Patrick J. Brenchley

Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period

The end Ordovician glaciation is distinct among Phanerozoic glaciations in that CO 2 , levels were generally high, yet major continental ice sheets accumulated on the Gondwana supercontinent. New oxygen isotopic data indicate substantial changes in sea-water temperatures and ice volume coinciding with glacio-eustatic changes in sea level reflecting the growth and decay of the Gondwana ice cap. Major glaciation was apparently confined to the Hirnantian and was 0.5-1 m.y. long, rather than the 35 m.y. of earlier estimates. Carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep-water circulation and then back again. We believe that the changes in the carbon cycle effected a reduction in P CO 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world.

Are we now living in the Anthropocene

The term Anthropocene, proposed and increasingly employed to denote the current interval of anthropogenic global environmental change, may be discussed on stratigraphic grounds. A case can be made for its consideration as a formal epoch in that, since the start of the Industrial Revolution, Earth has endured changes sufficient to leave a global stratigraphic signature distinct from that of the Holocene or of previous Pleistocene interglacial phases, encompassing novel biotic, sedimentary, and geochemical change. These changes, although likely only in their initial phases, are sufficiently distinct and robustly established for suggestions of a Holocene–Anthropocene boundary in the recent historical past to be geologically reasonable. The boundary may be defined either via Global Stratigraphic Section and Point (“golden spike”) locations or by adopting a numerical date. Formal adoption of this term in the near future will largely depend on its utility, particularly to earth scientists working on late Holocene successions. This datum, from the perspective of the far future, will most probably approximate a distinctive stratigraphic boundary.

Global carbon isotopic events associated with mass extinction and glaciation in the late Ordovician

Mass extinctions and glacioeustatic sea-level changes in the lower part of the Hirnantian (final stage of the Ashgill) are accompanied by shifts in marine stable-isotope compositions. Previously published stable-isotope changes have been used to identify the onset and demise of the Gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans. However, the existing isotopic data set had limitations because it was derived from Ordovician low-latitude settings and from carbonates or organic carbon in separate areas. We report new data from Ordovician high-latitude carbonates and demonstrate parallel shifts in organic and carbonate δ13C from Baltica. Brachiopod shells from a high-palaeolatitude, periglacial setting in Argentina have elevated δ13C values similar to those described previously from low-latitude sites. The new data demonstrate that the positive Hirnantian δ13C excursion, previously only recognised from low-palaeolatitude areas, was widespread and probably global in extent. The poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material. Preliminary carbon isotopic data from thermally immature organic matter from Estonia provide the first indication of a synchronous shift in organic and inorganic δ13C in sediments from the same basin. This work provides new data of critical importance for constraining models of end-Ordovician palaeoceanography and climate change.

Patterns in bioclastic accumulation through the Phanerozoic: Changes in input or in destruction?

Evolutionary changes in the ecology and diversity of organisms that produce and destroy calcareous skeletons suggest that bioclastic concentrations themselves might have changed in nature through the Phanerozoic. Empirical data from marine siliciclastic records of Ordovician-Silurian, Jurassic, and Neogene ages indicate a significant increase in the thickness of densely packed bioclastic concentrations over geologic time, from a primarily thin-bedded brachiopod-dominated record in the Ordovician-Silurian to a mollusk-dominated record with many more and thicker shell beds in the Neogene. Jurassic shell beds vary in thickness with the Paleozoic or modern affinities of the chief constituents, suggesting, along with other evidence, that the Phanerozoic increase was determined neither by diagenesis nor by a shift in taphonomic conditions on the sea floor but rather by the evolution of bioclast producers, namely, groups with (1) more durable low-organic skeletons, (2) greater ecological success in high-energy habitats, and (3) on the basis of indirect evidence, higher rates of carbonate production. These results suggest that (1) reproductive and metabolic output has increased in benthic communities over time and (2) the scale of time averaging in benthic assemblages has increased owing to greater hard-part durability of modern groups.

