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L. Brown

Publications and source records attributed to L. Brown.

7 recordsLinked to original sources
USGS2013

Enriquillo–Plantain Garden fault zone in Jamaica: paleoseismology and seismic hazard

The countries of Jamaica, Haiti, and the Dominican Republic all straddle the Enriquillo–Plantain Garden fault zone ( EPGFZ), a major left-lateral, strike-slip fault system bounding the Caribbean and North American plates. Past large earthquakes that destroyed the capital cities of Kingston, Jamaica (1692, 1907), and Port-au-Prince, Haiti (1751, 1770), as well as the 2010 Haiti earthquake that killed more than 50,000 people, have heightened awareness of seismic hazards in the northern Caribbean. We present here new geomorphic and paleoseismic information bearing on the location and relative activity of the EPGFZ, which marks the plate boundary in Jamaica. Documentation of a river bank exposure and several trenches indicate that this fault is active and has the potential to cause major destructive earthquakes in Jamaica. The results suggest that the fault has not ruptured the surface in at least 500 yr and possibly as long as 28 ka. The long period of quiescence and subdued geomorphic expression of the EPGFZ indicates that it may only accommodate part of the ∼7–9 mm=yr plate deformation rate measured geodetically and that slip may be partitioned on other undocumented faults. Large uncertainties related to the neotectonic framework of Jamaica remain and more detailed fault characterization studies are necessary to accurately assess seismic hazards.

Bulletin of the Seismological Society of America↗
USGS2010

Increase in lake trout reproduction in Lake Huron following the collapse of alewife: Relief from thiamine deficiency or larval predation?

In the Great Lakes there is still uncertainty as to the population level effects of a thiamine deficiency on salmonines caused by high consumption of alewives Alosa pseudoharengus . A resurgence of lake trout Salvelinus namaycush reproduction in Lake Huron following the crash of alewife stocks between 2002 and 2004 provided an opportunity to evaluate the relative effects of this crash on reproduction through relief from either alewife mediated thiamine deficiency or alewife predation on larval lake trout relative to possible changes in the size of the lake trout spawning stock. Changes in mean lake trout egg thiamine concentration post crash at one spawning reef in Parry Sound, where mean thiamine concentration increased by almost two-fold, were consistent with diet switching from alewives to rainbow smelt Osmerus mordax , the next most abundant prey fish in Lake Huron. Although thiamine levels for lake trout collected at a second reef in Parry Sound did not change post-crash, levels both pre- and post-crash were consistent with a rainbow smelt diet. A reef specific fry emergence index was found to be positively related to reef specific egg thiamine concentration but negatively related to reef specific occurrence of EMS, a thiamine deficiency related mortality syndrome. We found little evidence for overlap between the timing of spring shoreward migration of alewives and lake trout emergence, suggesting that relief from alewife predation effects had relatively little effect on the observed increase in lake trout recruitment. Numbers of spawners in the north, north-central, and southern zones of the lake increased from 2000 onwards. Overall the abundance post-2003 was higher than from pre-2004, suggesting that spawner abundance may also have contributed to increased lake trout reproduction. However, predicted numbers of spawners and measured abundance of wild recruits in assessment gear were poorly correlated suggesting that the increase in reproduction was not totally spawner dependent and hence relief from thiamine deficiency was also likely involved. We conclude from this that eliminating the effects of an alewife diet mediated thiamine deficiency can have positive effects on lake trout reproduction but more research is required to understand the effect of spawner number and the role of spawning habitat availability.

