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Continental Shelf Research 22 (2002) 1311–1323
Clay dispersal and the geochemistry of manganesein the Northern Gulf of California
L.W. Daessl!e*, S.E. Ramos, J.D. Carriquiry, V.F. Camacho-Ibar
Instituto de Investigaciones Oceanol !ogicas, Universidad Aut !onoma de Baja California, Apdo. 453, Ensenada, C.P. 22800 Baja California,
Mexico
Received 30 April 2001; received in revised form 11 December 2001; accepted 18 January 2002
Abstract
The regional distribution of Fe and Mn in bulk sediments, and in different geochemical fractions, has been
determined in surficial sediments from the Northern Gulf of California. The regional distribution of Fe (0.35–2.83%)
and Mn (97–922 mg g�1) in the sediments is mainly controlled by the distribution of fine-grained particles. Unlike silt (4–
62.5mm), clay (o4mm) in the NGC is mainly derived from the eroding Colorado River Delta and transported
southward adjacent to the Baja California coast, making up to 50% of the sediments. Normalisation of Fe and Mn
against the relative abundance of clay indicates that the main source of these metals toward the Central Gulf of
California is the re-suspension and re-deposition of the estuarine sediments derived from the Colorado River Delta.
Iron and Mn correlate statistically; their concentrations are similar to other coastal sediments in Baja California and in
seston from the Colorado River Delta. While the partition geochemistry of Fe suggests a dominant association of this
element with the lithogenic phases throughout the NGC, Mn is significantly partitioned in the exchangeable (average
15%) and carbonate (average 32%) sediment phases, especially in clayey sediments derived from the estuarine region.
The influence of Colorado River Delta-derived exchangeable Mn extends at least as far as the central Tibur !on Basin,
being probably responsible for the anomalously high Mn concentrations found in near-bottom waters in the Central
Gulf of California. r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Gulf of California; Colorado River; Sediment; Geochemistry; Manganese; Iron
1. Introduction
Almost one century has passed since thedamming of the Colorado River started, leadingto a progressive decrease of water and sedimentflux to the Gulf of California (van Andel, 1964;Baba et al., 1991a, b). At present, practically no
fresh water is reaching the delta from the ColoradoRiver stream, causing the development of a formerestuarine environment, to an ‘‘anti-estuarine’’,hypersaline, erosive setting (Vandivere and Vor-ster, 1984; Lav!ın et al., 1998). The sediment supplyto the estuarine basin is o0.5% of the originalsediment discharge calculated at 160� 106 metrictons (Milliman and Meade, 1983). Recentlypublished research on the impact of the changinghydrographic conditions in the area has focusedon aspects such as mineralogical facies and
*Corresponding author. PMB 133, P.O. Box 189003,
Coronado, CA 92178-9003, USA.
E-mail address: [email protected] (L.W. Daessl!e).
0278-4343/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 8 - 4 3 4 3 ( 0 2 ) 0 0 0 0 7 - 9
sediment transport models (Carriquiry and S!an-chez, 1999; Carriquiry et al., 2001). According tothese authors, sediment transport in the NorthernGulf of California (NGC) has changed from NW–SE trans-basinal to NE–SW cross-basinal. Theysuggest that the human induced changes areleading to the erosion of the estuarine basin andthe adjacent shallow platform, consequently lead-ing to the dispersal and re-deposition of sediments;mainly adjacent to the Baja Californian coast and
toward a depocenter located in the Delfin Basin, inthe southern NGC (Fig. 1). Earlier researchcarried out by Garc!ıa-Ballesteros and Larroque(1974), used water turbidity as a tracer for the N–Ssediment transport in the Upper Gulf of Califor-nia. Despite the current lack of a formerlyimportant sediment source to the area (as wasthe Colorado River), it appears that sedimentationrates in the deeper basins, south of the NGC, havenot decreased, but instead seem to be similar to
Fig. 1. General geomorphology of the NGC and sample location.
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–13231312
that existing prior to river damming (Baba et al.,1991b). Thus, two alternate sediment sources tothe area have been postulated: (1) reworking of thedeltaic structure and the shallow platform prox-imal to it (Baba et al., 1991a, b), and (2) therelative increase of aerosol input from the NWMexican deserts (Baumgartner et al., 1991; Carri-quiry et al., 2001).
