Theses and Dissertations - Department of Geological Sciences

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Aqueous geochemistry of a sulfurous freshwater spring: implications for sulfur cycling and resident microbial communities
(University of Alabama Libraries, 2014) Morrissey, Tacoma Nicole; Aharon, Paul; University of Alabama Tuscaloosa
Blount Springs offers a sulfide-rich environment inhabited by a diverse microbial community in which to study the sulfur redox reactions and the microbially mediated processes via the analysis of sulfur isotopes of sulfide and sulfate. The average δ34SH2S at the wellheads is +31.1 ±0.3 / (n=9) and the average δ34SSO4 in the biofilm and downstream locations is highly variable with a mean value of +16.9 ±7.5 / (n=9). Sulfur isotope fractionations from H2S to SO4 range from 7.1 to 13.9 /. Utilizing the sulfur isotope fractionations it is concluded that (i) the sulfide is most likely derived from Thermochemical Sulfate Reduction (TSR) in the subsurface; (ii) the sulfide is subsequently oxidized to sulfate via the microbially related process of chemosynthetic sulfide oxidation; and (iii) the sulfide is also likely consumed by the abiotic processes of sulfide oxidation and outgassing of H2S. The isotopic fractionations from H2S to SO4 corroborate the isotopic fractionations observed in the laboratory during chemosynthetic sulfide oxidation. The carbon isotopic composition of DIC and the concentration of DIC support the hypothesis of microbial consumption of organic matter. Visualization of the biofilm via macroscopic and microscopic imaging revealed a morphologically diverse community. Biofilm of white, pink, and orange color were observed over the course of the study. Microscopic images revealed rod-shaped, coccoid, and filamentous cells. PCR amplification confirmed the presence of bacterial DNA. Aerobic lithotrophs, such as Thioplaca and Beggiatoa are possible groups of bacteria responsible for the chemosynthetic oxidation of sulfide at Blount Springs.
Biomineralization of giant clam shells (tridacna gigas): implications for paleoclimate applications
(University of Alabama Libraries, 2016) Gannon, Michelle E.; Perez-Huerta, Alberto; Aharon, Paul; University of Alabama Tuscaloosa
The giant clam, Tridacna gigas, is an important faunal component of Indo-Pacific reef ecosystems, for which its shell is often used as an environmental archive for modern and past climates. This thesis is a study of the shell microstructure of modern specimens from Palm Island, Great Barrier Reef (GBR), Australia and Huon Peninsula, Papua-New Guinea (PNG), using a combination of petrography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and Raman spectroscopy, as well as a microstructural comparison of fossil T. gigas through 200 ka from PNG. Daily growth increments are recognizable in all specimens through ontogeny within the internal layer. For modern T. gigas from PNG, increments are composed of pairs of organized aragonitic needles and compact, oblong crystals, whereas modern specimens from GBR are composed of shield-like crystals. The combination of nutrient availability and rainfall are likely the most significant factors controlling shell growth and it may explain the observed differences in microstructure. The external layers are composed of a dendritic microfabric, significantly enriched in 13C compared to the internal layer, suggesting a different metabolic control on layer secretion. The internal and external layers are likely mineralized independent from each other, associated with the activity of a specific mantle organ. Furthermore, needles similar to those of modern T. gigas from PNG, are observed and the widths are measured in the set of fossil T. gigas. An exception includes two mid-Holocene-aged individuals, composed of elongated crystals, oblique to the outside of the shell. The results show that widths follows a cyclic pattern, similar to those of solar radiation variability, suggesting there is a relationship between solar activity and the width of aragonitic needles. Differences between modern and mid-Holocene T. gigas, are likely associated with fundamental environmental differences. The results of this study, pointing to locality and environmental dependence, layer specific mantle biomineralization, and co-variation between needle width and solar modulation, advance the potential of giant clam shells to assist in the reconstruction of many climate parameters that were previously limited to chemical analyses. Microstructural results are additionally applicable in engineering and medical research fields.
