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BIOGENIC SEDIMENTS, GEOPHYSIOLOGY AND EARTH'S ENVIRONMENTAL HISTORY
[Grimm, 1997. PALAIOS 12(4): 299-301]
A continuous record of Life on Earth since its inception nearly four
billion years ago is a testament to its resiliency, plasticity, and
ubiquity. As scientists, we are interested in fossils, sediments and
Earth's environmental history; we aim to comprehend and ultimately quantify
the coevolution of Life and Environment. Conventional approaches consider
sediments as recorders of Earth's environmental history; this is the
"sedimentology" of undergraduate textbooks, facies models,
and most economic applications. In this essay, I'll survey how sediments
- and in particular biogenic sediments - have been and are active participants
in governing Earth's environmental homeostasis and punctuated evolution.
Biogenic sediments - including the carbonate, phosphatic, and biosiliceous
sediments, as well as sedimentary organic carbon - record the spatial
and temporal heterogeneity of biologically-mediated sedimentary processes;
in turn, biogenic sedimentation influences local, regional, and global
atmospheric and oceanic environments, by acting as sinks for the nutrient
elements that drive biogeochemical cycles.
James Lovelock and Lynn Margulis fashioned an innovative model of Earth
they termed the Gaia Hypothesis, pointing to the persistent chemical
disequilibrium in Earth's atmosphere (O2 with CH4 for example), and
the biological modulation of trace greenhouse gases (CO2, CH4, N2O).
However, a teleological tone of their early writings stimulated a combative
debate of virtually mutual exclusion: Gaia is or isn't. The emergence
of the Daisyworld model appeased critics of "purposeful design"
- neither a necessary nor desirable aspect of a scientific Gaia - however,
the unfortunate [even if partly truthful?] equation of a profound scientific
worldview to the "living Earth" of New Age spiritualists caused
many to toss baby out with the bathwater. As a consequence, Gaia has
been widely dismissed as a curiosity, to be discussed in sound bites
on TV nature documentaries or perhaps as an aside in our introductory
lectures. Yet an important question remains: is the Gaian Earthview
scientifically viable and useful to scientists concerned with organisms,
sediments, and Earth's environmental evolution?
In my view, the answer is an enthusiastic yes. The Gaia Hypothesis -
entangled in ambiguities and hostilities of that earlier debate - may
be recast as Geophysiology, a view of Earth as a self-regulating environmental
system, where homeostatic governance of atmospheric temperature, chemistry,
and other oceanic factors is an emergent property deriving from intimate
coupling of Earth's constitutent subsystems (geosphere/hydrosphere/biosphere/atmosphere).
As Lee Kump noted in a recent essay, the recognition of mutual influence
between biology and the physical environment is ingrained in the physical
and life sciences, however the consequence of those interactions - self-regulation
- is not. Geophysiology is like any concept, tool, or theoretical construct
- some of us "get it" and some of us don't, some of us will
find it useful, and some of us won't........ hey, no problem! Nonetheless,
even if one argues that geophysiology is a non-falsifiable hypothesis
(I believe it is falsifiable and testable), is stubborn agnosticism
the only reasonable stance?
In my view, we humans are part of Earth's natural history, one marked
by symbiotic fusion and creative emergence. The symbiogenetic fusion
of prokaryotic cells gave rise to eukaryotes, evolutionary "Frankensteins"
manifesting capacities possessed by none of their formative components.
Alga plus fungus yields lichen, a derivative community that possesses
entirely new emergent properties, permitting lichens to thrive on bare
rock and play their essential weathering role at the interface between
the living and non-living realms. Two people meet, fall in love and
something greater emerges; the phenomenon that propels the destiny of
cultures and nations is a spontaneous product of interconnection. Emergent
properties are authentic and essential aspects of all natural systems
- as real as the nose on your face, but far more difficult to isolate
or measure. Where exactly do those uniquely "lichen-like"
properties reside ? Where dwells love or the planet's capacity for self-regulation?
We truly don't know. However, the admission that we don't understand
emergent properties very well - their study embodies the exciting new
discipline of complexity theory - doesn't negate their existence and
pervasive influence.
Reductionism focuses on the parts, systemism is concerned with interactions,
while vitalism includes interactions and emergent properties. Geophysiology
is a vitalistic, genuinely holistic approach to Earth system processes
and environmental evolution. Planetary homeostasis of atmospheric and
oceanic chemistry and temperature is an emergent property of vitalistic
Earth. As scientists, we practice rationalism - induction and deduction
- but the fuel for discovery and the brightest scientific careers are
those rare episodes of sparkling, intuitive insight, enriching the experience
and blossoming as innovative, testable hypotheses. Geophysiology - cast
in the language of general systems theory and biogeochemical cycles
- permits scientists to apply holistic intutition to Earth's environmental
evolution. Let's glance at a few examples.