The inorganic geochemistry of sediments de-rived from the estuarine basin and the shallowplatforms of the NGC is practically unstudied. Ageochemical study carried out by Baba et al.(1991a) in the Gulf of California (o6 samplesfrom the SW–NGC) briefly describes the fine-grained, metal-enriched nature of these sedimentsderived from the Colorado River and its estuarinebasin. Estuaries are known to function as sinks ofdissolved fluvial ions in the mixing zone, as well asthe catchment of particulate metal species, includ-ing Fe and Mn (Hunter, 1983; Chester, 1990;Haese, 2000). However, estuarine conditions in theColorado River delta no longer prevail, and thequestion arises as for the dispersal of formerlytrapped metal species in this erosive setting.Castro-Castro (1999) determined the metal con-centrations of suspended particulate matter flow-ing with the tidal currents within the modern deltachannel, and calculated a net ocean-ward flux ofparticulate Al, Fe, Mn, Cu and Zn. In the marineenvironment, the geochemistry of Fe and Mn is ofgreat interest due in part to their adsorptioncapability of other chemical elements such as P,Ni, Co, Cu, Zn, As, Cr, V, W (Glasby, 2000;Haese, 2000). Thus, the present research aims todescribe the regional extent and the Fe–Mngeochemistry of fine-grained sediments derivedfrom the Colorado River Delta. The rationale isto use the distribution pattern and geochemistry offine-grained sediment particles (characterised bytheir low settling velocities), as a regional tracerfor sediment and metal dispersion throughout theNGC.
2. Sampling and analytical techniques
Sediment samples were collected with a grabduring the ‘‘Gago’’ cruises on board the R.V. El
Puma and R.V. Francisco Ulloa on August 1995.Samples retrieved from the upper 5 cm of the grabsampler were frozen until analysis (Fig. 1). Thesampling locations are distributed evenly through-out the NGC, comprising sediments adjacent tothe Colorado River Delta to the North, and toAngel de la Guarda and Tibur !on Islands (MidriffIslands) to the South. The main deep basins in theNGC, Wagner, Delf!ın and Tibur !on, were alsosampled.
For bulk geochemical analyses, 0.500 g of driedsediment were finely ground with an agate mortarand pestle and dissolved using a combination ofHNO3, HF and HClO4 (Thompson and Walsh,1989). Eleven of the samples were analysed fortheir partition geochemistry in the followingphases: (1-Ex/Carb/Ox) loosely adsorbed (ex-changeable) ions, carbonates and amorphous Fe–Mn oxihydroxides, leached with 0.25 M hydro-xylamine HCl in 25% acetic acid, at roomtemperature to avoid dissolving the clay minerals(cf. Chester and Hughes, 1967; Chester et al.,1988); (2-Org/Sulph) organic matter and sulphideminerals, leached with a combination of dilutedHNO3 and H2O2 at 851C (Tessier et al., 1979); and(3-Residue) insoluble detritus, determined bysubtracting fractions 1–2 from the bulk concentra-tions. In addition, samples identified as delta-derived having a fine-grained sediment compo-nent, were analysed for their loosely adsorbed‘‘exchangeable’’ phase, after leaching with 1 Mammonium acetate at pH 7 (Kerstner and F-.orstner, 1986) and their carbonate phase, leachingwith 25% acetic acid. Analyses of the leachateswere carried out using a Thermo Jarrell Ash IrisICP-AES. The random analysis of duplicatesamples and certified reference sediments MESS-1 (estuarine; NRCC) and MAG-1 (marine;USGS), suggest that precision and analytical biasfor Fe and Mn are o75%.
High resolution grain-size distribution analyseswere carried out based on the cross sectional areaof the particles by means of a Horiba LA-910 laserparticle analyser (detection range 0.02–1000 mm),using the refractive index of Quartz in water, andsodium hexametaphosphate as a dispersant. Theultrasonic bath in the equipment was used for 30 sto ensure particle segregation. The analytical
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–1323 1313
efficiency of the equipment was checked by meansof reference material for sieve analyses (NIST 8010D) and 0.9 mm polystyrene spheres (NIST 1690).Replicate analyses and comparison with conven-tional grain size analyses of the same samples bysieves and settling tubes, indicates consistentresults between different runs of the same method,and between the two different methods, especiallyin the silt-clay size range. Laser particle sizedistribution analysers applied to environmentalanalyses have been used successfully before andcompared to other existing methods (cf. Singeret al., 1988).
