PP13B-1263
Opal burial in the Pacific Southern Ocean since the LGM: Implications for ocean circulation and nutrient cycles
230Th-normalized opal fluxes and 231Pa/230Th ratios were measured in over fifty cores in the Southern Pacific Ocean (100°W-160°W, 56°S-64°S) to reconstruct diatom paleoproductivity through the Last Glacial Maximum (LGM) and to test the Silicic Acid Leakage Hypothesis (SALH). The SALH suggests that during the LGM unused silicic acid escaped the Southern Ocean through intermediate and mode waters, and was transported to the equatorial oceans. The implications of this include an ecological shift in tropical regions from cocolithophorids to diatoms (and therefore increased LGM opal burial relative to the Holocene), and a drawdown of atmospheric CO2 due to calcium carbonate compensation. Conversely, the SALH predicts decreased total opal burial during the LGM in the Southern Ocean, as a result of silicic acid export to the tropics. Data from previous studies of the equatorial Pacific are not consistent with the SALH, in that they shows decreased LGM opal burial relative to the Holocene in most cores studied. Most Atlantic data, however, show increased LGM opal burial. Considering areas of equal size, the magnitudes of increased Atlantic and decreased Pacific LGM opal burial are equal (within error), implying some large-scale redistribution of silicic acid. Any mechanism to explain these opposite patterns is likely to be found in the Southern Ocean, as it is the direct connection between the two basins. A few distinct trends emerge in the South Pacific data. First, opal burial generally decreases from west to east. Second, the pattern of opal burial across the Antarctic Polar Front (APF) during the LGM is the opposite of that during the Holocene. Holocene opal burial is greatest south of the modern APF, while LGM opal burial was greatest north of the APF. However, while opal burial shifted north during the LGM, total opal burial in the Pacific Southern Ocean appears to have been less than today, consistent with the predictions of the SALH. Further implications for the SALH will be discussed, including the relationship of the Southern Ocean to the tropics, and what this relationship implies about ocean circulation over the last glacial-interglacial transition.
PP13B-1264
A Dynamic Explanation For The Origin Of The Western Mediterranean Organic Rich Layers
The eastern Mediterranean sapropels are amongst the most intensively investigated phenomena in the palaeoceanographic record , but relatively little has been written regarding the origin of the equivalent of the sapropels in the western Mediterranean, the Organic Rich Layers (ORL's). ORL's are recognised as sediment layers containing enhanced Total Organic Carbon that extend throughout the deep basins of the Western Mediterranean, and are associated with enhanced total barium concentration and a reduced diversity (dysoxic but not anoxic) benthic foraminiferal assemblage. Consequently, it has been suggested that ORL's represent periods of enhanced productivity coupled with reduced deep ventilation, presumably related to increased continental runoff, in close analogy to the sapropels. We demonstrate that despite their superficial similarity, the timing of the deposition of the most recent ORL in the Alboran Sea is different to that of the approximately coincident sapropel, indicating that there are important differences between their modes of formation. We go on to demonstrate, through physical arguments, that a likely explanation for the origin of the Alboran ORLs lies in the response of the Western Mediterranean basin to a strong reduction in surface water density and a shoaling of the interface between intermediate and deep water during the deglacial period. This moves the emphasis for forcing deep convection collapse in the western basins away from atmospheric forcing and towards changes in the residence time of water in the Mediterranean Sea (i.e. oceanic forcing). Furthermore, we provide evidence that deep convection had already slowed by the time of Heinrich Event 1, and explore this event as a potential agent for preconditioning deep convection collapse. Important differences between Heinrich-like and deglacial-like influences are highlighted, giving insight into the response of the western Mediterranean system to external forcing.
PP13B-1265
The Evolution of Surface and Deep Water Circulation in the Northern North Atlantic Over the Last 21,000 Years.
The northern North Atlantic is a critical location in the Atlantic Meridional Overturning Circulation (AMOC) where inflowing surface waters are converted into a deep water return flow. Foraminiferal stable isotope and Mg/Ca measuremnents from four sediment cores between 1237 m and 2303 m water depth on the South Iceland Rise document the changes in these water-masses through the deglaciation and Holocene. The evolution from the glacial mode of circulation to the present regime is punctuated by two periods with low benthic δ13C and δ18O values which do not lie on glacial or Holocene mixing lines. These periods correlate to the Younger Dryas (11.5-12.7 ka) and Heinrich Event 1 (16.8-14.5 ka) during which time meltwater input to the Nordic Seas and sea-ice formation led to brine rejection and export as an overflow into the northern North Atlantic. Planktonic paired δ18O – Mg/Ca measurements indicate that these periods of brine export are also accompanied by an increased influence of the inflowing, warm and saline, surface North Atlantic Current. Holocene circulation shows considerable variability, most notably periods of surface stratification and Polar Front advance, which show a 1500 year cyclicity and correlate to published records of drift ice and deepwater flow speed. The data presented provide evidence for the role of deepwater brine export and consequent surface inflow to the Nordic Seas in resuming overturning following meltwater inputs. The illustrated export of brine with low δ13C values from the Nordic Seas complicates traditional interpretations of low δ13C values during the deglaciation as incursions of Southern Source Water although the spatial extent of this brine is uncertain.
