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Geology topics

J. Platt Bradbury

Publications and source records attributed to J. Platt Bradbury.

At least 19 recordsLinked to original sources
USGS2004

The history of recent limnological changes and human impact on Upper Klamath Lake, Oregon

Hypereutrophic Upper Klamath Lake has been studied for almost 50 years to evaluate the nature, cause, and effects of its very productive waters. Mitigation of undesirable effects of massive cyanobacterial blooms requires understanding their modern causes as well as their history. Knowledge of the pre-settlement natural limnology of this system can provide guidelines for lake restoration and management of land and water use strategies to maximize the benefits of this aquatic resource. This investigation uses a paleolimnological approach to document the nature and chronology of limnological and biological changes in Upper Klamath Lake for the past 200 years, covering the time when the lake was first described until today. A 45-cm gravity core, dated by 210 Pb and diatom correlations, was analyzed for diatoms, pollen, akinetes (resting spores) of the cyanobacterium Aphanizomenon flos-aquae , reworked tephra shards, and sediment magnetic characteristics. Pollen profiles show little vegetation change during this time. In contrast, diatoms indicative of increased nutrient fluxes (P and Si) increase moderately, coinciding with the settlement of the region by Euro-Americans. Numerous settlement activities, including draining of lake-margin marshes, upstream agriculture and timber harvest, road construction, and boat traffic, may have affected the lake. Magnetic properties and reworked tephra suggest riparian changes throughout the basin and increased lithogenic sediment delivery to the lake, especially after 1920 when the marshes near the mouth of the Williamson River were drained and converted to agricultural and pasture land. Drainage and channelization also decreased the ability of the marshes to function as traps and filters for upstream water and sediments. Akinetes of Aphanizomenon flos-aquae record progressive eutrophication of Upper Klamath Lake beginning in the 20th century and particularly after 1920 when lake-margin marsh reclamation more than doubled. The coincidence of limnological changes and human activities following European settlement suggests a major impact on the Upper Klamath Lake ecosystem, although ascribing specific limnological changes to specific human activities is difficult.

Oregon↗
USGS2004

Paleolimnology and paleoclimate studies in Upper Klamath Lake, Oregon

The subsiding Upper Klamath Lake Basin contains sediments that were continuously deposited in a shallow, freshwater lake for more than 40 000 years. Well dated by radiometric methods and containing volcanic ashes of known age, these sediments constitute a valuable paleoclimate record. Sediment constituents and properties that reflect past climatic conditions in the area include pollen, diatoms, sediment geochemistry, and sediment magnetic properties. Many of these proxy measurements are also useful for comparing natural conditions in the lake to conditions following human settlement. Because of its location, the paleoclimate record from Upper Klamath Lake is valuable for comparisons to offshore marine records and as part of latitudinal transects of paleoclimate records along the west coast of the Americas.

Oregon↗
USGS2002

Early Holocene change in atmospheric circulation in the Northern Great Plains: An upstream view of the 8.2 ka cold event

Elk Lake, in northwestern Minnesota, contains numerous proxy records of climatic and environmental change contained in varved sediments with annual resolution for the last 10,000 years. These proxies show that about 8200 calendar years ago (8.2 cal. ka; 7300 radiocarbon years) Elk Lake went from a well-stratified lake that was wind-protected in a boreal forest to a well-mixed lake in open prairie savanna receiving northwesterly wind-blown dust, probably from the dry floor of Lake Agassiz. This change in climate marks the initiation of the widely recognized mid-Holocene “altithermal” in central North America. The coincidence of this change with the so-called 8.2 cal. ka cold event, recognized in ice-core and other records from the circum-North Atlantic, and thought by some to be caused by catastrophic discharge of freshwater from proglacial lakes Agassiz and Ojibway, suggests that the two “events” might be related. Our interpretation of the Elk Lake proxy records, and of other records from less accurately dated sites, suggests that change in climate over North America was the result of a fundamental change in atmospheric circulation in response to marked changes in the relative proportions of land, water, and, especially, glacial ice in North America during the early Holocene. This change in circulation probably post-dates the final drainage of proglacial lakes along the southern margin of the Laurentide ice sheet, and may have produced a minor perturbation in climate over Greenland that resulted in a brief cold pulse detected in ice cores.

