HR: 0800h
AN: OS21A-1194    [Abstracts]
TI: Marl-Chalk Rhymicity in Plio-Pleistocene Black Sea Sediments
AU: * Glenn, C R
EM: glenn@soest.hawaii.edu
AF: University of Hawaii, Department of Geology and Geophysics, Honolulu, HI 96822 United States
AU: Rajan, S
EM: rajan@ncaor.org
AF: National Center for Antarctic and Ocean Research, Vasco-da-Gama, Goa, 403804 India
AB: During Leg 42B of the Deep Sea Drilling Project in the Black Sea, two calcium carbonate-rich Plio-Pleistocene sequences totaling about 250 m in thickness were retrieved at Site 380 near the Bosporus Straits. These "Chalk units" are characterized by hundreds of rhythmically deposited, cm-to dm-thick marl-chalk couplets marked by varying amounts of carbonate and organic matter contents. Whereas the couplets in the Upper Chalk unit are largely structureless, in the Lower Chalk unit the chalk half-couplets occur as (1) finely laminated beds, (2) moderately bioturbated beds exhibiting a lensoidal pattern, or (3) intensely bioturbated homogeneous structureless chalk beds. In contrast, the marl half-couplets are by and large, structureless. Textural and mineralogical studies show that the carbonates in the marls and chalks in both the Chalk units are microcrystalline low-magnesian calcite. The paucity of biogenic and detrital carbonate in the sediments, the presence of bioturbation features along the contacts between the basal marl and the overlying chalk, and covariant increases in carbonate content and the $\delta$$^{13}$C values of the carbonate fraction across the couplets indicate that the carbonate phases in these rhythmicities are a direct result of authigenic precipitation from the surface waters. Because (i) the CaCO$_{3}$-C$_{org}$ systematics indicate that primary biological productivity variations during the times of deposition of these lithologies were not were not significant enough to promote the development of the rhythmites, and (ii) the Black Sea remained a gigantic, deep, hydrologically-closed lacustrine water body through most of the Plio-Pleistocene, we contend that the carbonate production in the basin was largely sustained by abiotic precipitation from the surface waters, with bio-induced carbonate precipitates constituting only a minor portion of the total carbonate fraction. Bedding thickness considerations indicate that systematic fluctuations in terrigenous flux rates and carbonate production could have been of significance in the formation of the couplets in the Lower and Upper Chalk sequences respectively. Published pollen records preserved in the sediments indicate that steppe pollen (reflecting cooler and more arid climatic conditions) is dominant in chalk half-couplets, whereas forest pollen (indicative of relatively warmer and more humid climate) dominates basal marl half-couplets. This, together with the observed systematic variations in the carbon and oxygen isotope composition across the couplets indicate the strong influence of climate in promoting the deposition of these rhythmicities. Our studies indicate that the individual marl-chalk couplets of both Chalk Units represent alternating warm-humid and cool-arid climatic conditions which in turn controlled rhythmic flucuations between evaporation and inflow rates, with average periods occurring approximately every 4200 years. However, whereas the short-term climatic fluctuations during the times of deposition of the Upper Chalk sequences were within an overall cold phase of early Pleistocene, similar perturbations which characterized the Upper Chalk times occurred during a relatively warm preglacial epoch in the mid-to late Pliocene.
DE: 4870 Stable isotopes
DE: 4239 Limnology
DE: 1845 Limnology
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting