Sediment core HLY0503-8JPC raised by the HOTRAX'05 expedition from the Mendeleev Ridge was analyzed for multiple lithological, paleontological, and stable-isotopic proxies to reconstruct paleoceanographic conditions in the western Arctic Ocean during the Late Quaternary. The core, extensively sampled in the upper 5 m, reveals pronounced changes in sedimentary environments during the ca. 250 kyr interval encompassing Marine Isotopic Stages (MIS) 1 to 7. An estimated average resolution of 500 yr/sample, at least for the last glacial cycle including the last interglacial, provides more detail than seen in other sedimentary records from the western Arctic Ocean. The age control is provided by 14C and amino acid racemization measurements on planktonic foraminifers and correlations with the stratigraphy developed for the central Lomonosov Ridge and with glacial events at the Eurasian Arctic margin. Cyclic variations in lithology combined with foraminiferal abundance and stable-isotopic composition indicate profound changes in hydrographic and depositional environments between interglacial-type and glacial-type periods apparently reflecting a combination of 100-kyr and precessional time scales. This periodicity is complicated by abrupt iceberg- and/or meltwater-discharge events with variable (Laurentide vs. Eurasian) provenance. The proxy record from the interval identified as the last interglacial (MIS 5e), which may aid in understanding the future state of the Arctic Ocean, indicates low ice conditions and possibly enhanced stratification of the water column.

Recent geologically-based reconstructions of the Eurasian ice sheet show that during the peak Saalian (approximate to 140 kya) the ice sheet was larger over Eurasia than during the Last Glacial Maximum (LGM) at approximate to 21 kya. To address this problem we use the LMDZ4 atmospheric general circulation model to evaluate the impact on the Saalian ice sheet's surface mass balance (SMB) from proglacial lakes. dust deposition on snow, vegetation and sea surface temperatures (SST) since geological records suggest that these environmental parameters were different during the two glacial periods. Seven model simulations have been carried out. Dust deposition decreases the mean SMB by intensifying surface melt during summer while proglacial lakes cool the summer climate and reduce surface melt on the ice sheet. A simulation including both proglacial lakes and dust shows that the presence of the former parameter reduces the impact of the latter, in particular, during summer. A switch from needle-leaf to tundra vegetation affects the regional climate but not enough to significantly influence the SMB of the nearby ice margin. However, a steady-state vegetation in equilibrium with the climate should be computed to improve the boundary conditions for further evaluations of the vegetation impact on the ice sheet's SMB. Finally, changes of the SST broadly affect the regional climate with significant consequences for the SMB. (C) 2009 Elsevier B.V. All rights reserved.

The Healy–Oden Trans-Arctic Expedition (HOTRAX'05) recovered 29 long piston cores and associated multicores along a transect from the Alaskan margin across the central Arctic Ocean to the northern Svalbard margin. The initial results focus on the age and stratigraphy of these cores using a variety of approaches including radiocarbon, paleomagnetic and other chronostratigraphic methods as well as detailed correlations of proxy records. High sedimentation cores from probable drift deposits are described from the shelf and continental slope north of Alaska. Provenance and sedimentary processes in these deposits include both currents and sea-ice rafting. Much lower sedimentation rates in the central Arctic Ocean show geographic variability consistent with the patterns of expected sediment distribution by ice and, to a smaller extent, subsurface currents. Insights into the Upper Quaternary stratigraphy and depositional history are provided in two papers on a core from the Mendeleev Ridge with sedimentation rates relatively elevated for this region. To compliment the results from the HOTRAX cores, two papers are included dealing with the stratigraphy and sedimentary processes on the Chukchi shelf and a modeling study addressing why the Marine Isotopic Stage 6 (ca. 140 ka) glaciation was substantially more extensive in the Arctic than the Last Glacial Maximum.

High resolution seismoacoustic chirp sonar data and piston cores were collected from the Lomonosov Ridge in the central Arctic Ocean (85degrees-90degreesN; 130degrees-155degreesE). The chirp sonar data indicate substantial erosion on the ridge crest above 1000 mbsl while data from deeper sites show apparently undisturbed sedimentation. Piston cores from both the eroded ridge crest and the slopes have been analyzed for a variety of properties, permitting inter-core correlation and description of paleoenvironmental change over time. Based on the evidence of extensive sediment erosion at depths above 1000 mbsl, we infer that the top of the Lomonosov Ridge has been eroded by grounded ice during a prominent glacial event that took place during MIS 6 according to a newly published age model. This event is coeval with a dramatic shift from low amplitude glacial-interglacial variability to high amplitude variability recorded in the sedimentary record. The new age model used in our study is based on nannofossil biostratigraphy and correlation between sedimentary cycles and a low-latitude oxygen isotope record and confirmed by paleomagnetic polarity studies where negative paleomagnetic inclinations are assigned to excursions. Due to the controversy between this age model and age models that assign the negative paleomagnetic inclinations to polarity reversals, we provide a correlation to Lomonosov Ridge core PS2185-6 [Spielhagen et al., Geology, 25 (1997) 783). According to the latter age models, the Lomonosov Ridge was eroded by ice grounding much earlier, at MIS 16. (C) 2001 Elsevier Science B.V. All rights reserved.

