Annual cyclicity in the Jurassic

BranscombeThis short article follows on from How old is the Earth? about the world-wide phenomenon of cyclical bedding in chalk formations – cycles where limestone repetitively alternates with marl. In the standard explanation such alternations reflect climate change caused by variation in the tilt of the Earth’s axis: limestones correlate with warmer periods, marls (mixtures of limestone and clay) with cooler periods and each cycle lasts around 20,000 years. The direct evidence speaks otherwise. Since experiments show that the worms and crustaceans which left burrow traces in these strata can destroy original layering within months, it is a safe inference that the lighter and darker sediments would have been homogenised into a uniform grey after a year or so. In reality, the burrows are still visible and the orderly alternation of limestone and marl still preserved. While an astronomical cause does seem required to explain such regularity, the couplets are best understood as reflecting alternations of summer and winter. Limestones formed when the seas were warm, marls when winter rains washed clays into the sea and productivity in the water column dropped off.

Chalk consists almost entirely of tiny platelets shed by species of algal plankton. The rock is characteristic of the Cretaceous period (the name means ‘chalky’). Here we look at putative expressions of the annual cycle from the late Triassic and early Jurassic, slightly earlier periods that, like the Cretaceous, are well represented along the Dorset coast of southern Britain (a UNESCO World Heritage Site).

Mercia Mudstone Group, Haven Cliff, Seaton

During the late Triassic age Britain was part of a supercontinent called Pangaea and lay at tropical latitudes. Here mudstones (top photograph) (top photograph) formed in a hot sabkha environment, where coastal mudflats were impregnated by salts from the sea. The rusty colour of the darker bands comes from oxidised iron. With less oxygen in the water, the iron oxide assumes a paler form. Evaporitic sulphates and silty dolomites also occur. Fossils – mostly spores – are common in the paler units; not much lived in the sediments themselves. The darker bands probably represent the cooler months of late autumn to early spring, the lighter bands the warmer months of late spring to early autumn.

The Blue Lias, Lyme Regis

Top Tape ammonite pavement, Lyme RegisLyme Regis, a fashionable seaside resort made famous by the novelist Jane Austen, is now celebrated for its fossils and fossil shops. To the west of the town the beach traces a succession of alternating limestones and marls or dark shales. These were deposited in the early Jurassic as Pangaea rifted apart into separate land areas and the margins of these areas became flooded. The gentle dip of the strata has now brought one limestone bed level with the beach to form a wave-washed pavement. It is littered with ammonites.

The concentration of ammonites takes some explaining. Although some researchers have inferred an exceptionally low rate of sedimentation, the enclosing limestone is not unusually thin. The most natural explanation is an environmental disturbance, such as a toxic algal bloom or an episode of anoxia. Ammonites propelled themselves through the water much as the modern nautilus does, but unlike some other species, the ammonites fossilised here lived close to the sea bottom. Clearly they did not die all at the same instant, for they are much more densely congregated than they could have been in life. Some overlap each other, and when directly superimposed they can be separated by sediment several centimetres thick. The sheer number of fossils in this bed is extraordinary.

Ammonite cross-sectionThe shells are uncrushed. Cross-sectional views show that, in contrast to the flattened state of the ammonites within the marls, most retained their shape and were only partly filled with sediment. The surrounding carbonate must have hardened quickly, before the weight of the sediments above could have compressed them and before their chemically unstable aragonitic shells could have dissolved back into the water. Since any mass-mortality event must have been brief, lasting, at most, months rather than thousands of years, the rate of sedimentation between the lower and upper ammonites must have been correspondingly rapid.