3. Results
3.1. The dispersion of fine-grained sediments from
the Colorado River Delta
The use of a particle size analyser has allowed usto study in fine detail the smallest grain sizes insediments from the NGC. Silt and clay sizedsediments (o62.5 mm) are abundant in the NGC,contributing up to 100% (average 66%) of thesediments (Table 1). However, the spatial distribu-tion in the NGC, is not equal for the different sizeclass components of which mud is composed (siltand clay; Fig. 2A and B). Silt (4o62.5 mm) areabundant adjacent to the Sonoran Coast as well asBaja California, forming up to 81% of thesediment. However, it does not appear to bederived at present from the delta region, where it isless abundant. Clay-sized sediment (here simplyreferred to as clay; o4 mm) constitutes up to 50%(but average 20%) of the sediments in the NGC.They are most abundant south of the delta andadjacent to the Baja California Coast, off Puerte-citos (Fig. 2B). Due to their small size, clay-sizedparticles have very low settling velocities and maybe transported far distances from their sourcebefore reaching their final deposition site. Theregional distribution of clay suggests a NW–SEgradient, also influencing the deeper sediments inthe southern NGC. Unlike silt, no significantenrichment of clay is seen in the proximities of theSonoran Coast.
The influence of delta-derived clay along thewestern coast of the NGC, and toward the CentralGulf of California (CGC), is best seen from thedistribution of clay relative to the abundance(given in %) of the o62.5 mm size fraction (Fig. 3).This normalisation is intended to correct for anydilution of the fine-grained component by coarser(sandy) admixed particles. Highest abundance ofclay relative to total mud content is found near thedelta region. From the regional distribution ofclay, a distinctive patterns extending NW–SE fromPuertecitos, pass the Delfin Basin is seen. Asuspended sediment plume that follows a similartrend between these two regions SE of Puertecitosis seen from airborne pictures taken by the authorin early October 2000 (Fig. 3). From here, clay richsediments (>15% clay in the o62.5 mm sizefraction) extend as far as the southern Angel dela Guarda Island, along the 400 m deep Tibur !onBasin. Similar to silt, clay is also abundant in theWagner Basin region, where its relative abundanceis >20%.
Relatively coarse sediments (o10% clay) aredominant along the Sonora Coast, mainly off R!ıoConcepci !on, where they extent offshore towardthe central NGC (Figs. 2A, B and 3). Shallowsediments (o100 m) adjacent to the MidriffIslands to the south are also composed of coarse,mainly clastic and biogenic particles. Bimodality iswell defined in samples from the southern NGC,adjacent to the Delfin-Tibur !on Basin sill (i.e., atstations HP=03, GP-04 and GP-05). The grainsize distribution modes of these samples lie in the8.2–9.4 mm interval (fine silt), and 142–244 mm (finesand). Coarse modes (108–322 mm) are found inuni-modal sediments nearby along the Tibur !onBasin South and East of HP-03 (HP04, JP-02, JP-03, KP-01 and LP-01).