Minnesota↗
USGS1999

Continental diatoms as indicators of long-term environmental change

It is curious that diatoms, whose short lifespans and capacity for rapid regeneration make them especially suitable for short-term paleoenvironmental studies, would also have a significant role as indicators of long-term environmental change. This chapter explores the nature of long diatom records, their relation to global environmental changes, guidelines for their interpretation, and problems common to such records.

Book chapter↗
USGS1997

Synthesis of the paleoclimatic record from Owens Lake core OL-92

During much of the late Quaternary, Owens Lake overflowed into one or more of four successively lower-elevation basins. Most of the water came from the high, eastern slopes of the southern Sierra Nevada, and changes in the volumes of that water reflect a dominant climatic cycle of ~100 k.y. Variations in the inflow to, and outflow from, Owens Lake since ca. 800 ka left biological, chemical, mineralogical, and geophysical evidence in the sediments of those changes. Biological evidence includes fossil ostracodes, diatoms, fish, and mollusks (and δ 18 O data from their shells) which indicate fresh or brackish lake water on the basis of their modern habitats. Fossil pollens indicate ~20 regional vegetation cycles during the same period. Chemical evidence of high inflow and, commonly, outflow volumes is provided by the low inorganic- and organic-C content of some sediments, reflecting short lake-water residence times; long residence times produced higher and more variable quantities of these components. Mineralogical variations in illite/smectite ratios indicate changes in weathering processes and glacial comminution. High magnetic susceptibility correlates with other criteria that indicate high runoff. Between 810 ka and 645 ka, Owens Lake was fresh, several meters deep, and depositing silt with a few beds of sand; it supported a flora and fauna now found in fresh, sometimes very cool, waters. (Note that most geologic ages describing the OL-92 chronology have been rounded to the nearest 5 or 10 ka.) A shallow-but-freshwater lake may have been the result of accelerated sedimentation during an earlier (>800 ka) glaciation in the Sierra Nevada, choking the basin with sediment nearly to its spillway level. Between 645 ka and 450 ka, the lake was probably even shallower, depositing beds of coarse to fine sand, but overflowing periodically allowing its water to remain fresh. Between 450 ka and 5 ka, Owens Lake was mostly deep, alternating between spilling and being closed part of the time. It deposited silt and clay on its floor, yet underwent detectable variations in salinity caused by climate changes; this part of the record is the most easily interpreted and constitutes the main basis for comparing this paleoclimatic record with other long records. From 5 ka to A.D. 1913, when the Owens River was diverted into an aqueduct, Owens Lake was shallow (~2 m to ~15 m), moderately saline (~5% to <15% salts), and depositing oolites. After 1913, the lake desiccated. Comparison of the Owens Lake water-depth record with that of Searles Lake, two-basins downstream during much of late Pleistocene time, shows that they underwent similar responses to climate, but sedimentation changes documenting those responses commenced thousands of years apart, apparently because changes in precipitation volumes occurred gradually. Owens Lake, at the base of high mountains, was the first to reflect increasing amounts of regional precipitation; Searles, in a more arid environment, was the first to reflect decreasing amounts of precipitation. Devils Hole, 150 km east of Owens Lake, has a well dated isotopic-temperature record that resembles the Owens Lake-depth record. Marine records of Pleistocene glacial fluctuations, which measure high-latitude ice-sheet volumes and thus both precipitation and temperature at those latitudes, also resemble the Owens Lake history. There are, however, differences between the ages of the maxima and minima of climatic events as reconstructed from the Owens Lake core and similar-appearing inflections in the other two records; the differences range from 0 to 33 k.y. and average ~15 k.y. The question arises whether the differences between those ages are results of errors in the time-scale used for the Owens Lake record, or were there significant differences in the times when atmospheric climate change began to affect its different elements. The three records compared here are measurements of different elements and combinations of elements in two latitude belts: the deep-sea marine records measure combinations of temperature and precipitation that determined global ice volumes (at mostly high latitudes), the Devils Hole record measures atmospheric temperatures (in its mid-latitude region), and the Owens Lake record measures effective precipitation (in the same mid-latitude region).

