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Figure 1 Biomass of chlorophyll from phytoplankton in surface waters of the northwest North
Atlantic Ocean, the area investigated by Li3, in two-week periods in 2000 and 2001. The amount of
chlorophyll is revealed by images produced from SeaWiFS data, collected by the OrbView-2 satellite,
resolved to about 1.5 km per pixel. The images show broad similarities in chlorophyll concentration,
as well as some significant differences, from year to year. They also show significant spatial
heterogeneity. Li3 has used macroecological principles to try to disentangle such broad patterns,
which probably result from large-scale climatic and physical oceanographic events, from the
organizing processes inherent to marine phytoplankton. (These images are composites for 16 May to
30 May in 2000 and 2001, and are available at http://oceanimage.mar.dfo-mpo.gc.ca)
This extends to marine phytoplankton
the ‘energy equivalence rule’ that has been
demonstrated previously for other groups of
organisms, including terrestrial animals and
plants4,5, suggesting that this rule is very
general, if not universal. If all the individuals
in a given size class do indeed use energy at
the same rate as all the individuals that make
up other size classes, this is an important
feature of ecological organization that begs
a mechanism. So far, it is not clear why
organisms from different size classes should
have access to, or be able to appropriate,
equal quantities of energy.
Li’s second important finding is that phytoplankton are most diverse at intermediate
levels of chlorophyll concentration, and
when the layers of the ocean water column
are mixed to a degree somewhere between
‘no mixing’ and ‘complete mixing’. Li
obtained this result by measuring phytoplankton diversity not by tallying the number of species — as is traditional in ecology —
but by creatively using flow cytometry to
quantify the variety of phytoplankton types
on the basis of their light-scattering properties. The results suggest that maximal diversity occurs when some intermediate degree of
water mixing allows diverse functional types
to coexist. (A high degree of mixing would
allow one or a few species that can cope well
with such disturbance to become dominant;
likewise, no disturbance at all would foster
competition, again allowing one or a few
superior competitors to become dominant.)
More generally, Li’s work3 shows that,
despite the enormous complexity and variety
of marine phytoplankton, there are emergent
features of phytoplankton abundance and
diversity that are related to cell size and overall
chlorophyll content, respectively. The implication is that it is possible to make mechanistic
connections between the metabolism of
individual organisms (in this case, phytoplankton) and the roles of those organisms
in ecosystems (here, in the productivity and
feeding dynamics of oceans). For example, a
recent study6 of marine food webs suggests
how oceanographic and climatic events,
through their effects on the abundance and
productivity of phytoplankton, can cause
fluctuations in commercial fish stocks.
Ecology has generally lacked unifying
theories. Much of the emphasis has been on
the seemingly large differences between different kinds of organisms and ecosystems,
and on the extensive spatial and temporal
variation within ecosystems. Now, however,
there is increasing evidence that some macroecological patterns and mechanistic processes hold across diverse taxa and ecological
systems. Such generality suggests exciting
prospects for conceptual unification.
■
Andrea Belgrano and James H. Brown are in the
Department of Biology, University of New Mexico,
167 Castetter Hall, Albuquerque, New Mexico
87131-1091, USA.
e-mail: [email protected]
1. Brown, J. H. Macroecology (Chicago Univ. Press, 1995).
2. Gaston, K. J. & Blackburn, T. M. Pattern and Process in
Macroecology (Blackwell, Oxford, 2000).
3. Li, W. K. W. Nature 419, 154–157 (2002).
4. Damuth, J. Nature 290, 699–700 (1981).
5. Enquist, B. J., Brown, J. H. & West G. B. Nature 395, 163–165
(1999).
6. Stenseth, N. C. et al. Science 297, 1292–1296 (2002).
Earth science
Baked Alaska
Peter Clift and Karen Bice
The warming of the Earth’s climate more than 50 million years ago is as yet
unexplained. Now the finger points to the heating of sediment in the Gulf
of Alaska as an important source of the greenhouse gas methane.
etween about 58 and 52 million years
ago, during the late Palaeocene and
early Eocene epochs, the Earth’s climate warmed considerably. What was the
cause of this unusually long warming trend?
