Ichnological analysis as a tool for assessing deep-sea circulation in the westernmost Mediterranean over the last Glacial Cycle

Highlights

• Ichnology at the Last Glacial Cycle is related to oxygenation and food availability.

• Ichnological data correlate with the climatic proxies SST and δ¹⁸O.

• Climatically induced ocean dynamics influenced the benthic habitat.

Abstract

During the Last Glacial Cycle (last ~130 kyr) climatically induced changes in the ocean dynamics affected the tracemaker habitat in the Alboran Sea Basin (westernmost Mediterranean), as observed in sediment records from ODP Leg 161 Sites 976 and 977. The trace fossil assemblage present is assigned to the Zoophycos ichnofacies and is of low/moderate diversity and comprises common Planolites, Chondrites and Thalassinoides, with occasional Scolicia and Zoophycos. Ichnodiversity, size of biogenic structures and percentage of bioturbational sedimentary structures clearly correlate. Fluctuations in ichnological features evidence a well-developed short-term cyclic pattern that could be related to environmental changes such as export production and oxygenation at the seafloor. The percentage of bioturbational sedimentary structures correlates well with sea-surface temperature (SST) records obtained for the Alboran Sea and Gulf of Lions, as well as with the δ¹⁸O profiles of Greenland ice cores. Correlation is seen for both the long-term (over the Last Glacial Cycle) and short-term changes, the latter comprising climate oscillations such as Heinrich Events, Younger Dryas, and periods of organic-rich layer deposition. Ichnological data also allow for a reconstruction of climatically induced changes in the ocean dynamics, which have a major incidence in the Western Mediterranean Deep Water that, in turn, affects deep-sea environmental conditions.

Introduction

The Mediterranean Sea (Fig. 1) has been particularly sensitive to palaeoenvironmental changes due to its semi-enclosed nature, with limited connection with the Atlantic Ocean through the Strait of Gibraltar. It has also proven to be an exceptional natural laboratory for Earth Sciences research (Bethoux et al., 1999; Krijgsman, 2002; Durrieu de Madron et al., 2011). Regarding climate conditions, its location is unique, affected by northern and mid-latitudes as well as tropical climate systems (e.g., Lionello, 2012). Due to short water residence times, climatically induced changes significantly affect the Mediterranean in relatively short periods (Durrieu de Madron et al., 2011), including the deep parts of the basin. Furthermore, the Mediterranean water dynamics also affect and modify the Atlantic Ocean Overturning Circulation (AMOC) in different ways contributing to the global oceanic circulation (e.g., Rogerson et al., 2012; Bahr et al., 2015).

At present, the Mediterranean Sea is subdivided into two main sub-basins separated by the shallow Sicily Channel: The Eastern Mediterranean (EMED) and the Western Mediterranean (WMED) (Fig. 2). In general terms, the Mediterranean Sea experiences anti-estuarine circulation (e.g., Millot, 1999; Millot and Taupier-Letage, 2005). Various water masses are distinguished (Fig. 2): a) At the surface, Atlantic Water (AW) enters as an inflow jet from the Atlantic through the Gibraltar Strait (e.g., Perkins et al., 1990; Millot, 1999; Macias et al., 2016). The AW flows to the East at shallow depth and is affected by mesoscale circulation that comprises two surface water gyres in the western and eastern Alboran area (Fig. 1) (e.g., Send et al., 1999; Pinardi et al., 2015); b) the Levantine Intermediate Water (LIW; Bethoux et al., 1999; Millot, 1999; Millot and Taupier-Letage, 2005; Pinardi et al., 2015) flows to the West in an anticlockwise pattern and modulates and contributes to the Mediterranean's Outflow Water (MOW) to the Atlantic Ocean, (Millot, 1999; Naranjo et al., 2012, Naranjo et al., 2015); c) in the deepest parts of the basins, trapped by the different sills, the Western Mediterranean Deep Water (WMDW) and Eastern Mediterranean Deep Water (EMDW) occur, they develop seasonally during winter in northern regions of the basins due to low temperature and evaporation inducing the production of deep convection, open-sea deep convection and cascading (Lacombe et al., 1970; Gascard, 1978; Durrieu de Madron et al., 2013), because of the loss of buoyancy of surface waters caused by cold and dry N-NW winds from the continent (Lacombe et al., 1970; Millot, 1999; Durrieu de Madron et al., 2013; Cisneros et al., 2019). In the Western Mediterranean Sea, the deep water mostly derives from the loss of buoyancy and sinking of more superficial water, both surface water and LIW, in the Gulf of Lions (Millot, 1990; Rhein, 1995) during winters. Thus, the WMDW overturning has been closely linked with climatic conditions in the northern hemisphere and Atlantic climate influence (Cacho et al., 2000; Sierro et al., 2005).

