Iberia, Land of Glaciers: How The Mountains Were Shaped By Glaciers

Iberia, Land of Glaciers: How The Mountains Were Shaped By Glaciers discusses the impact of past glaciers in the current landscape of Iberia. Currently, there are only small glaciers in the highest peaks of the Pyrenees that are the legacy of the last cold period that ended at the end of the 19th century: The Little Ice Age. However, an accurate observation of the landscape of the highest peaks and adjacent valleys of the Iberian Peninsula reveals a past shaped by the successive passage of glaciers with hundreds of meters of ice, similar to what happens today in the Alps or Patagonia.

Iberian glaciation has resulted in ice expansion through valleys that are now used by the road network and where important populations settle; in addition, large accumulations of sediments deposited by those glaciers are still unstable today and can trigger risks for mountain populations. Iberia, Land of Glaciers presents the impact of the glaciers in the landscape of mountains following a more educational perspective with examples of 21 Iberian massifs written by specialists from each of the areas.

• Assesses present-day Iberian Peninsula landscape trends by understanding the past behavior of glaciers
• Includes the latest findings of all the major Iberian mountains in a single book
• Includes quality, color figures to enhance understanding of glacier formations
• Provides a more educational and pedagogical perspective on glacial processes to reach an audience beyond academia

• Language: English
• Publisher: Elsevier Science
• Release date: Sep 22, 2021
• ISBN: 9780128219690

Book preview

Iberia, Land of Glaciers

How The Mountains Were Shaped By Glaciers

Editors

Marc Oliva

David Palacios

José M. Fernández-Fernández

Table of Contents

Cover image

Title page

Copyright

Contributors

Preface

Introduction

Chapter 1. The impact of the Quaternary ice ages on the landscape

Chapter 2. Quaternary ice ages in the Iberian Peninsula

1. Introduction

2. The Quaternary glacial cycles

3. Glacial–interglacial cycles in the context of the Iberian Peninsula

4. Rapid climatic variability of the last glacial period

5. The last deglaciation

6. The Holocene

7. Conclusions and reflections

Chapter 3. The glacial landscapes of the Iberian Peninsula within the Mediterranean region

1. The Mediterranean mountains

2. Quaternary glaciation in the Mediterranean mountains

3. Recent glaciers in the Mediterranean mountains

4. Iberia—a special place in the Mediterranean mountains

Chapter 4. The Iberian Peninsula: from palaeoglaciers to the current glaciers

Chapter 4.1. The glaciers of the Southeastern Pyrenees

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The La Màniga-La Feixa complex and key glacial landforms to understand the glacial evolution in the Southeastern Pyrenees

6. The significance of the glacial landforms of the Southeastern Pyrenees in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.2. The glaciers of the Central-Eastern Pyrenees

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The Unarre glacier complex

6. The significance of the glacial landforms of the Central-Eastern Pyrenees in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.3. The glaciers of the Central-Western Pyrenees

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The glacial valleys of Cinca and Alto Esera

6. The significance of the glacial landforms of the Central-Western Pyrenees in the context of glacial processes in the Iberian Peninsula

Chapter 4.4. The glaciers of the eastern massifs of Cantabria, the Burgos Mountains and the Basque Country

1. The glacial landscapes of the Eastern Cantabrian Mountains

2. The discovery and study of glacial landforms

3. The distribution of glacial landforms in the Eastern Cantabrian Mountains

4. Chronology of glacial landforms in the Eastern Cantabrian Mountains

5. The Trueba Valley: an example of the low-altitude Quaternary glacial development in the Cantabrian Mountains

6. The significance of the glacial landforms in the Pas and Basque Mountains in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.5. The glaciers of the Montaña Palentina

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The Carrión glacial valley: the largest glacier ice field tongue in Carrión-Peña Prieta Massif

6. The significance of the glacial landforms of the Montaña Palentina in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.6. The glaciers of the western massifs of Cantabria

1. The geographical framework

2. The discovery and study of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The glacial valley of Brañavieja, Alto Campoo

6. The significance of glacial landforms in the high valleys of the Ebro, Saja, and Nansa basins in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.7. The glaciers in the Redes Natural Park

1. The geographical framework

2. The discovery of the glacial landforms

3. The distribution of the glacial landforms

4. The chronology of the glacial landforms

5. The Monasterio River Valley: a geomorphological and geochronological reference point

6. The significance of the glacial landforms within the Redes Natural Park in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.8. The glaciers of the Picos de Europa

1. The geographical framework

2. The discovery of the glacial landforms

3. The distribution of the glacial landforms

4. The chronology of the glacial landforms

5. The environment of the Lagos de Covadonga, a representative enclave of past glaciations in the Picos de Europa

6. The significance of the glacial landforms of the Picos de Europa in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.9. The glaciers of the Central-Western Asturian Mountains

1. The geographical framework

2. The discovery and study of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The Puerto de Ventana, a representative site in the central-western sector of the Asturian Mountains

6. The significance of the glacial landforms of the central-western sector of Asturias in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.10. The glaciers of the Leonese Cantabrian Mountains

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The Sil glacier complex: ice field, transfluent glaciers, and alpine glaciers

6. The significance of the glacial landforms of the Leonese Cantabrian Mountains in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.11. The glaciers of the Montes de León

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The glacial morphology of the Vizcodillo Massif

6. The significance of the glacial landforms of the Montes de León in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.12. The glaciers around Lake Sanabria

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. Lake Sanabria: a geomorphological and geochronological reference point

6. The significance of the glacial landforms around Lake Sanabria in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.13. The glaciers in Western Galicia

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The Serra do Xistral, an exceptional example of glacial dynamics in low-altitude mountains

6. The significance of the glacial landforms of Western Galicia in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.14. The glaciers in Eastern Galicia

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The uniqueness of the ice fields of the Eastern Galician Mountains

6. The significance of the glacial landforms of the Eastern Galician Mountains in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.15. The glaciers of the Peneda, Amarela, and Gerês-Xurés massifs

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. Structural context and glaciers in the Serra do Xurés

6. The significance of the glacial landforms of the Peneda, Amarela and Gerês-Xurés massifs in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.16. The glaciers of Serra da Estrela

