PP41D-0774
Geochemical and Paleoceanographic Examination of the Cenozoic Arctic Ocean: Results from IODP ACEX 302.
We have analyzed sediments from Integrated Ocean Drilling Program Arctic Coring Expedition 302 in the central Arctic Ocean, for major, trace, and rare earth elements in order to assess detrital source changes to the Lomonosov Ridge throughout the Cenozoic. The terrigenous component consists of two compositional end- members, one with a shale-like composition and another with a more basaltic affinity. The shale end-member contributes the most over Unit 1 and the older portions of Unit 3, and may represent sediment supplied from the eastern Laptev Sea (drained by the Lena and Yana rivers). Therefore, even though transport mechanisms must have significantly changed from ice-free to ice-driven, these two units could have been under the influence of a single major terrigenous source. Input from the more basaltic end-member appears to be more important through Unit 2 and most likely represents material derived from the western Laptev Sea (drained by the Khatanga River). Thus, major changes in the biogenic system observed during the time of deposition of Unit 2 (Middle Eocene) appear broadly synchronous with a significant change in terrigenous provenance. Although the assignment of sources is constrained by the lack of data from other regions (e.g., eastern Siberian Sea) our data suggest changes in provenance that are linked to paleoceanographic changes in the Arctic Ocean.
PP41D-0775
Cyclicity in the Central Arctic Ocean Middle Eocene Sediment Record: Orbital Forcing and Environmental Response
Continuous X-ray fluorescence (XRF) scanning of middle Eocene (ca 46 Ma) ACEX (Arctic Coring Expedition, IODP 302) core 2A-55X revealed a strong cyclic signal in some of the major and trace geochemical elements. XRF (e.g., Fe, K, Ti, Al, Si) and physical properties (GRA, MagSus, PWL, and NGR) evidenced cyclicity with periods of about 50 cm and 100 cm. Using the age model of Backman et al. (Paleoceanography, accepted, 2007) and the derived sedimentation rate of 24.3 m/Myr, these frequency components are compatible with a Milankovitch type orbital forcing, with the oscillations at 50 cm and at 100 cm representing precession and obliquity, respectively. The longer 100 cm cyclicity is also present in the biological (pollen, dinocysts and siliceous microfossil) proxies and in the sedimentological (IRD) proxy. Results show that biological proxies seem to respond stronger to seasonal darkness (length of the growing season), whereas the terrigenous components, partly linked to sea-ice and/or glacial ice formation and extent, respond more directly to seasonal insolation.
PP41D-0776
A Siliceous Microfossil View of Middle Eocene Arctic Paleoenvironments
Integrated Ocean Drilling Program Expedition 302, "The Arctic Coring Expedition" (ACEX) made arguably the most significant discovery of Paleogene siliceous microfossils in nearly 2 decades. 100 m of mm to sub-mm laminated biosiliceous sediments of Middle Eocene age are rich in marine and freshwater siliceous microfossils allowing intriguing insights into central Arctic paleoenvironments during the start of Cenozoic cooling to icehouse conditions. Largely endemic assemblages of marine diatoms and ebridians are preserved along with very high abundances of chrysophyte cysts, the endogenously formed resting stage of freshwater algae. These siliceous microfossil groups imply an overall brackish environment, but variations in group dominance suggest episodic variations in salinity, stratification and trophic status. We synthesize the overall characteristics of the biosiliceous sediments by reporting on broad scale variations in siliceous microfossils and give some exciting insights into the composition of the laminae which may help explain the formation of these sediments. Our results indicate basin-wide paleo-environmental fluctuations on long- and possibly seasonal timescales.