California↗
USGS1997

A diatom-based paleohydrologic record of climate change for the past 800 k.y. from Owens Lake, California

A 323-m (~800 k.y.) core of lake deposits beneath Owens Lake playa, Inyo County, California, contains a nearly continuous paleolimnological record based on diatom assemblages. The core chronology is anchored by the Matuyama/Brunhes magnetostratigraphic boundary and the Bishop ash near the base of the record and by radiocarbon dates near the top. Throughout most of its history, Owens Lake was characterized by fresh-water diatoms, indicating a positive hydrologic input from the Owens River and overflow to lake systems downstream. Both benthic and planktic freshwater diatoms dominate in ashy and sandy sediments between 800 ka and 440 ka and suggest shallow, open-water environments in a basin where sedimentation and subsidence were approximately balanced. After 440 ka, freshwater planktic diatoms dominate, implying that the Owens basin became deeper, perhaps as a result of increased rates of tectonic subsidence. The stratigraphic distribution of saline benthic and planktic diatoms record comparatively short intervals when the lake was shallow and saline. Nevertheless, periodic overflow during these times prevented deposition of evaporites. According to a chronology based on sediment mass-accumulation rates, the alternation of saline and freshwater diatom assemblages approximately tracks the progression of oxygen isotope stages recorded in marine deposits. Even-numbered isotope stages representing glacial conditions are matched by episodes where freshwater planktic diatoms dominate, indicating abundant precipitation in the Sierra Nevada in response to a southward shift of storm tracks originating in the North Pacific around the Aleutian Low.

California↗
USGS1995

The diatom genus Actinocyclus in the Western United States

Ten new and four known taxa of the diatom genus Actinocyclus are described, illustrated, and (or) noted from middle Miocene lake deposits in the Western United States. A key is presented to help separate the taxa based on morphological criteria visible in the light microscope. The geologic ranges of Actinocyclus species in the Western United States are discussed based on examination of over 100 localities of diatomaceous lacustrine deposits.

Professional Paper↗
USGS1994

A synthesis of post-glacial diatom records from Lake Baikal

The biostratigraphy of fossil diatoms contributes important chronologic, paleolimnologic, and paleoclimatic information from Lake Baikal in southeastern Siberia. Diatoms are the dominant and best preserved microfossils in the sediments, and distinctive assemblages and species provide inter-core correlations throughout the basin at millennial to centennial scales, in both high and low sedimentation-rate environments. Distributions of unique species, once dated by radiocarbon, allow diatoms to be used as dating tools for the Holocene history of the lake. Diatom, pollen, and organic geochemical records from site 305, at the foot of the Selenga Delta, provide a history of paleolimnologic and paleoclimatic changes from the late glacial (15 ka) through the Holocene. Before 14 ka diatoms were very rare, probably because excessive turbidity from glacial meltwater entering the lake impeded productivity. Between 14 and 12 ka, lake productivity increased, perhaps as strong winds promoted deep mixing and nutrient regeneration. Pollen evidence suggests a cold shrub — steppe landscape dominated the central Baikal depression at this time. As summer insolation increased, conifers replaced steppe taxa, but diatom productivity declined between 11 and 9 ka perhaps as a result of increased summer turbidity resulting from violent storm runoff entering the lake via short, steep drainages. After 8 ka, drier, but more continental climates prevailed, and the modern diatom flora of Lake Baikal came to prominence. On Academician Ridge, a site of slow sedimentation rates, Holocene diatom assemblages at the top of 10-m cores reappear at deeper levels suggesting that such cores record at least two previous interglacial (or interstadial?) periods. Nevertheless, distinctive species that developed prior to the last glacial period indicate that the dynamics of nutrient cycling in Baikal and the responsible regional climatic environments were not entirely analogous to Holocene conditions. During glacial periods, the deep basin sediments of Lake Baikal are dominated by rapidly deposited clastics entering from large rivers with possibly glaciated headwaters. On the sublacustrine Academician Ridge (depth = 300 m), however, detailed analysis of the diatom biostratigraphy indicates that diastems (hiatuses of minor duration) and (or) highly variable rates of accumulation complicate paleolimnologic and paleoclimatic reconstructions from these records.

