3.6 Seawater evolution
Biogenic carbonates fulfil two of the
requirements of a sedimentary dating tool: they are fairly resistant to
diagenetic alteration, and since they are secreted directly from seawater by
the organism, they contain no detrital fraction. Unfortunately, the negligible
Rb content of carbonates precludes application of the conventional Rb)Sr dating method. However,
calibration of the seawater Sr isotope evolution path would allow the ‘initial’
87Sr/86Sr isotope ratios of carbonates to be used as an
indirect dating tool. In the following section we will assess the realisation
of this concept, as well as the application of Sr isotopes as an oceanographic
tracer.
3.6.1 Measurement of the curve
Interest in the strontium isotope composition
of seawater dates back to Wickman (1948). He argued that decay of 87Rb
to 87Sr in crustal rocks over geological time, and its subsequent
release into the hydrosphere by erosion, should lead to a 25% increase in
seawater Sr isotope composition over the last 3 Byr. This model was tested by
Gast (1955), who analysed carbonates of various ages as a means of
characterising the evolution of seawater through geological time. However, he
found that any natural variations were of the same order as the analytical
errors of 87Sr/86Sr analysis pertaining at that time (ca.
0.004), thus refuting Wickman’s model. Evidently the average crustal Rb/Sr
ratio assumed by Wickman was an over-estimate.
Resolution
of the actual extent of seawater Sr isotope variation through time had to wait
15 years for the advent of more precise mass spectrometry. Peterman et al. (1970) measured the 87Sr/86Sr
composition of macro-fossil shell carbonates with an order of magnitude
improvement in precision (" 0.0005, 2F). They found a total isotopic range of 0.0022 (4 H analytical error), which would have
been imperceptible using earlier equipment. Peterman et al. showed that, contrary to Wickman’s prediction, seawater Sr
isotope ratio actually decreased
during the Paleozoic, reaching a minimum during the Mesozoic before rising
quickly to a maximum at the present day.
In
order to avoid the effects of post-depositional alteration, Peterman et al. rejected any recrystallised shell
material, which they claimed to be able to recognise visually. The possibility
of Sr exchange between matrix and un-recrystallised shells was rendered
unlikely by the good compositional agreement between different shells in a bed.
A mixture of different types of molluscs was used (belemnites, bivalves and
brachiopods). Since no variation was seen between such classes at the present
day, they were assumed to behave in the same way as fossils.
Additional
data were collected by Dasch and Biscaye (1971) and Veizer and Compston (1974)
from different types of sample material. Dasch and Biscaye used
Cretaceous-to-Recent pelagic foraminifera, whereas Veizer and Compston analysed
‘sedimentary carbonate’ (in other words not macro-fossil carbonate) to test its
reliability for the determination of seawater Sr isotope ratios. In both cases
the authors found general agreement with the data of Peterman et al. (1970). This implies global
homogenisation of seawater Sr, which can be attributed to the very long
residence time of Sr in seawater (ca. 2.5 Myr; Hodell et. al., 1990) compared
with the average mixing time of oceanic water (ca. 1.6 kyr; section 14.1.7).
However, Veizer and Compston recognised that ‘sedimentary carbonate’ is more
susceptible to post-depositional exchange with pore-waters. They argued that
since detrital grains would normally have radiogenic Sr isotope signatures,
post-depositional exchange would normally be expected to raise 87Sr/86Sr
ratios. Therefore the minimum Sr isotope ratio found at any given time should
be the most reliable guide to contemporaneous seawater composition.
While
the analysis of whole-rock carbonate provides fewer constraints on
post-depositional processes, it provides more opportunity for sampling, and is
essential for Precambrian carbonates. Using the principles outlined above,
Veizer and Compston (1976) made a reconnaissance study of the Sr isotope
evolution of Precambrian seawater. They found uniformly unradiogenic Sr isotope
ratios in Archean carbonates, with values only slightly elevated over
contemporaneous upper mantle (Fig. 3.23). However, there was a substantial rise
in Sr isotope ratio during the Proterozoic, reaching a maximum in the early
Cambrian which was similar to the present-day composition.
Fig. 3.23. Sr isotope composition of marine
carbonates over the last 3.5 Byr, from which the isotopic evolution of seawater
is deduced (shaded band). After Veizer and Compston (1976).
