Environmental reconstruction - Fundamentals of the Physical Environment - page 577

The kingpin of multiple correlation is the tuning
of stratigraphic records with astronomic variations in
Earth's orbit (see p. 184), providing an internationally
accepted 'gold standard' of long-term global climate
change which informs both environmental reconstruc-
tion and IPPC's climate change forecasts. In essence,
differences in the accuracy and precision of each proxy
permit some 'wiggle room' for us to adjust broadly
convergent climate-induced changes to a common sig-
nal. MIS stages 1, 5e, 7, 9 and 11, corresponding to our
current and previous four temperate stages, show parallels
between patterns of orbital change and major climate-
linked responses. This allows us to 'tune' these sequences
with the 100 ka orbital (eccentricity) cycles for the
past 0.9 Ma, compared with earlier Quaternary 41 ka
(obliquity) cycles. This is a fitting conclusion to the prin-
ciples and methodologies of environmental recon-
struction and prelude to how they work in practice.

British sites reveal biogenic sediments underlying evidence
of subsequent glacier advance. Sharp stratigraphic bound-
aries and wood incorporated into the lower till suggest
that the glacier not only disturbed older organic layers but
may have bulldozed living trees. Lines of enquiry were
opened into the age and composition of the biogenic
assemblages and the sources and sequencing of glaciers.
Wood subfossils of pine, larch/spruce, juniper, fir and
yew ( Pinus, Larix/Picea, Juniperus, Abies and Taxus ) were
identified by their microscopic cell structure, and broad
vegetation assemblages were reconstructed by pollen
analysis from five separate organic lenses, suggesting at
least two distinct habitats ( Figure 23.10 ) The first is
indicative of a wetland/wetland-margin community with
some trees ( 10 per cent of total pollen) and the second

of a forested Pinus-Picea-Betula (birch) community
(50-75 per cent of total pollen). Other significant
subfossils present included algal and fungus spores, and
some beetle remains, whose species confirmed open
shallow water and wetland habitats respectively - typical
of modern Scandinavian communities. Absolute ages of
40,570 ± 860 / 760 and 41,160 ± 890 / 810 BP were obtained by
14 C dating for the dense peat and an initial age of 31,170
± 350 yr BP for the Abies subfossil. The latter is problem-
atic, however, since Abies was believed not to be native in
Britain during the past c. 400 ka - it requires warmer
conditions than found in Boreal forests - and could
have been derived from much older sediments by glaciers
and may have been contaminated by younger 14 C. A
radiocarbon recheck yielded an age of

ENVIRONMENTAL
RECONSTRUCTION IN PRACTICE

Devensian cold stage environmental
change: Irish sea basin
The modern Irish Sea basin acted as a major western
discharge route for British Quaternary ice sheets, fed by
Scottish, Lake District, Welsh and eastern Irish outlet
glaciers. Establishing the timing, sequence and source of
individual ice streams is complicated by incomplete
onshore sediment sequences, local diversity of glacigenic
sediments and postglacial sea-level rise which flooded
lower parts of the basin. Many different interpretations
existed until the 1990s, often assuming that each glacial
till separated by waterlain sediments represented a
previous cold stage separated by temperate stages.
Research, technological advances and the availability of
new sites have clarified the story at sites like Pen-y-bryn,
near Caernarfon, where the north-west Wales coastline
substantially narrows the Irish Sea basin (Addison and
Edge 1992; Chambers et al. 1995).
Quarrying Lower Palaeozoic shales for brick manu-
facture uncovers biogenic layers sandwiched between the
shale and overlying glacigenic sediments ( Plate 23.11a , b )
comprising three separate tills, glaciofluvial sands and
gravels of the Eryri and St Asaph Formations (Bowen,
1989). Biogenic layers are found in shallow, gravel-lined
channels and vary from heavily compressed amorphous
peat and individual large wood fragments ( Plate 23.11c ,
d ) to layers rich in conifer needles, cones and twigs. Few

46,630 BP by
a British laboratory ( infinite - beyond the accepted
radiocarbon range) but 60,600 ± 4500 / 2900 BP by a Dutch
laboratory ( minimum age). All these ages are significantly
younger than U/Th series dates for the peat, in the range
100-120 ka BP .
This range of absolute dates shows the benefits and
possible pitfalls of correlation and tuning, and the
influence of research on established ideas. All dates and
assemblages place the organic sequences in the most
recent, Devensian cold stage with more northerly climate
and habitat conditions than Britain experiences today.
The tree taxa, typical of interstadial (rather than full
interglacial) conditions, correlates broadly in assemblage
and age with other British sites (especially Chelford, near
Manchester), Grand Pile (France, see Figure 23.11 ) and
northern Europe. Lying close to acceptable limits, 14 C
dates probably underestimate the 'real' age and similar
northern European assemblages are tuned to globally
recognized early, rather than mid-Devensian, events. Abies
and some more thermophilous (warmth-preferring) plants
in the Caernarfon pollen record conflict in detail with a