Michael Bizimis

The drawdown of atmospheric CO ₂ likely triggered Antarctic glaciation during the Eocene‐Oligocene Transition (34 Ma). Accurately constraining this shift from “greenhouse” to an “icehouse” climate is essential for understanding global carbon cycle dynamics. However, conventional proxies often lack the sensitivity to capture the earliest, localized phases of continental ice‐sheet development. In this study, we combined radiogenic hafnium and neodymium isotopes on marine sediments from Ocean Drilling Program Site 689D to track changes in silicate weathering regimes and isolate the enhanced physical weathering signal uniquely associated with glacial grinding. By analyzing the detrital sortable silt fraction, we identified the onset of major Antarctic glaciation between 34.06 and 34.13 Ma. Additionally, our data reveal two distinct precursor glacial events occurring earlier, at approximately 34.19 and 34.41 Ma. These findings provide a more refined chronological framework for one of Earth's most significant climate shifts.

Volcanic whole-rock compositions from Buldir Island define a highly calc-alkaline (low-Fe) igneous series, with molar Mg/Mg + Fe* (total iron Mg#) mostly > 0.60; SiO ₂ from 50–64 wt.%; and distinctly low FeO*, CaO, CaO/Al ₂ O ₃ , and FeO*/Al ₂ O ₃ relative to MgO. Many Buldir samples have trace element characteristics similar to western Aleutian seafloor lavas along upward curving trends of increasing Sr and Sr/Y with increasing SiO ₂ and decreasing MgO. Least radiogenic Sr in andesites only (57–62 wt.% SiO ₂ with ⁸⁷ Sr/ ⁸⁶ Sr ~ 0.7029) requires a key role for source mixing in the origin of the low-Fe series at Buldir. Andesites and dacites falling outside these source mixing trends show evidence of fractional crystallization interpreted to be a late-stage (over-printing) effect. Partitioning data indicate that heavy REEs will behave compatibly in amphibole-saturated melts evolving at temperatures < 1000 °C. However, recent hydrous and high fO ₂ experiments show that Buldir melts with more than 4–5 wt.% MgO were unlikely to have reached amphibole saturation and cannot explain the decline in Yb concentrations starting at >9 wt.% MgO. Distinctive reverse sigmoidal (backward-S) REE pattern shapes in Buldir samples are also unlikely to be products of amphibole removal from evolving liquids and are instead interpreted to reflect melting of a light REE-depleted MORB source under eclogite conditions followed by interaction of the resulting hydrous-felsic slab melt with mantle peridotite upon ascent. Published pMELTS simulations and experimental observations indicate this style of melt-rock reaction will also create low FeO* and CaO in primitive (high-Mg#) melts over a wide range of SiO ₂ and MgO, as observed in Buldir and western Aleutian seafloor datasets. The implication is that distinctive aspects of low-Fe calc-alkalinity in most Buldir samples with SiO ₂ from 50 to 62% are created at sub-MOHO depths where cool-hydrous-siliceous partial melts of subducted-eclogitized MORB rise, decompress, and undergo thermal-chemical exchange with depleted peridotite that is hotter by hundreds of °C.

The temporal relationship between the Laacher See eruption (LSE), the Younger Dryas Boundary (YDB) layer, and the abrupt onset of Younger Dryas (YD) climate change has long been debated. The Körslättamossen fen in southernmost Sweden provides the first robustly dated Scandinavian record demonstrating that these events are clearly separated stratigraphically and temporally. The Laacher See Tephra (LST) layer at 288.5 cm (~13,006 cal BP) lies 6–10 cm below a discrete PGE-rich horizon (282–279 cm) marking YDB deposition, interpreted here as a 3-cm-thick impact fallout layer. This interval has an onset Bayesian-modeled age of 12,861 ± 54 cal BP, ~150 years after the eruption, indicating that the LST and YDB deposition represent separate events. Within this fallout layer, Fe–Si microspherules with dendritic quench textures peak at 282–281 cm, while maximum PGE-bearing nanoparticle mass occurs at 280–279 cm, consistent with delayed settling of submicron particles. LA-ICP-MS reveals Si-rich microspherules containing PGE-bearing microdomains but negligible Ni, Co, and Cr. These compositions indicate that the microspherules are dominated by terrestrial target material rather than meteoritic metal, while the associated PGE-rich microdomains and nanoparticle populations record a minor but detectable extraterrestrial component. Single-particle ICP-TOF-MS detects a sharp Pt anomaly with Pt-dominant elemental ratios (e.g., high Pt relative to Fe and Ir), consistent with the platinum enrichment reported in the YDB in the GISP2 ice core. Immediately above the fallout layer, a dark, organic-rich horizon records the abrupt hydroclimatic shift to cooler, wetter YD conditions. These observations document three stratigraphically distinct horizons: the LSE, a separate YDB fallout layer, and the near-synchronous onset of YD climate change, providing a well-constrained European stratigraphic framework for evaluating mechanisms proposed for the YD onset.