Lake Baikal↗
USGS1994

The volcanic, sedimentologic, and paleolimnologic history of the Crater Lake caldera floor, Oregon:Evidence for small caldera evolution

Apparent phreatic explosion craters, caldera-floor volcanic cones, and geothermal features outline a ring fracture zone along which Mount Mazama collapsed to form the Crater Lake caldera during its climactic eruption about 6,850 yr B.P. Within a few years, subaerial deposits infilled the phreatic craters and then formed a thick wedge (10-20 m) of mass flow deposits shed from caldera walls. Intense volcanic activity (phreatic explosions, subaerial flows, and hydrothermal venting) occurred during this early postcaldera stage, and a central platform of subaerial andesite flows and scoria formed on the caldera floor. Radiocarbon ages suggest that deposition of Iacustrine hemipelagic sediment began on the central platform about 150 yr after the caldera collapse. This is the minimum time to fill the lake halfway with water and cover the platform assuming present hydrologic conditions of precipitation and evaporation but with negligible leakage of lake water. Wizard Island formed during the final part of the 300-yr lake-filling period as shown by its (1) upper subaerial lava flows from 0 to -70 m below present water level and lower subaqueous lava flows from -70 to -500 m and by (2) lacustrine turbidite sand derived from Wizard Island that was deposited on the central platform about 350 yr after the caldera collapse. Pollen stratigraphy indicates that the warm and dry climate of middle Holocene time correlates with the early lake deposits. Diatom stratigraphy also suggests a more thermally stratified and phosphate-rich environment associated respectively with this climate and greater hydrothermal activity during the early lake history. Apparent coarse-grained and thick-bedded turbidites of the early lake beds were deposited throughout northwest, southwest, and eastern basins during the time that volcanic and seismic activity formed the subaqueous Wizard Island, Merriam Cone, and rhyodacite dome. The last known postcaldera volcanic activity produced a subaqueous rhyodacite ash bed and dome about 4,240 yr B.P. The late lake beds with base-of-slope aprons and thin, fine-grained basin-plain turbidites were deposited during the volcanically quiescent period of the past 4,000 yr. Deposits in Crater Lake and on similar caldera floors suggest that four stages characterize the postcaldera evolution of smaller (≤10 km in diameter) terrestrial caldera lake floors: (1) initial-stage caldera collapse forms the ring fracture zone that controls location of the main volcanic eruptive centers and sedimentary basin depocenters on the caldera floor; (2) early-stage subaerial sedimentation rapidly fills ring-fracture depressions and constructs basin-floor debris fans from calderawall landslides; (3) first-stage subaqueous sedimentation deposits thick flat-lying lake turbidites throughout basins, while a thin blanket of hemipelagic sediment covers volcanic edifices that continue to form concurrently with lake sedimentation; and (4) second-stage subaqueous sedimentation after the waning of major volcanic activity and the earlier periods of most rapid sedimentation develops small sili-ciclastic basin base-of-slope turbidite aprons and central basin plains. Renewed volcanic activity or lake destruction could cause part or all of the cycle to repeat.

Oregon↗
USGS1993

Elk Lake in perspective

Elk Lake is located in the forested region of north-central Minnesota at the headwaters of the Mississippi River and occupies one of countless basins left behind as the last great Pleistocene ice sheet retreated northward into Canada. In this respect it resembles many other moderately deep, dimictic, hard-water lakes in the north-central United States, the sediments of which contain a history of postglacial and Holocene climatic and environmental change. Elk Lake is different, however, because the Holocene sediments in the deeper part of the lake form an uninterrupted sequence of annual laminations or varves. The varves are a chronometer for timing precisely the biologic, geochemical, and sedimentological responses in the lake to cyclic and progressive changes in climate. The varves also, through profound changes in their composition, divide the history of Elk Lake into three, sharply defined episodes; a postglacial lake, a prairie lake, and a modern, mesic-forest lake. We use these episodes and the character of the varves as a framework to guide the reader to the chapters and discussions found in this volume.

Minnesota↗
USGS1993

Climatic and limnologic setting of Elk Lake

Elk Lake is located on the Itasca moraine near the source of the Mississippi River in northwestern Minnesota. The basin is in calcareous glacial drift, and the lake water is a dilute solution of calcium and magnesium bicarbonate. Low-magnesian calcite formed by precipitation from the lake water has been a major component of the sediment throughout the lake’s history. The sediment also is laminated with alternating light and dark, millimeter-thick layers containing diatoms, organic matter, Fe(OH) 3 , and CaCO 3 . The sediment microstratigraphy has been preserved because the lake is unusually deep (maximum depth is 30 m) for its size (surface area is 1.01 km 2 ). Oxygen is present in low concentrations or absent in the deepest water during summer and winter. Water movements in the deepest part of the lake are insufficient some years for the complete aeration of the deepst water during spring and autumn circulation periods. Phytoplankton photosynthesis, which occurs mostly in the surficial 6 m of water, typically removes 0.5 g C m −2 day −1 from the epilimnion, which becomes strongly oversaturated with calcite during late spring and summer as the pH increases above the equilibrium pH (7.73) for calcite saturation. Most of the CaCO 3 that makes up the light-colored layers of sediments probably is formed during the late summer, when concentrations of calcium in the epilimnion decrease most rapidly. The silica and organic matter that form the darker sediment laminae are deposited earlier in the year, during a period extending from April to late June, when silica decreases fastest in the epilimnion.

