Marine mollusk shells reveal how prehistoric humans adapted to intense climate change 

Peer-Reviewed Publication

UNIVERSITAT AUTONOMA DE BARCELONA

Current global climatic warming is having, and will continue to have, widespread consequences for human history, in the same way that environmental fluctuations had significant consequences for human populations in the past. The so-called ‘8.2 ka event’ has been identified as the largest and most abrupt climatic event of the past 11,700 years, caused by cool meltwater from North American lakes flooding into the North Atlantic and stopping ocean circulation systems. The cooling and drying effects of this event have been documented around the world, including along the Atlantic coast of Europe. Nevertheless, the sweeping impacts of the 8.2 ka (kilo annum i.e., thousand years ago) event on different environments and human societies are often assumed rather than proven. 

The journal Scientific Reports has published a paper led by Asier García Escárzaga, current researcher from the Institute of Environmental Science and Technology (ICTA-UAB) and the Department of Prehistory of the Universitat Autònoma de Barcelona, together with Igor Gutiérrez Zugasti, from the Universidad de Cantabria (UC). The study was coordinated from the Universidad de La Rioja (UR) and the Max Planck Institute (Germany) alongside members of other academic centres (Max Planck Institute, University of Burgos, Universidad Complutense de Madrid and University of Faro).    

The study applies a multidisciplinary toolkit of archaeomalacological studies and stable oxygen isotope analyses to shell remains recovered from the shell midden site of the El Mazo cave (Asturias, N Spain). With a long stratigraphic sequence of 1,500 years, El Mazo is a unique context along the European Atlantic coast, with especially high chronological resolution of each archaeological layer.  

The results obtained by these scientists allowed them to determine that colder seawater temperatures, deduced from stable oxygen isotope values measured on marine shells, led to changes in the availability of different shellfish species. For instance, one of the most commonly consumed species, the warm-adapted species P. lineatus, decreased during the 8.2 ka event, while populations of cold-adapted P. vulgata, another commonly exploited species, increased. Intriguingly, the warm-adapted limpet P. depressa also increased during this cool period, owing to a higher resistance to cold temperatures than other warm-water species. 

Their results also revealed an increase in the intensification of mollusc exploitation by humans, as indicated by a decrease in average mollusc size and evidence for increased harvesting in more dangerous coastal areas. The authors argued that this occurred because of human demographic growth in these Atlantic coastal settings which acted as refugia during this cold event, encouraging populations to move there from further inland. Nevertheless, populations around El Mazo managed to avoid over exploiting their coastal resources, as average mollusc size very rarely decreased below 20mm, the minimum size specified by modern regulations to guarantee long-term species survival. 

“Our results suggest an ongoing application of local marine ecological knowledge by some of the last foragers in western Europe, despite major changes to climate and demography” says Asier García-Escárzaga lead author of the current study.  

The resolution provided by the combination of taxonomic, geochemical and chronological analysis of molluscs from archaeological sites has major implications for other studies seeking to determine the significance of climate change on marine environments, and can provide detailed clues to the magnitude and nature of future climate changes and their impacts on human societies. 

JOURNAL

Scientific Reports

DOI

10.1038/s41598-022-10135-w 

METHOD OF RESEARCH

Experimental study

SUBJECT OF RESEARCH

Animals

ARTICLE TITLE

Human forager response to abrupt climate change at 8.2 ka on the Atlantic coast of Europe

From EurekAlert!

Here is the Abstract and Introduction

Abstract
The cooling and drying associated with the so-called ‘8.2 ka event’ have long been hypothesized as having sweeping implications for human societies in the Early Holocene, including some of the last Mesolithic hunter-gatherers in Atlantic Europe. Nevertheless, detailed ‘on-site’ records with which the impacts of broader climate changes on human-relevant environments can be explored have been lacking. Here, we reconstruct sea surface temperatures (SST) from δ18O values measured on subfossil topshells Phorcus lineatus exploited by the Mesolithic human groups that lived at El Mazo cave (N Spain) between 9 and 7.4 ka. Bayesian modelling of 65 radiocarbon dates, in combination with this δ18O data, provide a high-resolution seasonal record of SST, revealing that colder SST during the 8.2 ka event led to changes in the availability of different shellfish species. Intensification in the exploitation of molluscs by humans indicates demographic growth in these Atlantic coastal settings which acted as refugia during this cold event.

Introduction

Current global climate warming is having, and will continue to have, widespread consequences for humans. Looking to the past, multiple climatic and environmental changes have long been thought to have shaped human evolution and behaviour1,2,3. The Holocene (11.7–0 ka cal BP) is a geological epoch characterized by comparative stable climatic conditions. However, that stability was punctuated by a series of sudden climate changes, particularly during the Early Holocene4. Among these, the ‘8.2 ka event’ has been identified as the largest and most abrupt climatic event of the Holocene5,6. Climate scientists suggest that this ‘event’ was the result of an outburst of glacial meltwater from the Laurentide lakes in North America7. The influx of cold water into the Atlantic Ocean led to a reduction of sea surface salinity and a decline of the Atlantic meridional overturning circulation (AMOC), provoking a reduction in sea surface temperatures (SST) across the North Atlantic8. The cooling effects of this event have been documented in proxies from the Greenland ice cores6 and across Europe5,9,10. Short and sharp periods of colder or drier conditions have also been recorded ~ 8.2 ka throughout the northern11,12,13 and southern hemispheres11,12,14. Nevertheless, while now a well-established climatic phenomenon, the sweeping impacts of the ‘8.2 ka event’ on different environments often remain assumed rather than proven, and local records of marine or terrestrial conditions available at an appropriate resolution often remain lacking. Furthermore, while temperature or rainfall changes associated with the ‘8.2 ka event’ are often well-established, there are few palaeoclimatic proxies available that have enabled insights into the influence of the ‘8.2 ka event’ on the seasonality of climatic conditions in different parts of the world.