PP43B-01 INVITED 13:40h
Antarctic Timing of Surface Water Changes off Chile and Patagonian Ice-sheet Response Based on ODP Site 1233
ODP Site 1233 off southern Chile is situated in an ideal location to study Southern Hemisphere millennial-scale climate variations during the last glacial. We present records of surface ocean conditions in the southernmost Peru-Chile Current and associated changes in the extent of the Patagonian ice-sheet in adjacent southern Chile covering the last ca. 70,000 years. Sedimentation rates at this continental margin site are extraordinarily high (in the order of 1-3 m/kyr) consistent with strong fluvial discharge in response to heavy continental rainfall in southern Chile and the proximity to the Patagonian ice-shield (PIS) that advanced towards the Chilean Lake District (directly onshore Site 1233) during the last glacial providing terrigenous material via glacial erosion. This proximity to the ice-sheet provides the excellent opportunity to compare continental and marine paleoenvironmental signals within the same well-dated archive. Millennial-scale SST changes closely parallel the temperature changes recorded in Antarctic ice-cores. Including other records from the Southern Hemisphere mid-latitudes, our data suggest a quasi-hemisphere-wide response that is consistent with the bipolar see-saw mechanism but may also imply a more prominent role of the Southern Hemisphere in the origin and transfer of millennial-scale climate variations during the last glacial. Compositional changes in the terrigenous sediment input, interpreted to primarily represent changes in the extent of the PIS, show a similar pattern as SST changes but reveal a lagged response of the terrestrial signal most likely related to climate inertia of the PIS. During the deglaciation, paleosalinity data suggest that meltwater input from the ice-sheet started up to ca. 1000 years after the onset of deglacial warming. During early isotope stage 3. however, a lag between SST and PIS changes appears to be absent. Spectral analyses suggest that both proxy records are characterized by higher short-term variability in this interval. Though the general pattern of the records is still very similar to the Antarctic records, we speculate that the short-term variability may origin from the tropics during warmer intervals of the last glaciation.
PP43B-02 13:55h
Glacial History of Southernmost South America and Implications for Movement of the Westerlies and Antarctic Frontal Zone
The ~1 Myr glacial geologic record in southern South American is one of the few available terrestrial paleoclimate proxies at orbital and suborbital time scales in the middle latitudes of the Southern Hemisphere. Presently, southernmost Patagonia lies about 3\deg north of the Antarctic frontal zone and within the middle latitude westerlies and the climate is controlled by the surrounding maritime conditions. Thus, the long-term glacial record provides insight into the history of climatic boundaries over the middle and high latitude southern ocean, including the upwind SE Pacific Ocean, tectonic-glacial evolution of the Andes, and global climate. To date, cosmogenic nuclide and $^{14}$C dating have focused on glacial fluctuations between 51 and 53\deg S (Torres del Paine to northern Tierra del Fuego) during the last glacial cycle, including the late glacial period. At least 4 advances occurred between ca. 25 and 17 ka, with the maximum expansion of ice ca. 25-24 ka. Major deglaciation commenced after ca. 17.5 ka, which was interrupted by a major glacial-climate event ca. 14-12 ka. Modelling experiments suggest that the ice mass needed to form the glacial maximum moraines required about a 6\deg cooling and a slight drying relative to the present. Such a fundamental temperature reduction, despite high summer isolation, strongly suggests northward movement of the westerlies and the polar front on millennial timescales. The Patagonian record also indicates that on orbital timescales equatorward movement of climate boundaries and glacial growth was in phase with major Northern Hemisphere ice volume change, despite high local summer insolation. At suborbital timescales, the picture is more complex. While major facets of the last glacial maximum appear to be in phase between the hemispheres, at least some late glacial events may be in step with Antarctic climate change. Present and future research will further constrain the timing of glacial events over the last 1 Myr and the relation with mountain valley evolution and Southern Hemisphere climate change.
