Isotopic tracers of chemical weathering and consequences for marine geochemical budgets

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Isotopic tracers of chemical weathering and consequences for marine geochemical
budgets



Vance, D. (
Bristol Isotope Group,
Department of Earth Sciences, University of Bristol, Wills
Memorial Building, Bristol BS8 1RJ, UK. e
-
mail:
d.vance@bristol.ac.uk
; telephone: +44 117
954 5418; fax: +44 117 625 3385).


Abstract


The mechanical breakdown of rock by p
hysical weathering
exerts a significant control on

chemical weathering rates
because it p
roduces

surface area. During periods of icehouse
conditions on Earth, the grinding of rock by glacial processes should lead to faster chemic
al
weathering of the continents, perhaps particularly

d
uring periods of pronounced climatic
v
ariability, like the Qu
aternary
.

Evidence
is reviewed here
for both high and cyclical
chemical weather
ing rates during the Quaternary,
and the implications for both marine
geochemical budgets and climate
-
chemical weathering feedbacks are discussed.


Keywords:

chemical weathering
, marine geochemistry, soils, geochemical budgets



1.
Introduction


Chemical weathering of the continental crust is a key Earth System process, being one of the
prime mechanisms for the transfer of material between the solid Earth and
its

more dynamic
fluid envelope. For example, drawdown from the atmosphere via chemical weathering of
silicate rocks is the principal long
-
term

control (≥10
5

years) on the atmospheric concentration
of CO
2

(e.g. Walker et al. 1981
)
. Moreover, the
resultant
flu
x of dissolved material, fed
via
rivers, is the main input for many biogeochemically importan
t elements to the oceans.


Chemical weathering rates are positively correlated with Earth’s surface temperature and
with runoff.
The
paradigmatic

view of the long
-
term carbon cycle is

thus

that chemical
weathering and atmospheric CO
2

together thermostatically regulate the Earth’s surface
temperature via a negative fe
edback (e.g. Walker et al. 1981
)
. In this view,
the climatic
impact of any perturbation in the

volcan
ic
source
of CO
2

to the

surface

Earth

is
negate
d by

the response of the weathering sink from the atmosphere to changed greenhouse gas loading
.
Though the relative temporal stability of Earth’s climate
on the longest timescales
clearly
points to the importance of such a feedback, the detailed history of
climate and
atmospheric
chemistry also require
s

some variation in its efficiency.
The

rate at which rock is ground up
by physical weathering

(thus producing surface area)

is an ad
ditional variable that might be
key in explaining this detail.
In a popular set of ideas from the 1990s

(Raymo

and Ruddiman
1992
)
,

this additional control was invoked to explain one of th
e most prominent features of
atmospheric chemistry in recent Earth hi
story, the marked decline
in CO
2

concentrations
during the Cenozoic (e.g. Pagani et al. 2005).

These ideas
linked
Cenozoic
mountain
-
building, and the associated high rates of physical erosion, to fast chemical weathering that
could
potentially weaken

the
d
ominant
negative feedback.


Here,

the focus is o
n another
aspect of the link between
physical and chemical
weathering
rates: the potential role of repeated
continental
glaciation in
providing fine
-
grained substrate.
The central hypothesis is that cyclical

climate change in the icehouse world of the late
Cenozoic, and perhaps during earlier periods of icehouse conditions, leads to faster
chemical
weathering rates as a result of repeated
episodes of fresh sediment production during periods
of continental gla
ciation. In the extreme case, such a linkage could temporarily switch the
sign of the chemical weathering
-
CO
2
-
climate feedback.



