Research article
Spatial and temporal variability in Mediterranean climate over the last millennium from vermetid isotope records and CMIP5/PMIP3 models

https://doi.org/10.1016/j.gloplacha.2020.103159Get rights and content

Highlights

  • We study vermetid reefs records of the last millennium from the Mediterranean Sea.

  • We use CMIP5/PMIP3 models output to understand different trends in the records.

  • The South Asian Monsoon weakening may explain the recent Eastern Med. rapid warming.

  • North Atlantic Oscillation centers have latitudinally shifted during the last millennium.

Abstract

Stable isotope compositions of oxygen (δ18O) and carbon (δ13C) of the aragonite skeleton of the reef-building gastropod Dendropoma petreaum provide high-resolution records of the Mediterranean climate over the last millennium. In particular, the isotopic composition of vermetid cores collected from the west and east Mediterranean reveals that the different regions have had distinct thermal and primary production behaviors throughout the last millennium. The rate of warming in the recent Industrial Period is variable among the different regions of the Mediterranean Sea. The δ18O-derived Sea Surface Temperature (SST) anomalies show that the Eastern Mediterranean surface temperature is increasing much more rapidly than in the Western Mediterranean. Additionally, the signals of the Little Ice Age and of the Medieval Climate Anomaly are more apparent in the western and in the central Mediterranean while they are almost absent in the eastern Mediterranean.

We aim to reconcile the SST temporal and spatial pattern with the variability of the North Atlantic Oscillation (NAO) and the South Asian Monsoon (SAM) climate systems by analyzing coupled atmosphere-ocean models from the CMIP5/PMIP3 projects that simulate the global climate of the past 1000 years. We show that even though the NAO is more dominant in the western Mediterranean SST, its latitudinal movement, on a centennial time scale, is evident in the eastern Mediterranean SST signal. We also discuss the Atlantic water inflow role in the observed similarities of the surface productivity signals and the Suess effect that prevail the Industrial Period signal in all regions of the Mediterranean.

Introduction

The Mediterranean Sea climate, derived by its location between subtropical and mid-latitude climatic regimes, is characterized by hot and dry summers and wet, cool but mild winters. The North Atlantic Oscillation (NAO) from the northwest and the Asian Monsoon and its derivatives from the southeast are the main global systems that affect the Mediterranean surface water characteristics and influence its temporal and spatial variability. The NAO is the most pronounced pattern of variability in the North Atlantic sea-level pressure, with an anomalous low over Iceland and an anomalous high over the Azores Islands (Hurrell, 1995). This meridional bipolar pattern presents interannual to multidecadal trends in its intensity, position and shape (Jones et al., 1997, Jones et al., 2003; Trouet et al., 2009). A positive (negative) NAO phase is associated with warmer (cooler) conditions over the northwestern part of the Mediterranean Sea and cooler (warmer) conditions over the southeastern part (Hurrell, 1995; Pozo-Vázquez et al., 2001; Trigo et al., 2002; Luterbacher and Xoplaki, 2003). Another global scale climate system influencing the Mediterranean is the South Asian Monsoon (SAM) that remotely affects the eastern Mediterranean through a westward propagating Rossby wave (Rodwell and Hoskins, 1996; Rizou et al., 2018). It implies persistent northerly Etesian winds and strong subsidence over the Eastern Mediterranean (EM), both corresponding with the ascending air over the Bay of Bengal (Tyrlis et al., 2013; Rizou et al., 2018). The EM summer monotonic temperatures are associated with the balance between these two monsoonal effects (Ziv et al., 2004).

The Mediterranean Sea acts as an evaporation basin, water flows in from the Atlantic Ocean at the surface and leaves the Mediterranean at intermediate levels across the Strait of Gibraltar. Surface water, especially in the Western Mediterranean (WM), is strongly affected by the Atlantic Water properties. While this water propagates to the EM, its characteristic changes by becoming warmer, saltier and more oligotrophic. The anti-estuarine circulation evacuates intermediate, nutrient–rich water back to the Atlantic Ocean on a time scale of about 100 years (Roether and Schlitzer, 1991). This type of circulation controls the nutrient concentration, and hence, the primary productivity, which varies from being moderately productive in the WM (Ausín et al., 2015) to an extreme oligotrophic condition in the EM (Berman et al., 1986; Krom et al., 2005, Krom et al., 2014). A mass balance model shows that the bidirectional flows through the Strait of Gibraltar to the WM and through the Strait of Sicily to the EM are the dominant factors controlling the nutrient concentration of the two sub-basins (Powley et al., 2017), with the inflow to the WM being a few times higher in nutrients than the inflow to the EM. However, the presence of other sources of nutrients as riverine or atmospheric deposition can locally modify this general pattern (Herut et al., 1999; Sandroni et al., 2007; Schilman et al., 2001; Jalali et al., 2018).

