scholarly journals Atmospherically produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology

Geochronology ◽  
2021 ◽  
Vol 3 (1) ◽  
pp. 1-33
Author(s):  
Greg Balco ◽  
Benjamin D. DeJong ◽  
John C. Ridge ◽  
Paul R. Bierman ◽  
Dylan H. Rood
Keyword(s):  
High Resolution ◽  
North American ◽  
Ice Core ◽  
Glacial Sediment ◽  
Climate Proxy ◽  
Varve Chronology ◽  
The North ◽  
Proglacial Lakes ◽  
Short Period ◽  
Period Variations

Abstract. We attempt to synchronize the North American Varve Chronology (NAVC) with ice core and calendar year timescales by comparing records of atmospherically produced 10Be fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18 200 and 12 500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same timescale to facilitate high-resolution correlation of climate proxy variations in both. Total 10Be concentrations in NAVC sediments are within the range of those observed in other lacustrine records of 10Be fallout, but 9Be and 10Be concentrations considered together show that the majority of 10Be is present in glacial sediment when it enters the lake, and only a minority of total 10Be derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial experiment to determine whether or not 10Be fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in short varve sections sampled at high resolution was inconclusive: short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, longer varve sequences sampled at decadal resolution display centennial-period variations in reconstructed 10Be fallout that have similar properties as coeval 10Be fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes and are suppressed in paraglacial sections of the NAVC that were deposited in lakes lacking direct glacial sediment input. We attribute this difference to the fact that buffering of 10Be fallout by soil adsorption can filter out short-period variations in an entirely deglaciated watershed, but such buffering cannot occur in the ablation zone of an ice sheet. This implies that proglacial lakes whose watershed is mostly glacial may effectively record 10Be fallout variations. We attempted to match centennial-period variations in reconstructed 10Be fallout flux from two segments of the NAVC with ice core fallout records. For both records, it is possible to obtain matches that result in acceptable correlation between NAVC and ice core 10Be fallout records, but the best-fitting matches for the two segments disagree, and only one of them is consistent with independent calendar year calibrations of the NAVC and therefore potentially valid. This leaves several remaining ambiguities in whether or not 10Be fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could most likely be resolved by higher-resolution and replicate 10Be measurements on targeted sections of the NAVC.

scholarly journals Atmospherically-produced beryllium-10 in annually laminated late-glacial sediments of the North American Varve Chronology

2020 ◽  
Author(s):  
Greg Balco ◽  
Benjamin D. DeJong ◽  
John C. Ridge ◽  
Paul R. Bierman ◽  
Dylan H. Rood
Keyword(s):  
High Resolution ◽  
North American ◽  
Time Scale ◽  
Ice Core ◽  
The Other ◽  
Varve Chronology ◽  
The North ◽  
Proglacial Lakes ◽  
Period Variations ◽  
Climate Events

Abstract. We attempt to synchronize the North American Varve Chronology (NAVC) with the calendar year time scale by comparing records of atmospherically produced Be-10 fallout in the NAVC and in ice cores. The North American Varve Chronology (NAVC) is a sequence of 5659 varves deposited in a series of proglacial lakes adjacent to the southeast margin of the retreating Laurentide Ice Sheet between approximately 18,200–12,500 years before present. Because properties of NAVC varves are related to climate, the NAVC is also a climate proxy record with annual resolution, and our overall goal is to place the NAVC and ice core records on the same time scale to facilitate high-resolution correlation of climate events. Total Be-10 concentrations in NAVC sediments are within the range of those observed in other lacustrine records of Be-10 fallout, but Be-9 and Be-10 concentrations considered together show that the majority of Be-10 is present in glacial sediment when it enters the lake, and only a minority of total Be-10 derives from atmospheric fallout at the time of sediment deposition. Because of this, an initial strategy to determine whether or not Be-10 fallout variations were recorded in NAVC sediments by attempting to observe the characteristic 11-year solar cycle in high-resolution sections of varve sequences was inconclusive: observed short-period variations at the expected magnitude of this cycle were not distinguishable from measurement scatter. On the other hand, we did observe centennial-period variations in Be-10 fallout that are replicated between separate varve sections and have similar magnitude and frequency as coeval Be-10 fallout variations recorded in ice core records. These are most prominent in glacial sections of the NAVC that were deposited in proglacial lakes, but are suppressed in paraglacial sections of the NAVC deposited in lakes lacking direct glacial sediment input, which leads us to conclude that proglacial lakes whose watershed likely includes a large portion of the ablation area of an ice sheet can effectively record Be-10 fallout. We matched observed centennial-scale Be-10 fallout variations in two segments of the NAVC to ice core Be-10 fallout records. Although the calibration of the NAVC to the calendar year time scale implied by these matches is similar to that proposed previously in independent calibrations based on radiocarbon data and correlation of climate events, matches for the two different segments disagree with each other and with the independent calibrations by 50–200 years. One of these matches is not consistent with independent evidence and is probably not valid, but the other is consistent with most, although not all, evidence and may be valid. This leaves several remaining ambiguities in whether or not Be-10 fallout variations can, in fact, be used for synchronizing NAVC and ice core timescales, but these could likely be resolved by higher-resolution and replicate Be-10 measurements on targeted sections of the NAVC.

