Diurnal fluctuations of reduced iron concentrations, expected to occur in reduced sediments in the photic zone, were studied. Iron concentration was compared to O2-H2S, a microcanalysis of sulfate reduction was performed, as well as an examination of diurnal concentration of dissolved phosphate and changes in interstitial CO2. The iron profiles suggest a strong correlation between iron remobilization and processes occurring in the light. Phosphate profiles suggest the removal of phosphate is strongly correlated with precipitation of oxidized iron in the upper 2 mm to 5 mm of the sediments. Pore water CO2 concentrations and carbon isotope ratios are presented. These data are from the analyses of minisediment cores collected from the 42 per mil salt pond and incubated in the laboratory under light and dark conditions

Haddad, R.

Shaw, T.

English

NASA Technical Reports Server

N85 - 32 1 4  4 
-. .. 
Mi croanal yris of Di os01 ved Iron 
Hypersaline Sediment 
and Phosphate in Pore Waters of 
R. Haddad and  T. Shaw 
Int roduct i on 
Reduced it-on, s o l u b l e  ir, a c i d i c  pti. has. a n  i n t e r m e d i a t e  
r .eJux p u t e r t t i a l ;  i t  may serve  as an i n t e r m e d i a t e  e lectron 
a c c e p t o r  - .donor  betweerr ($2 and ’3-. F u r t h e r  more, 
r e d u c e d  iron is a b u n d a n t  i n  many .Fresh w a t e r  and m a r i n e  
s e d i m e n t s .  l r o n  may s e r v e  as a n  a l t e r n a t i v e  e lectron d o n o r  i n  
c y a n o b a e t e r i  a l  p h o t o s y n t h e s i s  ICohen, p e r s o n a l  c o m m u n i c a t i o n ) .  
D i u r n a l  f l u c t ~ i a t i o n s  of r e d u c e d  i r o n  c o n c e n t r a t i o n s ,  
e x p a c t e d  to o c c u r  i n  reduced s e d i m e n t s  i n  tk,e p h o t i c  z o n e ,  w e r e  
s t u d i e d .  I r o n  c 0 n c e n t . r - a t i a n  w a s  conipar-ed t o  
O*:B-+i&, a micr-oanaly5is o f  s u l f a t e  r e d u c t i o n  w a s  
per-Cor-med, ac-, we1 1 a5 an exant i rsat ion of  d i u r n a l  c o n c e n t r a t i o n  of 
d i s s u l v e d  p h o s p h a t e  and  c h a n g e s  i n  i n t e r s t i  t i a l  COz.  
Materials and Methods 
t o r e  Water I r o n  D e t e r m i n a t i o n  
D i s s o l v e d  i ron  c o n c e n t r s t i o n s  i n  s e d i m e n t  p o r e  waters w e r e  
tJeter-tnii.ic.d co1or ime t . r i ca . l  l y  by t h e  f e r r o z i n e  method iStoukey, 
197i:b. modi t i e d  b v  Mur-ray a n d  G i l l ,  1978). F u r t h e r  mod i+ ica t ions  
of t.he c r r i y i n a l  method were n e c e s s a r y  t o  w a r ) :  w i t h  t h e  l i m i t e d  
samplr .  s i z e  (0.5 m l i  i.i.jecf w i t - h  t h e  m i l l i m e t e r  p o r e  water method 
d e c x r i b e d  in the f o l l o w i n g  c i ec t ion .  4 s m a l l  amount (i3.3 m l )  o f  
s a m p l e  w a s  added  t a  0- 1 m l  oi: f e r r o r i n e  r e a g e n t ,  0 .1  m l  
tiydroxylarnii-re I iydroc t i lo r - ide ,  and  1 m l  of M i l l i Q  p u r i f i e d  w a t e r .  
The .jolc.ciion r e a c t e d  +or 11:) minutes i n  an  800C water b a t h .  
The samples w e r ’ e  a1  lowed t o  cool t o  room t e m p e r a t u r e  and  0.1 m l  
o f  ammuniuni a ce t a t e  b u f f e r  w a s  added .  The a b s o r p t i o n  of t h e  
r e s c i l  t i riq i r-on cump 1 e>: w a s  measured  a t  562 n a n o m e t e r s  1 n a 0.5 ml 
m i  c r u c e l  1 .  H stai rdar-d cur  v e  and b l a n k  w e r e  measu red  w i t h  e a c h  
set c s i  p o r e  water p r - o t i l e c .  
