Middle-Late Pleistocene Eastern Mediterranean nutricline depth and coccolith preservation linked to Monsoon activity and Atlantic Meridional Overturning Circulation

Highlights

• F. profunda in sapropel S5 is in line with maximum rates of δ¹⁸O Asian speleothems and with the CH₄ overshoot in EDC;

• F. profunda DCM development in glacials, due to sea-level lowering that reduced water-mass transport through straits;

• Holococcolith enhanced preservation during cold spells, in response to AMOC slowdown and weakened monsoon activity;

Abstract

The eastern Mediterranean Sea lies under the influence of high- and low-latitude climatic systems. The northern part of the basin is affected by Atlantic depressions and continental and polar air masses that promote intermediate and deep-water formation. The southern part is influenced by subtropical conditions and monsoon activity. Monsoon intensification results in enhanced freshwater discharge from the Nile River and other (now dry) systems along the North African margin. This freshwater influx into the Mediterranean Sea reduces surface water buoyancy loss. Disentangling the influences of these diverse climatic forcings is hindered by inherent proxy data limitations and by interactions between the climatic forcings. Here we use a wealth of published and new paleoclimate records across Termination II to understand the impacts of the higher latitude and subtropical/monsoon climate influences on coccolithophore ecology and holococcolith preservation in Aegean Sea sediment core LC21. We then use these findings to interpret coccolith assemblage variations at Ocean Drilling Program Site 967 (located nearby LC21, at the Eratosthenes Seamount) during multiple glacial-interglacial cycles across the Middle Pleistocene (marine isotopic stages 14–9). The LC21 analysis suggests that holococcolith preservation was enhanced during Heinrich Stadial 11 (∼133 ka) and cold spell C26 (∼119 ka). These two events have been previously linked to cold conditions in the North Atlantic and Atlantic Meridional Overturning Circulation weakening. We propose that associated atmospheric perturbations over the Mediterranean Sea promoted deep-water formation, and thus holococcolith preservation. Similarly, in the Middle Pleistocene (MIS 14-9) of Site 967, we observe temporal coincidence between ten episodes of enhanced holococcolith preservation and episodes of Atlantic Meridional Overturning Circulation slowdown. In Site 967, we also identified repeated fluctuations in placoliths and in Florisphaera profunda, which indicate nutricline depth variations. The development of a deep chlorophyll maximum is associated with the North Africa and wet phases, as recently observed using elemental proxy records at Site 967, during the deposition of sapropel layers. A further deep chlorophyll maximum development is identified during MISs 12 and 10, as a result of pycnocline and nutricline shoaling within the lower part of the photic zone due to glacial sea-level lowering and water mass transport reduction at both the Gibraltar and Sicily Straits. Finally, enhanced holococcolith preservation during cold/dry events is clearly correlated to weakened monsoon activity in both Africa and Asia.

Introduction

Paleoclimate reconstructions document the competing influence of southern versus northern climate systems on the hydrography and hydrology of the eastern Mediterranean Sea and its borderlands over a range of timescales (Emeis et al., 2000b; Grant et al., 2017, Grant et al., 2016; Lourens, 2004; Rohling et al., 2002b). During precession minima (Northern Hemisphere insolation maxima), the African monsoon intensified and shifted northward, with attendant enhancement of the freshwater release into the Mediterranean basin via large North African river systems and/or currently inactive wadis (Amies et al., 2019; Marino et al., 2009; Osborne et al., 2008; Rohling et al., 2002a; Rohling et al., 2015; van der Meer et al., 2007). This impacted the basin's hydrography and weakened or even shut down dense water formation, leading to oxygen starvation at depth and deposition of layers (sapropels) with elevated organic carbon concentrations (De Lange et al., 2008; Rohling et al., 2015; Rossignol-Strick et al., 1982). Millennial-scale climatic variations have been less well documented and appear to be associated with variations in the strength of the Atlantic Meridional Overturning Circulation (AMOC) (Grant et al., 2017, Grant et al., 2016; Stockhecke et al., 2016).

