Tbe contrasting biogeochemistryof iron and manganese in the Pacific Ocean
WILLIAMM. LANDING*and KENNETHw. BRULAND
Imtitute of MarineSciencu, Univerxityof California, Santa Crux, CA95064,U.S.A,
(ReceivedOctober28, 1985;acceptedin revisedformSeptember19, 1986)
Absira&-Vertical and bodxontal distributions of disoived and suspended part&late Fe and Mn, and
vertical &txes of these metals (obtaiues.4with sediment haps) were determined debut
the Pacific Ocean.
In general, disohd Fe is lowin surfaceand deep watem (0.1 to 0.7 amol/kgf, with maxima associated with
the intermediate depth oxygen minimum zone (2.0 to 6.0 nmol/kg). Verticaldistributionsof dissolvedMn
are similar to pwious mpott%,
exhibitiag a surfacemaximum, a subsurfaceminimum, a Mn maximum
layercoincident with the oxygen minimum zone, and lowest values in deep waters.
Nesr-sho~ removalproccws are more intense for dissolvedFe than for dissolvedMn. Dissolved Mn in
the swhcc mixed layer remains elevated much fhther offshorethan dissolved Fe. Elevatednear-suhce
dissolved h4n coacentratioasoccur in the North PacificEquatorialCurrent,sugg%hg tmospxt &om the
eastem boundary.Near-surf&xmixed-layerdissolved Mn ~n~n~tio~
are b&herin the North Pacific
gyrerekctiag eahaaced northerakmis@ere aeoliaa sourczs.
Reskhace time estimatesforthe set&g of refractorypsrticulateFe and Mn fromthe upperwater
column
are 62-220 days (Fe), and 105-235 days (Ma). In contrast,reskhw times for the scavengingof dissokd
Fe and Mn are 2-13 yeam(Fe) and 3-74 yeam (Mn). Scavengingtwidence times for dissolved Mn based
on horizontalmixing in the surf&. mixed layerof the northeastPa&c are0.4 years(near&ore)to 19 years
(1000 km oBihol+
Thereis no evideaca for itisituFe redoxdissoiution within suboxic waters in the tastern tropical North
Pacifk. Dissolved Fe appearedto be controlledby dishion
from sub-oxic sediments, with oxidative
scavengitq in the water column or upper sediment layers. However, insituMn dissoMion within the oxygen
minimum zone was evident.
dissolvedupon the mcyclin8of this material kith depth.
From a thermodynamic standpoint, dissolved Mn and
THEOCEANIC
DIsrRIB~IONS and biqwxhemicA beFe should have extremely low ~n~n~tions
(subhavior of diswlved and particulate Fe and Mu are cono~orno~~ under oxic marine conditions. However,
trolled by complex interactions among input, internal
reductions in oxygen concentration or pH will enhance
cyclin& and removal pmcesses coupled with physical
their solubiity (SrUr## and MORGAN,1981). Subsetransport and mixin in the oceans. Over the last ten
quent reoxidation of Mn(iI) to Mn(II1) or Ma(W)
yeam the use of relatively contamination-free mmplin8
oxyhydroxides is relatively slow (STWM and MORand analytical methods has resulted in a si@lkant
GAN, t981), but may be enhanced by bacterial proincrease in our understanding of the marine biogeocesses(NEALSON,1978).Oxidation of Fe(B) to Fe@)
chemistry of Mn. However, there is a paucity of reliable
is much faster, and may yield an amorphous colloidal
diseolved oceanic Fe data due principally to the often
Fe oxyhydroxide phase which passesthrough the lilters
severe contamination problems inherent in attempts
commonly used for sample l&ration (STUMMand
to measure sub-nanomolar concentrations of an eleMORGAN,198I ).
ment BSubiquitous as Fe (BRULAND,1983).
Reactive Mn and Fe oxyhydroxides associated with
Mn and Fe are brou~t to the oceans by fluvial,
kluvialparticles (Clm~!$,1977) and or@c-metal floce
aeolian, and submarine hy~~~~
pmcesses. Once
produced during estuarine mixin (SHOLKOVITZ,
1976,
introduced, they participate in a wide variety of bio1978) may be recyckd in estuarine or coastal sediments
geochemical processes. Mn and Fe may desorb from
(TREEREYand PRESLEY,1982). Mn and Fe oxyhyatmospherically introduced particles in the surface of
droxides may also be reduced in the ~o~h~i~
the ocean (HODGEef ai., 1978). Dissolved Mn and Fe
oxidation of dissolved organic matter (MILES and
may adsorb onto bio8enic and organically coated parBREEONIK,
1981; SWDA d af.. 1983), in orl@c-rich
ticles (BWSTREW@ai., 1981;MAR’ITN
and KNAUER,
reducing sediments (J;‘ROELIcH
e2 al., 1979), or under
1980, 1983, 1984) or oxidize during the formation of
suboxic water column conditions (KLINKHAMMER
oxyhydroxide coatings on bacterial capsules (COWEN
and BENDER.,
1980; LANorNo and BRULAND,1980;
and SILVER,1984; COWENand BRULWD, 1985). As
MARTINand KNAUER,1980,1982,1983,1984,1985;
required biockmical micro-nutrients, Mn and Fe may
GORDONet al., 1982; JONESand MURRAY,1985;
be inco~rated into organic tissue, perhaps to be reMARTINet at., 1985;MURRAY
et al., 1983;SYMESand
KELLER,1985; HONGand K~s+rf% 1986). Blevated
concentrations of reduced Mn and Fe are expelled from
* &SW address: Department of Oceanography, Florida submarine hydrothermal vents @UNKELWMER et al.,
state Uhrsity,
TN
FL 32306, U.S.A.
1977; WEISS,1977; -MER
el al., 1985; VON
INTRODUCI’ION
29
30
W. M. Landing and K. W. Bruland
DAMM et al., 1985). Within the oceans, dissolved and
particulate Mn and Fe will be mixed and transported
by turbulent diffusion and advection. Sediment resuspension will create nepheloid layers which can be
mixed both vertically and horizontally.
We wish to extract information regarding the oceanic
biogeochemistry of Mn and Fe from this complicated
background of physical, chemical, and biological processes. This paper presents the results of a comprehensive sampling and analytical program aimed towards
this goal. We have employed relatively contaminationfree sampling methods to collect dissolved, suspended
particulate, and sediment trap particulate Mn and Fe
samples from diverse regions of the Pacific Ocean. The
particulate samples were leached with 25% acetic acid
to solubiie reactive Mn and Fe associated with oxyhydroxides, carbonates, or adsorbed on particle surfaces; followed by total dissolution. By combining these
measurements of dissolved, reactive particulate, and
refractory particulate Mn and Fe with knowledge of
the hydrographic conditions in the study regions we
reach conclusions regarding the importance of some
of the physical mixing and biogeochemical processes
mentioned above.
THE STUDY AREAS
The samples for this research were collected on four oceanographic cruises in diverse regions of the Pacific Ocean as
shown in Fig. 1. The datesand coordinatesfor each vertical
profile station are given in the data tables (Appendices A
and B).
CEROP-I (Chemical Exchange Rates on Oceanic Particu&es) and VERTEX-I (Vettical Transport and Exchange) were
centered in the productive o!Fshore waters of the California
Current approximately I50 km west of the central Califomla
coast. The hydrographic features of this region have been described in detail by REID (1973a) and MARTIN et al. ( 1985).
In general, this region of the eastern North Pacific is characterized by seasonally h@h biological productivity in the photic
zone during the spring and summer months due to coastal
upwelling. In addition, continental drainage causes significant
input of dissolved and particulate fluvial material.
The hydrographic features of the VERTEX-II site In the
eastern tropical North Pacific have been described by WYRTKI
(1967). Compared to VERTEX-I, the surface waters of this
region can be characterized by relatively lower continental
input but comparable biological productivity, while the combination of respiration and sluggish vertical mixing creates an
intense oxygen minimum zone. Two stations were occupied
during the VERTEX-II cruise, VERTEX-II( 1). 5 km offshore
at the shelf/slope break; and VERTEX-II(Z). 350 km offshore.
The nearshore station was chosen to study the water column
directly overlying the reducing shelf/slope sediments located
within the intense oxygen minimum zone.
The Marine Chemistry 1980 cruise (MC-80) was a comprehensive sampling program covering major regions of the
Pacific Ocean. Samples from the surface mixed layer (upper
30 meters) were collected along an east-west horizontal transect
connecting the California Current region with the oligotrophic
central North Pacific gyre. This cruise track culminated with
a detailed vertical profile and sediment trap deployment at
station 17. The effects of several important biogeochemical
processes can be observed along this track. For example, the
effects of fluvial sources should diminish in importance with
increasing distance from the coast. In addition, reduced up
welling across the well-developed permanent pycnocline in
the North Pacific gyre limits primary productivity to a small
fraction of that observed nearer shore.
During the MC-80 cruise, samples were also collected from
the upper 30 meters in a latitudinal transect across the Equator
(20”N to 20’S along 160°W), connecting the North and South
Pacific central gyres, culminating in a detailed vertical profile
at station 31 in the central South Pacific. The hydro8raphic
features of this region have been described by REID (1973b).
At this station Antarctic Intermediate Water is present between
900 to 1000 meters, Pacific Deep Water between 1000 and
3500 meters, and Antarctic Deep and Bottom Water below
3500 meters. Surface waters of the central South Pacific gyre
are characterized by low productivity, extreme nutrient depletion, and minimal 5uvial input. Aeolian input is expected
to be diminished here due to the lower abundance of major
land masses in the southern hemisphere.
