Browsing by Subject "Paleoclimate"

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A 1,500-year record of late Holocene temperature variability and recent warming from Laguna Chingaza, Colombia
(2012-08) Bixler, Curtis William; Shanahan, Timothy M.
Rapid tropical glacier retreat over the last 50 years has been well documented, and has received significant media attention. Many studies suggest these changes are due to rising global surface air temperatures, however disentangling the effects of temperature and precipitation has hampered scientific consensus. Furthermore, because of the shortness of the instrumental record, it is difficult to assess the larger significance of the climate changes associated with the decline of tropical glaciers. Here, we present a locally calibrated, independent temperature reconstruction for the past 1,500 years from Laguna Chingaza, Colombia based on distributions of branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) in order to assess the controls on long term temperature variability in the tropical Andes, and their relationship with growth and demise of Andean glaciers. Comparison of reconstructed temperatures with the instrumental record suggests that our proxy record faithfully records decadal to century scale trends in temperature. The largest temperature decline over the last 1,500 years was a decrease of 2.5 ± 0.3 °C during the Little Ice Age (LIA), reaching lowest temperatures during the mid-17th century, and is broadly consistent with terrestrial temperature reconstructions throughout the tropics and the higher latitudes. The structure and timing of temperature changes at Laguna Chingaza are remarkably similar to recent terrestrial temperature reconstructions from elsewhere in the tropics, including sites in the tropical Pacific and equatorial Africa, suggesting that these changes are widespread in the tropics. Together, these records suggest that warming over the last few decades is unprecedented over the last 1,500 years, including the Medieval Climate Anomaly (800-1150 AD). Comparison of these temperature changes with records of Andean glacier limits suggests that temperature is the dominant driver of glacial retreat, particularly over the past few decades. Additionally, paleotemperatures inferred from LIA and recent glacial equilibrium line altitudes (ELAs) underestimate past changes in temperature when compared with brGDGTs reconstructions, suggesting that changes in precipitation complicate the use of glacier ELAs to reconstruct past temperatures. The coupling of temperature and ice extent in South America suggests that with projected future warming, the health of tropical glaciers could be in jeopardy, significantly impacting the communities and ecosystems that depend on them.
A central Texas drying event identified at the Younger Dryas-early Holocene transition using coupled speleothem 𝛿¹³C-¹⁴C analysis
(2017-08-29) James, Christina Danielle; Banner, Jay L.; Breecker, Daniel; Miller, Nathan
The Younger Dryas (YD, 12,900-11,700 years BP) is characterized by a return to near-glacial conditions in the northern hemisphere during the last deglacial period. Texas moisture proxy records support a general regional warming/drying trend from the YD through the Holocene, however, the timing and magnitude of changes in regional moisture and temperature conditions are poorly constrained. We use moisture proxies from a central Texas stalagmite record (McN-1) collected in the Edwards aquifer (an important human and ecological groundwater resource in central Texas) to assess how epikarst moisture conditions varied in the region during the YD-Holocene transition. The relatively high concentrations of ²³²Th (>1 ppb) in several horizons in this sample resulted in McN-1 U-series ages with high uncertainties (>10% of measured age). We use two isochrons and measured ²³⁰Th/²³²Th ratios from modern calcite grown in central Texas caves to estimate the McN-1 initial ²³⁰Th/²³²Th ratio and develop an age model with better constrained uncertainties. Strong correlations between speleothem 𝛿¹³C and ¹⁴C activities can result from changes in epikarst carbonate dissolution due to variable CO2 ventilation or pore space moisture. A decrease in the proportion of ¹⁴C-free carbon (the dead carbon proportion, DCP) to 0% is interpreted as a change from partial dissolution in a water-saturated closed system to dissolution in an open system, where carbon from limestone has no measurable effect on dissolved inorganic carbon ¹⁴C activities. A negative shift in 𝛿¹³C values of 2.8‰ coincident with a decrease in DCP from 7.5 to 0% occurs in McN-1 at the Younger Dryas-Holocene boundary in less than 230 years (as little as 50 years given age constraints). We attribute these parallel declines in 𝛿¹³C and DCP to changes in the epikarst dissolution system. If changes in the dissolution system are controlled by pore space moisture, the change to an open system at the YD-Holocene boundary indicates a rapid regional drying event. Application of a calcite dissolution model indicates that 25% of the negative 𝛿¹³C shift can be explained by change from a more closed to an open system in the epikarst dissolution region above the cave. We attribute the remaining shift in 𝛿¹³C values to reduced drip water pH, associated with an increased contribution of respired CO₂ to epikarst pCO₂. Speleothem growth rates decrease at the YD-Holocene boundary, consistent with our interpretation that carbon isotopes record a decrease in vadose zone moisture. This epikarst moisture interpretation is consistent with other Texas paleoclimate records, indicating a climate transition to drier early Holocene conditions. Compared with existing regional proxy records, the relatively high temporal resolution of the McN-1 𝛿¹³C record (inter-annual) indicates a rapid drying event concurrent with Greenland temperature increases, suggesting a contemporaneous climate response between regional and high latitude climate at end of the YD.
