[American Journal of Science, Vol. 309, October, 2009, P. 633– 660, DOI 10.2475/08.2009.01]
ELEMENTAL RELEASE RATES FROM DISSOLVING BASALT AND
GRANITE WITH AND WITHOUT ORGANIC LIGANDS
E. M. HAUSRATH*,**, A. NEAMAN***, and S. L. BRANTLEY*
ABSTRACT. Bacteria, fungi, lichen and plants all produce organic acids, which can
strongly affect weathering by increasing the solubility and mobility of elements.
Leaching by organic acids may therefore produce trace element signatures which could
record the presence of life in the rock record from early Earth. To elucidate this
effect, long term column experiments were performed with powdered granite and
basalt with and without 0.01 M citrate at pHⴝ6 for 45 weeks. Both granite (8.44 ⴛ
10ⴚ13, 3.39 ⴛ 10ⴚ13) and basalt (2.94 ⴛ 10ⴚ14, 6.47 ⴛ 10ⴚ14) dissolution rates mol (Ca,
Mg) (mⴚ2 sⴚ1 respectively) were enhanced in the presence of citrate relative to the
organic-free controls: granite (3.17 ⴛ 10ⴚ14, 4.4 ⴛ 10ⴚ15) and basalt (1.01 ⴛ 10ⴚ14,
1.04 ⴛ 10ⴚ14). Enhanced release of individual elements in the presence of citrate was
strongly correlated with the stability constant of the citrate-element complex. Elements
which might be useful as biosignatures are those elements that showed a strong
enrichment in the presence of citrate: Zr, Sc and Mn (basalt), V and Zn (granite), and Y,
La, Ce, Th, Ti, Al, P, Pb, Ni and Fe (both basalt and granite). Release of these elements
from the rock material in the columns is consistent with dissolution of apatite ⴙ Fe
sulfides ⴙ Fe oxides ⴙ augite in basalt and apatite ⴙ sphene ⴙ hornblende in granite.
Similar groups of elements have been reported to be enriched in organic-rich rivers,
suggesting leaching of strongly-complexed elements could be useful as biosignatures
and may have left mineralogical traces on early Earth.
Key words: Basalt, Granite, Weathering, Citrate, Biosignature, Trace elements.
introduction
Biota affect chemical and physical weathering by many mechanisms. One important mechanism for enhanced chemical weathering in the presence of biota is the
secretion of organic acids. Bacteria, fungi, lichen and plants all produce organic acids,
which can strongly affect weathering by increasing the solubility and mobility of
elements. However, the effects and mechanisms of organic acids remain an area of
active research and debate.
A significant body of work has focused on mineral dissolution in the presence of
organic acids (for example, Drever and Stillings, 1997, and references therein). Some
minerals which have been dissolved in the presence of citrate include feldspar, quartz,
augite, muscovite, kaolinite, illite, hornblende, apatite and phosphate rocks (Huang
and Kiang, 1972; Manley and Evans, 1986; Bennett and others, 1988; Lundstrom and
Ohman, 1990; Zhang, ms, 1990; Bennett, 1991; Welch and Ullman, 1993; Kpomblekou-a and Tabatabai, 1994; Stillings and others, 1996; Zhang and others, 1996; Blake
and Walter, 1999; Zhang and Bloom, 1999; Welch and others, 2002; Wang and others,
2005).
However, the mechanism by which organic ligands enhance dissolution is still not
known. The effect of ligands on dissolution may be due to the direct effect of the
ligands on the mineral surface, whereby they polarize and weaken the bond between
the cation and mineral lattice (Furrer and Stumm, 1983). According to this model, the
overall rate of dissolution is therefore the sum of proton-promoted dissolution and the
* Department of Geosciences, The Pennsylvania State University, University Park, Pennyslvania 16802,
USA
** Present address: Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, USA;
[email protected]
*** Facultad de Agronomı́a, Pontificia Universidad Católica de Valparaı́so, Casilla 4-D, Quillota, Chile
633
634
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
ligand-promoted dissolution. The equilibrium constant for sorption of ligands to a
mineral surface increases with the equilibrium constant for formation of the ligandmetal complex in solution (Sigg and Stumm, 1981). However, indirect mechanisms
have also been proposed, whereby the complexation of the ion in solution indirectly
affects the dissolution of the mineral surface (Oelkers and Schott, 1998; Kubicki and
others, 1999).
Drever and Stillings (1997) in their review of the effect of organic acids on mineral
dissolution, particularly feldspars, have found that a concentration of at least 1 ⫻ 10⫺3
M is necessary to cause a significant effect on dissolution. Concentrations of organic
acids in soils are difficult to measure (Banfield and others, 1991), but some reported
values for bulk soils include 1.5–3.0 ⫻ 10⫺5M (Shen and others, 1996) to ⬃1 ⫻ 10⫺3 to
⬃4 ⫻ 10⫺3 M for aliphatic organic ligands in modern soils (Stevenson, 1991). Values
measured near tree roots can be much higher (Jones and Edwards, 1998).
If organic acids are affecting mineral dissolution in field environments, such
weathering might be a valuable biosignature, or signature of life. Microbial biosignatures can take a variety of forms, as recently summarized by Fisk and others (2006),
including body fossils, stromatolites, trace fossils, reduced or oxidized minerals,
alteration of geochemical cycles, fractionation of stable isotopes, chirality, metabolic
byproducts, and organic molecules. Banfield and others (2001) have extensively
reviewed mineral biosignatures. They discuss the interpretation of phases, trace
element, major element, and isotopic compositions, surface composition and morphology, crystal morphology, particle size, spatial arrangement, aggregation state, and the
presence of organic molecules that may indicate a biogenic origin. Extensive evidence
has documented that micron-sized tunneling may be due to microbial action (Fisk and
others, 2006) and that mineral pitting (Buss and others, 2003) and element depletion
on the surface of a mineral (Kalinowski and others, 2000; Hausrath and others, 2008)
can result from biotic impacts. Hausrath and others (2007) also tested the effect of
methanogen growth on glass dissolution to determine whether a Ni biosignature
might occur. They demonstrated that the observed increase in Ni release in the
presence of methanogens was not due to the presence of cell exudates, low molecular
weight organic acids, lysates, direct cell-mineral reactions, such as biofilms or pitting,
but instead changes in pH.
To determine whether the presence of organic acids in paleosols might be a useful
biosignature distinguishable from reductive inorganic weathering, previous work has
examined the dissolution of basalt and granite in the presence of organic acids with
and without oxygen (Neaman and others, 2005a; Neaman and others, 2005b; Neaman
and others, 2006). Total elements removed were calculated from the rocks reacted
with a variety of organic acids, with and without oxygen present. Neaman and others
conclude that enhanced release of Fe, P and Y indicate organic-rich conditions, and
that enhanced release of Cu indicates oxygen-rich conditions (Neaman and others,
2005a).
Here we expand upon this previous work, to perform column dissolution experiments dissolving Columbia River Basalt and Tuolomne River Series granite under oxic
conditions with and without citrate. Column experiments using whole rock instead of
separate minerals were chosen because they most closely simulate a natural soil
environment. Citrate was chosen for these column dissolution experiments because it
is a common secretion product of prokaryotes, fungi, lichens, and plant roots, a
common degradation product of biomolecules, and a common constituent in modern
soil solutions (Baziramakenga and others, 1995; Krzyszowska and others, 1996; Neaman and others, 2005b). Citrate has also been previously demonstrated to have the
greatest effect on basalt dissolution from among aliphatic ligands acetate, formate,
fumarate, glutarate, lactate, malonate, oxalate, and succinate, and aromatic ligands
from dissolving basalt and granite with and without organic ligands
635
benzoate, gallate, phthalate, and salicylate (Neaman and others, 2005a; Neaman and
others, 2005b). The reason that citrate has this large effect is likely two-fold 1) it is able
to form a strong tridentate ligand and 2) aliphatic ligands have been previously
demonstrated to have a stronger effect than aromatic ligands (Neaman and others,
2005, 2006).
