HR: 11:50h
AN: PP32A-07    [Abstracts]
TI: Paleoceanographic Reconstruction of Productivity and Oxygenation Changes Across the Mediterranean Sea During the Deposition of two Late Pliocene Sapropels
AU: * Arnaboldi, M
EM: marna@umich.edu
AF: Department of Geological Sciences, The University of Michigan, 425 East University Avenue, Ann Arbor, MI 48105-1063 United States
AU: Meyers, P A
EM: pameyers@umich.edu
AF: Department of Geological Sciences, The University of Michigan, 425 East University Avenue, Ann Arbor, MI 48105-1063 United States
AB: The Mediterranean Sea is a marginal basin located at low latitudes that is extremely sensitive to climatic and environmental variations. In particular, climate oscillations associated with precessional cycles are amplified within this basin and drive dramatic oceanographic and environmental changes that result in the deposition of discrete layers of dark sediment at a 21-kyr periodicity. Sapropels are layers of organic-rich sediment deposited over the whole basin in correspondence with precessional minima, which are periods of increased humidity and seasonality over the Mediterranean area. Sapropel layers are interspersed in the light-colored, organic-poor calcareous marls that constitute the majority of the sedimentary record of the basin, which is 'normally' oligotrophic and fully oxygenated. We investigated the geochemistry of complete and continuous sequences containing two late Pliocene sapropels and intervening 'normal' sediment from six sites selected to cover an ample spectrum of oceanographic conditions from both eastern and western parts of the basin. The orbital signal that drives the formation of sapropels is modulated by local factors creating gradients and differences in the expression of sapropels across the basin. The geochemical composition of sapropels indicates they formed under oceanographic and environmental conditions extremely different from today. Results from bulk organic carbon and nitrogen isotopes in fact reveal that sapropels reflect an increase in the surface productivity coupled with extremely high rates of nitrogen fixation. Moreover, enrichments of chalcophile and redox-sensitive elements suggest the presence of anoxic conditions in the deep waters. Sapropels provide an ideal natural experiment to study the mechanisms and dynamics associated with climate changes because they allow the investigation of the same processes acting repetitively through time on a basin-wide scale. The use of proxies diagnostic of surface processes (carbon and nitrogen stable isotopes), such as productivity changes, and of bottom processes (trace and redox-sensitive elements), such as changes in the oxygenation state of deep waters, allows the evaluation of either a surface or bottom control on sapropel deposition. In addition, investigation of the concordant sapropel layers across the basin permits assessment of the spatial variability of sapropel expression caused by the interaction of local factors with the global signal and of the relative importance of water depth, proximity to land, and location in different sub-basins in modulating the orbital forcing. Finally, the presence of interruptions within sapropel layers suggests the existence of a delicate balance driving the formation of sapropels and allows the investigation of sub-orbital variations in the climate of the Mediterranean Sea.
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 1065 Trace elements (3670)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting