Fission-track Age Determinations Research Articles

LA-ICP-MS apatite fission track dating: A practical zeta-based approach

The LA-ICP-MS method is becoming increasingly popular for uranium determinations in fission track dating of apatite, zircon or titanite. This is because the approach has several advantages over the classical external detector method (EDM), including faster sample throughput, simultaneous acquisition of additional data (such as U-Pb age information and trace element abundances), while removing the need for neutron irradiation. Two different approaches are used to determine U contents in LA-ICP-MS fission track dating: an absolute dating approach, or a zeta-based determination analogous to the classical EDM. Absolute age dating by LA-ICP-MS potentially suffers from small but systematic deviations in apatite U contents, which in turn propagate through to minor systematic deviations in the accuracy of absolute fission track age determinations. A zeta-based approach typically requires time-consuming counting of large numbers of zeta-standard grains (usually Durango apatite) so as to yield a precise zeta factor for every LA-ICP-MS session containing unknowns. The modification of the zeta-based approach proposed here has two major advantages. Firstly, it employs just one large primary LA-ICP-MS session to determine a precise primary zeta factor on a large number of counted Durango primary zeta grains. During subsequent secondary LA-ICP-MS sessions with unknowns, no further fission track counting of the primary zeta standard is required. This is because we reanalyse a subset of the primary zeta grains to calculate a session-specific zeta fractionation factor, which is related to variations in the instrumental operating conditions (primarily plasma tuning) between primary and secondary LA-ICP-MS sessions. This enables us to ‘reuse’ the primary zeta factor, and thus avail of its precision derived from the large spontaneous track count. The second advantage is that reusing the primary zeta grains by applying a session-specific zeta fractionation factor allows us to verify that background and drift corrections applied during the secondary LA-ICP-MS session were fully appropriate. This method has been successfully tested by dating samples of known apatite fission track age, by comparing EDM and LA-ICP-MS data from the same sample and by participating in a round robin test between international fission track laboratories where ‘blind’ fission track dating of two unknown samples was undertaken. Our LA-ICP-MS apatite fission track dating approach is also easily modifiable for fission track dating of zircon or titanite if suitable age standards are employed.

Distinction between the Youngest Toba Tuff and Oldest Toba Tuff from northern Sumatra based on the area density of spontaneous fission tracks in their glass shards

Determination of the area density of spontaneous fission tracks (ρs) in glass shards of Toba tephra is a reliable way to distinguish between the Youngest Toba Tuff (YTT) and the Oldest Toba Tuff (OTT). The ρs values for YTT, uncorrected for partial track fading, range from 70 to 181tracks/cm2 with a weighted mean of 108±5tracks/cm2, based on 15 samples. Corrected ρs values for YTT are in the range of 77–140tracks/cm2 with a weighted mean of 113±8tracks/cm2, within the range of uncorrected ρs values. No significant difference in ρs exists between YTT samples collected from marine and continental depositional settings. The uncorrected ρs for OTT is 1567±114tracks/cm2 so that confusion with YTT is unlikely.The ρs values of the Toba tephra at Bori, Morgaon, and Gandhigram in northwestern India indicate a YTT identity, in agreement with geochemical data on their glass shards, the presence of multiple glass populations, and a glass fission-track age determination. Therefore, the view of others that OTT is present at these sites – and thereby indicates an early Pleistocene age for the associated Acheulean artifacts – is incorrect.

Novel calibration for LA-ICP-MS-based fission-track thermochronology

We present a novel age-equation calibration for fission-track age determinations by laser ablation inductively coupled plasma mass spectrometry. This new calibration incorporates the efficiency factor of an internal surface, [ηq]is, which is obtained by measuring the projected fission-track length, allowing the determination of FT ages directly using the recommended spontaneous fission decay constant. Also, the uranium concentrations in apatite samples are determined using a Durango (Dur-2, 7.44 μg/g U) crystal and a Mud Tank (MT-7, 6.88 μg/g U) crystal as uranium reference materials. The use of matrix-matched reference materials allows a reduction in the uncertainty of the uranium measurements to those related to counting statistics, which are ca. 1 % taking into account that no extra source of uncertainty has to be considered. The equations as well as the matrix-matched reference materials are evaluated using well-dated samples from Durango, Fish Canyon Tuff, and Limberg as unknown samples. The results compare well with their respective published ages determined through other dating methods. Additionally, the results agree with traditional fission-track ages using both the zeta approach and the absolute approach, suggesting that the calibration presented in this work can be robustly applied in geological context. Furthermore, considering that fission-track ages can be determined without an age standard sample, the fission-track thermochronology approach presented here is assumed to be a valuable dating tool.

