Geology topics

Yen Zheng

Publications and source records attributed to Yen Zheng.

8 recordsLinked to original sources

USGS2010

Intensification of the Northeast Pacific oxygen minimum zone during the Bölling-Alleröd warm period

Although climate records from several locations around the world show nearly synchronous and abrupt changes, the nature of the inferred teleconnection is still poorly understood. On the basis of preserved laminations and molybdenum enrichments in open margin sediments we demonstrate that the oxygen content of northeast Pacific waters at 800 m depth during the Bölling-Alleröd warm period (15–13 kyr) was greatly reduced. Existing oxygen isotopic records of benthic and planktonic foraminifera suggest that this was probably due to suppressed ventilation at higher latitudes of the North Pacific. Comparison with ventilation records for the North Atlantic indicates an antiphased pattern of convection relative to the North Pacific over the past 22 kyr, perhaps due to variations in water vapor transport across Central America.

Paleoceanography↗

USGS2009

Spatial pattern of groundwater arsenic occurrence and association with bedrock geology in greater augusta, maine

In New England, groundwater arsenic occurrence has been linked to bedrock geology on regional scales. To ascertain and quantify this linkage at intermediate (10 0 -10 1 km) scales, 790 groundwater samples from fractured bedrock aquifers in the greater Augusta, Maine area are analyzed, and 31% of the sampled wells have arsenic concentrations >10 ??g/L. The probability of [As] exceeding 10 ??g/L mapped by indicator kriging is highest in Silurian pelite-sandstone and pelite-limestone units (???40%). This probability differs significantly (p < 0.001) from those in the Silurian - Ordovician sandstone (24%), the Devonian granite (15%), and the Ordovician - Cambrian volcanic rocks (9%). The spatial pattern of groundwater arsenic distribution resembles the bedrock map. Thus, bedrock geology is associated with arsenic occurrence in fractured bedrock aquifers of the study area at intermediate scales relevant to water resources planning. The arsenic exceedance rate for each rock unit is considered robust because low, medium, and high arsenic occurrences in four cluster areas (3-20 km 2 ) with a low sampling density of 1-6 wells per km 2 are comparable to those with a greater density of 5-42 wells per km 2 . About 12,000 people (21% of the population) in the greater Augusta area (???1135 km 2 ) are at risk of exposure to >10 ??g/L arsenic in groundwater. ?? 2009 American Chemical Society.

Environmental Science & Technology↗

USGS2006

Sediment Cd and Mo accumulation in the oxygen-minimum zone off western Baja California linked to global climate over the past 52 kyr

Concentrations of organic carbon (orgC), cadmium (Cd), and molybdenum (Mo) were measured in two sediment cores raised from depths of 430 and 700 m within the oxygen-minimum zone (OMZ) off southern Baja California at a temporal resolution of e10.5 kyr over the past 52 kyr. These records are supplemented with diffuse spectral reflectance (DSR) measurements obtained on board ship soon after collection at a resolution of e10.05 kyr. In the core from 700 m depth, a component extracted from the DSR data and the three geochemical proxies generally vary in concert with each other and over a wide range (4-22% orgC; 1-40 mg/kg Cd; 5-120 mg/kg Mo). Intervals of increased orgC, Cd, and Mo accumulation generally correspond to warm periods recorded in the oxygen-isotopic composition of Greenland ice, with the exception of the Bolling/Allerod which is only weakly expressed off Baja California. Concentrations of the biogenic proxies are higher in the core from 430 m depth, but erratic sediment accumulation before 15 ka precludes dating of the older intervals that are laminated and contain elevated orgC, Cd, and Mo concentrations. The new data provide further evidence of an intimate teleconnection between global climate and the intensity of the OMZ and/or productivity along the western margin of North America. On the basis of a comparison with Cd and Mo records collected elsewhere in the region, we conclude that productivity may actually have varied off southern Baja California by no more than a factor of 2 over the past 52 kyr. Copyright 2006 by the American Geophysical Union.

Paleoceanography↗

USGS2004

Enhanced marine productivity off western North America during warm climate intervals of the past 52 k.y

Studies of the Santa Barbara Basin off the coast of California have linked changes in its bottom-water oxygen content to millennial-scale climate changes as recorded by the oxygen isotope composition of Greenland ice. Through the use of detailed records from a sediment core collected off the Magdalena Margin of Baja California, Mexico, we demonstrate that this teleconnection predominantly arose from changes in marine productivity, rather than changes in ventilation of the North Pacific, as was originally proposed. One possible interpretation is that the modern balance of El Nin??o-La Nin??a conditions that favors a shallow nutricline and high productivity today and during warm climate intervals of the past 52 k.y. was altered toward more frequent, deep nutricline, low productivity, El Nin??o-like conditions during cool climate intervals. ?? 2004 Geological Society of America.

