Application of Cosmogenic Nuclides and Argon Geochronology to Paleoclimate, Paleomagnetism, and Paleohydrology

This dissertation uses argon geochronology and cosmogenic nuclide surface exposure dating methods to address three research questions. The first question concerns a geomagnetic instability recorded in lava flows on the island of Floreana in the Galapagos Archipelago. Changes in the Earth’s magnetic field (intensity and orientation) occur frequently throughout geologic time for reasons not yet fully understood. Accurately dating these changes is important because these events provide critical stratigraphic markers in many natural archives, and constraints on the timing of geomagnetic change help better our understanding of the geodynamo. The lava flow on Floreana records an 86% reduction in field strength and intermediate virtual geomagnetic pole (VGP) orientations. Our age determination of 925.7 ± 4.6 ka (2σ; n = 6; MSWD = 1.23) indicates that this flow and the flows associated with it that record nearly identical VGP’s, record the Santa Rosa Excursion. We confirm that this site records the second reported terrestrial evidence of this geomagnetic excursion. Our age is in agreement with sediment records, which indicate this event spanned 3 kyrs. The Santa Rosa Excursion can now be characterized as a global short-lived geomagnetic anomaly associated with an 86% reduction in field strength in equatorial regions (as compared to modern day field strength). Similarities between the Santa Rosa and Laschamp excursions illustrate that such events can occur when the geomagnetic field is either in its reversed or normal polarity state. The second research question addresses the timing of the most recent polarity transition of Earth's magnetic field. The Matuyama-Brunhes (M-B) polarity transition is the most well-studied of all polarity transitions however it's timing remains in debate. A ~10 kyrs mismatch between volcanic and sedimentary archives that record the reversal have been reported. Important geodynamic insights can be gained by examining the structure of a reversing field through time, and the reliability of such studies requires accurate and precise timing of geomagnetic change. We use argon geochronology to date 13 Tahitian lava flows known to record the M-B reversal. A reconciliation between our new ages and those from previous literature from sedimentary and volcanic archives resolves this apparent temporal mismatch and suggests ~35 kyrs of instability and millennial to centennial timescale changes in VGP orientations prior to the Brunhes chron. This new illustration of the structure of a reversing field through time offers new insights into the short precursor history of a reversing field and the rapid -- if not erratic -- changing field dynamics during periods of low-dipole intensity. The final research question involves establishing the timing of intermittent catastrophic drainage of Glacial Lake Missoula in Montana and the associated large-scale flooding across the Columbia Basin and down the Columbia River to the Pacific Ocean during the last ice age. During the period from ~22,000 to ~13,000 years ago, there were perhaps 100 such flooding events that each lasted perhaps several weeks, with the amount of water in each flood being equivalent to ~20 times all of the world’s river flow combined. Surface exposure ages of boulders deposited by such floods at key geomorphic features can be used to determine flood routing, timing, and magnitude. This data may also suggest a source region of floods through time and help constrain the timing of Okanogan Lobe retreat. Eleven new site ages for flood related geomorphic features reveal the interplay between the Cordilleran Ice Sheet and the megafloods. We show that a large megaflood occurred at ~17.2 ka and traveled through an open northern Columbia River throughway before the advance of the Okanogan Lobe and impoundment of Glacial Lake Columbia. Okanogan Lobe retreat occurred between 14.0 to 14.8 ka and floods that occurred after Okanogan retreat were not as large as those before blockage of the northern Columbia. The relatively late retreat of the Okanogan Lobe as compared to the Puget Lobe may reflect independent controls on ice-sheet behavior. Radiocarbon ages indicating the minimum age of human occupation of the Pacific Northwest of the United States in combination with our most recent site ages suggest humans may have witnessed some of the youngest megafloods, which dramatic character likely influenced their oral traditions

Dating Clinopyroxene Phenocrysts in Submarine Basalts Using ^(40)Ar/^(39)Ar Geochronology