Revision of the tephrostratigraphy of the lower Sixtymile River area, Yukon Territory, Canada

Five distinctive tephra beds are preserved in fine-grained sediments on a prominent terrace strath along the lower Sixtymile River of the Yukon Territory. Commencing with the oldest unit, they are TA, SM3, Gold Run, Hollis 2, and Flat Creek tephra beds. The uppermost three tephra beds have rhyolitic compositions, the other two are bimodal with a much more basic glass phase. Three glass fission-track age determinations point to deposition during the early-Middle Pleistocene. We correct an error in an earlier study by clarifying that Flat Creek tephra is younger than Gold Run tephra, TA tephra is older.

Role of topography in isotherm perturbation: Apatite (U‐Th)/He and fission track results from the Malta tunnel, Tauern Window, Austria

Apatite (U‐Th)/He and fission track age determinations of samples from a 20‐km‐long, near‐horizontal tunnel in the Hochalm‐Ankogel Dome (eastern Tauern Window, Austria) are presented in order to determine the role of (paleo)‐topography in perturbing isotherms in the shallow crust. Apatite fission track ages (26–8 Ma) show no systematic correlation with distance along the tunnel or elevation. Two age components in the fission track data indicate cooling through ∼120°C at approximately 20 Ma and ∼80°C at approximately 6 Ma. Surface and tunnel (U‐Th)/He ages (17–9 Ma and 13–5 Ma, respectively) are consistently younger than the equivalent fission track ages. (U‐Th)/He ages vary systematically along the tunnel with older ages at the northern and southern tunnel portals and younger ages in the central section. Geological factors (faulting, lithology) appear to have had little effect on this age distribution. The (U‐Th)/He age pattern is inconsistent with rock cooling underneath the present‐day Hochalm‐Ankogel Dome topography. The age minimum is interpreted to coincide with a paleotopographic maximum in the Hochalm‐Ankogel Dome and suggests that the 40–60°C isotherms were warped sufficiently to affect apatite (U‐Th)/He ages. The tunnel apatite (U‐Th)/He ages match synthetic He ages derived from a two‐dimensional numerical model of landscape evolution. Integrating the thermal model results with the data allows the relief development to be constrained and we calculate that the present‐day Hochalm‐Ankogel Dome topography was formed at 7 to 10 Ma.

An Albian–Cenomanian unconformity in the northern Andes: Evidence and tectonic significance

The Villeta Group of Colombia and equivalent stratigraphic units of Venezuela and Ecuador comprise marine sequences ranging from Albian to Santonian in age. Deposition of the Villeta Group was presumed to take place entirely in quiet tectonic conditions in a passive margin setting that occupied NW South America. From a large database of 2D/3D seismic, well, surface geology, and biostratigraphic data, we present evidence for intra-Villeta (mostly late Albian–Cenomanian) deformation in parts of the Upper Magdalena Valley and Eastern Cordillera of Colombia, controlled by transpressional fault reactivation, produced by transpressional fault reactivation and thrusting that resulted in an angular unconformity. This event has been largely unnoticed in the literature, but previously scattered evidence supports our observations, suggesting regionally extensive tectonism. Published fission-track age determinations and other geologic evidence from Colombia and Venezuela suggest significant uplifts around 80–100 m.y., which may reflect changes in the subduction regime, with compressional deformation in certain regions and extensional deformation in others. A late Albian onset of compressional deformation along the Colombian and Peruvian segments of the Andes may be related to the opening of the South Atlantic Ocean at equatorial latitudes. Identification of tectonic activity with development of an unconformity in intra-Villeta times provides new insights into understanding the evolution of the Upper Magdalena Valley and adjacent areas of Colombia and western Venezuela and creates new possibilities for hydrocarbon exploration, with additional trapping phases, better reservoir preservation by early migration and secondary porosity, and ultimately facies changes with stratigraphic potential.

