Alan J. Busacca

Reconstruction of the late Pleistocene grassland of the Columbia basin, Washington, USA, based on phytolith records in loess

Abstract Silica phytoliths preserved in three loess sections in southeastern Washington State revealed a 100 000-year history of the Columbia Basin grassland. Changes in the proportion of different morphotypes indicate large shifts in vegetation composition during the last 100 ka. A low-elevation section (677 m asl) near the center of the basin provided a record of alternating xeric Festuca–Poa and mesic Festuca–Koeleria grassland. The middle-slope section (1095 m asl) supported Picea–Abies or Pinus ponderosa forest or non-analog parkland at different times. Some trees were present at or near the site even during the Last Glacial Maximum. The highest site (1220 m asl) supported Stipa-, Festuca- and Poa-dominated grassland with some Artemisia shrub during most of the late Pleistocene, but supports a coniferous forest today. Variations in vegetation can be explained as a response to changes in large-scale climatic controls. Grasslands and shrub steppe were apparently more widespread and forests more restricted than today during the marine isotope stages 2 and 4, probably as a result of cooler and drier conditions. The three new records are well correlated with previously published paleo-reconstructions based on phytolith, cicada burrow and stable isotope data from a nearby KP-1 loess section, Carp Lake pollen record, and global ice volume variations.

Long quaternary record in eastern Washington, U.S.A., interpreted from multiple buried paleosols in loess

Loess that is from a few meters to 75 m thick covers an area of more than 20,000 km2 on the Columbia Plateau in Washington, Idaho and Oregon. The region of deepest loess is in eastern Washington and is called “The Palouse”. The Palouse is downwind of the Channeled Scabland, through which massive outburst floods from Glacial Lake Missoula flowed repeatedly during glacial stages of the Pleistocene. The deepest stratigraphic section yet studied is in a remnant of the once continuous loess cover that is surrounded by Scabland channels. A normal over reverse magnetic polarity zonation in this section supports an age of more than 790,000 yr, but 15–35 m of loess may be present beneath this section, so a total of 1.5-2 million yr is possible. Upward fining of texture in some layers in the loess at this site and stratigraphic evidence at nearby sites suggest that at least some pulses of loess deposition were triggered by episodes of cataclysmic floods. Nineteen or more individual paleosols can be recognized in 26 m of section based on field morphology, physical and chemical properties, and micromorphology. The paleosols consist of calcic, petrocalcic and duripan horizons, many of which are associated with cambic horizons. Less strongly developed soils in this and other sections have been obscured by partial overlap of soil development in an episodically rising loess landscape. Paleosols in the loess at other sites reflect a dry-to-moist climatic gradient from west to east across the region during the Pleistocene that was grossly similar to todays. Accumulation of pedogenic carbonates and silica dominates in a western zone where the present-day mean annual precipitation is less than about 450 mm. Translocation of silicate clays and leaching of carbonates dominate in a central steppe zone where precipitation is between 450 and about 700 mm. Fragipans are common in an eastern zone along the steppe-forest transition at present-day precipitation of over 700 mm. Reconnaissance study suggests that these climatic-pedogenic zones have been somewhat stable during many of the episodes of soil development preserved in the loess, although some paleosols have different features from the majority of the paleosols in that zone, e.g., paleosols with fragipans in a sequence of paleosols with argillic horizons, which suggests that some episodes of soil development took place under different climatic conditions.

Interaction between aggrading geomorphic surfaces and the formation of a late pleistocene paleosol in the palouse loess of eastern Washington state

Abstract Variable rates of loess deposition contributed to dramatic regional variation in a soil-stratigraphic unit, the Washtucna Soil, in the Palouse loess deposits in the Channeled Scabland of eastern Washington state. Throughout most of the Channeled Scabland, the morphology of the Washtucna Soil is that of a single buried soil, but it bifurcates into two well-developed and pedologically distinct buried soils in areas immediately downwind of the major source of loessial sediment. Regional loess stratigraphy confirms that the two well-developed soils formed during the same interval of time during which only one soil formed in areas that are distal to loess source areas. The variable and perhaps rapid rates of soil formation suggested by the stratigraphy resulted from an interaction between variable rates of loess deposition and the formation of superimposed calcic soils. Petrocalcic horizons with weak Stage IV morphology formed as the zone of carbonate accumulation moved up into former A and cambic horizons that had been profusely burrowed by cicadas. The development of cicada burrows in one phase of soil development that were subsequently engulfed by pedogenic carbonate under a rising land surface seems to have greatly accelerated the development of the petrocalcic horizons. Accelerated rates of formation of the petrocalcic horizons occurred when extrinsic (pulses of loess deposition) and intrinsic (engulfment of burrowed horizons) thresholds were exceeded. Stratigraphic evidence suggests that the soil formation that accompanied the rise in the land surface due to additional loess deposition may have occurred during the late Wisconsin glaciation when giant glacial outburst floods in the channeled Scabland triggered a new cycle of loess deposition.

