Glenn W. Berger

The last glacial maximum in central and southern North Island, New Zealand: a paleoenvironmental reconstruction using the Kawakawa Tephra Formation as a chronostratigraphic marker

Abstract Kawakawa Tephra Formation, comprising Oruanui Ignimbrite flow member and Aokautere Ash airfall member, represents the products of an exceptionally large and widespread volcanic eruption from Taupo Volcanic Centre in the North Island of New Zealand. The eruption occurred during the Last Glacial Maximum, and is radiocarbon dated at c. 22.6 ka B.P. Thermoluminescence ages are in broad agreement with the radiocarbon age. The presence of Aokautere Ash in loess deposits, in alluvial gully-fills, on river terraces, and its absence from unstable sites, permits a detailed assessment of geomorphic activity during the Last Glacial Maximum. Widespread erosion of regolith, aggradation of river valleys, and deposition of loess, particularly in the period following eruption of the Kawakawa, point to a cold, dry, variable climate. A collation of pollen data for sediments containing Aokautere Ash, and those 14C dated in the range 17–23 ka, shows that tall forest was highly restricted in the central and southern parts of the North Island. An apparently subalpine grassland/shrubland was present at sites from present sea level to over 800 m elevation, suggesting that factors other than lower temperatures, such as exposure to wind and frost, fire and reduced rainfall, were important in controlling vegetation patterns. We conclude that the interval 23-13 ka B.P., broadly equivalent to oxygen isotope stage 2, represents the period of greatest environmental change in the North Island.

Initiation and development of the Laurentide and Cordilleran Ice Sheets following the last interglaciation

Abstract Fossil records from sites overridden by or adjacent to the Laurentide Ice Sheet indicate that the climate of the last interglaciation (Oxygen-Isotope Substage 5e, ca. 130-116 ka) was warmer than today. Following the last interglaciation, the Laurentide Ice Sheet first developed during Stage 5 over Keewatin, Quebec and Baffin Island. Along its northern margin, the ice sheet reached its maximum extent of the last glaciation during Stage 5. The ice sheet advanced across Baffin Island onto the continental shelf early during Stage 5 (5d?), whereas the advance into the western Canadian Arctic occurred late during Stage 5 (5b?). The ice sheet also may have advanced into the St Lawrence Lowland during Substage 5b, although this event may be younger (Stage 4). The Hudson Bay lowland became ice-free during Substage 5a. Retreat of the ice sheet on Baffin Island occurred during late Stage 5, probably Substage 5a. The exact timing of retreat from the western Canadian Arctic is unknown, but it occurred before 48 ka. The southern sector, including the St Lawrence Lowland, was ice-free during late Stage 5. The Hudson Bay lowland may have remained ice free through Stage 4 and much of Stage 3. Because of conflicting chronologies, however, it is more likely that this area was glaciated throughout Stage 3 and perhaps Stage 4. Nevertheless, the data demonstrate that the lowland was ice-free during part of the last glaciation. An independent ice cap developed over the Appalachian Uplands and advanced across Nova Scotia during Stage 4, perhaps as far as the edge of the continental shelf. The ice cap remained active over Nova Scotia as a setellite to the Laurentide Ice Sheet throughout the remainder of the last glaciation. The ice sheet advanced into the St Lawrence Lowland during Stage 4 and subsequently overwhelmed the local ice cap in the Appalachian Uplands, advancing perhaps into northern New England, but not farther south. The Lowland remained covered by the ice sheet until late Stage 2. The ice sheet may also have advanced into the Lake Ontario basin during Stage 4. The position of the northern margin of the Laurentide Ice Sheet during Stage 4 is not known, but it remained an unknown distance behind its maximum position reached during Stage 5. Cores from Baffin Bay indicate a substantial decrease in high-latitude glaciation during Stage 4. Following retreat, the Keewatin sector of the ice sheet may have remained over much of northwestern Canada as a quasi-stable ice mass until it readvanced during Stage 2. Similarly, the Baffin Island sector of the ice sheet may have remained largely intact. The southern margin of the ice sheet may have advanced into the Lake Ontario basin and upper Mississippi Valley during the middle of Stage 3 (ca. 50 ka), reaching its maximum extent of the last glaciation during Stage 2 (ca. 18–21 ka). Advance of the northern margin was younger (ca. 8–13 ka) than that of the southern margin; this advance was less extensive than the penultimate advance (Stage 5). Paleoenvironmental records indicate that the last interglaciation in areas covered by and near the Cordilleran Ice Sheet was as warm as, or warmer than, present. The Cordilleran Ice Sheet appears to have developed during Stage 5 or 4. At that time, it advanced over southern British Columbia and into the northern Puget Lowland. There is no record of this event in northern areas that were later covered by the ice sheet. The ice sheet disappeared before 59 ka, at the beginning of a lengthy nonglacial interval. Paleoenvironmental records indicate that climate was similar to the present during part of this interval. The ice sheet was absent, and glaciers probably were confined to mountain areas, throughout Stage 3. Climatic deterioration marking the end of this nonglacial interval may have begun as early as 29 ka. By 14–15 ka, the ice sheet had achieved its maximum extent of the last glaciation. Because there are few suitable dating methods capable of resolving events beyond the radiocarbon limit and because sites that preserve a record of events from the last glaciation are spatially restricted, we consider this synthesis as tentative and subject to significant revision as dating methods improve. Nevertheless, this perspective of the North American ice sheets through the last glaciation demonstrates their complex and dynamic behavior and attendant rapid fluctuations in ice volume.

