William D. McCoy

Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, Western United States

A substantially modified history of the last two cycles of Lake Bonneville is proposed. The Bonneville lake cycle began prior to 26,000 yr B.P.; the lake reached the Bonneville shoreline about 16,000 yr B.P. Poor dating control limits our knowledge of the timing of subsequent events. Lake level was maintained at the Bonneville shoreline until about 15,000 yr B.P., or somewhat later, when catastrophic downcutting of the outlet caused a rapid drop of 100 m. The Provo shoreline was formed as rates of isostatic uplift due to this unloading slowed. By 13,000 yr B.P., the lake had fallen below the Provo level and reached one close to that of Great Salt Lake by 11,000 yr B.P. Deposits of the Little Valley lake cycle are identified by their position below a marked unconformity and by amino acid ratios of their fossil gastropods. The maximum level of the Little Valley lake was well below the Bonneville shoreline. Based on degree of soil development and other evidence, the Little Valley lake cycle may be equivalent in age to marine oxygenisotope stage 6. The proposed lake history has climatic implications for the region. First, because the fluctuations of Lake Bonneville and Lake Lahontan during the last cycle of each were apparently out of phase, there may have been significant local differences in the timing and character of late Pleistocene climate changes in the Great Basin. Second, although the Bonneville and Little Valley lake cycles were broadly synchronous with maximum episodes of glaciation, environmental conditions necessary to generate large lakes did not exist during early Wisconsin time.

Fossil spring deposits in the southern Great Basin and their implications for changes in water-table levels near Yucca Mountain, Nevada, during Quaternary time

Fossil spring deposits are common in the southern Great Basin, and their distribution provides important constraints on the hydrologic response of the regional water table to climate change. This information is crucial, because the proposed high-level nuclear waste repository at Yucca Mountain will be located ∼200–400 m above the modern water table. Water tables will rise in response to a future return to glacial climates, but the magnitude of the change—and the consequences for radionuclide travel times and overall repository integrity—are key uncertainties. Increased recharge during past pluvial periods in the Spring Mountains and Sheep Range caused water tables to rise and ground water to discharge over broad expanses of the Las Vegas Valley system, and in nearby Pahrump, Sandy, and Coyote Springs Valleys. In contrast, other valleys in the region contain only small areas of Pleistocene discharge resulting from local damming of ground water by faults that cut valley alluvium. In these instances, which include the Valley Wells area, and Piute and northern Coyote Springs Valleys, smaller ranges such as the Clark and New York Mountains supplied the moisture. The change in water-table levels since the last full glacial period varies between and within valleys, from as little as 10 m in several areas to 95 m in the Coyote Springs Valley. At Yucca Mountain, the water table has probably changed by ≤115 m in response to climate change. The spring deposits and the mollusk faunas found with them, often misinterpreted as lacustrine in origin, share many essential features with active spring systems in northeast Nevada. Deposits associated with discharge mainly consist of pale brown silt and sand that is entrapped by dense stands of phreatophytes covering valley bottoms when water tables are high. Pale green mud, containing a mix of aquatic, semiaquatic, and moist terrestrial mollusks, accumulates in wet meadows and marshes associated directly with spring discharge. The record in subbasins of the Las Vegas Valley system is dominated by late Wisconsin–age sediments, although pond sediments and alluvium belonging to at least one older (pre-Wisconsin?) pluvial period are also locally exposed. Deposits from two even earlier episodes of spring discharge, both of which also occurred during Rancholabrean time (10 to <450 ka), are exposed in the Pahrump, Chicago, Piute, and Coyote Springs Valleys, and in the Valley Wells. The records from the Pahrump and Coyote Springs Valleys are especially well exposed and likely extend back to at least early Pleistocene time.

Malacological and sedimentological evidence for “warm” glacial climate from the Irig loess sequence, Vojvodina, Serbia

Four loess units and three paleosol layers are preserved in the Irig brickyard, Vojvodina, Serbia. Amino acid geochronology provides stratigraphic correlations between loess units V-L1 and V-L2 at the Irig section with loess of glacial cycles B and C, respectively, described from other central European localities. Luminescence dating results for the upper loess layers V-L1L1 and V-L1S1L1 confirm the geological interpretations, although in samples below paleosol V-L1S1S2, the age increase with depth is less than in our proposed age model. Magnetic susceptibility and sedimentological evidence from the Irig loess-paleosol sequence show general similarities with the MIS 6-1 pattern of the SPECMAP oxygen-isotope curve. Malacogical investigations at the Irig site reveal the continuous presence of the Chondrula tridens and Helicopsis striata faunal assemblages throughout the last glacial and final part of the penultimate glacial loess. The loess snail fauna, which is characterized by the complete absence of cold-resistant species, suggests a stable, dry, and relatively warm glacial climate, compared with other central European loess localities. Furthermore, these data suggest that the southern slope of Fruska Gora was a refugium for warm-loving and xerophilus mollusc taxa during the otherwise unfavorable glacial climates of the Late Pleistocene.

