Response of a modern cave system to large seasonal precipitation

Available online at www.sciencedirect.com
Geochimica et Cosmochimica Acta 91 (2012) 92–108
www.elsevier.com/locate/gca
Response of a modern cave system to large seasonal
precipitation variability
Jessica L. Oster a,⇑, Isabel P. Montañez b, Neil P. Kelley b
a
Department of Earth and Environmental Sciences, Vanderbilt University, United States
b
Department of Geology, University of California, Davis, United States
Received 17 October 2011; accepted in revised form 27 May 2012; available online 7 June 2012
Abstract
Speleothems are capable of providing information on the response of middle and low-latitude terrestrial environments to
global climate change during the Pleistocene and Holocene. Multiproxy speleothem studies, however, have demonstrated that
complex interactions can occur in cave settings between processes that are directly and indirectly related to climate change.
Thorough and extended monitoring of modern cave environments is necessary in order to fully understand how each cave
responds to these processes on seasonal and interannual timescales, and how environmental signals are preserved in speleothem carbonate.
Regular environmental monitoring began at Black Chasm Cavern in the Sierra Nevada foothills, California, during the
winter of 2006–2007. Monthly measurements of cave air temperature, humidity, and pCO2 in Black Chasm demonstrate that
the cave is ventilated in the winter months, when cold, dense surface air sinks into the cave. Cave drip-water flow nearly ceases
during the late summer and autumn, increases substantially during the winter and spring, and responds within hours to storm
events during the height of the rainy season. Rainwater and drip water d18O and d2H are controlled by variations in surface air
temperature and moisture source. While rainfall source influences rainwater isotopes through individual storm events, it has
less influence on drip water isotopic composition due to mixing of recharge waters delivered by different rainfall events in the
epikarst. Variations in drip water chemistry (d13C, Mg/Ca, and Sr/Ca) indicate that the greatest level of calcite precipitation
upflow from the drip water collection site (prior calcite precipitation) occurs during the autumn (October–November) when
drip rates are slow and cave air pCO2 is low. The least prior calcite precipitation occurs during the summer (July–August)
when drip rates are slow but cave air pCO2 is at a maximum. While pCO2 is a primary control on prior calcite precipitation
during all seasons, the predominant influence of drip rate variability on prior calcite precipitation is evident when considering
only those seasons (winter, spring, and autumn) characterized by low cave air pCO2. Thus, drip rate variability, and in turn
rainfall amount, should provide the primary control on trace element variations ultimately captured in speleothem calcite. The
isotopic and chemical variability observed in Black Chasm drip waters supports previous interpretations of speleothem paleoclimate proxy records from a nearby cave where such monitoring is not feasible. Observations of the modern cave environment at Black Chasm provide a reference point for the interpretation of stalagmite proxy records from similar seasonal
(Mediterranean) climates.
! 2012 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
⇑ Corresponding author.
E-mail address: jessi[email protected] (J.L. Oster).
0016-7037/$ - see front matter ! 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.gca.2012.05.027
Speleothem geochemical proxy records have the potential
to provide valuable archives of rainfall variability, atmospheric circulation changes, and vegetation response in low
and mid-latitude continental environments and have contributed significantly to our understanding of hemispheric
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
teleconnections linking distal regions (e.g. Wang et al., 2001,
2004, 2007; Partin et al., 2007; Oster et al., 2009; Asmerom
et al., 2010; Wagner et al., 2010). However, the interpretation of speleothem isotopic and geochemical proxies is rarely
straightforward (e.g. Fairchild et al., 2000; Genty et al.,
2001a; Mickler et al., 2004, 2006; Lambert and Aharon,
2011). Geochemical proxies (e.g. d18O, d13C, trace element
concentrations, 87Sr/86Sr) used in speleothem studies each
respond to a variety of environmental influences, and these
responses can vary significantly between caves, between speleothems within single cave, and through time. Although the
majority of speleothem studies have focused on developing
records of oxygen isotope variability, recent multiproxy
studies have highlighted the complex interactions that occur
in cave settings between processes that are directly related to
climate change (rainfall amount, temperature) and those
that are indirectly related (degree of water–rock interaction,
seepage-water degassing and prior calcite precipitation,
changes in cave air pCO2) (Banner et al., 1996; Baker
et al., 1997; Ayalon et al., 1999; Hellstrom and McCulloch,
2000; Johnson et al., 2006; Oster et al., 2010). Isolating those
processes that are directly and indirectly related to climate is
challenging, and this limitation can compromise the interpretation of speleothem proxy records. Rapid and extended
degassing of CO2 driven by decreased seepage water flow
rates, increased evaporation, and/or changing cave
air–water pCO2 can contribute to non-equilibrium isotopic
fractionation of stable isotopes during speleothem growth
(Hendy, 1971; Mickler et al., 2004). Such kinetic effects lead
to speleothem calcite that is enriched in 13C and 18O and possesses elevated concentrations of trace elements such as Mg,
Sr, and Ba (Lorens, 1981) that do not accurately reflect the
chemical signatures carried by the drip water into the cave.
In light of these complexities, it is necessary to perform
thorough and extended monitoring of cave environments in
order to fully understand how each cave responds to modern seasonal and interannual changes. Cave monitoring involves regular measurements of cave air temperature,
humidity, and pCO2 (e.g. Spötl et al., 2005; Banner et al.,
2007; Baldini et al., 2008), drip rate (e.g. Genty and Deflandre, 1998; Mattey et al., 2008), cave water and groundwater
chemistry (e.g. Bar-Matthews et al., 1996; Fairchild et al.,
2000; Musgrove and Banner, 2004; Spötl et al., 2005; Pape
et al., 2010; Wong et al., 2011), and soil chemistry (e.g.
Musgrove and Banner, 2004; Spötl et al., 2005).
Monitoring of modern cave environments provides a
more in-depth understanding of the processes that affect
speleothem paleoclimate proxies and allows for more accurate interpretations of speleothem time series. Relatively
few monitoring studies, however, have been conducted in
highly seasonal and semi-arid to arid environments
(Bar-Matthews et al., 1996; Banner et al., 2007; Fernandez-Cortes et al., 2008; Mattey et al., 2010; Pape et al.,
2010) despite an increase in the number of speleothembased paleoclimate records emerging from caves in these
environments (e.g. Neff et al., 2001; Bar-Matthews et al.,
2003; Auler et al., 2004; Genty et al., 2006; Vaks et al.,
2006), especially from the western United States (Asmerom
et al., 2007, 2010; Denniston et al., 2007; Oster et al., 2009;
Wagner et al., 2010). Speleothems provide a crucial window
93
into the history of these climatically sensitive regions where
other high-resolution, precisely dated paleoclimate archives
are generally scarce. Given the seasonal extremes of precipitation and temperature that can occur in these regions and
the potential differences in behavior between these caves
and those in humid and sub-humid environments, it is
essential to understand how conditions within these caves
respond to changing environmental conditions, and how
this response is captured in speleothem calcite over time.
Here we discuss the results of a five-year (2006–2011)
environmental monitoring program at Black Chasm Cavern, central Sierra Nevada foothills, California that provide
insight into the behavior of a cave dominated by fracture
flow and precipitation that is highly seasonally variable.