Lake Huron↗
USGS1997

Development of an IBI-based assessment of depressional wetlands in Maryland and Delaware

The hydrogeomorphic approach (HGM) of wetland assessment emphasizes functional components of wetlands such as water storage, transformation and cycling of elements, accumulation of sediments, and preservation of habitats. Many of the elements measured in HGM are physical rather than ecological or biological. The HGM approach, therefore, provides information on certain aspects of wetlands and omits other aspects. In contrast, the Index of Biological Integrity (IBI) approach focuses on biological components of wetlands such as species richness, the presence or proportion of certain 'indicator' species, representation of different trophic levels, and measures of wildlife or fish health. Here too, some aspects of a wetland are omitted and others not covered by HGM are included. We contend that these differences in focus add strengths and weaknesses to each method. This paper reviews progress on the development of IBIs for restored depressional wetlands in the Mid-Atlantic States, especially Maryland and Delaware. During our first field season we identified 25 wetlands ranging from 1-10 acres, most of which were restored with federal, state, and private landowner cooperation over hydric soils. Separate IBIs are being created for macrophytes, macroinvertebrates, amphibians, and data on mammal and avian populations are being collected. Simultaneously, chemical and physical data are being collected on water DO, turbidity, temperature, conductivity, nitrates, ammonia, chlorophyll, pH; soil metal levels and texture; and wetland size, configuration, hydrology, drainage area, and surrounding land use.

Book chapter↗
USGS1990

Brunhes chron excursion/polarity episode recorded during the late pleistocene, Albuquerque Volcanoes, New Mexico, USA

All basaltic lava flows of the Albuquerque Volcanoes (lat.: 35.2°N, long.: 253.2°E), Albuquerque-Belen Basin, New Mexico, record a short excursion/polarity episode. K-Ar isotopic age determinations (weighted average: 155 ± 47 ka) and evaluation of soil profiles on flow surfaces suggest the late Pleistocene ( circa between 250 and 80 ka) as the time of extrusion. Results from 63 sites in a minimum of eight flows yield a mean direction of D = 101.1°, I = -36.1°, α 95 = 1.2° (α 1 95 = 0.7, α 2 95 = 1.2), k = 2219 ( N = 8 flows) and a corresponding virtual geomagnetic pole position (VGP) of 354.1°E, 20.2°S, dp = 0.8°, dm = 1.4° ( A 1 95 = 0.5, A 2 95 = 1.4), and VGP angular standard deviation (ASD) = 1.8°. The unusual magnetization in flows of the Albuquerque Volcanoes is carried by fine-grained [single-domain (SD) and/or pseudo-single-domain (PSD)] low-Ti magnetite and is apparently not a function of complex sub-solidus alteration of magnetic phases. Underlying baked soils contain a relatively low coercivity magnetization with directions comprising two groups. One is roughly antipodal to the lava remanence; the other intermediate between the lava remanence and present field direction. Because all flows yield statistically indistinguishable directions, a field directional path clearly cannot be defined. At the precision level of our age determinations, the Albuquerque feature may correlate with other short polarity episodes or excursions of late Pleistocene age (e.g., Blake or Jamaica). Though characteristically limited in directional morphology, well-dated polarity episodes (i.e. sub-chrons) and excursions recorded in volcanic rocks may provide information on the frequency of significant dynamo instabilities.

New Mexico↗
USGS1986

10Be distribution in soils from Merced River terraces, California

The distribution and residence time of cosmogenic 10 Be in clay-rich soil horizons is fundamental to understanding and modelling the migration of 10 Be on terrestrial sediments and in groundwater solutions. We have analyzed seven profiles of clay-rich soils developed from terrace sediments of the Merced River, California. The terraces and soils of increasing age are used to compare the 10 Be inventory with a simple model of accumulation, decay and erosion. The data show that the distribution of 10 Be varies with soil horizon clay content, that the residence time of 10 Be in these horizons exceeds 10 5 years, and that to a rough approximation the inventory of 10 Be in a thoroughly sampled soil profile fits the equation: N = ( q − Em )(1 − e − λι )/ λ where q is delivery rate, E is erosion rate, m is the concentration of 10 Be in the eroding surface layer, λ is the decay constant, and t is the age of the depositional unit from which the soil has developed. The general applicability of this model is uncertain and warrants further testing in well-calibrated terrace sequences.

Geochimica et Cosmochimica Acta↗