Variations of Mediterranean–Atlantic exchange across the late Pliocene climate transition

Mediterranean-Atlantic exchange through the Strait of Gibraltar plays a significant role in the global ocean-climate dynamics in two ways. On one side, the injection of the saline and warm 10 Mediterranean Outflow Water (MOW) contributes to North Atlantic deep-water formation. In return, the Atlantic inflow is considered a sink of less saline water for the North Atlantic Ocean. However, while the history of MOW is the focus of numerous studies, the latter has received little attention so far. The present study provides an assessment of the Mediterranean–Atlantic exchange with focus on the Atlantic inflow strength and its response to regional and global climate from 3.33 to 2.60 Myrs. This time 15 interval comprises the mid-Pliocene warm period (MPWP, 3.29–2.97 Myr) and the onset of the Northern Hemisphere Glaciation (NHG). For this purpose, gradients in surface O records of the planktonic foraminifer Globigerinoides ruber between the Integrated Ocean Drilling Program (IODP) Hole U1389E (Gulf of Cadiz) and ODP Site 978 (Alboran Sea) have been evaluated. Interglacial stages and warm glacials of the MPWP revealed steep and reversed (relative to the present) W-E O gradients suggesting a 20 weakening of Mediterranean–Atlantic exchange likely caused by high levels of relative humidity in the Mediterranean region. In contrast, periods of stronger inflow are indicated by flat O gradients due to more intense arid conditions during the severe glacial Marine Isotope Stage (MIS) M2 and the initiation of the NHG (MIS G22, G14, G6–104). Intensified Mediterranean–Atlantic exchange in cold periods is linked to the occurrence of ice-rafted debris (IRD) at low latitudes and weakening of the Atlantic 25 Meridional Overturning Circulation (AMOC). Our results thus suggest the development of a negative feedback between AMOC and exchange rates at the Strait of Gibraltar in the latest Pliocene as it has been proposed for the late Quaternary. 1 Clim. Past Discuss., https://doi.org/10.5194/cp-2017-134 Manuscript under review for journal Clim. Past Discussion started: 20 October 2017 c © Author(s) 2017. CC BY 4.0 License.


Introduction
Mediterranean-Atlantic water mass exchange through the Strait of Gibraltar is driven by a two directional current system in which the westward outflow of warm and saline Mediterranean Outflow Water (MOW) at the bottom, driven by excess evaporation in the Mediterranean, is compensated by the 35 inflow of colder and less saline North Atlantic Central Water (NACW) at the surface (Bormans et al., 1986;Ochoa and Bray, 1991;Vargas-Yáñez et al., 2002). Mediterranean-Atlantic exchange plays an important role for regional and global climate in two respects. First, the injection of warm and salty MOW into the North Atlantic contributes to deep water formation at high latitudes, and thus to the dynamics of the ocean-climate system (Bryden and Kinder, 1991;Ivanovic et al., 2014;Reid, 1979;40 Rogerson et al., 2012;Voelker et al., 2006). Second, the eastward Atlantic inflow has been considered a freshwater sink for the North Atlantic since less saline North Atlantic Central Water is replaced with saltier MOW (Rogerson et al., 2010). As a result of this interaction, a negative feedback between exchange and Atlantic Meridional Overturning Circulation (AMOC) has been recognized during Heinrich Stadials in the late Quaternary (Rogerson et al., 2010(Rogerson et al., , 2012. 45 Previous research has focused strongly on MOW variability since the Pliocene (e.g. Bryden and Stommel, 1982;Hernández-Molina et al., 2014;Iorga and Lozier, 1999;Kaboth et al., 2016;Voelker et al., 2006) but has neglected the Atlantic surface water component (Rogerson et al., 2010). Furthermore, there are a number of studies on Mediterranean-Atlantic exchange during the early-mid Pliocene and early Pleistocene (e.g., Bahr et al., 2015;García-Gallardo et al., 2017;Grunert et al., 2017;Hernández-Molina 50 et al., 2014;Kaboth et al., 2017;Khélifi et al. 2009Khélifi et al. , 2014Van der Schee et al., 2016;Voelker et al., 2015) leaving a gap in our knowledge about the late Pliocene climate transition comprising the mid-Pliocene warm period (MPWP) and the initiation of the Northern Hemisphere Glaciation (NHG).

