PP42B-01
Information on the early Holocene climate constrains the summer sea ice projections for the 21st century
The summer sea ice extent strongly decreased in the Arctic over the last decades. This decline is very likely to continue in the future but uncertainty on projections is very large. However, summer sea ice changes for the early Holocene and for the 21st century are strongly linked, allowing to reduce this uncertainty. Using the limited information presently available for the early Holocene, simulations presenting very large changes for the 21st century could reasonably be rejected. On the other hand, simulations displaying low to moderate changes during the second half of the 20th century are not consistent with recent observations. Using this evidence based on observations during both the early Holocene and the last decades, the most realistic projection indicates a nearly disappearance of the sea ice at the end of the 21st century for a moderate increase in atmospheric greenhouse gas concentrations. For a faster increase in those concentrations, the Arctic Ocean would become almost ice-free in summer as early as 2060 AD.
PP42B-02
Sea Ice Entrainment and Sediment Sources
More than 20 sea ice sediment samples collected during the Healy-Oden Trans-Arctic Expedition (HOTRAX-'05) and the Lomonosov Ridge off Greenland Expedition (LOMROG-'07) along with 18 dirty ice samples from previous expeditions back to the early 1970's show a consistent trend of several sources especially the Laptev and Kara Seas and northern Canada. Modern sea ice back-trajectories from the coordinates of the dirty sea ice samples using past drifts of this ice from the International Buoy Drift Program indicate a close match to the Fe grain sources. Comparison with several sediment cores from across the Arctic and in Fram Strait indicate that Holocene sediment that is primarily transported by sea ice contains the same sources based on Fe oxide grain chemical fingerprint matches with circum-Arctic source areas previously characterized. Observations of encounters with dirty ice by HOTRAX and LOMROG icebreaker expeditions indicate that while a small percentage of sea ice contains sediment, this dirty ice is concentrated in bands extending several kilometers and consisting of irregularly spaced patches of dirty ice that are usually less than a 100 meters in width. Also, sea ice containing coarse sediment and/or large shells due to anchor ice are less than 10 percent of dirty sea ice but that they may contain much higher concentrations of sediment than sea ice containing finer sediment from suspension freezing.
PP42B-03
Organic carbon accumulation in the central Arctic Ocean during Cenozoic times and its paleoenvironmental significance
The paleoclimatic evolution of the Arctic Ocean and its surrounding continents during Cenozoic times is characterized by distinct changes in extent of continental glaciations, sea-ice cover, sea-surface-temperature, river discharge, and oceanic circulation. All these factors also influence the organic carbon (OC) input, preservation and burial in the continental margin and adjacent deep sea areas. Thus, records on amount and composition of OC in marine sediments cores yield important information on the long- and short-term variability of Arctic Ocean paleoclimate and paleoceanography. Within this presentation, two totally different paleoenvironmental situations will be discussed, the glacial cold Quaternary/ Neogene Arctic Ocean and the pre- glacial, warm Paleogene Arctic Ocean. During the Neogene/Quaternary, OC accumulation in the central Arctic Ocean is mainly controlled by terrigenous OC input by sea ice, turbidity currents, and river discharge, processes showing a strong glacial/interglacial variability. OC input by turbidity currents can be traced back to source areas and thus may give information about extent of continental glaciations. Marine OC contributions by primary production is only occasionally significant and related to periods of increased Atlantic-water inflow and reduced sea-ice cover. Based on organic-geochemical data sets (including bulk parameters as well as biomarker) determined in the Paleogene section of IODP Expedition 302 Holes M0002A and M0004A (ACEX drilling on Lomonosov Ridge), on the other hand, the early Cenozoic Arctic Ocean and its OC accumulation was characterized by predominantly anoxic (Black Sea-type) conditions, warm SST, moderate primary productivity, and high rate of river discharge. During Eocene times of anoxia, OC accumulation rates were 5-20 times higher than modern ones. Based on a study of the terrigenous coarse fraction in the ACEX section, a prominant Eocene cooling and onset of first significant IRD deposition near 45.4 Ma were recorded, related to iceberg and/or sea-ice transport (St. John, K., 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 (Weller and Stein, Paleoceanography, under revision; see AGU Poster Weller and Stein).
