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Search Completed | Title | Eastern Mediterranean Sea circulation inferred from the conditions of S1 sapropel deposition
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Clim. Past, 11, 855–867, 2015 www.clim-past.net/11/855/2015/ doi:10.5194/cp-11-855-2015
© Author(s) 2015. CC Attribution 3.0 License.
Eastern Mediterranean Sea circulation inferred from the conditions of S1 sapropel deposition
K. Tachikawa1, L. Vidal1, M. Cornuault1, M. Garcia1, A. Pothin2, C. Sonzogni1, E. Bard1, G. Menot1, and M. Revel2 1Aix-Marseille Université, CNRS, IRD, Collège de France, CEREGE UM34, 13545 Aix en Provence, France
2Geoazur, UMR 7329, 06560 Valbonne-Sophia Antipolis, France Correspondence to: K. Tachikawa (firstname.lastname@example.org)
Received: 24 November 2014 – Published in Clim. Past Discuss.: 20 December 2014 Revised: 26 March 2015 – Accepted: 2 May 2015 – Published: 11 June 2015
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 multi- proxy approach was applied to a core retrieved close to the 1800 m boundary (at 1780 m). We measured the bulk sedi- ment 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 13–12calkaBP, in concert with surface wa- ter freshening estimated from the G. ruber δ18O 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 re- sults and existing data, we suggest that S1 formation within the upper 1780 m of the eastern Mediterranean Sea was pre- conditioned 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 1780m. Short-term re-oxygenation in the Lev- antine Basin is estimated to have affected bottom water at least as deep as 1780m 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 wa- ter column structure in the eastern Mediterranean Sea since the last deglaciation.
The Mediterranean Sea is located in a transition zone be- tween subpolar depression and subtropical high pressure and is known to be sensitive to ongoing and past climate change (Bethoux and Gentili, 1999; Roether et al., 1996). Holocene sediments obtained from the eastern Mediterranean Sea of- ten contain the most recent organic-rich sapropel deposit, S1 (10.8±0.4 to 6.1±0.5 cal ka BP; De Lange et al., 2008), that formed due to a drastic decrease in labile organic-matter de- composition (Moodley et al., 2005). Reduced oxygen supply to bottom waters has been suggested to be a precondition for sapropel formation although increased biological productiv- ity further promoted S1 deposition (Bianchi et al., 2006; My- ers et al., 1998; Rohling, 1994; Stratford et al., 2000).
Surface water density decreases due to excess freshwater inputs have played a pivotal role in reducing Mediterranean Sea thermohaline circulation during sapropel formation (My- ers et al., 1998; Rohling, 1994; Stratford et al., 2000). By reinforcing Nile River discharge toward the Levantine Sea (Emeis et al., 2003; Kallel et al., 1997; Revel et al., 2010; Rohling, 1994; Rossignol-Strick et al., 1982), high summer insolation at minimum precession is known to have had a fundamental impact (Kutzbach et al., 2014; Rohling, 1994, and references therein; Ziegler et al., 2010). In parallel, due to an active Mediterranean storm track, reduced boreal win-
Published by Copernicus Publications on behalf of the European Geosciences Union.
Image | Eastern Mediterranean Sea circulation inferred from the conditions of S1 sapropel deposition
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