Pahnke, Katharina (2000) C org-reiche Sedimenthorizonte der Alboran See (ODP Site 976, westlichstes Mittelmeer) : Rekonstruktion der Ablagerungsbedingungen nach hochauflösenden Profilen stabiler Sauerstoff- und Kohlenstoffisotope und der Häufigkeitsverteilung benthischer Foraminiferen (Teil 2 : Laborarbeit). (Diploma thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 89 pp.

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Abstract

At Site 976 in the Alboran Sea, western Mediterranean, a high-resolution planktonic oxygen isotope record (G. bulloides) was established for the past 77,000 years with a time resolution of 7.5-1170 years. Short-lived δ180 anomalies document Dansgaard/Oeschger (D/O)-climatic oscillations and Heinrich (H)-events with δ180 shifts of up to 1.6 ‰. Glacial-interglacial δ180 amplitudes are between 2.5-3.2 ‰. The existence of D/O-oszillations and H-events at Site 976 document the influence of North Atlantic hydrographic and climatic conditions on the Mediterranean, transfered to the Alboran Sea via surface inflow and the atmosphere. Abundance distribution and assemblage structure of benthic foraminiferal populations are used to reveal environmental conditions during the formation of two organic-rich layers (ORLs) in the Alboran Sea. Bottom water oxygen concentrations were reconstructed using the benthic foraminiferal oxygen index (BFOI) as defined by Kaiho [1994]. Paleoproductivity was estimated using two independent methods, the benthic foraminiferal accumulation rate (BFARtotat sensu Herguera and Berger [1991] and Herguera [1992]) and TOC-contents ("total organic carbon"). Estimates from BF ARtota1/TOC and BFOI document low oxygen concentrations (0.46-1.01 ml/1) and paleoproductivities (103.05 gC/m2a via BFARtota1; 183.23 gC/m2a via TOC) for the early stage of ORL-Al deposition in the Younger Dryas. Estimated paleoproductivity gradually increases (168.82 gC/m2a from BFARtota1; 212.09 gC/m2a from TOC) while oxygen concentrations develop to suboxic to medium-oxic conditions (max. 3,21 ml/I). Reduced oxygen concentrations are also deduced from enhanced Corg-preservation throughout the entire ORL-Al. An influence of low oxygen conditions is also documented in the lower paleoproductivity values estimated from the BF ARtotat, which conceivably reflects stressed conditions for the benthic foraminiferal fauna. Changes in paleoproductivity are more sensibly traced by benthic foraminifera (BFARtotat) than by TOC as is inferred from larger amplitudes of paleoproductivity changes derived from the BFARtotat. Deposition of ORL-A6 during the penultimate glacial-interglacial transition i.e., Termination II, is inferred to have occurred under high-oxic conditions and enhanced biological productivity (452.71 gC/m2a from BFARtotat; 207.84 gC/m2a from TOC). Therefore, formation of ORL-A6 is suggested to be the result of a high productivity-event when near-bottom waters were well ventilated. Apparently, ORL formation in the Alboran Sea was influenced by the variable strength of Atlantic surface inflow, which effects the location and intensity of high-productivity fronts. Thus, other than sapropels in the eastern Mediterranean which formed during periods of enhanced insolation and concomitant warm-moist climates, ORL formation in the western Mediterranean likely was influenced by North Atlantic climate and ocean circulation.

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