Geoarchaeological investigations at site 44PY152 near Smith Mountain Lake, Virginia, document stratified Late Pleistocene–Early Holocene archaeological occupations preserved within ~1.7 m of floodplain alluvium. These deposits contain both in situ and eroding fluted Paleoamerican and Early Archaic projectile points. Here, we analyze a contiguous, high-resolution sedimentary sequence to evaluate the presence and stratigraphic integrity of microspherule and platinum-group element (PGE) anomalies associated with the onset of the Younger Dryas (YD). Analyses include Bayesian radiocarbon modeling, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS), single-particle inductively coupled plasma time-of-flight mass spectrometry (SP-ICP-TOF-MS), laser ablation ICP-MS (LA-ICP-MS), laser granulometry, bulk organic carbon and nitrogen, soot carbon, stable carbon isotopes (δ¹³C_OC and δ¹³C_soot), magnetic susceptibility, and mercury (Hg). Results reveal a narrow microspherule-rich interval centered within the 14–15 cmbd sample that is accompanied by pronounced PGE mass anomalies consistent with proxy assemblages reported near the Younger Dryas Boundary at multiple sites worldwide. Bayesian age modeling places this interval within the broader temporal range of the Younger Dryas onset, although the modeled age uncertainty exceeds the narrower age range proposed by Kennett et al. Chondrite-normalized elemental systematics indicate selective Pt-centered enrichment decoupled from comparatively conservative rare earth element behavior, arguing against simple sedimentological concentration processes. The stratigraphic position and internal coherence of this proxy suite identify a discrete marker horizon consistent with the onset of the Younger Dryas at 44PY152. Subtle excursions in δ¹³C_soot and Hg immediately above this interval may reflect associated environmental variability during the early Younger Dryas. Regardless of formation mechanism, the microspherule–PGE horizon provides a high-resolution stratigraphic marker confined to a 1-cm interval that may aid correlation and interpretation of pre- and post-12.8 ka archaeological sequences at stratified sites lacking abundant chronometric control.

We present new observations on the dynamics and locations of deep mantle reservoirs derived from the ages and compositions of Voyager Seamount Chain lava flows. The previously unexplored Voyager Seamount Chain trends NW–SE between the Mid‐Pacific Mountains and the Northwestern Hawaiian Ridge. Volcanic samples were recovered from the chain during the Ocean Exploration Trust expedition NA134. The lava flows are alkalic to highly alkalic in composition. Ages ranged from 81 to 86 Ma ( n = 8), with the oldest ages in the NW and an age‐progression toward the SE. The Voyager age‐progression continues southward through the Northern Line Islands region until at least 69 Ma. Mantle flowlines using absolute plate motion models indicate that the Voyagers were emplaced near the modern Marquesas Hotspot location approximately 86–69 Ma. The Sr‐Nd‐Pb‐Hf isotope systematics show the influence of an EMII component and overlap the compositions of Pliocene volcanism from the Marquesas Islands, consistent with the plate motion age model. These data imply a long‐lived plume as the source of the Voyager seamounts and the Marquesas. However, the lack of a clear and continuous seamount chain between the 86–69 Ma Voyager Seamount Chain and the 6–0 Ma Marquesas Islands implies that the mantle plume displays variable buoyancy flux over time. The surface expression of this mantle reservoir experienced a potential hiatus of up to ∼60 m.y. These new data indicate that the mantle beneath the Marquesas Islands region has been discontinuously producing age‐progressive, EMII‐like hotspot volcanism since at least the Late Cretaceous.

