THE GLACIAL WORLD ACCORDING TO WALLY W.S. BROECKER ELDIGO PRESS 2002 THE GLACIAL WORLD ACCORDING TO WALLY W.S. BROECKER LAMONT-DOHERTY EARTH OBSERVATORY OF COLUMBIA UNIVERSITY Palisades, NY 10964 September 1992 Second Revised Edition June 1995 Third Revised Edition August 2002 FOREWORD In 1990 I started a book on glacial climates. I planned four sections, one on indicators, one on clocks, one on records and finally, one on the physics of the system. After completing the first three of these, I got stuck in my attempt to come up with a conceptual view of what might be driving these changes. However, having taught a course on climate change for many years, I know that much of the information one needs is not available in textbook form. Instead, we all depend heavily on reprints. As this proves to be an inefficient route for students interested in learning the basics of our field, I have tried to capture many of these basics in the first three sections of my would-be book. As I realized that it would be several years before I completed this project, I thought it might be useful to the community if I were to make available the three largely completed sections. This was done in 1992. In preparation for use in a short course to be taught at the Geological Society of America in November 2002, I undertook an effort to update the first three sections and the references during the early part of 2002. I encourage purchasers of the book to make as many copies as they wish for use by students and colleagues. The spiral binding is easily removed so as to facilitate duplication. Library-bound copies are also available. DEDICATION To Gary Comer A newfound friend and patron of young abrupt-change researchers. ................................................................ This book is available at a cost of $50 for the spiral-bound version or $70 for the hard-bound version (shipping included) Pre-payment is preferred. Checks in US$ should be made out to Trustees of Columbia University. To purchase contact Patty Catanzaro Eldigio Press Lamont-Doherty Earth Observatory of Columbia University Palisades, NY 10964 USA Tel.: (845) 365-8515; Fax: (845) 365-8155; e-mail: [email protected] Copyright 1993 TABLE OF CONTENTS PROLOGUE INDICATORS INTRODUCTION...................................................................................... 1 ICE VOLUME............................................................................................ 1 Glacial Moraines............................................................................. 2 Shorelines........................................................................................ 4 Oxygen Isotopes.............................................................................. 13 Summary......................................................................................... 21 TEMPERATURE....................................................................................... 21 Mountaintop Temperatures............................................................. 23 Polar Temperatures......................................................................... 25 Sea Surface Temperatures............................................................... 31 Continental Temperatures............................................................... 45 The Tropical Temperature Dilemma.............................................. 55 ARIDITY.................................................................................................... 56 Precipitation.................................................................................... 58 Rock Varnish.................................................................................. 61 Dust................................................................................................. 63 ATMOSPHERIC GAS COMPOSITION................................................... 67 Carbon Dioxide............................................................................... 67 Methane........................................................................................... 75 Isotopic Composition of Atmospheric Gases................................. 77 OCEANIC CHEMISTRY........................................................................... 83 Trace Metals.................................................................................... 83 Carbon Isotopes.............................................................................. 96 RATE OF DEEP SEA VENTILATION..................................................... 105 CO3 Concentration......................................................................... 109 Paleo pH.......................................................................................... 119 CLOCKS INTRODUCTION...................................................................................... 1 THE RADIOCARBON CLOCK................................................................ 1 ANNUAL CLOCKS................................................................................... 5 THE URANIUM-THORIUM CLOCK...................................................... 15 CLOCKS READING MORE THAN 50,000 YEARS.............................. 29 THE POTASSIUM-ARGON CLOCK....................................................... 35 TABLE OF CONTENTS (continued) IN SITU PRODUCTION CLOCKS........................................................... 39 PLANETARY CLOCKS............................................................................ 40 SUNDIALS................................................................................................. 41 RECORDS 55 MILLION YEARS OF POLAR COOLING......................................... 1 MILANKOVITCH CYCLES..................................................................... 12 THE RECORD IN ANTARCTIC ICE....................................................... 24 THE LAST 160,000 YEARS...................................................................... 26 THE LAST TERMINATION..................................................................... 37 DANSGAARD-OESCHGER EVENTS..................................................... 63 HEINRICH EVENTS................................................................................. 69 RELATIONSHIP BETWEEN DANSGAARD-OESCHGER EVENTS AND HEINRICH EVENTS........................................................... 79 MECHANISMS INTRODUCTION...................................................................................... 1 FORCINGS................................................................................................. 1 INSOLATION SEASONALITY CYCLES............................................... 1 THERMOHALINE CIRCULATION REORGANIZATIONS.................. 2 FLUCTUATIONS IN SOLAR ENERGY OUTPUT................................. 3 THE BIPOLAR SEESAW.......................................................................... 6 AMPLIFIERS............................................................................................. 6 WATER VAPOR........................................................................................ 8 CLOUD COVER........................................................................................ 9 TERMINATIONS....................................................................................... 9 SUMMARY................................................................................................ 10 FOSSIL FUEL CO .................................................................................... 