METHODS
Sample collection
Ftci. l.Loationofsamp&ings@tionsinthePacificCkean
(+). Detaikd vcrGcal pm&s were c&le&d at CEROP-I,
VERTEX-I, VERTEX-II, MC-80(17), and MC-80(31) (A).
Sediment traps ‘R~FC
deployed at VERTEX-I, VERTEX-II(2),
and MC-80( 17).
Seawater samples for dissolved and suspended particulate
metal determinations were cobcted and processed usit@ the
techniques described by BRULANDel al. (1979) and BRUAND
( 1980). Samples were collected in 30 L, Teflon-lined, General
Oceanics Go-Ho bottles and were hlteredthnW& scid-wpeh#l,
0.3 rrn (pore diameter). Nuclepore polycarbollatc membrane
filters using N2 overpressure. All water samples were acidif#d
toapH<2with4mLofsub&oiIingquartzdistilbd6M
HCl (Q-Ha) per liter of sample and stored in pre-ckaned
conventional polyethylene (CPE) bottles. Tbc filters bearing
suspended particulates were rinsed with I5 mL of pH 8.5 Q
H20, folded, and stored frozen.
Settling particulate material was collected using 0.25 m2
surface area, cone-shaped traps designed and built by Andrew
Soutar of the Scripps Institution of Oceano8raphy, U.C. San
Diego. This trap design was tested during the !%diment Trap
IntercalibrationExperiment, and yielded total partkukte flux
and particlecomposition in good ag~men twithothersimilar
trap designs (SPENCER, 198 1). In other tests conducted in the
Southern California Bight, the traps yielded particle fluxes
Fe and Mn ~~herni~
31
Ail solutions wereamdysed for trace element concentrations
using a pezkia-Elmer 5000 Atomic Absorption Spectrophotometer equipped with a HGA-500 Heated Graphite Atomizer
and an AS40 AutoSampler. Gptimixed coditi~n~ closely
approximated those sugguted by the manuiactumr. The technique of standam additions was routinely used to compensate
for matrix effects.
To correct the particulate weights for residual sea salts and
to determine the amount of CaCGs present in the samples,
sodium and c&ium analyses on the digest solutions were
performed by atomic absorption using a VarianAA-6 in the
flame mcde. Lanthanum (500 ppm) was added to diluted
aliquots of the digested solutions to reduce &ractory compound inky
and ionization during analy& Thecarbon
and nitroaen content of the VERTEX-I and VERTEX-11121
sediment&p samples was determined using a Perkin/Elmer
240-B CHN Analyxer. Inorganic carbon was determined by
differena (alter loss of CQ) following HCl(1 .OM) treatment
of subsamples and subsequent CHN analysis
Analytical methoa2
To investiluue the solubilixation of particles while in the
trap cod-ends, the supernatant solutions from VERTEX-l,
Seawaterand particulate samples were further processedin VERTEX-II(Z),and MC-80(17)were analyzed for Na and Ca
by flame AA, and for Fe and Mn by direct injection flameless
our shore-based clean laboratory (l&era& positivepressum
air supply and Class-100 laminar tlow benches) using clean AA. The concentrations wem then compared to those in the
techniques and reagents. After thawing and rewe@ng of the surrounding water, and patticle solubilixation was estimated
Nuclepore liltem, particulate samples were subjected to a 2 from excessCa, Fe, and Mn. The trap material Born 250 and
750 meters at VERTEX-I showed little dissolution (4-1696
hour, room temperature leach usiog 25% (4.5 M) quartxdistilled acetic acid IWIAC). This selective leaching procedure for Ca, I-6% for Fe and 0.3% for Mn). At VERTEX-II, the
degree of Ca dissolution incmased with deptb (from 7% at
100 meteis to 49% at 1470 meters), while the solubilimtion
and dissolve
some fraction of Al, Fe, and Mn oxen
(W
and HWHQ 1967; BOWERet ol., 1978; STUMM of Fe and kin witbin the traps was 2-30% and 2-16%, reand MORGAW1981). The residue from this treatment was spectiveiy.At the MC8OfJ7) oentral gym station the shallower
then dissolved in a Teflon d&s&r bomb at 1OO’Cusing the traps showed more s&star&l solubilixation of Fe (70-8096)
and Mn (lO-30%). For further calculations the trap supersequentialstrongacid tmatment (HCZ,HNOs, HF)described
by ECKWANand BETZER( 1976).In addition, subsamples of natant values were combined with the HAc fractions.
Reasonable agreement was observed in a comparison of
the sediment trap material f&n VERTEX-I and VERTEXII were subjected to total dissolution for subsequent compar- the CaCOscon&t of the trap material from VERTEX-I and
ison with the sum ofthe weak acid and strong acid dissolution VERTEX-II determined bv CHN analvsis and lmrn the excess
treatments ([HAc + REFl/fbtal - 87 f 18% for Al, 101 Ca found in the leachatesand tmp sup&mtant solutions: [HAc
+ Supematant CGr]/CHN CO, = 120 * 37%).
It 14%for Ma, and 98 f 14%for PC).
Dissolved Fe was concentmted Born 300 gmm seawater
subsamples using a mod&d vuxlon ofthe dithiocmbamate
RESULTS AND DISCUSSION
~q~tiq~
organic extra&on technique developed by BRULANDef al. ( 1979).The back extraction step was omitted, and
the two chlorofbrm extracts were evaporated to dryness in a
quark beaker. The residue was oxklkui witb hvo separate
100 4 additions of concentrated Q-HNOs, redissolved in i
This research was primariiy intended to elucidate
M Q-I-IN03and analyzedby graphitetkmace atomic absoipxome
of the input, internal cycling, and removal
tion spectmphotometry. The overall extraction efficiency,determinedusing nFe radiotracer,was 97 zk 1%(all pi-e&ions mechanisms and pathways for dissolved aud parkulate
are exmsd
as f la). The pm&ions for Fe analysis (based
Fe and Mu in the Pacific Ocean. The sampling sites
upon pooled relative star&d deviations) for the various and aualytical treatments were therefore chosen to fuhil
sample sets was k5.296 for CEROP-I. f7.1% for VERTEXthis objective.
I, +4.2% for VERTEX-II, and f3.29dfor MC-80. Proceming
The treatment ofparticulate samples with 25% acetic
Ma& for dkclved Fe averaged2.3 + 0.4 (n = 21) ng Fe,
yielding an analytical detection limit of 0.05 nmol/kg Fe (2~ acid allows some estimation of the abundance of
f 0.3 kg).
amorphous Fe and Mn oxyhydroxides and adsorbed
DimolvaI Mn was concentrated from 300 gram seawater Fe and Mn. The abundance of these HAc-soluble par~~~~~~h~~~~
tiq~~~do~c
extraction tech&ue desuibed by LANDINGand BRUIAND ticulate forms should increase in areas where dissoived
Fe and Mn are being scavenged to particulate form,
(1980).
~o~~~on~~~
of97 + 1%wasde~~~~~~~~O~~Mn.~~~
High ratios of HAc particuhue metal to refractory
precisionof the method wasestimatedas +4%.The precision (REF) particulate metal will be especially indicative
of the analysis has slme improved to f3.2% for VERTEX-I,
23.3% for VERTEX-II, and *2.6% for MC-SO.Pmcessina of those areas. In regions where Fe and Mn oxyhydroxides are reductively dissolved (for example under
blanksfordissolvedMnaveraged1.0~0.7ngMn(n=22),
sub-oxic or reducing conditions) low HAc/REF metal
yielding a de&&on limit of 0.09 nmol/ka Mn.
Bothdlmolved metal extraction me&&s yield an opera- ratios would be expected. In the following sections
tionally delIned meawrem ent. For dissolved Fe this would muchofthedatawillbedkussedintermsofthese
include all Fe(R), Fe@), and colloidal Fe(W) species which
pass through tbe 0.3 run filter. For dissolved Mn this would ratios since they are, in a sense, normal&d, and are
in&& all Mn(I1)and any Mn(II1)or Mn(IV) associatedwith indicative of the varying chemical conditions (espccolloidal particles.
ciitiiy redox) mcountend
in the difkent study regions.
and trace element compositions nearly identical to the accumulation rates and c4unpositions of underlying sediment+
(BRULAND
et ai., 1981).
Sediment trapsamplesfor traceelement analysiswerepreservedin situ with a mixture of crysWine paraformaldehyde
andsodiumboratecontninedinaMmLCPEbottllwitha
perforated cap. After recovery of the trap arrays, trap samples
Born the VERTEX-I and VERTEX-II cruises were pmcemed
as follows The bulk of the supematant from the trap codend (2 L total volume) was decanted, filtered, and saved for
amdysis. The remain@seawater and particulates were centr&ged at 1425 G for 30 minutes. Again, the supernatant
(250 mL) was decanted, &red, and saved, while the centrifuged particles were frozen until subsequent processing. Trap
samples from station MC-80(17) were liltered rather than
cenuifbged and the 6lters pmcemed as described below. An
aliquot of the &ate was also saved for analysis.
32
W.
