 1992]. Recently,

the suggestionhat the creationof topography ssociatedwith

orogenesisan significantly ffect atesof chemicaldenudation

and therebyperturb the global carbonbudget and consequently

globalclimate Raymo t al., 1988;Raytooand Ruddiman,1992]

has further reinforced he need for a clear understanding f the

factorscontrolling enudationates.

Copyright 994by theAmericanGeophysical nion.

Papernumber 4JB00715.

0148-0227/94/94JB-00715505.00

Although range f approaches,ostnotablyhermo-

chronology nd calculations f offshoresedimentvolumes,can

provide stimatesf long-termenudationaies,onlydataon

present-dayatesderived rom sediment nd solutedischarges f

riversgenerally rovide he mostviablebasis or linking

variations n denudation ates to specificcontrollingvariables. A

numberf studiessinghis pproachave ocusedn he oleof

climate, r vegetationsmediatedyclimate, s hekeyvariable

determiningatesof denudationLangbein nd Schumm, 958;

Fournier, 1960; Corbel, 1964; Douglas, 1967; Wilson, 1973;

Jansenand Painter, 1974; Jansson,1982, 1988; Ohmori, 1983].

Althoughsome of these analysesalso considered opographic

controls n denudationa[es, he role of elevation nd relief has

beenemphasizedn relatively ew studies Ahnert,1970; Pinet

andSouriau,988; insele,992;Milliman ndSyvitski,992].

The aim here s to presenta new compilation f estimates f

rates of denudation or the world'smajor drainagebasinsand to

providean initial assessmentf the main controlsof denudation

rates at the regional to subcontinental calerelevant to the geo-

logical ime spans ppropriateor modellnghe interaction

between ectonicanddenudational rocesses.n order o produce

estimatesf denudationateswhichare moreappropi'iateo

modelingover geological imescaleswe have attempted o

exclude,s araspossible,nthropogenicffectshroughareful

assessment f the available sedimentand solute dischargedata.

In doing hiswe areaiming o provide stimatesf "natural"ates

of denudationand to assess he major "natural" controls hat

13,871


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13,872 SUMMERFIELD AND HULTON: FLUVIAL DENUDATION RATES

determine them. All externally drained basins with an area of

more han x 105 m are ncluded,lthoughomprehensiveata

are not available for all of these (Figure 1). Together hese 33

basins over n areaof 5.28 x 107 m ,representingver35% of

the Earth's land area. Three internal basins above the area

threshold the Okavango, he Volga, and the Chari) have been

excluded rom the analysis s they do not contribute o an overall

reduction f continental levation. Our reason or focusing n the

very largestbasins s that we are interestedat this stage n estab-

lishing first-order effects rather than examining small-scale

controls on denudation rates.

The main constraintson our analysis are the quality of

available sediment and solute load data, the extent to which

appropriate llowances an be made for anthropogenicmpacts,

and the accuracywith which likely controllingvariablescan be

characterized. n order to minimize these potential imitations,

sedimentand solute oad data have been carefully selectedso as

to represent, s far as possible, onditions rior to major human

modification of the basins concerned. We have also been careful

in our estimation of chemical denudation rates to make an

adjustment or the nondenudational omponent f solute load

introduced y atmosphericnputs. The data set we present s our

best estimate, within the limitations of the available data, of

natural rates of mechanical and chemical denudation under

prevailing basin conditionsof topography, limate, vegetation,

and lithology. This data set in turn provides he basis for our

exploratory nalysisof the main factorsdeterminingworldwide

variationsn denudationates or very argebasins.

In addition to our new estimates of natural denudation rates for

major drainagebasins,anothernovel component f this analysis

is our use of National GeophysicalData Center (NGDC) 10-

minutedigital topographic ata (seeCurningand Hawkins [1980]

for full documentationf thisdatabase)o calculate ccuratelyor

the first time a rangeof morphometric ariables or thesebasins.

In particular, we produce he first calculationsof mean local

relief for large drainagebasins.

