Beschreibung: Highlights • Kuroshio Current proxy was established by statistical analyses on grain size spectrum. • Sr–Nd isotope analyses on Kuroshio grain size spectrum reveals source of Taiwan. • Synchronous shift in ENSO and the North Pacific Gyre is subject to the insolation. • Earth System Modeling results confirm our proxies-indicated Kuroshio Current strength. Abstract The Kuroshio Current (KC) is the northward branch of the North Pacific subtropical gyre (NPG) and exerts influence on the exchange of physical, chemical, and biological properties of downstream regions in the Pacific Ocean. Resolving long-term changes in the flow of the KC water masses is, therefore, crucial for advancing our understanding of the Pacific's role in global ocean and climate variability. Here, we reconstruct changes in KC dynamics over the past 20 ka based on grain-size spectra, clay mineral, and Sr–Nd isotope constraints of sediments from the northern Okinawa Trough. Combined with published sediment records surrounding the NPG, we suggest that the KC remained in the Okinawa Trough throughout the Last Glacial Maximum. Together with Earth-System-Model simulations, our results additionally indicate that KC intensified considerably during the early Holocene (EH). The synchronous establishment of the KC “water barrier” and the modern circulation pattern during the EH highstand shaped the sediment transport patterns. This is ascribed to the precession-induced increase in the occurrence of La Niña-like state and the strength of the East Asian summer monsoon. The synchronicity of the shifts in the intensity of the KC, Kuroshio extension, and El Niño/La Niña-Southern Oscillation (ENSO) variability may further indicate that the western branch of the NPG has been subject to basin-scale changes in wind stress curl over the North Pacific in response to low-latitude insolation. Superimposed on this long-term trend are high-amplitude, large century, and millennial-scale variations during last 5 ka, which are ascribed to the advent of modern ENSO when the equatorial oceans experienced stronger insolation during the boreal winter.

Beschreibung: A distinct porcellanite layer from the Southwest Indian Ridge intercalated in Pleistocene diatom ooze was studied using nondestructive physical property measurements and sedimentological data. This bed was sampled by two piston cores at a water depth of 2615 m. The 3–5 cm thick porcellanite layer appears in the cores at a depth of 6.03 m (Core PS2089-2) and 7.73 m (Core PS2089-1) below the seafloor. Due to its characteristic physical properties the porcellanite bed can be detected with core measurements, and its distribution and lateral extent mapped with echosounding. The physical index properties, wet bulk density and electrical resistivity, increase significantly across this bed. Magnetic susceptibility is used to compare the lithological units of both cores and to distinguish whether resistivity anomalies are caused by a higher amount of terrigenous components or by the presence of porcellanite. The porcellanite has the special characteristic to affect a positive anomaly in resistivity but not in susceptibility. Most marine sediments, in contrast, show a positive correlation of magnetic susceptibility versus electrical resistivity; therefore a combination of electrical resistivity and magnetic susceptibility logs yields a definite detection of the porcellanite bed. Images from the X-ray CT survey indicate that the porcellanite is lithified and brittle and fragmented when the piston corer penetrated the bed.

Beschreibung: Production of biogenic silica and dissolution processes in the water column and surface sediment are important aspects for the investigation and reconstruction of present and past productivity of the ocean. Although the geological record of biogenic silica is often used as a proxy for paleoceanographic processes in the Southern Ocean, little is known about the present regional distribution of biogenic silica flux and accumulation and their relation to primary production in surface waters. Based on more than 130 sediment and pore water samples, the regional differences of the biogenic silica flux to the sea floor of the southern South Atlantic were investigated. In contrast to biogenic silica content, the dissolved Si-flux through the sediment/water interface, caused by intense dissolution of BSi in surface sediments, reflects biogenic production in surface waters. This was inferred by observed increases of Si-fluxes in regions of recurrent polynya formation or in the vicinity of Marginal Ice Zones as at the Weddell-Scotia Sea boundary. In the Scotia Sea, where no benthic fluxes were reported before, we found a considerable burial of biogenic silica and biogenic silica fluxes to the sea floor of ∼800–1300 mmol m-2 a-1. This is a significantly higher flux than derived for the known opal accumulation area in the SE Atlantic, further to the east in the Antarctic Circumpolar Current, where a flux of ∼600–767 mmol m-2 a-1 was observed. This shows that the Scotia Sea is not a gap within the Circumpolar Antarctic Opal Belt as previously assumed. The geochemical budget for different sub-regions of the South Atlantic was considered by a Geographic Information System. In contrast to most previous attempts, this ensures the accurate consideration of the spatial distribution of sampling sites, a crucial aspect for the accuracy of geochemical budgets. For the South Atlantic we calculated the flux of biogenic silica to the sea floor as ∼5.1×1012 mol a-1. Only ∼0.84×1012 mol a-1 is buried in these sediments, which is considerably less than previous estimates.