Trace elements were measured, to expand upon the work of Neaman and others
in determining biosignatures (Neaman and others, 2005a; Neaman and others, 2005b;
Neaman and others, 2006). We then compare our trace element results across a range
of scales to batch dissolution experiments and modern rivers to test whether laboratoryobserved trace element signatures may in fact yield an important indicator of biological factors in natural weathering environments.
methods
Material
Columbia River basalt, and granite from the Half-Dome Tuolumne River Series
were collected and powdered for dissolution experiments in deionized water with and
without 0.01 M citrate in flow-through column dissolution experiments. Basalt was
obtained from the same quarry as the BCR-1 basalt reference sample of the U.S.
Geological Survey (Flanagan, 1967), and the granite was sampled in Yosemite National
Park at Olmsted Point along Highway 120 (Neaman and others, 2004). Samples were
hammer-broken, cleaned with distilled water, crushed and powdered with a tungsten
carbide jaw crusher and disk mill, sieved to 100 to 200 mesh (75-150 ␮m), and cleaned
of fine particles by ultrasonication, acetone washing, and drying at 60 °C. The
concentrations of major and trace elements in the rocks were determined by X-Ray
Fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
(Neaman and others, 2004). The concentrations in the basalt are very similar to the
elemental compositions reported for the BCR-1 basalt reference sample (Govindaraju,
1994).
Mineralogy of Basalt and Granite
The mineralogy of the Half-Dome granite Tuolumne River Series has been
previously determined (Bateman and Chappell, 1979), and the chemical composition
of the minerals previously determined by electron microprobe microanalysis (Neaman
and others, 2005b) (table 1). A grain mount of the powdered granite was examined
optically and using Back-Scattered Electron Microscopy (BSE). Grains were determined to be primarily monomineralic (fig. 1). The powder used in the columns was
prepared identically to powders used in previous batch experiments (Neaman and
others, 2005a; Neaman and others, 2005b; Neaman and others, 2006), however, both
basalt and granite used in the columns have a higher surface area than the powders
previously dissolved in the batch experiments.
The normative mineralogy of the Columbia River basalt was determined from its
elemental composition (Neaman and others, 2005b). BCR-1 has previously been
petrographically characterized as an aphanitic, hypocrystalline basalt with an interstitial texture of plagioclase laths, interstitial augite, partially devitrified brown glass and
iron oxides (Flanagan, 1967). Previous X-ray diffraction (XRD) and electron probe
microanalyses (EPMA) of the Columbia River basalt have documented the presence of
the following major phases: plagioclase feldspar, alkali feldspar, quartz, and augite, as
well as minor phases ilmenite, and magnetite-ulvospinel solid solution, and trace
phases including fluorapatite, Fe/Ti oxides, Cu/Fe sulfide, and Fe-sulfide (Neaman
and others, 2005b).
The volume percent of the different minerals in the Columbia River basalt was
quantified using Energy Dispersive X-ray Spectroscopy (EDS) elemental maps of
636
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
Table 1
Granite and basalt rock mineralogy
1
The chemical composition of minerals was determined by electron microprobe microanalysis
(Neaman and others, 2005b).
2
Percentages of minerals determined by (Bateman and Chappell, 1979) by point counting.
3
Percentages determined from EDS images by image analysis as described in text.
4
Percentages determined from basalt normative calculation as described in text.
5
Simplified from (Neaman and others, 2005b).
polished and carbon coated sample and the INCA-mapping software (Buss, ms, 2006).
All magnesium was attributed to augite, all potassium to glass, and aluminum to
plagioclase (where Al maps did not overlap with other minerals). The percentage of
each of these volumes was determined, the total coverage of these maps scaled to 100
percent, and then averaged (table 1). Grain mounts of powdered basalt 75 to 150 ␮m
grain size identical to that used in the columns were examined optically and with BSE,
and grains were determined to be primarily polymineralic (fig. 1).
Surface Area
Surface area and pore size distribution of samples before and after dissolution
were measured by gas adsorption using a Quantachrome Autosorb-1 MP LP. Samples
were prepared for surface analysis by degassing at 250°C under vacuum overnight (at
least 12 hours or more) until the leak rate (pressure rise) was less than 30 micron/
minute. Specific surface area and pore structure of the minerals were determined by
N2 sorptometry (ASAP 2010, Micromeritics). Surface area was calculated using multipoint adsorption data from the linear segment of the N2 adsorption isotherms between
relative pressures of 0.05 and 0.3 (generally 5-6 points) using the Brunauer-EmmettTeller (BET) isotherm (Brunauer and others, 1938). If the y-intercept of the BET plot
was less than zero, the lowest points were eliminated. Pore size distributions were
calculated from desorption branch isotherms using the Barrett-Joyner-Halenda (BJH)
from dissolving basalt and granite with and without organic ligands
637
Fig. 1. Backscattered electron micrograph of the unreacted granite (A) and basalt (B) grains identical
to those within the dissolution columns. Granite grains appear to be primarily monomineralic while the
basalt grains appear to be primarily multimineralic. The higher the atomic number, the brighter the mineral
appears—therefore minerals such as magnetite and sphene appear white, hornblende, feldspars and quartz
gray, and the foliated grains are biotite. Scale bars are shown in the lower right corner.
method (Barrett and others, 1951), assuming the pores to be cylindrical, perpendicular to the mineral surface and closed on one end, and using the Halsey layer thickness
equation (Halsey, 1948). Only data from the desorption isotherm between relative
pressures of 0.995 and 0.3 were used (see table 2). According to these calculations,
638
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
Table 2
BET surface areas before reaction and after reaction with and without citrate1
Rock
Before reaction
Without citrate
With citrate
Basalt
12.03
12.06
9.54
Granite
0.193
0.33
0.3
1
m2/g.
about half of the basalt surface area is attributable to pores that are about 3.5 nm in size
(data not shown).
Examination of Reacted Grains
Although the columns had reached steady state by 45 weeks (fig. 2), and this paper
examines the water chemistry from the first 45 weeks, solutions continue to flow
through the columns. This is to allow future analyses of long term experiments similar
to White and others (1999). Therefore, in order to not disturb the water chemistry
analyzed in this work (the first 45 weeks), approximately 20 –30g of reacted powdered
basalt and granite was removed from inside the columns after 94 weeks. To remove the
sample, flow was stopped, the top of the column was removed, and sample scooped out
550
Basalt citrate
2500
500
2000
450
Si (µM)
Si (µM)
400
1500
350
1000
300
250
500
200
Basalt no ligand
0
0
10
20
30
40
50
0
10
20
30
4
0
140
Granite citrate
1200
Granite no ligand
120
1000
100
Si (µM)
Si (µM)
800
600
400
80
60
40
200
20
0
0
10
20
Time (week)
30
40
50
0
10
20
30
4
0
Time (week)
Fig. 2. Si concentrations corrected for background versus time reveal that the columns had reached
⬃steady state by weeks 40 to 45, which is the time period we are focusing on in this paper. Si concentrations
show three characteristic behaviors versus time (A) increasing with time (B) constant with time, and (C) and
(D) initially decreasing rapidly and then remaining steady.
from dissolving basalt and granite with and without organic ligands
639
for analysis. Because the samples were removed after the sampling for this manuscript
was finished (45 weeks), the flow rates and elemental concentrations were not
disrupted. These samples were rinsed twice with spectrophotometric grade acetone,
and once with MilliQ DDI water, frozen for 24 hours, and then freeze-dried. Samples
were examined under low vacuum Scanning Electron Microscopy (SEM), uncoated,
for areas of rock coating, and etch pit formation.
Experimental Setup
Six columns were prepared, 3 of which contained 0.01 M citrate, and 3 distilled/
deionized water (MilliQ) (ligand-free). Columns were designed to be similar to
columns in White and Brantley (2003). The input into each column was adjusted to pH
6 with concentrated high purity NH4OH, or 1 N HNO3, and was in equilibrium with
atmospheric oxygen. Each condition (citrate or ligand-free) consisted of one column
containing basalt, one containing granite, and one empty column (control). Basalt
(350g) and granite (330 g) were wet packed in the columns: each column was filled
with deionized water and the rock poured as a dry powder into the water-filled column.