PP13B-1266
Modulation of the Bipolar Seesaw in the Southeast Pacific During Termination 1
A well-dated and high resolution sea surface temperature (SST) record form the SE-Pacific (Ocean Drilling Project Site 1233 located within the northernmost Antarctic Circumpolar Current (ACC) off southern Chile (41°S)) provides new evidence on the timing and interhemispheric climate pattern during the termination of the last ice age (T1). Deglacial warming off southern Chile starts at 18.8 kyr BP with a ca. 2-kyr-long increase of nearly 5°C until 16.7 kyr BP. Thereafter, temperatures remain comparatively stable until the beginning of a second warming step of ca. 2°C between 12.7 and 12.1 kyr BP. A comparison of our SST record to different Antarctic ice-core records suggests a general correspondence in the major temperature trends, particularly the two-step warming over T1. However, the deglacial warming as documented in Antarctic ice-cores is substantially more gradual than observed in our SST record where most of the initial warming occurs over a time-interval of only ca. 1200 years (18.8 to 17.6 kyr BP). The timing of both the initial and the second warming step in our data, suggests that the SST response in the mid-latitude Southeast Pacific occurred quasi instantaneous to the starting slowdown of the Atlantic meridional overturning circulation consistent with the concept of the bipolar thermal seesaw. The occurrence of an "immediate" and high amplitude response in our SST record requires a rapid transfer of the Atlantic signal to the SE-Pacific without involving the thermal inertia of the Southern Ocean that contributed to the substantially more gradual deglacial temperature rise seen in Antarctic ice-cores. The most plausible mechanism for this rapid transfer is a seesaw induced change of the coupled ocean-atmosphere system of the ACC and the southern westerly wind belt. Model simulations suggest that in addition to the seesaw induced changes, SE-Pacific temperatures respond to orbital and greenhouse gas forcing, however, in a more gradual manner and distinct from the two-step warming observed in our proxy record. This two-step pattern is also apparent in the CO2 record from the Dome C ice-core. Both the initial warming (ca. 5°C) and the second major warming step during the NH YD (ca. 2°C) in our SST record coincide with the most significant increases in CO2 (ca. 35 ppmv and ca. 15 ppmv). Assuming that our record largely reflects shifts of the coupled ACC/westerlies system, this concurrence is consistent with the previously suggested important role of such latitudinal shifts in controlling atmospheric CO2 contents. Taken together, our results underline the importance of the superposition of seesaw related processes with other external forcings such as orbitally-induced seasonal variations of incoming solar radiation and atmospheric CO2 that were unique to T1. These processes involve regional and hemisphere-wide feedbacks through the southern westerly wind belt and the ACC system.
PP13B-1267
The Antarctic Intermediate Water During the Younger Dryas Time Period
Reorganization of intermediate and deep water masses during the last deglaciation is still a matter of debate. Two radiocarbon dated cores located in the southern Indian Ocean (core MD 79-257, 20°24' S ; 36°20' E ; 1262 m water depth and core MD 84-527 43°49'3 S ; 51°19'1 E ; 3269 m water depth ) were studied to reconstruct the physical characteristics (salinity and temperature) of the intermediate water and of the surface waters during the deglaciation. We used micropaleontologic transfer functions and the oxygen isotope of planktonic foraminifera to reconstruct Sea Surface Temperature and estimate the surface water oxygen isotopic composition (a proxy for salinity) in the area of intermediate water formation. We measured the oxygen isotopic composition of benthic foraminifera to estimate the temperature and salinity of the intermediate water mass near 20° S at 1260 m depth. Our data show that the Antarctic Intermediate Water in the Indian Ocean was saltier and warmer than the present during the Younger Dryas, between 12,5 and 11,5 kyr BP. The presence of warm saline surface waters near 44°S implies a southward migration of the subtropical front in the Southern Ocean compared to the present during the Younger Dryas. This warming may be explained by an enhanced inflow of warm saline waters carried by the retroflexion of the Agulhes Current following the southern shift of the Intertropical Convergence Zone during cold events in the North. A similar warming has been recorded in the Vostok and Dome C Antarctic ice cores during the Younger Dryas, which appears as a warm event by contrast with the climatic record of the North Atlantic. The opposite behaviour of Northern and Southern high latitudes suggests that the sea saw mechanism also affected the intermediate circulation at low latitudes.
PP13B-1268
Recent and Late Quaternary sedimentation processes in the area of the SE Mediterranean Sea
The present distribution of clay minerals in the southeastern Levantine Sea reveals a complex pattern of different source areas and dispersions of the main clay minerals smectite, illite, kaolinite, chlorite, and palygorskite. Smectite dominates the suspension load of the Nile River and rivers of the Near East. Sources of illite are dust- bearing winds blowing from the Saharan desert and the northern mainland of the Levantine Sea. Kaolinite is prevalent in the sediment yield of rivers which drain the Sinai, in sediments of wadis which discharge into the Nile River, and in dust loads of African storms. Palygorskite and chlorite are mainly derived from several dust bearing wind systems of the North and Central African continent, whereas chlorite is also provided by dust-bearing winds of the northeastern Mediterranean mainland. The clay mineral distributions and XRF element data of a sediment core from the southeastern Levantine Sea, spanning the last 27,000 years, show that the sediments are mainly affected by the discharge of the Nile River. The provenance of clay minerals provides a sensitive recorder of changes in the Nile River discharge and therefore in the climate conditions in the catchment of its tributaries. The glacial section of the core is characterized by low smectite concentrations, high K/Al ratios and low linear sedimentation rates when compared to the Holocene suggesting a low influence by the Nile outflow. Subsequently, during the early and middle Holocene, when the North African continent was influenced by more humid conditions during the African Humid Period, our record shows high proportions of smectite and low K/Al ratios which likely reflect higher discharge rates of the River Nile. In contrast to the sharp beginning and end of the African Humid Period as recorded in West African records at 15 and 5.5 kyr BP, our record in the southeastern Levantine Sea shows more transitional patterns and trend towards slightly lower Nile discharge rates at about 4 kyr BP. The similarity of the clay mineral pattern, in particular the smectite concentrations, with fluctuations in sea surface temperatures of the tropical western Indian Ocean indicate a tight relationship between the Indian Ocean Dipole and the discharge of the Nile River.