M. Landing
and
Refiactov pa~ic~iate AI. Mn, and Fe
High suspended REF metal concentrations are useful indicators of fluvial and atmospheric input, and
sediment resuspension and transport. A discussion of
the absolute concentrations, general distributions, and
vertical fluxes of these materials is therefore useful in
later descriptions of dissolved Fe and Mn biogeochemistry. The sediment trap results from the three trap
deployments are given in Appendix A (Table A-l to
A-3), and the vertical profiling data from the various
study areas are presented in Appendix B (Tabies B-l
to B-6).
In general, the vertical ~~~butions of REF metals
at all stations exhibited near s*
minima, consistent
with biological packaging and rapid aggregate removal.
Stations near the North American continent exhibited
subsurface REF metal maxima, probably caused by
offshore mixing of resuspended shelf sediments. The
intermediate depth REF metal maxima observed from
300 to 1300 meters in the eastern North Pacific (VERTEX-11(2), Table B-2; CERC?P-I, Table B-3; VBRTEXI, Table B-4) may aiso be due to offshore transport
processes. A resuspension maximum was observed at
all stations where near-bottom sampks were taken. Finally, the absolute con~n~tions
of suspended REF
metals generally reflected their proximity to ~ntinen~
sources, with the lowest values found in the central
South Pacific gyre and the highest values found along
the boundary of the North American continent.
In general, the vertical fluxes of REF Al, Mn, and
Fe also reflected proximity to continental sources. The
highest fluxes were observed at the VERTEX-I site
(Table A- 1), while the lowest fluxes were found at the
MC-80( 17) site in the central North Pacific gyre (Table
A-3). At all three sites, the shallowest traps had the
lowest REF trace element fluxes.
It is difficult to estimate natural material fluxes for
sediment traps deployed in or near the photic zone.
One probiem occurs because these traps routinely coliect large quantities of obvious “swimmers” (i.e. zooplankton which enter the trap and are subsequently
killed by the preservative in the cod-end). Despite efforts to remove them, their presence may introduce
errors in the trap data from the photic zone. Zooplankton vertical die1 mi~tion,
often covering
hundreds of meters, can tead to packaging of particies
in the surface waters and the release of fecal pellets
and other aggregates at greater depths (kWSONS et al.,
1977), thus couture
a bypass mechanism. Furthermore, aggregate formation in mid-depth zones of
intense biological activity, as suggested by KARL and
KNAUER (1984), may explain the high metal fluxes
observed within and beneath the mid-depth REF particle maxima found at alJ three sites where sediment
traps were deployed.
Finally, particle fluxes measured with sediment traps
are expected to vary seasonally with variations in particle input and packaging rates. We are forced to base
our conclusions on the short trap sampling intervals
K. W. Bruland
(less than 21 days) used in this work. For example.
REF Al fluxes at i 50 to 250 meters depth off the California coast range from 15 to 1100 rmol/m” 1day
(Table A-l; MARTIN and KNAUER, 1980, 1983:
LANDING,
1983; MARTIN et al.. 1985) with the highest
values found in December. From March to September,
values ranged from 15 to 100 gmol/m2 +day. Thus, the
uncertainty introduced into the various calculations
to follow is d&cult to estimate, but may be at least a
factor of 3 to 5, and perhaps much more. This is. of
course, true of all short-term sediment trap deployments. However, since sediment traps generally are
believed to collect rapidly settling large particles and
aggregates, they yield fluxes which are probably consistent in a vertical sense, and which reflect the processes occurring over smail spatial scales and short time
intervals. Therefore, the errors involved in estimating
process rates and species residence times for processes
operating over seasonal. or shorter, time scales may
not be so large.
At steady-state, or at least over seasonal or annual
time scales, the input of refractory materials to the
water column must be balanced by Emoval, much of
which is mediated by the activity of f&r-feeding and
mucou~pr~ucing
zooplankton. The water column
residence times of suspended REF particles can be estimated by dividing the dep~-int~at~
REF Al, Fe,
and Mn concentrations by their respective sediment
trap fluxes. Table 1 shows that REF metal residence
times for the upper 100 to 250 meters ranged from 39
to 201 days, Averaging over greater depth intervals
yields longer residence times showing much better internal consistency at each station, as one would expect.
It is probably reasonable to conclude that the average
refractory particle residence time in the upper lQo0
meters of the North Pacific Ocean is on the order of 1
to 3 years,
In addition, there was relatively good ~rneat
between the REF Al fluxes measured in the upper water
Table I.
ticulate
relative
in uoltr
Resldence tinas calculated
for P?J~refractory
(REF) Al. lln, end F8
All valun are
to vertical
ramval.
of days unless otherwlre noted.
Depth
Site
mn)
REF
*1
REF
@In
RTF
Fe
V-I
O-250
O-750
180
300
200
310
88
210
Hc-80
(178
o-135
O-260
o-570
ATn
2::
I50
240
88
100
140
220
74
4w
O-100
I50
110
62
440
3.4yr
5.9yr
V-II
510
86
440
O-470
3.3yr
O-970
O-1470
6.0~
320
2.9yr
4.qrr
l
* Restdance tflvs
for totat Al, Iln, wxl Fe
lrput
flux (Arrmto
hawed on the atm&Iartc
et al., 1905) and tha btagratecl
partkutate
losd fraa 0 to 260 emters.
33
Fe and Mn bi~misuy
column at MG80(17) in the central North Pa&k gym
and recent estimates for aeolian input (AR3MOTO
et
al., 1985;Table A-3). However, BEF Fe and Mn fluxes
at 570 metem were about HI-fold bigber than those
measured at shallowerdepths. The BEF Fe/Al and Mn/
Al ratios of the trapped material Born 570 meters also
far exceeded those observed in the water column, or
expected from aeolian input ratios. Mid-depth horixontal transport of resuspended refractory materiai
loom the Hawaiian Islands chain or liom the North
American continent (as suggested by Fii 1 of CODISPODand RICHARDS,1976) may be responsible for
the slightly elevated BEF metal concentrations found
from 300 to 1500 meters at this station (Table B-5).
The bigber BEF metal fluxes measured at 570 meters
would therefore imply high mid-depth aggregate formation rates. Explanations for these apparently anomalous data may become clear as more measurements
of the atmospheric input and vertical flux of materials
in the central oceans become available.
Rim&e supply of Fe and Mn is an important source
in tbe oceanic budgets of these elements. MARHHand
W~~(1983)~av~riverwatrr~lved
Fe and Mn concentrations of 73? and 150 nmol/kg,
impectively. SHOLKOvnZ(1976, 1978)has shown that
95% of the dissolved Fe, and from 25 to 45% of the
dissolved Mn, may be converted to a reactive particulate form during estuarine mixing. Some conversion
of this particulate Fe and Mn hack to the dissolved
state probably occurs in coastal regions, especially in
?I QO
.
1
160.
150.
.
PII
$
14w
Langltud.
I,
sub&c or anoxic estmuine and coastal sediments
(TREFRIZY
and PRESLEY,1982).
Tbe distribution and ol%lmre (east-west) mixing
cmcs
of dissolved Mn and Fe in the surface
mixed layer of the eastern North Pacific am shown in
Fig 2(A) and 2(B), mspectively. High concentrations
of dissolved Mn ( 10 nmol/kg) were observed in the
mixed layer at nearsbore stations, decreasing to around
1 nmol/kg approaching the central gyre (140” to
145OWLongitude). Sligbtly bigber dissolved Mn concentrations were observed in the center of the gym ( 1
to 2 nmollklgat 150” to 140”W) possibly representing
evidence for aeolian input, as suggnxed by KLINKHAMMER
and BEHDER
(1980).The of&boremixed layer
distributions of dissolved Fe exhibited features and
concentrations consistent with the aqueous chemistry
of reactive Fe(II1) species. For several samples, relatively l&h concentrations of dissolved Fe were observed at nearshore stations (0.75 to 1.25 nmol/kg),
but these values were still an order of magnitude lower
than dissolvedMn concentrations in the same samples.
While the estimated dissolved Fe:Mn input ratio from
rivers is approximately 5: 1 (MARTINand WI%I=FIE~D,
1983),the nearsbore inanition
ratio observed was
about 1:10, demonstrating that dissolved Fe removal
in estuarine and coastal waters is of major importance.
Dissolved Fe concentrations remained relatively low
and constant (approximately 0.5 nmol/kg) moving
offshore with the exception of possibly contaminated
samples at station MGSO(l4) (155”W, Fig 2B).
A quantitative description of tbe offshore horixontal
transport of dissolved Mn in the surface mixed layer
of the North Pacific can be obtained using a simplified
.e &XX0
I
130.
(W)
20-s
IO.
0.
Latltud.
IO.
20.N
FIG.2. DissolvedNn and Fe in the surfactmixedlayer (A)DissolvedMn vs.Longitudein the Northmst
Fkcific:(0): this work,(0): LANDING
and BRUUND(1980). (B)DkrolvedFe VS.Longitudein the Nortkast
Pacific.(C) DissolvedMa vs. Latitudeacrwsthe equatorat 16PW I.mghde. (D)DissolvedFe vs. Latitude
aems the equatorat 1f50°WLmgitude.
34
W. M. Landing
and K. W. B&and
model similar to that of WEISS
relationship is observed between In (Mn) and X (distance
offshore in km), contrary to what one would expect if
dissolved Mn exhibited simple &t-order removal kinetics, By transforming the data as shown in Fig. 3(B)
a linear relationship was obtained between In (Mn)
and In (X):
adv~ion/~~on
(1.977).Fiiure 3(A) i&r&rates that a non-linear
In (Mn), = In (Mnkr L;m)
- m In (X)
(11
yielding a slope (m) of 0.63 f 0.04 and an intercept at
1 km of 93 nmoI/kg. This empirical relationship can
then be used {after taking a second de~vative~ in a one
~rnen~on~
~v~on/~~u~on
equation:
gMn)fdt
= K&(Mn)jitX’
+(J),.
