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Ocean productivity before about 1.9Gyr ago limited by phosphorus adsorption onto iron oxides

Authors:
Christian J. Bjerrum
Christian J. Bjerrum
Christian J. Bjerrum
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Donald Canfield at University of Southern Denmark
Donald Canfield
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Abstract and Figures

After the evolution of oxygen-producing cyanobacteria at some time before 2.7 billion years ago, oxygen production on Earth is thought to have depended on the availability of nutrients in the oceans, such as phosphorus (in the form of orthophosphate). In the modern oceans, a significant removal pathway for phosphorus occurs by way of its adsorption onto iron oxide deposits. Such deposits were thought to be more abundant in the past when, under low sulphate conditions, the formation of large amounts of iron oxides resulted in the deposition of banded iron formations. Under these circumstances, phosphorus removal by iron oxide adsorption could have been enhanced. Here we analyse the phosphorus and iron content of banded iron formations to show that ocean orthophosphate concentrations from 3.2 to 1.9 billion years ago (during the Archaean and early Proterozoic eras) were probably only ~10-25% of present-day concentrations. We suggest therefore that low phosphorus availability should have significantly reduced rates of photosynthesis and carbon burial, thereby reducing the long-term oxygen production on the early Earth-as previously speculated-and contributing to the low concentrations of atmospheric oxygen during the late Archaean and early Proterozoic.
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sion line and nonlinear dependence of the intensity are clearly
observed up to a current of about 880 mA, where laser threshold is
reached. Lasing takes place at ,4.4 THz, on the high-energy side of
the luminescence line, probably owing to the reduced waveguide
losses at shorter wavelengths. Single-mode emission is obtained,
which is probably a consequence of the relatively narrow gain
spectrum and the wide Fabry–Perot mode spacing. A high-resol-
ution laser spectrum is shown in the inset; the measured linewidth is
limited by the resolution of the spectrometer (3.75 GHz).
Figure 4 shows the light–current (L–I) and voltage–current (V–I)
characteristics of a representative device. At a heat-sink temperature
of 8 K, the output peak power is estimated to be more than 2 mW,
with a threshold current density of 290 A cm
22
. The latter is a very
small value for quantum-cascade lasers and allows operation at high
duty cycles (up to 10%) even in this large device. We expect that
narrower stripes and appropriate changes in sample processing
would readily lead to continuous-wave operation. The initial high
resistivity in the V–Icharacteristics stems from misalignment of the
sub-bands at low field; at higher fields, the injector miniband lines
up with the second miniband of the following stage and carrier
injection into the upper laser level takes place. The electric field at
threshold is 7.5 kV cm
21
. This is larger than the design value of
3.5 kV cm
21
, probably as a result of the non-negligible contact
resistance. As expected, a negative differential resistance feature
was observed at about 850 A cm
22
.
These experimental results match well the theoretical predictions
of Fig. 1b. The V–Icurve has all the distinctive qualitative features
and the measured current densities are of the same order of the
computed ones, showing that even at these small energies carrier
relaxation and transport are dominated by electron–LO phonon
and electron–electron scattering. The discrepancy is a factor of
about 1.5, possibly related to acoustic phonon or impurity scatter-
ing or to a lower than specified free-carrier density in the sample. In
fact, the simulation indicates that a reduction in the doping density
of the injectors by 25% would lead to a reduction in current density
of 35%. From the theoretical values of population inversion and
confinement factor, we calculate a maximum modal gain of 23 cm
21
(ref. 18). This value compares well with the estimated cavity losses
aWþaM¼ð16 þ4Þcm21¼20 cm21,a
M
being the mirror out-
coupling of the 3.1-mm-long stripe. This consistency is confirmed
by the experimental observation that laser stripes shorter than
1 mm, with corresponding larger mirror losses, do not reach laser
threshold.
We believe that improved design of the active region of our device
(in particular aiming at the reduction of thermal backfilling),
together with optimized fabrication (junction-down mounting,
facet coating, lateral overgrowth), would rapidly lead to continu-
ous-wave emission and to operation at liquid-nitrogen tempera-
tures. The present demonstration of a terahertz quantum-cascade
laser, operating below the LO phonon band, represents a first step
towards the development of widely usable terahertz photonics. A
Received 14 January; accepted 19 February 2002.
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Acknowledgements
We thank S. Dhillon for discussions.This work was supported in part by the European
Commission through the IST Framework V FET project WANTED. R.K. was supported by
the C.N.R.; E.H.L. and A.G.D were supported by Toshiba Research Europe Ltd and The
Royal Society, respectively.
Competing interests statement
The authors declare that they have no competing financial interests.
Correspondence and requests for materials should be addressed to R.K.
(e-mail: koehler@nest.sns.it.).
..............................................................
Ocean productivity before about
1.9 Gyr ago limited by phosphorus
adsorption onto iron oxides
Christian J. Bjerrum*†‡ & Donald E. Canfield*
*Danish Center for Earth System Science, Institute of Biology, University of
Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark
†Danish Center for Earth System Science, Niels Bohr Institute for Astronomy,
Physics and Geophysics, University of Copenhagen, Juliane Maries Vej 30, DK-
2100 Copenhagen, Denmark
.............................................................................................................................................................................
After the evolution of oxygen-producing cyanobacteria at some
time before 2.7 billion years ago
1
, oxygen production on Earth is
thought to have depended on the availability of nutrients in the
oceans, such as phosphorus (in the form of orthophosphate). In
the modern oceans, a significant removal pathway for phos-
phorus occurs by way of its adsorption onto iron oxide depos-
its
2,3
. Such deposits were thought to be more abundant in the past
when, under low sulphate conditions, the formation of large
amounts of iron oxides resulted in the deposition of banded iron
formations
4,5
. Under these circumstances, phosphorus removal
by iron oxide adsorption could have been enhanced. Here we
analyse the phosphorus and iron content of banded iron form-
ations to show that ocean orthophosphate concentrations from
‡ Present address: Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350
Copenhagen, Denmark.
letters to nature
NATURE| VOL 417 | 9 MAY 2002 | www.nature.com 159
© 2002 Macmillan Magazines Ltd
3.2 to 1.9 billion years ago (during the Archaean and early
Proterozoic eras) were probably only ,10–25% of present-day
concentrations. We suggest therefore that low phosphorus avail-
ability should have significantly reduced rates of photosynthesis
and carbon burial, thereby reducing the long-term oxygen
production on the early Earth
—
as previously speculated
4
—
and
contributing to the low concentrations of atmospheric oxygen
during the late Archaean and early Proterozoic.
The oxidation of dissolved ferrous iron produces insoluble iron
oxyhydroxides, which strongly adsorb phosphates at pH values of
less than 9 (ref. 6). The extent of phosphorus adsorption can be
expressed as: [P
ads
]¼K
ads
[P
d
][Fe
3þ
], where [P
ads
] is the concen-
tration (
m
M) of phosphate adsorbed onto iron oxides, [P
d
] is the
concentration (
m
M) of dissolved orthophosphate (PO
4
32
) in sol-
ution, [Fe
3þ
] is the concentration (
m
M) of iron oxide particles, and
K
ads
(
m
M
21
) is the adsorption constant. Particles of iron oxide
formed from Fe
2þ
oxidation in submarine hydrothermal systems
adsorb phosphate with a K
ads
value of 0.07 ^0.01
m
M
21
(ref. 7). As
phosphorus adsorbs predictably onto newly formed iron oxide
surfaces, the phosphorus content of ancient sediments rich in
iron oxide indicates, in principle, the phosphorus concentration
of the water from which the oxides formed
7
.