Writing in Geology, Hudson and Magoon1
put forward an idea that adds to thinking on
the subject.
The marine geological record shows that
Earth’s climate has experienced swings of
cooling and heating over various timescales.
Change over thousands to hundreds of
thousands of years seems to be related to
orbital variations, but climate trends lasting
millions of years have been harder to explain.
The interval between the late Palaeocene and
early Eocene has attracted particular interest, not only because of the exceptional
B
NATURE | VOL 419 | 12 SEPTEMBER 2002 | www.nature.com/nature
© 2002 Nature Publishing Group
warming but also because this was a time of
widespread and intense tectonic activity at
the Earth’s surface.
Attempts to find out whether the two are
connected have usually centred on the
release of greenhouse gases, especially CO2,
generated by tectonic activity. In some
models the CO2 produced is related to volcanism, especially in the northeast Atlantic2.
In others, the suggestion is that CO2 was released from limestones originally deposited
on the northern coast of the Indian subcontinent, which were buried, heated and
then emitted CO2 on a large scale when this
landmass collided with the rest of Asia3.
Hudson and Magoon1 propose an alternative tectonic trigger for the global warming
of 58–52 million years ago, one that invokes
129
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methane, not CO2, as the key greenhouse gas.
Their model deals with the Gulf of Alaska,
where they suggest that large volumes of
sediment, eroded from freshly uplifted
mountains, were deposited in deep water off
the 2,200-km coast of Alaska. In this region
the oceanic Pacific plate is subducting below
Alaska but the overlying sediments are largely
scraped off against the edge of the continent
to form a ‘subduction–accretion complex’
(Fig. 1). As sediment becomes buried and
granite pushes up into the sedimentary layer,
the heat leads to the production of hydrocarbons, including methane, from organic
matter in the sediment. The large volumes
of methane released to the atmosphere,
increasing greenhouse-gas concentrations,
might explain the higher global temperatures
58–52 million years ago.
Dating of events is crucial in assessing
possible connections between tectonic
events and greenhouse-gas emissions in the
late Palaeocene and early Eocene. Take the
case of the models invoking CO2. Although
volcanism in the northeast Atlantic began
about 63 million years ago, the greatest
volumes of lava did not erupt until about 56
million years ago4 — too late to trigger the
initial warming. Analysis of the sedimentary
rocks in the ‘collision zone’ between India
and Asia5 shows that the earliest collision
between these land masses happened only
about 50 million years ago. In assessing
Hudson and Magoon’s hypothesis, a similar
caveat might apply. Age constraints indicate
that heating of the eastern part of Alaska6
might have occurred too late to have been
important in generating methane.
Another aspect of the new model1 deserving scrutiny is the estimated total methane
production. Hudson and Magoon’s estimate
might be too high because it is based on a
total sediment thickness of 25 km, assuming
that 10 km has been removed by erosion. The
eroded thickness has been estimated from
radioisotope dating of the cooling of rocks
through 300 °C, assuming a typical temperature gradient in the crust of 30 °C km11 (see,
for example, ref. 6). However, such cooling
could also be caused by the removal of the
overlying rocks by faulting, not erosion. In
this case, the ‘missing’ 10 km of sediment has
not been lost but merely displaced sideways.
In practice this would reduce the amount of
organic material available to be cooked into
methane. Nonetheless, so great is the
amount of methane predicted by Hudson
and Magoon1 that even if production were
halved this would still have resulted in a
methane concentration in the atmosphere of
several times the modern level.
Despite the apparent efficiency of Alaska
as a generator of greenhouse methane, there
is reason to wonder if this gas is really the
likely climate culprit here. The popularity of
methane as an agent in climate change has
grown since its implication in the brief and
130
Figure 1 The Alaskan continental margin during the late Palaeocene epoch. Sediment washed down
from newly lifted mountains collects at the sea floor. The subduction of the oceanic plate carries
that sediment down into the Earth’s crust, where, below the 150 °C level, the organic matter in it is
converted into methane and other hydrocarbons. Hudson and Magoon1 propose that the release of
methane — a greenhouse gas — from this ‘accretionary prism’ of rock contributed to the global
climate warming that happened over 50 million years ago.