Black Chasm provides a suitable setting for environmental
monitoring given that it was opened for tourism only recently (2002) and neither ventilation nor water flow within
the cave has been artificially modified. Observations drawn
from measurements of cave air and drip water at Black
Chasm confirm interpretations of speleothem proxy records
from other caves developed within proximal marble units in
the central Sierra Nevada foothills (e.g. Moaning Cave, Oster et al., 2009) where such monitoring programs cannot be
implemented due to anthropogenic modification of the cave.
2. SITE DESCRIPTION
Black Chasm Cavern (38.43"N, 120.63"W, 676 m elevation) (Fig. 1a) is developed along vertical fractures within
the Volcano carbonate unit, one of several discrete carbonate masses in the Sierra Nevada foothills. These lenticular
masses of metamorphosed Permo-Carboniferous limestone
and dolomite are intercalated with other metasedimentary
and metavolcanic rocks of the Calaveras Complex (Bowen,
1973). Black Chasm Cavern was opened for tourism in
2002.
Black Chasm Cavern (BLC) consists mainly of two adjacent, vertically oriented chambers, the Colossal Room, and
the Landmark Room (Fig. 1b). The Colossal Room is 30 m
long by 5 m wide with a ceiling height that ranges from 1.8
to 31 m, and the Landmark Room is !35 m long by 25 m
wide with a ceiling height that ranges from 1.2 to 12.5 m. A
series of smaller, short passages surround both rooms, and
much of the lowest level of the cave is occupied by standing
water (Barton, 2006). A moderately drained, immature soil
(Inceptisol) (10–20 cm thick) overlies 10–20 m of marble
overburden. During the winter (December through
February) and spring (March through May), water drips
within the cave near the entrance and at several locations
within the Colossal Room. Most drip locations are dry during the late summer and autumn (August–November).
Although the Landmark Room is currently dry year-round,
it is well decorated with stalactites, helictites, and
stalagmites, and the passages that surround this room do
experience seasonal drip water and possess many small,
ephemeral lakes. Speleothem growth is ongoing in the Colossal Room and in smaller chambers.
The climate above BLC is characterized by cool, wet
winters and warm, dry summers (Fig. 2). The University
of California Integrated Pest Management Program has
94
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
50 km
N
39°N
PL
BLC
OR
CA
Pacific
TC
NV
MC
38°N
3. METHODS
A
Black Chasm Cavern
Upper Level
Plan View
A
B C
Main
Entrance
B
than those at BLC due to the location’s lower elevation
and more open vegetation; the pattern of soil temperature
variation through the year, however, should be similar
(Fig. 2). Vegetation above BLC consists of C3 trees and
shrubs including California Black Oak (Quercus kelloggii),
Interior Live Oak (Quercus wislizenii), Ponderosa Pine
(Pinus ponderosa), Gray Pine (Pinus sabiniana), Incense Cedar (Calocedrus decurrens), Pacific Madrone (Arbutus menziesii), Scrub Oak (Quercus berberidifolia), Poison Oak
(Toxicodendron diversilobum), and Sword Fern (Polystichum munitum).
Colossal
Room
10 m
E
D
N
Landmark
Room
Fig. 1. (a) Map depicting location of Black Chasm Cavern (BLC),
the Tiger Creek Pump House (TC) where temperature and rainfall
data have been collected since 1951, and Plymouth (PL) where soil
temperature data was collected between 1990 and 2000 by
University of California Integrated Pest Management. MC is
location of Moaning Cave. Inset shows location of A in the western
United Sates. (b) Map of the upper level of Black Chasm Cavern.
Points A–D mark locations where cave air measurements were
taken. Point C is location of drip water collection. Point E is
location of HOBO data logger. Map after Barton (2006).
operated a weather monitoring station 12.4 km from BLC
at the Tiger Creek Pumping House (718 m elevation) since
January 1951 (TC: Fig. 1a). From 1951 to 2010, this site
experienced an average yearly rainfall of !1100 mm. On
average, 92% of this precipitation fell during the autumn,
winter, and spring months (October–April), and 70% of this
precipitation occurred between December and March.
Although Black Chasm is not located within the area of
the western United States that is significantly affected by
the North American monsoon (Higgins et al., 1999), on
average 2% of annual precipitation during this period occurred between July and September, suggesting either that
some monsoon precipitation reaches as far west as the central Sierra Nevada foothill region or that this area is occasionally affected by localized summer convective storms.
The University of California Integrated Pest Management
Program also monitored soil temperature between May
1990 and May 2000 at Plymouth, an irrigated grassland site
18 km northwest of BLC at 472 m elevation (PL: Fig. 1a).
Soil temperatures at Plymouth are likely slightly higher
Periodic measurements of cave air temperature, humidity, pCO2, and rainwater and drip water chemistry in BLC
were made between December 2006 and August 2011.
Monthly (winter and spring) water measurements began
in March 2007 and bi-weekly air and water measurements
occurred between January 2009 and December 2010. Drip
water flow within BLC slows substantially during the late
summer and early autumn, so it was not possible to collect
regular summer and early autumn water samples for all
geochemical measurements, however a series of summer
drip water samples was collected during July and August,
2011.
3.1. Air measurements
A HOBO# data logger was placed in a recess in the back
of the cave and away from the tourism walkway (Location
E, Fig. 1b) in October 2007. The logger performed hourly
measurements of air temperature and relative humidity between this date and September 2009. Bi-weekly to monthly
measurements of air temperature, humidity, and pCO2 in
BLC were taken between January 2009 and March 2011.
Air measurements were made between 10 am and 2 pm.
Air temperature, humidity, and pCO2 measurements were
conducted on the surface and at four platforms within the
cave (Locations A–D, Fig. 1). Temperature and humidity
measurements were made using a hand-held Traceable
hygrometer and thermometer. PCO2 measurements were
made using a Telaire portable CO2 meter.
3.2. Rain and drip water measurements
Rainwater was collected above the cave in an open area
approximately 8 m from the cave entrance. Following the
International Atomic Energy Agency (IAEA) protocol,
rainwater was collected in a 5 l LDPE carboy, and the bottom of the carboy was coated with approximately 0.5 cm of
mineral oil to prevent evaporation. The collector was emptied bi-weekly to monthly and sub-samples of the rainwater
were removed for d18O and d2H analysis.
Drip water samples were collected !30 m from the cave
entrance at Location C (Fig. 1b). Location C is the most
consistently wet area within the cave. Several drips fall from
draperies approximately 4 m above the collection site. Drip
water samples were collected during each cave visit from a
group of drips that cover an area of approximately 0.1 m2.
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
25
250
200
Avg T. (°C)
150
15
100
10
Avg. Precip. (mm)
20
5
95
50
Feb
Apr
Jun
Aug
Oct
Dec
0
Month
Fig. 2. Average monthly air temperature (white circles-black line) and rainfall (gray boxes-gray line) from 1951 to 2010 for the Tiger Creek
Pumping House (38.45"N, 120.48"W, 718 m elevation). Black circles show average soil temperatures (black circles-dashed line) for the
Plymouth site measured between 1990 and 2000.
Water samples for stable isotope analysis (d18O, d2H) and
d13CDIC were collected in pre-cleaned 30 ml LDPE bottles,
filled until there was no headspace and capped. Due to slow
drip rates, it was not always possible to collect samples for
d13CDIC analysis, especially during the late summer and autumn. Sub-samples (10 ml) for d13CDIC analysis were taken,
treated with HgCl2, and refrigerated for CO2 extraction.
