Beryllium isotope variations recorded in the Ad´elie Basin, East Antarctica reflect Holocene changes in ice dynamics, productivity, and scavenging efficiency

The Ad´elie Basin is a relatively small (~1600 km2), semi-enclosed continental shelf bathymetric depression located adjacent to the Wilkes Subglacial Basin, a basin underlying a sector of the East Antarctic Ice Sheet that contains ~3–4 m sea level equivalent of ice. Located within the Ad´elie Basin is a ~184 m thick laminated sediment deposit, the Ad´elie Drift, ideal for examining regional changes in ice sheet and ocean dynamics. Here, we examine the ratio of reactive beryllium-10 to reactive beryllium-9 ((10Be/9Be)reac) in a marine sediment core obtained from the Ad´elie Drift to assess these changes during the Holocene epoch (11.7 ka BP to present). The (10Be/9Be)reac record provides insight into changes in freshwater input, primary productivity, and scavenging efficiency, while removing the influence of particle size on 10Be concentration. During the early Holocene, (10Be/9Be)reac ratios indicate increased meltwater discharge from ca. 11.7 to 10 ka BP, as grounded ice retreated from the Ad´elie Basin and adjacent bathymetric highs. After ~10 ka BP, beryllium isotopes are influenced by scavenging efficiency and dilution controlled by ocean currents and accumulation rate, operating alongside meltwater input, suggesting there are additional factors to consider when using (10Be/9Be)reac as a proxy for ice shelf cover and glacial dynamics.Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) 20H00193Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) Japan Society for the Promotion of Science 20J21145 PE17712 P18791New Zealand Ministry of Business, Innovation and Employment (MBIE) ANTA1801Ministry of Science and Innovation, Spain (MICINN)Spanish Government CTM2017-89711-C2-1-PEuropean Union through FEDER fund

Great earthquakes along the Western United States continental margin: implications for hazards, stratigraphy and turbidite lithology

We summarize the importance of great earthquakes (<i>M</i><sub>w</sub> ≳ 8) for hazards, stratigraphy of basin floors, and turbidite lithology along the active tectonic continental margins of the Cascadia subduction zone and the northern San Andreas Transform Fault by utilizing studies of swath bathymetry visual core descriptions, grain size analysis, X-ray radiographs and physical properties. Recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California margins are analyzed using two methods: (1) radiometric dating (<sup>14</sup>C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The H method provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that, on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great earthquakes are approximately 300 yr for the Cascadia subduction zone and 130 yr for the northern San Andreas Fault, which indicates both fault systems are in (Cascadia) or very close (San Andreas) to the early window for another great earthquake. <br><br> On active tectonic margins with great earthquakes, the volumes of mass transport deposits (MTDs) are limited on basin floors along the margins. The maximum run-out distances of MTD sheets across abyssal-basin floors along active margins are an order of magnitude less (~100 km) than on passive margins (~1000 km). The great earthquakes along the Cascadia and northern California margins cause seismic strengthening of the sediment, which results in a margin stratigraphy of minor MTDs compared to the turbidite-system deposits. In contrast, the MTDs and turbidites are equally intermixed on basin floors along passive margins with a mud-rich continental slope, such as the northern Gulf of Mexico. <br><br> Great earthquakes also result in characteristic seismo-turbidite lithology. Along the Cascadia margin, the number and character of multiple coarse pulses for correlative individual turbidites generally remain constant both upstream and downstream in different channel systems for 600 km along the margin. This suggests that the earthquake shaking or aftershock signature is normally preserved, for the stronger (<i>M</i><sub>w</sub> ≥ 9) Cascadia earthquakes. In contrast, the generally weaker (<i>M</i><sub>w</sub> = or <8) California earthquakes result in upstream simple fining-up turbidites in single tributary canyons and channels; however, downstream mainly stacked turbidites result from synchronously triggered multiple turbidity currents that deposit in channels below confluences of the tributaries. Consequently, both downstream channel confluences and the strongest (<i>M</i><sub>w</sub> ≥ 9) great earthquakes contribute to multi-pulsed and stacked turbidites that are typical for seismo-turbidites generated by a single great earthquake. Earthquake triggering and multi-pulsed or stacked turbidites also become an alternative explanation for amalgamated turbidite beds in active tectonic margins, in addition to other classic explanations. The sedimentologic characteristics of turbidites triggered by great earthquakes along the Cascadia and northern California margins provide criteria to help distinguish seismo-turbidites in other active tectonic margins

Sea ice and biological production variability reconstructed in the Adélie Basin, East Antarctica, during the late Holocene