Minnesota↗
USGS1993

Holocene climatic and limnologic history of the north-central United States as recorded in the varved sediments of Elk Lake, Minnesota: A synthesis

Integration of the results and interpretations of geochemical, paleoecological, and sedimentological analyses of a varved sediment record provides a detailed chronicle of limnological and climatic changes for the past 10 ka at Elk Lake, west-central Minnesota. The early Holocene record at Elk Lake was controlled by circumstances of glacial history (e.g., basin morphometry and surrounding till lithology) in combination with global warming at the end of the Pleistocene. Later, the interplay of climate change and a disintegrating ice sheet determined the character of local environments that were affected by reduction of precipitation and increased windiness during the middle Holocene. Elk Lake became more productive and clastic sediment increased to dominate the record as the forests thinned and prairie vegetation characterized the region. During this prairie period, winters may have been cold because disintegrating northern ice sheets ceased to block winter outbreaks of Arctic air. Correlations of wind-deposited materials throughout much of the eastern two-thirds of the United States suggest that drought conditions and strong winds were widespread between 8 and 4 ka. The mid-Holocene was climatically variable, however, with strong fluctuations in varve thickness at decadal, centennial, and millennial scales testifying to rapid climatic changes. Although the cause of such climatic cycles is not yet clear, correlations between 14 C anomalies and varve thickness suggest that variations in solar flux and resulting magnetic storms and zonal winds may have induced strong climatic changes. A particularly strong 600 yr fluctuation to cool and wet climates that may document neoglacial conditions around 5 ka interrupted the prairie period. After 4 ka, the climate at Elk Lake was dominated by a tropical airstream during the summer, and dry arctic and Pacific airstreams during the winter. Large-scale variations ceased, although decadal and multi-decadal variations in varve thickness chronicle changes similar, but not clearly correlative, to historically documented climatic episodes such as the Medieval Warm Period and the Little Ice Age.

Minnesota↗
USGS1993

Chronology of Elk Lake sediments: Coring, sampling, and time-series construction

A 22 m series of cores from a continuously laminated sequence of postglacial sediment was recovered from 29.6 m of water from the deepest part of Elk Lake, Clearwater County, Minnesota, by piston and freeze-coring methods during the winters of 1978 and 1982. A varve time series constructed and used as a basis for subsampling the cores and samples, based on the varve chronology, allows precise determination of fluxes of geochemical and biological sediment components. Chronological and petrographic studies have shown that the laminations are varves and their measurement and enumeration has produced a 10,400 year time series that estimates the rates and timing of paleolimnologic and paleoenvironmental changes in Elk Lake and its drainage. A radiocarbon date from surface sediment is 850 years. The difference between radiocarbon and varve dates continues down core; varve dates are older than radiocarbon dates, probably because of systematic incorporation of dead carbon (as bicarbonate) in organic matter in the sediment. Varve-dated boundaries of pollen zones in the Elk Lake cores compare closely with the ages of the same zones in cores from nearby lakes that have been radiocarbon dated.