11 2 BIBLIOGRAPHY PROLOGUE In the debate regarding the possible consequences of the ongoing buildup of greenhouse gases, the climate record for the past hundred-thousand years sends us an important message. Contrary to the view held by those who downplay the consequences of fossil fuel burning, the paleorecord clearly demonstrates that the Earth’s climate system is far from self stabilizing. Rather, it has undergone large responses to seemingly small forcings. Not only have major changes occurred, but some of the largest have taken place on the time scale of a few decades. Further, the magnitude of these shifts is far greater than expected from any known forcing (i.e., fluctuations in solar output, variable interception of sunlight by dust and aerosols, changes in seasonality resulting from variations in the Earth’s orbital parameters...). Thus I am driven to the conclusion that the Earth’s climate system has several distinct modes of operation. To date, no Earth system model has been able to adequately account for the growth and maintenance of large ice sheets which occupied most of Canada and Scandinavia during peak glacial time. Nor has any model been able to adequately account for the atmosphere’s low CO content and high dust content during these times. 2 Nor has any model been able to reproduce the rectangular character of past climate changes. We do, however, have one valuable clue in this regard. Models clearly demonstrate that the ocean’s thermohaline circulation has a number of quasi-stable modes of operation. Further, these models show how the ocean might be triggered to jump from one of these modes to another. But to date, no clear explanation has been given as to how these changes in ocean operation translate into the globe-encompassing atmospheric changes recorded in the paleoclimate record. The paleoclimatic record challenges us by demonstrating that much more must be learned about the Earth’s climate system before we can have any confidence in model- based predictions of the changes which will result from the buildup of CO and other 2 greenhouse gases in our atmosphere. If we are to take full advantage of the information offered by experiments carried out in the past, we must develop the capability to properly read the record kept in sediment and ice. Such a reading requires that we fully understand how each of our proxies responds to changes in environmental conditions. It also requires that we develop the capability to precisely correlate records from different places. To this end, the first two sections of this book deal with “Indicators” and “Clocks”. In the first, I attempt to depict the strengths and weaknesses of the many climate proxies currently under investigation. In the second, I do the same for radioisotope dating methods. Then, in the third section, entitled “Records”, I summarize what I consider to be the most informative of the proxy versus time curves. Finally, in the fourth section, entitled “Mechanisms”, I provide a scenario designed to explain what I consider to be the most important features of the record. While in the text and captions I refer to by name some of the key contributors to this subject, these references are sparse compared to what would be found in a review article. While those of us in the field are extremely conscious of who did what and when they did it, my aim in writing this book is to introduce people to the subject. Detailed referencing would only interfere with this mission. Instead, I have put together a bibliography arranged by subject, and for a given subject, by publication date. By scanning these lists, one can see not only who did the work but also how the subject has evolved. As thousands of articles have been written on subjects included in this book, the bibliography is by no means comprehensive. Rather, it reflects my own reading. INDICATORS INTRODUCTION Except for the observations made over the last 130 or so years at weather stations and on ships, our knowledge of past climates is based on records kept in sediment and ice. The task of the paleoclimatologist is to decipher the proxies contained in these records. This has proven a complex task for every proxy is influenced by more than one climatic variable. While much progress has been made toward isolating the influence of these competing contributions, the task has proven to be a very tough one. For convenience, these proxies can be divided into five major groups; i.e., those which carry information regarding: 1) ice volume, 2) temperature, 3) aridity, 4) atmospheric composition, and 5) ocean chemistry. Emphasis is placed on reconstructing the conditions which prevailed during the peak of the last glaciation. Discussion of the time sequence of climate changes appears in the section entitled “Records”. ICE VOLUME When we speak of glaciation, we think mainly of the expansion and contraction of the great ice sheets which as recently as 20,000 years ago covered much of Canada and Scandinavia. Hence of primary interest to the study of glacial cycles is the volume of these ice masses. Emphasis is placed on volume rather than area because of its tie to sea level: the bigger the ice sheets, the lower the level of the sea. Estimates of the volume of ice on continents at any given time has been pieced together from three quite different sources of information, moraines which mark the perimeter of the ice sheets, shorelines which mark the level of the sea, and the ratio of 18O to 16O in benthic foraminifera which depend on ice volume. As each source has its limitations and ambiguities, only by considering the three together can we get the information we seek. 1 Glacial Moraines The primary information regarding the volume of ice comes from deposits produced by the glaciers themselves. Moraines, i.e. debris bulldozed into place by the advancing ice front, mark the perimeter of the ice. Most important in this regard are those features created when the ice sheets of the last glacial period reached their maximum extent. We know from marine oxygen isotope records (see below) that each of the great ice ages culminated with about the same ice volume. Assuming that for each cycle the geometry of the ice sheets was more or less the same, the moraines formed during the last glacial maximum tell us where the major ice masses were located, not only during the last cycle but also during the previous ones (see figure 1). Indeed, where moraines from earlier ice maxima are preserved, they support this supposition. While the location of the southern margins of the North American and Eurasian ice sheets is clearly defined by moraines, serious debate exists concerning the extent of ice along their northern perimeters. One school is convinced that large ice sheets were centered over the shelves of arctic Canada, Scandinavia, and Siberia. Another school is convinced that ice cover in these areas was minor, being confined to a few small island- based caps. The problem is that deposits similar to those that mark the edges of the southern margin of the Laurentide and Eurasian ice caps are absent in the Arctic. The big-ice school contends that they lie beneath the sea on the continental shelves. They point instead to the raised shorelines which ring the Arctic Islands. The height of these shorelines argues strongly for rebound triggered by the melting of large rather than small caps. Were no information other than the position of the ice margins available, estimates of the volume of ice at glacial maxima would have to be obtained from computer simulations designed to yield the height and contour of the ice caps fitted to these boundaries. Even though these simulations are constrained by studies of the existing ice caps on Greenland and Antarctica, the uncertainties associated with such 2