PP41C-0674
Quantitative reconstructions of oceanic paleoproductivity using dinocysts: the approach and application to the Last Glacial Maximum in the Northern North Atlantic
In order to develop a proxy of past productivity, we explored the relationship between the assemblages of organic- walled dinoflagellate cysts (or dinocysts) in marine sediment and productivity estimated from remote sensing. Dinoflagellates represent an important part of the primary production in the ocean. Their populations, which include both phototrophic and heterotrophic taxa, depend upon the trophic structure of upper water masses. Here, we used dinocyst data from 1171 surface sediment samples collected in the North Atlantic, Arctic and North Pacific oceans. For each sample site, we compiled two sets of primary productivity data derived from satellite observations: (1) The dataset from the Coastal Zone Color Scanner (CZCS) program applied to observations from 1978 to 1989 and (2) the data set from the MODerate resolution Imaging Spectroradiometer (MODIS) program using observations from 2002 to 2005. Canonical correspondence analyses (CCA) were performed with 57 dinocyst taxa and 8 sea-surface parameters (winter and summer salinity, winter and summer temperature, sea-ice cover, summer, winter and annual primary productivity). Results show that primary productivity is significantly correlated with the first or second component of dinocyst assemblages, depending upon the ocean basin considered, with the strongest relation being observed in the North Pacific. We tested the modern analogue technique (MAT) for quantitative reconstruction of productivity using dinocysts. The error of prediction is ±15-25 % depending upon the productivity dataset, with the best performance obtained for winter productivity using the MODIS data. It is noteworthy that the error of prediction for all estimated productivity parameters is lower than the differences between productivity data derived from the MODIS and CZCS datasets. Therefore, we conclude that dinocysts can be used to reconstruct productivity with reliability as good as possible given the uncertainty inherent to primary productivity estimates from satellite observations. MAT has been applied to northern North Atlantic time series, with special attention paid to the last glacial maximum (LGM). The reconstructions using MODIS and CZCS datasets indicates generally low productivity during the glacial stage, especially during the Younger Dryas and Heinrich events, with annual productivity of less than 100 gC m-2. Past productivity estimates for the LGM are consistent with simulation of the North Atlantic Ocean productivity in the hypothesis of reduced Atlantic Meridional Overturning as proposed by Schmittner (Nature, 2005) based on coupled climate-ecosystem model simulation.
PP41C-0675
Decadal Scale Variability in sub Antarctic Surface and Intermediate Water Properties Across the Laschamp Geomagnetic Excursion at ODP Site 1233
Here we report decadally resolved planktonic and benthic foraminiferal oxygen isotopic proxy reconstructions of surface and intermediate ocean properties across the Laschamp geomagnetic excursion (32-43 ka) from Ocean Drilling Program (ODP) Site 1233 (41°00'S, 74°27'W, 838m) on the Chilean continental margin. The sedimentation rates at this site spanning MIS 3 (~2.2 m/kyr) allow us to nominally achieve a temporal resolution of less than 20 years with 4cm sample spacing. Site 1233 is located on the northern margin of the Antarctic Circumpolar Current (ACC) and the seafloor lies in the core of Antarctic Intermediate Water (AAIW). Thus the site is ideally situated to reconstruct both near surface and AAIW variability in the high southern latitudes. The clearly defined Laschamp event in our core provides a stratigraphic reference for comparing the relative rate and timing of abrupt changes observed at our site to those observed globally. Our initial foraminiferal oxygen isotopic results show that variations in intermediate ocean properties and climate of the southeast Pacific closely align with those recorded in the EPICA ice core from Dronning Maud Land on millennial and even centennial timescales. The broad coherence of the observed Antarctic signal supports the concept of hemispheric thermal asynchrony on millennial timescales. The extension of this climate signal into the intermediate ocean demonstrates that AAIW is extremely sensitive and responds rapidly to climate variability in its source region on a broad spectrum of timescales. Using the Laschamp as a reference point, we compare the timing of our AAIW changes to those in other regions to evaluate the importance of AAIW in propagating climate signals.
PP41C-0676
Coupling Between Southern Hemisphere Climate and AAIW Revealed by Decadally Sampled Records of sub Antarctic Surface and Intermediate Water Property Changes from 20-75 ka
Ocean Drilling Program (ODP) Site 1233 (41°00'S, 74°27'W, 838m) provides an essential marine counterpart to the recent high resolution EPICA ice core from Dronning Maud Land: monitoring changes in circumpolar ocean circulation and water masses. ODP Site 1233 is situated beneath the northernmost reaches of the Southern Westerlies at a water depth intersecting Antarctic Intermediate Water (AAIW) and is thus perfectly located to fill crucial gaps in our knowledge of key Southern Hemisphere processes. Here we report decadally resolved planktonic and benthic foraminiferal isotopic records spanning MIS 3 (20-75 ka) from this exceptionally high sedimentation rate core (2.2m/kyr) retrieved from the Chilean slope. The benthic foraminiferal carbon and oxygen isotopic records monitor the physical and chemical properties of AAIW close to its source in the SE Pacific, while the planktonic isotopic records monitor the near surface water properties and the large scale atmospheric (Westerlies) and oceanic (ACC) systems influencing them. Our planktonic records suggest that surface water properties varied on multi-centennial to millennial timescales throughout MIS 3, while our bottom water proxy suggests that these surface water warmings were accompanied by concomitant changes in AAIW properties. As in the records of air temperature from Antarctica, warm events appear relatively symmetric; there is no systematic difference in the rate of warming versus cooling, and the bulk of both warming and cooling is often accomplished in as little as a few decades. The extremely rapid rate at which these surface and intermediate ocean changes are accomplished is indicative of their origin. Such abrupt and concurrent shifts in extratropical surface and intermediate ocean properties are most likely driven by changes in the major dynamical systems of Southern Hemisphere ocean and atmospheric circulation which govern their properties today (the Westerlies and the ACC). The similarity in timing and rate of these changes on the fringes of the Southern Ocean relative to those recorded closer to the pole in the new ice core records suggests that such changes are broadly felt and the involve the major circumpolar dynamical systems: resulting in their propagation into the ocean interior. We discuss what implications such abrupt, synoptic scale changes in Antarctic climate and dynamics have on the origin and propagation of abrupt climate change.
