Three fundamental changes in the global carbon reservoir, can be documented in western Tethys during Cretaceous time period: early Cretaceous -carbon was mostly buried as inorganic carbon in pelagic and shallow water carbonates; middle Cretaceous (Valanginian to early Turonian) - carbon was buried as organic carbon in deep ocean and margin basins, late Cretaceous (late Turonian to Paleogene) -no carbon was buried in the deep oceans, but some was deposited under special paleoceanograhic conditions as chalks on shelves. Such changes demonstrate complexity and interplay of various Earth Systems (plate tectonics, ocean circulation, bioproductivity and climate) on sediment accumulation in oceanic basins and mainly document heterogeneity and chemical fractionation not only between oceanic basins, but also within a sedimentary basin. A transition from dysoxic to oxic depositional setting on the floor of the North Atlantic oceanic basin occurred near to the Cenomanian-Turonian boundary. Such change is well documented by the deep sea cores from the North Atlantic (e.g. ODP Leg 103), which demonstrate rapid change from the OAE-2 event (Bonarelli horizon) with 30% of marine organic matter to overlying pelagic red clay, which lacks of any carbon. In some of the areas of the Tethys, the oxic bottom conditions during late Cretaceous and early Cenozoic expanded up the continental margins giving rise to pink colored marl deposits, also enriched in iron oxide (such as the Scaglia Rossa facie in Italy). Therefore, oxic conditions expanded from bottom to intermediate waters, which led to the organic carbon oxidation and destruction of potential source rock for hydrocarbon generation. Preliminary studies indicate that in an addition to the eustatic sea level and paleoclimate changes, it is tectonics and ocean circulation which play important roles in the variability of the hydrocarbon potential of the Atlantic continental margins.