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The major-element chemistry of suspended matter in the Amazon Estuary
The major-element chemistry of suspended matter in the Amazon Estuary E. R. SHOLKOVITZ and N. B. PRICE Grant
Institute
of Geology.
West Mains
Road.
Edinburgh.
EHY 3JW. U.K
(Rec,cr~>rr/ 6 Fc#~r~rtrr!. 197’): rrc,c,rpretl ,,I rc,riscd fornr 20 Srprenlhrr
1079)
Abstract--Suspended matter from the surface waters of the Amazon Estuary were collected durmg Ma! and June 1976 on the ‘R ‘V Alpha Hehx’. and their major-element compositions (Al. Si. TI. K. Mg. Ca. P. Fe and Mn) were measured. material. predominantly terrigenous III derlvatlon. Between salinities of 0 and lo”,,,,. the suspended decreases In load from 500 to 3 mgI but has a chemical composltlon which remains essentially constant With the onset of a large amount of biological productivity at approximately IO”,,,, salinity. there are large increases in the ratios of SI.!AI. P’AI. CasAl. Mg;AI. Ti:AI and Mn.AI which are mamtamed at higher salinities. Calculations of “excess” concentrations of elements held In the non-terrigenous components of the suspended material further support our main conclusion that Si. P. Ca. Mg. TI and Mn are Incorporated into the skeletal and organic phases of marine phytoplankton (predominately dIatomsI of the Amazon Estuary. The data suggest. but with less certainty. that Fe and K follow the above elements. This study has demonstrated that the chemical composition of river-introduced suspended matter can be significantly altered by biological activity within estuarme waters as can be the geochemical cycle of inorganic elements.
INTRODUCTION RIVERS through
their suspended loads supply the flux of solid material to the oceans (GARRELS and MACKENZIE. 1971). However. the chemical composition of river-borne suspended material is poorly documented as are the compositional changes that ma> occur as it enters the estuarine environment. greatest
(TUREKIAN. 1971;
GIBBS. 1977:
BECK. 1979; TREFR’I. and
MARTIN and
MAY-
PRESLEY, 1976). Biological,
chemical and physical processes operating in estuaries may result in the suspended material having compositions significantly different from those of the original river-borne suspended material. Hence. a complex relationship may exist between the composition of river-introduced suspended material and the material which leaves estuaries to become oceanic suspended material and sediments. In this paper we will report on the major element compositlon (5. Al. Ti. Fe. Mn. K. Ca, Mg and P) of suspended matter in the Amazon Estuary and shoM how this data can be used to elucidate the biological. chemical and physical processes occurring in the estuarine environment. In particular. the results will demonstrate that man> of these elements are incorporated into marine phytoplankton and therefore. are. involved in biogeochemical cycles. METHODS
OF
COLLECTION
AND
ANALYSES
Samples acre collected m Ma! June. 1976. during the ‘R \’ Alpha Helix’ expedition 10 the Amazon Estuar! and River. While the ship was underwa!. surface water (top 1m) \xaspumped through a plastic hose held out from the side of the ship dIrectI> In pol>eth!lene bottles. UsualI! althin three hours of collectlon. each bottle was shaken
thoroughly and an aliquot of water quickly transferred to a plastic filtration device and a measured volume filtered under pressure through a pre-weighed 0.4 pm Nuclepore filter (37 mm). Filtered distilled water (25-50 ml) was added to the filtration device and passed through the sample and filter to wash out the sea salt. In total about 140 samples of suspended material were collected from five separate transects of the Amazon Estuary (Fig. 1). On return to Edinburgh, these samples were desiccated and then weighed to determine the total suspended load. Chemical analyses were carried out b) X-ray fluorescence spectrometry (XRF). Details of the method are given in PRICE and CALVERT (1973)and SHOLR~VITZ (I 979). Briefly. standards and samples (both on Nuclepore filters) were mounted on special holders and placed in a Philips PWl450 X-ray spectrometer. Peak and background counts were measured for each major element. Chlorine concentrations were also measured and used tc correct the Ca. Mg and K concentrations for any sea-salt Ca. Mg and K not washed off the filters. Working standards were prepared by filtering suspensions of weighed amounts of finely ground USGS rock standards. To obtain a set of standards independent of the above XRF technique. a selected number of rock standards and Amazon samples were digested In HF and analyses bq spectrophotometric and atomic absorption methods. These two procedures yielded calibrations of element content and X-ray instrumental response. The analytical precision of the XRF analyses are referred to in the appropriate figures of this paper. Smce only samples with total weights of less than 3.0 mg are suitable for our XRF technique. the major element composition was determined for 75 out of the total of 140 samples collected RESULTS
AND
DISCUSSIONS
(a) Torcrl susper~dvd load The total suspended load (TSL) of the surface waters decreased dramaticall> from over 500 to 3 mg 1 163
164
E.R.
SHOLKOVITZ
I
P--
Fig.
1. The Amazon
PRKE ,
I
I
tea3
legs ? IX
Estuary and the lower reaches of the Amazon River, The depth The ‘legs’ covered by the ‘R/V Alpha Helix’ are shown.
the salinity increased from 0 to loo& (Fig. 2). At salinities greater than IO‘& the TSL varies between 1 and 3 mg/l. This rapid fall-off of TSL has been previously observed in the Amazon Estuary by MILLIMAN and BOYLE(1975). MILLIMANer al. (1975) and GIBBS (1976). The high suspended loads in the upper estuary result from the resuspension of bottom sediment as the surface water of the Amazon River itself only carried a suspended load of SO-70 mg/l in its lower reaches during our June 1976 study. The rapid fall-off in TSL implies deposition of particles.
as
(b) Visual observations-three
zones of’ the Amazon
Estuary
Visual observations
and N. B.
provide a valuable clue to the
4
contours
are in m.
variation in the type of suspended matter in the Amazon Estuary. During our cruise in May 1976 water with salinities between 0 and IO’&, were distinctly brown in colour and we have designated this water to lie within the ‘zone of terrigenous material’. At approximately loo,, salinity there was a sharp transition to green coloured water containing much diatomaceous debris. This material is most concentrated in waters within a IO-25’& salinity range; we designate this to be the ‘zone of biogenic material’. At salinities greater than approximately 25”&,,the water was distinctly blue; this ‘zone of blue water’ had a diffuse boundary with the ‘zone of biogenic material’. A division of the particulate matter in the Amazon Estuary into zones of terrigenous and biogenic
0
leg 1
2 x leg 3 8 leg 5 0 leg
4 leg
Fig. ?. Total
7
suspended load of the surface waters of the Amazon Estuary vs salimty. The Inset expands the scale of the total suspended loads for salinities greater than 4”,,,,.