Over time, this circulation pattern has significantly changed in response to climate forcing. In particular, during the Last Glacial Cycle (last ~130 kyr; Dansgaard et al., 1993), the Mediterranean Sea experienced rapid fluctuations in hydrographic conditions as the result of abrupt climatic changes in the Northern Hemisphere, which resulted from millennial-scale fluctuations between the penultimate and the current interglacials (Blunier and Brook, 2001; Siddall et al., 2003), that is, between Termination II (~130 ka) and Termination I (~11.7 ka) (Fischer et al., 1999; Hughes et al., 2013). Among them, Heinrich events (H events), Dansgaard-Oeschger Stadials (D-O) (cold) and Interstadials (warm) (Heinrich, 1988; Dansgaard et al., 1993; Cacho et al., 2002) are the most prominent changes. The H events resulted from oceanic cooling and increasing ocean ice cover extension over the North Atlantic, which subsequently resulted in distinctive layers of ice-rafted debris in the northern Atlantic (Hemming, 2004). In this regard, the Alboran Sea basin offers an exceptional record for reconstructing deep-sea circulation oscillations, which are in turn linked to sea surface temperature (SST) and atmospheric temperature variations (Martrat et al., 2004, Martrat et al., 2007, Martrat et al., 2014; Pérez-Folgado et al., 2004). In terms of deposition, these oscillations are linked to a recurrent accumulation of organic-rich sediments known as sapropels and organic-rich layers (ORLs) in the Eastern and Western Mediterranean, respectively. Sapropels have formed in the eastern basins cyclically over the last 13.5 million years (Rohling et al., 2015). Even though their origin has been a topic of intense debate, it is now widely accepted that productivity played a major role in their deposition, with enhanced organic matter fluxes (e.g., Calvert and Fontugne, 2001) and reduced oxygen levels promoted by stagnation and low ventilation of the deep water (Wüst, 1961; Cramp and O'Sullivan, 1999; Emeis and Weissert, 2009; Rohling et al., 2015).

In the Western Mediterranean, the ORLs are still poorly investigated and they do not correlate with warm insolation maxima periods like the eastern sapropels (von Grafenstein et al., 1999); yet, they could likewise be linked to low oxygenation during deglaciation periods and changes in productivity (Rogerson et al., 2008). Overall, freshwater input, deep-water ventilation and primary productivity are the main factors to be considered for ORL and sapropel formation (e.g., Cramp and O'Sullivan, 1999; Martínez-Ruiz et al., 2000; Rogerson et al., 2008; Rohling et al., 2015; Grant et al., 2016).

In the particular case of the Alboran Sea basin, high sedimentation rates (~3 ± 2 m/kyr) allow an exceptional high-resolution analysis for palaeoceanographic reconstructions (e.g., Cacho et al., 2000, Cacho et al., 2006; Martrat et al., 2007; Rodrigo-Gámiz et al., 2011, Rodrigo-Gámiz et al., 2014; Martínez-Ruiz et al., 2015). Furthermore, this basin is largely affected by the Atlantic inflow through the Gibraltar Strait (Naranjo et al., 2015). In turn, the MOW influences the AMOC (e.g., Hernández-Molina et al., 2014; Ivanovic et al., 2014; Jiménez-Espejo et al., 2015). Hence, the relationships between water mass exchange, circulation patterns, and climatic conditions hold keys for reconstructing deep-sea environmental parameters such as salinity, benthic food availability and, especially, oxygenation.