Alternative approaches to the identification and reconstruction of paleoecology of Quaternary mammals
(2012-12) George, Christian Owens; Bell, Christopher J., 1966-; Czaplewski, Nicholas J; Leibold, Matthew A; Rowe, Timothy; Sprinkle, James T
Since the 19th century the remains of Quaternary mammals were an important source of data for reconstructing past environmental conditions. I tested two basic assumptions that underlie Quaternary vertebrate paleoecology. The first assumption is that fossils mammals can be identified reliably to species. The second assumption is that correlations established between extant mammals and environmental parameters can be used to interpret reliably the paleoenvironment from the latest Pleistocene. Incorrect specimen identifications could lead to errors in paleoecologic interpretations. I explicitly tested an alternative to the traditional approach to identification by identifying fossil shrews based on apomorphies. My results indicated that some traditional characters are useful for identification, but only complete specimens with a combination of characters can be identified to species. This indicates that previous authors who identified shrews to species did not compare them to the full diversity of species. I tested the reliability of cenograms and species-richness models as approaches for the reconstruction of environmental conditions in the past. I used faunal data from Hall’s Cave, Kerr County, Texas to construct cenograms and species-richness models and compared the results to independent paleoclimate proxies. Neither species-richness models nor cenograms agree with paleoenvironmental reconstructions based on proxy data from the Late Pleistocene and Holocene. Cenograms and species-richness models are unreliable and fraught with problems, and both approaches should be abandoned as tools for paleoecological reconstruction. To test for potential geographic bias in the identification of Quaternary fossils I developed a GIS (geographic information systems) database of Quaternary paleontological sites within Texas. I was able to show that the identification of species of fossil soricids, heteromyids, Odocoileus, and Spilogale was influenced by geography. Those fossils should be treated as generic identifications until they are re-evaluated against the full diversity of species. Utilizing GIS I also developed a method of paleoecological analysis. My analysis showed that the environmental conditions found today in Texas might not be limiting the current range of shrews. Based on the known geographic range of shrew fossils, other ecological factors besides environmental conditions are shaping the current distribution of shrews.
Caves and Speleothems A Window into Today’s Aquifers and Past Climates
(2019) Maisano, John
Cave and Speleothem Formation Caves can form when slightly acidic water dissolves a soluble rock, such as limestone. Water that originates as rainfall can drip into a cave, losing dissolved carbon dioxide to the cave air. This forms a layered deposit, called a speleothem, usually composed of the mineral calcite (CaCO3). As they grow (typically less than one hundredth of an inch per year), speleothems encode a history of the cave and of environmental conditions at the surface. Paleoclimate Research Stalagmites are the kind of speleothem that grows upward from the floor of a cave. They can actively grow for hundreds of thousands of years. The shape and chemical composition of a stalagmite depends in part on the environmental conditions above a cave (e.g. temperature, rainfall, and vegetation type). Precise measurement of a growth layer from a stalagmite for isotopes that decay over time, such as carbon-14, uranium-238, and thorium-230, allow for the age of that growth layer to be calculated. These measurements require small samples of the stalagmite calcite to be extracted, either by an automated micromill or by dental drill and a researcher with a very steady hand. A stalagmite cross-section, sampled for uranium, thorium, and other isotopes, can be seen on the right. Glass plates, placed on top of actively growing stalagmites for a month at a time (seen below), allow us to isolate and catalog short intervals of calcite growth in a cave. This helps geoscientists determine how changes at the surface are reflected in the growth and composition of stalagmites. Speleothem Growth A stalactite is a kind of speleothem that grows from the cave ceiling downward. Soda-straw stalactites are thin tubes that grow downward as water flows through their center. When a droplet of water lands on a solid surface in the cave, degassing of carbon dioxide from the water drives the growth of calcite upward in layered stalagmites. Eventually, a straw may become blocked, forcing water to flow down the outside of the soda straw. This forms a type of stalactite called a “carrot.” Over time, the stalagmite will grow taller, as new calcite is added to the top and sides of the formation. The shape of a stalagmite is dependent on how far the drops of water fall from the cave ceiling, the amount of time between drips, the drip water’s chemical composition, and the composition of the cave air. As water continues to flow, this carrot will become a large stalactite, well-cemented to the ceiling. When a stalactite, growing downwards, meets a stalagmite growing upwards, they begin to form a column. A cut and polished column (right) records a complicated visual history of this union. Can you see the top of the stalagmite at the time the column formed? Can you see the trace of the original soda straw at the center of the stalactite? Vadose or Phreatic? Calcite can grow above the water table (in the vadose zone) or below the water table (in the phreatic zone). What are the characteristics of each? All stalagmites and stalactites form from dripping water in the vadose zone. “Cave popcorn,” seen below and on the drapery formation in the upper left, form in areas of the cave that undergo evaporation—especially where air is moving. An abundance of cave popcorn might be a sign of a nearby entrance or small passageway! Needle-like acicular crystals (below) and calcite spar (right) are seen growing in all directions, immune to the influence of gravity. These crystals only grow underwater, in the phreatic zone. Identifying where vadose and phreatic features occur can help unravel a region’s geologic history. Imposters! These two “stalagmites” did not grow in a cave. Leaking water from a nearby fountain percolated through the fountain’s cement foundation. This seeping water dissolved calcium oxide (CaO) from the cement, forming a very alkaline calcium hydroxide (CaOH) solution. When this water dripped into a basement room below the fountain, it absorbed carbon dioxide in the air to form these calcite formations. Acknowledgements We would like to thank the following people and organizations for the contributions that made this exhibit possible: Watershed Protection Department of The City of Austin, Water and Environmental Research Institute of the Western Pacifi¬c at the University of Guam, Environmental Science Institute of UT Austin, ACI Consulting, Cambrian Environmental. The owners, managers and staff at Inner Space Cavern, Natural Bridge Caverns, Westcave Preserve, and Cave Without a Name. Design and science by members of the Banner Research Group. Background Photo: Natural Bridge Caverns, Daniel Felan, 2015
Coral-based reconstruction of surface salinity at Sabine Bank, Vanuatu
(2011-05) Gorman, Meaghan Kathleen; Quinn, Terrence M.; Shanahan, Timothy M.; Breecker, Daniel O.