The time problem presented by these beds has not gone unremarked. As Paul et al. remark, the deposition rate implied by the conventional timescale is a mere 6 mm per thousand years, whereas

It is impossible to imagine an aragonitic shell exposed on the sea floor for even 5000 years without gaining any encrusting epifauna or indeed the shell dissolving away completely. The bivalve Pinna had a [chemically stable] calcitic shell and was semi-infaunal in life. We have seen examples of Pinna in life position in Bed H42, but truncated at the inferred top of the limestone bed, suggesting that even exposed calcite fossils did not survive long unless buried within the sediments, which in turn were probably rapidly cemented. Ammonites with encrusting epifauna that could have grown while the ammonite shell was exposed on the sea floor are rare.  The simplest explanation of the lack of encrusting organisms is that most large cephalopod shells were buried rapidly, before any encrusting epifauna could become established.  The conclusion that limestone beds with large ammonites or nautiloids in them were deposited very rapidly seems inescapable.

C R C Paul et al (2008), Palaeogeography, Palaeoclimatology, Palaeoecology 270.

The direct evidence, therefore, indicates that the radioactivity-based geological timescale is grossly inflated.

Although, as in the Cretaceous, much of the limestone may have derived from algal plankton, its lumpy texture suggests that inorganic precipitation was also involved, accelerated by sulphur-reducing bacteria that here and there left patches of pyrite. Another difference is that here the warmer seasons are represented by the darker beds – the marls and shales. With Britain in the early Jurassic lying at a lower latitude than in the late Cretaceous, summer temperatures in the upper water column were uncomfortably high for carbonate-producing algae. Temperatures at the sea bottom were cooler, allowing molluscs, sea-lilies, and various burrowing animals to thrive. On land, the wettest season – the period of enhanced runoff – was now the warmer months. Occasionally, an algal bloom consumed most of the oxygen and the laminated shales were hardly burrowed at all.

Belemnite Marls, east of Charmouth

The belemnite Passaloteuthis bisulcata, HolzmadenThe Belemnite Marls are named after the cephalopods that inhabited the near-shore and mid-shelf waters of this time, the early Pliensbachian. Their nearest living relatives are squid. They had a sharp beak, an ink sac, and ten arms projecting from their head. The arms were studded with hooks rather than suckers for catching prey. Calcitic bullet-shaped guards acted as a counterweight at the rear of the body. After their bodies decayed, these alone remained as fossils of the whole animal. Belemnites are especially numerous in these marls.

Cyclicity in the Belemnite Marls

As in the Blue Lias, these blue-grey muds alternate between dark and light, but the amount of carbonate (the lighter-coloured component) is smaller, and the boundaries between one bed and another more diffuse. Belemnite guards occur throughout, but burrows occur mostly in the lighter beds. Eighty-nine couplets have been counted in this 25-metre thick formation, not counting intervals at the top and bottom when sedimentation was too slow to produce clearly differentiated couplets. On the basis that the cycles were annual, this translates into an average sedimentation rate (after compaction) of 25 cm a year – not an unreasonable estimate for sediments that are only partially burrowed. On the basis that they reflect a 38,000-year cycle in the tilt of the Earth’s axis, the sedimentation rate would have been a miniscule 0.007 mm per year.

Bridport Sands, West Bay

BridportThese are of Toarcian age and thus somewhat younger than the Blue Lias. On a sunny day the cliffs east of West Bay present a majestic series of siltstones coloured deep gold. Strata honeycombed with well-cemented burrows alternate with thicker, more erodible layers in which burrowing is scarce. The original environment consisted of a relatively deep-water sand body migrating gradually southwards. Again, the cyclicity is probably annual, with the more carbonate-rich beds forming in the summer. The undeformed condition of mica flakes in the harder beds, contrasting with the crumpled flakes in the softer ones, shows that cementation took place early, preventing compaction of the sand grains. Sedimentation itself was rapid – otherwise the cement-rich fluids would have diffused evenly through the whole fabric. Likewise, few remains of seafloor-dwelling organisms occur in the poorly consolidated silts because sedimentation was too rapid for the seafloor to be a comfortable environment. Given that the cyclicity reflects the alternation of warm and cool seasons, the greater thickness of the cement-poor beds also suggests a higher rate of sedimentation.