A
major expansion of the seawater Sr data set was achieved by Burke et al. (1982), who presented 786
isotopic analyses of marine carbonates, phosphates and evaporites, with good
coverage of all of Phanerozoic time except the Lower Cambrian (Fig. 3.24). In
addition, work by Derry et al.
(1989), Asmerom et al. (1991) and
Kaufman et al. (1993) extended the
curve back to the Late Proterozoic. In the absence of fossil material, the
latter studies were made principally on whole-rock carbonates, which are
susceptible to contamination by fluid-borne Sr during post-depositional
alteration. Therefore, Sr was extracted from bulk carbonates by leaching with
dilute acetic acid, to reduce the amount of contamination from detrital phases
containing radiogenic Sr.
Fig. 3.24. Sr isotope data for Phanerozoic
carbonates. Solid line indicates the lower bound of most of the data, which is
the most probable seawater Sr composition. After Burke et al. (1982).
Following
the wide-ranging study of Burke et al.
(1982), subsequent work was generally devoted to improving precision on small
segments of the curve. This requires material to be well-dated
stratigraphically and carefully screened before analysis in order to exclude
the possibility of post-depositional alteration. In Paleozoic rocks, this
screening is best achieved chemically. Brand and Veizer (1980) showed that open-system
diagenesis of carbonates is accompanied by a decrease in Sr/Ca ratio and an
increase in Mn content (Fig. 3.25). However, Mn-enriched calcite can be
detected by cathodoluminescence, so that sections of shell can be screened for
alteration before sample analysis. Popp et
al. (1986) showed that samples of brachiopod shell prepared in this way
gave more reliable results than whole brachiopod shells (which were sometimes
contaminated by unradiogenic Sr) or whole-rock carbonates (which were usually
contaminated by radiogenic Sr).
Fig. 3.25. Summary of diagnostic chemical
changes which occur during the diagenetic alteration of carbonates. Boxes
represent primary fields. After Brand and Veizer (1980).
The
use of high quality brachiopod shells and belemenites from around the world
allowed Veizer et al. (1999) to present a complete Sr evolution curve for the
Mesozoic and Paleozoic, based on 1450 new analyses. They utilised the interior
shell layers from brachiopods and single laminae of belemnites, and much of
their material showed excellent preservation of textures on a sub-micron scale,
as demonstrated by examination under the scanning electron microscope (SEM).
This study therefore represents a successor to that of Burke et al. (1982) in giving an overview of
seawater Sr evolution between 100 and 500 Myr ago.
Construction
of a very precise seawater Sr isotope evolution curve for the last 100 Myr was
made easier by the availability of numerous Deep Sea Drilling Project (DSDP)
cores. These cores provide overlapping continuous sections with well preserved
microfossils such as foraminifera. Relatively constant sedimentation rates in
these sections are used to interpolate between biostratigraphic and
magnetostratigraphic calibration points. This avoids the age uncertainty
involved in correlating short stratigraphic sections from different localities.
Two
different sampling approaches have been adopted for DSDP core material. DePaolo
(1986) made a study on a single DSDP hole reaching back to the Early Miocene,
but with duplicate analysis of all samples to improve analytical precision. In
his approach, bulk samples of foram)nano-fossil ooze were analysed by direct acetic
acid leaching of washed whole-rock samples. This necessitates a correction for
post-depositional exchange, in order to determine original seawater
compositions. These corrections were based on the analysis of pore waters.
However, pore waters displayed relatively small deviations in 87Sr/86Sr
from the carbonate fraction (< 0.0001), and were also found to have Sr
contents an order of magnitude lower (Richter and DePaolo, 1987). Hence it was
argued that corrections for Sr exchange were smaller than mass spectrometric
reproducibilities.
In
the other approach (Hess et al.,
1986), hand-picked whole foram tests were analysed. These were screened for
secondary alteration by SEM examination and chemical analysis (e.g. Mn and Sr
content). Fig. 3.26 shows data from eight partially overlapping DSDP sections.
Slight scatter is seen, but much of this can be attributed to analytical error
rather than diagenetic effects. In selected samples from two sites, pore-waters
had very similar isotope ratios to forams. At one other site, pore waters were
somewhat more radiogenic, but there was no evidence that the foram data had
been perturbed. Most subsequent studies have also employed hand-picked forams.
