Gifford H. Miller

The Last Great Ice Sheets

Late Weichselian Ice Sheets in Eurasia and Greenland (B. G. Andersen). Late Wisconsin Ice Sheets of North America (P. A. Mayewski, G. H. Denton, & T. J. Hughes). Late Wisconsin-Weichselian Mountain Glaciers and Small Ice Caps (J. T. Hollin & D. H. Schilling). Numerical Reconstruction of Valley Glaciers and Small Ice Caps (D. H. Schilling & J. T. Hollin). Numerical Reconstruction of Paleo-Ice Sheets (T. J. Hughes). The Last Great Ice Sheets: A Global View (T. J. Hughes, G. H. Denton, B. G. Andersen, D. H. Schilling, J. L. Fastook, & C. S. Lingle). The History of the Marine Ice Sheet in West Antarctica During the Last Glaciation: A Working Hypothesis (M. Stuiver, G. H. Denton, & T. J. Hughes). The Arctic Ice Sheet: An Outrageous Hypothesis (G. H. Denton & T. J. Hughes).

The Laurentide and Innuitian ice sheets during the Last Glacial Maximum.

The Late Wisconsinan advance of the Laurentide Ice Sheet started from a Middle Wisconsinan interstadial minimum 27–30 14 C ka BP when the ice margin approximately followed the boundary of the Canadian Shield. Ice extent in the Cordillera and in the High Arctic at that time was probably similar to present. Ice advanced to its Late Wisconsinan (stage 2) limit in the northwest, south, and northeast about 23–24 14 C ka BP and in the southwest and far north about 20–21 14 C ka BP. In comparison to some previous reconstructions of ice extent, our current reconstruction has substantially more Late Wisconsinan ice in the High Arctic, where an Innuitian Ice Sheet is generally acknowledged to have existed, in the Atlantic Provinces, where ice is now thought to have extended to the Continental Shelf edge in most places, and on eastern Baffin Island, where ice probably extended to the fiord mouths rather than to the fiord heads. Around most of the ice margin, the Late Wisconsinan maximum ice extent either exceeded the extent of earlier Wisconsinan advances or it was similar to the Early Wisconsinan advance. Ice marginal recession prior to 14 14 Ck a BP occurred mainly in deep water and along the southern terrestrial fringe. However, Heinrich event 1 probably drew down the entire central ice surface at 14.5 14 C ka BP sufficiently to displace the Labrador Sector outflow centre 900 km eastward from the coast of Hudson Bay. The onset of substantial ice marginal recession occurred about 14 14 C ka BP in the northwest, southwest, and south but not until about 10–11 14 C ka BP in the northeast and in the High Arctic. Thus, the period of maximum ice extent in North America generally encompasses the interval from B24/21 to 14 14 C ka BP, or considerably longer than the duration of the LGM defined as occurring during a period of low global sea level as well as during a time of relative climate stability B18 14 C ka BP. The interval of advance of much of the Laurentide Ice Sheet to its maximum extent (between B27 14 C ka BP and B24 14 C ka BP) coincides with a suggested interval of rapid fall in global sea level to near LGMlevels. r 2001 Elsevier Science Ltd. All rights reserved.

Recent warming reverses long-term arctic cooling.

Climate Reversal The climate and environment of the Arctic have changed drastically over the short course of modern observation. Kaufman et al. (p. 1236) synthesized 2000 years of proxy data from lakes above 60° N latitude with complementary ice core and tree ring records, to create a paleoclimate reconstruction for the Arctic with a 10-year resolution. A gradual cooling trend at the start of the record had reversed by the beginning of the 20th century, when temperatures began to increase rapidly. The long-term cooling of the Arctic is consistent with a reduction in summer solar insolation caused by changes in Earths orbit, while the rapid and large warming of the past century is consistent with the human-caused warming. A 2000-year-long Arctic cooling trend seen in a surface air temperature reconstruction was reversed during the last century. The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

Simulating Arctic climate warmth and icefield retreat in the last interglaciation

In the future, Arctic warming and the melting of polar glaciers will be considerable, but the magnitude of both is uncertain. We used a global climate model, a dynamic ice sheet model, and paleoclimatic data to evaluate Northern Hemisphere high-latitude warming and its impact on Arctic icefields during the Last Interglaciation. Our simulated climate matches paleoclimatic observations of past warming, and the combination of physically based climate and ice-sheet modeling with ice-core constraints indicate that the Greenland Ice Sheet and other circum-Arctic ice fields likely contributed 2.2 to 3.4 meters of sea-level rise during the Last Interglaciation.

Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise

Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.