Beschreibung: Since the Last Glacial Maximum, ice has retreated through the fjords of the South Shetland Islands leaving a valuable record of submarine landforms behind. In this study, glacial landforms and sub-bottom characteristics have been mapped to investigate the late Holocene retreat behaviour of the Fourcade Glacier and to delineate past environmental processes in Potter Cove, King George Island. The comprehensive datasets include high-resolution swath bathymetry, shallow seismic profiling and one sediment core. Moraines, moraine incisions and glacial lineations were mapped on the sea floor in the inner part of the cove, whereas pockmarks, ice scour marks and channel structures were identified in the outer part. Sub-bottom characteristics have been assigned to different acoustic facies types indicating different depositional settings. The results reveal glacial recessions as well as stillstands and potential readvances during the late Holocene. Furthermore, the sediment record indicates that the Fourcade Glacier was situated inside the inner cove during the Little Ice Age (500–100 cal yr bp).

Beschreibung: Highlights • First fission track and (U-Th-Sm)/He data from eastern Marie Byrd Land • First direct dating of Cenozoic WARS activity outside the Ross Sea area • Structural model kinematically linking areas of extended crust within the WARS • Data on paleotopographic evolution providing boundary conditions for glaciation Abstract The West Antarctic Rift System is one of the largest continental rifts on Earth. Because it is obscured by the West Antarctic Ice Sheet, its evolution is still poorly understood. Here we present the first low-temperature thermochronology data from eastern Marie Byrd Land, an area that stretches ~ 1000 km along the rift system, in order to shed light on its development. Furthermore, we petrographically analysed glacially transported detritus deposited in the marine realm, offshore Marie Byrd Land, to augment the data available from the limited terrestrial exposures. Our data provide information about the subglacial geology, and the tectonic and morphologic history of the rift system. Dominant lithologies of coastal Marie Byrd Land are igneous rocks that intruded (presumably early Paleozoic) low-grade meta-sedimentary rocks. No evidence was found for un-metamorphosed sedimentary rocks exposed beneath the ice. According to the thermochronology data, rifting occurred in two episodes. The earlier occurred between ~ 100 and 60 Ma and led to widespread tectonic denudation and block faulting over large areas of Marie Byrd Land. The later episode started during the Early Oligocene and was confined to western Pine Island Bay area. This Oligocene tectonic activity may be linked kinematically to previously described rift structures reaching into Bellingshausen Sea and beneath Pine Island Glacier, all assumed to be of Cenozoic age. However, our data provide the first direct evidence for Cenozoic tectonic activity along the rift system outside the Ross Sea area. Furthermore, we tentatively suggest that uplift of the Marie Byrd Land dome only started at ~ 20 Ma; that is, nearly 10 Ma later than previously assumed. The Marie Byrd Land dome is the only extensive part of continental West Antarctica elevated above sea level. Since the formation of a continental ice sheet requires a significant area of emergent land, our data, although only based on few samples, imply that extensive glaciation of this part of West Antarctica may have only started since the early Miocene.

