PP51F-1365 0800h
Alkenone Sea-surface Temperature Variations in the Southeast Pacific Ocean Inferred from a High-Resolution Record of the past 200,000 Years (ODP Site 1234)
High sedimentation rate records with millennial to centennial resolution were recovered from ODP Site 1234 in the southeast Pacific. SST reconstructions by using alkenone unsaturation indices measured from the sediments recovered from this site allow assessment of climate variability pattern in the surface ocean of the southeastern Pacific. Site 1234 SST records also provide opportunity to establish a high-resolution record that is sensitive to eastern Pacific climate, and allow a comparison with a SST record from a more southern (ODP Site 1233) near the glacial extension of Patagonian Ice Sheet in southern Chile. The record from Site 1234 can be compare further with a western Pacific record (MD972151) over the last two glacial-interglacial cycles and be used to identify any basin-scale type of climate variability pattern in the Pacific. In this study, we present (~10cm mcd sampling intervals, ~200 years resolution) alkenone SST reconstruction that covers the past ~200,000 years. Based on our preliminary results, the SST record from Site 1234 reveals large oscillations from ~17-11>FXC and patterns of high frequency changes which open a room for further correlations with Greenland and Antarctic ice cores. Large amplitude oscillations in SSTs are also observed from the MIS 5 interval of the record and appear to parallel to the variations of benthic oxygen isotopes measured from the same core. A long-term trend of alkenone concentration decreases is clearly observed from MIS 5 to the present, suggesting that at this site calcareous productivity was decreased or siliceous productivity / terrestrial sediment supply was increased since MIS 5. Accurate chronological controls by using AMS 14C, paleomagnetic and oxygen isotope stratigraphies will provide opportunity to assess the timing of these climatic events into a global framework.
PP51F-1366 0800h
Alkenone and planktic foraminifer sea-surface temperature reconstructions at ODP Site 1240: 500-900 kya
One of the objectives of ODP Leg 202 is to retrieve high-resolution sediment cores from the eastern equatorial Pacific, an area in which modern interannual-scale climate are dynamically fluctuating. In this study we aimed to reconstruct paleoceanographic records from the ODP Site 1240 (0o1.311'N; 86o27.758'W; WD: 2921m) from the Leg 202. One of our objectives in the paleoceanographic reconstructions is to better understand how tropical oceans interact with global climate over orbital time scales, and focus on testing hypotheses of ENSO-like oscillation and of extra-tropical forcing prevailed in long-term climate variations in the tropics. We are especially interested in examining an abnormally "warm and wet" climate in glacial MIS 14 shown in Chinese loess and South China Sea marine records and looking for any tropical origin of forcing or feedback for explaining the abnormality. Site 1240 was chosen for this purpose as this site is ideal for monitoring upwelling and circulation changes in the eastern equatorial Pacific, and is likely very sensitive to if any ENSO-like oscillations occurred in this particular time interval. In this preliminary study, the age model of cores from Site 1240 was constructed primarily based upon 4 calcareous nannofossil datums at the top 80 meters composite depth (mcd) and the average sedimentation rate is ~10cm/kyr. We have also generated alkenone SSTs from an interval of 500-900 kya of the cores. Initial observations indicate that high alkenone concentrations are associated low SSTs and vice versa at this interval. Alkenone SSTs calculated from Prahl's equation show an amplitude ~$4-5\deg$C variations over major glacial-interglacial periods in this interval and suggest a dynamic climate pattern in the EEP. Comparison of the alkenone SSTs and relatively low-resolution planktic foraminifer fauna MAT-SSTs also indicates that these two estimates appear to be of similar amplitudes, though the MAT-SSTs are apparently warmer than alkenone SSTs by ~$1-2\deg$C. In the interval equivalent to MIS 13-15 based on our very initial age model, both SST proxies show large cooling (~$4\deg$C) which suggest a strong equatorial circulation at this time.