Data Sourcesand Quality

Potential Controlling Variables

Data are presented or 11 potential controllingvariables for

which measurement s possible at the interval or ratio scale

(Tables 1 and 2). (Full documentationof data sources s avail-

able on request rom the authors.) Variable selectionwas aimed

at including hose actors houghtmost ikely to play an mportant

role in controlling denudation rates and for which data of

adequate ualityare available. In order o use the NGDC digital

topographic atabase or the calculationof morphometricvari-

ables,all basinperimeterswere digitized. Where the locationof

basin perimeters s unambiguous ur basin area estimatesare

within 5% of thosegiven n existingpublished ources. Mean

local relief for large basinshas previouslybeen estimated rom

Dnepr

Tocantlns

Francisco

' -- 0 10 25 50 100 250

lOOO o lOOO

km Denudationatemmka1)

500

Figure 1. Locations ndestimatedotaldenudationates or major externallydrainedbasins.


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13,874 SUMMERFIELDNDHULTON: LUVIALDENUDATIONATES


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SUMMERFIELD AND HULTON: FLUVIAL DENUDATION RATES 13,875


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partially, the effects of pollution by not incorporating ecent

analyses rom industrializedcatchments. They do, however,

include atmospheric nd recycled components. Previousesti-

matesof chemicaldenudation ave generallybeenbaseddirectly

on this raw data, but in order to assess orrectly he contribution

of dissolvedoad to denudationas opposedo total solute rans-

port) it is necessaryo deduct hesecomponents.Detailedmass

balancestudies re not available or large drainage asins,so we

have assumed,on the basis of the global mean estimatesof

Bernerand Berner 1987], that 4.5% of TDS is contributed y

precipitationnd hat64%of HCO 'originatesromatmospheric

CO . Values or HCO3' andTDS havebeenderivedixan

Meybeck [1979], except for the Amazon [Berner and Berner,

1987], Chiang Jiang [Hu et al., 1982] and Rio Grande

[Livingstone,963].Forbasinsorwhich odataonHCOz' re

available we have estimated that bicarbonate constitutes 52% of

(unpolluted)TDS (global mean value according o Meybeck

[1979]). Where not directly available, TDS values have been

calculated rom dissolved oad data. The denudational ompo-

nent for the Zaire Basin s a specific stimate y Nkounkou nd

Probst [ 1987].

Mean annual specific solid load and mean annual specific

denudational issolved oad were derived for each basin using

our digitizeddrainage reas. In all cases he lowestpoint n the

basin for which solid and solute load data are available have been

used, although his is not alwaysat the basin outlet. However,

the discrepancies etween the upstreamarea from these meas-

urementpointsand the digitizedbasin areasare small. Ratesof

mechanical, chemical, and total denudation were calculated

assuming mean ockdensity f 2700 kg m z (Table3). It is

important o note, however, that calculations f denudation ates

from mass ransport ata involve assumptionsbout he density

changeshat occurduring ock weatheringprior to the removalof

solidanddissolvedmaterial rom a drainage asin Surmneeld,

1991a]. In usingbedrockdensityas the volumetricconversion

factorwe are assuming steady tate egolith hickness.

Resultsand Analysis

The data or potential ontrollingariables enerally howat

least an order of magnitudevariation (Table 2). For instance,

maximum alues or relief ratio andmean ocal elief attained y

the Brahrnaputraasin 0.00554 and992 m, respectively) anbe

comparedwith the minimumvalues or the Dneprbasin 0.00039

and 30 m). Mean annual unoff s similarlyvariable, anging

from 5 mm for the Rio Grande to 1244 mm for the Orinico. The

Brahrnaputra asinhas he highest atesof both mechanical670

mm kyr ) and total denudation688 mm kyr ) (Table 3 and

Figure1), although recentsediment ischarge stimate f 540

Mt yr4 cited romunpublishedataby MillimanandSyvitski

[1992] indicates hat our estimate may be too high. The

Brahmaputra comes only second, however, in chemical

denudation, eingexceeded y the ChiangJiangbasinwith a rate

of 27 mm kyr4. The Dnepr and St. Lawrence asinshave he

lowest atesof mechanicalenudationt lmm kyr , while our

basins hareheminimum hemical enudationateof lmm kyr

- the Kolyma,Niger, Nile andRio Grande. The Kolymaalsohas

the lowest ate of total denudationt 4 mm kyr . As found n

previousstudies, he proportionof total denudation ontributed

by chemicaldenudationdecreases s total denudation ncreases

(Figure 2). This means that in basins experiencing ery high

total denudation ates chemicaldenudation s high in absolute

terms,but low in relative terms. In basinswith very ow overall

denudation ates, such as those of the major Siberian rivers,

chemical denudation can account for more than 50% of the total

(Table 3).

