[Invited] Atmosphere-ocean changes in the Pacific Southern Ocean over the past 1 Million years and implications for global climate

第6回極域科学シンポジウム分野横断セッション：[IG] 全球環境変動を駆動する南大洋・南極氷床11月17日（火）　国立極地研究所 2階 大会議

Bottom‐Current Variability and the Relationship With Topography and Sedimentary Processes in the Drake Passage

Bottom-current related sediments have been commonly used for paleoceanographic reconstructions. However, the strength and variability of bottom currents are poorly understood and thus the processes that control sedimentation in deep environments are not clear. In this study, we focus on the Drake Passage, which is connected to the Antarctic Circumpolar Current, that has a major impact on the global climate. We studied the intensity and variability of bottom currents and how they are related to sedimentary processes. For this purpose, we used 27-years from GLORYS12 Mercator Ocean reanalysis at high resolution to evaluate the bottom current dynamics. Geophysical data and surface grain size measurements were used to identify the type of sediment deposits. Our results show that the dynamics of bottom currents is disconnected from the sea surface dynamics, and bottom circulation is strongly controlled by the rough topography of the Drake Passage. The patterns for the first modes of bottom-current variability are related to the local topography and seem to generally control the distribution of contourites. The second and third EOF modes show patterns in the bottom currents that differ from the mean field, and they may affect the rate of erosion and deposition differently. Time series of bottom currents reveals multiple high-speed current events, but contourite drifts seem to accumulate preferentially in zones of slow and stable bottom currents. Our study highlights the potential of using ocean reanalysis to better constrain bottom currents in zones of scarce data and to plan future campaigns of direct measurements

Holocene Event Record of Aysen Fjord (Chilean Patagonia): An Interplay of Volcanic Eruptions and Crustal and Megathrust Earthquakes

In the first months of 2007, the Aysen region in southern Chile was affected by a crustal seismic swarm. Its largest earthquake (M-w 6.2) occurred in April and had its epicenter in Aysen Fjord. Seismic intensities became so high that hundreds of onshore mass movements were triggered, several of which entered into the fjord, resulting in mass transport deposits (MTDs) preserved at the fjord bottom. Here we present a Holocene record of paleo-earthquakes in the previously unstudied Patagonian fjordland based on MTD stratigraphy. High-resolution seismic data retrieved using two different seismic systems (sparker and TOPAS) reveal multiple older MTDs on different stratigraphic levels. Correlation of the seismic stratigraphy with sedimentological data obtained from a long Calypso core (MD07-3117) allows conclusion on the seismic origin of these deposits. Additionally, radiocarbon dating permits constructing an age model, validated by tephrochronology, providing an age for the different MTD levels. We thus present a highly detailed paleoseismological history of the Aysen region, including at least six major Holocene earthquakes, one of which is likely related to a known megathrust earthquake. Other earthquakes are related to activity of the Liquine-Ofqui Fault Zone (LOFZ), forming the main source of seismic hazard in the area. We can infer a general average recurrence time for LOFZ earthquakes of -2,100years in the vicinity of Aysen Fjord with clustered events during the early and late Holocene. Finally, we argue that cascading events (causal link between volcanic and seismic events) may be a frequent phenomenon along the LOFZ

Evidence for late glacial oceanic carbon redistribution and discharge from the Pacific Southern Ocean

Southern Ocean deep-water circulation plays an important role in the global carbon cycle. On geological time-scales, upwelling along the Chilean continental margin likely contributed to the deglacial atmospheric carbon dioxide rise, but little quantitative evidence exists of carbon storage. Here, we use a new X-ray Micro-Computer-Tomography method to assess foraminiferal test dissolution as proxy for paleo-carbonate ion concentrations [CO3^2−]. Our subantarctic Southeast Pacific sediment core depth transect shows significant deep-water [CO3^2−] variations during the Last Glacial Maximum and Deglaciation (10 – 22 ka BP). We provide evidence for an increase in [CO3^2−] during the early deglacial period (15-19 ka BP), followed by a ca. 40 µmol kg^-1 reduction in Lower Circumpolar Deepwater (CDW). This decreased Pacific to Atlantic export of low-carbon CDW contributed to significantly lowered carbon storage within the Southern Ocean, highlighting the importance of a dynamic Pacific–Southern Ocean deep-water reconfiguration for shaping late-glacial oceanic carbon storage, and subsequent deglacial oceanic-atmospheric CO2 transfer

Late Quaternary terrigenous sediment supply in the Drake Passage in response to Patagonian and Antarctic ice dynamics