Geology↗

USGS2004

Precarious rock and overturned transformer evidence for ground shaking in the Ms 7.7 Kern County earthquake: An analog for disastrous shaking from a major thrust fault in the Los Angeles basin

Precariously balanced rocks and overturned transformers in the vicinity of the White Wolf fault provide constraints on ground motion during the 1952 Ms 7.7 Kern County earthquake, a possible analog for an anticipated large earthquake in the Los Angeles basin (Shaw et al., 2002; Dolan et al., 2003). On the northeast part of the fault preliminary estimates of ground motion on the footwall give peak accelerations considerably lower than predicted by standard regression curves. On the other hand, on the hanging-wall, there is evidence of intense ground shattering and lack of precarious rocks, consistent with the intense hanging-wall accelerations suggested by foam-rubber modeling, numerical modeling, and observations from previous thrust fault earthquakes. There is clear evidence of the effects of rupture directivity in ground motions on the hanging-wall side of the fault (from both precarious rocks and numerical simulations). On the southwest part of the fault, which is covered by sediments, the thrust fault did not reach the surface ("blind" thrust). Overturned and damaged transformers indicate significant transfer of energy from the hanging wall to the footwall, an effect that may not be as effective when the rupture reaches the surface (is not "blind"). Transformers near the up-dip projection of the fault tip have been damaged or overturned on both the hanging-wall and footwall sides of the fault. The transfer of energy is confirmed in a numerical lattice model and could play an important role in a similar situation in Los Angeles. We suggest that the results of this study can provide important information for estimating the effects of a large thrust fault rupture in the Los Angeles basin, specially given the fact that there is so little instrumental data from large thrust fault earthquakes.

Bulletin of the Seismological Society of America↗

USGS2003

A rapid procedure for the determination of thorium, uranium, cadmium and molybdenum in small sediment samples by inductively coupled plasma-mass spectrometry: Application in Chesapeake Bay

This paper describes a rapid procedure that allows precise analysis of Mo, Cd, U and Th in sediment samples as small as 10 mg by using a novel approach that utilizes a "pseudo" isotope dilution for Th and conventional isotope dilution for Mo, Cd and U by ICP-MS. Long-term reproducibility of the method is between 2.5 and 5% with an advantage of rapid analysis on a single digestion of sediment sample and the potential of adding other elements of interest if so desired. Application of this method to two piston cores collected near the mouth of the Patuxent River in Chesapeake Bay showed that the accumulation of authigenic Mo and Cd varied in response to the changing bottom water redox conditions, with anoxia showing consistent oscillations throughout both pre-industrial and industrial times. Accumulation of authigenic U shows consistent oscillations as well, without any apparent increase in productivity related to anoxic trends. Degrees of Mo and Cd enrichment also inversely correlate to halophilic microfaunal assemblages already established as paleoclimate proxies within the bay indicating that bottom water anoxia is driven in part by the amount of freshwater discharge that the area receives. ?? 2002 Elsevier Science Ltd. All rights reserved.

Applied Geochemistry↗

USGS2003

On the preservation of laminated sediments along the western margin of North America

Piston, gravity, and multicores as well as hydrographic data were collected along the Pacific margin of Baja California to reconstruct past variations in the intensity of the oxygen-minimum zone (OMZ). Gravity cores collected from within the OMZ north of 24??N did not contain laminated surface sediments even though bottom water oxygen (BWO) concentrations were close to 5 ??mol/kg. However, many of the cores collected south of 24??N did contain millimeter- to centimeter-scale, brown to black laminations in Holocene and older sediments but not in sediments deposited during the Last Glacial Maximum. In addition to the dark laminations, Holocene sediments in Soledad Basin, silled at 290 m, also contain white coccolith laminae that probably represent individual blooms. Two open margin cores from 430 and 700 m depth that were selected for detailed radiocarbon dating show distinct transitions from bioturbated glacial sediment to laminated Holocene sediment occurring at 12.9 and 11.5 ka, respectively. The transition is delayed and more gradual (11.3-10.0 ka) in another dated core from Soledad Basin. The observations indicate that bottom-water oxygen concentrations dropped below a threshold for the preservation of laminations at different times or that a synchronous hydrographic change left an asynchronous sedimentary imprint due to local factors. With the caveat that laminated sections should therefore not be correlated without independent age control, the pattern of older sequences of laminations along the North American western margin reported by this and previous studies suggests that multiple patterns of regional productivity and ventilation prevailed over the past 60 kyr. Copyright 2003 by the American Geophysical Union.

Paleoceanography↗

USGS2000

Authigenic molybdenum formation in marine sediments: A link to pore water sulfide in the Santa Barbara Basin

Pore water and sediment Mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the Santa Barbara Basin to examine the connection between authigenic Mo formation and pore water sulfide concentration. Only at the deepest site (580 m), where pore water sulfide concentrations rise to >0.1 μM right below the sediment water interface, was there active authigenic Mo formation. At shallower sites (550, 430, and 340 m), where pore water sulfide concentrations were consistently <0.05 μM, Mo precipitation was not occurring at the time of sampling. A sulfide concentration of ∼0.1 μM appears to be a threshold for the onset of Mo-Fe-S co-precipitation. A second threshold sulfide concentration of ∼100 μM is required for Mo precipitation without Fe, possibly as Mo-S or as particle-bound Mo.

Geochimica et Cosmochimica Acta↗