A monthly resolved coral δ18O record from Sabine Bank, Vanuatu (SBV; 166.04° E, 15.94°S), extending from 2006 to 1929 CE, is used to assess the influence of sea surface salinity (SSS) on the oxygen isotopic composition of coral aragonite at this location. Monthly SSS anomalies at SBV between 2006 and 1970 are strongly correlated with monthly anomalies in sea surface temperature (SST) variations in the central Pacific cold tongue, as recorded by SST anomalies in the Niño 3.4 grid box (i.e., canonical record of ENSO variability, r = 0.68, p < 0.01; lag of 6 months). This relationship demonstrates that SSS in the waters offshore of Vanuatu respond to ENSO-driven changes in the coupled ocean-atmosphere system in the tropical Pacific. SBV coral δ18O is also strongly correlated with monthly instrumental SSS anomalies at Vanuatu (r = 0.71, p < 0.01), therefore SBV coral δ18O variations are driven by the ENSO-related changes in surface ocean conditions. A calibration-verification exercise using SBV coral δ18O values and instrumental SSS was performed over the period 2006-1970 CE. A statistically robust transfer function was determined and used to predict SSS at SBV back to 1929 CE. The coral δ18O and SSS relationship at Vanuatu is further evaluated via comparison with a coral δ18O record from Malo Channel, Vanuatu, a site that is 130 km to the east of SBV. The strong correlation between the two coral δ18O records (r = 0.70; p < 0.01) suggests that ENSO drives regional changes in SSS in this region and that such changes can be reconstructed using variations in skeletal δ18O of corals.
Geochemistry and high-resolution chemostratigraphy of the Haynesville Formation, East Texas
(2015-05) Bitar Nehme, Rita Abdo; Rowe, Harry; Fisher, W. L. (William Lawrence), 1932-; Kerans, Charles
The Upper Jurassic Haynesville Formation of East Texas and Louisiana is an organic-rich calcareous mudrock that is Kimmeridgian in age. It underlies the less calcareous Bossier Shale, and it overlies the Smackover Formation limestone. The Haynesville has low permeability, but a relative high porosity, compared to other mudrock formations. Mudrocks are the most common sedimentary rock and some of the most challenging to study, analyze and understand. Despite having a homogeneous appearance on a macroscopic scale, mudrocks often have high variability in facies and composition on the microscopic scale and elemental level. Many studies and methods have been developed to identify facies and stratigraphic variations in mudrocks. A complete understanding of these variations is valuable to comprehend paleoenvironments, paleoclimate and paleoceanographic conditions. Mudrocks studies are also beneficial to shale exploration because these formations, which have a high hydrocarbon content, can be targeted by oil and gas companies for exploration and production. Geochemical methods, chemostratigraphy in particular, will be used in this thesis to complement core description, petrophysical studies and sedimentological studies. This thesis focuses on acquiring chemostratigraphic data from X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) measurements to identify elemental and mineralogical variations in the T. W. George core, from the Haynesville Formation in Harrison County, Texas. The data are linked to core description and are analyzed using Hierarchical Cluster Analysis (HCA) to acquire a better understanding of the paleoceanographic conditions and depositional environments that controlled the sediment deposition of the Haynesville Formation. The Haynesville Formation comprises a Ca-rich lower Haynesville, a more Ca-rich upper Haynesville, and underlies the Si-rich and Ca-poor Bossier. The dominant condition during deposition is anoxic/euxinic in the lower Haynesville becoming dysoxic in the upper Haynesville and more oxygenated in the Bossier Formation. The greenhouse climate of the Late Jurassic led to the deposition of strata yielding petroleum source rocks such as the Haynesville Formation that today have great economic value. Thus, studying the Haynesville has both academic and economic importance.
Glacial to Holocene climate variability in the southern mid latitudes
(2022-12-05) Piatrunia, Natallia; Shanahan, Timothy M.; DiNezio, Pedro N; Partin, Jusdon W; Breecker, Daniel O; Martindale, Rowan C
The Southern Hemisphere westerly winds play an important role in global climate, by influencing the weather in the mid latitudes, transmitting climate changes between high and low latitudes, and have been proposed to influence the carbon storage in the Southern Ocean. Changes in the position and intensity of the westerlies are anticipated to have a large impact in the future and have also been proposed to play significant role in past climate changes. Paleoclimate proxy records of past variation in the westerlies, however, are controversial, and models suggest that they may depend on a complex interplay between high and low latitude drivers as well as factors such as the mean state of the climate system. This dissertation seeks to improve our understanding of the Southern Hemisphere westerly wind and climate variations. To do so, I generate a 45,000-year record of Southern Hemisphere westerly wind and temperature variability on the North Island of New Zealand using compound-specific hydrogen isotope in sedimentary leaf waxes and branched GDGT-derived temperature using lake records. To improve our interpretations of the hydrogen isotope data in proxy records, I first examine climatic controls on the modern isotopic composition of rainfall on the North Island of New Zealand. This work indicates that isotopic ratios of precipitation mainly reflect regional-scale changes in the atmospheric circulation, with higher δ¹⁸Oₚ values reflecting an increased influence of the tropical anticyclonic circulation and lower δ¹⁸Oₚ ratios reflecting a greater contribution from the Southern Hemisphere westerly winds. Based on our improved understanding of isotopic controls, our multi-proxy record shows that Northern Hemisphere climate exerts a dominant control over the westerlies, irrespective of the changes in the southern Hemisphere mid to high latitude climate conditions. Finally, I examine the distribution of isoprenoid GDGT lipids and elemental ratios from the same lake to provide a complimentary set of proxies of lake stratification and mixing.
Human-environment interactions on the desert South Coast, Peru : a review of paleoclimate proxies and archaeological evidence
(2017-05-05) Weinberg, Camille Dominique Starczak; Covey, R. Alan, 1974-; Rosen, Arlene
The dynamic environment of the Peruvian Andes has always been conceptualized as central to human lifeways. A growing body of paleoclimate research provides new evidence to improve our understanding of the environmental processes that interact throughout this region. This has important archaeological implications to both generate new data on specific human-environment interactions, and more critically assess how the environment has influenced cultural transitions, and how humans have adapted and modified their surroundings. This report discusses existing paleoenvironmental proxy data for the desert Peruvian South Coast cultural region, and focuses on archaeological faunal assemblages, sediment cores, and geomorphological and sedimentological evidence. Each of these proxies differs in terms of scale, coherency, and temporal resolution. Combined, they provide evidence of long-term hyper-aridity in this region, influenced by El Niño Southern Oscillation (ENSO) fluctuations since the Late Pleistocene. ENSO has been the predominant climate driver for the Eastern Pacific, affecting coastal Peru to varying degrees on a multi-decadal scale since it onset at modern frequencies during the Middle Holocene. At local scales, geomorphological evidence and archaeological settlement pattern data suggest changes in precipitation, temperature, and climate variability, with specific relevance for human subsistence and resource access within the coastal desert. This report also summarizes the development of sociopolitical complexity within the South Coast region, which played out within this sequence of environmental change and was closely linked to agricultural developments and marine resource access because past climates created fertile ecosystems within the desert and influenced ranges of marine food species.
