Wolfgang Kuhnt

Impacts of orbital forcing and atmospheric carbon dioxide on Miocene ice-sheet expansion.

The processes causing the middle Miocene global cooling, which marked the Earths final transition into an ‘icehouse’ climate about 13.9 million years ago (Myr ago), remain enigmatic. Tectonically driven circulation changes and variations in atmospheric carbon dioxide levels have been suggested as driving mechanisms, but the lack of adequately preserved sedimentary successions has made rigorous testing of these hypotheses difficult. Here we present high-resolution climate proxy records, covering the period from 14.7 to 12.7 million years ago, from two complete sediment cores from the northwest and southeast subtropical Pacific Ocean. Using new chronologies through the correlation to the latest orbital model, we find relatively constant, low summer insolation over Antarctica coincident with declining atmospheric carbon dioxide levels at the time of Antarctic ice-sheet expansion and global cooling, suggesting a causal link. We surmise that the thermal isolation of Antarctica played a role in providing sustained long-term climatic boundary conditions propitious for ice-sheet formation. Our data document that Antarctic glaciation was rapid, taking place within two obliquity cycles, and coincided with a striking transition from obliquity to eccentricity as the drivers of climatic change.

Fundamental Modes and Abrupt Changes in North Atlantic Circulation and Climate over the last 60 ky — Concepts, Reconstruction and Numerical Modeling

Centennial- to millennial-scale changes in global climate over the last 60 ky were first documented in ice cores from Greenland, with ice sheets around the North Atlantic and its thermohaline circulation (THC) as prime candidates for a potential trigger mechanism. To reach a new quality in understanding the origin and causal links behind these changes, two strategies were intimately tied together in this synthesis, high-resolution 3-D ocean modeling and paleoceanographic reconstructions. Here, five time series with a time resolution of several decades and various time slices of surface and deep-water paleoceanography were established from hundreds of deep-sea cores for the purpose of monitoring rapid changes across the North Atlantic and testing or initiating model results. Three fundamental modes were found to operate Atlantic THC. Today, mode I shows intensive formation of North Atlantic Deep Water (NADW) and strong heat and moisture fluxes to the continents adjacent to the North Atlantic. Peak glacial mode II leads to a reduction in NADW formation by 30-50%, in line with a clear drop in heat flux to Europe. The glacial Nordic Seas, however, remain ice-free during summer and little influenced by meltwater, in contrast to the sea west ofIreland, where iceberg meltwater blocks an eastbound flow into the Norwegian Sea and induces a cold longshore current from Faeroe to the Pyrenees. The subsequent Heinrich 1 (HI) meltwater mode III leads to an entire stop in NADW and intermediate-water production as well as a reversed pattern of THC, stopping any heat advection from the central and South Atlantic to the north. In contrast to earlier views, the Younger Dryas, possibly induced by Siberian meltwater, began with mode I and ended with mode III, continuing into the Preboreal. Modeling the impact of modes I to III on the global carbon budget, we find that the atmosphere has lost 34-54 ppmv CO2 from interglacial to glacial times, but has gained 23-62 ppmv CO2 at the end of HI within a few decades, equivalent to 33-90% of modem, man-made CO2 release. The robust 1500-y Dansgaard- Oeschger (D-O) cycles and their multiples of as much as 7200 years, the Heinrich event cycles, are tied to periodical changes between THC modes I/II and II/III. In the Irminger Sea rapid D-O coolings are in phase with initial meltwater injections from glaciers on East Greenland, here suggesting an internal trigger process in accordance with binge-purge models. Ice rafting from East Greenland and Iceland occurs only 240-280 y later, probably inducing a slight sea-level rise and, in tum, Heinrich ice rafting from the Laurentian ice sheet during H1, H2, H4, H5. At H1 a major surge from the Barents shelf has lagged initial cooling by 1500 y and entails the most prominent and extended reversal in Atlantic THC over the last 60 ky (probably also at the end of glacial stage 4, at H6). Meltwater stratification in the Inninger Sea reaches its maximum only 640 y after initial meltwater injection and induces, via seasonal sea-ice formation, brine-water injections down to 4 km water depth, signals leading the classic D-O jump to maximum warmth by only 125 y. It may be inferred from this short-phase lag that brine water-controlled deep-water formation probably entrains warm water from further south, thereby forming the key trigger mechanism for the final tum-on of the Atlantic THC mode II roughly within a decade (or mode I, in case of favorable Milankovitch forcing).