Cave air and dripwater variability in Cathedral Caverns, Alabama
(University of Alabama Libraries, 2016) McKay, Kathleen Kingry; Lambert, William J.; Andrus, C. Fred T.; University of Alabama Tuscaloosa
Monthly monitoring of dripwater (δ18O, δD, [DIC], δ13CDIC, and pH) and air (pCO2 and δ13CCO2) chemistry from within Cathedral Caverns (Grant, AL) was conducted for 12 months (January 2015-December 2015) to better characterize the factors influencing deposition and δ18O chemistry of speleothems within the cave. Cave dripwater (δ18O and δD) isotope values for the Southeast, US are thought to be consistent with a yearly average. Cave monitoring of Cathedral Caverns, however, indicates that dripwater values are biased towards the winter season. This winter signal is emphasized through the study of the cave air pCO2, which shows a maximum during the month of October (7691 ppmV) and minimums during the colder, winter months. The max pCO2 value indicates that less CO2 is degassing from the dripwater during the hot summer months while during the colder winter months, more CO2 is degassed leaving less [DIC] to remain in the dripwater and more potential calcite deposition onto the stalagmite. The [DIC] and δ13CDIC which range from 0.6 to 6.0 mM and -4.7 to -14.7‰, respectively, show that [DIC] is at a maximum and δ13CDIC is 13C-depleted during summer months. These results indicate that the paleoclimate record in Cathedral Cavern’s speleothems and possibly most SE U.S. caves is biased towards a winter climatic signal. This conclusion is supported by: (i) a strong coupling between the timing of karst aquifer recharge (winter) and increased dripwater flow rates, (ii) cave dripwater δ18O (-5.7‰ (±0.2)) and δD (-32.1‰ (±2.6)) being similar to winter rainwater (-5.1‰ (±1.4) for δ18O and -27.8‰ (±15.1) for δD) collected at nearby Tuscaloosa, AL, and (iii) more favorable chemical conditions for calcite deposition to occur during winter months. These data illustrate that seasonal cave air exchange with the outside atmosphere is an important control on cave-specific periods of enhanced calcite deposition as well as the effect on the chemistry of dissolved inorganic carbon within the dripwater. This work demonstrates the utility of monitoring dripwater chemistry before conducting on paleoclimate reconstructions and furthermore, serves as a precursor for paleoclimate reconstruction of δ18O in speleothems from Cathedral Caverns.
Cave air C O_2 and drip-water geochemical variability at Desoto Caverns: implications for speleothem-based paleoclimate studies
(University of Alabama Libraries, 2017) Dhungana, Rajesh; Aharon, Paul; University of Alabama Tuscaloosa
This study has addresses the question whether speleothems from DeSoto Caverns (Childersburg, AL) can be used as paleoclimate archives for the Southeast USA. The monitoring program encompassed determination of cave air CO2, cave ambient conditions (i.e., air temperature, humidity), drip-water geochemistry and local rainfall amount, and stable isotopes of oxygen and hydrogen (Tuscaloosa, AL). The substantial attenuation of drip water isotope ranges (-3.1 to – 5.3 ‰ V-SMOW) relative to rainwater (-1.2 to -6.4 ‰ V- SMOW) is likely caused by mixing of freshwater with residual evaporated water in the epikarst zone. The cave drip water δ18O shows an interannual negative trend from the warm/dry year (2012) to the relatively cool/wet year (2013) suggesting that evapotranspiration above the cave plays an important role in drip water δ18O variability. Drip water Ca, Mg and Sr and Mg/Ca and Sr/Ca ratios exhibit lower values and higher ratios, respectively, during the warm/dry relative to the cool/wet year. The interannual rainfall amount variability likely exerts a dominant control on the elemental concentrations and elemental ratios of the drips. Cave air pCO2 varies seasonally with high values (up to 5.0 atm ×103) during summer when cave air flow is in stagnation mode and low values (down to 0.48 atm ×103) during winter when cave air flow is in ventilation mode. The data suggest that seasonal variations in the concentration of cave air CO2 affect the δ13C of drip water and by extension that of speleothem δ13C values. The documented abrupt hydroclimate changes at ~5 ka in a DeSoto stalagmite is synchronous with the reduction of the North Atlantic Deep Water (NADW) production suggesting the latter being the likely controlling factor. The periodic (68 ± 4 yrs periodicity) switches of seasonal rainfall amount dominance from winter to summer and back are a prominent feature of the mid-to-late Holocene δ18O time series of the speleothem. The observed 68 ± 4 yrs periodicity in stalagmite 18O agrees well with the ~ 70 yrs periodicity of the Atlantic Multidecadal Oscillation (AMO) suggesting the latter played a dominant role in the hydroclimate changes in the southeastern US during the late Holocene.