PP41D-0777
The silicoflagellates and ebridians from the central Arctic Ocean in the early middle Eocene
The early middle Eocene sediments from the central Arctic Ocean obtained by IODP Expedition 302 (ACEX) were studied for the siliceous microfossils of silicoflagellates and ebridians in order to establish the biostratigraphy and to decipher the paleoceanographic changes of the upper water column. Seven silicoflagellate taxa of the total of 56 taxa and three ebridian taxa of the total of 30 taxa were previously unknown and they were newly described as new species. Silicoflagellate and ebridian assemblages in lower part of Lithologic Unit 2 are endemic compared to the assemblages of the outside of the Eocene Arctic Ocean. Temporal intervals of the silicoflagellate and ebridian assemblages were categorized to several assemblage groups according to the variation in the assemblage characteristics. Changes in characteristic assemblage is probably due to the habitat modulation governed by the extent of mixing of significantly different water masses between the low salinity waters derived from the Arctic region and relatively high salinity waters supplied from the outside of the Arctic Ocean. The low salinity water in the Eocene Arctic is suggested from the co-occurrence of freshwater and blackish water microfossils. The origin of the freshwater is attributed to the rainfall and river influx during the rainy Eocene Arctic summer. The circulation of the Arctic Ocean in the early middle Eocene probably corresponds to an estuarine type, which includes the Black and the Baltic Seas today. The high abundance of ebridians may reflect the presence of hypoxic waters in or near the euphotic layer based on the extant ebridian ecology of Hermesinum adriaticum with symbiotic algae, which is present in the Black Sea today.
PP41D-0778
Paleoceanography Of The Middle Eocene Arctic Ocean Based On Geochemical Measurements Of Biogenic Matter
The IODP Expedition 302, Arctic Coring Expedition (ACEX), recovered 428 m long sediment cores on the Lomonosov Ridge in the central Arctic Ocean. Chemical analyses for biogenic opal, total organic carbon (TOC), total sulfur (TS), and stable sulfur isotopic composition were conducted on the middle Eocene section of the ACEX cores. The previous study for microfossil assemblages on this section indicated the presence of low- salinity water mass in the Arctic Ocean. However, % TS contents were high in all intervals, indicating that abundant sea water was present in the deep layer of the paleo Arctic Ocean in contrast with low salinity surface water. The light sulfur isotope composition indicates the microbial sulfate reduction in an open system. This supports the continuous supply of sea water from the outside of the Arctic Ocean. The euxinic condition of the bottom water is suggested by the TOC-TS diagram. The anoxic environment was brought about by salinity stratification like the modern Black Sea. The high values of the accumulation of biogenic opal and TOC indicate high productivity which continued for nine myr. The high productivity was related to the estuarine type circulation in the semi-closed Arctic Ocean.
PP41D-0779
Reconstruction of the Eocene Arctic Ocean Using Ichthyolith Isotope Analyses
Nd, Sr, O and C isotopic compositions of Eocene fish debris (teeth, bones, scales), and their reduced organic coatings, have been used to reconstruct water mass composition, water column structure, surface productivity and salinities of the Arctic Ocean Basin at Lomonosov Ridge between 55 and 44 Ma. Cleaned ichthyolith samples from IODP Expedition 302 (ACEX) record epsilon Nd values that range from -5.7 to -7.8, distinct from modern Arctic Intermediate Water (-10.5) and North Atlantic Deep Water. These Nd values may record some exchange with Pacific/Tethyan water masses, but inputs from local continental sources are more likely. Sr isotopic values are consistent with a brackish-to-fresh water surface layer (87Sr/86Sr = 0.7079-0.7087) that was poorly mixed with Eocene global seawater (0.7077-0.7078). Leaching experiments show reduced organic coatings to be more radiogenic (>0.7090) than cleaned ichthyolith phosphate. Ichthyolith Sr isotopic variations likely reflect changes in localized river input as a function of shifts in the Arctic hydrologic cycle, and 87Sr/86Sr values might be used as a proxy for surface water salinity. Model mixing calculations indicate salinities of 5 to 20 per mil, lower than estimates based on O isotopes from fish bone carbonate (16 to 26 per mil). Significant salinity drops (i.e., 55 Ma PETM and 48.5 Ma Azolla event) registered in oxygen isotopes do not show large excursions in the 87Sr/86Sr data. Carbon isotopes in fish debris record a spike in organic activity at 48.5 Ma (Azolla event), and otherwise high-productivity waters between 55 and 44 Ma. The combined Sr-Nd-O-C isotopic record is consistent with highly restricted basin-wide circulation in the Eocene, indicative of a highly stratified water column with anoxic bottom waters, a "fresh" water upper layer, and enhanced continental runoff during warm intervals until the first appearance of ice rafted debris at 45 Ma.