2.
Temporal dynamism of physical
and chemical
weathering rates during late Cenozoic
climate change


It is well
-
established

that glaciation of the continents leads to substantially increased rates of
physical erosion

at high latitudes

(e.g. Bell and Laine 1985)
.
Related

phenomena have also
been documented in
mid
-

to
low
-
latitude mountain belts. In the European Alps,
records of

sedimentation rates in peri
-
Alpine basins
document dramatic changes
during the
last two
glacial cycles

(e.g. Hinderer 2001)
,
with particularly

prominent pulses of sediment
production at deglaciatio
ns as mountain glaciers retreat.
At lower latitudes,
inclu
ding the
Himalaya and perhaps the Andes, a similarly dramatic deglacial increase in sediment
production may be related to both
glacial advance and retreat

a
s well as

temporal
ly
-
linked

variation in the strength of monsoonal rainfall

(e.g. Goodbred and Kuehl

2000).


Laboratory experiments, as well as s
tudies of soil chronosequences

(e.g. Taylor and Blum
1995; White and Brantley 2003; Porder et al. 2007)
, demonstrate that chemical weathering
rates show a power
-
law dependence on substrate age, with rates declin
ing by 2
-
3 orders of
magnitude
,

from high values on first exposure of the substrate
,

to much lower values over the
subsequent 10
4
-
10
5

years
. Tak
en together, all these fi
ndings point to the possibility
of

both
generally high and
temporal
ly dynamic
chemical weathering rates during
t
he Quaternary,
because of the repeated physical renewal of landscapes and the re
-
supply of fresh fine
-
grained sediment during successive glacial periods.



3
.
Evidence

for temporal dynamism in chemical weathering rates dur
ing the Late
Cenozoic


Three
approaches are discussed here that provide tests of the importance of the ideas put
forward above.


3.1 The isotope geochemistry of modern rivers


Early interglacials should see very rapid weathering rates when fresh glacially
-
ground
substrate is first exposed, suggesting that the dissolved load of modern rivers, just 10
-
15 kyr
out of the last glacial period, may be perturbed from the long
-
term average.
The dissolved
major and trace element chemistry of major rivers is controlle
d by a complex interplay of
processes that are not often easy to tease apart. However, two types of isotopic study have
revealed features that are
of significance here
. Rate information is accessible through U
-
Th
isotopic studies of the dissolved and suspe
nded load of major rivers. A number of such
studies have been conducted in the last decade (many summarised in
Dosseto et al. 2008
)
,
and have revealed
departures from st
eady state weathering

that
necessitate a step increase in
chemical weathering rate

in the past 5

20 kyr, a timescale that encompasses the last
deglaciation. Importantly, this finding holds for low latitude river systems that drain young
mountain belts (like the Andes) that were glaciated during the last glacial cycle. It is
also
clear t
hat some lowland tropical rivers are close to steady state at

the present day
, including
parts of the Amazon catchment that do not drain the Andean highlands.


With reference to other isotope studies, the
assessment of the extent of recent d
isequilibrium
is often rendered difficult by uncertaint
ies over the variability of the
rock source material.
This uncertainty is perhaps least serious for some of the new metal isotope systems where
values for rocks are homogeneous and predictable. Thus, a recent study
of molybdenum
isotopic compositions of

major rivers

(Archer and Vance 2008)
, and the finding that the
dissolved load is universally heavier than rocks, clearly points to isotopic fractionation at
some point during weathering and transport. The exact origin

of these fractionations,
however, and the degree to which they are supportive of significant isotopic disequilib
rium
during chemical weathering

as opposed to transport,
is still the subject of ongoing studies of
molybdenum

and other

isotopes in soils.


3.2 The strontium isotope budget of the modern and recent oceans


Though one of the best
-
studied isotope systems in the marine realm

(see Vance et al. 2009 for
a summary)
, and one that has previously been used extensively in attempts to understand the
larg
e scale pattern of

chemical weathering rates on the continents, detailed work on the
strontium

(Sr)

isotope budget of the modern ocean reveals a significant imbalance. We know
the riverine flux of Sr to the oceans and its isotopic composition better than a
ny other
element.
We

have

also been
able to place increasingly tight constraints on other fluxes
of Sr
to the oceans, such as the
unradiogenic
exchange flux at mid
-
ocean ridges that partially
balance the radiogenic flux from the continents.
Finally, we