Luminescence dating of loess from the Northwest United states

Thermoluminescence (TL) and infra-red stimulated luminescence (IRSL) ages have been determined for loess from the Columbia Plateau region of southeast Washington and northeast Oregon, U.S.A. The IRSL ED values and ages appear to be tending towards a maximum level lower than that of the TL ED values and ages. In general, the TL ages are in stratigraphic order and agree well with the age control, in the form of 14C and TL dated tephra units derived from various eruptions of Mount St Helens.

Record of pre-late Wisconsin giant floods in the Channeled Scabland interpreted from loess deposits

Loess deposits adjacent to flood coulees on the Columbia Plateau in eastern Washington State record multiple episodes of pre-late Wisconsin cataclysmic floods. Evidence in the loess consists of flood-cut unconformities, upward fining in individual loess strata, and flood deposits that consist of basaltic gravel, fragments of calcified paleosols, and slackwater sediment. At least six episodes of floods during the Pleistocene are indicated by paleomagnetic measurements, tephra correlations, and the stratigraphic position of flood indicators. One episode of the six occurred prior to 790 ka; the most recent episode before the last glacial floods occurred at 36 ka, at the latest.

Late Quaternary stratigraphy of loess in the Channeled Scabland and Palouse Regions of Washington State

Abstract Loess deposits of late Quaternary age in the Channeled Scabland and western Palouse regions of Washington state contain distinctive sequences of calcic buried soils, loess layers, tephra layers, and stratigraphic unconformities caused by cataclysmic flooding. Four new soil-stratigraphic units, the Sand Hills Coulee Soil, Washtucna Soil, Old Maid Coulee Soil, and the Devils Canyon Soil, are recognized across a 100-km transect. The pedostratigraphic correlations are strengthened by two tephra layers found at several sites that are correlative with the Mount St. Helens set S (radiocarbon ages about 13,000 yr B.P.), Mount St. Helens set C (radiocarbon ages about 36,000 yr B.P.), and by a third one that may be from an earlier (pre-set C) Mount St. Helens eruption. The set S tephra layer separates the Sand Hills Coulee and Washtucna Soils; the set C tephra layer separates the Old Maid Coulee and Devils Canyon Soils; and the pre-set C tephra layer underlies the Devils Canyon Soil. The well-known late Wisconsin episode of Scabland flooding and two older episodes of flooding are marked by regional stratigraphic unconformities that lie between pedostratigraphic units. One unconformity that underlies the Washtucna and Old Maid Coulee Soils was caused by a major episode of Scabland flooding that occurred before 35,000 yr B.P., probably during the early Wisconsin (80,000 to 65,000 yr B.P.). A possible second and older episode of flooding, recognized by an unconformity that underlies the Devils Canyon Soil, may be Illinoian (ca. 130,000 to 200,000 yr B.P.) in age.

Correlation of distal tephra layers in loess in the Channeled Scabland and Palouse of Washington state

Abstract Thirty-two distal tephra layers that are interbedded in Quaternary loess at 13 sites in the Channeled Scabland and Palouse were sampled as part of a regional study of the stratigraphy and chronology of dominantly windblown sediments on the Columbia Plateau. An electron microprobe was used to determine the elemental composition of volcanic glass in all of the samples and also to determine the composition of ilmenite in 14 of them. Two of the distal tephra layers correlate with Glacier Peak eruptions (11,200 yr B.P.), five with Mount St. Helens tephra set S (13,000 yr B.P.), and nine with Mount St. Helens tephra set C (ca. 36,000 yr B.P.) based on analysis of glass and ilmenite in reference pumices from Glacier Peak, Mount St. Helens, Mount Mazama, Mount Rainier, and Mount Jefferson, on the calculation of similarity coefficients for comparisons of both glass and ilmenite reference compositions with those of distal tephras, and on considerations of stratigraphic position. The composition of glass and ilmenite and the stratigraphic position of one distal tephra layer in the loess suggests that it is from an eruption of Mount St. Helens at least several thousand years older than the set C eruptions. Glass composition and stratigraphic position of a distal tephra at another site in loess suggested a possible correlation with some layers of the Pumice Castle eruptive sequence at Mount Mazama (ca. 70,000 yr B.P.), but similarity coefficients on ilmenite of only 45 and 48 fail to support the correlation and show why multiple correlation methods should be used. Similarity coefficients higher than 96 for both glass and ilmenite establish a correlation with Mount St. Helens layer Cw for distal layers in two widely separated sites. These layers are in sedimentary successions that are closely associated with giant floods in the Channeled Scabland. The 36,000 yr B.P. radiocarbon age of the Mount St. Helens set C establishes a minimum limiting date for an episode of flooding that predates the widespread late Wisconsin floods. A correlation of distal tephra layers at two other sites in the Scabland and Palouse establishes a chronostratigraphic link to a still-older episode of flooding within the Brunhes Normal Polarity Chron. Six distal tephra layers in pre-late Quaternary loess that are not correlated with known or dated eruptions have compositions and distinctive stratigraphic positions relative to magnetic reversal boundaries that make them key markers for future work.