第3回極域科学シンポジウム　横断セッション「海・陸・氷床から探る後期新生代の南極寒冷圏環境変動」11月26日（月）、27日（火） ２階ラウン

The past : a compass for future earth

Antarctic sea ice impacts on the ocean-atmosphere heat and gas fluxes, the formation of deep and intermediate waters, the nutrient distribution and primary productivity, the so-called &#8216;biological carbon pump&#8217;, one of the most active in the global ocean. In this study, we explore the link between sea ice dynamic, biological production and nutrient cycling during the late Holocene (the last 2,000 yrs) in the Adélie Basin, East Antarctica, from the well-dated sediments of the Ocean Drilling Program (ODP) Site U1357. This archive, composed from ~32 meters of seasonal to annual laminated diatomaceous sequences, allows reconstructions at an unprecedented time resolution (5-10 yrs). Our study combines records of diatom census counts and diatom-specific biomarkers (a ratio (D/T) of di- and tri-unsaturated Highly Branched Isoprenoid lipids (HBI)) as indicators of sea ice and biological production changes, XRF data as markers for terrigenous inputs and bulk nitrogen isotopes (d15N) and d15N on chlorins as proxies for reconstructing nitrogen cycle. The diatom and HBI records reveal five distinct periods. From 0 to 350 yrs AD, decreasing occurrences of sea ice-related diatom species (e.g. Fragilariopsis curta + F. cylindrus) together with low D/T values and increasing open ocean diatom species (large centrics, Chaetoceros Resting Spores (CRS)) document a progressive decline of sea ice presence during the year (>9 months per year) with spring melting occurring earlier in the year and autumn sea ice formation appearing later. In contrast, between 350 and 750 yrs AD, high production of open ocean diatom species and low low D/T values and sea ice related species indicate a short duration of sea ice cover (~10 months per year) is illustrated by a pronounced increase of sea ice-associated diatom species and high D/T values. Between ~1400 and 1850 yrs AD, seasonal sea ice strongly declines (<~7 months per year) as a result of early spring melting (increasing CRS production) and late autumn waxing (high occurrences of Thalassiosira antarctica). Longer growing seasons promoted a substantial development of phytoplankton communities (especially large centric diatoms) that conducted to lower D/T values. Consistent with diatom and HBI reconstructions, XRF data show higher Fe/Al and Zr/Al ratios values during inferred warmer periods and lower ratio values during inferred cooler and icier periods, thus supporting a strong impact of the sea ice seasonal cycle on glacial runoffs. The link between sea ice conditions, biological production and nutrient cycling is still being explored and we will discuss its relationship by combining all the cited records cited above with the d15N records that we are currently generated. Based on our results, we find that sea ice dynamic and associated diatom production in the Adélie Basin revealed an opposite climatic trend than that identified in the Northern Hemisphere for the last 2000 years. The 'Little Ice Age' (1400-1850 yrs AD) or the 'Dark Ages' (400-750 yrs AD) corresponded to warmer climate conditions in the Adélie Basin, while the 'Roman Warm Period' (0-350 yrs AD) or the 'Medieval Warm Period' (900-1200 yrs AD) were associated to colder conditions. We therefore emphasize that Northern and Southern Hemisphere climate evolved in anti-phase seesaw pattern during the late Holocene

Paleoceanography and ice sheet variability offshore Wilkes Land, Antarctica – Part 3: Insights from Oligocene–Miocene TEX86-based sea surface temperature reconstructions

The volume of the Antarctic continental ice sheet(s) varied substantially during the Oligocene and Miocene ( 34–5 Ma) from smaller to substantially larger than today, both on million-year and on orbital timescales. However, reproduction through physical modeling of a dynamic response of the ice sheets to climate forcing remains problematic, suggesting the existence of complex feedback mechanisms between the cryosphere, ocean, and atmosphere systems. There is therefore an urgent need to improve the models for better predictions of these systems, including resulting potential future sea level change. To assess the interactions between the cryosphere, ocean, and atmosphere, knowledge of ancient sea surface conditions close to the Antarctic margin is essential. Here, we present a new TEX86- based sea surface water paleotemperature record measured on Oligocene sediments from Integrated Ocean Drilling Program (IODP) Site U1356, offshore Wilkes Land, East Antarctica. The new data are presented along with previously published Miocene temperatures from the same site. Together the data cover the interval between 34 and 11 Ma and encompasses two hiatuses. This record allows us to accurately reconstruct the magnitude of sea surface temperature (SST) variability and trends on both million-year and glacial–interglacial timescales.Julian D. Hartman, Francesca Sangiorgi, Henk Brinkhuis, and Peter K. Bijl acknowledge the NWO Netherlands Polar Program project number 866.10.110. Stefan Schouten was supported by the Netherlands Earth System Science Centre (NESSC), funded by the Dutch Ministry of Education, Culture and Science (OCW). Peter K. Bijl and Francien Peterse received funding through NWO-ALW VENI grant nos. 863.13.002 and 863.13.016, respectively. Carlota Escutia and Ariadna Salabarnada thank the Spanish Ministerio de Econimía y Competitividad for grant CTM2014-60451-C2-1-P. We thank Alexander Ebbing and Anja Bruls for GDGT sample preparation during their MSc research. This research used samples from the Integrated Ocean Drilling Program (IODP). IODP was sponsored by the US National Science Foundation and participating countries under management of Joined Oceanographic Institutions Inc

