Crustal storage and transit play a critical role in the compositional evolution of arc magmas; however, the enigmatic nature of lower crustal magma storage and early differentiation limit our understanding of the connections between the physical processes of subduction zones and the architecture of the arc crust. We present new geochemical data and applications of existing barometric and chronometric tools to interrogate the mantle source compositions, crustal storage depths, and ascent timescales of a primitive, high-Mg, clinopyroxene-bearing cinder cone, the basaltic andesite of Box Canyon, located in the vicinity of Lassen Volcanic National Park, CA, in the southern Cascades. Petrographic examination in addition to bulk and in situ geochemical analyses (XRF, LA-ICP-MS, and EPMA) of tephra and lava-derived samples reveals co-crystallization of clinopyroxene and olivine as phenocrysts and glomerocrysts with ~ Mg# 80 and > Fo 85 , respectively. Phase equilibria experiments of analogous Cascade Arc magma compositions estimate crustal storage of the observed phase assemblage at pressures in the lower crust > 700 MPa. Reverse zonation in olivine and clinopyroxene phenocryst interiors from core-rim analytical profiles record a lower crustal mafic mixing event. Results from one-dimensional, multi-elemental olivine and clinopyroxene diffusion models fit to these interior mixing zones provide an assessment of available trace element (Ti, La, Yb, Ce, and Nd) diffusion chronometers in clinopyroxene by considering multiple element profiles within two phases that experienced the same pre-eruptive conditions. While multi-elemental and multi-phase diffusion timescales span two to three orders of magnitude, Ni in olivine profiles provide the most-robust estimate of 19.1 ± 8.6 years from mixing-to-eruption. Our results provide new constraints on arc crustal differentiation processes, indicate rapid crustal transit timescales in agreement with a growing database of diffusion-based ascent timescales of basaltic magmas, and demonstrate significant, systematic deviation of diffusion timescales calculated from natural zonation of rare earth elements in clinopyroxene and Ti in clinopyroxene and olivine.
The land surface beneath the Greenland and Antarctic Ice Sheets is isostatically suppressed by the mass of the overlying ice. Accurate computation of the land elevation in the absence of ice is important when considering, for example, regional geodynamics, geomorphology, and ice sheet behaviour. Here, we use contemporary compilations of ice thickness and lithospheric effective elastic thickness to calculate the fully re-equilibrated isostatic response of the solid Earth to the complete removal of the Greenland and Antarctic Ice Sheets. We use an elastic plate flexure model to compute the isostatic response to the unloading of the modern ice sheet loads, and a self-gravitating viscoelastic Earth model to make an adjustment for the remaining isostatic disequilibrium driven by ice mass loss since the Last Glacial Maximum. Feedbacks arising from water loading in areas situated below sea level after ice sheet removal are also taken into account. In addition, we quantify the uncertainties in the total isostatic response associated with a range of elastic and viscoelastic Earth properties. We find that the maximum change in bed elevation following full re-equilibration occurs over the centre of the landmasses and is +783 m in Greenland and +936 m in Antarctica. By contrast, areas around the ice margins experience up to 123 m of lowering due to a combination of sea level rise, peripheral bulge collapse, and water loading. The computed isostatic response fields are openly accessible and have a number of applications for studying regional geodynamics, landscape evolution, cryosphere dynamics, and relative sea level change.
As anthropogenic warming continues to melt continental ice and deliver it to the global oceans, mitigation plans will require more confident projections of sea-level rise. The capability of coupled ice-Earth-sea-level models to predict the amplitude and timing of sea-level rise during this century relies on uncertain calibrations of ice sheet sensitivity to variable amounts of warming in Earth's past DeConto et al., 2021;Fischer et al., 2018). While the mid-Pliocene Epoch has already been used as a calibration target that significantly affects projections of future sea-level change DeConto et al., 2021), the early Pliocene Epoch (5.3-3.6 Ma) has so far received less attention. This time consisted of interglacial global mean temperatures around 4°C higher than today and CO 2 concentrations above 400 ppm (
<div> <p>Bivalve and gastropod shell beds deposited during the Early Pliocene (4.69-5.23 Ma) occur in uplifted outcrops (36 &#8211; 180 m above sea level) along the east coast of Patagonian Argentina. These rock units provide a record of sea level during a geologic period when atmospheric CO2 and temperatures were higher than today. As such, reconstructing the elevation of global mean sea level (GMSL) during this time allows us to better understand how sensitive ice sheets are to increased past and future warming. However, reconstructing GMSL from local sea level indicators is hindered by effects such as mantle dynamic topography and glacial isostatic adjustment (GIA) that cause local sea level to deviate from the global mean. Here we use geodynamic modeling to better understand this complex dynamic setting and quantify the amount of uplift along this coastline.</p> </div><div> <p>Despite being located on a relatively stable passive margin, significant variations in the elevation of the paleo shoreline indicators imply that the underlying convecting mantle is deforming the coastline. In particular, the subduction of the Chile Rise beginning ~18 Ma beneath Patagonia has generated a slab window underneath this region through which hot asthenosphere ascends. However, the former slab is still present deeper in the mantle, which causes a complex interplay between the downwelling slab and the upwelling asthenosphere. To quantify the effects of dynamic topography change since the Pliocene, we run 3D mantle convection simulations using the code ASPECT. We initialize our global model with a composite temperature structure derived from recent tomographic studies and a calibrated parameterization of upper mantle anelasticity. Independent estimates of pressure and temperature from thermobarometric calculations of proximal Pali-Aike xenoliths agree with the thermal structure of the tomography-based Earth model. We back-advect temperature perturbations and extract the resulting change in dynamic topography. Pairing GIA models and a suite of convection simulations in which we vary the viscosity and buoyancy structure with the observed differential paleo shoreline elevations allows us to forward model the most likely scenario for uplift along this coast.</p> </div>
Global geological evidence from marine and terrestrial systems has demonstrated that prior to the onset of vast northern hemisphere glaciation in the Pleistocene, early Pliocene atmospheric CO 2 levels were near 400 p.p.m.v., and global mean temperatures were approximately 3°C warmer than present (Raymo et al., 1996; Pagani et al., 2010). These conditions offer a powerful analogue in the paleoclimate record that can be used to inform modern anthropogenic warming and sea level rise. Well-dated marine lithostratigraphic constraints from the Ross Sea by the ANDRILL program have linked 40-kyr fluctuations in the ice sheet extent and sediment record to orbitally-driven cycles in solar insolation (changes in Earth's obliquity; Naish et al., 2009). This project utilizes Ocean Drilling Program (ODP) cores from Expedition 113 in the Weddell Sea and Integrated Ocean Drilling Program (IODP) and ODP geophysical down-hole well logs from Expeditions 119 and 188 from Prydz Bay, Expedition 178 from the Bellingshausen Sea, and Expedition 318 offshore Wilkes Land to study the connection between nearshore diatomite-diamicton sequences (ANDRILL) and distal circum-Antarctic continental shelf sediments. While the ANDRILL program was able to study glacial cycles by directly identifying lithological facies diagnostic of either glacial advance or retreat, this project uses sediment cores (XRF) and borehole measurements (gamma ray and resistivity) in its analysis. LOCALITIES This study considers four circum-Antarctic marine ODP/ IODP locations: the Weddell Sea, Prydz Bay, the Bellingshausen Sea, and offshore Wilkes Land (see O'Connell, this volume for location map). Expedition 113 Site 697 is located northeast of the Antarctic Peninsula in the northwestern Weddell Sea, close to where the Weddell Gyre joins the Antarctic Circumpolar Current, both of which flow clockwise.
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