PP23A-1388 1340h
10Be dating of late-glacial moraines near the Cordillera Vilcanota and the Quelccaya Ice Cap, Peru
The surface exposure method, based on the measurement of cosmogenic 10Be produced in quartz, is applied to determine the age of deposition of glacial moraines near the Cordillera Vilcanota and the Quelccaya Ice Cap (about 13° S, 70° W) in southeastern Peru. These data are useful for examining the timing of past glaciation in the tropical Andes and for comparison with chronologies of glaciation at higher latitudes. The preliminary data set consists of more than ten surface exposure ages. Samples used for dating are from the surfaces of boulders on a set of prominent moraines about four kilometers away from the present ice margins. The age of the moraine set was previously bracketed by radiocarbon dating of peat associated with the glacial deposits. Based on radiocarbon ages, these moraines were formed during the late-glacial period, just prior to the last glacial-interglacial transition. The surface exposure dating method enables the direct dating of the moraines. Surface exposure dates are cross-checked with the previously existing radiocarbon dates and provide a means to improve the chronology of past glaciation in the tropical Andes.
PP23A-1389 1340h
Evidence from Rodent Middens for Summer Rainfall Variability over the Last 22,000 Years from Northern Chile's Rio Salado (22§ S)
The timing and relative contributions of precipitation versus temperature changes is crucial to understanding the tropical hydrological cycle over the central Andes since the Last Glacial Maximum. Despite its location on the lee side of the Andes on the western margin of the tropical rainfall belt, the Rio Salado basin receives almost 90% of its scant precipitation during the summer months, depending on upper-air conditions that either favor or suppress transport of moist air masses from the Amazon and Gran Chaco Basins across the Altiplano onto the Pacific slope. Previous results from the central Atacama Desert indicate that century to millennial-scale variability of summer rainfall have been shown to produce dramatic elevational displacements in plant distributions. Plant macrofossils were analyzed in 35 rodent middens from the lower Rio Salado basin that span the last 22,000 calendar years (22 ka). Middens were collected at three different localities, two from between 2950-3050 m with almost exclusively riparian vegetation with sparse low shrubs, and one between 3100-3150 m with diverse dry slope plant communities. Based on the presence of steppe grasses (some of which are found several degrees further north today) and high species richness, we infer wet phases between 17.4-16.4, 11.2-9.6, 7.4-6.7, and 0.7-0.5 ka. Dry phases, characterized by low diversity and/or dominance of riparian species, occurred at 21.9, 14.2 and 4.8-2.3 ka. Summer precipitation variability inferred from Rio Salado middens generally agrees with other midden and lake records throughout the central Andes between 22-10 ka. Increased rainfall inferred from middens dated between 7.4-6.7 ka is in conflict with high Andean lake records which show evidence of severe drought beginning at 8 ka. Increased temperature at the end of the Pleistocene, however, was followed by colonization of the high Andes by considerably more productive plant communities. This may have had a major impact on lake hydrology budgets through increased evaporation, evapotranspiration and decreased surface runoff. Thus, part of the discrepancy between paleoclimate records in the central Andes during the early to middle Holocene could stem from confounding climate factors. Funding: FONDECYT 3030062, NSF-ESH, ICM P02-051-FICM, and CASEB
PP23A-1390 1340h
Large Temperature Variability in a Southern Tropical Continental Setting Through the Last Glacial Maximum
The magnitude and timing of temperature change since the Last Glacial Maximum (LGM) on the tropical continents is becoming better understood as the box of independent paleotemperature tools for continental systems expands. Here we present a paleotemperature record derived from the application of TEX$_{86}$ to lacustrine sediments for Lake Malawi, East Africa through the LGM. We find a $\sim$4 $\deg$C overall warming since the LGM, with temperature reversals of more than 2 $\deg$C during the Younger Dryas (12.5 ka BP) and in the early Holocene. The warmest Holocene temperatures in this part of Africa appear to have occurred around 5000 years ago. The onset of warming in the Lake Malawi basin coincides with the BYRD oxygen isotope record of warming in Antarctica. While the range of temperatures observed in this study is not surprising, the timing of post-glacial warming, the thermal response to the YD, and the Holocene history of warming and cooling trends are providing important new insights into tropical climate dynamics on centennial to millennial scales.