1. Geographical setting

2. History of the research on glacial landforms

3. Distribution of the glacial landforms

4. Chronology of the glacial landforms

5. The upper catchment of the Zêzere glacial valley

6. The significance of the glacial landforms of the Serra da Estrela in the context of the Iberian Peninsula

Chapter 4.17. The glaciers of the Iberian Range

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The glacial valley of Laguna Negra in the Sierra de Urbión, the most representative glacial valleys of this mountain range

6. The significance of the glacial landforms of the Iberian Range in the context of the glacial development of the Iberian Peninsula

Chapter 4.18. The glaciers of the Sierra de Gredos

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The Cuerpo de Hombre valley, the most representative of the glacial evolution in the Sierra de Gredos

6. The significance of the glacial landforms of the Sierra de Gredos in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.19. The glaciers of the Sierras de Guadarrama and Somosierra

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glacial landforms

4. The chronology of glacial landforms

5. The Peñalara Glacier cirques, the most representative of the Sierras de Guadarrama and Somosierra

6. The significance of the glacial landforms of Guadarrama and Somosierra in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.20. The glaciers of the Sierra Nevada

1. The geographical framework

2. The discovery of glacial landforms

3. The distribution of glaciers and their associated landforms

4. The chronology of glacial landforms

5. The Poqueira glacial system

6. The significance of the glacial landforms of the Sierra Nevada in the context of the climatic evolution of the Iberian Peninsula

Chapter 4.21. The existing glaciers of the Iberian Peninsula: the Central Pyrenees

1. The geographical framework of the Pyrenean glaciers

2. The discovery and study of the Pyrenean glaciers

3. The Pyrenean glaciers

4. Chronology and retreat of the Pyrenean glaciers

5. Evolution and regression in the historical period: the case of the Monte Perdido Glacier

6. The significance Pyrenean glaciers in the context of climate change

Chapter 5. Iberia: land of the ancient glaciers

1. The singularity of the Iberian Peninsula in the context of the world's recent glacial evolution

2. The uniqueness of the Iberian glacial landscapes

3. The legacy of previous glaciations to the last glacial cycle in Iberia

4. The maximum extent of glaciers in Iberia during the last glacial cycle

5. The Iberian evidence of glacial oscillations during Termination I

6. Glacial activity during the Holocene in Iberian mountains

7. The Little Ice Age in Iberia: the gateway to the current glacial retreat

8. Pyrenean glaciers, the last Iberian ice

9. The anthropogenic impact on glacial morphologies

List of abbreviations

Index

Copyright

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Contributors

Nuria Andrés,     Department of Geography, Universidad Complutense de Madrid, Madrid, Spain

Isabel Cacho,     GRC Geociències Marines, Dept. de Dinàmica de la Terra i de l'Oceà, Facultat de Ciències de la Terra, University of Barcelona, Barcelona, Spain

David Gallinar Cañedo,     Facultad Padre Ossó, University of Oviedo, Oviedo, Spain

Rosa M. Carrasco,     Department of Geological and Mining Engineering, University of Castilla-La Mancha, Toledo, Spain

Alipio García-de Celis,     Department of Geography, University of Valladolid, Valladolid, Spain

María José Domínguez-Cuesta,     Department of Geology, University of Oviedo, Oviedo, Spain

José M. Fernández-Fernández,     Centre of Geographical Studies, IGOT, University of Lisbon, Lisboa, Portugal

Cristina García-Hernández,     Departmento de Geografía, Universidad de Oviedo, Oviedo, Spain

José M. García-Ruiz,     Instituto Pirenaico de Ecología, Consejo Superior de Investigaciones Científicas (IPE-CSIC), Zaragoza, Spain

Manuel Gómez-Lende,     GIR Pangea, University of Valladolid, Valladolid, Spain

Antonio Gómez-Ortiz,     Department of Geography, University of Barcelona, Barcelona, Spain

Amelia Gómez-Villar,     Department of Geography and Geology, Universidad de León, León, Spain

María José González-Amuchastegui,     Department of Geography, UNED, Madrid, Spain

Benjamín González-Díaz,     Departmento de Geografía, Universidad de Oviedo, Oviedo, Spain

Rosa Blanca González-Gutiérrez,     Department of Geography and Geology, Universidad de León, León, Spain

Saúl González-Lemos,     ASCIEM Consulting S.L.P., Avilés, Spain

Philip D. Hughes,     Department of Geography, School of Environment, Education & Development, The University of Manchester, Manchester, United Kingdom

Montserrat Jiménez-Sánchez,     Department of Geology, University of Oviedo, Oviedo, Spain

Marc Oliva,     Departament of Geography, University of Barcelona, Barcelona, Spain

David Palacios,     Departament of Geography, Universidad Complutense de Madrid, Madrid, Spain

Javier Pedraza,     Department of Geodynamics, Stratigraphy and Paleontology, Universidad Complutense de Madrid, Madrid, Spain

Ramón Pellitero,     Department of Geography, National Distance Education University (UNED), Madrid, Spain

Augusto Pérez-Alberti,     CRETUS Institute, Departamento de Edafoloxía e Química Agrícola, Facultade de Bioloxía, Universidade de Santiago de Compostela (USC), Santiago de Compostela, Spain

Alfonso Pisabarro,     Department of Geography and Geology, University of León, León, Spain

José María Redondo-Vega,     Department of Geography and Geology, Universidad de León, León, Spain

Laura Rodríguez-Rodríguez,     Department of Earth Sciences and Condensed Matter Physics, University of Santander, Santander, Spain

Jesús Ruiz-Fernández,     Departmento de Geografía, Universidad de Oviedo, Oviedo, Spain

Ferran Salvador-Franch,     Department of Geography, University of Barcelona, Barcelona, Spain

Javier Santos-González,     Department of Geography and Geology, Universidad de León, León, Spain

Enrique Serrano,     Department of Geography, University of Valladolid, Valladolid, Spain

Valentí Turu

Marcel Chevalier Foundation, Andorra la Vella, Principat d'Andorra

Department of Earth and Ocean Dynamics, University of Barcelona, Barcelona, Spain

Department of Geological and Mining Engineering, University of Castilla–La Mancha, Toledo, Spain

Marcos Valcarcel,     Department of Geography, University de Santiago de Compostela (USC), Santiago de Compostela, Galicia, Spain

Josep Ventura,     ANTALP (Antarctic, Arctic and Alpine Environments), Department of Geography, University of Barcelona, Barcelona, Spain

Gonçalo Vieira,     Centre of Geographical Studies, IGOT, University of Lisbon, Lisboa, Portugal

Barbara Woronko,     Faculty of Geology, University of Warsaw, Warsaw, Poland

Preface

Standing between two seas and two continents, the Iberian Peninsula is home to high mountains, both in the periphery and at the center, with a mixed orography characteristic of its Mediterranean location. The mountains run along almost all of the peninsula's boundaries as well as the high inland areas (the Meseta), which share many of the distinctive features of mountain environments, and high peaks are found only a few tens of kilometers from the coast. The complexity of the relief results in a diversity of climates and a wide variety of landscapes, particularly in the mountains, where high vertical gradients accentuate contrasts and create unique ecosystems.