Fission-track analysis of meteorites: Dating of the Marjalahti pallasite

The results of the Marjalahti pallasite fission-track age determination are presented. Thorough examination of fossil tracks in the phosphate (whitlockite) crystals coupled with U-content determination in whitlockites can make it possible to estimate the contributions of all possible track sources to the total track density and to calculate a model fission-track age. It is found that whitlockite crystals of the Marjalahti pallasite contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); fission of U and Th induced by cosmic rays; spontaneous fission of 238U; and spontaneous fission of extinct, short-lived 244Pu present in significant quantities in the early solar system. A great track density attributed to the extinct 244Pu testifies to the high fission-track age. The model fission-track ages of ( 4.31 ± 0.02 ) × 10 9 yr for the Marjalahti pallasite are calculated. Petrographic studies allow us to interpret the fission-track age as the time of the last shock/thermal event in the cosmic history of the pallasite.

Igneous rocks emplacement and exhumation of sedimentary basement: Fission track age determination on the Osuzuyama volcano-plutonic complex and surrounding rocks, Miyazaki prefecture, Southwest Japan

Fourteen fission track apatite ages and thirteen zircon ages are measured for the Osuzuyama volcanoplutonic complex (OVPC) and the surrounding Hyuga unit in Kyushu, Southwest Japan. The average apatite OVPC age is 12.6 ± 0.9 (±1σ) Ma, slightly younger than the average zircon age of 15.2 ± 0.5 (±1σ) Ma which is consistent with K-Ar ages reported before. Apatite ages of the nearby Hyuga unit are more or less older than the average apatite OVPC age, suggesting that the OVPC age would represent time of slow cooling after intrusion rather than the time of exhumation. Compared to the previously reported apatite data from the Shimanto accretionary complex at the north of the studied area, the age here is older and indicates less exhumation of the studied region. Given reasonable assumptions, the difference in the amount of exhumation since Miocene is estimated being larger than ∼1 km, probably controlled by the material to be exhumed and local geological setting.

Fission Track Dating of Phosphate Minerals and the Thermochronology of Apatite

Several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method. Of these, apatite [Ca5(PO4)3(F,Cl,OH)], has proved pre-eminently suitable for this purpose for reasons discussed below. Apatite was one of the first minerals, amongst many others, to be investigated for fission track dating by Fleischer and Price (1964) and has subsequently become by far the most important of all the minerals used for dating by this method. The usefulness of apatites for fission track dating arises from their near-universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups. Price and Walker (1963) first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238U within their lattices could be used for geological dating. They also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope (Price and Walker 1962b, 1963). This simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses (e.g., Fleischer et al 1975), the most important of which was fission track dating. Fleischer et al. (1964) also showed that spontaneous, or fossil, fission tracks provided an explanation for various ‘anomalous’ etch pits in apatite which had puzzled crystallographers over many years. The essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present. Knowing the rate of spontaneous fission decay, a geological age can be calculated. Detailed …

APATITE FROM AN OLIVINE PYROXENITE OF THE CANAÃ VALLEY, ESPÍRITO SANTO, BRAZIL: FISSION-TRACK AGE DETERMINATION AND CONFINED TRACK LENGTH MEASUREMENTS IN ORDER TO ESTABLISH AN INTERNAL STANDARD

We present the first results of fission-track age determinations and natural confined track length measurements in apatite crystals of an olivine pyroxenite from the Canaa Valley, Espirito Santo, Brazil. This apatite was chosen for its high abundance in the rock and also for its high uranium content. Confined track length measurements were performed by three different analysts, whereas fission-track ages corresponding to two different irradiations were determined by two analysts. Confined track length distributions as well as ages showed good reproducibility. The characteristics of this apatite (its abundance in the rock and the relatively large number of confined tracks) make it a very promising internal standard which experienced a more complex thermal history in comparison with the volcanic apatite international age standards. The fission-track age of around 95 Ma of the apatite sample from the olivine pyroxenite is consistent with the age interval 65-110 Ma previously measured by Fonseca and Poupeau (1984), Fonseca (1989) and Fonseca et al. (1992) in the Ribeira Belt. This age interval corresponds to the basaltic and alkaline volcanic activities related to the opening of the Atlantic Ocean.