(2)
In this model, a first-order functionality in dissolved
Mn (by adsorption or oxidation/precipitation
reactions) is not specified. The horizontal eddy dit%sivity,
Kn, is dependent upon scale-length (OKUBO, 1971).
The xero-order scavenging removal term, (& is determined by assuming steady-state conditions (d(Mn)/
dt = 0). All other mixing terms am implioitly combined
with the J term. Longshore advective and di&&ve
terms could be neglected by assuming that the longshore Mn inaction
gradient is small, however
these terms are also implicitly combined in J.
Table 2 contains the rest&s of this mode& Residence
times of dissolved Mn in the surface mixed layer as a
function of distance otiore are obtained by dividing
(Mn), by (J), and are on the order of years, increasing
from 0.4 years nearshore to about 19 years at I OOOkm
oft&ore.
The no~-~u~
~butio~
of dissolved Mn and
Fe in the surface mixed layer along 16O”W are shown
in Fig. 2(C) and 2(D). Dissolved Mn concentrations
were slightly higher in the North Pacific gyre relative
to the South Pacific gym, consistent with an enhanced
aeoiian source in the northern hemisphere. Rapid offshore advective transport from the eastern boundary
by the North Paciftc Equatorial Current is apparent
From the low salinity (not shown) and high dissolved
Mn concentrations (> 1 nmol/kg) at 7’N. In contrast,
-101
0 05
IO
Dtsm”S.
1.s
~1o’xml
2.0
2.5
I
40
so
6.0
L” ci~.,one*
TO
80
IkIn)
FIG.
3.(A)ln(~~
Mn) vs. Distaumfromsborcin
the surf&e mixed layer from MC-80(l) to (10). (B) tn 0%
sohed Mn) VS.In (Distance) from the ssme stations.
2. Relfdence
tim uicui4tlms
for dlSsolved
rrt4ttve to offshorehorfzont41diffuslm Fran
the cmtml Cellfomla -t
fnto the worth titflf Qyrc.
curv4turc
in tha carautr4tlon
gr4dimt
w41 dmtwmfned
ustng ths eflwlrIut fit desermed
in the text. V4Iw,
of the horizontateddy dlffuT4bim
tin
sfon cocfflcicnt
(It-c bl)5ed on ok&o
d2C/dX2
xfkm)
10
40
100
400
1000
~rmI/kg*kmz~
2.24XlO”~
5.80X10-!
5 2iix10-*
1:35x10-5
I.tlXlO-6
Kli
(kni/yr)
0.26~103
1.27x10!
3.76x10’
17.9
Xl03
51.4
x103
(1971).
tf-I
$nrcl/kg)
22
9
5
2.1
1.2
T
(yews)
0.38
1.2
2.5
8.7
!9
the north-south distribution of dissolved Fe shows no
discernable trend. Dissolved Fe concentrations were
~0.5 nmol/kg, indicating that dissolved Fe distributions in open ocean surface waters am governed by
insolubility and particle scavenging behavior.
The vertical profiles of dissolved Mn from the five
oi%hore stations occupied during this work atl exhibhd
dma
in the upper IO0 meters which were generally
associated with lower HAc/REF particulate Mu ratios
(Tables B-Z to B-6, Figs. $7,891.
The dissolved Mn
~~n~tio~
decmamd towards the base of the photic
zone as the HAc/REF Mn ratios inemaaed. Some of
the high dissolved Mn in the surface waters of the
northeast Pa&c stations is clearly due to fluvial input,
as described above. However, SUNDAet al. ( 1983) suggeated that photochemical reduction of particulate Mn
oxyhydroxides associated with organic matter oxidation may be responsible for sustaining the dissoiv&
Ma maximum and our data are consistent with this
pnX&?ss.
The vettic& pro&s of dissolved Fe from the five
o@hom stations generally exhibited tow concentmtions
in the surface mixed layer in association with rehitivcly
constant HAcfREF Fe ratios, suggesting that photochemical Fe reduction may be unimportant in open
Pacific surface waters.
The supply of dissolved Fe and Mn to the surface
mixed layer should be partly balanced by adsorption
or oxidative conversion to particulate forms (scavenging) and subsequent (vertical) removal by biokrgical
and physical processes. Dissolved Fe and Mn residence
times relative to particle scavenging can be estimated
horn the vertical fluxes of HAc Fe and Mn by assuming
that such conversion results in an HAc-sohrble particulate phase (MARTINand KNAUER,
1982). By dividing
the dep~-inky
dissohed Fe and Mn concentrations by their respective HAc Fe and Mn fluxes, the
residence times shown in Table 3 are obtained. These
are minimum estimates, since particles arrive to the
upper water column already accompanied by some
fraction of metallic oxyhydroxides (GIBBS, 1977; COLLIERand EDMOND,~ 983).
The residence times of particles were estimated earher from the drinks
REF Fe, Mn and Al
concentrations and their vertical fluxes (TabIe If.
35
Fe and Mn biogecckmistry
t
z
O-250
O-750
1.73
1.34
380
350
3.1
7.9
1.31
I.96
190
670
4.7
6.0
ffC-sO
($71
O-135
O-260
O-570
1.13
0.71
0.49
3.6
If
27
74
45
29
1.29
1.20
I.05
74
67
120
6.4
13
14
V-II
(21
O-100
O-470
O-970
O-1470
2.47
3‘93
2.56
1.82
62
22
23
21
2::
300
350
0.26
1.79
1.31
1.23
35
140
110
88
2.0
16
31
58
V-1
Comparing Table 1 with Table 3 for all stations and
all depth intervals shows that the dissolved metal res-
50
“,
8
100
I
200
i
sooov M
Dfmun
kao1/hg~
024660
HAMREf
fed
-
2
4
MswtW
6
Fe
%v-ie
0
Hle/REF
F.
fnmoflk91
FIG.4. %rticai pro&3
of dissolvedI&, particulateWAC/
REP Mn ratios,dissolvedFe, and particulateHAc/REF Fe
ratiosfrom VERTEX-II(
1) in the easterntropicalNorth PaCifiC.
idence times are substantially longer than the particle
residence times, suggestingthat the mte-~rniti~ step
in the scavenging and removal of dissolved Mn and
Fe is conversion of Fe and Mn to HAc-soluble forms.
The residence times for dissolved Fe are generally
shorter than those of dissolved Mn, once again consistent with the oxidation kinetics of Mn(II) and the
aqueous chemistry ofFe(III). The scavengingresidence
times for Mn at VERTEX-II(Z) am artificially long
due to redox dissolution of HAc particulate Mn as described in a later section.
In the surface mixed layer at VERTEX-I the residence time for dissolved Mn relative to replacement
via horizontal mixing input (>2.5 years) agrees well
with the dissolved Mn residence time relative to scavenging and particle removal output (3.1 years). The
higher level of dissolved Mn and Fe inmu balanced by
the high rate of particle scavenging and removal in the
VERTEX-I area results in shorter residence times relative to the North Pacific central gym regions.
Mn and Fe in oxygen minimum zones
Oxypn concentrations at mid-depth fell to extremely low levels in the eastern tropical North Pacific
(VERTEX-II) (Fig. 5). At the nearshore station, VERTEX-W 11,the oxygen minimum zone extended from
75 meters to the bottom (a~ro~mately 250 meters).
Vertical profiles of diilved Mn and Fe in this suboxic water column show evidence of dissolved Mn and
Fe release from the underlying reducing sediments.
Near-bottom water contained elevated inanitions
of dissolved Mn ( 12 nmol&g) and Fe (5 nmol/kg)
compared to near-sutiace water values (2.5 and 1.O
nmol/kg mspectively; Table R-l, Fig, 4).
The REF Al, Mn, aud Fe profiles at this station provided evidence of sediment resuspension in this relatively high-energy coastal environment. The HAc Mn/
REF Mn ratios in Fig. 4 show that the resuspended
particles am depleted in HAc Mn with respect to REF
Mn in the sub-oxic zone. The distributions of the Fe
forms are, however, quite different. The ~n~n~tio~
of suspended HAc Fe are large in the sub-oxic zone,
leading to high HAc Fe/REF Fe ratios.
At the of&ore station, VERTEX-R(2), a broad oxygen ~~rnurn layer was observed from 100 to 600
meters, mmpanied by elevated dissolved Mn (about
4 nmol/kg). Evidence for in situ Mn oxyhydroxide reduction can be shown by the extremely low HAc Mn
concentrations and low HAc Mn/REF Mn ratios (less
than 1) (Fig. 5, Table R-2). Results kom the sediment
traps deployed at this station (Table A-2) demonstrate
tbe recycling of l&genie materials, leading to loss of
organic matter and dissolution of carbonates with
depth. Reductive dissolution of HAc Mn within the
sub-oxic zone leads to a large decrease in the HAc Mn
flux between 100 and 470 meters. From Table A-2,
1.04 mmol of organic carbon and 51 nmol of HAc
Mn (per square meter per day) were recycled between
36
W. M. Landing and K. W. Bruland
100 and 470 meters. If the Mnforganic C relationship
summarized by LANDING and BRULAND(1980) and
measured by COLLIERand EDMONJJ (1983) (approximately 1 X 10e5 mol Mn/mol organic C) is multiplied
by the recycling rate of organic C it represents an additional dissolved Mn input of 10.4 nmol/m* *day
which should remain soluble under these suboxic
conditions. Below the suboxic zone, increased fluxes
of HAc Mn reflect slow dissolved Mn oxidative scavenging (from 6 to 13 nmol Mn/m* - day).
