Jason P. Briner

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.

Cosmogenic exposure dating of late Pleistocene moraine stabilization in Alaska

Seventy-three new 1 0 Be/ 2 6 Al ages from 57 moraine boulders and 2 tors, together with 43 previously published cosmogenic exposure ages from 41 moraine boulders, allow us to critique the use of cosmogenic exposure (CE) dating of moraine boulders in Alaska. Boulder exhumation during moraine degradation likely gives rise to the largest uncertainty in constraining the timing of initial moraine stabilization following ice retreat. Isotopic inheritance appears to be most important for moraines deposited close to their cirque headwalls. Boulder-surface (bedrock) erosion rate can be roughly constrained and leads to a range in moraine stabilization ages. Snow-cover history is difficult to constrain, but its effect is thought to be minor for the tall boulders sampled. Despite these complications, the CE ages provide important new information regarding the timing of the last and penultimate glaciations in Alaska. Three penultimate moraines yielded CE ages that overlap with marine isotope stage (MIS) 4/early MIS 3 (45-65 ka) rather than MIS 6 (ca. 140 ka). Based on a combination of our new CE chronologies and existing 1 4 C ages from six study areas, glaciers retreated from their local late Wisconsin maxima: ca. 24-27 ka, Kokrines Hills (west-interior Alaska); ca. 24-26 ka, northeastern Brooks Range (NE Alaska); ca. 21-23 ka, Yukon Tanana Upland (east-interior Alaska); ca. 22 ka, Ahklun Mountains (SW Alaska); ca. 20 ka, western Alaska Range (central Alaska); ca. 16-18 ka, Chuilnuk Mountains (SW Alaska). Overall, glacier retreat was concurrent with the peak of the last global glacial maximum, probably in response to limited moisture availability.

Last Glacial Maximum ice sheet dynamics in Arctic Canada inferred from young erratics perched on ancient tors

Along-standing debate regarding the reconstruction of former ice sheets revolves around the use of relative weathering of landscapes, i.e., the assumption that highly weathered landscapes have not been recently glaciated. New cosmogenic isotope measurements from upland bedrock surfaces and erratics along the northeastern margin of the Laurentide Ice Sheet (LIS) shed light on this debate. 10 Be and 26 Al concentrations from three perched erratics, yielding cosmogenic exposure ages of 17–11 ka, are much lower than those measured in two unmodified, highly weathered tors upon which they lie, which yield cosmogenic exposure ages of >60 ka. These findings suggest that non-erosive ice covered weathered upland surfaces along the northeastern margin of the LIS during the last glacial maximum. These data challenge the use of relative weathering to define the margins of Pleistocene ice sheets. The juxtaposition of non-erosive ice over upland plateaus and erosive ice in adjacent fiords requires strong gradients in basal thermal regimes, suggestive of an ice-stream mode of glaciation. r 2003 Elsevier Science Ltd. All rights reserved.

Cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets

The interpretation of differentially weathered mountainous areas along the fringes of Pleistocene ice sheets is fundamental for determining ice-sheet behavior and thickness during the last glaciation. Two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice, but that it was differentially erosive as a function of its spatially variable basal thermal regime. Cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern Baffi n Island shed light on Laurentide Ice Sheet (LIS) dynamics and thickness. Tors on weathered upland surfaces have minimum 10 Be ages between ca. 50 and ca. 170 ka (n = 12), whereas the majority of erratics perched in the uplands range from ca. 10 to ca. 13 ka (n = 14), indicating that the whole landscape was glaciated during the Last Glacial Maximum (LGM). Glacially sculpted bedrock (n = 7) near sea level refl ects the age of deglaciation, and increasing amounts of isotopic inheritance in highelevation sculpted bedrock (n = 3), higher elevation intermediately weathered bedrock (n = 11), and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus. Cold-based glaciation of uplands was contemporaneous with warm-based glaciation in fi ords, suggesting the presence of ice streams. 26 Al and 10 Be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure, burial, and limited glacial erosion. The paired isotope data reveal a minimum duration of burial of 80‐500 k.y., presumably by cold-based ice, and indicate that the tors have an age of at least 150‐580 ka. These data, together with other reconstructions of ice streams in the eastern Canadian Arctic, suggest that ice streams were a large infl uence on northeastern LIS dynamics throughout the late Quaternary. Thus, the northeastern LIS was sensitively tied with North Atlantic thermohaline circulation and abrupt climate change.

Using inherited cosmogenic 36Cl to constrain glacial erosion rates of the Cordilleran ice sheet

Cosmogenic 36 Cl/Cl ratios measured from glacially eroded bedrock provide the first quantitative constraints on the magnitude, rate, and spatial distribution of glacial erosion over the last glacial cycle. Of 23 36 Cl/Cl ratios, 8 yield exposure ages that predate the well-constrained deglaciation of the Puget Lowland, Washington, and are inferred to result from 36 Cl inherited from prior exposure during the last interglaciation where ice did not erode enough rock (~1.80‐2.95 m) to reset 36 Cl/Cl ratios to background levels. Surfaces possessing inherited 36 Cl

