Available online http://amq.aiqua.it ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, Vol. 31 (Quaternary: Past, Present, Future - AIQUA Conference, Florence, 13-14/06/2018), 105 - 108 CLIMATIC AND OCEANOGRAPHIC CHANGES IN THE AZORES REGION DURING THE LAST 74.7 KA Alessandro Bonfardeci 1, Antonio Caruso 1, Annachiara Bartolini 2, Franck Bassinot 3, Marie-Madeleine Blanc-Valleron 2 1 Dipartimento di Scienze della Terra e del Mare, Università degli studi di Palermo, Palermo, Italy 2 CNRS - UMR 7207 CR2P, MNHN, Paris, France 3 Institut Pierre-Simon Laplace, Lab. des Sciences du Climat et de l'Environnement, UMR8212, CEA-CNRS-UVSQ, Gif-sur-Yvette, France Corresponding author: A. Bonfardeci ABSTRACT: In this study, we reconstruct the complex palaeoclimatic and palaeohydrographic history of the North Atlantic Ocean during the Upper Pleistocene-Holocene, through a high-resolution Globigerinoides ruber - Globigerinoides elongatus plexus study. The studied core (ATA13-OF-KT1) was collected southwest of the Azores islands near the present-day boundary of the Subtropical Gyre/Azores Front Current System (STG/AFCS). Quantitative and stable isotope data of the G. ruber - G. elongatus plexus chromotypes and selected morphotypes showed cyclic oscillations of the STG/AFCS boundary linked to climatic variability at orbital and millennial scales, during the last 74.7 ka. KEYWORDS: Foraminifera, stable isotopes, palaeoclimatology, palaeoceanography, subtropical Gyre, Azores Front/Current System 1. INTRODUCTION The Atlantic Meridional Overturning Circulation (AMOC) is known to be the surface and deep-water movements system that controls the North Atlantic hy- drography (Fig. 1; Lynch-Stieglitz et al., 2007; Bon- fardeci et al., 2018). The Azores Current (AC), originat- ing as the eastern branch of the Gulf Stream, flows to the south of the Azores archipelago throughout the year, centred at 34°N (Schiebel et al., 2002a-b). This current is limited to the north by the so-called Azores Front (AF) that separates the warmer and oligotrophic waters belonging to the Subtropical Gyre (STG) from the northernmost colder and nutrient-rich Eastern North Atlantic Central Water (ENACW) (Gould, 1985; Schiebel et al., 2002a-b; Schwab et al., 2012; Repschläger et al., 2015). Moreover, the AC represents the main surface waters input for the Mediterranean basin and its intensity is strongly forced by the Mediter- ranean Outflow Water (MOW) (Schwab et al., 2012). Globigerinoides ruber (d’Orbigny, 1839) gr. alba and rosea, and Globigerinoides elongatus (d’Orbigny, 1826) are common species in tropical/subtropical bio- geographic provinces (Bé and Torderlund, 1971; Bé, 1977) and they have been extensively used to recon- struct past sea-surface temperature and salinity. How- ever, these two species exhibit morphological variability linked to different environmental conditions (Aurahs et al., 2011; Schiebel & Hemleben, 2017). In this study, we test the link between environmental conditions and mor- phological variability of the G. ruber - G. elongatus plexus and its response to climatic changes at orbital and millennial-scale (Bonfardeci et al., 2018). The main aim of this work was to analyse latitudinal shifts in the Subtropical Gyre/Azores Front Current System (STG/ AFCS), using the quantitative abundance and isotopic variations of the different G. ruber - G. elongatus (eco-) morphotypes as sensors. 2. MATERIAL AND METHODS The ATA13-OF-KT1 core (35°24.956’N - 37° 15.749’W; bathymetry= 3,431 m; length= 4.03 m), col- lected with a Kullenberg gravity corer during the Ocean- ograflu 2013 cruise, was sub-sampled every 1 cm, with a total of 402 samples. The quantitative analyses on G. ruber - G. elongatus plexus of ATA13-OF-KT1 core were performed on the >125 and 250-315 μm size-fractions. Stable isotope analyses were performed every 2 centimetres, in the size -fraction 250-315 μm, on about ten specimens of G. ru- ber s.s., whereas further analyses on different morpho- types were carried out in two strategic intervals: 0-18.5 cmbsf and 42.5-60.5 cmbsf (Bonfardeci et al., 2018). The isotope analyses were performed at the LSCE (Laboratoire des Sciences du Climat et de l’Environne- ment - Gif-sur-Yvette, France). 