Drip water samples for cation (Ca, Mg, Sr) analysis were
collected biweekly to monthly between November 2008
and December 2010 and during July and August 2011.
Samples were collected in filter flasks and acidified on site.
Measurements of pH were taken prior to acidification using
an Orion portable pH meter. Drip rate (in drips/minute)
was measured at Location C using a stopwatch for one
minute per visit from December 2006 to August 2011. In
August 2009, a Stalagmate acoustic drip rate counter was
installed in the cave. The counter downloaded drip count
measurements every hour from August 2009 until April
2011, when it was removed from the cave.
Oxygen isotope measurements of drip waters were performed on a Finnigan MAT 251 IRMS, and hydrogen isotope measurements were performed on an IsoPrime IRMS.
d18O and d2H values are reported relative to Standard
Mean Ocean Water (V-SMOW) and d13C values are reported relative to Pee Dee Belemnite (V-PDB) using standard delta notation. Analytical precision for d18O is
±0.05&, for d2H is ±1.0&. The d13CDIC for water samples
collected after January 1, 2010 was analyzed at the Stanford
School of Earth Sciences Stable Isotope Biogeochemistry
Laboratory. During the analysis, 0.5 ml samples were reacted with 30 ll of phosphoric acid for 12 h at room temperature in 12 ml Labco vials. The headspace gas was
analyzed in continuous-flow mode using a Finnigan GasBench-II headspace sampler interfaced with a Finnigan
Delta+XL mass spectrometer. The precision, based on 6 repeat analyses of an in-house standard (absolute value
"5.53& d13C, calibrated against NBS-18, NBS-19, NBS20, L-SVEC), is 0.22&. The d13CDIC for water samples
collected prior to January 1, 2010 was analyzed in the
Stable Isotope Laboratory, Department of Geology, UC
Davis. For interlaboratory calibration of the d13CDIC measurements, the Stanford in-house standard was dissolved in
degassed, weakly acidified deionized water, and the
d13C RCO2 was determined by acid-stripping CO2 from
5 ml of the solution under vacuum (using 105% orthophosphoric acid) and purifying the CO2 cryogenically prior to
mass spectrometric analysis. The average of three analyses
of the Stanford in-house standard is "5.5&, overlapping
with the mean value ("5.53&; n = 3) obtained on the powdered Stanford standard in the same laboratory. The longterm precision of the d13C RCO2 replicates was ±0.03&.
Analysis to quantify concentrations of Ca, Mg, and Sr,
in water samples collected between November 2008 and
June 2010 were performed on an Agilent 7500 series quadrupole ICPMS at the Interdisciplinary Center for Plasma
Mass Spectrometry, UC Davis. Analyses of waters collected between June 2010 and August 2011 were performed
on a Thermo-Scientific X-series 2 quadrupole ICP-MS in
the Stanford School of Earth Sciences Environmental Measurement 1 laboratory. External calibration standards were
prepared using the Spex2a multi-element standard from
0.01 to 100 ppb for minor elements (Sr) and the Spex
Instrument Check Standard 3 multi-element standard from
1000 to 50,000 ppb for major elements (Ca, Mg). Analytical
error on cation analyses ranges from <1–3%.
3.3. Statistical tests
In order to examine the sign and strength of relationships among drip water geochemical parameters and environmental factors, Pearson’s product moment correlation
coefficients (“r”) and one-tailed tests of significance (“p”)
between pairs of measured parameters were calculated
using the statistics program R 2.14.1 (http://www.r-project.org/). Data with non-normal distributions were log10transformed prior to statistical calculations in order to meet
the assumptions of the Pearson’s correlation tests. Data
with zero or negative values were rescaled with the addition
of a constant prior to log-transformation. “P” values of
<0.05 were accepted as statistically significant.
96
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
4. RESULTS
4.1. Cave air
Biweekly cave air temperature at the site of drip water
collection (Location C) varied between 9.4 and 18.3 "C
(mean 12.3 "C) between January 2009 and August 2011
(Fig. 3a), with peak temperatures occurring from the late
spring to early autumn (May–October) and minimum temperatures occurring during the late autumn to early spring
(November–April). Air temperature in BLC is most variable at Locations A and B (Fig. 1b), which are closest to
the cave entrance. At Location A, temperature varied between 9.4 and 18.9 "C (mean 14.3 "C), and Location B temperature varied between 9.1 and 19.4 "C (mean 13.5 "C).
Near the back of the cave (Location D, Fig. 1), temperature
was less variable, ranging from 10.7 to 16.1 "C (mean
12.8 "C). At Location E, in the back of the cave removed
from the tourist path, HOBO# logger data indicates temperature varied the least, ranging from 11.8 to 13.3 "C between October 2007 and September 2009.
Air pCO2 at Location C in BLC is positively correlated
with surface air temperature (r = 0.74, p < 0.001) and negatively correlated with surface air humidity (r = "0.68,
p < 0.001), and is at its maximum in the summer (July–September), begins a steady decline in late October and early
November, and is at its minimum in the late autumn
(late November) through early spring (Fig. 3b). PCO2 at
Location C varied between 685 and 8020 ppm between January 2009 and March 2011. Cave air pCO2 did not vary significantly (±150 ppm) between measurement locations
within the cave with the exception of slightly lower values
at Location A, which is closest to the cave entrance.
Relative humidity at the drip collection site (Location C
on Fig. 1) varied between 67% and 100% (mean 94%) between January 2009 and August 2011 (Fig. 3c), with minimum values during the late spring to early autumn (May
to October), and maximum values during the winter and
early spring (December through March). Elsewhere in the
cave, relative humidity varied between 64% and 100%, with
the lowest values recorded at Locations A and B, and the
highest values near the back of the cave (Location D). Surface air humidity is also higher during the wet winter
months.
4.2. Water measurements
BLC rainwater d18O varies between "16.5& and
"4.6&, d2H varies between "121.7& and "28.2&
(Fig. 4) (Table S1), and values for rainwater d18O and
d2H fall on the global meteoric water line (Fig. 4a). Overall,
Black Chasm rainwater stable isotopes are more negative
during the winter and spring and less negative during the
summer and autumn (Fig. 4), with the exception of individual Pineapple Express precipitation events (see Section 5.2).
Rainwater d18O values display a positive correlation with
Fig. 3. (a) Air temperature measurements from the surface above Black Chasm (open circles, gray line) and from Location C within Black
Chasm (closed circles – black line) for the period January 2009 through August 2011. (b) Measured pCO2 at Location C for Jan 2009 to Mar
2011. Damage to pCO2 meter prevented further measurements, but a short (May–Aug 2011) pCO2 data set for a similar cave 10 km W of
Black Chasm (gray line) shows a similar trend of increasing summer pCO2. Continuous pCO2 for this cave was measured with a
CO2Meter.com data logger. (c) Relative humidity at Location C within Black Chasm for January 2009 to August 2011.
