Comparative evolution of material flux through two DSDP successions from Central Atlantic and Gulf of Mexico. Preliminary results of a new approach to sedimentary dynamics in pelagic environments

Abstract

A method for gaining knowledge of sedimentary fluxes is applied to Early Cretaceous sequences containing high-frequency sedimentary cycles. The cycles are used to determine the relative fluctuations of (1) the sedimentation rate and (2) carbonate and silicate terrigenous fluxes expressed as linear values proportional to the real fluxes. Visual and automatic image analysis of black-and-white core photographs have been used to calculate the masses of different materials deposited in equal time intervals and to determine the distribution of the material within lithological units of cores according to a scale of grey intensities. Characters of these distributions allow definition of a flux variability index, assumed to be negatively correlated to ocean-continent distance related itself to sea level. At DSDP Sites 534 (western Atlantic off Florida) and 535 (Gulf of Mexico), the Valanginian-Hauterivian interval has been chosen for determining the relative variations of carbonate and silicate terrigenous linear fluxes, their variability and their distribution within lithological cycles. These data provide information about the relative importance of carbonate sources (lands, platforms and plankton) and their evolution, and of other forcings upon planktonic productivity such as supply of nutrients from the continents and upwelling. By comparing the data in the two sites, it is possible to detect common (climate and eustasy) and local factors (tectonics and upwelling) which have controlled the material fluxes. Compared with Site 534, Site 535 was supplied by more numerous, active and close carbonate sources and less active but closer terrigenous sources. At the two sites, carbonate flux was controlled by common factors, sea level being the most prominent one. Terrigenous flux was submitted to more local controls. This new method for investigating pelagic deposits should be tested on other deep-sea sedimentary sequences.