PP51F-1367 0800h
Phosphorus and Barium Tracers of Productivity, Nazca Ridge ODP Site 1237, Southeast Pacific Paleoceanographic Transects
Phosphorus, biogenic barium, and trace metal concentrations have been measured in sediments recovered from Ocean Drilling Program (ODP) Leg 202 Site 1237, Nazca Ridge. Our work has been to understand how these tracers of productivity and sedimentary redox conditions may have responded to the tectonic and climatic evolution of the Southeast Pacific. Changes in lithology as the site moves from a pelagic to hemi-pelagic depositional setting with past plate motion are important when applying paleoproductivity proxies. This unique record extends to 31 Ma over 360 mcd, with a sampling resolution of $\sim$ 0.2 m.y. throughout the sequence. Reactive phosphorus, an indicator of nutrient burial, shows increasing concentrations toward the modern, peaking at 25.3 umol P/g at 15.34 mcd. Biogenic barite, a proxy for export productivity, peaks at 36.90 mcd at a concentration of 3.8 umol Ba/g. Reactive P/Ba excess ratios display lower than expected values for the upper $\sim$ 180 mcd with a sharp increase to higher ratios starting at $\sim$ 270 mcd. Biogenic barite (determined by sequential extraction) defined as Ba barite, were performed on select samples to compare with Ba excess measurements. We found that estimates of biogenic barite from Ba excess are greater than those from barite extraction for these samples. Uranium and manganese enrichment factors (EF) were determined for the entire record. U EF all exceeds crustal values. Dramatic changes in Mn EF at $\sim$ 162 mcd and the observed color change from grayish white to a fairly homogeneous pale brown at this depth lead us to believe that a change in source material, not redox conditions caused this event. We will need an improved age model to generate accumulation rates and to test nutrient burial history at this site.
PP51F-1368 0800h
Continental and Marine Climate Records from Chile and the Southeast Pacific: Joint Pollen, Oxygen Isotope, and Radiolaria Records from ODP Sites 1233 and 1234
Rapidly-deposited sediments (1-3m/kyr) from ODP Site 1233 (41°0.005'S, 74°26.992'W, 838m water depth) document millennial-scale changes in the response of Chilean plant communities to atmospheric circulation in the southeast Pacific over the past 60ka. Pollen data from the upper 95m of Site 1234 (36°13.153'S, 73°40.902'W) replicate and extend the terrestrial record through the last glacial cycle (MIS 1-MIS5e), thus providing the first record of vegetation and climate from the last interglacial in temperate South America. Both Sites monitor a major discontinuity in Chilean climate - the transition between northern semi-arid, summer dry-winter wet climate and southern year-round, rainy, cool temperate climate. Downcore changes in diagnostic pollen from xeric and mesic vegetation (Lowland Deciduous Beech forest, Valdivian Evergreen Forest, and Subantarctic Evergreen Forest and Parkland) reflect frequent latitudinal shifts of the southern westerlies during MIS 2 and MIS 3. During most of the last glacial, cool, mesic rainforests composed of evergreen beech (Nothofagus dombeyi) and conifers, such as the endangered Prumnopitys andina, characterized coastal Chile. Late Pleistocene, intervals of heavy rainfall and lower temperatures inferred from expansion of Subantarctic Parkland correspond with regional glacial events, and the structure and variability of southern Chilean vegetation and climate correspond with changes in marine surface waters offshore and in Antarctic ice core data. In the last 140,000 years, only during MIS 5e was vegetation of southern Chile similar to that of the Holocene. At the MIS 6/5e transition, coeval with the rapid shift to light isotopic values, glacial vegetation was rapidly replaced by plant communities associated with Mediterranean climate. An increased prominence of halophytic vegetation suggests that MIS 5e was more arid and possibly warmer than MIS 1. Lowland Deciduous Beech Forest (N. obliqua) extended well into the interval of ice growth of MIS 5d, thus interglacial conditions in southern Chile were not synchronous with global ice volume.