Each of the Pyrex columns had a total volume of 246 cm3, and therefore, assuming a
density of 3 g/cm3 basalt and 2.75 g/cm3 granite, the pore volume of the columns is
129 cm3 (basalt) and 126 cm3 (granite).
The input solutions flowed from input reservoirs above the columns down
through 1/16⬙ (0.16cm) tubing into the bottom of the columns and then up through
the columns. At the top of each column was a 0.2 ␮m filter, which was connected by
1/16⬙ (0.16cm) tubing to the outlet which drained into another set of reservoirs, at
which point samples were collected. Since the input reservoirs were higher than the
columns, the effect of gravity was sufficient to cause the solutions to flow, which
allowed there to be no moving parts, an important consideration in a long-term
experiment such as this. In addition, since columns were wet-packed and flow occurred
upwards, flow through the columns was saturated and development of preferential
flow paths was assumed to be minimal. Flow rates were controlled by adjusting the
height of the outlet, and clamps on the tubing leading to the column. Filters were
changed as needed to prevent a decrease in flow rate due to blocking of the pores in
the filters or when visual inspection indicated evidence of precipitation on the filter.
For the first 3 months filters on the basalt and granite-containing columns with citrate
were changed weekly. All solutions were made up to contain 0.05 percent azide, to
prevent microbial growth (Na-azide for 0-14 weeks, Li-azide after 14 weeks). The
solutions contained no pH buffer, as such buffers have previously been shown to
impact mineral dissolution experiments (Brantley, 2008).
Sampling
The effluent discharge from the top of each column passed through a 0.2 ␮m
filter as previously described and was collected in bottles. A flow rate was determined
by weighing the effluent discharged since the last sampling period and dividing it by
that time period (1-2 weeks). Because the effluent collected for flow rates flowed into
an open bottle with a large fluid surface area, the measured flow rate (mass of
water/time between samples) was corrected for evaporation (0.21 ml/hr). Since
samples collected for chemical analysis had a small fluid surface area, no correction
was deemed necessary or performed. Approximately 10 to 15 ml of effluent was
collected for major and trace element analysis by Inductively Coupled Plasma Mass
Spectrometry (ICP-MS) and Inductively Coupled Plasma Atomic Emissions Spectrometry (ICP-AES). Elements analyzed include Rb, Sr, Y, Zr, Mo, Ba, La, Ce, W, Pb, Th, Na,
Mg, Al, P, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, K and Si. Sampling occurred
approximately weekly throughout the first 3 months of the experiment, and at least
every 2 weeks thereafter. Samples were first analyzed for pH, and then acidified with
640
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
high purity HNO3 to 3%v/v. Input solutions were sampled and analyzed by ICP-MS
beginning in the 19th week. Concentrations used for calculations were generally
analyzed by ICP-MS except for non-dilute samples for which ICP-AES concentrations
were used. In some cases the sample concentrations were below detection, in which
case a value of 10⫻ the instrument detection limit (IDL) was used in calculations of
cumulative release, and zero for release rate calculations. We considered this to be the
most realistic detection limit (the instrument detection limit is very low), and also the
most conservative value for calculating the difference between the elemental concentrations released in the citrate-containing columns and the citrate-free columns. Using
a smaller detection limit the difference could have been calculated to be larger than it
actually was, whereas this is unlikely to occur if we use a conservative detection limit. In
some cases results were determined to be anomalous, or were not measured, in which
case a value of the mean of the remaining values was used in calculations.
results
Concentrations
Elemental concentrations with time displayed three characteristic behaviors,
either increasing, decreasing or remaining steady with time (fig. 2). However, it
appeared that concentrations reached approximate steady state by weeks 40 to 45 (fig.
2). Steady state was defined as a constant concentration for a number of residence
periods. The residence period of the water in the columns was approximately 3 pore
volumes per week, or approximately 2 days (determined by dividing pore volume by
flow rate). Since steady state concentrations had been reached, the average concentrations for weeks 40 to 45 (table 3) corrected for input concentrations were used in
calculations.
Flow Rates and pH
Flow rates were variable with time. Flow rates in the granite citrate containing
column increased slightly with time, and flow rates in the rest of the columns decreased
slightly with time. (See table 4 for average flow rates weeks 40-45).
The pH of the effluent from the columns decreased in the order: basalt citratecontaining ⬎ basalt ligand-free ⬎granite ligand-free⬃ control ligand-free ⬎granite
citrate-containing ⬎citrate control (table 5). pH values for the basalt citrate column
increased and then decreased through time, and the rest of the columns showed an
initial drop in pH and then steady state pH values for the remainder of the experiment.
Reacted Grains
No pitting or coating was observed on reacted samples removed from inside the
columns, and no difference was observable between unreacted samples, samples
reacted with deionized water, and samples reacted with citrate for either basalt or
granite. Measured surface areas of reacted grains (table 2) were used for calculation of
release rates.
calculations
Release Rates
Release rates were calculated as described in White and Brantley (1995) using the
expression
R⫽
共Cout ⫺ Cin兲Q
,
Am
(1)
where Cout is the measured output concentration, Cin is the inlet concentration, Q is the
flow, A is the specific surface area, and m is the mass. Here Cin was assumed to equal the
14,000
3,200
5.66
448,000
203,000
141
799
362
11,900
915
38.2
16.8
17.3
2,390
20,000
1,610,000
1,600
323
11
1,060
178,000
2,170,000
130,000
41,900
822
534
608
1,240
3.7
99.8
1
SD is one standard deviation.
bd indicates below detection.
Na
K
Rb
Mg
Ca
Sr
Ba
Sc
Ti
V
Cr
Mo95
Mo97
W
Mn
Fe
Co
Ni
Cu
Zn
Al
Si
P
S
Y
Zr
La
Ce
Pb
Th
1,300
1,100
0.69
18,000
14,000
12
330
26
330
48
4.3
2.4
2.3
300
1,100
920,000
41
37
17
150
12,000
480,000
3,200
1,400
33
51
26
58
1.1
4.2
Basalt citrate
concentration SD1 (x10-3)
(µMx10-3)
12,700
3,700
14.1
44,700
112,000
193
0
4.61
2,030
86
2.5
4.5
4.6
50.6
1,500
35,400
29.2
354
92
420
82,000
105,000
86,700
510
30.9
6
156
207
8.7
270
1,900
1,200
2.2
7,300
7,100
16
0
0.6
210
12
1.9
4.7
4.8
7.5
200
4,800
3.5
770
41
560
11,000
38,000
5,600
330
1.5
3
10
12
7.7
39
Granite citrate
concentration SD1 (x10-3)
(µMx10-3)
11,500
bd
0.57
71,200
69,300
bd
bd
2.75
1.55
26.3
bd
5.9
6
900
320
90
42.5
0.017
7.7
180
1,120
430,000
bd
59,000
bd
1.05
bd
0.0071
0.37
0.069
0
0.13
0.075
0.86
15,000
1.4
1.4
250
19
45
2.6
0
9.9
150
160
110,000
0.7
0.68
4.4
0.34
5,800
2,300
990
Basalt no ligand
concentration SD1 (x10-3)
(µMx10-3)
Average concentrations and standard deviations (␮M) (Weeks 40 – 45)
corrected for background
Table 3
4,600
5,800
21.3
650
4,800
110
bd
0.21
0.61
5.7
1.4
2.7
2.7
46.6
56
99
22.3
0.61
74
87.3
56.8
38,100
76
273
bd
0.59
bd
0.071
0.55
0.048
0
0.78
0.041
0.44
0.17
0
1.2
1.9
1.5
1.5
4.5
11
0
3.7
0.3
19
0
5.9
3,700
40
18
1,600
1,800
3.2
270
1,300
53
Granite no ligand
concentration
SD1 (x10-3)
-3
(µMx10 )
from dissolving basalt and granite with and without organic ligands
641
642
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
Table 4
Flow rates weeks 40 – 45 corrected for evaporation (0.21 ml/hr)
Column
Basalt with citrate
Granite with citrate
Control with citrate
Basalt no ligand
Granite no ligand
Control no ligand
Average flow rate (ml/hr)
1.7
2.7
2.5
2.2
2.6
2.7
Standard deviation (ml/hr)
0.2
0.4
0.7
0.6
0.3
0.7
measured concentrations of elements in effluent from the control columns (citrate
and ligand-free). This accounts for any leaching from any part of the columns,
although such leaching appears to be negligible. Release rates and standard deviations
for each element for weeks 40 to 45 are reported in table 6.