Dissolved Fe also showed a maximum in the suboxic zone at this offshore site, while the HAc Fe/REF
Fe ratios were relatively unchanged from the nearshore
site (Fig. 5, Table B-2). The vertical fluxes of HAc Fe
increased dramatically into the sub-oxic zone, while
the HAc Fe/REF Fe ratios of the trapped particles increased slightly (Table A-2). This suggests that the horizontal transport and packaging processes responsible
for the increased REF metal concentrations and fluxes
at 470 meters are also responsible for the increased
HAc Fe (and HAc Al) fluxes. The pE and pH conditions in this region must be such that Fe is quickIy
reoxidized and scavenged to a reactive particulate form
in the surficial sediments and sub-oxic water column.
The dissolved Fe maximum appears to be the result
of horizontal transport. Mn, in contrast, remains soluble at both stations and appears to be rapidly reduced
in situ from settling particles.
HONG and ~TER (1986) have recently reported
persuasive evidence for the presence of dissolved Fe(B)
in the water column overlying reducing sediments in
the Peru Upwelling region. Under environmental conditions very similar to those found in the eastern trap
ical North Pacific, this ferrous Fe accounted for as
much as 80% of the total dissolved Fe measured. Unfortunately, our dissolved Fe measurements represent
the sum of Fe(B), Fe(III), and any colloidal Fe (~0.3
pm), while their measurements did not include a study
of the solid phase chemistry of the particles.
The dissolved Mn maximum in the sub-oxic zone
at VERTEX-11(2) can be described using a vertical advection/d&ision model (CRAIG, 1974) across the linear
mixing region between overlying Subtropical Subsurface Water ( 150 meters) and Pacilic Intermediate Water
(750 meters) (WYRTKI, 1967; LANDING, 1983):
d(Mn)/dr = K&Mn)/az’+
wa(Mn)/az + J.
recycling and in situ dissolution of HAc Mn from rap
idly settling particles provides approximately 6 1 pmol/
kg - year or 54% of the necessary input required to balance the model.
An additional source of dissolved Mn to the suboxic zone must come from offshore horizontal mixing
from the Mn enriched sub-oxic waters overlying nearshore sediments. This source can be estimated by two
independent methods. The input term, J, is in fact an
implicit combination of several terms, which are constants at any given point (at steady state):
J= Jo + K,,[aZ(Mn)/aX2 f a2(Mn)/aY*]
+ ua(Mn)/aX + t@(Mn)jaY
(4)
where Jo represents a true input/scavenging term, Ku
is the horizontal eddy di&ivity (treated as a constant
and equal in x and y directions), and u and u are advective velocities. If dissolved Mn input from the
breakdown of organic matter and the redox dissolution
of HAc Mn (quantihed from the sediment trap results)
approximates JO,then the combined horizontal mixing
terms can be estimated by di&rence: 113 - 6 1 = 52
pmol Mnlkg - year.
Using the horizontal advection/ditR&on
model
(Eqn. 2), the offshore mixing input of dissolved Mn
can be calculated using three dissolved Mn pro&s
from the suboxic zone along 18“N latitude. Figure 6
illustrates that a linear relationship exists between In
(Mn) and In (x) using data from VERTEX-II( 1) and
VERTEX-ll(2)
(this work), and GEGSECS (343)
(KLINKHAMMERand BENDER, 1980). Using Eqn. 1, a
slope (m) of 0.206 f 0.003 and a (Mn), Lmof 14 nmol/
kg are obtained. This relationship is then used to calculate the curvature in the horizontal dissolved Mn
gradient at VERTEX-II(Z) (350 km oflkhorc):
$(Mn)/aX’
= 8.5 X 10e6 nmol/kg km’.
(5)
Using a horizontal d&sivity of 15,390 km’/year
(OKUBO, 197 I), an offshore diffusive input of 13 1 pm01
Mn/kg . year is obtained. This value is about a factor
of 2.5 higher than that estimated by difference for the
3.01-----l
1
(3)
Using steady-state assumptions, and the temperature/
salinity distributions reported by BROENKOW and
I&ENTZ ( 1982), a value for Z*, the ratio of the vertical
diffusivity (K,) to the upwelling velocity (w), was obtamed. Data from eight stations in the immediate vicinity of the sediment trap array gave a value for Z*
of 0.58 -t 0.06 km. Assuming an upwelling velocity of
3 meters/year (CLINEand RICHARDS,1972) and solving
for J/w, the rate of change (input) of dissolved Mn at
3OOmetersisJ= 113+12X 10-‘2mol/kg~yearQmtol/
kg. year) which must be balanced by loss through vertical mixing. Dissolved Mn input from organic matter
Fro 6. (A) hi (Dindved MD)vs. Diaana from s&se (km)
in the sub-&c zone from VERTEX-II(I A VERTEX-II@).
and GEOSBCS(343). (B) In (Dbolvad Mn) VS.In @irona)
from the same stations.
balaua among all horixomal mixing terms. However,
the nature of the sub-oxic zone in the eastern tropical
North Pa&c is such that horizontal mixing in the
longshore (y) direction must result in a net losp of dissolved Mn from the mid-depth hum,
as it would
also in the casts of the nitrate defkkncy and oxygen
minimum itself(W
and RICBARDS,1972; COLW~TI and RIQWWS, 1976). U~o~~~ly,
the sampling conducted for this work caunot be used to quanmixiug losses.
tifyl
W
various input and removal estimates for dissolved Mn in the sulxnck zone at VERTIIX-II(Z) are
clearly within the same o~~f-~tu~*
This suggests that the most important input, mixing, and removal terms have been identified and accounted for.
These results may be compared with the box model
presented by MAR’~~N
and KNALJFZRf1984) for Mn in
this same area. While the rate-of+hange estimates for
dissolved Mn in the sub-oxic zone were greater in the
box-model (due to the depth intervals chosen and use
of a faster upwelling veloeity)~MARL and KNAI.JHX
(1984) arrive at the same conch&on that offshore mixiug is at least of the same o~~~f-~~de
as in situ
Mn redax dissolution.
The residence time for dissolved Mn in the sub=oxic
xone mlative to horizontal difIhsiveinput at VERTEXR(2) is 30 years (-4 nmol/kg 4 131 pmol/kg*year),
in reasonabk agteement with the 20 year estimate
made by MURRAY etal.(1983) using a similar modeling treatment on data from the suboxic xone in the
Guatemala Basin, approximately 1000 km soutlt~
of the ORYX-II(2) site. The residence timer&sive
to vertical mixing loss (also relative to Jo plus all horizontal mixing terms) is about 35 years (-4 nmol/kg
KNAUER(1984)
meinthesesame
yq
yielding a weighted
avemge of 38 yeam over the 106 to 700 meter depth
interval.
The o&horn gradient of dissolved Fe in the sub-oxic
layer implies that horixomal mixing is also a source
for dissolved Fe at ~R~-~(2~.
~nfo~u~~ly, we
only have data from the two VIIRTEX-II stations so
that question
of this process is not possible.
In the northeast Pacific, at the ofkhore California
Current CEROP-I and VERTEX-l sites, the oxygen
minimum zone (between 500 and 1500 meters) is
rn~~~~~~~~~~c
(Pii 7). As might be expected, dissolved Mn exhibited
only a slight or no mid-depth maximum (Tabks R-3
and I%+ F& 7), while the HAc Mn/REF Mn ratios
wem slightly lower within the oxygen minimum zone.
Thessdiment tmpsamplea from VERTEX-I showed
some similar katums to those from the VERTEX-R(2)
site (Tabk A-l). The total part&da
carbon, and orpanic nitrogen &es
as mate&s were recycled within the water coiumn. Retween 250 and 750 meters, a factor of 3 to 4
increase in the RIIF material fluxes (as described earlier), a amaU deqreese in the HAe Mu flux, and a 1~
increase in the HAc Fe flux was observed. As at VERTEX-II(2), oue can calculate the minimum dissolved
Mn input in the inter@ between 250 and 750 meters
due to organic matter meyding (50 nmol M4rn**~y~
and in situ HAc Mn slisolution
(48 mu01 Mnl
m’ - day).If this dissolved Mn input estimate (98 nmd
Mn/m2 9day)once again approximates JOthen a steadystate baknce can be set up where all vertical and horizontal mixing terms must be equal and opposite to
Jo.This~el~adi;lrsohrcdMnresidenoetimewithrespect to in sitardissolution inpuh or mixing outpu& of
16 years, about a factor oftwo higher than the 8 year
residence time caI&ti Born dissolved Mn scavenging and HAc Mn partick mmoval for the entire O-750
meter depth interval (Tabk 3). Other estimates for d&
solved Mn residence times in the intermediate waters
of the Cal%ornia current
from lSto33year-s
(MARTIN and IbrAuER, 1980,
198% MARTIN et id.,
1985).
