Coccolithophorid ecostratigraphy and multi-proxy paleoceanographic reconstruction in the Southern Adriatic Sea during the last deglacial time (Core AD91-17)

摘要

A very detailed environmental history of the last deglaciation and the Holocene is recorded in a high sedimentation rate core collected in the Southern Adriatic Sea (Core AD91-17). The stratigraphic framework, based on radiocarbon dating (~(14)C AMS) and the oxygen isotope record, allows recognition of the paleoceanographic changes of the last 16 200 years. Fluctuations within the coccolithophorid assemblage identify five intervals that can be correlated to major changes in the planktonic foraminifera records. Recognition of the same zonation in the eastern Mediterranean Sea suggests that they are truly basin-wide ecozones rather than local events. Interpretation of the paleoceanographic meaning of these coccolithophorid ecozones, together with planktonic and benthic foraminifera, dinocyst, pollen, magnetic parameters, oxygen isotopes and alkenone unsaturation indexes, outlines fluctuations related to different paleoclimatic phases. In particular, from 16 200 to about 11 670 yr BPnc all proxies register cold conditions with a well-ventilated sea bottom. A gradual sea-surface temperature (SST) increase characterized the period between 11 670 and 10 800 yr BPnc, followed by a slight cooling coincident with the beginning of the Younger Dryas. Between 8650 and 6560 yr BPnc, corresponding to sapropel S1 formation, all proxies register a transition to warmer climate. Sea-surface productivity reached maximum values, while evidence for development of low salinity superficial waters and of a deep chlorophyll maximum is observed. The multi-proxy analysis suggests the presence of at least three phases in the S1 sapropel itself. The first part of the sapropel is characterized by high nutrient availability, warm stratified waters and severe bottom anoxia. Between 7650 and 7500 yr BPnc, corresponding to an interruption of the sapropel, we observe a rapid reoxygenation at the seafloor when SST warmed. Just after the sapropel interruption, and particularly between 7400 and 7250 yr BPnc, a slight climatic deterioration, an increase of salinity and a decrease of runoff are observed. At the top part of the sapropel, eutrophic environments, a well-stratified euphonic zone and dysoxic conditions at the bottom are identified. From 6560 to 5080 yr BPnc, conditions became more oligotrophic and SST reached a maximum, while a slight increase of superficial water salinity may suggest the end of stratified waters and the beginning of water column mixing. Finally, between 5080 and 2240 yr BPnc all proxies indicate warm and normal salinity waters. The bottom environment returned to normal oxygenated conditions.