The major-element
165
chemistry of suspended matter III the Amazon Estuary
Table 1. Amazon Estuary: element/Al ratios”’ 0
A
--
Zone of terrigenous material
Zones of biogenic
(S
nz 21)(2) -__
material
(S>lOO/,,;
-.-.--
Ratio
water
of
WA
n = 40) ._~
2.0 0.006 0.102 0.022 0.022 0.0048 0.42 0.190
Si/Al ?/Al Mg/Al Ca/Al Ti/Al MniAl fe/Ai K/Al
and blue
+ 5 7 5 5 5 5 -c -
0.1 0.002 0.006 0.002 0.003 0.0007 0.04 0.009
4.6 0.047 0.126 0.036 0.031 0.0079 0.46 0.200
+ : 5 5 T 5 5 7 -----
2.2 7.6 1.24 1.64 1.41 1.65 1.07 1.05 --
1.9 0.021 0.022 0.016 0.009 0.0017 0.05 0.015
(I 1Mean values and standard deviations. (2) Ratios used as values for terrigenous material in the calculation of ‘excess’ concentrations.
influence has been previously made (MILLIMANet al., 1975; MILLIMA~ and BOYLE, 1975) although the chemical character of the particles was not determined. These authors suggested that the onset of biological productivity occurred where there is a marked increase in light transparency as a result of rapidly decreasing TSL. IC) ElenwztiAl ratio qf the suspetlded material In this paper many of the chemical data on susAMAZON P/Al
pended matter are presented as elemental ratios to Al. Aluminium is a major and chemically unreactive constituent of clay minerals which, in themselves, are the dominate component of river-borne particles. By comparing element/Al ratios in the three zones of the Amazon Estuary it is possible to determine if estuarine processes cause certain elements to be enriched or depleted relative to the terrigenous suspended matter. Moreover, ratios to Al are useful for this purpose since they are independent of variations in the con-
ESTUARY . leg 1 0 leg 2 . leg 3 m leg 5 a leg 7 *
0.08 .’
.
.
tOr
.
Fig. 3. The P Al and Si Al ratios of surface suspended matter in the Amazon Estuary vs salinity. The precision of the ratlo measurement is P Al i 0.001 and Si’Al & 0.1.
E. R. SHOLKWITZ
166
centration of biogenic material (e.g. Si02. CaCO,. organic matter). This is particularly important when considering marine sediments and suspended matter which contain large amounts of biogenic material (SPENCER and SACHS, 1970: PRICE and CALVERT. 1973: SHOLKOVITZ, 1979). One outstanding ion of suspended
feature
of the chemical
matter
composit-
in the Amazon Estuary is that all the element/Al ratios have almost constant values between salinities 0 and lo”,,,, (‘zone of terrigenous material’) even though the total suspended load decreases from values of near 500 mg/l to 3 mgl (Figs Z-6). This lack of chemical variation provides evidence that no chemical, biological or physical processes in the low salinity region of the Amazon Estuary are capable of altering the major element composition of the surface suspended matter. The elementiA1 ratios change markedly at salinities greater than about lo’<, (Table 1 and Figs 3-6). The large increases in Si;Al and P/AI ratios (Fig. 3) coincide with the onset of biological productivity. The P,‘Al ratios show the greatest increase (factors of up to 16) as would be expected since P is clearly incorporated into living organisms and occurs only in small concentrations in clay minerals (P!AI = 0.006). The increases in SiiAl and P,:Al ratios also are
and N. B. PRICE
accompanied by smailer but significant Increases in the ratios of Co ‘Al. 418 AI. Mn Al and Ti A1 (Table I and Figs 4 and 5). Our data shokvs that these increases are caused by increased concentrations of ;I particular element and not from a decrease in Al concentration. It appears that not only are Si. P and C;i incorporated into marine ph~topiankton (e.g. diatoms) but so are ,Mg, Ti and Mn. In contrast both the K.Al and Fe Al ratios show only a slight increase between the two zones (Table I and Fig. 61. K Al increases from 0.190 + 0.009 (mean and standard deviation) to 0200 + 0.015 while Fe Al increases from 0.42 & 0.04 to 0.45 i: 0.05. Increases of this magnitude are greater than the analytical precision of the measurements (Fe!AI. iO.01: K Al. iO.004). suggesting that K and Fe also are incorporated into phytoplankton. However. as this conclusion rests on our interpretation of very small changes m the K Al and Fe:AI ratios. it should be viewed critically. The increase in element, Al ratios in waters above IO’,,, salinity are consistently observed in the various transects of the Amazon Estuary. There is some suggestion of a broad maximum in many of the ratios between salinities of IO and 25’:,,1,and this is most evident in P!AI and Si Al ratios from leg I samples (Fig. 3). This trend suggests that the formation of bio-
Co/AI 0.10 -
. 0.08 0.06 -
MS/AI
.
0.20
Fig. 4. The Ca:AI and Mg;AI ratios of surface suspended matter in the Amazon Estuary vs salinity. See the key in Ftg. 3 to refer to samples from ditferent legs. The precisron of the ratio measurement is Ca, Al k 0.003 and Mg;AI & 0.05.
The major-eJement chemistry of suspended matter m the Amazon Estuary
167
Mn/Al
T,/At
Fig. 5, The Mn’AI and Ti:AI ratios of surface suspended matter in the Amazon Estuary vs sahntty. See the key in Fig. 3 to refer to samples from different legs. The precision of the ratio measurement IS Mn:Al 5 0.001 and Ti/‘AI k 0.001.
genie matter is most intense in the intermediate salinity range (l@-25?,,). This conclusion is supported by the variation of ‘excess’ P (see below) with salinity (Fig. 7) and by the particulate organic carbon data of EDMUND er al. (in prep). tncreases in the element/Al ratios within the zone of biogenic material’ are mirrored by the dissolved nutrient data of EDMOND er at. (in prep). They show that extensive removal of phosphate and nitrate
occurs at approximately IO’& salinity and continues until 15%, where the concentration of nitrate is almost zero. Some comment should be made on the large scatter in many of the element/AI ratios (e.g. Si/AI. P/AI. Ca/AI) of samples collected in the ‘zones of biogenic material and blue waters’. We suspect that this is caused by inhomogenous mixtures of terrigenous and biogenic material as indicated by the visual patchiness
Fq 6. The Fe Al and K Al rattos of surface suspended matter m the Amazon Estuary vs salinity. See the key in Fig. 3 to refeF to samples from different legs. The precision of the ratio measurement IS Fe Al & 0.01 and K;Al i 0.004.
168
E. R.
SHOLKOVITZ
and N. B. PRICE
AMAZON
ESTUARY 0 leg 1 0 leg 2 x leg 3 l leg 5 A leg 7
excess P’crg/li 6-
‘0
s4-
l
%
32-
.
l0
0
.
A *
00
‘rn
0
.
.
.o
.
.