3. RESULTS For the age model developing four AMS 14C dating and the δ18O G. ruber s.s. record were combined. The AMS 14C analyses were preferentially carried out on surface- dwelling species (Globigerinoides spp.). A reservoir age correction (Rsurf) of 400 a was used, except for the 39-40 cmbsf level, corresponding to the Heinrich event 1 (H1), where a Rsurf of 800 was adopted (Bard et al., 1998; Siani et al., 2001). According to the tuning strategy, G. https://doi.org/10.26382/AIQUA.2018.AIQUAconference mailto:alessandro.bonfardeci@unipa.it� ruber gr. alba and δ18OG.ruber s.s. maxima and minima were aligned to the NGRIP δ18O (North Greenland Ice Core Project members, 2004) and the MD95-2042 δ18OG. bulloides records (Lisiecki & Stern, 2016). Mean sedimentation rate of 5.6 cm ka-1 (temporal resolution of 178.5 a cm-1) was calculated, with an uncertainty of 2-3 ka (Lisiecki & Stern, 2016). The time interval covered by the ATA13-OF-KT1 core record spans from 74.7 to 2.75 ka BP (Bonfardeci et al., 2018). The d18O G. ruber s.s. reached maxima values (from 2.1 to 0.8‰) during MIS 4 and 2 stages (Fig. 2), whereas decreased in correspondence with warmer MIS 1 and MIS 5.1 phases (from 1.7 to 0.4‰). High fre- quency/low amplitude fluctuations observed in the d18OG.ruber s.s. record were linked to sub-orbital climatic oscillations, such as Heinrich and Dansgaard-Oeschger events. G. ruber gr. alba increased in abundance during warm climate phases (MIS 5.1 and MIS 1) with maxima percentages (10-20 %) during the Holocene (Fig. 2), whereas minima (2-12.5 %) were observed during MIS 4 and MISs 3.1/2 transition. Within the G. ruber - G. elongatus population, G. elongatus became dominant (up to 50 %) during warmer periods (MISs 5.1, 3.3, 3.1 and 1) and during the Bøl- ling/Allerød–Younger Dryas interval, G. ruber cyclo- stoma type reached maximum abundances during inso- lation minima and glacial periods (MIS 4 and MIS 2), with a remarkable peak for H7, and it was almost absent in interglacial MIS 5.1 and especially in MIS 1 (Bonfardeci et al., 2018). 4. DISCUSSION AND CONCLUSIONS Abundance fluctuations of G. ruber gr. (alba and rosea), as well as of the G. ruber - G. elongatus plexus morphotypes, testify that the Azores region climate and hydrography were strictly controlled by orbital and sub- orbital forcing, during the last 74.7 ka. In particular, G. ruber gr. (alba and rosea) reached highest abundance during climatic phases in which the insolation signal was closely in phase with the obliquity (Laskar et al., 2004). Therefore, this latter group of species has been used as monitor for the expansion/reduction of the North Atlantic STG (Repschläger et al., 2015; Bonfardeci et al., 2018). G. ruber gr. alba (Fig. 2) increases in abundance, coe- val with δ18OG. ruber s.s. lightening, indicate that during the final part of MIS 5 and the Holocene the Azores region was interested by warmer, stratified and oligotrophic surface waters, due to the STG expansion. Conversely, during the glacial stages (MISs 4 and 2) and the Heinrich events, cooler, nutrient-richer and saltier wa- ters, forced by STG reduction, dominated this region, as testified by G. ruber gr. alba decreases and δ18OG. ruber s.s. weighting (Bonfardeci et al., 2018). Analysing the quantitative distribution of each eco- morphotype in the G. ruber - G. elongatus plexus, G. ruber cyclostoma type represents the best cooler waters indicators, reaching maxima abundance during the MISs 4 and 2, whilst G. elongatus, increasing in abundance during interglacial periods (latest part of MIS 5.1 and MIS 1), can be considered as the best warmer waters indicator in the Azores region. According to Bonfardeci et al., (2018), the difference between the amount of G. elongatus and G. ruber cyclostoma type represents a 106 Fig. 1 - Present North Atlantic surface hydrography and ATA13-OF-KT1 core location (modified from Bonfardeci et al., 2018). GS=Gulf Stream; NAC=North Atlantic Current; AC=Azores Current; AF=Azores Front; PC=Portuguese Current; CC=Canary Current; MOW=Mediterranean Outflow Water; STG=Subtropical Gyre; ENACW= Eastern North Atlantic Central Water. 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