3.2. Mn and Fe geochemistry in the NGC
3.2.1. Bulk metal concentrations
Results for Fe and Mn concentrations are givenin Table 1. Both elements correlate significantly(a ¼ 0:01; inset in Fig. 4A). The bulk concentra-tions of Fe range from 0.35% to 2.83% and of Mnfrom 97 to 922 mg g�1. Both, Fe and Mn aredominantly enriched SW of the delta region and
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–13231314
Table 1
Sample location, relative abundance of silt and clay, and metal concentrations in sediments from the NGC
Sample Depth (m) Long (1W) Lat (1N) Silt (%) Clay (%) Fe (%) Mn (mg g�1)
a-01 12 31136.80 114125.21 45 17 1.22 228
a-03 13 31134.98 114130.03 51 44 1.23 235
a-06 9 31131.02 114137.14 59 30 2.30 400
a-11 4 31124.72 114148.27 54 29 2.44 409
b-02 14 31133.05 114122.01 25 11 1.48 299
b-05 16 31129.96 114129.93 51 30 2.51 444
b-09 15 31124.05 114138.97 47 35 1.89 314
b-12 9 31120.99 114146.90 46 37 1.44 256
c-01 8 31131.60 114114.74 3 1 0.56 132
c-05 23 31127.01 114123.97 53 22 2.13 368
c-09 18 31121.94 114134.03 54 38 2.54 427
c-14 9 31117.94 114147.47 37 31 2.08 299
d-04 28 31121.01 114127.02 56 25 2.10 350
d-06 20 31116.22 114137.46 46 42 2.25 358
d-08 8 31111.67 114146.58 44 29 2.08 340
e-04 36 31124.93 114109.99 45 12 1.13 200
e-08 18 31115.12 114130.01 81 13 1.00 202
e-10 24 31109.84 114139.77 47 37 2.38 378
e-13 10 31104.86 114149.67 22 4 2.69 358
f-04 21 31120.00 114100.20 4 1 0.50 97
f-06 62 31115.03 114109.93 40 10 1.10 218
f-08 44 31109.99 114119.89 41 13 1.34 258
f-12 21 31100.50 114140.00 49 27 2.38 401
bda-02 19 31117.59 113145.08 19 5 0.80 98
bda-04 10 31124.97 113159.16 1 0 0.35 98
BP-02 48 31110.02 113159.95 65 11 2.02 333
BP-04 49 31100.00 114119.97 29 11 1.44 296
BP-05 25 30154.94 114129.94 56 37 1.95 301
CW-03 212 30159.97 114110.38 72 28 2.34 493
CP-01 35 31104.92 113129.96 56 11 1.04 196
CP-02 65 30159.83 113140.00 76 21 2.42 279
CP-04 88 30150.07 114100.01 52 12 1.52 239
CP-05 195 30145.12 114110.09 61 34 2.63 417
CP-07 39 30135.09 114130.07 50 50 2.71 441
DP-01 52 30149.89 113119.84 59 10 1.46 227
DP-04 85 30135.27 113149.97 28 6 1.24 263
DP-06 143 30125.09 114109.99 68 31 2.36 382
DP-08 40 30115.01 114129.90 69 26 2.83 373
EP-01 35 30135.02 113109.92 24 3 1.37 204
EP-02 140 30130.00 113120.00 56 10 1.91 270
EP-04 129 30120.12 113140.20 40 7 1.13 179
EP-06 330 30109.85 114100.00 55 26 2.27 610
EP-08 109 30100.00 114120.02 65 35 2.56 278
FP-01 52 30120.12 112159.87 33 5 1.05 140
FP-02 115 30115.11 113110.03 81 19 1.76 230
FP-04 145 30105.01 113129.98 44 11 1.47 269
FP-06 460 29154.58 113149.77 60 39 2.49 560
FP-08 180 29145.04 114110.05 58 25 2.67 346
CD-06 460 29148.13 113142.92 68 13 2.38 464
GP-01 75 30105.03 112149.96 69 9 1.81 263
GP-03 145 29155.08 113109.87 62 14 2.04 264
GP-04 190 29150.06 113119.51 45 17 2.18 243
GP-05 230 29144.99 113129.89 34 13 2.35 213
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–1323 1315
along the Baja California Coast (Figs. 4A and B).While Fe concentrations are highest immediatelyadjacent to the Baja Californian Coast, Mn ispreferentially enriched in the deeper Delfin Basin.The bulk concentrations of Fe and Mn in surficialNGC sediments (except for Mn in HP-04) aresimilar to other near-shore sediments from theBaja California Gulf and Pacific regions andseston from the Colorado River Delta (Baba
et al., 1991a; Castro-Castro, 1999; Daessl!e et al.,2000).
In order to better understand the significance ofdelta derived clay-sized particles, relative to othersediment sources in the dispersion of metals in theNGC, Fe and Mn are normalised against theabundance of clay (Fig. 5A and B). Except for anoutlier for Mn, both elements correlate signifi-cantly with the abundance of clay in the NGC
Table 1 (continued)
Sample Depth (m) Long (1W) Lat (1N) Silt (%) Clay (%) Fe (%) Mn (mg g�1)
HP-00 47 29135.00 112129.94 48 4 1.54 198
HP-01 82 29132.60 112141.88 34 5 0.78 146
HP-02 260 29130.80 112156.02 73 12 2.07 316
HP-03 285 29130.83 113107.69 45 11 2.14 232
HP-04 154 29130.43 113120.22 16 5 1.65 922
JP-02 220 29109.89 112140.01 44 6 1.70 267
JP-03 450 29104.88 112150.05 70 11 2.21 258
LP-01 390 28155.13 112149.92 38 7 1.98 216
KP-01 510 28150.02 112139.99 6 0 1.86 198
Fig. 2. (A) Regional distribution (%) of silt-sized sediments (4–62.5mm) in the NGC and (B) regional distribution (%) of clay-sized
sediments (o4 mm) in the NGC.