The mountains in Iberia have long been part of the idiosyncrasy of its inhabitants thanks to the mountain–interior–coast triad; the mountains provided water, pastures, medicinal plants, wood, etc., while the plains offered the basis of the Mediterranean diet—bread, wine, and oil. Trade and fishing have flourished along its coasts, which are calmer on the Mediterranean side and wilder and more rugged on the Atlantic side. Therefore, one cannot comprehend the uniqueness of the current landscape of the entire Iberian Peninsula without understanding the role that the mountainous areas have played in its evolution.

As in other Alpine regions, the perception of high peaks has varied over time. Their caves and shelters were the refuges of Neanderthal men during the cold Quaternary phases, from where they hunted and gathered wild fruits on the ice-free slopes. The high peaks and valley bottoms were occupied by glaciers. With the deglaciation and progressive human occupation of the highest lands, particularly during the Neolithic in the Mediterranean (7–5   ka), the mountains began to be domesticated; mining and, above all, transhumance, facilitated a gradual colonization of the Iberian mountains by incipient human communities settled in its vicinity. The fear of the dangers posed by the mountains gradually disappeared as knowledge of their landscapes and the resources they offered increased, particularly since the Age of Enlightenment, which encouraged the study of alpine environments from a rational and academic perspective. Gradually, respect for the mountains turned into admiration for their landscapes, as seen in the first illustrated attempts to explain the reasons for their attractiveness and the boom in hiking during the 19th century. Since then, and at a very accelerated pace during the last century, society has surrendered to the romanticism of its landscapes, sacralizing the high mountains and the harmony of its inhabitants' lifestyles.

The current landscape of the Iberian mountains is, fundamentally, a consequence of the intense reshaping of the Quaternary ice carved on structures sculpted by tectonics over millions of years. The book that you have in your hands aims, therefore, to bring you closer to the times when Iberian mountains were stained with perpetual white. Referring to all the mountain areas that were flooded by ice in the Last Ice Age, this book brings together the current state of the glacial knowledge pertaining to our mountains as well as the climatic conditions that favored the development and disappearance of glaciers. Experts in the glacial geomorphologies of some 20 mountain regions drew up detailed descriptions of the impacts of the glaciers on their landscapes, from the glaciations that occurred more than 100,000 years ago to the last glacial strongholds that persist today in the Pyrenees as a legacy of those Quaternary cold phases.

From the viewpoint of glacial geomorphology, the Iberian Peninsula is one of the best studied areas of the planet. For this reason, the editors hope that this book, along with its conceptual analyses and case studies, will constitute a reference manual for Iberian researchers and those from other regions affected, in the past and present, by glacial processes. The results presented here should be a starting point inspiring a new generation of researchers to address the gaps that still exist in our knowledge of the glacial paleoenvironments of the Iberian mountains as well as the deglaciation process that continues to this day.

It is to be expected that, within a few decades, the current glacial trend will see the end of the last vestiges of glacial ice that are preserved in the highest mountains of the peninsula and bear witness to a recent past in which most of these mountains were extensively covered with ice. The accelerated retreat of the remaining mountain glaciers underscores the fragilities of their ecosystems and provides proof of the need to preserve their landscapes for generations to come.

Marc Oliva

David Palacios

José M. Fernández-Fernández

Introduction

Scientific knowledge of glacial landscapes in the Iberian mountains has advanced significantly over the last few decades. Since the publication of the book Las Huellas Glaciares de las Montañas Españolas in 1998, coordinated by professors Antonio Gómez Ortiz and Augusto Pérez-Alberti (Editorial Universidade de Santiago de Compostela), the scientific vision of the glacial geomorphology of the high peaks of the Iberian Peninsula has changed substantially. Over more than two   decades, new scientific advances have made possible a detailed spatial characterization of the distribution of glacial landforms on Earth's surface and of their paleoenvironmental significance. Moreover, notable progress has been made with regard to the chronology of the formation of glacial landforms with new (and more refined) techniques of absolute dating.

Like few other regions of the planet, all these advances have been translated into an exponential increase in our knowledge of the role of the Quaternary glaciers in shaping the Iberian mountains. With the aim of updating the vast amount of information generated during the last few decades, this book seeks to synthesize the most recent advances on glacial and climatic evolution in these mountains over hundreds of thousands of years to the present day.

The book includes three initial chapters that deal with general aspects. First, the editors present the features that characterize the impacts of glacial development in the Iberian mountains, with an introduction to the climatic dynamics and glacial response that occurred during the Quaternary on a global scale, as well as certain concepts and chronologies that will be used in the subsequent chapters. Later, paleooceanographer Isabel Cacho contextualizes the climatic variability registered during the Late Quaternary, which was responsible for the glacial oscillations in the Iberian mountains, from the study and characterization of the marine sediments that surround the peninsula. Next, Phil Hughes frames the spatial and temporal patterns of the glacial phases that occurred in Iberia against the events in the other massifs of the Mediterranean alpine fringe.

The second part of the book comprises 21 chapters. The glacial footprints in each of the mountain areas of the Iberian Peninsula are examined by the foremost researchers in those regions. Each chapter presents the geography of the glacial process and the gradual discovery of its glacial landforms, as well as a detailed explanation of its distribution and chronology. Finally, each chapter ends with a representative example of the relevant area and highlights its uniqueness in the context of Iberian glacial development.

The book concludes with a section where the evidence described for each of the areas examined in detail in the previous chapters is integrated with its relevance for Iberia as a whole, allowing us to understand the temporal and spatial frameworks of the different climatic phases that conditioned the development and retreat of the ice masses. Thus, the climatic sensitivity of the Iberian Peninsula, namely the small variations in its temperature and rainfall regimes, which determined the advances or retreats of the ice during the Quaternary, is highlighted, although these glacial oscillations were not synchronous and also responded to the orographic complexity of the peninsula. All these aspects demonstrate the uniqueness of glacial landscapes and their need for protection in a land, Iberia, whose mountains were shaped by the ice that almost no longer exists.