PP51F-1369 0800h
A Reconstruction of Millennial-scale Changes in Sub-surface Watermasses Off of Central Chille
Here we examine millennial scale changes in subsurface watermasses off central Chile during the last ice age, based on stable carbon and oxygen isotopes from benthic foraminifera. ODP sites 1235: (489m water depth) and 1234 ( 985 m water depth) bracket the modern extent of Antarctic Intermediate Water. The core top \delta 18O at 1235 and 1234 are 2.7 \permil and 3.3 \permil respectively. This 0.6 \permil difference between the two cores corresponds to a modern 2.4 $^{\circ}$ C temperature offset between sites. Variations in \delta 18O from Site 1234 resemble similar high-resolution records from the deep Atlantic, which suggests Southern Ocean linkage of deep ocean temperatures in the two ocean basins. High amplitude \delta 18O variations at Site 1234 indicate a larger change in temperature in the intermediate Southeast Pacific than the deep North Atlantic, whereas Site 1235 varies with lower amplitudes. If glacial ice volume accounts for 1.1 - 1.3 \permil of the observed change in benthic foraminiferal \delta 18O, Site 1235 may have experienced temperature changes of 2 - 3 $^{\circ}$ C and that Site 1234 experienced changes of 3-4 $^{\circ}$ C.
PP51F-1370 0800h
Metal Deposition Along the Peru Margin Since the Last Glacial Maximum: Evidence For Regime Change at \sim 6ka
The Peru Margin upwelling zone plays a key role in regulating marine biogeochemical cycles, particularly the fate of nitrate. High biological productivity and low oxygen waters fed into the oxygen minimum zone result in intense denitrification in the modern system, the consequences of which are global in nature. It has been very difficult, however, to study the paleoclimatic history of this region because of the poor preservation of carbonate in Peru Margin sediments. Here we present records of trace metal accumulation from two cores located in the heart of the suboxic zone off the central Peru coast. Chronology comes from multiple AMS $^{14}$C dates on the alkenone fraction of the sediment, as well as correlation using major features of the \delta $^{15}$N record in each core. ODP Site 1228 provides a high resolution, continuous sediment record from the Recent to about 14ka, while gravity core W7706-41k extends the record to the Last Glacial Maximum. Both cores were sampled at a 100 yr resolution, then analyzed for % N, \delta $^{15}$N, alkenones, and trace metal concentration. Analysis of redox-sensitive metals (Mo and V) alongside metals associated with changes in productivity (Ni and Zn) provides perspective on the evolution of the upwelling system and distinguishes the two major factors controlling the intensity of the oxygen minimum zone. The trace metal record exhibits a notable increase in the intensity and variability of low oxygen waters and productivity beginning around 6ka and extending to the present. Within this most recent 6ka interval, the data suggest fluctuations in oxygenation and productivity occur on 1000 yr timescales. Our core records, therefore, suggest that the Peru Margin upwelling system strengthened significantly during the mid to late Holocene.
PP51F-1371 0800h
Sedimentary Characteristics through Glacial/Interglacial Transitions at ODP Site 1233
ODP Site 1233, located shoreward of the Chile Trench on the Chilean continental slope ($\sim$38 km offshore; $41\deg$0.005'S, $74\deg$26.992'W, 838 mbsl), recovered a 136 mcd-thick continuous and extremely high-resolution sedimentary sequence that extends through 70 ka, providing the potential to significantly enhance our knowledge of Southern Hemisphere climates. The sediment collected originates in an area where the erosion and transport of sediment to the continental margin is enhanced by alpine glaciation and heavy rainfall. This study investigates glacial/interglacial sediment variations in through the use of magnetic fabric (anisotropy of magnetic susceptibility, AMS), terrigenous grain size, and flux calculations. Site 1233 consists primarily of homogenous nannofossil clay, nannofossil silty clay, and nannofossil-bearing silty clay. Ninety-one samples were taken at approximately 1.5-meter intervals from a composite splice. On average, one sample was obtained for every $\sim$800 years. Bulk magnetic susceptibility values range from 3.4 x 10$^{-4}$ to 4.6 x 10$^{-3}$ SI with an average of 3.0 x 10$^{-3}$ SI. Values of AMS parameter P' (the strength of the magnetic fabric) range from 1.005 through 1.058, with an average of 1.025 while T values (shape factor) fall almost exclusively within the oblate realm, especially when samples from noted intervals of diagenesis are disregarded. The depositional environment, as indicated by P' and T, plots within the field of low to mid-velocity currents, indicating some current influence at this site. The strength of the magnetic fabric (P') varies through time, with maxima present at $\sim$38 and 13 ka, and may represent fluctuations or changes in the surrounding AAIW. Analyses of terrigenous grain size and flux are on-going and will facilitate further interpretation of the magnetic fabric results.
