Julie Brigham-Grette

2.8 Million Years of Arctic Climate Change from Lake El’gygytgyn, NE Russia

Crater Core The high-northern latitudes of the Arctic have an important influence on climate and constitute a region with a unique array of complex feedbacks that make it difficult to understand the workings of its climate. Melles et al. (p. 315, published online 21 June) developed a 2.8-million-year record of Arctic climate, using a sediment core from a lake in northeastern Russia that was formed more than 3.5 million years ago by a meteorite impact. Pronounced glacial episodes began 2.6 million years ago but did not achieve orbital pacing for another 700,000 years. A sediment core from a Russian lake provides a high-latitude climate record where prior terrestrial records have been sparse. The reliability of Arctic climate predictions is currently hampered by insufficient knowledge of natural climate variability in the past. A sediment core from Lake El’gygytgyn in northeastern (NE) Russia provides a continuous, high-resolution record from the Arctic, spanning the past 2.8 million years. This core reveals numerous “super interglacials” during the Quaternary; for marine benthic isotope stages (MIS) 11c and 31, maximum summer temperatures and annual precipitation values are ~4° to 5°C and ~300 millimeters higher than those of MIS 1 and 5e. Climate simulations show that these extreme warm conditions are difficult to explain with greenhouse gas and astronomical forcing alone, implying the importance of amplifying feedbacks and far field influences. The timing of Arctic warming relative to West Antarctic Ice Sheet retreats implies strong interhemispheric climate connectivity.

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.

Rapid sea-level rise and Holocene climate in the Chukchi Sea

Three new sediment cores from the Chukchi Sea preserve a record of local paleoenvironment, sedimentation, and flooding of the Chukchi Shelf (50 m) by glacial-eustatic sea-level rise. Radiocarbon dates on foraminifera provide the first marine evidence that the sea invaded Hope Valley (southern Chukchi Sea, 53 m) as early as 12 ka. The lack of significant sediment accumulation since ca. 7 ka in Hope Valley, southeastern Chukchi Shelf, is consistent with decreased sediment supply and fluvial discharge to the shelf as deglaciation of Alaska concluded. Abundant benthic foraminifera from a site west of Barrow Canyon indicate that surface waters were more productive 4‐6 ka, and this productivity varied on centennial time scales. An offshore companion to this core contains a 20 m record of the Holocene. These results show that carefully selected core sites from the western Arctic Ocean can have a temporal resolution equal to the best cores from other regions, and that these sites can be exploited for high-resolution studies of the paleoenvironment.

Aminostratigraphic correlations and paleotemperature implications, Pliocene-Pleistocene high-sea-level deposits, northwestern Alaska

Abstract Multiple periods of Late Pliocene and Pleistocene high sea level are recorded by surficial deposits along the coastal plains of northwestern Alaska. Analyses of the extent of amino acid epimerization in fossil molluscan shells from the Nome coastal plain of the northern Bering Sea coast, and from the Alaskan Arctic Coastal Plain of the Chukchi and Beaufort Sea coasts, allow recognition of at least five intervals of higher-than-present relative sea level. Three Late Pliocene transgressions are represented at Nome by the complex and protracted Beringian transgression, and on the Arctic Coastal Plain by the Colvillian, Bigbendian, and Fishcreekian transgressions. These were followed by a lengthy period of non-marine deposition during the Early Pleistocene when sea level did not reach above its present position. A Middle Pleistocene high-sea-level event is represented at Nome by the Anvilian transgression, and on the Arctic Coastal Plain by the Wainwrightian transgression. Anvilian deposits at the type locality are considerably younger than previously thought, perhaps as young as Oxygen-Isotope Stage 11 (∼410,000 BP). Finally, the last interglacial Pelukian transgression is represented discontinuously along the shores of northwestern Alaska. Amino acid epimerization data, together with previous paleomagnetic measurements, radiometric-age determinations, and paleontologic evidence provide geochronological constraints on the sequence of marine deposits. They form the basis of regional correlations and offer a means of evaluating the post-depositional thermal history of the high-sea-level deposits. Provisional correlations between marine units at Nome and the Artic Coastal Plain indicate that the temperature difference that separates the two sites today had existed by about 3.0 Ma. Since that time, the effective diagenetic temperature was lowered by about 3–4°C at both sites, and the mean annual temperature was lowered considerably more. This temperature decrease was largely accomplished by the close of the Fishcreekian = Beringian III transgression (ca. 2.5-2.1 Ma). Since then, intervals of warm temperature must have been extremely brief. These data suggest that the steep latitudinal temperature gradient and the frigid temperatures that characterize the high latitudes of Alaska today are ancient features of Arctic climate.

