Mediterranean circulation perturbations over the last five centuries: Relevance to past Eastern Mediterranean Transient-type events

Abstract

The Eastern Mediterranean Transient (EMT) occurred in the Aegean Sea from 1988 to 1995 and is the most significant intermediate-to-deep Mediterranean overturning perturbation reported by instrumental records. The EMT was likely caused by accumulation of high salinity waters in the Levantine and enhanced heat loss in the Aegean Sea, coupled with surface water freshening in the Sicily Channel. It is still unknown whether similar transients occurred in the past and, if so, what their forcing processes were. In this study, sediments from the Sicily Channel document surface water freshening (SCFR) at 1910 ± 12, 1812 ± 18, 1725 ± 25 and 1580 ± 30 CE. A regional ocean hindcast links SCFR to enhanced deep-water production and in turn to strengthened Mediterranean thermohaline circulation. Independent evidence collected in the Aegean Sea supports this reconstruction, showing that enhanced bottom water ventilation in the Eastern Mediterranean was associated with each SCFR event. Comparison between the records and multi-decadal atmospheric circulation patterns and climatic external forcings indicates that Mediterranean circulation destabilisation occurs during positive North Atlantic Oscillation (NAO) and negative Atlantic Multidecadal Oscillation (AMO) phases, reduced solar activity and strong tropical volcanic eruptions. They may have recurrently produced favourable deep-water formation conditions, both increasing salinity and reducing temperature on multi-decadal time scales.

Figure 1

(A) Bathymetric map of the Mediterranean Sea (from Ocean Data View, version ODV 4.7.4, https://odv.awi.de/), showing location of the sites analysed for this study: Sicily Channel box-cores St 342 (36°42’N, 13°55’E, 858 m depth; red dot) and St 407 (36°23’N, 14°27’E, 345 m depth; blue dot) and the Aegean Sea multicore Athos-M2 (40°5’N, 24°33’E, 1018 m depth; black dot). Black arrows show the path of surface water circulation and white squares with numbers refer to 1-Gibraltar Strait, 2-Ligurian Basin, 3-Tyrrhenian Basin, 4-Sicily Channel, 5-Adriatic Basin, 6-Adriatic-Ionian Bimodal Oscillating System, 7-Cretan Channel, 8-Aegean Basin and 9-Levantine Basin. Dashed dark grey areas show sites of deep and intermediate water formation: GL, Gulf of Lions; AD, southern Adriatic Sea; AE, Aegean Sea; LIW, Levantine Intermediate Water. Other sites previously studied are the Ocean Drilling Program Site 963 (yellow dot; ref. 59) and cores in Taranto (GT89-3, GT91-3, NU04 and GeoB 10709-4; orange dot; refs 60,61). (B) Evolution of key diagnostics from the NEMOMED8 hindcast since the 1960s during winter months, when deep water formation takes place: winter heat flux and mixed depth layer (MDL) in the Aegean, the South Adriatic and the Gulf of Lions regions. The time evolution of sea surface temperatures (SST; MEDAR/MEDATLAS2002 database; ref. 62) in these areas and sea surface salinity in the Sicily Channel are also shown. The Eastern Mediterranean Transient (EMT) is shaded grey.

Figure 2. Paleo-dataset in Sicily and Aegean Sea sediments over the last five centuries.

Time progression in years of the Common Era (CE). (A) δ¹⁸O_calcite of Globigerinoides ruber and (B) Uvigerina mediterranea reflecting Sicily Channel surface and bottom water properties, respectively. Sicily Channel SSTs derived from (C) the Mg/Ca ratio and (D) alkenone palæothermometers Normalized δ¹⁸O_sw profiles in the Sicily Channel after temperature correction of (E) G. ruber Mg/Ca-δ¹⁸O_calcite and (F) alkenones. (G) Resultant salinity anomaly, shown as a 20-year Gaussian low pass filter of the normalized δ¹⁸O_sw composite, based on reconstructed oxygen isotopic composition of seawater δ¹⁸O_sw in the Sicily Channel (details in Materials and Methods) derived from (E) and (F). Values above average (−0.1‰) are filled in. Relative proportion of n-hexacosan-1-ol (C₂₆OH) to the sum of C₂₆OH plus n-nonacosane (C₂₉) in (H) the Sicily Channel and (I) the Aegean Sea sediments. The Eastern Mediterranean Transient (EMT) detected since1988 and similar past anomalies (SCFR1 1910 ± 12, SCFR2 1812 ± 18, SCFR3 1725 ± 25 and SCFR4 1580 ± 30) are shown by bands shaded in grey. In panels (A–F,G), dashed lines refer to St 407 and solid lines to St 342. Bands in panels (C,D,H) are drawn by using measurement replicates. Core depth-age equivalences are at the bottom of the figure. The box in red shows the time interval within the ²¹⁰Pb range (see Fig. 4).

Figure 3. Scatter plots of surface salinity in the Sicily Channel and mixed layer depth in deep water formation sites (Gulf of Lions, Adriatic and Aegean) during winter.

Data are obtained from the NEMOMED8 hindcast.

Figure 4. Eastern Mediterranean Transient (EMT) and SCFR events over the last two centuries (within the ²¹⁰Pb range interval).

Time progression in years of the Common Era (CE). (A) The 20-year Gaussian low pass filter of the normalized δ¹⁸O_sw composite is shown (values above average are filled in) in comparison with (B) the n-hexacosan-1-ol index, as a proxy for deep ventilation in the Aegean (this study) (higher ventilation downwards). (C) NAO and AMO indices, both instrumental and reconstructed. (D) Total solar irradiance with 1-sigma uncertainty in watts per square meter (right axis; shown as the difference of total solar irradiance from the value of the Physikalisch-Metorologisches Observatorium Davos composite during the solar cycle minimum of the year 1986, i.e. 1365.57 watts per square) and annual total sunspot numbers (left axis) as provided by the Royal Observatory of Belgium, Sunspot Index Data center (http://www.sidc.be/silso/datafiles); note the three periods of low solar activity: Dalton minimum from 1790 to 1830, Gleissberg minimum around the 1900s and a subdued minimum over the 1970s. (E) Two estimations for explosive volcanism: global total injection of sulphate aerosol (teragrams; right axis) and annual-mean volcanic radiative forcing (watts per square meter; left axis) derived from aerosol optical depth data (a measure of stratospheric transparency to incoming solar radiation; ref. 63), which has been used for the volcanic-only forcing LOVECLIM simulation performed for the global compilation of marine sediment reconstructions over the past two millennia. The Eastern Mediterranean Transient (EMT, since 1988) event and SCFR anomalies are shown by grey-shaded bands (SCFR1 1910 ± 12 and SCFR2 1812 ± 18). Note that the lightest peaks in δ¹⁸O_sw are in line with a decrease in solar irradiance, increase in volcanic activity, positive NAO phases and are preceded by negative AMO phases and higher deep ventilation in the Aegean.