High resolution, U/Th dated (32,000 to 11,000 years), oxygen and carbon isotope proxy climate records from a stalagmite in Desoto Caverns, Alabama, USA
(University of Alabama Libraries, 2010) Lambert, William Joseph; Aharon, Paul; University of Alabama Tuscaloosa
This study addresses the question whether speleothems from DeSoto Caverns (Childersburg, AL) can serve as archives of paleoclimate conditions for the Southeast, USA. The focus of the study involves determining present-day controls of cave water δ^18 O and δ^13 C followed by interpretation of stalagmite δ^18 O and δ^13 C variability in comparison to climate events of the past. The monitoring program involved a 3-year study of cave waters and local rainfall (Tuscaloosa, AL) during years characterized by a significant trend from wet to dry conditions. Decreasing recharge of the cave aquifer was expressed as an interannual trend of declining drip flow rates, which was punctuated by seasonal oscillations due to varying rates of evapotranspiration. Amount-weighted mean monthly rainwater δ^18 O range from -1.5 to -8.3 /, show a mean seasonal amplitude of ~4 /, and exhibit an interannual trend toward ^18 O-enrichment that I interpret as being governed by global atmospheric circulation patterns. The cave's aquifer attenuates seasonal δ^18 O variability, records 20% of rainfall's interannual ^18 O-enrichment, and is biased toward winter rainfall δ^18 O. Cave waters display strong seasonal variability in dissolved inorganic carbon (DIC) and δ^13 C, which range from 0.2 to 6.0 mM and 2.7 to -12.9 / (VPDB), respectively. The data suggest the strongest seasonal controls are cave air ventilation/stagnation and varying CO_2 fluxes through the soil horizon and epikarst. δ^13 C of active speleothems imply the precipitating aragonite captures the seasonality observed in source dripwaters and time-series δ^13 C records of stalagmites carry the imprints of drip annual means entailing climate-driven δ^13 C seasonal biases. A fossil stalagmite provided a high-resolution proxy record of rainfall variability between 31.9 and 11.3 ka. I propose a more southerly polar jet stream (PJS) promoted increased winter rainfall amounts during cold phase events while warm phases result in a higher PJS position and decreased winter rainfall. The Younger Dryas was characterized by a dramatic change in the PJS path as warm air from the Gulf of Mexico infiltrated deep into the continent's interior and substantially decreased winter rainfall. Establishment of near modern climate conditions greatly enhanced deposition rates before changes in flow paths through the epikarst prevented stalagmite deposition since 11.3 ka.
A high-resolution hydroclimate record of the last three millennia from a cored stalagmite at Desoto Caverns (Alabama, USA)
(University of Alabama Libraries, 2010) Dhungana, Rajesh; Aharon, Paul; University of Alabama Tuscaloosa
Late Holocene climate changes in the Southeast USA are poorly documented due to the paucity of high-resolution paleo-records. This study provides high-resolution records of rapid hydroclimate changes in the Southeast over the last three millennia. The records are based on stable isotope rainfall proxies whose time series are constrained by precise U/Th dates from a stalagmite sampled at DeSoto Caverns. The average growth rate of the stalagmite was 149 µm/yr prior to 1400 years and it has been growing with an average growth rate of 42 µm/yr in the last 1400 years. During the past three thousand years stable isotope time series document six wet episodes (at ~ 2950, 2450, 1675, 1200, 700 and 70 years ago) alternating with six drier periods (at ~ 3100, 2800, 1900, 1500, 800 and 300 years ago). The biannually resolved 18O record agrees well with the contemporaneous SST record from the Sargasso Sea cores suggesting that changes in moisture availability in the Southeast are likely linked to subtropical North Atlantic SST variability. Power spectra analysis of the stalagmite-based oxygen isotope record reveals statistically significant periodicities at 24±1 and 36±1 year that are consistent with those observed in the contemporaneous atmospheric 14C production record. The 24 years periodicity is also consistent with the 24-year NAO Index periodicity. On the basis of our analysis we propose that the hydroclimate in the Southeast USA over the last three millennia was intimately linked to NAO variability powered by solar activity fluctuations.