Chapter 1: The impact of the Quaternary ice ages on the landscape

David Palacios ¹ , Marc Oliva ² , and José M. Fernández-Fernández ³       ¹ Departament of Geography, Universidad Complutense de Madrid, Madrid, Spain      ² Departament of Geography, University of Barcelona, Barcelona, Spain      ³Centre of Geographical Studies, IGOT, University of Lisbon, Lisboa, Portugal

Abstract

The existence of ice ages has been extensively studied over the last two centuries focusing on the chronology and spatial extent of glaciers extending across northern Europe and the highest ranges, including the Iberian mountains. The alternation of long glacial cycles (i.e., glaciations) and short interglacial periods, together with the existence of human beings, characterizes the Quaternary (the last 2.6   ka). Glacial cycles and interglacials, with average duration of 100 and 10–15   ka, respectively, have alternated during the last 700   ka. The origin of these glacial cycles was directly related to the cyclical changes of seasonal and latitudinal insolation derived from Earth's orbital changes. The Last Glacial Cycle (115–11.7   ka) was a period of large temperature variability, with shifts (8–12°C) that allowed glaciers to reach their maximum extent during or slightly prior to the Last Glacial Maximum (26–19.5   ka). From 19   ka, glaciers retreated dramatically until the beginning of the last interglacial (the Holocene; since 11.7   ka). During the deglaciation, the warm periods (Bølling–Allerød interstadial; 14.6–12.9   ka) were interrupted by cold stages, when glaciers either readvanced or stopped retreating, i.e., the Oldest Dryas (17.5–14.6   ka) and the Younger Dryas (12.9–11.7   ka) in the Northern Hemisphere. Conversely, temperatures were higher again during the Holocene, and especially during the Holocene Thermal Maximum (c. 9–5   ka), when they were higher than the present-day average. After that, the climate cooled again during the last millennia until the Little Ice Age (1300–1850 AD), the coldest period during the Late Holocene, just before the last temperature rise of the 20th century.

Keywords

Glaciations; Holocene; Interglacials; Last glacial cycle; Quaternary

Glaciers in modern-day Iberia occupy a tiny proportion of the highest peaks in the Pyrenees. Nonetheless, this has not been the case for the last few 100,000 years. The current warm period is one of the shortest phases along the entire Quaternary (the last 2.6 million years) that has alternated with much longer periods of time during which glaciers occupied a vast proportion of the continents in Iberia and the entire planet.

The Quaternary is divided in the Pleistocene (2.6 million years–11.7   ka), which comprises most of the era, and the Holocene, which is actually the last warm period (comprising 11.7   ka to the present). Progress in the natural sciences during the 18th century helped us realize that Earth was not stable and that it exhibited great dynamism in the distribution of lands and seas as well as its climatic conditions. This recognition is more formally called the Huttonian Theory in honor of James Hutton, one of its main proponents (Hutton, 1795). It was observed that the current relief was the result of a long process of changes, wherein many landforms were inherited from the past. Specifically, geomorphology, the science that sheds light on the origin of reliefs, is based on observations of current landforms and sediments, the aim being to deduce the processes, origins, and climate that led to the creation of the relief landforms inherited from past times. Thus, the first glacial geomorphologists observed the deposits and erosive landforms left in front of the current glaciers in the Alps and the Scandinavian mountains and deduced that these glaciers must have occupied much larger extensions in the Alps, the British Isles, central Europe, and Scandinavia (see Woodward (2014) for details of the discovery of the Ice Ages).

After a major advance in the 17th century, the glacial fronts of the Alps started a long retreat during the 18th and 19th centuries. It was the end of the so-called Little Ice Age (LIA), which lasted from 1300 to 1850 AD, one of the coldest periods of the Holocene. The first glaciologists observed how these glacial fronts left very heterometric and scattered deposits, called till, which formed crests, called moraines. Sometimes they also left huge standalone blocks, called erratic blocks. Finally, they observed that these glaciers, by transporting these large blocks as well as fine sand, exerted a peculiar erosion on the rock at their base, polishing the surfaces and scratching striations and channels. Scientists such as Venetz (1821) in the Alps or Esmark (1824) in Norway realized that these same landforms were now many kilometers away from today's glacial fronts. On the other hand, the existence of large glaciers was related to changes in sea level, especially in the Scandinavian Peninsula and the British Isles, where the loss of weight due to the melting of a large mass of ice must have caused its recent rise (Jamieson, 1865; Lyell, 1835). From these and similar observations in the Alps and Scotland, Agassiz (1840) proposed his theory of the Ice Ages periods during which ice occupied a large part of the mountains as well as northern and central Europe. The theory became established in the second half of the 19th century and observations multiplied. By the end of the 19th century and the beginning of the 20th century, the extents of the glacial reliefs were fairly well known, and they had even been delimited quite precisely on maps of Europe (Geikie, 1894) and North America (Wright, 1889). Soon, it was observed that not all landforms of these glacial landscape belonged to the same period; some showed great differences in their ages. Albrecht Penck and Eduard Brückner observed glacial landforms in the valleys of the Alps that they considered to belong to four consecutive glaciations, to which they gave the names of the rivers in which the respective landforms of each glaciation were found (Gunz, Mindel, Riss, and Würm, in order from more to less ancient) (Penck and Brückner, 1901–1909). Later, Penck visited several Iberian mountains and ascribed many of their glacial landforms to his polyglacialist theory. The start of the glacial geomorphology studies in Iberia can be attributed to his influence and the recent discoveries in the Alps. Thus, since the beginning of the 20th century, the first glacial geomorphologists were well aware of the real extension of the glacial features in Iberia from the delimitation of the moraines and other glacial landforms.

