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UNIVERSITY of CALIFORNIA, SAN DIEGO Temperature Reconstruction UNIVERSITY OF CALIFORNIA, SAN DIEGO Temperature reconstruction at the West Antarctic Ice Sheet Divide, for the last millennium, from the combination of borehole temperature and inert gas isotope measurements. A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Oceanography by Anais J. Orsi Committee in charge: Jeffrey P. Severinghaus, Chair Bruce D. Cornuelle Helen Fricker Dan Lubin Lynne D. Talley William Trogler 2013 Copyright Anais J. Orsi, 2013 All rights reserved. The dissertation of Anais J. Orsi is approved, and it is ac- ceptable in quality and form for publication on microfilm and electronically: Chair University of California, San Diego 2013 iii EPIGRAPH Fortitudine Vincimus By endurance we conquer - Ernest Shackelton iv TABLE OF CONTENTS Signature Page................................... iii Epigraph....................................... iv Table of Contents..................................v List of Figures.................................... xii List of Tables.................................... xv Acknowledgements................................. xvi Vita, Publications, and Fields of Study....................... xix Abstract of the Dissertation............................. xxi Chapter 1 Introduction.............................1 Chapter 2 Magnitude and Temporal Evolution of DO 8 Abrupt Temperature Change Inferred From Nitrogen and Argon Isotopes in Greenland Ice Using a New Least-Squares Inversion.............4 2.1 Introduction..........................5 2.1.1 Reconstructing polar surface temperature......5 2.1.2 Abrupt climate changes................6 2.1.3 Noble gas isotopes..................8 2.1.4 Temperature reconstruction.............9 2.2 Methods............................9 2.2.1 Laboratory measurements..............9 2.2.2 Timescale....................... 10 2.2.3 Isolation of the thermal signal............ 12 2.2.3.1 Gas loss correction............. 13 2.2.3.2 Using two pairs of isotopes........ 14 2.2.3.3 Using a densification model........ 16 2.2.4 Forward densification and heat diffusion model... 16 2.2.4.1 Improvement of the Goujon model.... 18 2.2.4.2 Linearization of the forward model.... 18 2.2.4.3 Linearity of the model........... 19 2.2.4.4 Choice of the basis functions....... 20 2.2.5 Inverse model.................... 22 2.2.5.1 Least-squares inversion.......... 22 2.2.5.2 Uncertainty estimation........... 24 v 2.2.5.3 Influence of the parameters used in the in- version................... 25 2.3 Results............................ 25 15 2.3.1 First, and simplest, approach with δ Nxs ...... 27 2.3.2 Second approach, with a diffusive column height fit to data......................... 27 2.3.3 Third approach, with a densification model..... 29 2.3.3.1 Accumulation rate scenarios........ 29 2.3.3.2 Surface temperature reconstruction.... 30 2.3.3.3 Gas Age calculation............ 32 2.4 Discussion........................... 34 2.4.1 Accumulation history................. 34 2.4.2 Amplitude of the warming.............. 36 2.4.3 Calibration of the δ18O thermometer........ 37 2.5 Sensitivity analysis...................... 39 2.5.1 Sensitivity to the initial conditions.......... 39 2.5.2 Sensitivity to the initial temperature history..... 39 2.5.3 Comparison of the Goujon and Spencer densifica- tion models...................... 40 2.5.4 Sensitivity to the accumulation history........ 43 2.5.5 Sensitivity to the convective zone.......... 43 2.5.6 Sensitivity to the lock-in depth............ 44 2.6 Conclusions.......................... 47 Chapter 3 Little Ice Age Cold Interval in West Antarctica: Evidence from Borehole Temperature at the West Antarctic Ice Sheet (WAIS) Di- vide.................................. 48 3.1 Introduction.......................... 49 3.1.1 The last 1000 years................. 49 3.1.2 Site description.................... 49 3.2 Method............................ 51 3.2.1 Sampling method................... 51 3.2.2 Forward model.................... 51 3.2.3 Inverse model..................... 53 3.2.3.1 Linearization................ 55 3.2.3.2 Least-squares regression.......... 56 3.2.4 Uncertainty estimation................ 57 3.2.4.1 Uncertainty associated with the least- squares optimization............ 57 3.2.4.2 Uncertainty in the timing of the tempera- ture minimum............... 58 3.2.4.3 Uncertainty associated with the boundary conditions................. 58 vi 3.2.4.4 Uncertainty associated with the model pa- rameters.................. 58 3.3 Results and discussion.................... 59 3.3.1 Seventeenth century minimum............ 59 3.3.2 Recent warming................... 62 3.4 Sensitivity analysis...................... 64 3.4.1 Influence of the initial boundary conditions..... 