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Abstract Drill cores from the Antarctic continental shelf are essential for directly constraining changes in past Antarctic Ice Sheet extent. Here, we provide a sedimentary facies analysis of drill cores from International Ocean Discovery Program (IODP) Site U1521 in the Ross Sea, which reveals a unique, detailed snapshot of Antarctic Ice Sheet evolution between ca. 18 Ma and 13 Ma. We identify distinct depositional packages, each of which contains facies successions that are reflective of past baseline shifts in the presence or absence of marine-terminating ice sheets on the outermost Ross Sea continental shelf. The oldest depositional package (>18 Ma) contains massive diamictites stacked through aggradation and deposited in a deep, actively subsiding basin that restricted marine ice sheet expansion on the outer continental shelf. A slowdown in tectonic subsidence after 17.8 Ma led to the deposition of progradational massive diamictites with thin mudstone beds/laminae, as several large marine-based ice sheet advances expanded onto the mid- to outer continental shelf between 17.8 Ma and 17.4 Ma. Between 17.2 Ma and 15.95 Ma, packages of interbedded diamictite and diatom-rich mudstone were deposited during a phase of highly variable Antarctic Ice Sheet extent and volume. This included periods of Antarctic Ice Sheet advance near the outer shelf during the early Miocene Climate Optimum (MCO)—despite this being a well-known period of peak global warmth between ca. 17.0 Ma and 14.6 Ma. Conversely, there were periods of peak warmth within the MCO during which diatom-rich mudstones with little to no ice-rafted debris were deposited, which indicates that the Antarctic Ice Sheet was greatly reduced in extent and had retreated to a smaller terrestrial-terminating ice sheet, most notably between 16.3 Ma and 15.95 Ma. Post-14.2 Ma, diamictites and diatomites contain unambiguous evidence of subglacial shearing in the core and provide the first direct, well-dated evidence of highly erosive marine ice sheets on the outermost continental shelf during the onset of the Middle Miocene Climate Transition (MMCT; 14.2–13.6 Ma). Although global climate forcings and feedbacks influenced Antarctic Ice Sheet advances and retreats during the MCO and MMCT, we propose that this response was nonlinear and heavily influenced by regional feedbacks related to the shoaling of the continental shelf due to reduced subsidence, sediment infilling, and local sea-level changes that directly influenced oceanic influences on melting at the Antarctic Ice Sheet margin. Although intervals of diatom-rich muds and diatomite indicating open-marine interglacial conditions still occurred during (and following) the MMCT, repeated advances of marine-based ice sheets since that time have resulted in widespread erosion and overdeepening in the inner Ross Sea, which has greatly enhanced sensitivity to marine ice sheet instability since 14.2 Ma. 

Abstract We present a full characterization of a 20 cm‐thick tephra layer found intercalated in the marine sediments recovered at Site U1524 during International Ocean Discovery Program (IODP) Expedition 374, in the Ross Sea, Antarctica. Tephra bedforms, mineral paragenesis, and major‐ and trace‐element composition on individual glass shards were investigated and the tephra age was constrained by⁴⁰Ar‐³⁹Ar on sanidine crystals. The⁴⁰Ar‐³⁹Ar data indicate that sanidine grains are variably contaminated by excess Ar, with the best age estimate of 1.282 ± 0.012 Ma, based on both single‐grain total fusion analyses and step‐heating experiments on multi‐grain aliquots. The tephra is characterized by a very homogeneous rhyolitic composition and a peculiar mineral assemblage, dominated by sanidine, quartz, and minor aenigmatite and arfvedsonite‐riebeckite amphiboles. The tephra from Site U1524 compositionally matches with a ca. 1.3 Ma, rhyolitic pumice fall deposit on the rim of the Chang Peak volcano summit caldera, in the Marie Byrd Land, located ca. 1,300 km from Site U1524. This contribution offers important volcanological data on the eruptive history of Chang Peak volcano and adds a new tephrochronologic marker for the dating, correlation, and synchronization of marine and continental early Pleistocene records of West Antarctica. 