Bob Garrels pointed the way, recognizing that a hunk of granite is raw
material to be devoured by biologically-mediated, chemical weathering
processes. Crunch a pluton through a soil profile a few times to produce
quartz arenite (insoluble residue), mudrock (chemical by-product) and
a suite of dissolved species (dissolved Si, P, bicarbonate and nutrients).
Let rivers and groundwater titrate these into the ocean to create biogenic
sediments, including carbonate, phosphatic, and biosiliceous sediments
along with sedimentary organic carbon.
Preston Cloud formulated early insights that photosynthesis and Paleoproterozoic
iron formation- source of 90% of Earth's iron ore - participated in
transforming anoxic Earth into one that was fully aerobic. Photosynthetically-derived
O2 in the early aerobic atmosphere was a prerequisite to formation of
the stratospheric ozone layer, a UV "umbrella" that permitted
the evolution of eukaryotes via symbiogenesis. Furthermore, organisms
have been recently implicated in propelling oceanic chemical changes
that accompanied the explosion of biomineralization in the early Cambrian.
We've come to deeper appreciation of coccolithophores as dimethyl sulfide-spewing
protists (cloud condensation nuclei, thus influencing planetary albedo,
ocean-to-continent moisture transfer and oceanic nutrient influx) and
as predominant sediment producers in modern and ancient marls and chalks.
Their photosynthetic growth and sedimentation participates in biological
pumping of CO2 from the atmosphere to the ocean and sediments; the dissolution
of their calcareous tests in the deep sea buffers ocean alkalinity and
thus solubility of CO2 in seawater. These planktic critters are no small
potatoes: John Milliman recently estimated that the mass of modern aragonite
and Mg-calcite being preserved in reefs, banks and tropical shelves
is equalled by pelagic calcite burial.
On a related theme, Karl Föllmi is bringing a stratigraphy of Alpine
limestones and condensed phosphatic beds to life for us, illustrating
how a vast Middle Cretaceous carbonate platform met its eutrophic demise,
as enhanced tectonic rates propelled increased greenhouse forcing, with
resultant increases in weathering rate and nutrient flux to the epeiric
carbonate community. Mark Caplan's research on a Late Devonian carbonate
platform in western Canada blanketed in a global (Chattanooga/Woodford
shale equivalent) black shale similarly points to resolution of Schlager's
paradox of "drowned" carbonate platforms: they eutrophy and
die. These innovative studies document links between global climates,
nutrient inputs and changing mechanisms and rates of carbonate and organic
carbon burial, illustrating how organism growth, biomineralization,
and burial are products of and governing participants in profound regional
and global change.
The intimate interplay of chemical, physical and biological processes
are well-represented by actualistic study of coastal upwelling systems.
Sediments influenced by coastal upwelling are commonly organic-rich
and finely laminated, are richly micro-fossiliferous and are deposited
at high sedimentation rates. As components of Earth's climate system,
upwelling systems contribute large volumes of organic carbon - and thus
atmospheric CO2 - into hemipelagic sedimentary sinks (the "biological
pump") , and they commonly yield a thinly-laminated, high-resolution
record of oceanic, biological, and sedimentary processes. In addition,
the organic-richness of upwelling deposits is responsible for their
economic prominence as hydrocarbon source rocks and as the birthplace
for many sedimentary phosphorites.
Biologist Alice Alldredge and her colleagues have recently demonstrated
that some marine phytoplankton actively govern their own sedimentation
by the formation of sticky transparent gels that facilitate rapid aggregation,
accelerated sinking and efficient export flux. We find that these "self-sedimenting"
phytoplankton blooms are well-preserved as monospecific flocs and laminae
in laminated diatomaceous sediments, suggesting that the life history
strategy of algal individuals and populations governs accelerated organic
carbon and opal burial and the development of many distinct sedimentary
laminae. Presently, my students and I are quantifying the changes in
export efficiency of organic carbon, biosilica and other biophile elements
that result from the self-sedimentation phenomenon. We speculate that
some short-duration variations in organic carbon burial rates in the
coastal ocean - and thus some abrupt atmospheric CO2 variations - may
be governed by the ecology of phytoplankton populations, rather than
exclusively by nutrient-dependent changes in primary production. Testing
these unconventional hypotheses will require a geophysiological approach
and furtherance of projects like the Joint Global Ocean Flux Study (JGOFS).