In order to explore the relationships oth betweendenuda-

tional and potential controlling variables, and among the

controlling ariables hemselves, catterplots were produced or

all variablepairings. In addition,Pearsonian orrelation oeffi-

cientswere calculatedor all variablepairings or Y = mX + c,

and or selected ariablepairings or log Y = mX + c and og Y =

mlog X + c. Partial and multiple correlation oefficients ere

800 600 400 200 120

MECHANICAL CHEMICAL

100 80 60 40 20 0 20 40

Kolma

Dnepr

Ob

Niger

Yenisei

Nile

Murray

S.Lawrence

La Plato Parana)

Zambezi

Rio Grande

Shatt-E1-Arab

Orange

Columbia

Mackenzie

Yukon

Danube

HuangHe

Orinoco

Mississippi

Amazon

Colorado

Mekong

Chiang iang

Indus

Ganges

Brahmaputra

DENUDATIONATEmm a1)

Figure 2. Histogramsomparingmechanicalndchemical enudationates or majorexternally rained asins.


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also calculated for selected variable pairings. Unlike in the

recentanalysisof Milliman and Syvitski 1992], all data points

were included in the calculation of correlation coefficients and

regression quations inceusingan arbitrarycutoffpoint n terms

of standarddeviation would have masked the real degree of

scatter in the data.

For mostvariablepairings his degreeof scatterwas high, an

unsurprisinginding given the wide range of factors ikely to

control rates of mechanical and chemical denudation. However,

an importantadditional eason or high data scatter s probably

low data quality, since there is no reason to suspect that

inaccuraciesn the data would be systematically iased so as to

enhance any correlationsbetween controlling and denudation

variables. On the contrary, data errors are likely to be

approximately andom, and therefore the true strength of

statistical associations between controlling and denudation

variablesdetermined rom the data presented ere is likely to be

underestimated. In other words, our interpretations of the

strengthof associations etween controllingand denudational

variablesare nherentlyconservative.

The relatively low amount of variance accounted or among

most of the pair-wise associations ean that a range of best fit

regression elationshipsdescribe them about equally well.

Nevertheless, close examination of the data indicates that

associations etween the potential controllingvariables are best

described n terms of linear relationships Table 4), whereas

associations between denudation rates and basin variables are

moreadequately escribed y the form og Y = mX + c (Table 5).

Among the potential controllingvariables there is a relatively

high degree of correlation between those expressing various

aspects f topography,hat s, mean runkchannelgradient, asin

relief, relief ratio, mean local relief, and mean modal elevation

(Table 4). However, basin hypsometry, s representedby the

hypsometricntegral, is weakly correlatedwith the other basin

variables xcept or basin area and meanmodalelevationwhere

there is a moderatedegreeof association.None of the climatic

variables (including ranoff and ranoff variability) is strongly

associated with the other basin variables.

Table 5 andFigure3 illustrate he relativelystrong orrelation

between he topographic ariables,exceptingbasin hypsometry,

and rates of both mechanical and total denudation. Of

considerablenterest s the fact that mean local relief appears o

be no better as a predictorof denudation ate than basin relief

ratio, in spite of the former providing a much more detailed

representation f averagebasin relief and slope characteristics.

Given that mean local relief is a time-consuming ariable to

quantify, his s a helpful finding or those equitinga simplebut

effectivemeansof characterizing asin elief in modelingstudies.

However, this result may be scale-specificand does not

necessarilymean hat relief ratio providesan equally appropriate

representation f basin relief characteristicsor smaller basins

than those considered here. Chemical denudation rates are more

weakly associated with these topographic variables than

mechanical enudationates. Interestingly, iven he findingsof

some other studies [Milliman and Syvitski, 1992], only an

extremely weak negative correlationwas found between basin

area and denudation rates. In terms of the "climatic" variables

only mean annual ranoff and to a lesser extent mean annual

precipitation re at all stronglyassociated ith denudation ates

(Figure 3). Runoff variability, at least as defined n this study,

shows no association with denudation rates.