APATITE FROM AN OLIVINE PYROXENITE OF THE CANAÃ VALLEY, ESPÍRITO SANTO, BRAZIL: FISSION-TRACK AGE DETERMINATION AND CONFINED TRACK LENGTH MEASUREMENTS IN ORDER TO ESTABLISH AN INTERNAL STANDARD

We present the first results of fission-track age determinations and natural confined track length measurements in apatite crystals of an olivine pyroxenite from the Canaã Valley, Espírito Santo, Brazil. This apatite was chosen for its high abundance in the rock and also for its high uranium content. Confined track length measurements were performed by three different analysts, whereas fission-track ages corresponding to two different irradiations were determined by two analysts. Confined track length distributions as well as ages showed good reproducibility. The characteristics of this apatite (its abundance in the rock and the relatively large number of confined tracks) make it a very promising internal standard which experienced a more complex thermal history in comparison with the volcanic apatite international age standards. The fission-track age of around 95 Ma of the apatite sample from the olivine pyroxenite is consistent with the age interval 65-110 Ma previously measured by Fonseca and Poupeau (1984), Fonseca (1989) and Fonseca et al. (1992) in the Ribeira Belt. This age interval corresponds to the basaltic and alkaline volcanic activities related to the opening of the Atlantic Ocean.

山形県新庄盆地に分布する折渡層のフィッション・トラック年代

Fission-track age determination on the Cenozoic marine sedimentary sequences distributed in the Shinjo Basin, Yamagata Prefecture was carried out. Though the upper Sakekawa and Oriwatari Formations have been considered to be late Pliocene-Pleistocene in previous researches, the present result of FT dating on the volcanic tuffs intercalated in the basal part of the Oriwatari Formation showed 4.3-4.6 Ma the early Pliocene age.

Multi-method dating comparison for mid-pleistocene Rangitawa Tephra, New Zealand

The Rangitawa Tephra, a widespread mid-Pleistocene marker in the New Zealand region, is dated by fission-track, thermoluminescence (TL), infrared stimulated luminescence (IRSL), K Ar and single crystal laser fusion (SCLF) 40 Ar 39 Ar methods, as well as by matching to the astronomical timescale via oxygen isotope stratigraphy in deep sea cores. Fifty one independent numerical age estimates for Rangitawa Tephra range from 173 to 456 ka. However, only 15 of these results are considered to be reliable: five zircon fission-track results, four isothermal plateau fission-track (ITPFT) age estimates on glass shards, four SCLF 40 Ar 39 Ar results, and two astronomically calibrated age estimates from deep sea cores. The weighted mean of the nine reliable fission-track age determinations is 345 ± 12 ka, in excellent agreement with the weighted mean of 340 ± 7 ka for the two reliable astronomically calibrated age estimates. The weighted mean of the four reliable 40 Ar 39 Ar results (302 ± 8 ka) is significantly younger, and further Ar Ar dating is recommended to investigate this difference. Since the TL and IRSL dating methods are experimental for sediments beyond 200 ka, we have excluded these from the group of reliable age results. TL and IRSL results on dunesand appear to be systematically younger than TL results on loess, and loess results depend upon analytical procedures.