Here we use the phosphorus and Fe
3þ
content of Archaean and
early Proterozoic banded iron formations (BIFs), and the value for
K
ads
determined for modern hydrothermal systems
7
, to estimate the
[P
d
] of contemporaneous sea water:
½Pd¼ð1=KadsÞðPads =Fe3þÞð1Þ
The ratio P
ads
/Fe
3þ
is directly available, in many instances, from BIF
analyses (Fig. 1). An average Archaean and early Proterozoic
phosphorus concentration of 0.15 ^0.15
m
M is calculated (ranging
from 0.03 to 0.29
m
M) for the waters where BIFs deposited (Fig. 1).
Generally lower values of [P
d
] are indicated at 1.9–2.0 Gyr ago,
compared to earlier BIFs.
We have avoided siderite-rich BIFs in the analysis as some contain
unusual
13
C-depleted carbonates (
d
13
C,25‰)
8,9
, indicating the
oxidation of sedimentary organic carbon, possibly by Fe
reduction
10
, and the probable inclusion of original organic P into
the total P pool. This would complicate our calculation. Further-
more, iron oxide (re)crystallization or iron reduction during early
diagenesis could have induced a loss of P from the oxide-rich BIFs.
In modern sediments preserving hydrothermally derived iron
oxides, a loss of up to 50% of the originally adsorbed P is indicated
11
.
A 50% loss in P
ads
would mean that our calculated [P
d
] could be too
low by as much as a factor of two (ref. 11). However, this is probably a
maximum correction, because hydrothermal sediments with higher
iron content, approaching that of BIFs, have experienced ,25%
P loss
11
. Acknowledging that further tests of our approach are
required, we conservatively estimate that BIFs deposited in sea
water containing between, on average, 0.15 and 0.6
m
M dissolved P.
We note that P retention in sediments under anoxic ocean
conditions in the Archaean and early Proterozoic was probably
different from today. This is indicated by the preservation of Fe
oxides in BIFs, which deposited from anoxic ocean waters contain-
ing dissolved Fe
2þ
(ref. 4). Today, ocean anoxia results in a sulphidic
water column which effectively reacts all of the available reactive Fe
to form Fe sulphide minerals
5
. Thus, today, anoxia leads to some P
loss from sediments
12–15
, whereas anoxia during BIF deposition
probably led to P retention on Fe oxides, and as ferrous phosphate
minerals when there was Fe reduction in the sediment. The main
difference between now and then was probably low sulphate
concentrations in Archaean and early Proterozoic oceans, signifi-
cantly reducing rates of sulphate reduction
5
. Even in the modern
world, controlled sediment incubation experiments show that loss
of adsorbed P is minimal under sediment diagenesis with low
Figure 1 Element ratios in BIFs and calculated dissolved phosphate concentrations. Mole
ratios of Fe
3þ
/Fe
tot
(a) and P/Fe
tot
(b) are from the literature (see Supplementary
Information). c, Dissolved phosphate concentrations are calculated from equation (1).
Phosphate loss during early diagenesis can result in up to a factor of two error in P
d
(see
text and ref. 11). Standard deviation for each BIF unit is shown as solid error bars. Dashed
error bars indicate that no standard deviation could be calculated from the literature. In
cases where Fe
3þ
content is not available, we use the mean Fe
3þ
/Fe
tot
ratio of 0.43.
Figure 2 Simplified phosphate and iron cycle model of the Archaean and early
Proterozoic. Dissolved ferrous iron (Fe
2+
) is oxidized at the base of the oxic mixed layer,
leading to iron oxide burial (F
Fe,ox
¼aF
Fe,in
) and BIFs with adsorbed P (F
P,ads
). Organic
matter export production (EP) is limited by upwelled P and leads to the burial of organically
derived phosphate (F
P,org+CFA
). For simplicity, ferrous iron is assumed to be buried
(F
Fe,red
) without associated phosphate. The total reactive iron input, F
Fe,in
, is the sum of
iron from land (dissolved+particulate) and hydrothermal iron.
letters to nature
NATURE| VOL 417 | 9 MAY 2002 | www.nature.com160 © 2002 Macmillan Magazines Ltd
sulphate concentrations
16
. The fact that BIFs are preserved, and
contain phosphate, shows that the usual view of anoxia leading to P
release is probably not valid during the periods in Earth’s history
when BIFs formed.
BIFs deposited in environments ranging from shelf and upper
slope to the abyssal plain
17
. The Barberton BIF, associated with, and
probably related to, local volcanism, was deposited at a water depth
of approximately 900 m, the deepest water yet recognized for BIFs
18
.
The other generally larger BIFs analysed here, lacking an obvious
local volcanic source, deposited on the outer shelf or slope well
below storm wave influence
19
. The Fe-oxide-rich facies of the
Kuruman Iron Formation, a reasonable representative of the large
2.6-Gyr and younger BIFs in the data set, was deposited in deeper
water than the contemporaneous siderite-enriched BIF and shales
19
.
Furthermore, the siderite-enriched BIF formed in water with dis-
solved inorganic carbon highly
13
C-depleted compared to surface
ocean water
8,20
. Thus, the siderite-enriched BIF, and by extension
the Fe-oxide-rich facies, deposited well below a pronounced che-
mocline where the accumulation of phosphate should have
accompanied the steep gradients in isotopic composition of dis-
solved inorganic carbon. Therefore, our calculated phosphate
concentrations characterize a region well below the chemocline
and may represent the deep ocean. Phosphate concentrations of
0.15 to 0.6
m
M are considerably less than the average modern ocean
value of 2.3
m
M.
Low phosphate concentrations in the Archaean and early Proter-
ozoic oceans could have arisen from changes in the weathering flux
of dissolved and reactive phosphate (F
P,in
) to the ocean and/or
significant changes in the ocean phosphate sinks (Fig. 2). We have
no evidence supporting significantly reduced weathering fluxes of
reactive phosphate. Continents may have reached their present size
early in Earth’s history and the intensity of chemical weathering,
controlled by temperature and soil pH, might have been high in the
Archaean and early Proterozoic. Alternatively, and more probably,
low P concentrations originated from a strong P sink owing to
significant adsorption onto, and removal by, iron oxide particles.
We use a simple ocean model to explore the possible influence of
BIF deposition on ocean P concentrations (Fig. 2). P is delivered to
the oceans from rivers (F
P,in
) and is removed by adsorption onto
iron oxides (P
ads
) and as organically derived reactive P (P
orgþCFA
),
which includes organic-bound P and P liberated from organic
matter and mainly reprecipitated into carbonate fluorapatite
(CFA) (refs 12, 13). Of the total reactive iron flux (F
Fe,in
) to the
ocean
5
, only a fraction (a) is buried as Fe oxides with adsorbed P;
the rest is assumed to be buried as ferrous iron phases to which P
does not adsorb (that is, sulphides and siderite). The burial flux of
adsorbed P is then:
FP;ads ¼ðKads½PdÞðaFFe;in Þð2Þ
The burial flux of P
orgþCFA
(designated as F
P,orgþCFA
) depends on
[P
d
], the upwelling velocity, u, of deep water supplying P
d
to the
euphotic zone, and the relationship, g, between the export of
organic P from the surface ocean and its burial
4,21
:
FP;orgþCFA ¼gðu½PdÞ ð3Þ
At steady state FP;in 2ðFP;orgþCFA þFP;adsÞ¼0;allowing a solution
for [P
d
]:
½Pd¼ FP;in
guþaKadsFFe;in ð4Þ
Figure 3 shows [P
d
] contoured as a function of the fraction, a,of
total reactive iron buried with adsorbed P, versus reactive phosphate
input (F
P,in
). [P
d
] increases as F
P,in
increases, and decreases with
increasing a. With today’s range in F
P,in
(ref. 12) and F
Fe,in
(ref. 5)
we find a [P
d
] similar to that obtained in the above BIF analysis if
approximately half of the reactive iron flux is buried with adsorbed
P (0.4 ,a,0.6). Thus, low values of ocean P
d
can be maintained
if about half of F
Fe,in
is removed as freshly precipitated iron oxides.