Dissolved Fe concentrations also exhibited slight
maxima within the oxygen minimum, associated with
incmasing HAc/REF Fe ratios. This provides no evidence for in situFe dissolution, and the generally increasing HAc/REF Pe ratios in the suspended particulate material suggestongoing scavenging(Fs 7). The
HAc/REF Fe ratios in the sediment trap material also
increased into the oxygen minimum, supporting t&s
~o~ntioo, The dissolved Fe maxima am therefore
believed to refkct of&bore transpofi which is clearly
shown by the REF metal inanitions
(Table B-3
and B-4). The residence times for dissolved Fe relative
to removal as HAc particulate Fe from the upper 750
meters at VEiRTEX-Iwere approximately 6 years (Table 3).
The M~~ql7~ site southwest of Hawaii in the tentral North pacific gym has an even less intense oxygen
meters
aslight
maximum between 500 and 1000 metws, but once
again the HAc/RRF metaI ratios showed evidence of
38
W. M. Landing and K. W. Bndand
(weak-acid soluble) Mn and Fe solid phases. in the
bottom waters of the central gyres, resuspended HAc
Mn and Fe account for 35% and f 78, respectively, of
SO
what would be considered “total dissolvable” Mn or
100
Fe (TIN or TDFe) (Tables B-5 and B-6). At the
CEROP-I and VERTEX-I sites these percentages in3
crease to 70 to 90% for TDM, and 30 to 60% for TDFe
c
,p so0
(Tables B-3 and B4). SYMES and I&.STER(1985) found
1000
that about 80% of the TDFe in the deep waters of the
mid-Atlantic bight slope region was due to resuspension, The need for sample filtration is obvious. if one
woo
wishes to obtain this most basic “speciation” da@ on
Ll-4..uAi..,,hU
Mn and Fe in the oceans. In addition, careful study of
0 loo 200 0 03
I
0
20
40 0 03
1 0
0.2 0.4
oluomd@
5bwhudkta
WAc/f@f Mn -d
h
HA.c.‘REF F.
the filtered particles themselves (such as the selective
t~ml/kgb
Imnol/hg~
Inmallkgl
leaching treatment used in this work) yields useful info~ation
regarding the im~~ance
of various biof%G. 8. Vertical profilesof dissolvedoxygen,averagedissolvedMn, par&late HAc/REF Mn rati% averagedissolved geochemical processes throughout the water column,
Fe, and particulate HAc/REF Fe ratios from MC-80(17) in such as possible photochemical reduction of Mn and
the central North Pacific gyre.
Fe oxyhydroxides in surface waters, dissolved Mn and
Fe scavenging under oxie conditions, and reductive
ongoing Mn and Fe scavenging.The verticaifhtxesof Mn and Fe dissolution under sub-oxic conditions.
Little evidence for hydrothe~~
injection of disHAc and REFAI,Ma and Fe aIIincreasedwith depth,
so&d Mn or Fe was observed in the deep waters at
although the HAcmF Mn and Fe ratios decreased
VERTEX-H(2) despite its proximity to the East Pacific
somewhat at 570 meters (Table A-3). From these results
Rise (Tabie 3-2, Fig. 5). The I-IAc/REF Mn and Fe
we conclude that the diqht dissolved Mn and Fe maxratios in the suspended matter were somewhat etevated
ima are supported by horizontal mixing input. This
in the 2ooO to 3ooO meter depth interval, consistent
supgwxts
the
cuncltions
drawn
by MARTIN el al.
with hydrothe~~
input coupled with water column
(1985) for Mn, where the verti& fluxes of reactive Mn
oxidative scavenging, or with resuspension of hydroincteased with depth at a station in the central northeast
thermally enriched sediments (cl: BCXGER etal.,1978).
Pacific, while the REF Mn fluxes remained nearly
A similar TDM enrichment can be seen in the data of
constant.
~IN~AM~ER
and BENDER(1980) from the nearby
The re&denee t&es for dissolved Mn and Fe relative
GEOSECS(324) station.
to removal as HAc soiubie
Etam the upper 570
The apparent dissofved Fe maximum at 2500 meters
meters ranged from 30 to 70 yean for Mn, and from
may be real, in which case we would have expected a
6 to 14 years for Fe (Table 3). hdAltTlNetaf.(1985)
much larger signal in the dissolved Mn profile. It may
calculated a dissolved Mn residence time of 10 years
more iikely be due to reactive Fe oxyhyd~~de coUoids
in the dissolved Mn ~rn~rn of&e centralnortheast
which pass through the 0.3 pm filter and are subsePacific, with ho&o&al diilved Mn input as the
quently determined as “dissolved” Fe. This process
dominant source term. Using their data in the same
manner as in Table 3, we cakulate a dissokd ‘Mn
residence time rciative to scavet?gingand vertical removal of 11 yearsfor the 0 to 500 meterdepth interval.
These values are &mcrallylonger than the residence
times c&uWed near shore, and most likely re&ct reductions in dissofved metai input, biological activity,
and suspended particle loed at these ceWa! gyre sites.
In genera& the lowest dissolved Mn captions
were found in the kttomwaters
atatl
of the offshore
@rationsalso
sites (Figs. 5,7,8,9)
at these sites.
generallydecreaxd
Thii simply reflects the ongoing scaveuging and removal of these metals from the water column. The
concentrations of HAc Mn and Fe also increased in
0
loo 200
Oi,,cttwd~
t~wl/kq)
0
0.4
0.e 0
OlwokdUn
tnmoilkq)
so loo
WAeIREF
u,
0
0.1
Oluohd
Fe
Inmollkq)
WcloeF Fe
the bottom waters, ~~
that *
FW. 9. Vertical profiles ofdissolved
oxygen, average dii
Mn and Fe near-bottom maxima SK&
~~M~,~~M~~~a~
by ~~~A~ER
and BENDER (1980~ and SYMES Fe, and particulate HAc/REF Fe ratios from MC-W31) in
and KESTER (1985)are due to ~n~on
OfrractiVe the central South Pacific gyre.
Fe aad Mu biogeochemistry
may also have contributed to the dissolved Fe maxima
found in the sub-oxic zone at this site and at the nearshore VERTEX-II(l) site. However, resuspension of
deep-sea sediments rich in HAc Fe at this station, sad
at CEROP-I aad l&K-80( 17), did not contribute to the
measured dissolved Fe. Therefore the presence of colloidal Fe aray only be important in regions where active
Fe(R) oxidative scavenging is occurring.
The vertical pro&s of Mn and Fe fonns in deep
water from station MC-80(31) deserve special notice
(Fg. 9). Roth dissolved Fe aad Mn showed broad maxirna between 1000 and 3500 meters in the southwardflowing Paciftc Deep Water m
which lies between
Antarctic Intemrediate Water (at 1000 meters) aad
Antarctic Deep Water (at 3500 meters) (&ID, 1973b).
These consistent enrichments are higher than at comparable depths in the North pacific gym, as are the
HAc/REF Mn and Fe ratios, precluding in situ redox
dissolution in these well-oxygenated waters. As this
station lies approximately 100 ion southwest of the
Cook Islands, it is possible that these enrichments reflect horizontal transport from the flask sediments of
these i&ads, or from the many other seamounts aad
islands in the region.
CONCLUSIONS
Dissolved Fe
The data presented here represent some of the tirst
detailed, open-ocean dissolved and particulate Fe
meesurements. Improvements in sampling aad aaelytical procedures have reduced contamination problems such that reliable values can now be obtained.
Thus, it is possible to provide aa initial description of
the distribution sad biogeochearical behavior of dissolved Fe in the Pacilic Ocean.
The vertical distribution of dissolved Fe in the California Current and central North Pacific gyre appears
to be coatrolled by pert&-reactive
scavenging behavior throughout the water column, uptake sad removal
from the photic zone, and little or no regeneration with
depth. A small mid-depth dissolved Fe maximum, associated with the oxygen minimum zone, appears to
result from horizontal transport.
In the eastern tropical North Pa&c, input from sub
oxic or reducing shelf/slope sediments produces high
dissolved (or perhaps colloidal) Fe coacentrations. Recause %spet&d perticulate HAc Fe concentrations
were also high in these weters, we believe that elevated
dissolved Fe concentratioas reflect Fe reduction in sub
oxic sediments followed by rapid oxidation in surlicial
sediatents or in the water column, leading towards
quasi-equilibrium conditions between dissolved Fe(III),
colloidal Fe(III), end amorphous (HAc-soluble) oxyhydroxide phases. The slight maximum in dissolved
Fe at 300 meters depth further otlhhore (VERTEXII(2)) indicates that lateral transport can mix this feature over limited distances.
The lowest near-surf&e dissolved Fe conceatratioas
were observed in the central South Pacific gyre, con-
39
tit
with reduced ae&m and fluvial input. Elevated
dissolved Fe (and dissolved Mn) coacentrations were
observed between 1000 and 3500 meters in the Pacilic
jeep Water ma~~. exceeding ceatral North Pacilic deep
water gyre~vahes, aad suggesting a horizontal trenspott
source.
Dissolved Fe measuremeats sad HAc Fe fluxes from
the sediment traps were used to estimate dissolved Fe
residence times with respect to particle scavenging and
removal from the upper 500 meters of the water column. In the California Current residence times were
short, from 1 to 6 years, while in the eastern tropical
Pacific residence times were 16 to 20 years. In the central North Pacific gyre, dissolved Fe residence times
ranged from 6 to 14 years. These values are short relative to oceanic mixing time scales aad rellect the high
reactivity of dissolved Fe toward oxyhydroxide formation and particle scavenging.
