Browsing by Subject "Paleoclimatology"

Now showing 1 - 6 of 6

A 20,000 year flowstone record of Gulf of Mexico sourced moisture in Texas
(2016-05) Charlton, Timothy Callison; Banner, Jay L.; Breecker, Dan O.; Musgrove, MaryLynn
Understanding how future atmospheric warming may affect moisture distribution in the American Southwest is becoming increasingly important. To this end, various paleoclimatic proxies have been used to investigate the range of climatic fluctuations forced by natural processes during past periods of warming. As part of this effort, speleothem-based oxygen isotope records have been used to hypothesize the contribution of either Gulf of Mexico (GoM) or Pacific Ocean sourced moisture since the Last Glacial Maximum (LGM) at cave sites across the Southwest. In this study, a new oxygen isotope time series is developed from a flowstone in central Texas as a means to test this hypothesis. Due to significant age reversals in the U-series chronology, multiple age models were created that use a series of assumptions to exclude questionable age determinations. By comparing these age models, portions of the flowstone that are most likely to produce robust chronologies were identified. The rate, timing, and amplitude of oxygen isotope shifts in all of the age models are consistent with the only other Texas speleothem and one of the few GoM sediment cores that cover the same time period and have an oxygen isotope record. These results support the hypothesis that central Texas had a dominant GoM moisture source since the LGM and that speleothems in the region might record this signal. Furthermore, this study developed a novel workflow that could be applied to other flowstone samples, which would allow for greater spatial coverage of speleothem-based paleoclimate reconstructions when high-quality stalagmite samples are not available.
Interpreting the difference in magnitudes of PETM carbon isotope excursions in paleosol carbonate and paleosol organic matter : oxidation of methane in soils versus elevated soil respiration rates
(2016-05) Cacciatore, Christopher Guy; Breecker, Dan O.; Shanahan, Timothy M; Cotton, Jennifer M
The Paleocene-Eocene Thermal Maximum (PETM) was a rapid global warming event at ~56 Ma that was driven by a rapid release of carbon into the ocean-atmosphere system. The most recognizable feature marking the PETM in the rock record is a negative carbon isotope excursion (CIE) recorded in organic carbon and calcium carbonates deposited in both marine and terrestrial environments. Differences among excursion magnitudes (ΔCIE) exist between marine and terrestrial proxies, and between carbonates and organic carbon. We evaluated the plausibility of two hypothetical mechanisms behind the observed ~ 1.9‰ difference between the magnitude of the CIE as recorded by paleosol carbonate and paleosol organic matter (ΔCIEpc-som). Specifically, we test whether 1) oxidation within soils of isotopically light methane or 2) increases in soil respiration rates are plausible explanations for the observed ΔCIE. A production-diffusion model used to simulate carbon isotope compositions of soil CO₂ and paleosol carbonates is coupled with a box model that constrains methane flux from hydrates into atmosphere. The box model simulates atmospheric CO₂ concentrations, the δ¹³C values of atmospheric CO₂ and of plants, and the methane flux into soils, which are all used in the production-diffusion model to simulate the δ¹³C value of paleosol carbonate. Given conservative prior distributions for model inputs grounded in previous empirical studies, model output demonstrates that oxidation of atmospheric methane in soil pore space is unlikely to cause the ΔCIEpc-som even for rapid methane release rates. However, increased respiration rates during the PETM could explain the observed ΔCIE, with a minimum approximate doubling of respiration rates required to reproduce a ΔCIEpc-som ≥ 2‰.
Multidecadal rainfall variability in the South Pacific convergence zone using the geochemistry of stalagmites from the Solomon Islands
(2016-08) Sekhon, Natasha; Quinn, Terrence M.; Partin, Judson W.; Jackson, Charles S.; Shanahan, Timothy M.
