Atmospheric forcing over Holocene productivity in the Alboran Sea (W Mediterranean) from coccolithophores and Mg/Ca-estimated SST records (стр. 4 )

5.3.3.  Proposed forcing mechanism

Based on these outcomes, past productivity in the Western Alboran Sea has been seen influenced by several interrelated processes: WMDW production rate, volume of inflowing AJ, nutricline oscillations and mixing conditions of the water column. In turn, all these processes are driven by two pathways: inflowing NASW and/or changes of atmospheric circulation. Parrilla and Kinder (1987) pointed to atmospheric sea-level pressure over the Mediterranean region as responsible for present-day changes in both pathways. Higher sea-level pressure triggers a decrease in the NASW inflow and weaker westerlies; on the contrary, lower sea-level pressure results in stronger westerlies and strengthened inflow. Nowadays, the pressure gradient formed by the Azores high and the Icelandic low-pressure centres modulates climatic variability in the North Atlantic region through the NAO. Due to the well-known climatic teleconnection of the Western Mediterranean with northern latitudes (Cacho et al., 1999; Martrat et al., 2004), a NAO-like mechanism could be also responsible for variations in productivity and climate variability at millenial-centennial time-scale. Olsen et al., (2012) reconstructed the NAO for the last 5,200 years (Figure 6) through principal component analyses (PCA). Mg/Ca-estimated SST general variability for the last 5,200 years is very similar to that of PCA3 (Olsen et al., 2012), which is a proxy for dominant NAO circulation pattern (Figure 6d). However, the lower resolution of the former only allows a roughly comparison. Despite of the different resolution employed, general trend of reworked nannoliths (Figure 6c) is in very good agreement with this NAO reconstruction. Reworked nannoliths have been proposed here to reach the study area by river erosion and drainage of the fluvial basin during more arid periods. The good agreement between this record and PCA3 suggests that climatic conditions in relation to arid periods in the study area were highly influenced by persistent phases of NAO pattern. A good match is also found between PCA3 and UP10 fraction (Frigola et al., 2007) from Minorca basin. This points to NAO as the main forcing mechanism accountable for the intensity of westerly winds that drives the WMDW formation rate in the Gulf of Lions, a physical process that is in turn proposed here to influence to a large degree ocean productivity in the “Malaga upwelling” from its onset at 7.7 ka cal. BP at millennial-centennial time-scale. Olsen et al., (2012) also inferred a positive/negative type circulation pattern (Figure 6a) that has been compared with the occurrence of M events. M3, M2 and M0 coincide with negative-NAO index periods. In exchange, M5, M4 and M1 coincide with positive-NAO index periods. From this, a clear relationship between M events and NAO index cannot be established. Nevertheless, the good match between the NAO reconstruction by Olsen et al., (2012) and general variability of some of our proxies highlights the role of the NAO as an important component of the climatic and oceanographic signal recorded in deep-sea sediments from the Western Mediterranean, influencing physical and biological processes that took place in different locations of the basin. Nowadays, a positive-NAO phase is translated into the intensification of the westerly winds that lead to the formation of the WMDW in the Gulf of Lions, while negative-NAO phase becomes weaker northwesterly winds and an increase in precipitation in the Mediterranean region. Derived from our results and from present-day NAO functioning, two scenarios are proposed to explain the behaviour of this productive cell at millenial-centennial time-scale from its onset at 7.7. ka cal. BP: [1] persistent lower-NAO index conditions over the region would cause weaker westerlies in the Gulf of Lions leading to a reduction of WMDW formation. This fact would promote both the deepening of the nutri-thermocline in the Alboran Sea and the entrance of a weaker and warmer AJ influx, with the ensuing reduction of vertical mixing (Figure 7a). This situation along with higher SST would trigger long term stratification events while wet climate conditions prevail in the study area. [2] Higher-NAO index conditions would encourage an intensification of westerlies in the Gulf of Lions and the ensuing WMDW reinforcement. This would generate the rise of the nutricline in the Alboran Sea and the entrance of a stronger AJ. Vertical mixing would increase, which in turn would prompt long-term upwelling events of cold waters while dry climate conditions prevails in the area and aridity affects the continental margin (Figure 7b). Several authors have already suggested a NAO-like mechanism acountable for climate variabilty over the Western Mediterranean region (Sбnchez-Goсi et al., 2002; Goy et al., 2003; Moreno et al., 2004; Frigola et al., 2007; Fletcher et al., 2012). However, these interpretations were limited by the absence of a robust paleo NAO reconstruction covering the study period. Although the present available comparisons only span the last 5,200 years, the results shown here shed some light about the role of NAO as forcing mechanism accountable for many physical and biologycal processes that take place in the Western Mediterranean at millenial-centennial time-scale.