The temporal relationship between the Laacher See eruption (LSE), the Younger Dryas Boundary (YDB) layer, and the abrupt onset of Younger Dryas (YD) climate change has long been debated. The Körslättamossen fen in southernmost Sweden provides the first robustly dated Scandinavian record demonstrating that these events are clearly separated stratigraphically and temporally. The Laacher See Tephra (LST) layer at 288.5 cm (~13,006 cal BP) lies 6–10 cm below a discrete PGE-rich horizon (282–279 cm) marking YDB deposition, interpreted here as a 3-cm-thick impact fallout layer. This interval has an onset Bayesian-modeled age of 12,861 ± 54 cal BP, ~150 years after the eruption, indicating that the LST and YDB deposition represent separate events. Within this fallout layer, Fe–Si microspherules with dendritic quench textures peak at 282–281 cm, while maximum PGE-bearing nanoparticle mass occurs at 280–279 cm, consistent with delayed settling of submicron particles. LA-ICP-MS reveals Si-rich microspherules containing PGE-bearing microdomains but negligible Ni, Co, and Cr. These compositions indicate that the microspherules are dominated by terrestrial target material rather than meteoritic metal, while the associated PGE-rich microdomains and nanoparticle populations record a minor but detectable extraterrestrial component. Single-particle ICP-TOF-MS detects a sharp Pt anomaly with Pt-dominant elemental ratios (e.g., high Pt relative to Fe and Ir), consistent with the platinum enrichment reported in the YDB in the GISP2 ice core. Immediately above the fallout layer, a dark, organic-rich horizon records the abrupt hydroclimatic shift to cooler, wetter YD conditions. These observations document three stratigraphically distinct horizons: the LSE, a separate YDB fallout layer, and the near-synchronous onset of YD climate change, providing a well-constrained European stratigraphic framework for evaluating mechanisms proposed for the YD onset.

Geoarchaeological investigations at site 44PY152 near Smith Mountain Lake, Virginia, document stratified Late Pleistocene–Early Holocene archaeological occupations preserved within ~1.7 m of floodplain alluvium. These deposits contain both in situ and eroding fluted Paleoamerican and Early Archaic projectile points. Here, we analyze a contiguous, high-resolution sedimentary sequence to evaluate the presence and stratigraphic integrity of microspherule and platinum-group element (PGE) anomalies associated with the onset of the Younger Dryas (YD). Analyses include Bayesian radiocarbon modeling, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS), single-particle inductively coupled plasma time-of-flight mass spectrometry (SP-ICP-TOF-MS), laser ablation ICP-MS (LA-ICP-MS), laser granulometry, bulk organic carbon and nitrogen, soot carbon, stable carbon isotopes (δ ¹³ C_OC and δ ¹³ C_soot), magnetic susceptibility, and mercury (Hg). Results reveal a narrow microspherule-rich interval centered within the 14–15 cmbd sample that is accompanied by pronounced PGE mass anomalies consistent with proxy assemblages reported near the Younger Dryas Boundary at multiple sites worldwide. Bayesian age modeling places this interval within the broader temporal range of the Younger Dryas onset, although the modeled age uncertainty exceeds the narrower age range proposed by Kennett et al. [1]. Chondrite-normalized elemental systematics indicate selective Pt-centered enrichment decoupled from comparatively conservative rare earth element behavior, arguing against simple sedimentological concentration processes. The stratigraphic position and internal coherence of this proxy suite identify a discrete marker horizon consistent with the onset of the Younger Dryas at 44PY152. Subtle excursions in δ ¹³ C_soot and Hg immediately above this interval may reflect associated environmental variability during the early Younger Dryas. Regardless of formation mechanism, the microspherule–PGE horizon provides a high-resolution stratigraphic marker confined to a 1-cm interval that may aid correlation and interpretation of pre- and post-12.8 ka archaeological sequences at stratified sites lacking abundant chronometric control.

Characterizing the mantle transition zone (MTZ) is essential for understanding Earth’s evolution and dynamics. Petrological, geochemical, and geophysical evidence suggests that the MTZ may host subducted slabs containing sediments and volatiles, potentially forming enriched mantle 1 (EM1) reservoirs. However, the links between volatiles, subduction components, and mantle endmembers stored therein remain unclear. Here we present mercury isotope data for Cenozoic intraplate EM1-type basalts from Northeast Asia, derived from volatile-rich MTZ. These basalts exhibit negative mass-independent fractionation (Δ¹⁹⁹Hg, Δ²⁰¹Hg), and they also display mass-dependent fractionation (δ²⁰²Hg) that correlates with geochemical indicaters of mantle heterogeneity. The results indicate incorporation of recycled ancient terrigenous sediments with anomalous Hg signatures into the MTZ, later tapped during hydrous upwellings or entrained into ascending mantle plumes to form EM1-type basalts. Our findings demonstrate that distinct Hg-isotope anomalies can persist in the MTZ for over a billion years, recording long-term volatile recycling in Earth’s deep mantle.