Dating submarine basalts using ^(40)Ar/^(39)Ar geochronology is often hindered by a lack of potassium‐bearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to low‐temperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for ^(40)Ar/^(39)Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112 Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002–0.4, suggesting that other K‐bearing phases hosted within the clinopyroxene are likely degassing to yield the ^(40)Ar/^(39)Ar age information. There are three possible origins for the K and corresponding ^(40)Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (>60% ^(39)Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values >0.05. We conclude that clinopyroxene dating by the ^(40)Ar/^(39)Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents

Dating Clinopyroxene Phenocrysts in Submarine Basalts Using ^(40)Ar/^(39)Ar Geochronology

Dating submarine basalts using ^(40)Ar/^(39)Ar geochronology is often hindered by a lack of potassium‐bearing phenocrystic phases and severe alteration in the groundmass. Clinopyroxene is a common phenocrystic phase in seafloor basalts and is highly resistive to low‐temperature alteration. Here we show that clinopyroxene phenocrysts separated from marine basalts are a viable phase for ^(40)Ar/^(39)Ar incremental heating age determinations. We provide results from a pilot study comprising 16 age experiments from nine clinopyroxene separates, five of which from samples with dated coeval phases. The clinopyroxene ages range from 11.5 to 112 Ma with relatively high uncertainties (ranging from 0.8% to 7.1%; median of 1.9%) compared to more traditional phases. The clinopyroxene age plateaus form at low to moderate temperature steps and are characterized by relatively elevated K/Ca of 0.002–0.4, suggesting that other K‐bearing phases hosted within the clinopyroxene are likely degassing to yield the ^(40)Ar/^(39)Ar age information. There are three possible origins for the K and corresponding ^(40)Ar* including films of trapped melt/nanomineral inclusions along grain defects, secondary melt inclusion bands, or variations in degassing behaviors between lower and higher crystalline Ca pyroxene phases. Regardless of the source of the K, the age determinations are successful with 75% of the experiments producing long plateaus (>60% ^(39)Ar released) with mean square of the weighted deviations ranging from 0.6 to 1.5 and probability of fit values >0.05. We conclude that clinopyroxene dating by the ^(40)Ar/^(39)Ar method has the potential to provide a wealth of information for previously undated, altered seafloor lithologies and continental equivalents

^(10)Be dating of late Pleistocene megafloods and Cordilleran Ice Sheet retreat in the northwestern United States

During the late Pleistocene, multiple floods from drainage of glacial Lake Missoula further eroded a vast anastomosing network of bedrock channels, coulees, and cataracts, forming the Channeled Scabland of eastern Washington State (United States). However, the timing and exact pathways of these Missoula floods remain poorly constrained, thereby limiting our understanding of the evolution of this spectacular landscape. Here we report cosmogenic ^(10)Be ages that directly date flood and glacial features important to understanding the flood history, the evolution of the Channeled Scabland, and relationships to the Cordilleran Ice Sheet (CIS). One of the largest floods occurred at 18.2 ± 1.5 ka, flowing down the northwestern Columbia River valley prior to blockage of this route by advance of the Okanogan lobe of the CIS, which dammed glacial Lake Columbia and diverted later Missoula floods to more eastern routes through the Channeled Scabland. The Okanogan and Purcell Trench lobes of the CIS began to retreat from their maximum extent at ca. 15.5 ka, likely in response to onset of surface warming of the northeastern Pacific Ocean. Upper Grand Coulee fully opened as a flood route after 15.6 ± 1.3 ka, becoming the primary path for later Missoula floods until the last ones from glacial Lake Missoula at 14.7 ± 1.2 ka. The youngest dated flood(s) (14.0 ± 1.4 ka to 14.4 ± 1.3 ka) came down the northwestern Columbia River valley and were likely from glacial Lake Columbia, indicating that the lake persisted for a few centuries after the last Missoula flood