Hydroclimate changes over the Great Plains since the Last Glacial Maximum
(2021-08-05) Sun, Chijun; Shanahan, Timothy M.; DiNezio, Pedro N; Partin, Judson W; Breecker, Daniel O; Quinn, Terrence M
The weather extremes associated with mesoscale convective systems (MCSs) over the Great Plains have substantial socio-economic impacts on the regions’ diverse and growing society. It remains unclear how anthropogenic warming will affect the climate of this region in the future due to large uncertainties in simulated precipitation changes by climate models and a lack of knowledge about the long-term variability of these storms. This dissertation seeks to improve our understanding of the climate dynamics of the Great Plains, focusing on using past changes in MCS activity and hydroclimate to understand their responses to external forcings. To do so, I generate a 20,000-year record of MCS activity and hydroclimate in central Texas using the D/H ratios of sedimentary leaf waxes coupled with trace element analysis and carbon isotope composition of bulk organic matter. To improve our confidence in the interpretation of hydrogen isotope data, I first investigate the meteorological controls on the isotopic composition of precipitation in the Great Plains, which suggests that the isotopic ratios of rainwater mainly reflect the magnitude of the dominating MCS storms over this region, with larger and stronger MCSs producing more negative isotopic values and vice versa. Based on our improved understanding of the regional isotope systematics, our multi-proxy record shows pronounced responses of MCS to external forcings (i.e., retreating ice sheets, solar insolation) on orbital to centennial timescales. Results from transient climate model simulations indicate that variations in the intensity of the Great Plains Low-Level Jet in response to springtime land surface warming can account for these changes in MCS activity. Differential springtime warming over the continent and the North Atlantic drives steeper zonal temperature and pressure gradients over the Great Plains, intensifying the Great Plains Low-Level Jet which consequently enhances the magnitude of Great Plains MCSs via increased moisture availability. These results provide important implications for the potential responses of Great Plains MCS activity to anthropogenic warming, where more pronounced land surface warming relative to the ocean may dynamically intensify future storms with increased risks for extreme weather in the Great Plains
Insights into circum-Arctic sea ice variability from molecular geochemistry : the IP₂₅ index
(2012-05) Stoynova, Vera Petrova; Shanahan, Timothy M.; Quinn, Terrence M.; Breecker, Daniel O.
Geological records of past sea ice, such as those contained in Arctic marine sediments, offer an opportunity to strengthen our understanding of long-term sea ice variability, provided unambiguous paleo-sea ice proxies can be developed. One such recently proposed proxy is IP₂₅, a highly branched isoprenoid alkene biosynthesized exclusively by sea-ice dwelling diatoms (Haslea spp.), which is well preserved in marine sediments and could be used to reconstruct past changes in spring sea-ice extent. However, little is known about regional-scale controls on IP₂₅ production in sea ice, limiting its wider applicability as a paleo-sea-ice proxy. To address this issue we examined the distributions of IP₂₅ and the marine productivity biomarkers dinosterol and brassicasterol in a suite of surface sediment samples distributed across the Arctic. We find a statistically significant, logarithmic relationship between IP₂₅ and spring sea ice cover in samples from arctic and subarctic sites in the Pacific (n = 96, r² = 0.67, P < 0.0001) and the Atlantic n = 25, r² = 0.50, P < 0.0001), though the absolute concentrations of IP₂₅ are generally higher in the Atlantic (1.6 - 166.4 [mu]g/g OC) than in the Pacific (0 - 38.5 [mu]g/g OC) for equivalent sea-ice cover, and there are regional and basin-specific differences in the slope of the IP₂₅ - sea ice relationship. After normalization of IP₂₅ concentrations to that of a productivity biomarker (e.g., dinosterol; the PDIP₂₅ index) the proxy-sea ice relationship in greatly improved for all regions (r² = 0.86 and r² = 0.75 for Atlantic and Pacific, respectively) and most of the basin specific differences in the rate of change of IP₂₅ with sea ice are removed. This suggests that productivity plays an important secondary role in controlling IP₂₅ concentrations. However, the use of the PDIP₂₅ index does not change the absolute differences in concentrations seen in the Atlantic and the Pacific, and previously published data from Fram Strait remain anomalous when compared to the rest of our data. This suggests that there are additional, yet unidentified controls on the IP₂₅ proxy - sea ice relationship, which may hinder the development of an Arctic-wide calibration but that the PDIP₂₅ index is a viable tool for local and regional sea ice reconstructions.
