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1. Inventory of ice-rafted clasts and sediment constituents that track with dynamic ice-margin processes and biological productivity, Amundsen Sea region, Antarctica

   Siddoway, Christine S. ( April 2023 , EGU General Assembly 2023)

   Marine sediments, obtained from cores and captures from deep sea and continental shelf sites of West Antarctica, contain rich records of latest Miocene to Present glacial and deglacial processes and conditions at the margin of the West Antarctic ice sheet (WAIS). The materials we are investigating were recovered from a) Resolution Drift on the Amundsen Sea continental rise (water depths >3900m), b)the continental shelf in the Amundsen Sea, Wrigley Gulf, and Sultzberger Bay (water depths <1000m). Resolution Drift cores were drilled by IODP Expedition 379 (Gohl et al., doi:10.14379/iodp.proc.379.2021) in sediments dominated by compacted clay and silty clay, with conglomeratic intervals of ice-rafted detritus (IRD) and downslope deposits. The shelf sediments were recovered by piston core, trigger core, and Smith McIntyre Grab (SMG) during USA research cruises of the RVIB Nathaniel B Palmer (1999, 2000, 2007) and USCGC Glacier (1983). The shelf samples are non-compacted clay, containing abundant cobbles, pebbles and biogenic fragments. Our research focuses upon rock clasts, detrital apatite and zircon, felsic volcanic tephra, and micro-manganese nodules separated from marine and glaciomarine clay. The rock clasts and detrital minerals represent samples of continental crust that we characterise according to rock type, petrology, geochemistry, and geo-thermochronology [U-Pb, (U-Th)/He, and fission track methods]. These characteristics illuminate solid Earth processes, including the development of subglacial topography . We compared clasts’ petrology and age data to the exposed onshore geology and thermochronology of bedrock, and determined that ≥90% of clasts likely originated in West Antarctica. Therefore the materials can be used to assign roughness, erodibility, and heat production factors for subglacial bedrock, which constitute boundary conditions used by ice sheet modelers. Rhyolite ash and fragments provide new evidence for explosive eruptions (dated ca. 2.55 to 2.92 Ma; feldspar 40Ar/39Ar) delivered to sea as airfall, IRD, and possible subglacial water transport. Silicic eruptions produce ash and aerosols that may screen solar energy, and provide bio-available nutrients that produce phytoplankton blooms leading to sequestration of carbon. The rhyolite dates coincide with the end of a Pliocene warm period recorded in IODP379 cores (Gille-Petzoldt et al., 10.3389/feart.2022.976703). Our work in progress seeks to obtain higher resolution geochronology in order to determine whether silicic continental volcanism occurred in response to ice unloading due to deglaciation (cf. Lin et al., 10.5194/cp-18-485-2022) and whether erupted products contributed to latest Pliocene significant cooling and WAIS re-glaciation. Another distinctive sediment constituent is micro-manganese nodules of unusual form. Whereas typical micro-MN nodules are dark, formed of concentric layers, this form is pale in color, ‘barbell’ shaped, and transparent in transmitted light. Scanning electron microscopy shows these to be microcrystalline Mn-oxide with embedded grains of quartz and feldspar, which likely served as seed material. Mn-oxides form by authigenesis at/near the seafloor surface, requiring high oxygen concentrations in the bottom water and low sedimentation rates, generally associated with the end of glacials/during interglacials (Hillenbrand et al. 2021, 10.1029/2021GL093103). Work is in progress to determine whether Mn oxides formed through passive accretion upon seed grains or microbially-mediated precipitation from Mn-oxyhydroxides or colloids, of possible relevance for coastal carbon budgets. https://doi.org/10.5194/egusphere-egu23-9728 

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2. Tephra glass chemistry provides storage and discharge details of five magma reservoirs which fed the 75 ka Youngest Toba Tuff eruption, northern Sumatra

   https://doi.org/10.1002/jqs.3149  

   Pearce, Nicholas J. G. ; Westgate, John A. ; Gualda, Guilherme A. R. ; Gatti, Emma ; Muhammad, Ros F. ( October 2019 , Journal of Quaternary Science)