PP42B-04
Application of Foraminiferal Stable Isotopy to Quaternary Stratigraphy and Paleoceanography of the Central Arctic Ocean
We synthesize new and published foraminiferal stable-isotopic (d18O and d13C) data from the Arctic Ocean's Northwind, Mendeleev, Alpha and Lomonosov ridges, including measurements from recently recovered HOTRAX'05 sediment cores. The majority of this data has been obtained on the >150 um tests of the dominant Arctic planktonic foraminifer, left-coiling Neogloboquadrina pachyderma. These adult specimens characterize subsurface waters, primarily the lower halocline that is ventilated by water mixing processes at the Eurasian Arctic shelves. Generally, stable-isotopic records from the central Arctic Ocean differ significantly from the typical oceanic stable-isotopic curves, especially with respect to d18O. Due to the relatively small size, nearly land- locked nature and the voluminous river input to the Arctic Ocean, d18O of foraminiferal calcite is controlled primarily by the composition of ambient water in which the principal end members are oceanic water and runoff during interglacials, or meltwater during glacial periods. As a result, Arctic stable-isotopic curves cannot be used directly as a chronostratigraphic tool; however, understanding their patterns is important for reconstructing paleoceanographic environments. Comparison of stable-isotopic records across the Arctic Ocean shows a consistent, dramatic change in their pattern. Whereas records from the Fram Strait and adjacent areas are similar to those from the North Atlantic by having temperature-controlled d18O variations, records from the Amerasia Basin have a very different pattern, with low d18O and d13C values characterizing glacial periods, and high values during interglacials/interstadials. Especially high d13C values occur in the interstadials such as OIS 3, while full interglacials such as OIS 5e seem to have intermediate values. We infer that glacial/deglacial stable-isotopic compositions are controlled by (1) high amounts of meltwater pooled in the Amerasia Basin and (2) a lack of halocline ventilation during low sea levels and glacial build-up on the Barents- Kara shelf. The opposite end member is represented by interstadials when water mixing was active on the Eurasian shelves, when freshwater fluxes were reduced due to the inundation of the Bering Strait and possibly reduced riverine discharge. Full interglacials are characterized by a somewhat intermediate regime, with considerable haline stratification and moderate subsurface ventilation.
PP42B-05
Sedimentary 231Pa/230Th records of 231 Pa export and sedimentation from the East and West Central Arctic
The radionuclides protactinium-231 and thorium-230, produced at constant rate and ratio in seawater by uranium decay, are scavenged by particles in the water column and buried in seafloor sediments. The concentration and ratio of 231Pa and 230Th in these sediments reflect both sedimentation processes and water column processes such as advection in the North Atlantic and eddy diffusion in the Pacific. Thus far, however, the deposition of 231Pa in the Arctic has been poorly constrained. We present high-resolution 231Pa/230Th profiles dating from marine isotope stage 3 through the Holocene, measured in box cores from the Eastern and Western Arctic Ocean. These profiles suggest regional differences in 231Pa deposition in the Arctic. To the east, in the central Nansen Basin, 231Pa/230Th ratios have stayed well below the production ratio for the last 30 kyr, suggesting that 231Pa has been consistently exported from this region over that time. At Lomonosov Ridge, glacial 231Pa/230Th ratios were slightly less than the production ratio during the late glacial, dropped very low at the start of the deglacial period, and then rose to above the production ratio at the end of the deglacial, suggesting that export and scavenging have varied considerably there as climatic conditions changed. At a deep site in the Western Arctic, ratios have remained below the production ratio throughout the glacial and for much of the Holocene, with two jumps towards the production ratio in the deglacial and early Holocene which may correspond to times of increased particle supply to the region. A shallower slope core in the same region shows evidence for boundary scavenging of 231Pa during the Holocene but not during the deglacial, when increased particle rain elsewhere may have increased local deposition and decreased 231Pa export from the central basins. These records thus offer insight into the history of regional changes in Arctic sedimentation and water column export of radionuclides over the glacial/interglacial transition.
PP42B-06 INVITED
Arctic Ocean: Glacial History From Multibeam Mapping and Coring During the HOTRAX (2005) and LOMROG (2007) Expeditions.