Multi-decadal trends in Antarctic sea-ice extent driven by ENSO–SAM over the last 2,000 years

Antarctic sea ice has paradoxically become more extensive over the past four decades despite a warming climate. The regional expression of this trend has been linked to changes in vertical redistribution of ocean heat and large-scale wind-field shifts. However, the short length of modern observations has hindered attempts to attribute this trend to anthropogenic forcing or natural variability. Here, we present two new decadal-resolution records of sea ice and sea surface temperatures that document pervasive regional climate heterogeneity in Indian Antarctic sea-ice cover over the last 2,000 years. Data assimilation of our marine records in a climate model suggests that the reconstructed dichotomous regional conditions were driven by the multi-decadal variability of the El Niño Southern Oscillation and Southern Annular Mode (SAM). For example, during an El Niño/SAM– combination, the northward sea-ice transport was reduced while heat advection from the subtropics to the Southern Ocean increased, which resulted in reduced sea-ice extent in the Indian sector as sea ice was compacted along the Antarctic coast. Our results therefore indicate that natural variability is large in the Southern Ocean and suggest that it has played a crucial role in the recent sea-ice trends and their decadal variability in this region.This research was funded by the ERC StG ICEPROXY project (203441), the ANR CLIMICE project, FP7 Past4Future project (243908), the RCN OCTEL project (248776/ E10), the Belgian Research Action through Interdisciplinary Networks Mass2Ant project (BR/165/A2/Mass2Ant), the JSPS KAKENHI (grants 23244102 and 17H06318), the Royal Society Te Apārangi Marsden Fund (MFP-VUW1808) and the MBIE NZ Antarctic Science Platform (ANTA1801). It also benefited from the ESF PolarClimate HOLOCLIP project. D.S. benefited from the Blue-Action project (European Union’s Horizon 2020 Research and Innovation Program, grant number: 727852) and the French LEFE-IMAGO programme. Hole U1357B samples and data were provided by the International Ocean Discovery Program (IODP)

Pliocene deglacial event timelines and the biogeochemical response offshore Wilkes Subglacial Basin, East Antarctica

Significantly reduced ice coverage in Greenland and West Antarctica during the warmer-than-present Pliocene could account for ∼10m of global mean sea level rise. Any sea level increase beyond this would require contributions from the East Antarctic Ice Sheet (EAIS). Previous studies have presented low-resolution geochemical evidence from the geological record, suggesting repeated ice advance and retreat in low-lying areas of the EAIS such as the Wilkes Subglacial Basin. However, the rates and mechanisms of retreat events are less well constrained. Here we present orbitally-resolved marine detrital sediment provenance data, paired with ice-rafted debris and productivity proxies, during three time intervals from the middle to late Pliocene at IODP Site U1361A, offshore of the Wilkes Subglacial Basin. Our new data reveal that Pliocene shifts in sediment provenance were paralleled by increases in marine productivity, while the onset of such changes was marked by peaks in ice-rafted debris mass accumulation rates. The coincidence of sediment provenance and marine productivity change argues against a switch in sediment delivery between ice streams, and instead suggests that deglacial warming triggered increased rates of iceberg calving, followed by inland retreat of the ice margin. Timescales from the onset of deglaciation to an inland retreated ice margin within the Wilkes Subglacial Basin are on the order of several thousand years. This geological evidence corroborates retreat rates determined from ice sheet modeling, and a contribution of ∼3 to 4m of equivalent sea level rise from one of the most vulnerable areas of the East Antarctic Ice Sheet during interglacial intervals throughout the middle to late Pliocene.Provenance analysis was supported by a Kristian Gerhard Jeb-sen PhD Scholarship and NERC UK IODP grants (NE/H025162/1 and NE/H014144/1). Biogenic silica data was supported by a Royal So-ciety of New Zealand Marsden FastStart grant (#UOO-1315) and a University of Otago PhD Scholarship. Support for sedimentol-ogyanalysis was provided by the Royal Society of New ZealandRutherford Discovery Fellowship (RDF-13-VUW-003). XRF work was supported by the Ministry of Science and Innovation Grant CTM2014-60451-C2-1-P co-financed by the European Regional De-velopment Fund (FEDER). Samples were provided by the Integrated Ocean Drilling Program