PP41D-0780
Bulk Mineral Assemblage of the PETM and Other Extreme Warm Events in the IODP Arctic Ocean Coring Expedition's Sediments - Weathering vs. Transport
The Arctic Coring Expedition (ACEX) recovered several meter of sediment containing the Paleocene and Eocene extreme warm events from the central part of the Lomonosov Ridge. Here we present a 20 cm sample resolution study of these sediments. The bulk mineral assemblage of these samples will be compared to our Arctic Ocean and shelf region surface sediment dataset of more than 2000 samples. All these samples have been investigated during the last 15 years by X-ray diffraction with the same sample preparation, measurement equipment and full-pattern quantification techniques. Therefore, results are fully comparable. Sources of the terrigenous components of the ACEX sediments are not well known yet, in particular of the Tertiary section. The presentation will try to enlighten the question if the mineral assemblage presents rather changes in transportation processes and pathways and/or different weathering conditions in the surrounding hinterland. Changing weathering conditions to warmer and wet conditions for example should show up in increases of mineral contents of clay minerals and in particular kaolinites. But, despite the cold and rather physical weathering conditions in the today Polar Arctic region, there are source rock regions with very high contents of kaolinite. As these sediments are in particular of Triassic and Jurassic age, it might not be easy to relate high kaolinite values in the ACEX sediments to the suggested warmer and wet weathering conditions during the Paleocene and Eocene warm events.
PP41D-0781
Paleoceanography of the Eocene Arctic Basin Reconstructed With Chemical Parameters and Siliceous Microfossils
With the currently predicted drastically different future environments of the Arctic Ocean without perennial sea-ice cover, it is important to learn how the ice-free Arctic basin behaved in the past. The environmental reconstruction of the middle Eocene Arctic basin explored by the IODP Arctic Coring Expedition (ACEX) provides excellent opportunity to advance our knowledge on the ice free Arctic basin. The early to middle Eocene represents the warmest interval of the Cenozoic Era. Of this ACEX retrieved approximately 120 m long more or less continuous Eocene section from the Lomonosov Ridge located in the central Arctic. The measured parameters includes total organic carbon, total sulfur, stable sulfur isotopic composition (δ34S), nitrogen, biogenic opal, silicoflagellates, ebridians, chrysophytes, and diatoms. The interpreted environments of the middle Eocene arctic basin are represented by the presence of fresh water chrysophytes near the seasurface, brackish waters in the subsurface indicated by the presence of diatoms, silicoflagellares, and ebridians, followed by anoxic waters lying below the brackish waters but within the euphotic layers indicated by ebridians. The position of the boundary between the oxygenated and anoxic layers reconstructed by ebridians conforms with those obtained by organic compounds such as lycopane and isorenieratene derivatives (Dr. R. Stein, Pers. Comm., 2007). High biological productivity and poor basin water circulation was evident from abundant pyrites. Although the seasurface was covered by fresh waters, the middle Eocene Arctic basin contained ample amount of seawaters indicated by the continuous high abundance of sulfur mainly in the form of pyrites as well as sporadic occurrences of radiolarians. Based on the silicoflagellate paleotemperature reconstruction, 20-25°C have been obtained for the middle Eocene Arctic. This range is conformable with those of 19-23°C SST estimated by TEX86 (Sluijs, et al., Nature, 441, 2007) as well as surrounding continental temperatures of 18-25°C shown by MPT as an indicator of soil bacteria (Dr. H. Brinkhuis, Pers. Comm., 2007).
PP41D-0782
Pollen evidence for Thermophilic and Hydrophilic Terrestrial Vegetation in the Central Arctic During the Paleocene/Eocene Thermal Maximum
Rapid increases in sea-surface and continental temperature and moisture delivery to the Arctic during the global warming called the Paleocene/Eocene Thermal Maximum (PETM) have been documented by dinoflagellate cysts and organic geochemical proxies. We compare new pollen data with published dinocyst, higher-plant n-alkane isotope, TEX86 sea surface- and MBT continental-temperature records from PETM sediments recovered from Lomonosov Ridge in the central Arctic Ocean by Integrated Ocean Drilling Program Expedition 302. Substantial increases in abundance of Taxodiaceae and angiosperm pollen occur at the onset of the PETM, when Apectodinium, a subtropical dinoflagellate restricted to low latitudes before the PETM, first appears. Pollen assemblages during the PETM indicate temperate to subtropical atmospheric temperatures and greater moisture availability during this interval. A shift back to conifer-dominated assemblages later in the PETM coincides with atmospheric and sea-surface cooling, decreased abundances of Apectodinium, the termination of photic zone euxinia, and increased salinity. We will discuss new pollen evidence that is consistent with published subtropical sea-surface and atmospheric temperature reconstructions in the Arctic. These data address hypotheses on the synchroneity of ocean-land climate changes and provide new constraints on potential vital effects on higher-plant n-alkane isotope records.
