Eastern Mediterranean Sea circulation inferred from the conditions of S1 sapropel deposition

Abstract

Abstract. Holocene eastern Mediterranean Sea sediments contain an organic-rich sapropel S1 layer that was formed in oxygen-depleted waters. The spatial distribution of this layer revealed that during S1 deposition, deep waters were anoxic below a depth of 1800 m. However, whether this boundary permanently existed from the early to the mid-Holocene has not been examined yet. To answer this question, a multiproxy approach was applied to a core retrieved close to the 1800 m boundary (at 1780 m). We measured the bulk sediment elemental composition, the stable isotopic composition of the planktonic foraminifer Globigerinoides ruber and the abundance of benthic foraminifera since the last deglaciation. The result indicates that authigenic U and Mo accumulation began around 13-12 cal ka BP, in concert with surface water freshening estimated from the G. ruber δ 18 O record. The onset of bottom and pore water oxygen depletion occurred prior to S1 deposition inferred from barium enrichment. In the middle of the S1 deposition period, reduced authigenic V, Fe and As contents and the Br / Cl ratio indicated short-term bottom-water re-oxygenation. A sharp Mn peak and maximal abundance for benthic foraminifera marked a total recovery for circulation at approximately 7 cal ka BP. Based on our results and existing data, we suggest that S1 formation within the upper 1780 m of the eastern Mediterranean Sea was preconditioned by reduced ventilation, resulting from excess freshwater inputs due to insolation changes under deglacial conditions that initiated between 15 and 12 cal ka BP within the upper 1780 m. Short-term re-oxygenation in the Levantine Basin is estimated to have affected bottom water at least as deep as 1780 m in response to cooling and/or the reduction of freshwater inputs. We tentatively propose that complete ventilation recovery at the S1 termination was depth-dependent, with earlier oxygenation within the upper 1780 m. Our results provide new constraints on vertical water column structure in the eastern Mediterranean Sea since the last deglaciation.

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

“…Furthermore, the premature oxic conditions in the Aegean Sea, as deduced from previous mostly benthic foraminifera-based studies, have led to the conclusion that sapropel formation terminated earlier than elsewhere, i.e. at 7.1 cal ka BP [Kuhnt et al, 2007;Kotthoff et al, 2008a;Schmiedl et al, 2010;Tachikawa et al, 2015]. From our record it is clear that indeed the resumption of deep-water formation has occurred at ca 7.4 cal ka BP, as inferred from the Mn/Al ratio and the benthic foraminifera record (Fig.…”

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

“…It has been previously suggested that processes leading to sapropel formation, that is, humid climate conditions, water column stagnation and/or increased primary productivity, started earlier than the reported age based on sedimentary records (de Lange et al, 2008;Rohling et al, 1993;Tachikawa et al, 2015;van Helmond et al, 2015). Northward migration of ITCZ promoted moisture transfer above the basin as well as enhanced runoff, thus leading to the hampering of deepwater ventilation.…”

“…Recent studies have indicated that sapropel deposition may not have occurred exclusively under permanently anoxic conditions (Casford et al, 2003;Siani et al, 2013) at least not for shallower depths (Filippidi et al, 2016). The sporadic renewal of the water column at intermediate water depths has been suggested (Casford et al, 2003;Rohling et al, 2015;Tachikawa et al, 2015), which could explain a continuous supply of riverine and oceanic-derived trace metals to these levels (Nijenhuis et al, 1999). In addition, distinct major sapropel interruption events have been reported for 8.2 and 7.4 cal.…”

Hydrological variations of the intermediate water masses of the western Mediterranean Sea during the past 20 ka inferred from neodymium isotopic composition in foraminifera and cold-water corals

“…In addition, the WIW, flowing between the MAW and the LIW, has also been observed along the Channel (Sammari et al, 1999). The core of the LIW is located at 400-450 m water depth in the Tyrrhenian Sea (Hopkins, 1988;Astraldi et al, 2002b), which is the depth range of CWC samples from the Sardinia Channel (RECORD 23; 414 m) (Taviani et al, 2015). The youngest CWC sample dated at ∼ 0.1 ka BP has an εNd value of −7.70 ± 0.10 (Table 1, Fig.…”

“…Nineteen fragments were collected at various core depths from a coral-bearing sediment core (RECORD 23;38 • 42.18 N,08 • 54.75 E; Fig. 1) retrieved from 414 m water depth in the "Sardinian cold-water coral province" (Taviani et al, 2015) during the R/V Urania cruise RECORD in 2013. The core contains well-preserved fragments of M. oculata and L. pertusa embedded in a brownish muddy to silty carbonate-rich sediment.…”