High-Resolution carbon isotope stratigraphy of the basal Silurian Stratotype (Dob's Linn, Scotland) and its global correlation

Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dobs Linn, Southern Scotland, has received criticism due to the difficulties of relating its well-constrained graptolite biostratigraphy to shallow-water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dobs Linn have yielded carbon stable-isotope signatures consistent with those recorded elsewhere, in particular showing a clear positive δ13C excursion in the terminal Ordovician. The architecture of the δ 13C curve from Dobs Linn enables very high-resolution stratigraphic subdivision and direct correlation between the deep water Dobs Linn section and time-equivalent carbonate shelf deposits. An integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites, conodonts and shelly faunas has been constructed. This direct correlation shows that the shallow water successions, including the former stratotype candidate at Anticosti Island, are generally incomplete, with hiatuses related to the rapid sea-level changes during the Hirnantian stage. This confirms and greatly increases the global utility of Dobs Linn as a boundary stratotype.

A facies analysis of upper Ordovician regressive sequences in the Oslo Region, Norway — A record of glacio-eustatic changes

Abstract Detailed sedimentological studies of Upper Ordovician rocks of the Oslo Region, and comparative studies in Wales, document Late Ordovician glacio-eustatic sea-level changes more specifically than hitherto. On the Baltic Platform we recognise one main regressive event which promoted the spread of coarse clastics into some parts of the area and the development of oolites, calcarenites and bioherms elsewhere. The distribution of these facies was determined partly by the regional palaeogeography of the platform, and partly by block movements of the basement which created tectonic highs on which oolites accumulated and which determined the form of one major sand bar. At the regressive maximum there was widespread emergence of the platform and deeply incised tidal channels were formed. Studies in Wales suggest that some channels fed a contemporaneous influx of turbidites into basin regions. We estimate that the Hirnantian glacial maximum lasted less than 1 m.y., that sea level dropped by 50–100 m, and that the subsequent transgression was an extremely rapid event which drowned all the preceding facies to relatively deep shelf depths. We conclude that the glacio-eustatic sea-level rise caused synchronous facies changes of major chronostratigraphic value which should help to determine the best location of the Ordovician/Silurian boundary.

A storm surge origin for sandstone beds in an epicontinental platform sequence, Ordovician, Norway

Abstract The upper Ashgillian (Ordovician) sediments of the Oslo area were deposited on an extensive open marine platform. Within the sequence thin sandstones (0.5–10 cm thick) are interbedded with bioturbated silty shales. The beds commonly have sharp lower surfaces and more diffuse tops which suggest sand was rapidly introduced into a tranquil environment. Estimates suggest that the deposition of a sand bed was a very infrequent event, recurring every few thousand years. The sandstones show lateral changes in frequency of occurrence, and total cumulative thickness, which, taken together with palaeocurrent evidence, indicate derivation from the west. Vertical changes in thickness of sandstone per metre, mean bed thickness, bedforms and grain size all reflect an increasing proximality to a westerly sediment source and can be related to a shoreface with eastward longshore drift developed during the upper Ordovician regression. The general reconstruction of Ashgillian palaeogeography in the Oslo region suggests that the sand beds were deposited by currents flowing offshore, obliquely down a low palaeoslope. Most normal currents on modern continental shelves flow parallel to the shoreline, so we have considered other mechanisms which might have produced low-frequency, high-energy currents which flowed offshore and which could have been responsible for the deposition of the thin, discrete sandstones. Three models are discussed for the formation of the thin sandstones, and an ebb current generated by a storm surge is considered the most likely explanation.

Flume experiments on the orientation and transport of models and shell valves

Abstract Several series of flume experiments were performed in order to investigate the current response and orientation of models and shells over different substrata. Eighteen different models, of simple symmetrical shapes, and eight species of modern shells, six pelecypods and two gastropods, were used on two different sediment bases, medium sand and mud. All movements of objects were recorded photographically and final measurements were made from the photographs. The experiments were begun with model sets in random orientation, all objects being in the concave-up attitude. From the experiments it was concluded that most objects take up a preferred final orientation of longest axes across the current. Only exceptionally, for example, in the gastropod, Turritella , and the very light weight pelecypod, Cultellus , do current parallel orientations result. Some of the models were loaded with lead to produce an eccentric centre of gravity and to simulate the effect of the thickened umbonal regions of shells. These loaded models indicate the tendency for an up-current preferred orientation of centre of gravity (= load). Some non-loaded models produced unidirectional patterns, reflecting the relative ease of transport in certain positions. By analysing the behaviour of the objects during the experiments it was possible to break down the final orientation patterns into their component parts. This operation demonstrated that the objects are most perfectly oriented during transport and that finally, this pattern can become more or less dispersed due to impedence between objects, irregularities on the substrata and inversion of objects to the convex-up attitude. Experiments on the two substrata indicate that objects can be moved more easily and for greater distances, and are less likely to be inverted on sand than on mud. This is considered to be due, at least in part, to the relative mobilities of the two sediments: sand sized particles are more easily eroded at a given current velocity than are mud sized particles. Observations on the burial of objects indicate that burial begins at lower velocities on sand, and that, by scour, objects can become buried in bizarre attitudes to the sedimentary interface. It is restated that objects are most stable in the convex-up position. They are soon inverted to this position on mud and because of this it seems difficult to develop drifted assemblages on this substratum.