PP41D-0783
Paleogene Record of Orbital Variations, Time Scales and Elemental Distribution in Sediments from the Arctic Ocean Obtained by XRF Analyses.
We present a high resolution X-Ray Fluorescence (XRF) core scanner record for the expanded middle Eocene section from IODP Expedition 302 (ACEX) drilled on the Lomonosov Ridge. The division of the middle Eocene into two units (sub-unit 1/6 and unit 2) is seen in both cyclical behavior of the elements and the changing inter- elemental correlations and their relationship to physical properties. Al, Ti and K strongly correlate throughout the record while the behavior of Fe, Mn and Si seems to be more complex. These variations in elemental concentrations, which occur on depth scales from decimeters to meters, correspond to the varying contribution of detrital minerals and biogenic silica in a likely euxinic basin setting. We also highlight the diagenetic effect on Fe within the record reflecting the decoupling of this record from the other elements. Additionally we show a coherent and cross-correlated pattern between physical property data and our XRF derived elemental concentrations for calculating sedimentation rates for selected intervals in the early and middle Eocene. We confirm and refine sedimentation rates obtained independently through bio- and magnetostratigraphic means to be of the order of 10 to 25 m/Myr, and observe a strong imprint of astronomically forced cycles, particularly through climatic precession which allows us to test recently proposed new insolation calculations.
PP41D-0784
Biomarker records and paleoenvironment of the central Arctic Ocean during Paleogene times
During IODP Expedition 302 (Arctic Coring Expedition – ACEX), a more than 200 m thick sequence of Paleogene organic-carbon (OC)-rich (black shale-type) sediments has been drilled. Here, we present new biomarker data determined in ACEX sediment samples to decipher processes controlling OC accumulation and their paleo- environmental significance during periods of extreme global warmth and proposed increased freshwater discharge in the early Cenozoic. Specific source-related biomarkers including n-alkanes, fatty acids, isoprenoids, carotenoids, steranes/sterenes, hopanes/hopenes, hopanoic acids, aromatic terpenoids, benzohopanes, long- chain alkenones and organic sulfur compounds show a high variable of compounds, derived from marine, terrestrial and bacterial origin. Based on the biomarker data, the terrestrial OC supply was significantly enriched during the late Paleocene and part of the earliest Eocene, whereas n-alkanes and n-fatty acids in samples from the PETM and Elmo events as well as the middle Eocene indicate increased aquatic contributions. For the latter, an anoxic environment similar to the modern Black Sea, and moderate primary productivity are proposed. The occurrence of C37-alkenenones, which were first determined in the middle part of the Azolla Freshwater Event (about 49 Ma), suggests that significant amounts of the OC is of marine origin during in middle Eocene. During the Eocene, a prominant cooling and onset of first significant IRD deposition near 45.4 Ma were recorded in the terrigenous coarse fraction of the ACEX sequence, related to iceberg and/or sea-ice transport (K. St. John, Paleoceanography, in press). This cooling trend is also reflected in the alkenone SST, showing a temperature decrease of about 10°C between about 49 and 44 Ma.