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
δ 2H (
) rain
-20
97
A
-40
-60
-80
-100
-120
-16
) rain
-8
δ 18O (
-4
-12
-14
-12
-10
δ 18O (
-8
-6
-4
) rain
B
C
300
200
100
avg. monthly temp (C)
30
25
0
D
total monthly rain (mm)
400
-16
20
15
10
5
12/06
8/07
4/08
12/08
8/09
4/10
12/10
8/11
Fig. 4. (a) Black Chasm rainwater data plotted with global meteoric water line, open circles indicate late spring–autumn rain events (May–
November). Closed boxes are winter–early spring rain events (December–April). Stars indicate strong North Pacific-sourced storms. Open
boxes indicate Pineapple Express events. (b) d18O for rainwater from December 2006 to August 2011. Stars indicate strong North Pacificsourced storms. Open boxes indicate Pineapple Express events. (c) Monthly rainfall totals (black boxes, dashed line), and (d) average monthly
temperature (gray boxes and line) at Tiger Creek Pumping House for December 2006 through August 2011. Gray bars highlight winter
months, December through February.
average air temperature of rain days at the Tiger Creek
Pumping House during the collection period (r = 0.42,
p = 0.013), and show no significant relationship with rainfall amount during the collection period (r = "0.09,
p = 0.61).
Drip rate at BLC is highly variable, and ranges from 1
drip per 10 min during the dry summer and autumn months
to up to 60 drips/minute following winter and spring storm
events. As drip rate is so variable and sensitive to storm
events during the rainy season, we have normalized drip
rates to each winter maximum for the purpose of reducing
noise and allowing a more clear comparison of the timing
of drip rate variability between years (Fig. 5). The pattern
of this normalized drip rate closely resembles the pattern
of monthly rainfall at the Tiger Creek Pump House. Comparison of the Stalagmate logger data from BLC for October 2009 through 2010 with rainfall data demonstrates that
drip response to the first storm of the season (e.g. October
24th, 2010, Fig. 6) was somewhat muted, leading to drip
rates that were increased by !1500 drip/day over pre-storm
values. Storms later in the season (e.g. January) caused drip
rate to increase by !3000 drips/day, with rainfall events
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J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
1
0.6
400
total monthly rainfall (mm)
0.4
normalized drip rate
0.8
0.2
300
0
200
100
0
12/06
8/07
4/08
12/08
Date
8/09
4/10
12/10
8/11
Fig. 5. Normalized drip rate measurements (gray points and line) at Location C within Black Chasm and total monthly rainfall at the Tiger
Creek Pumping House (black points and dashed line) for December 2006 through August 2011.
a
b
Fig. 6. Daily drip rate at location C within Black Chasm (open
squares) and daily rainfall (black bars) for two time intervals. (a)
January 14–February 4, 2010, (b) October 21–November 16, 2010.
Note the relatively small and brief response to the much larger
rainfall event in October (early in the rainy season) vs. the larger
and sustained response to the smaller, but longer-lived events in
January (middle of the rainy season).
that lasted multiple days causing drip rates to increase by
up to 5000 drips/day within 2–3 days of the onset of the
storm event.
Drip water d18O values range from "11.0& to "8.0&
(Fig 7b) (Table S2), and d2H values range from "76.0&
to "54.7&. Cave drip water d18O and d2H follow rainwater
d18O and d2H and are generally more negative during the
winter and spring months, and less negative during the
summer and autumn months. The least negative drip water
d18O and d2H values occur during the late autumn (October
and November) following 6–8 months of minimal precipitation in the region. Drip water d13C varies between "12.5&
and "9.1& (Fig. 7c) (Table S2), and displays a strong negative correlation with cave air pCO2 (r = "0.81, p < 0.001),
and a positive correlation with drip rate (r = 0.59, p = 0.01)
(Table 1).
Drip water Mg/Ca ranges from 0.026 to 0.054 and Sr/Ca
ranges from 0.00089 to 0.0020 (Table S2). Drip waters collected during the summer (June–August) have the lowest
Mg/Ca and Sr/Ca and highest Ca concentrations, while
samples collected in the autumn (October–November) have
the highest Mg/Ca and Sr/Ca and the lowest Ca concentrations (Fig. 8). For the entire drip water data set, Mg/Ca and
Sr/Ca, display significant negative correlations with pCO2
(r = "0.56, p = 0.0064; and r = "0.55, p = 0.0081, respectively) (Table 1), and show no significant relationship with
drip rate. However, when the drip water samples collected
during the summer months are removed from the data
set, the correlation between pCO2 and Mg/Ca and Sr/Ca
is greatly reduced (r = "0.38, p = 0.108; r = "0.30,
p = 0.214, respectively) and negative correlations between
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
99
Fig. 7. From December 2006 through August 2011, (a) BLC rainwater d18O. As in Fig. 4b, stars indicate strong North Pacific-sourced storms,
and open boxes indicate Pineapple Express events. (b) drip water d18O, (c) drip water d13C, (d) drip water Mg/Ca, and (e) normalized drip rate
at time of water collection. Gray bars highlight winter months, December through February.
Table 1
Pearson’s product moment correlation coefficients (r-values) for
drip water d13C, Mg/Ca, and Sr/Ca at BLC.
Parameter
pCO2
Drip rate
d13C
Mg/Ca (all)
Sr/Ca (all)
Mg/Ca (winter, spring, autumn)
Sr/Ca (winter, spring, autumn)
"0.81**
"0.56*
"0.55*
"0.38
"0.30
0.59*
"0.19
"0.20
"0.60*
"0.59*
**
*
p < 0.001.
p < 0.05.
drip rate and Mg/Ca and Sr/Ca emerge (r = "0.60,
p = 0.024; r = "0.59, p = 0.0257, respectively).
5. DISCUSSION
Prior calcite precipitation (PCP), which can occur within
the epikarst between the soil zone and the cave passage or
on the cave ceiling any time prior to drip water reaching
a collection site or stalagmite, has been demonstrated to
significantly influence speleothem proxy records such as
carbon isotopic compositions and trace element contents
(Baker et al., 1997; Fairchild et al., 2000; Johnson et al.,
2006; Cruz et al., 2007; Fairchild and Treble, 2009; Oster
et al., 2009; 2010). Variations in PCP have been linked to
changes in cave water supply and water residence time in
the epikarst, leading to the use of speleothem Mg/Ca and
Sr/Ca as proxies of paleo-rainfall (Fairchild et al., 2000;
Johnson et al., 2006; Karmann et al., 2007; McDonald
et al., 2007). However, recent cave monitoring studies have
documented the importance of cave ventilation in controlling calcite precipitation in both the epikarst and the cave,
with enhanced calcite precipitation occurring during times
when the cave is ventilated and cave air pCO2 is reduced
(Spötl et al., 2005; Banner et al., 2007; Kowalczk and Froelich, 2010; Mattey et al., 2010; Boch et al., 2011; Frisia
et al., 2011; Lambert and Aharon, 2011; Tremaine et al.,
2011; Wong et al., 2011). Thus, it has been suggested that
cave ventilation, rather than water supply to the cave,
may provide the primary source of variability for speleothem growth rates as well as speleothem d13C, Mg/Ca,
and Sr/Ca.