PP23A-1391 1340h
A Mechanism For Amplifying Obliquity Forcing In Low-Latitude Oceans During The Pleistocene
Since the introduction of Milankovitch's astronomical theory of the Ice Ages, Earth's orbital fluctuations have been suspected of driving Pleistocene climate variability. All three orbital parameters, eccentricity, obliquity, and precession, are directly linked to climate change through their influence on the solar insolation received by Earth. By altering the annual meridional distribution of insolation on Earth, obliquity is potentially important to the climate system. In comparison to times of low obliquity (i.e., axial tilt of 22.2°), at times of high obliquity (i.e., axial tilt of 24.5°) mean-annual insolation at high latitudes is 1.5% greater at 65° and 0.4% less at the equator. Despite the very small change in insolation at low latitudes, early Pleistocene climate indices of low-latitude eastern Pacific sea-surface temperature and total alkenone abundance demonstrate a 41 k.y. cyclicity. Using a coupled ocean-atmosphere model (the Fast Ocean-Atmosphere Model), we have identified an oceanic mechanism that amplifies the obliquity signal in the low-latitude Pacific. In response to an increase in axial tilt, subsurface waters in the eastern equatorial Pacific increase by up to 1 °C in the model. Using a Lagrangian transport model, we determine that ventilation of warm subtropical waters, heated by enhanced insolation during periods of high axial tilt, is the primary cause of the low-latitude subsurface warming. In contrast to subsurface waters, sea-surface temperature changes are consistent with variations in local insolation; surface temperatures cool at low-latitudes and warm at high latitudes. This subsurface ocean mechanism is consistent with early Pleistocene proxy data and explains the anti-phase relationship between obliquity insolation forcing and seawater temperature in the eastern tropical Pacific.
PP23A-1392 1340h
African Climate Change Leads Deglaciation at Termination II
The onshore lake-sediment sequence exposed in the Ol Njorowa Gorge south of Lake Naivasha in the Central Kenya Rift together with previously published sediment cores from the modern lake area represents a unique record of East African climate during the last two glacial-interglacial cycles. Based on sediment characteristics, diatoms, authigenic mineral assemblages and 17 single-crystal 40Ar/39Ar age determinations we found that Lake Naivasha was subjected to dramatic water level and hydrochemical changes during the last 175 kyrs. Assuming that the fluctuations recorded in the sediments reflect climate changes, the Lake Naivasha record demonstrates that between 146 and 60 kyr BP, periods of increased humidity in East Africa mainly followed maximum equatorial solar radiation in spring causing periods of more intense April-May rains every 23 kyr similar to the present. Interestingly, the most dramatic change in the Naivasha Basin occurred as early as 146 kyr BP and the highest lake level was recorded at about 139 to 133 kyr BP. This is consistent with other low-latitude climate records, but does not correspond to peaks in Northern Hemisphere summer insolation as the trigger for ice-age cycles. The Naivasha record therefore provides further evidence for a low-latitude forcing of the ice-age climate cycles. The new "Lake Naivasha Coring Project" now aims to test the hypothesis that East African climate also leads at Termination I (see abstract by Bergner et al., this session).
