Geoscience Reference
In-Depth Information
The present is the key to the past - and
the future?
KEY CONCEPTS
Environmental reconstruction relies on the modern relevance and application of principles and laws governing
environmental processes which guided the emergence of Earth science after AD 1800. Early geology held catastrophic
events, often literally of biblical origin and proportions such as Noah's Flood, responsible for environmental processes
and change. This catastrophist or diluvialist thinking was replaced by widespread acceptance that geological
processes operate continuously, over long time scales, largely by currently observable processes and unconstrained
by impossibly short biblical time scales such as Archbishop Ussher and John Lightfoot's seventeenth-century date
for the Creation of 26 October 4004 BC (at 09.00 hours). Drawing on belief that the laws of physics do not change
over time, and preference for empirical science over speculation and myth, the work principally of three internally
renowned British geologists (James Hutton, 1795; Charles Lyell, 1830 and Archibald Geikie, 1882) developed this
principal of uniformityor uniformitarianism . Despite disputing early views of catastrophism in environmental change,
Earth scientists now recognize the role of sudden, high-energy, high-magnitude, low-frequency events -
neocatastrophism- in environmental change. Their role in non-linear changes and more turbulent episodes of Earth
history is increasingly accepted as fundamental to, not at odds with, uniformitarian views of environmental change.
Geikie's paraphrase of uniformitarianism as 'the present is the key to the past' remains essentially true, at the heart
of environmental reconstruction. It also provides our best chance of predicting environmental futures, although a
case is being made for regarding the modern scale of anthropogenic impact to have initiated the Anthropocene epoch
(Crutzen and Stoermer 2001) with Earth in a 'no-analogue state' (Steffen et al. 2003).
We now turn to general principles underpinning the relationship between properties of Earth materials and the
stratigraphic record of past environments they may reveal. Sediments are 'landfill' products of continental erosion
and atmospheric and oceanic processes. They are deposited by or through water, ice or air, as layers (strata) in
sedimentary basins, often reflecting strong tectonic and/or climate influences. The location and processes of
accumulation, and diagnostic properties of their provenance (source), internal form and structure are set out in Chapter
12 (pp. 265-72), where they describe 'live' sediments forming in contemporary basins. Accuracy and completeness
of subsequent environmental reconstruction depend on the extent and clarity with which these diagnostic properties
survive subsequent burial, tectonic deformation, relocation, exhumation and subaerial geomorphic processes.
Sedimentological and stratigraphic principles governing their study focus on the stratigraphic 'law' of superposition
and differences between sediment packages. Superposition recognizes that each stratum (single layer) must be
younger than the base on which it is superimposed but older than the stratum above - establishing its relative age
in the sequence or succession ( Plate 23.1 ). This remains evident even when strata are subsequently highly folded
or even overturned, with sediment structures or fossils buried in their life positions showing which way is up! The
predominance of minerogenic (inorganic) or biogenic (organic) material differentiates between lithostratigraphy and
biostratigraphy, with both branches reunited in the field of chronostratigraphy, focusing on the age of rocks.
Stratigraphy embraces all rock types and not just sediments. Intrusive igneous rocks, datable by their radioisotopes,
also provide approximate or minimum ages for other rocks they were injected into, just as eruptive igneous or
metamorphic rocks may be age-constrained (identified by a particular time band) by underlying or overlying datable
organic sediments.
Finally, the very environmental changes we try to reconstruct prevent continuous rock successions from developing
or surviving for long periods of geological time, placing reliance on the correlation of successions across space
and time. That is, recognizing similarities in indicator fossils, distinctive mineral assemblages or structures in rocks
from place to place, with sufficient evidence that they reflect common environmental events. These may be time-
equivalentin age, or reflect the same event (glacier advance, sea-level rise etc.), spreading gradually from one place
to another and therefore time-trangressive in age. These principles are illustrated in practical case studies later in
this chapter.
 
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