While some cave monitoring studies have been carried
out in monsoonal regions (Johnson et al., 2006) and subhumid regions that experience significant interannual precipitation variability (e.g. McDonald et al., 2007; Wong et al.,
2011), to date, few studies have been conducted in semi-arid
or arid regions that experience extreme seasonal drying
(Bar-Matthews et al., 1996; Banner et al., 2007; FernandezCortes et al., 2008; Mattey et al., 2010). Furthermore, the
majority of cave monitoring studies are carried out in caves
hosted in large limestone or dolomite units that have high
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J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
western Sierra Nevada foothills offers a stark seasonal contrast in precipitation, with virtually all of the recharge to
the aquifer occurring during the winter and spring. Thus,
BLC offers an environment in which to investigate processes
of ventilation and drip rate variability and their relative effects on geochemical proxies of stalagmites that form in cave
systems in which local recharge is episodic and transmitted
rapidly into the cave.
5.1. Cave ventilation
Fig. 8. (a) 1000 Mg/Ca vs. Ca concentration for Black Chasm drip
waters shown by season over the period 2007–2011; blue
squares = winter (December–February); green diamonds = spring
(March–May); red triangles = summer (June–August); orange
circles = autumn (September–November). (b) 1000 Mg/Ca vs.
1000 Sr/Ca for the same waters. Lines represent model of prior
calcite precipitation created using the software Geochemists
Workbench. The model was run by moving CaCO3 saturated
water with initial Mg = 2400 ppb and Sr = 85 ppb from an
environment with high (soil) CO2 fugacity (10"2.1) to lower (air)
fugacity (10"3.5). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this
article.)
capacity to store and mix water before it enters the cave and
in which fracture flow may not be the primary method of
transport through the aquifer (e.g. Baker et al., 2000; Banner
et al., 2007; Kowalczk and Froelich, 2010; Mattey et al.,
2010; Boch et al., 2011). In these cave environments, variations in water supply may have little effect on drip rate within
the cave, or may affect only drip sites that are fed by fracture
flow and not others that are fed by more diffuse flow through
the epikarst (Mattey et al., 2010; Wong et al., 2011). In contrast to these caves, Black Chasm (BLC) is located in a small
(!2.5 km2 of surface exposure) limestone unit that is
bounded by surface drainages, and the cave is mainly fed
by fracture flow through a relatively thin epikarst (!15–
20 m). Additionally, the Mediterranean climate of the
Variations in air temperature, humidity, and pCO2 in
BLC suggest that the cave is most dynamically ventilated
from the late autumn through the spring (November–
May). During these months, variations in cave air temperature most closely follow changes in average surface air
temperature and cave air pCO2 fluctuates in response
(Fig. 3). The monitoring site closest to the cave entrance
experiences the largest fluctuations in temperature and
humidity, and the overall lowest pCO2 values (i.e. closest
to surface air pCO2) during this time period. As the spring
months progress, temperature in the cave stabilizes, humidity declines, and pCO2 begins to rise. CO2 accumulates in
the cave as it seeps into the cave from the soil zone and is
degassed from drip waters, due to a lack of exchange between cave air and low pCO2 surface air (Banner et al.,
2007). Cave air pCO2 reaches a maximum in the summer
and early autumn (July–September). While pCO2 within
BLC may be modified by tourism, this modification should
occur to some degree during all seasons, as the cave is open
to tourism year-round. However, increased visitation during the summer could lead to increased summer pCO2. As
a comparison, a brief (4 month) pCO2 time series collected
in a nearby (10 km W of Black Chasm), non-commercial
cave using a CO2meter data logger displays a similar summer increase in pCO2 (Fig. 3b).
These trends support increased exchange between cave
and surface air during the cold winter months into the
spring due to sinking of cold, low pCO2 surface air into
the cave. This ventilation continues through the spring, as
the rainy season is ending but surface temperatures are still
cool. Ventilation slows in early summer (June), and cave air
pCO2 builds within the cave to a summer maximum (July–
September). The winter ventilation observed at BLC is consistent with observed seasonal changes in cave ventilation
and pCO2 in many monitored caves including those in the
southern United States (Banner et al., 2007; Kowalczk
and Froelich, 2010; Lambert and Aharon, 2011), Austria
(Spötl et al., 2005; Boch et al., 2011), and Italy (Frisia
et al., 2011) that are driven by changes in the relative density of surface and cave air that occur due to seasonal temperature variability.
5.2. Rainwater variability
The observed trend of more negative d18O and d2H values of BLC rainwater during the colder and wetter winter
and early spring, and less negative values during the warmer and drier late spring through late autumn/early winter
suggests that rainwater isotopes are influenced by seasonal
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
variation in surface air temperature or rainfall amount (Fig
4). However, the direct relationships between rain d18O, air
temperature on rainy days, and rainfall amount during the
collection period suggest that temperature exerts a stronger
control on rain isotope values than does rainfall amount.
These observations agree with a similar study that suggests
temperature rather than rainfall amount controls rainfall
d18O in the Pacific Northwest (Ersek et al., 2010). However,
in contrast to observations from that region, and in agreement with a study of rainfall d18O in southern California
(Berkelhammer et al., 2012), the isotopic composition of
rainwater from BLC also appears to vary with water vapor
source, with North Pacific-sourced vapor associated with
more depleted rainwater isotope values, and tropicalsourced water vapor associated with less depleted values.
For example, several strong storms sourced in the far North
Pacific passed over Northern California during the study
interval. The rainwater taken from the collector for these
periods had the most depleted d18O and d2H values measured in this study (Fig. 4). In addition, two Pineapple Express storms passed over the cave during the spring of 2009
and 2010. Pineapple Express events occur when the jet
stream taps into warm, wet air from the tropical Pacific
and funnels it up to the west coast of North America. These
storms are uncharacteristically warm and wet for California
winter precipitation (Dettinger, 2004). Rainwater collected
at BLC after these Pineapple Express events had d18O
and d2H values ("8.2&, "51.4& and "4.6&, "28.2&,
respectively) that were less depleted than other early spring
precipitation events, as would be expected of precipitation
derived from a more tropical source.
A
Dec. 28, 2010
δ18O = -16.5
101
Back-calculated particle trajectories for rainfall events
over the central Sierra Nevada foothills using the NOAA
Hybrid Single Particle Lagrangian Integrated Trajectory
Model (HYSPLIT) and global reanalysis data indicate that
the stable isotopic composition of rainfall in this region is
influenced by water vapor source. These models, each run
at 1500 m above ground level, the approximate height of
the 850 hPa level from where rain is expected to originate
(http://www.ready.arl.noaa.gov/HYSPLIT.php), simulate
the trajectory that an air parcel followed over 72 h before
reaching BLC on the day of a particular storm event. Particle trajectories for December 28th, 2010 (Fig. 9a), when
rain d18O was very depleted, suggests a source in the Bering
Sea, while the trajectory for March 3rd, 2009 (Fig 9b), when
rain d18O was the most enriched of the four events shown,
shows the trajectories dipping southward before reaching
California. Trajectories for the rainfall events of December
17th, 2010 (Fig. 9c) and March 20th, 2011 (Fig. 9d) are similar to one another and remain between !30 and 40"N. The
rainfall event of December 17th (Fig. 9c) occurred just eleven days prior to the event pictured in Fig. 9a, and average
air temperatures for these two days were similar (within
1 "C). Thus the observed difference in d18O between the
December 17th and 28th events most likely reflects different
vapor sources. However, different d18O values between the
December 17th, 2010 (Fig. 9c) and March 20th, 2011
(Fig. 9d) events could best be explained by variables other
than moisture source, as the particle trajectories for these
events are similar. Indeed the average air temperature at
the weather station was 3.5 "C colder on March 20th
2011 than on December 17th, 2010. Thus, on event to
B
Mar. 3, 2009
δ18O = -8.2
C
D
Dec. 17, 2010
δ18O = -10.2
Mar. 20, 2011
δ18O = -12.5
Fig. 9. Reverse particle trajectories for Black Chasm for (a) December 28th, 2010, (b) March 3rd, 2009, (c) December 17th, 2010, and (d)
March 20th, 2011. All trajectories were generated with the NOAA HYSPLIT model using global reanalysis data and run for 72 h at 1500 m
height above ground level (!850 hPa – where rain likely originates).
