GEOLOGICAL CHARACTERISTICS OF DESERT
AND UPPER DESERT CAVES (NE BLUE
DIAMOND HILL, NEVADA, USA
ABSTRACT
Desert Cave (72 m) in Upper Desert Cave (62 m) sta najdaljši znani jami na
Blue Diamond Hill in se nahajata znotraj enega od več kanjonov. Jami sta razviti
v apnencu iz zgornjega dela spodnjega permija (Kaibab formacija). V najnižjem
delu Desert Cave, ki je 12,6 m globoka, jamski rov doseže litološko mejo med
Kaibab apnencem in sedimentacijsko brečo (spodnji del Kaibab formacije). Stik
breča-apnenec je ena od inicialnih struktur iz freatičnega obdobja razvoja jame,
vendar ne pred mezozojskimi narivnimi deformacijami. Površinska razdalja med
vhodoma v Desert in Upper Desert Cave je 81 m, jami nista povezani. Desert
Cave se nahaja 2 km zahodno od nariva Bird Spring. Prevladujoče smeri razpok
v jami so NW-SE in NE-SW. Smer rova je skoraj vzporedna s smerjo razpok
NE-SW. Obstaja jasna povezava med javljanjem podorov v jami in moćneje
izraženimi razpokami v smeri NW-SE.
Desert Cave (72 m) and Upper Desert Cave (62 m) are the longest known caves
in Blue Diamond Hill and are situated in one of its canyons. Caves are
developed in limestone of latest early Permian age (Kaibab formation). In lowest
part of Desert Cave, which is 12,6 m deep, cave passage reaches lithological
contact between Kaibab limestone and sedimentary breccia (lower part of
Kaibab formation). The contact breccia-limestone is one of initial structures in
phreatic period of cave development but not before Mesozoic thrusting tectonics.
Surface distance between entrances to Desert and Upper Desert Cave is 81 m,
the caves are not connected. Desert Cave is situated 2 km W from Bird Spring
thrust. Prevailing fissure directions in the cave are NW-SE and NE-SW. Passage
direction is almost parallel with the NE-SW fissures direction. There is obvious
connection in occurrence of breakdown in the cave with strongly expressed
fissures in NW-SE direction.
Precipitation pulses and soil CO2 flux in a
Sonoran Desert ecosystem
Abstract
Precipitation is a major driver of biological processes in arid and semiarid
ecosystems. Soil biogeochemical processes in these water‐ limited systems are
closely linked to episodic rainfall events, and the relationship between
microbial activity and the amount and timing of rainfall has implications for
the whole‐ system carbon (C) balance. Here, the influences of storm size and
time between events on pulses of soil respiration were explored in an upper
Sonoran Desert ecosystem. Immediately following experimental rewetting in
the field, CO2 efflux increased up to 30‐ fold, but generally returned to
background levels within 48
h. CO2 production integrated over 48
ranged from 2.5 to 19.3
C
g
h
m−2 and was greater beneath shrubs than
in interplant spaces. When water was applied on sequential days, postwetting
losses of CO2 were only half a large as initial fluxes, and the size of the second
pulse increased with time between consecutive events. Soil respiration was
more closely linked to the organic matter content of surface soils than to
rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size,
reaching an asymptote at approximately 0.5
cm simulated storms. This
nonlinear relationship stems from (1) resource limitation of microbial activity
that is manifest at small time scales, and (2) greatly reduced process rates in
deeper soil strata. Thus, beyond some threshold in storm size, increasing the
duration or depth of soil moisture has little consequence for short‐ term losses
of CO2. In addition, laboratory rewetting across a broad range in soil water
content suggest that microbial activity and CO2 efflux following rainfall may
be further modified by the routing and redistribution of water along hillslopes.
Finally, analysis of long‐ term precipitation data suggests that half the
monsoon storms in this system are sufficient to induce soil heterotrophic
activity and C losses, but are not large enough to elicit autotrophic activity and
C accrual by desert shrubs.
A proposed mechanism for the formation of ‘Fertile
Islands’ in the desert ecosystem
Movement of nitrogen atoms throughout the environment occurs by (1)
physical transport such as molecular diffusion and hydrodynamic flow,
mechanisms that always disperse and (2) biological transport such as sap
flow and animal movement, mechanisms that most often lead to concentration.
