Contribution of calcareous nannoplankton to carbonate deposition: a new approach applied to the Lower Jurassic of central Italy Emanuela Mattioli  , Bernard Pittet UMR Pale ¤oenvironnement et Pale ¤obiosphe 're, Sciences de la Terre, Universite ¤ Lyon 1, 29^43 Bv. du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 18 March 2001; received in revised form 25 October 2001; accepted 28 October 2001 Abstract The Upper Pliensbachian^Lower Toarcian Somma section (Central Italy), which displays alternation of limestone^marl lithotypes, was investigated for its calcareous nannofossil and CaCO 3 contents. In order to evaluate the contribution of calcareous phytoplankton to the carbonate fraction of rocks a new method was applied combining a recently developed technique for nannofossil absolute quantification and measurements of nannofossil size. Our study suggests that nannofossils did not produce the bulk of carbonate mud in the studied section, although they contribute to different extents to the pelagic carbonate fraction. The contribution of both coccoliths and (mainly) the Incertae sedis Schizosphaerella to the carbonate fraction was important only in periods of reduced accumulation rates and/or during clay deposition. The highest estimated carbonate production by nannofossils in the studied section is around 83% of the total carbonate content, but in a sample containing only 12.8 wt% CaCO 3 . The observed limestone^marl alternations therefore result from cyclical export of carbonate mud from the shallow Latium^Abruzzi carbonate platform to the Umbria^Marche Basin and, only subordinately, from changes in nannoplankton productivity. ß 2002 Published by Elsevier Science B.V. Keywords: Lower Jurassic; Central Italy; calcareous nannofossils; carbonate production; quanti¢cation 1. Introduction In modern and past oceans, calcareous nanno- plankton (unicellular golden brown algae) is well documented as being one of the major pelagic producers converting dissolved CO 2 into carbon- ate. Both coccoliths and nannoliths are recorded in pelagic and hemipelagic sediments in rock- forming proportions since the Early Cretaceous. This is the reason why in recent years many au- thors have attempted to estimate extant £uxes of coccolith-produced carbonate to the sedimentary reservoir (Sprengel et al., 2000; Zeltner et al., 2000; Zivieri et al., 2000). A quanti¢cation of car- bonate production by nannofossils has recently been attempted also in Cretaceous sediments (Erba and Tremolada, 2000), whilst no study has yet been carried out to investigate Jurassic lithotypes. 0377-8398 / 02 / $ ^ see front matter ß 2002 Published by Elsevier Science B.V. PII:S0377-8398(02)00039-7 * Corresponding author. Tel.: +33-4-72-44-58-00; Fax: +33-4-72-44-83-82. E-mail address: mattioli@cismsun.univ-lyon1.fr (E. Mattioli). MARMIC 872 4-6-02 Marine Micropaleontology 45 (2002) 175^190 www.elsevier.com/locate/marmicro