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–13231316
(insets in Figs. 5A and B). For the purpose of thiswork, the lower Fe/o4 mm and Mn/o4 mm ratiosin Figs. 5A and B, are interpreted as the areaswhere Fe and Mn are dominantly associated withthe clay sediment fraction (western NGC); whereasthe relatively higher ratios suggest an increasingassociation of these metals with particles >4 mm
(eastern NGC). Both, Fe and Mn show adistinctive distribution pattern, which suggestssimilar metal deposition processes that extend asfar south as the Midriff Islands. Minimumnormalised metal concentrations (i.e., strongermetal/clay association) occur along the fine-grained deltaic sediment path adjacent to Baja
Fig. 3. Distribution of fine-grained sediments in the NGC as a function of the % of clay-sized material (o4 mm) contained in the mud
fraction (o62mm) fraction and the sediment plume SE of Puertecitos as seen from Airborne pictures. Arrows indicate the gyre (circle;
Lav!ın et al., 1997) and the generalised geostrophic circulation (ellipsis; Carrillo et al., in press). Inset: Bivariate correlation of clay and
silt in sediments from the NGC.
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–1323 1317
California, and also extends to the East along twoprominent lobes toward the coast of Sonora,indicating the influence of Fe and Mn dispersedeastward as part of clay dispersal. In addition, Feand Mn associated to the fine-grained sediment,appear to disperse toward the south along Tibur !onBasin, where normalised values tend to increasegradually, thus indicating a decrease in theassociation between metal contents and clay there.Iron and Mn concentrations in sediments from theshallow near-shore sediments along Sonora, aswell as in one sample adjacent to Angel de laGuarda Island (HP-04), are mainly associated withsediments containing o10% clay.
3.2.2. Partition geochemistry
In order to understand the geochemical associa-tion of Fe and Mn in the NGC, partition analyseswere carried out on two transects across the NGCand three samples proximal to the estuarine basin(Table 2; Fig. 5A). The geochemical partitioningof Fe reveals a strong lithogenic association,regardless of the relative distance to coastal or
delta derived sediment sources. On average, 84%Fe is associated with the residual phase for allsamples. Unlike Fe, the partitioning of Mn isvariable across the NGC, signalling a moresignificant influence of different Mn-carryingsediment components. Manganese is mainly parti-tioned (13–76%) in the Ex/Carb/Ox sedimentphase. The proportion of Ex/Carb/Ox bound-Mnin the NGC is highest along the fine-grained,metal-rich path south of the delta and toward theCGC. In contrast, it is lowest (o30%) insediments along the near-shore areas, especiallythe Sonora Coast. Due to the abundance oforganic matter in Delfin Basin and more reducingconditions, Org/Sulph bound Mn proportions(25%) are highest there, with Fe also beingenriched in this phase (12%).
Considering the possibility that labile Mn issignificantly being exported from the erodingdelta, a few samples were analysed to investigatetheir exchangeable Mn (1 M ammonium acetatesoluble), and their carbonate (25% acetic acidsoluble) partitioning (Table 3; Fig. 5B). For 16
Fig. 4. (A) Regional distribution of Fe (%) and (B) regional distribution of Mn (mg g�1) in the surficial sediments of the NGC. Inset:
Bivariate correlation of Fe and Mn in surficial sediments from the NGC.