Minnestoa↗
USGS1993

Modern sedimentation in Elk Lake, Clearwater County, Minnesota

The varved sediments of Elk Lake, Clearwater County, Minnesota, contain a 10,000 year record of climatic and limnologic events. Sediment traps deployed in the lake’s water column from 1979 to 1981 and from 1983 to 1984 collected samples that permitted us to identify materials, to see the timing of sedimentation events, and to deduce processes that form the microlaminae within varves. Fall and spring microlaminae consist mainly of sequential accumulations of biogenic silica and resuspended calcific and siliceous materials. Precipitates of iron, manganese, and organic detritus dominate the thin winter microlaminae. Calcific microlaminae are deposited in summer. Concentrated iron and manganese precipitates form when the onset of seasonal circulation (especially in autumn) oxygenates the lower water column, but precipitation of these metals also continues throughout periods of seasonal stratification, when these dissolved elements migrate upward and are converted to particles that rain back to the bottom. Mineraloids dominate the sediment; minerals compose only a minor part and include quartz, calcite, rhodochrosite, and rockbridgeite (iron phosphate). The bulk of the bottom accumulation occurs during the longer, calmer summer and winter periods, but important contributions are also made during spring and autumn overturn events. Sediment resuspended from the shallows accumulates together with newly formed endogenic sediment, and can even briefly dominate the seston in autumn and spring. Vigor and duration of seasonal circulations in the upper water column dictate the amount of resuspended sediment contributed annually to a varve. When abrupt warming within days after ice-out stratifies the lake, sedimentation in that year is diminished by resultant suppression of plankton blooms and lack of vernal resuspension that would normally move sediment from the littoral areas into the deep parts of Elk Lake. Thin sections of varves confirm that resuspension during autumn and spring is a varve-forming process that has probably varied in importance as a function of climate and changing morphometry due to infilling. Through an entire year, sediment traps catch a greater proportion of material from spring and autumn overturns than accumulates on the bottom. Lake morphometry is the most important factor governing sediment resuspension and associated annual accumulation rates in traps.

Minnesota↗
USGS1990

Environmental changes in the Tule Lake basin, Siskiyou and Modoc Counties, California, from 3 to 2 million years before present

Pollen and diatom analyses of a core from the town of Tulelake, Siskiyou County, California, for the period between 3 and 2 Ma reveal a paleoclimatic and paleolimnologic sequence recording a long, warm time interval that lasted from about 2.9 to 2.6 Ma and had a short, cooler interval within it. During this warm interval, the regional vegetation surrounding ancient Tule Lake was a mixed coniferous forest, and Tule Lake was a warm monomictic lake. Approximate modern analogs for this Pliocene fossil record at Tulelake are found at least 2 degrees farther south. The Tulelake warm interval appears to have correlatives in the North Atlantic oxygen isotope record and in the pollen record of the Reuverian in the Netherlands. An interval beginning at about 2.4 Ma was characterized at Tule Lake by slow sedimentation, by changes in the relative amounts of algae in the lake, and by an increase in the maximum percentages of Artemisia pollen.

California↗
USGS1988

Depositional environments of the Cache, Lower Lake, and Kelseyville Formations, Lake County, California

We describe the depositional environments of the Cache, Lower Lake, and Kelseyville Formations in light of habitat preferences of recovered mollusks, ostracodes, and diatoms. Our reconstruction of paleoenvironments for these late Cenozoic deposits provides a framework for an understanding of basin evolution and deposition in the Clear Lake region. The Pliocene and Pleistocene Cache Formation was deposited primarily in stream and debris flow environments; fossils from fine-grained deposits indicate shallow, fresh-water environments with locally abundant aquatic vegetation. The fine-grained sediments (mudstone and siltstone) were probably deposited in ponds in abandoned channels or shallow basins behind natural levees. The abandoned channels and shallow basins were associated with the fluvial systems responsible for deposition of the bulk of the technically controlled Cache Formation. The Pleistocene Lower Lake Formation was deposited in a water mass large enough to contain a variety of local environments and current regimes. The recovered fossils imply a lake with water depths of 1 to 5 m. However, there is strong support from habitat preferences of the recovered fossils for inferring a wide range of water depths during deposition of the Lower Lake Formation; they indicate a progressively shallowing system and the culmination of a desiccating lacustrine system. The Pleistocene Kelseyville Formation represents primarily lacustrine deposition with only minor fluvial deposits around the margins of the basin. Local conglomerate beds and fossil tree stumps in growth position within the basin indicate occasional widespread fluvial incursions and depositional hiatuses. The Kelseyville strata represent a large water mass with a muddy and especially fluid substrate having permanent or sporadic periods of anoxia. Central-lake anoxia, whether permanent or at irregular intervals, is the simplest way to account for the low numbers of benthic organisms recovered from the Kelseyville Formation. Similar low-oxygen conditions for benthic life are represented throughout the sedimentary history of Clear Lake. Water depths for the Kelseyville Formation of 10 to 30 m and 12 m near the margins of the basin are inferred both before and after fluvial incursions. These water-depth fluctuations cannot be correlated with major climatic changes as indicated by pollen and fossil leaves and cones; they may be due to faulting in this technically active region.

GSA Special Papers↗
USGS1984

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.

Minnesota↗