Genesis and relationship of macromorphology and micromorphology to contemporary hydrological conditions of a welded Argixeroll from the Palouse in Idaho

Abstract Soils formed in distal loess of the Palouse region have welded profiles that make assessment of their genesis and pedostratigraphic relationships difficult. These soils typically have a mollic-cambic horizon sequence that is underlain by a well-expressed albic E horizon and a strongly developed argillic horizon. In this study, we utilized hydrological monitoring combined with micromorphological analysis and new stratigraphic data to help establish the genesis of a representative Argixeroll of the region and the relationship of its morphology to ancient and contemporary hydrological conditions. Monitoring results indicate that the Argixeroll is seasonally episaturated for approximately 5–6 months per year. Ferrimanganiferous nodules, segregations, hypocoatings, and depletion mottles are observed in thin section throughout the pedon, but dominate within, just above, and below the present-day E horizon. Measured soil Eh is sufficiently low for Fe and Mn reduction to occur for 1–3 months during the period of episaturation, suggesting that at least some of these redoximorphic features are likely to be contemporary. Other compound redoximorphic features are clearly relict, however, as they are coated by illuvial clay which is unaffected by Fe Mn redistribution processes. Thick illuvial clay coatings on interpedal fissures of the argillic horizon postdate pockets of Holocene tephra within deeply bioturbated aggregates, while other channel argillans in the same part of the pedon are clearly older. This indicates that at least two distinct phases of clay accumulation have occurred, the most recent being contemporaneous with loess aggradation during the Holocene. Our results suggest that these welded profiles consist of a mollic-cambic horizon sequence that has formed primarily in Holocene loess and an albic-argillic sequence that has formed in Wisconsinan loess. The modern-day E horizon was initiated as the clay-eluvial surface horizon of the regionally recognized Washtucna paleosol that continued forming into the late-Wisconsinan cold phase and, upon burial by Holocene loess, it has been transformed into an albic horizon and even enlarged as a result of redoximorphic processes associated with seasonal saturation.

Pedogenesis of a chronosequence in the Sacramento Valley, California, U.S.A., I. Application of a soil development index

Abstract A quantitative soil development index based on field morphologic data has been applied to a chronosequence of six soils at Honcut Creek in central California. The soils range from 600 yr to 1.6 m.y. in age and exist in a xeric-thermic soil climatic regime. Plots of the development of individual properties versus the log of soil age are generally linear. Of the eight soil properties used in the index, clay films, texture plus wet consistence, structure, and rubification (color hue and chroma), are most highly correlated with soil age (correlation coefficients for linenar regression of 0.96−0.87). Dry consistence, moist consistence, melanization (color value), and pH lowering are not as highly correlated with soil age (correlation coefficients of 0.68−0.55), although correlation coefficients for all eight soil properties are significant at α = 0.01 or 0.05 level. Depth functions of a horizon index that incorporates all eight soil properties chart the development of the A-horizons in Holocene soils and the development of the cambic and then argillic horizons in older soils. Total depth of soil development increases from about 60 cm in Holocene soils to more than 5 m in the oldest soils. Lithologic discontinuities and stratification are brought out in the horizon index because of their effects on soil development. Soils on the alluvial terraces of Honcut Creek have developed systematically over the 1.6 m.y. age span of the chronosequence: correlation coefficients for regressions of the profile index versus the log of soil age are ⩾0.97 (significant at α = 0.01 level). The coefficient of variation of replicate profiles decreases with increasing soil age. The soil development index substantiates the geologic and pedologic correlations made during geologic mapping and soil survey of the area, although differences among soils in parent material and topographic factors complicate the age trends.

Glacial anticyclone recorded in Palouse loess of northwestern United States

Climate simulations for the Last Glacial Maximum using general circulation models typically show a glacial anticyclone that produced an easterly wind anomaly south of the North American ice sheets. Evidence of this phenomenon has not been found in eolian sedimentary deposits that record surface wind patterns. Luminescence ages of loess and accompanying paleoecologic records from opal phytoliths and paleosol units across the Columbia Plateau, Washington State, United States, document a decrease, up to fivefold, in regional dust production and accumulation from 35 ka to 15 ka. This interval corresponds to simulations indicating that a glacial anticyclone weakened prevailing south-southwesterly winds that have shaped the eolian landscape of the Columbia Plateau since at least 75 ka. At the same time that the glacial anticyclone suppressed dust production and accumulation, enhanced cold and dry conditions resulted in soil formation dominated by intense bioturbation in periglacial steppe accompanied by shallow calcium carbonate precipitation. This is the first evidence from eolian deposits in North America verifying that the glacial anticyclone altered surface wind patterns and affected eolian systems during the Last Glacial Maximum.