PP23A-1393 1340h
Detrital Sediment Supply And Late Quaternary Environmental Changes Off Taiwan, ODP Site 1202
During Leg 195 of the Ocean Drilling Program, Site 1202 was drilled in the subtropical northwestern Pacific Ocean beneath the Kuroshio ("Black Current") between northern Taiwan and the Ryukyu Island Arc on the northern flank of the Ilan Ridge at 1275 m water depth. The ridge separates the southwestern end of the Okinawa Trough from the basins of the Philippine Sea. The upper 120 m of the Site 1202 section, composed of dark grey calcareous silty clay, provide a high-resolution record of the last 30 kyr, deposited at high sedimentation rates between 3.0 and 5.0 m/kyr and peak values of 9,0 m /kyr between 15.5 and 11 ka. Variations in the modes and sources of detrital sediment input, as inferred from sediment granulometry, mineralogy, and element-scanner data, reflect complex changes in environmental boundary conditions related to postglacial sea-level rise, monsoon climate, tectonics, and Kuroshio variability. Between 30 and 11 ka, low portions of sortable silt show that the Kuroshio did not enter the Okinawa Trough because of low sea level. Clay-mineral tracers point to increased sediment supply from mainland China by the discharge of Yangtze sediments directly across the East China Shelf edge into the Okinawa Trough. Consistent with maximum sedimentaion rates, Yangtze discharge reached a maximum between 15.5 and 11.0 ka. High fluvial sediment supply apparently was triggered by both postglacial sea-level rise and strengthened summer monsoon in response to precessional forcing of insolation. The Holocene was dominated by sediment supply from Taiwan since 11 ka, as indicated by clay-mineral tracers and geochemistry. High proportions of sortable silt point to strong bottom currents, mediated by sea-level rise and enhanced Kuroshio activity at Site 1202. Element-scanner data reveal the presence of 200-yr cycles, possibly related to short-term climate perturbations.
PP23A-1394 1340h
Southern Hemisphere mid-Latitude Sea Surface Temperature Changes 0-150,000 years BP: Low- {\it Versus} High-Latitude Orbital Forcing
The climatic and oceanographic history of the Southern Hemisphere, particularly the mid-latitudes, is still largely unknown and the understanding of mid- to high-latitude climate coupling consequentially incomplete. We have generated three new alkenone sea surface temperature (SST) records from mid-southern latitudes that reveal differential subtropical and subantarctic climatic changes over the past 150,000 years. Subantarctic SST variability mirrors changes in global ice volume that are linked to Northern Hemisphere summer insolation. Subtropical SST, on the other hand, show a strong eccentricity modulation of the precession cycle and warmer conditions during glacial stage 6 than during the last glacial maximum. Consistent subtropical warming between 20-45 ky coincides with an eccentricity minimum and a decrease in orbital precession (Northern Hemisphere insolation maximum). These patterns highlight the impact of orbital eccentricity on climate through the modulation of the precession cycle at low latitudes. The apparent differential response of subtropical and subantarctic summer SST to changes in earth's orbital parameters has implications for the meridional temperature gradient and hence changes in atmospheric circulation and moisture transport to high latitudes.
PP23A-1395 1340h
Do Tropical SST Changes Lead High Latitude Climate Change, Or Are Our Proxies Misleading Us?