The instrumental record of hydroclimate variability in the South Pacific is sparse and of short duration; rainfall observations span on average 60-70 years, albeit, non-continuously. Therefore, proxy records of rainfall variability are needed to extend the hydroclimate record into the pre-instrumental period. In this study, we investigate the multidecadal timescale variability in rainfall using stable isotopic variations in three absolutely dated stalagmites (cave deposits) from the Central Province (CP) and Western Province (WP) of the Solomon Islands (SB) (~9.5°S, ~160°E). Hydroclimate variability in the SB is associated with the position and intensity of rainfall of the South Pacific Convergence Zone (SPCZ). The relatively high temporal resolution of the three stalagmites directs the focus on decadal-scale variability using oxygen isotope (δ18O) variations in the stalagmites, which are a proxy for rainfall variability in the tropics. We compare the three Solomon Island (SB) stalagmite δ18O records with previously published stalagmite δ18O records from Santo, Vanuatu and Guadalcanal, Central Province, Solomon Islands (CPSB) to assess the regional coherency amongst the various proxy rainfall records. The three new SB stalagmite δ18O records from this study are dominated by abrupt and large amplitude changes (~1-2‰) on multidecadal timescales. Similar patterns of multidecadal stalagmite δ18O variability have previously been identified in the stalagmite δ18O from Guadalcanal (CPSB) and Vanuatu and implies that multidecadal rainfall variability occurred across the South Pacific Ocean over the last millennium. However, the multidecadal changes do not occur in phase with time, which is in part due to age errors, but is also greatly influenced by the different seasonality of rainfall across the sites that bias which months of rainfall change will be recorded by a stalagmite. Our results also show that large changes in δ18O may not necessarily translate to equally large changes in rainfall amount as the modern rainfall - δ18O relationships at the various sites do not have equivalent slopes on multidecadal timescales. After deriving rainfall- δ18O transfer functions for the different sites, we observe that South Pacific stalagmite δ18O records translate to roughly similar amounts of rainfall change on multidecadal time scales for Guadalcanal, CPSB, Suku, CPSB and Santo, Vanuatu. However, Western Province, Solomon Island (WPSB) site appears to have the largest rainfall amount change (~2.0m/year) on multidecadal timescale. We also demonstrate that it is challenging to precisely date decadal and multidecadal scale variability in young stalagmites (past 1000 years) that have very low 238U concentrations and are very short in length. These characteristics lead to larger temporal uncertainty than solely analytical error. This will be of importance to future studies that may overlook the difficulties with young and short stalagmites records with low 238U concentrations.
Paleodistributions and Comparative Molecular Phylogeography of Leafcutter Ants (Atta spp.) Provide New Insight into the Origins of Amazonian Diversity
(Public Library of Science, 2008-07-23) Solomon, Scott E.; Bacci, Mauricio Jr; Martins, Joaquim Jr; Vinha, Giovanna Gonçalves; Mueller, Ulrich G.
The evolutionary basis for high species diversity in tropical regions of the world remains unresolved. Much research has focused on the biogeography of speciation in the Amazon Basin, which harbors the greatest diversity of terrestrial life. The leading hypotheses on allopatric diversification of Amazonian taxa are the Pleistocene refugia, marine incursion, and riverine barrier hypotheses. Recent advances in the fields of phylogeography and species-distribution modeling permit a modern re-evaluation of these hypotheses. Our approach combines comparative, molecular phylogeographic analyses using mitochondrial DNA sequence data with paleodistribution modeling of species ranges at the last glacial maximum (LGM) to test these hypotheses for three co-distributed species of leafcutter ants (Atta spp.). The cumulative results of all tests reject every prediction of the riverine barrier hypothesis, but are unable to reject several predictions of the Pleistocene refugia and marine incursion hypotheses. Coalescent dating analyses suggest that population structure formed recently (Pleistocene-Pliocene), but are unable to reject the possibility that Miocene events may be responsible for structuring populations in two of the three species examined. The available data therefore suggest that either marine incursions in the Miocene or climate changes during the Pleistocene—or both—have shaped the population structure of the three species examined. Our results also reconceptualize the traditional Pleistocene refugia hypothesis, and offer a novel framework for future research into the area.