PP13B-1269
A multi-phased ~17-15.5 ka BP (~H1 age) advance of the last British Ice Sheet into the North Sea – linking paleoceanographic and continental records
The last British Ice Sheet's (BIS) relatively small size and maritime setting suggests it was strongly susceptible to changes in the North Atlantic oceanic circulation, through associated fluctuations in precipitation and temperature. Marine records have shown clear evidence of recurrent BIS instabilities, however firm data on ice sheet extent for the period are sparse. We present seismostratigraphical and chronological data from the central/northern North Sea showing that after disconnecting from the Fennoscandian Ice Sheet around ~24 ka BP (cal. yrs) the BIS reached the central/northern North Sea again only between ~17-15.5 ka BP (cal. yrs). This is based on the identification and radiocarbon dating of ice-marginal grounding-line wedges interstratified with marine sediments spanning the last 24 ka. The grounding-line wedges indicate at least two major oscillations of the ice front between ~17-15.5 ka BP, implying that the British Ice Sheet was significantly larger at a much later stage than previously thought. Recent work published on North Atlantic marine cores from positions near the western margin of the BIS suggest a strongly falling trend in sea surface temperatures after 18 ka BP, accompanied by an increased IRD flux, lasting until the abrupt warming after H1 at around 15 ka BP. Based on the radiocarbon chronology we explore the possibilities that our ice marginal positions are correlative to the IRD depositional events, and that the IRD signal reflects oscillations of an active ice sheet culminating at ~17-15.5, rather than ice sheet disintegration, as has previously been suggested.
PP13B-1270
Different Strengths of the Bipolar See-saw During Glacial and Interglacial times
We present results from two experiments using the National Center for Atmospheric Research (NCAR) coupled Community Climate System Model, version 3. In these experiments, we applied a 1 Sverdrup freshwater addition over the North Atlantic between 50 and 70 degrees N for 100 years. The two experiments used boundary conditions relevant for 8.5 ka and 21 ka (Last Glacial Maximum). The climate response in the Southern Hemisphere was significantly different between the two simulations. Mean annual 2-meter air temperature increased about 1.5 degrees C in the 21 ka simulation, while no change occurred in the 8.5 ka simulation. Temperature increases in the 21 ka simulation were even greater during the austral winter, with regional warming of up to 4 degrees C. The difference between the two simulations cannot be explained by differences in ocean heat transport; in each simulation there is an increase in southward ocean heat transport in the Atlantic at 29S of about 0.55 petawatts. Instead, sea ice feedbacks play a major role in determining the climate response. In the 21 ka simulation, there is enhanced insolation during austral summer due to orbital forcing and a more equatorward location of sea ice. These factors lead to stronger positive sea-ice feedbacks that amplify the initial warming caused by the increase in southward heat transport.
PP13B-1271
A Bayesian Algorithm for Reconstructing Spatially Averaged Temperature
The determination of spatially averaged temperature from point estimates is a non-trivial statistical problem. In the paleo-climate context, the additional need to convert proxy time-series into temperature estimates presents a serious challenge. Most estimates of spatially averaged temperature at paleo-climate time-scales address these two issues sequentially: the proxy values are first averaged through space, and these estimates are then transformed onto the temperature scale via some form of regression. This two step approach distances the final estimate of temperature from the underlying data, complicating estimates of the associated uncertainty. Our approach is to model the relationship between the true temperature field and the noisy, localized measurements of it using a hidden Markov model. We use a fully Bayesian algorithm to simultaneously estimate the coefficients linking the proxy values to temperature units, the parameters associated with both the temporal and spatial covariance structures, the observational error variances, the temperature values at a large number of uniformly distributed spatial locations, and the average of these estimated temperature values. We assume proper but weakly informative priors for all unknowns. We have, when possible, used conjugate priors - many of them not normal. A major benefit of this Bayesian approach is that, by drawing repeatedly from the full conditional posterior distributions, we obtain an estimate of the uncertainty covariance structure. This allows us to make quantitative statements about both the relative contributions of the different proxies to the spatial average, and the extent to which the model can constrain the various parameters. In particular, the model outputs the uncertainty in the coefficients of the transformation linking the proxy values to temperature units. The model can easily be generalized to accommodate different categories of proxy data assumed to have different uncertainty properties. We first apply the model to surrogate data to test the method and develop intuition about the convergence properties of the Monte-Carlo sampling procedure. We then apply the model to instrumental data, before extending the reconstruction back in time using proxy data.
PP13B-1272
Assessment of radiocarbon-based deep-ocean ventilation proxies using an Earth-system model of intermediate complexity
A common method to reconstruct past deep-water ventilation is the calculation of the radiocarbon age difference of planktonic and benthic foraminifera ("top-to-bottom age"). Adkins and Boyle (1997) developed an alternative method, the so-called "projection age" method, to eliminate the circulation-induced time lag, which originates from atmospheric Δ14C variations. So far, the uncertainties of both methods have not been quantified. We use the University of Victoria Earth System-Climate Model including carbon, radiocarbon and idealized ventilation-age tracers to assess which method is more accurate. Due to the large volume of the ocean and the mixing of water masses of different age, the sub-surface ocean Δ14C evolution over time is smoother than the atmospheric Δ14C evolution, with some local variations. We show that the assumption of closed system radiocarbon decay (i.e. without mixing) is nearly valid in the northern North Atlantic Ocean. As the deep water moves along the global conveyor belt, mixing becomes more important and reaches a maximum in the deep Pacific Ocean. The use of a constant reservoir age in the projection-age method appears as another source of error, as our model predicts reservoir-age changes by hundreds of years. The model results suggest that simple benthic-planktonic age differences are more reliable than projection ages in regions where the ventilation age is small (a few hundred years) and the reservoir age variation are comparably large (also a few hundred years). With increasing ventilation age, the correction of the time lag between the signals improves the results of the projection-age method, while errors due to enhanced damping and mixing increase uncertainties in both methods. This leads to errors of similar magnitude for regions such as the deep Pacific Ocean. We present a potential improvement of the projection- age method based on the use of a smoother marine Δ14C record instead of the atmospheric one and the possibility to employ reservoir-age variations in the source areas of deep-water masses.