Investigating pedogenic carbonate formation by measuring the stable isotope composition of water in Vertisols
(2014-05) Okafor, Brandon Jerrod; Breecker, Dan O.; Young, Michael H; Banner, Jay
The oxygen isotope compositions of pedogenic carbonates in paleosols are used to reconstruct paleoelevations, paleoatmospheric circulation, paleotemperatures, and paleoprecipitation. The oxygen isotope compositions of pedogenic carbonates are controlled by temperature and the oxygen isotope composition of soil water, which predominately originates from precipitation. In most calcic soils studied, pedogenic carbonates record the oxygen isotope composition of summer precipitation and/or mean annual precipitation subjected to evaporation. However, due to the complex hydrological properties of Vertiols, which are abundant in the rock record, the isotopic composition of soil water could potentially vary and could influence the isotopic composition of pedogenic carbonate. Furthermore, it is well established that soils contain multiple pools of water with different stable isotope compositions but little work has been done to investigate which pools are recorded by pedogenic carbonates. Therefore, the isotopic composition of soil water in modern Vertisols was monitored and compared with the oxygen isotope composition of pedogenic carbonate in the same soils to investigate if the oxygen isotope composition of pedogenic carbonates in Vertiols record mobile or immobile water. The isotope composition of soil water was determined in four ways: 1) measurement of isotope composition of water collected by vacuum distillation of soil samples collected by auger, 2) calculation from measured oxygen isotope compositions of soil CO₂, 3) calculation from measured oxygen isotope compositions of pedogenic carbonate, and 4) measured isotope compositions of water collected under tension in a soil solution sampler. The oxygen isotope compositions of water in equilibrium with CO₂ and water from the solution sampler were indistinguishable at 140cm and were interpreted as mobile water in macropores. The vacuum distilled water (which includes water from a mixture of macropores and micropores) always had lower δ ¹⁸O values than the macropore water and the other sampling methods, implying the presence of water with low δ ¹⁸O values. These oxygen isotope compositions of soil water pools were compared with δ ¹⁸O values of local precipitation (GNIP data from nearby Waco, TX). Below ~100cm, total soil water δ ¹⁸O values converge to -6.3 ± 0.7 %₀ (1σ, n=20), which is isotopically lighter than the δ ¹⁸O of mean annual precipitation (MAP) of Waco, Texas (-3.8 ± 2.7 %₀, 1σ, n=96). This could result from recharge of isotopically light September precipitation (SEPT); (-5.9 ± 2.4 %₀, 1σ, n=8)) replenishing the soil after dry periods and/or the contribution of winter precipitation (WP) (-5.5 ± 2.4 %₀, 1σ, n=25). The δ ¹⁸O values of soil water in equilibrium with soil CO₂ ((-4.1 ± 0.8%₀) are isotopically similar to or heavier than the isotopic composition of MAP. The δ ¹⁸O values of soil water in equilibrium with pedogenic carbonate (-2.7 ± 0.9%₀) are also isotopically similar to the isotopic composition of summer precipitation (SP, including June, July, and August) (-2.0 ± 2.9 %₀, 1σ, n=8). This suggests that, despite the more complex hydrology of Vertisols compared with other soils orders, the δ ¹⁸O values of pedogenic carbonates formed in central Texas Vertisols record SP and/or mean annual precipitation that has been subjected to evaporation, just as they do in other soils. If this holds true for Vertisols formed in other climates, then this facilitates the comparison among δ ¹⁸O values of paleosol carbonates from various soil orders, which is common practice in vertical successions of paleosols. Furthermore, the observation that the σ ¹⁸O values of water in equilibrium with pedogenic carbonate are more similar to the σ ¹⁸O values of macropore than micropore water suggests that pedogenic carbonates in central Texas Vertisols may form in macropores. Formation in macropores is more consistent with CO₂ degassing and/or evaporation, rather than root water uptake, as a proximal driver of calcite precipitation.
New insights into the carbon isotope composition of speleothem calcite : an assessment from surface to subsurface
(2012-05) Meyer, Kyle William; Banner, Jay L.; Breecker, Dan O.; Musgrove, MaryLynn
The purpose of this study was to provide new insights into the interpretation of speleothem (cave calcite deposit) δ13C values. We studied two caves in central Texas, which have been actively monitored for over 12 years. We compared δ13C values of soil CO2 (δ13Cs), cave drip water (δ13CDIC), and modern cave calcite (δ13Ccc). Measured average δ13C values of soil CO2 were -13.9 ± 1.4‰ under mixed, shallowly-rooted C3-C4 grasses and were -18.3 ± 0.7‰ under deeply-rooted ashe juniper trees (C3). The δ13CDIC value of minimally-degassed drip water in Natural Bridge Caverns was -10.7 ± 0.3‰. The carbon isotope composition of CO2 in equilibrium with this measured drip water is -18.1 ± 0.3‰. The agreement between juniper soil CO2 and drip water (within ~0.2‰) suggests that the δ13C value of drip water (δ13CDIC) that initially enters the cave is controlled by deeply-rooted plants and may be minimally influenced by host-rock dissolution and/or prior calcite precipitation (PCP). At Inner Space Caverns, δ13CDIC values varied with vegetation above the drip site, distance from the cave entrance, and distance along in-cave flow paths. Whereas CO2 derived from deeply-rooted plants defines the baseline for drip water δ13CDIC entering the caves, kinetic effects associated with the degassing of CO2 and simultaneous precipitation of calcite account for seasonal variability in δ13CDIC and δ13Ccc. We documented increases in δ13CDIC at a rate of up to 0.47‰/hour during the season of peak degassing (winter), suggesting that δ13CDIC variations may be controlled by total elapsed time of CO2 degassing from drip water (Ttotal). We also observed seasonal shifts in the δ13C values of modern calcite grown on glass substrates that are correlated with shifts in drip water δ13CDIC values and drip-rate. Therefore, we suggest that increased aridity at the surface above a given cave results in, slower drip-rates, higher Ttotal, and therefore higher δ13CDIC values. We propose that large variability (>2‰) in speleothem δ13Ccc values dominantly reflect major vegetation changes, and/or increasing Ttotal by slowing drip-rates. Based on these findings, variability in speleothem carbon isotope records may serve as a proxy for paleoaridity and/or paleovegetation change.