Initial reconstruction of the last glacial maximum (LGM) sea surface temperature (SST) estimates showed little to no change in the world's tropical oceans. More recently, several studies have shown that the tropics were 1 to $6\deg$C cooler during the LGM and previous ice ages. These observations, as well as the timing of the SST changes, has led some researchers to hypothesize that the tropical Pacific may have played an important role in glacial/interglacial (G/IG) climate change during the late Quaternary. Uncertainty remains about the magnitude of the SST change, the spatial pattern of the changes, and the timing of tropical SST changes relative to high latitude climate transitions. For example, in the eastern equatorial Pacific (EEP) geochemical proxy data indicate that during the LGM SST was only 1 to $2\deg$C cooler at the equator , but was 2 to $3\deg$C cooler just north of the equator and in the central equatorial Pacific compared to today. Additionally, the sites on the equator record a significantly larger lead in SST compared to ice volume than one site just north of the equator. These records have been reconstructed using different SST proxies however, leading us to question if the observed geographical differences in the magnitude and timing of SST changes during recent glacial cycles could be explained by the different uncertainties inherent in each of the proxies. The EEP is a critical region because SST in this region is sensitive to changes in upwelling, and is therefore indicative of the state of the tropical Pacific, which has known air sea feedbacks that can affect global climate. A clear picture of the timing of SST changes and ice volume is needed if we are to understand the potential role of the tropical Pacific in G/IG climate change. We have generated high resolution ($\sim$2-4 k.y.) SST records at ODP site 847 ($0\deg$12'N, $95\deg$19'W, 3346m water depth) using the U$^{k'}$$_{37}$ and Mg/Ca paleothermometers extending back 500 k.y. This site provides an excellent opportunity to compare Mg/Ca and alkenone records, as it is located above the lysocline, thereby minimizing the effect of dissolution on Mg/Ca in foraminifera, and has relatively high organic matter content. The U$^{k'}$$_{37}$ SST record shows a G/IG amplitude of $\sim$$2\deg$C through MIS 8, and a larger ($\sim$5-$6\deg$C) amplitude from MIS 9-12. SST leads ice volume changes at all glacial to interglacial transitions, but the magnitude of the lead varies. Comparing the variability in these two proxies at this site over the last ~500 k.y. will increase our understanding of the behavior of these two proxies in this region, and dramatically increase our confidence in our climatic interpretations.
PP23A-1396 1340h
Early Onset and Origin of 100-kyr Cycles in a Tropical Pleistocene SST Record
The large 100-kyr cycles evident in most late-Pleistocene paleoclimatic records still lack a satisfactory explanation. Many climatic records show that the period of Pleistocene glacial cycles changed from 41-kyr, dictated by orbital obliquity, to 100-kyr during the mid-Pleistocene transition (MPT). Careful documentation and consideration of how this transition occurred may help resolve the puzzle of late Pleistocene 100-kyr cycles. One hypothesis is that strengthened semi-precession cycles (about 10-kyr) originating in the tropics propagated to high latitudes around 1.5 Ma and triggered the sustained 100-kyr glacial cycles. A 2-Myr SST record from ODP Site 846 in the eastern equatorial Pacific (EEP) indicates that an early onset of 100-kyr cycles did occur in the tropics. However, the record also provides an alternative view of the MPT. Substantial spectral power near the 100-kyr period occurs in early Pleistocene tropical, but not extra-tropical SST records. This spectral power is derived from the subharmonics (multiple integers, about 120-kyr and 80-kyr) of obliquity cycles. In addition, as shown in evolutionary SST spectra, the two subharmonics converge into the late-Pleistocene pseudo 100-kyr period. Therefore, we argue that the subharmonics of obliquity cycles in the early Pleistocene represent a prototype of the late Pleistocene 100-kyr cycles, and that the pseudo 100-kyr cycles are a nonlinear response to orbital obliquity forcing. This view can potentially resolve some existing Pleistocene climate puzzles: (1) the transition from 41-kyr to 100-kyr periodicity during the MPT, (2) the dominance of late-Pleistocene 100-kyr cycles, and (3) weakened late Pleistocene 41-kyr power in some climate records, despite the expected ice-albedo amplification of high latitude insolation forcing.