I”hospha tc nssjays 
I ) i ssc i lvcd  p h o s p h d t e  c ~ ~ - t c : e n t r a t i o n s  i n  p o r e  w a t e r - s  w e r e  
t J t - ! t e r m i n e d  c o l o r - . t r ; i t . t . r  i r a l  1 y b y  t h e  s t a n n o u s  c h l o r i d e  methad .  
!:)ev.er*i*l. modif i c e t i ~ n ~ j  t o t h e  method w e r e  x lopt rd  t o  a l low 
d e t e r m i n a t i o n  us.ing ci .  1 ml ot sample .  An a c i d i c  i i o l u t i o n  
c o n s i s t i n q  of ( 3 .  1 m l  of cob-lcentr-ated b . C l  i i i  100 m l  of d i s t i l l e d  
water w d ! ~  u s e d  t o  d i l u t e  the p o r e  water s a m p l e .  The s t a r t n o u s  
c h l o r - i d e  s o l u t i o n  was d i l u t e d  1:5 w i t h  e t h a n o l  t o  allow more 
acc.cir-L+te h d d i t i c m  04 5o.Lut ion.  1’0 1 m l  of acidic s o l u t i o n  w e r e  
added 0. 1 m l  c ~ f  s a m p l e ,  ~ > . ~ 3 2  m i of m o l y b d a t e  r - e a g e n t ,  and  0.03 m l  
ut k;tr\nncxis c h l o r i d e  s a l u t i u n .  A f t . a r  ten m i n u t e s  the a b s c w p t i o i i  
ot: the . ;o lu t : ion  was measured a t  690 n m .  
183 
https://ntrs.nasa.gov/search.jsp?R=19850024431 2020-03-20T18:19:58+00:00Z
Carbon Isotope Rat io  Determination Methods 
Sampling Procedure 
3 t o  5 cm-long sediment cores were co l l ec ted  usinq 1 1.2 cm 
diameter a c r y l i c  core tubes. These core tubes were s p e c i a l l y  
designed; one end was tapered f o r  easier sediment penet ra t ion  and 
the other end w a r  faced o f f  f o r  p rec ise  sect ioning. The cores 
were returned t o  the  San J o s e  State Un ive rs i t y  l abo ra to r ies  
w i t h i n  th ree  hours o f  c o l l c c t i o n  and incubat ion processes begun 
i mmedi ate; y. 
Extruding Procedure 
For s u m  COZ and fe r rous  i r o n  analyses, the  cores were 
t rans fe r red  i n t o  a n i t rogen atmosphere glove bag f o r  processing. 
Procedural blanks ind ica ted  t h a t  t he  carbon d iox ide  concentrat ion 
w i t h i n  the  qlove bag was l e s s  than 0.6 nM. I ns ide  the glove bag, 
cores were extruded and sectioned i n t o  1 mm or 2 mm sect ions 
using a .036 cm giasis re in fo rced  epoxy sheet. The e r r o r  was 
estimated t o  be 1 mm p lus  o r  m i n u s  0.5 mm sections. Pore water 
was obtained v i a  u l t r a c e n t r i f u g a t i o n  f o r  IC1 minutes of the  s l i c e d  
sections, t rans fe r red  and cent r i fuged a second t ime t o  insure  
p a r t i c u l a t e  removal. 
1 o t a l  Carbon D i  o x  i de Fore Water Froredure 
CI known volume of t he  p a r t i c u l a t e  f r e e  pore water was 
t rans fer red  by syr inge from the  cen t r i f uge  tubes i n t o  15.75 m l  
Fierce  crimp-top v ia l s .  These were immediately crimped us ing 
septa and tear-away aluminum crimp taps. The stoppered v i a l s  
were removed from the  glove bag and stored a t  -76 deqrees 
cent igrade u n t i l  i s o t o p i c  analyses could be done (0.5-1 day). 
Vacuum L ine  Procedures 
P r i o r  t o  I so top ic  analysis,  t he  f rozen pore water samples 
were allowed t o  e q u i l i b r a t e  t o  ambient room temperature 
(approximately 25 "C). Two m l  a f  1 M s u l f u r i c  a c i d  were 
added t o  l i b e r a t e  t h e  t o t a l  carbon d iox ide  as carbon d iox ide  gas. 