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13,878 SUMMERFIELDAND HULTON:FLUVIAL DENUDATIONRATES

Table 5. Pearsonian orrelationMatrix for Denudational ersusMorphometric,Hydrologic,and

Climatic Variables

Log. Mechanical Log. Chemical Log.Total

Denudation Rate Denudation Rate Denudation Rate

Area -0.11 -0.16 -0.18

Mean tnmk channel radient 0.67 0.36 0.66

Basin relief 0.80 0.51 0.79

Relief ratio 0.78 0.50 0.79

Mean modal elevation 0.66 0.36 0.66

Mean local relief 0.68 0.54 0.71

Hypsometricntegral -0.03 -0.06 0.03

Mean annual ranoff 0.45 0.52 0.54

Runoff ariability -0.04 0.08 -0.05

Meanannualemperature 0.41 0.09 0.34

Meanannual recipitation 0.42 0.32 0.52

Interpretationand Discussion

Mechanical Denudation Rates

The importance f basin topography, nd to a lesserextent

rimoff, in influencing ates of mechanicaldenudation s sup-

ported by the relatively strongstatisticalassociation etween

these variables. On the other hand, the very weak correlation

between mechanicaldenudation ate and rimoff variability does

not support he importance f "storminess"n influencing enu-

dation ates [Molnar and England, 1990] for the scaleof basins

consideredere. The strong tatistical ssociationetween elief

and mechanical enudationate may, itself, be partly a function

of other factors elated to relief, suchas high levels of seismicity

and the prevalenceof fractured ock in high-relief orogenic

terrains Milliman and Syvitski,1992].

The significant ata scatter, ven n the relationships etween

mechanical denudation rate and relief and ranoff, could be due to

a number of factors n addition to errors in specific sediment

yield andwaterdischargestimates.One s the ackof anydirect

assessmentf variations rt erodibility,suchas thoseassociated

with lithology,but for very large basinserodibilityvariations

might be expectedo average ut and thusnot play a majorrole

in contolling asin-wide enudationates. A secondactorwhich

is potentiallymoresignificant, speciallyor thevery argebasins

considered ere, is variablestorage ffects. In very large basins

sediment may be "temporarily"stored in floodplains for

thousands f years or more. Clearly, for such river systems,

sediment ampling t thebasinmouth,evenoverseveral ecades,

mayprovidean unrepresentativenapshot f the ong-termmean

sediment lux. Given theseeffects, t is perhapssurprising hat

the datado not, in fact, showan evengreaterdegreeof scatter.

The very weak negative orrelation etweenmechanical enu-

dation rate and basin area observedhere contrastswith findings

in a number f previous tudies,most ecentlyhat of Milliman

and Syvitski 1992]. This maybe because nly a relativelysmall

size range of basinswas considered, panningonly about one

order of magnitude. However,basin area itself can have no

causal link with denudation rates since area itself cannot be a

determining actor. The associationdentified in previous

studies, herefore, robablyarises rom the fact that basin area s

negatively orrelatedwith otherpotentialcausalvariables,such

asrelief ratio andmean ocal relief (Table 4). This is a complex

issue,however, as demonstrated y data from westernCanada

which shows that denudation rates are lowest in the smallest

basins100,000 km2) were found to have intermediate enudation

rates.

The idea that mechanical, and indeed total, denudation rates

vary as a function of mean elevation would appear to be

supportedy the datapresentedere Table5). This association

has been applied n a numberof studiesmodeling nteractions

between tectonics and denudation Lainbeck and Stephenson,

1986;Slingerland nd Furlong, 1989;Pitman and Golovchenko,

1991; Lorenzo and Vera, t992] either through a

misunderstandingf the studies y Ahnert 1970] andRuxton nd

McDougall [1967] (whichdemonstrated relationshipwith local

relief rather than elevation), for mathematical convenience,or

from an assumed causal link between elevation and denudation

rate. But clearly,as with basinarea,elevation tself cannot e a

direct determinant of denudation rate. The flux of sediment at a

specific ocationwill be a functionof the gradientat that point,

irrespective f its elevation bovemeansea evel [Summerfield,

1991b].

