Roger Y. Anderson

Lacustrine varve formation through time

Abstract Studies using sediment traps in lakes reveal that the seasonal flux of sediment regulates both the composition and timing of deposition of materials that reach the bottoms of lakes. If the bottom waters of a lake are partly or totally anoxic, the seasonally deposited materials are preserved as annual groupings of laminae (varves). Common components that form individual laminae consist of allochthonous clastic material derived from the drainage basin, precipitated carbonate minerals, diatom frustules, iron-rich and manganese-rich flocs, autochthonous organic detritus, and autochthonous and allochthonous materials resuspended from the bottom. The “style” of varving has changed over geologic time, reflecting changes in biologic evolution and types of materials available. Precipitated iron-rich laminations were common in the middle Precambrian. Graded sets of clastic organic laminations persisted through the Precambrian, prior to the evolution of bioturbating benthic organisms. Glaciolacustrine varves appear to have retained their distinctive character through time. Carbonate-rich varves occurred sporadically in the Precambrian and Phanerozoic. With the exception of diatoms, major components of modern lacustrine varves were present through the Paleozoic and Mesozoic, and yet varves are rare in strata of these ages, and may have accumulated in marine to brackish-water environments. Diatoms were introduced into lacustrine systems in Early Tertiary time and are common components of varves from then on. Diatom laminae, combined with a greater chance for geologic preservation of younger lake deposits, have increased the number of geologically young occurrences of varved sediments. However, seasonal associations of modern varve components, and the processes they represent, are present in ancient deposits and provide clues to the interpretation of ancient environments.

Regional aridity in North America during the middle Holocene

Increased aridity throughout the Great Plains and Rocky Mountain region during the middle Holo cene has been documented from pollen records, aeolian proxy variables in lake cores, and active sand dune migration. Varve calibration provided by a continuously varved record of the Holocene from a core from Elk Lake, northwestern Minnesota, shows that the influx of aeolian clastic material increased beginning about 8 ka and ended about 3.8 ka, with peak aeolian activity at about 6 ka. If aeolian influx to Elk Lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in Minnesota dune fields, then the varve calibration in Elk Lake provides precise time calibration of periods of peak aeolian activity in Minnesota. Palaeowind studies from the Minnesota dune fields show that the dominant wind direction when the dunes were active was from the northwest, the same as the dominant wind direction in dune fields throughout the Great Plains and Rocky Mountains. If the mid-Holocene aeolian activity in Minnesota was driven by an increase in westerly zonal winds, then the varve calibration can be extended to more precisely determine the timing of activity of dunes over a much broader area. We suggest that an increase in the westerly zonal wind field might have a solar-geomagnetic cause.

A continuous, high-resolution record of late Pleistocene climate variability from the Estancia basin, New Mexico

Lake sediments that accumulated in the Estancia basin, central New Mexico, provide a detailed record of submillennial latest Pleistocene climatic oscillations. Sedimentologic, biologic, and geochemical proxies for changes in salinity and lake level were used to identify episodes of wet and dry climate that occurred between 24 and 12 k.y. B.P., as dated by 14C by means of accelerator mass spectrometry from shoreline and basin-center deposits. These dates determine the timing and duration of the episodes of wet and dry climate. The lake expanded and contracted repeatedly during the last glacial maximum and fluctuated near its highest elevation several times during the interval ca. 20 to 15 k.y. B.P. A pronounced lowstand lasting ∼1 k.y. occurred between ca. 15 and 14 k.y. B.P., followed by two more highstands of the lake between ca. 14 and 12.5 k.y. B.P. Desiccation of the perennial lake after 12 k.y. B.P. was followed by a final, poorly dated highstand at ca. 10 k.y. B.P. The record of lake-level fluctuations from the Estancia basin provides a high-resolution record of changes in water budget in a basin, the comparatively simple physiography and hydrogeology of which ensured a direct response to the strong climatic fluctuations that occurred during the last glacial maximum and termination.

Evidence from Western North America for Rapid Shifts in Climate During the Last Glacial Maximum

The Estancia basin in southwestern United States contains evidence for strong and rapid pulsations in the supply of moisture brought into the region during the last ice age. The pulses were recorded during episodes of stream discharge that spread plumes of fresh water laden with quartz sand over the saline lake. The largest pulses in stream discharge lasted only a few decades, were organized into cycles that were spaced approximately 200 to 250 and 2000 years apart, and were of sufficient magnitude to freshen and maintain the lake at its maximum recorded elevation.