Hydroclimate time-series archived in a 4300 year old stalagmite from Desoto Caverns (Alabama, USA)
(University of Alabama Libraries, 2014) Aldridge, David Edward; Aharon, Paul; University of Alabama Tuscaloosa
Recently published climate studies have implicated the Atlantic Multidecadal Oscillation (AMO) as the dominant factor modulating the precipitation in the regions adjacent to the Atlantic Ocean and the Gulf of Mexico but long range, land-based paleoclimate proxies, are notably lacking. Here I report the results of a new stalagmite- derived ä18O and ä13C ( / VPDB) time series and petrographic study from DeSoto Caverns, Alabama, spanning the interval from recent to 4025 years BP. The new data document AMO as having a pervasive controlling influence on the Southeastern USA region's hydroclimate during the Late Holocene. Ten precise 230Th/234U age determinations, from an 11.3 cm section of stalagmite, spanning the interval from 1883 to 4025 years BP, were used to construct an age model for the stable isotopes time-series containing 882 determinations. Analysis of the stalagmite's ä18O time-series in the frequency domain exhibits dominant periodicities of 30.8 ± 1.4 years and 27.4 ± 0.8 years at the Chi Squared 95% confidence interval that match the instrument-derived AMO half cycle periodicity of approximately 30 years. At the Chi Squared 90% confidence interval, the stalagmite's ä18O time-series frequency analysis reveal a periodicity of 58.0 ± 2.7 years, matching the instrument derived AMO full cycle of approximately 60 years. Starting about 1883 years BP a series of anomalous black laminations appear in the stalagmite and continue, with short interruptions, until a return to normal deposition at approximately 49 years BP. Petrographic investigations reveal that the section of the stalagmite that contains the black laminations also features intense dissolutional unconformities that are predominately composed of detrital material. Both the detrital material and dissolutional unconformities, likely resulted from intense landscape modifications by the pre-Columbian Native American and European societies inhabiting the region during the time.
Insights into aragonite diagenesis as evidenced from spelean carbonate
(University of Alabama Libraries, 2011) Phillips, Joseph Hunter; Aharon, Paul; University of Alabama Tuscaloosa
Cave deposits are increasingly being used as paleoclimate archives. Though a majority of cave deposits are composed of calcite, cave deposits composed of aragonite are quite common. Aragonite, being unstable in the cave environment, will readily alter to stable calcite. Paleoclimatologists are reluctant to sample in areas where aragonite has altered to calcite lest the data derived be a result of diagenesis and not indicative of the original paleoclimate signal. The avoidance of these altered areas comes at the expense of potentially important paleoclimate data. Because aragonite is unstable near the surface of the earth and readily alters to calcite, studying the transformation of aragonite to calcite is difficult as instances where aragonite is adjoined to secondary calcite are extremely rare. DeSoto Caverns allow an opportunity to comment on the aragonite to calcite transformation as primary aragonite is adjoined to secondary calcite. Aragonite adjoined to secondary calcite provides an opportunity to study whether the original paleoclimate signal in aragonite can be preserved in secondary calcite. Two cored stalagmites, DSSG-5 and DSSG-6, were sampled in order to comment on the aragonite to calcite transformation and its chemical implications. Stalagmites from DeSoto Caverns reveal the aragonite to calcite transformation to be variable under similar physicochemical conditions. Two neomorphic calcite fabrics, equant and columnar, were observed. Equant calcite inherits textural and chemical features of primary aragonite, namely the ä13C, ä18O, Mg, Sr, and Ba. Columnar calcite is fabric destructive and gains Mg and but loses Ba and Sr from aragonite. The carbon and oxygen isotope composition of columnar calcite, relative to aragonite, agrees well with experimental stable isotope fractionation studies. Equant calcite and columnar calcite represent replacement in a closed system and open system respectively. Equant calcite is favored where fluid-filled pores and permeability of aragonite are low. Columnar calcite, however, is favored where fluid-filled pores and permeability of aragonite are high. Open system and closed system replacement fabrics observed in such close proximity have important implications with respect to the stabilization of carbonate units with time. The use of altered speleothems in the study of paleoclimate may be possible if the system in which the alteration occurs is well known.