PP51F-1372 0800h
High-Resolution Accumulation Rate and Authigenic U Variations On The Nazca Rise , Peru Margin, Over the last 25ka
As part of a larger regional study into ocean circulation changes along the Peru Margin over the last 25 ka, we have a developed an excess Th-230 (xsTh-230) record of sediment mass accumulation rates (MAR) and authigenic U (Uauth) at an offshore site (140km off margin) on the Nazca Rise. Over an interval between 7 and 25 ka, these profiles consists of 50 xsTh-230 and U measurements for an average resolution of ~350 yr/sample. This has allowed us to evaluate concerns about sediment redistribution affecting interpretations of other proxies, like $\delta$N-15, as well as make more direct comparisons with shallower marginal sites. The Site RR9702A-69TC (3.2 km water depth) is located at $16\deg$S $76\deg$W and has an average sedimentation rate of 5.4 cm/ka based on AMS C-14 dates and correlation to nearby locations. The xsTh-230 profile combined with estimates of dry bulk density and sedimentation rates indicates that sediment focusing (a surplus of Th-bearing sediments) is about 1.5 to 2x the expected throughout the record. Th-230 data and %CaCO$_{3}$ analyses enabled us to produce detailed records of carbonate and non-carbonate MAR. The Th-normalized detrital MAR (primarily eolian) decreases by a factor of two (1.2 to 0.6 g/cm$^{2}$/ka) across the deglacial (15 to 12 ka). In contrast, Th-normalized CaCO$_{3}$ MAR shows values increasing slowly through the LGM (0.1 to 0.15 g/cm$^{2}$/ka) then decreasing from maximum at 15ka to no Holocene accumulation by 11 ka. In addition to xsTh-230, measurements of Th-232 and Uauth (often interpreted as a total organic carbon (TOC) proxy provide us independent measures of detrital flux and productivity variations, respectively. The Th-232 derived detrital flux is consistent with estimates of eolian accumulation from nearby site ODP Site 1237 and is relatively constant through the Last Glacial Maximum (LGM). Uauth varies from 4 to 10 ppm during the LGM, decreasing through the deglacial to Holocene values of 1 to 2 ppm. However, the Th-232 detrital flux does not co-vary with Uauth concentrations (R$^{2}$ =0.2) . Instead, there is a much better correlation between CaCO$_{3}$ flux and Uauth (R$^{2}$=0.5). This indicates that the variability (factor of 4-5x) observed in Uauth was not caused primarily by dilution by terrigenous inputs.
PP51F-1373 0800h
Surface and Thermocline Variability of the Eastern Equatorial Pacific During the Last 80,000 yr: Results from ODP Site 1240
Recent hypothesis consider low latitudes as the source region for past global rapid climatic changes. In this sense, the Eastern Equatorial Pacific (EEP) is an area with great potential to trigger global changes due to its active role in the El Niño-Southern Oscillation (ENSO). Here we present results obtained from ODP 1240 (Leg 202, 0° 01.31'N, 86° 27.76'W, 2921 mbsl) on paired stable isotopes (d18O, d13C) and trace elements (Mg/Ca) on planktonic foraminifera. Multiple species with different dwelling preferential depths were studied to reconstruct both surface and thermocline characteristics. Core-top Mg/Ca inferred temperatures are in good agreement with current mean annual sea surface temperature (SST) and deep thermocline temperatures (DTT) as recorded from available data from hydrographical stations. Our reconstruction covers the last 80 kyr, including the entire last glacial period, the glacial-interglacial transition and the Holocene. Chronostratigraphy is currently in progress, but initial results evidence strong changes in sedimentary rates. Higher accumulation rates are recorded during the glacial time and deglaciation, likely reflecting changes in the intensity of the upwelling system (cold tongue expansion). In general, sedimentation rates are relatively high for the region (8-15 cm/kyr) which makes Site 1240 particularly valuable for the study of rapid climatic variability. Results are still preliminary but several rapid oscillations of about 1-2C are observed during MIS 3 in both, SST (G. ruber) and DTT (N. dutertrei) records. Thermocline structure also changed in relation to these oscillations. Last deglaciation is seen as a SST (G. ruber) warming of about 4°C, while lower DTTs (N. dutertrei) increase only about 2°C. This early warming preceded the isotopic depletion of the last deglaciation. A middle deglaciation abrupt cooling event of about 3°C (SST) and 1°C (DTT) interrupted the gradual warming and nearly returning to full glacial conditions. During the Holocene two main periods are easily distinguishable in both d18O and Mg/Ca records. Early Holocene is characterized by higher Mg/Ca and relatively high d18O values whereas Late Holocene presents lower Mg/Ca and lower d18O levels.