PP13B-1273
Ar-Ar Ages of Detrital Hornblendes from Glacial Sediments of the North Sea Trough Mouth Fan
Determining the relative timing of major iceberg calving from different ice sheet margins around the North Atlantic remains an important goal that will lead to a better understanding of causes and consequences of rapid climate variability during the last glacial period. Characterization of the composition of potential contributors is a necessary step towards this goal. The North Sea trough mouth fan is one of the largest glaciogenic debris flow complexes in the North Atlantic/Arctic region, with an approximate area of 142,000 square km (King et al., 1998, Marine Geology v. 152, pp. 217-246; Nygard et al., 2007, Geology, pp. 395-398). The large ice stream trough crosses the shelf along the southern margin of Norway. The crystalline rocks along the southern margin of Norway are Grenville (approximately 1 Ga old orogen). We undertook a study of the Ar-Ar age populations of individual detrital hornblende grains from a sediment sample of the glacigenic debris lobe created during the last phases of the last glacial maximum from the North Sea trough mouth fan. The goal is to test the hypothesis that the ice stream that fed this fan is the source of abundant Grenville age grains found on Bjorn drift site ODP984, at times when North American Grenville sources are not found in the North Atlantic ice rafted detritus belt (Hemming et al., 2005, AGU Spring meeting, PP23A-04). Hornblende grains from North Sea TMF core NH071-B01\SC1 (1) (63.24N, 3.36E, 1049m) have a dominant age population of Grenville (921 Ma, 19 of 48 grains) with subordinate populations of 1108 Ma (n=3) and 1779 Ma (n=4). Accordingly they lend support to the hypothesis that this ice stream could be the source of IRD on the Bjorn drift. These results could additionally shed light on the pathways of fine grain sediment transport to the Bjorn drift which would contribute a better understanding of sediment processes in the region. For example, the provenance implied for the IRD by the Ar-Ar hornblende ages is consistent with that implied by the Nd isotope data from fine grained sediments reported for glacial intervals by DePaolo et al. (2006, EPSL, v. 298, pp. 394- 410). It is well known that much of the debris carried by icebergs is fine grained, and thus this provenance match suggests that it is likely that much of the fine grained material may also be carried to this site by icebergs.
PP13B-1274
Groundtruthing the Neodymium Isotope Proxy in Deep-Sea Corals
The Nd isotopic composition of marine precipitates is increasingly recognized as a powerful tool in paleoceanography. Unlike nutrient proxies such as δ13C or Cd/Ca, Nd isotopes are not thought to be altered by biological processes, and thus may serve as a quasi-conservative water mass mixing tracer. However, any archive, which is used to extract authigenic Nd isotopes, needs careful examination, to test the integrity of the inferred seawater signal. Here we present first data on cleaning experiments and modern calibration experiments on different species of deep-sea corals. Seven different coral samples ranging in age from modern to ~220ka were selected for experiments designed to remove ferromanganese crusts and / or organic residues that may contain high concentrations of Nd and Th. The aim was to determine whether the rigorous chemical procedure we use to remove Th associated with these crusts is effective at removing Nd, and whether it causes any fractionation in the Nd isotopic composition of the coral aragonite. Crusts were found to contain Th-232 concentrations of up to ~160ppm, with 232Th/230Th ratios dependent on the oceanic location of the coral. Un-cleaned corals had Th-232 concentrations of up to 8ppb and the cleaning procedure reduced these values to less than 0.2ppb in both modern and fossil specimens. Neodymium isotopic compositions reveal that for modern corals, with no visible coating, a pre-cleaning step is sufficient to yield the isotopic composition of ambient seawater. The ferromanganese coating around fossil corals however may have a very different isotopic composition than the coral aragonite since it may be a time-integrated signal biased towards modern values. This bias is observed for intermediate water depth D. dianthus corals from stage 3 in the northwest Atlantic. Modern D. dianthus skeletons from the northwest Atlantic and the Drake Passage reflect the seawater Nd isotopic composition, and we are extending this modern calibration to include five different species from diverse oceanographic settings.
PP13B-1275
Shallow Ocean Overturning and the Heat Flux and Carbon Content of the Glacial Warm Pool Since the Last Glacial Maximum
The correspondence between temperature and carbon dioxide during the Quaternary glacial cycles is one of the most remarkable aspects of the paleoclimate record, directly relevant to prediction of future climate change due to carbon dioxide. Increased ocean stratification at high latitudes appears to play a part, reducing mixing between the carbon-poor surface ocean and the carbon-rich abyss during glacial times. Little evidence exists regarding the possible contribution of the shallow overturning circulation that ventilates the upper thermocline. Our investigation reveals that vertical ocean gradients in temperature and carbon in the upper kilometer were steeper at the Last Glacial Maximum, and turbulent diffusion at the base of the warm pool was reduced by 20 percent or more. The steeper vertical gradients, unexpectedly-small surface cooling, and lower carbon content of the glacial warm pool may be explained by reduced shallow overturning. The shallow overturning hypothesis links the carbon and heat content of the tropical ocean with the shallow overturning and hence the winds and the low-high latitude thermal gradient. This is significant for the future because warmer climates mix carbon out of the deep sea at a faster rate.
PP13B-1276
Upper water column hydrology changes off Cape Hatteras and Gulf Stream activity over the Holocene
Modern oceanographic studies show that most of the ocean heat content in the North Atlantic Western Boundary Current region is stored in the upper 400 meters. To constrain past ocean heat content is important to understand past climate changes as this parameter controls heat flux to the atmosphere. We developed proxies for the upper water column hydrology in this area and used them to reconstruct the changes, over the Holocene, of the upper water column temperature and salinity off the Cape Hatteras. Deep-dwelling planktonic foraminifera living at different water depths may provide a proxy for upper water column hydrological changes. Calcification depths of Globorotalia inflata, Globorotalia truncatulinoides and Pulleniatina obliquiloculata have been constrained by correlating modern hydrographic data to oxygen isotopic measurement of late Holocene North Atlantic core-top samples. We found that the three deep-dwelling foraminifera species have a preferred habitat at the base of the seasonal thermocline (Cleroux et al, 2007). The same set of North Atlantic core-tops has been used to define relationships between trace elemental compositions and temperature. Relationships found for Globigerina bulloides and Globigerinoides ruber agree with previous calibrations for these species. We established calibrations between Mg/Ca ratio and temperature for the three deep-dwelling foraminifera species and between Sr/Ca ratio and temperature for G. inflata and G. truncatulinoides (Cleroux et al, submitted). To study Holocene heat content and Gulf Stream activity, we performed coupled analyses of oxygen isotopic and trace elemental composition on several foraminifera species from core MD99-2203 located off Cape Hatteras where the Gulf Stream separates from the United States coast. This location is the meeting point of three water masses: the southward flowing Labrador current with fresh and cold water; the warm Gulf Stream water circulating toward north-east and the deep-mixed layer Mode Water (18° C water) originating from the Sargasso Sea. Under modern condition, the core site is at the western limit of the Mode Water extension. We analysed G. ruber to constrain past surface water condition, P. obliquiloculata and G. truncatulinoides dextral to reconstruct seasonal and main thermocline conditions. High-resolution surface reconstructions over the Holocene show low amplitude periodic temperature and salinity changes that could be related to NAO type mechanisms. Large hydrological changes in sub-surface reflect variations of Labrador current and Mode Water influences. We conclude that the Gulf Stream activity was low from 14 to 8 ka off Cape Hatteras, with a strong Labrador current and mode water system. From 8 to 4 ka, the Gulf Stream activity gets stronger decreasing the contribution of the Labrador and mode waters. The modern situation was established around 4 ka.