Beschreibung: Glacial/interglacial changes in Southern Ocean's air-sea gas exchange have been considered as important mechanisms contributing to the glacial/interglacial variability in atmospheric CO2. Hence, understanding past variability in Southern Ocean intermediate to deep water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial/interglacial changes in the global carbon cycle. Our study focused on the glacial/interglacial variability in the vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW). We compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern AAIW and Upper Circumpolar Deep Water (UCDW; 943-2066 m water depth) to monitor changes in water mass circulation spanning the past 350,000 years. We propose that pronounced freshwater input by melting sea ice into the glacial AAIW significantly hampered the downward expansion of southwest Pacific AAIW, consistent with climate model results for the Last Glacial Maximum. This process led to a pronounced upward displacement of the AAIW-UCDW interface during colder climate conditions and therefore to an expansion of the glacial carbon pool.

Beschreibung: This article presents a new comprehensive assessment of the Holocene hydrological variability of Lake Ladoga, northwest Russia. The reconstruction is based on oxygen isotopes of lacustrine diatom silica (δ18Odiatom) preserved in sediment core Co 1309, and is complemented by a diatom assemblage analysis and a survey of modern isotope hydrology. The data indicate that Lake Ladoga has existed as a freshwater reservoir since at least 10.8 cal. ka BP. The δ18Odiatom values range from +29.8 to +35.0‰, and relatively higher δ18Odiatom values around +34.7‰ between c. 7.1 and 5.7 cal. ka BP are considered to reflect the Holocene Thermal Maximum. A continuous depletion in δ18Odiatom since c. 6.1 cal. ka BP accelerates after c. 4 cal. ka BP, indicating Middle to Late Holocene cooling that culminates during the interval 0.8–0.2 cal. ka BP, corresponding to the Little Ice Age. Lake‐level rises result in lower δ18Odiatom values, whereas lower lake levels cause higher δ18Odiatom values. The diatom isotope record gives an indication for a rather early opening of the Neva River outflow at c. 4.4–4.0 cal. ka BP. Generally, overall high δ18Odiatom values around +33.5‰ characterize a persistent evaporative lake system throughout the Holocene. As the Lake Ladoga δ18Odiatom record is roughly in line with the 60°N summer insolation, a linkage to broader‐scale climate change is likely.

Beschreibung: Sedimentary architecture and morphogenetic evolution of a polar bay-mouth gravel-spit system are revealed based on topographic mapping, sedimentological data, radiocarbon dating and ground-penetrating radar investigations. Data document variable rates of spit progradation in reaction to atmospheric warming synchronous to the termination of the last glacial re-advance (LGR, 0.45–0.25 ka BP), the southern hemisphere equivalent of the Little Ice Age cooling period. Results show an interruption of spit progradation that coincides with the proposed onset of accelerated isostatic rebound in reaction to glacier retreat. Spit growth resumed in the late 19th century after the rate of isostatic rebound decreased, and continues until today. The direction of modern spit progradation, however, is rotated northwards compared with the growth axis of the early post-LGR spit. This is interpreted to reflect the shift and strengthening in the regional wind field during the last century. A new concept for the interplay of polar gravel-spit progradation and glacio-isostatic adjustment is presented, allowing for the prediction of future coastal evolution in comparable polar settings.

Beschreibung: The ‘greenhouse climate’ of the Late Cretaceous epoch was one of the Earth’s warmest periods of the past 140 Ma, particularly at high latitudes. However, records allowing insights into terrestrial environmental conditions south of the Antarctic circle during that time are extremely rare. Hence, it remains highly elusive how the sensitive South Polar environment may have been impacted by such an extreme climate. Here we report a unique sedimentary sequence that was recovered with the MeBo-70 sea floor drill rig from the central Amundsen Sea Embayment shelf, West Antarctica. The record contains ~26 m of quartzitic sandstone underlain by a lithified swamp deposit that consists of a ~2 m-long complex and intact network of in-situ fossil plant roots embedded in a mudstone matrix. The lower ~1.5 m of this mudstone contain a highly diverse pollen and spore assemblage, documenting a temperate coastal lowland rain forest environment with mean annual temperatures of 11-15°C at a palaeolatitude of 77°S. Hence, the drill record provides the hitherto southernmost evidence of Cretaceous terrestrial environmental conditions and reveals a ‘greenhouse climate’ that was capable of maintaining a temperate environment much further south than previously known. The predictive capabilities of model simulations for high-latitude climate and environment characteristics for this critical period of Earth’s climatic history can therefore now be evaluated more reliably.