PP41C-0677
Decadal to Million-Year Sedimentation Rates and the Ultra-High-Resolution Marine Record From Santa Barbara Basin
An inherent feature of the sedimentary record is that it is incomplete. Sadler (1981) showed that sediment accumulation rates are inversely related to the time-span of measurement for nearly all depositional environments - sediment accumulation rates decrease with larger time frames of interest because of the inclusion of more and larger undetected intervals of nondeposition or erosion. Nonetheless, investigation of the mechanisms and timing of climate change at high-resolution relies on the assumption of continuous, rapidly accumulating sedimentary successions. This creates a serious problem that is rarely addressed. Quaternary sedimentation in Santa Barbara basin (SBB) stands in marked contrast to this general trend by maintaining similar high rates of sediment accumulation spanning 5 orders of magnitude in time over the past million years. ODP Site 893 recovered the past ~160 kyr with 200 m of core. We have extended this record further in time by recovering more than thirty short piston cores from a breached anticline in the eastern SBB where older Quaternary strata have been uplifted and eroded. These cores demonstrate the persistence of alternating laminated and massive/bioturbated sediment in the older deposts similar to that observed at Site 893, as well as abundant microfossils suitable for ultra-high-resolution paleoclimate and paleoceanographic studies. Biostratigraphic and tephrachronologic age datums confirm that strata cropping out on the anticline extend back to 700 ka, an age previously unreachable by conventional deep coring methods. Linear sedimentation rates (LSR) estimated from varve-counts in laminated intervals are remarkably uniform across the entire age range sampled, and vary from about 80 to 130 cm/kyr, similar to the average LSR at Site 893 (120 cm/kyr) derived from radiocarbon dates and oxygen isotope stratigraphy. Correlated, dated, and depth-converted seismic stratigraphy indicates that such unusually high sedimentation rates are continuous for at least the last 1 m.y., with the average depth (bsf) to a dated 1-Ma horizon varying from 1.2 to 1.8 km (120 to 180 cm/kyr) over much of the deep SBB. These initial data suggest a remarkably uniform and rapid sedimentation process in spite of inclusion of evitable hiatuses or condensed sections. These older strata have already yielded paleoclimatic findings of similar resolution to ODP Site 893. Isotopic compositions of benthic and planktonic foraminifera reveal millennial and sub-millennial scale climate fluctuations in the climate record back as far as ~500 ka that include abrupt (decadal) climate jumps. SBB is thus quite capable of extending its usefulness as an ultra-high-resolution paleoceanographic record back further in time with continuous deeper drilling. Sadler, P.M., Sediment accumulation rates and the completeness of stratigraphic sections, Journal of Geology (September 1981), 89(5):569-584.
PP41C-0678
The marine and freshwater stages of the Black Sea: initial results on sediment cores from the SE Black Sea covering the last ca. 140 kyrs
Coring efforts during the last years rather suggested an extensive glacial sediment cover on most of the Black Sea slope areas not reachable with conventional gravity and piston coring devices. Here we present new sediment cores retrieved from the tectonically formed Archangelsky Ridge in the southeastern Black Sea during this year's RV Meteor cruise M72/5, which provide a first view into a most probably complete and undisturbed section of the last glacial period. Initial results from continuous XRF scanning on the split sediment cores suggest strong and immediate responses of the glacial Black Sea freshwater lake to the abrupt D-O climate oscillations of the last glacial period. At the base of one of the cores the complete marine unit deposited during the last global sea level highstand in Marine Isotope Stage 5 - the "Eemian" sapropel - is exposed. The intercalation of dark organic-rich intervals with light layers of aragonite precipitates and coccolith ooze in this unit demonstrates that the last marine stage of the Black Sea must have had a more complex history than we know from the Holocene. Despite the fact that the exact dating of this sequence will be challenging, the "Eemian" sapropel will provide the opportunity not only for a comparative study to the Holocene, but will also allow to understand how the marine Black Sea returned to a glacial freshwater lake. Furthermore, the finely laminated "Eemian" sapropel will provide insights into interannual to decadal-scale climate variability of the previous interglacial.