PP43B-03 14:10h
Pliocene Shoaling of the Central American Seaway and its Effect on Caribbean and Tropical East Pacific Upper Ocean Stratification
Multispecies planktonic \delta$^{18}$O records and Mg/Ca-temperature reconstructions from ODP sites 999, 1000 and 1241 were used to assess changes in Caribbean and tropical East Pacific upper ocean stratification in response to the progressive closure of the Central American Seaway. Our studies focus on the time interval from 5.5 to 2.5 million years. Previous studies suggested that the sill depth of the Panamanian Gateway shoaled to less than 50 m by about 4.6 Ma as indicated by an increase in Caribbean sea surface salinity (SSS), reflecting the development of the modern Atlantic-Pacific SSS contrast. At site 1241, the general increase of \delta$^{18}$O and Mg/Ca temperature gradients between the mixed-layer dweller {\it G. sacculifer} and deeper dwelling foraminifers {\it N. dutertrei}, {\it G. limbata} and {\it G. tumida} suggests a shoaling of the tropical East Pacific thermoline from 5.5 Ma to 3.3 Ma. At Caribbean sites 999 and 1000, both \delta$^{18}$O and Mg/Ca temperature gradients indicate a warming of subsurface water masses after 4.4 Ma. The direct comparison of subsurface water signatures from Caribbean sites (999, 1000) and tropical East Pacific site 1241, however, points to a deeper leakage in the Central American Seaway that persisted at least until 3.3 Ma, just before the onset of the Northern Hemisphere Glaciation. The increase in Caribbean SSS after 4.6-4.2 Ma is interpreted to reflect a diminished eastward volume transport of less saline Pacific water masses through the gateway, rather than a shoaling of the gateway to a water depth of less than 50 m. Similar and stable Mg/Ca temperatures at sites 999 and 1241 paralleled by a \delta$^{18}$O increase at both sites display a salinity increase in Caribbean and tropical E-Pacific subsurface waters from 4.4 Ma to 3.3 Ma, possibly indicating reversed throughflow at greater water depths.
PP43B-04 INVITED 14:25h
Changes in Southeastern Pacific Circulation and Productivity on Tectonic, Orbital and Millennial Timescales in Simulations with a Global Climate Model
I present simulations with the UVic Earth System Climate Model addressing the closure of the Panama seaway, glacial-interglacial changes and millennial scale variability associated with Dansgaard/Oeschger events. The focus will be on the simulated changes in the Southeastern Pacific circulation and productivity in the context of the global climate system. Attempts will be made to compare the model results with proxy records from this area including leg 202 in order to gain insight into the role of the Southeast Pacific within the global climate system.
PP43B-05 14:40h
Regime Shifts in Climate Forcing of Peru Denitrification
Water column denitrification occurs in suboxic intermediate waters when bacteria transform nitrate to N2 gas, thereby removing it from the ocean's combined N inventory. Combined N availability is an important controlling factor for organic C production across much of the ocean, such that past variations in oceanic denitrification likely had repercussions for marine productivity and perhaps, at appropriate time scales, atmospheric CO2. Prior studies in the Arabian Sea and E. Tropical N. Pacific have demonstrated climatically-forced oscillations in denitrification on orbital to millennial time scales. Here we examine the Peru denitrification zone, the last of major water column denitrification zones to be studied and the only one in the S. Hemisphere. It is also the most likely to be influenced by ENSO variability. We have examined a series of high resolution cores from the upper Peru margin and developed a chronostratigraphy covering the last 60 kyr overcoming difficulties of common hiatuses and little or no preserved foraminifera. We have also examined ODP Site 1237 and associated site survey cores located in deep waters near the margin on the Nazca Ridge. A multi-proxy approach is taken including N isotopic composition to record denitrification intensity; major and minor elemental composition for sediment provenance, water column redox state, and productivity; alkenone UK37 for SST. Denitrification intensity is observed to vary at a variety of time scales. The lower resolution, 2 Ma record from site 1237 shows large orbital-scale shifts with the dominant mode shifting at the MPT. The high resolution records from the margin elucidate the nature of important forcings. The last deglaciation experienced a sharp and early rise in Peru denitrification that preceded by 2 kyr any major changes in local productivity. Forcing appears to be remote from the Peru upwelling zone, likely due to changes in either the ventilation of source intermediate waters in the Subantarctic and/or changes in the relative isolation of the `shadow zone' from the subtropical gyre circulation. The Holocene is also marked by large excursions in denitrification that are only centennial scale in duration that are unique to this region. A regime shift appears to have occurred in which local, productivity-driven forcing appears to dominate.
