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The Azores Front Since the Last Glacial Maximum

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The Azores Front Since the Last Glacial Maximum Earth and Planetary Science Letters 222 (2004) 779–789 www.elsevier.com/locate/epsl The Azores Front since the Last Glacial Maximum M. Rogerson*, E.J. Rohling1, P.P.E. Weaver2, J.W. Murray3 Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK Received 14 July 2003; received in revised form 25 March 2004; accepted 29 March 2004 Abstract The spatial distribution of warm surface water in the Atlantic Ocean reflects the state of the thermohaline circulation. The Azores Current/Front, which is a recirculation of the Gulf Stream, marks the northeastern boundary of the North Atlantic subtropical gyre. Its position is therefore diagnostic of the width of the Atlantic warm water sphere. Here we report high resolution stable isotope and faunal abundance records of planktonic foraminifera in a sediment core from the Gulf of Cadiz (southwest Spain) which reflects shifting of the Azores Front since the Last Glacial Maximum (LGM). Today, the Azores Front does not penetrate into the Gulf of Cadiz, even though the front resides at the same latitude as the Gulf of Cadiz in the Atlantic. Our results indicate that the Azores Front is a robust feature of the Atlantic surface circulation, and that is present both in interglacial times and during the LGM at roughly the same latitude. However, during the LGM prior to 16 ka BP and during the Younger Dryas, the Azores Front did penetrate eastward into the Gulf of Cadiz. D 2004 Elsevier B.V. All rights reserved. Keywords: Palaeoceanography; foraminifera; Stable isotopes; Azores Front; Last Glacial Maximum 1. Introduction coast of Morocco [1]. South of the Azores, the AF coincides with the Azores Current, a strong eastward Close to the Azores Islands lies the Azores Front flow that is generated by water mass transformation in (AF) (see Fig. 1). It marks the boundary between the the Gulf of Cadiz (GoC), southwest of Spain [3]. The European and African surface water masses [1,2] and AF marks a zone of strong hydrographic transition, in extends across the Atlantic between latitudes of 30j terms of both temperature ( f 4 jC) [1] and water and 40jN from the Newfoundland GrandBanks to the column structure [4], and it is characterised by locally intense upwelling [5,6]. The plankton assemblage changes substantially * Corresponding author. Faculty of Earth Sciences, University across the AF, particularly below the seasonal ther- of Utrecht, Budapestlaan 4, 3584 CD Utrecht, The Netherlands. mocline, as a result of higher overall productivity and Tel.: +31-30-2535148. a deeper Deep Chlorophyll Maximum north of the E-mail addresses: [email protected] (M. Rogerson), front [2,4,7,8]. The local thermal and ecological [email protected] (E.J. Rohling), [email protected] signature of the AF/AC close to the Azores Islands (P.P.E. Weaver), [email protected] (J.W. Murray). 1 Tel.: +44-23-80593042. allows evidence of its past behaviour to be preserved 2 Tel.: +44-23-80596020. in the sediment record. The AC/AF is strongest in the 3 Tel.: +44-23-80592617. spring [6] and shows random variability in strength 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.03.039 780 M. Rogerson et al. / Earth and Planetary Science Letters 222 (2004) 779–789 Fig. 1. General surface circulation in the North Atlantic today. Core locations are shown. and position throughout the year as a result of increased velocity, which turns south to flow between meandering [9]. The sediment record will therefore the Canary Islands and North Africa. record the long-term average position of the AF We here report stable isotope (C and O) and meander belt, rather than that of the AF itself. planktonic foraminiferal assemblage records for a core Towards the East Atlantic Margin, the geographic from the GoC in comparison with core MD952042 positions of the AC and AF are separated. The AC from offshore southern Portugal [12], to investigate flows into the Gulf of Cadiz to replace water lost during the history of the Azores Current/Azores Front in the water mass transformation. The AF resides further to GoC region. the south, between the Canary Islands and Madeira [10]. A high-resolution circulation model captures observations made during the CANIGO project and 2. Modern planktonic foraminiferal assemblage so offers comprehensive insight into the oceanography of the area between the Azores and the Strait of Both the GoC and the southern Portuguese margin Gibraltar [11]. The model shows the AC continuing have mean summer temperatures of 22 jC, and eastwards from the Azores to the Strait of Gibraltar, similar mean winter temperatures (16 jCforthe while the AF (a sharp f 4 jC transition at the Azores) GoC, 15 jC for southern Portugal) [13]. degenerates