0
n
. O
I-
Fig. 7. The concentration
of ‘excess’ P vs salinity for surface suspended matter in the Amazon Estuary
in the ‘zone of biogenic material’. Another consideration is that the suspended material contains plankton in various states of growth and decomposition. As these processes are known to take-up and release Si, P.K.Mg and Ca (GRILLand RICHARDS, 1964;BISHOP rcul.. 1977,1979). variations in the relative proportion of living to dead plankton could add to the scatter of the results. (d) ‘E.ucess’ concentrations:
element
incorporation
(2-4 times) less than their concentrations in the terrigenous component. For Si. P, Mg, Ca. Mn and Ti their ‘excess’ concentrations are greater than or equal to their concentrations in the terrigenous component: this gives us confidence in using the ‘excess’ concentrations of these elements as interpretive tools. As P is the most diagnostic element indicating organic matter, ‘excess’si, Mg. Ti. Ca. K and Fe concentrations have been plotted against ‘excess’ P
into
marine phpoplankton
700 r
From the patterns of element/Al ratios it appears that P. Si, Ca, Mg, Ti, Mn (to a lesser certainty K and Fe) are associated with or incorporated into the phytoplankton of the Amazon Estuary. To support this claim, ‘excess’ concentrations of these elements were computed and their distributions with respect to salinity examined. ‘Excess’ concentrations may exist in association with organic matter, within skeletal matter or as authigenic substances such as metal oxides. For example. ‘excess’ Si is calculated as follows: ‘excess’ Si(pg/l)
0;
Ao
AMAZON
Ll
ESTUARY
= total Si(pg/l) _
Si !-IAl TSM
where TSM = terrigenous
x total Al(pg/l)
suspended
0
1
2
3
excessP
4
5
6
pg/l
material.
As seen from this calculation, the element/Al ratios of the terrigenous constituents must be established within the particulate matter. For the Amazon Estuary. the constant element/Al ratios in the ‘zone of terrigenous material’ (GlCr’,,,) give a good estimate of the element/Al ratios of the terrigenous component (see Figs 3-6 and Table I) and these ratios have been employed for computing the ‘excess’ element concentrations Caution must prevail when interpreting ‘excess’ concentrations as they are computed by subtracting two variables. For the majority of our samples only Fe and K have ‘excess’ concentrations significantly
Fig. 8. The concentration of ‘excess’ P vs the concentrattons of ‘excess’ Si and ‘excess’ Mg for surface suspended matter of the Amazon Estuary (S > IO”,,,,). The linear correlation coefficients are 0.6X and 0.44 respectively.
The ma,ior-element chemtstry of suspended matter I” the Amazon Estuaq 0.5 excess
a
excess
K i&Ii
Mn (p,9/1) 0.4
‘3
4.0 -
: .
0.3 -
.
0.2 Ol. x -ai
3.0 -
00. .
n
0
1
.
.
l
2.0 -
s:.
.
on. l em .O q O e 0 0’ 0 00 I I 1 2 3 4 excess P (&Is/l)
1.0 excess
.
.
;
.
. 1.0 -
I 5
.
I 6
0L-I 0
3
0
l
.
om.*,* 0 . . ’ “cl’ 1.0 2.0 3.0 4.0 excess P 1rJg/l,
. 1 6.0
I 5.0
3
n
Ti(crg/l) 0.8
l
excess 4.0 -
. .
..
%9/l)
n
.
.
2.0 -
o.+. ,Of‘1:;.., 2
3
4
excess
P(pg/l)
e
.
.
R
1
.
0
.
3.0 -
0.6
0
169
,
1
5
6
0 0
.
. .
l
iIELcL0
1
2 excess3P (pg/Tj
5
6
FIN. 9. The concentration of ‘excess’ P vs the concentratlons of ‘excess’ Ti and ‘excess‘ Mn for surface suspended matter of the Amazon Estuary (S > 10”,,,,). The linear correlation coefficients are 0.55 and 0.57 respectively.
Fig. 10. The concentration of ‘excess’ P vs the concentrations of ‘excess’ Fe and ‘excess’ K for surface suspended matter of the Amazon Estuary (S r lo”,,,,). The linear correlation coefficients are 0.40 and 0.47 respectively.
(Figs 8-10). These figures show that ‘excess’ Si. Mg, Ti. Mn, K and Fe concentrations are broadly proportional (linear correlation coefficients of 0.68, 0.44. 0.55, 0.57. 0.47 and 0.40 respectively) to that of ‘excess’ P. suggesting that these elements are incorporated into phytoplankton. In contrast. the linear correlation coefficient between ‘excess’ Ca and P is only 0.03; our calcium results indicate that calcareous plankton are present in minor amounts in these waters. a conclusion consistent with previous phytoplankton studies in the Amazon Estuary (TEIXEIRA and TUNDIS. 1967; HULRERT and CORWIN. 1969; MILLIMAN er al., 1975). ‘Excess’ Si IS obvious& in the skeletal material of the diatoms. The high ratios of ‘excess’ Mg!‘excess’ Ca (i-9 mol ratio) suggests that Mg is not held in calclum carbonate but in an organic phase. A similar conclusion has been reached by BISHOP er al. (1977, 19791. These authors also conclude from the distribution of Mg. Ca and K in particulate matter in the upper 400 m of water in the eastern equatorial Atlantic Ocean and off southwestern Africa that ‘excess’ Ca and K are associated with the organic phase of marme phytoplankton. Their data shows a positive correlation between the extent of biological productivit) and the concentration of ‘excess’ K. Mg and Ca. Distribution of ‘excess’ Ti. Mn and Fe do not allow determining whether these elements are preferentially associated with skeletal materials or held m the orga-
nit matter of the phytoplankton since ‘excess’ Ti. Fe and Mn all show a directly proportional relationship with both ‘excess’ P and ‘excess’ Si. The data of MARTIN and KRAUER (1973) suggest that Ti is preferentially associated with siliceous frustules, that Mn is associated with the organic matter. and that Fe is associated with both organic matter and siliceous frustules with the former phase being more important. An important point is that the concentrations of ‘excess’ elements in the zones of biogenic material and blue water (Table 2) are much higher than total or excess concentrations of corresponding elements (Fe. Si, P, Mg. K and Mn) reported for oceanic particles (COPIN-MONTEGUT and COPIN-MONTEGUT. 1972. 1978: FEELY, 1975: KRISHNASWAMIand SARIN. 1976; WALLACE ef al.. 1977; BISHOP et al., 1977. 1979). Only in the highly productive regions ofi southwest Africa (BISHOP et al., 1979) do the ‘excess’ concentrations of Si. Mg, Ca and K approach those of the ‘zone of biogenic material’ in the Amazon Estuary. (e) The chemical composition
qf suspended material
Chemical compositions. calculated as percentage oxides on an organic matter- and water free-basis, of the suspended material of the Amazon River and Estuary are summarized in Table 3. In this way they provide a ready means of comparing them with sediments and fine-grained sedimentary rocks.
E. R. SHOLKOVITZ and N. B. PRICE
170
Table 2. Concentrations
(pgil) of ‘excess’ elements in the combined blue water (S > lO”,,,,) Mean value
Element
“excess”
174 2.8 2.0 1.0 0.36 0.16 1.8 1.2
Si P 4 Ca Ti Mn Fe K
of
Standard
concentration
(n (n (n (n (n (n (n (n
= E = = = : = I
deviation
3. Chemical
composition
Amazon~Ri;e;)!‘j’2’ n
SiO2 Ti02
A1203 Fe203 MnO
58.0 0.79 23.5 10.5 0.088
of suspended
Zone
Total “excess”
+ 149 -e 1.4 T 1.5 + 0.8 + 0.26 T 0.10 5 1.5 5 1.3 -
41 40 36 41 38 39 29 36
Samples from the ‘zones of biogenic material and blue water’ have high concentrations of silica (total SiOZ = 71.599 due to the biogenic component. This biogenic silica masks the enrichments of Ti, Mn, Ca and Mg previously documented from the element/Al ratios. These enrichments become evident if the compositions of these samples are converted to a biogenic silica-free basis. One feature merits a brief discussion. Th.e Amazon River suspended matter collected approximately 200 km above the head of the estuary has significantly higher contents of Fe,O, and TiOz than do the low salinity estuarine samples. The general impoverishment of these elements in the latter samples may result from a prior deposition of Fe- and Ti-rich heavy minerals near the head of the estuary. EDMOND et ul. (in prep.) also show the concentrations of several dissolved constituents (e.g. nutrients and trace metals) to be very different in the river water and in zero-salinity estuarine waters. It follows that important physical and chemical processes occur between the lower reaches of the river and upper reaches of the estuary.