64 3.4.2 Influence of the bottom boundary conditions.... 64 3.4.2.1 Bottom temperature............ 65 3.4.2.2 Bottom temperature gradient....... 65 3.4.2.3 Bottom depth................ 65 3.4.3 Influence of the vertical velocity parameterization.. 67 3.4.4 Influence of the accumulation rate.......... 67 3.4.5 Influence of the thermal conductivity of the ice... 69 3.4.6 Influence of the basis functions used in the inversion. 69 3.4.6.1 Influence of the prior covariance of the model parameters............. 69 3.4.6.2 Influence of the signal to noise ratio.... 73 3.4.6.3 Influence of the basis functions...... 73 3.4.7 Uncertainty in the measurements........... 75 3.4.7.1 Uncertainty in the calibration....... 75 3.4.7.2 Systematic biases............. 76 3.4.7.3 Random noise............... 77 3.5 Conclusion.......................... 77 Appendix 3.A Forward model description............ 78 3.A.1 Model parameters................... 79 3.A.2 Boundary conditions................. 79 Chapter 4 Analytical Methods......................... 81 4.1 Introduction.......................... 82 4.2 La Jolla Air standard..................... 83 4.2.1 Pier extraction.................... 84 4.2.2 Aliquot extraction.................. 85 4.2.3 Working standard................... 85 4.2.4 Stability of the standard............... 86 4.3 Nitrogen and argon isotope measurements.......... 87 4.3.1 Set up description................... 89 4.3.2 Oxygen removal................... 90 4.3.2.1 Optimisation of the oxygen removal pro- cess with hot copper............ 90 4.3.2.2 Tests with Ridox.............. 91 4.3.3 Air extraction from ice samples........... 92 4.3.3.1 Flask design................ 92 vii 4.3.3.2 Evacuation time.............. 93 4.3.3.3 Melting and stirring............ 94 4.3.4 Mass spectrometry.................. 96 4.3.4.1 Gas configuration............. 96 4.3.4.2 Source focusing.............. 96 4.3.4.3 Aliquot expansion............. 97 4.3.4.4 Quality Control.............. 99 4.3.4.5 Argon isotopes............... 101 4.3.4.6 Nitrogen isotopes............. 104 4.3.4.7 Ar/N2 ratio................. 105 4.3.4.8 Kr/N2 ratio................. 109 4.3.5 Ice core measurements of WDC05A......... 110 4.3.5.1 WDC05A samples............. 110 4.3.5.2 Evaluation of the precision........ 111 4.3.5.3 Gas loss.................. 113 4.3.5.4 Argon isotopic offset............ 114 4.3.5.5 Comparison with the getter method.... 114 4.3.5.6 Comparison with WDC06A........ 117 4.4 Noble gas measurements in firn air.............. 117 4.4.1 Experimental protocol................ 118 4.4.2 Mass Spectrometry.................. 120 4.4.2.1 Optimisation of the performance of the MAT-253.................. 120 4.4.2.2 Sample handling.............. 122 4.4.2.3 Standard.................. 122 4.4.2.4 Argon isotopes............... 124 4.4.2.5 Krypton isotopes.............. 124 4.4.3 Elemental ratios................... 125 4.5 Noble gas measurements in ice cores............. 126 4.5.1 Sample preparation.................. 127 4.5.1.1 Ice extraction................ 127 4.5.1.2 Gettering.................. 127 4.5.2 Mass spectrometry.................. 129 4.5.2.1 Background................ 129 4.5.2.2 Pressure imbalance sensitivity....... 129 4.5.2.3 Chemical slope............... 130 4.5.3 WDC05A samples.................. 131 4.5.3.1 La Jolla Air calibration.......... 131 4.5.3.2 Evaluation of the precision........ 133 4.5.3.3 Fractured ice................ 133 Appendix 4.A Protocol for extracting La Jolla Air........ 134 4.A.1 In the laboratory................... 134 4.A.2 At the pier...................... 134 viii 4.A.3 Take down...................... 135 Appendix 4.B Protocol for filling working standard cans..... 136 4.B.1 Ar-N2 standard.................... 136 4.B.1.1 Preparation................. 136 4.B.1.2 Addition of argon............. 136 4.B.1.3 Addition of nitrogen............ 137 4.B.1.4 Correct the Ar/N2 ratio by adding Ar... 138 4.B.2 Ar-Kr-Xe standard.................. 139 4.B.2.1 Handling of Kr and Xr tanks........ 139 4.B.2.2 Setup.................... 139 4.B.2.3 Calculations................ 140 4.B.2.4 Operation................. 140 4.B.2.5 Notes.................... 141 Appendix 4.C Protocol for extraction of Ar and N2 in ice sam- ples: Copper method..................... 142 4.C.1 Preparation of the method.............. 142 4.C.1.1 Chest freezer................ 142 4.C.1.2 Getting the flasks ready.......... 142 4.C.2 The night before................... 142 4.C.3 In the freezer..................... 143 4.C.4 In the lab....................... 143 4.C.5 Clean up....................... 147 4.C.6 Mass spectrometry.................. 148 Appendix 4.D Notes on Focusing the MAT-252......... 149 4.D.1 Settings that should be left alone........... 149 4.D.2 Settings that should be adjusted........... 149 Appendix
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