Gulf of Cadiz -IODP Hole U1389E
IODP Hole U1389E is located in the northern Gulf of Cadiz (36°25.515′N, 07°16.683′W; Fig. 1) at 644 m 90 water depth under direct influence of MOW. It constitutes a key site for the recovery of an upper Pliocene contourite succession (Stow et al., 2013). Once LIW and WMDW exit the Strait of Gibraltar and form MOW, this water mass splits into two plumes due to the complex morphology of the continental slope in the Gulf of Cadiz. The upper plume flows between 500 and 800 m while the lower plume flows between 800 and 1400 m (Ambar and Howe, 1979;Borenäs et al., 2002;García et al., 2009;Llave et al., 95 2007;Madelain, 1970;Marchès et al., 2007;Serra et al., 2005;Zenk, 1975). Surface circulation in the Gulf of Cadiz is governed by the Gulf of Cadiz slope current, flowing eastward along the western Iberian margin and the offshore inflow which meet at the Strait of Gibraltar and enter the Mediterranean basin (Peliz et al., 2009).

Sample material and data collection
This study relies on published and newly acquired stable oxygen isotope 18 O) records of the planktonic foraminifer Globigerinoides ruber from upper Pliocene (3.33-2.60 Myrs) sediments at Site 978 (Alboran Sea), Hole U1389E (Gulf of Cadiz) and the Rossello section in Sicily (Fig. 1 Grunert et al. (2017) and adopted for this study.
The stacked  18 O record of Lourens et al. (1992Lourens et al. ( , 1996 from the Rossello outcrops in Sicily has been adopted for stratigraphic calibration of the new  18 O record at Site 978. 115

Stable isotope analysis
Details on laboratory protocols and isotopic analyses performed by Grunert et al. (2017), Khélifi et al.
(2014) and Lourens et al. (1996) can be found in the respective publications. For the continuation of the Site 978 record, samples from core sections 19R-5 to 16R-4 were analyzed every 50 cm. Sediment samples were dried, weighed, washed through sieves 250 and 63 µm and dried. Whenever possible, 10 120 to 20 well-preserved specimens of Gdes. ruber > 250 µm were picked for isotopic analysis. Crushed and cleaned shells were reacted with 100% phosphoric acid at 70 °C using a Gasbench II connected to a ThermoFisher Delta V Plus mass spectrometer at GeoZentrum Nordbayern (Erlangen). All values are reported in per mil relative to VPDB. Reproducibility and accuracy were monitored by replicate analysis of laboratory standards calibrated by assigning δ 13 C values of +1.95‰ to NBS19 and -46.6‰ to LSVEC 125 and δ 18 O values of -2.20‰ to NBS19 and -23.2‰ to NBS18.

Age model
Age constraints for Site 978 are established from 3.6 to 2.8 Myrs by Khélifi et al. (2014) and those for Hole U1389E are reported in Grunert et al. (2017) Lourens et al., 1996). Identification and denomination of Marine Isotopic Stages (MIS) has been established according to Lourens et al. (1996;135 Figs. 3, 4-a).