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–13231318
samples along the clay dispersion path in theNGC, exchangeable Mn averages 15% of the bulkMn concentrations in these samples, with highestproportions (21–32%) found proximal to the
delta, the Wagner Basin, the central NGC, andthe southernmost sample (LP-01) recovered SEfrom Angel de la Guarda Island. In average,carbonate-bound Mn represents 32% of the bulk
Fig. 5. (A) Regional distribution of Fe normalised against the content of particles o4 mm (in %). The histograms show the percentage
of Fe (black) and Mn (white) associated with the residual sediment phase. (B) Regional distribution of Mn normalised against the % of
particles o4 mm. The pie diagrams show the partitioning of Mn in: exchangeable phase (white), carbonate phase (grey), and the other
organic-sulphide-residual phases (black). Darker areas represent lower normalised Fe and Mn values and their stronger association
Table 2
Bulk concentrations and general partitioning of Mn and Fe in sediments collected along three transects in the NGC
Sample Mn (mg g�1) Resid (%) Org/Sulph (%) Ex/Carb/Ox (%) Fe (mg g�1) Resid (%) Org/Sulph (%) Ex/Carb/Ox (%)
A-06 400 18.6 5.0 76.4 23000 85.0 6.5 8.5
E-04 200 44.8 7.9 47.3 11290 80.2 12.2 7.6
E-10 378 21.2 6.1 72.6 23840 81.8 6.6 11.6
CP-01 196 65.7 4.1 30.2 10380 80.8 8.3 11.0
CW-03 493 20.2 5.6 74.2 23390 84.1 8.3 7.6
CP-05 417 40.7 16.4 42.8 26260 92.1 6.2 1.7
CP-07 441 21.1 7.6 71.4 27110 80.9 8.7 10.4
FP-01 140 79.9 7.2 12.9 10460 78.7 16.3 5.0
FP-04 269 33.6 8.8 57.6 14660 91.1 6.8 2.1
CD-06 464 31.2 24.6 44.3 23770 85.8 12.4 1.8
FP-08 346 60.5 10.9 28.6 26700 84.6 12.2 3.2
Std. dev. 121 20.9 6.0 21.5 6870 4.3 3.6 3.7
Resid=residual, Org/Sulph=organic and sulphide, Ex/Carb/Ox=Exchangeable, carbonate and Fe–Mn oxihidroxide phases.
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–1323 1319
Mn concentrations and >40% in samples directlyalong the fine-grained sediment path, and thecoarse ‘‘anomalous’’ sample (HP-04) adjacent toAngel de la Guarda Island. In this sample (whichcontains abundant shell fragments) Mn is dom-inantly associated with the carbonate fraction,with only a low proportion in the exchangeablefraction.
4. Discussion
4.1. The dispersion of metal-rich clayey sediments
From the regional distribution of clay, the bulkFe and Mn concentrations, and the partitioning ofthese metals, it can be seen that the dispersion ofmetal-rich clay in the western NGC originatedfrom the eroding delta and the shallow platformsouth of it. The normalisation of Fe and Mnagainst the relative abundance of clayey sediments(% o4 mm), indicates that the distribution of thesemetals in the western NGC is dominantly afunction of the abundance of the clayey sedimentsthere; unlike the eastern NGC, where Fe and Mn
are mainly associated with the >4 mm sedimentfractions (Figs. 5A and B). Metal-rich clay, whichis transported along the Baja California Coast,also appears to be partially dispersed toward theWagner and Delf!ın Basins. However, unlike theoffshore W–E dispersion pass the Delfin Basin,clay proximal to the Wagner Basin may also bederived either from the Sonoran Coast (Carriquiryet al., 2001) and/or the eroding delta via thesouthward flowing (high salinity) gravity currentsderived from the estuarine basin (Lav!ın et al.,1998; !Alvarez, 1999). The high proportion ofexchangeable Mn in sample CW-03 supports thelatter as the dominant source of clay towardWagner Basin.
The cross-basinal dispersion of fine-grainedsediments in the southern NGC may be attributedto the development of a gyre in the central region(Fig. 3; Lav!ın et al., 1997; Mart!ınez-D!ıaz-de-Le !on,2001). The circulation of this gyre varies fromanticyclonic (November to March) to cyclonic(June to September), with transition periodsbetween them (Lav!ın et al., 1997 and referencestherein), and may be the dominant force influen-cing the sediment re-distribution in the NGC.Surface (0–70 m) geostrophic circulation patternsdescribed by Carrillo et al. (in press), suggest aninfluence of the gyre on sediments at least 70 mdeep, with velocities of up to 70 cm s�1 duringsummer (cyclonic) and 22 cm s�1 during winter(anticyclonic). Carriquiry et al. (2001) describedsediment transport vectors in the area, suggestingthat in response to the higher velocities of thecyclonic gyre, sediment transport in the northernNGC is mainly NE–SW cross-basinal. However, itis possible that the relatively weaker velocities ofthe winter gyre and the gravity currents may driveonly the smallest particles from the Baja Califor-nian platform and Colorado Delta toward WagnerBasin.