Dating loess up to 800 ka by thermoluminescence

Thermoluminescence (TL) ages agreeing with expected ages have been obtained for 13 loess samples spanning the age range from 20 to 800 ka. Our samples are from Alaska and North Island, New Zealand, and are unusual in TL dating studies of loess older than 80-100 ka by having independent age assignments that are generally well constrained, from ages of associated tephra beds. With the polymineral fine-silt-sized (4-11 μm) grains, the partial-bleach TL technique yielded expected ages up to about 350 ka, whereas the total-bleach method gave accurate ages in the range 100 to 800 ka. Thus, the much disputed upper age limit of 100-150 ka for the TL dating of loess now appears to be sample and worker dependent, rather than a global property of the TL signals in the TL-dominant feldspars.

Test of thermoluminescence dating of loess from New Zealand and Alaska

Abstract The accuracy of thermoluminescence (TL) ages for loess (and sediments in general) greater than ∼100 ka is disputed. We tested the accuracy of three common TL sediment-dating techniques applied to 4–11 μm sized polymineral grains from known-age loess, using 16 samples from North Island and South Island, New Zealand, and 2 samples from central Alaska. Estimated sample ages range from 20–26 to ∼800 ka. We varied the optical bleaching spectrum, the selected window of the emission spectra (ultraviolet or uv, blue and green wavelengths), the pre-readout heat treatment, and the TL equivalent-dose measurement technique. Most of the 20–26 ka samples gave TL age underestimates of 3–6 ka that may be attributed to post-burial open system behavior. For the older samples, the partial-bleach TL method gave expected ages up to ∼300 ka, and the total-bleach TL method produced expected ages above ∼100 ka. The partial-bleach regeneration TL method gave significant age underestimates for samples older than ∼100 ka, with a maximum TL age of ∼250 ka for samples having expected ages up to ∼350 ka. Two 300–360 ka,samples which gave total-bleach age underestimates with use of uv TL and green TL, gave expected ages with use of blue TL. These results demonstrate that reliable TL ages for loess from New Zealand and Alaska up to ∼800 ka can be obtained if uv (and green?) emissions and the various regeneration methods are avoided. This age range is well above the former putative 80–100 ka upper age limit for TL dating of loess from other regions, which was thought to be a global limit. Application of our successful procedures to such loess is encouraged.

Thermoluminescence chronology of Toronto-area Quaternary sediments and implications for the extent of the midcontinent ice sheet(s)

We report direct dating, by thermoluminescence (TL) methods, of key Quaternary stratigraphic horizons in the Toronto area. TL ages indicate that (1) most of the exposed sediments of the Don Formation at the sampled site were probably deposited during the closing phase of the Sangamon Interglacial (∼80 ka; oxygen isotope stage 5a), not the climatic optimum substage 5e (∼120∼130 ka),as generally supposed; (2) overlying deltaic strata of the Scarborough Formation, which accumulated in an ice-dammed ancestral Lake Ontario, were deposited between 60 and 50 ka (early Wisconsin, oxygen isotope stage 4); (3) the Sunnybrook diamict, overlying the Scarborough Formation, was deposited in the middle Wisconsin at ∼40-45 ka (oxygen isotope stage 3), likely by glaciolacustrine processes, rather than before 65 ka, as previously supposed; (4) sediments of the Upper Thorncliffe Formation lying stratigraphically just beneath the last-glacial-maximum Hallton Till were deposited at ∼23 ka, consistent with reliable 14 C dating. Thus, the earliest Wisconsin incursion of the (proto) Laurentide Ice Sheet into the Lake Ontario basin, as recorded by the Scarborough Formation, probably did not take place until at least 60 ka, and did not extend south and southwest of the Toronto area until after ∼25 ka. Ice-free conditions probably persisted over large areas of midcontinent North America from the end of the Illinoian glaciation at ∼130 ka to the late Wisconsin glacial maximum at ∼20 ka.