Table
zones of bio_eenlc material
59.1
-+ 0.3 + 0.4
25.8
0.49
of
concentration
2 -700 0.2 -6 0 -6 0 -2 0.1 -1.0 0.01-0.5 0.4 -4.2 0.2 -4.2
CONCLUSIONS
Our main conclusion is that Si. P. Ca. Mg. Ti and Mn (and to a lesser certainty K and Fe) are incorporated into the skeletal and organic phases of phytoplankton of the Amazon Estuary. Evidence for this conclusion is three-fold and comes from the measurement of surface suspended matter:
(1) In the salinity range @IO”,,,, the major-element/Al ratios remain constant even though the particles (predominately terrigenous) decrease in load from 500 to 3 mg;l (indicating resuspension and deposition). (2) With the onset of biological productivity at approximately loo,,,, there are large increases in the Si;AI and P/Al ratios accompanied by smaller but significant increases in the ratios of Ca,AI. Mg;AI, T&Al and MnjAI: the ratios of K/Al and Fe:AI show very slight increases. (3) A broadly proportional relationship exists between the concentrations of ‘excess* P and ‘excess‘ matter
from the Amazon
Amazon Estuary(‘) of terrigenous material (So/,,
-+ 0.7 -+ 0.01
range
and
+ 1.08 _c 0.07 -+ 0.5
River and Estuary
Amazon Estuary(” of biogenic material and blue water >lO: n = 41) (SO/,,
Zones
71.5 0.43 15.7
-+ 7.3 + 0.18 -+ 4.4 2 1.4
8.13
-+ 0178
5.3
0.083
+ 0.009
0.080
0.41
0.50
-+ 0.20 -+ 0.44
2.32
-+ 0.03 + 0.14
1.81
-c 0.62 -+ 0.53 -+ 0.73
CaO
1.04
-+ 0.001 -+ 0.02
MgO
1.68
-+ 0.01
3.08
_c 0.02
3.12
2 0.15
1.99
0.18
-+ 0.01
n.20
-+ 0.05
1.03
Ya20 K2° p2°s ( I) From
the muin river channel approximately 200 km upstream from the head of the Estuary (Station S202). (7) Calculated on an organic matter-. and water-fret hasis: umts. “,, as oxides. Mean values and standard deviations given.
The major-element
Si. Mg. Ti. Mn. K and greater than I O”,,,,.
Fe. in water
chemistry
with
of suspended
salinity
It could be argued that there are alternative, nonbrologrcal processes which could explain our observations. There could for Instance be differential settling of particles with different chemical and mineralogical compositions. Point number (I ) given above seems to rule thus out. In addition. our unpublished results show that the ratios of Si Al. TVAI. K,AI and Fe/Al actually decrease slightly with decreasing particle size. SABLES and MANC~ELSDORF (1979) have shown through cation-exchange studies that Amazon River suspended matter take-up K” and Mg” from seawater (salinity of 36”,,,,) to the extent of approximately 2 and 7 m-equiv IOOg respectively. We can apply these values IO the ‘zone of biogenic material’ where the total suspended load (TSL) is 3 mgil. However. of this TSL only about Y,, is terrigenous. the rest being organic matter and diatom frustules (MILLIMAN et al., 1975: MILLIMA~ and BOYLE. 1975). Using the above mformation there is an additional 0.1 pg/l of particulate K and Mg due to cation-exchange involving terripenous material. This is less than IO”,, of our mean ‘excess‘ concentration (Table 2). Our evrdence. when weighted against alternative explanations. indicates that biologically mediated processes play an important role in the geochemical cyclmg of inorganic elements. The nature and magnitude of these processes. however. requires additional mvestigation. .~~.~,I~~~,/L’~H~~~I~~I~s-OU~ greatest thanks goes to Dr JOHN Er>~oun who invited us to participate in this Amazon Stud!. Thanks also goes to Dr ED BOYLE and BARRY Ga.4~1 for their help at sea. This paper was greatly improved by the comments and suggestions of three reviewers to whom we would like to offer our appreciation. Our participation in the Amazon expedition was funded by the Natural Environment Research Council of Great Britam
REFERENCES BISHOP J
K. B.. ED~O~D J. M.. KETTEN D. R.. BACON M. P and SILKER W. B. (1977) The chemistry. biology and vertical flux of particulate matter from the upper 4OOm of the equatorial Atlantic Ocean Deep-Seu Res. 24. 51 I-548. BISHOP J. K. B.. KETTEV D. R. and EDMOND J. M. (1979) The chemistry. biology and vertical flux of particulate matter from the upper 400m of the Cape Basin in the S.E. Atlantic Ocean. Deep-Sea Res. 25. 1121-l 162. COPII-MOUTEGCT C. and COPIN-MOI‘;TEGUT G (1972) Chemical analyses of suspended particulate matter collected in the north-east Atlantic. Deep-Sea Res. 19, 445452. COPI\-MONTEC;~T C and COPIN-MONTEGUT G. (1978) The chemistry of particulate matter from the south Indian and Antarctic ocean. Drrp-Sea Res. 25. 91 l-931.