Towards a revised chronostratigraphy of loess in Austria with respect to key sections in the Czech Republic and in Hungary

Abstract The stratigraphic ages of some well-known and characteristic paleosols in Austrian and Hungarian loess sections have been questioned by several workers since the mid-1970s. In particular, a generally accepted correlation of Austrian and Hungarian loess stratigraphies with the well-established Czech loess stratigraphy has not been put forward. This paper presents 41 new thermoluminescence (TL) dates and the first results from amino acid chronology of key sections in these areas. The results are in agreement and suggest a new interregional correlation of loess-paleosol sequences. For loess older than the last interglacial, quartz and feldspar inclusions yield more reliable TL ages than the polymineralic fine grain fraction.

Quaternary aminostratigraphy of the Bonneville Basin, western United States

The deposits of at least four distinct deep-lake cycles of Pleistocene Lake Bonneville are recognized on the basis of isoleucine epimerization in fossil gastropod shells. Deposits of the earliest recognized cycle predate the Lava Creek ash (600,000 yr B.P.). Three younger cycles of Lake Bonneville are represented by lacustrine deposits of the Pokes Point Alloformation (∼200,000 yr B.P.), the Little Valley Alloformation (∼140,000 yr B.P.), and the Bonneville Alloformation (∼30,000 to 11,000 yr B.P.). Analyses of fossil shells from the Main Canyon Formation in the Gentile Valley of southern Idaho support the contention that those deposits span an interval of time from before 2 m.y. B.P. to sometime between the last two cycles of Lake Bonneville, perhaps as recent as 30,000 yr B.P. It is concluded that isoleucine epimerization in non-marine gastropods can provide a useful stratigraphic tool for the study of Quaternary lacustrine deposits.

The precision of amino acid geochronology and paleothermometry

Abstract Uncertainties in the measurements necessary for the calculation of amino acid age estimates and paleotemperatures can be propagated through age and temperature equations to yield estimates of precision. It is found that aIle/Ile ratios can be used to calculate numerical ages with a precision generally no better than ±40–50% if the age equation is not calibrated locally and with a precision approaching 15% if appropriate calibration samples are available locally. The effective diagenetic temperature of a sample of known age can be calculated to within about 2°C, although the local relationship between ground and air temperatures may add additional uncertainty when interpreting the effective temperature in terms of climate. The effective temperature for an interval of time bracketed by samples of known age can be known to within about 2–4°C under favorable circumstances. The change in effective temperature between an interval of time bracketed by two samples of known age and the period of time following that interval can be calculated to within 1–2°C in may instances.

Sequence stratigraphy of lacustrine deposits: A Quaternary example from the Bonneville basin, Utah

The late Quaternary lacustrine sedimentary record in the Bonneville lake basin in the eastern Great Basin provides an excellent example of sequence stratigraphy. Two sequences, referred to as the Little Valley and Bonneville Alloformations, are exposed in the bluffs of the Sevier River where it has entrenched its Pleistocene delta between Leamington and Delta, Utah. Both alloformations contain offshore marl units and fine-grained deltaic or underflow-fan deposits. They can be identified and mapped by tracing the unconformity separating them and employing a number of geochronometric tools, including amino acid epimerization in fossil gastropods, radiocarbon and thorium-230 ages, and basaltic tephrochronology. Thin transgressive sand of the Little Valley Alloformation is overlain by deeper-water marl and down-lapping regressive-phase deltaic silt. The Bonneville Alloformation lies unconformably above the Little Valley deposits. Fine-grained deltaic sediments deposited during the transgressive phase of Lake Bonneville fill the entrenched Sevier River valley that was eroded subsequent to the Little Valley lake cycle. Marl deposited during the deep-water phase is overlain by down-lapping deposits of the regressive phase below the Provo shoreline but is the uppermost unit in the altitudinal range where the lake was lowered catastrophically during the Bonneville Flood. The sequence stratigraphic interpretation leads to the conclusion that the Sevier River delta as a whole is probably made up of a number of sediment sequences, each composed of several facies. Recognition of this complexity could be important in potential applications of the stratigraphic model.