Cumulative Release
The cumulative release (B) of each element from each column was also calculated
over the duration of the experiment (30 samples from 45 weeks) using the expression
冘 共C
30
B⫽
n
out
n
⫺ Cin
兲Qn tn,
(2)
n⫽1
where Cout is the measured output concentration, Cin is the inlet concentration (again
estimated as the output of the control columns), Q is the flow, and t is the time between
sampling. The percentage (P) of the total element released in the rock-containing
columns is calculated using the expression
P⫽
B
Cm
(3)
where B is the cumulative release over the 45 weeks of the experiment, C is the
concentration of the element in the basalt or granite rock, and m is the mass of the rock
in the column (table 7).
Element Mobility
To assess elemental mobility in a soil, element concentration in a soil can be
normalized by the concentration of an assumed immobile element (often Ti, Zr, and
Nb) to calculate the fractional mineral loss or enrichment (Anderson and others,
2002)
␶i,j ⫽
Cj,w Ci,p
⫺ 1.
Cj,p Ci,w
(4)
Table 5
pH values weeks 40 – 45
Citrate
Ligand-free
Basalt
7.33
7.04
Granite
6.19
6.66
Control
6.07
6.61
from dissolving basalt and granite with and without organic ligands
643
Table 6
Average element release rates and standard deviations week 40 – 45
Basalt citrate
Granite citrate
Basalt no ligand
Granite no ligand
RR1x1016 SD2x1016 RR1x1016 SD2x1016 RR1x1016 SD2x1016 RR1x1016 SD2x1016
Rb
Sr
Y
Zr
Mo95
Mo97
Ba
La
Ce
W
Pb
Th
Na
Mg
Al
P
S
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
K
Si
1
2
0.0082
0.205
1.19
0.770
0.0240
0.0250
1.17
0.88
1.80
3.43
0.0044
0.140
20.3
647
256
189
60.6
294
0.520
17.3
1.32
0.0550
29.0
2400
2.32
0.470
0.010
1.55
4.6
3120
0.0013
0.032
0.15
0.042
0.0035
0.0034
0.59
0.12
0.24
0.31
0.0025
0.018
3.1
57
10
21
6.7
34
0.020
1.7
0.05
0.0030
3.7
1400
0.26
0.063
0.022
0.33
1.8
700
1.05
14.6
2.35
0.40
0.32
0.33
0.10
1.9
0.40
0.33
0.30
0.31
0.0083
0.0025
1.42
7.4
0.33
4.1
0.0012
0.0083
0.0084
0.0012
0.0017
0.0018
0.013
0.18
0.18
0.024
0.10
0.10
11.9
15.8
3.8
0.71
20.3
953
3390
6200
6600
21
8440
0.35
153
6.5
0.21
114
2680
2.21
26
7.2
20
280
7900
1.9
2.5
0.6
1.24
0.16
0.72
0.00030
0.00034
3.0 0.000046 0.000061
57
16.9
4.6
670
104
28
1100
1.61
0.37
1200
30
76.8
6.4
940
101
28
0.05
0.0041
0.0018
10
0.0010
0.0013
1.1
0.0376
0.0086
0.17
21
0.47
0.12
460
0.11
0.11
0.36
0.062
0.016
63
4.1
0.007
0.015
33
0.11
0.22
110
2900
604
190
0.0015
3.09
0.018
0.0031
300
44
1.3
1.7
5.9
317
0.012
0.007
0.38
0.031
3.2
1.1
1.48
0.020
4.9
1.0
380
2510
0.0026
0.56
0.037
0.0025
110
19
2.1
2.8
9.1
99
0.012
0.016
0.11
0.076
1.6
2.6
0.30
0.026
1.3
2.4
120
280
Release rate (mol m⫺2 s⫺1).
Standard deviation (mol m⫺2 s⫺1).
Here ␶i,j is the fraction of mobile element or mineral j lost (␶i, ⬍ 0) or gained (␶i, ⬎ 0)
assuming that element or mineral i is immobile. C is the concentration of the immobile
and mobile elements in the parent and weathered materials (w and p refer to
weathered and parent material respectively).
Ti was used as an immobile element to calculate ␶ for each of the elements in
each of the columns. Despite the fact that Ti is much more mobile in the presence
of citrate than without, it is still very immobile, with only 0.14 percent removed
from the basalt and 0.35 percent from the granite in the presence of citrate (table
7). The cumulative loss from each of the columns (B from equation 2) was
subtracted from the total elemental content of the original columns (Cp, also
represented as the product Cm in equation 3) to obtain Cw for both mobile and
immobile elements (i,j) (table 8). The percentages removed (table 7) can also be
644
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
Table 7
Percentages of element removed and the ratio of percent removed with citrate to percent
removed without citrate
Na
K
Rb
Mg
Ca
Sr
Ba
Sc
Ti
V
Cr
Mo95
Mo97
W
Mn
Fe
Co
Ni
Cu
Zn
Al
Si
P
S
Y
Zr
La
Ce
Pb
Th
Basalt
citrate %
removed
0.43
0.071
0.46
2.3
0.64
0.36
0.4
1.6
0.14
0.42
0.24
bd
bd
bd
4.6
6.4
bd
4.6
1.3
2.2
0.29
0.93
8.2
bd
7.5
0.98
13
11
0.37
13
Granite
Basalt no Granite no
Basalt
citrate % ligand % ligand % citrate:no
removed removed removed
ligand
0.55
2.2
1.5
0.19
0.1
0.2
0.2
0.36
0.23
0.27
0.41
1.7
1.6
0.61
0.13
3.7
1.8
0.31
0.16
2
0.69
0.036
0.084
10
0.091
bd
0.008
NA
0.27
0.028
0.039
55
0.35
4.4E-05
0.00015
3200
0.63
0.052
0.017
8.1
0.17
0.018
0.065
14
bd
bd
bd
NA
bd
bd
bd
NA
bd
bd
bd
NA
2.1
0.093
0.25
50
0.64
0.0002
0.00069 33000
bd
bd
bd
NA
3
0.056
0.05
82
12.4
0.047
4.7
27
7.2
0.36
0.039
6.2
0.28
0.0032
0.0018
91
0.13
0.3
0.03
3.1
87
0.023
0.058
360
bd
bd
bd
NA
2
0.0038
0.0079
2000
0.065
0.0056
0.016
170
7.9
0.0092
0.0048
1400
4.9
0.0064
0.0036
1800
1.5
0.011
0.011
32
31
0.07
0.01
190
Granite
citrate:no
ligand
0.38
0.52
0.56
13
11
8.2
11
6.8
2300
36
2.7
NA
NA
NA
8.3
930
NA
59
2.6
190
160
4.4
1500
NA
250
4.1
1700
1400
150
3200
Calculated percentages of less than 1 ⫻ 10⫺5 are reported as below detection (bd).
used to determine which elements might indicate the presence of organic acids
over geologic time.