Since less than 50 ng of Sr is now needed for a precise analysis, this may be
possible on a few or even a single foram. As an additional precaution, Martin
and Macdougall (1991) were able to break open large Cretaceous forams to
examine them by SEM for internal calcite growth.
Fig. 3.26. Plot of Sr isotope ratio against
age for forams from eight DSDP holes (distinguished by symbol shape). Solid
symbols and crosses indicate most reliable data; open symbols may be slightly
disturbed. After Hess et al. (1986).
The
high-precision seawater Sr isotope evolution curve can be used as a
stratigraphic dating tool, with a (conservative) precision as good as 0.5 Myr
for periods of rapid Sr isotope evolution, but as bad as 2 Myr during periods
of slow isotopic evolution. This precision cannot compete with biostratigraphic
dating in the Cretaceous and Tertiary periods, but it may be useful for
calibration of un-fossiliferous borehole sections (e.g. Rundberg and Smalley,
1989; McArthur et al., 2001).
An
interesting observation by Hess et al.
(1986) in their Cretaceous)Tertiary data set was a ‘spike’ in seawater Sr isotope ratio at the
so-called ‘K)T boundary’ (Fig. 3.26). They speculated as to whether a meteorite
impact could have released sufficient Sr, either from the bolide or the
terrestrial impact ejecta, to explain this peak. If the spike in 87Sr
is attributed to a meteorite, it is critical to demonstrate that it occurred at
exactly the correct stratigraphic level. Martin and Macdougall (1991) collected
data from four widely spaced localities around the world which appeared to
support the model. However, detailed analysis of sample suites close to the K)T boundary in
Fig. 3.27. Variation of Sr isotope ratio in
the vicinity of the K-T boundary, showing data of McArthur et al. on macro-fossils and leached chalk ( !) in comparison with data from Martin
and Macdougall, 1991 ( " ). After McArthur et al.
(1998).
To
examine this problem further, MacLeod et
al. (2001) studied Sr isotope variation in a section across the K)T boundary in a DSDP hole off
MacLeod
et al. considered two alternative
explanations for these observations. One is that the Tertiary taxa (solid
symbols in Fig. 3.28) were more susceptible to contamination by overgrowths
because they have thinner tests. However, an analysis of such overgrowths
suggested that this might lower rather than increase the 87Sr/86Sr
ratio (Fig. 3.28). The other explanation is that the Tertiary strata were
contaminated by Cretaceous forams reworked elsewhere from below the boundary
and then carried into the section as clastic sediment. The fact that the drill
hole comes from part way down the continental slope makes this a significant
possibility. This would imply that the Tertiary taxa best represent the
composition of seawater Sr after the impact, and hence that a very small
(0.00003) increase in seawater 87Sr/86Sr ratio occurred
across the boundary. This suggests that the impact event at the boundary did
have a small effect on seawater Sr. However, this evidence needs to be tested
at a site less susceptible to sedimentary reworking.
Fig. 3.28. Variation in St isotope ratio in
different sample types at the K-T boundary, from a DSDP site off Florida. ( " ) = Cretaceous foram taxon; ( ! ) = Tertiary foram taxon; (
<> ) = dolomite rhomb overgrowths; ( + ) = bulk carbonate; ( x ) =
silicate. After MacLeod et
al. (2001).
Neogene
seawater evolution has provided a challenge to geochemists to find the most
short-term variations in Sr isotope evolution which can be documented. In early
work on this question, Hodell et al.
(1990) determined a smooth evolution curve. However, Dia et al. (1992) and Clemens et
al. (1993) claimed to observe changes in 87Sr/86Sr,
correlated with d 18O, with a periodicity of about 0.1 Myr (Fig. 3.29 a, b,
c). This was surprising, in view of the long seawater residence of Sr. However,
subsequent work by Henderson et al.,
(1994) and Clemens et al., 1995)
failed to reproduce these cycles in three drill cores (including two used in
the original work). Instead, the new data fell on a linear evolution path
defined by Hodell et al. (1990).
Hence, the apparent periodicity in the earlier work is attributed to analytical
artefacts and does not reflect seawater Sr isotope evolution.