into two weaker transitions, one lying to Plankton tows in the Gulf of Cadiz indicate that the the west of the GoC but not penetrating into it (f 1j) dominant foraminiferal species is Globigerina bul- and one lying further to the south (f 3j) (see Fig. 2). loides, which makes up between 30% and 54% of The southern branch coincides with a second zone of the individuals in the study area. Globigerinoides M. Rogerson et al. / Earth and Planetary Science Letters 222 (2004) 779–789 781 Fig. 2. Present day oceanographic setting from CANIGO regional model [11]. (a) Velocity at 36.6 m depth (after [11]). (b) Temperature at 202 m depth (after [11]). ruber, Globorotalia inflata, Globigerinella siphon- scitula, Globorotalia truncatulinoides, Neogloboqua- ifera and Globigerinoides sacculifer are also com- drina pachyderma dextral and Orbulina universa mon, each contributing 5–15% of the living fauna [14–16]. [14]. Core top records indicate a similar dominance A similar planktonic foraminiferal assemblage is of G. bulloides (30%) and G. ruber and G. saccu- found in core tops from the Portuguese margin, lifer together make up f 23% of dead fauna [15]. though it is more dominated by G. bulloides (25– Also present in low abundances in plankton tows 75%) [17,18]. N. pachyderma (d) is more common and core tops are Turborotalita quinqueloba, Globi- than in the GoC (15–40%) and G. ruber and G. gerinita glutinata, Globigerinita humilis, Globiger- sacculifer are generally less abundant north of Cape inoides tenellus, Globorotalia hirsuta, Globorotalia St. Vincent [17]. Other important species (5–15%) are 782 M. Rogerson et al. / Earth and Planetary Science Letters 222 (2004) 779–789 G. inflata, G. siphonifera and O. universa.This 4. Material and methods assemblage is comparable to that reported in plankton tows immediately north of the AF [2] and in the We present high-resolution data for core D13898, BIOTRANS sector off northwest Iberia [19,20] indi- to assess changes in the regional oceanography of the cating that a fairly consistent bioprovince extends GoC between the LGM and present. This is a f 16 m from north of Iberia to the AF. long core recovered using the SOC Giant Piston Corer from water depths of f 1250 m in the central part of the Gulf of Cadiz. D13898 is positioned on the most 3. Modern distribution of G. scitula distal and muddy parts of the Gulf of Cadiz sediment drift and consists of homogenous and massive, brown G. scitula is generally placed with the temper- coloured, clay rich mud with no sand/silt layers or ate–sub-polar assemblage [21], as in the Atlantic it other evidence for turbidite activity apparent. The is uncommon in the low latitudes and found in chronostratigraphic framework for D13898 is based relatively high abundance north of 35–40j [22].It on 10 AMS radiocarbon dates performed on >6 mg of is also frequent in temperate regions elsewhere, shallow-dwelling (i.e. excluding all Globorotaliids, G. such as the Indian Ocean [23,24] and the North siphonifera and G. calida) planktonic foraminiferal Pacific [25–27]. Though G. scitula is consistently tests picked from the >150 Am fraction. The radiocar- found in the Mediterranean during the last glacia- bon analyses were undertaken via the NERC Radio- tion, it is not found in this region today [28], which carbon Laboratory (NERC-RCL), at the University of also seems to indicate a preference for cool surface Arizona NSF-AMS facility. The datings have been water conditions. calibrated using the Calib 4.3 program, and the Bard High abundances of G. scitula have been recorded et al. (1998) ‘‘glacial’’ polynomial [31,32] in the case in upwelling cells in the Panama Basin [29] and of the oldest two (see Table 1). As the aim of this associated with periods of upwelling in the Central study is to compare the Gulf of Cadiz records to that Pacific [30]. Kuroyanagi et al. (2002) [27] have of MD952042, the chronostratigraphy is ‘‘tuned’’ to shown that G. scitula is part of the Spring bloom that of MD952042 by correlation of tie-points fauna when the surface waters are at their most mixed, concerning global events such as the deglaciation which also indicates an association with poorly strat- and Heinrich Events. ified/enhanced productivity settings. In the modern For the planktonic foraminiferal abundance study, North Atlantic G. scitula reaches its maximum abun- samples were disaggregated, washed and sieved to dance around the Azores Islands [22], and plankton remove all material finer than 150 Am. Where neces- tows indicate that it is present in high abundances north of and within the Azores Front where frontal Table 1 upwelling causes high productivity, but in very low Stratigraphic position of radiocarbon datings abundances to the south [2]. Bradshaw (1959) [25] Sample Conventional 1r Mean shows peak numbers of G.
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