Today, our knowledge of the number and extent of the Ice Ages on our planet throughout the Quaternary has improved substantially, especially through the study of marine sediments (see Chapter 2). In addition, glacial sediments have been observed interspersed among significantly old sediments. It is now known with certainty that Earth went through at least four major ice-house periods before the Quaternary (2400–2100, 720–630, 460–420, and 360–260 million years ago). After >250 million years without extensive glaciers on Earth, the climatic conditions started trending toward cooling. The Antarctic continent has been glaciated since 34   Ma at that led the planet into the current ice-house period. Glaciers began to appear again in different places on Earth, as the Arctic Ocean became isolated between the continents and large mountain ranges started to rise. It was with the arrival of Greenland permanent glaciation and the onset of the Quaternary (2.6   Ma) that a cyclic series of intense cold periods dominated Earth’s climate and the development of large glacial extensions began. These periods are called glacial cycles or glaciations and were interrupted by short warm periods referred to as interglacials. The study of marine sediments has made possible to delimit more than 50 Quaternary glacial cycles. Seasonal and latitudinal distribution of insolation depends on the evolution of Earth's orbital movements (see Chapter 2), which are cyclical at the timescale of tens of thousands of years and were the trigger of the glacial ages. These orbital cycles have been constant along the Quaternary, but the climate record has revealed that the cyclicity of the glacial–interglacial cycles has changed along the Quaternary. In the early part of the Quaternary, the glacial cycles coincided with orbital cycles spanning 41 ka, when a few consecutive cold periods allowed the formation of extensive glaciers in the mountains and the polar regions. After the Mid-Pleistocene Transition, from c. 800   ka, the glacial cycles become longer following a dominant cyclicity of 100   kyr, while ice sheets become thicker. In other words, long glacial cycles of c. 100 ka occurred for the past 700   ka periods, alternating with interglacials of c. 10–15   ka. Within these glacial cycles occurred a rapid succession of cold (stadial) and warm (interstadial) periods oscillating at millennial timescale, the extreme cold conditions during the stadials caused glaciers to expand successively in the mountain and polar regions as the glacier cycle progresses. At high latitudes, the growth was such that the glaciers extended into the temperate areas, especially on the northern continents, until they reached a maximum critical size. Thus, glaciers fixed large amounts of water in the form of ice, causing the volume of water stored in the seas to drop (i.e., the seas reached their lowest levels). An interglacial period began when the glacial cycle ended abruptly. In these past 700   ka, five glacial cycles have followed one another until the Holocene, the last interglacial. The disposition in large high-latitude areas (50–65°N) of North America and Europe in the temperate zones of the Northern Hemisphere and the decrease in the solar radiation along these high latitudes allowed the formation of enormous ice sheets on these continents during the glacial periods (Fig. 1.1), unlike the case of the Southern Hemisphere, where the oceans predominated in the temperate zones. For this reason, the last glacial cycles were marked by a great glacial imbalance between the two hemispheres (for more information on the Ice Ages, see Ehlers et al., 2018).

During glacial cycles, snow and ice covered the large continents of the Northern Hemisphere, causing an albedo effect on solar radiation that helped preserve lower temperatures and enhanced the cycle. The sea ice also expanded. The polar anticyclones were reinforced and the great extension of the cold polar air masses forced the polar front to migrate toward the south; even the Intertropical Convergence Zone migrated toward the south, weakening or even annulling the Asian monsoon, while it reinforced the monsoons of the Southern Hemisphere (Novello et al., 2017), a notable feature given the glacial asymmetry between the continents. The intensity of the large marine currents (surface, deep, and vertical ocean movements) that exchanged heat and salinity throughout the Atlantic, the Atlantic Meridional Overturning Circulation (AMOC), was drastically reduced, causing the storage of large masses of carbon dioxide (CO2) in the Southern Ocean, thus reducing its greenhouse effect on the atmosphere (Böhm et al., 2015). Again, the glacial cycle fed back on itself. Further cooling, evaporation, and therefore precipitation, decreased dramatically. Vegetation was very limited; arid regions increased considerably and, with them, atmospheric dust. Sea levels fell by tenths, even hundreds of meters, as water accumulated mainly on the ice sheets that covered the continents of the Northern Hemisphere (Figs. 1.2 and 1.3). The drop in sea level meant that the distribution between land and sea also changed completely with the appearance of land that was previously flooded by the sea. Therefore, the landscape was not only altered by the presence of glaciers during the glacial cycles, but it also changed globally. Animals and humans had to adapt to these circumstances.

Figure 1.1 Number of times that the ice sheets have covered the European continent over the last 950   ka (elaborated using data from Batchelor et al., 2019). The map is based on the ice sheet extents (best estimates) from MIS 2, 4, 6, 8, 10, 12, 16, and 20–24. 

Figure adapted from Batchelor, C.L., Margold, M., Krapp, M., Murton, D.K., Dalton, A.S., Gibbard, P.L., Stokes, C.R., Murton, J.B., Manica, A., 2019. The configuration of Northern hemisphere ice sheets through the Quaternary. Nat. Commun. 10, 1–10. https://doi.org/10.1038/s41467-019-11601-2 and Palacios, D., Hughes, P. García-Ruiz, J.M., Andrés, N. (Eds), 2021. European Glacial Landscapes. Maximum Extent of Glaciations. Elsevier.

Figure 1.2 Estimated sea level evolution relative to the present-day conditions (normalized to 0), based on the accumulated ice volume on North Hemisphere ice sheets over the last 850 ka according to different models. Extent (gray) and cumulative (black) data (synthetized from Spratt and Lisiecki, 2016). The black dot refers to the present sea-level equivalent estimated by Batchelor et al. (2019). Dark gray (light gray) bands highlight cold (warm) periods in the Northern Hemisphere. 

Figure adapted from Batchelor, C.L., Margold, M., Krapp, M., Murton, D.K., Dalton, A.S., Gibbard, P.L., Stokes, C.R., Murton, J.B., Manica, A., 2019. The configuration of the Northern Hemisphere ice sheets through the Quaternary. Nat. Commun. 10, 1–10. https://doi.org/10.1038/s41467-019-11601-2.

Figure 1.3 Reconstruction of the temperature anomaly evolution on Earth compared to the present-day conditions (normalized to 0), based on the data obtained from the EPICA Dome C ice core (Antarctica) for the last 800 ka. Dark gray (light gray) bands highlight cold (warm) periods in the Northern Hemisphere. 

Modified from Masson-Delmotte, V., Stenni, B., Pol, K., Braconnot, P., Cattani, O., Falourd, S., Kageyama, M., Jouzel, J., Landais, A., Minster, B., Barnola, J.M., Chappellaz, J., Krinner, G., Johnsen, S., Röthlisberger, R., Hansen, J., Mikolajewicz, U., Otto-Bliesner, B., 2010. EPICA Dome C record of glacial and interglacial intensities. Quat. Sci. Rev. 29, 113–128. https://doi.org/10.1016/j.quascirev.2009.09.030.

