Past surface temperatures at the NorthGRIP drill site from the difference in firn diffusion of water isotopes

Abstract

Abstract.A new ice core paleothermometer is introduced based on the temperature dependent diffusion of the stable water isotopes in the firn. A new parameter called differential diffusion length is defined as the difference between the diffusion length of the two stable water isotopologues 2 H 1 H 16 O and 1 H 2 18 O. A model treatment of the diffusion process of the firn and the ice is presented along with a method of retrieving the diffusion signal from the ice core record of water isotopes using spectral methods. The model shows how the diffusion process is highly dependent on the inter-annual variations in the surface temperatures. It results in a diffusion length longer than if the firn was isothermal. The longer diffusion length can be explained by the strong nonlinearly behaviour of the saturation pressure over ice in the range of the surface temperature fluctuations.The method has been tested on δ 18 O and δD measurements, spanning the transition from the last glacial to the holocene, from the NorthGRIP ice core. The surface temperature reconstruction based on the differential diffusion resembles other temperature reconstructions for the North-GRIP ice core. However, the Allerød warming is seen to be significantly warmer than observed in other ice core based temperature reconstructions. The mechanisms behind this behaviour are not fully understood.The method shows the need of an expansion of high resolution stable water isotope datasets from ice cores. However, the new ice core paleothermometer presented here will give valuable insight into past climate, through the physical process of isotope diffusion in the firn column of ice sheets.

Transient climate simulations of the deglaciation 21–9 thousand years before present (version 1) – PMIP4 Core experiment design and boundary conditions

“…The next event of particular interest is the Younger Dryas cooling, when Northern Hemisphere temperatures are thought to have dropped by several degrees at 12.8-11.7 ka and most prominently in high latitudes Heiri et al, 2007;Lea et al, 2003;Liu et al, 2012;Simonsen et al, 2011;Steffensen et al, 2008). The event presents a conceptual paradox; the magnitude of the cooling is difficult to reconcile with rising atmospheric CO 2 (approximately +10 ppm compared to the earlier Bølling period ∼ 14.5 ka; Bereiter et al, 2015) and increasing boreal summer insolation (Berger and Loutre, 1991).…”

“…Certainly, others have shown that the location of injection is a controlling factor on the impact of freshwater delivery to the ocean, not just laterally (e.g. Condron and Winsor, 2012;Smith and Gregory, 2009), but also in terms of depth (e.g. Roche et al, 2007).…”

A brief history of ice core science over the last 50 yr

“…The difference in firn diffusion of water isotopes (Johnsen et al, 2000) offers the possibility to estimate past J. Jouzel: A brief history of ice core science over the last 50 yr temperature changes from very detailed δD and δ 18 O profiles in ice cores (Simonsen et al, 2011). Co-isotopic measurements (δD and δ 18 O) have also been applied to study basal ice as a result of the fractionation processes taking place during melting/refreezing processes (Jouzel and Souchez, 1982;Souchez and Jouzel, 1984;Jouzel et al, 1999).…”

Interaction of ice sheets and climate during the past 800 000 years

“…The rapid, sub-millennial fluctuations, however, are not present in the model output because of lacking short-timescale forcing variations. Furthermore, the model seems to underestimate the amplitude of the temperature record by approximately 5 K. Some uncertainty resides in the interpretation of the data, as Greenland temperature records inferred from different water isotope data show different glacial-interglacial variability (Kindler et al, 2014;Simonsen et al, 2011). However, most likely this underestimation of temperature variability is predominantly a consequence of the zonal averaging in our model, which distributes temperature anomalies over the entire meridional band.…”

“…These indicate local temperature variability, although their interpretation is ambiguous (see e.g. different glacial-interglacial reconstructed temperature variability in Kindler et al, 2014, andSimonsen et al, 2011). Greenland and Antarctic climate have been shown to correlate closely to each other (EPICA community members, 2006).…”