PP23A-1397 1340h
High Latitude and Tropical Climates Linked Prior to the Onset of Northern Hemisphere Glaciation: Evidence From the Eastern Tropical Pacific
The last major climatic transition in Earth's history occurred between the Pliocene and Pleistocene epochs. This transition witnessed a substantial increase in the variance of Earth's climate in response to changes in orbital obliquity at $\sim$3 Ma, approximately coincident with the onset of Northern Hemisphere glaciation. A recent theory suggests that the start of this transition also marks a fundamental shift in the ocean's heat budget from a state of local equilibrium prior to $\sim$3 Ma, to a state in which heat absorbed in low latitude upwelling zones is balanced by heat removed at high latitudes after $\sim$3 Ma. In this remotely balanced state, changes in the distribution of high latitude insolation due to changes in orbital obliquity can be communicated from the high to low latitudes via the thermocline. To evaluate this theory, we use the alkenone organic proxy on sediments from Ocean Drilling Program (ODP) Site 846 ($3\deg$S, $91\deg$§W) in the Eastern Equatorial Pacific (EEP), documenting orbital scale variations in sea surface temperature (SST) and productivity in the largest upwelling zone on Earth over the past 5 Myrs. Our U$^{k'}$$_{37}$ temperature record shows that SST decreased by $\sim$$1\deg$C/Myrs from a high of $28\deg$C in the early Pliocene to a low of $20\deg$C in the late Pleistocene. A dramatic increase in paleoproductivity occurred $\sim$3 Ma followed by a sharp decline $\sim$1.5 Ma. Evolutive spectral analysis of sea surface indices and benthic oxygen isotope data from ODP Site 846 show that before 1 Ma all three proxies were dominated by obliquity variations and that these variations were modulated by the 1.2 Myr obliquity envelope. Evolutionary cross-spectra further indicate that in the obliquity band, the phase relationships between these three climatic indices remain essential constant throughout the entire 5 Myr interval, with EEP SST and paleoproductivity essentially in phase and both of these proxies leading benthic $\delta$$^{18}$O by $\sim$4 kyrs. The constancy of phase between SST, paleoproductivity, and ice volume implies that the physical mechanisms governing the climate of the EEP have persisted for at least the past 5 Myrs and that the high latitudes and the tropics were linked at least 2 Myrs before the onset of Northern Hemisphere glaciation. These results do not rule out a major reorganization of the sources and sinks of the Earth's heat budget, but suggest that if it occurred, it took place prior to 5 Ma. However, the EEP was not entirely unaffected by the onset of Northern Hemisphere glaciation. The major increase in productivity and cooling of SST at $\sim$3 Ma suggest a dramatic strengthening of the EEP upwelling system at this time.
PP23A-1398 1340h
Orbitally-paced paleoproductivity variations in the Timor Sea and Indonesian Throughflow variability during the last 460-ky
A high-resolution ($\sim$ 1-2 ky) multi-proxy record from the Timor Sea in the easternmost Indian Ocean (IMAGES Core MD01-2378, Lat. 13$ \deg$ 04.95' S, Long. 12$ \deg$ 47.27' E, 1783 m water depth) closely tracks changes in intermediate water ventilation and paleoproductivity over the last 460 ky within one of the main outflow passages of the Indonesian Throughflow. Spectral analysis of five different types of flux-based productivity proxies indicates spectral power concentrated in the 100 ky (glacial-interglacial) and 23 ky and 19 ky (precessional) periods. The productivity indicators are coherent and in phase with the precession controlled equatorial winter insolation and generally lead the benthic $\delta$$^{18}$O record by 2-3 ky. At terminations, rising sea-level (driven by high latitude forcing) and decreasing NW monsoon (driven by precessional forcing) probably triggered an intensification of the Indonesian Throughflow, stabilizing the upper water column and prohibiting vertical mixing. Thus, productivity fluctuations in the Timor Sea over the last 460 ky were strongly influenced by monsoonal flow patterns in the eastern equatorial Indian Ocean (23 and 19 ky) and were also modulated by the intensity of the Indonesian Throughflow (100 ky).
PP23A-1399 1340h
Delayed onset of the South American Monsoon during the Last Glacial Maximum
Regional climate model simulations support geological evidence that the Amazon basin was drier at the time of the Last Glacial Maximum (LGM). The model grid spacing of 60 km reveals the regional dependence of precipitation patterns with, for example, wet LGM conditions in some parts of the high Andes. The primary cause of drying throughout the Amazon Basin is a 2-3 month delay in the onset of the summer monsoon, in association with cool sea surface temperatures in the tropical Atlantic. Once the monsoon is finally triggered, LGM rainfall rates are similar to those of the present day. A shorter, but equally intense, South American monsoon during the LGM is consistent with geological proxy data that indicate that the Amazon rainforest persisted through the glacial period but with modified species populations. Evidence from the charcoal record of more fires and higher atmospheric dust loading is also consistent with the simulations of an extended dry season in South America during the LGM.