Such a high Fe
3þ
/Fe
tot
ratio is indicated for BIFs
8,19
(Fig. 1a), and if
this ratio represents the removal of F
Fe,in
in general, then the initial
adsorption of P into BIF explains the low [P
d
] inferred for Archaean
and early Proterozoic sea water.
Low values of ocean [P
d
] would have probably limited rates of
primary production, and by extension, organic carbon burial, and
the input of oxygen to the atmosphere
4,20,22
. The relationship
between [P
d
] and the burial rate of organic carbon (F
C,org
)is
given as the product of equation (4) and the ratio of organic carbon
to organically derived P (h
c:p
¼C
org
/P
orgþCFA
):
FC;org ¼gðu½PdÞhc:p ð5Þ
Figure 3 Modelled mean ocean phosphate concentration [P
d
] from equation (4). a, The
standard case with K
ads
¼0.07
m
M
21
;b, The possible case of a 50% P
ads
loss during
early diagenesis with K
ads
<0.035
m
M
21
. The [P
d
] values (in
m
M) are shown on the
contours as a function of the fraction (a) of total iron buried with adsorbed P, and reactive
phosphate input (F
P,in
). Very low ocean phosphate concentrations occur if a large fraction
of reactive iron input is buried with adsorbed P. Boxed regions correspond to the
diagnosed Archaean conditions with the pre-industrial range of F
P,in
(refs 12, 30) and
present F
Fe,in
(ref. 5). At present P, burial fluxes after early diagenesis are partitioned into:
FP;org ¼3:6£1010 mol P yr21;FP;CFA ¼3:6£1010 mol P yr21and FP;ads ¼
0:7£1010 mol P yr21(refs 12, 30). Part of the F
P,CFA
is originally reprecipitated from P
liberated from decaying organic matter as well as P from reduction of iron oxides.
Assuming, conservatively, that ,50% of the CFA burial flux originates from decay of
organic matter, then FP;orgþCFA ¼5:4£1010 mol P yr21:With a burial efficiency g¼
1:9£1022;the above F
P,org+CFA
is reproduced for the present mean ocean P
concentration (2.3
m
M) and mixing u¼1:26 £1018 lyr
21:At present F
P,org+CFA
is
associated with an organic carbon burial of ,1£10
13
mol P yr
21
(ref. 23), which gives a
C
org
/P
org+CFA
ratio of 185.
letters to nature
NATURE| VOL 417 | 9 MAY 2002 | www.nature.com 161
© 2002 Macmillan Magazines Ltd
With values of g,uand h
c:p
comparable to today, a reduction in [P
d
]
to 10–25% of present-day concentration (as we infer for the late
Archaean and early Proterozoic) implies a 75–90% reduction in the
rate of organic carbon burial. By contrast, if much less P
orgþCFA
was
buried with organic carbon, as has been argued for modern euxinic
sediments
14
(see also in ref. 15), then low [P
d
] would support
substantial organic carbon burial, and oxygen production
4
. Thus,
with h
c:p
<1,000, carbon burial would approach modern values
23
.
However, we believe that such high h
c:p
ratios are probably in-
appropriate for an anoxic Fe-containing Precambrian ocean for two
reasons. First, the high h
c:p
ratios in modern euxinic sediments are
partly a result of P loss
13–15
under high sulphate conditions that were
not present during BIF deposition
24
. Second, even in modern
settings, h
c:p
may be much lower than previously thought
15
, with
ratios of 150–200 recently reported from the euxinic, and rapidly
depositing, Saanich Inlet sediments
13
.Thus,low[P
d
]inlate
Archaean and early Proterozoic oceans should have resulted in
reduced rates of carbon burial. Higher heat flow and tectonic
activity could have accelerated rates of geochemical cycling includ-
ing rates of all of the processes discussed here, although the
quantitative influence of heat flow on geochemical cycling rates is
poorly known
25,26
.
Previous analysis of the marine isotope record of organic and
inorganic carbon suggests that for the Archaean (.2.5 Gyr ago)
around 11% of the total carbon buried in ocean sediments was
removed as organic carbon
27
. By comparison, today, organic
carbon represents 20% of the total carbon removal
27
, or a burial
percentage about twice that in the Archaean. Our results are
generally consistent with this, and suggest reduced rates of organic
carbon burial in the late Archaean and early Proterozoic. Further
modelling studies, and a better resolution of the carbon isotope
record, would better help to establish the concordance between our
reconstruction of late Archaean and early Proterozoic nutrient
chemistry and the carbon burial history as revealed from carbon
isotope studies.
In addition to indicating relatively reduced rates of organic
carbon burial in the Archaean, the isotope record of marine
carbonates demonstrates at least one, and perhaps several, very
large positive isotope excursions between 2.4 and 2.0 Gyr ago (refs
20, 27). These excursions indicate large increases in the relative
proportions of organic carbon burial in ocean sediments, and
possibly also indicate increases in the rate of organic carbon
deposition
27
. Recent compilations of the timing of BIF deposition
indicate a curious lack of BIFs during this same time from 2.4 to
around 2.0 Gyr ago (ref. 28). Furthermore, the isotope record of
sedimentary sulphides
24
shows the first occurrence of highly
34
S-
depleted marine sulphides around 2.4 Gyr ago. The sulphur isotope
record indicates an increase in seawater sulphate concentrations to
,1 mM at around 2.4Gyr ago, and a consequent increase in rates of
sulphate reduction due to more sulphate availability
24
. We propose
that the burial pulse of organic carbon between 2.4 and 2.0 Gyr ago
could have been driven, at least in part, by the P made available by
increasing rates of sulphate reduction. Therefore, the rise in atmos-
pheric oxygen concentrations
4,20
that apparently accompanied the
burial pulse of organic carbon
27
could also, in part, have resulted
from a reduction in the Fe oxide sink and a consequently larger
availability of P.
The decline of BIFs between 2.4 and 2.0 Gyr ago (ref. 28) could
have been a logical consequence of the titration of deep-ocean Fe
with sulphide owing to increasing rates of sulphate reduction
5
. For
some reason not yet clear, BIF deposition, decreased organic carbon
burial, and low P concentrations were reinitiated around 2.0 Gyr
ago, persisting until around 1.8 Gyr ago (Fig. 1). Thereafter, con-
sistent with an earlier proposal
5
, sulphidic conditions reoccurred.
There is no indication of high burial fluxes of organic carbon after
1.8 Gyr ago, even though the iron oxide sink for phosphorus would
have been substantially reduced under sulphidic ocean conditions.
Another nutrient may then have limited ocean primary pro-
ductivity
29
.A
Received 23 August 2001; accepted 26 March 2002.