Black shale deposition on the northwest African Shelf during the Cenomanian/Turonian oceanic anoxic event: Climate coupling and global organic carbon burial

High-resolution geochemical records from a depth transect through the Cenomanian/Turonian (C/T) Tarfaya Basin (northwest African Shelf) reveal high-amplitude fluctuations in accumulation rates of organic carbon (OC), redox-sensitive and sulphide-forming trace metals, and biomarkers indicative of photic zone euxinia. These fluctuations are in general coeval and thus imply a strong relationship of OC burial and water column redox conditions. The pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher-frequency forcing. Determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event (OAE2) and its duration. We propose that eccentricity is the main forcing factor at Tarfaya and controlled fluctuations in wind-driven upwelling of nutrient-rich, oxygen-depleted intermediate waters from the adjacent Atlantic and the periodic development of photic zone and bottom water euxinia on the mid-Cretaceous northwest African shelf. Accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six-fold increase in OC burial from similar to2 g/m(2) . yr prior to the OAE2 to similar to12 g/m(2) . yr during the OAE2. Photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing. Mass balance calculations imply that similar to2% of the overall global excess OC burial associated with the OAE2 was deposited in the Tarfaya Basin, an area that represented only similar to0.05% of the total global C/T ocean floor. In fact, the lateral extent of similar black shales along the African continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the OAE2.

The response of benthic foraminifers to carbon flux and primary production in the Arctic Ocean

Abstract We examine the quantitative composition of benthic foraminiferal assemblages of Rose Bengal-stained surface samples from 37 stations in the Laptev Sea, and combine this data set with an existing data set along a transect from Spitsbergen to the central Arctic Ocean. Foraminiferal test accumulation rates, diversity, faunal composition and statistically defined foraminiferal associations are analysed for living (Rose Bengal-stained) and dead foraminifers. We compare the results of several benthic foraminiferal diversity indices and statistically defined foraminiferal associations, including Fishers alpha and Shannon–Wiener diversity indices, Q-mode principal component analysis and correspondence analysis. Diversity and faunal density (standing stock) of living benthic foraminifers are positively correlated to trophic resources. In contrast, the accumulation rate of dead foraminifers (BFAR) shows fluctuating values depending on test disintegration processes. Foraminiferal associations defined by Q-mode principal component analysis and correspondence analysis are comparable. The factor values of the correspondence analysis allow a quantitative correlation between the foraminiferal fauna and the local carbon flux, which may be used as a tool to estimate changes in primary productivity.

Paleoecology, biostratigraphy, paleoceanography and taxonomy of agglutinated foraminifera