Mineral Dissolution Rates
Mineral dissolution rates were calculated for the basalt and granite columns by
inverse modeling using PHREEQC (Parkhurst and Appelo, 1999). The input solution
was assumed to be pure water at pH ⫽ 6 (input pH) in contact with atmospheric CO2,
and the output solution was the average concentration of weeks 40 to 45 corrected for
the concentrations in the control columns (table 9). An effort was made to obtain the
simplest model that fit the data, and therefore the only minerals allowed to precipitate
645
from dissolving basalt and granite with and without organic ligands
Table 8
t values less than ⫺0.1
La
Ce
Th
P
Cu
Basalt Citrate
-0.13
-0.11
-0.13
NA
NA
Basalt no ligand
-0.000091
-0.000063
-0.00070
NA
NA
Granite citrate
NA
NA
-0.31
-0.86
-0.12
Granite no ligand
NA
NA
-0.000095
-0.000095
-0.047
NA indicates that those elements did not have t values less than ⫺0.1 in those rocks.
were amorphous silica, gibbsite, amorphous iron oxide, Mg-containing saponite, and
kaolinite. Similarly, minerals allowed to dissolve included plagioclase (of the specific
composition in each rock), hornblende, quartz, fluorapatite, magnetite, and augite as
well as secondary kaolinite (see table 10 for mineral reactions). Since the reacting
solutions entered the columns in equilibrium with atmospheric oxygen, and the
oxygen participates in mineral reactions, it was necessary to artificially allow Fe2⫹ to
precipitate, as if it had first been oxidized to Fe3⫹ and then precipitated (see table 10).
In order to allow fluorapatite to dissolve, it was necessary to input fluoride concentrations into PHREEQC for the input and effluent solutions. However, since fluoride had
not been measured during the experiment, fluoride concentrations were unknown.
Therefore, effluent fluoride concentrations of 10 ␮M were used for each column, but
uncertainty on the fluoride concentrations was set to 1000 percent to allow the model
to adjust fluoride concentrations during model calculations based on measured
elemental concentrations.
As an estimate of uncertainty, 1 standard deviation around the average concentration during weeks 40 to 45 (table 3) was assumed. Uncertainty was estimated at 40
percent for Ca (the granite-containing column with no ligand) and Ca and Mg
concentrations (the granite-containing column with citrate). The increased uncertainties in Ca and Mg were necessary to allow PHREEQC to successfully produce models
for the granite-containing columns, and are likely due to the low concentrations of Mg
and Ca in the effluent from the granite.
Moles of mineral dissolved per kg of water as calculated by PHREEQC were
converted to mineral dissolution rates using the expression
V⫽
共MQ兲
,
Am
(5)
Table 9
Modeled solutions in PHREEQC
Condition
T(ºC) pH
Basalt citrate
20
7.3
Basalt no ligand
20
6.1
Granite citrate
20
6.2
Granite no ligand 20
6.7
1
2
Na2
14
11.1
12.7
4.55
K2
P2
Ca2
3.17 130
203
4.61 0.0051 69.3
3.65 86.7 112
5.79 0.756 4.75
Below detection, therefore the value used was 10⫻ the IDL.
All concentrations in ␮M.
Si2
2170
427
105
38.1
Al2
178
1.12
81.5
0.0568
Fe2
1610
0.09
35.4
0.0987
Mg2
448
71.2
44.7
0.645
646
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
Table 10
Mineral dissolution and precipitation reactions in PHREEQC
Mineral phase
labradorite
oligoclase
hornblende
quartz
fluorapatite
magnetite
augite
kaolinite
amorphous silica
gibbsite
amorphous
amorphous
glass
Mg-saponite
Mineral reaction
Ca0.5Na0.5Si2.5Al1.5O8 + 6H+ +2H2O = 0.5Ca2+ + 0.5Na+ +2.5H4SiO4 +1.5Al3+
Ca0.3Na0.7Si2.7Al1.3O8 + 5.2H+ + 2.8H2O = 0.3Ca2+ + 0.7Na+ + 2.7H4SiO4 + 1.3Al3+
Ca2Mg3.5Fe1.5Si8O22(OH)2 + 14H+ + 8H2O = 2Ca2+ + 3.5Mg2+ + 1.5Fe2+ + 8H4SiO4
SiO2 + 2 H2O = H4SiO4
Ca5(PO4)3F + 3H+ = F- + 3HPO42- + 5Ca2+
Fe3O4 + 8H+ = 2Fe3+ + Fe2+ + 4H2O
MgFe0.5Ca0.5Si2O6 + 4H+ + 2H2O = Mg2+ + 0.5Fe2+ + 0.5Ca2+ + 2H4SiO4
Al2Si2O5(OH)4 + 6H+ = H2O + 2H4SiO4 + 2Al3+
SiO2 + 2H2O = H4SiO4
Al(OH)3 + 3H+ = Al3+ + 3H2O
Fe+3 oxide Fe(OH)3 + 3H+ = Fe3+ + 3H2O
Fe2+ oxide Fe(OH)2 + 2H+ = Fe2+ + 2H2O
K0.125Na0.125Al0.25SiO2.5 + 1.5H2O + H+ =0.125K+ +0.125Na+ + 0.25Al3+ +H4SiO4
Mg3.165Al0.33Si3.67O10(OH)2 + 2.68H2O + 7.32H+ = 0.33Al3+ + 3.165Mg2+ + 3.67H4SiO4
where V is the dissolution rate in moles m⫺2s⫺1, M is the moles of mineral dissolved per
kg of water as calculated by PHREEQC, Q is the flow rate in Ls⫺1, A is the specific
surface area of the rock in the columns (m2g⫺1), and m is the mass of mineral in the
columns (g). Surface area was assigned to the different minerals based on mass as
shown in equation (5). In some cases PHREEQC produced more than one value for
moles of mineral dissolved, in which case we report the full range in calculated rates
(table 11 and fig. 3).
Experimental and Computational Uncertainties
These column experiments were designed after White and others (1999) to flow
for long periods of time. They were therefore designed with no moving parts (such as
peristaltic pumps) that can break under such long collection times. This experimental
design, while it allows the collection of data that would not otherwise be possible,
results in some uncertainties. Here we discuss some of these uncertainties, including
variations in flow rate, secondary mineral precipitation, and oxidation state.
Although the flow rates are low and somewhat variable, the columns and the
residence times are relatively long (⬃2 days). During the period that we report
release rates, flow rates were particularly carefully controlled (table 5). Therefore,
the effects of the variations in flow rates on mineral dissolution rates are likely to be
minimal.
Secondary mineral precipitation was observed on the filters, although not on the
grains by SEM, as discussed above. We included this effect in our PHREEQC modeling,
by allowing the precipitation of secondary phases. Ideally, this precipitation would
have been quantified. However, since the precipitation was primarily observed on the
columns containing citrate, it does not change the conclusions: the presence of
precipitates merely means that the observed enrichments in the presence of citrate are
minima.
Entering solutions were in equilibrium with atmospheric O2; however, mineral
reactions within the columns (for example, oxidation of Fe2⫹ present in the parent
minerals) likely lowered the oxygen concentration within the columns. These columns
were designed to be similar to long-term columns used by White and others (1999),
who document the oxidation of sulfide throughout their similar granite-filled column.
In addition, our PHREEQC modeling requires the oxidation and precipitation of iron.
1
Dissolution rate (mol m⫺2 s⫺1).
NP indicates that mineral is not present in that rock.