Historical developments and recent advances in amino acid geochronology applied to loess research: examples from North America, Europe, and China

Abstract Amino acid geochronology provides important chronostratigraphic insight in the regional correlation and paleoclimatic evaluation of loess–paleosol sequences in the midwestern US, throughout western, central, and eastern Europe, and in China. In general, loess of the last four glacial cycles, corresponding to marine oxygen-isotope stages (OIS) 2–4, 6, 8, and 10, respectively, can be distinguished on the basis of alloisoleucine/isoleucine (A/I) ratios measured in fossil gastropod shells preserved in the loess. The racemization reaction is much slower in older samples, resulting in decreased temporal resolution. In the midwestern US and Europe, it is not possible to confidently subdivide the last glacial cycle on the basis of aminostratigraphic data. However, in China, where effective temperatures were higher during the Late Pleistocene, loess above and below L1SS1, the OIS-3 interstadial paleosol, can be distinguished using amino acid racemization data. With improvements in analytical methods, multiple amino acid d / l- enantiomers are now being measured using reverse-phase liquid chromatography. Aspartic acid racemizes at a higher rate than the traditionally measured alloisoleucine/isoleucine diastereomers and can be measured more rapidly. This development offers hope for better temporal resolution and enhanced stratigraphic subdivision of loess units than has been achieved previously.

Evidence for a shallow early or middle Wisconsin-age lake in the Bonneville Basin, Utah

Abstract Relatively complete stratigraphic records of the Bonneville cycle and of at least one and probably two earlier lacustrine are exposed along the Bear River below Cutler Dam in northern Utah between altitudes of 1290 and 1365 m. In most exposures the unconformity between the Bonneville Alloformation and the underlying unit, herein named the Cutler Dam Alloformation, is marked by slight erosional relief and by a weakly to moderately developed buried soil, herein named the Fielding Geosol. In truncated profiles, the Fielding Geosol reaches a maximum of stage II carbonate morphology. Wood from near the base of the Cutler Dam Alloformation yielded a 14 C date of >36,000 yr B.P. (Beta-9845). Alloisoleucine/isoleucine (aIle/Ile) ratios of Sphaerium shells from the Cutler Dam beds average 0.15 ± 0.01 in the total hydrolysate, which is significantly greater than the average for Sphaerium shells of Bonneville age elsewhere in the basin. Therefore, the Cutler Dam Alloformation is older than 36,000 yr B.P., but much younger than deposits of the Little Valley lake cycle (140,000 yr B.P.?) which bear shells having significantly higher aIle/Ile ratios. The Cutler Dam Alloformation along the Bear River may be broadly correlative with marine oxygen-isotope stages 4 or 3. Fine-grained, fossiliferous, marginal-lacustrine facies of the Cutler Dam Alloformation are exposed at altitudes near 1340 m, and are probably the highest exposures of sediments deposited in the early or middle Wisconsin lake in the Bonneville basin.

Amino acid estimates of latitudinal temperature gradients and geochronology of loess deposition during the last glaciation, Mississippi Valley, United States

Principles of amino acid paleothermometry are used to estimate paleotemperatures and latitudinal temperature gradients for the period during and following the last glacial maximum in the Mississippi Valley. Gastropod shells were collected from the Peoria Loess for amino acid analysis, and Arrhenius parameters of isoleucine epimerization were determined experimentally for the gastropod genera Catinella and Hendersonia . There are 37 radiocarbon and 5 thermoluminescence ages that constrain the paleotemperature estimates and provide additional chronological data for loess deposition in the region. Amino acid paleotemperature estimates suggest that the north-south temperature gradient was significantly depressed in the Mississippi Valley for a considerable period during the past ≈25 k.y. Effective diagenetic temperature estimates indicate that at some time during or following the last glaciation, the effective-temperature gradient was ≈0.3–0.6 °C/degree of latitude, which is significantly lower than the modern mean annual air-temperature gradient of ≈0.9 °C/degree of latitude. Calculated effective paleotemperatures for three localities in Tennessee and Mississippi suggest that temperatures were ≈7–13 °C lower than present during the period from ca. 24 to 16 ka in the lower Mississippi Valley. These results provide additional evidence for a significant cooling in southern United States continental temperatures during the last glacial maximum.