The Drake Passage, as the narrowest passage around Antarctica, exerts significant influences on the physical, chemical, and biological interactions between the Pacific and Atlantic Ocean. Here, we identify terrigenous sediment sources and transport pathways in the Drake Passage region over the past 140 ka BP (thousand years before present), based on grain size, clay mineral assemblages, geochemistry and mass-specific magnetic susceptibility records. Terrigenous sediment supply in the Drake Passage is mainly derived from the southeast Pacific, southern South America and the Antarctic Peninsula. Our results provide robust evidence that the Antarctic Circumpolar Current (ACC) has served as the key driver for sediment dispersal in the Drake Passage. High glacial mass accumulation rates indicate enhanced detrital input, which was closely linked to a large expansion of ice sheets in southern South America and on the Antarctic Peninsula during the glacial maximum, as significantly advanced glaciers eroded more glaciogenic sediments from the continental hinterlands into the Drake Passage. Moreover, lower glacial sea levels exposed large continental shelves, which together with weakened ACC strength likely amplified the efficiency of sediment supply and deposition in the deep ocean. In contrast, significant glaciers' shrinkage during interglacials, together with higher sea-level conditions and storage of sediment in nearby fjords reduced terrigenous sediment inputs. Furthermore, a stronger ACC may have induced winnowing effects and further lowered the mass accumulation rates. Evolution of ice sheets, sea level changes and climate related ACC dynamic have thus exerted critical influences on the terrigenous sediment supply and deposition in the Drake Passage region over the last glacial-interglacial cycle

Precipitation as the main driver of Neoglacial fluctuations of Gualas glacier, Northern Patagonian Icefield

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Climate of the Past 8 (2012): 519-534, doi:10.5194/cp-8-519-2012.Glaciers are frequently used as indicators of climate change. However, the link between past glacier fluctuations and climate variability is still highly debated. Here, we investigate the mid- to late-Holocene fluctuations of Gualas Glacier, one of the northernmost outlet glaciers of the Northern Patagonian Icefield, using a multi-proxy sedimentological and geochemical analysis of a 15 m long fjord sediment core from Golfo Elefantes, Chile, and historical documents from early Spanish explorers. Our results show that the core can be sub-divided into three main lithological units that were deposited under very different hydrodynamic conditions. Between 5400 and 4180 cal yr BP and after 750 cal yr BP, sedimentation in Golfo Elefantes was characterized by the rapid deposition of fine silt, most likely transported by fluvio-glacial processes. By contrast, the sediment deposited between 4130 and 850 cal yr BP is composed of poorly sorted sand that is free of shells. This interval is particularly marked by high magnetic susceptibility values and Zr concentrations, and likely reflects a major advance of Gualas glacier towards Golfo Elefantes during the Neoglaciation. Several thin silt layers observed in the upper part of the core are interpreted as secondary fluctuations of Gualas glacier during the Little Ice Age, in agreement with historical and dendrochronological data. Our interpretation of the Golfo Elefantes glaciomarine sediment record in terms of fluctuations of Gualas glacier is in excellent agreement with the glacier chronology proposed for the Southern Patagonian Icefield, which is based on terrestrial (moraine) deposits. By comparing our results with independent proxy records of precipitation and sea surface temperature, we suggest that the fluctuations of Gualas glacier during the last 5400 yr were mainly driven by changes in precipitation in the North Patagonian Andes.This research was supported by an EU FP6 Marie Curie Outgoing Fellowship to S.B. Cruise NBP0505 was funded by the US National Science Foundation, Office of Polar Programs grant number NSF/OPP 03-38137 to J. Anderson (Rice University) and J. Smith Wellner (University of Houston). The Cimar-7 Program was supported by the Chilean National Oceanographic Committee (CONA, Grant C7F 01-10 to S. Pantoja)

The Ventilation and Circulation of the Southern Indian Ocean on Glacial / Interglacial Timescales

With this project, we want to enhance our knowledge of the global carbon cycle on glacial/ interglacial time-scales. To achieve this objective, it is of crucial importance to understand the role of the Southern Ocean on the release and uptake of greenhouse gases. As the southern Indian Ocean is currently fundamentally underrepresented in paleoceanographic reconstructions, it is our aim to reconstruct the contribution of this ocean to the atmospheric pattern of CO2. Therefore, we plan to use a novel multiproxy-approach, combining stable (δ13C) and radiogenic (d14C) isotope reconstructions with analyses of B/Ca-derived carbonate ion concentrations on a sediment core depth transect of the Kerguelen Islands. These analyses will provide a detailed insight into the history of water mass ventilation in the Indian Ocean on glacial/interglacial timescales. Ultimately, we want to combine the findings of this project with other water mass ventilation studies (e.g. Skinner et al., 2010; Sarnthein et al., 2013; Ronge et al., under review) and Earth System Modeling. These findings, in combination with previous studies from the Atlantic and Pacific Oceans will for the first time allow a comprehensive reconstruction of CO2-enriched deep-water during the last glacial, the ventilation throughout the deglaciation and the contribution to the atmospheric CO2-level