Augite
Glass
Plagioclase
Fluorapatite
Magnetite
Biotite
Quartz
K-feldspar
Hornblende
Basalt citrate
Max
Min
dissolution
dissolution
rate1 x1012
rate1 x 1012
0.18
0.17
0.016
0.0078
0.012
0.0079
0.014
0.015
0.59
0.17
NP
NP
NP
NP
NP
NP
NP
NP
Basalt no ligand
Min
Max
dissolution
dissolution
rate1 x 1012
rate1 x1012
0.077
0.069
0
0
0.0099
0.0083
0
0
0.0033
0.0011
NP
NP
NP
NP
NP
NP
NP
NP
Granite citrate
Max
Min
dissolution
dissolution
rate1 x 1012
rate1 x1012
NP
NP
NP
NP
0.38
0.28
183
176
19
7.1
0.89
0
0
0
0.15
0
3.7
2.5
Dissolution rates from PHREEQC
Table 11
Granite no ligand
Max
Min
dissolution
dissolution
rate1 x1012
rate1 x 1012
NP
NP
NP
NP
0.074
0.063
0.23
0.071
0.0012
0.00040
0.73
0
0.78
0
0.12
0
0.36
0.25
from dissolving basalt and granite with and without organic ligands
647
648
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
Fig. 3. Dissolution rates in the presence of (filled symbols) and without citrate (open symbols). All
values from this study are plotted versus final pH. In each case, the range in dissolution rates represents the
largest and smallest values of the range in models (table 11), and are compared to data from the literature
(line art symbols). Dissolution rates for (A) bulk basalt rock and basalt glass (calculated as described in
methods), (B) fluorapatite, (C) magnetite, (D)augite, (E)hornblende, and (F) oligoclase. The fact that the
magnetite dissolution rates are smaller than other values from the literature suggest that Fe may be
precipitating.
from dissolving basalt and granite with and without organic ligands
649
Therefore, these columns are assumed to be oxic, although not in equilibrium with
oxygen throughout the length of the columns.
discussion
Implications for Biosignatures
If microorganisms, lichens or plants produced organic acids which leached rocks
and soil, the differential mobility of trace elements might provide a biosignature of
their presence. Previous batch experiments suggest that differential mobility of the
elements P and Y in paleosols can be attributed to the presence of organic acids and
host mineral phase (Neaman and others, 2005a; Neaman and others, 2005b). Since
geochemical behavior is observable at multiple scales from the molecular to the
laboratory and field scale, we here compare our results to phenomena observed at the
molecular, laboratory and field scales to determine whether observed behaviors are
consistent across these scales, and therefore indicative of organomarkers, observed at
the field and laboratory scales and predicted by the molecular mechanisms of organic
acids.
Metal Complexation
Elements that are preferentially removed by citrate in these column experiments
(elements with a ratio of B with citrate to B with no ligand ⱖ 30) are Zr, Sc and Mn
from basalt, Y, La, Ce, Th, Ti, Al, P, Pb, Ni and Fe from both basalt and granite, and V
and Zn from granite (fig. 4). Figure 5 indicates a plot of the ratio of the percentage of
the element released (P) with citrate to without citrate, versus the NIST stability
constant of the metal with citrate (Martell and Smith, 2001). Some have argued that
the release rate of elements from minerals in the presence of a metal-complexing
ligand may be controlled by the stability constant of the metal with the ligand (Casey
and Westrich, 1992; Neaman and others, 2005a; Neaman and others, 2005b; Goyne
and others, 2006). Figure 5 suggests a strong correlation between the ratio of the
release with citrate to that without citrate with the NIST stability constant of the metal
with citrate in solution. The labeled elements (Th and Al in basalt) do not follow the
trend as strongly as other elements. This may be due to reprecipitation, or differences
in mineralogy between basalt and granite. Other elements not present in the NIST
database (Zr, Mo, W, P, S, Sc, Ti, V, Cr, Si) are not compiled in this figure.
In addition to the complexing effect of the ligand, the elemental release rate is
strongly affected by the host mineral phase. Due to the fine grained nature of the
basalt, no attempt was made to analyze mineral grains. Apatite is assumed to host La,
Ce, Sr, P, Y, Th based on previous work (Neaman and others, 2005b; Prowatke and
Klemme, 2006). Augite is assumed to host Mg, Sc, V, Cr, Mo, W, Mn based on previous
work (Hart and Dunn, 1993), as well as knowledge of chemical reactivity. Glass is
assumed to host K, Rb, Ba, based on previous work (Ho and Cashman, 1997) as well as
assumptions as to the similarity of the elements. The Fe-Ti minerals were assumed to
host Ti and Zr based on previous work (Neaman and others, 2005b). The copper iron
sulfides were assumed to host Co, Ni, Zn, and Pb, as well as Cu and S, based on the
chalcophyle and siderophile nature of these elements.
Bateman and Chappel (1979) have extensively characterized the Tuolomne river
series granite. They document that Ni, Cr, V, Sc, Co, Mn, and Zn are present in biotite
and hornblende, Rb and Pb in both biotite and feldspar, Mn, V and Cr in magnetite, Sr
in alkali feldspar, Th in sphene, and La, Ce, and Y in sphene, hornblende, and apatite
in that relative order. Zr is primarily present in zircons, and Cu and S in Cu-Fe sulfide
inclusions in Fe oxide phases (Neaman and others, 2005b). Based on other work on
the Sierra Nevada batholiths, we infer that Ba is present primarily in biotite (Dodge
and others, 1982). Molybdenum is assumed to be present in magnetite (Kuroda and
Sandell, 1954).
650
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
10
Ratio citrate: no ligand
10
10
10
10
10
5
Basalt
Granite
4
3
2
1
0
-1
10
Na K RbMgCa Sr Ba Sc Ti V CrMnFe Ni CuZn Al Si P Y Zr LaCePb Th
Element
Fig. 4. Ratio of percent of element released with citrate (Pcitrate) to percent of element released without
citrate (Pnoligand) for basalt and granite. High ratios (⬎30) indicate elements that may be potential
biosignatures (for example, Y, La, Ce, Th, Al, P, Ti, Fe, Ni for both basalt and granite, Zr, Sc, and Mn for
basalt, and Pb, V, and Zn for granite). Line is drawn at a ratio of 30. Elements are organized after the periodic
table, with first s block elements, from smallest to largest atomic radius, then d block elements, then p block
and f block elements. This allows the comparison of elements with similar properties.
Therefore, broadly, the elemental organomarkers documented here may also be
consistent with mineralogical biosignatures. In basalt, if P, Y, La, Ce, and Th are
present in apatite, then their coupled enhanced release suggests that the dissolution of
apatite may be an indicator of organic acids in the soil environment. Apatite also
dissolves in inorganic solutions, although more slowly. If Pb and Ni are present in
Cu-Fe sulfides, then their coupled release may reflect the oxic dissolution of these
phases. The enhanced release of Zr and Ti may reflect the enhanced dissolution of
Fe-Ti and Fe oxides. Sc and Mn are both inferred to be present in augite, and are
significantly enhanced in the presence of organic acids.
In granite, the large percentage of phosphate lost documents the loss of significant apatite. The large percentage of Th (in sphene) documents the large percentage
of sphene lost; La and Ce are both present in apatite and sphene as well as hornblende.
V, Zn, Pb and Ni are all present in biotite and hornblende. Therefore, the trace
elements released are consistent with the enhanced loss of apatite, sphene, hornblende and biotite.
Laboratory Scale: Elemental Release With and Without Citrate
Basalt batch experiments.—Elemental release from our column experiments can be
compared to release from previous batch experiments with basalt (Neaman and
651
from dissolving basalt and granite with and without organic ligands
10000
A) Basalt
Ratio of release citrate:noligand
1000
Al
100
10
Th
1
0.1
10000
0
2
4
6
8
10
12
2
4
6
8
10
12
B) Granite
1000
100
10
1
0.1
0
log K1 for metal-citrate
Fig. 5. The ratio of the percentage released with citrate (Pcitrate) to without citrate (Pnoligand) versus the
stability constant for the element with citrate (K1 ⫽ [ML]/[M][L] where [ML], [M] and [L] are
concentrations of the element-ligand complex, free ion, and free ligand respectively). The stability constant
for Fe3⫹ was used as the columns were exposed to the atmosphere. Stability constants were obtained from
the NIST data base. Elements not included in the NIST database include: Zr, Mo, W, P, S, Sc, Ti, V, Cr, and
Si. The pK3 value of citric acid ([HL]/[L][H]) is at pH⫽5.75 (NIST Database 46). Therefore, in all of the
columns, citrate should be fully deprotonated, and present as citrate ligand.
others, 2005a; Neaman and others, 2005b) and granite (Neaman and others, 2006).
These previous batch experiments were also designed to test the effect of citrate in
creating biosignatures. Therefore, a comparison as to whether the same biosignatures
were inferred in the oxic batch experiments as in the present study is an interesting
study in scaling.