Mineralogy and seasonal growth of South Pacific mussel valves
(University of Alabama Libraries, 2010) Jones, Christie Ann; Andrus, C. Fred T.; University of Alabama Tuscaloosa
Choromytilus chorus and Aulacomya ater are among the most common mussel species in the South Pacific. However, little sclerochronological analysis has been conducted on them, even though they are found in both archaeological sites and sub-fossil deposits and could potentially be useful paleoclimate proxies. One valve from each species was analyzed via x-ray diffraction (XRD) and Raman spectroscopy to determine the shell's mineralogy. C. chorus is mostly calcite with aragonite in some zones, while A. ater is mostly aragonite with less calcite. There are abrupt and continuous boundaries between the two calcite layers and across the aragonite-calcite interface of C. chorus. All of the boundaries in C. chorus can be seen in cross section under reflected or transmitted light along the longest growth axis. The boundaries between mineralogical and structural variations in A. ater are not as visually distinct, and thus may render the species problematic for oxygen isotope analysis. Comparison of C. chorus oxygen isotope profiles to regional temperature records and local water δ18O suggest the sampled shells' lifespan was between one and two years of age. Analyses of the oxygen isotope profiles indicate nearly continuous shell growth throughout the first year of life, with a marked decrease or periods of cessation in shell growth in the second year in the larger specimens analyzed. This growth pattern may make the valves useful as proxies for at least one year of paleoclimatic/paleoenvironmental data. Three out of the four specimens of C. chorus analyzed for oxygen isotopes display a prominent growth break on the outer surface of their valves and each appear roughly contemporaneous. Oxygen isotope analysis indicates that the prominent growth breaks in each of the three valves were formed just before the peak SST of January of 2006 occurred. Qualitative seasonal variations are recorded in the δ18O profiles of the four shells, and median annual δ18O values are in general agreement between individuals. However, the full range of the winter to summer seasonal shift in SST was not recorded by all C. chorus valves. Therefore, caution should be exercised when utilizing C. chorus valves to interpret changes in seasonality.
Paleoclimate reconstructions over the last century from a tropical speleothem on Niue Island, South Pacific
(University of Alabama Libraries, 2010) Murgulet, Valeriu; Aharon, Paul; University of Alabama Tuscaloosa
The study examines the question whether speleothems from Niue Island (19°00'S, 169°50'W), a large carbonate platform located at the edge of West Pacific Warm Pool, can serve as archives of hydroclimate controlled by El-Niño/Southern Oscillation (ENSO) and of catastrophic cyclones that frequent the island. Niue Island is heavily karstified, with modern and fossil speleothems hosted by coastal and inland caves. The flank margin caves on Niue are shown to be formed by the action of corrosive groundwaters on uplifted Pleistocene-age reef carbonates in a tectonically active region. The focus of this study is an actively growing stalagmite sampled from a flank margin cave (Avaiki Cave) that contains about 146 years of deposition (2002-1856 AD). The stalagmite consists of sub-annual couplets alternating between white porous calcite laminae deposited during the austral summer and dark, compact calcite laminae deposited during the austral relatively dry winter. High resolution (sub-annual) stable isotope and trace element profiles accompanied by trace element X-ray mapping were used to test the validity of ENSO-controlled hydroclimate and tropical cyclones archived in the stalagmite. The results show that interannual variability in the stalagmite d18O and d13C time series agrees well with instrumental-derived ENSO phases (El Niño and La Niña events during 1866-2002) and the sea level pressure differential (Samoa-Fiji)-based SPCZ index (SPI) that controls the interdecadal hydroclimate variability. Severe cyclones that directly impacted Niue Island over the last century are recorded by abrupt, large increases in trace element concentration values of Mg, and S accompanied by stable isotope positive excursions bearing seawater-derived signatures. Application of selected trace elements (i.e., Mg, Na, S, P) as proxies of severe storms is a novel technique that can be successfully applied in carbonate coastal areas with flank-margin caves impacted by severe cyclones. This study also demonstrates that sub-annual geochemical cycles in trace element laminae, unresolved by analytical linear transects due their complex distribution pattern, are successfully imaged by large area X-ray mapping of the stalagmite