For
the data of Dia et al. (1992), the
analytical artefact was apparently a breakdown in the accuracy of the
fractional correction. Thus, Clemens et
al. (1995) were able to reproduce the temporal periodicity using a
linear-law fractionation correction, but this also generated a positive
correlation between 88Sr/86Sr and fractionation-corrected
87Sr/86Sr ratio, indicative of a fractionation bias
(section 2.2.3). After correction of this bias, the periodicity disappeared
(Fig. 3.29d). The data of Clemens et al.
(1993) were not subject to this bias, since the more accurate exponential law
was used. However, Henderson et al.
(1994) showed that only eight of 75 samples analysed by Clemens et al. (1993) lay outside 2F (95%) confidence limits from the
linear evolution path of Hodell et al.
(1990). Since four outliers would be expected at this confidence limit, the
apparent periodicity in this data set is probably not statistically significant
(Fig. 3.29b).
Farrell et al. (1995) carried out a study with similar sampling density and
analytical precision to the above work, but using 455 samples extending over
the past 6 Myr. These data constrain the seawater evolution curve to an average
confidence limit of " 0.00002 (2F). The curve shows undulations with a 1 – 2 Myr periodicity which are
realistic reflections of changing Sr fluxes, given a 2.5 Myr residence time of
Sr in the ocean system.
Fig. 3.29. Comparison between seawater Sr and
oxygen isotope data for the past 400 kyr. a) oxygen isotope record; b) data of
Clemens et al. (1993) expressed by 2F error limits; c, d) data of Dia et al. (1991) and Henderson et al. (1994) on the same drill core.
Modified after Henderson et al.
(1994).
3.6.2 Modelling the fluxes
The first model for the Sr isotopic composition
of seawater was constructed by Faure et
al. (1965) to explain the present-day Sr isotope ratio of the North
Atlantic. They suggested that there was a balance between the supply of
unradiogenic Sr by erosion of young volcanics, radiogenic Sr from old crustal
rocks, and Sr of intermediate composition from the erosion of carbonates. This
model was adopted by Peterman et al.
(1970) to explain the rise and fall of seawater Sr isotope ratio during the
Phanerozoic. Armstrong (1971) supplemented this model, suggesting that peaks in
seawater Sr isotope ratio during the Carboniferous and Tertiary periods were
due to enhanced glacial erosion of old shields with elevated 87Sr
contents (Fig. 3.30). However, in other ways the model remained largely
unchallenged.
Fig. 3.30. Illustration of a glacial)erosional model to explain the
seawater Sr evolution curve of Peterman et
al. (shaded band). After Armstrong (1971).
A
major advance in modelling seawater Sr evolution was the proposal of Spooner
(1976) that the unradiogenic Sr flux was due to submarine hydrothermal exchange
with basaltic crust, rather than sub-aerial erosion of basic rock. Spooner
calculated that the hydrothermal flux must be six times the magnitude of the
river water Sr flux. However, this was based on high estimates of the isotopic
composition of run-off (0.716) and hydrothermally buffered water (0.708).
Subsequent analysis of hydrothermal vent waters from the East Pacific Rise
(Albarede et al., 1981) indicated
much less radiogenic compositions. Albarede et
al. estimated the flux of hydrothermally recycled Sr as less than
one-quarter of the flux due to continental run-off. This model predicted an
average Sr isotope composition of between for run-off between 0.710 and 0.711,
in good agreement with major rivers such as the Amazon (Brass, 1976). The
magnitudes of present-day Sr fluxes were further refined by Palmer and Edmond
(1989), who measured the Sr budget and isotope composition of hydrothermal vent fluids and of most of the
world’s major rivers. Taken together, the complete data set of Palmer and
The
recognition of competing riverine and hydrothermal fluxes raises the question
of how these fluxes interacted in the past to cause variations in seawater
isotope ratio with time. Spooner (1976) assumed that the hydrothermal Sr flux
was fairly constant over time. Therefore, he attributed the increase in 87Sr/86Sr
since the Cretaceous principally to an increase in continental exposure (and
hence Sr run-off) over the last 85 Myr (Fig. 3.31). In contrast, Albarede et al. (1981) argued that a drop in the
ocean ridge Sr exchange flux from a Mesozoic value nearly four times higher was
more important than a rise in the flux of continental run-off. However, these
two effects are difficult to separate, since they are bound together as a
system. A drop in spreading rate causes ridge collapse and a consequent fall in
sea-level, so that continental exposure should increase as hydrothermal
buffering of seawater decreases.