6.  Conclusions

Down core variations in NAR are tightly linked to changes in marine paleoproductivity. A drastic change observed in NAR of all coccolithophore taxa at 7.7 ka cal. BP has been related to the latest prominent fast-flowing AJ and the ensuing establishment of the WAG. The onset of the semi-permanent productive cell “Malaga upwelling” is proposed at that time. From that time across the Holocene, the area would have been affected by a general cooling trend as revealed by both Mg/Ca and alkenone-estimated SST. Nannofossil records evidence variability in past productivity based on upwelling and stratification events alternation since Malaga upwelling onset. These productivity variations have been found to be related to M events (periods of WMDW reinforcement in the Gulf of Lions) reported by Frigola et al., (2007), in such a way that whenever an M event occurs a productive period is recorded in the Alboran Sea. This provides a new insight about what procesess account for past productivity variations. These are: intensity of westerlies and WMDW formation rate in the Gulf of Lions, nutricline depth, AJ inflow and mixing proceses in the water column in the Alboran Sea. A good agreement has been found between these M events, upwelling pulses, arid conditions and colder SST in the Alboran Sea. According to the suggested interconnections existing between these procesess two likely oceanographic-climatic scenarios have been projected from our interpretations in order to describe the main physical features prevailing over Western Mediterranean across the Holocene at millennial-centennial time-scale. The comparison between proxies described above and a NAO reconstruction by Olsen et al., (2012) for the las 5,200 yr reveals a strong relationship, thus NAO circulation pattern is proposed to be the main forcing mechanism driving the alternation between both scenarios as follows: [1] lower-NAO index prevailling over the Western Mediterranean would lead to wetter climate conditions and weaker westerlies, this last one resulting in a reduction of WMDW formation in the Gulf of Lions. As a consequence in the Alboran Sea, nutri-thermocline would deep and the AJ volume influx would decrese. As a result, stratification episodes would occur in the area; [2] higher-NAO index would cause the opposite situation: drier climate conditions and stronger westerlies blowing over the Gulf of Lions, wich in turn would lead to a major WMDW formation. In consequence, nutri-thermocline would rise in the Alboran Sea and the AJ influx would be strenghtened. The ensuing vertical mixing in the water column would prompt long term upwelling pulses of colder waters and productive episodes in the study area. These results highlight the sensitivity of the Western Mediterranean to high-latitude climatic system. Besides, they point to the “Malaga upwelling” area as a location of high interest in the Alboran Sea; their oceanographic particularities regarding to ocean productivity are recorded in the deep-sea sediments, providing an information that in turn can be used to unravel the climatic and oceanographic patterns that influenced the Western Mediterranean through time.

Acknowledgements

B. Ausнn is sincerely grateful to I. Ruiz (Department of Stratigraphy, Paleontology and Marine Geosciences, University of Barcelona) and to Y. Gonzбlez and B. Hortelano (Department of Environmental Chemistry, IDAEA-CSIC, Barcelona) for their guidance and supervision during geochemical treatment of the samples. This study was supported by the FPU grant AP2010-2559 of the Ministry of Education of Spain given to B. Ausнn and by the Spanish project CGL2009-08651-CLI of the Ministry of Economy and Competitiveness of Spain, and CONSOLIDER GRACCIE VACLIODP339 and MINECO CTM2012-38248 projects of the Ministry of Science and Innovation.

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