International Ocean Discovery Program (IODP) Expedition 374 drilled Site U1521 (75°41.0′S, 179°40.3′W; 562 m water depth) on the middle to outer continental shelf in the Pennell Basin, Ross Sea (Figure F1), to test hypotheses related to Neogene evolution of the West Antarctic Ice Sheet, particularly in the Miocene (McKay et al., 2019a). Site U1521 was drilled to a total depth of 650.1 meters below seafloor (mbsf) and recovered 411.5 m (63% recovery) of Lower Miocene to Pleistocene sediments. The sedimentary sequence includes seven lithostratigraphic units (McKay et al., 2019b) containing diamictites, mudstones, diatom-rich mudstones, and diatomites that provide insight into past depositional environments (Marschalek et al., 2021; Pérez et al., 2022; McKay et al., 2024). X-ray fluorescence (XRF) core scanning of Site U1521 sediments provides semiquantitative data useful for further characterizing sedimentary units, as well as for paleoceanographic and terrigenous provenance studies (e.g., Grützner et al., 2003; Monien et al., 2012; Cook et al., 2013; Patterson et al., 2014; Croudace and Rothwell, 2015; Bertram et al., 2018; Penkrot et al., 2018; Wilson et al., 2018; Jimenez-Espejo et al., 2020). Here, we report XRF data from Site U1521. Raw XRF counts are influenced by sediment density and atomic weight (e.g., Tjallingii et al., 2007; Lyle et al., 2012; Kasuya et al., 2023) such that elements with higher atomic weights (e.g., Fe and Ba) emit higher energy X-rays compared to elements with lower atomic weights (e.g., Al and Si), resulting in attenuation of signals for elements with lower atomic weights (Tjallingii et al., 2007). To account for these effects, XRF data were (1) scaled to the median elemental concentrations of sediment collected from Site U1521, which were measured shipboard using inductively coupled plasma–optical emission spectroscopy (ICP-OES) to transform peak areas to weight percents and (2) normalized to eliminate variations caused by changes in sediment density, porosity, and cracking (Lyle et al., 2012). Normalized median-scaled (NMSc) XRF data are plotted against shipboard lithology and natural gamma radiation (NGR), which reflects relative amounts of fine-grained terrigenous sediment (clays) (Figure F2) (McKay et al., 2019b; Blum, 1997). 

International Ocean Discovery Program (IODP) Expedition 374 drilled Site U1521 on the outer shelf of the Ross Sea, Antarctica. Site U1521 is composed of seven lithostratigraphic units consisting of diamictites, mudstones, and diatomites. We present low to moderate resolution records of bulk sediment-derived total organic carbon (TOC), total nitrogen (TN), and carbon and nitrogen isotopes (δ13Corg and δ15N, respectively) from the uppermost ~300 m of Site U1521. Measured TOC values range 0.39–1.55 wt%, and TN values range 0.03–0.14 wt%. δ13Corg values range −28‰ to −22‰, and δ15N ranges 2‰–8‰. Generally, geochemical trends correspond with lithologic variations at Site U1521. Diamictites contain lower %TOC, %TN, and δ13Corg and higher δ15N and TOC/TN ratios. In contrast, diatom-bearing/-rich mudstones contain higher and more variable %TOC, %TN, and δ13Corg and lower and more variable δ15N and TOC/TN. These general trends indicate that organic matter sources shifted as depositional environments changed during the Miocene and Pliocene. 