Fig. 3.31. Plot of seawater Sr isotope composition
over the last 85 Myr against % continental flooding (relative to the present
land area). After Spooner (1976).
In
addition to run-off and hydrothermal exchange, two other fluxes have been
proposed to control seawater Sr. One which has been widely accepted, although
small in size, is the Sr released from ocean floor carbonates by diagenetic
recrystallisation (Elderfield and Gieskes, 1982). This is estimated at about
10% of the run-off flux and tends to dampen isotopic fluctuations because it recycles
old seawater Sr.
Another
proposed flux is the sub-surface outflow of continental groundwater, from below
the water table, into the sea (Fig. 3.32). This flux was termed ‘run-out’ by
Chaudhuri and Clauer (1986), who proposed that it could explain seawater Sr
isotope fluctuations that are not in harmony with variations in sea-level. For
example, run-out would be affected by the length of the continental perimeter
as well as the extent of continental uplift, so plate tectonic configurations
which form super-continents would be characterised by low run-out, whereas
fragmented continents (such as those existing at the present day) should be
characterised by high run-out. This model attributes the rising Sr isotope
ratio during the early Cretaceous (despite rising sea-level) to progressive
continental break-up at that time.
Fig. 3.32. Simplified circulation model for
the present day seawater Sr budget. Modified after DePaolo (1987).
Chaudhuri
and Clauer suggested that the run-out (continental groundwater) Sr flux could
be almost as large as the riverine run-off flux. This proposal has received
very little attention over subsequent years, but a similar model was recently
proposed by Basu et al. (2001) based
on studies of groundwater flow in the Bengal Fan. Basu et al. cited evidence that this groundwater flux could supply as
much strontium to the sea as the riverine Sr flux of the Ganges-Brahmaputra
system. If this flux was extrapolated world-wide, it might imply a doubling of
the continental Sr flux, as proposed by Chaudhuri and Clauer (1986). This would
have a fairly dramatic effect on calculations of global Sr fluxes, including a
reduction of the estimated oceanic residence time of Sr to 2 Myr. However, our
present knowledge of groundwater Sr fluxes is too poor to further constrain the
importance of this process.
3.6.3 The effects of Himalayan erosion
The
best opportunity to study the interaction of competing fluxes in the buffering
of seawater Sr is during periods of rapid change in isotope ratio with time.
The Tertiary represents one such period, which is characterised by an overall
trend of increasing Sr isotope ratio, on which several smaller steps are
superimposed. These variations can be represented in terms of the rate of
change of Sr isotope ratio with time (Fig. 3.33).
Raymo
et al. (1988) attributed the general
trend of seawater evolution over the last 40 Myr to increased rates of uplift
of the Himalayas, Tibet and Andes. This could have caused a substantial
increase in the supply of radiogenic Sr to the oceans, since the rivers which
rise in these regions (Ganges-Brahmaputra, Yangtze and Amazon) together supply
20% of the total solid load to the oceans. On the other hand, changes in the
hydrothermal Sr flux are not thought to have occurred during the Neogene, since
ocean spreading rates have been nearly uniform during this time.
Fig. 3.33. Representation of seawater Sr
isotope variation in the last 100 Myr in terms of the rate of change per
million years. After Richter et al.
(1992).
Additional
evidence for the control of seawater Sr by Himalayan erosion rates was provided
by Richter et al. (1992). Ar)Ar thermochronology was used to date
the sudden unroofing of the Quxu granite pluton, corresponding to a period of
exceptionally rapid erosion of the Tibetan plateau. The timing of this event,
which began 20 Myr ago, matches exactly with the peak rate of change in the
seawater Sr isotope record (Fig. 3.33). However, Harris (1995) claimed that
there was no evidence in the Bengal Fan for increased Himalayan erosion 20 Myr
ago. Instead, he suggested that the inferred 87Sr spike in river
water at that time was due to the exposure and chemical weathering of
meta-sedimentary rocks with a large budget of leachable 87Sr.