PP41C-0679
Drawdown Episodes of the Labrador-Quebec Ice Dome Through Ungava Bay During Heinrich Events? Insights From the Sediment Sources of Ice Streams
Glacial terrains surrounding the Ungava Bay region are characterized by abundant drumlins, crag-and-tails, and other lineations indicating convergent ice flow towards the bay. These geomorphologic features have been interpreted to represent a complex network of fast-flowing ice (ice streams), which drained the eastern sector (Labrador-Quebec ice dome) of the Laurentide ice sheet at some time during the last glacial cycle. It was recently proposed that the Ungava Bay ice streams may be responsible for the massive iceberg discharges in the North Atlantic that are at the origin of Heinrich events (e.g. Jansson et al., Boreas, v. 32, p. 256, 2003). Several other provenance studies have attributed the characteristic high detrital carbonate content of these anomalously thick layers of ice rafted deposits (IRD) to a calving ice margin that overrode the carbonate-floored Hudson Bay/Strait, while lead isotopes and 40Ar/39Ar ages measured on feldspar and hornblende grains have identified the rock-source area of Heinrich layers as being the Paleoproterozoic Churchill province of the Canadian Shield. Although the Laurentide ice sheet was unequivocally the main contributor of detrital material to Heinrich layers, the exact sector of the Laurentide ice sheet that collapsed and drained through Hudson Strait has yet to be identified, and is currently ascribed to the broader Hudson Bay/Strait region. Here we evaluate the possible contribution of the Labrador-Quebec ice dome to these iceberg discharges by documenting the provenance of glacial sediments (tills) representing the catchment (source) area of the Ungava Bay ice streams. Specifically, we used the 40Ar/39Ar fusion method to date about 10 individual hornblende grains in 13 samples distributed along a latitudinal transect (lat. 58°) extending east and west of Ungava Bay. We also date hornblende grains from IRD and bounding sediments in a North Atlantic deep-sea core (Orphan Knoll) to evaluate whether the provenance signature of Ungava Bay glacial deposits can be recognized in sediments spanning the time interval comprised between H4 to H0. The results show that most tills located east of Ungava Bay are characterized by grains with early Paleoproterozoic ages (2.0 to 1.8 Ga), with a few till samples containing a small population of grains with Archean ages (>2.6 Ga). Tills west of Ungava Bay are largely dominated by Archean-age grains. These results contrast with the predominant late Paleoproterozoic 40Ar/39Ar ages (1.8 to 1.6 Ga) that characterize hornblende grains in typical Heinrich layers, thereby indicating that drawdown episodes of the Labrador-Quebec ice dome through Ungava Bay during Heinrich events are unlikely. Hornblende grains from marine deposits are currently being analyzed and the results should provide information on when these ice streams were active during the last glaciation.
PP41C-0680
Response of the Surface and Deep Ocean to Abrupt Climate Events in the South East Atlantic Ocean During the Middle Brunhes
Recent work has shown that millennial-scale climate events punctuated many of the most recent glacial periods, and may have involved re-organisations of the Atlantic Meridional Overturning Circulation (AMOC). While numerical models often seek to understand how these events may have been manifested in locations outside of the North Atlantic Ocean, relatively little proxy data exists to test these simulations. New data has been produced from ODP-1085B (1713m water depth) in the South East Atlantic Ocean, and includes information on sea-surface temperatures, intermediate water hydrology, bottom water nutrients, and biological productivity. A preliminary age model is based on a visual correlation to the EPICA Dome C deuterium record plotted on the EDC-3 timescale and shows that the dataset extends from Marine Isotope Stage (MIS)-9 to MIS-13. Abrupt climate events punctuated MIS-12 and MIS-10, and resulted in moderate surface-water warming in the South-East Atlantic Ocean, associated with a southwards shift in the southern edge of the trade-wind belt (Intertropical Convergence Zone) and a reduction in the influence of southwards-flowing North Atlantic Deepwater (NADW) on bottom water hydrography. Carbon isotope values of deep-dwelling planktonic foraminifera and fine-grained bulk carbonates also suggest a possible shoaling of Antarctic Intermediate Waters in response to reduced NADW export during these events, but this remains a testable hypothesis with the current dataset. Within the constraints of the age- model the abrupt events are seen to be coincident with surface water cooling in the North Atlantic Ocean, suggesting a link to hydrographic forcing at sites of NADW production. The events are superimposed on glacial- interglacial shifts in the Southern Hemisphere frontal systems which promote large-scale changes in the mode of sedimentation at ODP-1085B.
PP41C-0681
Late Holocene Radiocarbon Variability in Northwest Atlantic Slope Waters
Deep-sea gorgonian corals secrete a two-part skeleton of calcite and proteinaceous gorgonin. The calcite is derived from dissolved inorganic carbon at depth, while the gorgonin is tightly coupled to recently fixed and exported particulate organic matter. Radiocarbon content of the calcite and gorgonin provide direct measures of seawater radiocarbon at depth and in the overlying surface waters, respectively. Using colonies of Primnoa resedaeformis and Keratoisis ornata, we have developed radiocarbon timeseries records for surface and intermediate waters at 3 sites in the Northwest Atlantic slope: 1) Northern Labrador Sea (approx. 61°N/61°W, 500 m depth), (2) Southwest Grand Banks (44°N/53°W, 700 m), 3) Northeast Channel (42°N/65°W, 400 m). Pre-bomb surface water Δ14C averaged -69 ± 1 ‰ (1sd, n = 5) at site 1, -70 ± 4 ‰ (n = 8) at site 2, and -76 ± 4 ‰ at site 3. Corresponding reservoir ages averaged 401 ± 30 yrs, 451 ± 68 yrs, and 471 ± 28 yrs, respectively. Pre-bomb intermediate water Δ14C averaged -65 ± 5 ‰ (n = 2) at site 1, -73 ± 3 ‰ (n = 8) at site 2, and -90 ± 10 ‰ (n = 13) at site 3. At sites 1 and 2, surface and deep water Δ14C values were statistically equivalent, reflecting rapid vertical mixing of Labrador Sea Water. At site 3, pre-bomb intermediate-water Δ14C was significantly lower than at sites 1 and 2, oscillating quasi-decadally between -105 and -80 ‰. These changes probably reflect North Atlantic Oscillation-driven shifts in the relative dominance of younger Labrador Sea Water versus older Warm Slope Water originating from further south. We also analysed 3 subfossil K. ornata colonies that date to between 1050 and 1280 calendar BP from the Northern Labrador Sea. During this interval, intermediate waters (1000 m) were 80 yrs older than contemporaneous surface waters, reflecting greater stratification than at present. These results demonstrate the utility of deep-sea gorgonians in tracking the mixing of water masses in dynamic regions.