Phosphatic sediments are a characteristic yet poorly understood aspect
of coastal upwelling deposits. Most phosphatic grains are the microbially-mediated
product of diagenesis in organic-rich , suboxic sediments; their formation
accompanies the transfer of nutrient phosphate from the oceanic pool
into the sediment reservoir. The residence time of nutrient P in the
ocean is only about 70,000 years; consequently, changes in P burial
rates can profoundly influence oceanic primary production and thus atmospheric
CO2. John Compton and colleagues are employing stable isotopic approaches
(?13C, ?180 and ?87Sr) to co-occuring Tertiary calcites and phosphates,
linking episodes of phosphogenesis and reworking to changes in global
carbon burial. The enigma of "phosphorite giants" such as
the Permian Phosphoria Complex remains a puzzling fascination, perhaps
more so since Gabe Filipelli and Peggy Delaney estimated that P burial
rates in the Phosphoria are comparable to those seen in the modern Peru
Margin. We've learned a great deal about phosphatic sediments in recent
years, but "the phosphorite problem" is alive and well !
As a final illustration, Rick Behl and Jim Kennett recently presented
an astounding correlation between isotopic proxies of North Atlantic
climate in Greenland ice cores and sediment fabric and microfossil records
of seafloor oxygenation in diatomaceous hemipelagites of the Santa Barbara
Basin, coastal California, USA. Conceptually, their ongoing, high-resolution
studies complement the astonishing zig-zag of Charles Keeling's 40 year
time series of atmospheric CO2 from Mauna Loa (and son Ralph Keeling's
confirmation of reciprocal changes in atmospheric O2 !). Climate modulation
operates via tight mutualistic coupling and planetary teleconnections
amongst the geosphere, hydrosphere, biosphere and atmosphere.
Many readers will agree that the single most important challenge facing
the Earth sciences is forecasting the rate and trajectory of human-induced
regional and global climatic change. In the sediments we possess the
incomplete, imperfect and only "control experiment" of Earth
unperturbed by anthropogenic factors. The value of our contributions
to global change science will be measured by their accuracy, insight,
and timeliness.
In my opinion, Life cannot be reduced to mathemetics, chemistry and
physics, because Life at cellular, ecosystem, and planetary scales envelops
and transcends these disciplines. As Ken Wilber admonishes, a "heap"
of interdisciplinary science is an improvement over outdated atomistic
approaches, but it is not holism. A truly holistic - vitalistic - approach
fully embraces the intutition of a seasoned naturalist, in particular,
the breed of scientists that study and contribute to this fine journal.
Simply stated, geophysiology provides a template for innovative thinking
and communication.
In sum, Planet Earth has evolved and continues to function as a self-regulating
environmental system. Homeostasis of an organism, ecosystem, or planet,
is an emergent property of integrated subsystems. Biogeochemical transfer
of matter and energy amongst the geosphere, hydrosphere , atmosphere,
and biosphere constitutes a readily measurable aspect of Earth's self-regulatory
processes. Internally and externally rooted perturbations to biogeochemical
cycling have cascaded into a 4 billion year evolutionary history of
distinct environmental modes. Sediments and sedimentary rocks are sensitive
and interactive recorders of environmental homeostasis and change. Biogenic
sediments record the spatial and temporal heterogeneity of biologically
mediated sedimentary processes; in turn, they govern the coevolution
of Life and Environment, as interactive repositories for the nutrient
elements that fuel biogeochemical cycles.
Spaceborne imagery provides magnificent intutitive confirmation of a
vitalistic Earthview. The holistic examples described above suggest
a fruitful, vitalistic trajectory for the Earth sciences. The perspective
of geophysiology - including organisms, sedimentary processes, biogeochemistry,
and emergent properties - promises new insights at scales spanning intercontinental
correlations, seismic lines, outcrops, hand samples, thin sections,
and geochemical proxy measurements. As researchers and educators, we
share a deep enthusiasm for understanding Earth and sharing our experience
with others. In the final analysis, Geophysiology simply enriches and
enlightens what is already a passionate curiosity.
Kurt Grimm
Assistant Professor
Department of Earth and Ocean Sciences
University of British Columbia
Vancouver, BC V6T 1Z4 CANADA
Voice (604) 822-9258
FAX (604) 822-6088
e-mail: kgrimm@ eos.ubc.ca
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