Both types of transport are accelerated by moisture, but the physical
mechanisms are more sensitive.
In the desert ecosystem, micro-, meso-, and macro-fauna are all attracted to
the canopies of whatever plants exist, the place of highest moisture, lowest
daytime temperature and most abundant food sources. This paper presents
the hypothesis that under conditions of desert aridity the concentrating
biological transport mechanisms dominate over the dispersive physical
mechanisms. This causes the phenomenon known as ‘fertile islands.’
Soil Properties in a Mesquite-Dominated Sonoran
Desert Ecosystem
Abstract
Soil was collected in 30-cm depth increments to 90 cm from beneath Prosopis glandulosaTorr.
var. glandulosa (L. Benson) M. C. Jtn. (mesquite) and from the nonvegetated area between mesquite
trees in a phreatophytic stand in the Sonoran Desert of southern California. Total N, NO -3-N, NH+4-N,
organic C, NaHCO3-extractable PO3-4-P, and saturation extract K+were significantly higher beneath
mesquite, while Na+ and Cl- were significantly higher between mesquite trees. Differences in pH, the
osmotic potential of saturation extracts, saturation percent, and SO 2-4-S were nonsignificant. Large
amounts of N (1.68 g kg−1 of soil) have accumulated in the surface 30 cm beneath mesquite. This N most
likely had been symbiotically fixed by mesquite. Over 20% of the N in this ecosystem occurred as NO -3.
This unusual NO-3 accumulation was possible since leaching and denitrification were probably limited by
aridity. The Na+ adsorption ratio of saturation extracts (SAR), 0–30 cm, was significantly lower at the
center of tree canopies (7.9) than in soil between trees (17.3) and also was lower than the groundwater
(12.7). Foliar analysis indicated mesquite was excluding Na +. Consequently, the decomposition of
mesquite litter produced a soil with a lower SAR than the nonvegetated soil. Soil N was determined at six
additional Sonoran Desert sites. Nitrogen accumulation beneath mesquite was related to soil texture and
water regime. Total soil N was highest on low relief phreatophytic sites with high clay content (1.34 g of N
kg−1 of soil, 0–15 cm) and lowest in aeolian sand dunes (0.18 g of N kg −1 of soil, 0–15 cm). Woody
legumes such as mesquite, through the accumulation of symbiotically fixed N and the accretion of other
nutrients in the surface beneath their canopies, may be important in maintaining the long-term
productivity of some desert ecosystems.
Precipitation pulses and soil CO2 flux in a
Sonoran Desert ecosystem
Abstract
Precipitation is a major driver of biological processes in arid and semiarid
ecosystems. Soil biogeochemical processes in these water‐ limited systems are
closely linked to episodic rainfall events, and the relationship between
microbial activity and the amount and timing of rainfall has implications for
the whole‐ system carbon (C) balance. Here, the influences of storm size and
time between events on pulses of soil respiration were explored in an upper
Sonoran Desert ecosystem. Immediately following experimental rewetting in
the field, CO2 efflux increased up to 30‐ fold, but generally returned to
background levels within 48
h. CO2 production integrated over 48
ranged from 2.5 to 19.3
C
g
h
m−2 and was greater beneath shrubs than
in interplant spaces. When water was applied on sequential days, postwetting
losses of CO2 were only half a large as initial fluxes, and the size of the second
pulse increased with time between consecutive events. Soil respiration was
more closely linked to the organic matter content of surface soils than to
rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size,
reaching an asymptote at approximately 0.5
cm simulated storms. This
nonlinear relationship stems from (1) resource limitation of microbial activity
that is manifest at small time scales, and (2) greatly reduced process rates in
deeper soil strata. Thus, beyond some threshold in storm size, increasing the
duration or depth of soil moisture has little consequence for short‐ term losses
of CO2. In addition, laboratory rewetting across a broad range in soil water
content suggest that microbial activity and CO2 efflux following rainfall may
be further modified by the routing and redistribution of water along hillslopes.
Finally, analysis of long‐ term precipitation data suggests that half the
monsoon storms in this system are sufficient to induce soil heterotrophic
activity and C losses, but are not large enough to elicit autotrophic activity and
C accrual by desert shrubs.