Evidence for the Presence of Oxygen-Depleted Sapropel Intermediate Water across the Eastern Mediterranean during Sapropel S1

Abstract

In the Eastern Mediterranean Sea (EMS), natural climate-driven changes that impacted its physical circulation and created deep-water anoxia have led to periods of distinctive deposits referred to as sapropels. The most recent sapropel event (S1) occurred between ∼6 and 10.8 kyr BP. Use of a global climate model has suggested that there was a previously unrecognized mid-depth (500−1800 m) water mass present at that time. Here, a broad review of cores previously analyzed from across the EMS was undertaken to determine if the field evidence was compatible with the presence of this intermediate water mass. The proxy records document a widespread oxygenated interruption centered around 8.2 ka BP in water depths between 500 and 1800 m, but not present in shallower or deeper water. We attribute this pattern to the formation and spread of a water mass, which we refer to as Sapropel Intermediate Water (SIW). It is shown that this water mass was formed in the Aegean Sea and became more depleted in oxygen, to anoxicity but not euxinicity, as it flowed west to the Adriatic and east across the Levantine basin. The rate of formation and flow of this water mass was estimated to be slow but not zero. An evolving oxygen minimum zone in the SE Levantine basin is posited where oxygen was depleted first in shallower water and expanded with time into deeper water. The presence of SIW obviates the need for the previously suggested thin anoxic blanket, since the 1300 m-thick SIW can be fully deoxygenated in 200−500 years.

The Postglacial Isotopic Record of Intermediate Water Connects Mediterranean Sapropels and Organic‐Rich Layers

“…In the Levantine Basin, the base of this interval marks the end of the Nile River maximum flooding (Figure 6l) (Weldeab et al, 2014). The 8.2 ka event, coinciding with the 8.5–7.8 ka sapropel interruption and seafloor reventilation at intermediate depths (Abu‐Zied et al, 2008; Casford et al, 2003; Filippidi & De Lange, 2019; Incarbona, Abu‐Zied, et al, 2019; Le Houedec et al, 2020; Rohling et al, 2015; Zirks et al, 2019), is characterized by increasing LIW density and improved bottom ventilation in the Sicily Channel (Figures 6e, 6f, and 6i). The final alkenone accumulation in the ODP Site 976 occurred between7.3 and 6.8 ka and matches with a prominent peak in the Aegean Sea oxyphilic benthic foraminifera (Figure 6k) (Abu‐Zied et al, 2008; Casford et al, 2003), with geochemical and micropaleontological evidence of deep water formation in the Adriatic Sea (Di Donato et al, 2019; Filippidi & De Lange, 2019; Maiorano et al, 2019) and again with increasing LIW density and improved seafloor ventilation in the Sicily Channel (Figures 6e, 6f, and 6i).…”

Deoxygenation dynamics above the western Nile deep-sea fan during sapropel S1 at seasonal to millennial time-scales

“…At deep site MS27PT offshore from the Nile River mouth, suboxic conditions occurred between 12 and 10 ka BP and were accompanied by higher primary productivity (Ménot et al, 2020). A peak of productivity was also identified in some records from the SE Levantine Basin prior to the onset of S1 deposition at depths similar to MS27PT (>1000 m) (Zirks et al, 2021;Zwiep et al, 2018) but not at shallower depths (Castañeda et al, 2010;Mojtahid et al, 2015). Revel et al, 2015).…”

“…At present, the core site is ventilated by formation of intermediate and deep-water masses as it lies at the transition be-tween LIW and LDW flows (Kubin et al, 2019). During S1 deposition, it was suggested that a sluggish hypoxic water mass occupied the depth range 500-1800 m, originating from the Aegean Sea and labelled "Sapropel Intermediate Water" (SIW) (Zirks et al, 2019). The persistence of a yearly cycle of fluvial input during S1, with a marked low-flow regime in winter as observed by our data, fits previous observations blue), (c) GeoB7702-3 (black) and MS27PT (grey) on an age scale (0-10 ka BP).…”

“…Based on the observations of seasonal sublayer deposits and in particular the occurrence of authigenic calcite, we derived that bottom waters offshore from the Nile mouth were anoxic almost all year round during sapropel S1 deposition and potentially reached euxinic and methane-rich states in spring. This indicates that even though the Nile flow decreased during the winter-spring months, potentially causing the formation of the SIW in the Aegean Sea (Zirks et al, 2019), the circulation of this intermediate water mass was too sluggish to efficiently ventilate the basin. During the large summer Nile floods, efficient vertical mixing was probably able to draw low-salinity nutrient-rich and sediment-laden water masses down to 700 m w-d. By comparing available markers of deoxygenation in the three sediment cores (in particular the S/Cl records and occurrence of laminations), we can determine the changes in seawater chemistry at centennial to millennial scale along a bathymetric transect (Fig.…”

“…Such conditions have been detected only in deeper parts of the Levantine Basin during S1 (>2000 m; (Azrieli-Tal et al, 2014), and shallower sites located along the path of the Levantine Jet were found to be anoxic but not euxinic between 10 and 7 ka BP (Hennekam et al, 2014;Matthews et al, 2017;Zirks et al, 2021). Recent results hypothesize that high input of iron through enhanced Nile runoff during S1 likely prevented the bottom waters on the coast of Israel (>1000 m w-d) from becoming sulfidic, and instead they remained anoxic (Zirks et al, 2021). Our core sites on the Nile DSF are shallower and under direct influence of fluvial nutrient release compared to the more distal sites of Zirks et al (2021).…”