matter
in the Amazon
Estuary
171
EI)MONI) J. M.. BOYLI E. D.. GRANT B. and STALLAWUR. F. The chemical mass balance in the Amazon Plume I: The Nutrients. (In prep). Ft.t~v R. A. (1975) MaJor-element composition of the particulate matter in the near-bottom nepheloid layer 01 the Gulf of Mexico. Mur. Chmr. 3. 121&156. GARRELS R. M. and MACKENZII. F. T. ( 1971 ) T/w Etwluziot~ of Sedirmwrur~ Roc,k.~ 397 pp. Norton. Glees R. J. (1976) Amazon River sediment transport in the Atlantic Ocean. Gcolocrr 4. 45-48. GIBRS R. J. (1977) Transport phases of transition metals In the Amazon and Yukon Rivers. Grol. Sot,. Ani. Bull. 88. X29-X43. GRILL E. V. and RI(.HARI)S F. A. (1964) Nutrient regeneration from phytoplankton decomposing in seawater. J. Mu. Res. 22. 5 I-69. HULBERT E. M. and CORWII\. N. (1969) Influence of the Amazon River outflow on the ecology of the western tropical Atlantic. III. The planktonic flora between the Amazon River and the Windward Islands. J Mar. Res. 21, 55-73. KRISHNASWAMI S. and SARIN M. M. (1976) Atlantic surface particulates: composition, settling rates and dissolution in the deep sea. Eurrh Pkmer Sci. Lrrr. 32, 43@440. MARTIN J. H. and KNAUER G. A. (1973) The elemental composition of plankton. Geochim. Cosmochim. Actu 37, 1639-1653. MARTIX J. M. and MAYBET~ M. (1979) Elemental massbalance of material carried by world major rivers. Mar. Chem. 7, 173-206. MILLIMA~ J. D. and BOYLE E. (1975) Biological uptake of dissolved silica m the Amazon River estuary. Science 189, 995-997. MILLIMAN 1. D.. SUMMERHAYESC. P. and BARRETTO H. T. (1975) Oceanography and suspended matter off the Amazon River February-March, 1973. J. Sediment. Perrol. 45. 189-206. PRICE N. B. and CALVERT S. E. (1973) A study of the geochemistry of suspended particulate matter in coastal waters. Mu. Chew 1, 169-189. SAYLES F. L. and MANGELSDORF P. C. JR (1979) Cationexchange characteristics of Amazon River suspended sediment and its reaction with seawater. Geochim. Cosmochim. Acta 43, 767-780. SHOLKOVITZ E. R. (I 979) The physical and chemical processes controlling the chemical composition of suspended matter of the Tay River and Estuary. Estuarinr Coasr. Mar. Sci. 8. 523-545. SPENCER D. W. and SACHS P. 1. (1970) Some aspects of the distribution. chemistry and mineralogy of suspended matter in the Gulf of Maine. Mar. Geol. 9, 117-I 36. TEIXEIRA C. and TUNDIS J. (1967) Primary production and phytoplankton in equatorial waters. Bull. Mar. Sci. 17, 884-89 1. TREFRY J. H. and PRESLEY B. J. (1976) Heavy metal transport from the Mississippi River to the Gulf of Mexico. In Marine Polluranr Transfer (eds H. L. Windom and R. A. Duce). pp. 39-76. Lexington Books. TUREKIAN K. K. (1971) River. tributaries and estuaries. In Inipingernern of bJau on rhe Oceans (ed. D. W. Hood). Chap. 2. pp. 9-73. Wiley. WALLACE G. T.. HOFFMAK G. L. and DUCE R. A. (1977) The influence of organic matter and atmospheric deposition on the particulate trace metal concentration of north-vvest Atlantic surface sea water. Mar. Chem 5. 143-170.
Institute
of Geology.
West Mains
Road.
Edinburgh.
EHY 3JW. U.K
(Rec,cr~>rr/ 6 Fc#~r~rtrr!. 197’): rrc,c,rpretl ,,I rc,riscd fornr 20 Srprenlhrr
1079)
Abstract--Suspended matter from the surface waters of the Amazon Estuary were collected durmg Ma! and June 1976 on the ‘R ‘V Alpha Hehx’. and their major-element compositions (Al. Si. TI. K. Mg. Ca. P. Fe and Mn) were measured. material. predominantly terrigenous III derlvatlon. Between salinities of 0 and lo”,,,,. the suspended decreases In load from 500 to 3 mgI but has a chemical composltlon which remains essentially constant With the onset of a large amount of biological productivity at approximately IO”,,,, salinity. there are large increases in the ratios of SI.!AI. P’AI. CasAl. Mg;AI. Ti:AI and Mn.AI which are mamtamed at higher salinities. Calculations of “excess” concentrations of elements held In the non-terrigenous components of the suspended material further support our main conclusion that Si. P. Ca. Mg. TI and Mn are Incorporated into the skeletal and organic phases of marine phytoplankton (predominately dIatomsI of the Amazon Estuary. The data suggest. but with less certainty. that Fe and K follow the above elements. This study has demonstrated that the chemical composition of river-introduced suspended matter can be significantly altered by biological activity within estuarme waters as can be the geochemical cycle of inorganic elements.
INTRODUCTION RIVERS through
their suspended loads supply the flux of solid material to the oceans (GARRELS and MACKENZIE. 1971). However. the chemical composition of river-borne suspended material is poorly documented as are the compositional changes that ma> occur as it enters the estuarine environment. greatest
(TUREKIAN. 1971;
GIBBS. 1977:
BECK. 1979; TREFR’I. and
MARTIN and
MAY-
PRESLEY, 1976). Biological,
chemical and physical processes operating in estuaries may result in the suspended material having compositions significantly different from those of the original river-borne suspended material. Hence. a complex relationship may exist between the composition of river-introduced suspended material and the material which leaves estuaries to become oceanic suspended material and sediments. In this paper we will report on the major element compositlon (5. Al. Ti. Fe. Mn. K. Ca, Mg and P) of suspended matter in the Amazon Estuary and shoM how this data can be used to elucidate the biological. chemical and physical processes occurring in the estuarine environment. In particular. the results will demonstrate that man> of these elements are incorporated into marine phytoplankton and therefore. are. involved in biogeochemical cycles. METHODS
OF
COLLECTION
AND
ANALYSES
Samples acre collected m Ma! June. 1976. during the ‘R \’ Alpha Helix’ expedition 10 the Amazon Estuar! and River. While the ship was underwa!. surface water (top 1m) \xaspumped through a plastic hose held out from the side of the ship dIrectI> In pol>eth!lene bottles. UsualI! althin three hours of collectlon. each bottle was shaken
thoroughly and an aliquot of water quickly transferred to a plastic filtration device and a measured volume filtered under pressure through a pre-weighed 0.4 pm Nuclepore filter (37 mm). Filtered distilled water (25-50 ml) was added to the filtration device and passed through the sample and filter to wash out the sea salt. In total about 140 samples of suspended material were collected from five separate transects of the Amazon Estuary (Fig. 1). On return to Edinburgh, these samples were desiccated and then weighed to determine the total suspended load. Chemical analyses were carried out b) X-ray fluorescence spectrometry (XRF). Details of the method are given in PRICE and CALVERT (1973)and SHOLR~VITZ (I 979). Briefly. standards and samples (both on Nuclepore filters) were mounted on special holders and placed in a Philips PWl450 X-ray spectrometer. Peak and background counts were measured for each major element. Chlorine concentrations were also measured and used tc correct the Ca. Mg and K concentrations for any sea-salt Ca. Mg and K not washed off the filters. Working standards were prepared by filtering suspensions of weighed amounts of finely ground USGS rock standards. To obtain a set of standards independent of the above XRF technique. a selected number of rock standards and Amazon samples were digested In HF and analyses bq spectrophotometric and atomic absorption methods. These two procedures yielded calibrations of element content and X-ray instrumental response. The analytical precision of the XRF analyses are referred to in the appropriate figures of this paper. Smce only samples with total weights of less than 3.0 mg are suitable for our XRF technique. the major element composition was determined for 75 out of the total of 140 samples collected RESULTS
AND
DISCUSSIONS
(a) Torcrl susper~dvd load The total suspended load (TSL) of the surface waters decreased dramaticall> from over 500 to 3 mg 1 163
164
E.R.
SHOLKOVITZ
I
P--
Fig.