PP41C-0682
North Atlantic Glacial-Interglacial Ostracode Faunal Patterns through the Mid-Pleistocene Transition at ODP Site 980 and 982
Glacial-interglacial cycles through the Mid-Pleistocene Transition (MPT; 1.2 to 0.6 Ma) are characterized by a frequency decrease from 41 to 100 kyr and an amplitude increase via more extreme glaciations. This major transition has been the focal point of much North Atlantic paleoceanographic and micropaleontological research, yet little is know about North Atlantic deep-ocean benthic ostracode faunas through the MPT. One might predict such glacial-interglacial cycles to affect ostracode assemblages, yet North Atlantic ODP Sites 980 (1,145 m) and 982 (2,168 m) reveal no major or consistent changes in ostracode accumulation rates, resampled (abundance- corrected) richness, or taxon percent abundances, with the exception of an increase in ostracode accumulation rates late in the MPT at the deeper Site 980. This ostracode faunal stability is consistent with Raymo et al.'s (2004) interpretation of relatively stable North Atlantic thermohaline circulation through the MPT based on carbon-isotope depth-profiles. However, this ostracode faunal stability stands in stark contrast to other North Atlantic ostracode studies of the latest Pleistocene (Didie and Bauch, 2000) and Late Pliocene (Cronin et al., 1996), which show strong faunal variations in response to presumed oscillations in surface export production and intermediate water ventilation associated with glacial-interglacial cycles. Late in the MPT interval (0.8 to 0.6 Ma), benthic foraminifera experience a major decline and preferential extinction of infaunal taxa at Site 980 and 982 (Kawagata et al., 2002). Coeval ostracode assemblages show no comparable taxonomic loss through this enhanced extinction interval, but ostracode accumulation rates do markedly increase at the deeper Site 980. Such differential biotic response under presumably stable benthic environmental conditions remains difficult to reconcile with current paleoceanographic data.
PP41C-0683
Fresh water forcing hypothesis of abrupt climate change: A test of consistency with sea level reconstructions
A synthesis is made of observational and modeling perspectives of abrupt climate change in order to test their consistency with the fresh water forcing hypothesis. The synthesis is carried out using stochastic inverse modeling of a 4-box model of the ocean's meridional overturning circulation (MOC) in order to infer the volume of fresh water input to the North Atlantic that is required to explain the Greenland ice core proxy records of abrupt climate change from 39 ka to 30 ka. The similarity in the amplitude and timing of predicted sea level change with reconstructions from the geologic record give added support to the fresh water forcing hypothesis. In particular, the inversion suggests that the duration of the abrupt warmings should be directly related to the volume of land ice growth. This conclusion is unaffected by uncertainties in the processes that govern mixing of deep polar waters into the tropical thermocline. The fresh water forcing solutions were also able to reproduce the Greenland data within the seasonally-resolving Bern climate model (Schmittner and Stocker, 1999, J Clim), suggesting that the solutions may be valid forcing scenarios for more realistic ocean GCMs.
PP41C-0684
Environmental Magnetism From the Gardar Drift Since MIS5e
A series of cores along NE-SW transect across the Gardar Drift were collected during a cruise of the RV Knorr in 2002. U-channel samples taken from three cores (JPC2, JPC5 and JPC13) show significant changes in magnetic properties (magnetic susceptibility, magnetic grain size) along the drift transect. Changes in magnetic (magnetite) grain size, as indicated by karm/k, may provide a proxy for strength of bottom currents moving over the drift. On the northern section of the drift (Core JPC2), changes in magnetic grain size can be seen to track glacial/interglacial cycles, with coarser grain sizes in MIS1 and MIS5 and finer grain sizes in MIS2 and MIS4, with a coarsening trend with age in MIS3. On the southern end of the drift (Core JPC13), magnetic grain size fluctuations in MIS3 accompany melt-water events identified by low planktic oxygen isotope values. This may be interpreted in terms of bottom-current strength variations in response to surface melt-water events. Volume magnetic susceptibility decreases southwest along the drift and cannot be correlated amongst the cores. The benthic oxygen isotope record from Core JPC13 can be clearly correlated to the benthic isotope record from Core MD95-2042 from the Portuguese Margin that has been correlated to the GISP ice core via its planktic oxygen isotope record (Shackleton et al., 2004, SFCP04). This allows Core JPC13 to be placed on the SFCP04 GISP chronology.
