Stephen W. Robinson

WISCONSIN--HOLOCENE PALEOENVIRONMENT OF THE BERING SEA: EVIDENCE FROM DIATOMS, POLLEN, OXYGEN ISOTOPES AND CLAY MINERALS

Abstract A core from the southeastern Bering Sea contains a depositional record of the Wisconsin glacial interval and the Holocene. Four lithologic units show distinctive changes in diatom species, palynomorphs, oxygen isotopes in diatom silica, and clay minerals. Unit 4, the oldest, probably represents the glacial advance of the early Wisconsin and records extensive sea ice cover and a soggy, herbaceous tundra on the exposed continental shelf. Unit 3 represents the stades and interstades of the middle Wisconsin, including an early period of high seasonal variability and a later period with expansion of winter sea ice in the basin and of tundra onshore. Unit 2 represents the late Wisconsin glacial maximum and subsequent deglaciation with a tundra-covered shelf. Sea ice cover increased to a maximum, during which the surface waters were probably covered by sea ice throughout the year. Subsequent to the sea ice maximum the isotope ratios record a major meltwater event. Unit 1 represents the Holocene, with expansion of shrub tundra or scrub forest in the coastal upland and waters of increased temperature and salinity in the Bering Sea.

Deglaciation and postglacial timberline in the San Juan Mountains, Colorado

Lake Emma, which no longer exists because of a mining accident, was a tarn in a south-facing cirque near the headwaters of the Animas River in the San Juan Mountains of southwestern Colorado. During the Pinedale glaciation, this area was covered by a large transection glacier centered over the Lake Emma region. Three radiocarbon dates on basal organic sediment from Lake Emma indicate that by ca. 15,000 yr B.P. this glacier, one of the largest in the southern Rocky Mountains, no longer existed. Twenty-two radiocarbon dates on Picea and Abies krummholz fragments in the Lake Emma deposits indicate that from ca. 9600 to 7800 yr B.P., from 6700 to 5600 yr B.P., and at 3100 yr B.P. the krummholz limit was at least 70 m higher than present. These data, in conjunction with Picea:Pinus pollen ratios from both the Lake Emma site and the Hurricane Basin site of J. T. Andrews, P. E. Carrara, F. B. King, and R. Struckenrath (1975, Quaternary Research 5, 173–197) suggest than from ca. 9600 to 3000 yr B.P. timberline in the San Juan Mountains was higher than present. Cooling apparently began ca. 3000 yr B.P. as indicated by decreases in both the percentage of Picea pollen and Picea:Pinus pollen ratios at the Hurricane Basin site (Andrews et al., 1975). Cooling is also suggested by the lack of Picea or Abies fragments younger than 3000 yr B.P. at either the Lake Emma or the Hurricane Basin site.

A fan dam for Tulare Lake, California, and implications for the Wisconsin glacial history of the Sierra Nevada

Historic fluctuations and late Quaternary deposits of Tulare Lake, in the southern San Joaquin Valley, indicate that maximum lake size has depended chiefly on the height of a frequently overtopped spillway. This dependence gives Tulare Lake a double record of paleoclimate. Climate in the Tulare Lake region has influenced the degree to which the lake fills its basin during dry seasons and dry years: during the past 100,000–130,000 yr, incidence of desiccation of Tulare Lake (inferred from stiffness, mud cracks, and other hand-specimen properties) has been broadly consistent with the lake9s salinity and depth (inferred from diatoms and ostracodes) and with regional vegetation (inferred from pollen). Climate, however, also appears to control basin capacity itself: Tulare Lake becomes large as a consequence of glacial-outwash aggradation of its alluvial-fan dam. Late Wisconsin enlargement of Tulare Lake probably resulted from the last major glaciation of the Sierra Nevada. The lake9s spillway coincides with the axis of the glacial-outwash fan of a major Sierra Nevada stream; moreover, sediment deposited in the transgressive lake resembles glacial rock flour from the Sierra Nevada. Differential tectonic subsidence and deposition by a Coast Range creek facilitated the building of Tulare Lake9s fan dam during the late Wisconsin but were less important than deposition of Sierra Nevada outwash. Four stratigraphically consistent 14 C dates on peat and wood give an age of 26,000 yr B.P. for the start of Tulare Lake9s late Wisconsin transgression. The last major Sierra Nevada glaciation (Tioga glaciation) thus may have begun about 26,000 yr B.P., provided that vigorous glacial-outwash deposition began early in the glaciation. Onset of the Tioga glaciation about 26,000 yr B.P. is consistent with new stratigraphic and radiocarbon data from the northeastern San Joaquin Valley. These data suggest that the principal episode of glacial-outwash deposition of Wisconsin age began in the San Joaquin Valley after 32,000 yr B.P., rather than at least 40,000 yr B.P., as previously believed. An earlier enlargement of Tulare Lake probably resulted from a fan dam produced by the penultimate major (Tahoe) glaciation of the Sierra Nevada. Average sedimentation rates inferred from depths to a 600,000-yr-old clay and from radiocarbon dates indicate that this earlier lake originated no later than 100,000 yr B.P. The Tahoe glaciation therefore is probably pre-Wisconsin.