Marine δ¹⁸O data reveal astronomical forcing of the climate and cryosphere during the Miocene, when atmosphericPco₂was on par with emissions scenarios over the next century. This inspired hypotheses for how Milankovitch cycles, ice-ocean interactions, and greenhouse gases influence ice volume. Mass balance controls for marine and terrestrial ice sheets differ, and proxy data collected far from Antarctica provide valuable but limited insight into regional processes. We evaluate clast abundance data from Antarctic marine sedimentary records, observing a strong signal of eccentricity and precession coincident with a terrestrial ice sheet and a clear obliquity signal at the margins of a marine ice sheet. These analyses are integrated with a synthesis of proxy data, and we argue that high variance in obliquity forcing (mediated and enhanced by the ocean and atmosphere) can inhibit ice sheet growth, even when insolation forcing is conducive to glaciation. This “obliquity disruption” explains cryosphere variability before the existence of large northern hemisphere ice sheets. 

Abstract. The Ross Sea record of the Miocene Climatic Optimum (MCO; ∼ 16.9–14.7 Ma) and the Middle Miocene Climate Transition (MMCT; ∼ 14.7–13.8 Ma) provides critical insights into Antarctic ocean–cryosphere interactions during a time of extreme warmth and subsequent cooling. Here we report on Lower to Middle Miocene foraminiferal assemblages from the International Ocean Discovery Program (IODP) Site U1521 on the outer shelf of the central Ross Sea to identify regional shifts in environmental and water mass conditions and trace continental shelf evolution. We identified seven benthic biofacies clusters, dominated by abundant Globocassidulina subglobosa (a proposed indicator of proto-Circumpolar Deep Water, pCDW), Uvigerina cf. U. fueguina (high productivity and enhanced bottom-water currents), Nonionella spp. (high productivity), or Melonis spp. (high productivity) using a Q-mode cluster analysis to develop preliminary regional paleoenvironmental interpretations. Four unique assemblages, including Globobulimina cf. G. auriculata (high productivity and low oxygen), are also identified. Unit IV (representing the early MCO event) is a short-lived (∼ 80 ka), progradational, clast-poor sandy diamictite, likely deposited during deglaciation; the upper part of Unit IV is transitional with overlying Unit III. Unit IV sediments contain the most persistently abundant and diverse foraminiferal assemblages recovered at U1521 because they are mud-rich and diatom-poor, despite very high sedimentation rates. The benthic assemblages shift between Globocassidulina and Uvigerina dominance, suggesting changes in the pCDW influence relative to productivity and/or current activity. We suggest the abundance of Uvigerina (a shelf-edge proxy) in Unit IV records the northward progradation of the Ross continental shelf at this location during the late Early to Middle Miocene. Unit III (MCO) was deposited in an open-marine setting, evident by the ice-rafted detritus or debris (IRD) clast-free, diatom-rich/diatom-bearing muds. The sporadic nature of foraminiferal abundances in Unit III is likely due to intervals of terrigenous mud alternating with more diatom-rich/diatom-bearing muds. As in Unit IV, the muddier lithologies (higher natural gamma ray (NGR) values) are more likely to preserve calcareous foraminifera, whereas the most diatom-rich sediments (lower NGR values) are more corrosive to carbonate. We interpret the muddier intervals as interglacials with incursions of pCDW, as indicated by increased Globocassidulina subglobosa, and sporadic occurrences of rare warmer-water planktic foraminifera. Collectively, these multiple incursions of warmer-water planktic foraminifera provide evidence for polar amplification in the Ross Sea during the MCO and MMCT. The diatom-rich muds are interpreted as glacials during the MCO with open-marine conditions and higher productivity. The dominance of Globobulimina in the upper part of Unit III corresponds with the carbon maximum of Carbon Maxima 2 (CM2) and low-oxygen conditions in the sediments at ∼ 16.1 Ma. Subsequent glaciation (including Mi2, Miocene Isotope event 2), marine-based ice sheet grounding, and erosion on the shallow shelf are recorded by the widespread Ross Sea Unconformity 4 (RSU4; ∼ 15.95–14.2 Ma) at Site U1521. Unit II (MMCT) likely represents sedimentation in the interval between the RSU4 and the Mi3 (Miocene Isotope event 3) glaciation at ∼ 13.9–13.8 Ma. The benthic biofacies composition of Unit II shows a further increase in neritic taxa, including Elphidium magellanicum and Epistominella vitrea, suggesting continued shoaling of the continental shelf, which facilitated the growth of marine-based ice sheets during the Middle Miocene. Our initial correlation between Site U1521 and the ANtarctic geological DRILLing Project (ANDRILL) site, AND-2A, yields similar environmental interpretations, including peak warm events 3 and 4 during the MCO, supported by the foraminifera and unit lithologies. Suspected glacial intervals during the MCO, including Mi2 at the top of Unit III, correlate well with the reconstructed deep-sea estimates of ice volume changes (seawater δ18Osw record) from the Ocean Drilling Program (ODP) Site 1171 on the South Tasman Rise. 