Recent climate records from a near-entrance stalagmite
(2018-08-14) Carlson, Peter Eric; Breecker, Dan O.; Banner, Jay L.; Mickler, Patrick J; Shanahan, Timothy M; Affek, Hagit P
Sub-annually resolved environmental proxies can be valuable archives of climate change, but they are rare in terrestrial settings, and it can be difficult to verify their annual nature. We suggest that speleothems that grow in well-ventilated (near-entrance) zones of caves may preserve such high-resolution records. Previous monitoring studies of a well-ventilated, temperate-latitude cave (Westcave Preserve, central Texas) have documented seasonal variations in the oxygen isotope composition of calcite grown on glass substrates. We extend this work to provide a high-resolution (weekly to monthly) 52-year record of δ¹⁸O, Mg, Sr, and Ba in Westcave stalagmite WC-3, using the temperature-dependent variation of calcite δ¹⁸O and Mg concentrations as a precise seasonal chronometer. We confirm this chronology using measurements of radiocarbon in the stalagmite, which show an attenuated, lagged, and spread record of the atmospheric “bomb pulse” from nuclear weapons testing. We develop and test a new inverse modeling framework using these results, and show that stalagmite WC-3 carbon is sourced primarily from decomposition of subsurface organic matter. Finally, we correlate the WC-3 geochemical records to global and local climate parameters, finding that the ability of the records to respond to local annual-scale climate signals (particularly of aridity and precipitation) is highly dependent on the multi-decade phase of the Pacific Decadal Oscillation (PDO). We find that water residence times are low during cold-phase PDO, and high during warm-phase PDO. Additionally, we find that summer Mg concentrations increase with drought conditions (Palmer Drought Severity), especially during warm-phase PDO, and summer δ¹³C values decrease with increasing precipitation, during both cold- and warm-phase PDO. As PDO phases can last for decades, proxy calibration using cave monitoring studies may only be valid for a portion of the climate record. Throughout the entirety of this work, we show comprehensively and for the first time that high-resolution near-entrance speleothems can serve as an important intermediary between short-term monitoring and longer speleothem records.
Reconstructing late Holocene Artctic climate change using high resolution sediment records from Simpson Lagoon, Alaska and the Colville River alluvial valley
(2015-12) Hanna, Andrea Jo Miller; Goff, John A.; Shanahan, Timothy M.; Allison, Mead A. (Mead Ashton),; Quinn, Terrence M; Mohrig, David; Marcantonio, Franco
Arctic nearshore environments proximal to large rivers, like Simpson Lagoon, Alaska, potentially contain high-resolution sediment archives that can be utilized to reconstruct paleoclimate variability over the late Holocene. The ongoing, rapid environmental changes recently observed in the Arctic highlight the need for high-resolution records of pre-industrial climate change in this climatically sensitive region; such records are fundamental for understanding recent anthropogenic changes in the context of natural variability. This dissertation utilizes a suite of geochemical and sedimentological proxies in combination with age-constrained, shallow acoustic reflection data to demonstrate that these underutilized coastal sediment archives are capable of generating high-resolution paleoclimate records on par with other terrestrial climate archives (i.e. lake sediments, ice cores, tree rings) and provides the first ~1650-year long record of climate variability from the inner shelf of the Alaskan Beaufort Sea. An analysis of sedimentation patterns within Simpson Lagoon using CHIRP seismic data and radioisotope geochronology reveals that sediment infilling in Simpson Lagoon began ~3500 y BP, creating a primary depocenter with mm/y sediment accumulation in western Simpson Lagoon. The interbedded sediments suggest that major sediment reworking from ice processes, a common occurrence in Arctic shelf environments, does not disrupt the sediment archive contained within the lagoon. Quantitative reconstructions of surface air temperature are obtained using the brGDGT-derived MBT’/CBT paleothermometer. A comprehensive study of lagoon and river sediments and catchment soils demonstrate that brGDGTs are primarily soil-derived, and yield reconstructed temperatures consistent with instrumental summer temperature observations from Alaska’s North Slope. Temperature reconstructions from Simpson Lagoon also show similarities with regional and pan-Arctic climate records over the last few millennia, with evidence of temperature departures correlative with noted climate events (i.e., Little Ice Age, Medieval Climate Anomaly). In addition, temporal variability in sediment sourcing to the lagoon, determined using a multi-proxy approach (i.e., granulometry, elemental analysis, clay mineralogy), broadly corresponds with temperature fluctuations, indicating relative increases in fluvial sediment discharge during colder intervals and decreased river discharge/increased coastal erosion during warmer periods. This paleoclimate variability may be driven by variations in solar output and/or shifts in the regional ocean-atmosphere circulation patterns (e.g., the Aleutian Low).
Reconstructing past changes in El Niño-Southern Oscillation variability using geochemical proxies from corals
(2020-05-06) Lawman, Allison E.; Quinn, Terrence M.; Partin, Judson W.; Shanahan, Timothy M; Martindale, Rowan C; Di Nezio, Pedro N
The El Niño-Southern Oscillation (ENSO) is the leading mode of interannual (>1-9 year) climate variability and has far-reaching impacts on global temperature and rainfall patterns. That said, the instrumental record of ENSO does not have the temporal coverage necessary to characterize the full range of natural variability. Uncertainties about how ENSO variability may change in the future with anthropogenic warming motivates the study of past climate conditions when the Earth experienced different climate conditions compared to the warm, rapidly changing climate of today. Climate scientists use two primary tools to study past climate. First, general circulation models use mathematical equations based on physical principles to simulate different aspects of the climate system, including the ocean, atmosphere, land, and ice sheets. Second, various geological archives like corals, cave stalagmites and lake and ocean sediment also provide clues about how the climate varied in the past. The paleoclimate proxy records generated from geological archives thus provide independent validation of paleoclimate model simulations. Geochemical proxy records from corals are particularly well-suited for investigating past (paleo) ENSO variability as corals can provide decades to hundreds of years of climate information from the tropics at sub-annual resolution. This dissertation seeks to quantify changes in paleo-ENSO variability using geochemical proxies from corals. I first generate replicated coral records from Vanuatu in the southwest Pacific to investigate the range of ENSO variability during the 20th century and ~900 years during a time interval called the Medieval Climate Anomaly. The paleo-temperature records are based on the ratio of strontium to calcium (Sr/Ca) of massive Porites corals from Vanuatu. Next, I develop various computational algorithms to investigate how different uncertainties inherent to the coral archive impact the ability of a coral to capture changes in ENSO variability. The algorithms incorporate the impact of variable growth rates, analytical and calibration errors, and age model assumptions, and are collectively referred to a coral proxy system model (PSM). The utility of the coral PSM is demonstrated using unforced climate model output for the last millennium. I then use the PSM and apply the analytical techniques developed in the first two projects to investigate simulated and coral proxy-inferred changes in paleo-ENSO variability during the Holocene (11.65 thousand years ago to the present). The resulting model-data comparison provides a new perspective on long-term changes in ENSO variability.
