Multiproxy approach for determination of nature and origin of carbonate micro-particles so-called “micarb” in pelagic sediments

Abstract

Carbonate fractions of pelagic and hemipelagic sediments may contain significant proportions of microcrystalline (≤ 6 µm) calcitic particles with no specific structure so-called micarb. Depending on the authors, this term is related to different descriptions and yet there is no clear consensus on their origin. Although, micarb can dominate the sediment compositions in the pelagic realm, their isolation from the other carbonate particles is not easy due to their small size. The use of a granulometric separation method of the main carbonate particles allows the characterization and quantification of the compositions of the carbonate phases of the sediments and gives access to the geochemical signatures (oxygen and carbon stable isotopes) and mineralogical characteristics of each type of carbonate particles in the sediments. This approach was applied to three sections: the Mid-Pliocene Punta Piccola section (South Sicily), the Upper Campanian Bidart section (SW France) and the Aptian sediments from La Marcouline Quarry (SE France). The quantifications showed quite important proportions (up to 70% of the carbonated phases) of micarb in the sections studied excepted in the mid-Pliocene Punta Piccola samples. However, depending on the sedimentary context, it appears that those fine calcitic particles may result from fine biogenic fragmentation (Aptian, La Marcouline Quarry), authigenic precipitation in the water column (Mid-Pliocene, Punta Piccola section), or they may have an allochthonous origin (Upper Campanian Bidart section). Based on these interpretations, it is then possible to estimate precisely the composition of the carbonate fractions in each studied section. This work shows that except for the Bidart sediments which are dominated by fine exported carbonate mud, the other sections mainly consist in biogenic carbonates.