3,836 research outputs found
The Ka'ena Highstand of O'ahu, Hawai'i: Further Evidence of Antarctic Ice Collapse during the Middle Pleistocene
Marine isotope stage (MIS) 11 may well represent one of the most
significant interglacial highstand events of the past million years. Ocean volume
changes charted from coastal exposures imply partial or complete melting of
some of the world's major ice caps during a middle Pleistocene interglacial. The
coastal geology of both Bermuda and the Bahamas yields evidence of an MIS 11
highstand 20 m higher than present. Further support for this catastrophic episode
in sea-level history is revealed in subtidal and intertidal deposits at +28 ± 2
m in O'ahu, Hawai'i. The stratigraphy, petrology, and uplift history of the Hawaiian
deposits strongly suggest a correlation with MIS 11, and a compilation of
amino acid racemization, uranium/thorium (alpha and mass spectrometry), and
electron spin resonance ages shows a scatter between 300 and 550 kyr. When
corrected for uplift, the Ka'ena Highstand succession at Wai'anae Health Center
(OWH1) reveals a "stepping up" of sea level through the interglaciation,
similar to that described in the Bahamas. Previous studies on O'ahu attributed
all 28 m elevation of the Ka'ena Highstand to uplift since 0.5 Ma, but now it
appears that only 8 m of that was caused by uplift, and the remaining 20 m by
eustatic sea-level rise. These findings from O'ahu strengthen evidence for the
complete disintegration of the Greenland and West Antarctic ice sheets and
partial melting of the East Antarctic ice sheet during the middle Pleistocene. If
the instability of polar ice sheets can be linked to prolonged warm interglaciations
as the data suggest, then existing conservative predictions for the magnitude
of sea-level change by future "greenhouse" warming are seriously underestimated
The biogeography and geomorphology of the Niobrara River Valley near Valentine, Nebraska.
Within the study area located near Valentine, Nebraska, the Niobrara River is deeply entrenched in Tertiary siltstones and sandstones, and covered with Pleistocene and Holocene eolian deposits of sand and loess. The 100 m depth of the valley reflects the entrenchment. Periods of equilibrium of the river are indicated by the numerous benchlands that lie within and adjacent to the valley. Mass wasting in the forms of landslides and creep significantly alter the form of these terracelands when it is combined with the shifting of the river and saturation by groundwater. Many of the terraces have been correlated with climatic events of the Pleistocene and Holocene epochs. There is some evidence of uplift in the area. The stratigraphy of the low terraces reflects the overall downcutting interspersed with periods of aggradation. Some elements of vegetative community are unique to the grasslands, to Nebraska, and to the Great Plains. Representatives of the Rocky Mountain, eastern deciduous, and northern forests interact with variables of topography, stratigraphy, and microclimate. Active landslides and newly exposed terraces are revegetated in a definite sequence from annuals to hardwood forests or to grasslands in some cases. The parallel zones of woody vegetation on the right bank is disrupted by mass wasting events. The paper birch is a Pleistocene relic and thrives in the valley under stringent habitat requirements such as north facing slopes, springs, and shade of other trees. The results of this study are numerous and diverse. However, in a synthesis, they present a concept of the dynamic interaction of the physical and biological factors in the area. From these individual factors, a grander hypothesis was formulated that describes the Niobrara River as the principal contributor of sand in the formation of the Nebraska Sandhills
Uplift of Oahu, Hawaii, during the past 500 k.y. as recorded by elevated reef deposits: REPLY
On the MBM12 Young Association
I present a comprehensive study of the MBM12 young association (MBM12A). By
combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS)
survey with new optical imaging and spectroscopy, I have performed a census of
the MBM12A membership that is complete to 0.03 Msun (H~15) for a 1.75deg X
1.4deg field encompassing the MBM12 cloud. I find five new members with masses
of 0.1-0.4 Msun and a few additional candidates that have not been observed
spectroscopically. From an analysis of optical and IR photometry for stars in
the direction of MBM12, I identify M dwarfs in the foreground and background of
the cloud. By comparing the magnitudes of these stars to those of local field
dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM12 cloud;
it is not the nearest molecular cloud and is not inside the local bubble of hot
ionized gas as had been implied by previous distance estimates of 50-100 pc. I
have also used Li strengths and H-R diagrams to constrain the absolute and
relative ages of MBM12A and other young populations; these data indicate ages
of 2 +3/-1 Myr for MBM12A and 10 Myr for the TW Hya and Eta Cha associations.
MBM12A may be a slightly evolved version of the aggregates of young stars
within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2
Myr).Comment: to be published in The Astrophysical Journal, 41 pages, 14 figures,
also found at http://cfa-www.harvard.edu/sfgroup/preprints.htm
Interferometric Observations of the T Tauri Stars in the MBM 12 Cloud
We have carried out a millimeter interferometric continuum survey toward 7
YSOs in the MBM 12 cloud. Thermal emissions associated with 2 YSOs were
detected above the 3- level at 2.1 mm, and one also showed a 1.3 mm
thermal emission. Another object was marginally detected at 2.1 mm. Spectral
energy distributions of the YSOs are well fitted by a simple power-law disk
model. Masses of the circumstellar disks are estimated to be an order of 0.05
M_{\sun}. The circumstellar disks in the MBM 12 cloud have properties in
common with the disks in nearby star-forming regions, in terms of disk
parameters such as a disk mass, as well as an infrared excess.Comment: 9 pages, 3 figures, accepted by ApJ Letter
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