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J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
monthly timescales, it appears that both temperature and
storm source influence rainwater isotopes at BLC.
5.3. Drip water variability
Drip rate in Black Chasm is strongly tied to rainfall
above the cave, with drip rates near zero (!1 drip/10 min)
in the dry summer months due to the lack of water recharge
from the surface. The first rainfall event of the wet season,
usually in October or November, has little effect on cave
drip rate, suggesting that the majority of the water from
these events is absorbed by the low moisture soil zone,
and does not infiltrate into the epikarst. Rainfall events that
occur later in the wet season, after the soil has been wetted,
trigger up to fourfold increases in drip rate that last for several days, depending on the duration of the rain event
(Fig. 6).
Shifts in drip water d18O and d2H tend to lag behind
analogous changes in rainwater d18O and d2H by up to
three weeks during the rainiest months, indicating some
storage of recharge water in the epikarst. Furthermore,
lower variability of drip water d18O and d2H relative to that
of the rainwater suggests there is some mixing of rainwater
recharge with the older stored waters within the epikarst.
Such rainwater storage, however, likely consists of waters
recharged during each rainy season given that drip water
ceases to enter the cave during the late summer months.
The storage and mixing of rainwater in the epikarst within
each season, however, appears to dampen the effect of singular variations in water vapor source (i.e. North Pacific
versus Pineapple Express storms, compare Figs. 4b and
7a and b). Sustained differences in the stable isotopic composition of autumn and winter-spring rainfall due to temperature have a greater influence on drip-water stable
isotopes. This would suggest that, in order for changes in
vapor source to have a significant long-term effect on
drip-waters and in turn be recorded by speleothem stable
isotope compositions, the shifts would need to be sustained
at least throughout a rainy season, and possibly for several
years depending on stalagmite growth rate.
BLC drip water d13C ("8.6& to "12.5&) is less negative than the d13C composition of DIC anticipated for drip
waters in fully open caves systems overlain by C3 vegetation
and saturated with respect to CaCO3 ("14& to "18&) (cf.
Hendy, 1971; Dulinski and Rozanski, 1990). In a fully open
cave system, the cave seepage water is always in contact
with an excess of soil CO2. Less negative d13C values of
DIC are expected for closed cave systems ("11&) where
seepage waters are isolated from soil CO2 after entering
the epikarst (Hendy, 1971). Observation of modern cave
systems has shown, however, that most cave systems are
at least partially open (McDermott, 2004). It is possible,
therefore, that the overall 13C-enriched BLC drip waters
can be explained by a higher percentage of carbon contribution from the host limestone due to dissolution that occurs
in partially rather than fully open system conditions. This
scenario, however, cannot explain the highest drip water
d13C values that are observed at BLC ("8.6& to
"10.9&), and geochemical modeling of C isotope systematics for a proximal cave (Moaning Cave), which is developed
in marble of similar age and depositional and diagenetic
history within the Calaveras Formation, suggests that the
contribution of host rock dissolution to DIC d13C variability is minimal (Oster et al., 2010). Thus, it is unlikely that
drip-water d13C at BLC is controlled by dissolution of the
host limestone. We thus conclude that the 13C-enriched drip
waters at BLC most likely reflect a combination of variation in soil respiration and precipitation of calcium carbonate in the epikarst, upflow from the drip collection site.
The strong positive correlation of Mg/Ca with Sr/Ca
(r = 0.96, p < 0.001) further argues for a PCP influence on
the trace element contents of drip waters and in turn stalagmite calcite. If differential dissolution of calcite and dolomite provided a primary control on elemental
concentrations, Mg/Ca should be negatively correlated
with Sr/Ca given that dolomite has lower concentrations
of Sr than calcite (Fairchild et al., 2000; Hellstrom and
McCulloch, 2000). Rather, the positive correlation between
drip water Mg/Ca and Sr/Ca and the relationship between
Mg/Ca and Sr/Ca and calcium concentration (Fig. 8), as
well as a negative correlation between drip water d13C
and calcium concentration (r = "0.83, p = 0) suggests that
drip water d13C, and Mg and Sr concentrations are best explained by degassing and precipitation of calcite upflow
from the drip site. During degassing and calcite precipitation, the d13C value of the HCO"
3 ðaqÞ reservoir follows Rayleigh-type enrichment in 13C with progressive degassing, as
the loss of isotopically light 12C from degassing cannot be
offset by the loss of isotopically heavier 13C through calcite
precipitation (Mickler et al., 2004), assuming a one for one
loss of CO2 and CaCO3. Likewise, as calcite is precipitated
from drip water, the remaining water possesses higher concentrations of Mg and Sr, as the distribution coefficients for
these elements in calcite are much less than 1 (Lorens,
1981). Prior calcite precipitation (PCP) in a cave will increase due to increased degassing that occurs when slower
seepage rates within the epikarst allow saturation to be
reached earlier in the water travel path (Fairchild et al.,
2000) and when seepage waters encounter air pockets of
lower pCO2.
In order to further evaluate this hypothesis that PCP is
the predominant influence on the observed trace element
compositions of drip waters at BLC, we modeled PCP in
a CaCO3 saturated drip water, with Mg and Sr contents
equal to the maximum measured values of BLC drip waters
using Geochemist’s Work Bench (Fig. 8). The model curves
were created by moving water from an environment characterized by a CO2 fugacity representing the soil environment
(10"2.3) to an environment characterized by a CO2 fugacity
representing the cave environment (10"3.5). All of the measured drip waters at BLC plot on or proximal to these model curves, further supporting the hypothesis that PCP is the
primary control on trace element variation in BLC drip
waters.
The significant negative correlation between drip water
Mg/Ca and Sr/Ca and cave air pCO2 year-round
(r = "0.56, p = 0.0064; and r = "0.55, p = 0.0081, respectively) reflects the overall influence of pCO2 driven by
seasonal cave ventilation on PCP in this cave system. However, the negative correlation between drip rate, Mg/Ca and
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
Sr/Ca during the winter, spring, and autumn (r = "0.62,
p = 0.0187; r = "0.60, p = 0.0235, respectively) supports
the hypothesis that water supply is the predominant influence on PCP when overall cave pCO2 is low. Thus, elemental concentrations in Black Chasm drip waters indicate that
both pCO2 and water supply influence prior calcite precipitation in this cave system, with drip rates becoming more
important during periods of lower cave air pCO2. Given
that speleothem precipitation in this cave system most likely
occurs during the low pCO2 months we conclude that variations in Mg/Ca and Sr/Ca of BLC stalagmites will primarily record changes in water supply to the cave.
