Atmospheric forcing over Holocene productivity in the Alboran Sea (W Mediterranean) from coccolithophores and Mg/Ca-estimated SST records (стр. 1 )

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

B. Ausнn1; J-A. Flores1; F-J. Sierro1; I. Cacho2; I. Hernбndez-Almeida3; B. Martrat4; J. O. Grimalt4

1Department of Geology, University of Salamanca. Pza/ de la Merced s/n, Salamanca 37008, Spain.

2Department of Stratigraphy, Paleontology and Marine Geosciences, University of Barcelona, C/Martн i Franquиs s/n, 08028 Barcelona, Spain.

3Institute of Geography and Oeschger Centre for Climate Change Research, University of Bern, Erlachstrasse 9a, CH-3012 Bern, Switzerland.

4Department of Environmental Chemistry. Institute of Environmental Assessment and Water Research. Jordi Girona,18. 08034 Barcelona, Spain.

Correspondence to: *****@***es

Abstract

High-resolution paleoproductivity variations in a coastal upwelling system in the Alboran Sea during the Holocene have been reconstructed from the study of fossil coccolithophore assemblages and linked to Mg/Ca and Uk’37-estimated Sea Surface Temperature (SST) and other paleoenvironmental proxies. The onset of this productive cell is suggested at 7.7 ka cal. BP. and linked to the establishment of the anticyclonic gyres in the Western Mediterranean. From that time to present, the N ratio and the accumulation rate of Florisphaera profunda show the alternation between upwelling and stratification episodes simultaneous to changes in Mg/Ca-estimated SST and relative abundance of reworked nannoliths, also coincident with variations in the formation rate of the Western Mediterranean Deep Waters (WMDW). Two scenarios modulated by North Atlantic Oscillation (NAO) operating at millennial-centennial time-scale are proposed to be the main climatic and oceanographic conditions prevailing in the Alboran Sea during the late Holocene and driving its productivity. [1] Persistent lower NAO index corresponds with weaker westerlies blowing over the Gulf of Lions and the ensuing slackening of the WMDW formation. This together with a lesser and warmer Atlantic Jet (AJ) entering the Alboran Sea leads to a deepening of the nutricline and hence, long term stratification events. [2] Prevailing higher NAO index enables stronger westerlies and the ensuing reinforcement of the WMDW in the Gulf of Lions, favouring a shoaling of the nutricline in the study area. This along with a major AJ triggers vertical mixing, which in turn promotes upwelling events in the study area.

1.  Introduction

Climatic variability along the Holocene (11.7 kyr to present) has been a challenging question for the last years. Recent studies have demonstrated that several abrupt climatic events as well as pervasive short-term oscillations characterizes this period (Mayewski et al., 2004; Wanner et al., 2011), dismissing the perception of the Holocene as warm and stable. As the most recent geological period, variability recorded by natural system during this time must be taken into account when doing any effort aimed at projecting future climatic scenarios (IPCC, 2007). Causes of this variability as well as the mechanisms transferring from one region to another become crucial in order to have an overall understanding of the system. The Western Mediterranean is an essential region in relation to determine climatic teleconnections with the North Atlantic region (Cacho et al., 1999; Sierro et al., 2005). High-resolution paleorecords have recently allowed recognizing millenial-centennial time-scale paleoclimatic variations in the Western Mediterranean across the Holocene (Jalut et al., 1997; Jalut et al., 2000; Goy et al., 2003; Frigola et al., 2007; Fletcher and Zielhofer, 2011; Fletcher et al., 2012). These latest works highlight the North Atlantic imprint over the Western Mediterranean and propose the intensity of atmospheric pressure gradient in the North Atlantic region as a likely transmitting mechanism. However, little evidence exists for the impact of short-term climatic oscillations on past ocean productivity in the Mediterranean Sea (Lionello, 2012). The Alboran Sea is considered an exception within the generalized oligotrophy in the Mediterranean Sea, counting with quasi-permanent areas of upwelling (Sarhan et al., 2000). Atlantic water entering the Alboran Sea has been seen as nutrient-depleted, and it has been demonstrated that the local vertical mixing is the main factor controlling marine productivity in the region (Dafner et al., 2003). Today in the Western Mediterranean, insolation from summer and early-fall causes the stratification of the water column, separating rich-nutrient deep and intermediate waters from nutrient-depleted surface waters. During winter, vertical mixing is intense providing nutrients to the surface layer (Dafner et al., 2003). Nevertheless, vertical mixing in this quasi-permanent productive is also influenced by local hydrology and atmospheric circulation as follows: eddy-induced upwelling offshore, close to the northern limit of anticyclonic gyres in the Alboran Sea; and wind-induced coastal upwelling, on the shore, promoted by westerlies along the Spanish coast (Garcнa-Gorriz and Carr, 1999). Moreover, its latitudinal position, its strong connection with the Atlantic Ocean and its semi-enclosed features lead to partial isolation of these phenomena. All these features turn the Alboran Sea into an ideal region for the study of the impact of short-term climatic oscillations on ocean productivity. Besides, it provides a reasonable area for the study of ocean-climate teleconnections between high and low latitudes. The main objective of this study is to reconstruct past productivity in an upwelling area of the Alboran Sea during the Holocene as well as to elucidate the climatic and oceanographic mechanisms involved in past productivity variations, especially those related to the North Atlantic climatic and oceanographic dynamics. Coccolithophores are haptophyte algae possessing calcified scales (coccoliths). Fossil forms of coccoliths preserved in deep-sea sediments are commonly used as a widespread proxy in paleoenvironmental reconstructions since their living forms are strongly influenced by nutrients, SST and dissolved CO2 concentration in sea water among others (Thierstein and Young, 2004). In this study we show a marine productivity record reconstructed from fossil coccolithophores along with Mg/Ca and alkenones-estimated SST records for the last 12 kyr and correlate them with those of paleoenvironmental variations.