Continuous 150 k.y. monsoon record from Lake Eyre, Australia: Insolation-forcing implications and unexpected Holocene failure

Our reconstructed history of Lake Eyre provides the first continuous continental proxy record of Australian monsoon intensity over the past 150 k.y. This continental records broad correspondence to the marine isotope record demonstrates that this very large catchment, with its hydrology dependent on a planetary-scale climate element, responds to Milankovitch-scale climate forcing. Abrupt transitions from dry phases to wet phases (ca. 125 and 12 ka) coincide with Northern Hemisphere winter insolation minima rather than Southern Hemisphere summer insolation maxima, indicating that Northern Hemi- sphere insolation exerts a dominant control over the intensity of the Australian monsoon. Stratigraphic and dating uncertainties of other wet phases preclude conclusive correlation to specific insolation signals but, within the uncertainties, are consistent with Northern Hemisphere forcing. Regardless of the hemispheric forcing, the low intensity of the early Holocene Australian monsoon—by comparison with the last interglacial and particularly the last high-level lacustrine event at 65-60 ka when all forcing elements were modest— is an enigma that can be explained by a change in boundary conditions within Australia.

Amino acid geochronology: Resolution and precision in carbonate fossils

Changes in indigenous proteins preserved in carbonate skeletons can be used to estimate the time elapsed since death of the organism. Racemization, the most widely used reaction in amino acid geochronology, refers to the inversion of l-amino acids to their d-configuration. For the protein amino acid l-isoleucine, racemization occurs about only one of the two chiral carbon atoms; this reaction (epimerization) proceeds from an initial dl ratio near zero to an equilibrium ratio of about 1.30. The reaction rate is controlled primarily by temperature, and to a lesser degree by the nature of the proteins themselves, a variable related to taxonomy and the extent of protein degradation. Analyzing monospecific samples minimizes the taxonomic variable. The resolving power of dl ratios as relative-age indices is a function of the reaction rate, degree of natural variability and analytical uncertainty. Natural variability is defined as the spread in ratios in five well preserved shells of the same age from a single stratum, and is primarily related to taxonomy. It can range from ±20% or more in taxa of low reliability to ±1% in taxa of greatest integrity; 5 to 10% is to be expected. Analytical uncertainty within a single laboratory is relatively insignificant, amounting to no more than ±2%, but interlaboratory comparisons suggest caution in comparing ratios determined at different laboratories or by different techniques. Sampling appropriate portions of a shell is essential to minimize variability in dl ratios. Site temperature exerts the greatest control on reaction rate, hence resolving power. For tropical sites (>25°C) age differences <1 ka can be resolved within the Holocene and Ca. 5 to 10 ka for older samples as the rate decreases; equilibrium is attained after 150 to 300 ka. The reaction rate is substantially lower at mid latitude sites (ca. + 10°C), where equilibrium requires about 2 Ma and resolution within the last glacial cycle is no better than 10 to 20 ka. Arctic sites (<−10°C) show almost no measurable racemization within the Holocene, and in extreme cases even within the last 100 ka; equilibrium is not attained in 10 Ma. The extent of amino acid racemization/epimerization in carbonate fossils provides chronological information for the critical time period beyond radiocarbon dating. dl ratios can be used directly as relative age indices, and with suitable calibration can provide reasonable estimates of absolute age and/or effective diagenetic temperature. Improving the accuracy of racemization-based age estimates requires a better understanding of racemization kinetics in carbonate systems.

Stratigraphy, sedimentology, chronology and palaeohydrology of Quaternary lacustrine deposits at Madigan Gulf, Lake Eyre, south Australia

Abstract Madigan Gulf is a large bay at the southern end of Lake Eyre North, a major ephemerally flooded playa in arid central Australia at the southwestern margin of a vast (1,300,000 km2) internal drainage basin. The stratigraphy and chronology of the Quaternary sequence in the gulf is described from 8 cores and a cliff exposure at the gulf margin. A number of depositional environments are recognised and their distinctive sedimentary components are described. Facies recognised include deep- and shallow-water lacustrine environments, dominated by surface-water processes, and dry or ephemerally flooded playa environments dominated by groundwater-controlled processes. Sedimentary components include terrigenous clastics from river inflow and shoreline erosion, carbonates of detrital, inorganic or biological origin and gypsum and halite evaporites. Carbonates and gypsum evaporites, precipitated within the basin, are frequently reworked as clastic components. The establishment of a preliminary chronology for the sequence, by the application of thermoluminescence, uranium/thorium disequilibrium, amino acid racemization and radiocarbon dating techniques, has allowed a reconstruction of the last 130 ka of Lake Eyre palaeohydrology. The wettest phase occurred during the last interglacial (early in oxygen isotope stage 5) when an enlarged Lake Eyre was up to 25 m deep. Subsequently there has been a number of dry periods separating successively less effective wet phases culminating in the deposition of a substantial halite salt crust around the time of the glacial maximum. The dry interludes are characterised by deflation of salts and sediment from the basin, a process controlled by lowering of the watertable. The record from Madigan Gulf demonstrates the dramatic and repetitive impact of lake deflation on the Quaternary record of Lake Eyre. In the early Holocene a minor, but mostly perennial, lacustrine event was terminated at about 3–4 ka when the modern ephemeral playa regime was established. The major catchment of Lake Eyre is located in the monsoon-watered areas of northern Australia. As demonstrated by large floodings of the modern ephemeral regime, major lacustrine episodes must indicate enhanced monsoon precipitation in northern Australia. In the Holocene the lake has not risen to levels achieved during the early stage 5 lacustral phase, indicating a marked reduction in the effectiveness of the monsoon in the present interglacial by comparison with its predecessor.