The percentage of each element released with and without citrate is relevant to
whether that element may serve as a useful biosignature for citrate. However, since the
previous batch experiments and present column experiments were performed for
different lengths of time, the percentages of each element released cannot be simply
compared. Therefore, we normalize each element released from the basalt dissolution
experiments to the percentage of silicon released. When we compare the normalized
elemental release from the oxic basalt batch experiments and the oxic basalt dissolution columns, we find that the normalized percentage is within a factor of two for the
following elements: Al, Fe, Mg, P, Ti, V, Y, Zr, and Ba. In contrast, Cr, Cu and Rb are
much more strongly enhanced relative to Si in the batch experiments than in the
column experiments.
The basalt dissolution rates measured by Neaman and others (2005b) are significantly higher than the rates calculated in these column experiments (table 6), which is
not surprising given the different durations of the experiments, the different waterrock ratios, and the fact that the batch experiments include initial rates. Release rates
652
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
with citrate from Eick and others (1996a, 1996b) were not compared to our results
because only parabolic release rates were given.
Granite batch experiments.—Similarly, we here compare normalized total element
release for the granite dissolution experiments. Silicon was not reported for the
granite batch dissolution experiments, therefore we normalized to Mg. The normalized percentage is within a factor of two for the columns and batch experiments for: Fe,
P, Ti, V, Cu and Rb. Much less Al, Cr, and Y, and significantly more Ba was released
from the batch experiments than the column experiments normalized to Mg.
Laboratory Scale: Dissolution Rates With and Without Citrate
The logarithm of the basalt rock dissolution rate (mol m⫺2 s⫺1) calculated from
the Si release rate (equation 1) is significantly higher (⫺12.51 log dissolution rate in
mol m⫺2 s⫺1) than the log basalt dissolution rate without citrate (⫺13.22 log dissolution rate in mol m⫺2 s⫺1) (fig. 3A). This trend was also observed in previous studies
comparing basalt dissolution rates with and without citrate in batch experiments
(Neaman and others, 2005b). However, dissolution rates from this study are much
slower than other basalt dissolution studies (fig. 3A) (Gislason and Eugster, 1987; Eick
and others, 1996a; Eick and others, 1996b; Oelkers and Gislason, 2001; Gislason and
Oelkers, 2003; Wolff-Boenisch and others, 2004). Such a discrepancy is likely due to
the long duration of our experiments and also to the column method itself. In
dissolution columns, solutions can become relatively concentrated compared to batch
experiments, slowing the rate of dissolution.
In basalt, the mineral dissolution rates calculated by PHREEQC using equation 5
for augite, glass, magnetite and apatite were enhanced by more than a factor of two in
the presence of citrate in comparison to the absence of citrate (table 11). Similarly, for
the granite, the mineral dissolution rates calculated using PHREEQC for plagioclase,
apatite, magnetite and hornblende were enhanced by more than a factor of two in the
presence of citrate compared to the absence of citrate (table 11). Dissolution rates of
these minerals are shown in figure 3, compared to previous literature values. Data are
shown either as rate laws derived from multiple experiments (Bandstra and Brantley,
2008), or from individual experiments (Siegel and Pfannkuch, 1984; Schott and
Berner, 1985; Holdren and Speyer, 1987; Sverdrup, 1990; Casey and others, 1991;
White and others, 1994; Stillings and Brantley, 1995; Stillings and others, 1996; Hoch
and others, 1996; van Hees and others, 2002). Dissolution rates are also compared to
rates from the literature for experiments containing citrate for basalt (Neaman and
others, 2005b), oligoclase (Lundstrom and Ohman, 1990; Stillings and others, 1996),
apatite (Welch and others, 2002), and hornblende (Zhang, ms, 1990; Zhang and
others, 1996; Zhang and Bloom, 1999). More than one data point for a condition
indicates a range in values (table 11). Mineral dissolution rates enhanced in the
presence of citrate calculated from major element chemistry using PHREEQC are:
augite, apatite, glass, and magnetite (in basalt) and apatite, hornblende, magnetite
and plagioclase (in granite). These minerals can be compared to the trace element
hosts significantly enhanced in the presence of citrate (basalt: augite, apatite, Cu-Fe
sulfides, and Fe-Ti oxides and granite: apatite, hornblende, sphene, and biotite). The
overlap in the minerals showing enhanced dissolution when calculated from major
element chemistry by PHREEQC or inferred from the trace element hosts builds
confidence in the observation that organic acids enhance mineral dissolution rates
either by indirect mechanisms such as changing the complexation of the ions in
solution (Oelkers and Schott, 1998; Kubicki and others, 1999), or by directly affecting
the mineral surface (Furrer and Stumm, 1983).
from dissolving basalt and granite with and without organic ligands
653
Field Scale: Rivers
We also compare our results from the column experiments to observations from
organic-rich and -poor rivers. Multiple comparisons of element concentrations between organic-rich rivers (or “black” rivers), and organic-poor rivers indicate increased
concentrations of elements such as Al, Fe, Th, Zr, Y, and rare earth elements (REE) in
the organic-rich waters as compared to the organic-poor waters, but low concentrations
of Ca, Na, Sr, K, Ba, Rb, and U (Dai and Martin, 1995; Dupré and others, 1996; Viers
and others, 1997; Braun and others, 1998; Oliva and others, 1999; Viers and others,
2000; Millot and others, 2003; Tosiani and others, 2004; Braun and others, 2005;
Pokrovsky and others, 2006). The enhanced dissolution and mobility of these elements
in the organic-rich rivers supports the observations of these experiments that this
element list is more mobile in the presence of citrate, and may therefore be useful as
biosignatures (table 7).
Viers and others (1997) observe that in the comparison between organic-poor and
organic-rich waters in a tropical watershed in Cameroon, the organic-poor waters
contain low concentrations of major and trace elements [as well as Dissolved Organic
Carbon (DOC)], and that the elements are in a true dissolved form except for Al and
REE. In contrast, in the organic-rich waters, concentrations of elements usually
considered immobile (Al, Ga, Fe, Ti, Th, Zr, Y and the REE) are increased. In addition,
when these organic-rich waters were successively filtered, thereby removing organic
carbon colloids, the relationship between the elements Al, Ga, Fe, Ti, Th, Zr, Y and the
REE, and DOC form a straight line, suggesting that the concentrations are controlled
by the organic colloidal fraction. When we compare the elements enhanced with DOC
in the rivers to the elements enhanced in the presence of the granite columns
containing citrate (Y, La, Ce, Pb, Th, Al, P, Ti, V, Fe, Ni and Zn), we notice that the
enhanced elements are very similar. This builds confidence that these elements may be
useful as biosignatures.
Similar results are observed in a more spatially detailed study of the Nyong basin
rivers in Cameroon (Viers and others, 2000) in which Al, Fe, Y, Zr, Th and the REE
were found to exhibit a strong correlation with dissolved organic carbon. Similarly,
Dupré and others (1996) observe in the Congo Basin that both organic-rich and
organic-poor rivers contain very low concentrations of very soluble elements such as
Ca, Na, Sr, K, Ba, Rb, and U, and very high concentrations for trace elements and REE,
particularly in the organic-rich rivers. These authors again attribute the high concentrations of these trace and REE to transport in a colloidal phase.
In a study of groundwater in the tropical watershed Nsimi-Zoetele in Cameroon,
Oliva and others (1999) have found that organic acids enhance dissolution of minerals
and transport of the generally insoluble elements Al, Fe, Ti, Zr and REE. In further
studies of the same watershed, Braun and others (2005) have similarly found that
degradation of swamp organic matter produces organic colloids, which mobilize Fe,
Al, Zr, Ti, and Th from the top meter of the swamp. When the organic colloid content
is low, Th and Zr are close to detection, controlled by the solubility of the secondary
mineral thorianite (ThO2) and the primary mineral zircon (ZrSiO4). Tosiani and
others (2004) also observe that enhanced transport of Al and Fe correlates with DOC,
and attribute the mobility of these elements (as well as La) to transport by organic
colloids in the Cuyuni basin in Southern Venezuela.