Agglutinated Foraminifera: An Introduction.- Why are Foraminiferida Foraminifers ?.- Composition and Microstructure of Agglutinated Foraminifer Wall.- Wall Structures of Palaeotextulariid Foraminifers and Discussion of Microgranular Test Walls.- Partitions and Fistulose Chamberlets in Textulariina.- Abyssal Agglutinates: Back to Basics.- On the Way to the Optimal Suprageneric Classification of Agglutinating Foraminifera.- Revision of the Trochamminacea and Remaneicacea of the Plymouth District S.W. England, Described by Heron-Allen and Earland (1930).- Agglutinated Foraminifera from the Palaeogene of the North Sea.- Gerochammina N.G. and Related Genera from the Upper Cretaceous Flysch-Type Benthic Foraminiferal Fauna, Eastern Carpathians - Romania.- Recent Deep-Sea Agglutinated Foraminifera: A Brief Review.- The Ecology, Distribution and Taxonomy of Crithionina Hispida Flint, 1899.- High Latitude Agglutinated Foraminifera: Prydz Bay (Antarctica) vs. Lancaster Sound (Canadian Arctic).- Biostratigraphy and Paleoecology of Deep-Water Agglutinated Foraminifera at ODP Site 643, Norwegian-Greenland Sea.- Danian Deep-Water (Bathyal) Agglutinated Foraminifera from Bavaria and Their Comparison with Approximately Coeval Agglutinated Assemblages from Senegal and Trinidad.- Paleoecology of Late Cretaceous to Paleocene Deep-Water Agglutinated Foraminifera from the North Atlantic and Western Tethys.- Deep Water Agglutinated Foraminiferal Assemblages from Upper Cretaceous Red Shales of the Magura Nappe / Polish Outer Carpathians.- The Oldest Assemblages of Agglutinated Foraminifers of the Polish Flysch Carpathians.- Faunal Trends and Assemblages of the Northern South China Sea Agglutinated Foraminifera.- Agglutinated Foraminifera in Organic-Rich Neritic Carbonates (Upper Cretaceous, Israel) and Their Use in Identifying Oxygen Levels in Oxygen-Poor Environments.- Agglutinated Foraminifera from the Albian and Cenomanian of Jordan.- Facies Controlled Distribution of Foraminifera in the Jurassic North Sea Basin.- Variations in Estuarine Foraminiferal Biofacies with Diminishing Oxygen Conditions in Drammensfjord, SE Norway..- Seasonality in the Benthic Foraminiferal Community and the Life History of Trochammina Hadai Uchio in Hamana Lake, Japan..- Recent Marsh Foraminifera from the East Coast of South America: Comparison to the Northern Hemisphere.- Estuarine and Marsh Foraminifera from the Lower Cretaceous of the Lusitanian Basin, West Portugal.- Recent Marsh-Type Agglutinated Foraminifera from Inland Salt Springs, Manitoba, Canada.- Biogeographic Distribution of Modern Thecamoebians in a Transect Along the Eastern North American Coast.- Thecamoebians from the Early Cretaceous Deposits of Ruby Creek, Alberta (Canada).- Fossil Thecamoebians: Present Status and Prospects for the Future.- Stratigraphically Important Agglutinated Foraminifera in the Badenian (Miocene M4) of Poland.- Foraminiferal Biostratigraphy and Seismic Sequences - Examples from the Cenozoic of the Beaufort-Mackenzie Basin, Arctic Canada.- Deep-Water Agglutinated Foraminifera from the Massignano Section (Ancona, Italy), a Proposed Stratotype for the Eocene-Oligocene Boundary.- Agglutinated Foraminifera, Biostratigraphy and Intraregional Correlation of Upper Cretaceous Deposits of Eastern Urals.- Cretaceous Agglutinated Foraminifera of the UK: A Review.- Agglutinated Foraminiferida from the Albian / Cenomanian Boundary in SE England.- The Application of Middle Jurassic-Early Cretaceous Agglutinated Foraminifera to the Offshore Correlation of Humber Group Sediments in the North Viking Graben.- Agglutinated Foraminiferal Stratigraphy of Middle Jurassic to Basal Cretaceous Shales, Central Spitsbergen..

Neogene History of the Indonesian Throughflow

The Indonesian Throughflow acts as a major switchboard in the global thermohaline circulation, and its variability is strongly related to tropical climate dynamics on shorter and longer timescales. During the Holocene and Pleistocene, fluctuating sea surface temperature and salinity patterns in the Western Pacific Warm Water Pool and Indonesian Seas and variations in East Asian monsoon strength mainly controlled the intensity and hydrological characteristics of the throughflow. Additionally, glacial/deglacial sea-level change strongly influenced throughflow volume in shallow sections of many passages (i.e. the southern part of the Timor passage on the NW Australian shallow shelf) thus altering the related heat transfer between oceans. The tectonic history of the Indonesian Gateway ultimately controlled the long-term evolution of the throughflow. During the Pliocene, changes in the position and geometry of the inflow passages (Mindanao Passage to the North and Halmahera Passage to the south) in relation to the tropical Pacific front significantly modified the climatic role of the tropical Indian and Pacific Oceans, resulting in reduced atmospheric heat transport from the tropics to high latitudes. However, the precise timing of major restriction in the surface and thermocline water flow is difficult to ascertain. The early evolution of the Indonesian Gateway was characterized by tectonic restriction of the deep water pathway between the Pacific and Indian Oceans at approximately 25 Ma. By the early Miocene, the Indonesian Gateway was already closed as a deep water pathway between the Pacific and Indian Oceans.