Peruvian mollusk shells as multi-proxy archives: late Holocene upwelling variation and El Niño-induced biomineralization effects on trace elements
(University of Alabama Libraries, 2010) Etayo-Cadavid, Miguel Fernando; Andrus, C. Fred T.; University of Alabama Tuscaloosa
The purpose of this research is to characterize Peruvian upwelling during the late Holocene (last 2000 years) using molluscan proxies. Peruvian upwelling is a key component of El Niño/Southern Oscillation (ENSO) cycle, an important factor introducing interannual variability to Earth's weather. Thus by studying Peruvian paleoupwelling a better picture of past ENSO conditions can be inferred. High resolution sampling for radiocarbon and stable oxygen isotopes in modern pre-bomb Donax obesulus and Protothaca asperrima shells revealed sub-seasonal variations in Peruvian upwelling. Based on the shells' radiocarbon data a new reservoir effect correction (ΔR) was calculated for the Peruvian coast. ΔR, the radiocarbon age difference between global and local marine reservoirs, is also a qualitative proxy for deep water upwelling. A Trachycardium procerum shell that survived the 1982-1983 El Niño revealed that biomineralization changes induced by this event likely affected trace element incorporation into molluscan aragonite. Detected variations in mollusk biomineralization linked to El Niño suggest the need for coupled structural and chemical analyses in environmental proxy studies. Comparison between modern pre-bomb and archaeological ΔR obtained from D. obesulus shells revealed similar upwelling rates in northern Peru for the 20th and 16th centuries and lower rates for the 6th century. Low upwelling rates in northern Peru in the 6th century are in agreement with reported Mega- El Niño events that contributed to the political decline of Moche society.
Sulfur, carbon, and oxygen isotopes of coexisting sulfides and carbonates in gas and oil seeps from the Gulf of Mexico
(University of Alabama Libraries, 2019) Morelli, Erica C.; Wielicki, Matthew; Aharon, Paul; University of Alabama Tuscaloosa
Microbial processes consisting of bacterial sulfate reduction (BSR), bacterial disproportionation of sulfur (BDS), and BSR coupled with anaerobic oxidation of methane (BSR-AOM) occur in the Gulf of Mexico (GOM) at sites where hydrocarbon oil and gas seep through conduits to the seafloor. Thus far, no studies have measured the solid-phase sulfides from GOM seeps in-situ that provide a superior analogue to sulfides from the geologic record. This study employs in-situ sulfur isotope measurements by secondary ion mass spectrometry (SIMS) in authigenic sulfides associated with barites and carbonates, and isotope measurements of carbon and oxygen by isotope ratio mass spectrometry (IRMS) in coexisting carbonates from 5 sites (GC-140, GC-185, GC-272, MC-929, and GB-382) in the GOM. Pyrite grains (FeS₂) yield variable δ³⁴S values and are considerably more ³⁴S-depleted in seeps with dominant carbonate phase (GC-140 & GC-185: range of -50.5 to -9.0‰ CDT) compared to seeps with minor to dominant barite phase (GC-272 & MC-929: range -23.9 to 19.5 ‰ CDT; GB-382: range 5.3 to 25.9‰). Measurements of carbonate reveals variably ¹³C-depleted and ¹⁸O-enriched δ¹³C and δ¹⁸O values in all seeps (GC-140 & GC-185: -36.7 to -3.5‰ VPDB and 2.5 to 5.3‰ VPDB, respectively; GC-272 & MC-929: -38.0 to -1.2‰ VPDB and 1.6 to 2.5‰ VPDB, respectively; GB-382: -31.4 to -1.2‰ VPDB and -0.5 to 4.1‰ VPDB; respectively). The isotope data suggest a strong influence of: (i) BSR and BDS processes using crude oil and unlimited SO₄²⁻ supply near the sediment-water interface (GC-140 & GC-185); (ii) BSR using crude oil/ non-methane sublimated gas hydrate and variable availability of SO₄²⁻ at depth within the sediment column (GC-272 & MC-929; GB-382), or (iii) possible BSR-AOM using sublimated methane hydrate and SO₄²⁻ near the sulfate-methane transition zone (GC-272 and GB-382). Additionally, Fe-oxides reported in association with pyrites at GC-140, GC-185, and GC-272, could potentially influence fractionation of sulfur isotopes during microbial processes by promoting greater sulfur recycling and thus muting fractionation effects. The results of this study offer important insights on the high variability of sedimentary pyrites in cold seeps that has implications for the biogeochemical cycling of sulfur and carbon in marine environments.

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