The Cretaceous stratigraphy and geochemistry of the mixed siliciclastic-carbonate system of Saudi Arabia : implications for global correlation and paleoclimate
(2023-08-04) Fallatah, Mohammed Ibrahim; Kerans, C. (Charles), 1954-; Mohrig, David; Steel, Ronald; Olariu, Cornel; Alnazghah, Mahmoud
The Cretaceous was a unique time period characterized by greenhouse conditions with multiple climatic perturbations. The interplay between the tumultuous Cretaceous climate and the depositional systems generated complex stratigraphic architectures. Such a complex stratigraphy is manifested in the interior of the Arabian Plate. Excellent outcrops of the Cretaceous succession in central Saudi Arabia offers an opportunity to document the response of the depositional system to the Cretaceous climatic changes. This study aims to investigate the evolution of the depositional system throughout the Cretaceous Period, including the interaction between siliciclastic and carbonate sedimentation in a greenhouse system, and the effect of any autogenic forces dominating over the Cretaceous extrinsic parameters on the resultant stratigraphy. These objectives will be achieved by adopting a multi-disciplinary integration of sedimentology, sequence stratigraphy, biostratigraphy, and high-resolution geochemistry (chemostratigraphy and trace elements). During the Late Jurassic–Early Cretaceous (late Tithonian–Valanginian), a shift from an arid to humid climate is recognized based on an influx of organic matter attributed to accelerated hydrological cycle, coinciding with the formation of a diachronous drowning unconformity in the early Valanginian. Following the shift to a humid climate, terrigenous sediment influx intensified and tidal siliciclastic sedimentation became prevalent during the Early to Late Cretaceous (Hauterivian–Turonian). The siliciclastic succession closely resembles modern tide-dominated depositional systems, which allowed to document the development of double clinoform deltas in an epeiric sea. It also allowed to conclude that lithological boundaries in greenhouse systems do not represent unconformities. From the Turonian to Maastrichtian, the climate shifted back to arid and evaporitic conditions marking the cessation of siliciclastic influx. Due to these conditions, seawater composition was altered such that it inhibited coccolith proliferation. This provides an explanation for the lack of chalk deposits over the Arabian Plate during the Campanian– Maastrichtian. The novelty of the study is also highlighted by recognizing a positive carbon isotope excursion at the J-K boundary. This signal is vastly identified in the Boreal Realm and is absent in the Tethys. Moreover, the study records the deposition of mud-mounds at K-Pg boundary, which could have an implication for the Cretaceous mass extinction
Toward a mechanistic understanding of silicate weathering and Li transfer across landscapes, past and present
(2021-08-09) Ramos, Evan Joseph; Breecker, Daniel O.; Barnes, Jaime Danielle; Rempe, Daniella M; Johnson, Joel PL; Foreman, Brady Z
Silicate weathering is a fundamental process that sculpts landscapes, provides nutrients for organisms, underpins life-sustaining biogeochemical cycles on Earth, and keeps Earth from entering a runaway greenhouse state. This process is influenced by the geologic and environmental conditions at the Earth's surface, therefore uniting many traditional geoscience subdisciplines and being an area of active research for nearly a century. It has been studied through observations of drainage catchments on continents, where most silicate weathering on Earth happens today. The geologic record also served as an important archive of this process, invariably recording the conditions and processes occurring at the Earth's surface in the past and allowing for observations on timescales that studies of modern environments do not permit. Despite advancements in the process-based understanding of silicate weathering, considerable debate remains regarding its primary drivers, the relevance of silicate weathering in the evolution of Earth's climate, and the degree with which known modulators of silicate weathering can be constrained independently of one another. The central focus of this dissertation is to expand upon the utility of lithium (Li) isotopes: a burgeoning isotope system that, when measured in geologic material, allows for a quantitative assessment of silicate weathering over a range of timescales. The dissertation is composed of three case studies which test the viability of Li isotopes in discerning the controls of silicate weathering, both in past and present environments. In turn, these studies help identify controls of silicate weathering in different Earth surface environments. The outcome of these studies are conceptual or mechanistic models for silicate weathering that either i) provide constraints for its controls or ii) yield a template with which to probe it with Li isotopes and other measurements
Trace element incorporation in modern speleothem calcite and implications for paleoclimate reconstruction
(2014-12) Hatch, Rosemary; Banner, Jay L.
Trace element compositions, expressed as ratios relative to Ca (Mg/Ca, Sr/Ca and Ba/Ca), in drip water and modern speleothem calcite were measured at multiple sites within a single cave system (Natural Bridge Caverns) in central Texas. These measurements are used to investigate how water and calcite compositions respond to changes in climate. Drip water trace element ratios respond to changes in climate and in soil, vadose zone and/or in-cave processes, which are in turn influenced by climate. It is commonly assumed that speleothem calcite directly reflects these changes in the drip water composition. To test this assumption, this study quantifies the partitioning of trace elements into speleothem calcite in a natural cave setting. To determine the controls on calcite trace element ratios, empirical partition coefficients (K [subscript D]) for Mg, Sr, and Ba are measured using a unique time series of water and modern calcite geochemistry at two drip sites. One drip site, dominantly supplied by conduit flow, has relatively invariant calcite trace element compositions that reflect correspondingly small variations in drip water chemistry. A second drip site, supplied by a combination of conduit and diffuse flow, exhibits a seasonal change in drip water composition due to changes in cave-air CO₂ concentrations. The drip water seasonality at this site is recorded in the calcite trace element compositions; however the partitioning of Mg/Ca from drip water to calcite is not controlled by the same mechanism(s) that control Sr/Ca and Ba/Ca partitioning. Results of this study indicate that the partitioning of Mg changes with drip water Mg concentration, temperature and location of the calcite relative to the point of drip impact. Calcite Sr/Ca and Ba/Ca ratios are more strongly influenced by changes in cave-air CO₂ that cause changes in CO₂ degassing, affecting calcite precipitation. This element specific partitioning complicates the interpretation of speleothem trace element records, since calcite Mg/Ca and Sr/Ca may not always covary even at drip sites that experience PCP. Although there is a strong correlation between Mg/Ca and Sr/Ca ratios in the drip water at these sites, there is considerable scatter between these two values in calcite. Average K [subscript D] values at both sites fall within the range of previous theoretical and empirical studies and are 0.025, 0.12 and 0.15 for Mg, Sr and Ba respectively. It is important to note that not all trace element partitioning is controlled by the same mechanism, since this has implications for interpretations of hydrologic processes from speleothems.