Isothermal plateau fission track age determinations on volcanic glass from pleistocene tephra in New Zealand : Alloway, B.V., Sandhu, A.S., Westgate, J.A., Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 3B1; Walter, R.C. Geochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, CA 94709

Isothermal plateau fission track age determinations on volcanic glass from pleistocene tephra in New Zealand : Alloway, B.V., Sandhu, A.S., Westgate, J.A., Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 3B1; Walter, R.C. Geochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, CA 94709

Structure and deformation history of the northern range of Trinidad and adjacent areas

Conflicting models have been proposed for both the evolution of northern South America and the neotectonics of the south Caribbean plate boundary zone. The Trinidadian portion of the margin is particularly controversial, but surprisingly it has been little studied. We present a structural analysis of Trinidad's Northern Range, pertinent updates of the island's stratigraphy and sedimentology, and new zircon fission track age determinations, and use them to constrain Trinidad's geologic history, and to better understand the controlling tectonic processes. In our interpretation Trinidad's three E‐ENE striking ranges, which are separated by late Neogene‐Recent depocenters, expose (1) the Northern Range Group, generally greenschist‐metamorphosed Upper Jurassic to Cretaceous north facing continental slope sediments of the Northern Range, deposited on the northern South American passive margin 200–400 km to the WNW, and (2) the Trinidad Group, Cretaceous‐Paleogene shelf slope sediments of the central and southern Trinidad deposited less than 100 km WNW of their present location. A small allochthon composing the Sans Souci Group Cretaceous tholeiitic volcaniclastic, basaltic, and gabbroic rocks (Sans Souci Formation) and sediments (Toco Formation) now in the northeastern Northern Range, has been transported hundreds of kilometers from the west with the Caribbean Plate. Despite earlier references to Cretaceous orogenesis, all deformation in Trinidad is of Cenozoic age. The first deformation in the Northern Range (D1) formed north vergent nappes and induced greenschist metamorphism, probably in the Late Eocene or Oligocene. The nappes developed either by the underthrusting of the Proto‐Caribbean crust beneath South America due to convergence between North and South America, or as gravity slides caused by oversteepening induced by this convergence and/or the passage of the Caribbean Plate's peripheral bulge and arrival of its foredeep. Northern Range D2 deformation is south vergent and represents the incorporation of Northern Range metasediments into the Caribbean accretionary prism. The transition to D3 brittle transpressive right‐lateral strike‐slip faulting is interpreted to be due to the uplift and east‐southeastward transpressive emplacement of Northern Range/Caribbean prism rocks onto the South American stepped shelf. This emplacement formed the Miocene transpressive thrust belts and foreland basin in central and southern Trinidad. In the final phase of Northern Range deformation (D4) ∼E‐W normal faults and shear zones and conjugate NNW‐SSE and NE‐SW normal faults developed, and displacement on preexisting ∼E‐W right‐lateral strike‐slip faults continued. The 11 Ma Northern Range zircon fission track ages suggest rapid uplift from the Late Miocene to Recent. Late Miocene subsidence of the Tobago platform immediately to the north of the Northern Range, and greater than 3 km of normal, down to the north, displacement indicated for the North Coast Fault Zone separating the Northern Range and Tobago platform, leads us to postulate that the rapid uplift of the Northern Range was in response to the northward detachment of the Tobago platform from above the Northern Range, along the north‐dipping transtensional North Coast Fault Zone. This Late Miocene change in deformation style can be explained by a change from Caribbean/South American right‐lateral transpression to right‐lateral strike‐slip generally striking 080°. This has generally induced a component of extension on pre‐existing faults striking at greater than 080°, and a component of compression on faults striking at less than 080°.

Correction methods in fission track dating

Fission track technique is in use for dating minerals and rocks for more than two decades. Most of the ages reported in the literature are the observed ages and need corrections due to geothermal events in the geological past. The annealing effect on fission track age determination is discussed in this paper. A new annealing correction method based on the etch rate studies is given. The results are compared with the existing standard techniques viz. size correction and age plateau methods.