During the Healy-Oden Trans-Arctic 2005 (HOTRAX) expedition, a transect from Bering Strait across the central Arctic Ocean to Svalbard was accomplished. Multibeam mapping and chirp sonar profiling along this transect revealed an abundance of glaciogenic bedforms on the Chukchi Borderland including iceberg keel scours, mainly at water depths shallower than 350-400 m, flutes and mega-scale glacial lineations extending as deep as 900 m below the present sea level, small drumlin-like features, and morainic ridges and grounding-zone wedges. The Lomonosov Ridge off Greenland 2007 (LOMROG) expedition with Swedish icebreaker /Oden/ supported by the new Russian nuclear icebreaker /50 Let Pobedy/ reached the previously unexplored areas of the southernmost Lomonosov Ridge. Ice erosion was mapped on the ridge crest above 800 m water depth with /Oden's/ newly installed multibeam and chirp sonar system. From this ice eroded area two cores were taken that contained a stiff diamicton below a slightly less than 2 m thick drape of mud deposited after the ice erosional event. These cores will allow dating of the ice erosional event. After mapping portions of the Lomonosov Ridge, the Morris Jesup Rise protruding from the Northern Greenland Continental shelf, was investigated for glacial features. Remarkably large iceberg scours as deep as 1050 m below present sea level were mapped crossing the Morris Jesup Rise from West to East. These new glaciogenic data suggest that large ice shelves occupied parts of the Arctic Ocean during glacial maxima, ice rises were formed over the Chukchi Borderland and portions of the Lomonosov Ridge, and icebergs with drafts deeper than 900 m scoured the Morris Jesup Rise. This presentation is on behalf of the entire LOMROG Scientific Partly and the coring group of the HOTRAX scientific party.
PP42B-07 INVITED
North Atlantic Deepwater Responses to Obliquity and Precession
Many studies have shown that times of increased ice volume, particularly the last glacial maximum (LGM), correspond to decreases in North Atlantic deepwater (NADW) at depths below ~2000 m. Here we investigate the different effects of obliquity and precession forcing on NADW using 30 benthic δ13C records spanning the last 425 kyr. We find that obliquity-forced minima in high latitude summer insolation correlate with lower benthic δ13C values at Atlantic sites below 2500 m, consistent with reduced production of lower NADW as at the LGM. In contrast, precession-forced minima in northern hemisphere summer insolation correspond with greater benthic δ13C values at Atlantic sites from 2500-4000 m water depth, suggesting increased production of lower NADW. Because obliquity and precession produce different responses to the same change in high-latitude northern insolation at the summer solstice, lower NADW production is likely more sensitive to insolation at a different latitude or season, perhaps due to sea ice formation, freshwater flux, or meridional heat transport.
PP42B-08
The High Latitude Glacial Ocean as Viewed Through the Sea of Okhotsk Window
The physical and biological properties of the Sea of Okhotsk's upper kilometer are probably more similar to comparable depths of the high latitude glacial ocean than to any other modern sea other than the Arctic. Evidence for this stems from the fact that in Holocene sediments of other modern seas the accumulation rates of shallow- living far exceed those of deep-living radiolarians. In Sea of Okhotsk Holocene, and high latitude glacial sediments of the North Pacific, Bering Sea and Antarctic, the accumulation rates of deep- may equal or surpass shallow-living radiolarians, producing sea floor assemblages with greater than 20 percent of the deep-living { \it C. davisiana}. Glacial sediments with such high { \it C. davisiana} percentages are ubiquitous in these seas as well as the Atlantic at latitudes above 45 degrees. In the Sea of Okhotsk these unusual accumulation rates result from low radiolarian concentrations in cold (-1 to 0 C) near-surface water between 20 and 200m, and higher concentrations, including { \it C. davisiana}, in warmer (1 to 2 C) subsurface water between 200 and 1000m (Nimmergut and Abelmann, 2002). This is evidence that water column properties similar to the modern Sea of Okhotsk and the Arctic were widespread in high latitudes of the glacial ocean. Cold winter air and the nonlinear behavior, at near freezing temperatures, of both heterotrophic metabolism and seawater's coefficient of thermal expansion, produce these unusual physical and biological properties in the Sea of Okhotsk today as they probably did in latitudes above 45 degrees in the glacial Atlantic, Pacific and Antarctic Oceans. Such water column properties promote the spread of sea ice, reduce winter heat flux from ocean to atmosphere and provide for the efficient transfer of carbon from the atmosphere to the deep-ocean. Strengthening or weakening these water column properties, in response to changing winter air temperatures, probably amplified glacial interglacial climate change.