PP41C-0685
Climatic Shifts and Geomagnetic Excursions
Studies arguing for a link between geomagnetic field variability and climate change have been inconclusive due to a variety of reasons including low-resolution paleomagnetic or climatic records, age uncertainties, and strong lithologic biases in relative paleointensity records. We present an improved chronology of a high-resolution paleomagnetic record recovered from Ocean Drilling Program Site 1061 (Leg 172) on the Blake Outer Ridge (BOR) in the western North Atlantic Ocean in order to test for such a relationship. Sediments from the BOR are established recorders of climate change and have the high sedimentation rate (10-25 cm/k.y.) necessary to document geomagnetic variability on a scale that allows for the accurate comparison of timing with proxies of climate change. Additionally, the sediments contain a large number of excursions that we use here to resolve short-term geomagnetic variations. This is important, as our correlation to climatic change is not dependent on relative paleointensities alone. Our age model refined the existing astronomically-tuned model for the area via high-resolution stable isotope measurements over intervals containing excursions, which were then correlated to the global isotopic stack of Lisiecki and Raymo [2005]. Using the resulting complete and continuous well-dated record of geomagnetic field variability we found excursions to occur predominantly after or synchronous with large isotopic changes and to occur preferentially within interglacials.
PP41C-0686
Increased Sensitivity of Northern Component Water on Millennial Scales During Glacial Intervals
The North Atlantic is a highly sensitive recorder of the Earth's climatic history partially due to Northern Component Water (NCW) formation in the Norwegian, Greenland and Labrador seas. Previous studies show strong correlations between abrupt climatic events such as Heinrich Events and perturbations in thermohaline circulation, particularly in areas regionally influenced by NCW formation. We present a record of NCW flow during the late Pleistocene from two sites drilled to the south of Greenland during IODP Expedition 303. Sites U1305 and U1306 are located on the flank of Eirik Drift at depths of 3459 m and 2274 m, respectively. The combination of mean sortable silt records at Sites U1305 and U1306 yield a high-resolution record (100 to 300 yr temporal resolution) of NCW flow strength and position from the last glacial maximum to present. Our results show a bimodal relationship in NCW buoyancy and flow position on Eirik Drift over the last glacial cycle. The glacial interval is characterized by high sedimentation rates over Site U1306 and is interpreted as a more buoyant NCW water mass. The interglacial interval shows high sedimentation rates over Site U1305 and is interpreted to be a less buoyant NCW water mass. An important observation in the sedimentation patterns on Eirik Drift is that the deepening of NCW over Eirik Drift lagged the onset of the interglacial by ~2 kyr. The expanded glacial interval (MIS 2-4) at Site U1306 records a series of cycles in the sortable silt record. These cycles are well defined and have durations of 1 to 3 kyr. In contrast, the Holocene sortable silt record from the deeper site shows cycles of similar duration, but with smaller amplitudes. The glacial cycles infer that the intensity and locus of NCW was more sensitive to climate variability. We attribute this sensitivity to the proximity of large terrestrial ice sheet and its capability to alter the fresh water contribution to this region.
PP41C-0687
Climatic variability during MIS 7 in the Norwegian Sea; based on benthic and planktonic foraminiferal fauna and stable oxygen isotopes
A new record (MD99-2288) from the Vøring Plateau in the Norwegian Sea has been investigated with emphasis on surface and intermediate water masses during stage MIS 7. The core was raised from a water depth of 1262 m situated under the influence of the Norwegian Sea Arctic Intermediate Water (NSAIW) (and the Norwegian Sea Deep Water (NSDW). A total of ca. 140 samples are analysed with regard to foraminiferal assemblages (benthic and planktonic), stable oxygen/carbon isotopes (Neoglobigerina pachyderme s., Cassidulina neoteretis), grain size distribution and MSCL-analyses. The age model of MIS 7 is based on the stable oxygen isotopes of benthic and planktonic foraminifera and the oxygen isotope curves are compared with recent records from the North Atlantic The MIS 7 record in the Norwegian Sea is found to be in general accordance with other records further south. The stage is however characterised by one very cold stadial, MIS 7.4, with high d18O values. Towards the end of the stadial the oxygen isotopes are characterised by some very low d18O values, which imply sever conditions during that event. This event falls together with faintly laminated sediments deposited at a sedimentation rate of approximately 11 cm per ka. Further, MIS 7.3 is found to be warmer than MIS 7.5. Both interstadials show low d18O values and a high percentage of Bulimina and Cibicides species. This suggests a stronger influence of warm water masses. Compared to records further south MIS 7.2 and 7.1 show higher d18O values earlier in time and the d18O values are nearly comparable to the values during MIS 6. The sedimentation rate (terrigenous input) is also increasing during this time period (ca. 7.5 cm per ka) compared to MIS 7.3 (2.2 cm per ka). The significant increase in the percentage of Cassidulina neoteretis (ca. 80 percent) suggests that shift to cooler climate.