PP51F-1374 0800h
Late Pleistocene-Holocene Paleoenvironmental Setting in the SE Pacific Inferred From Paleoproductivity Variations
A multiparameter investigation of several sediment cores collected along the Chilean continental margin between $24\deg$S and $44\deg$S served to reconstruct the spatial and temporal variability in the (paleo)environmental setting through the last 40,000 years. The faunal and isotopic composition of planktic foraminifera suggest that marine productivity was relatively high between 40,000 and 32,000 cal yr BP, highest between 32,000 and 16,000 cal yr BP, lowest between 16,000 and $\sim$4000 cal yr BP, and relatively low from 4000 cal yr BP towards the present. The comparatively consistent pattern of planktic foraminifera along $20\deg$ of latitude points to changes in the oceanic circulation affecting the entire study area. Thus, periods of higher paleoproductivity appear during stronger advection and/or a northerly position of the Peru-Chile Current as the dominant oceanographic current off Chile, and vice versa. The relative variations in the paleoproductivity calculated from accumulation rates of organic carbon, opal, and calcium carbonate generally support these findings, although with some local differences mainly during the last deglaciation. We suppose that only a shift in the position of the zonal systems (the Antarctic Circumpolar Current, ACC, and the Southern Westerly Belt, SW) is capable of provoking such nearly synchronous large-scale changes in the marine productivity. The ACC is the main macronutrient source off Chile, and its position is strongly tied to the location of the SW, which contributes the supply of micronutrients to the coast by precipitation onshore and the successive continental runoff. Displacement of these circulation systems in the short run occurs during ENSO events, as observed presently and for the Late Holocene, when changes in the strength of the Hadley Cell and the South Pacific High parallel the movement of the ACC and SW. During the Last deglaciation, our data suggest that the reorganisation of the thermo-haline circulation led to the southward return of the ACC and the SW with the ensuing lowest marine productivity during the Early and Middle Holocene.
PP51F-1375 0800h
the Pliocene Evolution of E-Pacific and Caribbean SST Reveals the Final Phase of the Closure of the Panamanian Gateway.
We used samples from ODP Sites 999 ($12\deg$44N, $78\deg$44W, 2828m waterdepth) and 1241($5\deg$50N, $86\deg$26W, 2027m waterdepth) to investigate the development of the sea surface temperature (SST) gradient between the East-Pacific and the Caribbean for the Pliocene interval 2.2-4.8 Ma. For Site 1241 we constructed new benthic and planktonic $\delta$$^{18}$O-records. The benthic $\delta$$^{18}$O-record was used to construct an orbitally tuned age model to which the age model of Site 999 was tuned. SST was determined by applying Mg/Ca-analyses on the planktic foraminifer Globigerinoides sacculifer. The spatial resolution for the SST-records averages 5 kyr, for the $\delta$$^{18}$O-records 3 kyr. An open gateway, permitting the throughflow of water from the Pacific into the Caribbean, leads to equal SSTs on both sides of the Isthmus. When the gateway reached the final phase of the closure, the Caribbean started to warm up and the E-Pacific cooled down, resulting in an increasing SST-gradient (E-Pacific-Caribbean SST difference). Before 4.6 Ma the Caribbean was slightly warmer than the E-Pacific by about $1\deg$C, suggesting the inflow of Pacific water into the Caribbean. After 4.6 Ma, average SST at Site 999 decreased by about $2\deg$C, while at Site 1241 average SST remained the same. This implies that no longer the same watermass is bathing both Sites, suggesting a cooler watermass, probably originating from the Peru-Chile Current, to penetrate into the Caribbean. The final phase of the closure is indicated by a change in Caribbean-Pacific SST relationships. Positively correlated SST fluctuations persisted from 4.8Ma until about 3.2Ma, before larger-scale sea level fluctuations became important in response to the amplification of the northern hemisphere glaciation. After about 3.2Ma, glacial stages (indicative of sea level low-stand) are associated with Pacific minima in SST (Site 1241) and maxima in Caribbean SST (Site 999). Thus, the cessation of the throughflow during glacials terminated the imprint of cooler Pacific surface waters in the Caribbean and allowed Caribbean temperatures to increase. This documents the final phase of the closure of the Central American Isthmus, when sea level fluctuations on the order of 50-70 m start to control the Pacific water inflow into the Caribbean.