Glacial/interglacial δ 18 O gradients
Interglacial periods MIS G7, G5 and G3 as well as glacial period MIS G4 show negative  18 O gradients between -0.03 and -0.10 ‰ degree -1 ). In contrast, glacial periods MIS G6, G2 and 104 reveal positive  18 O gradients ranging from + 0.02 to + 0.06 ‰ degree -1 (Fig. 4-b3). late Pliocene, however, our data suggest that the  18 O gradient was considerably more variable, particularly during glacial stages (Figs. 4-a, 4-b). While all studied interglacial stages and glacial stages G22-G16 (except G14) of Interval II show a reversed gradient with respect to the present, the strong glacials M2, G14, G6, G2 and 104 show a gradient in line with present-day observations (Figs. 4-b, 5b). 195 Seasonal variations of Atlantic inflow reported in previous studies (e.g. Bormans et al., 1986;Ovchinnikov, 1974;Parada and Cantón, 1998;Vargas-Yáñez et al., 2002) can be the cause of Sea Surface Salinity (SSS) and Sea Surface Temperature (SST) variability. While seasonal changes of SSS are < 0.5, SST variability is more prominent and may result in brief temporary reversals of SST gradients (MEDATLAS, 2002;Rogerson et al., 2010). The Alboran Sea shows colder temperatures than the Gulf of Cadiz during 200 all seasons due to upwelling, with occasional exceptions in summer under the influence of easterly winds (Bakun and Agostini, 2001;Folkard et al., 1997;Peeters et al., 2002;Rogerson et al., 2010;Sarhan et al., 2000;Shaltout and Omstedt, 2014). The  18 O composition of present-day seawater is thus considered to be largely determined by changes of SST (Rogerson et al., 2010).
Reversed gradients in the late Pliocene could imply a different SST gradient due to a different current 205 regime and/or lack of upwelling in the Alboran Sea in the Pliocene. Unfortunately, there is little data on SST available from the late Pliocene which would allow further evaluation. Khélifi et al. (2014) provide an alkenone-based SST record from Site 978 from ~3.6 to 2.7 Myrs (Intervals I and II of this study) which indicates SSTs ranging from ~ 26.5 to 27.5 °C. A comparable alkenone-based SST reconstruction is available from IODP Site U1387 in the Gulf of Cadiz and suggests an SST-range from ~ 26 to 27 °C from ~6 210 to 2.7 Myrs (Tzanova and Herbert, 2015). The comparison thus suggests that the difference in SST between the two sites was little to none, with surface waters in the Gulf of Cadiz probably slightly colder than in the Alboran Sea. Given a temperature sensitivity of ~ 0.23 ‰ °C -1 for planktonic foraminifera (O'Neill et al., 1969), estimated SST offsets cannot fully explain δ 18 O gradients < -0.05 ‰ degree -1 in our case. 215 Variations of δ 18 O gradients between the Mediterranean and Atlantic have also been explored in modeling studies by Rohling (1999). This model indicates that the direction of the gradient is largely influenced by relative humidity in the Mediterranean. While relative humidity values similar to presentday (which also persisted during the LGM; Rohling, 1999;cf. Rogerson et al., 2010) recreate  18 O gradients as observed in Holocene and late Pleistocene foraminiferal records accurately, an increase of 220 5% in relative humidity is sufficient to reverse the  18 O gradient due to isotopic depletion in the Mediterranean (Rohling, 1999). Pollen-based data suggest that annual precipitation and humidity in the Mediterranean were considerably higher relative to present-day levels during most of the late Pliocene with the exception of the strong glacial periods M2, G22, and G6-104 (Bertini, 2010;Fauquette et al., 1998). A warmer and more humid climate implies higher runoff and freshening of the Mediterranean 225 surface waters leading to depleted δ 18 O records as reported from Pliocene to Holocene in previous studies (e.g. Gudjonsson and van der Zwaan, 1985;Kaboth et al., 2017;Thunell and Williams, 1989;Van Os et al., 1994;Vergnaud-Grazzini et al., 1977). We thus consider δ 18 O depletion of Mediterranean waters during a warm and humid paleoclimate as the most likely explanation for the reversed gradients observed for interglacial stages from all three studied intervals as well as for the comparably warm 230 glacial periods MIS G22, G20, G16, and G4 ( Fig. 4-b). Conversely, arid conditions are indicated by pollen data only for the strong glacial stages M2, G6, G2, and 104 which herald increasing Northern Hemisphere Glaciation and for which our data suggest δ 18 O gradients similar to the present (Fauquette et al., 1998;Figs. 4-b, 5-b).