Further south, along the Tibur !on Basin, otherhydrographic conditions may dominate the sedi-ment transport and re-deposition. Baba et al.(1991a) described the sediments along the Tibur !onBasin as part of a sandy facies. Our results arebased on a denser sampling grid and indicate arelatively high abundance of sands there, althoughthe proportion of particles o62.5 mm is generally
Table 3
Manganese concentrations associated with the carbonate and
exchangeable phases in sediments from a transect along the
NGC
Sample Mn (mg g�1)
Carbonate-bound Exchangeable
A06 134 113
GP04 73 47
KP01 25 17
E10 162 44
JP02 75 27
HP04 489 78
GP03 73 24
CP07 143 51
HP03 51 21
EP04 63 57
LP01 40 45
CW03 211 136
JP03 61 24
CD06 142 51
EP08 100 24
EP06 248 90
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>45%. Adjacent to the Delf!ın–Tibur !on Basin sill,sediment samples (HP-03, GP-04 and GP-05) arebimodal, probably in response to a combination ofsediments sources there: foraminifera-rich sands,with superimposed delta-derived clay and SonoranCoast derived silt. The finer material transportedtoward the Tibur !on Basin from the North, isprobably re-suspended by a strong near-bottomcurrent (at ca. 317 m depth), which flows NW at anaverage speed of 30 cm s�1 through the sill betweenTibur !on and Delfin Basins (L !opez and Garc!ıa,1999). However, below the surface, and down to260 m, the currents are more variable flowing NE,turning SE with increasing depth. Although thecurrents above 260 m are weaker than the near-bottom current, it remains to be studied how thiscurrent regime may affect the dispersion of clayeysediments near Angel de la Guarda Island.Delgadillo-Hinojosa (2000) and Delgadillo-Hino-josa et al. (2001) studied the distribution of Mn,Cd and suspended material in the water-column ofthe central and Southern Gulf of California.Unlike the water profiles studied by these authorsalong the Southern Gulf of California, theabundance of suspended particles SE of Angel dela Guarda Island (West of our sample LP-01)increases slightly with depth (up to 1 mg l�1),suggesting a local near-bottom flux of particulatematter, probably through the Tibur !on Basin andits sills. Thus, at present the distribution of claythroughout the NGC appears to be a function ofdelta erosion and its re-deposition. Water depthplays a secondary role in the accumulation of fine-grained particles at present, as seen for the deeperDelfin and Tibur !on Basins, where the percentageof o4 mm particles is o20%.
4.2. Manganese: a geochemical tracer in the NGC
Based on the chemical composition of a fewsediment samples clustered north of Isla Angel dela Guarda (adjacent to our sample EP06), Babaet al. (1991a) suggested that the fine-grainedsediments in this area comprise a combination ofthree statistically defined sediment factors: terrige-nous, biogenic silica and Mn. However, their workonly focuses on the Central and Southern Gulf ofCalifornia, their main sampling area. A high metal
content in fine-grained sediments is usually asso-ciated with the adsorption of metal ions ontoparticles with a high surface area (Horowitz, 1991)or, in bulk analyses, mainly with the mineralcomposition of these fine-grained sediments. Un-like Fe in the NGC, our results show that Mn isconsistently associated with the non-residualphases in the clayey sediments that are dispersedfrom the delta region toward the CGC, domi-nantly with the carbonate and exchangeablesediment fractions. Sediments immediately adja-cent to the Baja California and Sonora coasts havea much lower proportion of carbonate/exchange-able associated Mn, which occurs mainly as part ofthe lithogenic fraction that is probably derivedfrom continental (aeolian/fluvial) sources.
That the eroding delta is the most likely sourceof Mn to the NGC is further supported bypartition results for seston in the main channel ofthe estuarine basin (Castro-Castro, 1999). Thepartition geochemistry of particles suspended bythe action of tidal currents there, shows a similarmetal partitioning to that described in the presentwork. Of the metals studied in deltaic seston, onlyMn is significantly associated with the exchange-able plus carbonate phases 44–48% (soluble in 1 Msodium acetate at pH 5). Furthermore, Delgadillo-Hinojosa (2000) described a strong near-bottomdissolved Mn anomaly (Mn>10 nM) in a waterprofile adjacent to Tibur !on Basin at ca. 300 mdepth (West of our sample LP-01). His study alsodescribes relatively high particulate Mn contents inthe deeper waters in the same region. If thesouthward dispersion of fine-grained sedimentsderived from the delta region and the shallowplatforms in the NW–NGC is held to be respon-sible for the high near-bottom Mn concentrationsreported for the CGC, o45 mg g�1 of Mn could bepotentially exchangeable between the sedimentsand the marine environment in this area (based onexchangeable Mn concentrations in sample LP-01).