In a very different climate regime in the Mackenzie River Basin in Canada, Millot
and others (2003) have observed that the presence of organic matter in the northern
watersheds increases weathering by a factor of approximately 3 to 4 between the low
and high DOC rivers. Trace elements were not measured, so it is not possible to
compare the effect of DOC on trace elements in these environments. In two arctic
rivers, Dai and Martin (1995) have also concluded that the low metal concentrations
654
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
Fig. 6. Basalt column discharge concentrations and rivers draining a similar lithology (average weeks
40-45) normalized to BCR-1. Column discharges are indicated by heavy black lines (solid ⫽ citratecontaining, dotted⫽ligand-free), and the rivers by the gray solid lines. River concentrations are from Tosiani
and others (2004), and Pokrovsky and others (2006). The higher the concentration of organic carbon in the
rivers, the thicker the gray line. In most cases, the normalized citrate-containing and ligand-free column
discharges bracket the normalized concentrations of the rivers. Where elements are not plotted, concentrations were unavailable for the rivers (for example, P), or below detection (ligand-free column). Elements are
organized after the periodic table, with first s block elements, from smallest to largest atomic radius, then d
block elements, then p block and f block elements. This allows the comparison of elements with similar
properties.
found in these rivers are largely controlled by the colloidal fraction, although they have
not determined whether the colloids are organic, inorganic, or both.
These results span a wide range of climatic conditions. The similarity of the
elements that appear to be enhanced by the presence of organics in natural environments (Al, Fe, Th, Zr, Y and REE) to those enhanced in the column experiments (Y,
La, Ce, Th, Al, P, Ti, Fe, Pb and Ni in both granite and basalt-containing columns, Zr,
Sc, and Mn in basalt-containing columns, and V and Zn in granite-containing columns) is additional evidence that these metals may function as important biosignatures.
We have compared the average elemental concentrations for weeks 40 to 45 of the
basalt and granite-containing columns with and without citrate to river concentrations
draining basalt and granite lithologies (figs. 6 and 7). To make the comparison we
normalized the river concentrations either to the basalt BCR-1 or the granite from the
Tuolomne River Series. In most cases, the citrate-containing column and the ligandfree column bracket the normalized river values (figs. 6 and 7). The heavier lines
represent higher concentrations of dissolved organic carbon, and in some cases it
from dissolving basalt and granite with and without organic ligands
655
Fig. 7. Granite column discharge (average of weeks 40-45) and rivers draining a similar lithology
concentrations normalized to Tuolomne river granite. Column discharges are indicated by heavy black lines
(solid ⫽ citrate-containing, dotted⫽ligand-free), and the rivers by the gray solid lines. River concentrations
are from Tosiani and others (2004), Viers and others (1997), and Viers and others (2000). The higher the
concentration of organic carbon in the rivers, the thicker the gray line. In most cases, the normalized
citrate-containing and ligand-free column discharges bracket the normalized concentrations of the rivers.
Where elements are not plotted, concentrations were unavailable for the rivers (for example, P), or below
detection (ligand-free column). Elements are organized after the periodic table, with first s block elements,
from smallest to largest atomic radius, then d block elements, then p block and f block elements. This allows
the comparison of elements with similar properties.
appears that the increase in elemental concentration corresponds to an increase in
dissolved organic carbon (for example, elements such as Zr) and in other cases it
appears that other factors such as climate, vegetation, or relief might be playing a
stronger role.
If the organic content of the river is controlling the elemental concentrations in
the river water, and the stability complex with citrate were similar to the stability
complex of the natural organic matter, we would expect increasing concentrations
with increasing strength of the stability constant as was observed for the different
elements in the basalt and granite-containing columns (fig. 8). To normalize the river
concentrations for effects that might change all elemental concentrations (for example, water-rock ratios), we divided each normalized elemental concentration by the
concentration of another element (Ca). The concentrations from the citratecontaining columns increase with increasing stability constant (fig. 8), while the
ligand-free columns display no apparent trend. However, when we plot the river data
in the same manner, we observe that the river concentrations, instead of increasing
with increasing stability constant as is observed in the citrate-containing columns,
instead display a decreasing trend more typical of the ligand-free columns.
656
E.M. Hausrath, A. Neaman, and S.L. Brantley—Elemental release rates
Fig. 8. Elemental concentrations normalized by Ca for (A) basalt citrate and ligand free columns (B)
granite citrate and ligand-free columns, (C) an example organic-rich river draining basaltic lithology and
(D) an example organic-rich river draining granitic lithology. Normalized elemental concentrations
increase in the citrate-containing columns with increasing NIST stability constant, whereas normalized
concentrations in the citrate-free columns and rivers show no trend or decrease with increasing NIST
stability constant. Concentrations were first normalized to elemental concentrations in BCR-1 (if basalt were
present), or granite Tuolumne series (if granite were present).
This decreasing trend could be explained since the organic matter in the rivers is
much more dilute than the organic acids used in our experiments. Alternately, it is
possible that the soils may already have been depleted by organic-rich weathering
solutions. If the soils drained by these rivers have been leached by organic-rich
solutions, the depletions in the soils and sediments might prove biosignatures. This has
previously been tested in paleosols (Neaman and others, 2005a), and it would be
interesting to test it in modern soils as well.
Field Scale: Implications for Element Normalization
Elements often used as an assumed immobile element for mass balance calculations include Zr, Ti, Fe, and Al. In the absence of citrate, Ti is the least mobile element
from both basalt and granite. In the presence of citrate, Ti is still relatively immobile
(⬍0.14% removed from basalt and ⬍0.35% removed from granite), and is still a good
choice as an immobile element for basalt weathered in the presence of citrate.
However, Zr (0.065% removed) and Al (0.28%) are both less mobile from granite in
the presence of citrate than Ti. This suggests that Zr and Al might both be better
choices as an immobile element for mass balance calculations in weathering granite.
from dissolving basalt and granite with and without organic ligands
657
conclusions
Citrate clearly enhances the dissolution of both basalt and granite, and the
chemical signatures of this enhancement may prove a useful biosignature of biological
activity. Elemental releases indicate a strong correlation with the stability constant of
the element with citrate, and this suggests that values of association constants may be
used to infer the presence of other organic acids. Elements which showed a strong
enrichment in the presence of citrate, and might therefore be useful biosignatures
include: Zr, Sc and Mn (basalt), V and Zn (granite), and Y, La, Ce, Th, Ti, Al, P, Pb, Ni
and Fe (both basalt and granite). Release of these elements is consistent with
enhanced dissolution of apatite ⫹ Fe sulfides ⫹ Fe oxides ⫹ augite in basalt and
apatite ⫹ sphene ⫹ hornblende in granite suggesting mineralogical as well as
elemental biosignatures. Compilation of elemental concentrations in river waters
indicates that enhanced concentrations of Al, Fe, Ti, Th, Zr, Y and the REE correlate
with higher DOC in natural waters: these elements were also enhanced by the presence
of citrate in our column experiments. Differences in mineral reactivity between
ligand-free and ligand-containing solutions suggest that the presence of organic acids
in paleosols from early Earth may indeed be documented in soil minerals and element
compositions.
acknowledgments
We thank B. Alexander, J. Kittleson, H. Gong, M. Angelone, J. Catalina, T. Rusnak,
D. Voigt, L. Liermann, A. Zimmerman, R. Conrey, Maya Bhatt, and D. Eggler for their
assistance. This work was supported by funding from the NASA grant NAG5-12330,
National Science Foundation Integrative Graduate Education and Research Traineeship grant DGE-9972759, the Penn State Biogeochemical Research Initiative for
Education and the NASA Astrobiology Institute, Grant # NNA04CC06A. E.M.H. is
grateful for support from the National Science Foundation Graduate Research Fellowship Program. S.L.B. acknowledges support from the NSF-funded Center for Environmental Kinetics Analysis NSF-CHE-0431328. We appreciate the thoughtful reviews by
K. Maher and M. Velbel.
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