Temperatures recorded by cosmogenic noble gases since the last glacial maximum in the Maritime Alps

Hans Renssen,Marissa M Tremblay,Matteo Spagnolo,David L Shuster,Adriano Ribolini

doi:10.1017/qua.2018.109

Abstract

AbstractWhile proxy records have been used to reconstruct late Quaternary climate parameters throughout the European Alps, our knowledge of deglacial climate conditions in the Maritime Alps is limited. Here, we report temperatures recorded by a new and independent geochemical technique—cosmogenic noble gas paleothermometry—in the Maritime Alps since the last glacial maximum. We measured cosmogenic 3He in quartz from boulders in nested moraines in the Gesso Valley, Italy. Paired with cosmogenic 10Be measurements and 3He diffusion experiments on quartz from the same boulders, the cosmogenic 3He abundances record the temperatures these boulders experienced during their exposure. We calculate effective diffusion temperatures (EDTs) over the last ∼22 ka ranging from 8°C to 25°C. These EDTs, which are functionally related to, but greater than, mean ambient temperatures, are consistent with temperatures inferred from other proxies in nearby Alpine regions and those predicted by a transient general circulation model. In detail, however, we also find different EDTs for boulders from the same moraines, thus limiting our ability to interpret these temperatures. We explore possible causes for these intra-moraine discrepancies, including variations in radiative heating, our treatment of complex helium diffusion, uncertainties in our grain size analyses, and unaccounted-for erosion or cosmogenic inheritance.

Highlights

Data from paleoclimate proxies constrain the dynamics of Earth’s climate system on timescales inaccessible with the instrumental record
Proxy observations from key intervals of Earth’s past climate, such as the last glacial maximum (LGM) and subsequent deglaciation, allow us to understand how the climate system responds to a change in forcing (e.g., Schmittner et al, 2011; Annan and Hargreaves, 2013; von der Heydt et al, 2016) and evaluate the ability of climate models to simulate the climate system’s response (e.g., Schmidt et al, 2014; Annan and Hargreaves, 2015; Harrison et al, 2015) both of which improve our ability to forecast future climate change (e.g., Masson-Delmotte et al, 2013)
Over the historical 62 period, the Maritime Alps have been characterized by warmer mean annual temperatures (MAT), smaller annual temperature amplitudes, lower mean annual precipitation (MAP), and snow cover that is thinner and lasts for a smaller fraction of the year than in other Alpine sectors (Durand et al., 2009a, 2009b)

Summary

Introduction

Data from paleoclimate proxies constrain the dynamics of Earth’s climate system on timescales inaccessible with the instrumental record. An extensive network of terrestrial climate reconstructions since the last glacial maximum (LGM) exists across the European Alps from pollen, chironomids (midges), organic biomarkers, and inorganic isotope systems (e.g., Bartlein et al, 2011; Blaga et al, 2013; Heiri et al, 2014; Luetscher et al, 2015; Mauri et al, 2015) Despite such an extensive proxy network, our knowledge of deglacial climate conditions in the southern sector of the French and Italian Alps (hereafter referred to as the Maritime Alps) from proxy records is limited. Pollen-based climate reconstructions from four high-elevation sites in the Maritime Alps suggest trends in temperature and precipitation anomalies since the LGM that are broadly similar to those of other Alpine regions (Ortu et al, 2008). Neither of these studies interprets the proxies in terms of quantitative climate parameters

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