2.  Area of study: modern water masses and climatic dynamics

Core HER-GC-T1 has been recovered off the coast of Malaga (Figure 1b) in a very productive coastal upwelling cell in the Alboran Sea named as the Malaga upwelling (Bбrcena and Abrantes, 1998). This is a transitional region where North Atlantic Surface Water (NASW) enters through the Strait of Gibraltar to the Mediterranean Sea as a jet named Atlantic Jet (AJ) (Sarhan et al., 2000), describing two quasi-permanent anticyclonic gyres: the Western Anticyclonic Gyre (WAG) and the eastern one (EAG) (Heburn and La Violette, 1990). Interaction between NASW and more saline and warmer (Figure 1b) Mediterranean water results in the formation of geostrophic fronts in the northern limits of both gyres, called Alboran Front (where HER-GC-T1 is located) and Almeria-Oran Front (Minas et al., 1991) (Figure 1b), forming productive cells where upwelling occurs. Zonal vertical circulation belt can be simplified in a two-layer model: in surface, NASW mixes with Mediterranean water (warmer and saltier) forming the Modified Atlantic Water (MAW) and occupying the photic zone (100-200 m). In depth, Levantine Intermediate Water (LIW), formed in the eastern part of the Mediterranean Sea, flows in the opposite direction. Besides, in the Gulf of Lions (Figure 1c) dry and cold northwesterly winds induce the evaporation of MAW, which becomes saltier and colder and finally sinks because of its high density, forming the Western Mediterranean Deep Water (WMDW) (MEDOCGROUP, 1970) which flows below LIW at 800-3000 m, the two together named as the Mediterranean Outflowing Water (MOW) once in the Atlantic Ocean. Present climate conditions in the region are influenced by stable atmospheric high-pressure centre (Azores). Summers are usually dry and hot; but during winter, the subtropical high-pressure centre migrates to a southerly position favouring an increase in rainfall (Sumner et al., 2001). At decadal-time scale, climate variability is modulated by the North Atlantic Oscillation (NAO) (Rodу et al., 1997). NAO is a natural mode of variability of the atmosphere that operates through fluctuations in sea-level atmospheric pressure between the Icelandic low and the Azores high-pressure centres (Hurrell, 1995). Positive-NAO phase means stronger latitudinal pressure gradient: stronger-than-average northwesterly winds across middle latitudes and dry conditions over southern Europe. Specifically over the Western Mediterranean, the intensification of the westerly winds leads to the formation of the WMDW aforementioned, which plays a crucial role in the thermohaline dominated circulation of the Mediterranean Sea (Millot, 1991). By contrast, negative-NAO phase becomes weaker northwesterly winds and an introduction of moist air that causes an increase in precipitation in the Mediterranean region (von Grafenstein et al., 1999).

3.  Material and methods

We analyze the top 183 cm of a gravity core HER-GC-T1 (Lat. 36°22’12’’N, Long. 4°17’57’’W), 658.9 depth meters below sea level (mbsl) recovered by the BIO Hespйrides during the research cruise Hermesione in 2009. Sediment of the core is mainly composed of dark greenish gray mud rich in foraminifers.

3.1.  Age model

Six picked foraminiferal samples (Table 1) were considered to measure accelerator mass spectrometry (AMS) radiocarbon ages at the Poznan Radiocarbon Laboratory and Woods Hole Oceanographic Institution. Conversion from radiocarbon ages to calibrated calendar years was performed using the CALIB REV6.0.0 online software (Stuiver and Reimer, 1993) and the curve of the calibration dataset Marine09 (Reimer et al., 2009), which includes the correction of 400 yr for the global marine reservoir effect. The regional difference from this global reservoir correction (ΔR) (Stuiver and Braziunas, 1993) resulted in -22± 35 years and was also considered. The age model for the last 12 ka is based on linear interpolation between these six calendar ages (Figure 2) performed with the AnalySeries Version 1.1 (Paillard et al., 1996). Average sedimentation rate is 16.4 cm*ka-1 and the resultant average time resolution is ~140 years.