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Supplementary Information accompanies the paper on Nature’s website
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Acknowledgements
We thank J. Hayes and T. Lenton for comments and suggestions. This work was funded by
the Danish National Research Foundation (Danmarks Grundforskingsfond) and the
Danish National Science Foundation (SNF).
Competing interests statement
The authors declare that they have no competing financial interests.
Correspondence and requests for materials should be addressed to C.J.B.
(e-mail: cjb@geo.geol.ku.dk).
letters to nature
NATURE| VOL 417 | 9 MAY 2002 | www.nature.com162 © 2002 Macmillan Magazines Ltd
... Phosphorus therefore directly controls rates of photosynthetic oxygen production, and indirectly the redox state of the atmosphere and oceans (Van Cappellen and Ingall, 1996;Lenton and Watson, 2000b), and even the tempo of biological evolution (Elser et al., 1996;Elser et al., 2006). Thus, oceanic phosphate availability through time is of profound importance for understanding the co-evolution of life and Earth environments (Bjerrum and Canfield, 2002;Planavsky et al., 2010;Reinhard et al., 2017), and considered to be a driving force for periods of major environmental and evolutionary change (e.g. Papineau, 2010;Bekker and Holland, 2012;Lenton et al., 2012;Li et al., 2018). ...
... Unlike nitrogen, phosphorus only has one stable isotope, making it impossible to track mass fluxes by isotope mass balance methods. Alternative methods such as phosphorus speciation (Ruttenberg, 2003;Thompson et al., 2019), P/Fe ratios in iron formations (Bjerrum and Canfield, 2002;Planavsky et al., 2010) and bulk-rock phosphorus concentrations have been developed to constrain its distribution in the geological record (Reinhard et al., 2017). While these techniques can provide valuable information on the phosphorus cycle, they also have limitations. ...
... Speciation data provide valuable information regarding phosphorus cycling within sediments (Filippelli and Delaney, 1995;Bowyer et al., 2020) which influences oceanic phosphate availability (Van Cappellen and Ingall, 1996;Delaney, 1998;Lenton and Watson, 2000a), but this technique cannot track phosphorus cycling within the water column (Ruttenberg, 2003). Application of P/Fe ratios is limited by the sparse temporal occurrences of iron formations, uncertainties about the silica concentration of ancient oceans (Konhauser et al., 2007;Jones et al., 2015), and diagenetic effects related to iron-oxide reduction and phosphorus remobilisation (Bjerrum and Canfield, 2002). Bulk-rock concentrations can document delivery of phosphorus to the sediment but are not necessarily tightly coupled to oceanic phosphate concentrations due to bottom-water redox influences on phosphorus retention in the sediment (Ingall et al., 1993;Slomp and van Raaphorst, 1993;Ingall and Jahnke, 1997;Algeo and Ingall, 2007), and potential variation in nutrient uptake ratios by primary producers (Reinhard et al., 2017). ...
Development of carbonate-associated phosphate (CAP) as a proxy for reconstructing ancient ocean phosphate levels
Article
  • Mar 2021
  • GEOCHIM COSMOCHIM AC
Phosphorus is considered the ultimate limiting nutrient in the oceans over geological timescales. Phosphate (PO4³⁻) availability consequently exerts control on the global carbon cycle and atmospheric/oceanic redox conditions over million-year timescales. Despite its importance, there are no established tools that can directly and continuously reconstruct oceanic phosphate concentrations through time. Here, we report a new approach to carbonate-associated phosphate (CAP) extraction and a series of experiment-based constraints for using CAP as a tool for reconstructing ancient oceanic phosphate concentrations. Experimental work shows that phosphate is incorporated into carbonate at various concentrations as a function of solution phosphate concentration, pH, temperature, and carbonate mineralogy. It is recommended here that in order to selectively extract CAP, carbonate sediments should be partially leached in order to avoid contamination from non-carbonate phases. Analyses of recent and ancient carbonates indicate that CAP values may shift during neomorphic aragonite-to-calcite transformations and secondary alteration. Diagenetic carbonate concretions in the organic- and phosphorus-rich Monterey Formation yield elevated CAP values that correlate with indicators of significant organic matter oxidation; however, this pattern was not observed for geologically young, weakly altered carbonates in the Marion Platform (north-eastern Australian shelf). These observations suggest CAP can yield reliable information on secular variation in oceanic phosphate concentrations and semi-quantitative reconstructions, if samples are characterised mineralogically and screened for secondary alteration.
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... For most of Earth's history, the ocean interior is thought to have been predominantly anoxic (Fig. 1;Lyons et al., 2014), which implies that reduced forms of iron and sulphur would have dominated the marine redox landscape (Poulton and Canfield, 2011;Raiswell and Canfield, 2012). 15 Whether an anoxic water body becomes ferruginous or euxinic can have significant impacts on the availability of nutrients, with ferruginous conditions potentially leading to phosphate limitation, and euxinic conditions potentially leading to depletion of key biological trace elements (Van Cappellen and Ingall, 1996;Bjerrum and Canfield, 2002;Reinhard et al., 2013Reinhard et al., , 2017Wallmann et al., 2019). Furthermore, before the advent of oxygenic photosynthesis, the productivity of marine ecosystems was likely, at least partly, fuelled by the oxidation of H 2 S or F e 2+ to their oxidised counterparts (Kharecha et al., 2005;Canfield 20 et al., 2006;Ozaki et al., 2018;Thompson et al., 2019). ...
... Whether the ocean interior is ferruginous or euxinic can significantly impact the supply of essential nutrients to surface ocean environments, and thus nutrient availability for primary producers (Van Cappellen and Ingall, 1996;Bjerrum and Canfield, cGEnIE is an earth system model of intermediate complexity (EMIC) which comprises a modular framework that incorporates different components of the Earth system, including ocean circulation and biogeochemical cycling, ocean-atmosphere and ocean-sediment exchange, and the long-term (geological) cycle carbon and various solid-Earth derived tracers Colbourn et al., 2013;Adloff et al., 2020). Here, we use the current 'muffin' release that 5 encompasses a range of developments and/or additions in the representation of: temperature-dependent metabolic processes in the ocean (Crichton et al., 2020), ocean-atmosphere cycling of methane (Reinhard et al., 2020a), marine ecosystems (Ward et al., 2018), organic matter preservation and burial in marine sediments (Hülse et al., 2018), and geological cycles of weatheringrelevant trace-metals and isotopes (Adloff et al., 2020). ...
... We anticipate that this will both improve the realism of tracer fields within the ocean interior and will make comparisons between predicted sedimentary signals and observations from Earth's sedimentary rock record more accurate and robust. Second, there are likely to be important mechanistic links between the 5 biogeochemistry of Fe and S within the ocean and the local and global recycling and bioavailability of key nutrient species for the biosphere (Bjerrum and Canfield, 2002;Laakso and Schrag, 2014;Jones et al., 2015;Reinhard et al., 2017). Future work will thus also focus on explicitly coupling the anoxic Fe and S biogeochemistry to the phosphorus (P) and nitrogen (N) cycles, and in particular the scavenging and remobilisation of P under different redox states and the impact of dissolved Fe availability on nitrogen fixation. ...