Steepness of  18 O gradients 235
While humidity likely explains large-scale variations of normal and reversed δ 18 O gradients in the late Pliocene relative to the present-day trend, slope steepness is considered sensitive to regional signals, i.e. it responds to the strength of surface water exchange across the Strait of Gibraltar (Rohling, 1999;Rogerson et al., 2010). Whereas flat gradients are indicative of well-connected basins and enhanced exchange, steeper gradients suggest more restricted conditions and reduced exchange (Rogerson et al., 240 2010).
The steepness of present-day gradients varies from 0.05 to 0.13 ‰ degree -1 depending on the time of calcification of Gdes. ruber and G. bulloides (Fig. 5-b; this study; Rogerson et al., 2010; see chapter 5.1).

Support for our interpretation comes from available studies on MOW development during the late
Pliocene to which the strength of AMOC is directly linked. The onset of long-term intensification of MOW has been linked to arid conditions during glacial stage M2, a second pulse occurs at ~ 2.8-2.7 Hernández-Molina et al., 2014;Khélifi et al., 2009;2014;Sarnthein et al., 2017). Both pulses occur during these glacial periods in which we observe gradients similar to today and their succession is  Fig. 4-a). Thus, enhanced Mediterranean-Atlantic exchange suggested for MIS M2 265 parallels a drop of NADW production and weakened AMOC (DeSchepper et al., 2009;Khélifi et al., 2014;Sarnthein et al., 2017). Continuous occurrences of IRD at low mid-latitudes and reduced AMOC during glacial periods G14 to 104 fall together with intensified Mediterranean-Atlantic exchange during strong glacial periods at the onset of the NHG. In contrast, the lack of IRD and increased  13 C observed during interglacials (Kleiven et al., 2002) is in accordance with more restricted exchange during MIS MG1, G21, 270 G19, G15, G5, and G3 ( Fig. 4-a).The observed negative feedback between AMOC and Mediterranean-Atlantic exchange at the onset of the NHG seems to work similarly as during Heinrich Stadials from the Holocene as reported by Rogerson et al. (2010). However, higher resolution of our Pliocene records would be necessary to establish an accurate assessment of the timing of these feedback mechanisms within the glacial periods. Intensified MOW has been reported during M2 and from 2.8 Ma coinciding with intense glacial stages.
Strengthened Mediterranean-Atlantic exchange occurs at times of reduced AMOC and NADW formation, when a higher influx of IRD arrived to lower latitudes causing the freshening of Atlantic surface waters. Our results thus suggest a negative feedback between AMOC and exchange rates at the 290 Strait of Gibraltar in the late Pliocene as it has been proposed for the late Quaternary. Data availability. Data will be archived at PANGAEA, data publisher for Earth and Environmental Science.

Competing interests. The authors declare that they have no conflict of interest. 295
Acknowledgments. This project is funded by the Austrian Science Fund (FWF; project P25831-N29). We thank the Expedition 339 from the Integrated Ocean Discovery Program for providing the samples used in this study. The Bremen Core Repository is acknowledged for the sampling performed on sediment cores from ODP Leg 161. Michael Joachimski from GeoZentrum Nordbayern (Erlangen) is acknowledged 300 for performing isotopic analyses.    (Lourens et al., 1996). (b) Calculated sedimentation rates.   ruber between core-top samples from the Gulf of Cadiz (M39022-1; Salgueiro et al., 2008) and the Alboran Sea (stations KS82-30 and KS-82-31;Rohling, 1999). Red circles represent mean values of the 320 respective data-sets, the red line denotes the gradient between both basins. For comparison, the gradient obtained from  18 O composition of G. bulloides by Rogerson et al. (2010) has been added.  (Fig. 3). Comas, M.C., Zahn, R., Klaus, A., et al., (1996)