Surface-ocean variability in the northern Gulf of Mexico during the late Holocene
(2016-05) Thirumalai, Kaustubh Ramesh; Quinn, Terrence M.; Shanahan, Timothy M; Gulick, Sean; Okumura, Yuko; Partin, Judson W; Frohlich, Cliff
The surface waters of the Gulf of Mexico (GOM) are a major moisture source for North America and play an important role in modulating the hydroclimate of the region. Predictions of future changes in surface-ocean variability in the GOM and hydroclimatic changes in response to greenhouse gas forcing must be placed in context of past changes. However, the instrumental record of sea-surface temperature (SST) and salinity (SSS) observations in the GOM is too brief to examine climate variability on multidecadal-to-centennial timescales; thus, proxy records of SST and SSS variability as encoded in marine sedimentary archives must be used to extract information about climate change on these timescales. In this work, I produce a near-decadal-resolution record of SST and SSS variability in the northern GOM over the last 4,400 years. These paleo-records are based on the measurement of the stable isotopic and trace metal composition of planktic foraminifer Globigerinoides ruber (White) shells in a suite of multicores from the Garrison Basin, northern GOM (26º 40.19’N,93º 55.22’W). The fidelity of this proxy is assessed by performing geochemical measurements on in-situ samples from a nearby sediment trap and by performing statistical data-model comparisons with a foraminiferal forward model that can simulate different calcification depth habitats and seasonal productivity. Next, I construct a computational algorithm that characterizes uncertainty in foraminiferal reconstructions including age, analytical, calibration, ecological, sampling, and preservation errors. The utility of this algorithm is shown by applying it to several previously published records. It is also used to place the new Garrison Basin SST and SSS reconstructions in a quantitative uncertainty framework. I diagnose the controls of multidecadal-to-centennial-scale SST and SSS variability in the GOM (and establish its relationship with Atlantic Ocean circulation) by performing correlation analyses using observations, reanalysis datasets, and transient models. Several other marine and terrestrial proxy records spanning the last millennium are synthesized to document a coordinated linkage between Atlantic Ocean circulation and Western Hemisphere precipitation. This work hypothesizes that a century-scale linkage between ocean circulation and precipitation variability occurred over the last millennium, and perhaps the late Holocene, thereby providing a new perspective on long-term climate change.
The response of central Texas cave drip sites to extreme events : implications for paleoclimatology
(2019-05) Bunnell, Kendra Elizabeth; Banner, Jay L.
As global climate changes, extreme weather events such as droughts and floods are hypothesized to become more frequent and extreme. Therefore, understanding the geographic range, intensity, and frequency of extreme events in the past is important to provide context for future changes. Speleothems have the potential to record these events. This study investigates the response of central Texas deep-cave drip sites to extreme weather events. Deep caves are defined in this study as those caves with high winter speleothem calcite growth rates and lower summer speleothem calcite growth rates. From the resulting observations, it was hypothesized how such events may be preserved in a speleothem record, if at all. To investigate this, the drip rates and oxygen isotope (δ¹⁸O) responses of cave drip water sites, as well as calcite growth rates for calcite precipitated on artificial substrates below the drip sites, of eleven sites in four central Texas caves over eight years (2010-2018) were analyzed. The study period includes three extreme events: a historic drought (2011), a hurricane (2017), and a historically wet fall season (2018). Using a maximum drip rate versus coefficient of variability curve the sites used in this study were defined as diffuse (long water residence-time in the epikarst), conduit (short residence time), or intermediate (intermediate residence time). A weighted running mean model using the unique isotopic signature of Hurricane Harvey (2017) was used to assign residence times. Diffuse sites were shown to have a 34.5-month residence time on average, conduit sites were shown to have a residence time of 18 months on average, and intermediate sites have a 27-month average residence time. This study shows that these short-term events are only likely to be preserved in conduit to intermediate sites. Events that occur in the summer, a regional period of slow to no calcite deposition or speleothem growth, are less likely to be preserved. As would be expected, those events that last longer, or have a longer recovery time, are more likely to be preserved. The most likely preservation styles of the studied events, based on a combination of the magnitude and duration of the drip water response, as well as the statistical significance and commonality of the response, are as follows: 1) droughts show statistically significant longer-than-average periods of low to no growth, with no isotopic response , at 2 of the 11 sites; 2) tropical storms show an abrupt and historic isotopic low (drip water depleted in δ¹⁸O) followed by a multi-month return to baseline isotopic values with no growth rate response, seen at 2 of the 11 sites; and 3) abnormally wet periods show a statistically significant isotopic low, observed at 5 of the 11 sites, which can be coupled with a growth rate increase, this combination was observed at 2 of the 5 sites showing a significant isotopic low. These results indicate that in paleoclimatology studies investigating extreme climatic events, deep-cave temperate speleothems chosen should be those that are supplied by conduit drip sites and have higher growth rates. Monitoring studies of cave drip sites can be used to identify sites that fit these criteria and thus have application to paleoclimate studies.