hav
e very good records of the
secular evolution of the Sr isotopic composition of the oceans over long timescales.
All of
these constraints together suggest that the

marine isotopic composition

of Sr should be
evolving

to radiogenic values

at a rate almost an

order of magnitude faster than we observe

in
real records
.
A

possible solution to this problem that has recently been suggested

(Vance et
al. 2009)

is that the dissolved chemistry of modern rivers does not accurately reflect the long
-
term flux (ie. the mu
lti
-
million year timescale

on which strontium in the ocean responds).
If
correct, this finding is wholly consistent with dynamic Quaternary chemical weathering rates
and the suggestion that modern (early interglacial)

rivers

are
delivering more
dissolved
m
aterial to the oceans than over the long
-
term
.
The impact on marine Sr isotopes is
potentially re
-
enforced by a further isotopic phenomenon.
The

same studies of soil
chronosequences that established the power law dependence of chemical weathering rate on
s
ubstrate age have also
shown

that early chemical weathering of silicate soils preferentially
leaches radiogenic Sr held in the interlayers of biotite

(Blum and Erel 1997)
, and that isotopic
compositions evolve back to the bulk soil value over the same time
scales as overall
weathering rates decrease. Thus, modern rivers
,

are
not only delivering
more Sr to the oceans
than the long
-
term average, but are perhaps also delivering Sr that is more radiogenic.


3.3 Cyclical changes in the marine isotope geochemistry

of short residence time
elements


The

picture presented above also predicts glacial
-
interglacial cyclicity in marine

Sr isotopes
,

but this

is so massively dampened by th
e large marine reservoir of Sr
that it is not resolvable
with our current analytical techniques. However, this is not the case for
the isotopic systems
of other

elements that have much smaller oceanic inventories and shorter residence times.
One such system is lead (Pb). For Pb isotopes
, like Sr, soil chronosequence studies have
revealed

time
-
dependent

incongruent release of isotopes

(Harlavan et al. 1998)
. In this case,
early
rapid
weathering leaches radiogenic Pb from damaged sites in the lattices of U
-
Th
-
rich
accessory phases, while s
ubsequently the Pb isotopic composition of the weathering products
evolve back to the bulk soil. Unlike for Sr, however, the cyclical changes in the marine
isotopic composition of Pb that
should result
are

easily discernible if the right location is
chosen
. Several recent studies
(e.g. Foster and Vance 2006)
have documented quite
substantial fluctuations in the Pb isotope geochemistry of the deep North Atlantic that
are
consistent with
pulses of radiogenic Pb during the early part of interglacial periods, f
ollowed
by
a slow return to unradiogenic values as ice builds up again

on the continents
. Modelling of
these isotope variations,
and the
accompanying changes in
absolute
weathering rate,
has
suggested consistency with the idea of dynamic Quaternary chemica
l weathering rates driven
by changes in physical weathering rates.


4. Conclusions


This paper has summarised recent findings that all point to the possibility that chemical
weathering rates in the Late Cenozoic are significantly impacted by fluctuations i
n the rate of
physical rock breakdown. The emphasis above has been on the geochemical tests of this idea
that derive from the predicted cyclicity in
Quaternary
chemical weathering rates. But a
corollary of these ideas is that overall rates across several g
lacial cycles are higher than in the
absence of glaciation. In this respect, and though likely to be controlled by numerous other
processes, the widespread evidence for departures from steady
-
state in secular records of the
chemistry of the
l
ate Cenozoic o
ceans is significant. Indeed, the start of the Cenozoic rise in
marine Sr isotopes occurs at a time of incipient glaciation
in another part of the Earth,
Antarctica (Zachos et al. 1999). It seems feasible
, therefore, that the long
-
term decline in
atmospher
ic carbon dioxide through the Cenozoic might be a consequence of changes in the
nature of the chemical weathering
-
CO
2
-
climate feedback that controls the Earth’s climate
through much of its history.



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