Response of Jakobshavn Isbrae, Greenland, to Holocene climate change

Rapid fluctuations in the velocity of Greenland Ice Sheet (GIS) outlet glaciers over the past decade have made it difficult to extrapolate ice-sheet change into the future. This significant short-term variability highlights the need for geologic records of preinstrumental GIS margin fluctuations in order to better predict future GIS response to climate change. Using 10 Be surface exposure ages and radiocarbon-dated lake sediments, we constructed a detailed chronology of ice-margin fluctuations over the past 10 k.y. for Jakobshavn Isbrae, Greenland9s largest outlet glacier. In addition, we present new estimates of corresponding local temperature changes using a continuous record of insect (Chironomidae) remains preserved in lake sediments. We find that following an early Holocene advance just prior to 8 ka, Jakobshavn Isbrae retreated rapidly at a rate of ∼100 m yr −1 , likely in response to increasing regional and local temperatures. Ice remained behind its present margin for ∼7 k.y. during a warm period in the middle Holocene with sustained temperatures ∼2 °C warmer than today, then the land-based margin advanced at least 2–4 km between A.D. 1500–1640 and A.D. 1850. The ice margin near Jakobshavn thus underwent large and rapid adjustments in response to relatively modest centennial-scale Holocene temperature changes, which may foreshadow GIS response to future warming.

The Goldilocks dilemma: big ice, little ice, or “just-right” ice in the Eastern Canadian Arctic ☆

Our conceptions of the NE sector of the Laurentide Ice Sheet (LIS) at the Last Glacial Maximum (LGM) have evolved through three major paradigms over the past 50 years. Until the late 1960s the conventional view was that the Eastern Canadian Arctic preserved only a simple deglacial sequence from a LIS margin everywhere at the continental shelf edge (Flint Paradigm). Glacial geologic field studies began in earnest in this region in the early 1960s, and within the first decade field evidence documenting undisturbed deposits predating the LGM led a pendulum swing to a consensus view that large coastal stretches of the Eastern Canadian Arctic remained free of actively eroding glacial ice at the LGM, and that the most extensive ice margins occurred early in the last glacial cycle. This Minimalist Paradigm dominated until the late1980s when an expanded data set from shallow marine studies indicated LGM ice at least locally reached the continental shelf. Within the past decade the marine data, coupled with new evidence from lake sediments and cosmogenic exposure dates on moraines and glaciated terrain in the Eastern Canadian Arctic has led to a new paradigm that better reconciles the terrestrial and marine evidence. Collectively, these lines of evidence indicate that all of southern Baffin Island was glaciated at the LGM, but that some coastal uplands north of Cumberland Sound remained above the limit of actively eroding glacial ice, even though outlet glaciers reached the continental shelf in front of most fiords and sounds. The most plausible explanation for the observed glacial limits is that low-gradient, relatively fast-moving outlet glaciers sliding on deformable sediments occupied marine embayments and fiords, contrasting with slower moving ice frozen to its bed on the intervening crystalline terrain. Slow-moving ice frozen to its bed would have had steeper surface gradients, hence would have terminated inland from the coast. This scenario is consistent with observations indicating high-elevation coastal terrain remained unglaciated even though outlet glaciers reached the continental shelf. LGM summer temperatures were apparently much lower than present, as lakes in the ice-free regions were permanently frozen.

Recent changes in a remote Arctic lake are unique within the past 200,000 years

The Arctic is currently undergoing dramatic environmental transformations, but it remains largely unknown how these changes compare with long-term natural variability. Here we present a lake sediment sequence from the Canadian Arctic that records warm periods of the past 200,000 years, including the 20th century. This record provides a perspective on recent changes in the Arctic and predates by approximately 80,000 years the oldest stratigraphically intact ice core recovered from the Greenland Ice Sheet. The early Holocene and the warmest part of the Last Interglacial (Marine Isotope Stage or MIS 5e) were the only periods of the past 200,000 years with summer temperatures comparable to or exceeding todays at this site. Paleoecological and geochemical data indicate that the past three interglacial periods were characterized by similar trajectories in temperature, lake biology, and lakewater pH, all of which tracked orbitally-driven solar insolation. In recent decades, however, the study site has deviated from this recurring natural pattern and has entered an environmental regime that is unique within the past 200 millennia.

Glacier readvance during the late glacial (Younger Dryas?) in the Ahklun Mountains, southwestern Alaska

An expansion of alpine glaciers during the latest Pleistocene produced an extraordinarily well defined end moraine system in the Ahklun Mountains, southwestern Alaska. These moraines, deposited during the Mount Waskey advance, are several kilometers beyond modern glacier termini, and ∼80 km upvalley of the late Wisconsin Ahklun Mountains ice cap terminal moraine. Eleven cosmogenic 10 Be and 26 Al exposure ages on moraine boulders, combined with radiocarbon ages from a lake core upvalley of a moraine deposited during the Mount Waskey advance, suggest that the advance culminated between 12.4 and 11.0 ka, sometime during, or shortly following, the Younger Dryas event (ca. 12.9–11.6 ka). We believe that the Mount Waskey advance was a consequence of cooling during the Younger Dryas. These data further strengthen emerging evidence for Younger Dryas–age cooling of the North Pacific region.

Response of a marine‐terminating Greenland outlet glacier to abrupt cooling 8200 and 9300 years ago

Greenland’s largest outlet glacier, using 10 Be surface exposure ages and 14 C‐dated lake sediments. Our chronology of ice‐ margin change demonstrates that Jakobshavn Isbrae advanced to deposit moraines in response to abrupt cooling recorded in central Greenland ice cores ca. 8,200 and 9,300 years ago. While the rapid, dynamically aided retreat of many Greenland outlet glaciers in response to warming is well documented, these results indicate that marine‐terminating outlet glaciers are also able to respond quickly to cooling. We suggest that short lag times of high ice flux margins enable a greater magnitude response of marine‐terminating outlets to abrupt