In
order to reach a better understanding of how variations in riverine Sr
influence seawater Sr, Palmer and
Fig. 3.34. Plot of Sr isotope ratio against
reciprocal of Sr concentration. a) For the world’s major rivers; b) for tributaries
of the Ganges. After Palmer and Edmond (1989).
Further
examination of these data (Palmer and Edmond, 1992) showed that the mixing line
for the Ganges system had a more elevated intercept (Sr isotope ratio) than for
other rivers of the world, as well as a steeper trend. Palmer and Edmond
attributed this pattern to the presence of carbonate rocks in the Ganges
watershed with abnormally radiogenic Sr isotope ratios. They speculated that
these carbonates had become enriched in radiogenic Sr by exchange with the
surrounding very radiogenic silicate rocks.
Subsequent
to this work, more detailed studies have been made of the rivers draining the
High Himalayas, which are tributaries to the Indus and Ganges-Brahmaputra river systems. For example, Blum et al. (1998) analysed river water, rock
outcrops, and river-bed sands from the Raikot watershed in northern Pakistan.
They showed that river and stream waters define a positive trend on a plot of
Sr isotope ratio against Ca/Sr ratio (Fig. 3.35). This trend runs from the
composition of marbles and marble sands at the unradiogenic end, to a
radiogenic end-member with a much higher Ca/Sr ratio than silicate rocks. Blum et al. speculated that this unknown
end-member might be calcite that had inherited its radiogenic Sr during
hydrothermal alteration of the surrounding silicate rocks.
Fig. 3.35. Plot of Sr isotope ratio against
Ca/Sr ratio for samples from the Raikot river watershed, northern Pakistan. ( ! ) = waters; ( + ) = silicate rocks;
( " ) = carbonate rocks; boxes = proposed end-members. After Blum et al. (1998).
Further
study (Jacobson and Blum, 2000) identified disseminated calcite interstitially
within silicates, at grain boundaries, and in fracture fillings. This calcite
makes up less than 0.5% of the orthogneissic rocks in the Raikot watershed, but
appears to dominate the Sr budget of streams draining this terrain. Similar
conclusions about the role of carbonate dissolution were reached by English et al. (2000), based on a study of the
Seti watershed in western Nepal. In addition, carbonates in the Seti watershed
have also been proposed as a source of radiogenic osmium in Himalayan rivers
(Pierson-Wickmann et al., 2002).
An
alternative explanation for recent increases in the Sr isotope signature of
seawater is the onset of Tertiary glaciation, as originally proposed by
Armstrong (1971). Hodell et al.
(1990) revived this model, attempting to link inflections in the Tertiary Sr
isotope evolution path to glacial advances and retreats, and this type of
argument has been examined in several subsequent papers. The basis of this
model is that glaciation creates rock flour, which is then more susceptible to
chemical weathering than in situ
crystalline rocks.
Blum
and Erel (1995) attempted to quantify the amount of radiogenic Sr that could be
released by glacial erosion. In order to do this, they used ammonium acetate
leaching to analyse the isotopic composition of exchangeable Sr in glacial
moraines. Weathered soils from six moraines in the Wind River Range, Wyoming,
displayed a negative correlation between the isotopic composition of leachable
Sr and the age of the soil (Fig. 3.36). Most notably, a very radiogenic 87Sr/86Sr
ratio of 0.795 was obtained by leaching soil from the youngest (400 yr old)
moraine. Blum and Erel used these data to argue that a spike of radiogenic Sr
is released by weathering of moraines immediately after glaciation. Modelling
of this spike suggested that it could yield an incremental increase in 87Sr/86Sr
of 0.00005 for each 100 kyr glacial cycle of the Quaternary period, thus
reproducing (within error) the seawater evolution curve for this interval.
Fig. 3.36. Sr isotope ratio in ammonium
acetate soil leachates plotted against the geological age of the moraines on which
the soils were formed. After Blum and Erel (1995).
The
glacial erosion model was further developed by Jacobson et al. (2002), who showed that this could also be a facet of
Himalayan erosion. Analysis of exchangeable Sr from weathered moraines, together
with the carbonate and silicate fractions, showed that a carbonate fraction as
low as 1% by weight could nevertheless supply as much as 90% of soluble Sr to
streams that are tributaries to the
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