Continent‐ocean interactions within the East Asian Marginal seas

Interactions between continents and oceans are a frontier area for the Earth sciences in the 21st century. An AGU Chapman Conference, Continent-Ocean Interactions within the East Asian Marginal Seas, examined the nature of these interactions in the marginal seas of east Asia. The objective was to highlight both recent advances, and especially the contributions made by the Ocean Drilling Program (ODP)semi; as well as to identify key future science goals. The types of continent-ocean interactions discussed were wide-ranging, including climate-tectonic interactions, continental-oceanic climate linkages, and the material flux from the rivers of Asia to the ocean, as well as how continental tectonic evolution since the India-Asia collision has influenced the tectonics of the western Pacific and vice-versa. The marginal seas of east Asia form the transition between the worlds largest continent and its largest ocean, and are major repositories of information on the interaction between the two.

Benthic foraminiferal paleoceanography of the South China Sea over the last 40,000 years

Benthic foraminifera in gravity and piston cores from two sites of the northern and southern slopes of the South China Sea (SCS) were analyzed to evaluate changes in surface productivity and deep-water mass characteristics over the last 40,000 years. Our observations suggest that changes in organic carbon flux, that is food supply, and chemical and=or physical properties of the ambient water mass may be the two primary and intercorrelated factors controlling the distribution patterns of benthic foraminifera. When organic carbon flux increased above 3.5 g C m 2 yr 1 in the southern SCS during the last glacial maximum and in the northern SCS during the first part of the Holocene around 10 ka B.P., a group of detritus feeders including Bulimina aculeata and Uvigerina peregrina dominated the benthic foraminiferal assemblage as shown by relative abundance (%) and accumulation rates. This may reflect episodes of increased surface productivity, possibly induced by increased input of nutrients from nearby river runoff. Suspension feeders such as Cibicidoides wuellerstorfiand a group of ‘opportunistic’ species including Oridorsalis umbonatus, Melonis barleeanum and Chilostomella ovoidea gradually became more abundant than detritus feeders as soon as the organic carbon flux decreased to 2.5‐3.5 g C m 2 yr 1 . Similar glacial to interglacial changes in relative abundance and accumulation rates were observed in both cores for a number of species, including Eggerella bradyi, Globocassidulina subglobosa , Astrononion novozealandicum , Sphaeroidina bulloidesand Cibicidoides robertsonianus. These changes were not correlated to the distribution patterns of organic carbon in both cores and may have been related to yet unspecified changes in chemical and=or physical properties of the ambient water mass, independent of changes in organic carbon flux.

Deep-sea benthic foraminiferal recolonization of the 1991 Mt. Pinatubo ash layer in the South China Sea

A census count of Rose Bengal stained benthic foraminifera from the surface area on top of a 2 to 6 cm thick ashfall layer at three deep water stations along the western margin of the Philippines exhibits a unique assemblage composition of benthic foraminifera. The total number of benthic foraminifera is low and the ratio of living individuals to empty tests is high. Specific diversity is low, with a significant dominance of infaunal morphotypes including species of the genus Reophax (R. scorpiurus, R. bilocularis and R. dentaliniformis), which are regarded as successful recolonizers. Assemblages below the ash layers are diverse and contain many epifaunal suspension-feeding agglutinated and calcareous foraminifera. The 1991 Mt. Pinatubo eruption caused mass mortality of benthic foraminifera in a vast area of the eastern South China Sea followed by step-wise recolonization of the ash substrate. Three years after the eruption, the benthic foraminiferal community structure is still far from recovery to background levels.

Changes in Arctic Ocean paleoproductivity and hydrography during the last 145 kyr: The benthic foraminiferal record

The benthic foraminiferal record of two sediment cores at 900 and 2500 m water depth in the marginal Arctic Ocean reflects fluctuations in paleoproductivity, ice sheet extent, and Atlantic water inflow. Highest paleoproductivity is observed in interglacial periods, and at the termination of interstadials to stadials within oxygen isotope substages (OIS) 6.3, 5.3, 5.1, 3.1, and Termination Ia. Stable and high paleoproductivity at the ice-edge during terminations is indicated by the Melonis zaandami benthic foraminiferal association. Periods of glacial ice sheet advance or retreat from the core site are dominated by benthic foraminiferal associations related to seasonal organic carbon flux. Temperate saline Atlantic water entered the Arctic Ocean within OIS 6.3, 5.5 and the Holocene. Coincident paleoproductivity maxima are related to extended seasonal ice retreat. During glacial periods of stagnating Atlantic water advection unusual low paleoproductivity values are observed in the Arctic Ocean, indicating increasing ice coverage. Deep water was poorly ventilated during these periods and the deeper site became exposed to corrosive bottom waters.