scholarly journals Vegetation Dynamics within the North American Monsoon Region

2011 ◽  
Vol 24 (6) ◽  
pp. 1763-1783 ◽  
Author(s):  
Giovanni Forzieri ◽  
Fabio Castelli ◽  
Enrique R. Vivoni
Keyword(s):  
North American ◽  
Vegetation Dynamics ◽  
Vegetation Index ◽  
North American Monsoon ◽  
Terrain Attributes ◽  
The North ◽  
Short Period ◽  
Long Term Trends ◽  
Northwestern Mexico

Abstract The North American monsoon (NAM) leads to a large increase in summer rainfall and a seasonal change in vegetation in the southwestern United States and northwestern Mexico. Understanding the interactions between NAM rainfall and vegetation dynamics is essential for improved climate and hydrologic prediction. In this work, the authors analyze long-term vegetation dynamics over the North American Monsoon Experiment (NAME) tier I domain (20°–35°N, 105°–115°W) using normalized difference vegetation index (NDVI) semimonthly composites at 8-km resolution from 1982 to 2006. The authors derive ecoregions with similar vegetation dynamics using principal component analysis and cluster identification. Based on ecoregion and pixel-scale analyses, this study quantifies the seasonal and interannual vegetation variations, their dependence on geographic position and terrain attributes, and the presence of long-term trends through a set of phenological vegetation metrics. Results reveal that seasonal biomass productivity, as captured by the time-integrated NDVI (TINDVI), is an excellent means to synthesize vegetation dynamics. High TINDVI occurs for ecosystems with a short period of intense greening tuned to the NAM or with a prolonged period of moderate greenness continuing after the NAM. These cases represent different plant strategies (deciduous versus evergreen) that can be adjusted along spatial gradients to cope with seasonal water availability. Long-term trends in TINDVI may also indicate changing conditions favoring ecosystems that intensively use NAM rainfall for rapid productivity, as opposed to delayed and moderate greening. A persistence of these trends could potentially result in the spatial reorganization of ecosystems in the NAM region.

High-Resolution Atmospheric Modeling of Fluorotelomer Alcohols and Perfluorocarboxylic Acids in the North American Troposphere

2007 ◽  
Vol 41 (16) ◽  
pp. 5756-5762 ◽  
Author(s):  
Greg Yarwood ◽  
Susan Kemball-Cook ◽  
Michael Keinath ◽  
Robert L. Waterland ◽  
Stephen H. Korzeniowski ◽  
...  
Keyword(s):  
High Resolution ◽  
North American ◽  
Atmospheric Modeling ◽  
The North ◽  
Perfluorocarboxylic Acids ◽  
Fluorotelomer Alcohols

scholarly journals Characterizing the North American Monsoon in the Community Atmosphere Model: Sensitivity to Resolution and Topography

2019 ◽  
Vol 32 (23) ◽  
pp. 8355-8372 ◽  
Author(s):  
Arianna M. Varuolo-Clarke ◽  
Kevin A. Reed ◽  
Brian Medeiros
Keyword(s):  
High Resolution ◽  
Annual Cycle ◽  
North American ◽  
Horizontal Resolution ◽  
North American Monsoon ◽  
Monsoon Precipitation ◽  
Grid Spacing ◽  
Low Level ◽  
The North ◽  
Community Atmosphere Model

Abstract This work examines the effect of horizontal resolution and topography on the North American monsoon (NAM) in experiments with an atmospheric general circulation model. Observations are used to evaluate the fidelity of the representation of the monsoon in simulations from the Community Atmosphere Model version 5 (CAM5) with a standard 1.0° grid spacing and a high-resolution 0.25° grid spacing. The simulated monsoon has some realistic features, but both configurations also show precipitation biases. The default 1.0° grid spacing configuration simulates a monsoon with an annual cycle and intensity of precipitation within the observational range, but the monsoon begins and ends too gradually and does not reach far enough north. This study shows that the improved representation of topography in the high-resolution (0.25° grid spacing) configuration improves the regional circulation and therefore some aspects of the simulated monsoon compared to the 1.0° counterpart. At higher resolution, CAM5 simulates a stronger low pressure center over the American Southwest, with more realistic low-level wind flow than in the 1.0° configuration. As a result, the monsoon precipitation increases as does the amplitude of the annual cycle of precipitation. A moisture analysis sheds light on the monsoon dynamics, indicating that changes in the advection of enthalpy and moist static energy drive the differences between monsoon precipitation in CAM5 1.0° compared to the 0.25° configuration. Additional simulations confirm that these improvements are mainly due to the topographic influence on the low-level flow through the Gulf of California, and not only the increase in horizontal resolution.

