Geographic distribution of modern coccolithophorids in the Mediterranean Sea and morphological evolution of Calcidiscus leptoporus

The seasonal distribution patterns of modern and Holocene cocco¬ lithophorids in the Mediterranean Sea during late summer (September-October, 1986) and late winter (February-March, 1988) and the morphological evolution of one extant species, Calcidiscus leptoporus, were investigated in this study. In the Meditenanean Sea, a total of 72 living species were identified, which is 52.2 % of the calcareous nannoflora known. In the Mediter¬ ranean Sea, only 7 species are responsible for about 90% of the entire living Meditenanean coccolithophore stock. The main fluctuations of the composition and diversity of the Meditenanean calcareous phyto¬ plankton are due to the seasonal, wind induced hydrological changes from a highly stratified upper water mass during summer to an enti¬ rely homogenized watercolumn during winter. During winter E. huxleyi dominated the calcareous phytoplankton Community by up to 80 %. The main coccolithophore production was observed during late winter. Typical cell concentrations in the upper water column were 10^ to 3 x 10-5 cells per liter, which is more than 5 to 10 times the concentra¬ tion in summer. The second important factor influencing the taxonomic composition is water depth. The deep-dwelling species Florisphaera profunda reached high concentrations during winter and during sum¬ mer, and is an important contributor to the sediment accummulation too. The wide occunence of this species during the whole year is pro¬ bably due to a less pronounced seasonality in the lower photic layer. The geographic changes in the taxonomic composition are linked to surface cunent patterns. Regional differences appear to be more pronounced during winter than during summer. Gephyrocapsa oceanica and Gephyrocapsa ericsonii are abundant in the Western Meditenane¬ an Sea, but very rare in the Eastern Meditenanean Sea. G.oceanica is a tracer for Atlantic waters entering the Meditenanean Sea through the Strait of Gibraltar. The mean annual coccolith carbonate flux from the photic zone to the sediment surface in the Meditenanean Sea is 8.89 g m-2yr-l. During summer, a coccolith carbonate flux of 1.4 g m_2yr-l (15.8 % of the total annual flux), was estimated. During winter, the coccolith carbonate flux was 7.48 g nr^yr-1, (84.2 % of the total annual coccolith carbonate flux). Coccolith assemblages in surface sediments reflect the calcareous nannoplankton composition of the homogenized water column during winter. The most important contributors are coccoliths of E. huxleyi, F. profunda and the gephyrocapsids. Coccoliths of G. oceanica can be found abundantly in surface sediments of the Sou¬ thern Algerian Basin. Umbellosphaera tenuis, which dominates the summer plankton assemblages, are undenepresented in the sedimen¬ tary record, probably due to the decreased coccolith carbonate flux during summer. The taxonomic investigations showed, that polymorphism is a most widespread feature among coccolithophorids. Only taxa of the Braarudosphaeraceae, Helicosphaeraceae and parts of the Coccolithaceae do not produce polymorph coccoliths. Highly variable groups are the Syracosphaeraceae (dithecatism and endothecal dimorphism) and the Calyptrosphaeraceae. The morphological evolution of C. leptoporus during the past 24.6 million years (Upper Oligocene to Recent) was investigated in the Atlantic, Pacific and Indian Oceans. In total, three extant morphotypes P (24.6 Ma-Recent), T (24.5 Ma-Recent), and F (Holocene) and five extinct morphotypes A (4.2 Ma-1.5 Ma), B (2.7 Ma-0.8 Ma), C (22.6 Ma5.9 Ma), D (10.7 Ma-8.4 Ma) and E (8.9 Ma-0.8 Ma) were distinguished by coccolith size and element number in the distal shield. Morphotypes P and T could be identified in the living plankton and F was only found in Holocene surface sediments of the tropical Pacific. In Holocene sedi¬ ments, the size and number of elements of morphotypes P and T are conelated to sea-surface temperatures. Coccoliths from areas above a winter temperature of 23.5° C are significantly larger and conespond to morphotype T, while coccoliths from areas below a winter tempera¬ ture of 23.5° C conespond to morphotype P. At 1.5 Ma morphotypes A and B followed also a latitudinal gradient, but with inverse morpholo¬ gical trends compared to the Holocene. In addition, three morphoclines PTC, ABCD and PTEF were identified, along which morphotypes P, T, C and A, B, C D and P, T, E, F had evolved during different geological time intervals. The morphoclines are interpreted as three different species, which have repeatedly developped similar coccoliths at different stratigraphic levels. Morphocline PTC can be observed from 24.5 Ma to 12.3 Ma. It consisted of a continuous plexus of coccoliths of morphoty¬ pes P, T and C, which developped by anagenic evolution between 24.6 Ma and 21.9 Ma. Morphocline PTC is distinguished from the other two morphoclines by its steeper slope (6.0 in PTC, 0.9 in ABCD and PTEF) in the morphospace defined by the coccolith diameter and its number of elements. Morphocline ABCD occuned from 12.3 Ma to 0.8 Ma and morphocline PTEF from 12.3 Ma to Recent. It appears, that morpho¬ clines ABCD and PTEF evolved during a time-transgressive sympatric cladogenic event between 16.2 Ma and 12.3 Ma, which could be due to the formation of an oceanic boundary of watermasses. This event is expressed in a bifurcation of morphocline PTC into ABCD and PTEF, and in a simultaneous rotation of the slope of morphoclines from 6.0 (mor¬ phocline PTC) to about 0.9 (morphoclines ABCD and PTEF). The three morphoclines seem to minor the prefened direction along which coccolithophores developped new morphotypes due to ecophenotypic Variation. Morphotypes A, B, E and F are interpreted to have evolved along a specific morphocline by branching and subsequent phyletic divergence in local, bathymetrically or geographically isolated groups.