PP13B-1277
The Flooding of Long Island Sound
Between the Last Glacial Maximum (22-19 ka) and the Holocene (10 ka) regions marginal to the Laurentide Ice Sheets saw complex environmental changes from moraines to lake basins to dry land to estuaries and marginal ocean basins, as a result of the interplay between the topography of moraines formed at the maximum extent and during stages of the retreat of the ice sheet, regional glacial rebound, and global eustatic sea level rise. In New England, the history of deglaciation and relative sea level rise has been studied extensively, and the sequence of events has been documented in detail. The Laurentide Ice Sheet reached its maximum extent (Long Island) at 21.3-20.4 ka according to radiocarbon dating (calibrated ages), 19.0-18.4 ka according to radionuclide dating. Periglacial Lake Connecticut formed behind the moraines in what is now the Long Island Sound Basin. The lake drained through the moraine at its eastern end. Seismic records show that a fluvial system was cut into the exposed lake beds, and a wave-cut unconformity was produced during the marine flooding, which has been inferred to have occurred at about 15.5 ka (Melt Water Pulse 1A) through correlation with dated events on land. Vibracores from eastern Long Island Sound penetrate the unconformity and contain red, varved lake beds overlain by marine grey sands and silts with a dense concentration of oysters in life position above the erosional contact. The marine sediments consist of intertidal to shallow subtidal deposits with oysters, shallow-water foraminifera and litoral diatoms, overlain by somewhat laminated sandy silts, in turn overlain by coarser-grained, sandy to silty sediments with reworked foraminifera and bivalve fragments. The latter may have been deposited in a sand-wave environment as present today at the core locations. We provide direct age control of the transgression with 30 radiocarbon dates on oysters, and compared the ages with those obtained on macrophytes and bulk organic carbon in the same samples (calibrated with CALIB 5.1 using the Intcal 04 data set). The carbonate ages for most samples are considerably younger than those of the bulk carbon and plant fragments: the organic matter must have resided on land, possibly stored in periglacial lake beds, for up to several millennia prior to deposition in Long Island Sound. The carbonate ages indicate that the main marine transgression occurred at 11-10 ka, at the end of the Younger Dryas (Melt Water Pulse 1B), when glacial rebound was waning and the rate of sea level rise accelerated. It is possible that earlier inundation led to deposition of estuarine clays in deeply incised channels. We estimate that the maximum crustal depression of Long Island Sound was about 40 m, and rebound started at about 15 ka (Melt Water Pulse 1A). We thus conclude that Long Island Sound became a marine estuary at the beginning of the Holocene, much later than had been assumed. The earliest native Americans reached the area during the Younger Dryas and may have witnessed the relatively rapid inundation by the sea of a large section of the Long Island Sound basin. http://ethomas.web.wesleyan.edu/lisweb
PP13B-1278
Change of ocean surface circulation at 5-6 ka BP in the northeastern North Atlantic
A micropaleontological investigation was conducted on two sediment cores from the Reykjanes Ridge area (LO09-14, 59°12.30 N, 31°05.94 W) and the Faroe-Shetland Channel (HM03-133-25, 60°06.55 N, 06°04.18 W) in order to document hydrographical changes during the Holocene. Dinocyst and coccolith assemblages were analyzed and discussed in view of their modern biogeographical distribution, and quantitative reconstructions of sea surface temperature (SST) and salinity (SSS) were conducted based on the best analogue technique applied to dinocyst assemblages. Both proxy records exhibit significant changes that allow the definition of three main intervals. The earliest Holocene (10.3-9.3 ka BP) was characterized by a SST optimum in the Faroe-Shetland Channel, whereas conditions were still cold on the Reykjanes Ridge, suggesting a time transgressive thermal optimum spreading westward. The remainder of the Holocene can be divided into two main periods with a transition phase at 5-6 ka BP. At both sites, SSS before 5-6 ka BP was generally lower than during the early to mid-Holocene, especially in core LO09- 14. However, long term trends of SST show contrasted patterns between the Reykjanes Ridge and the Faroe- Shetland Channel domains. Summer SST was higher than present on the Reykjanes Ridge from 9.3 to 5-6 ka BP, accordingly with a gradual decrease of summer insolation. In the Faroe-Shetland Channel, SST was lower than present until ca. 5.5 ka BP. The contrasted SST trends on the Reykjanes Ridge and in the Faroe-Shetland Channel indicate that decreasing summer insolation was not the only forcing behind changes in sea-surface conditions in the northeastern North Atlantic. Decoupling of the two currents influencing the Faroe-Shetland Channel, the North Atlantic Current (NAC) and the Slope Current (SC) is thus proposed as a possible mechanism explaining the different records at the two sites. We hypothesize that a strong NAC during the early-middle Holocene resulted in a SST increase on the Reykjanes Ridge and decrease in the Faroe-Shetland Channel. Inversely, as a result of a weaker NAC after 5-6 ka BP, SST decreased on the Reykjanes Ridge whereas an enhanced relative contribution of the warmer, saltier SC in the Faroe-Shetland Channel resulted in a SST and SSS increase.