Dissolved Mn
The distributions of dissolved Ma presented here
illustrate sad quantify a number of input, cycling, and
removal pmcesses which govern oceaaic Ma distributions. Residence times of dissolved Mn in the upper
30 meters of the North Pacific, relative to horizontal
mixing input, were extremely short nearshore (ap
proximately 138 days at 10 km o@hore), sad increased
to about 20 years in the central gym. Good agreeareat
was obtained between these values aad those estiarated
for particle scavenging aad removal using HAc Mn
vertical flux data.
The latitudinal distribution of dissolved Ma in the
upper 30 meters demonstrated two phenomena. First,
concentrations were about a factor of two higher in
the central North Pacific gyre than in the central South
Pacific gyre, reflecting diminished aeolian and fluviel
sources to the southern oceaa. Second, dissolved Mn
concentrations were eievated in surf&e waters in the
salinity minimum associated with the North Pacific
Fquatoriel Current, suggesting rapid advectioa from
the eastern boundary (where high near-surface Ma
values were observed).
The vertical distributions of dissolved Ma exhibited
elevated concentrations at intermediate depths where
dissolved oxygen minima occumd, substaatiating
earlier reports. Elevated dissolved Mn ia the oxygen
minimum zone in the northeast Pacific is, in part, due
to in situ recycling of organic matter aad dissolution
of Mn oxyhydroxide phases, in approximately equal
proportions. The dissolved Mn residence time relative
to nxlox dissolution input or mixing output is 16 years.
Advectioa/difRrsion modelling predicts that vertical
mixing loss of dissolved Ma in the sub-oxic layer in
the eastern tropical North Pacilic is balanced by equal
amounts of in situ dissolution (lYom rapidly settling
trap materiel) and horizontal mixing input. Dissolved
Mn input from the recycling of organic matter accounted for less than 209b of the in situ dissolved Mn
production, with redox dissolution of Mn oxyhydrox-
40
W. M. Landing and K. W. Bruland
ide phases accounting for the remainder. The dim&&
Mn residence time in this sub-oxic zone is 35 years
relative to vertical mixing removal. A horizontal model
to quantify the offshore difbive mixing terms yields
a residence time relative to horizontal diffusive input
of 30 years.
work was made possible by grants
from the National Science Foundation (Gram Numbers GCE
79-19928, GCE 79-23322, and GCE 82-16672)for which the
authors am grateful. Most of these results were coBectedand
utilized by W.M.L. during the completion of his Ph.D. in
Chemistry at U.C. Santa Crux. ‘Ibe ~n~butions and comments from past and present members of the “Marine Biogeochemistry Collective” at U.C. Santa Crux including Dave
Ameberg, Kenneth &ale, Jim Cowen, Greg Cutter, Geoff
Smith, and Chtistv Weeks-Tabor am also matefullv acknowiedged.Hein de B& Harry E&&&Id, R&Franks;Gary Gii,
Kristin orians, and Peter Statham provided constructive editorial comments. We am also grateful for the amistance rendered by the captains and crews of the R/V Wecoma and the
R/V Thomas G. Thompson during the sampling cruises.
Acknowledgements-T
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G. P.. BENDERM. L..
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331
<I
73
477
4.6
NAC m (mall
r5lEerMtvlt
m WwlJ
REEF m (-1)
Tota1
I& In&l,
HAc/REF
m
MAC Fe f-1)
Sqerrat&nt
Fe
REF fe (muI)
Total
Fe @mot)
HAc/EF
Fe
A-2.
vEEX;f”“J’
18%.0~N
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S. A. and HARVEY
G. R. ( 1983)
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J. L. and KESrERD. R. (1985) The distribution of
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B. J. (1982) Manganese fluxes
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Vertical fluxes of particulate materials measured with sediment tmps for this study. Total Fe, Mn, and Al fluxes were
determined on separate subsamples.
FIuX
M%TEX-I;
9-07-60.
36°30.0’N
IZ4°01LIYlfr
Inwp*rlc
hrtfCUtrt0
SworM
8-27-80
1470
(IMJ
73
1.44
0.17
36
0.40
0.03
29
0.25
0.02
16
0.14
0.01
(IvaJ
c (rmlJ
(-1)
MC Al olmol)
AEF Al (MD11
Total
Al (~1)
(al)
w
ca$
c
f-1)
Iall
750
W
250
5.19
1.09
95
0.828
0.116
I.0
0.812
0.041
0.379
0.366
0.071
285
a640
15500
(-1)
c&Q
cd
0.28
0.24
0.02
0.12
0.13
0.04
0.15
0.06
0.05
0.06
r-1)
(al)
NAc Al WmoIJ
REF Al (ml)
Total
Al (reel)
124
1366
1466
@fw=Fm
tlAc Fe (md)
swwmant
fe
Xizf fe (mml)
Totai
FC frail
IUefREF
Fe
REF
UEF
16.3
0.9
4.3
18.1
3.93
0.096
170
3.3
13fO
1710
0.132
76.6
4.6
796
945
a.164
35.2
4.3
730
935
0.034
0.27
0.002
0.29
0.002
0.26
0.002
0.46
0.003
2:::
1.40
z
Fe/Al
m/At
I31
Fiw
9.9
1.4
24.0
11.2
rmt)
cd2dny-‘J
1245
z:
HAc
XEF
(M
(ml)
(~1)
)(Ac m fr*ptf
suwrwtant
=f
m I~IJ
lMc/REF
m
tIAc Fe (mlJ
wwIuwlt
XEF Fe (ml)
Hk/REF
Fn
REF
REF
133
fm6)
Al
Al
fe/Al
m/Al
3::
m
Fe
onco~
UwlJ
182
1580
2030
23.4
0.6
1.3
36.1
3.29
52.7
9.7
3.0
s8.s
20.8
NAC m (mot)
swememtmlmlJ
RET ?h (-1)
Total
m (ml)
267
3070
3300
411
4470
5000
for
flux
total
14°40.S’N
160aOl.3’W,
dmta -.
taken
Al.
Fa. and m.
3.8
1.9
s.4
I.1
10.3
36.8
330
0.14
0.99
0.016
73
20
56
305
220
1330
15.3
2.
3.4
5.1
fnn
570
hJ
260
h)
to
250
frnf
c
Inorgmfc
Pa-tfcul8t~
swwMblt
Rrtlcln
(n**-‘J
Rrtk1n
owmtc
WltlqN!
fllpl
(ralj
(and)
to
11-10-81
970
MJ
Tale
A-3.
Mmrln
ChrIstry-BO(I7):
IO-03-60
ta 10-09-60.
A-k
Arimoto
ti al.
(I%51
ud
WV
APPENDIX A
i&t8 A-l.
c
108°00.0wi
470
(SJ
(d2dsy-‘)
PartlCIes
Q-anrc
Nitmgnn
0.329
0.002
(I@)
100
Flux
0.467
0.003
4040
3445
REF Fe/AI
REF m/Al
Table
0.047
900
11
t2000
12400
0.076
(nnolf
Planet.Sci. L&t. 41,77-86.
SPENCER
13. W.
750
(mJ
250
(MJ
(m-‘dw-‘J
I
19.7
40.2
6W
0.10
I.20
0.007
36.3
3.5
37.0
1.1
133.1
33.7
5940
0.03
4.47
0.026
I260
4.5
275
0.22
0.004
42
W. M. Landing and K. W. Bruiand
Vertical pr5fites of: total suspended matter (TSM): 25%
acetic acid soluble (MAC)and rn~~~~ (RI%‘)part&date Al,
Mn, and Fe: dissolved Mn and Fe from stations occupied far
Table
B-1.
VEWIEX-II(f);
19%2.Y’N
104°34.9*W,
Ai
t(nJ
IS
30
30
75
lO&
130
230
Talc
REF
nhc
rief
228
237
77
3.26
4.29
0.66
0.84
2.42
4.91
3.t4
164
236
32
37
62
169
98
1.75
2.65
0.42
0.20
0.14
0.15
0.10
0.31
0.38
0.05
a.07
O.OP
0.32
a. I8
~mTEJl-tlf2)~
lE”PB.O’N
lB8*OB,O%t
IS
TSH
f&c
tfJ7
0.03
a.99
0.03
O.Ln
0.06
o*B4
0.07
0.0x
0.14
a.29
0.29
0.3%
0.33
5.23
0.13
0.17
0.18
d.22
0.21
0.20
0.29
0.21
0.24
0.36
0.45
O”57
l .Of
:x v8
04
45
106
z
13
tBB
125
lS0
z
39
68
63
72
fE
300
400
500
MI0
VSO
lOBB
1250
IVW
2Bw
226B
2500
2730
3BBo
3250
3500
Tfa
z
:t
35
43
46
38
6s
29
36
5s
49
31
38
32
27
mrrvr
2s
SO
75
800
250
!5aO
1%
IWO
13Bo
l3BO
1750
At1
All
2.w
J2.B
0.33
1.32
1.07
0.43
t.tll
0.77
1.11
3.71
3.72
4.63
7.23
3.12
2.97
3.39
3.04
s.12
3.71
4.05
4.30
3.61
3.34
4.32
4.69
4.93
7.65
uiss.
arr*.
1.11
i.60
0.32
0.47
2.69
IO,6
3.6Y
31.1
60.3
f.3
8.6
14.0
44.9
26.5
4.14
3.14
2.8!
4.14
10.3
12.1
7.3D
1.13
I.13
Q.66
1.1s
2,711
5.71
4.46
II-IQ-81
fa
II-IWII.