dramatic Palaeocene–Eocene Thermal Maximum7 (around 55 million years ago), when
temperatures in the deep oceans and at high
latitude increased by 5–7°C in less than 10,000
years. Here, the abrupt release of methane,
previously stored in frozen form beneath the
sea floor, provides a viable explanation for
observed changes in marine and terrestrial
carbon isotope records. But the direct climatic
effects of methane that escapes to the atmosphere remain a matter of debate. Other new
work8 shows that, because of its much longer
residence time in the ocean–atmosphere
system, CO2 might be responsible for the
formation of polar stratospheric clouds
(whose insulating effects could explain the
especially elevated temperatures at high
latitudes1,9). The uncertainty over this issue is
all the more severe because most climate
models currently lack the chemical detail and
the resolution to predict the occurrence and
radiative effects of these clouds.
There is also a long-standing problem in
understanding the climate in continental
interiors, for which models predict lower
winter temperatures than those inferred
from fossil evidence. Neither the direct radiative effects of increased greenhouse gases nor
polar stratospheric clouds can account for
this discrepancy. The cause of model underpredictions of temperatures both in polar
regions and in continental interiors might
lie in inadequate sensitivity of the modelled
feedbacks from water vapour and cloud to
increased greenhouse-gas concentrations. If
so, then these same models are probably
underestimating the strength of the hydrological cycle and the warming effects of the
current increases in greenhouse-gas levels.
Hudson and Magoon’s work1 is notable in
highlighting a possible overlooked influence
on climate change. In attempting to understand past climate trends, episodes of faster
or slower carbon recycling at subduction–
accretion complexes require more attention,
as Hudson and Magoon have shown that in
certain cases this process can influence the
global greenhouse-gas budget to the same
degree as does volcanic activity. Sedimentation and tectonic activity in the Gulf of
Alaska is probably inadequate to explain
the entire warming trend during the late
Palaeocene and early Eocene by itself,
although it might have contributed. Despite
this advance in relating solid-Earth evolution to climate change, we are still struggling
to understand the disparity between climatemodel temperature estimates and those
inferred from the fossil record, for polar
regions and continental interiors during past
warm intervals. A satisfactory, comprehensive mechanism has yet to be proposed. But
when it emerges, we can hope that it will
solve both model problems simultaneously,
and apply more generally to warming
episodes in the Earth’s history.
■
Peter Clift and Karen Bice are in the Department
of Geology and Geophysics, Woods Hole
Oceanographic Institution, Woods Hole,
Massachusetts 02543, USA.
e-mail: [email protected]
1. Hudson, T. L. & Magoon, L. B. Geology 30, 547–550 (2002).
2. Eldholm, O. & Thomas, E. Earth Planet. Sci. Lett. 117, 319–329
(1993).
3. Kerrick, D. M. & Caldeira, K. Chem. Geol. 108, 201–230 (1993).
4. Saunders, A. D., Fitton, J. G., Kerr, A. C., Norry, M. J. & Kent,
R. W. in Large Igneous Provinces (eds Mahoney, J. J. & Coffin,
M. L.) 45–94 (Am. Geophys. Union Monogr. 100, 1997).
5. Clift, P. D., Carter, A., Krol, M. & Kirby, E. in The Tectonic and
Climatic Evolution of the Arabian Sea Region (eds Clift, P. D.,
Kroon, D., Craig, J. & Gaedicke, C.) (Geol. Soc. Lond. Spec.
Publ. 195, in the press).
6. Sisson, V. B., Hollister, L. S. & Onstott, T. C. J. Geophys. Res. B
94, 4392–4410 (1989).
7. Dickens, G. R., O’Neil, J. R., Rea, D. K. & Owen, R. M.
Paleoceanography 10, 965–971 (1995).
8. Kirk-Davidoff, D. B., Schrag, D. P. & Anderson, J. G.
Geophys. Res. Lett. 29, 10.1029/2002GL014659 (2002).
9. Sloan, L. C. & Pollard, D. Geophys. Res. Lett. 25, 3517–3520
(1998).
© 2002 Nature Publishing Group NATURE | VOL 419 | 12 SEPTEMBER 2002 | www.nature.com/nature