PP13B-1279
Air-Sea Feedbacks onto the NAO on Decadal Timescales in Present Day and Last Glacial Maximum Simulations
The meridional overturning circulation (MOC) in several well-regarded global climate models contains a prominent 20-25 year oscillation. Previous studies suggest that the timescale for the oscillation is set by the ocean while the forcing is atmospheric, with an association to North Atlantic Oscillation (NAO) fluctuations. The NAO in these models also has an approximate 20-year oscillation. However, when the ocean in the Community Climate System Model (CCSM3) is replaced with a simple, slab-ocean mixed layer, the NAO lacks any self- sustaining oscillations, and thus, the oscillation requires feedback from ocean circulation. Other previous research shows that the NAO forces a delayed flux of subtropical gyre water into the Nordic seas, where deep water forms, due to subpolar gyre adjustments. We suggest here that the resulting SST and sea ice anomalies in the Nordic seas provide a feedback to the atmosphere, which, being delayed from the initial NAO forcing, may set the longer timescale variability for the NAO. Additionally, the timescale for the MOC oscillation in a last glacial maximum (LGM) simulation of the CCSM3 is approximately half that found in the modern day simulation. Whether the same SST and sea ice feedback processes are active in the LGM simulation compared to the modern day simulation are investigated in this study. The technique used to uncover these relationships is linear inverse modeling (LIM). Using LIM, we are able to decipher which variables are important to the longer timescale NAO fluctuations and what the optimal patterns for the growth of that oscillation are.
PP13B-1280
Northern Component Water Variability Over the Past 21 kyr: High-resolution Records From Eirik and Gardar Drifts
Northern Component Water (NCW; analogous to modern NADW) circulation has been described as bi-modal with shallow flow associated with the last glacial maximum (LGM) and a deeper current associated with the Holocene. We have generated foraminiferal stable isotope records as well as grain size records from cores collected on the Eirik and southern Gardar Drifts. Our records show that the shallow mode of NCW circulation persisted through the deglacial and into the early Holocene. Beginning around 9 ka (calendar age), the core of NCW began to descend, reaching its present position by 8 ka. This is consistent with Fagel et al. (2002) who argued that Denmark Strait overflow, which is the densest contributor to NCW, began around 8.5 ka. We note also that benthic foraminiferal oxygen isotope data from Gardar diverged from the Eirik data during the late deglacial and early Holocene, and may indicate the resumption of flow over the Iceland-Faroe Ridge and/or through the Faroe Bank Channel. Higher frequency changes in the benthic foraminiferal carbon isotopes during the late glacial through deglacial record interruptions in NCW flow. A 1 per mil decrease occurred from 18 to 16 ka. This decrease was associated with a 4-fold decrease in sedimentation rates and is interpreted as a further shoaling of NCW and influx of AABW. The start of this decrease predates the H1 event but the carbon isotope decrease continued through the H1 event, reaching minimum values at 16.3 ka. Between 16.3 and 14.0 ka, carbon isotope values fluctuated between 0 and 0.75 per mil. Sedimentation rates increased sharply to rates >25 cm/kyr, indicating that the current was actively supplying sediments with minimal winnowing. At 14 ka, carbon isotope values increased to ~1 per mil, associated with the Bolling-Allerod warming, and fluctuated about a mean of 1 per mil until the beginning of the Younger Dryas at 12.5ka. During the Younger Dryas, benthic carbon isotope values decreased by 0.5 per mil, signaling a minor reduction in NCW flux.
PP13B-1281
Circulation and Property Changes in the North Pacific Ocean for the Last Glacial Maximum
The property and circulation changes in the North Pacific Ocean during the last glacial maximum (LGM) are investigated using a coupled ocean-atmosphere-sea ice climate model. In the LGM, an increase in potential density in upper layers of the northern North Pacific makes the water column highly unstable and eventually results in the enhancement of the North Pacific Intermediate Water (NPIW) production. The NPIW outflow reaches deeper layers than in present, but confined to the North Pacific, in consistent with proxy evidence. The increase in potential density is predominantly due to the increase in salinity and secondarily decreases in temperature. The increase in surface salinity is especially high in the Sea of Okhotsk and the western Bering Sea, which are the possible source areas of the glacial NPIW production. In these regions, an increase in brine release due to the marked increase in sea ice, the excessive evaporation over precipitation, and the reduction in river discharge contribute to the increase in surface salinity. In short, reduction in freshwater input to the northern North Pacific is mainly responsible for the increase in the production and outflow of glacial NPIW.
PP13B-1282
The Oxygen-isotope Compositions of Holocene Lake Water Based on Study of Ostracodes from Four Sediment Cores, Lake Superior
An 18m-long piston cores were obtained from the Duluth Basin, Thunder Bay Trough, Caribou Basin, and Ile Parisienne Basin in Lake Superior. The valves of ostracodes, Candona subtriangulata, were collected from 10 cm intervals, and analyzed for their oxygen isotope compositions. Using the oxygen isotope compositions of the valves and a lake bottom temperature of 4 °C, we have calculated the oxygen isotope compositions of water in early Lake Superior. From 18 to 7 m (Duluth Basin), 12 m (Thunder Bay Trough), and 17 m (Ile Parisienne Basin) depth, the oxygen- isotope values of the water decrease from -23 to -28 ‰. This suggests that the lake water at this time was dominated by glacial meltwater. Isotopically depleted meltwater amount in the lake was likely increased by ice melting in the basin and near Lake Agassiz, causing lower oxygen-isotope values of lake water. Then the water isotopic values increase to -23, -21, and -15 ‰ at 6 m, 10 m, and 15 m (Duluth Basin, Thunder Bay Trough, Ile Parisienne Basin, respectively) around 8,000 14C years BP. probably due to decreasing of glacial meltwater supply after glaciers retreated from the basin. The variation in the values may be resulted from the degree of stratification of Lake Superior water depending on the water depths. From 6 m (Duluth Basin), 10 m (Thunder Bay Trough), and 15 m (Ile Parisienne Basin) to 0 m depth, no ostracodes were found in the sediments, suggesting they are postglacial sediments that ostracodes were not preserved in because of the low sedimentation rate and no delivery of calcareous till eroded from Paleozoic rocks. From the 18 to 6 m, the oxygen-isotope compositions of the lake water calculated from ostracodes from Caribou Basin sediment core varied little (-26 to -24 ‰). Because the sediment is much older than those from other three basins, it recorded lake water of glacial time, which may have fairly constant isotope values. From 6 to 3 m depth, then, the values decrease to -27 ‰, which shows the same trend with other cores. From 3 to 0 m depth, sediments from Caribou Basin were lost when sampling. The oxygen-isotope compositions of ostracodes are a very useful proxy to assess glacial meltwater amount and its effect to the lake water.