   The Youngest Toba Tuff contains five distinct glass populations, identified from Ba, Sr and Y compositions, termed PI (lowest Ba) – PV (highest Ba), representing five compositionally distinct pre‐eruptive magma batches that fed the eruption. The PI–PV compositions display systematic changes, with higher FeO, CaO, MgO, TiO₂and lower incompatible element concentrations in the low‐SiO₂PIV/PV, than the high‐SiO₂PI–PIII compositions. Glass shard abundances indicate PIV and PV were the least voluminous magma batches, and PI and PIII the most voluminous. Pressure estimates using rhyolite‐MELTS indicate PV magma equilibrated at ~6 km, and PI magma at ~3.8 km. Glass population proportions in distal tephra and proximal (caldera‐wall) material describe an eruption which commenced by emptying the deepest PIV and PV reservoirs, this being preferentially deposited in a narrow band across southern India (possibly due to jet‐stream and/or plinian eruption transport), and as abundant pumice clasts in the lowermost proximal ignimbrites. Later, shallower magma reservoirs erupted, with PI being the most abundant as the eruption ended, sourcing the majority of distal ash from co‐ignimbrite clouds (PI‐ and PIII‐dominant), where associated ignimbrites isolated earlier (PIV‐ and PV‐rich) deposits. This study shows how analysis of tephra glass compositional data can yield pre‐eruption magma volume estimates, and enable aspects of magma storage conditions and eruption dynamics to be described.

    

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3. Geochronological Constraints on the Volcanic and Topographic Evolution of Central Baja California, Mexico

   Hausback, B ; Schmitt, A ; Heizler, M ; Dolby, G ; Dorsey, R ; Bennett, S ; Darin, M ; Grandy, S ; Sawlan, M ; Gardner, K ; et al ( December 2021 , American Geophysical Union)

   Central Baja California (BC) experienced tectonism and volcanism that shaped the landscape from the Miocene to Recent. One important feature is the San Ignacio trough (SIT) that hosted a marine seaway or embayment and acted as a physical barrier to animal and plant migration. This barrier may be responsible for a well-known break in the DNA, N and S of this region. Central BC has also hosted contemporary voluminous and chemically diverse volcanism. Radiometric ages provide important constraints on the origins and longevity of critical topographic features. The Baja GeoGenomics research group is investigating the nature and timing of Pliocene marine and tidal deposits in the NE-oriented, low-lying SIT, located W of the peninsular divide. These new data reveal that the Sierra San Francisco, a highland volcanic area immediately N of the SIT, is a series of volcanoes constructed of dacitic and andesitic Peleean domes with voluminous lahar and pyroclastic flow deposits. These calcalkaline rocks were previously thought to be subduction-related magmatism and part of the early to middle Miocene (~2412 Ma) Comondu Group. However, zircon U-Pb and 40Ar/39Ar dates yield ages of 11-9 Ma. These data indicate the Sierra San Francisco erupted post-subduction and is not part of the lithologically similar but older Comondu Group. Within the SIT, 12km NE of San Ignacio at 200 m asl, newly mapped marine tidal deposits, informally called the San Regis beds, indicate that the SIT has been significantly uplifted. Mafic scoria interbedded in tidal deposits yield a groundmass 40Ar/39Ar age of about 4.2 0.1 Ma. San Regis tidal beds are unconformably overlain by a rhyolite ash-flow tuff from the Quaternary La Reforma caldera situated to the E, on the Gulf of California coast. The highly mobile ash cloud flowed W into the SIT at least as far as the San Regis beds locality NE of San Ignacio. The tuff yielded a preliminary U-Pb zircon age of 1.09 0.04 Ma and an 40Ar/39Ar anorthoclase age of 1.11± 0.01 Ma. These dates indicate that the ash-flow was one of the latest erupted from the caldera and its distribution was in part controlled by the SIT. In BC genetic diversity along the peninsula appears to change at the latitude of the SIT. Tidal and volcanic deposits suggest this topographic low persisted for over 4Ma and remains a distinctive feature in the topography today. 