Permian Castile Varved Evaporite Sequence, West Texas and New Mexico

Laminations in the Upper Permian evaporite sequence in the Delaware Basin appear in the preevaporite phase of the uppermost Bell Canyon Formation as alternations of siltstone and organic layers. The laminations then change character and composition upward to organically laminated claystone, organically laminated calcite, the calcite-laminated anhydrite typical of the Castile Formation, and finally to the anhydrite-laminated halite of the Castile and Salado. Laminae are correlative for distances up to 113 km (70.2 mi) and probably throughout most of the basin. Each lamina is synchronous, and each couplet of two laminated components is interpreted as representing an annual layer of sedimentation—a varve. The thickness of each couplet in the 260,000-varve sequence (a total thickness of 447.2 m, 1467 ft) has been measured individually and recorded and provides the basis for subdividing and correlating major stratigraphic units within the basin. The uppermost 9.2 m (30.3 ft) of the Bell Canyon Formation contains about 50,850 varve couplets; the Basal Limestone Member of the Castile about 600; the lowermost anhydrite member of the Castile (Anhydrite I) contains 38,397; Halite I, 1,063; Anhydrite II, 14,414; Halite II, 1,758; Anhydrite III, 46,592; Halite III, 17,879; and Anhydrite IV, 54,187. The part of the Salado collected (126.6 m) contains 35,422 varve couplets. The Bell Canyon-Castile sequence in the cores studied is apparently continuous, with no recognizable unconformities. The dominant petrologic oscillation in the Castile and Salado, other than the laminations, is a change from thinner undisturbed anhydrite laminae to thicker anhydrite laminae that generally show a secondary or penecontem-poraneous nodular character, with about 1,000 to 3,000 units between major oscillations or nodular beds. These nodular zones are correlative throughout the area of study and underly halite when it is present. The halite layers alternate with anhydrite laminae, are generally recrystallized, and have an average thickness of about 3 cm. The halite beds were once west of their present occurrence in the basin but were dissolved, leaving beds of anhydrite breccia. The onset and cessation of halite deposition in the basin was nearly synchronous. The Anhydrite I and II Members thicken gradually across the basin from west to east, whereas the Halite I, II, and III Members are thickest in the eastern and northeastern part of the basin and thicken from southeast to northwest. This distribution and the synchroneity indicate a departure from the classical model of evaporite zonation.

Short-Duration Holocene Lakes in the Mojave River Drainage Basin, Southern California

Stratigraphic, sedimentologic, and pedologic studies of beach ridge and lacustrine deposits indicate that up to five times during the Holocene, shallow lakes covered Silver Lake playa in southeastern California for periods of years to decades. The two youngest lacustrine events (at about 390 ± 90 yr B. P. and 3620 ± 70 yr B. P.) coincide with the early and late Neoglacial episodes of North America. Increasing evidence in recent years from other nonglaciated areas leads us to conclude that the effects of these climatic episodes were much more widespread than previously thought. The climate during these episodes was characterized by an increased frequency of winter storms in the southwestern United States, causing wetter conditions that affected diverse, hyperarid environments in the Mojave Desert and adjacent regions. We propose that this wide areal coverage was caused by large-scale, winter atmospheric circulation patterns, which are probably related to changes in sea-surface temperatures and oceanic circulation in the eastern North Pacific Ocean.

The variability of holocene climate change: evidence from varved lake sediments.

Varved sediments from a lake near the present forest-prairie border in northwestern Minnesota provide an annual record of climate change for the last 10,400 years. Climate-sensitive mineral, chemical, and biological components show that the mid-Holocene dry interval between 8500 and 4000 years ago is asymmetrical and actually consists of two distinct drier pulses separated by a moister interval that lasted about 600 years. Cyclic fluctuations with periods of several hundred years were abrupt and persistent throughout the Holocene and are most clearly recorded within the two drier pulses.

Geochemical evidence for enhanced preservation of organic matter in the oxygen minimum zone of the continental margin of northern California during the Late Pleistocene

The present upper water mass of the northeastern Pacific Ocean off California has a well-developed oxygen minimum zone between 600 and 1200 m wherein concentrations of dissolved oxygen are less than 0.5 mL/L. Even at such low concentrations of dissolved oxygen, benthic burrowing organisms are abundant enough to thoroughly bioturbate the surface and near-surface sediments. These macro organisms, together with micro organisms, also consume large quantities of organic carbon produced by large seasonal stocks of plankton in the overlying surface waters, which are supported by high concentrations of nutrients within the California Current upwelling system. In contrast to modern conditions of bioturbation, laminated sediments are preserved in upper Pleistocene sections of cores collected on the continental slope at water depths within the present oxygen minimum zone from at least as far north as the California-Oregon border and as far south as Point Conception. Comparison of sediment components in the laminae with those delivered to sediment traps as pelagic marine “snow” demonstrates that the dark-light lamination couplets are indeed annual (varves). These upper Pleistocene varved sediments contain more abundant lipid-rich “sapropelic” (type II) organic matter than the overlying bioturbated, oxidized Holocene sediments. The baseline of stable carbon isotopic composition of the organic matter in these slope cores does not change with time, indicating that the higher concentrations of type II organic matter in the varved sediments represent better preservation of organic matter rather than any change in the source of organic matter.

Dissolution of salt deposits by brine density flow

The origin of collapse structures and breccias that vertically penetrate or occur within impermeable evaporites has never really been understood. The density of the brine that develops as salt deposits are dissolved can generate continuous gravitational brine movement. If the source of the dissolving water is artesian, or continuous, a flow cycle is developed in which the salt itself supplies the density gradient that becomes the vehicle of its own dissolution. The Delaware Basin in western Texas and southeastern New Mexico provides a particularly good example of how brine density flow can produce dissolution chambers that collapse to form breccias. The potential for dissolution by brine flow is an inherent property of partly exhumed evaporites and may constitute a risk factor in the storage of radioactive waste in evaporite deposits.

Contributions of La niña and El Niño to middle Holocene drought and late Holocene moisture in the American Southwest

Eolian landforms in Estancia basin, central New Mexico, record two episodes of extreme drought and low groundwater levels during the middle Holocene (7000-5400 1 4 C yr B.P.), followed by a rise in the water table throughthe late Holocene. Blowouts and associated lunettes formed when water levels fell below the desiccated floor of pluvial Lake Estancia, allowing widespread deflation. Elevation of the water table in the basin is regulated by a balance between evaporation from playas that occupy the blowouts and recharge of aquifers in the adjacent Manzano Mountains. Isotopic analyses of modern precipitation and groundwater reveal that recharge originates primarily as winter moisture, which is amplified during El Ninos and diminished during La Ninas. Thus, changes in the elevation of a reconstructed Holocene water table may reflect systematic changes in El Nino-Southern Oscillation (ENSO) circulation.