PP41D-0785
Towards a Neogene Palynostratigraphy of the Arctic Ocean
The Neogene paleoenvironmental history of the Central Arctic Ocean is largely unknown despite many efforts to recover sequences older than the Pleistocene in the past 40 years. Although a number of studies claimed that some sediment cores comprise the Late Pliocene to Pleistocene, the age control of these sediments remained questionable. On this background, IODP Expedition 302 was a major step forward because sediments of undoubtedly Neogene age were drilled for the first time in the Central Arctic Ocean. In the course of shorebased biostratigraphic work, palynomorphs are studied at a relatively high resolution to establish a biostratigraphic framework for the Neogene of the Central Arctic Ocean. Numerous potentially valuable palynomorph datums have been identified in Hole 2A but comparison with occurrences at other high latitude sites is presently hampered by taxonomically problematic taxa and an inconsistent stratigraphic framework of a number of ODP holes from the Atlantic sector of the high northern latitudes. A number of taxa must be restudied and age models of ODP holes must be revised and adjusted to ATNTS2004 before palynomorph datums can be calibrated and a comprehensive and consistent zonation for the polar domains can be established. These specific problems of high latitude palynostratigraphy are illustrated by describing the biogeographic and stratigraphic distribution of a number of palynomorph taxa and by discussing the implications of revised datums for high latitude chronostratigraphy.
PP41D-0786
The radiogenic isotope record of Arctic Ocean circulation and weathering inputs of the past 15 million years
Analyses were made of lead (Pb), neodymium (Nd) and strontium (Sr) isotopes in bulk digests and Pb and Sr isotopes in leachates (reflecting the isotope composition of past seawater) of Neogene sediments recovered during "ACEX" IODP Leg 302, and of Late Quaternary sediments from the neighboring site PS2185. The Nd and Sr isotopic compositions of these bulk sediments have been remarkably constant over the entire past 15 Ma at the core locations (on the Lomonosov Ridge). The source provenance can be readily identified as being the E. Laptev and Kara Sea region, where the sediments are entrained by sea ice, or as IRD. This implies that there has been a surface ocean circulation pattern similar to today, as well as continuous sea ice transport of sediment in the Arctic Ocean for the past 15 Ma. That is, the TransPolar Drift (TPD) appears to have transported sea ice from the N. Eurasian shelf regions across the North Pole for most of the latter Neogene until present. Although possibly and artifact of the leaching procedure, the leachate Sr isotope data differ significantly from the global ocean values only before 13 Ma, and may indicate that Arctic-Atlantic exchange was somewhat restricted prior to this time. The Pb isotope record of Antarctic Intermediate Water (AIW) reflects a local signal, largely formed through exchange with settling sediment. Thus the AIW Pb isotope record is best interpreted as reflecting terrestrial erosion processes. In this light, the record indicates that, consistent with the Sr and Nd isotopic data, changes in the Arctic Ocean over the past 15 Ma have been closely linked to erosive processes on the continental regions of its Eurasian margin, rather than continental North America and Greenland. For example, the early growth of northern Eurasian ice sheets during glacial inception is clearly documented in the Pb isotopic record of the Late Quaternary.
PP41D-0787
A middle Miocene onset of a permanent sea-ice cover in the Arctic Ocean
The Arctic Coring Expedition (ACEX) penetrated over 400 m of Cenozoic sediments from the central part of the Lomonosov Ridge in the Arctic Ocean. The pervasive occurrence of dropstones and sand lenses in the Neogene ACEX sediments, which clearly are of terrigeneous origin, must have been brought to this mid-oceanic location by sea-ice and icebergs. Critical to understanding the paleoclimate history of the Arctic is a reconstruction of the magnitude and timing of the occurrence of perennial sea-ice. Unambiguously determining whether or not the sea-ice was seasonal or perennial is a challenging undertaking. One approach to addressing this challenge is to determine the source areas of the mineral assemblages that were deposited at the ACEX drill site. To establish the source areas for the terrigeneous material, the mineralogical composition of the upper 300 m of drilled sequence was investigated. Heavy and clay mineral associations indicate a major and consistent shift in provenance that occurs across a boundary at ca. 156 meters below seafloor. This boundary defines a change in source region from the Barents-Kara-western Laptev Sea, identified by the presence of common clinopyroxene and smectite below 156 m, to the eastern Laptev–East Siberian seas above 156 m, identified by common hornblende (amphibole) and illite. Bccause of the geographic distance, sea-ice originating from the eastern Laptev–East Siberian seas source region would have survived at least one summer melt cycle in order to reach the ACEX drill site, based on modern sea-ice trajectories and velocities. This shift in mineral assemblages, and thus source areas, is interpreted to represent the onset of a perennial sea-ice cover in the Arctic Ocean, which occurred at about 13 Ma, indicating a coeval cooling in the Arctic and Antarctic regions.