Anoxic iron and sulphur cycling in the cGENIE.muffin Earth system model (v0.9.16)
Preprint
Full-text available
  • Nov 2020
The coupled biogeochemical cycles of iron and sulphur are central to the long-term biogeochemical evolution of Earth's oceans. For instance, before the development of a persistently oxygenated deep ocean, the ocean interior likely alternated between states buffered by reduced sulphur ('euxinic') vs. buffered by reduced iron ('ferruginous'), with important implications for the cycles and hence bioavailability of dissolved iron (and phosphate). Even after atmospheric oxygen concentrations rose to modern-like values, the ocean continued, episodically, to develop regions of euxinic or ferruginous conditions, 5 such as associated with past key intervals of organic carbon deposition (e.g. during the Cretaceous) as well as extinction events (e.g. at the Permian/Triassic boundary). A better understanding of the cycling of iron and sulphur in an anoxic ocean, how geochemical patterns in the ocean relate to the available spatially heterogeneous geological observations, and quantification of the feedback strengths between nutrient cycling, biological productivity, and ocean redox, requires a spatially-resolved representation of ocean circulation together with an extended set of (bio)geochemical reactions. 10 Here, we extend the 'muffin' release of the intermediate-complexity Earth system model cGENIE, to now include an anoxic iron and sulphur cycle, enabling the model to simulate ferruginous and euxinic redox states as well as the precipitation of reduced iron and sulphur minerals (pyrite, siderite, greenalite) and attendant iron and sulphur isotope signatures, which we describe in full. While we cannot make direct model comparison with present-day (oxic) ocean observations, we use an idealized ocean configuration to explore model sensitivity across a selection of key parameters. We also present the spatial patterns of 15 concentrations and δ 56 F e isotope signatures of both dissolved and solid-phase Fe species in an anoxic ocean as an example application. Our sensitivity analyses show how the first-order results of the model are relatively robust against the choice default kinetic parameters within the Fe-S system, and that simulated concentrations and reaction rates are comparable to those observed in process analogues for ancient oceans (i.e., anoxic lakes). Future model developments will address sedimentary recycling and benthic iron fluxes back to the water column, together with the coupling of nutrient (in particular phosphate) 20 cycling to the iron cycle.
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... Because organic carbon burial is generally more efficient in the reducing marine environments that were pervasive in the Precambrian, the most obvious mechanism for dramatically reducing organic carbon burial-required to satisfy isotope mass balance given our revised ¾ org estimates for a low-oxygen Precambrian world-is to reduce nutrient availability for oxygenic phototrophs (see Derry, 2014Derry, , 2015. There are several scenarios whereby a largely anoxic ocean-a consequence of low atmospheric pO 2would be expected to trigger a nutrient crisis (Bjerrum and Canfield, 2002;Fennel and others, 2005;Laakso and Schrag, 2014;Derry, 2015;Reinhard and others, 2017). Foremost, there is likely to be enhanced abiogenic phosphorous scavenging in an anoxic ocean (Bjerrum and Canfield, 2002;Derry, 2015;Reinhard and others, 2017). ...
... There are several scenarios whereby a largely anoxic ocean-a consequence of low atmospheric pO 2would be expected to trigger a nutrient crisis (Bjerrum and Canfield, 2002;Fennel and others, 2005;Laakso and Schrag, 2014;Derry, 2015;Reinhard and others, 2017). Foremost, there is likely to be enhanced abiogenic phosphorous scavenging in an anoxic ocean (Bjerrum and Canfield, 2002;Derry, 2015;Reinhard and others, 2017). Consistent with this view, a fundamental shift in the P cycle roughly coincident with Neoproterozoic oxygenation was proposed based on a record of P burial in marine sediments (Reinhard and others, 2017). ...
View
... Phosphorous has been argued to be a critical factor to maintain the widespread ferruginous condition in the ocean before the Ediacaran Period (Bjerrum and Canfield, 2002; Poulton and Canfield, 2011). It has been suggested that low phosphorus availability may have significantly reduced the rates of photosynthesis and organic carbon burial, thereby resulting in a long-term delay in the rise of atmospheric oxygen rise during the late Archaean and early Proterozoic (Bjerrum and Canfield, 2002). ...
... Phosphorous has been argued to be a critical factor to maintain the widespread ferruginous condition in the ocean before the Ediacaran Period (Bjerrum and Canfield, 2002; Poulton and Canfield, 2011). It has been suggested that low phosphorus availability may have significantly reduced the rates of photosynthesis and organic carbon burial, thereby resulting in a long-term delay in the rise of atmospheric oxygen rise during the late Archaean and early Proterozoic (Bjerrum and Canfield, 2002). In contrast to the general absence of phosphorites in early Earth's history, widespread phosphogenesis during the Ediacaran Period stands out, and is likely coupled with profound changes in atmospheric oxygenation, as well as the evolution of seawater redox conditions (Cook and Shergold, 1984; Brasier, 1992; Cook, 1992; Papineau, 2010; Planavsky et al., 2010). ...
Phosphogenesis associated with the Shuram Excursion: Petrographic and geochemical observations from the Ediacaran Doushantuo Formation of South China
Article
  • May 2016
  • SEDIMENT GEOL
The Ediacaran Period witnessed one of the largest phosphogenic events in Earth's history. Coincidently, some phosphorite deposits in South China are associated with the largest carbon isotope negative excursion in Earth history (i.e., Shuram Excursion), suggesting an intimate coupling of the biogeochemical carbon and phosphorous cycles. However, the geomicrobiological linkage between these anomalies remain poorly understood. In this study, we investigated the petrography and geochemistry of phosphorite samples collected from the uppermost Doushantuo Formation in South China. Carbon isotope compositions of authigenic calcite cements and nodules in the phosphorites samples are as low as − 34‰ (V-PDB). Petrographic and geochemical investigations indicate that the 13C-depleted carbonates likely formed as the result of microbial sulfate reduction that released phosphorous from iron oxyhydroxide, concentrating phosphorous in pore waters, and thereby promoting phosphate mineralization. The timing of this event appears to coincide with enhanced sulfate delivery to seawater through continental weathering. The basin-scale distribution of Doushantuo phosphorites suggests a redox control associated with the availability of iron oxyhydroxide and the recycling of pore water phosphorous. Both inner and outer shelf regions were likely characterized by an oxic water column, and were the main loci for phosphogenesis; on the contrary, intra-shelf and slope regions, which are lean in phosphorite, were subjected to euxinic or ferruginous water column conditions. The intimate coupling between Ediacaran phosphogenesis and the Shuram Excursion suggests strong links among seawater redox conditions, carbon–sulfur–phosphorous cycling, and fossil phosphatization. Increased microbial sulfate reduction driven by enhanced sulfate reservoir in the Ediacaran ocean may played an essential role on these biogeochemical events.
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... For example, Boyle et al. (2015) used P profiles from lake sediments in the UK to infer catchment P inputs over the last 10 000 years and linked that to the historical evolution in population density. Similarly, banded iron formations in deep oceanic waters allowed the inference of oceanic P concentrations of over 2 billion years ago (Bjerrum and Canfield, 2002). Likewise, the sediments deposited by rivers or lakes react with surface water PO 4 and are deposited in regularly flooded areas. ...