Aminostratigraphy of European marine interglacial deposits

Molluscan fossils collected from shallow water marine sediment across NW Europe and nearby Arctic regions have been analysed for the extent of isoleucine epimerization (DL ratio) in indigenous protein residues. The DL ratios confirm that essentially all ‘classical’ Eemian sites from NW Europe are of the same age, and are correlative with the type locality near Amersfoort in the Netherlands; shells from interglacial marine sediment beneath the type Weichselian till in Poland also correlate with the type Eemian site. DL ratios in Holsteinian marine shells (0.29) are substantially higher than in their Eemian counterparts (0.17); ‘Late Cromerian’ shells yield even higher ratios (0.46). DL ratios in late glacial shells (0.06) and Middle Weichselian shells (0.09) permit differentiation from modern (0.01) and last interglacial material. Based on the position of the Matuyama-Brunhes boundary and the differences in DL ratios, the Eemian must correlate with isotope substage 5e, whereas the Holsteinian is most likely substage 7c, possibly stage 9 but certainly younger than stage 11. Intra-Saalian warm periods may be terrestrial equivalents of the younger substages of stage 7. Extensive pre-Eemian marine sediments along the SW coast of Denmark previously correlated with the Holsteinian are shown to be of ‘Late Cromerian’ age. The underlying till there is the first widespread evidence of a pre-Elsterian till in NW Europe. DL ratios in molluscs from last interglacial sites along the Arctic coast of the USSR, the Arctic Islands and eastern Greenland are substantially lower than in their European counterparts due to their low thermal histories. The combined mid- and high-latitude data are used to develop a predictive model for the expected DL ratio in any of several moderate epimerization-rate taxa for last interglacial sites with mean temperatures between −20 and +15°C. Not all sites could be unambiguously assigned to an established interglacial. The Fjosanger (Norway) and Margareteberg (Sweden) sites previously thought to be Eemian, yield DL ratios higher than in secure nearby Eemian material. It is yet unresolved whether these are aberrant sites or if they predate the last interglacial. In situ shoreline deposits encountered in borings in SW Belgium and in exposures on the Belgium coastal plain contain molluscs that yield DL ratios intermediate between secure Eemian and Late Weichselian ratios, raising the possibility that a late stage 5 high-sea-level event attained near-modern levels in the southern North Sea basin. Resolution of these uncertainties is the focus of future work.

Aminostratigraphy of Quaternary shorelines in the Mediterranean basin

The age and correlation of shorelines around the Mediterranean basin have been addressed by analyzing the extent of isoleucine epimerization (aIle/Ile ratio) in protein preserved in molluscan fossils collected from raised marine deposits. The taxodont genera Glycymeris and Arca were selected as the primary taxa for this study because of their simple shell structure, reproducible aIle/Ile ratios, and ubiquitous occurrence. Direct comparison of aIle/Ile ratios in associated mollusks allows correlation of disjunct marine deposits and relative dating of sequential marine units in nearby areas that have similar thermal histories. The thermal gradient across the Mediterranean basin is, however, sufficiently high that shells from isochronous shorelines have significantly higher ratios at warmer than at cooler localities. Absolute dating, primarily U-series dates on corals directly associated with molluscan samples, provides an independent calibration of the amino acid data and compensates for dissimilar thermal histories. AIle/Ile ratios in shells from 46 marine units cluster into 5 discrete groups (C, E, F, G, and K) that are related to positive sea-level events (interglacials/interstadials) associated with odd-numbered deep-sea isotopic stages. The most complete sequences are in southern Italy, where group C ratios in Glycymeris that average 0.30 are associated with the Neotyrrhenian, a post–last-interglacial (late stage 5), high–sea-level event. Group E ratios (0.38) are associated with classical Eutyrrhenian deposits from which four U-series coral dates (126 ± 7 ka) substantiate the correlation to isotope substage 5e. Group F ratios (0.50) are associated with U-series coral and mollusk dates between 200 and 300 ka; the deposits are tentatively correlated with stage 7. Deposits that have group G ratios (0.58) are correlated with stage 9. Shells from lower Pleistocene marine deposits that have ratios between 1.0 and 1.2 (group K) constrain the ages of the younger groups. An exponential decrease in the epimerization rate inhibits resolution of the older events. AIle/Ile ratios in last-interglacial deposits are similar in the northern and central region of the study area but increase sharply in southern Sicily, North Africa, and Crete, similar to the modern thermal gradient. A lacuna between group G and group K is similar to gaps identified in California and Alaska, suggesting generally lower interglacial sea levels between stage 11 and sometime before the Brunhes/Matuyama boundary.