A 2 M s u l f u r i c  ac id  p l u s  2 M cuprous; s u l f a t e  s o l u t i o n  was 
experimented w i th  i n  a n t i c i p a t i o n  of h iqh  concentrat ions of 
su l f i de ;  however, the lack  of any C:uS p r e c i p i t a t e  demonstrated 
t h a t  the concentrat ion o f  hydrogen s u l f i d e  i n  the  upper 1 c m  of 
sediment would not i n t e r f e r e  w i t h  the  i * s C / ~ T  
i so top i c  measurements. The COz produced upon 
a c i d i f i c a t i o n  VJas p u r i f  isd.  iso la ted,  measured .for volume. and 
co l l ec ted  using v a c u L i m  l i n e  techniques i n  Dr . David DesMarai~,' 
carbon isotope laboratory  a t  the IWASU A m e s  Research Center (see 
PBME, 1382, NASG Techn:l ca l  Memorandum 86043 f o r  de ta l  1s) . 
184 
m m A L  PAQEvG 
OE MOR QUALITY S t a b l e  Isotope Mass Spectrometer Procedure 
ixC/*’C i s o t p i c  n. asurements warm made on a 6” 
r iucl ide s t a b l e  i sotope mass spectrometer and are pr-esentrd i n  the  
usual de l  r io ta t ion re.frrenced t o  the  Pr~edeea belemnite (PUB) 
standard (See Table 1-2 p 1 .  
Paraeatramitus and Manganese-Oxidizing Bacterra 
For methods 5ee Read, e t  51, 1983. Manganese? acetate p l a t e s  
were inoculated i n  the cci?nt.er w i t h  1 mm” a+ sediment 
sample and f looded w i t h  1 m l  dirati1lei.l water f a r  each 1 mm depth 
fur the 42 per m i l  s i te .  
Results 
Pore water f r o m  the  upper centimeter of the 42 per m i l  pond 
was analyzed to determine the  cuncentrat ione of dissolved i r o n  
and phosphate. The i r o n  p ro f  i les suggest a stronq core1 a t  i on 
between i r o n  remobrl izcit ion end processes occur.ring i n  the l i g h t  
(see Fig. I V - 1 3 ) .  Phosphate prof  1 les (Fig. I V - - 1 4 )  suggest t he  
removal af phosphate 1 %  L;troogly cor re la ted  w i t h  pr-ecipi t a t i o n  af 
oxid ized ir-on i n  the  upper- 2 - 5  mm 0.C the sediments. Phosphate 
concentrat ions w e r e  hiqher i n  the  dark cyc le  cures where the  i r o n  
d i f f u s i o n  yr-adient to the uxiciized zone of  the sediment i s  a t  a 
m i n i m u m .  A1 thoiryh phosphate removal is associated w i th  i r o n  
removal, there i s  l i t t l e  evidence f o r  phosphate rcntobi l i - .-at ion 
associated w i t h  iron r t~mobi l iz tat icm. Fhoephate ..,C)ear~ 4.0 dif+use 
upward from below. T h i s  observation suygests that  the mechanism 
of  i r o n  r -e tnu t i l i ra t ion  dur ing  t.he 1 i g h t  c y c l e  rJues nut  i nvo l ve  
reduct ion of ox id ized i ron.  Further., the+ c u r e l a t i o n  of i ron 
rewubi 1 i z a t i o n  w i t h  1 i g h t  c.ytrlu proces.;oo Lauygeit t h a t  tlis 
tiret.hanism i).F i r o n  mobLl i ra t ian may be the r e s u l t  of an oxidative 
pr cxesis. 
Pore w a t e r - z C O z  concerrtr-ations and carbun isotape 
;.-atiCjs are pr-es-nted CFiaure I V - 1 5  and I’d-17).  These data ar.e 
Fr o m  the  anaiyfies of m i r i i  -ssdilnont: cor-Ezs cu l l ec ted  from the 42 
per s i 1  salt pond and incubated i n  the ldboratury  under l i q h t  and 
dark t:a~iditrons. The dc.1 ta 13C values i n  the  
l ight - incubated core docrease from -5.25 0.19 per mi1 at 0 - 1  mm 
to ..7.i13 per m i l  a t  : ! -3  n11 and iricr’edse back to abaut - b per m i l  
a t  6--10 mm. iiJtre!n sampling t r s o l u t i u n  i s  decreased to 2 mm 
i n t e r v a l s  i f iyures LV-15. 1.6, and 171 the niaJor dawn core t rends 
Fine-scale rstructure of the pro41 le chanaos. 
at heavv 002 # a t  the 51JlF.fiiCf2 dt-e st.1 11 trvldeflt bu t  the 
T h e  l a r y e  rJi f turence between t h e  d e l t a  13C values i n  
the 0 - 1  and 1-2 nlm sect iuns meascrrcd in the l iyht - - incubated cores 
i s  not presunt i n  t.he dar 1. -.icicuba.teJ cores. I n  these cures, the 
isotope values are  . t i n i r l y  cunatant over the  upper 5 mml thev  
i nc rea~ct  from 5-7 mm arid l e v e l  o f f  i n  ,the 7-10 mm region. 
