Isotope calibrated Greenland temperature record over Marine Isotope Stage 3 and its relation to CH₄

Abstract

Large temperature variations on millennial time scales in Greenland characterised the last ice age. Abrupt warmings, known as Dansgaard–Oeschger (DO) events, can be traced in the δ¹⁸O_ice record of Greenland ice cores. However, it has been shown that δ¹⁸O_ice is not a direct temperature proxy. Measurements of the isotopic composition of gases trapped in the ice can be used to calibrate the paleothermometer. Here we present a continuous temperature record based on high resolution δ¹⁵N measurements and firn model studies. It covers a sequence of 9 DO events (9–17) during the time period from 38 to 64 kyr BP for which temperature changes of 8 to 15 °C were estimated. The difference between the modern and the glacial δ¹⁸O_ice–T relationship can be explained by a combination of source temperature changes and changes in the annual distribution of precipitation. A detailed comparison of the temperature evolution with reconstructions of the atmospheric methane (CH₄) concentration shows that CH₄ rises lag temperature increases at the onset of DO events by 25 to 70 yr within data resolution. The strong correlation between Greenland temperature and CH₄ on millennial and submillennial time scales suggests that variations on these time scales were probably of hemispheric extent.

Introduction

The climate over the last glacial period was characterised by numerous abrupt climate changes known as Dansgaard–Oeschger (DO) events [1]. They can be traced in paleorecords from the Arctic ice sheets, as well as from tropical and subtropical regions [2], [3], [4], [5]. DO events are most prominently represented in δ¹⁸O_ice, the oxygen isotope ratio in Greenland ice cores. They have been related to shifts of the ocean thermohaline circulation (THC) [6], [7], [8]. DO events in Greenland typically start with a rapid warming of about 8 up to 16 °C within a few decades [9], [10], [11], [12] followed by a more gradual cooling phase over several centuries and a rapid drop back towards cold stadial conditions. The long lasting DO events were preceded by massive ice surges from the northern ice sheets, documented by debris deposits at the ocean floor known as Heinrich (H) events [13].

Water isotopes in ice cores (δ¹⁸O_ice or δD) are useful temperature proxies because changes in isotopic composition of precipitation in polar regions are mainly related to variations of temperatures at the site of precipitation. The present day spatial relationship between δ¹⁸O_ice and temperature, α_spatial, is estimated at 0.67‰/K for Greenland [14]. α depends on variations of the seasonal precipitation distribution [17] and/or changes occurring at the source region [18], [19] of precipitation. Therefore, the present day α_spatial cannot be used to quantitatively interpret past climate shifts [15], [16].

Measurements of the isotopic composition of nitrogen δ¹⁵N and/or argon δ⁴⁰Ar on air trapped in ice cores can be used to calibrate the δ¹⁸O_ice to temperature relation, during an event of rapid temperature change [9], [10], [11], [12], [20], [21], [22]. Because atmospheric δ¹⁵N is constant [23], changes of this air parameter trapped in ice indicate fractionations due to gravitation and thermal diffusion [24]. Rapid warming or cooling at the surface produces a temperature gradient in the firn that forces the heavier isotopes to migrate towards the cold end. This results in an alteration of the isotope signal trapped in the ice core. The surface temperature change can be reconstructed by comparing the measured isotope fractionation with firn gas diffusion model calculations. This approach has been used to deduce rapid temperature changes for several DO events [9], [10], [11], [12], [20], [24], [25] during the last glacial epoch.

Here we present a reconstruction of the temperature evolution over 9 consecutive DO events (events 9 to 17) during Marine Isotope Stage (MIS) 3, based on high resolution δ¹⁵N measurements on the ice core from the North Greenland Ice Core Project (NorthGRIP) [26], using a new on-line technique [27], [28]. This record is compared with existing high resolution CH₄ measurements from the NorthGRIP [29] (DO 9–12), and the GISP2 [31], [30] (DO 9–17) ice cores. Additionally, we present new highly resolved CH₄ measurements from the NorthGRIP ice core for the time period of DO 15–17. This allows us to compare in detail methane and reconstructed temperature evolutions over a longer time period including rapid temperature variations (DO 9–17).