Abstract. International Ocean Discovery Program (IODP) Expedition 374 sailed to the Ross Sea in 2018 to reconstruct paleoenvironments, track the history of key water masses, and assess model simulations that show warm-water incursions from the Southern Ocean led to the loss of marine-based Antarctic ice sheets during past interglacials. IODP Site U1523 (water depth 828 m) is located at the continental shelf break, northeast of Pennell Bank on the southeastern flank of Iselin Bank, where it lies beneath the Antarctic Slope Current (ASC). This site is sensitive to warm-water incursions from the Ross Sea Gyre and modified Circumpolar Deep Water (mCDW) today and during times of past warming climate. Multiple incursions of subpolar or temperate planktic foraminifera taxa occurred at Site U1523 after 3.8 Ma and prior to ∼ 1.82 Ma. Many of these warm-water taxa incursions likely represent interglacials of the latest Early Pliocene and Early Pleistocene, including Marine Isotope Stage (MIS) Gi7 to Gi3 (∼ 3.72–3.65 Ma), and Early Pleistocene MIS 91 or 90 (∼ 2.34–2.32 Ma) and MIS 77–67 (∼ 2.03–1.83 Ma) and suggest warmer-than-present conditions and less ice cover in the Ross Sea. However, a moderately resolved age model based on four key events prohibits us from precisely correlating with Marine Isotope Stages established by the LR04 Stack; therefore, these correlations are best estimates. Diatom-rich intervals during the latest Pliocene at Site U1523 include evidence of anomalously warm conditions based on the presence of subtropical and temperate planktic foraminiferal species in what likely correlates with interglacial MIS G17 (∼ 2.95 Ma), and a second interval that likely correlates with MIS KM3 (∼ 3.16 Ma) of the mid-Piacenzian Warm Period. Collectively, these multiple incursions of warmer-water planktic foraminifera provide evidence for polar amplification during super-interglacials of the Pliocene and Early Pleistocene. Higher abundances of planktic and benthic foraminifera during the Mid- to Late Pleistocene associated with interglacials of the MIS 37–31 interval (∼ 1.23–1.07 Ma), MIS 25 (∼ 0.95 Ma), MIS 15 (∼ 0.60 Ma), and MIS 6–5e transition (∼ 0.133–0.126 Ma) also indicate a reduced ice shelf and relatively warm conditions, including multiple warmer interglacials during the Mid-Pleistocene Transition (MPT). A decrease in sedimentation rate after ∼ 1.78 Ma is followed by a major change in benthic foraminiferal biofacies marked by a decrease in Globocassidulina subglobosa and a decrease in mud (< 63 µm) after ∼ 1.5 Ma. Subsequent dominance of Trifarina earlandi biofacies beginning during MIS 15 (∼ 600 ka) indicate progressive strengthening of the Antarctic Slope Current along the shelf edge of the Ross Sea during the mid to Late Pleistocene. A sharp increase in foraminiferal fragmentation after the MPT (∼ 900 ka) and variable abundances of T. earlandi indicate higher productivity, a stronger but variable ASC during interglacials, and/or corrosive waters, suggesting changes in water masses entering (mCDW) and exiting (High Salinity Shelf Water or Dense Shelf Water) the Ross Sea since the MPT.