Iron and Mn are susceptible to diageneticremobilisation and internal cycling due to chan-ging redox conditions within marine sediments(Froelich et al, 1979; Shaw et al., 1990). Asoxidation of organic matter proceeds within thesediment and dissolved oxygen becomes depleted,
L.W. Daessl!e et al. / Continental Shelf Research 22 (2002) 1311–1323 1321
nitrate, Mn and later Fe oxides become reducedand remobilised. Unlike Fe, which typically hasonly a small fraction (B10%) available forreduction (Shaw et al., 1990), a relatively largefraction of Mn can be remobilised by this process.The enrichment of Mn in the surficial sedimentsfrom the NGC due to recycling and precipitationwithin the oxic surface layer may represent analternate mechanism for the relatively higherconcentrations of Mn found in the westernNGC. However, the regional distribution andgeochemical partitioning of Mn as part of theclayey mud derived from the Colorado RiverDelta (including deltaic seston; Castro, 1999) andits statistical correlation with detrital Fe, suggeststhat this element within surficial sediments isdominantly related to source material rather thanto suboxic diagenesis. Nevertheless, it is likely thatin the deeper Delfin Basin, where Mn and organicmatter concentrations are relatively high, thesurficial enrichments in Mn have a partial diage-netic source.
According to Carriquiry et al. (2001), the‘‘Colorado River Delta’’ mineralogical provinceextends to the southern NGC, where sedimentsattributed to the ‘‘Baja California-Sonora’’ pro-vince become increasingly dominant. Based ongeochemical and high-resolution particle sizeanalyses, it appears that the Baja California-Sonora province represents a relatively coarsesediment component with a superimposed clayeydelta-derived component. An additional aeoliansource for clay minerals in the southern NGC hasalso been postulated (Carriquiry et al., 2001). Thedeltaic sediment component is best identified bythe abundance of exchangeable and carbonate-bound Mn, which may be affecting the particulateand dissolved Mn fluxes toward the CGC. Thisgeochemical tracer may be useful to identify thetemporal variability in sediment transport priorand after river flux interruption.
5. Conclusions
In response to the hydrographic conditions inthe NGC and to their low settling velocities, claysized particles (o4 mm) derived from the Colorado
River Delta are actively being re-suspended,dispersed and re-deposited adjacent to the coastof Baja California. Gravity currents appear to bepartially responsible for the clay-sized sedimentenrichment in the Wagner Basin. S–SE of theDelfin Basin, clayey sediments are further dis-persed southward along the Tibur !on Basin, whereclay abundance relative to total mud content is>15%. The strong bottom currents along theTibur !on Basin and the sill connecting it with theDelfin Basin are probably responsible for furthersediment re-suspension and dispersion there. Inthe NGC Mn and Fe are dominantly enriched dueto their association with the clay-sized sedimentfraction south of the Colorado River Delta.Unlike Fe, Mn is mainly partitioned in the moresoluble, non-residual sediment phases, mainly as acarbonate phase (average 32%) and looselyadsorbed to particles (average 15%) as anexchangeable phase. The Colorado River Delta—derived exchangeable Mn, extends at least as far asthe Central Tibur !on Basin. Exchangeable andcarbonate-bound Mn in sediments from theNGC are postulated as geochemical tracers forthe regional dispersion of sediments derived fromthe Colorado River Delta.
Acknowledgements
We thank the captain, crew and staff on boardR/V El Puma and R/V Francisco Ulloa for theirassistance during the Gago cruises. Thanks to E.S!anchez, P. Castro and A. Siqueiros for their helpduring laboratory work, as well as the reviewersfor helping to improve this paper. Funding for thisproject was provided by UABC and CONACYT.The analytical equipment used for this researchwas obtained through support from the MexicanMinistry for Education through CONACYT andFOMES.
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