Two long Holocene piston cores (HLY0501-06JPC and -08JPC; herein after referred to as 6JPC and 8JPC) were raised from high sediment accumulation areas in the Arctic Alaskan margin in order to reconstruct the millennial- to centennial-scale behavior of Earth’s magnetic field and to better constrain the regional chronostratigraphy of the Western Arctic. Paleomagnetic analyses using a u-channel cryogenic magnetometer (natural, anhysteretic and isothermal remanent magnetizations: NRM, ARM and IRM) and a vibrating sample magnetometer indicate that a strong and stable single component characteristic remanent magnetization carried by low coercivity pseudo-single domain (PSD) grains such as magnetite can be isolated in the postglacial unit of both cores, where the inclination values vary around the geocentric axial dipole (GAD) for the latitude of the coring sites and where the maximum angular deviation (MAD) values are generally lower than 5 degrees. Apart from one interval in each core, all the derived relative paleointensity proxies (NRM/k(LF), NRM/ARM and NRM/IRM) yield similar results. NRM/IRM was used as the preferred proxy because, based on cross-spectral analysis, it is not coherent with its normalizer. Based on the comparison with paleomagnetic records from Western North America. the paleomagnetic and physical analyses indicate that both sedimentary sequences have recorded some of the first reliable Arctic high-resolution records of paleomagnetic secular variation (inclination and declination) and relative paleointensity during the Holocene. In addition, full vector paleomagnetic correlations (inclination, declination and relative paleointensity) were used to constrain the chronology of core 6JPC, using core 8JPC and other previously published and independently dated sedimentary and volcanic records from Western North America. The Accelerator Mass Spectroscopy (AMS) radiocarbon-based postglacial chronology of core 8JPC indicates sedimentation rates higher than 300 cm/kyr on the continental shelf near Barrow Canyon from approximately 8000 to 5000 cal BR followed by a major decrease in sediment deposition. In contrast, the postglacial deposition on the slope at core site 6JPC is relatively constant and sedimentation rates are nearly three times lower. (C) 2009 Elsevier B.V. All rights reserved.

Sediment cores from the western Arctic Ocean obtained on the 2005 HOTRAX and some earlier expeditions have been analyzed to develop a stratigraphic correlation from the Alaskan Chukchi margin to the Northwind and Mendeleev-Alpha ridges. The correlation was primarily based on terrigenous sediment composition that is not affected by diagenetic processes as strongly as the biogenic component, and paleomagnetic inclination records. Chronostratigraphic control was provided by C-14 dating and amino-acid racemization ages, as well as correlation to earlier established Arctic Ocean stratigraphies. Distribution of sedimentary units across the western Arctic indicates that sedimentation rates decrease from tens of centimeters per kyr on the Alaskan margin to a few centimeters on the southern ends of Northwind and Mendeleev ridges and just a few millimeters on the ridges in the interior of the Amerasia basin. This sedimentation pattern suggests that Late Quaternary sediment transport and deposition, except for turbidites at the basin bottom, were generally controlled by ice concentration (and thus melt-out rate) and transportation distance from sources, with local variances related to subsurface currents. In the long term, most sediment was probably delivered to the core sites by icebergs during glacial periods, with a significant contribution from sea ice. During glacial maxima very fine-grained sediment was deposited with sedimentation rates greatly reduced away from the margins to a hiatus of several kyr duration as shown for the Last Glacial Maximum. This sedimentary environment was possibly related to a very solid ice cover and reduced melt-out over a large part of the western Arctic Ocean. Published by Elsevier B.V.

Neogene marine sediments can be dated via decay of the cosmogenic radionuclide Be-10. Two cores from the Alpha and Mendeleev Ridges in the Arctic Ocean have been analyzed for seawater-derived beryllium (Be) isotopes in order to date the sediments and to calculate sedimentation rates. The decrease of Be-10 concentration in the cores was used to calculate first order sedimentation rates. To eliminate the dilution effect of beryllium caused by short-term changes in sedimentation rate and grain size, the Be-10 concentrations were normalized to the terrigenous stable isotope Be-9 determined in the same sample aliquot. The measured Be-10 concentrations yield low average sedimentation rates for the Alpha and Mendeleev Ridges of 2.3 mm ka(-1) and 2.7 mm ka(-1), respectively. Sedimentation rates calculated from the Be-10/Be-9 ratios result in similarly low values, ranging from 0.2 to 6.8 mm ka(-1) for the Alpha Ridge core and from 1.9 to 6.9 mm ka(-1) for the Mendeleev Ridge core. However, amino acid racemization dating for the past 150 ka of a core adjacent to the Mendeleev Ridge core studied here indicates significantly higher sedimentation rates than calculated from the downcore decrease of Be-10 and Be-10/Be-9. If such higher rates also prevailed at the locations of our cores, for which there is biostratigraphic evidence, either the supply of Be-10 was much lower than assumed or that of Be-9 was much higher. This could imply that the signature of the deep waters in this part of the Arctic Ocean compared to today was largely different for most of the past approximately one million years with a significantly lower Be-10/Be-9 ratio. Our study also addresses the variability of beryllium isotopes in sediment cores across the Arctic Ocean through a comparison of previously published results. Calculated Be-10 fluxes reveal low values in the Amerasian Basin and highest values in the Eurasian Basin, near the Fram. Strait. The decrease of Be isotopes in the two studied Amerasian Basin cores may thus have been caused by environmental factors such as significantly reduced inflow of Atlantic waters in the past, reduced input of Be-10 and/or increased input of Be-9 from the shelves, combined with a more efficient sea ice shielding in this part of the Arctic Ocean. (C) 2009 Elsevier B.V. All rights reserved.