PP51F-1376 0800h
Orbitally Tuned Stratigraphies From 2.5 to 8 Ma at Sites 1237 and 1241
The astronomical tuning technique is at present the most accurate absolute dating method for sediment records spanning the time interval of the last 35 Ma for which astronomers provide a valid and precise orbital solution for variations in Earth's orbital parameters (eccentricity, obliquity, precession). The orbitally tuned geological time scale already became the standard chronology for the last 5 Ma. The late Miocene magnetic Polarity Times Scale (PTS) was tuned by two groups. Hilgen et al. (1995, 2003) used land-based sediment sections and Shackleton et al. (1995) used marine sediment records from Leg 138 to calibrate the PTS. Both time scales, however, differ by about 180 kyr. More recently, Shackleton and Crowhurst (1997) and Shackleton et al. (1999) established a Miocene astronomical time scale at sediment records from Leg 154 (Ceara Rise). The lack of magnetostratigraphic information, however, complicates comparisons with the tuned magnetic polarity timescales. Our goal is to expand and to astronomically calibrate the 'Magnetic Polarity Time Scale', the oxygen isotope stratigraphy and biostratigraphy to 12 Ma at east Pacific Sites 1237 and 1241. So far, we developed an oxygen isotope stratigraphy from 2.5 to 6 Ma at Site 1241 that was based on tuning cyclic fluctuations in benthic d13C values and sand percentages of the carbonate fraction to orbital obliquity and precession, respectively. The isotope stratigraphy is nearly identical with that from Leg 154 (Sites 925/926). Site 1237 was orbitally tuned from 2 to 6.5 Ma. The tuning was mainly based on the GRAPE-density record although other proxy records were considered (magnetic susceptibility, color, carbonate sand fraction, isotopes). The astronomically calibrated ages for geomagnetic reversal boundaries are almost identical with those from the Cande and Kent timescale for the time interval from 2 to 5.2 Ma. Although geomagnetic reversal boundaries are not well defined between 5.2 and 6.5 Ma, our tuning suggests considerable differences to earlier APTS. Orbital tuning at Site 1237 suggests ages for late Miocene magnetic reversal boundaries to be about 150 kyr older than those from the Cande and Kent time scale and to be about 80 kyr older than those from Hilgen et al. (1995).
PP51F-1377 0800h
Orbital and suborbital variability in Coccolithophore assemblages in the eastern tropical and equatorial Pacific Ocean (7°N to17°S) for the last 500 ka (ODP Leg 202)
Ocean Drilling Program Leg 202 covered a South-North transect in the eastern Pacific Ocean, providing excellent material to reconstruct variations in surface and subsurface water dynamics in selected time-slices. The main goal here is to investigate the evolution of coastal and equatorial upwelling systems and their relationship to global and/or regional climate processes such as El Nino or related features. Based on a preliminary age-model using calcareous nannofossil-calibrated datums, a quantitative analysis in the Coccolithophore assemblages was carried out to investigate orbital variations in surface water dynamics during the last 500 kys. Here we present data from four ODP sites: Site 1237 situated in the west South America upwelling system; Sites 1240 and 1241 in the equatorial Pacific upwelling system, and Site 1242 close to the Costa Rica margin. The ratio between the shallow and deep coccolithophore dwellers is used to reconstruct fluctuations in the nutricline/thermocline position, and consequently, monitor variations in the upwelled waters. Our preliminary results show a orbital periodicity (ca. 100, ca. 40 and ca. 20 ka periodicity), together with an obvious millennial signal in the above mentioned proxy. Moreover, other well-defined patterns are overprinted in all sites, allowing us to differentiate three intervals without a clear orbital relationship. Periods of well defined upwelling alternate with other intervals where surface and subsurface waters appear better stratified. This model seems to respond to regional and almost synchronous variations linked to variations in like-El Nino-La Nina periods of orbital range.