Due to extremely slow drip rates, it was not possible to
sample waters for d13CDIC during the late summer and fall
(between late August and December). Thus, it is not possible to fully assess the relationship between d13C, drip rates,
and pCO2 variability at BLC. The strong negative correlation between d13C and pCO2 in the winter through early
summer (r = "0.81, p = < 0.001) argues that cave air
pCO2 provides the dominant control on drip water d13C
during the wetter part of the year. Drip water d13C should
also be influenced by soil respiration, with periods of increased respiration leading to more negative drip water
d13C. Soil respiration in arid and semi-arid environments
is controlled by both temperature and soil moisture, and
soil CO2 concentrations are generally at a maximum during
the spring and early summer when soil temperatures are rising but soils are still moist (Amundson et al., 1988; Quade
et al., 1989; Terhune and Harden, 1991). Although soil CO2
was not measured during this study, it is likely that soil respiration rates at BLC reach their maximum during the
spring and early summer as soil temperatures increase
(Fig. 2) but before soil moisture levels substantially
decrease. Indeed, there is a significant negative correlation
between drip water d13C and surface air temperature
(r = "0.62, p = 0.0046). Thus, it is likely that the minimum
d13C values for spring and summer drip waters reflect enhanced soil respiration, in addition to reduced PCP, during
this interval. The positive correlation observed between
d13C values and drip rate (r = 0.0.59, p = <0.05) is likely
a result of the strongly seasonal nature of drip rate variability in this cave, given that both d13C values and drip rate
are negatively correlated with surface air temperatures. If
it were feasible to measure d13C during the driest season
above the cave, it is possible that a relationship between
drip rate and d13C would emerge that is parallel to what
is observed with Mg/Ca and Sr/Ca, where low fall drip
rates are associated with drip waters with less negative
d13C values reflecting the combined effects of reduced soil
respiration and increased PCP relative to summer values.
Observations of cave air and drip water chemistry at
BLC illustrate how ventilation and drip rate variability
should influence geochemical proxies of stalagmites that
form in cave systems in winter-precipitation dominated climates where local recharge is episodic and transmitted rapidly into the cave. Variations in the d18O recorded by a
stalagmite from BLC should reflect changes in surface air
temperature and/or sustained shifts in moisture source,
while d13C changes should reflect a combination of PCP
variability and soil respiration. As stalagmite precipitation
103
in the cave is most likely to occur during periods of lower
cave air pCO2, speleothem paleoclimate archives will most
likely record autumn through spring climates, rather than
annually averaged climate variability, and variations in
Mg/Ca and Sr/Ca recorded by a stalagmite are likely to reflect changes in drip rate, and therefore changes in water
availability. These conclusions may apply to other central
Sierra Nevada foothills caves given their similar geologic
setting, paragenetic history, and environmental and climatic conditions.
5.4. Implications for paleoclimate studies
Cave air ventilation and pCO2 have been shown to exert
a dominant control on calcite precipitation rates in caves in
humid to sub-humid environments, and in those where significant water storage occurs in the epikarst (Banner et al.,
2007; Baldini et al., 2008; Kowalczk and Froelich, 2010;
Mattey et al., 2010; Frisia et al., 2011; Lambert and
Aharon, 2011; Tremaine et al., 2011; Wong et al., 2011).
In these systems, enhanced calcite deposition in the cave
and in the epikarst occurs when the difference between
the cave air pCO2 and drip water pCO2 is greatest such as
when cave air pCO2 is low due to increased exchange between the cave and surface atmosphere (Banner et al.,
2007), and/or when drip water pCO2 is high as a result of
increased soil pCO2 during the growing season (Genty
et al., 2001b; Kowalczk and Froelich, 2010). Seasonal
changes in water supply and drip rate have been shown
to exert a secondary control on calcite deposition rates
within caves, mainly when comparing calcite precipitation
under different drips within the same cave (Banner et al.,
2007; Mattey et al., 2010).
Chemical variations of drip water at BLC indicate that,
while both pCO2 and drip rate variability have a significant
influence on PCP, drip rate, and therefore water supply,
provides a dominant influence during periods of low
pCO2 when stalagmites are likely to grow, and thus we predict that speleothem proxy records that are sensitive to PCP
should respond primarily to variations in water supply to
the cave. The observed relationships between stable isotopes and trace elements in BLC rainwater and drip water
on a seasonal-scale provide a high-resolution analogue for
the decadal to multi-centennial variability inferred from
speleothem proxy records from another central Sierran
foothill cave, Moaning Cave (Oster et al., 2009). Although
BLC is open to tourism, ventilation and water flow within
the cave have not been modified. This is not the case for
Moaning Cave, 42 km southeast of BLC (MC: Fig. 1a),
where spring water from below the cave is artificially
pumped through the cave during the dry summer months
precluding monitoring of the cave and surface conditions.
Like BLC, Moaning Cave (MC) is a vertically oriented cave
developed along fractures in a metamorphosed limestone
unit within the Calaveras Formation. MC is located at a
slightly lower elevation than BLC (520 m a.s.l.) and receives
slightly less yearly rainfall (!850 mm/year from 1951 to the
present compared to !1100 mm/year at BLC). Similar to
BLC, 75–100% of the yearly rainfall above MC occurs during the winter and early spring.
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J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
Fig. 10. For 17 to 8 ka, (a) Insolation at 38"N for July (red) and January (blue) with proxy records from the Moaning Cave speleothem. (b)
Mg concentration, (c) Sr concentration, (d) d13C, (e) d18O, compared to (f) the d18O record from the NGRIP ice core (NGRIP Members, 2004;
Svensson et al., 2008). Blue bars highlight the Older Dryas and Inter-Allerød Cold Period. See Oster et al. (2009) for further interpretations.
(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Comparison of the seasonal-scale variability observed in
BLC drip water with the decadal to multi-centennial
variability noted in the deglacial to early Holocene proxy
records from MC suggests that similar environmental
J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
processes drive changes on both timescales. Crystal fabrics
indicative of continuous precipitation in the Moaning Cave
speleothem are associated with lower [Mg] and [Sr] and
more negative d13C and d18O values. U-series ages indicate
these intervals of stalagmite calcite precipitated during
Northern Hemisphere cold periods (Fig. 10). Conversely,
crystal fabrics in the MC stalagmite that indicate intermittent growth are associated with higher [Mg] and [Sr], and
less negative d13C and d18O values. These layers of calcite
precipitated during Northern Hemisphere warm periods between 16.5 and 8.7 ka (Oster et al., 2009). Enhanced cave
ventilation leading to increased calcite precipitation in the
epikarst (PCP) on long timescales should be driven by large
seasonal temperature differences above the cave (Banner
et al., 2007). The largest difference between winter and summer insolation at the Moaning Cave latitude during the period of 16.5–8.7 ka occurs from !12 to 10 ka (Fig. 10a),
from which we infer that the largest seasonal temperature
differences above the cave occurred during this interval.