The origins of these glacial cycles were directly related to the different levels of insolation received by Earth's surface over time due to the orbital changes of the planet around the Sun (see Chapter 2). However, one of the great mysteries is why the glacial cycles ended so abruptly (i.e., 100–115   ka) after they began. The glacial cycle always ended with an increase in solar radiation in summer for orbital reasons, especially for latitudes above 65°N, precisely the average latitude of the great northern ice sheets during the glacial cycles (Denton et al., 2010). However, this increase also occurred on other occasions throughout glacial cycles without causing their termination. In fact, the increase in solar radiation only had a key effect at the end of the glacial cycle, when it coincided with the maximum critical growth of the Northern Hemisphere's ice sheets (Denton et al., 2010). Cheng et al. (2016), for example, proposed that five cycles of Earth's precession were necessary for the northern ice sheets to reach their maximum critical size. The conditions must have been extreme, given the maximum sea ice and minimum CO2 in the atmosphere (Shakun et al., 2012) leading to minimum precipitation and minimum vegetation cover. Ellis and Palmer (2016) proposed that the amount of dust in the atmosphere at these times must have been high enough to reduce the albedo drastically, a key factor in the end of glacial cycles. In any case, the maximum critical growth of the ice sheets translated to the maximum isostatic depression over the continents and, therefore, most of the ice sheets became marine-based at this time. Under these conditions, an increase in solar radiation drastically reduced the areas of glacial accumulation and facilitated, with a slight increase in sea level, the collapse of the large northern glacial masses (Denton et al., 2010).

Once the deglaciation of the northern ice sheets began, the whole process accelerated and the glacial cycle ended dramatically within a few thousand years. This event is called Glacial Termination, the end of the glacial cycle, which marks the stabilization of the climate on the planet and the beginning of the warm interglacial period. Denton et al. (2010) proposed a concatenation of subprocesses, which fed back and occasionally interrupted the overall process. This phenomenon occurred inversely within the two hemispheres (see Chapter 2), so that while one warmed up, the other cooled down, and vice versa, although the final result by the end of the termination was overall warming in both hemispheres.

All our knowledge about the succession of glacial cycles comes, as we have already explained (and detailed in Chapter 2), from the information gained from the large accumulations of sediments on ocean floors. Nonetheless, the advance of glaciers over mountain valleys and continents largely erases all previous traces. The last glaciation retained most of the current glacial landscapes, and glacial landforms from previous cycles remain only in some enclaves. From the viewpoint of the landscape, it is very important to define the Last Glacial Cycle (LGC), which ran from the end of the penultimate interglacial (Eemian) to the beginning of the present one (Holocene), namely from 115 to 11.7   ka (Fig. 1.4). This was a period of highly fluctuating temperatures, with variations of up to 8–12°C within a few decades at high latitudes, and stages of extreme cold alternating with warmer stages, with temperatures almost as high as those of the present day (Chapter 2). As in previous glacial cycles, the combined effect of decreased solar radiation in summer at high northern latitudes and lower atmospheric CO2 concentrations resulted in the Northern Hemisphere's ice sheets reaching their maximum critical extents and the sea being at its lowest level. These circumstances occurred around c. 25   ka. Clark et al. (2009) proposed a limit of between 26.5 and 19   ka to determine the period during which these extreme circumstances were maintained with some balance and called it the Global Last Glacial Maximum (GLGM). However, many mountain and continental glaciers reached their respective maximums before 26.5   ka or began their retreats before 19   ka (Hughes et al., 2016). For this reason, it is possible to differentiate when the glaciers reached their last maximum extensions in each region, a period known as the Local Last Glacial Maximum (LLGM).

The last Glacial Termination or Termination I is considered to have started and ended around 19 and 11.7   ka, respectively, the latter coinciding with the beginning of the Holocene (Fig. 1.4). The orbital changes that took place around 21   ka conditioned an increase in sunshine during the warm season of the year in the high latitudes of the Northern Hemisphere. Greenland's ice cores indicate a clear warming and increased evaporation since 19   ka (Lambeck et al., 2014). This increase in temperature led to the rapid retreat of glaciers worldwide around 18   ka (Clark et al., 2012). Glacier retreat caused a rise in the sea level, a reorganization of ocean and atmospheric circulation patterns on a global scale, a redefinition of the coastline, ecosystem changes, as well as variations in greenhouse gas concentrations (Denton et al., 2010). The millennial warming trend was cut off by climate fluctuations on a century-to-millennium scale during Termination I, which again favored the expansion of ice masses as a result of abrupt glaciological events that led to massive freshwater discharges in the North Atlantic, inducing large-scale changes in ocean circulation (Clark et al., 2012). This situation was prolonged with large fluctuations occurring between 17.5 and 14.6   ka. This period is called the Heinrich-1 Stadial or Oldest Dryas (OD) (Denton et al., 2006). Temperatures fell dramatically in winter, sea ice expanded, the Asian monsoon disappeared, and the Northern Hemisphere glaciers advanced, but the concentration of CO2 in the atmosphere continued to increase, probably because it was a warm period in the Southern Hemisphere and the oceans continued to release this gas into the atmosphere with an intense thawing process in Antarctica (Denton et al., 2010).

Figure 1.4 A 15 ka reconstruction of: (A) insolation in June at 60°N (data from Paillard, 2001); (B) CO2 concentration in the atmosphere (ppm   =   parts per million; data from Monnin, 2001); (C) temperature anomaly on Earth with respect to the current one (normalized to 0), based on the data obtained from the EPICA Dome C ice core (Antarctica; data from Jouzel et al., 2007); (D) evolution of Earth's temperature anomaly with respect to the current one (normalized to 0), based on the data obtained from the GRIP ice core (Greenland; data from Cuffey and Clow, 1997). Black double arrows highlight periods of inverse temperature change in the two hemispheres, but always with a constant increase in the concentration of CO2 in the atmosphere (dashed arrow). Dark gray (light gray) bands highlight cold (warm) periods in the Northern Hemisphere. 

Figure adapted from Severinghaus, J.P., 2009. Climate change: Southern see-saw seen. Nature 457, 1093–1094. https://doi.org/10.1038/4571093a.