1. The Amazon
PRKE ,
I
I
tea3
legs ? IX
Estuary and the lower reaches of the Amazon River, The depth The ‘legs’ covered by the ‘R/V Alpha Helix’ are shown.
the salinity increased from 0 to loo& (Fig. 2). At salinities greater than IO‘& the TSL varies between 1 and 3 mg/l. This rapid fall-off of TSL has been previously observed in the Amazon Estuary by MILLIMAN and BOYLE(1975). MILLIMANer al. (1975) and GIBBS (1976). The high suspended loads in the upper estuary result from the resuspension of bottom sediment as the surface water of the Amazon River itself only carried a suspended load of SO-70 mg/l in its lower reaches during our June 1976 study. The rapid fall-off in TSL implies deposition of particles.
as
(b) Visual observations-three
zones of’ the Amazon
Estuary
Visual observations
and N. B.
provide a valuable clue to the
4
contours
are in m.
variation in the type of suspended matter in the Amazon Estuary. During our cruise in May 1976 water with salinities between 0 and IO’&, were distinctly brown in colour and we have designated this water to lie within the ‘zone of terrigenous material’. At approximately loo,, salinity there was a sharp transition to green coloured water containing much diatomaceous debris. This material is most concentrated in waters within a IO-25’& salinity range; we designate this to be the ‘zone of biogenic material’. At salinities greater than approximately 25”&,,the water was distinctly blue; this ‘zone of blue water’ had a diffuse boundary with the ‘zone of biogenic material’. A division of the particulate matter in the Amazon Estuary into zones of terrigenous and biogenic
0
leg 1
2 x leg 3 8 leg 5 0 leg
4 leg
Fig. ?. Total
7
suspended load of the surface waters of the Amazon Estuary vs salimty. The Inset expands the scale of the total suspended loads for salinities greater than 4”,,,,.
The major-element
165
chemistry of suspended matter III the Amazon Estuary
Table 1. Amazon Estuary: element/Al ratios”’ 0
A
--
Zone of terrigenous material
Zones of biogenic
(S
nz 21)(2) -__
material
(S>lOO/,,;
-.-.--
Ratio
water
of
WA
n = 40) ._~
2.0 0.006 0.102 0.022 0.022 0.0048 0.42 0.190
Si/Al ?/Al Mg/Al Ca/Al Ti/Al MniAl fe/Ai K/Al
and blue
+ 5 7 5 5 5 5 -c -
0.1 0.002 0.006 0.002 0.003 0.0007 0.04 0.009
4.6 0.047 0.126 0.036 0.031 0.0079 0.46 0.200
+ : 5 5 T 5 5 7 -----
2.2 7.6 1.24 1.64 1.41 1.65 1.07 1.05 --
1.9 0.021 0.022 0.016 0.009 0.0017 0.05 0.015
(I 1Mean values and standard deviations. (2) Ratios used as values for terrigenous material in the calculation of ‘excess’ concentrations.
influence has been previously made (MILLIMANet al., 1975; MILLIMA~ and BOYLE, 1975) although the chemical character of the particles was not determined. These authors suggested that the onset of biological productivity occurred where there is a marked increase in light transparency as a result of rapidly decreasing TSL. IC) ElenwztiAl ratio qf the suspetlded material In this paper many of the chemical data on susAMAZON P/Al
pended matter are presented as elemental ratios to Al. Aluminium is a major and chemically unreactive constituent of clay minerals which, in themselves, are the dominate component of river-borne particles. By comparing element/Al ratios in the three zones of the Amazon Estuary it is possible to determine if estuarine processes cause certain elements to be enriched or depleted relative to the terrigenous suspended matter. Moreover, ratios to Al are useful for this purpose since they are independent of variations in the con-
ESTUARY . leg 1 0 leg 2 . leg 3 m leg 5 a leg 7 *
0.08 .’
.
.
tOr
.
Fig. 3. The P Al and Si Al ratios of surface suspended matter in the Amazon Estuary vs salinity. The precision of the ratlo measurement is P Al i 0.001 and Si’Al & 0.1.
E. R. SHOLKWITZ
166
centration of biogenic material (e.g. Si02. CaCO,. organic matter). This is particularly important when considering marine sediments and suspended matter which contain large amounts of biogenic material (SPENCER and SACHS, 1970: PRICE and CALVERT. 1973: SHOLKOVITZ, 1979). One outstanding ion of suspended
feature
of the chemical
matter
composit-
in the Amazon Estuary is that all the element/Al ratios have almost constant values between salinities 0 and lo”,,,, (‘zone of terrigenous material’) even though the total suspended load decreases from values of near 500 mg/l to 3 mgl (Figs Z-6). This lack of chemical variation provides evidence that no chemical, biological or physical processes in the low salinity region of the Amazon Estuary are capable of altering the major element composition of the surface suspended matter. The elementiA1 ratios change markedly at salinities greater than about lo’<, (Table 1 and Figs 3-6). The large increases in Si;Al and P/AI ratios (Fig. 3) coincide with the onset of biological productivity. The P,‘Al ratios show the greatest increase (factors of up to 16) as would be expected since P is clearly incorporated into living organisms and occurs only in small concentrations in clay minerals (P!AI = 0.006). The increases in SiiAl and P,:Al ratios also are
and N. B. PRICE
accompanied by smailer but significant Increases in the ratios of Co ‘Al. 418 AI. Mn Al and Ti A1 (Table I and Figs 4 and 5). Our data shokvs that these increases are caused by increased concentrations of ;I particular element and not from a decrease in Al concentration. It appears that not only are Si. P and C;i incorporated into marine ph~topiankton (e.g. diatoms) but so are ,Mg, Ti and Mn. In contrast both the K.Al and Fe Al ratios show only a slight increase between the two zones (Table I and Fig. 61. K Al increases from 0.190 + 0.009 (mean and standard deviation) to 0200 + 0.015 while Fe Al increases from 0.42 & 0.04 to 0.45 i: 0.05. Increases of this magnitude are greater than the analytical precision of the measurements (Fe!AI. iO.01: K Al. iO.004). suggesting that K and Fe also are incorporated into phytoplankton. However. as this conclusion rests on our interpretation of very small changes m the K Al and Fe:AI ratios. it should be viewed critically. The increase in element, Al ratios in waters above IO’,,, salinity are consistently observed in the various transects of the Amazon Estuary. There is some suggestion of a broad maximum in many of the ratios between salinities of IO and 25’:,,1,and this is most evident in P!AI and Si Al ratios from leg I samples (Fig. 3). This trend suggests that the formation of bio-
Co/AI 0.10 -
. 0.08 0.06 -
MS/AI
.
0.20
Fig. 4. The Ca:AI and Mg;AI ratios of surface suspended matter in the Amazon Estuary vs salinity. See the key in Ftg. 3 to refer to samples from ditferent legs. The precisron of the ratio measurement is Ca, Al k 0.003 and Mg;AI & 0.05.