Estimation of the natural background of phosphate in a lowland river using tidal marsh sediment cores
Article
Full-text available
  • Feb 2022
  • BG
Elevated phosphate (PO4) concentrations can harm the ecological status in water by eutrophication. In the majority of surface waters in lowland regions such as Flanders (Belgium), the local PO4 levels exceed the limits defined by environmental policy and fail to decrease, despite decreasing total phosphorus (P) emissions. In order to underpin the definition of current limits, this study was set up to identify the pre-industrial background PO4 concentration in surface water of the Scheldt River, a tidal river in Flanders. We used the sedimentary records preserved in tidal marsh sediment cores as an archive for reconstructing historical changes in surface water PO4. For sediment samples at sequential depths below the sediment surface, we dated the time of sediment deposition and analysed the extractable sediment P. The resulting time series of sediment P was linked to the time series of measured surface water-PO4 concentrations (data 1967–present). By combining those datasets, the sorption characteristics of the sediment could be described using a Langmuir-type sorption model. The calibrated sorption model allowed us to estimate a pre-industrial background surface water PO4 levels, based on deeper sediment P that stabilized at concentrations smaller than the modern. In three out of the four cores, the sediment P peaked around 1980, coinciding with the surface water PO4. The estimated pre-industrial (∼1800) background PO4 concentration in the Scheldt River water was 62 [57; 66 (95 % CI)] µg PO4-P L−1. That concentration exceeds the previously estimated natural background values in Flanders (15–35 µg TP L−1) and is about half of the prevailing limit in the Scheldt River (120 µg PO4-P L−1). In the 1930s, river water concentrations were estimated at 140 [128; 148] µg PO4-P L−1, already exceeding the current limit. The method developed here proved useful for reconstructing historical background PO4 concentrations of a lowland tidal river. A similar approach can apply to other lowland tidal rivers to provide a scientific basis for local catchment-specific PO4 backgrounds.
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... Critical to the genetic material (nucleotides), membranes (phospholipids) and energy transfer (ATP) in cells, its availability is thought to limit primary productivity on geological timescales ( Laakso and Schrag, 2018;Ruttenberg, 2013;Tyrrell, 1999). Temporal variations in the P content of sedimentary rocks such as iron formation and carbonaceous shale suggest low levels of phosphate in Archean marine sediments (Bjerrum and Canfield, 2002;Planavsky et al., 2010;Reinhard et al., 2017), but quantification of the P cycle and how it changed through a billion years of recorded Archean history remain a challenge, hindering our understanding of the role played by P in biosphere/geosphere co-evolution on the early Earth. ...
Cycling Phosphorus on the Archean Earth: Part I. Continental weathering and riverine transport of phosphorus
Article
  • Jan 2020
  • GEOCHIM COSMOCHIM AC
Phosphorus (P) is the key nutrient thought to limit primary productivity on geological timescales. Phosphate levels in Archean marine sediments are low, but quantification of the P cycle and how it changed through a billion years of recorded Archean history remain a challenge, hindering our understanding of the role played by P in biosphere/geosphere co-evolution on the early Earth. Here, we design kinetic and thermodynamic models to quantitatively assess one key component of the early P cycle – continental weathering – by considering the emergence and elevation of continents, as well as the evolution of climate, the atmosphere, and the absence of macroscopic vegetation during the Archean Eon. Our results suggest that the weathering rate of apatite, the major P-hosting mineral in the rocks, was at least five times higher in the Archean Eon than today, attributable to high levels of pCO2,g. Despite this, the weathering flux of P to the oceans was negligible in the early Archean Eon, increasing to a level comparable to or greater than the modern by the end of eon, a consequence of accelerating continental emergence. Furthermore, our thermodynamic calculations indicate high solubilities of primary and secondary P-hosting minerals in the acidic weathering fluids on land, linking to high Archean pCO2,g. Thus, weathering of P was both kinetically and thermodynamically favorable on the Archean Earth, and river water could transport high levels of dissolved P to the oceans, as also supported by the observed P-depletion in our new compilation of Archean paleosols. Lastly, we evaluated the relative rates of physical erosion and chemical weathering of silicates during the Archean Eon. The results suggest that continental weathering on the early and middle Archean Earth might have been transport-limited due to low erosion rates associated with limited subaerial emergence and low plateau elevations; by the late Archean, however, continental weathering would have transited to kinetically-limited state because of continental emergence, increased plateau elevation, and weakening weathering rates. Overall, our weathering calculations together with paleosol evidence indicate an increasing flux of bioavailable P to the oceans through time, associated with late Archean continental emergence, reaching levels comparable to or higher than modern values by the end of the eon. Increased P fluxes could have fueled increasing rates of primary production, including oxygenic photosynthesis, through time, facilitating the irreversible oxidation of the Earth’s atmosphere early in the Proterozoic Eon.
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... In the Fe-rich oceans of the Archaean, P burial rates could have been universally high due to extensive sorption onto Fe oxides (e.g. [48,49]) and a paucity of electron donors for organic matter remineralization [40]. The redox-stratified oceans that characterized the Proterozoic [50] could have promoted high P burial in ferruginous deep-water sediments [51] (cf. ...
Biogeodynamics: bridging the gap between surface and deep Earth processes
Article
Full-text available
Life is sustained by a critical and not insubstantial set of elements, nearly all of which are contained within large rock reservoirs and cycled between Earth's surface and the mantle via subduction zone plate tectonics. Over geologic time scales, plate tectonics plays a critical role in recycling subducted bioactive elements lost to the mantle back to the ocean–biosphere system, via outgassing and volcanism. Biology additionally relies on tectonic processes to supply rock-bound ‘nutrients’ to marine and terrestrial ecosystems via uplift and erosion. Thus, the development of modern-style plate tectonics and the generation of stable continents were key events in the evolution of the biosphere on Earth, and similar tectonic processes could be crucial for the development of habitability on exoplanets. Despite this vital ‘biogeodynamic’ connection, directly testing hypotheses about feedbacks between the deep Earth and the biosphere remains challenging. Here, I discuss potential avenues to bridge the biosphere–geosphere gap, focusing specifically on the global cycling and bioavailability of major nutrients (nitrogen and phosphorus) over geologic time scales. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.
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... Lower terrestrial weathering fluxes of phosphorus (relative to present) have been predicted, due to a shift from terrestrial to seafloor weathering to balance the carbon cycle earlier in Earth history, and this would have tended to reduce ocean phosphorus concentration, because seafloor weathering is not a source of phosphorus (Mills et al., 2014 ). Initial work estimated only ∼ 10–25 % of today's phosphorus concentration in the Late Archean ocean (Bjerrum and Canfield, 2002), however subsequent studies have revised this upwards to ∼ 1–4 times present-day phosphorus concentration (Planavsky et al., 2010 ). Furthermore, nutrient recycling by the microbial loop within the surface ocean (Azam et al., 1983 ) was conceivably more efficient than today because eukaryotic mechanisms of exporting organic matter out of the surface ocean were absent. ...
Revolutions in energy input and material cycling in Earth history and human history
Article
Full-text available
  • Apr 2016
Major revolutions in energy capture have occurred in both Earth and human history, with each transition resulting in higher energy input, altered material cycles and major consequences for the internal organization of the respective systems. In Earth history, we identify the origin of anoxygenic photosynthesis, the origin of oxygenic photosynthesis, and land colonization by eukaryotic photosynthesizers as step changes in free energy input to the biosphere. In human history we focus on the Palaeolithic use of fire, the Neolithic revolution to farming, and the Industrial revolution as step changes in free energy input to human societies. In each case we try to quantify the resulting increase in energy input, and discuss the consequences for material cycling and for biological and social organization. For most of human history, energy use by humans was but a tiny fraction of the overall energy input to the biosphere, as would be expected for any heterotrophic species. However, the industrial revolution gave humans the capacity to push energy inputs towards planetary scales and by the end of the 20th century human energy use had reached a magnitude comparable to the biosphere. By distinguishing world regions and income brackets we show the unequal distribution in energy and material use among contemporary humans. Looking ahead, a prospective sustainability revolution will require scaling up new renewable and decarbonized energy technologies and the development of much more efficient material recycling systems – thus creating a more autotrophic social metabolism. Such a transition must also anticipate a level of social organization that can implement the changes in energy input and material cycling without losing the large achievements in standard of living and individual liberation associated with industrial societies.