PP13B-1283
The Late Quaternary Oxygen Isotope Composition of Lake Michigan
We present stable isotope records for porewater (oxygen, hydrogen) and biogenic carbonates (oxygen, carbon; ostracode and clam shells) in sediment cores from the Chippewa, Milwaukee and South Chippewa Basins of Lake Michigan. The oxygen and hydrogen isotope compositions of porewater from the South Chippewa Basin core showed very little variation with depth. At the maximum depth of 16.6m, δ18O values were within 2‰ and δD values were within 12‰ of modern Lake Michigan water (average δ18O = -5.9‰; average δD = -45‰); original porewater compositions have not been preserved. The oxygen isotope results for the biogenic carbonates, by comparison, provide a record of the isotopic composition of Lake Michigan over the last ~11,000 years, including significant incursions of very low-18O water, as first reported by Colman et al. (1990) and Forester et al. (1994). The low-18O waters originated from the retreating Laurentide ice sheet and may have been routed through Lakes Agassiz and Superior and discharged as large volumes over very short intervals of time. Periods characterized by much higher oxygen isotope compositions likely record the isotopic composition of regional precipitation over the catchment area. In summary, the large variations in the oxygen isotope composition of early Lake Michigan water arose from regional climate change and changing water sources during the times of ice-sheet retreat.
PP13B-1284
Anti-phase variability of North Pacific ventilation and Atlantic overturning circulation during the last deglaciation
Modern North Pacific Intermediate Water (NPIW), the densest water formed in the North Pacific, is one of key water masses in the Pacific overturn circulation1. climate-driven fluctuations in the glacial NPIW (GNPIW) circulation held the clue to whether the millennial-scale global climate change during glacial times was initiated through ocean-atmosphere feedbacks or by changes in the Atlantic meridional overturning circulation(AMOC). Here we present high resolution records of ventilation change estimated from the 14C age difference between coexisting benthic and planktic foraminifera and sea surface temperatures and intermediate water temperature from two mid-depth cores in the northwestern Pacific near NPIW source region. During the last deglaciation, the GNPIW ventilation history was drastically changed with millennial time scales and also synchronous with the collapse and rapid resumption of the AMOC during cooling intervals of Heinrich event 1(H1). Ventilation rates were increased during cooling intervals of the H1 and the Younger Dryas. Conversely, during warming intervals of the preBorial and the Bølling-Ållerød (B/A), they were significantly decreased. Both warm periods coincide with maxima in surface ocean productivity reported along the western and eastern margins of the North Pacific2. On the other hand, alkenone surface water temperature in the deglacial period was coincidently changed with ventilation change and substantially dropped during Younger Dryas, synchronous with shutdown and reduction of the AMOC. These results suggest that the GNPIW ventilation was probably controlled by millennial fresh water cycle and the atmospheric teleconnections over the North Pacific driven by reorganization of oceans' overturn circulation.
PP13B-1285
Millennial-Scale Variation of Intermediate Water Intensity in the Bering Sea During the Last Glacial-Interglacial Cycle
Knowledge on past deep and intermediate water circulations in the North Pacific is important to understand the dynamics of climate changes and feedback processes. Yet, ventilation changes in the North Pacific basin are still poorly understood. It is widely accepted that at present North Pacific Intermediate Water (NPIW) forms in the Okhotsk Sea. However, according to studies on radiolarian assemblage, the Bering Sea might have been the dominant site of NPIW production during the glacial period. It is therefore critical to identify intermediate water flow and reconstruct its variations in the Bering Sea. During Mirai cruise MR06-04 Leg2, three pairs of piston cores, each is approximately 18m in length, have been recovered along a depth transect at the northeastern part of the Bering Sea continental slope. Preliminary studies on lithology and age model (based on last occurrences of radiolarian species) suggest that the two deeper cores (1002m and 1158m water depth) preserve continuous climatic records covering deglaciation and MIS 2-4, while the shallowest core (852m water depth) seems to preserve a record back into the last interglacial. We conducted lithological observation utilizing soft-X ray radiographs and high resolution major element analysis by XRF micro-scanner to reconstruct changes in intermediate water circulation in the Bering Sea during the last Glacial-Interglacial Cycle. The results advocate for a close relationship between grain size and compositional variation of the detrital component, which is best represented by changes in Ti/Al and Si/Al ratios. The changes in these elemental ratios suggest millennial-scale variation in bottom current velocity, which probably reflects NPIW intensity changes. We also examined profiles of redox sensitive elements such as S and Mn. We assume S/Fe ratio to represent the degree of pyritization, which is commonly employed as a proxy for the bottom water oxygenation level. S/Fe ratio also shows millennial-scale changes with decreases in S/Fe associated with increases in Ti/Al and Si/Al, suggesting enhanced intermediate water ventilation associated with intensified flow speed. The results support the idea that NPIW was present in the Bering Sea and featured millennial-scale variations during the last glacial to deglaciation period.
