Proxy Description & Simulations Used

A combination of both ice core records as well as ocean sediment core records was used in order to construct the data compilation. A pre-condition of selecting the data is the availability of measurements at a temporal resolution of at least 2 ka or higher, in order to ensure that each individual record could be synchronized to the common temporal framework. A total of five records of surface air temperature deduced from stable water isotopes measured in polar ice cores as well as 42 SST records from marine sediment cores were used to create the time composite snapshots of the LIG climate..

The ice core records used include 4 from Antarctica: EPICA Dome C (EDC), EPICA Dronning Maud Land (EDML), Vostok Ice Core (VK), and Dome F (DF). One core from Greenland is included; the NEEM ice core [NEEM community members, 2013],

4.1. COMPARISON AGAINST NORTH ATLANTIC MULTI-PROXY SUMMER SEA SURFACE TEMPERATURE RECONSTRUCTIONS

extends into the LIG from 116 ka B.P. to 128 ka B.P.¹. The sediment cores are located primarily in the North Atlantic. Furthermore, the SST reconstructions are made from a variety of proxies. In total, 3 records are based upon Mg/Ca ratios, 3 on alkenone unsaturation ratios, and 30 on faunal assemblages, and 6 records are based upon the relative abundance of the polar foraminifera speciesNeogloboquadrina pachyderma. As such, a majority of the reconstructions are based upon faunal assemblages, since very few records with geochemical methods have been published.

How does each proxy reconstruct SST?

Mg/Ca Ratios The SST values reconstructed with the aid of Mg/Ca ratios utilized a temperature calibration by Kozdon et al. [2009]. This calibration is commonly used for the speciesNeogloboquadrina pachyderma:

M g

Ca =0.13(±0.037)T+0.35(±0.17)

where T is the temperature of Calcification. As N. pachyderma is a planktic foraminifera species, these temperatures are assumed to correspond to SST.

Alkenone Unsaturation Index Prahl and Wakeham [1987] discovered a physiological response inEmiliania huxleyi’s membrane chemical composition. The degree of unsaturation of alkenones is related to temperature changes, and can be expressed as:

U₃₇^K=0.043+0.033×T

Faunal Assemblages Here, a Modern Analogue Technique (MAT) is applied [Hutson, 1980], which relies on the similarity of faunal species distribution to a modern-day core top sample. The modern samples are thought to be analogues of the oceanographic conditions which produced a particular faunal distribution, and

1All five of these ice cores are compared against in Section 4.4

therefore, the similarity between the modern and past sample can be used to estimate changes in sea surface temperature.

Foraminifera Abundance The abundance ofN. pachydermaalso relies on MAT esti-mations (based upon the method described by Prell [1985]) to reconstruct SST. An example of such reconstructions, along with a detailed methodology, can be found in Bauch et al. [2012].

Determining Ages

The LIG presents a further problem, namely that it is beyond the age range that can be directly determined by radiocarbon dating. As¹⁴Chas a half life of 5730±40 years [Godwin, 1962], it is an unsuitable tool to determine exact ages of measurements during the LIG, which covered a time period from 115 ka B.P. to 130 ka B.P.. Therefore, other tools must be used.

Chronostratigraphic age scales are constructed to assist in anchoring proxy data – which are measured on relative depths of a sediment or ice core – to ages in the Earth’s past. This would allow proxy data points to be tied to specific ages. Constructing such a chronology can be accomplished by a variety of methods. As glacial ice accumulates in a sequence of annual increments, it is possible to generate a chronology based upon the visible banding in the ice; and such a layer counting technique has been employed for dating parts of the the GRIP and NGRIP ice cores [Rasmussen et al., 2006, Svensson et al., 2008].

At deeper levels within the ice sheet, defomation makes the individual layers more difficult to recognize. This problem is further complicated at low accumulation sites, such as over Antarctica, where the seasonal layers are not clearly deposited, and further smoothed during the conversion of snow to ice (known as firnification). While some absolute age markers can be used, such as tying ¹⁰Be peaks to magnetic reversals

4.1. COMPARISON AGAINST NORTH ATLANTIC MULTI-PROXY SUMMER SEA SURFACE TEMPERATURE RECONSTRUCTIONS

[Raisbeck et al., 2007], ⁴⁰Ar/³⁹Ar ratios [Dunbar et al., 2008], or U/Th ratios [Aciego et al., 2011], such absolute dating markers are sparse, necessitating additional methods.

One such approach of dating relies on the fact that certain atmospheric trace gases concentratations (such as methane) vary in correlation to variations in the earth’s orbital parameters, which can be independently dated based upon astronomical calculations [Ruddiman and Raymo, 2003]. Other measurements of air content properties trapped in the ice cores are also useful. Long records ofδ¹⁸O_atm [Bender et al., 1994, Petit et al., 1999],δO₂/N₂ [Bender, 2002, Kawamura et al., 2007], and total air content [Raynaud et al., 2007] have been found to be highly correlated with the insolation variations in Earth’s orbit, thereby providing additional constraints on ice core chronologies.

All of these orbital tuning constraints rest upon the principle that variations in the Earth’s orbit can be precisely calculated. The basis for this theory was shown earlier in Chapter 1.2, and calculations for the various oscillations of Earth’s orbit have been performed by Berger [1978] as well as by Laskar et al. [2011].

Synchronizing Proxy Records

The age scale chosen was the Antarctic Ice Core Chronology 2012 (AICC2012) chronology, initially published by Bazin et al. [2013] and Veres et al. [2013]. This time scale is the first of its kind, integrated over the LIG based upon multiple locations, and provides age/depth relationships both for Greenland (NGRIP) as well as Antarctic ice cores (EDC, EDML, TALDICE, Vostok).

Capron et al. [2014] transferred the records onto the AICC2012 age scale utilizing a method outlined by Govin et al. [2012]. This synchronization bases upon the assumption that surface water temperature changes in the North Atlantic occurred simultaneously with air temperature variations over Greenland. A detailed description of this method-ology is presented in Capron et al. [2014], along with a summary of which events are considered to be synchronous between the NGRIP ice core and the sediment cores. In

principle, age models are built based upon linear interpolation operates under the as-sumption that the sedimentation rate remains constant. The resulting synchronized records include errors based upon Monte-Carlo analysis to propagate the errors asso-ciated with both the uncertainty linked to the SST reconstruction as well as the age uncertainties of the AICC2012 chronology. The Monte-Carlo analysis utilizes 1000 age model simulations, adding random noise to the SST reconstruction values within the space of the method’s error, and randomly perturbing the age of each tie-point within the age uncertainty. This method resulted in a combined uncertainty for both reconstruction method and age. The data compilation claims to reconstruct boreal summer sea surface temperature, as is given by the respective authors of each of the individual records

Model Runs Employed for Comparison

These proxy compilations are compared to simulations of the early and mid LIG, LIG-130 and LIG-125, as already described in Chapter 3. Additionally, as the records are available as time series as well as time composites, some of the records are compared against a transient comparison of the LIG. This simulation was created using the greenhouse gas (GHG) concentrations described in the Paleoclimate Model Intercomparison Project (PMIP) project as well as orbital parameters calculated using the Berger and Loutre [1991] routine. Both of these forcings were accelerated by a factor of 10, following a method initially documented by Lorenz and Lohmann [2004]. This simulation is hereafter referred to as LIG-T.