The reasonwhy mean basin elevation s stronglyassociated

with denudation ate is that elevation s itself stronglycorrelated

with other topographicactors which are causally elated to

specific ediment ield. This is evident rom Table 4 where t

can be seen hat trunk channelgradient, elief, relief ratio, and

mean ocal relief are all either moderately r strongly elated o

mean modal elevation. These relationships,however, must be

examinedwith care. Although here is a clear relationship

between mean modal elevation and mean local relief for basins as

a whole (Figure 4), when intrabasinpatterns are examined

important ifferences merge. For a numberof basins,suchas

the Yukon, the intrabasin pattern replicates the interbasin

associationetween levation nd ocalrelief (Figure5). But for

basinssuch as the Ganges he data points cluster around a

distinctcurvilinear rend causedby a decline n local relief at

high elevations>3500 m). This kind of pattern s also ound n

other basins, such as the Indus, Brahmaputra, and La Plata


9/13


1000.

1000'

E

E

,,, 100'

z

0

lO-

z

lOOO.

1oo.

lO.

z

o

lOOO

E

E

Lul O0

z

o

1o

z

o

0.0 d.4 0:8 1:2 1:6 l.0 .2:4 0.000

MEAN GRADIENTOF TRUNK CHANNEL(mkm")

4(0 8(0 12'00 1100 2(00 2,100 0

MEAN MODAL ELEVATION m)

,

* .

* , *

,

coo

1,1% *

0 2(0 4b0 66o 86o 1(oo 12bo 0

MEAN ANNUAL RUNOFF (mm)

,

, *

,

, , ,

,

t

, *

*

,

MEAN ANNUALTEMPERATURE C)

o.D01 0.602 o.b03 o.b04

RELIEF. RATIO

,

.

,

,

OIiO,

*

,0.

*

200 400 600

MEAN LOCAL RELIEF (m)

0305

800

lb 2'o o ,{o o go io 8b

RUNOFF VARIABILITY %

Q ,

,

,

8

e

e

, * * ,

,

1 ooo

0 260 4bo 6bo 8b0 ld0012'00 1,001001100 2600

MEAN ANNUAL PRECIPITATION mm)

Figure 3. Scatter lotsof totaldenudationatesversus ariousmorph,metric,hydrologic ndclimaticvariables.

(Parana), which have part of their upper catchments n high

mountain plateaus. Another kind of intrabasin pattern is

characteristic f basins which drain to high elevation passive

margins, such as the Orange and Zambezi. Here there is a

clusteringof very low local relief values at moderateelevations

of around 1000 m. This pattern s also suggested y the high

hypsometric ntegrals for these basins (Table 2). The

significance f thesecontrastingelationshipss that local relief

is stronglycorrelatedwith local slopegradients Ahnert, 1970],

so such intrabasin variations in local relief are likely to be

mirroredby contrastsn denudationates [Summerfield, 991b].

In basinswhere there s a very low correlationbetweenelevation

and ocalrelief, suchas the Orangeand Zambezi (Figure 5), ele-

vation may provide a very poor basis for predictingdenudation

rates.

lOOO,

a: OOO,

40O,

'OO,

C

0 4 8 12:X) 130 2(:30 200 X)

MEAN MODAL ELEVATION m)

Figure 4. Scatterplot of mean modal elevationversusmean

local relief.


10/13

13,880 SUMMERFIELDAND HULTON:FLUVIAL DENUDATIONRATES

4000

3500

3000

'-' 2500

,,, 2000

1500

1000

5OO

YUKON

GANGES '

50001

45001

4000'

. 3500,

u_ 3000'

,,, 2500'

< 2000

8

_a 1500

1000 ::: ; . '.. .

'.i'.:::..::: . : . .

500 ..:::.' '"-:-:::.:'

0 500 1000 1500 2000 2500 3000 3500 4000

MEANMODALELEVATION (rn)

..... : : ' : .- . . ....

; :.... : : : . . . : i ..: :-..

.