The smallest difference between winter and summer insolation occurs from 16 to 14 ka. Therefore, if variations in
cave pCO2 due to changes in cave ventilation were the only
control on PCP in the Moaning Cave system we would
expect to see the highest speleothem Mg and Sr concentrations during the period of enhanced seasonality
(!12–10 ka). While this interval does correspond to crystal
fabrics that are indicative of continuous stalagmite precipitation, it also corresponds to some of the lowest observed
Mg and Sr concentrations. Crystal fabrics indicative of
intermittent calcite precipitation as well as higher stalagmite Mg and Sr concentrations and d13C values are observed when seasonality is reduced (15–14 ka).
The documented relationship between drip rates and
trace element ratios of BLC drip waters during periods of
low pCO2 suggests that, for intervals of stalagmite growth,
higher Mg and Sr concentrations in the MC stalagmite
should indicate periods of relatively slower drip rate and
lower water availability. Taken together, these results suggest that Northern Hemisphere warm periods recorded in
the MC stalagmite were characterized by little or no stalagmite growth, punctuated by intervals where the cave was
ventilated enough to allow some calcite precipitation. Yet
even during these growth intervals, drip rates remained
low enough to promote high amounts of PCP in the epikarst, leading to higher concentrations of Mg and Sr in the
stalagmite. Conversely, Northern Hemisphere cold periods
were characterized by ventilation of the cave that in turn
promoted continuous stalagmite growth. Given that crystal
fabrics indicate a relatively consistent water supply to the
drip site and growing MC stalagmite during these periods,
the lower Mg and Sr concentrations are interpreted to record decreased PCP in the epikarst, even during periods
of enhanced ventilation, due to high water availability
and drip rates.
The d13C values of BLC drip waters reflect changes in
PCP, and possibly also variations in soil respiration rate,
as the lowest d13C values occur during the spring and early
summer when soil respiration should be at a maximum.
Less negative d13C values in the MC stalagmite during
Northern Hemisphere warm periods could reflect more
105
PCP. If variations in cave air pCO2 were the dominant control on stalagmite d13C, then, as with the trace element records, we would expect to see the highest d13C values
during the periods of increased seasonality. However, we
see the lowest stalagmite d13C values between 12 and
10 ka, when summer insolation is at its peak, and seasonality is enhanced, indicating that cave ventilation is not the
primary control on d13C in the Moaning Cave record.
Rather, this relationship suggests that increased water supply to the cave during overall cold periods as is suggested by
the stalagmite trace element records, could have lead to decreased d13C values in stalagmite calcite. This decrease in
the d13C values of stalagmite calcite may have been enhanced by increased soil respiration due to increased water
supply.
In comparison with observations from BLC rainwater
and drip water, less negative d18O values in the MC speleothem during Northern Hemisphere warm periods are consistent with warmer temperatures and potentially an
enhanced contribution of water vapor from a more subtropical source, possibly from more frequent Pineapple Express events. Recent climate models investigating the effects
of future global warming on the occurrence of Pineapple
Express events in the western United States suggest that
the frequency of years with a larger number of these storms
increases with global warming (Dettinger, 2011). It is also
possible that moisture transport from the tropics to the
mid-latitudes would be enhanced during warmer periods
due to the increased water-holding capacity of warmer
tropical air, leading to increased d18O values of precipitation in this region (Schneider et al., 2010; Berkelhammer
et al., 2012). Thus a change in the relative contribution of
subtropical moisture reaching the central Sierra Nevada
during past warmings of the last deglaciation could have
amplified the increase in precipitation d18O driven by increased temperatures in the region.
Observations of drip water chemistry and cave ventilation at Black Chasm provide a useful modern analogue
for the interpretation of paleoclimate proxy records from
nearby Moaning Cave, and will be essential for the interpretation of speleothem proxy records from other caves in
the central Sierra Nevada and similar semi-arid and arid regions that experience large seasonal precipitation variability. The BLC drip water and cave air pCO2 variability,
coupled with the MC paleoclimate records, suggest that
in this region, evidence of variation in PCP can provide a
proxy for changes in drip rate, and thus in rainfall amount,
above the cave. This is especially true in caves such as those
in the Sierra Nevada foothills in which ground water flow is
dominated by fracture-flow, and long-term groundwater
storage is minimal.
6. CONCLUSIONS
Regular environmental monitoring at Black Chasm
Cavern in the Sierra Nevada demonstrates that the cave is
ventilated in the winter months, when cold, dense surface
air sinks into the cave. Large seasonal rainfall variability
causes cave drip-water to slow substantially during the late
summer and autumn months, increase significantly during
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J.L. Oster et al. / Geochimica et Cosmochimica Acta 91 (2012) 92–108
the wet winter and spring, and respond within hours to
storm events during the height of the rainy season. Rainwater d18O and d2H are controlled by a combination of surface
air temperature and water vapor source. Due to mixing of recharge from different events in the epikarst, seasonal changes
in rainwater d18O have a larger influence on drip water d18O
than single storm events with distinct isotopic signatures.
However, moisture source could provide a more significant
influence on drip water values if vapor sources shift more
permanently. Cave drip-water Mg/Ca and Sr/Ca are highest
in the autumn when drip rates and cave air pCO2 are low,
and lowest during the summer when drip rates are low, but
cave air pCO2 is at its highest. Drip water Mg/Ca and Sr/
Ca display a significant correlation with pCO2 values
throughout the year, but this relationship becomes less significant than the correlation with drip rate if summer data
is excluded. This suggests that drip rate may be the primary
control on drip water elemental ratios and prior calcite precipitation during the seasons when speleothem growth is
most likely to occur. Drip water d13C values are most negative during the late spring and early summer and likely reflect
both prior calcite precipitation and soil respiration rates.
The isotopic and chemical variability observed in Black
Chasm drip waters is consistent with interpretations of speleothem paleoclimate proxy records from a nearby cave
where such monitoring is not feasible and suggests that speleothems from these caves can be used to investigate past
precipitation variability. While a complete understanding
of speleothem proxy records is limited by the inability to
observe cave systems under different boundary conditions,
such as those that existed during the last glacial period,
careful and long-term monitoring of modern cave environments offers the best approach to accurately interpreting
these records. Observations of the modern cave environment at Black Chasm provide an analogue for the interpretation of stalagmite proxy records from similar fractureflow dominated cave environments characterized by large
seasonal differences in rainfall and minimal long-term
groundwater storage and should be useful as more speleothem records from arid regions are developed. Continued
cave monitoring work in this and other caves in this region
will better constrain the effects of longer-term environmental changes on cave air circulation and water chemistry as
well as the degree of variability in the response among caves
in this region to these changes. Paleoclimate records from
caves in these environments will shed valuable light on
how rainfall varies with climate change in regions with
highly sensitive water resources.
ACKNOWLEDGEMENTS
We thank Steven Fairchild, President of the Sierra Nevada Recreation Corporation, and Shaundy Farley, manager of Black
Chasm Cavern for providing access to the cave. We also thank
E. Kleber, M. Kelley, and N. Marks for field assistance. P. Green
and Guangchao Li assisted with cation measurements, and H.
Spero, D. Winter and. P. Blisnuik assisted with stable isotope measurements. This work benefitted from comments by S. Zimmerman
and three anonymous reviewers. This work was supported by a
Cave Research Foundation grant to Jessica Oster and NSF grant
NSF-ATM0823656 to Isabel P. Montañez.
APPENDIX A. SUPPLEMENTARY DATA
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.gca.2012.05.027.
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Associate editor: Sidney Hemming