Using the present as an analogue for the past

Making the assumption that Living and Recent calcareous nannoplankton are an adequate analogue for the past, Cretaceous calcareous nannoplankton can greatly help in our understanding o f the palaeoceanography for this time period.

Nannoplankton in the past were most likely restricted to the photic zone i.e. in the upper 50 - 200m o f the water column - as is seen in the present day (Jordan, 1988). At present, with cold polar seas and warm tropical waters, nannofloral distribution and species diversity is largely controlled by surface water temperature patterns. Differences in nutrients and salinity have, in the past, appeared to be only

minor factors affecting coccolithophorid distributions in the water column (Honjo, 1977). Carbonate dissolution at depth is seen to strongly affect coccolith distribution in sediments (McIntyre & McIntyre, 1971; Schneidermann, 1973, 1977; Roth & Berger, 1975; Roth & Coulbourne, 1982).

The uncertainty about the physical and chemical conditions in ancient oceans increases with geological age - but despite this many similarities can be noted.

At present, latitudinal differences in the oceans, cold polar seas and warm tropics, seems to exert dominant control on the distribution o f coccolithophores. A similar effect is seen in coccolith assemblages o f Cenozoic age (Haq & Lohmann, 1976) and Late Cretaceous age (Thierstein, 1979) although there appears to be marked provincialism in the Late Jurassic and Early Cretaceous (Rutledge, 1994). The influence o f latitude is, however, less pronounced in Mid-Cretaceous times and earlier. This is to be expected, as the Mid-Cretaceous world was one where global temperature gradients were much reduced and polar ice caps were negligible, if non-existent (See Chapter 4 for details). Nevertheless, distinct Mid-Cretaceous biogeographic provinces can be identified and areas defined for specific species assemblages.

Mid-Cretaceous oceans and nannoplankton assemblages appear to have been strongly controlled by nutrient availability rather than latitude and temperature (Roth, 1981, 1986 & 1989). This is discussed in more detail in Chapter 4.

Despite temperature and climatic differences, other similarities with present day oceans can be made. Just as EniiUania h uxleyi is ubiquitous today W atznaueria

bam esae appears to have played a similar rôle in the past being found in most mid-

Cretaceous deposits in high abundances.

It is important we use all our knowledge o f present day oceans and their chemical and physical conditions along with nannoplankton distribution to help us decide the ecological controls that may have exerted themselves upon the nannoplankton in the past.

In the present day oceans, various ecological controls appear to be exerted upon the physiology o f many coccolithophores. Placolith type coccolithophores appear to dominate upwelling, eutrophic and high productivity zones whilst umbelliforms seem to prefer oligotrophic, open ocean, nutrient depleted waters o f mid-ocean gyres.

Floriform coccolithophores appear to inhabit deeper waters, below the mixed layer or thermocline, and they seem to prefer high nutrient concentrations (Young, 1994) (Figure 3.15a).

Just as Em iliania huxleyi, Gephyrocapsa oceanica, and G ephyrocapsa ericsonii

dominate equatorial divergence zones and areas o f upwelling today, and are indicative o f nutrient rich upwelling zones and central gyres (warm water conditions and all eutrophic areas) in which light levels are adequate for algal growth and nutrient supply exceeds the demands o f the crop giving rapid population growth (Kilham & Kilham, 1980), B i scutum constans and some zeugrhabdotids are believed to have been indicative o f high surface water productivity in the past. Biscutum constans and

Zeugrhabdotus erectus are suggested to have been controlled less by temperature and

latitudinal effects in the Cretaceous due to a shallower gradient from pole to equator and more by nutrient availability, closely associated with upwelling centres (Erba, 1986, 1988, 1992; Roth, 1981, 1986, 1989 and Roth & Bowdler, 1981).

Just as Florisphaera profttnda has been found (Jordan, 1988, M olfino & McIntyre, 1989) as indicative o f water depths and low surface water nutrients in present day and recent oceans, nannoconids have been proposed by Erba (1992) and Mutterlose & Erba (1993) to prefer similar conditions in the Lower and Mid- Cretaceous. Rutledge (1993) looked at early Barremian nannofossils in a darkening upwards section from Speeton, North Yorkshire, England and noted that nannoconids reached bloom proportions towards the top o f the section and were in opposition to

Biscutum constans and Zeugrhabdotus noeliae (proposed high fertility indicators). The

nannoconids therefore, were proposed to be indicators o f oligotrophic conditions and their palaeogeographic distribution, mainly temporal and concentrated within carbonate-rich units o f pale/dark rhythmic sections indicated that they appeared to prefer warmer waters.

A similar scenario can be reconstructed from this information for the oceans in Mid-Cretaceous times. Although different species, it appears that similar ecological controls affected their distribution and Figure 3.15b. helps to demonstrate this. Chapter 4 elucidates on possible scenarios for the Mid-Cretaceous including the climate, palaeogeographic setting and oceanic conditions at that time.

m

s Placolith bearing coccolithophorids such as

Umilicosphaera, Coccolithus pelagicus,Emiliania

huxleyi, Gephyrocapsa oceanica & Calcidiscus

leptoporus.

Umbelliform coccolithophorids such as

U.irregularis, U.tenuis & Discosphaera tubifera.

Floriform coccolithophorids such as Florisphaera

profunda & Thorosphaeraflabellata.

Miscellaneous group including Syracosphaera,

Helicosphaera & Calyptrosphaera amongst

others.

[T] Coastal & upwelling environments (T| Oligotrophic mid-ocean environments

l3l Deep stable stratified waters

Present Day

m

E

^ Braarudosphaera and Broinsonia

Biscutum constans, Z.noeliae, Z.erectus

and other Zeugrhabdotids Watznaueria barnesae Nannoconus

[Â] Coastal and nearshore environments [B] Upwelling and nutrient rich environments fCl Oligotrophic mid-ocean environments

fPl Deep stable stratified waters

Mid-Cretaceous

Figure 3.15 b) Ecological distribution of Mid-Cretaceous nannoplankton. Information collated from a variety of sources including Roth (1981 & 1986), Roth & Bowdler (1981), Roth & Krumbach (1986), Crux (1989) and Erba (1986 & 1992).