Terrestrial Palaeoecology and Global Change

Chapter 2. Taphonomy
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Upland runoff increases with a drop of sea level (depressing the basis of erosion)
and with seasonality of precipitation causing high floods. A significant upland contribution
to fossil plant assemblages is first recorded in association with the Late Palaeozoic
Hercynian orogeny. Thick allochthonous coals are confined to the post-orogenic
molassoid facies where peat accumulation was enhanced by rainwater runoff over
vegetated slopes, bearing low sedimentary load and sustaining a permanently high
water table in the foredeeps.
Although some dead mass is exported from oceanic ecosystems to land (in the dung
accumulations of sea birds, stranded algal mats, etc.), the terrestrial export is overwhelmingly
larger. On balance, continents are exporters but these relationships might
have been different before the advent of higher land plants or even for some time after
that. Terrestrial vegetation is not only the major exporter of dead mass but it also controls
transport (of different rates over vegetated or barren landscapes for both wind-borne
and water-borne material) and deposition. Terrestrial organic material is scarcely incorporated
in near-shore deposits unless it is trapped by littoral vegetation that reduces
wave action. Since the Late Silurian on, littoral vegetation plays a key role in dead mass
transfer from land to sea.
Owing to its wide distribution and rapid recycling, terrestrial organic matter is a significant
factor of oceanic productivity and sedimentation (De Leeuw et al., 1995; Opsahl
& Bonner, 1997). Thus, lignine is found throughout the water column of both the Atlantic
and Pacific oceans, its concentration correlates with riverine discharge, hence higher in
the Atlantic. Palynological studies reveal a correlation of palynomorphs and structureless
kerogens with oil/gas accumulation. In particular, the Botryococcus-type unicells
and amorphous kerogens occur in oil source deposits, whereas terrestrial palynomorphs
and “fibrous” kerogens associate with gas sources (Batten, 1982 and elsewhere).
Differences in kerogen microstructures are conventionally ascribed to their marine
or terrestrial origins. However, most kerogens are produced at river mouths that export
many times more dead mass than is produced in the oceans. The fibrous “land plant”
kerogens originate from organic debris of deltaic facies while the amorphous “algal”
kerogens are estuarine production (algal blooms in hypertrophic estuarine ecosystems
are the most abundant source of organic matter: Alimov, 1989). Thick deltaic deposits
typically correlate with regressions, whereas estuarine contributions increase with ponding
of river mouths by rising sea.
In tectonically active marginal zones (foredeeps of coastal ranges, fore-arc depressions),
terrestrial dead mass brought in with continental runoff is redeposited downslope
by turbid flows inflicting short-term anoxic conditions and fluctuations of carbonate
dissolution depths. The dead mass is intermittently deposited in organic-rich bands
of turbiditic flysch sequences, sometimes containing structurally preserved plant fossils.
Periodic influxes of suspended organic material enhance precipitation/diagenetic
redistribution of silica over the banded silicites (cherts). The radiolarian cherts of ophiolite
assemblages sometimes contain fossil wood (Abbate et al., 1980) and other ter-