Chlorine-36 and 14C chronology support a limited last glacial maximum across central chukotka, northeastern Siberia, and no Beringian ice sheet

Abstract The Pekulney Mountains and adjacent Tanyurer River valley are key regions for examining the nature of glaciation across much of northeast Russia. Twelve new cosmogenic isotope ages and 14 new radiocarbon ages in concert with morphometric analyses and terrace stratigraphy constrain the timing of glaciation in this region of central Chukotka. The Sartan Glaciation (Last Glacial Maximum) was limited in extent in the Pekulney Mountains and dates to ∼20,000 yr ago. Cosmogenic isotope ages > 30,000 yr as well as non-finite radiocarbon ages imply an estimated age no younger than the Zyryan Glaciation (early Wisconsinan) for large sets of moraines found in the central Tanyurer Valley. Slope angles on these loess-mantled ridges are less than a few degrees and crest widths are an order of magnitude greater than those found on the younger Sartan moraines. The most extensive moraines in the lower Tanyurer Valley are most subdued implying an even older, probable middle Pleistocene age. This research provides direct field evidence against Grosswald’s Beringian ice-sheet hypothesis.

Last Interglacial (isotope stage 5) glacial and sea-level history of coastal Chukotka Peninsula and St. Lawrence Island, Western Beringia

Abstract Study of glacial and marine sequences along the outer coast of Chukotka Peninsula, Bering Strait, is the basis for major revisions of the regional stratigraphy. The Val’katlen Suite at its type section at the mouth of the Enmelen River records the youngest high sea stand of the last interglaciation on Chukotka Peninsula. Marine deposits and glacial diamicton stratigraphically below the type section record, respectfully, the peak of the last interglacial (marine oxygen-isotope substage 5e) and rapid glacierization of the coastal mountains probably during substage 5d or 5b. Correlative deposits recording a similar sequence of intra-stage 5 events, but once thought to be of early and middle Pleistocene age, include the Upper and Lower Pinakul’ Suite at Cape Pinakul’ and marine deposits near the Nunyamo River on the Gulf of Anadyr, based upon amino-acid analyses, biostratigraphy, and supporting geochronology. These deposits enclose warmer-than-present faunas and floras comparible to last interglacial Pelukian marine deposits found along the coast of Alaska. The last major advance of glacial ice from Chukotka Peninsula across Anadyr Strait and onto St. Lawrence Island was likely initiated during the later part of stage 5 based on our reinterpretaion of the stratigraphy at Cape Pinakul’, near the Nunyamo River, and on northwestern St. Lawrence Island. Chukotkan ice probably reached St. Lawrence Island sometime during oxygen-isotope stage 4. Ice extend across Chukotka during the LGM was very imited.

The last interglaciation in Alaska: Stratigraphy and paleoecology of potential sites

At least 20 localities in Alaska contain deposits that may provide information on the last interglaciation (Oxygen-Isotope Substage 5e). These widely dispersed localities include river bluffs, coastal bluffs and terraces, elevated marine shorelines, lake basins, and artificial excavations. Most of the inferred interglacial deposits contain macrofossils or pollen that are older than the range of radiocarbon dating and commonly indicate climate as warm as or warmer than the present. At a few localities, evidence for deep thaw of permafrost also indicates a warm paleoclimate. At eight localities, the Old Crow tephra occurs at or below organic deposits that may represent Substage 5e. The tephra occurs beneath conspicuous organic deposits at Fairbanks, the Yukon Palisades, and Holitna lowland, and directly above a peat bed at Hogatza Mine. At Birch Creek, Halfway House, Ky-11, and Imuruk Lake, the tephra occurs within a paleosol or organic deposit, but other organic horizons that more likely indicate interglacial conditions occur at higher stratigraphic levels. The varied stratigraphic relations of the Old Crow tephra suggest that it may have been deposited close to the boundary between Isotope Substages 6 and 5, which is dated at about 130 ka in the marine record and between 132 and 140 ka on land. These age relations suggests that the tephra may have been deposited about 135 ± 5 ka, validating the recent fission-track age determination of 140 ± 10 ka for this deposit. Six coastal localities contain deposits of probable interglacial age, and these commonly are associated with evidence for eustatic sea levels higher than those of the present. Beach and sublittoral sediments of the Pelukian transgression occur up to 12 m asl along the northwest coast of Alaska, and are correlative with barrier island and lagoonal sediments on the Alaskan Arctic Coastal Plain. Both sets of deposits commonly contain extralimital mollusks and microfauna that indicate marine water slightly warmer than present and suggest that seasonal sea ice did not extend south of Bering Strait during the last interglacial as it does today. Farther south, elevated marine-terrace deposits on Amchitka Island contain marine invertebrates that indicate a climate warmer than at present. Peat horizons in coastal exposure at Goose Bay and coastal terraces at Lituya Bay contain pollen spectra that suggest forests like those of the present day, and spruce macrofossils exposed on Baldwin Peninsula indicate boreal forest more extensive than at present. Sediments from several lakes in northwestern Alaska may contain continuous records of the last interglaciation. A major warm interval, possibly Isotope Substage 5e, has been identified in a core from Squirrel Lake by a peak in Picea pollen that indicates forest extension beyond present limits. Similar pollen records are potentially available from two maars which formed in the Cape Espenberg area more than 125 ka. Terrestrial organic deposits thought to record the last interglaciation occur interstratified with marine and glaciogenic sediments in the Nushagak Lowland of southwest Alaska and on Baldwin Peninsula in Kotzebue Sound. Extensive exposures along the Copper and Nenana Rivers may also contain organic deposits that record the last interglaciation.