PP41C-0688
Glacial-Interglacial Variations in Ostracod Assemblage Composition at IODP Site U1314 in the North Atlantic
North Atlantic climate changes have affected surface water temperature, primary productivity, and deep water production and circulation patterns. These changes are recorded in deep sea sediments as fluctuations in the proportions of planktonic and benthic microfossils. Here, we analyzed sediments from IODP Site U1314 (56.36° N; 27.88° W, water depth: 2820 m) in the North Atlantic for glacial-interglacial changes in ostracod species composition, total CaCO3 content and the presence of ice-rafted debris (IRD) during marine isotope stages (MIS) 1 to 6. Our results show a diverse ostracod fauna (more than 70 species) that exhibits long and short term variations apparently associated to glacial and interglacial climatic conditions. The most abundant genera are Krithe, Rockallia, Henrihowella, Oxycythereis, Cytheropteron and Thalassocythere. Krithe is the dominant genus (~40-90%), but exhibits relatively lower abundances during MIS 2, and during intervals of high CaCO3 content in MIS 5. Henrihowella and Thalassocythere do not seem to be associated with any particular climatic condition, however Rockallia is practically absent from MIS 1 and 5e. Among the rest of the taxa, two assemblages were identified to be associated to interglacial and glacial conditions. The interglacial assemblage occurs during MIS 1 and 5, and is characterized by Oxycythereis, Ambocythere, Pelecocythere, Bradleya and Echinocythereis. The glacial assemblage is primarily composed of Bythocythere, Polycope, Swainocythere and several species of Cytheropteron, and occurs during MIS 2 and 6. Detailed comparison of ostracod abundances and the IRD record also revealed short-term climate related changes related to ice-sheet instability in the North Atlantic. These are represented by intervals of increased terrigenous sediments (Heinrich Events) which yielded ‘shallow water' taxa (e.g., Heterocyprideis sorbyana, Eucytherura calabra), both suggesting increased iceberg discharge. A qualitative ostracod dissolution index was used to determine carbonate preservation patterns. The index documented a general pattern showing enhanced corrosion during interglacial times and good preservation during glacial times. The observed dissolution pattern appears to be out of phase with the general rhythm of glacial-interglacial carbonate preservation in other regions of the Atlantic, but in phase with that in the Nordic Seas and the Pacific Ocean. The impact of dissolution on ostracods was also observed as a reduction in their abundance.
PP41C-0689
The Biomarker Inventory, Trace, and Source of Heinrich Events and Heinrich-type Layers (MIS 8-16) in the North Atlantic
Multiple cores from different locations in the North Atlantic were recovered during IODP Expeditions 303/306. We have investigated the biomarker distributions of identified Heinrich layers (HL) and ambient glacial/interglacial samples from Site U1305, located in the Labrador Sea in proximity to the former major iceberg discharge pathway derived via the Hudson Strait from the Laurentide-Ice-Shield (LIS).A unique association of a multitude of "petrogenic" compounds such as benzohopanes, D-ring monoaromatic 8,14-secohopanes, rearranged diasterenes, mono- and triaromatic steranes, isorenieratene-derivatives as well as characteristic pristane/n- C17 and pristane/phytane ratios ideally allows to distinguish organic matter from HL compared to adjacent samples. They therefore are regarded and can be used as organic-geochemical tracers not only for the occurrence of Heinrich Events of the last glacial cycle but also for older Heinrich-like events.We present a biomarker based high resolution record of such events during a time interval from about 320-460 kyr at Site U1313 (MIS 10-12) and an extended record (MIS 8-16) of Heinrich-type events from Site U1308, i.e. from positions near the Mid-Atlantic Ridge and distal to the LIS.In addition, the HL-specific biomarker association provides circumstantial evidence to derive from a relatively immature, Paleozoic marine rock deposited under occasional photic zone anoxic conditions, that is today located on the Laurentide/Canadian shield. Reinvestigation of available geologic and organic-geochemical data allowed narrowing down this assumed source to an Ordovician oil shale close to Hudson Strait, that indeed bears a striking resemblance in terms of biomarker distributions when compared to the specific association of compounds from samples of Heinrich Events.
PP41C-0690
Synchronic Response of Arctic Ice-Sheets to Global Climate Deterioration During the Plio- Pleistocene
Here, we report the revised chronostratigraphic and new sedimentological data of various ODP boreholes (Legs 151, 162) in the Atlantic-Arctic gateway region to present a coherent glaciation model for the Barents Sea ice sheet over the past 3.5 Ma. Pulses of IRD during the Northern Hemisphere Glaciation (NHG) at ~2.7 Ma indicate consistency of glacial dynamics of circum-Arctic ice sheets to global climate deterioration. However, large-scale expansion of the Barents Sea ice sheet towards the shelf edge are not evident before ~1.5 Ma. This is inferred not only from IRD pulses but also from a step-wise decrease of Siberian river supplied smectite-rich sediments likely caused by ice-sheet blockade and glacigenic wedge growth along the northern and western Barents Sea margin. Large-scale glaciation in the Barents Sea occurs since ~1 Ma (Mid-Pleistocene Transition) and repeatedly advanced to the shelf edge at least 7 times thereafter. The timing is inferred from ice grounding on the Yermak Plateau at ~0.9 Ma attributed to a pronounced ice sheet advance or deep-draft icebergs and higher frequencies of gravity-driven mass movements along the western Barents Sea margin associated with glacial growth.
PP41C-0691
Millennial-Scale Climate Variability During the Mid Pleistocene
We use the oxygen isotopic composition of planktonic foraminifera Globigerinoides ruber (white) from Ocean Drilling Program ODP Site 1058 in the subtropical northwestern Atlantic to construct a high-resolution (~800 year) climate record spanning the mid-Pleistocene transition (~0.41 Ma to 1.35 Ma). We test the hypothesis that millennial-scale climate instabilities are associated with the extent of continental glaciation. We apply two age models in order to assess the effect of the chronology on the timing of suborbital variations. The first age model is derived by tuning the isotope record to the G. ruber record from ODP Site 677, and the second by tuning to Northern Hemisphere (NH), summer insolation. Independent of age control, results indicate the appearance of significant (95% level) millennial-scale (4-8 kyr) variability at ~880 Ka (Marine Isotope Stage (MIS) 20). These types of instabilities show the largest amplitude during MIS 12 supporting the idea that millennial-scale fluctuations are associated with the presence of a large NH ice sheet. Higher frequency (2-4 kyr), but lower amplitude, variations occur during glacial and interglacial intervals throughout the mid Pleistocene.