PP41D-0788
Paleo-Currents and -Ice, Using Grain Size Analyses in the ACEX and ARCTIC '91 Cores
The Arctic Coring Expedition (ACEX), a milestone research endeavor that took place during the summer of 2004, obtained a sediment core record from the central Arctic Ocean. The cored sediments, taken from the Lomonosov Ridge, provided the first paleo-record extending throughout the Cenozoic Era. Grain size analyses of samples from roughly every 0.5 m from the ACEX cores are used to examine a variety of paleo-climate variables. The low- resolution ACEX samples are complemented in the Holocene by high-resolution sampling taken from an ARCTIC `91 piston core from a location nearby the ACEX site. This study is concerned with our ability to resolve and decouple paleo-currents from the paleo-ice climate. Relative paleo-current magnitudes can be ascertained using a measure of sortable silt (SS), the mean of the silt fraction in a sample (McCave et al., 1995). Paleo-ice climate can be estimated from the ice rafted debris (IRD), specifically the sand fraction percentage of a sample. The grain size analysis, which was completed using the Malvern Mastersizer 2000, shows a linear relationship between SS and IRD. The slope of the relationship is consistent regardless of the glacio-marine location, suggesting that paleo-ice climate intensity can be overestimated with higher paleo-current velocities. Employing the linear relationship, the contribution of paleo-currents can be removed from the IRD record and the two paleo- indicators examined separately. A weak paleo-ice climate and strong paleo-current velocities characterize the Paleogene. In contrast, the Neogene has a stronger paleo-ice climate and weaker paleo-current velocities, with a notable change at the MIS 6 boundary where both current velocities and ice climate were significantly strengthened to levels higher than at any other time in the Cenozoic record recovered from ACEX and the central Arctic Ocean.
PP41D-0789
The Arctic perennial ice cover over the last 14 million years
Knowledge of the long-term history of the perennial ice is an important issue that has eluded study because Cenozoic core material needed has been unavailable until the recent Arctic Coring Expedition (ACEX). Detrital Fe oxide mineral grains analyzed by microprobe from the last 14 Ma (164 m) of the ACEX composite core were matched to circum-Arctic sources with the same mineral and 12 element composition. These precise source determinations and estimates of drift rates were used to determine whether these sand grains could be rafted to the ACEX core site in less than a year. The abundance of Fe grains from sources more than a year drift indicate a perennial ice cover has existed since 14 Ma, except for the unlikely rapid return to seasonal ice between the average sampling interval of about 0.17 Ma. The older ACEX core contains more Fe grains from Ellesmere Island where other Arctic cores less than 50 ka in age show higher numbers of grains from the shelf near Ellef Ringnes Island in the Queen Elizabeth Islands. Both North America and Russia contributed significant Fe grains to the ACEX core (Lomonosov Ridge) during the last 14 Ma.
PP41D-0790
Petroleum Generation in the Central Arctic Ocean: How, Where and When?
Results of seismic interpretations and published sedimentological and organic geochemical data from IODP Expedition 302 provided the framework for the first quantitative assessment of source rock quality and distribution of the Palaeogene sediments in the central Arctic Ocean. In addition a simple burial history and thermal modelling was performed to evaluate if hydrocarbon generation is likely to occur in these early Tertiary sediments. The modelling results can be summarized as follows: An approximately 100 m thick Early to Middle Eocene sedimentary sequence of good to very good source rock is suggested to occur along a 75 km long transect across the Lomonosov Ridge. In-situ generation of hydrocarbons is rather unlikely as overburden (~200-250 m) and thermal maturity are too low. Burial history and thermal modelling revealed that an additional overburden of at least 1000 m is necessary to start generation in this area. However, source rock modelling results show the possibility for good source rock potential in lateral equivalents in the adjacent Amundsen Basin. Simulated organic carbon contents of 1.5 – 5 %, coupled with an overburden of ~1000 – 1200 m and heat flow anomalies due to the vicinity to the Gakkel Ridge spreading centre, indicate that necessary conditions for hydrocarbon expulsion are already reached and point to viability of a potential petroleum system. Our results support the hypothetical deposition of a good potential hydrocarbon source rock across the entire Arctic Basin and adjacent margins during the early Tertiary.