40Ar/39Ar dating of Quaternary feldspar: Examples from the Taupo Volcanic Zone, New Zealand

Using a continuous laser and resistance furnace, we have measured ages on Quaternary plagioclase with an absolute precision of about ±30 ka and on Quaternary sanidine with a relative precision of better than 1%. Such precision was achieved by using low-temperature heating steps to remove much of the nonradiogenic argon contamination. Plagioclase is one of the most common mineral phases in volcanic rocks; thus, these procedures will be widely applicable to many problems for which precise radiometric age control has not been available. We studied plagioclase and plagioclase-sanidine concentrates from the oldest and the three largest silicic ash-flow deposits of the Taupo Volcanic Zone, New Zealand, one of the world9s largest and most active volcanic systems. The results are in close agreement with new magnetostratigraphic data, suggest that existing fission-track age determinations significantly underestimate the age of older units, and shift the inception of Taupo Volcanic Zone volcanism back to at least 1600 ka. The improved precision has permitted the first correlations between proximal and distal units; previous correlations with deep-sea ash flows in the western Pacific require major revision.

Major Late Miocene cooling of the middle crust associated with extensional orogenesis in the Funeral Mountains, California

The Funeral Mountains of the Death Valley region consist of a metamorphic core complex that underlies the Miocene Boundary Canyon detachment fault. Fissiontrack age determinations on sphene, zircon, and apatite from the highest grade portions of the Funeral Mountains (collected in Monarch Canyon) indicate rapid cooling from above ∼285°C at 9–10 Ma. The steep dT/dt slope implied by the cooling path envelope as well as its synchroneity with development of the Boundary Canyon detachment suggests tectonic unroofing of between 9.5 and 17 km since ∼10 Ma (assuming typical geothermal gradients prior to detachment development of between ∼20–30°C/km). Previous studies report muscovite 40Ar/39Ar plateau ages from the same area indicating that the Funeral Mountains core may have cooled below ∼350°C between 110‐55 Ma. These data, when combined with the fission track ages, suggest residence of the metamorphic terrain at 285–350°C from Late Cretaceous to late Miocene time. Recent studies, using petrologic constraints, have suggested that much of the unroofing of the Funeral Mountains, as well as development of tectonite fabrics, occurred during Mesozoic extension. Recognition of temperatures in the 300°C range during Miocene time suggests some of the ductile extensional fabrics may be late Miocene in age.

Magmatism and deformation, southern Revillagigedo Island, southeastern Alaska

The mid-Cretaceous, cale-alkaline, LREE-enriched plutons on southern Revillagigedo Island, southeastern Alaska, form two distinct groups: (1) biotite-epidote-hornblende tonalite of the Moth Bay pluton and (2) biotite-bearing, hornblende-absent leucotonalite. Higher total REE and lower Na 2 O, Al 2 O 3 , and Sr distinguish the Moth Bay tonalite from the leucotonalite. The plutons are surrounded by narrow contact metamorphic aureoles containing kyanite and staurolite. Pressure estimates for emplacement based on metamorphic conditions of contact rocks and from igneous amphibole compositions are ∼8-9 kbar. Detailed analyses of pluton/host rock relationships indicate that pluton emplacement was synchronous with orogen deformation. Pre-emplacement thrusting placed the metavolcanic and metasedimentary rocks of the Taku terrane structurally above the younger metasedimentary rocks of the Gravina belt. The contact between these units was subsequently cut by the Moth Bay pluton. The southern margin of the Moth Bay pluton has been transformed to blastomylonite. Contact metamorphic textures and undeformed dikes which cut the blastomylonite indicate that shearing did not outlast crystallization of the pluton. A still later phase of deformation transposed the pluton-related structures and generated a northeast-dipping, ductile thrust fault (Southern Revillagigedo shear zone) along the southern shore of Revillagigedo Island. The shear zone and earlier structures all trend approximately northwest. Fission-track age determinations and track-length measurements in apatite were used to determine the cooling histories of the plutonic units. Fission track ages from the western part of the study area are 51.3 ± 4.0 Ma (apatite), 57.8 ± 4.4 Ma (zircon), and 69.8 ± 4.2 Ma (sphene); eastward, toward the Coast batholith, fission track ages are ∼10 m.y. younger. These younger ages may result from a tectonic/thermal event associated with batholith emplacement. This same event may have generated north-trending structures that overprint older northwest-trending structures on the east side of the study area.

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.