From 14.6 to 12.9   ka, the climate in the Northern Hemisphere returned to a warm one, with a sharp warming of up to 9°C according to Greenland's ice records and c. 3–5°C in Western Europe (Clark et al., 2012). This was the Bølling–Allerød (B–A) interstadial. During this period, the AMOC intensified with a marked increase in atmospheric greenhouse gases and a strengthening of the Asian monsoon. This warming affected the entire North Atlantic region and led to a massive retreat of the high-latitude ice sheets (Clark et al., 2012) as well as the virtual disappearance of the glaciers in the mid-latitude mountains or their retreat to the higher cirques of the higher massifs. The period was only interrupted by a few cold events in the Northern Hemisphere, but ice cores in Antarctica showed widespread cooling during the B–A, exhibiting a thermal situation opposite to that in the Northern Hemisphere. Thus, this period is called the Antarctic Cold Reversal (ACR) in the Southern Hemisphere. In this way, the glaciers retreated drastically in the Northern Hemisphere, while they advanced in the Southern, although the exact latitude that separated these two behaviors is not well defined to date (Pedro et al., 2015).

Between 12.9 and 11.7   ka, the Northern Hemisphere underwent another intense cooling. This period is called the Younger Dryas (YD). Once again, the AMOC was reduced, the sea ice expanded, the Asian monsoon weakened, and the winter and spring temperatures fell sharply. Ice cores from Greenland suggest that the YD was c. 4.5   ±   2°C warmer than the OD, with temperatures in Western Europe being up to 5–10°C lower than those recorded during the B–A (Clark et al., 2012). Cooling in Europe was more pronounced in the north (4–5°C) than in the south (2–3°C) (Moreno et al., 2014). As a consequence of all these processes, the glaciers advanced in the Northern Hemisphere. As in the case of the OD, the cooling of the Northern Hemisphere during the YD was accompanied by a warming in Antarctica and a persistent increase of CO2 in the atmosphere. Glaciers retreated during the YD in many areas of the Southern Hemisphere.

After the YD, from 11.7   ka, temperatures increased dramatically across the globe, with values up to 10°C warmer in Greenland and c. 4°C higher in Western Europe (Clark et al., 2012). This was the beginning of the Holocene (Fig. 1.5). This increase in temperature led to the prevalence of cold geomorphological processes at higher altitudes, briefly interrupted at the beginning of the Holocene by short cold periods. The Holocene Thermal Maximum (HTM), which occurred between 9 and 5   ka, was a period of higher temperatures associated with the maximum summer radiation in the Northern Hemisphere (Renssen et al., 2018). Consequently, the glaciers that existed during YD progressively reduced in size, and many disappeared during the HTM. The last remaining glacier strongholds of the large Pleistocene ice sheets in North America (Stokes et al., 2016) and Fennoscandia (Stroeven et al., 2016) also disappeared. Climate variability was accentuated in the North Atlantic during the Middle and Late Holocene, with temperature oscillations of c. ±2°C (Mayewski et al., 2004). Climate variability during the Late Holocene included several warm and cold phases, but with a clear general trend toward cooling. Indeed, the LIA has been defined as one of the coldest stages in the Northern Hemisphere in the last 10,000 years.

Figure 1.5 Evolution of: (A) Earth's axial tilt obliquity; (B) Earth's temperature anomaly referred to the present-day one (normalized to 0), based on an integration of multiproxy data for the last 11.3   ka (synthetized from Marcott et al., 2013). The approximate duration of the main Holocene climatic periods is highlighted as dark gray (light gray) bands, referred to cold (warm) periods in the Northern Hemisphere.

However, the expansion of the glaciers during the LIA was much less than during the LGC, which, in turn, had a maximum size very similar to those in previous glacial cycles. Above all, the northern continental ice sheets reached almost the same critical maximum size during each of the cycles, a situation that, in turn, led to the beginning of the Terminations. For this reason, as has already been stated, the LGC glaciers erased most of the imprints on the landscape of the previous cycles. Glacial geomorphologists seek the remains of these possible tracks, for example, in the most remote and outer moraines of the glacial landscape. It is much easier to delimit the glacial landforms, especially the moraines, of the maximum advance of the LGC, which still exhibit clearly preserved morphologies and entities in the landscape. Geomorphologists investigate the ages of the corresponding LLGM moraines in each region and explore whether they are synchronous with the GLGM. Younger moraines that demonstrate great variations in glacial evolution during Termination I often appear behind these LLGM moraines. It is common to find more inner moraines that represent advances or stabilizations of the glacial fronts in many glacial regions during the OD and the YD or even the small advances in the Holocene (as, e.g., during the LIA).

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Chapter 2: Quaternary ice ages in the Iberian Peninsula

Isabel Cacho     GRC Geociències Marines, Dept. de Dinàmica de la Terra i de l'Oceà, Facultat de Ciències de la Terra, University of Barcelona, Barcelona, Spain

Abstract

This chapter reviews the evolution of the glacial–interglacial cycles of the Quaternary on a global scale and then focuses on the context of the Iberian Peninsula, mainly reviewing studies based on marine records and complemented by some terrestrial records, particularly those of lakes and speleothems. After briefly describing some of the most remarkable results from records covering several glacial–interglacial cycles, this chapter analyzes the climate variability of the Last Glacial Cycle, deglaciation, and the Holocene. The discussed results focus on analyzing the changes in the evolution of the marine temperature around the peninsula and its hydrologic availability. These results are complemented with other relevant variables to understand the functioning of the oceanography of the North Atlantic and the Mediterranean. This chapter tries to frame the changes described for the Iberian Peninsula with the climatic evolution on a global scale.

Keywords

Iberian Peninsula; Palaeoceanography; Palaeoclimatology; Quaternary

1. Introduction

The science of palaeoceanography attempts to unravel and understand the evolution of the ocean before man was able to take direct measurements of different ocean variables. The ocean and atmosphere are two key elements of the climate system, continuously interacting at different time scales; therefore, palaeoclimatology also includes studying the ocean of the past. This chapter provides an overview of the information about the Iberian Peninsula from marine records and contextualizes it with that offered by some palaeoclimate archives of the peninsula, focusing on lake and speleothem records (Fig. 2.1). The current climate change situation has increased the interest of the scientific community for better understanding the climate variability of our planet and the key processes that have played a role in past climate transitions. Such research attempts have greatly increased the volume and quality of available information on this subject. This chapter does not aim to provide an exhaustive review of all the existing work around Iberian Peninsula; rather, it presents a general overview of this research area, highlighting some records or aspects that have been considered to be of special interest. It must be said that many other works of excellent quality are not discussed here because of space limitations.