185 
40- 
30- 
20- 
60- 
so- 
w -  
W t h  Y Depth Y 
186 
0 
187 
' 2  
Y 1 4  
r)) 
6 
8 
delta G O J  per 
2 
4 
4 
B 
10 
m i  1 
1 
2 
n 
E 3  
" 4  
(u 
S 
6 
7 
8 
9 
ml 1 
R 
light 8' hours 
mil) 
dark 6 hours 
Figure IV-IS. hmpariwn of concontration of d i r w l v d  
Fo** and delta ~ ~ C O Z ;  i n  the. $ntcN.tit$.l W r t W .  
189 
0 
1 
? 
3 
CI 
f :  4 
Y 
i: 
4 
111 
0 
a s  
b 
7 
8 
9 
10 
I9 -h 10 
. c  
1 
2 
3 
e 
Y $ 4  
r 
4 a 5  
P 
m 
4 
7 
F i g u r m  IV-16. Light cyc18 delta CCOa 
prO$i18. 
F i g u r m  IV-17. Dark cyclm delta <COa 
prof i lm. 
190 
ThecCOa pore water concentretions arm presented for 
the 2 mm sectioned cores only in Figure IV-1s. In the 
light-incubated core, there is P gradual d-reaw in=- 
concentrations from 3.89% 0.14 raM at 0-2 mn to 2.112 0.43 mM at 
7-8 mm. No such trend is evident in theCCOp 
concentration profile in the dark-incubated core. In this core, 
CO~C ranges from 2.742 0.86 mM to 4.082 0.37 mM. 
Colonies of manganese-oxidizing bacterie appeared in all 
samples studied. Millimeter samples showed the most activity for 
both Parsfetramifus and manganese oxidizers at 6 millimeters 
below the surface (Table IV-4). 
Partial oxidation of sulfide in the form of iron monosulfide 
may explain these iron profiles. The presence of elemental 
sulfur in the upper centimeter of these sediments (Chapter 111) 
confirms the assumption that the sulfide is being oxidized in 
this zone but the presence of dissolved iron to depths of 8-9 
millimeters suggests that very little free sulfide is available 
for oxidation in this tone. 
An important aspect of the proposed mechanism is that iron 
monosulfide in the presence of elemental sulfur reacts to form 
pyrite, the ultimate sulfur sink in the sediments; these two 
reactants, however, are not usually found in the s a m e  sedimentary 
regime. The mechanism suggests that in this environment 
elemental sulfur and iro? mono=iAlf ide can exist concurrently. 
Furthermore, rater of pyrite Armation in these sediments are 
greatest in the top centimeter (see Chapter 111). 
Recent studies on microbial mat communities using 
microelectrodes have demonstrated that large gradients in oxygen 
(0-200 uM), sulfide (0-200 uM) and pH (1-9) exist within 
the upper 1-3 mm of the mat (Jorgensen 4et al., 1983). These 
profiles have been shown to be supported by microbially mediated 
processes ( @ . g o ,  photosynthetic COS fixation, Wa 
respiration, sulfate reduction, etc. 1 and they exhibit diurnal 
variations. One interestinq observation from the6Q studies is 
that both oxygen and sulfide coexist in low concentrations (200 
uM and 50-100 uM respectively) in a narrow zone within 
the upper few mm of the mats. This, chemical boundary is usually 
exploited by sulfur-oxidizing bacteria such as Beggietoe and 
flcxibacteria yet results from recent experiments using a new 
35-S-sulfotc reduction determination technique 
qualitatively demonstrate that sulfate reduction is occurring in 
thihi microaerophilic environment at significant rates (Y. Cohen, 
personal communication). In this microenvironment, 
photosynthetic products (amino acidr, short-chained f8tty acids, 
etc.) excreted by the phototrophic bacteria may be utilized as 
carbon sources by sulfate-reducing bacteria. 