'' ':' ': '" : ';'3' : :.. : -

..

.....

..

. . .

o6 ' "'ioO0 260036004600 5000" 6007600

MEAN MODALELEVATION (m)

32o

2800

2400

2000

--n 16oo

LIJ

< 12002

8OO

400

ORANGE

::iiiiiiii:.i i i : '

- '500 1600 1500 2000 2500

MEANMODALELEVATION (rn)

Frequencysarea,km )

1000

4000

9000

* 16000

25000

" 36000

3000

2000:

1800

1600'

1400

E

'"1200,

-hi000

' 800

0

..j 600

4O0

ZAMBEZI

.;::.::. :.:: .:. .: .

o ....................

.

: :' :: ::::.': :':::;:::;:::' :

0 ..... eelee ....

O - ' ;40'0' ' -800' ' i200' ' 1600 ' 2000 ' 24'00

MEANMODALELEVATtON (m)

Figure5. Scatter lotof intrabasinariationsf local eliefandarea requencyeanmodal levationorthe

Yukon,Ganges, range, ndZambezi asins.

Chemical Denudation Rates

'rhe statisticalassociationsecorded n Table 5 support he

conclusion that chemical denudation rates, like those for

mechanical enudation, re more strongly nfluencedby relief

factors han by climatic controls Summerfield, 991a;Raytoo

and Ruddiman, 992]. In particular, t the scaleof this study,

temperature ppears o play no role in controllingates of

chemicaldenudation.This supportshe idea that the efficient

removal of weathered egolith and the resulting continuous

advectionof bedrock nto the weatheringzone is the critical

determinant f the rate of chemicalweathering. n fact, our data

suggest hat chemical denudation ates are more clearly

influencedby relief variables han (indirectly)by elevation.

Thick weatheringmantlesdevelopedn areasof minimal ocal

relief are likely to be inimical to high rates of chemical

denudation,rrespective f elevation. The correlationof

chemicaldenudation ates with runoff suggestshat maximum

rateswill occurwherehigh runoff s coupledwith high relief, a

conclusionupported y the rates observedn basinsdraining

A factor not considered n the correlation and regression

analysispresented ere is that of the lithologiccontrol of

chemical denudation rates. A detailed study of very small

(median area 7.8 km ) ,artpollutedatchments f uniform

lithology by Meybeck [1987] has demonstrated ignificant

variations n chemicaldenudationate as a functionof rock type.

It is probable hat the otherwise nomolouslyigh chemical

denduationate for the ChiangJiang Table 3) canbe explained

by the extensive utcrop f carbonateockswithin ts catchment

area. The potential ole of lithology aises he question f the

extent to which high rates of chemicaldenudationn orogenic

belts are a function of the exposureof readily weathered

sedimentarytrata ather han relief itself [Bernerand Berner,

1987,pp.225-226;Raytoo ndRuddiman, 992].

Irrespective f the specific ontrols n chemical enudation

rates t is clear hat for basinswith high overalldenudationates

chemicaldenudation onstitutes nly a small proportionof the

total (Figure2 and Table 3). Both mechanical nd chemical

denudationatesare positively ssociated ith relief variables,


11/13


topography. This is dramatically llustrated by comparing he

Brahmaputraand Ob basins. Although chemical denudation

accounts or only 2.6% of the total in the former basin, the

absoluteateof 18 mm kyr1 s the second ighestn the basins

consideredere. Nearly 65% of the denudationn the Ob basin,

however, is attributable to solute loss, but the absolute rate is

less hana quarterof thatof theBrahmaputra.

Interaction of Relief and Runoff

The individual ole of two, at leastpartly, ndependentactors

(relief and runoff) in influencing denudation ates raises the

question s o how stronglyheir combined ffectcontrols enu-

dation ates. Taking relief ratio to characterizeasin opography,

the linear partial correlationcoefficient or denudation ate and

relief ratiokeeping unoffconstants 0.732. This is only slightly

lower than the correlation coefficient for relief ratio and denuda-

tion rate (r = 0.757) indicating hat the latter relationships not

significantlynflated by the impact of interbasindifferences n

runoff. The joint effect of relief and runoff can be clearly seen

through he linear multiple correlation oefficient or denudation

rate against he relief ratio and ranoff (r = 0.792). Thus these

two variables alone account for over 62% of the variance in

denudationate. This is a remarkably igh amountof explained

variance given the low data quality and the fact that only

erosivityvariablesare ncluded.