Mapping mantle flows underneath the North American and Caribbean Plates

2021 ◽  
Author(s):  
Hejun Zhu
Keyword(s):  
North American ◽  
Subduction Zones ◽  
Current Model ◽  
Waveform Inversion ◽  
Body Waves ◽  
Plate Motion ◽  
Cascadia Subduction Zone ◽  
Fast Axis ◽  
The North ◽  
Short Period

<p>In this talk, I will present a new 3-D azimuthally anisotropic tomographic model, namely US32, for the North American and Caribbean Plates. This model is constrained by using seismic data from USArray and full waveform inversion. The inversion uses data from 180 regional earthquakes recorded by 4,516 seismographic stations, resulting in 586,185 frequency-dependent phase measurements. Three-component short-period body waves and long-period surface waves are combined to simultaneously constrain deep and shallow structures. The current azimuthally anisotropic model US32 is the result of 32 pre-conditioned conjugate-gradient iterations. In the current model, I observe a complex depth-dependent pattern for fast axis directions across the North American and Caribbean Plates.<span>  </span>At shallow depths, these fast axis directions delineate local geological provinces, such as the Snake River Plain, Cascadia subduction zone, Rio Grand Rift, etc. At greater depths, the fast axis directions follow the absolute plate motion trajectories at most places. At depths around 700 km, the fast axis directions are perpendicular to the strikes of the mapped Farallon slab, suggesting the presence of 2-D corner flows induced by this ancient subduction underneath the mantle transition zone. In addition, underneath the Cascadia and Cocos subduction zones at depths from 250 to 500 km, the fast axis directions suggest the presence of toroid-mode mantle flows, following the geometry of fast downwelling materials.</p>

scholarly journals Anatomy of a Dansgaard‐Oeschger warming transition: High‐resolution analysis of the North Greenland Ice Core Project ice core

2009 ◽  
Vol 114 (D8) ◽  
Author(s):  
Elizabeth R. Thomas ◽  
Eric W. Wolff ◽  
Robert Mulvaney ◽  
Sigfus J. Johnsen ◽  
Jorgen P. Steffensen ◽  
...  
Keyword(s):  
High Resolution ◽  
Ice Core ◽  
The North ◽  
High Resolution Analysis ◽  
Resolution Analysis ◽  
Core Project ◽  
North Greenland

scholarly journals Long-Term Regional Estimates of Evapotranspiration for Mexico Based on Downscaled ISCCP Data

2010 ◽  
Vol 11 (2) ◽  
pp. 253-275 ◽  
Author(s):  
Justin Sheffield ◽  
Eric F. Wood ◽  
Francisco Munoz-Arriola
Keyword(s):  
Remote Sensing ◽  
High Resolution ◽  
North American ◽  
Remote Sensing Data ◽  
Surface Model ◽  
Spatial Average ◽  
Sensing Data ◽  
The North ◽  
True Value

Abstract The development and evaluation of a long-term high-resolution dataset of potential and actual evapotranspiration for Mexico based on remote sensing data are described. Evapotranspiration is calculated using a modified version of the Penman–Monteith algorithm, with input radiation and meteorological data from the International Satellite Cloud Climatology Project (ISCCP) and vegetation distribution derived from Advanced Very High Resolution Radiometer (AVHRR) products. The ISCCP data are downscaled to ⅛° resolution using statistical relationships with data from the North American Regional Reanalysis (NARR). The final product is available at ⅛°, daily, for 1984–2006 for all Mexico. Comparisons are made with the NARR offline land surface model and measurements from approximately 1800 pan stations. The remote sensing estimate follows well the seasonal cycle and spatial pattern of the comparison datasets, with a peak in late summer at the height of the North American monsoon and highest values in low-lying and coastal regions. The spatial average over Mexico is biased low by about 0.3 mm day−1, with a monthly rmse of about 0.5 mm day−1. The underestimation may be related to the lack of a model for canopy evaporation, which is estimated to be up to 30% of total evapotranspiration. Uncertainties in both the remote sensing–based estimates (because of input data uncertainties) and the true value of evapotranspiration (represented by the spread in the comparison datasets) are up to 0.5 and 1.2 mm day−1, respectively. This study is a first step in quantifying the long-term variation in global land evapotranspiration from remote sensing data.

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