The major-eJement chemistry of suspended matter m the Amazon Estuary
167
Mn/Al
T,/At
Fig. 5, The Mn’AI and Ti:AI ratios of surface suspended matter in the Amazon Estuary vs sahntty. See the key in Fig. 3 to refer to samples from different legs. The precision of the ratio measurement IS Mn:Al 5 0.001 and Ti/‘AI k 0.001.
genie matter is most intense in the intermediate salinity range (l@-25?,,). This conclusion is supported by the variation of ‘excess’ P (see below) with salinity (Fig. 7) and by the particulate organic carbon data of EDMUND er al. (in prep). tncreases in the element/Al ratios within the zone of biogenic material’ are mirrored by the dissolved nutrient data of EDMOND er at. (in prep). They show that extensive removal of phosphate and nitrate
occurs at approximately IO’& salinity and continues until 15%, where the concentration of nitrate is almost zero. Some comment should be made on the large scatter in many of the element/AI ratios (e.g. Si/AI. P/AI. Ca/AI) of samples collected in the ‘zones of biogenic material and blue waters’. We suspect that this is caused by inhomogenous mixtures of terrigenous and biogenic material as indicated by the visual patchiness
Fq 6. The Fe Al and K Al rattos of surface suspended matter m the Amazon Estuary vs salinity. See the key in Fig. 3 to refeF to samples from different legs. The precision of the ratio measurement IS Fe Al & 0.01 and K;Al i 0.004.
168
E. R.
SHOLKOVITZ
and N. B. PRICE
AMAZON
ESTUARY 0 leg 1 0 leg 2 x leg 3 l leg 5 A leg 7
excess P’crg/li 6-
‘0
s4-
l
%
32-
.
l0
0
.
A *
00
‘rn
0
.
.
.o
.
.
0
n
. O
I-
Fig. 7. The concentration
of ‘excess’ P vs salinity for surface suspended matter in the Amazon Estuary
in the ‘zone of biogenic material’. Another consideration is that the suspended material contains plankton in various states of growth and decomposition. As these processes are known to take-up and release Si, P.K.Mg and Ca (GRILLand RICHARDS, 1964;BISHOP rcul.. 1977,1979). variations in the relative proportion of living to dead plankton could add to the scatter of the results. (d) ‘E.ucess’ concentrations:
element
incorporation
(2-4 times) less than their concentrations in the terrigenous component. For Si. P, Mg, Ca. Mn and Ti their ‘excess’ concentrations are greater than or equal to their concentrations in the terrigenous component: this gives us confidence in using the ‘excess’ concentrations of these elements as interpretive tools. As P is the most diagnostic element indicating organic matter, ‘excess’si, Mg. Ti. Ca. K and Fe concentrations have been plotted against ‘excess’ P
into
marine phpoplankton
700 r
From the patterns of element/Al ratios it appears that P. Si, Ca, Mg, Ti, Mn (to a lesser certainty K and Fe) are associated with or incorporated into the phytoplankton of the Amazon Estuary. To support this claim, ‘excess’ concentrations of these elements were computed and their distributions with respect to salinity examined. ‘Excess’ concentrations may exist in association with organic matter, within skeletal matter or as authigenic substances such as metal oxides. For example. ‘excess’ Si is calculated as follows: ‘excess’ Si(pg/l)
0;
Ao
AMAZON
Ll
ESTUARY
= total Si(pg/l) _
Si !-IAl TSM
where TSM = terrigenous
x total Al(pg/l)
suspended
0
1
2
3
excessP
4
5
6
pg/l
material.
As seen from this calculation, the element/Al ratios of the terrigenous constituents must be established within the particulate matter. For the Amazon Estuary. the constant element/Al ratios in the ‘zone of terrigenous material’ (GlCr’,,,) give a good estimate of the element/Al ratios of the terrigenous component (see Figs 3-6 and Table I) and these ratios have been employed for computing the ‘excess’ element concentrations Caution must prevail when interpreting ‘excess’ concentrations as they are computed by subtracting two variables. For the majority of our samples only Fe and K have ‘excess’ concentrations significantly
Fig. 8. The concentration of ‘excess’ P vs the concentrattons of ‘excess’ Si and ‘excess’ Mg for surface suspended matter of the Amazon Estuary (S > IO”,,,,). The linear correlation coefficients are 0.6X and 0.44 respectively.
The ma,ior-element chemtstry of suspended matter I” the Amazon Estuaq 0.5 excess
a
excess
K i&Ii
Mn (p,9/1) 0.4
‘3
4.0 -
: .
0.3 -
.
0.2 Ol. x -ai
3.0 -
00. .
n
0
1
.
.
l
2.0 -
s:.
.
on. l em .O q O e 0 0’ 0 00 I I 1 2 3 4 excess P (&Is/l)
1.0 excess
.
.
;
.
. 1.0 -
I 5
.
I 6
0L-I 0
3
0
l
.
om.*,* 0 . . ’ “cl’ 1.0 2.0 3.0 4.0 excess P 1rJg/l,
. 1 6.0
I 5.0
3
n
Ti(crg/l) 0.8
l
excess 4.0 -
. .
..
%9/l)
n
.
.
2.0 -
o.+. ,Of‘1:;.., 2
3
4
excess
P(pg/l)
e
.
.
R
1
.
0
.
3.0 -
0.6
0
169
,
1
5
6
0 0
.
. .
l
iIELcL0
1
2 excess3P (pg/Tj
5
6
FIN. 9. The concentration of ‘excess’ P vs the concentratlons of ‘excess’ Ti and ‘excess‘ Mn for surface suspended matter of the Amazon Estuary (S > 10”,,,,). The linear correlation coefficients are 0.55 and 0.57 respectively.
Fig. 10. The concentration of ‘excess’ P vs the concentrations of ‘excess’ Fe and ‘excess’ K for surface suspended matter of the Amazon Estuary (S r lo”,,,,). The linear correlation coefficients are 0.40 and 0.47 respectively.
(Figs 8-10). These figures show that ‘excess’ Si. Mg, Ti. Mn, K and Fe concentrations are broadly proportional (linear correlation coefficients of 0.68, 0.44. 0.55, 0.57. 0.47 and 0.40 respectively) to that of ‘excess’ P. suggesting that these elements are incorporated into phytoplankton. In contrast. the linear correlation coefficient between ‘excess’ Ca and P is only 0.03; our calcium results indicate that calcareous plankton are present in minor amounts in these waters. a conclusion consistent with previous phytoplankton studies in the Amazon Estuary (TEIXEIRA and TUNDIS. 1967; HULRERT and CORWIN. 1969; MILLIMAN er al., 1975). ‘Excess’ Si IS obvious& in the skeletal material of the diatoms. The high ratios of ‘excess’ Mg!‘excess’ Ca (i-9 mol ratio) suggests that Mg is not held in calclum carbonate but in an organic phase. A similar conclusion has been reached by BISHOP er al. (1977, 19791. These authors also conclude from the distribution of Mg. Ca and K in particulate matter in the upper 400 m of water in the eastern equatorial Atlantic Ocean and off southwestern Africa that ‘excess’ Ca and K are associated with the organic phase of marme phytoplankton. Their data shows a positive correlation between the extent of biological productivit) and the concentration of ‘excess’ K. Mg and Ca. Distribution of ‘excess’ Ti. Mn and Fe do not allow determining whether these elements are preferentially associated with skeletal materials or held m the orga-
nit matter of the phytoplankton since ‘excess’ Ti. Fe and Mn all show a directly proportional relationship with both ‘excess’ P and ‘excess’ Si. The data of MARTIN and KRAUER (1973) suggest that Ti is preferentially associated with siliceous frustules, that Mn is associated with the organic matter. and that Fe is associated with both organic matter and siliceous frustules with the former phase being more important. An important point is that the concentrations of ‘excess’ elements in the zones of biogenic material and blue water (Table 2) are much higher than total or excess concentrations of corresponding elements (Fe. Si, P, Mg. K and Mn) reported for oceanic particles (COPIN-MONTEGUT and COPIN-MONTEGUT. 1972. 1978: FEELY, 1975: KRISHNASWAMIand SARIN. 1976; WALLACE ef al.. 1977; BISHOP et al., 1977. 1979). Only in the highly productive regions ofi southwest Africa (BISHOP et al., 1979) do the ‘excess’ concentrations of Si. Mg, Ca and K approach those of the ‘zone of biogenic material’ in the Amazon Estuary. (e) The chemical composition
qf suspended material
Chemical compositions. calculated as percentage oxides on an organic matter- and water free-basis, of the suspended material of the Amazon River and Estuary are summarized in Table 3. In this way they provide a ready means of comparing them with sediments and fine-grained sedimentary rocks.
