Figure 1 in the article shows a series of chalk/marl couplets from the Cenomanian of the Crimea. Grey-scale fluctuations track the proportion of clay in the chalk, from 5-15% in the light-coloured beds to 10-30% in the darker ones, and the proportion of clay corresponds to the intensity of run-off from the land: the wetter the weather, the more clay eroded, thereby diluting the microfossils of calcareous plankton.
Similar couplets are exposed on the east coast of the Isle of Wight (Culver Cliff), as pictured below:
The grey beds represent the Chalk Marl formation. Higher up they pass into the Grey Chalk, which formed as sea-level rose – quite rapidly – and continental sources of sediment became more distant. Because the darker beds have more clay than the lighter beds, they erode more easily, producing on the wave-scoured foreshore a ribbed pattern. The Chalk Marl interval is a useful section for understanding what is going on in the chalk because the marl component makes the body and trace fossils in the sediment much more visible.
Left: Dark patches and streaks in the less eroded chalk-rich beds record the activity of crustaceans and other animals burrowing down from the marl-rich beds – also vice versa with the lighter patches and streaks. The dark beds become less mixed with chalkier sediment towards their middle and are more bioturbated because richer in nutrients (more attractive to the burrowers). Note that the bedding is not homogenised. In relation to a timescale which attributes 20,000 years to each couplet this is problematic, since burrowers can homogenise a 10 cm depth of sediment in a matter of weeks. The direct geological evidence suggests that the cyclicity is annual rather than precessional (‘Milankovitch’).
The other striking evidence of sedimentation rates being higher than the supposed 0.04 mm per year is the state of fossil preservation. Among the most distinctive fossils is the meandriform sponge Exanthesis. The next photograph shows a large example, 25 cm in diameter. The shape and form of this specimen – siliceous in life but subsequently calcified – are well preserved. After infill of its cavities, the weight of the sponge caused it to depress and sink into the mud, after which more sediment draped over it. That the sponge could have remained intact for thousands of years is difficult to believe.
Unlike most benthic fauna, sponges are immobile, so are unable to keep themselves above sediment that might be ‘rapidly’ collecting around them. On the other hand, Exanthesis could not have cemented itself to the surface, because the substrate was soft. Probably it would have tried to attach itself by filamentous ‘roots’ but was dislodged from time to time by sea-bottom currents and rolled along. In this way, sponges could reach ages much greater than one year. The danger was that their pores became clogged and the animals asphyxiated.
Chalk and marl beds are typical of background sedimentation – indications of what was normal, not catastrophic. Sedimentation rates were many thousands of times higher than modern rates because in the Cretaceous, as in other periods, the Earth was geologically much more active. Because modern rates are infinitesimally slow, it was possible for them to be thousands of times faster in the past and still permit seafloors to sustain flourishing communities.
A note on flint bands
Why was there so much silica in the ocean?
The silica derived from siliceous organisms such as glass sponges and radiolaria (a kind of plankton). As the buried skeletons of sponges broke down, the silica dissolved into a gel and recrystallised, encapsulating other fossil fauna or migrating into the fills of disused crustacean burrows, which were more porous than the surrounding chalk. In this way flint nodules formed. More fundamentally, silica-producing organisms flourished because of the high rates of hydrothermal circulation at the mid-ocean ridges. In this respect the present is not the key to the past:
Radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint. … Oceanic chemistry and productivity, as well as patterns of circulation/upwelling have changed radically. … In addition to plate drift, hypersiliceous domains and intervals are explainable mostly by a large-scale volcano–hydrothermal activity during major plate-boundary reconfigurations. … The present biogeochemical cycle is representative only for the overall silica-depleted post-Eocene oceanic ecosystems, which broadly correlates with a major expansion of diatom groups extremely efficient in silica removal.
G Racki & F Cordi (2007). Earth-Science Reviews 52:83
Chert bands did not develop in the marly successions because silica concentrations were lower in cool water, radiolaria preferred warm water, and cherts were more readily preserved in deep water. Hence, as a general rule, the incidence of chert bands increased as the marl component decreased and sea-levels rose. Cherts can themselves vary from pure to marly.
Why is the chert concentrated in bands and why is their spacing so regular?
Paradoxically, where there is a marl admixture, the flints tend to occur at the level of the marls rather than the purer chalks, probably because of the higher concentration of nutrients in the water column and the higher concentration of organic matter in the sediment (since lower pH promotes silica precipitation). This also promoted the concentration of chert in disused burrows. In the purer successions, the flint bands formed when production rates of radiolaria were high and those of calcareous nannoplankton were low. Silica-rich fluids most easily passed along bedding planes.
Modern radiolaria tend to bloom in the late summer, about the same time as the coccolith flux falls off. Flint bands formed at regular intervals, it is reasonable to infer, because the radiolaria blooms were seasonal. The intervals were generally annual.