Unravelling the oxygen isotope signal (δ¹⁸O) of rodent teeth from northeastern Iberia, and implications for past climate reconstructions

Highlights

• Oxygen isotopes analyses on rodent contributes to continental environment knowledge.

• Iberia requires a specific regional equation for past temperatures estimation.

• Prey-predator interactions influence oxygen isotope temporal information recorded.

• Seasonality bias and Iberian context are considered in the methodological proposal.

• Complementary palaeoenvironmental methods yielded matching results at Xaragalls cave.

Abstract

Small mammals, especially rodents, constitute valuable proxies for continental Quaternary environments at a regional and local scale. Recent studies have demonstrated the relation between the stable oxygen isotope composition of the biogenic phosphate from rodent teeth (δ¹⁸O_p), and the oxygen isotope composition of meteoric waters (δ¹⁸O_mw), which is related to air temperatures at mid and high latitudes. This work explores the δ¹⁸O_p of rodent tooth enamel (from Murinae and Arvicolinae subfamilies) to investigate the palaeoenvironmental conditions in northeastern Iberia during Marine Isotope Stage 3 (MIS 3; ca. 60-30 ka). Fourteen new δ¹⁸O_p analyses from modern samples in conjunction with forty-six δ¹⁸O_p analyses previously published are used to decipher the isotope record of present-day rodent teeth in this region. Two main factors should be considered in Iberian palaeoenvironmental reconstructions: the singular nature of Iberian δ¹⁸O_mw records and the potential seasonality bias of small-mammal accumulation. Methodological proposals are made with a view to ensuring the correct interpretation of the δ¹⁸O_p of small mammals in reconstructing past air temperatures. This methodology is applied to the MIS 3 sequence of the Cova dels Xaragalls site (Vimbodí-Poblet, Tarragona, Spain), where fifty-one δ¹⁸O analyses were performed on wood mouse (Apodemus sylvaticus) lower incisors. A spring-early summer accumulation of small mammals is suggested for the layers at Cova dels Xaragalls. In agreement with previous environmental studies of the site, variations in the δ¹⁸O_p values suggest slight fluctuations in the climatic conditions throughout the sequence, which are consistent with the stadial-interstadial alternations that characterized MIS 3. Complementary palaeoenvironmental methods determine cooler conditions than nowadays, but within a globally stable climatic period.

Introduction

The Iberian Peninsula is located at mid latitudes, with an area of 582,000 km² and an average altitude of 660 m. This peninsula constitutes the southwestern extremity of the Eurasian continent, from which it is partially isolated by the Pyrenean Mountains. Today, Iberia experiences temperate climatic conditions, which nonetheless vary spatially due to the proximity of the Atlantic Ocean, the Mediterranean Sea and the Pyrenean Mountains. The specific orography of the Iberian Peninsula adds further complexity by structuring the climate and the landscape, with the Central Meseta located at high altitudes (660 m), surrounded by several ranges such as the Sierra Nevada and the Cantabrian Mountains. The Galician-Cantabrian and Pyrenean mountain systems shelter the rest of the peninsula from the Atlantic Ocean and the continental cold air masses coming from the northwest and the northeast, respectively. The combination of all these factors gives the Iberian Peninsula exceptional environmental conditions in relation to the rest of the Eurasian continent.

The Late Pleistocene glacial regime and subsequent glacial-interglacial fluctuations had an impact both on flora and fauna, including human populations throughout western Europe and the Iberian Peninsula (Uriarte, 2003; Arrizabalaga, 2004). In southern Europe, Marine Isotope Stage 3 (MIS 3; ca. 60-30 ka) was characterized by climate dynamics that alternated between warming and cooling periods of sea-surface temperatures, which on the continent generated phases of forest development and expansion of semi-arid areas, respectively (Fletcher et al., 2010; Harrison and Sanchez Goñi, 2010). However, pollen and small-mammal studies have shown that these alternations are not easily resolved in regional and local contexts, suggesting Iberia never underwent a complete loss of woodland, even during stadials or Heinrich events (Fletcher et al., 2010; López-García et al., 2014). Small-vertebrate assemblages, which reflect local environments, reveal cooler and wetter conditions for MIS 3. It has been observed that the coexistence of cold- and temperate-adapted species varies with the stadial and interstadial fluctuations, although woodland-dwelling species are always frequent (Fernández-García et al., 2016; López-García et al., 2014). The Iberian Peninsula served as a macro-refugium for many floral and faunal species during stadial periods, sheltering them from harsher climatic episodes until more favourable environmental conditions allowed their geographic radiation (Hewitt, 2000; Sommer and Nadachowski, 2006).

To obtain a more complete view of past changes in climate and biodiversity, it is necessary to expand the continental palaeoenvironmental record, which is underrepresented compared to marine and ice-core records. Geochemical approaches based on stable oxygen isotope compositions (δ¹⁸O) represent a promising tool for obtaining high-resolution environmental records from regional and local contexts. Isotope compositions measured from mammalian bone and tooth fossils permit the quantification of climatic parameters, due to the interdependence of climatic variables such as air temperatures, the oxygen isotope composition of meteoric waters (δ¹⁸O_mw) and the oxygen isotope composition of body tissues (Dansgaard, 1964; Kolodny et al., 1983; Longinelli, 1984; Longinelli and Nuti, 1973; Luz et al., 1984; Rozanski et al., 1993). These isotopic data can be combined with palaeoecological data inferred from vertebrate assemblages in order to obtain a coherent and refined picture of continental palaeoenvironments (e.g. Freudenthal et al., 2014; Royer et al., 2013b, 2014). Advances in chemical techniques and analytical methodologies now offer an opportunity to perform geochemical analyses on small-mammal teeth, which are particularly relevant in documenting high-resolution climatic changes at a regional or a local scale (e.g. Barham et al., 2017; Freudenthal et al., 2014; García-Alix, 2015; Grimes et al., 2003; Héran et al., 2010; Jeffrey et al., 2015; Leichliter et al., 2017; Lindars et al., 2001; Navarro et al., 2004).

Unfortunately, interpreting the oxygen isotope composition of small-mammal skeletal apatite is not straightforward, and several questions concerning its application to the fossil record have recently been raised (Barham et al., 2017; Jeffrey et al., 2015; Peneycad et al., 2019; Royer et al., 2014, 2013a, 2013b). The question of prey-predator interactions is considered to be of paramount importance in deciphering the isotopic signal recorded in accumulated fossil rodent remains, especially for regions with a specific climatic mode such as the Iberian Peninsula. The main purpose of this work is thus to develop an isotopic framework for environmental and climatic reconstructions for the Iberian Peninsula, based on the oxygen isotope composition of phosphate (δ¹⁸O_p) from present-day rodent teeth recovered from pellets. A first step aims to assess the specific role of the climatic context of the Iberian Peninsula, whilst a second one seeks to achieve a better understanding of δ¹⁸O from small-mammal teeth, focusing on the origin of the remains and their period of production. A refinement of the interpretation of the δ¹⁸O_p of fossil rodent tooth phosphate in terms of palaeotemperature reconstructions is finally applied to Cova dels Xaragalls assemblage. This Iberian Late Pleistocene sedimentary sequence, rich in small-vertebrate remains, thus offers the opportunity to reconstruct climate parameters in a continental environment contemporaneous with MIS 3.