New isotopic data solving an old biostratigraphic problem: the age of the upper Ordovician brachiopod Holorhynchus giganteus

Recent work has shown that there is a pronounced positive late Ordovician excursion in both δ13C and δ18O which appears to be global and identifies a clear chemostratigraphic interval that is confined to part of the Hirnantian Stage. Pre-Hirnantian (Rawtheyan) brachiopod carbonate values of δ13C are typically in the range – 1 to + 1‰ PDB and lower Hirnantian values are typically in the range of +4 to +7%o PDB. This contrast in isotopic values has been used to assess the age of the distinctive late Ordovician brachiopod Holorhynchus giganteus. The age of the Holorhynchus association is important in terms of its relationship to the major late Ordovician extinction that was initiated at the start of the Hirnantian. The species, which traditionally was regarded as uppermost Ordovician (Hirnantian) in age has been shown on stratigraphic evidence to be pre-Hirnantian at many locations. If this is so it predates the first phase of extinction and is not associated with the Hirnantia fauna, which characterizes the interval between the two main episodes of late Ordovician extinction. However, at its type locality in the Asker district of Norway the stratigraphic evidence has pointed to it being Hirnantian in age. To resolve this ambiguity, analyses have been made on Holorhynchus at three locations, (1) in the east Baltic region where Holorhynchus occurs below demonstrable Hirnantian rocks, (2) in the Boda Limestone of central Sweden where Holorhynchus occurs high on a carbonate mud mound where its age is unclear and (3) in the type area of the species in Norway. At all three locations the δ13C values are low, indicating a pre-Hirnantian, pre-extinction, age for Holorhynchus. The re-assessment of the age of the Asker sequence in Norway implies the presence of a cryptic unconformity there and suggests uplift rather than subsidence during the latest Ordovician. This study emphasizes the value of chemostratigraphy in high-resolution stratigraphy.

Storm-Influenced Inner-Shelf Sand Lobes in the Caradoc (Ordovician) of Shropshire, England

ABSTRACT A sequence through the middle Caradoc (Ordovician) of Shropshire shows the Horderley Sandstone with thick beds of hummocky cross-stratified sandstone, succeeded by the Cheney Longville Flags consisting of interbedded mudstones, siltstones, and sandstones, some of which also exhibit hummocky bedding. In the upper formation the mudstones are commonly bioturbated but also have many wave-rippled silty horizons. The interbedded thick siltstones and sandstones all lack current ripples and appear to have been rapidly deposited as a result of storm episodes. The siltstone and sandstone beds can be separated into two types: type 1 beds are massive or faintly planar-laminated, and have good lateral continuity; type 2 beds are lenticular, coarser, well laminated, and have hummocky bedding. The f rmer are believed to have been deposited from suspension, and the latter from powerful bottom traction currents flowing offshore during storm surges. Two varieties of hummocky bedding have been recognized within the sequence. The more usual type has laminae mantling a hummocky erosional topography. A second variety confined to the Cheney Longville Flags has laminae that accreted upwards from a planar base to produce undulating bedforms (wavelength up to 3 m; wave height up to 15 cm) that have symmetrical internal laminae that are form-concordant. The bedforms resemble wave ripples in cross section but are about an order of magnitude larger than usual wave ripples. In the Cheney Longville Flags, the argillaceous sediments and their fauna show virtually no change through 80 m of the succession. The type 1 beds, however, increase in proportion upwards whereas the type 2 beds show two strong maxima. Because the overall environment apparently remained nearly constant, and there is no evidence for progradation, we interpret the two developments of type 2 beds as reflecting two localized lobes of sand carried offshore into an inner-shelf environment by strong laterally restricted offshore currents, analogous to, but on a larger scale and more powerful than, rip currents. The independent upward increase in type 1 beds might reflect regional changes in paleogeography which caused a progressive exposure of the shoreface to an oceanic wave climate.