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4. Geochronological Constraints on the Volcanic and Topographic Evolution of Central Baja California, Mexico

   Brian Hausback, California State ( October 2021 , AGU Fall 2021 Meeting)

   Central Baja California (BC) experienced tectonism and volcanism that shaped the landscape from the Miocene to Recent. One important feature is the San Ignacio trough (SIT) that hosted a marine seaway or embayment and acted as a physical barrier to animal and plant migration. This barrier may be responsible for a well-known break in the DNA, N and S of this region. Central BC has also hosted contemporary voluminous and chemically diverse volcanism. Radiometric ages provide important constraints on the origins and longevity of critical topographic features. The Baja GeoGenomics research group is investigating the nature and timing of Pliocene marine and tidal deposits in the NE-oriented, low-lying SIT, located W of the peninsular divide. These new data reveal that the Sierra San Francisco, a highland volcanic area immediately N of the SIT, is a series of volcanoes constructed of dacitic and andesitic Peleean domes with voluminous lahar and pyroclastic flow deposits. These calcalkaline rocks were previously thought to be subduction-related magmatism and part of the early to middle Miocene (~24–12 Ma) Comondú Group. However, zircon U-Pb and 40Ar/39Ar dates yield ages of 11-9 Ma. These data indicate the Sierra San Francisco erupted post-subduction and is not part of the lithologically similar but older Comondú Group. Within the SIT, 12km NE of San Ignacio at 200 m asl, newly mapped marine tidal deposits, informally called the San Regis beds, indicate that the SIT has been significantly uplifted. Mafic scoria interbedded in tidal deposits yield a groundmass 40Ar/39Ar age of about 4.2 ± 0.1 Ma. San Regis tidal beds are unconformably overlain by a rhyolite ash-flow tuff from the Quaternary La Reforma caldera situated to the E, on the Gulf of California coast. The highly mobile ash cloud flowed W into the SIT at least as far as the San Regis beds locality NE of San Ignacio. The tuff yielded a preliminary U-Pb zircon age of 1.09 ± 0.04 Ma and an 40Ar/39Ar anorthoclase age of 1.11± 0.01 Ma. These dates indicate that the ash-flow was one of the latest erupted from the caldera and its distribution was in part controlled by the SIT. In BC genetic diversity along the peninsula appears to change at the latitude of the SIT. Tidal and volcanic deposits suggest this topographic low persisted for over 4Ma and remains a distinctive feature in the topography today. 

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5. Discovery of the Youngest Toba Tuff in South Africa; Challenges of Extremely Low Abundance Cryptotephra

   Smith, E ; Johnsen, R ; Ren, M ; Hirniak, J ; Fisher, E ; Campisano, C ; Marean, C. ( July 2021 , Annual VM Goldschmidt Conference)

   null (Ed.)

   The 74 ka Youngest Toba Tuff (YTT) was discovered as cryptotephra in South African archaeological sites at Pinnacle Point (PP) 5-6N, Vleesbaai [1] and Klasies River on the Indian Ocean, and the Diepkloof Rock Shelter on the Atlantic coast nearly 750 km west of PP. The YTT eruption distributed tephra across eastern and southern Africa and provides a widespread isochron useful for dating archaeological deposits, testing age models, and precisely determining the timing of changes in human behavior. At PP, we demonstrated that the MIS 4-5 transition began just before the YTT eruption. Humans thrived both through the YTT event and the changing climate, and important changes in technology occurred just after the Toba eruption [1]. Controversy related to trapped charge age models at the Diepkloof rock shelter [2,3] were resolved by identifying YTT at a location in the stratigraphic section that confirmed the Jacobs et al. [2] model for the site, confirming that technological changes similar to those observed at PP occurred synchronously to those at Diepkloof, not substantially before as suggested by a prior published age model [3]. Processing samples with very low abundance cryptotephra, such as that found in South Africa, is a challenge and requires revision of standard laboratory techniques. Samples with high organic or clay content benefit by being treated with 10% HCl and 3% hydrogen peroxide (H2O2). This step does not degrade shard integrity or affect chemistry and is useful in separating shards from clay and organic particles allowing better recovery in heavy liquids. We also modified the heavy liquid density range from 1.95 - 2.55 to 2.2 - 2.5 g/cm3 to more effectively remove quartz and feldspar as well as biogenic silica. This density range captures shards ranging from rhyolite to dacite in composition. [1] Smith, E. I. et al. Nature 555, 511, 2018 [2] Jacobs, Z. & Roberts, R. G. Journal of Archaeological Science 63, 175-192, 2015. [3] Tribolo, C. et al. Journal of Archaeological Science 40, 3401-3411 2013. 

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