The use of δ]¹³C values of leporid teeth as indicators of past vegetation
(2013-05) Wicks, Travis Zhi-Rong; Shanahan, Timothy M.; Bell, Christopher J., 1966-
Records of change of [delta]13C values in vertebrate teeth offer an opportunity to gain insight into changes in past vegetation. Increasingly, teeth from small mammals are used for such purposes, but because their teeth grow very rapidly, seasonal changes in vegetation potentially provide a large source of variability in carbon isotope composition, complicating interpretations of small mammal tooth isotope data. To investigate the controls of seasonality on the stable isotope composition of fossil teeth, we constructed a Monte-Carlo-based model to simulate the effects of changes in the seasonal pattern of diet in leporid lagomorphs (rabbits and hares) on the distribution of [delta]¹³C values in random populations of leporid teeth from the Edwards Plateau in central Texas. Changes in mean-state, seasonal vegetation range, and relative season length manifest themselves in predictable ways in the median, standard deviation, and skewness of simulated tooth [delta]¹³C populations, provided sufficient numbers of teeth are analyzed. This Monte Carlo model was applied to the interpretation of a 20,000 year record of leporid tooth [delta]¹³C values from Hall's Cave on the Edwards Plateau in central Texas. Variations in the [delta]¹³C values of teeth deposited at the same time (standard deviation = 1.69%) are larger than changes in the mean vegetation composition reconstructed from bulk organic carbon [delta]¹³C, indicating the influence of short-term variability, making it difficult to assess changes in mean C3/C4 vegetation from the tooth [delta]¹³C data. However, populations of teeth from different climate intervals (e.g., the late Glacial, Younger Dryas, and the Holocene) display changes in the shape of the tooth [delta]¹³C distributions. Interpretation of these changes as shifts in seasonal vegetation patterns that are based upon results from our model are consistent with hypothesized climatic changes. An increase in the standard deviation of the tooth population between the late Glacial and the Younger Dryas -- Holocene is consistent with an increase in seasonality. Furthermore, a shift to more C3-dominated vegetation in the tooth [delta]¹³C distribution during the Younger Dryas is accompanied by a more skewed population -- indicative of not only wetter conditions but an increase in the duration in the C3 growing season. However, late Holocene changes in vegetation are not clear in the tooth data, despite the evidence from bulk organic carbon [delta]¹³C values for an increase in % C3 vegetation of 57%. Small mammal teeth can potentially provide unique insights into climate and vegetation on seasonal and longer timescales that complement other data, but should be interpreted with a careful consideration of local conditions, taxon ecology and physiology, and the dominant timescales of isotope variability.
West Pacific warm pool, South Pacific convergence zone and El Niño : southern oscillation variability in the common Era
(2014-12) Maupin, Christopher Robert; Quinn, Terrence M.; Banner, Jay L.; Jackson, Charles L.; Okumura, Yuko M.
Observations and reconstructions of climate variability are necessary to place predictions of future global climate change into temporal context (Goddard et al., 2012). I focus here on the western tropical Pacific (Solomon Islands; ~9.5°S, ~160°E), a region directly influenced by: the South Pacific Convergence Zone (SPCZ), the West Pacific Warm Pool, the Pacific Walker Circulation, and the Hadley Circulation. The West Pacific Warm Pool (WPWP) is an integral source of heat and moisture to the atmosphere. It is part of both Walker and Hadley circulations, and the El Niño-Southern Oscillation (ENSO), which is the largest source of interannual climate variability on Earth. Modern observations of changes in the mean state of the WPWP, Walker circulation, and ENSO are limited in temporal scale and overprinted with the signal of anthropogenic climate change; hence, proxy-based observations in the pre-instrumental period are needed to place any future changes in these systems, and attribution of these changes, into context. I present evidence for large (~1.5 m), abrupt, and periodic changes in total annual rainfall amount on decadal to multidecadal timescales since 1423±5 CE (Year Common Era) in the Solomon Islands utilizing a fast growing (>2 mm yr⁻¹) stalagmite sample and an empirical relationship developed between stalagmite δ¹⁸O and local rainfall amount. The ~600-yr Solomon Islands stalagmite δ¹⁸O record indicates that decadal oscillations in rainfall are a persistent characteristic of SPCZ-related climate variability, and available evidence points to these changes arising from internal forcing. Such changes have important implications to water resource management in this region, in addition to contextualizing models and future predictions of rainfall amount changes and latent heat release into the atmosphere via anthropogenic climate change. I also present a reconstruction pre-instrumental variability utilizing a monthly δ¹⁸O time series generated series from the long lived, slow-growing coral species Diploastrea heliopora spanning 1716-2008 CE. The record demonstrates strong coherence with modern instrumental variability, and in addition, demonstrates variance in centennial-multicentury, decadal-multidecadal and interannual bands. Quantitative comparison of the interannual component of the record to instrumental ENSO indices, and application of the resulting metrics to previously undocumented, potentially extreme ENSO events occurring during the early eighteenth and nineteenth centuries. These extreme events are absent from existing multiproxy reconstructions due to temporal limitations of extant proxies that have the greatest ENSO sensitivity. The intensity of these events may rival or exceed the largest in the instrumental record. The record also contains significant but transient decadal to multidecadal variability. The coral record exhibits a trend toward isotopic depletion, implying significant changes in SST and/or SSS over the entirety of the record. I also document the unique behavior of the SPCZ and the Intertropical Convergence Zone (ITCZ) over the past six centuries, present evidence of influence by decadal-scale advection processes on Solomon Islands SSS, and explore basin-scale changes in the tropical Pacific conditions potentially related to global average temperature change over the past three centuries.