Implications

The datapresented bove ndicate he degreeof variabilityof

denudation ates for very large drainagebasins and the major

factors hatappear o controlsuchvariations. A critical question,

however, s to what extent these ates, and the interpretationof

the factors that control them, can be extrapolated to the

geological imescales elevant o the interactionof denudation

and tectonics. A common view is that modem denudation rates

have limited applicability to these long timescalesbecause

sediment ields since he beginning f farming are probably ar

in excessof preagricultureates (see Milliman and Syvitski

[1992] for discussion). Although this is undeniably rue in

specific nstances,t is less certain hat such a generalisations

valid at the global scale. Our reason or suggestinghis is that

presentdenudationates for major basinsare in some cases

comparableo long-term ates estimated rom sedimentvolume

and thermochronologic ata. For instance, preliminary

calculationsor the Orangebasin rom offshore ediment olumes

indicate rates of denudation since the formation of the South

Atlantic ranging rom 7 to 114 mm kyr1 (depending n

assumptions ade aboutchangesn drainagearea through ime)

[Rustand Summerfield, 990]. These ates, which have varied

both temporally nd spatially,are compatiblewith estimates f

post-riftingdepthsof denudation f up to 3-4 km from apatite

fission track analysis Brown et al., 1990], but they are also

comparableo the modern stimate f 28 mm kyr for the

Orangebasin. The greatextentof the basinsconsideredn this

studyprecludes irectcomparisons ith thermochronologicata

which provide denudation estimates for limited areas.

Nevertheless, vailabledata certainly ndicate denudation ates

which are at least of the same order of magnitude as modern

rates. For instance, hermochronologicata from the Alps and

Himalayasndicate enudationates anging p to 1000mm yr1

and more over the past few million years Brownet al., 1994;

Clark andJiiger, 1969;Hurford, 1991].

A more specificcomparison an be made for the Gangesand

Brahrnaputra asinssince the accumulation f sediment n the

Himalayan molasse oredeep and alluvial plains, the subaerial

part of the GangesDelta and the Bengal Fan can be used to

estimate atchmentmechanical enudationatesover the past20

m.y. Assumingno change in catchmentboundaries, his

indicates a mean combined denudation rate for these two basins

of 435 mm kyr 1 from20 to 7.7 Myr ago,and300 mm kyr 1 from

7.7 Myr ago to the present [Einsele, 1992]. These figures

comparewith the presentcombined ate estimatedhere for the

BrahrnaputrandGanges f 420 mm kyr 1. The dataof Milliman

and Syvitski 1992] yield a similarly comparable igure of 242

mmkyr 1

If the grossvariations demonstrated ere at the regional to

subcontinental calebetweendenudation ates n drainagebasins

of contrasting elief and runoff characteristics re acceptedas

broadly valid over geological timescales, then present-day

denudation ates and the factors hat appear o control hem can

be used as a basis for modeling nteractions etween tectonics

and denudation. The careful applicationof such data should

greafiy mprove he calibrationof tectonicmodels ncorporating

the effectsof subaerialprocesses, lthough he scaledependence

of the factorscontrollingdenudationmustbe considered.

Two main avenuesof future researchare indicated by the

conclusionsrom this preliminarysurveyof the world'smajor

drainagebasins. One is the need for an application f the kinds

of morphometric ata presented ere. In particular, t would be

interesting o see to what extent variations n denudation ates n

smallerbasinscouldbe "predicted"rom a combination f basin

morphometry ndrtmoffdata. The otherpotentially ruitful line

of enquiry would be a more detailed comparisonof modem

denudation ates with estimatesof long-term rates culled from

thermochronologicndsediment olumedata.

Acknowledgments. Texaco nc. providedprimary funding or this

research. Computingwas carried out in the GIS Laboratoryof the

Department f Geography, niversity f Edinburgh, nd we are very

grateful o S. Dowers or hisassistancen datamanagement.

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daniel doido

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