Over the last millennium climatic changes in the western Mediterranean basin versus those reported in the eastern basin have not always been synchronous (Metaxas et al., 1991; Roberts et al., 2012). The different regions of the Mediterranean Sea respond differently to the external forces influencing them. Satellite observations show opposite trends in Sea Surface Temperature (SST) between the two basins (Macias et al., 2013). The eastern basin is warming at an unprecedented high rate (Luterbacher et al., 2004; Xoplaki et al., 2006; Skliris et al., 2012; Sisma-Ventura et al., 2014), while the western basin exhibits no warming or even a cooling trend, which is associated with the Atlantic Multidecadal Oscillation natural variability (Macias et al., 2013; Vargas-Yáñez et al., 2008). Paleoclimate reconstructions, which are based on marine sediments cores, show centennial SST fluctuations in the Gulf of Lion (Sicre et al., 2016), the Balearic basin (Moreno et al., 2012; Cisneros et al., 2016), Strait of Sicily (Incarbona et al., 2016), and the Aegean sea (Gogou et al., 2016). Some of the observed trends are consistent with our results and a detailed comparison of the regional records is described in the results section.

Changes in productivity were also reported during the last millennium in the WM (Ausín et al., 2015) and the EM (Schilman et al., 2001; Hennekam et al., 2014; Sisma-Ventura et al., 2014). However, the effects described to cause this variability are local, as studied on different time scales. Ausín et al. (2015) proposed that the strength of the Atlantic Jet entering the Alboran Sea governed the WM basin productivity during the Holocene. Schilman et al. (2001) showed a coincidence between Nile floods and EM productivity in the last 1400 years from the analysis of planktonic and benthic foraminifera in sediment cores.

The vermetid Dendropoma petraeum is a marine gastropod, adapted to a sessile life style and is abundant around the Mediterranean Sea. The vermetids build reefs composed of dense aggregations of individual aragonite shells, cemented by algae and sets its colonies on rocky shores in the intertidal zone (Safriel, 1975; Calvo et al., 2009). Vermetids precipitate their aragonite skeleton in isotopic equilibrium with the ambient surface seawater (Sisma-Ventura et al., 2014) and serve as archives of high-resolution trends in sea surface properties. Vermetid reefs have been used previously to study climate over the last millennium in the Mediterranean Sea (Antonioli et al., 1999; Silenzi et al., 2004; Montagna et al., 2008; Sisma-Ventura et al., 2009, Sisma-Ventura et al., 2014). Antonioli et al. (1999) established vermetid reefs as indicators for sea-level fluctuations in tectonically stable areas. Silenzi et al. (2004) showed that vermetid reefs capture SST trends and provide paleoclimatic indicators over the last 500 years. Sisma-Ventura et al., 2009, Sisma-Ventura et al., 2014 calibrated δ18O and δ13C records from EM vermetid reefs to study climate change at high resolution.

In this paper we use the vermetid based records to suggest a climatic framework for the observed temporal and spatial variability. We compare the skeletal δ18O and δ13C composition of fourteen vermetid cores collected from four sampling sites along the Mediterranean Sea and examine the climatic influence on the ambient surface water properties. Earth System Models of the fifth phase of the Coupled Modelling Intercomparison Project (CMIP5) and the third phase of the Paleoclimate Modelling Intercomparison Project (PMIP3) outputs are used to interpret the causes of the observed variations and to separate the externally forced climate signal from the internal variability. The novelty of this study is to compare proxy-based records with outputs from paleoclimate Earth System Models to study the differences between the western and eastern Mediterranean basin. We identify shifts in large-scale climatic systems from west (NAO) and east (SAM) of the Mediterranean Sea by looking at the isotopic differences in the vermetid cores. We suggest that long-term trends in the observed record from the Mediterranean are related to global scale atmospheric patterns and their decadal variability.

Section snippets

Vermetid cores

Fourteen cores of the reef-building gastropod Dendropoma petreaum (vermetid) were collected from the Alboran basin (Spain hereafter), the Algerian basin (Tunis hereafter), the Tyrrhenian basin (Sicily hereafter) and the Leventine basin (EM hereafter). They were retrieved using a pneumatic drill from the different basins of the Mediterranean Sea by the late S.Silenzi, G.Sisma-Ventura and Y.Jacobson (Fig. 1; Table 1). Prior to isotope analysis, the cores were cleaned following the method reported

Results and discussion

The composite δ18O and δ13C records from the four main locations along the Mediterranean Sea (Spain, Tunis, and Sicily in the WM, and Israel in the Levantine basin of the EM) are shown in Fig. 2. The differences between the oxygen isotopic composition records are well evident, with the EM and Sicilian cores showing the lowest and highest δ18O values, respectively. The δ18O records from the WM exhibit a trend toward higher values from the beginning of the last millennium until approximately

Summary and conclusions

Vermetid cores collected from four different locations along the Mediterranean Sea were analyzed for δ18O and δ13C, and the results were compared to simulated physical parameters in the broad frame of Earth System models. These organisms live in the intertidal zone and their aragonite skeleton can potentially record the variability of both atmospheric and oceanic parameters. Hence both atmospheric and oceanic parameters were tested and analyzed using the CMIP5/PMIP3 models, according to the

Acknowledgements

We are thankful to S.J. Fallon for the extensive dating analysis of the vermetid samples following his collaboration with the late Prof. S. Silenzi and to Y. Jacobson for his help in sample preparation for NOSAMS 14C dating. We also thank I. Brailovsky for technical support. This study was supported by the de Botton center for Marine Research of the Weizmann Institute and by the Israel Science Foundation grant 1246/12 to A.S. Support provided to Y. Amitai by the Mediterranean Sea Research

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