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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 Antarctica’s continental margins pose an unknown submarine landslide-generated tsunami risk to Southern Hemisphere populations and infrastructure. Understanding the factors driving slope failure is essential to assessing future geohazards. Here, we present a multidisciplinary study of a major submarine landslide complex along the eastern Ross Sea continental slope (Antarctica) that identifies preconditioning factors and failure mechanisms. Weak layers, identified beneath three submarine landslides, consist of distinct packages of interbedded Miocene- to Pliocene-age diatom oozes and glaciomarine diamicts. The observed lithological differences, which arise from glacial to interglacial variations in biological productivity, ice proximity, and ocean circulation, caused changes in sediment deposition that inherently preconditioned slope failure. These recurrent Antarctic submarine landslides were likely triggered by seismicity associated with glacioisostatic readjustment, leading to failure within the preconditioned weak layers. Ongoing climate warming and ice retreat may increase regional glacioisostatic seismicity, triggering Antarctic submarine landslides. 

The Maumee River is the primary source for nutrients fueling seasonal Microcystis-dominated blooms in western Lake Erie’s open waters though such blooms in the river are infrequent. The river also serves as source water for multiple public water systems and a large food services facility in northwest Ohio. On 20 September 2017, an unprecedented bloom was reported in the Maumee River estuary within the Toledo metropolitan area, which triggered a recreational water advisory. Here we (1) explore physical drivers likely contributing to the bloom’s occurrence, and (2) describe the toxin concentration and bacterioplankton taxonomic composition. A historical analysis using 10-years of seasonal river discharge, water level, and local wind data identified two instances when high-retention conditions occurred over ≥ 10 d in the Maumee River estuary: in 2016 and during the 2017 bloom. Observation by remote sensing imagery supported the advection of cyanobacterial cells into the estuary from the lake during 2017 and the lack of an estuary bloom in 2016 due to a weak cyanobacterial bloom in the lake. A rapid-response survey during the 2017 bloom determined levels of the cyanotoxins, specifically microcystins, in excess of recreational contact limits at sites within the lower 20 km of the river while amplicon sequencing found these sites were dominated by Microcystis. These results highlight the need to broaden our understanding of physical drivers of cyanobacterial blooms within the interface between riverine and lacustrine systems, particularly as such blooms are expected to become more prominent in response to a changing climate. 

Abstract Oscillations in ice sheet extent during early and middle Miocene are intermittently preserved in the sedimentary record from the Antarctic continental shelf, with widespread erosion occurring during major ice sheet advances, and open marine deposition during times of ice sheet retreat. Data from seismic reflection surveys and drill sites from Deep Sea Drilling Project Leg 28 and International Ocean Discovery Program Expedition 374, located across the present-day middle continental shelf of the central Ross Sea (Antarctica), indicate the presence of expanded early to middle Miocene sedimentary sections. These include the Miocene climate optimum (MCO ca. 17–14.6 Ma) and the middle Miocene climate transition (MMCT ca. 14.6–13.9 Ma). Here, we correlate drill core records, wireline logs and reflection seismic data to elucidate the depositional architecture of the continental shelf and reconstruct the evolution and variability of dynamic ice sheets in the Ross Sea during the Miocene. Drill-site data are used to constrain seismic isopach maps that document the evolution of different ice sheets and ice caps which influenced sedimentary processes in the Ross Sea through the early to middle Miocene. In the early Miocene, periods of localized advance of the ice margin are revealed by the formation of thick sediment wedges prograding into the basins. At this time, morainal bank complexes are distinguished along the basin margins suggesting sediment supply derived from marine-terminating glaciers. During the MCO, biosiliceous-bearing sediments are regionally mapped within the depocenters of the major sedimentary basin across the Ross Sea, indicative of widespread open marine deposition with reduced glacimarine influence. At the MMCT, a distinct erosive surface is interpreted as representing large-scale marine-based ice sheet advance over most of the Ross Sea paleo-continental shelf. The regional mapping of the seismic stratigraphic architecture and its correlation to drilling data indicate a regional transition through the Miocene from growth of ice caps and inland ice sheets with marine-terminating margins, to widespread marine-based ice sheets extending across the outer continental shelf in the Ross Sea.