PP51F-1378 0800h
The Terrestrial Paleoclimatic Record of the Late Quaternary as Revealed by Drilling Lake Titicaca, Peru/Bolivia
Seven drill cores were recovered from Lake Titicaca during the NSF/ICDP/DOSECC drilling expedition of 2001; our most detailed multi-proxy analyses have been done on Core 2B raised from 232 m water depth in the central basin of the lake. This site was drilled to 139 mblf with 141 m of total sediment recovered (101%). The recovered sediments consist of two main lithologies, organic- and inorganic-carbon-rich (often-laminated) muds that alternate with detrital-rich muds. These lithologies represent respectively low and high lakestand deposits. Proxies for water level include planktic-to-benthic diatom ratio, sedimentary carbonate content, and stable isotopic ratio of organic carbon. There are six highstand intervals separated by five lowstand intervals indicating that the level and volume of Lake Titicaca underwent several large changes during the late Quaternary. We infer from high values of magnetic susceptibility in most highstand muds that glacial advances in the surrounding Andes coincided with periods of relative wetness. During the most recent lowstand, in the early and middle Holocene, Lake Titicaca fell to 85 m below its modern level, salinity increased several-fold, and the downstream Salar de Uyuni desiccated. By contrast, throughout the LGM from ca. 25,000 cal BP to 15,000 cal BP, Lake Titicaca was deep and fresh, and overflowed southward to the Salar de Uyuni. Prior to the LGM, back to ca. 53,000 BP, the lake was predominantly fresh and overflowing. Pulses of increased benthic diatom abundance and inorganic carbon concentration during that time were likely due to episodes of downslope transport. We believe (based on U-Th dates of authigenic carbonate layers) that the penultimate lowstand of Lake Titicaca (seismic evidence indicates a lake level 200 m lower than today) was coincident with MIS 5. We recovered sediments recording three older lowstands, each separated by periods in which the lake freshened dramatically and when glaciers apparently advanced in the Andes. We tentatively correlate these lowstands with MIS 7, 9, and 11. The high abundance of benthic diatoms in the lowermost 40 m of drill core may imply that the Altiplano was persistently drier than today, prior to and including MIS 9. Alternatively, basin morphometry may have been significantly different than today because of geomorphic or tectonic effects. This period apparently coincides with a period when the Salar de Uyuni was mostly dry and dominated by salt deposition.
PP51F-1379 0800h
Continental and marine climate records from the Southeast Pacific as inferred from Radiolaria and pollen records from 0 to 90ka at ODP Site 1233
Site 1233 drilled during Leg 202 of the Ocean Drilling Program provides a detailed record of oceanographic and continental climate change in the Southeast Pacific and South American continent. Splits from over 500 samples from Site 1233 were used for detailed radiolarian and pollen species population analysis. Samples were taken at 20cm interval equivalent to temporal resolution of 200 to 400 years. In each sample splits used for pollen analysis abundances of 25 species were determined while in the splits used for radiolarian studies abundances of 41 species were evaluated. Age control is provided by 25 AMS 14C dates (Lamy et al., 2004). From detailed multivariate analysis of these data sets we state the following conclusions: 1) During the past 75kyr the region of the southern Chile coast is not directly influenced by polar water from the Antarctic region. 2) Changes in ocean conditions during this time interval reflect small north-south shifts in the south Pacific transition zone and its impact on the coastal waters of Chile. 3) The ocean and terrestrial climate records extracted from Site 1233 show remarkable similarities to each other as well as to temperature records from the Antarctic. These records suggest that climate variability during the past 75kyr in the Southeast Pacific is tightly coupled at periods longer than 3000 years and not linked directly to changes in the Northern Hemisphere and; 4) the marine and terrestrial climate records from Site 1233 studied here, show no phase shift in response suggesting that ocean/continental vegetation is much more tightly coupled with regional atmospheric changes as indicated by the Antarctic temperature proxy record while the results of Lamy et al., (2004) show a time lag between ocean responses and the Patagonian Ice Sheet suggesting possibly reflecting the long response time of ice versus continental vegetation and oceanic systems.