http://www.smast.umassd.edu
PP43B-06 14:55h
Paleoproductivity and Paleoclimate off Southern Chile During the last Deglaciation. Diatom and Phytoliths Records from Site 1233
Diatom and Phytolith Accumulation Rates (DAR and PhAR) and diatom assemblage composition were determined between 23 and 8 Cal kyr B.P. for Site 1233 recovered off Southern Chile (41°S, 74.45°W, 837 m) during ODP Leg 202. At present, the site is located in an area where the surface forcing is dominated by strong poleward winds and heavy precipitation that generates a tongue of low-salinity water that spreads northward from the fjord region (43°S). Oceanic circulation is dominated by the spliting of the West Wind Drift (WWD) into the Peru Current (PC) and the Cape Horn Current (CHC). Primary productivity in the region increases in winter/spring possibly associated to river input of nutrients and/or micronutrients. During the 22-8 Cal kyr B.P.time interval, both diatoms and phytoliths abundance show centennial to millennial variability. Marine Diatom AR increases as alkenone SST decreases and \delta$^{18}$O becomes heavier. Highest AR (10$^{8}$ valves/cm$^{2}$ kyr) occurs between 20 and 22 Cal kyr BP. The diatom assemblage is dominated by the genus Chaetoceros, which may be interpreted to reflect an intensification of the PC. A new average increase in DAR (to values 8*10$^{7}$ valves/cm$^{2}$ kyr) is noted from 11 to 9 Cal kyr B.Pfollowed by a sharp decrease to a minimum centered at about 8.4 Cal kyr B.P. This second maximum is accompanied by an increase in Thalassiosira spp indicating an increase in river induced productivity, and consequently stronger precipitation over southern Chile. Oceanic warm water forms reach maximum AR between 10 and 9 Cal kyr B.P. indicating a possible strong southward penetration of relatively warm subtropical waters or the poleward flowing undercurrent in the earliest Holocene. Increased PhAR occurs between 17 and 12 Cal kyr B.P. In general good agreement with a decrease in isotopic and alkenone derived salinity, this data points to an increase in grass source and continental input. Past sea-surface temperatures and productivity will be estimated on the basis of tranfer functions generated from a multivariate analysis of 232 SE Pacific surface sediment samples.
PP43B-07 15:10h
Linking Global Climates Between Hemispheres and Ocean Basins: Millennial-Scale Temperature and Isotopic Variability of Intermediate and Mid-Depth Watermasses of the Equatorial and Southeast Pacific.
Analysis of stable isotopes and trace metals in benthic foraminifera from rapidly accumulating sediments in the Equatorial and Southeast Pacific recovered on ODP Leg 202, reveal remarkably high amplitude variations on millennial scales at water depths of $\sim$500-1500 m. Presence of climate signals that mimic variability in ice cores provides a strategy for inter-hemispheric and interocean correlation. Surprisingly large variability in subsurface watermass properties in this region mimics signals found in the mid-depth Atlantic. This variability in the Pacific suggests a role for global-scale ocean circulation in interhemispheric and interocean linkage.
PP43B-08 INVITED 15:25h
Paleomagnetic Results From ODP Leg 202: The Chilean Margin Sites
ODP Leg 202 sediments provided the unique opportunity to derive paleomagnetic records that range from sub-centennial to tectonic timescales. Paleomagnetic studies are ongoing with the most complete results to date from the Chilean Margin Sites (1233, 1234 & 1235) and Site Survey cores. Shipboard data from these sites suggest that they contain an unprecedented record of geomagnetic field behavior for the last 70 kyr. Progressive alternating field (AF) demagnetization of u-channel paleomagnetic samples from the Holocene section of Site 1233 (41.0. S, 74.26W, water depth 838 m) indicated that these sediments preserve a strong (10-2 A/M), stable, well-defined, low coercivity magnetization. Component directions show that inclinations vary around the expected GAD values for the site latitude, declinations show variations consistent with PSV. The u-channel results attest to the general robustness of the shipboard data. Comparisons between ODP cores, multi cores and gravity cores demonstrate that Site 1233 preserves the upper most sediment and records the distinct shallow inclination and eastward declinations of the historical field off South America. By splicing u-channel and shipboard data and calibrating it to the historical record a long geomagnetic timeseries is reconstructed. Augmented by twenty-seven radiocarbon dates from Site 1233 and nearby cores show that the normalized intensity records capture millennial scale variability that can be correlated to the highest resolution relative paleintensity records globally. We have developed a composite chronostratigraphy for the last 75 kyr and will begin the exploration of the spatial coherence of the paleomagnetic secular variation record. These and other results will be discussed.