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Controls on the Sediment Geochemistry of a Low-Oxygen Precambrian Analogue
Thesis
  • Jan 2019
With early life presumed to have evolved in ancient oceans and lakes, identifying the availability of oxygen (e.g. redox chemistry) in these environments is necessary to establish the environmental conditions required for early biosphere evolution. However, the sediments of ancient oceans are the only relics of their existence, making sediment geochemical analyses critical tools for analyzing ancient biogeochemistry. In particular, the geochemistry of elements that are sensitive to oxygen (e.g. redox-sensitive metals, such as iron, manganese, and molybdenum) have proven useful for considering the biogeochemical cycling of modern environments. Subsequently, these geochemical tools have been considered robust proxies for identifying the redox chemistry of ancient systems such as Proterozoic (~0.5–2.3) oceans, wherein microbial life diversified and eukaryotic life evolved with limited atmospheric and aquatic oxygen. This dissertation uses a Proterozoic ocean analogue—the Middle Island Sinkhole (MIS)— to characterize the sediment geochemistry of a modern low-oxygen aquatic environment, and consider implications for the biogeochemical cycling of ancient oceans and lakes. This is achieved by 1) testing various metal redox proxies in an iron-rich environment (such as is inferred for Proterozoic oceans), and 2) assessing how the presence of a cyanobacterial microbial mat in MIS impacts macronutrient and metal burial in sediments. In Chapter II, I explore macronutrient and iron geochemistry in MIS sediments and a fully oxygenated Lake Huron control site (LH). Differences in redox between the two locales drive the enhanced burial of macronutrients in MIS, with iron speciation results consistent with known water chemistry: MIS is ferruginous and LH is oxic. Given that iron speciation in MIS is only recording a small portion of the water column, these results indicate that we must take caution when using iron geochemistry to interpret water column redox in the fossil record. In Chapter III, I use sediment redox-sensitive trace metal contents and microbial community composition in MIS and LH to consider how trace metals do or do not reflect the presence of a microbial mat. Results indicate that bulk sediment trace metal abundance cannot be used as a biosignature for the community composition of microbial mats in the analogue site. Additionally, I establish that the relationships between trace metals and organic carbon in MIS and LH are not consistent with our expectations based on their use as paleo-redox and paleo-productivity proxies. Therefore, this work impacts how we use proxy metals to interpret redox chemistry within the fossil record. In Chapter IV, I compare the sediment geochemistry of MIS to that of Proterozoic lake sediment—the Nonesuch Formation (~1.1 billion years old)—in order to determine the redox chemistry of this Proterozoic lake, and to gauge whether or not the abundance of redox-sensitive metals can help to elucidate biological productivity or atmospheric oxygen levels. Iron geochemistry describes fluctuating oxic and anoxic redox chemistry within the Nonesuch Formation, with molybdenum and uranium covariation confirming that euxinia is not necessary for moderate molybdenum burial. Altogether, the comparison of Nonesuch Formation to the modern analogue data indicates that elemental abundance is unlikely to record atmospheric oxygen, with no clear indicator for abundant biological productivity. Taken together, these results demonstrate that we need critical evaluations of metal burial mechanisms in modern ferruginous environments before we can confidently use these metals as proxies to constrain paleoenvironmental biogeochemical cycling.
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Minimal biomass deposition in banded iron formations inferred from organic matter and clay relationships
Article
Full-text available
  • Nov 2019
The cycling of iron and organic matter (OM) is thought to have been a major biogeochemical cycle in the early ferruginous oceans which contributed to the deposition of banded iron formations (BIF). However, BIF are deficient in OM, which is postulated to be the result of near-complete oxidation of OM during iron reduction. We test this idea by documenting the prevalence of OM in clays within BIF and clays in shales associated with BIF. We find in shales >80% of OM occurs in clays, but <1% occurs in clays within BIF. Instead, in BIF OM occurs with 13C-depleted carbonate and apatite, implying OM oxidation occurred. Conversely, BIF which possess primary clays would be expected to preserve OM in clays, yet this is not seen. This implies OM deposition in silicate-bearing BIF would have been minimal, this consequently stifled iron-cycling and primary productivity through the retention of nutrients in the sediments.
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Plume-related mafic volcanism and the deposition of banded iron formation
Article
Full-text available
  • Jul 1999
We have compiled a record of the geochronology of mantle plume activity between 3.8 and 1.6 Ga. Over this time period, the ages of komatiites, and those of global plumes, correlate strongly, at the 99% confidence level, with the ages of banded iron formations (BIFs). The ages of continental plumes correlate more weakly, at an overall 85% confidence level. Using the geochronological records of these events, we can define four periods characterized by mantle superplume activity. Three of these periods are also times of enhanced BIF deposition. The fourth mantle plume period may similarly be coeval with increased BIF accumulation, but the BIF chronostratigraphic resolution is not accurate enough to test this rigorously. Mantle superplume volcanism may promote BIF deposition by increasing the Fe flux to the global oceans through continental weathering and/or through submarine hydrothermal processes. It may also be enhanced by increasing the number of paleotectonic environments appropriate for BIF deposition (particularly plume-induced ocean plateaus, seamounts, and intracratonic rifts) and by promoting global anoxic, Fe-rich hydrothermal plumes in the shallow to intermediate marine water column.
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A Model of Biogeochemical Cycling of Phosphorus, Nitrogen, Oxygen, and Sulphur in the Ocean' One Step Toward a Global Climate Model
Article
Full-text available
  • Feb 1989
A ocean model has been developed which, for prescribed physics, deals with interrelationships between chemical distributions, biogeochemical sinks and sources, chemical reactions at redox fronts, and trans-ports across the air-sea and sediment-water interfaces. In its first application here, the model focuses on biogeochemical cycling of phosphorus, nitrogen, oxygen, and sulphur in an ocean forced by river input of nutrients. This is a natural starting point for a global climate model since ocean circulation and biology determine atmospheric CO 2 concentrations for a given inventory of inorganic C and oceanic production is controlled mainly by the availability of inorganic P and/or N. A general approach is taken to look at oxic versus anoxic conditions, P versus N limitation of primary production, with or without inorganic removal of phosphate to the sediments. As demanded by this approach, the model is nonlinear and continuous in a vertical coordinate. To focus on the biogeochemical aspects, ocean physics are kept as simple as possible. Cold, oxygen-rich water sinks at high latitudes and is upwelled with a constant velocity. Turbulent mixing is parameterized with a constant, vertical diffusion coefficient. The biogeoche-mical processes considered are new production, burial, nitrogen fixation, phosphorite formation, and three types of organic decomposition: oxidation with 0 2, denitrification, and sulphate reduction. Organic matter is taken to consist of a high-and a low-reactive fraction. The chemical species considered explicitly are PO,, 3--P, NO 3 --N, 0 2, NH 4 +-N and H2S-S. Results indicate that a change from oxic to weakly anoxic conditions at middepths in a P-limited ocean would lead to strong local denitrification and low nitrate concentrations throughout the water column. New production would also become dominated by nitrogen fixers. Geological evidence implies that anoxic conditions in the water column have been rare in the Phanerozoic ocean. Both phosphorite formation (for P limitation) and denitrifica-tion (for N limitation) can strongly constrain primary production and the development of anoxia. N limitation, i.e., negligable nitrogen fixation, practically precludes anoxia but is unlikely for very long times scales. For P limitation and no phosphorite formation the model indicates that the redox state of the ocean may be most sensitive to changes in ocean biology followed by changes in ocean circulation and mixing and finally by changes in ocean temperature.