F*
t?w
IiAe
REF
0.02
0.07
0.02
0.10
O.tl
0.15
0.13
0.2D
I)“02
<a.001
0.0@2
<o.oui
O.Wl
0.040
iwo9
o&54
lLw3
o.all2
0.01
0.38
It.26
s*z
o:oB4
z:
0:007
:z
0:w
B.BB6
O.il
0.13
0.13
0.13
8.16
0.32
0.29
0.66
0.62
0.61
0.93
:z
a:fJm
0.006
0.005
0.813
o.w#l
0.001
a.006
6.OllP
B.Bo6
O.D20
0.012
0.020
0.079
0.04
o.us
Nn
fl)
RiSS.
Itfss.
3.69
0.21
t::
0.21
0.22
a.04
0.03
0.02
0.87
0.2a
0.13
0.20
0.80
0.22
a.18
0.27
6.20
0.21
0.24
0.49
0.23
0.50
P.%
0.86
0.82
0.74
0.78
0.19
0.22
0.17
0.76
I .25
1.36
I .63
2.75
0.93
0.90
1.m
0.76
1.31
0.94
0.79
0.99
0.w
1.12
I.17
1.14
1.34
z.4a
2.66
2.41
2.17
I.86
2m
3.M)
3.70
4.32
3.w
4.13
3.13
1.09
0.37
0.29
0.38
2.30
4.01
3.18
2.49
2.27
2.34
2.i4
2.18
2.13
l.74
3.87
3.83
4.32
3.99
3.93
3.14
1.29
iJ.37
0.43
0.33
0.25
0.38
0.32
0.28
0.14
0.11
0.13
r%.%J 0.38
0.02
il.SYJ
0.33
f0ASJ
(11.3)
3.32
(4.W
1.29
2.67
2.44
1.74
1.00
1.10
1.2s
Lt.84
I.Si
3*28
B&3
B.tQ
0.01
0.32
0.37
9.29
(9.19)
3.27
I.33
2.72
1.49
2.36
2.15
1.92
D&4
0.91
1.29
I.23
1.02
1.61
1.33
2.63
1.04
0.79
0.93
0.72
ml/kg
3.96
49.7
26.8
13.3
7.49
0.03
0.0s
0.36
0.36
0.10
It.07
0.56
o*s9
d.40
0.91
0.82
0.44
13.8
15.2
22.1
16.2
13.5
10.7
:=g
Oh
B.Be
0. IO
0.14
B.79
D.$B
12,4
11.2
:::
0.93
i*O?
16.9
16.8
OtherI
Fe
R&F
MC
a.07
1.52
0.6%
0.43
0.29
0.31
69
68
81
54
36
34
37
9V
3u
79
E
3poo
3580
3980
ml
ethwr
il2f
Ml
MAC
m
Al
Ztaf
F.?
liAc
B2.
11-07-81.
ttn
7%
2:
134
87
this study. paired data represent replicate G&h samples.
NA: sample not anaIyze& ( ): sample beiicved to be contaminatwi; 42 sample known to be co~~rnina~.
f.43
Z:Z
mt/*a
a.004
P.oos
0.099
o&33
0.042
B&23
0.691
fJAt36
0.092
B.03lJ
0.034
0.032
0.831
O.lUP
O*Wf
0.035
0.46
0.49
le.2
8.3f
3.28
3.111
S:Z
0.61
0.22
0.17
0.18
0.34
0.3%
0.52
0.46
0.4t
0.24
0.36
0.34
0.44
0.36
::f;:
8.47
6.41
4.85
4.21
3.96
3.9a
6.20
4.115
3.59
4.10
0.60
0.38
2.49
0.86
0.64
l&4
0.93
0.82
0.82
0.75
0.55
0.62
0.55
0.31
0.35
0.20
::g
1.25
I.66
f.%6
I .81
2.13
l.96
1.93
I .61
1.64
I.78
I.12
0.72
43
Fe and Mn bi~hemi~
T&l* 54.
WtTEx-lg 36*3o.ow
124%.0wr
25
it
25
SO
7S
loo
IS0
250
SOS
730
loo0
IO00
IS00
2000
2300
En
20s
194
222
183
64
43
41
12s
39
*07-80.
FC
WC
REF
MAC
REF
ffAc
fEF
<O.OI
0.87
1.33
0.62
3.67
2.07
1.07
0.52
0.94
7.62
a.77
9.56
22.9
21.4
17.7
10.4
9.21
10.9
0.021
OmS
-C0.059
0.021
0.2%
0.622
0.457
0.092
0.051
0.046
o.ff70
0.074
0.073
O.lSO
0.070
0.005
0.003
0.005
oJfe4
0.002
0.003
0.033
0.021
O.Of6
0.017
0.044
0.037
0.031
0.017
0.014
0.017
0.007
0.012
-C0.010
o.oa5
o.ofm
0.017
a.112
a.133
0.165
0.407
0.176
0.210
0.172
0.179
0.309
0.30
0.19
0.39
0.25
0.19
0.15
0.31
2.02
2.05
2.39
5.32
4.82
3.81
2.27
2.03
2.49
-c-
&* I4
0.07
0.02
0.10
0.22
0.25
0.33
i.07
0.85
0.80
0.37
0.37
0.63
::
16
34
40
40
23
to
nil
AI
Lwl)
6-27-90
m
fc
010,.
Diss,
NA
3.03
3.28
3.e9
2.97
2.27
1.04
1.47
I.11
1.14
1.20
0.96
a.83
0.43
0.51
0.41
2.77
2.09
1.02
1.40
I.31
1.17
1.10
O.%
0.54
0.61
0.41
1.19
0.78
i4A
0.20”0.14
NA
1.81
1.49
1.89
1.49
3.10
2.49
1*77)(Lil.7s
1.41~1.26
I.84
I.41
1.22
1.36
734:
lrble
E+5.
!fwin
Cfmmlstry-8Wl7~1
l4*4#.W*
f60°01.38Wl
tO-03-W
to
le-o9-8&
--__
?fn
Al
2Wf
7w
Fe
tUc
REF
If&
REF
fUc
REF
0.03
-c-c0.13
0.10
0.09
0.12
0.05
0.10
0.08
0.06
0.m
O.II
0.29
0.14
0.02
0.57
1.30
1.00
O.%
2.44
0.88
0.84
0.68
0.55
0.99
1.50
3. I4
0.002
-c-C0.091
0.083
0.093
0.138
0.048
0.037
0.031
0.0%
0.024
O.ff44
0.077
0.000s
0.023
0.14
0.42
0.42
0.32
0.45
0.46
0.89
other:
nmrlclj
sfc
20
24
30
::
:
13
300
500
loo0
iSo6
2ooe
2soo
3OW
3see
4000
4Soo
4900
=*
fable
2:
I2
IO
8
9
13
59
32
w/k9
8-6.
All
Warin@ Chanlstry-8of31);
l9059.lf’S
AI
zm
m
If&
0.0007
0.0012
0.0036
0.0031
0.0027
0.0065
e.oo2f
O&S6
O.fmlS
e.ee39
O.#e16
O.w(16
0.0135
:;_
0.043
0.063
O.iOff
::z
0,120
0.093
0.120
O.%i
0.061
0. I38
fSS”59.3W
en
le-PI-80
F*
REF
HAAC
REf
tf&
REF
o.eo4
0.004
0.046
0.069
0.064
0.059
o.oss
0.0019
0.0013
O.eoll
o.oois
em07
0.0010
O.eoI
0.0019
O.eoPO
O.ool8
0.0016
e.owcI
0.014
0.019
0.017
0.140
0.013
0.033
O.OlS
0.070
0.113
0.071
o.om
0.132
e.io
0.21
0.08
O.fS
0.13
0.17
0.14
0.23
0.16
0.15
0.14
0.50
34
If
22
(0.01
0.03
0.02
0.23
0.21
0.07
ft
I9
16
I1
I3
IS
IS
7
0.01
0.15
0.02
0.03
0.05
0.05
0.1
0.05
0.11
0.15
0.19
0.30
0.34
0.39
0.31
0.36
0.31
1.29
0.m
0.m
o.eSs
0.039
0.099
1
Fe
OISS.
Di%*
0.73
0.97
0.77
O.%
1.20
0.J2
0.19
0.42
0.37
0.22
0.21
O‘lf3
0.19
0.16
0.16
0. I3
a. 14
0.43
O.%
0.23
Ef
0.35
0.30
0.33
0.31
I.25
sfc
30
loo
zoo
300
500
7%
I000
IS00
2lW
2eoa
3Sea
ffn
0.19
to
0.51
0.30
I.00
1.33
Y4) -c‘a20
0.92
(2.20)
1.M
o.82
-c-C0.73
0.19
-c0.6s
0.56
0.89
1.25
1.06
0.75
0.95
em
0.79
0.39
0.59
0.44
0.55
10-24-80.
?fn
fC
Dfss.
@I%,
0.49
0.53
0.69
0.31
0.26
0.16
0.20
0.19
0.2s
0.24
0.26
0.23
0.18
O.%
O.%
0.63
0.32
0.24
0.18
0.14
0.13
0.23
0.26
D.26
0.22
0.20
0.52
0.65
0.38
0.12
0.55
0.39
0.55
0.47
0.06
I.01
0.97
I.02
(6.23)
0.51
0s
0.37
fL2o
0.62
0.37
0.63
0.0
e.%
0.80
1.12
O.%
0.77