Among the diverse proxies that serve for reconstructing environmental conditions, ichnological analysis has proven particularly useful. Trace fossils record the behaviour of tracemakers in response to the environment and provide valuable information regarding the depositional and environmental conditions. Ecological and depositional parameters affecting the marine realm (i.e., oxygenation, nutrients, hydrodynamic energy, rate of sedimentation, etc.) can be addressed through a detailed ichnological analysis (i.e., Wetzel, 1983; Savrda and Bottjer, 1986, Savrda and Bottjer, 1987, Savrda and Bottjer, 1989, Savrda and Bottjer, 1991; Savrda, 2007; Rodríguez-Tovar and Uchman, 2010; Wetzel, 2010; Buatois and Mángano, 2011; Uchman and Wetzel, 2011, Uchman and Wetzel, 2012; Wetzel and Uchman, 2012). Thus, ichnological information reveals as a key proxy for environmental conditions, particularly, when it is applied in sediment core records for palaeoclimate and palaeoceanographic studies (Wetzel, 1991; Savrda, 1995; Rodríguez-Tovar et al., 2015). Notably, ichnological information has been incorporated in several multi-proxy studies to investigate, for instance, glacial/interglacial transitions, Heinrich events, and diverse Marine Isotope Stages (MIS) (Baas et al., 1997, Baas et al., 1998; Löwemark et al., 2004, Löwemark et al., 2006, Löwemark et al., 2008, Löwemark et al., 2012; Rodríguez-Tovar et al., 2015, Rodríguez-Tovar et al., 2019, Rodríguez-Tovar et al., 2020; Dorador et al., 2016; Hodell et al., 2017; Sánchez Goñi et al., 2019; Evangelinos et al., 2020; Pérez-Asensio et al., 2020). Besides, the ichnofabric approach, as a paradigm, addresses ichnological features such as the amount of bioturbational sedimentary structures, their size and penetration depth, to assess environmental parameters including oxygenation and benthic food availability (Ekdale and Bromley, 1983, Ekdale and Bromley, 1991; Bromley and Ekdale, 1986; Taylor and Goldring, 1993; Ekdale et al., 2012; Savrda, 2016).

However, ichnological research in cores entails particular limitations due to features such as the narrow exposed surface, two-dimensional core slabs, or the usual absence of complete structures, making the characterization of trace fossils and ichnofabrics more complicated (e.g., Knaust, 2017; Dorador and Rodríguez-Tovar, 2014). The ichnotaxonomical classification of bioturbational structures may be further impeded by scarce information on diagnostic criteria or ichnotaxobases (Bromley, 1996; Bertling et al., 2006; Knaust, 2017), so that a precise ichnotaxonomical classification at the ichnospecies level is often impossible (usually conducted at the ichnogenus level; Bromley, 1996). Ichnological research in cores from modern marine deposits is particularly difficult when the material is unconsolidated, and the visual differences between biogenic structures and host sediment are weak (Dorador and Rodríguez-Tovar, 2015). To improve trace fossil visualization and the analysis of ichnological attributes in marine cores from modern deposits, a new high-resolution image treatment has emerged (see Dorador and Rodríguez-Tovar, 2018 for a recent review of the methodology). This novel method has proven to be a powerful tool for the ichnofacies model (Dorador and Rodríguez-Tovar, 2015) and ichnofabric approach (e.g., Rodríguez-Tovar and Dorador, 2014, Rodríguez-Tovar and Dorador, 2015), and its usefulness has been demonstrated in numerous studies involving paleoenvironmental reconstructions, ocean-atmosphere dynamics and sedimentary basin analysis (e.g., Rodríguez-Tovar et al., 2015; Dorador and Rodríguez-Tovar, 2016, Dorador and Rodríguez-Tovar, 2018; Hodell et al., 2017).

Within this context, a detailed ichnological analysis was conducted on sediment cores from the Alboran Sea Basin to evaluate the impact of environmental variations in deep-sea settings and the tracemaker habitat over the Last Glacial Cycle, in order to reconstruct oxygen conditions and deep-sea circulation patterns. Such study aims to contribute to our understanding of the deep-sea dynamics in the westernmost Mediterranean Sea by adding detailed ichnological data to the currently available knowledge as well as describing the tracemaker community variations and correlating them with paleoclimate and palaeoceanographic changes.