PP41C-0692
Significant differences between the millennial variability in IRD of the last and penultimate glaciations in the North Atlantic
The complete, high-resolution (approx. 250 year sampling interval) MIS-6 record of hematite-stained grains (HSG) and Icelandic volcanic glass at IODP Site U1308 (reoccupied DSDP 609) indicates that significant differences in millennial climate variability existed between the last and penultimate glaciations. Relative to the last glaciation, 1) HSG cyclicity is lower in amplitude and not as well developed; 2) HSG and glass do not covary; 3) Glass is virtually absent during several intervals. While both the HSG and glass records exhibit 3-sigma significant periodicity at ~ 950 and ~ 1075 years, respectively, without further evaluation of age model uncertainties, it is unclear whether these records are truly periodic.
PP41C-0693
5Ma Record of extraterrestrial 3He flux, sediment provenance, and detrital fluxes at IODP Site 1313 in western North Atlantic
During Expedition 306, DSDP Site 607, which is located along the western flank of the Mid-Atlantic Ridge (41° N, 32°W; water depth = 3426m), was reoccupied and new cores collected at a nearby location IODP Site 1313. Site 1313 has sedimentation rates that average ~4.5 cm/ka for more than 5Ma, which is unusual considering the changes observed in isotopic records, detrital inputs, and other proxy records (Ruddiman et al. 1989, Raymo et al. 1989). The remarkably constant sediment deposition during the Plio-Pleistocene, along with excellent age control, makes it an ideal location to analyze sedimentary fluxes and inputs quantitatively in a climatically sensitive region of the North Atlantic. Helium isotopes can provide important new information on sedimentary fluxes, detrital inputs and sources, and extraterrrestrial 3He particle fluxes from a well-constrained system with relatively high sedimentation rates. Previously, fluxes of extraterrestrial 3He have been linked to climate variability and to evidence for ocean circulation as well as serving as a long-term constant flux proxy (Muller and MacDonald, 1995; Farley and Patterson, 1995;Marcantonio et al. 1996). Almost all previous 3He studies on long timescales have come from cores located in the Pacific basin that have lower sedimentation rates (<2cm/ka). The results of our helium isotope analyses to date have focused on two intervals in the late Quaternary (0.5 to 1.3Ma) and the Pliocene (3.4 to 5.0 Ma). The Late Quaternary section is dominated by changes from 41ka world to 100ka world and Northern Hemisphere ice sheets. Based on 4He and other proxies, detrital inputs are 10X higher than the earlier Pliocene interval. 3He/4He ratios are 3X lower than the Pliocene interval and indicative of the expected IRD and sediment source areas from Greenland and Canadian provinces. The extraterrestrial 3He flux varies by a factor of 3 to 4 in the Late Quaternary but this variability is driven by carbonate dilution as previously found in cores from Equatorial Pacific (Marcantonio et al., 1996). The average flux over last 1.3 Ma, inclusive of Farley and Patterson's (1995) earlier 3He work at Site 607 (200-450 ka), is ~1.0±0.35×10-12cm3STPcm-2ka-1, which is quite consistent with many studies of Late Quaternary sediments. The Pliocene interval has the same sedimentation rates as the Quaternary, much lower detrital content, a different source/proportion of detrital sources, and, most significantly, has a marked ~3X decrease in average extraterrestrial 3He flux and variability. Ongoing work will provide a complete 5Ma record.
