Geology topics

R. J. Spencer

Publications and source records attributed to R. J. Spencer.

7 recordsLinked to original sources

USGS2011

The rise and fall of Lake Bonneville between 45 and 10.5 ka

A sediment core taken from the western edge of the Bonneville Basin has provided high-resolution proxy records of relative lake-size change for the period 45.1–10.5 calendar ka (hereafter ka). Age control was provided by a paleomagnetic secular variation (PSV)-based age model for Blue Lake core BL04-4. Continuous records of δ18O and total inorganic carbon (TIC) generally match an earlier lake-level envelope based on outcrops and geomorphic features, but with differences in the timing of some hydrologic events/states. The Stansbury Oscillation was found to consist of two oscillations centered on 25 and 24 ka. Lake Bonneville appears to have reached its geomorphic highstand and began spilling at 18.5 ka. The fall from the highstand to the Provo level occurred at 17.0 ka and the lake intermittently overflowed at the Provo level until 15.2 ka, at which time the lake fell again, bottoming out at ∼14.7 ka. The lake also fell briefly below the Provo level at ∼15.9 ka. Carbonate and δ18O data indicate that between 14.7 and 13.1 ka the lake slowly rose to the Gilbert shoreline and remained at about that elevation until 11.6 ka, when it fell again. Chemical and sedimentological data indicate that a marsh formed in the Blue Lake area at 10.5 ka. Relatively dry periods in the BL04-4 records are associated with Heinrich events H1–H4, suggesting that either the warming that closely followed a Heinrich event increased the evaporation rate in the Bonneville Basin and (or) that the core of the polar jet stream (PJS) shifted north of the Bonneville Basin in response to massive losses of ice from the Laurentide Ice Sheet (LIS) during the Heinrich event. The second Stansbury Oscillation occurred during Heinrich event H2, and the Gilbert wet event occurred during the Younger Dryas cold interval. Several relatively wet events in BL04-4 occur during Dansgaard-Oeschger (DO) warm events. The growth of the Bear River glacier between 32 and 17 ka paralleled changes in the values of proxy indicators of Bonneville Basin wetness and terminal moraines on the western side of the Wasatch Mountains have ages ranging from 16.9 to 15.2 ka. This suggests a near synchroneity of change in the hydrologic and cryologic balances occurring in the Bonneville drainage system and that glacial extent was linked to lake size.

Quaternary International↗

USGS2005

An ostracode based paleolimnologic and paleohydrologic history of Death Valley: 200 to 0 ka

Death Valley, a complex tectonic and hydrologic basin, was cored from its lowest surface elevation to a depth of 186 m. The sediments range from bedded primary halite to black muds. Continental ostracodes found in the black muds indicate that those sediments were deposited in a variety of hydrologic settings ranging from deep, relatively fresh water to shallow saline lakes to spring discharge supported wetlands. The alkaline-enriched, calcium-depleted paleolake waters indicate extrabasinal streamflow and basin-margin spring discharge. The alkaline-depleted, calcium-enriched paleowetland waters indicate intrabasinal spring discharge. During Marine Isotope Stage 6 (MIS 6, ca. 180-140 ka) the hydrologic settings were highly variable, implying that complex relations existed between climate and basin hydrology. Termination II (MIS 6 to MIS 5E) was a complex multicyclic sequence of paleoenvironments, implying that climates oscillated between high and low effective moisture. MIS 4 (ca. 73-61 ka) was a spring discharge supported wetland complex. During MIS 2 (ca. 20-12 ka) the hydrologic settings were variable, although they are not fully understood because some black muds deposited during that time were lost during coring. ?? 2005 Geological Society of America.

Geological Society of America Bulletin↗

USGS1991

Change in the size of Walker Lake during the past 5000 years

In 1984, a 12-m sediment core (WLC84-8) was taken from the deepest part of Walker Lake. Samples of the core were analysed for diatoms, pollen, carbonate mineralogy, magnesium content, δ 18 O and δ 13 C values of the total inorganic fractin, δ 18 O and δ 13 C values of Limnocythere ceriotuberosa , δ 13 C values of the total organic fraction, grain size, and magnetic susceptibility. The data indicate that Walker Lake became shallow and probably desiccated between ⩾5300-4800 and 2700-2100 yr B.P.. Each of the organic and inorganic proxy indicators of lake size discussed in this paper was useful in determining the presence of the shallow-lake intervals. However, none of the indicators was useful in determining the cause of the shallow-lake intervals. Instead, the types of fish living in Walker Lake prior to 1940 were used to demonstrate that shallow-lake intervals resulted from diversion of the Walker River and not from climatic aridity. Major changes in mineralogy and magnesium content of carbonates and major changes in diatom populations with time were found to be a function of the chemical evolution of Walker Lake combined with changing lake size. The stable isotopes of oxygen and carbon were found to be good indicators of lake volume changes. A lake-level record for Walker Lake constructed from stable-isotope data was found to be similar to a lake-level record constructed using tufa and tree-stump data. Both records indicate relatively high lake levels between 4800−2700 yr B.P., at 1250 yr B.P., and within the last 300 yr. Substantial declines in lake level occurred ∼2000 and ∼1000 yr B.P.