Southernmost Patagonia, located at the relatively narrow passage between Antarctica and South America, is a highly sensitive region for recording meridional and zonal changes in the pattern of oceanic and atmospheric circulation. The island of Isla de los Estados, situated at 54.5 degrees S, 64 degrees W, east of Argentinean Tierra del Fuego, provides an exceptional possibility, to investigate terrestrial records of atmospheric conditions in an oceanic setting during the last deglaciation and the Holocene. Here we present geochemical and diatom analyses from 10600 to c. 1500 cal BP of one sequence (LGB) with peat, lake sediments and lagoon deposits at the northern coast of the island, and a lake sediment sequence (CAS) 3 km further inland. The data comprise TC, TN, loss on ignition analyses and continuous XRF scanning as well as age-depth modeling based on AMS-C-14 dating on both cores. Diatom analysis of the CAS record complements the geochemical proxies. During the Holocene, our two sites have been impacted by two different forcings: changes in the regional climate regime largely influenced by the varying strength and position of the Southern Hemisphere Westerlies (SHW), while relative sea-level changes affected the deposits of the coastal site. In concert with the onset of the Antarctic thermal optimum, our data suggest fairly warm conditions and the establishment of denser peat and forest vegetation on the island c. 10600 cal BP. Between 8500 and 4500 cal BP geochemistry and diatoms from the CAS record indicate stronger Westerlies at this latitude, which means higher wind speed or higher storm frequency and more precipitation, resulting in more pronounced surface run-off. After 4500 cal BP, the geochemical proxies and large changes in diatom assemblages indicate a decrease in precipitation, weaker winds and possibly cooler conditions, probably as an effect of weaker SHW and/or a latitudinal shift. The depositional environment of CAS changed from gyttja to peat around 1000 cal BP. At LGB, the onset of gyttja sedimentation around 7900 cal BP shows that the former wet land with peat accumulation had become a lake with a fairly rapid sedimentation. The chemical data from LGB imply a gradually increasing marine influence, probably as an effect of both rising sea level and increased impact of storms and maximum high tides. After a marine high-stand during the mid-Holocene, the LGB site returned into a peat bog again around 3400 cal BP. Our data suggest that since then relative sea level first dropped back below present sea level followed by a rise to present day sea level. (C) 2010 Elsevier B.V. All rights reserved.

Marine benthic ecosystems collapsed during the catastrophic end-Permian mass extinction, and subsequently endured a protracted phase of biotic recovery under harsh environmental conditions. In particular, metazoan reef communities almost totally disappeared and were replaced by microbe-dominated mounds during the latest Permian-earliest Triassic. Here we report the stratigraphically oldest exclusively metazoan bioconstructions from earliest Triassic (mid-Induan) strata in East Greenland - these formed within the first ca 300 ka after the Permian-Triassic boundary. Unlike the multitaxic sponge-microbe and bivalve-based buildups recorded from the Early Triassic peri-paleoequatorial Panthalassan and Tethyan margins, the East Greenland bioaccumulations developed within a restricted Boreal mid-paleolatitude seaway, and comprised a monospecific primary framework of microconchid 'lophophorate tubes with shell fragments and phosphatic debris cemented by biogenic calcite. Prostrate growth of the microconchids likely facilitated their accretion into successive sheet-like biostromes and small bioherms. These are associated with a regional paleoenvironmental shift towards well-oxygenated bottom waters, and locally punctuated sedimentation that created a favorable habitat. Although microconchids were both abundant and geographically widespread throughout the earliest Triassic, such buildups formed solely by these metazoans have not been reported from that time frame outside the Boreal Realm. These apparently flourished in the absence of more stable complex communities, and suggest that a locally variable, rather than ubiquitously sequential revival of metazoan bioconstruction activity took place in the immediate aftermath of the end-Permian extinction. However, these may also suggest that ecological recovery of benthic marine ecosystems following the end-Permian mass extinction might have started earlier in higher paleolatitudes.