Diatom evidence on Wisconsin and Holocene events in the Bering Sea

Abstract Previous work on surface (modern) sediments has defined diatom species which appear to be good indicators of various oceanographic/ecologic conditions in the North Pacific Ocean and marginal seas. Three long cores from the eastern and northern sides of the Aleutian Basin show changes in species assemblage which can be interpreted in terms of changes in the ocean environment during the last glaciation (Wisconsin) and the Holocene. The early and late Wisconsin maxima were times of prolonged annual sea-ice cover and a short cool period of phytoplankton productivity during the ice-free season. The middle Wisconsin interstade, at least in the southern Bering Sea, had greater seasonal contrast than today, with some winter sea-ice cover, an intensified temperature minimum, and high spring productivity. Variations in clastic and reworked fossil material imply varying degrees of transport to the basin by Alaskan rivers. The results of Jouse from the central Bering Sea generally correspond with those presented here, although there are problems with direct comparison.

Anomalous radiocarbon ages from a Holocene detrital organic lens in Alaska and their implications for radiocarbon dating and paleoenvironmental reconstructions in the arctic

Abstract Eleven radiocarbon age determinations clearly show that a lens of Holocene fluvial organic debris on the Alaskan Arctic Coastal Plain contains mostly pre-Holocene organic material. Radio-carbon ages of identified plant macrofossils indicate the material was deposited about 9000 to 9500 yr B.P. Radiocarbon analyses of bulk samples from this deposit, however, range from 13,300 to 30,300 yr B.P. Most of the old organic matter seems to be in the smaller size fractions in the deposit, particularly in the fraction between 0.25 and 0.5 mm, but all size fractions are contaminated. Particular caution must be exercised in submitting bulk samples for radiocarbon dating from areas where conditions favor redeposition of isotopically “dead” carbon.

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.

Hydrogeochemistry of Big Soda Lake, Nevada: An alkaline meromictic desert lake

Big Soda Lake, located near Fallon, Nevada, occupies an explosion crater rimmed by basaltic debris; volcanic activity apparently ceased within the last 10,000 years. This lake has been selected for a detailed multidisciplinary study that will ultimately cover the organic and inorganic hydrogeochemistry of water and sediments because the time at which chemical stratification was initiated is known (~1920) and chemical analyses are available for a period of more than 100 years. Detailed chemical analyses of the waters show that the lake is at present alkaline (pH = 9.7), chemically stratified (meromictic) and is extremely anoxic (total reduced sulfur—410 mg/L as H2S) below a depth of about 35 m. The average concentrations (in mg/L) of Na, K, Mg, Ca, NH3, H2S, alkalinity (as HCO3), Cl, SO4, and dissolved organics (as C) in waters of the upper layer (depth 0 to 32 m) are 8,100, 320, 150, 5.0, < 0.1, < 0.5, 4,100, 7,100, 5,800, and 20 respectively; in the deeper layer (depth 37 to 64 m) they are 27,000, 1,200, 5.6, 0.8, 45, 410, 24,000, 27,500, 6,800, and 60, respectively. Chemical and stable isotope analyses of the waters, δ13C and Δ14C values of dissolved total carbonate from this lake and surface and ground waters in the area together with mineral-water equilibrium computations indicate that the waters in the lake are primarily meteoric in origin with the present chemical composition resulting from the following geochemical processes: 1. (1) evaporation and exchange with atmosphere, the dominant processes, 2. (2) mineral-water interactions, including dissolution, precipitation and ion exchange, 3. (3) inflow and outflow of ground water and 4. (4) biological activity of macro- and microorganisms, including sulfate reduction in the water column of the deeper layer at a very high rate of 6.6 μmol L−1 day−1.

Slip Rate and Rare Large Prehistoric Earthquakes of the Red River Fault, Southwestern China

The Red River fault is an important plate-boundary fault that has played a significant role in the tectonic evolution of northern Southeast Asia. Nonetheless, its millennial slip rate and earthquake recurrence behavior are poorly constrained. Analysis of 5and 30-m-resolution topography reveals right-lateral offsets that range from 60 m to 24 km along its “mid-valley” trace but none along its “range-front” trace. This strongly implies that the range-front fault has experienced very little lateral slip for millions of years, even though it is the geologically more significant fault. Stratigraphic and geomorphologic investigation of the mid-valley fault within a small channel near Gasa yields a C-based slip rate of 1.1 ± 0.4 mm/year, averaged over the last 30,000–50,000 years. Three-dimensional paleoseismic excavation of colluvial wedges produced by collapse of shutter ridges into the channel shows that sudden dextral ruptures of the fault have occurred every 6,000 ± 1,000 years over the past 30,000 years. Two and possibly three large surface ruptures occurred in the past 13,500 years, and two previous ones occurred at 18,500 and 24,500 calendar years before present (cal yr BP). The oldest one in the section likely occurred at or a few 1,000 years before 29,800 ± 2,000 cal yr BP. The 3-D extent of the colluvial wedges implies dextral offsets of ≥4.5 m, amounts that are consistent with the slip rate and recurrence interval. The evidence for low slip rate and rare large seismic events is consistent with the lack of large historical earthquakes along the fault and the low Global Positioning System (GPS)-derived slip rate, but is much lower than widely cited geological slip rates.