Microbially-mediated degradation processes such as sulfate 
reduction and fermentation are assumed to be primarily by organic 
19 1 
matter produced i n  s i f u  in the mat communities. 
The results of the O a  end sulfide profiles and pore 
wrtersCCap concentrations and carbon isotopic values do 
not support this hypothesis. Instead they argue that .vqanic 
matter from the overlying water column limits degradative 
processem. During the light cycle, oxygen is present only in the 
upper 1-2 mm where active photosynthesis occurs. Concurrently, 
sulfide does not appear to be present in the pore water until 6-8 
mm depth and even then only in trace quantities (less than 0.1 
mM). During the dark period, oxygen is present in much lower 
concentrations, and sulfide apparently is present in trace 
amounts up to the sodiment/water interface. 
A comparison of the light and d a r k z  COa carbon 
isotope profiles (Fig IV-15) demonstrates a siqnif icant diurnal 
variation in the upper 1-2 mm due to the photosynthetic activity. 
During the light cycle, relatively light carbon dioxide (carbon 
dioxide enriched in "C with respect to lSC) is 
fixed, thereby enriching the remaining sum carbon dioxide pool in 
ascarbon dioxide 4 t h  respect to '"carbon dioxide. 
This results in an increase in the delta 13C value. 
During the dark cycle, carbon dioxide is not fixed and 
respiration by heterotrophs produces light carbon dioxide thereby 
decreasing the delta '=C sum carbon dioxide value in the 
pore water. 
Directly below the photosynthetic zone, the proposed 
coupling of sulfate reduction with photosynthetic activity should 
result in lighter delta 13C z C O z  values in the 
pore water during the light cycle (when the light - roughly 30 
per mil - photosynthate is being produced). The light/d--rk 
profiles demonstrate that the pore water delta 13C 
COZ is actually lighter during the dark cycle presumably 
due to heterotrophic (fermentative) activity, and the absence of 
photosynthetic activity. 
These isotopic data suggest that the degradation processes 
rrediated b y  microbes are driven not by in situ 
photosynthetically derived organic matter, but by allochthonouc 
organic matter (presumably from the overlying water column). 
This view is supported by t h e r  COS and delta 13C 
values in the overlying water column. Comparing the measured 
values with predicted values based on concentrating seawater to 
42 per mil, the measured COS concentration values range 
from 10-25 percent greater than the predicted values. Also, the 
delta '=C values in the water column are 5-7 per mil 
lighter than the predicted value of about 0 per mil. In 
addition, there appears to be a net export of>zCO= from 
the sediment to the overlying water column, completing a type of 
mini-carbon cycle which, for the most part, bypasses the 
processes of sedimentary COa fixation. 
192 
A final curiosity is tho increase in theCCOa delta 
profile in the, 6-8 inm interval in which no diurnal 
variation is seen. These isotope values may be indicative of 
some type of chemoautotrophic COS fixation. Although thio 
w w l d  explain tho heavy shift in isotope, no significant 
concentration change is seen. There, appears to be a slight 
decrease inlCOI concentration in the dark profile# the 
precision of the light profile io such that no trend is clear. 
If those isotope profiles are superimposetd upon those of the 
dissolved ferrous iron it can be 0een that the maximum pore water 
ferrous iron concentration occurs in a zone where 
chemoautotrophic uptake of COn is proposed. This maximal 
zone (6-7 mm in the light) seems to support the highest 
concentration of manganese-oxidizing bacteria and their 
predators, P .  jugosus (an amoebomastigote, table I V - 5 )  . 
In summery, the)3COz, isortope, and concentration 
profiles measured in a mm interval demonstrate that the upper 1 
cm of this environment is extremely dynamic. There appears to be 
no direct relation between photosynthesis occurring in the mat 
community and microbially mediated degradation processes. 
Rather, the sedimentary microbial processes appear to be driven 
by allochthonous organic matter. TheZC01 concentration 
and isotopic composition reflect microbially mediated degradation 
processes occurring within the sediment. In conclusion, w e  
propose that the upper 10 mm of this environment can be viewed as 
having three geochemically distinct zones: 1) the upper 0-2 mm 
dominated by the diurnal variations in photosynthesis, 2) a 
middle 2-4 mm zone dominated by heterotrophic activity, and 3) a 
deeper 4-10 mm zone in which some chemoautotrophic activity is 
occurring together with ferrous iron and phosphate geochemical 
reac t i ons. 
193 


Microanalysis of dissolved iron and phosphate in pore waters of hypersaline sediment

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