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Phosphate Mobilization in Iron-Rich Anaerobic Sediments: Microbial Fe(III) Oxide Reduction Versus Iron-Sulfide Formation
Article
Full-text available
  • Jun 1997
  • Arch Hydrobiol
Mechanisms of phosphate (PO43-) mobilization and retention were examined in iron-rich anaerobic freshwater wetland, lake, and coastal marine sediments. Direct microbial Fe(III) oxide reduction solubilized only 3-25% of initial solid-phase PO43 during sulfate-free sediment incubation experiments. Experiments with reduced, non-sulfidic solid-phase Fe(II) rich sediment demonstrated PO43- sorption by the solid-phase, and chemical equilibrium calculations indicated that conditions were favorable for precipitation of Fe(II)-PO4 minerals [e.g. Fe3(PO4)2] in such sediments. These results suggested that much of the PO43- released from Fe(III) oxides during microbial Fe(III) reduction was captured by solid-phase reduced iron compounds (Fe(II) hydroxide-PO4 complexes and/or Fe(II)-PO4 minerals). Enhanced liberation of PO43- to sediment porewaters (33-100 % of initial solid-phase PO43-) occurred during anaerobic incubation in the presence of abundant sulfate and was directly correlated with sulfate reduction and iron-sulfide mineral formation. Incubation of PO43 amended sediment with different amounts of sulfate demonstrated a linear correlation between PO43 release and sulfate reduction. Release of PO43 to sediment porewaters during decomposition of fresh organic matter (freeze-dried cyanobacteria) was more extensive in sulfate-amended (67% of added organic P) than in sulfate-free sediment (17% of added organic P), and the ratio of dissolved PO43- released to organic carbon oxidized was seven-fold higher in sulfate amended sediment despite a common level of overall organic C and P mineralization in the two treatments. Our results demonstrate that iron rich anaerobic sediments can immobilize substantial amounts of PO43- under Fe(III) oxide reducing conditions but thai extensive PO43- release will take place if sediment Fe compounds are converted to iron-sulfides via bacterial sulfate reduction.
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Plume-related mafic volcanism and the deposition of banded iron formation
Article
Full-text available
  • Jul 1999
We have compiled a record of the geochronology of mantle plume activity between 3.8 and 1.6 Ga. Over this time period, the ages of komatiites, and those of global plumes, correlate strongly, at the 99% confidence level, with the ages of banded iron formations (BIFs). The ages of continental plumes correlate more weakly, at an overall 85% confidence level. Using the geochronological records of these events, we can define four periods characterized by mantle superplume activity. Three of these periods are also times of enhanced BIF deposition. The fourth mantle plume period may similarly be coeval with increased BIF accumulation, but the BIF chronostratigraphic resolution is not accurate enough to test this rigorously. Mantle superplume volcanism may promote BIF deposition by increasing the Fe flux to the global oceans through continental weathering and/or through submarine hydrothermal processes. It may also be enhanced by increasing the number of paleotectonic environments appropriate for BIF deposition (particularly plume-induced ocean plateaus, seamounts, and intracratonic rifts) and by promoting global anoxic, Fe-rich hydrothermal plumes in the shallow to intermediate marine water column.
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Redfield revisited: 2. What regulates the oxygen content of the atmosphere?
Article
Full-text available
  • Mar 2000
The continuous charcoal record, interpreted with the aid of the results of combustion experiments, indicates that the mixing ratio of atmospheric oxygen has varied remarkably little over the past 350 Myr. We develop a dynamic feedback model of the coupled P, N, C, and O2 cycles and use perturbation analysis and a case study of the past 40 Myr to test various feedback mechanisms that have been proposed to stabilize atmospheric oxygen. These mechanisms involve alterations in nutrient driven productivity and the subsequent burial flux of organic carbon, which provides the main source of atmospheric oxygen. Suppression of the burial of phosphorus sorbed to iron minerals under anoxic conditions in ocean bottom waters tends to increase the ocean nutrient inventory and provide negative feedback against declining oxygen [Holland, 1994]. However, denitrification is enhanced by anoxia, tending to reduce the nutrient inventory and amplify declining oxygen [Lenton and Watson, this issue]. If organic phosphorus removal from the ocean is also suppressed under anoxic conditions, this improves oxygen regulation [Van Cappellen and Ingall, 1994], as does direct enhancement of organic carbon burial due to reduced oxygen concentration in bottom waters [Betts and Holland, 1991]. However, all of the ocean-based feedback mechanisms cease to operate under increases in oxygen that remove anoxia from the ocean. Fire frequency is extremely sensitive to increases in oxygen above 21% of the atmosphere, readily suppressing vegetation on the land surface. This should transfer phosphorus from the land to the ocean, causing less carbon to be buried per unit of phosphorus and providing a weak negative feedback on oxygen [Kump, 1988]. However, a new proposal that increases in oxygen suppress the biological amplification of rock weathering and hence the input of phosphorus to the Earth system provides the most effective oxygen regulation of all the mechanisms considered. A range of proxies suggests that the input of available phosphorus to the ocean may have been significantly reduced 40 Myr ago, suppressing new production and organic carbon burial in the model. With only ocean-based feedback, the atmospheric oxygen reservoir is predicted to have shrunk from ~26% of the atmosphere 40 Myr ago. However, when land plant mediated negative feedback on phosphorus weathering is added, oxygen is regulated within 19-21% of the atmosphere throughout the past 40 Myr, in a manner more consistent with paleorecords.
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Carbon isotopes and the rise of atmospheric oxygen
Article
Full-text available
  • Oct 1996
  • GEOLOGY
New data for the isotopic composition of carbon in carbonate sediments deposited between 2.6 and 1.6 Ga indicate that the value of δ13C in these sediments underwent a very large positive excursion between 2.22 and 2.06 Ga. A reassessment of the earlier δ13C data for carbonate sediments shows that this excursion was probably worldwide, and that it was preceded and followed by several hundred million years during which the δ13C of carbonate sediments differed little from that of modern carbonates. The large δ13C excursion between 2.22 and 2.06 Ga was probably related to an abnormally high rate of organic carbon deposition, which generated an abnormally high rate of O2 production. We estimate that the total excess O2 produced during the excursion was between 12 and 22 times the present atmospheric O2 inventory. The δ13C data therefore suggest that the O2 content of the atmosphere increased very significantly between 2.22 and 2.06 Ga. This inference is supported strongly by several other lines of evidence.
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The relationship between P/Fe and V/Fe ratios in hydrothermal precipitates and dissolved phosphate in seawater
Article
A strong positive relationship between the molar P/Fe ratio in fresh hydrothermal Fe ferrihydrite precipitates and seawater-dissolved phosphate has been demonstrated using samples from the Atlantic and Pacific Oceans. We ascribe this relationship to scavenging of dissolved phosphate from seawater during the formation of hydrothermal Fe-rich particles above hydrothermal vents. In contrast, molar V/Fe ratios in hydrothermal particles are inversely correlated with dissolved phosphate. These results indicate that inter-ocean variations in dissolved phosphate control the P/Fe and V/Fe ratios in hydrothermal Fe precipitates, one of the precursors of metalliferous sediment.
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