California, Idaho, Nevada, Oregon↗

USGS1990

Accelerator-mass spectrometer (AMS) radiocarbon dating of Pleistocene lake sediments in the Great Basin

Pleistocene lake sediments in the Great Basin typically contain little organic carbon, and thus are difficult to date reliably by conventional radioccarbon methods. Paleoenvironmental data are abundant in these sediments, but are of limited value without adequate age controls. With the advent of accelerator-mass spectrometer (AMS) radiocarbon dating, it is now possible to date these paleolacustrine sediments. AMS dates were obtained on sediment cores from the Bonneville, Franklin, and Lahontan Basins. In the Bonneville Basin, the AMS-based chronology compares well with other chronologies constructed from dated shore-zone features. In the Bonneville and Franklin basins, AMS dates delimit unconformities not apparent by other means. We found that dispersed organic carbon from sediments deposited during relatively freshwater intervals provided apparently reliable AMS radiocarbon dates. Carbonate microfossils from the Lahontan Basin also produced results that appear reasonable, while bulk carbonate yielded erroneous results.

California, Nevada, Oregon, Utah↗

USGS1985

Geochemistry of Great Salt Lake, Utah I: Hydrochemistry since 1850

The hydrochemistry of Great Salt Lake, Utah, has been defined for the historic period, 1850 through 1982, from published data combined with new observations. The water balance depends largely on river inflow, atmospheric precipitation onto the lake surface and evaporation. Input of the major solutes can best be accounted for by mixing dilute calcium-bicarbonate type river waters with NaCl-dominated hydrothermal springs. Prior to 1930, lake concentrations fluctuated inversely with lake volume in response to small climatic variations. Since then, salt precipitation and dissolution have significantly modified lake brine compositions and have led to density stratification and the formation of brine pockets of differing composition. Brine mixing has become an important component of brine evolution. We have used calculated evaporation curves with mineral precipitation and dissolution to clarify these processes. Pore fluids represent important storage for solutes. Solute profiles can be modeled by simple one-dimensional diffusion calculations. Short-term historic variations in lake composition affect shallow pore fluids in the upper 2 metres of sediment. ?? 1985.

Utah↗

USGS1985

Geochemistry of great Salt Lake, Utah II: Pleistocene-Holocene evolution

Sedimentologic and biostratigraphic evidence is used to develop a geochemical model for Great Salt Lake, Utah, extending back some 30,000 yrs. B.P. Hydrologie conditions as defined by the water budget equation are characterized by a lake initially at a low, saline stage, rising by about 17,000 yrs. B.P. to fresh water basin-full conditions (Bonneville level) and then, after about 15,000 yrs. B.P., dropping rapidly to a saline stage again, as exemplified by the present situation. Inflow composition has changed through time in response to the hydrologie history. During fresh-water periods high discharge inflow is dominated by calcium bicarbonate-type river waters; during saline stages, low discharge, NaCl-rich hydrothermal springs are significant solute sources. This evolution in lake composition to NaCl domination is illustrated by the massive mirabilite deposition, free of halite, following the rapid drawdown until about 8,000 years ago, while historic droughts have yielded principally halite. Hydrologic history can be combined with inferred inflow composition to derive concentration curves with time for each major solute in the lake. Calcium concentrations before the drawdown were controlled by calcite solubility, and afterwards by aragonite. Significant amounts of solutes are removed from the lake by diffusion into the sediments. Na+, Cl- and SO42- are also involved in salt precipitation. By including pore fluid data, a surprisingly good fit has been obtained between solute input over the time period considered and the amounts actually found in lake brines, pore fluids, salt beds and sediments. Excess amounts are present for calcium, carbonate and silica, indicating detrital input.

Utah↗

USGS1984

Great Salt Lake, and precursors, Utah: The last 30,000 years

Sediment cores up to 6.5 m in length from the South Arm of Great Salt Lake, Utah, have been correlated. Radiocarbon ages and volcanic tephra layers indicate a record of greater than 30,000 years. A variety of approaches have been employed to collect data used in stratigraphic correlation and lake elevation interpretation; these include acoustic stratigraphy, sedimentologic analyses, mineralogy, geochemistry (major element, C, O and S isotopes, and organics), paleontology and pollen. The results indicate that prior to 32,000 year B.P. an ephemeral saline lake-playa system was present in the basin. The perennial lake, which has occupied the basin since this time, rose in a series of three major steps; the freshest water conditions and presumably highest altitude was reached at about 17,000 year B.P. The lake remained fresh for a brief period, followed by a rapid increase in salinity and sharp lowering in elevation to levels below that of the present Great Salt Lake. The lake remained at low elevations, and divided at times into a north and south Basin, until about 8,000 year B.P. Since that time, with the exception of two short rises to about 1290 m, the lake level has remained near the present elevation of 1280 m. ?? 1984 Springer-Verlag.

Contributions to Mineralogy and Petrology↗