PP41C-0694
Surface and Deep Water Variations During MIS 11-15 at IODP Site U1313 in the Mid-Latitude North Atlantic: Insights Into Mid-Pleistocene Rapid Climate Change
IODP site U1313, the recording of DSDP site 607, is nowadays located within the transitional waters between the subpolar and the subtropical fronts and influenced by the North Atlantic Drift (NAD). Studies on DSDP site 607 (e.g. Raymo et al., 1989) further revealed that the prevailing deep water mass changed from North Atlantic Deep Water (NADW) to Antarctic Bottom Water (AABW) on interglacial to glacial time scales, respectively. To contribute to the better understanding of past climate variability within interglacials and of rapid climate change during glacial inceptions and glacials we produced planktonic and benthic stable isotope and ice-rafted debris (IRD) records with a temporal resolution of 250 to 600 years for the interval from 355 to 640 ka. In addition to the glacial/ interglacial deep-water variations previously seen in the DSDP site 607 records, our high-resolution benthic isotope records reveal short-lasting excursions. Within glacial MIS 12 these excursions to lighter oxygen and heavier carbon isotope values are "bundled" into three intervals and are interpreted as a deepening of the NADW/ AABW interface and thus the presence of glacial NADW. During MIS 14 and the section of MIS 16 covered by our record such excursions are not revealed. MIS 15.4 to 15.1 and also the glacial inception of MIS 10 are, on the other hand, periods with higher variability again. During MIS 15 the benthic carbon isotope data reveals that the variations are linked to changes in the NADW composition. The changes within MIS 11, on the other hand, are caused by shoaling of the NADW/ AABW interface and thus incursions of AABW, like during the MIS 3 Heinrich ice- rafting events. In the surface water records higher variability is also observed during MIS 15 and 12. During MIS 12, three longer lasting periods with IRD deposition occurred with peak IRD deposition at the beginnings and ends of these intervals. Thermocline conditions as revealed by the G. inflata records, on the other hand, experienced 6 oscillations with 2-3 degree temperature changes and warmer conditions between the IRD events; again reminding of the MIS 3 stadial/ interstadial variability. Glacial MIS 14 is standing out as the "warmest" glacial in the studied interval and thermocline temperatures were not much colder than during MIS 13, which itself experienced colder temperatures than the other interglacial and interstadial periods. Peak interglacial values were similar during MIS 11 and 15 but thermocline nutrient levels were higher during MIS 15. During terminations nutrient levels gradually decreased and heaviest planktonic carbon isotope values at site U1313 were always reached during the second interglacial temperature rise observed during MIS 15.5, 13.3 and 11.3 in the EDC dD record. The onsets of the glacial inceptions were marked by IRD and cooling events. After the cold event, however, one or in the case of MIS 11 two warmings of thermocline waters are observed. These warmings lasted several thousand years and temperatures were nearly as warm as those recorded during the preceding interglacial (MIS 11.3) or interstadials (MIS 13.1, 15.1). Thus the modern flow path of the NAD persisted during the early phases of glacial inceptions allowing heat transport from the tropics to the mid-latitude North Atlantic and keeping conditions in the western basin much more stable and warmer than those observed along the eastern margin at ODP site 980 (McManus et al., 1999) or off Portugal.
PP41C-0695
Relaxation Oscillations as a Mechanism of Abrupt Climate Change
Climate variability at the millennial time scale is difficult to rationalize, as the frequency 0.001 1/yr falls near the middle of a wide gap in the spectrum of external forcing on the climate system (between the low-frequency orbital components and the high-frequency tidal components). This situation prompted interest in the possibility for the climate system to undergo self-sustained or self-excited oscillations. Our contribution will comprise two parts. First, we will review elements from the theory of non-linear vibrations that provide a framework for the discussion of the fundamental mechanisms responsible for millennial-scale climate variability. Particular emphasis will be put on the self-sustained oscillations that occur in physical systems with one degree of freedom. In such systems self-sustained oscillations arise from the nonlinear dependence of the damping force on velocity. In the limit of very large nonlinearity, the oscillator stores energy for a relatively long period of time and releases this energy in a relatively short time, i.e., the oscillations are strongly asymmetric (relaxation oscillations). Second, we will examine the self-sustained oscillations of the meridional overturning circulation simulated by an ocean circulation model when subject to large freshwater forcing (salt addition at low latitudes and salt extraction at high latitudes). A scaling analysis provides evidence that these oscillations can be fundamentally interpreted as relaxation oscillations : the model ocean stores potential energy in the form of an unstable vertical temperature gradient for a relatively long period of time (phase of reduced MOC) and converts this potential energy into kinetic energy (phase of intense MOC) when the unstable vertical temperature gradient dominates the stable vertical salinity gradient in the density stratification. The merits and weaknesses of the hypothesis of relaxation oscillations as a mechanism of abrupt climate change will be discussed.
PP41C-0696
Objectively Derived Age Models for Marine Isotope Stage 3 Paleoclimate Records From Ice, Land and Sea
A primary goal of paleoclimate studies is to determine the mechanisms and dynamics that are responsible for climate change on long time scales and during times of very different Earth boundary conditions. To achieve this goal, we must be able to determine the spatial and temporal relationships between all elements of the climate system. Recently Clark et al. (2007) combined the spatial and temporal modes of variability defined from a set of 38 paleoclimate time series with ocean and atmospheric models to develop a scenario for the sequence of system responses to climate change observed during MIS-3. Clark et al. (2007) used the published time scales for these records in their data analysis. Empirical Orthogonal Function (EOF) analysis showed that there were two dominant spatial/temporal modes in these diverse time series. Clark et al. (2007) referred to the first EOF as the Northern Signal, which is most important in paleoclimate records from much of the Northern Hemisphere, and the second EOF as the Southern Signal, which is most important in most Antarctic and Southern Ocean records. We will show that the EOF analysis can also provide a framework for refining the chronologies of the paleoclimate records used in these types of studies. Numerical experiments show that the time series of the modes extracted by EOF analysis average out random chronologic errors in the individual time series used in the analysis. Thus it is possible, by mapping the original time series into simple weighted combinations of the extracted EOF modes, to remove these random errors from the chronology of each time series. Applying this strategy to 38 time series from MIS-3 shows that: 1) average errors in chronologies range from 200 to 1800 years with a mean of 620 years; 2) that the modes of variability extracted by EOF analysis are reasonably robust to these chronologic uncertainties; and 3) most of the time series analyzed are not simply one mode or the other but are a complex pattern of combination of the Southern and Northern modes of climate variability. This work was supported by an NSF grant, PALEOVAR to Oregon State University, University of Oregon and University of Minnesota. Clark, P.U., S.W. Hostetler, N.G. Pisias, A. Schmittner, and K.J. Meissner, 2007. Mechanisms for a ~7-kyr Climate and Sea-Level Oscillation During Marine Isotope Stage 3, AGU Monograph, in press