PP23A-1400 1340h
Geochronology of Tropical Alpine Glaciations From the Cordillera Huayhuash, Peru
The Cordillera Huayhuash of the Central Peruvian Andes (10.3$\deg$S, 76.9$\deg$W) is an ideal range to study regional climate signals and variations in paleo-ice volumes. Located between the Cordillera Blanca to the north and the Junin region to the south, the range trends nearly north-south with modern glaciers confined to the high peaks ($ > $4800 m). Cross-cutting relationships, geomorphology, and correlation with surface exposure dated moraines in the nearby Cordillera Blanca suggest the region preserves a rich record of tropical glaciation. In order to determine the glacial chronology we mapped and dated glacial features of the Jahuacocha valley (which drains the western side of the range) and two eastern drainages, the Mitococha valley, and the Carhuacocha valley. At each locality we used ASTER data, aerial photographs, and GPS to map glacial features both within main valleys and tributaries. We sampled quartz-bearing erratics on moraine crests as well as ice-polished bedrock surfaces for exposure age dating using in situ produced cosmogenic $^{10}$Be and $^{26}$Al. In the Jahuacocha valley, the greatest ice extent reached an elevation of $\sim$4090m and moraine crest boulders yield and age of $\sim$11.2 $\pm$0.6 ka suggesting a significant late Glacial ice advance or stillstand. A younger cluster of moraines exists $\sim$1 km up-valley at an elevation of $\sim$4100m. These moraines, dated at $\sim$8.0 $\pm$1.0 ka, suggest an early Holocene advance. In the Mitococha valley, a young moraine and polished bedrock dated at $\sim$0.2 ka and $\sim$11.4 $\pm$0.4 ka respectively span the late Glacial through recent. The late Glacial features of this eastern drainage occur at an elevation of $\sim$4100m while the recent events occur at an elevation of $\sim$4380m. Our preliminary results suggest that all three valleys experienced a very similar glacial history with minor differences likely due to the variations in valley morphology. Comparing the chronology of glaciation in the Cordillaera Huayhuash with that in regions to the south and north will provide a means of evaluating the degree of synchroneity of glaciation and climate change across 5$\deg$ of latitude in the tropics.
PP23A-1401 1340h
Response of East African climate to orbital forcing during two glacial-interglacial cycles: The Lake Naivasha Coring Project
High-quality chronologies of late Pleistocene tropical climate have become increasingly important in discussions concerning tropical forcing of deglaciation, i.e., the transition from a glacial to an interglacial. The key argument of this hypothesis is that tropical climate leads high-latitude ice regression by several thousand years. A tropical forcing of deglaciation would also help to explain why ice ages occur in both hemispheres simultaneously, although the changes in solar irradiance from orbital variations have opposite effects in the two hemispheres. The Central Kenya Rift provides a unique opportunity to study continuous records of tropical climate changes during the last two glacial-interglacial cycles (approximately 150 kyr) through sedimentological and paleoecological changes documented in lake sediments. We have assessed the amplitude and timing of East African climate change during Termination II from Ar/Ar-dated onshore lacustrine deposits in the Lake Niavasha basin. Our results show that local precipitation increased by up to 30% already between 145-135 kyr BP. Now we track similar environmental changes at Termination I using two 40-kyr-old sediment cores from Lakes Naivasha and Nakuru. The new sediment records are expected to prove the hypothesis that hydrological changes in East Africa follow early warming of the tropics due to strengthen of local solar radiation.