PP13B-1286
Frontal movement of the subtropical convergence south of Tasmania over the last 60,000 years
The northern boundary of the Southern Ocean is the subtropical convergence (STC), which sits just to the south of Tasmania today. Alkenone temperature data was used to reconstruct paleo-ocean circulation and frontal movement of the STC over the last 60 Kyrs on the South Tasman Rise (STR). Alkeonone data was collected from four gravity cores that provide a latitudinal transect across the STR and a longitudinal perspective at the north of the Rise with the eastern core location tracking possible influence from warm waters of the East Australian Current (EAC). Frontal movement can be determined by changes in the temperature gradient between the cores through time. Frontal movement can be determined by changes in the temperature gradient between the cores through time. The STC front moved numerous times across the STR area during the last 60 kyrs as reflected by temperature fluctuation across the latitudinal transect. The STC was at its most southerly position , south of 46S, between MIS stage 3 and early in the last glaciation (40 to 20 ka). After that time (20 to 13 ka) it moved to a more northerly position in the late glacial and early deglacial period , to between 45 and 46S. In the early deglaciation,(12 to 11.5 ka) the STC was located more northerly still (around 45S) and to the east cold temperatures indicate diminished EAC influences and/or that the front sat north of 44S. In the latest deglaciation (11.5 to 10 ka), the STC was located was in a similar position over the STR but to the east the EAC influence was strong or the STC sat south of 44S. In the early Holocene, (10-7ka), the STC was located further south , between 46 and 45 S but to the east moved north of 44S . In the late Holocene, (7-0ka), the STC retreated south of 46 S but remained more northerly to the east of the STR . During MIS stages 3 and 4 (45 and 60 ka) the STC sat as far north as in the early deglaciation. These reconstructions suggest that Southern Ocean conditions to the south of Tasmania were most extreme in late stage in the early glaciation and was in a similar location in the mid to late Holocene.
PP13B-1287
Sedimentary Redox Conditions, Biogenic Production, and Oxygenation of Southeast Pacific Intermediate Waters Over the Past 30 ky.
We present data from SE Pacific sediments in an effort to characterize the paleo-redox conditions of shallow (~400-1000 m) subsurface sediments, and the water masses that overlie them, over the past 30 ky. The sediments were recovered during ODP Leg 202, and come from three sites: 1233, 1234, and 1235; together these three sites constitute a vertical transect of Antarctic Intermediate Water (AAIW). Site 1233, at 41°S, sits in the core of AAIW at 838 m depth. Site 1234, at 36°S, is located between AAIW and the Pacific Central Water (PCW) mass. Site 1235 (489 m) is located close to Site 1234, but is between AAIW and the overlying low- dissolved-oxygen Gunther Undercurrent (GUC) water mass. Recent sediments for sites 1234 and 1235 contain a rich signature of biogenic opal production and enrichments of iron and the authigenic metals U, Mo, and Re. At site 1234 there is a minimum in biogenic Si at approximately 20 ky followed by a slight increase and a second minimum between 12 and 5 ky. At both sites 1233 and 1234 the trace metals exhibit more structure during the period of roughly 15 to 30 ky as compared to the most recent 15 ky. The trace metal data at site 1234 shows a sharp reducing signature at ~17 ky (higher Mo, lower U:Mo ratios), bracketed by periods of more oxygenated conditions (lower Mo, higher U:Mo) back to ~22 ky, and forward to ~12 ky. This combination of low biological production and more oxygenated bottom water may suggest a period of increased AAIW ventilation as far north as site 1234. Although it is difficult at this point to unequivocally separate the impact of ventilation from production using our current data base, it does appear that some of the observed changes in sedimentary character may be ventilation-driven rather than driven by local production.
PP13B-1288
Deglaciation in the High Andes – a Record from Laguna Piuray (Cusco, Peru)
The Peruvian Andes lie in a crucial location for paleoclimate investigation. Fluctuating Pacific and Atlantic air masses compete for long-term dominance of the region, with the El-Nino Southern Oscillation (ENSO) system causing further variability. A laminated glacial/interglacial sediment sequence (6m) exposed around the shores of Laguna Piuray, near Cusco, offers not only the potential to reconstruct the climate history of the area but also to test for strength and frequency of the Atlantic monsoonal and Pacific ENSO influence. A suite of continuous cores was collected from deep trenches. The sedimentary record is characterized by postglacial diatom-rich chalk overlying organic-rich clayey chalk. Between these units are 3 distinct organic layers (80% TOC) deposited between 12-14 cal. kyr BP (14C). The base of the record is probably as old as 25kyrs (U/Th). We obtained a multi- proxy record of the section including continuous XRF scanning data of the entire sequence, and stable isotopes, XRF, XRD, TOC, biogenic opal, and carbonate analysis of discrete samples as well as a relative paleotemperature record from analyses of soil biomarkers. All the data profiles we obtained show a pronounced increase in temperature and decrease in precipitation at 13.8kyrs and are in good correlation with published regional Andean records using single proxies. Our results confirm that the Deglaciation Cold Reversal in central South America is not identical to the Younger Dryas event in the Northern Hemisphere.
PP13B-1289
Abrupt climate change and collapse of deep-sea ecosystems during the last 20,000 years
We investigated high-resolution deep-sea sediment core records of benthic ostracodes during periods of rapid climate and oceanographic change of the last 20,000 years in the Ocean Drilling Program Hole 1055B, western subtropical North Atlantic (32° 47.041'N, 76°17.179'W; 1798 m water depth). Age control was established with radiocarbon and oxygen isotope. Results show that deep-sea benthic community collapses occur with faunal turnover of up to 50 % during all major climatically-driven oceanographic changes. The Shannon-Wiener species diversity index falls from 3 to as low as 1.6 at minimum during these events. Major disruptions of benthic communities occurred during Heinrich Event 1 (16.8 ka), the Intra-Allerød Cold Period (13.1 ka), the Younger Dryas (YD: 12.9-11.5 ka), and several Holocene Bond events. The largest collapse is identified as an abrupt two- step decrease of the North Atlantic Deep Water assemblage and species diversity that occurred at 13.1 and 12.2 ka and is associated with the YD. This collapse of deep-water assemblage and species diversity did not fully recover until ~8 ka. Reduced deep-water formation at intermediate depths (<2000 m) during the deglaciation may have caused the collapse. Similar deep-water slowdowns may also have caused benthic community collapses during Holocene climatic oscillations. These results indicate that deep-sea ecosystems are not immune to the effects of rapid climate changes occurring over centuries or less.