E. R. SHOLKOVITZ and N. B. PRICE
170
Table 2. Concentrations
(pgil) of ‘excess’ elements in the combined blue water (S > lO”,,,,) Mean value
Element
“excess”
174 2.8 2.0 1.0 0.36 0.16 1.8 1.2
Si P 4 Ca Ti Mn Fe K
of
Standard
concentration
(n (n (n (n (n (n (n (n
= E = = = : = I
deviation
3. Chemical
composition
Amazon~Ri;e;)!‘j’2’ n
SiO2 Ti02
A1203 Fe203 MnO
58.0 0.79 23.5 10.5 0.088
of suspended
Zone
Total “excess”
+ 149 -e 1.4 T 1.5 + 0.8 + 0.26 T 0.10 5 1.5 5 1.3 -
41 40 36 41 38 39 29 36
Samples from the ‘zones of biogenic material and blue water’ have high concentrations of silica (total SiOZ = 71.599 due to the biogenic component. This biogenic silica masks the enrichments of Ti, Mn, Ca and Mg previously documented from the element/Al ratios. These enrichments become evident if the compositions of these samples are converted to a biogenic silica-free basis. One feature merits a brief discussion. Th.e Amazon River suspended matter collected approximately 200 km above the head of the estuary has significantly higher contents of Fe,O, and TiOz than do the low salinity estuarine samples. The general impoverishment of these elements in the latter samples may result from a prior deposition of Fe- and Ti-rich heavy minerals near the head of the estuary. EDMOND et ul. (in prep.) also show the concentrations of several dissolved constituents (e.g. nutrients and trace metals) to be very different in the river water and in zero-salinity estuarine waters. It follows that important physical and chemical processes occur between the lower reaches of the river and upper reaches of the estuary.
Table
zones of bio_eenlc material
59.1
-+ 0.3 + 0.4
25.8
0.49
of
concentration
2 -700 0.2 -6 0 -6 0 -2 0.1 -1.0 0.01-0.5 0.4 -4.2 0.2 -4.2
CONCLUSIONS
Our main conclusion is that Si. P. Ca. Mg. Ti and Mn (and to a lesser certainty K and Fe) are incorporated into the skeletal and organic phases of phytoplankton of the Amazon Estuary. Evidence for this conclusion is three-fold and comes from the measurement of surface suspended matter:
(1) In the salinity range @IO”,,,, the major-element/Al ratios remain constant even though the particles (predominately terrigenous) decrease in load from 500 to 3 mg;l (indicating resuspension and deposition). (2) With the onset of biological productivity at approximately loo,,,, there are large increases in the Si;AI and P/Al ratios accompanied by smaller but significant increases in the ratios of Ca,AI. Mg;AI, T&Al and MnjAI: the ratios of K/Al and Fe:AI show very slight increases. (3) A broadly proportional relationship exists between the concentrations of ‘excess* P and ‘excess‘ matter
from the Amazon
Amazon Estuary(‘) of terrigenous material (So/,,
-+ 0.7 -+ 0.01
range
and
+ 1.08 _c 0.07 -+ 0.5
River and Estuary
Amazon Estuary(” of biogenic material and blue water >lO: n = 41) (SO/,,
Zones
71.5 0.43 15.7
-+ 7.3 + 0.18 -+ 4.4 2 1.4
8.13
-+ 0178
5.3
0.083
+ 0.009
0.080
0.41
0.50
-+ 0.20 -+ 0.44
2.32
-+ 0.03 + 0.14
1.81
-c 0.62 -+ 0.53 -+ 0.73
CaO
1.04
-+ 0.001 -+ 0.02
MgO
1.68
-+ 0.01
3.08
_c 0.02
3.12
2 0.15
1.99
0.18
-+ 0.01
n.20
-+ 0.05
1.03
Ya20 K2° p2°s ( I) From
the muin river channel approximately 200 km upstream from the head of the Estuary (Station S202). (7) Calculated on an organic matter-. and water-fret hasis: umts. “,, as oxides. Mean values and standard deviations given.
The major-element
Si. Mg. Ti. Mn. K and greater than I O”,,,,.
Fe. in water
chemistry
with
of suspended
salinity
It could be argued that there are alternative, nonbrologrcal processes which could explain our observations. There could for Instance be differential settling of particles with different chemical and mineralogical compositions. Point number (I ) given above seems to rule thus out. In addition. our unpublished results show that the ratios of Si Al. TVAI. K,AI and Fe/Al actually decrease slightly with decreasing particle size. SABLES and MANC~ELSDORF (1979) have shown through cation-exchange studies that Amazon River suspended matter take-up K” and Mg” from seawater (salinity of 36”,,,,) to the extent of approximately 2 and 7 m-equiv IOOg respectively. We can apply these values IO the ‘zone of biogenic material’ where the total suspended load (TSL) is 3 mgil. However. of this TSL only about Y,, is terrigenous. the rest being organic matter and diatom frustules (MILLIMAN et al., 1975: MILLIMA~ and BOYLE. 1975). Using the above mformation there is an additional 0.1 pg/l of particulate K and Mg due to cation-exchange involving terripenous material. This is less than IO”,, of our mean ‘excess‘ concentration (Table 2). Our evrdence. when weighted against alternative explanations. indicates that biologically mediated processes play an important role in the geochemical cyclmg of inorganic elements. The nature and magnitude of these processes. however. requires additional mvestigation. .~~.~,I~~~,/L’~H~~~I~~I~s-OU~ greatest thanks goes to Dr JOHN Er>~oun who invited us to participate in this Amazon Stud!. Thanks also goes to Dr ED BOYLE and BARRY Ga.4~1 for their help at sea. This paper was greatly improved by the comments and suggestions of three reviewers to whom we would like to offer our appreciation. Our participation in the Amazon expedition was funded by the Natural Environment Research Council of Great Britam
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