This chapter is structured chronologically, starting with a general description of the glacial and interglacial cycles of the Quaternary and ending with the evolution of the last few millennia. The time scales and the units used, therefore, vary throughout the chapter. Changes beyond the Quaternary are described in millions of years (Ma), while the discussion on the Quaternary uses the unit kilo years before the present (ka BP), placing the present in the year 1950. The chronologies used combine different dating methods, including ¹⁴C, but these ages are always calibrated to calendar ages according to the original references in the papers. Finally, the discussion dedicated to the recent millennia uses the unit common era (CE) years, which correspond to the years of our calendar.

Figure 2.1 Map locating the different palaeoclimatic records discussed in the text and shown in the figures.

2. The Quaternary glacial cycles

The Quaternary, the most recent geological era, began in 2.63   Ma (Gibbard et al., 2010). It commenced when a permanent ice sheet existed over Greenland in the Northern Hemisphere (2.8   Ma) (Tan et al., 2018), while in the Southern Hemisphere, Antarctica was already covered by permanent ice sheets since 34   Ma (Coxall et al., 2005). These large ice sheets oscillated in volume and extent throughout the Quaternary glacial and interglacial cycles but did not melt completely (Zachos et al., 2001).

The sequence of glacial and interglacial cycles has been established in base to the study of seabed sediments (Lisiecki and Raymo, 2005). Specifically, these sediments store the carbonate skeletons of microorganisms called foraminifera, which capture chemical signals from marine waters when they calcify. In particular, the evolution of the planetary ice volume has been studied from oxygen isotopic ratio (δ¹⁸O) measurements in foraminifera. The ice sheets store water previously evaporated from the ocean, which causes the sea level to fall while also changing the δ¹⁸O ratio of the ocean, since the lighter isotope (¹⁶O) is distilled preferentially in the process of evaporation and precipitation, and it accumulates in these ice sheets. Consequently, during glacial periods, the ocean is enriched with the heavy isotope (¹⁸O).

Cesare Emiliani was a pioneer in analyzing the δ¹⁸O relationship in foraminifera and established a numbering system for the identified cycles, giving odd numbers to the periods with light values (interglacial periods) and even numbers to those with heavy values of δ¹⁸O (glacial periods) (Emiliani, 1957). These so-called Marine Isotopic Stages (MIS) are still used in palaeoclimatological studies, the most modern being MIS 1, which corresponds to our current interglacial known as the Holocene. Later, the innovative application of palaeomagnetism in marine sediments allowed researchers to establish a chronology for the isotopic stages defined by Emiliani (Shackleton and Opdyke, 1973). Since then, isotopic stratigraphy, based on the correlations of the marine records of δ¹⁸O, has become a fundamental tool for the dating of paleoceanographic records, a method that has been refined over the years. The current global marine record of δ¹⁸O (LR04 benthic stack) is based on the superposition of 57 globally distributed benthic records. The benthic foraminifera live on the ocean floor, and thus they preserve the global continental ice volume signal with less interferences from regional climate changes at the surface (Fig. 2.2, Lisiecki and Raymo, 2005).

Long before such marine records were obtained, it had been hypothesized that planetary orbital movements were the origin of these glacial and interglacial cycles. In particular, Milutin Milankovitch first calculated their impact on the insolation received by the planet (Milankovitch, 1930). Milankovitch estimated the effects induced by the Earth's orbital eccentricity, obliquity, and precession over time on the insolation received by the Northern Hemisphere at 65°N during the summer season. He postulated that the insolation that reaches these high latitudes is a determining factor in the growth or retreat of the large ice sheets, particularly during summer, since this season establishes whether the ice formed in winter is preserved or destroyed. This hypothesis could not be tested until continuous climate records with chronological control were available, which made possible to analyze the relationship between changes in insolation and the volume of ice on the planet. Hays, Imbrie, and Shackleton published a paper in 1976 in which they tested Milankovitch's hypothesis for the first time using the marine records of δ¹⁸O. The result was totally unexpected; indeed, it was possible to identify the dominant frequencies from the three orbital movements in the palaeoclimatic record, 21, 40, and 100   ka, corresponding to the precession, obliquity, and eccentricity respectively and, therefore, to confirm the influence of the orbital movements on the glacial–interglacial cycles, as proposed by Milankovitch. However, an even greater surprise involved the revelation that the most recent Quaternary deglaciations (in the last 500   ka) followed a cycle of approximately 100   ka, comparable to the eccentricity movement, which hardly induced changes in the solar irradiance balance (Fig. 2.2). As described by Hays, Imbrie, and Shackleton in 1976 and validated in subsequent studies, orbital movements were responsible for setting the pace of the glacial–interglacial cycles; however, these results also clarified that the climate system does not force or trigger a linear climate change response (Hodell, 2016). The dominance of the 100   ka cycle could only be the result of the interaction of feedback mechanisms of the climate system itself.

Figure 2.2 (A) Time evolution of the insolation received by the planet at 65°N during the summer months; and (B) global records of δ¹⁸O measured in benthic foraminifera (LR04), representing the change in the ice volume along the glacial–interglacial cycles of the Quaternary (Lisiecki and Raymo, 2005). MPT, The Middle Pleistocene Transition.

The abovementioned phenomenon has been called the mystery of the 100 ka, and it has been and continues to be one of the fundamental objectives of recent palaeoclimatic research, since it reveals the complexity of the climate system. Great progress has been made in understanding the nature of nonlinearity in the climate system, which goes hand in hand with intense feedback processes, among which we could list the changes in the terrestrial albedo associated with the dimensions of the ice sheets, variations in the atmospheric concentrations of greenhouse gases, and changes in ocean circulation. Understanding these processes is key, not only for palaeoclimatology as a whole, but also for improving our predictive capacity in the climate change scenario we currently live in.

3. Glacial–interglacial cycles in the context of the Iberian Peninsula

One of the major limitations in the analysis of palaeoclimatic archives of terrestrial origin is the continuity of the records, which rarely cover more than one glacial–interglacial cycle continuously and often only part of one. Most of the available lake records in the Iberian Peninsula are concentrated