Late Cenozoic Arctic Ocean sea ice and terrestrial paleoclimate.

Sea otter remains found in deposits of two marine transgressions (Bigbendian and Fishcreekian) of the Alaskan Arctic Coastal Plain which occurred between 2.4 and 3 Ma suggest that during these two events the southern limit of seasonal sea ice was at least 1600 km farther north than at present in Alaskan waters. Perennial sea ice must have been severely restricted or absent, and winters were warmer than at present during these two sea-level highstands. Paleomagnetic, faunal, and palynological data indicate that the later transgression (Fishcreekian) occurred during the early part of the Matuyama Reversed-Polarity Chron. Amino acid diagenesis in fossil mollusks suggests that since the later transgression the effective diagenetic temperature (EDT) in the deposits has been about −16 °C, which is about 7 °C colder than modern values and slightly colder than the EDT calculated for the past 125 ka. Such a low EDT suggests that permafrost and perennial sea ice have been present nearly continuously since this transgression. Permafrost probably was absent, however, during the earlier (Bigbendian) transgression. Permafrost and extensive perennial sea ice may have been initiated during the late stages of climatic cooling that spanned the Gauss Normal-Matuyama Reversed-Polarity Chron boundary and led into the first major late Cenozoic glaciation of the Northern Hemisphere.

Evidence for restricted ice extent during the last glacial maximum in the Koryak Mountains of Chukotka, far eastern Russia

Field evidence in the Koryak Mountains‐ Lake Mainitz region of far eastern Russia supports three Pleistocene glacial advances. The early Wisconsinan and pre-Wisconsinan glaciations are represented by broad lobate moraines extending as much as 30 km north of the Koryak Mountains. Field evidence demonstrates that the terminal, lateral, and medial moraines, as well as meltwater channels, dead ice topography, kettles, and outwash plains mark the extent of ice during the last glacial maximum (LGM), during which glaciers reached no more than 20 km beyond their present limits. Those emanating from the southern Koryak Mountains may have reached the Bering Sea. Numerical and relative dating techniques support these results and test the theoretical models of the LGM in western Beringia. Erratics on moraines and glaciofluvial terraces, common to all valleys at 13‐13.8 m above river level, yield 36 Cl exposure ages ranging from 10.08 to 25.78 ka. The Koryak Mountains‐Lake Mainitz record of glaciation is spatially and temporally consistent with the glaciation pattern across central Beringia found in other terrestrial and marine records. Glacier growth in the Koryak Mountains was sustained by possible increased summer sea surface temperatures and precipitation in the northwest Pacific region. Evidence from the northern Koryak Mountains (lat 64°N, long 177°E) indicates that the extent of ice in western Beringia was limited to mountain and valley glaciers during the LGM. This fieldbased research contradicts M.G. Grosswald’s theoretical Beringian ice sheet hypothesis.

Pleistocene raised marine deposits on Wrangel Island, northeast Siberia and implications for the presence of an East Siberian ice sheet

Two previously undocumented Pleistocene marine transgressions on Wrangel Island, northeastern Siberia, question the presence of an East Siberian or Beringian ice sheet during the last glacial maximum (LGM). The Tundrovayan Transgression (459,000–780,000 yr B.P.) is represented by raised marine deposits and landforms 15–41 m asl located up to 18 km inland. The presence of high sea level 64,000–73,000 yr ago (the Krasny Flagian Transgression) is preserved in deposits and landforms 4–7 m asl in the Krasny Flag valley. These deposits and landforms were mapped, dated, and described using amino acid geochronology, radiocarbon, optically stimulated luminescence, electron spin resonance, oxygen isotopes, micropaleontology, paleomagnetism, and grain sizes. The marine deposits are eustatic and not isostatic in origin. All marine deposits on Wrangel Island predate the LGM, indicating that neither Wrangel Island nor the East Siberian or Chukchi Seas experienced extensive glaciation over the last 64,000 yr.