Eastern-Mediterranean ventilation variability during sapropel S1 formation, evaluated at two sites influenced by deep-water formation from Adriatic and Aegean Seas

Abstract

a b s t r a c tPresent-day bottom-water ventilation in the Eastern Mediterranean basin occurs through deep-water convection originating from the two marginal basins, i.e. Adriatic and Aegean Seas. In the paleo record, long periods of enhanced deep-water formation have been alternating with shorter periods of reduced deep-water formation. The latter is related mainly to low-latitude humid climate conditions and the enhanced deposition and preservation of organic-rich sediment units (sapropels). This study focuses on sedimentary archives of the most-recent sapropel S1, retrieved from two sites under the direct influence of the two deep-water formation areas. Restricted oxygen conditions have developed rapidly at the beginning of S1 deposition in the Adriatic site, but bottom-water conditions have not persistently remained anoxic during the full interval of sapropel deposition. In fact, the variability in intensity and persistence of sedimentary redox conditions at the two deep-water formation sites is shown to be related to brief episodes of climate cooling. In the Adriatic site, sapropel deposition appears to have been interrupted twice. The 8.2 ka event, only recovered at the Adria site, is characterized by gradually increasing suboxic to possibly intermittently oxic conditions and decreasing C org fluxes, followed by an abrupt re-establishment of anoxic conditions. Another important event that disrupted sapropel S1 formation, has taken place at ca. 7.4 cal ka BP. The latter event has been recovered at both sites. In the Adriatic site it is followed by a period of sedimentary conditions that gradually change from suboxic to more permanently oxic, as deduced from the Mn/Al pattern. Using the same proxy for suboxic/oxic sedimentary redox conditions, we observe that conditions in the Aegean Sea site shift to more permanently oxic from the 7.4 ka event onwards. However, at both sites the accumulation and preservation of enhanced amounts of organic matter have continued under these suboxic to intermittently oxic sedimentary conditions. It seems thus, that after 7.4 cal ka BP sapropel-like surface or deep-chlorophyllmaximum conditions including enhanced productivity continued, whereas bottom-water conditions were at least intermittently oxic. The latter is related to decreasing precipitation, i.e. run-off, and thus a progressive development and deepening of deep-water formation. The shallower Aegean site, would be affected earlier by such deepening ventilation than the slightly deeper Adriatic site. Finally, termination of sapropel S1 formation as deduced from diminished organic matter contents and Ba/Al, appears to have occurred almost simultaneously in the two areas, namely at 6.6 ± 0.3 and 6.3 ± 0.5 cal ka BP in Adriatic and Aegean sites, respectively.

Reventilation Episodes During the Sapropel S1 Deposition in the Eastern Mediterranean Based on Holococcolith Preservation

“…The subsequent step at 10.5 cmbsf, at the end of Sapropel S1, led to persistent oxygen availability on the seafloor, repopulation of oxyphilic benthic foraminifera assemblages, and the preservation of holococcoliths that were resistant to prediagenetic dissolution (Kleijne, ). The sequence described above is perfectly compatible with the occurrence of distinct reventilation episodes at 8.2 and 7.4 ka in the Aegean and Adriatic Seas, before the termination of sapropel deposition at 6.6‐6.3 ka (Filippidi et al, ). Both the 8.2‐ and 7.4‐ka events would be caused by cool and arid conditions that led to improved deep‐water oxygenation and benthic foraminiferal repopulation.…”

Eastern Mediterranean Deep Water Formation During Sapropel S1: A Reconstruction Using Geochemical Records Along a Bathymetric Transect in the Adriatic Outflow Region

“…ka BP is also recorded at these two sites indicating that once again a rapid cooling of surface waters triggered the restart of DW‐formation to water depths at least as deep as MP39PC site (1,359 m). This cooling event is detectable at these sites, which are under the immediate influence of DW‐formation, whereas it has been previously reported also for the Aegean Sea (e.g., Filippidi et al, ; Kouli et al, ; Triantaphyllou et al, ).…”

“…This is thought to relate to the diminished rate of sulfate reduction in the shallow core due to lower reactive organic matter content and the relatively large amount of Fe oxides due to a high sedimentation rate. This lower reactive organic matter content is related to a lower degree of preservation in the more shallow bottom waters that are thought not to have been permanently anoxic (De Lange et al, ; Filippidi et al, ; Reitz et al, ).…”

“…The second cold episode, at 7.4 ka BP, prominently expressed in a number of marine and terrestrial records coincides with a northerly cold outbreak and the beginning of the gradual retreat of the ITCZ and thus the weakening of humid conditions. This event also triggered the resumption of DW overturning at intermediate water depth (~700 m), temporarily in the Adriatic Sea and more permanently in the Aegean Sea (Filippidi et al, ; Goudeau et al, ; Kouli et al, ; Triantaphyllou et al, ). Consequently, the wide range of sapropel S1 deposits studied throughout the Eastern Mediterranean basin have revealed that sapropel formation has taken place under variable depositional settings.…”

Large-scale response of the Eastern Mediterranean thermohaline circulation to African monsoon intensification during sapropel S1 formation

“…78a) prior to the 419 S1 termination. This was a rather short-lived event but widespread over the EMS as previously observed 420 in the Aegean and Ionian regions (Filippidi et al, 2016).…”