Coccolithophores are marine unicellular phytoplankton organisms living in the upper part of the water column. The ecology of coccolithophore species shows a strong sensitivity to modern gradients within the Mediterranean Sea and different species thrive in different areas, mainly in response to West-East temperature and nutrient gradients, water column dynamics, and meso-scale oceanographic features (Bonomo et al., 2012; D'Amario et al., 2017; Knappertsbusch, 1993; Oviedo et al., 2015). In the sedimentary archive, calcite coccolithophore remains (coccoliths) have been used successfully to infer orbital and suborbital variations in climate, productivity, and nutricline depth in oceans and marginal seas (Beaufort et al., 1997; Flores et al., 1997; Incarbona et al., 2013, Incarbona et al., 2010a; Marino et al., 2008; Molfino and McIntyre, 1990a; Rogalla and Andruleit, 2005). In the eastern Mediterranean Sea, coccolith-based paleoenvironmental reconstructions have been mostly aimed at assessing the shallow versus deep position of the nutricline within the photic layer and its relationship with the basin's freshwater budget, water mass circulation, and deep-sea ventilation during sapropel deposition (e.g., Grelaud et al., 2012). These studies attest to the development of a deep chlorophyll maximum (DCM) while organic carbon-rich layers were accumulating on the oxygen-starved eastern Mediterranean seafloor (Castradori, 1993; Giunta et al., 2003; Grelaud et al., 2012; Incarbona et al., 2019, Incarbona et al., 2011; Incarbona and Di Stefano, 2019; Maiorano et al., 2013; Negri et al., 1999; Principato et al., 2006; Triantaphyllou et al., 2009b, Triantaphyllou et al., 2009a), corroborating findings based on other marine planktonic groups (Kemp et al., 1999; Meier et al., 2004; Rohling and Gieskes, 1989).

Here we present new data that complement a previous dataset of coccolith assemblages from south-eastern Aegean Sea core LC21 (Grelaud et al., 2012), across the penultimate glacial termination (termination II, T-II) and the last interglacial period, with a precise, radiometrically constrained chronology (Grant et al., 2012). This allows comparison of LC21 “coccolith proxies” with time series of palaeoclimate variability in the monsoon and the North Atlantic region (Cheng et al., 2009; Hodell et al., 2013), as well as atmospheric methane (CH₄) concentrations. Our combined dataset is probabilistically evaluated to decipher the amplitude and timing of change by quantitatively assessing the impact of chronological, analytical, and proxy uncertainties. We use this analysis as a proof of concept for new, highly resolved coccolith data from Ocean Drilling Program (ODP) Site 967 from the Eratosthenes Seamount, South of Cyprus, within the Nile Delta Basin province (Emeis et al., 1996). The new ODP 967 time series spans, at centennial-scale resolution, three glacial/interglacial cycles of the Middle Pleistocene, from glacial Marine Isotope Stage (MIS) 14 to interglacial MIS 9. Collectively, our new data and analyses provide insights into climate variability at orbital and sub-orbital timescales during both glacial and interglacial periods, complementing a wealth of existing knowledge of the intervals of sapropel deposition. Specifically, we explore modifications in nutrient dynamics and holococcolith preservation during the Middle Pleistocene. These changes are compared with recently acquired variations in elemental abundances, elemental ratios, and climate indices for ODP Site 967 (Section 6.3) that portray the alternation of wet and dry North Africa periods at both orbital and sub-orbital timescales (Grant et al., 2017). Finally, we centre on the correlation between holococcolith preservation, AMOC, and boreal monsoon activity (both in Africa and in a wider Asian context) to assess: (i) the atmospheric impact of continental/polar air outbreaks on the eastern Mediterranean deep-sea ventilation and seafloor calcite preservation during cold stadials; and (ii) impact of millennial-scale atmospheric perturbations on the eastern Mediterranean Sea.