Dating volcanic ash by use of thermoluminescence

The fine-silt-sized (4-11 {mu}m) grains of glass separated from four samples of independently dated, 8 to 400 ka, tephra beds provide accurate thermoluminescence (TL) ages. This demonstration of reliable TL dating of volcanic glass provides a new tephrochronometer for deposits spanning the Holocene to middle Pleistocene age range.

TL dating studies of tephra, loess and lacustrine sediments

Abstract A brief review is presented of my efforts to improve the accuracy of dating unheated sediments by TL and of the development of a new tephrochronometer. Specific applications to known-age deposits are outlined, as are studies of the depositional environment of subaqueous sediments. Removal of anomalous fading in loess by storage at 75°C for 8 days is demostrated. Throughout, for unheated sediments the preferential use of the partial bleach (R-gamma or R-beta) technique is emphasized, especially in situations where the growth curves are sublinear. Contrary to a widespread misconception, sublinearity does not invalidate the partial bleach method. Furthermore, examples are given of TL ‘sensitivity’ (or ‘efficiency’) changes occurring with the regeneration technique — changes that are variable and seemingly sample dependent.

Test of thermoluminescence dating of buried soils from Mt. Kenya, Kenya

We have applied the partial bleach (R-Gamma) thermoluminescence (TL) dating method to four organic-rich Ab horizons of buried soils from Mount Kenya. Our objective was to test the feasibility of directly measuring the time of last exposure to sunlight of the fine-grained silicate minerals within these horizons. The resulting apparent ages for three samples of 1.2±0.3, 6±2 and 10±2 ka (1 std. dev.) are in general agreement with associated 14C age estimates from buried whole soil (paleosol) samples. The fourth sample, of an age (though unknown) presumed to lie beyond the 14C dating limit, yielded a TL age estimate of 60±20 ka. Notwithstanding some limitations, these results are encouraging and imply that with simple refinements, this TL method is capable of directly dating accurately such buried soils, probably to well beyond the ≈ 40 ka 14C dating limit.

Test of thermoluminescence dating with coastal sediments from northern California

Abstract Application of the thermoluminescence (TL), fine-grain (4–11 μm) dating method to modern coastal deposits and to tectonically displaced sediments near the Mendocino Triple Junction in northern coastal California demonstrates the suitability and limitations of dune sand, buried soil, estuarine mud, and beach deposits for TL dating. TL measurements on modern estuarine mud, modern beach sediments, and radiocarbon-dated dune sand and soils show that these deposits are suitably zeroed for TL dating under some conditions. A weighted mean TL age of 16.4 ± 1.3 ka was obtained for four samples (two from a buried soil and two from the straddling terrace dune sand), in close agreement with a radiocarbon age of 16.84 ± 0.06 ka B.P. on charcoal from this paleosol. Application to older sediments and soil profiles of similar origin may therefore provide useful geological ages. Deposits associated with deformed marine terraces yielded TL ages of 176 ± 33 ka for a shallow marine mud at the Mad River site and 119 ± 31 ka for nearshore deposits at the Southport Landing site. These age estimates are consistent with the stratigraphic positions of the TL samples relative to deposits having previously inferred ages based upon soil profile correlations to local age assignments, which are in turn based upon uplift rates relating to eustatic sea level stands. These single TL age results suggest that more detailed TL dating applications in this region, and in similar geological settings, could provide accurate estimates of geologic ages and rates of deformation. However, the polymineralic fine grains from two other nearshore deposits (at the Table Bluff and Centerville Beach sites) yielded ambiguous plateau plots and thus can not be considered datable without additional TL experiments. This difficulty may be due to a high quartz/feldspar ratio in sediments from this area.

Dating volcanic ash by thermoluminescence: Test and application

Abstract We have developed a new physical dating method for Late Pleistocene tephrochronometry. Tests of laboratory procedures applied to independently dated samples show that the thermoluminescence (TL), additive-dose dating method can yield accurate ages for both proximal and distal tephra from a few hundred years to at least 400 ka. The technique involves purification of 4–11 μm glass by heavy liquid centrifugation, pre-TL-readout heating at 50–75°C for eight days and careful regression analysis of TL growth curves. Five unknown-age tephra beds from Summer Lake, Oregon, were dated directly by this method for the first time. The TL ages for these tephra are: 67.3 ± 7.5 ka (ash 2), 102 ± 11 ka (ash N), 165 ± 19 ka (ash R), 200 ± 27 ka (ash KK), and 162 ± 35 ka (ash LL). These results indicate a nonuniform sedimentation rate for the enclosing pluvial lake deposits, a sedimentation hiatus in the interval 100–160 ka, and maximum ages roughly half of the previous indirectly inferred ages.